Investigation on Water Vapor Adsorption of Silica-Phosphonium Ionic Liquids Hybrid Material - MDPI
Page content transcription
If your browser does not render page correctly, please read the page content below
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; firstname.lastname@example.org (C.L.); email@example.com (M.Z.); firstname.lastname@example.org (S.Z.); email@example.com (X.H.) * Correspondence: firstname.lastname@example.org; 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  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  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  studied that CO2 adsorption of activated alumina strongly decreased with increasing water content in the vapor-gas. Xu  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  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  reported that water adsorption at low relative pressures occurred mainly in the micro-porosity of silica. Saliba  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.
Materials 2019, 12, 1782 3 of 13 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 . In thisIL-modified silica using work, another kindgrafted method of silica with was carried different out, pretreated according to a previous report . 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 188.8.131.52.3.1. 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) . 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 .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. 184.108.40.206. 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 220.127.116.11. 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 . The shoulder at 956 cm−1 indicates the existence of the Si-OH asymmetric vicinal, and geminal groups . The shoulder at 956 cm-1 indicates the existence of the Si-OH stretching band . Based on the comparison of the intensity of the characteristic peaks of Si-OH in asymmetric stretching band . 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, . 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 . 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 . 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  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) . 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) . ∂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) . ∞ 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. References 1. Ramdin, M.; Amplianitis, A.; Bazhenov, S.; Volkov, A.; Volkov, V.; Vlugt Thijs, J.H. Solubility of CO2 and CH4 in ionic liquids: Ideal CO2 /CH4 selectivity. Ind. Eng. Chem. Res. 2015, 53, 15427–15435. [CrossRef] 2. WlazlO, M.; Karpińska, M.; Domańska, U. Separation of water/butan-1-ol mixtures based on limiting activity coefficients with phosphonium-based ionic liquid. J. Chem. Thermodyn. 2017, 113, 183–191. [CrossRef] 3. Zhao, Y.S.; Pan, M.G.; Kang, X.J.; Tu, W.H.; Gao, H.S.; Zhang, X.P. Gas separation by ionic liquids: A theoretical study. Chem. Eng. Sci. 2018, 189, 43–55. [CrossRef] 4. Brennecke, J.F.; Gurkan, B.E. Ionic liquids for CO2 capture and emission reduction. J. Phys. Chem. Lett. 2010, 1, 3459–3464. [CrossRef] 5. Makino, T.; Kanakubo, M. Absorption of n-Butane in imidazolium and phosphonium ionic liquids and application to separation of hydrocarbon gases. Sep. Purif. Technol. 2019, 214, 139–147. [CrossRef] 6. Zhou, L.Y.; Fan, J.; Shang, X.M. CO2 capture and separation properties in the ionic liquid 1-n-butyl-3-methylimidazolium nonafluorobutylsulfonate. Materials 2014, 7, 3867–3880. [CrossRef]
Materials 2019, 12, 1782 12 of 13 7. Akhmetshina, A.I.; Gumerova, O.R.; Atlaskin, A.A.; Petukhov, A.N.; Vorotyntsev, I.V. Permeability and selectivity of acid gases in supported conventional and novel imidazolium-based ionic liquid membranes. Sep. Purif. Technol. 2016, 176, 92–106. [CrossRef] 8. Wang, S.B.; Wang, X. Imidazolium Ionic liquids, imidazolylidene heterocyclic carbenes and zeolitic imidazolate frameworks for CO2 capture and photochemical reduction. Angew. Chem. Int. Edit. 2016, 55, 2308–2320. [CrossRef] 9. Ferreira, A.F.; Simoes, P.N.; Ferreira, A.G.M. Quaternary phosphonium-based ionic liquids: Thermal stability and heat capacity of the liquid phase. J. Chem. Thermodyn. 2012, 45, 16–27. [CrossRef] 10. Oster, K.; Goodrich, P.; Jacquemin, J.; Hardacre, C.; Ribeiro, A.P.C.; Elsinawi, A. A new insight into pure and water-saturated quaternary phosphonium-based carboxylate ionic liquids: Density, heat capacity, ionic conductivity, thermogravimetric analysis, thermal conductivity and viscosity. J. Chem. Thermodyn. 2018, 121, 97–111. [CrossRef] 11. Keglevich, G.; Grun, A.; Hermecz, I.; Odinets, I.L. Quaternary phosphonium salt and 1,3–dialkylimidazolium hexafluorophosphate ionic liquids as green chemical tools in organic syntheses. Curr. Org. Chem. 2011, 15, 3824–3848. [CrossRef] 12. Tsunashima, K.; Ono, Y.; Sugiya, M. Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond. Electrochim. Acta. 2011, 56, 4351–4355. [CrossRef] 13. Bradaric, C.J.; Downard, A.; Kennedy, C.; Robertson, A.J.; Zhou, Y.H. Industrial preparation of phosphonium ionic liquids. Green Chem. 2003, 5, 143–152. [CrossRef] 14. Rzelewska, M.; Baczyńska, M.; Wiśniewski, M.; Regelrosocka, M. Phosphonium ionic liquids as extractants for recovery of ruthenium(III) from acidic aqueous solutions. Chem. Pap. 2017, 71, 1065–1072. [CrossRef] 15. Deferm, C.; Malaquias, J.C.; Onghena, B.; Banerjee, D.; Luyten, J.; Oosterhof, H.; Fransaer, J.; Binnemans, K. Electrodeposition of indium from the ionic liquid trihexyl(tetradecyl)phosphonium chloride. Green Chem. 2019, 21, 1517–1530. [CrossRef] 16. Zhang, Y.; Zhang, S.; Lu, X.; Zhou, Q.; Fan, W.; Zhang, X.P. Dual amino-functionalised phosphonium ionic liquids for CO2 capture. Chem. Eur. J. 2010, 15, 3003–3011. [CrossRef] [PubMed] 17. Balsamo, M.; Erto, A.; Lancia, A.; Totarella, G.; Montagnaro, F.; Turco, R. Post-combustion CO2 capture: On the potentiality of amino acid ionic liquid as modifying agent of mesoporous solids. Fuel. 2018, 218, 155–161. [CrossRef] 18. Ansaloni, L.; Nykaza, J.R.; Ye, Y.S.; Elabd, Y.A.; Baschetti, M.G. Influence of water vapor on the gas permeability of polymerized ionic liquids membranes. J. Membran. Sci. 2015, 487, 199–208. [CrossRef] 19. Chanut, N.; Bourrelly, S.; Kuchta, B.; Serre, C.; Chang, J.S.; Wright, P.A.; Llewellyn, P.L. Screening the effect of water vapour on gas adsorption performance: Application to CO2 capture from flue gas in metal-organic frameworks. ChemSusChem 2017, 10, 1543–1553. [CrossRef] [PubMed] 20. Mirzaei, M.; Badiei, A.R.; Mokhtarani, B.; Sharifi, A. Experimental study on CO2 , sorption capacity of the neat and porous silica supported ionic liquids and the effect of water content of flue gas. J. Mol. Liq. 2017, 232, 462–470. [CrossRef] 21. Li, G.; Xiao, P.; Webley, P. Binary adsorption equilibrium of carbon dioxide and water vapor on activated alumina. Langmuir 2009, 25, 10666–10675. [CrossRef] [PubMed] 22. Xu, D.; Xiao, P.; Zhang, J.; Li, G.; Xiao, G.K.; Webley, P.A.; Zhai, Y.C. Effects of water vapour on CO2 capture with vacuum swing adsorption using activated carbon. Chem. Eng. J. 2013, 230, 64–72. [CrossRef] 23. Uehara, Y.; Karami, D.; Mahinpey, N. Effect of water vapor on CO2 sorption-desorption behaviors of supported amino acid ionic liquid sorbents on porous microspheres. Ind. Eng. Chem. Res. 2017, 56, 14316–14323. [CrossRef] 24. Alcañiz-Monge, J.; Pérez-Cadenas, M.; Materials, D. Influence of pore size distribution on water adsorption on silica gels. J. Porous. Mat. 2010, 17, 409–416. [CrossRef] 25. Saliba, S.; Ruch, P.; Volksen, W.; Magbitang, T.P.; Dubois, G.; Michel, B. Combined influence of pore size distribution and surface hydrophilicity on the water adsorption characteristics of micro-and mesoporous silica. Micropor. Mesopor. Mat. 2016, 226, 221–228. [CrossRef] 26. Zhu, J.M.; He, B.T.; Huang, J.H.; Li, C.C.; Ren, T. Effect of immobilization methods and the pore structure on CO2 separation performance in silica-supported ionic liquids. Micropor. Mesopor. Mat. 2018, 260, 190–200. [CrossRef]
Materials 2019, 12, 1782 13 of 13 27. Kim, J.M.; Chang, S.M.; Kong, S.M.; Kim, K.S.; Kim, J.; Kim, W.S. Control of hydroxyl group content in silica particle synthesized by the sol-precipitation process. Ceram. Int. 2009, 35, 1015–1019. [CrossRef] 28. Shiflett, M.B.; Yokozek, A. Solubilities and Diffusivities of carbon dioxide in ionic liquids: [bmim][PF6 ] and [bmim][BF4 ]. Ind. Eng. Chem. Res. 2005, 44, 4453–4464. [CrossRef] 29. Liu, H.; Huang, J.; Pendleton, P. Experimental and modelling study of CO2 absorption in ionic liquids containing Zn (II) ions. Energy Procedia. 2011, 4, 59–66. [CrossRef] 30. Detallante, V.; Langevin, D.; Chappey, M.; Métayer, M.; Mercier, R.; Pinéri, M. Kinetics of water vapor sorption in sulfonated polyimide membranes. Desalination 2002, 148, 333–339. [CrossRef] 31. Zeng, Q.; Xu, S.L. A two-parameter stretched exponential function for dynamic water vapor sorption of cement-based porous materials. Mater. Struct. 2017, 50, 128–141. [CrossRef] 32. Yang, H.; Wang, D.K.; Motuzas, J.; Diniz, da.; Costa, J.C. Hybrid vinyl silane and P123 template sol−gel derived carbon silica membrane for desalination. J. Sol-Gel. Sci. Technol. 2018, 85, 280–289. [CrossRef] 33. Firoozmandan, M.; Moghaddas, J.; Yasrebi, N. Performance of water glass-based silica aerogel for adsorption of phenol from aqueous solution. J. Sol-Gel. Sci. Technol. 2016, 79, 67–75. [CrossRef] 34. Mcdaniel, J.G.; Son, C.Y.; Yethiraj, A. Ab initio force fields for organic anions: Properties of [BMIM][TFSI], [BMIM][FSI], and [BMIM][OTF] ionic liquids. J. Phys. Chem. B 2018, 122, 4101–4114. [CrossRef] [PubMed] 35. Akbari, Z.; Ghiaci, M. Heterogenization of a green homogeneous catalyst: Synthesis and characterization of imidazolium ionene/Br–Cl–@SiO2 as an efficient catalyst for the cycloaddition of CO2 with epoxides. Ind. Eng. Chem. Res. 2017, 56, 9045–9053. [CrossRef] 36. Landers, J.; Gor, G.Y.; Neimark, A.V. Density functional theory methods for characterization of porous materials. Colloids Surf. A Physicochem. Eng. Asp. 2013, 437, 3–32. [CrossRef] 37. Ma, Y.C.; Foster, A.S.; Nieminen, R.M. Reactions and clustering of water with silica surface. J. Chem. Phys. 2005, 122, 144709. [CrossRef] [PubMed] 38. Mahadevan, T.S.; Du, J.C. Evaluating Water Reactivity at Silica Surfaces Using Reactive Potentials. J. Phys. Chem. C 2018, 122, 9875–9885. [CrossRef] 39. Ruthven, D.M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, NY, USA, 1984. 40. Crank, J. The Mathematics of Diffusion, 2nd ed.; Oxford University Press: London, UK, 1975. 41. Oehler, A.; Tomozawa, M. Water diffusion into silica glass at a low temperature under high water vapor pressure. J. Non-Cryst. Solids 2004, 347, 211–219. [CrossRef] 42. Aristov, Y.I.; Tokarev, M.M.; Freni, A.; Glaznev, I.S.; Restuccia, G. Kinetics of water adsorption on silica Fuji Davison RD. Micropor. Mesopor. Mater. 2006, 96, 65–71. [CrossRef] 43. Mohammed, R.H.; Mesalhy, O.; Elsayed, M.L.; Hou, S.; Su, M.; Chow, L.C. Physical properties and adsorption kinetics of silica-gel/water for adsorption chillers. Appl. Therm. Eng. 2018, 137, 368–376. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
You can also read
NEXT SLIDES ... Cancel