Investigation on Water Vapor Adsorption of Silica-Phosphonium Ionic Liquids Hybrid Material - MDPI

 
materials
Article
Investigation on Water Vapor Adsorption of
Silica-Phosphonium Ionic Liquids Hybrid Material
Cancan Li, Jiamei Zhu *, Min Zhou, Shuangquan Zhang and Xiaodong He
 School of Chemical Engineering and Technology, China University of Mining & Technology,
 Xuzhou 221116, China; 17851145012@163.com (C.L.); zmcumt@126.com (M.Z.); cumtzsq@126.com (S.Z.);
 researchcumt2017@126.com (X.H.)
 * Correspondence: zhujiamei1991@cumt.edu.cn; Tel.: +86-516-83591109
                                                                                                      
 Received: 7 May 2019; Accepted: 30 May 2019; Published: 1 June 2019                                  

 Abstract: Adsorption and diffusion of water vapor in phosphonium ionic liquid modified silica gel
 were studied, aiming to reduce the loading of water vapor in porous materials. The modified silica
 gel was prepared through a grafting method and characterized by FTIR, thermal gravity analysis and
 X-ray photoelectron spectroscopy. N2 sorption isotherms at −196 ◦ C and CO2 sorption isotherms
 at 0 ◦ C were also measured to analyzee the porosity. Water vapor adsorption equilibriums at 25 ◦ C
 up to 30 mbar were tested. The results indicate that the ionic liquids (ILs) phase acts as a protecting
 film which decreases water vapor adsorption. The improvement of water-resistant performance is
 also attributed to the decrease of micro-porosity and silanol groups on the silica surface. Diffusion
 behavior of water vapor on modified silica was determined on the basis of the adsorption equilibrium.
 The effective diffusivity of water vapor in modified silica is almost the same as in bare silica and
 decreases with the increasing of water vapor loading.

 Keywords: ionic liquids; water vapor; modified silica; adsorption

1. Introduction
     Ionic liquids (ILs) are salts with a melting point below 100 ◦ C, which are commonly composed
of organic cations and inorganic or organic anions, typically. ILs have been considered as promising
solvents in separation [1–3], chemical reaction, or catalytics, due to their unique properties, such as
low volatility, chemical stability, high gas selectivity, and tunable properties [4,5]. A lot of research into
imidazolium-based ILs have been reported [6–8], while phosphonium-based ILs are currently appear
more attractive, owing to their unique advantages in many aspects. Phosphonium salts are generally
more thermally stable and have lower densities than nitrogen-based ILs [9,10]. Moreover, compared
with imidazolium and pyridinium ILs, phosphonium ILs have no acidic proton or aromatic ring,
which makes them stable towards nucleophilic and basic conditions [11,12]. In addition, the kinetics
of phosphonium salt formations are much faster than those of nitrogen-based salts, implying higher
productivity and less expense than their equivalent imidazolium ILs [13–15]. Based on the characteristics
of ILs, there have been many studies on IL-modified porous materials, such as activated silica, carbon,
and alumina for CO2 adsorption. Zhang [16] showed that CO2 capture capacity increased markedly
for grafting amino-functionalized phosphonium ILs ([aP4443 ]AA) onto silica gel than in a bulk liquid
and bare support. Balsamo [17] reported that supported [Emim][Gly] ionic liquid on mesoporous
gamma-Al2 O3 at different loadings had better CO2 sorption capacity. These experiments reveal
that CO2 capture performance is greatly enhanced owing to the fact that ILs were supported on the
porous support.
     A noticeable influence of water on the physical characteristics of adsorbents has been also reported,
due to the presence of a small amount of water vapor (8–15%) in the power plant flue gas, which tends

Materials 2019, 12, 1782; doi:10.3390/ma12111782                                www.mdpi.com/journal/materials
Materials 2019, 12, 1782                                                                             2 of 13

to significantly reduce CO2 adsorption capacity [18–20]. Because of the competition adsorption of
CO2 and H2 O, the adsorbed water occupies a part of adsorption sites, which is unfavorable for
CO2 adsorption. Li [21] studied that CO2 adsorption of activated alumina strongly decreased with
increasing water content in the vapor-gas. Xu [22] investigated the effect of water on CO2 adsorption
properties in a commercial activated carbon, and cyclic adsorption experiments with wet feed gas
showed that the CO2 adsorption capacity of activated carbon was negatively affected by the presence
of water. Uehara [23] studied CO2 sorption behaviors of supported amino acid ionic liquid [EMIM]AA
sorbents on poly(methyl methacrylate) (PMMA) under a humidified gas flow condition (2.3% CO2 ,
2.1% H2 O and balance N2 ), and the results showed that CO2 sorption capacities of [EMIM][Gly]- and
[EMIM][Lys]-PMMA decreased in the presence of water vapor as the adsorbed water inhibited their
reaction with CO2 .
      Meanwhile, the water vapor adsorption performance of the pores adsorbent, especially silica gel,
has been investigated extensively. Alcañiz-Monge [24] reported that water adsorption at low relative
pressures occurred mainly in the micro-porosity of silica. Saliba [25] also showed that the adsorption
of water vapor on porous silica depended upon the concentration of surface hydroxyl groups (silanol)
and the porosity. At present, it has been reported that the water vapor adsorption capacity was affected
by the pore size and surface hydroxyl groups, but little research has been done to evaluate the surface
properties and pore structure of silica gel modified by ILs and different pretreated methods for the
supporter, which shows further influence of water vapor adsorption and its water vapor adsorption
performance. Thus, we considered surface-modifying and regulating the pore size of silica to be an
effective way of controlling the amount of adsorbed water vapor in competitive adsorption.
      The water hold-up needs to be addressed in adsorption processes under humid conditions, as this
can reduce the adsorption capacity for CO2 . Based on this starting point, in this work, phosphonium
ILs, especially hydrophobic ionic liquids, were chosen to be supported on pretreated silica to prevent
the water vapor entering effectively. According to the literature [18–23], the surface performance and
porosity of porous materials are main effects for competitive adsorption of H2 O and CO2 . We assume
that pore structure and surface hydroxyl content of silica gels functionalized with ILs will be changed,
which can effectively play the advantages to reduce water vapor sorption capacity of porous materials,
aiming to prepare a new adsorbent with better solid supported ILs which achieve better water-resistance
performance for being more suitable for post-combustion CO2 capture applications. Simultaneously,
the diffusion of water vapor in IL-modified silica was investigated in order to characterize the influence
of the carrier surface and porosity.

2. Materials and Methods

2.1. Materials
      Tri-n-octylphosphine (purity: 97%) was purchased from Stream Chemicals, Inc. (Cambridge, UK).
Bistrifluoromethanesulfonimide lithium (purity: 99%) was purchased from J&K Scientific (Beijing, China).
3-Chloropropyltriethoxysilane (purity: 97%) was purchased from Beijing Fengtuo Chem Co., Ltd., Beijing,
China. Toluene (dried by Na), acetone, and HCl were all analytical grade. All chemicals were purchased
from a commercial supplier and used as received. Silica gel with 0.5–1.5 mm in particle size was purchased
from Shanghai Silica Gel Factory, Shanghai, China. Silica was pretreated before use.

2.2. Synthesis of grafted silica gel
     The pretreated methods of silica affect the pore structure and hydroxyls of the silica surface,
thus influencing water vapor sorption capacity. In this study, two different treatement methods were
used. Silica was calcined at 150 ◦ C for 3.5 h (coded as SiO2 -C) or acid treated with hydrochloric acid
(8 mol/g) at 70 ◦ C for 12 h (coded as SiO2 -H). It was then washed with distilled water until neural, and
dried at 353 K under vacuum for 8 h before further use. The pretreated silica is referred to as bare silica
in the following text.
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       Materials 2019, 12, x FOR PEER REVIEW                                                                              3 of 14
      The preparation of phosphonium IL-modified silica using grafted method was carried out,
accordingThe   to apreparation    of phosphonium
                     previous report        [26]. In thisIL-modified   silica using
                                                             work, another        kindgrafted     method
                                                                                          of silica    with was     carried
                                                                                                               different      out,
                                                                                                                            pretreated
      according to a previous report [26]. In this work, another kind of silica with different pretreated
methods was selected for further controlled amounts of IL. The general procedure is as follows.
      methods was selected for further controlled amounts of IL. The general procedure is as follows. Tri-
Tri-n-octylphosphine was added to 3-chloropropyltriethoxysilane and anhydrous toluene and stirred
      n-octylphosphine was added to 3-chloropropyltriethoxysilane and anhydrous toluene and stirred
underunder
        nitrogennitrogen    110at◦ C
                    gas atgas      110for°C40for
                                              h. 40
                                                  Phosphonium
                                                      h. Phosphoniumchloride    was was
                                                                          chloride     thenthen
                                                                                              stirred   withwith
                                                                                                    stirred    an aqueous
                                                                                                                     an aqueoussolution
of bistrifluoromethanesulfonimide              lithium atlithium
      solution of bistrifluoromethanesulfonimide             room at temperature
                                                                        room temperaturefor 30 for
                                                                                                 h. 30
                                                                                                    The h. resulting
                                                                                                            The resultingcrude  IL was
                                                                                                                            crude
extracted
      IL was extracted from the aqueous phase using dichloromethane. The dichloromethane phase was then
              from   the  aqueous      phase     using    dichloromethane.         The    dichloromethane           phase    was
washedthen with   distilled
              washed   withwater     forwater
                              distilled   severalfortimes
                                                     severaluntil
                                                             timesnountil
                                                                      residual   chloride
                                                                           no residual        salt was
                                                                                           chloride    saltdetected    with the
                                                                                                             was detected     withuse of
AgNO  the
        3 . use
             TheofILAgNO   3. The in
                     was dried      IL vacuum
                                       was driedatin333 vacuum
                                                           K for at 333The
                                                                  8 h.   K for  8 h. The pretreated
                                                                              pretreated     silica was silica
                                                                                                            added was to added  to
                                                                                                                         the prepared
      the prepared
grafting    agent in grafting
                       anhydrous  agent   in anhydrous
                                       toluene     under toluene    under
                                                           a nitrogen       a nitrogen atmosphere
                                                                         atmosphere         and stirredand      stirred
                                                                                                             at 90        at 90
                                                                                                                    ◦ C for  24°C
                                                                                                                                h. The
      for   24 h. The  solid  was   collected  and   washed   with  acetonitrile    and   distilled
solid was collected and washed with acetonitrile and distilled water and then dried at 80 C overnightwater    and  then  ◦dried at
      80 °C overnight under vacuum to obtain an IL-modified silica. The product prepared by calcined
under vacuum to obtain an IL-modified silica. The product prepared by calcined silica was labeled as
      silica was labeled as Si-P8TFSI/SiO2-C, when the molar ration of silica and IL is 1:1. The product
Si-P8 TFSI/SiO2 -C, when the molar ration of silica and IL is 1:1. The product prepared by acid-treatment
      prepared by acid-treatment silica was coded as Si-P8TFSI/SiO2-H(I) and Si-P8TFSI/SiO2-H(II),
silicarespectively,
       was codeddepending
                        as Si-P8 TFSI/SiO      2 -H(I) and
                                     on the different    molarSi-P 8 TFSI/SiO
                                                               ratio             2 -H(II),
                                                                      of silica and          respectively,
                                                                                      the grafting              depending
                                                                                                      agent (1:0.5              on the
                                                                                                                      and 1:1), as
different    molar   ratio of
      illustrated in Figure 1. silica  and   the   grafting  agent   (1:0.5  and    1:1),  as illustrated     in Figure    1.

                Figure 1. Illustration
                    Figure               of of
                            1. Illustration synthesizing
                                               synthesizing(EtO)
                                                            (EtO)33Si[P
                                                                   Si[P8883
                                                                       8883  ]TFSI
                                                                           ]TFSI   ionic
                                                                                 ionic    liquid-modified
                                                                                       liquid-modified silica.silica.

2.3. Characterization
      2.3. Characterization

2.3.1.2.3.1.
       Chemical  Analysis
             Chemical Analysis
            Fourier
      Fourier        transform
                transform        infrared
                              infrared      (FTIR)spectra
                                          (FTIR)      spectra were
                                                               were recorded
                                                                     recordedonona aThermoThermo    Scientific Nicolet
                                                                                                       Scientific       380 IR
                                                                                                                   Nicolet    380 IR
      Spectrometer
Spectrometer          (Thermo
                 (Thermo         Scientific,
                             Scientific,      Massachusetts, USA),
                                           Massachusetts,        USA), and
                                                                        andthe thesamples,
                                                                                    samples,    in in
                                                                                                   powder
                                                                                                      powderform,  along
                                                                                                                form,     with with
                                                                                                                        along
      KBr, were
KBr, were         compressed
             compressed     intointo  tablets.
                                  tablets.   TheThe    spectra
                                                    spectra  (32(32 scansfor
                                                                  scans    forthe
                                                                                thepure
                                                                                     pureionic
                                                                                           ionic liquid
                                                                                                    liquid and
                                                                                                           and 128
                                                                                                                 128scans
                                                                                                                      scansforfor the
      the IL-modified silica) were collected with a spectral resolution of 4 cm          −1  −1.
IL-modified silica) were collected with a spectral resolution of 4 cm .
            An x-ray photoelectron spectroscopy measurement was carried out on an X-Ray Photoelectron
      An x-ray photoelectron spectroscopy measurement was carried out on an X-Ray Photoelectron
      Spectrometer ESCALAB 250Xi (Thermo Fisher Co, Massachusetts, USA). All binding energies were
Spectrometer
      referencedESCALAB         250Xi
                   to the neutral       (Thermo
                                   C1 speak           Fisher
                                                at 284.8  eV toCo.,  Franklin,
                                                                 compensate     forMA,    USA). All binding
                                                                                    the surface-charging            energies were
                                                                                                               effects.
referencedThermogravimetric
              to the neutral C1analysis
                                    speak at(TGA)284.8waseV to  compensate       for  the  surface-charging
                                                             performed at a heating rate of 10 °C/min in the range effects.
      Thermogravimetric        analysis   (TGA)      was performed      at a heatingusingrate of       ◦ C/min in the range from
      from room temperature         to 700    °C under     nitrogen atmosphere,                  a 10
                                                                                                    STA409C    DTA/DSC-TG
room(Netzsch,Selb,
       temperature to          ◦
                          700 C under
                        Germany).     The nitrogen
                                            hydroxylatmosphere,
                                                          group contentsusingwerea STA409C
                                                                                      estimated   DTA/DSC-TG
                                                                                                      by assuming(Netzsch,
                                                                                                                      that the Selb,
      condensation
Germany).             occurredgroup
              The hydroxyl       based on    the reaction
                                         contents     wereof   two hydroxyl
                                                             estimated           groups onthat
                                                                          by assuming          the silica surface and can
                                                                                                     the condensation        be
                                                                                                                          occurred
      calculated  as  follows  in Equation     (1) [27]. Moreover,    according    to  the  thermogravimetric
based on the reaction of two hydroxyl groups on the silica surface and can be calculated as follows in              curve,  the
      total (1)
Equation    IL load
                [27].on the surfaceaccording
                      Moreover,       of silica geltoisthe
                                                        determined    by using Equation
                                                           thermogravimetric        curve, the (2). total IL load on the surface
of silica gel is determined by using Equation (2).2[W(T0 )-W(Tfinal )]
                                                 nOH (SiO2 ) =                                                              (1)
                                                                      100MH2O
                                                              2[W(T0 ) − W(Tfinal )]
                                            nOH (SiO2 ) =                                                                            (1)
                                                                    100M
                                                             W(T0 )-W(T   H)2 O
                                                                       final
                                                 GR(%) =                         ×100                                       (2)
                                                                  W(Tfinal )
                                                          W(T0 ) − W(Tfinal )
    Where n (SiO ) indicates the GR      (%)of=hydroxyl groups on the×silica
                                     moles                                 100 particles W(T ) and W(T ) (2)
                                                       W(Tfinal )
    are the weight of the silica particles (g) at the temperatures 200 °C and 700 °C, respectively, MH2O is
wherethe
       nOH (SiO2 ) weight
         molecular indicates   the moles
                          of water  and GR of (wt%)
                                               hydroxyl
                                                     is thegroups   onofthe
                                                            total load    ILssilica
                                                                              on theparticles   (T0 ) gel.
                                                                                              Wsilica
                                                                                     surface of       and W(Tfinal )
are the weight of the silica particles (g) at the temperatures 200 ◦ C and 700 ◦ C, respectively, MH2O is
the molecular weight of water and GR (wt%) is the total load of ILs on the surface of silica gel.
Materials 2019, 12, x FOR PEER REVIEW                                                                               4 of 14
  Materials 2019, 12, 1782                                                                                             4 of 13

2.3.2. Pore Structure Characteristics
   2.3.2.
     ThePore   Structure
           adsorption       Characteristics
                       isotherm    of N2 at −196 °C and CO2 at 0 °C were measured by an Autosorb-1-MP
(Quantachrome,
         The adsorption isotherm ofan
                    Florida,   USA),     N2automatic
                                              at −196 ◦ Csurface
                                                          and COarea      and
                                                                          ◦      pore size analyzer. The porous
                                                                   2 at 0 C were measured by an Autosorb-1-MP
particles  were driedFlorida,
   (Quantachrome,      and degassed
                                USA), an atautomatic
                                             100 °C under   higharea
                                                        surface   vacuum
                                                                      and porefor 5size
                                                                                     h toanalyzer.
                                                                                           remove moisture
                                                                                                    The porous or particles
                                                                                                                  other
volatile  contaminants.    As for N
   were dried and degassed at 100 C under high vacuum for 5 h to remove moisture or other 0.001
                                    2 adsorption,
                                          ◦          the relative pressure     (P/P 0 ) range was   set  between   volatile
and  0.995, and theAsCO
   contaminants.       for2 adsorption   isotherms
                            N2 adsorption,            were pressure
                                              the relative  collected(P/P
                                                                       within    the relative pressure (P/P0) range
                                                                           0 ) range was set between 0.001 and 0.995,
of and
   0.0001~0.03
        the CO2 for  restricting
                 adsorption       pores narrower
                              isotherms               than within
                                            were collected 2 nm. A thedensity
                                                                        relativefunctional    theory
                                                                                  pressure (P/P         (DFT) analysis
                                                                                                 0 ) range of 0.0001~0.03
wasfor restricting pores narrower than 2 nm. A density functional theory (DFT) analysis wasusing
      used  to  calculate   the pore   size   distribution  (PSD)   and   the   pore    structure  parameters      used to
Quantachrome       ASiQwin
   calculate the pore           software (Quantachrome,
                        size distribution      (PSD) and the poreFlorida,
                                                                      structureUSA).     based on
                                                                                   parameters    usingtheQuantachrome
                                                                                                            adsorption
isotherms.
   ASiQwin software (Quantachrome, Florida, USA). based on the adsorption isotherms.

2.4.2.4.
     Adsorption  Studies
         Adsorption Studies
      Adsorption
         Adsorptionand anddiffusivity       watervapor
                           diffusivity of water     vapor   were
                                                          were      measured
                                                               measured     on anonIntelligent
                                                                                      an Intelligent   Gravimetric
                                                                                                Gravimetric  Analyzer
Analyzer   (IGA-003,    Hiden   Isochema    Ltd., Manchester,    UK).   Detailed   information   on  the
   (IGA-003, Hiden Isochema Ltd., Manchester, UK). Detailed information on the apparatus and analyzing   apparatus
and   analyzing
   methods       methods
              have          have beenreported
                    been previously     previously    reported
                                                  [28,29]. Water[28,29].
                                                                   vaporWater
                                                                          uptakevapor    uptake was
                                                                                    was measured    in measured   in
                                                                                                       the Analyzer’s
theHumidifier
     Analyzer’sconfiguration,
                 Humidifier configuration,
                                  using similarusing    similar
                                                  methods   at 25  ◦ C. Theat
                                                                methods        25 °C. The
                                                                            samples    weresamples
                                                                                             dried bywere  dried bydry
                                                                                                       circulating
circulating
   nitrogen dry
              untilnitrogen  until
                    a constant     a constant
                                 weight         weight was
                                         was obtained.        obtained.
                                                           Vapor   pressureVapor
                                                                               was pressure   was then
                                                                                    then controlled   by controlled
                                                                                                         circulating a
by mixture
    circulating a mixture
             of dry         of dry
                     nitrogen  andnitrogen    and vapor-saturated
                                    vapor-saturated                    nitrogen
                                                        nitrogen to reach    the to  reach the
                                                                                  required     requiredAt
                                                                                            humidity.    humidity.
                                                                                                            a required
At humidity,
    a requiredthe
                humidity,
                   mass gainthewasmass  gain was
                                    measured   as ameasured
                                                     function ofastime
                                                                    a function   of timeconstant.
                                                                         until reaching   until reaching  constant.
                                                                                                   More details   have
More
   beendetails haveinbeen
         reported           reported
                       references     in references [30,31].
                                   [30,31].

   3. Results
3. Results andand Discussion
               Discussion

3.1.3.1. Characterization
     Characterization      of the
                       of the     Samples
                               Samples
   3.1.1. FTIR Analysis
3.1.1. FTIR Analysis
         The FTIR spectra of bare and grafted silica are shown in Figure 2. The typical peaks for SiO2 -H
      The FTIR spectra of bare and grafted
                                        −1 for silica are shown in Figure 2. The typical peaks for SiO2-H
   and SiO2 -C were found at 1098 cm           asymmetric stretching vibration and 800 cm−1 for symmetric
and SiO2-C were found at 1098 cm-1 for asymmetric stretching vibration and 800 cm-1 for symmetric
   stretching vibration of Si-O-Si. The vibration bands of OH groups are located at different frequencies,
stretching vibration of Si-O-Si. The vibration bands of OH groups are located at different     frequencies,
   which depend on the bonding configuration. The broad band between 3800 cm-1−1 and 3050 cm                −1 is
which depend on the bonding configuration. The broad band between 3800 cm and 3050 cm-1 is
   attributed to the stretching of silanol associated with surface silanols, including isolated silanol, vicinal,
attributed to the stretching of silanol associated with      surface silanols, including isolated silanol,
   and geminal groups [32]. The shoulder at 956 cm−1 indicates        the existence of the Si-OH asymmetric
vicinal, and geminal groups [32]. The shoulder at 956 cm-1 indicates the existence of the Si-OH
   stretching band [33]. Based on the comparison of the intensity of the characteristic peaks of Si-OH in
asymmetric stretching band [33]. Based on the comparison of the intensity of the characteristic peaks
   Figure 2a,b, Figure 2 reviews that hydroxyl content on the surface of SiO2 -H treated by hydrochloric
of Si-OH in Figure 2a,b, Figure 2 reviews that hydroxyl content on the surface        of SiO2-H treated by
   acid and SiO2 -C treated by calcined are a little different. The hydroxyl content was calculated in the
hydrochloric acid and SiO2-C treated by calcined are a little different. The hydroxyl content was
   scetion of thermogravimetry analysis.
calculated in the scetion of thermogravimetry analysis.

                                                          (a)

                                                          (b)
                                         %Transmittance

                                                          (c)

                                                          (d)

                                                    4000        3500   3000   2500   2000   1500   1000   500

                                                                        Wavenumber(cm-1)

        Figure
     Figure    2. FTIR
            2. FTIR    spectra
                    spectra    of SiO
                            of SiO     -H (a), SiO -C (b), Si-P8 TFSI/SiO
                                   2-H2 (a), SiO2-C2(b), Si-P8TFSI/SiO    2 -H(I)
                                                                       2-H(I) (c), (c),
                                                                                   andand
                                                                                        neatneat ionic
                                                                                             ionic     liquid
                                                                                                   liquid  (IL)(IL)
                                                                                                                (d).(d).

       As shown in Figure 2, the infrared spectrum of Si-P8 TFSI/SiO2 -H(I) shows the appearance of the
    As shown in Figure  2, the infrared spectrum of Si-P8TFSI/SiO2-H(І) shows the appearance of the
  bands at 2924 cm−1 and 2856 cm−1 due to C-H aliphatic stretching vibration, and the peak at 1465
bands at 2924 cm-1 and 2856 cm-1 due to C-H aliphatic stretching vibration, and the peak at 1465 cm-1
Materials 2019, 12, x FOR PEER REVIEW                                                                                                            5 of 14
Materials 2019, 12, 1782                                                                                                                         5 of 13

belongs to P-C stretching vibration. Figure 2 appears to show that broad peak characteristic bands
cm−1 belongs
between    1024~1246
                to P-Ccm-1, due to Si-O-Si and S=O, and the peak at 664 cm-1 belongs to S-C stretching
                       stretching   vibration. Figure 2 appears to show that broad peak characteristic
vibration.  The success
bands between           of the
                  1024~1246    cm −1 , due
                                 neat  phosphonium    IL grafted
                                           to Si-O-Si and         ontothe
                                                           S=O, and     thepeak
                                                                            silicaatsurface  −1 belongs
                                                                                     664 cmhas   been assessed
                                                                                                        to S-C
based   on the appearance   of the characteristic bands  for neat IL at 2926, 2855,   1466, and
stretching vibration. The success of the neat phosphonium IL grafted onto the silica surface has1227~1060,663
cm −1.
been   assessed based on the appearance of the characteristic bands for neat IL at 2926, 2855, 1466,
and 1227~1060,663 cm−1 .
3.1.2. XPS Analysis
3.1.2. XPS Analysis
      The XPS spectrum of SiO2-H, neat [Si-P3888][TFSI], and Si-P8TFSI/ SiO2-H(І) is shown in Figure 3.
      The XPS
The peaks        spectrum
              of C  1s, O 1s,ofand SiOSi2 -H,
                                           2p neat  [Si-P
                                               appear     3888 ][TFSI],
                                                        only             and Si-P
                                                               in spectrum         8 TFSI/
                                                                               (a) of      SiO2 silica.
                                                                                      the bare    -H(I) isItshown
                                                                                                              can bein    Figure
                                                                                                                        seen   from3.
The peaks
Figure        of Ca1s,
         3b that         O 1s, and
                     binding    energy Si 2pof appear
                                               168.4 eV,only   in spectrum
                                                           397.4  eV, and 688.9(a) ofeV
                                                                                      the
                                                                                        is bare   silica. to
                                                                                           attributed      It Scan
                                                                                                                 2p,beNseen    from
                                                                                                                         1s, and   F
Figure
1s       3b neat
    for the  that aIL,binding   energyinof[TFSI]
                        respectively,           168.4 anion
                                                      eV, 397.4    eV,
                                                               [34]. Inand    688.9 eV
                                                                         addition,      is attributed
                                                                                     there  is a P 2p peakto S 2p,
                                                                                                                 withNa1s,    and F
                                                                                                                          binding
1s for the
energy   of neat
             131.0IL,
                    eV,respectively,
                          which further    in [TFSI]
                                               provesanion    [34]. In addition,
                                                       the successful      synthesisthere   is aionic
                                                                                       of the     P 2pliquid
                                                                                                         peak [Si-P
                                                                                                                 with3888
                                                                                                                        a binding
                                                                                                                           ][TFSI].
energy of 131.0
Compared            eV, which
               to spectrum     (a),further
                                      the C proves
                                               1s peaktheinsuccessful
                                                             spectrum synthesis        of the ionic
                                                                           (c) is significantly        liquid [Si-P
                                                                                                   enhanced,      and3888two][TFSI].
                                                                                                                                new
Compared
peaks         to spectrum
        attributable          (a),(binding
                        to F 1s     the C 1senergy
                                                peak in689.0
                                                         spectrum
                                                               eV) and(c) P
                                                                          is 2p
                                                                             significantly  enhanced,
                                                                                (binding energy       131.1andeV)two   new in
                                                                                                                   appear     peaks
                                                                                                                                 the
attributable    to F  1s (binding     energy     689.0 eV)   and   P 2p   (binding   energy   131.1
XPS spectrum. This shows that the ionic liquid has been successfully immobilized on the surface of     eV)  appear     in the   XPS
spectrum. This shows that the ionic liquid has been successfully immobilized on the surface of silica.
silica.

                                                                                                             O1s
                                                    Counts/s

                                                               (a)                                                            Si2s Si2p
                                                                                                                    C1s

                                                                                                    F1s     O1s    C1s    P2p
                                                               (b)                                              N1s    S2p Si2p
                                                                                                             O1s
                                                                                                                                     P2p
                                                                                                                        C1s
                                                               (c)                                  F1s
                                                                                                                              Si2s Si2p

                                                                     1200     1000            800     600         400         200          0
                                                                                        Binding Energy(ev)

      Figure 3. Full-range X-ray photoelectron spectroscopy spectra of SiO -H (a), neat IL (b), and Si-P TFSI/
      Figure 3. Full-range X-ray photoelectron spectroscopy spectra of SiO22-H (a), neat IL (b), and Si-P88TFSI/
      SiO -H(I) (c).
      SiO22-H(І) (c).
3.1.3. Thermogravimetric Analysis
3.1.3. Thermogravimetric Analysis
      Thermal gravimetric analysis (TGA) is performed to determine the quantitative information of
      Thermalofgravimetric
the contents        silanol andanalysis      (TGA)
                                  ionic liquids    onisthe
                                                         performed
                                                             surface oftothe
                                                                          determine     the quantitative
                                                                             porous silica                 information
                                                                                               products. TGA    patterns of
the contents    of  silanol  and  ionic  liquids   on  the   surface of  the porous    silica  products. TGA
bare silica and modified silicas are depicted in Figure 4. It is found that there are two distinct mass-loss   patterns    of
bare
stepssilica
       in theand    modified silicas are
               thermogravimetric        curvesdepicted   in Figureespecially
                                                  of all samples,    4. It is found
                                                                                  the that
                                                                                      abrupt there are two
                                                                                                change      distinct
                                                                                                        of the        mass-
                                                                                                               first step  at
loss steps in the
temperatures      belowthermogravimetric
                           200 ◦ C for SiO2 -Hcurves
                                                 and SiO of 2all
                                                              -C.samples,   especially
                                                                  The removal             the abrupt change
                                                                                 of the physiosorbed     surfaceofadsorbed
                                                                                                                   the first
step
wateratand
         temperatures
              gas are lostbelow
                              before200
                                      200°C◦ C.
                                              forThe
                                                  SiOcondensation
                                                      2-H and SiO2-C.   of The   removal of
                                                                           the hydroxyl         the physiosorbed
                                                                                              group  and the loaded surface
                                                                                                                        ionic
  Materials 2019, 12, x FOR PEER REVIEW                                                                            6 of 14
adsorbed
liquid on thewater     and
                  silica    gas are
                          surface arelost
                                       lost before   200 °C. The ranging
                                            at the temperature       condensation
                                                                               from 200of the
                                                                                           ◦ C tohydroxyl
                                                                                                  700 C. group and the
                                                                                                      ◦

loaded ionic liquid on the silica surface are lost at the temperature ranging from 200 °C to 700 °C.
                                                              100
                                                                                  (e)
                                                               96
                                        Weight retention(%)

                                                                                  (d)
                                                               92                 (c)

                                                                                  (a)
                                                               88
                                                                                  (b)
                                                               84

                                                               80

                                                               76
                                                                    0       100         200     300       400      500         600         700
                                                                                               Temperature(°C)

        Figure4.4. Thermogravimetric
      Figure                             curvesofof
                   Thermogravimetric curves          SiO
                                                   SiO2-H2 -H
                                                            (a),(a),
                                                                 SiO2SiO 2 -C Si-P
                                                                      -C (b),  (b),8TFSI/SiO
                                                                                    Si-P8 TFSI/SiO  2 -H(I)
                                                                                             2-H(I) (c), Si-P(c), Si-P8 TFSI/
                                                                                                             8TFSI/SiO  2-

      SiO  -H(II)  (d), and Si-P
          2 (d), and Si-P8TFSI/SiO
        H(II)                   8 TFSI/SiO
                                     2-C (e).
                                           2 -C (e).

      Silanol group content for silica and the total load of phosphonium ILs on the surface for modified
  samples determined by TGA techniques are listed in Table 1. The condensation of all kinds of silanol
  groups occurs by the reaction of two silanol groups on the silica surface, resulting in the release of
  one molecule of water and the formation of one siloxane group. It is assumed that no other groups
Materials 2019, 12, 1782                                                                                                  6 of 13

     Silanol group content for silica and the total load of phosphonium ILs on the surface for modified
samples determined by TGA techniques are listed in Table 1. The condensation of all kinds of silanol
groups occurs by the reaction of two silanol groups on the silica surface, resulting in the release of one
molecule of water and the formation of one siloxane group. It is assumed that no other groups than
water were released from the sample. As shown in Table 1, for SiO2 -H and SiO2 -C, TGA shows that
the hydroxyl content of SiO2 -C is significantly higher than that of SiO2 -H, which is probably because
the H+ in the hydrochloric acid activating solution destroys the hydroxyl groups on the surface of
the silica gel [35]. Moreover, IL loading calculated for the Si-P8 TFSI/SiO2 -H(I), Si-P8 TFSI/SiO2 -H(II),
and Si-P8 TFSI/SiO2 -C was 4.92 wt%, 10.62 wt%, and 10.79 wt%, respectively, whereby the ionic liquid
loading depends on the limited number of active silanol groups on the supporter surface.

      Table 1. Silanol group content for bare silica and the total load of phosphonium ILs onto silica surface
      determined by TGA techniques.

                      Sample                      OH Group Content(mmol/g)               Amount of Grafting (wt%)
                      SiO2 -H                                  3.867                                    -
                      SiO2 -C                                  5.479                                    -
              Si-P8 TFSI/SiO2 -H(I)                              -                                    4.92
              Si-P8 TFSI/SiO2 -H(II)                             -                                   10.62
                Si-P8 TFSI/SiO2 -C                               -                                   10.79

3.1.4. Pore size distribution
     There is a certain correlation between pore structure and water adsorption. The use of both
adsorbates, N2 and CO2 , provides complementary information about the porous texture of the
samples. Isotherms measured with N2 often lead to an underestimation of the microporosity due to
the lower mobility of N2 at −196 ◦ C and whose diffusion in small size pores is inhibited. To access the
microporosity, isotherms can be measured under CO2 at 0 ◦ C, since the CO2 molecules are more mobile.
Based on the N2 and CO2 adsorption isotherms at −196 ◦ C and 0 ◦ C, respectively, CO2 adsorption
capacity at 0 ◦ C and the data to evaluate their surface areas and pore structures of the samples were
obtained using the DFT method [36] in Table 2. The DFT model is based on the molecular statistical
thermodynamics equation that calculates the specific adsorption amount in an individual pore of a
given adsorbate–adsorbent system at a given experimental temperature and pressure by solving the
function of the grand thermodynamic potential in terms of the distribution of gas density in a specific
pore space.

      Table 2. Pore structure parameters of samples based on N2 sorption isotherms at −196 ◦ C and CO2
      sorption isotherms at 0 ◦ C.

                                     N2 Sorption Isotherms                         CO2 Sorption Isotherms             V (CO2 )
         Sample
                                                                                                                       (wt%)
                            CPV ×   10−1   (cm3 ·g−1 )   CSA   (m2 ·g−1 )   CPV × 10−1 (cm3 ·g−1 )   CSA (m2 ·g−1 )
           SiO2 -H                  4.001                  525.3                    1.058                306.8          5.51
           SiO2 -C                  4.149                  635.6                    1.241                362.3          6.47
   Si-P8 TFSI/SiO2- H(I)            3.303                  425.8                    0.860                263.2          4.78
   Si-P8 TFSI/SiO2 -H(II)           3.263                  373.1                    0.801                238.7          4.27
     Si-P8 TFSI/SiO2 -C             3.347                  455.4                    0.980                283.9          4.96

     The cumulative pore volume (CPV) and cumulative surface area (CSA) of SiO2 -C shown in Table 2
are considerably higher than those of SiO2 -H, and the surface modified silica presents a significantly
lower pore volume and specific surface area as compared to the bare silica, which leads to a decrease
in the adsorption of CO2 . On the basis of structural characteristics, silica impregnated with ILs results
in the blockage of pores. Not surprisingly, Si-P8 TFSI/SiO2 -C prepared by grafting ILs on calcined
supporter has a higher pore volume and specific surface area than Si-P8 TFSI/SiO2 -H(II).
higher IL loading and less micropores should exhibit lower water adsorption capacity, which will be
discussed later.

3.2. Water Vapor Adsorption
Materials 2019, 12, 1782                                                                                                                                                   7 of 13
      The mechanism of water adsorption on silica is very complex. It is known that the surface of
silica contains silanol or siloxane groups. The Si-OH shows a specific interaction with water
      The pore size
(hydrophilic),     anddistribution    based
                        it is a typical      on N
                                         polar     2 sorption
                                                  group   and isotherms
                                                                has some at    −196 ◦properties
                                                                            similar   C and CO2 withsorption   isotherms
                                                                                                           water,  so the
at 0 ◦ C are   shown    in  Figure  5.  According     to the  pore   size distribution     (PSD)   of the  micropore      in
hydrogen bonds were formed by the reaction of water and the hydroxyl group [37,38]. It is reasonable
Figure
to think5b,  thewater
           that   PSD of   samples
                        vapor         mainly concentrated
                                adsorption,    depending oninthe  theamount
                                                                       range ofof0.4–1.0    nm. Thegroup
                                                                                   the hydroxyl       proportion
                                                                                                            on theof    the
                                                                                                                     silica
micropores
surface and for theSi-P   TFSI/SiO
                    adsorption
                        8             -H(II)
                                  capacity,
                                    2        is lower
                                              increasedthan   that
                                                           with  theof Si-P  TFSI/SiO
                                                                       hydroxyl
                                                                           8     content.
                                                                                        2 -H(I)  and
                                                                                              Water   Si-P
                                                                                                      vapor
                                                                                                          8 TFSI/SiO   2 -C,
                                                                                                              adsorption
which   is  mainly   due   to the clogging    of a part  of the  pores   after the  introduction    of
isotherms, corresponding to all samples, are illustrated in Figure 6. SiO2-H has better water resistancethe  ionic liquid.
Combined
than SiO2-C.  with
                 SiOIL
                     2-Hloading
                          displaysobtained
                                    a lowerby     TGA data,
                                               quantity        it hydroxyl
                                                          of the  is expected   that the
                                                                              group,        Si-P8 TFSI/SiO2by
                                                                                       as demonstrated        -H(II) with
                                                                                                                the FTIR
higher
and TGA IL loading     and less to
            data. According     micropores      should exhibit
                                   the pore structure            lower water
                                                          parameters,    SiO2-Hadsorption
                                                                                  also presents capacity,
                                                                                                   lower which
                                                                                                           CSA and willless
                                                                                                                         be
discussed
micropores,  later.
                which is unfavorable for the adsorption of water vapor.

                  0.03
                         (a)                                                                                        (b)                        SiO2-C
                                                                         SiO2-C                              0.10
                                                                         SiO2-H                                                                SiO2-H
                                                                                                                                               Si-P8TFSI/SiO2-H(Ⅰ
                                                                                                                                                                )
                                                                         Si-P8TFSI/SiO2-H(Ⅰ)                 0.08
                  0.02                                                                                                                         Si-P8TFSI/SiO2-H(Ⅱ)
                                                                         Si-P8TFSI/SiO2-H(Ⅱ)
                                                                                                                                               Si-P8TFSI/SiO2-C
        dvp/drp

                                                                                                   dvp/drp
                                                                         Si-P8TFSI/SiO2-C                    0.06

                                                                                                             0.04
                  0.01

                                                                                                             0.02

                  0.00                                                                                       0.00
                               1                                          10                                     0.2      0.4   0.6   0.8      1.0   1.2   1.4       1.6
                                       rp(nm)                                                                                         rp(nm)

      Figure 5. Pore size distribution of silica and modified silica based on: (a) N2 adsorption isotherms at
      Figure 5. Pore size distribution of silica and modified silica based on: (a) N2 adsorption isotherms at
      −196 ◦ C and (b) CO2 adsorption isotherms at 0 ◦ C.
      −196 °C and (b) CO2 adsorption isotherms at 0 °C.
3.2. Water Vapor Adsorption
       The mechanism of water adsorption on silica is very complex. It is known that the surface of silica
contains silanol or siloxane groups. The Si-OH shows a specific interaction with water (hydrophilic),
and it is a typical polar group and has some similar properties with water, so the hydrogen bonds
were formed by the reaction of water and the hydroxyl group [37,38]. It is reasonable to think that
water vapor adsorption, depending on the amount of the hydroxyl group on the silica surface and
the adsorption capacity, increased with the hydroxyl content. Water vapor adsorption isotherms,
corresponding to all samples, are illustrated in Figure 6. SiO2 -H has better water resistance than SiO2 -C.
SiO2 -H displays a lower quantity of the hydroxyl group, as demonstrated by the FTIR and TGA data.
According to the pore structure parameters, SiO2 -H also presents lower CSA and less micropores,
Materials
which is  2019, 12, x FOR PEER
            unfavorable     for REVIEW
                                the adsorption of water vapor.                                       8 of 14

                                                                30
                                                                            SiO2-C
                                                                25          SiO2-H
                                        Amount adsorbed (wt%)

                                                                            Si-P8TFSI/SiO2-H(I)
                                                                20          Si-P8TFSI/SiO2-C
                                                                            Si-P8TFSI/SiO2-H(II)
                                                                15

                                                                10

                                                                5

                                                                0
                                                                     0         5    10      15     20          25         30    35
                                                                                         Pressure(mbar)

                                Watervapor
                      Figure6.6.Water vaporadsorption
                                            adsorptionisotherms
                                                       isothermsat
                                                                 at25  ◦ C on silica and modified silica.
                                                                    25°C
                     Figure                                               on silica and modified silica.

      The results in Figure 6 also indicate that water adsorption of silica modified by ionic liquid reduces
     The results in Figure 6 also indicate that water adsorption of silica modified by ionic liquid
 significantly, and Si-P8 TFSI/SiO2 -H(II) with relatively higher IL content and a lower microporous
reduces significantly, and Si-P8TFSI/SiO2-H(II) with relatively higher IL content and a lower
 structure shows better water resistance. Due to grafting the hydrophobic ionic liquid onto the silica
microporous structure shows better water resistance. Due to grafting the hydrophobic ionic liquid
onto the silica gel, there is an ionic liquid film formed by reacting with the silanol groups on the silica
surface, and the schematic is shown in Figure 7. According to the previous PSD data, the CPV,
especially of the micropores, and CSA of Si-P8TFSI/SiO2-H(II) are lower, resulting in a slight decrease
in water adsorption capacity.
     The pore structure parameters and surface property show a correlation with water vapor
will be coadsorbed with CO2 at the high relative humidities than can be expected in post-combustion
applications.

3.3. Water Diffusion Coefficient
     The2019,
Materials diffusion
              12, 1782 coefficient
                                calculation presented a great significance toward the evaluation of   the
                                                                                                   8 of 13
industrial application value of the adsorbent. The diffusion in porous solids is more complicated than
in liquids. For bare silica and silica-phosphonium ionic liquid hybrid material prepared with the
gel, theremethod,
grafting   is an ionic  liquid film
                   the particle      formed byasreacting
                                is considered    a uniformwith  the silanol
                                                              adsorbent.    groups of
                                                                         Diffusion  onwater
                                                                                       the silica
                                                                                            vaporsurface,
                                                                                                  into a
and  the schematic  is shown   in Figure 7. According  to the  previous
porous spherical particle can be described by Equation (3) [39].         PSD data, the CPV, especially  of
the micropores, and CSA of Si-P8 TFSI/SiO2 -H(II) are lower, resulting in a slight decrease in water
                                                                    2
adsorption capacity.                (1 − )    +      =      (     +      )                           (3)

            Figure 7.
            Figure    Schematic diagram
                   7. Schematic diagram of
                                        of the
                                            the ionic
                                                 ionic liquid
                                                        liquid membrane
                                                               membrane on
                                                                        on the
                                                                           the surface
                                                                               surface of
                                                                                       of the
                                                                                          the carrier.
                                                                                              carrier.

     The pore structure parameters and surface property show a correlation with water vapor
sorption capacity, suggesting that porosity and the IL film play an important role in reducing a
small amount of water vaporin adsorption of the power plant flue gas for the bare and modified
SiO2 , thereby resulting in decreased CO2 adsorption capacity. The maximum adsorption capacity
of water vapor on silica-phosphonium ionic liquid hybrid material is lower than that for bare silica,
with a higher pore volume and quantity of the hydroxyl group, which is advantageous, as less
water vapor will be coadsorbed with CO2 at the high relative humidities than can be expected in
post-combustion applications.

3.3. Water Diffusion Coefficient
      The diffusion coefficient calculation presented a great significance toward the evaluation of the
industrial application value of the adsorbent. The diffusion in porous solids is more complicated than
in liquids. For bare silica and silica-phosphonium ionic liquid hybrid material prepared with the
grafting method, the particle is considered as a uniform adsorbent. Diffusion of water vapor into a
porous spherical particle can be described by Equation (3) [39].

                                                  ∂q    ∂C        ∂2 C 2 ∂C
                                        (1 − ε)      +ε    = εDe ( 2 +      )                              (3)
                                                  ∂t    ∂t        ∂R   R ∂R

where De is the effective porous diffusion coefficient (m2 /s), ε is the material porosity, q is the adsorbate
concentration in the solid phase (wt%), C is the adsorbate concentration in the gas phase (wt%), R is
the particle of radius (m), and t is the adsorption time(s).
     Initial conditions (IC) and boundary conditions (BC):

                                           C(Rp , 0) = C0              q(RP , 0) = q0                      (4)

                                         C(Rp , ∞) = C∞                q(RP , ∞) = q∞                      (5)
                                                   ∂C              ∂q
                                                              =               =0
                                                   ∂R   R=0        ∂R   R=0
     The solution of the equation is given by Equation (6) [40].
                                                                   ∞
                      m         q−q0                          6    P 1      −n2 π2 (εDe /(ε+(1−ε)K))t
                      m∞   =   q∞ −q0              = 1−       π2    n2
                                                                       exp(
                                                                                         R2p
                                                                                                      )
                                                                n=1                                        (6)
                                                                          ∞           −n2 π2 Dap t
                                                               = 1 − π62     1
                                                                          P
                                                                             n2
                                                                                exp(               )
                                                                                           R2p
                                                                         n=1
Materials 2019, 12, 1782                                                                                9 of 13

where
                                         Dap = εDe /[ε + (1 − ε)K]                                         (7)

Dap is the apparent diffusion coefficient, q is the quantity adsorbed at time t, and K is the slope of
adsorption isotherm.
     A simplified equation for the diffusion is given by Equation (8):
                                                     r
                                          m    2A        Dap t     √
                                             ≈                   =S t                                      (8)
                                          m∞   V          π

where A is the particle surface area (m2 ) and V is the particle volume (m3 ). In case of spherical particles:
2A/V = 3/R.
    From equation (8) it can be obtained that:
                                                         s
                                                  6           Dap
                                               S= √                                                        (9)
                                                   π          R2p

where S is the slope of the curve m/m∞ versus t0.5 . Further, from Equation (9), Dap can be obtained as:

                                                       π 2 2
                                               Dap =     R S                                              (10)
                                                       36 p
      Finally, the effective diffusion coefficient (De ) can be calculated with the relationship:

                                                      ε + (1 − ε)K
                                           De = Dap                                                       (11)
                                                            ε
where,
                                                      Vp × ρsi
                                              ε=                                                          (12)
                                                   1 + Vp × ρsi
Vp is the pore volume and ρsi is the apparent density.
      Examples of the experimental data and fitting curves for SiO2 -H, SiO2 -C, and Si-P8 TFSI/SiO2 -H(I)
at different pressures and at 25 ◦ C are illustrated in Figure 8. A good agreement between the
experimental values and the fitting curves was obtained.
      Diffusion coefficients obtained from the curve fitting are listed in Table 3. It can be obtained from
Table 3 that the calculated effect diffusion coefficients of silica gel are at a level of 10−7 and these values
are close to reports on silica [41–43]. Comparing two kinds of bare silica, the diffusion coefficient of
SiO2 -C is lower than that of SiO2 -H. The reason may be that the diffusion resistance of water vapor is
increased by an excessive amount of water adsorbed by SiO2 -C with a higher hydroxyl group, CPV,
and CSA. Moreover, diffusion coefficients in Si-P8 TFSI/SiO2 -H(I) are about 10−8 ~10−7 m2 /s and slightly
lower than those of SiO2 -H, which implies that the thin IL coating on the silica surface and a loss of
micro-porosity impedes water diffusion.
      Figure 9 shows the relationship between water vapor uptake and De . Effective diffusion coefficients
of all the samples show a decreasing trend with the increase of water vapor adsorption, which, due to
the porosity available for water vapor transport, decreases at higher loading as adsorbed water vapor
occupies a part of porous sites.
Materials 2019, 12, 1782                                                                                                                                                                                                                      10 of 13

                                                             Table 3. The diffusion coefficient of water vapor sorption in samples at 25 ◦ C.

                                                                                       De × 10−7 (m2 /s)                         De × 10−7 (m2 /s)                                                           De × 10−7 (m2 /s)
             Pressure (mbar)
                                                                                           SiO2 -C                                   SiO2 -H                                                               Si-P8 TFSI/SiO2 vH(I)
                       5 PEER REVIEW
Materials 2019, 12, x FOR              4.76                  5.75                       4.41            10 of 14
                   8                   4.25                  5.62                       4.32
                  10
     Diffusion coefficients            4.03 the curve fitting5.46
                             obtained from                     are listed in Table 3. It4.11
                                                                                         can be obtained from
                  12                   3.89                  5.37                       4.36-7
Table 3 that the calculated
                  15        effect diffusion
                                       3.12 coefficients of silica
                                                             4.37 gel are at a level of3.82
                                                                                          10 and these values
are close to reports
                  18 on silica [41–43].2.38
                                        Comparing two kinds  3.61 of bare silica, the diffusion
                                                                                        2.67     coefficient of
                  20 that of SiO2-H. The
SiO2-C is lower than                   2.41 reason may be that
                                                             3.82the diffusion resistance
                                                                                        2.43 of water vapor is
                  22
increased by an excessive   amount of1.94
                                        water adsorbed by 3.51                          2.71
                                                              SiO2-C with a higher hydroxyl      group, CPV,
                  25                   1.54                  3.42                       2.55
and CSA. Moreover,28
                       diffusion coefficients
                                       1.44
                                              in Si-P 8TFSI/SiO2-H(I) are about 10-8~10-7 m2/s and slightly
                                                             2.64                       2.98
lower than those30 of SiO2-H, which implies
                                       1.38    that the thin IL
                                                             2.13coating on the silica1.84
                                                                                         surface and a loss of
micro-porosity impedes water diffusion.

                                                            5.9 (a)                                                                                                           13.1 (b)

                                                                                                                               Water vapour uptake(wt%)
                                 Water vapour uptake(wt%)

                                                            5.8                                                                                                               13.0

                                                                                                                                                                              12.9
                                                            5.7
                                                                                                                                                                                                                P=22mbar
                                                                                               P=8mbar                                                                        12.8
                                                            5.6                                                                                                                                                     Experimantal data
                                                                                         Experimantal data                                                                                                          Theoretical curve
                                                                                                                                                                              12.7
                                                            5.5                          Theoretical curve
                                                                                                                                                                              12.6
                                                            5.4
                                                                                                                                                                              12.5
                                                                   550    555   560    565     570     575     580       585                                                     1645 1650 1655 1660 1665 1670 1675 1680
                                                                                      t(min)                                                                                                              t(min)

                                               8.0                (c)                                                                                                                 (d)
                                                                                                                                                                         19.2
                                                                                                                                   Water vapour uptake(wt%)
                  Water vapour uptake(wt%)

                                               7.9                                                                                                                       19.0

                                               7.8                                        P=8mbar                                                                        18.8                                        P=22mbar
                                                                                        Experimantal data                                                                                                          Experimantal data
                                               7.7                                      Theoretical curve                                                                18.6                                      Theoretical curve
                                               7.6
                                                                                                                                                                         18.4
                                               7.5
                                                                                                                                                                         18.2
                                                            130     135   140   145     150     155     160        165                                                                640     645   650   655      660   665   670      675
                                                                                      t(min)                                                                                                              t(min)

                                                                  (e)                                                                                                                   (f)
                                                  3.45                                                                                                                         8.65
              Water vapour uptake(wt%)

                                                                                                                                                   Water vapour uptake(wt%)

                                                  3.40                                                                                                                         8.60

                                                  3.35                                     P=8mbar                                                                             8.55                                  P=22mbar
                                                                                         Experimantal data                                                                                                         Experimantal data
                                                  3.30                                   Theoretical curve                                                                                                         Theoretical curve
                                                                                                                                                                               8.50

                                                  3.25                                                                                                                         8.45

                                                                   370    375   380      385     390         395     400                                                               1480 1485 1490 1495 1500 1505 1510 1515
                                                                                       t(min)                                                                                                              t(min)

                  Theexperimental
                       experimentalcurves
                                       curvesatat      ◦ C and the theoretical curves, according to Equation (6),
     Figure 8.
     Figure   8. The                            2525 °C and  the theoretical curves, according to Equation (6), for
     for 2SiO
     SiO       -H (a,b),
          -H 2(a,b), SiO2SiO2 -C (c,d),
                         -C (c,d), and and
                                        Si-P8Si-P 8 TFSI/SiO
                                             TFSI/SiO        2 -H(I)
                                                         2-H(I) (e,f). (e,f).
30                            1.38                 2.13                 1.84
     Figure 9 shows the relationship between water vapor uptake and De. Effective diffusion
coefficients of all the samples show a decreasing trend with the increase of water vapor adsorption,
which, due to the porosity available for water vapor transport, decreases at higher loading as
Materials 2019, 12, 1782                                                                      11 of 13
adsorbed water vapor occupies a part of porous sites.

                                                    9
                                                    8                   SiO2-C
                                                                        SiO2-H
                                                    7
                                                                        Si-P8TFSI/SiO2-H(I)
                                                    6

                                    De×10-7(m2/s)
                                                    5
                                                    4
                                                    3
                                                    2
                                                    1
                                                    0
                                                        0   5     10     15     20      25
                                                                water vapor (wt%)

            Figure 9.
            Figure    Thecurve
                   9. The curveof
                               of the
                                   the effective
                                       effective diffusion
                                                 diffusion coefficients
                                                           coefficients D
                                                                        Dee versus
                                                                            versus water
                                                                                   water vapor
                                                                                         vapor uptake.
                                                                                               uptake.

4. Conclusions
4. Conclusions
      Phosphonium-based ILs were successfully immobilized on bare silica pretreated by hydrochloric
     Phosphonium-based ILs were successfully immobilized on bare silica pretreated by
acid or calcination. Adsorption and diffusion of water vapor on bare and modified silica were
hydrochloric   acid or calcination. Adsorption and diffusion of water vapor on bare and modified silica
investigated. It can be seen from the analyses of FTIR, TG, and PSD that SiO2 -H has less hydroxyl
were investigated. It can be seen from the analyses of FTIR, TG, and PSD that SiO2-H has less
groups and micropore volumes, which is unfavorable for the adsorption of water vapor, thus it has
hydroxyl groups and micropore volumes, which is unfavorable for the adsorption of water vapor,
better water resistance. After the grafting of ILs on silica, the sorbents form a hydrophobic film on
thus it has better water resistance. After the grafting of ILs on silica, the sorbents form a hydrophobic
the surface to block the entry of water molecules and retain partial pore characteristics of the support.
film on the surface to block the entry of water molecules and retain partial pore characteristics of the
Therefore, Si-P8 TFSI/SiO2 -H(II), with relatively higher IL loading and lower micropore, increases the
support. Therefore, Si-P8TFSI/SiO2-H(II), with relatively higher IL loading and lower micropore,
water-resistance performance. Fitting the model with experimental data is beneficial and suggests
increases the water-resistance performance. Fitting the model with −7        experimental data is beneficial
that the effective diffusion coefficients of bare silica are at a level of 10 , while SiO2 -H, with relatively
and suggests that the effective diffusion coefficients of bare silica are at a level of 10-7, while SiO2-H,
rapid diffusion, as lower content of water vapor is benefit for diffusivity. The diffusion of immobilized
with relatively rapid diffusion, as lower content of water vapor is benefit for diffusivity. The diffusion
ionic liquids on silica did not change much and most of the values under different water vapor
of immobilized ionic liquids on silica    did not change much and most of the values under different
pressure were also at a level of 10−7 , while a slight reduction of the diffusivity was observed due to
water vapor pressure were also at a level of 10-7, while a slight reduction of the diffusivity was
the partial impairing of the pore structure and the forming of a hydrophobic ionic liquid film on the
observed due to the partial impairing of the pore structure and the forming of a hydrophobic ionic
surface. Furthermore, with the increase of water vapor loading, effective diffusion coefficients give a
liquid film on the surface. Furthermore, with the increase of water vapor loading, effective diffusion
decreasing trend.
coefficients give a decreasing trend.
Author Contributions: C.L. and J.Z. conceived the research topic and wrote the manuscript. C.L. and X.H.
designed and carried out the experiments. M.Z. and S.Z. participated to discuss and analyze the data.
Funding: This research was funded by Fundamental Research Funds for the Central Universities,
No. 2019XKQYMS14.
Acknowledgments: The authors gratefully acknowledge the Australia-China Joint Collaboration Group Fund for
Clean Coal Technology Round 2.
Conflicts of Interest: The authors declare no conflict of interest.

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