Nonuniform Chain-Length-Dependent Diffusion of Short 1 Alcohols in SAPO-34 in Liquid Phase
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Article
pubs.acs.org/JPCC
Nonuniform Chain-Length-Dependent Diffusion of Short 1‑Alcohols
in SAPO-34 in Liquid Phase
Julien Cousin Saint Remi, Gino V. Baron, and Joeri F. M. Denayer*
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
*
S Supporting Information
ABSTRACT: Liquid-phase diffusion of 1-alcohols in SAPO-34 was
explored by batch experimentation. The uptake of pure and binary
mixtures of 1-alcohols, dissolved in tert-butanol, was obtained for C1−C8
1-alcohols at temperatures between 25 and 80 °C, concentrations varying
between 0.5 and 10 wt %, and crystal sizes between 7.5 and 20 μm. The
experimental uptake data were fitted with an intracrystalline diffusion
model and a linear driving force model. The intracrystalline diffusion
coefficient showed a nonuniform stepwise decrease with chain length,
ranging from 10−12 m2/s for methanol to 10−20 m2/s for 1-pentanol. No
effect of the external concentration on the intracrystalline diffusion
coefficient was observed. Variation of the crystal size showed that the
intracrystalline diffusion is the rate-limiting step. On the basis of the
Arrhenius equation, the activation energies of diffusion of ethanol, 1-
propanol, and 1-butanol were determined, being, respectively, 27.8, 47.8,
and 47.2 kJ/mol. Co-diffusion occurred in the uptake of binary mixtures of methanol/ethanol, methanol/1-propanol, and
ethanol/1-propanol, where mutual effects could be noticed. From this experimental work, it could be concluded that the small
dimensions of the SAPO-34 framework generate a very sterically hindered diffusion of 1-alcohols into the crystals, resulting in a
chain-length-dependent behavior, interesting to obtain efficient kinetic-based separations.
■ INTRODUCTION
The depletion of fossil reserves makes our petroleum-based
with a low adsorption capacity. Activated carbons and carbon
molecular sieves show a better adsorption capacity, but the
society search and invest in alternative processes to produce desorption of 1-butanol from the adsorbent after adsorption
energy and transport fuels and chemicals. Fermentation is an remains a problem.3 The effect of the fermentation substrates
example of such a process because it can be executed with and side-products on the adsorption of 1-alcohols has been
renewable feedstocks while generating a wide range of studied by Nielsen5 Bowen and Vane.6 During the past decade
products, like organic acids and 1-alcohols.1 Because of its new adsorbents have become available. There has been an
favorable properties, including low hygroscopicity, large-energy enormous growth of interest in metal−organic frameworks
density, easy transportation, and minor volatility and viscosity, (MOFs) because of their unprecedented capacities and
1-butanol obtained in fermentation forms a very promising chemical and structural tunability.7 The literature concerning
molecule as an alternative chemical and fuel. Nevertheless, the adsorption of hydrocarbon and alcohols on MOFs is well-
significant improvements are still needed to make the reviewed by Wu, Kärger, and coworkers.8,9 For applications in
fermentative production of 1-butanol economically viable. an adsorption technology, chemically stable MOFs are desired.
One of the major challenges for the production of 1-butanol Although most MOFs cannot meet this requirement, the
is its recovery from the fermentation medium. This zeolitic-imidazolate frameworks (ZIFs) have attracted much
fermentation produces acetone, 1-butanol, and ethanol, with a attention due to their high chemical and hydrothermal
productivity of only 20 g/L 1-butanol diluted in the aqueous stability.10
fermentation medium. Acetone, 1-butanol, and ethanol (the Recently, we studied adsorption and separation of butanol on
“ABE” products) are formed at a ratio of 3:6:1.2 The huge ZIF-8, a member of the family of zinc-imidazolate frameworks,
amount of water, the higher boiling point of 1-butanol, and the and compared it with silicalite-1 and active carbon.11 It was
formation of a water-1-butanol azeotrope makes distillation found that ZIF-8 is a promising adsorbent for this application
cost- and energy-intensive to recover pure 1-butanol. Among because of its high adsorption capacity, selectivity, and easy
the alternative recovery techniques, like gas-stripping, pervapo- desorption of 1-butanol. In older work, we also studied the
ration, and extraction, adsorption was identified as the process
with the lowest energy consumption.3,4 Received: December 13, 2012
Previous studies have shown that among the zeolites, Revised: March 4, 2013
silicalite-1 has the best selectivity toward 1-butanol, however, Published: April 15, 2013
© 2013 American Chemical Society 9758 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C Article
separation and adsorption of 1-alcohols with chabazite-type System, FID detector, HP-5 column). This procedure was
adsorbents. Liquid-phase batch adsorption equilibrium experi- repeated at different adsorbate concentrations (1, 2, 4, and 10
ments of 1-alcohols on K-CHA and on SAPO-34 clearly show wt %) and at different temperatures (25, 40, 55, and 70 °C).
that the short 1-alcohols, methanol and ethanol, have higher The vessels were placed in an oil bath, and the temperature was
adsorption capacities than the longer 1-alcohols.12,13 Binary controlled by a RCT basic IKAMAG system (IKA, Germany).
mixtures isotherms indicate a full exclusion of the longer 1- The uptake measurements were done for pure compounds
alcohol at equilibrium, in exception of the methanol−ethanol (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-
mixture, where no clear selectivity appears. This was also hexanol, 1-heptanol, and 1-octanol) and also for mixtures of
observed by molecular simulations on the adsorption and these components: methanol/ethanol, methanol/1-propanol,
separation of 1-alcohols with CHA zeolite.14 Furthermore, and ethanol/1-propanol. The following equation, derived from
SAPO-34 could efficiently separate short 1-alcohols from 1- the total and component mass balance, was used to calculate
butanol,13 which is of great interest for the separation and the amount adsorbing for each time interval (t − 1, t), between
purification of biobutanol − fermentative produced 1-butanol.15 two sampling events:
SAPO-34, a member of the silicon aluminophosphates
t t
minerals, exhibits the same framework structure as the naturally (xi , t − 1 − xi , t ). (ml,0 − mads,0 ∑1 qt − 1 − yl . ∑1 msample, t − 1)
occurring chabazite. The chabazite topology consists of a 3D qt* =
(1 − xi , t ). mads,0
pore system of ellipsoidal cages (6.8 × 10 Å2) interconnected
by eight-membered windows (3.8 × 3.8 Å2), where each cage where q*t is the amount adsorbing between time t − 1 and t, xi,t
possesses six windows. Because of this small pore system and its is the fraction of component i at time t, ml,0 is the initial total
catalytic properties, SAPO-34 is a very interesting material for
mass of fluid, mads,0 is the initial adsorbent mass, ∑t1qt−1 is the
molecular sieve separations and selective catalysis. Much
total amount of adsorbed phase at time t − 1, yl is the fraction
attention has been given to the potential of SAPO-34 to
of liquid phase in the system, and ∑t1msample,t−1 is total amount
separate small gaseous molecules16,17 and to catalyze the MTO
of sample removed from system at time t − 1
process,18,19 but a study of the transport properties of larger
This equation incorporates the change of total mass of the
alcohol molecules is missing.
system and the change of concentration of the fluid due to
Therefore, this work focuses on the liquid phase uptake of 1-
adsorption (Supporting Information). By summing qt* for each
alcohols in SAPO-34. Single compound and binary mixture
time interval, the total amount adsorbed is determined as a
uptake experiments of C1−C8 alcohols in liquid phase were
function of time, generating the uptake curves.
performed for different concentrations, temperatures, and
Linear Driving Force and intracrystalline Diffusion
crystal sizes. The corresponding mass-transfer coefficients
Model. The overall mass transfer and intracrystalline diffusion
were determined by fitting a linear driving force (LDF)
coefficient were determined by fitting the experimental uptake
model and an intracrystalline diffusion model to the data.
■
curves to both the LDF model and the intracrystalline diffusion
MATERIALS AND METHODS model using Athena Visual Studio v14.2.
The LDF model, also called the pseudo-first-order (PFO) or
Materials. Tianjin Scientific provided two SAPO-34 Lagergren first-order (LFO) model, was introduced by
samples with different crystal sizes. The average crystal size Lagergren in 1898 for the adsorption of oxalate and malonate
was calculated from a set of SEM (scanning electron on active carbon.21 This equation describes the adsorption rate
microscope) pictures as an average of 150 different crystals of liquid−solid systems by an overall mass transfer coefficient:
(Supporting Information). The unit cell formula of SAPO-34 is
Si4.02Al18.32P14.58O72, as obtained by inductively coupled plasma ∂q(t )
atomic emission spectroscopy.13 The nitrogen adsorption = kLDF. [q∞ − q(t )]
isotherms were determined with a Quantasorb Autosorb AS-1 ∂t
(Quantachrome Instruments) at 77 K. Micropore volume was where q(t), kLDF, and q∞ are, respectively, the concentration of
determined from the intercept of a standard t plot,20 assuming the adsorbed phase at time t, the overall mass transfer
adsorbed nitrogen to have the density of liquid nitrogen (808 coefficient, and the adsorption capacity at equilibrium for the
kg/m3) (Supporting Information). given concentration.
Uptake Measurements. All 1-alcohols and tert-butanol The intracrystalline diffusion model was derived from Fick’s
were bought from Sigma Aldrich as ACS reagent grade. tert- second law of diffusion. The diffusion of the adsorbate into the
Butanol was used as a nonadsorbing solvent (because it is too adsorbent for a step change in adsorbate concentration,
large to pass through the windows of SAPO-34), wherein the 1- assuming that the adsorbent crystals are spherical and the
alcohols were dissolved for 1, 2, 4, and 10 wt %. The uptake diffusion coefficient is constant and working isothermally, can
measurements were carried out in VWR borosilicate 3.3 glass be written as follows:
vessels of 250 mL. whereupon a polypropylene (PP) cap (VWR
International, USA) combined with a Versilic silicone stop
∂q ⎛ 2 ∂q ∂ 2q ⎞
(Saint-Gobain, France) was placed. About 10 g adsorbent, in = Dc . ⎜ . + 2⎟
powder form, was added to the vessel after activation in a ∂t ⎝ r ∂r ∂r ⎠
Carbolite furnace (Analis SA, Belgium) at 1 °C/min to 550 °C
for 20 h and cooled in a N2-chamber. Then, the vessel with the where q is the concentration of the adsorbed phase, Dc is the
activated adsorbent was filled with a 1-alcohol solution (about intracrystalline diffusion coefficient, and r is the radius. The
250 g) and stirred at 360 rpm during all measurements. The equation was solved with the approximation that the effect of
uptake curves were obtained by taking samples (ca. 1g each the external concentration change due to adsorption was
sample) at different moments and by determining the 1-alcohol negligible, such that the next initial and boundary conditions
concentration of the samples via GC analysis (Agilent 6890 could be used:
9759 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C Article
⎛ ∂q ⎞ ethanol, 1-propanol, 1-butanol, and 1-pentanol. 1-Alcohols
q(r , 0) = 0 q(rc , t ) = qmax ⎜ ⎟ =0 longer than 1-pentanol did not show any measurable
⎝ ∂r ⎠r = 0
adsorption after 1 day, so the diffusion rate should be at least
where rc is the average radius of the adsorbent crystals and qmax as slow as 1-propanol or even slower.
is the equilibrium concentration of the adsorbed phase The correct determination of diffusion coefficients from
■ RESULTS AND DISCUSSION
Single-Component Diffusion. In this study, linear 1-
uptake curves of macroscopic experimental methods is always a
challenge. Because each system behaves differently and various
resistances (bulk phase diffusion, film diffusion, intraparticle
alcohols have been used to investigate the adsorption behavior diffusion), heat effects, and crystal size distribution effects are
of the cage-window system of SAPO-34 in liquid phase. The potentially present, careful analysis is required. In a first
dimensions of the studied 1-alcohols were determined using approach, we fitted the LDF model and the intracrystalline
Chemdraw, where the molecules were surrounded by the diffusion model to the experimental data of the uptake
“Connolly surface”, which is a good measure of the accessible measurements. (See Figures S5A and S5B in the Supporting
surface of the molecule (Table 1). These values are to be Information). The so-obtained parameters are shown in Figure
compared with the size of the windows and the cages of SAPO- 2.
34, which correspond to, respectively, 3.8 × 3.8 and 6.8 × 10
Å2.
Table 1. Length and Width of Linear 1-Alcohols
molecule formula length (Å) width (Å)
methanol CH3OH 5.2 4.0
ethanol C2H5OH 6.3 4.5
1-propanol C3H7OH 7.6 4.5
1-butanol C4H9OH 8.8 4.5
1-pentanol C5H11OH 10.1 4.5
Figure 1 shows the uptake of the different 1-alcohols, diluted
in tert-butanol, as a function of time. Clear differences between
Figure 2. Intracrystalline diffusion coefficient (Dc) and LDF rate
constant (k) as a function of carbon number Nc of 1-alcohols, obtained
from fitting intracrystalline diffusion and LDF models to the uptake
curves (Figure 1).
Although both models fit efficiently to the experimental data
for methanol, ethanol, and 1-propanol, the fitting is less good
for 1-butanol and 1-pentanol (Figures S5A and S5B in the
Supporting Information). This is due to the very slow uptake of
these two 1-alcohols, such that during a long time the amount
adsorbed is too low to be detected accurately, which makes it
very difficult to fit the data to a model. The best fitting was
obtained with the intracrystalline diffusion model derived from
the second law of Fick. Therefore, the further discussion of the
Figure 1. Single-component uptake curves of (●) methanol, (○) results will be based on the intracrystalline diffusion coefficient
ethanol, (■) 1-propanol, (◇) 1-butanol, and (▲) 1-pentanol at room obtained with the latter model. Because microporous crystals
temperature and an external concentration of 0.02 g/g (diluted in tert- were used and no film diffusion can generate such alcohol chain
butanol) on SAPO-34 (crystal size of 20 μm).
length dependence, neither a macropore diffusion model nor a
film diffusion model was fitted to the experimental data.
the 1-alcohols are observed. Methanol fills up the pore volume The intracrystalline diffusion coefficients of 1-alcohols on
withinThe Journal of Physical Chemistry C Article
Figure 3. Uptake curves at different concentrations for (a) methanol, (b) ethanol, (c) 1-propanol, and (d) 1-butanol at room temperature on SAPO-
34 (crystal size of 20 μm).
propanol on SAPO-34 in vapor phase observed by Remy et suggests that the cutoff is dictated by a change in intracage
al.13 were similar to this study. By fitting an appropriate model mechanism. The longer molecules typically interact more
to their uptake curves, the diffusion coefficient matched strongly with the framework.31 Because of more specific
perfectly with those obtained in this research. These previous packing and configuration inside the cages, the longer
studies consolidate the magnitude of the obtained intracrystal- molecules are more likely to follow a preferential path through
line diffusion coefficients and the nonuniform stepwise decrease the crystal. This anisotropic diffusion was also observed by Bär
as a function of the chain length of the 1-alcohols. This et al. in a PFG NMR study of water in chabazite.32 An even
behavior should not be confounded with the window effect, more drastic effect that could arise is that some cages of SAPO-
which arises at a longer chain length, as demonstrated by 34 could never be reached by the longer molecules due to the
molecular simulations on the diffusion of nC8 and nC12 combination of a preferential diffusion path with internal and
alkanes in chabazite.25 The oscillatory relation between the surface barriers that makes some parts of the crystal
intracrystalline diffusion coefficient and the chain length, unavailable.33 Consequently, a less efficient filling of the
introduced as the “window effect” by Gorring in 1973,26 is crystals for the longer molecules could influence the adsorption
still under debate. Although this was later attributed to an equilibrium.
experimental artifact,27 other studies support this nonmono- Furthermore, the adsorption capacities of the different 1-
tonic chain-length-dependent diffusion of molecules in porous alcohols have been compared with those found in the literature
media.25,28,29 Further work should be devoted to the diffusion (Figure S6 in the Supporting Information). The adsorption
of longer 1-alcohols into SAPO-34 to investigate if window capacities of C1−C5 1-alcohols obtained in this study are in
effects occur, as proposed by molecular simulations.25 better agreement with the theoretical capacity obtained in
The intracrystalline diffusion rate is affected by two mass- molecular simulations14 than those determined in previous
transfer processes: first the entry of the adsorbed molecules experimental work.12,13 The difference in adsorption capacity
through the windows (intercage motion) and second the for methanol can be attributed to the fact that in this study the
configurational or intracage diffusion.30 The tight passage external concentration is too low to reach maximal capacity. For
formed by the window exhibits a strong steric hindrance that 1-alcohols longer than ethanol, the adsorption capacities from
must be overcome for the diffusion of the molecules. When the previous experimental studies are underestimated. The amount
diameter of the molecules becomes similar or even bigger than adsorbed was calculated after only 24 h, while the uptake curves
the diameter of the window, diffusion becomes extremely of the present study evidenced that equilibrium is reached
slow.22 Ethanol is a little wider than methanol; therefore, much later. Although the diffusion into the cages of SAPO-34 is
passage through the window is more difficult, leading to slower very slow for 1-propanol and 1-butanol, our experimental
diffusion. For molecules longer than ethanol, a clear cutoff in results prove that long 1-alcohols can adsorb with two
uptake rate occurs. Comparing the dimensions of the 1-alcohols molecules per cage, as previously determined by Krishna et
with those of the SAPO-34 cages, the shorter 1-alcohols al.14 Besides, the curve of the adsorption capacity as a function
methanol and ethanol can fit in any direction in the cage, while of the chain length follows the same trend as that observed for
the longer 1-alcohols fit only in the longitudinal axis. This the diffusivities. A clear cutoff between ethanol and 1-propanol
9761 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C Article
occurs. When the molecule is longer than ethanol, it is forced to obtained at higher temperature was in better agreement with
adsorb along the longitudinal axis of the cages, suggesting a the theoretical capacity obtained in molecular simulations.14
radical change in intracage mobility and packing inside the cage, On the basis of the Arrhenius equation, the activation
resulting in a nonuniform stepwise decrease in, respectively, the energies and pre-exponential factors were determined for the
intracrystalline diffusivities as well as the adsorption capacities different 1-alcohols (Figure 4, Table 4).
as a function of chain length.
Effect of Concentration. Figure 3 shows the different
uptake curves of methanol, ethanol, 1-propanol, and 1-butanol
for concentrations from 0.5 to 10 wt % (diluted in tert-butanol).
It can be seen that in this concentration range the different
uptake curves vary in concentration of the adsorbed phase at a
particular time, but no change of shape of the uptake curves
occurs. The difference in concentration of adsorbed phase at a
particular time is only the expression of the adsorption
isotherm. By dividing the concentration of the adsorbed
phase by the equilibrium concentration it can be seen that the
different uptake curves clearly superpose (Figure S7 in the
Supporting Information), indicating that the external concen-
tration has no influence on the uptake rate. Therefore, the
intracrystalline diffusion coefficient does not vary with the
concentration of the 1-alcohol in the bulk fluid (Table 2). This
is attributed to the fact that the pores of the adsorbent are close
to saturation at all experimental concentrations.
Figure 4. Arrhenius representation of intracrystalline diffusion
Table 2. Intracrystalline Diffusivities as a Function of Liquid- coefficients for (□) ethanol, (▲) 1-propanol, and (○) 1-butanol at
Phase Concentration a liquid phase concentration of 0.02 g/g (diluted in tert-butanol) on
SAPO-34 (crystal size of 7.5 μm).
Dc (m2/s)
C (g/g) methanol ethanol 1-propanol Table 4. Activation Energies (Ea) and Pre-Exponential
0.005 1.7 × 10−12 Factors (Do) for C2−C4 1-Alcohols
0.01 1.4 × 10−12 7.2 × 10−14 adsorbate Ea (kJ/mol) Do (m2/s)
0.02 1.6 × 10−12 6.3 × 10−14 8.5 × 10−18
ethanol 27.8 1.6 × 10−9
0.04 7.1 × 10−14 1.1 × 10−17
1-propanol 47.8 8.4 × 10−10
0.1 4.7 × 10−14 7.5 × 10−18
1-butanol 47.2 5.5 × 10−12
Effect of Temperature. The adsorption and transport
parameters obtained from the uptake curves of ethanol, 1- The observed activation energies are high compared with
propanol, and 1-butanol at temperatures of 25, 40, 55, and 70 those determined for much less polar molecules like ethane,
°C (Figure S8A−C in the Supporting Information) on SAPO- propane, n-alkanes, and n-alkenes in SAPO-34, ranging from
34 (crystal size 7.5 μm) at an external alcohol concentration of 4.2 to 11.71 kJ/mol.34,35 However, other similar eight-
0.02 g/g are given in Table 3. membered ring window materials, like LTA or DDR materials,
The intracrystalline diffusion coefficients (Dc) exhibit a also show high activation energies for linear hydrocarbons.9 ter
temperature dependency for all 1-alcohols. For the equilibrium Horst et al. stipulated that the energy barrier for the diffusion of
adsorbed phase concentration (q∞) the 1-alcohols behave C3 and C4 hydrocarbons from cage to cage, passing through an
differently. At this external concentration, ethanol shows no 8M-ring window in a cage-window type adsorbent, can be
variation of the equilibrium adsorbed phase concentration as a written as the difference between the energy of a molecule in
function of temperature, whereas a decrease was observed for 1- the cage and the energy in the ring.36 Because ethanol, 1-
propanol with increasing temperature. For 1-butanol, an propanol, and 1-butanol have a similar diameter (Table 1), the
increase in the diffusion rate was noticed together with an energy barrier from passing through the window of a SAPO-34
increase in the adsorption capacity. The uptake of 1-butanol is cage is affected by the chain length. Beside that, the intracage
so slow at room temperature that equilibrium was never behavior also influences the activation energy. Ethanol is small
reached. By increasing the temperature, a better assessment of enough to move in any direction in the cage. The longer 1-
the adsorption capacity could be obtained. The capacity alcohols can only arrange themselves in the longitudinal axis
Table 3. Parameters Obtained from the Uptake Curves of 1-Alcohols at Different Temperatures
ethanol 1-propanol 1-butanol
T (°C) Dc (m2/s) q∞ (g/g) Dc (m2/s) q∞ (g/g) Dc (m2/s) q∞ (g/g)
−14 −18 −20
25 1.6 × 10 0.14 3.5 × 10 0.12 3.0 × 10 0.06
40 5.3 × 10−14 0.14 9.5 × 10−18 0.09
55 6.8 × 10−14 0.14 1.8 × 10−17 0.08
70 7.4 × 10−14 0.14 4.7 × 10−17 0.08 3.6 × 10−19 0.11
9762 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C Article
and interact more with the adsorbent surface.31 These by the second type of molecules that diffuses more slowly.37
differences could result in a different temperature dependency This behavior within crystals is governed by a combination of
of the diffusion coefficient. mixture adsorption thermodynamics and mixture diffusion.25
Effect of Crystal Size. In Figure 5 the uptake of ethanol For all binary mixtures, the fastest molecule is the shortest
and 1-propanol by SAPO-34 can be viewed for two different molecule, which is consistent with the differences in diffusion
coefficients obtained via the single-component uptake measure-
ments.
The mutual effect of the molecules on their intracrystalline
diffusion has also been investigated (Figure 7). In a first
Figure 5. Uptake curves of ethanol and 1-propanol at room
temperature and liquid phase concentration of 0.02 g/g for two
different SAPO-34 crystal sizes (filled symbols: crystal size of 7.5 μm,
empty symbols: 20 μm). Figure 7. Comparison of time constants kFD of single-component
uptake (from Figure 2) and binary mixture uptake in SAPO-34 at
room temperature.
crystals sizes. By decreasing the crystal size the equilibrium of
adsorption is reached more quickly. Even the small molecule
ethanol exhibits a clear dependence of crystal size, showing that approach, time-constants kFD have been determined by fitting
the diffusion through the framework of SAPO-34 is a slow and the intracrystalline diffusion model to the uptake curves of both
hindered process. No change in magnitude of intracrystalline mixture components separately and compared with the pure
diffusion coefficient was observed. These experiments prove component uptake. The time constant of the fastest diffusing
that intracrystalline diffusion is the rate-limiting step in the molecule was obtained by fitting the model until the maximum
diffusion of 1-alcohols in SAPO-34. of the uptake curve. This approach implies that no mixture
Binary Mixture Diffusion. The uptake of binary alcohol thermodynamics have been incorporated and no mixture
mixtures, ethanol/1-propanol (Figure 6), methanol/ethanol, diffusivities have been determined but only time constants.
and methanol/1-propanol (Figure S9A,B in the Supporting The correct determination of mixture diffusivities would also
Information), was studied. For all mixtures, codiffusion was require, besides the uptake curves, the complete binary
observed. In other words, a first type of molecules diffuses into isotherm combined with a data analysis based on a Maxwell−
the adsorbent crystal, reaches a maximum, and is then replaced Stefan approach, as proposed by Krishna et al.38 Such an
analysis goes beyond the scope of this study.
Depending on the mixture, different scenarios take place.
When methanol is present in the binary mixture, the uptake of
the longer 1-alcohol is enhanced as compared with the single-
component uptake. Especially for 1-propanol, the time
constants increase by almost two orders of magnitude. As
methanol adsorbs first, its interactions with the cage-window
surface can affect the intraporous environment, making it easier
and more favorable for the slower molecules to diffuse into
SAPO-34. The presence of methanol in the cages can reduce
the available space and interactions for 1-propanol, such that
the latter molecule will retain a higher mobility. As pure
component, 1-propanol fills up the SAPO-34 in such an
effective way that it loses a large part of its mobility. To
determine whether the mixture diffusion is enhanced compared
with pure component diffusion or if this is only an effect of
mixture thermodynamics, a more detailed analysis as
Figure 6. Uptake curves of a binary mixture of ethanol and 1-propanol mentioned above should be carried out.
(diluted in tert-butanol) compared with single component uptake at In the case of an ethanol/1-propanol mixture, a mutual slow
room temperature and an external concentration of 0.02 g/g (with down of the uptake takes place. The dimensions of the 1-
(○) pure ethanol, (●) ethanol in binary mixture, (△) pure 1- alcohols are probably the determining factor for both
propanol, and (▲) 1-propanol in binary mixture). phenomena (Table 1). The smaller the molecule, the higher
9763 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C
■
Article
the chance the molecule can visit all of the spaces in a cavity. ASSOCIATED CONTENT
Therefore, if a larger molecule like 1-propanol is already present
in a cavity, then the motion through the spaces left could be
* Supporting Information
S
more difficult for ethanol than for methanol. Consequently a Additional information on the crystal size distributions and
decrease in uptake of ethanol is observed, while this is not the nitrogen isotherms of the two SAPO-34 samples used in this
case for methanol. Equivalent effects have been noticed for study. A detailed development of the equation used for the
different adsorbate−adsorbent systems.37,39−43 Molecular sim- calculation of the adsorbed phase concentration as a function of
ulations highlighted that this mutual slowdown emerges from time is given. The fitting of the LDF model and the
hydrogen-bonding effects in the uptake of water−alcohol intracrystalline diffusion model to the uptake curves can be
mixtures in zeolites.44 found herein. The comparison between the adsorption
Although all different binary systems behave differently in equilibrium of this study and those found in the literature.
their uptake, they converge to similar equilibrium selectivity Finally, the effects of the external concentration and temper-
(Figure 8). The systems shows a coadsorption of both ature on the uptake curves are presented as well as the uptake
of the binary mixtures of methanol/ethanol and methanol/1-
propanol. This material is available free of charge via the
Internet at http://pubs.acs.org
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: joeri.denayer@vub.ac.be. Tel: +32.2.629.17.98. Fax:
+32.2.629.32.48.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
J.C.S.R. is grateful to the Agency for Innovation by Science and
Technology in Flanders (IWT) for the financial support.
Figure 8. Selectivity for the shorter 1-alcohol in function of time in the
uptake of binary mixtures at room temperature (■ ethanol/1-
■ REFERENCES
(1) Antoni, D.; Zverlov, V.; Schwarz, W. Biofuels from Microbes.
propanol, □ methanol/1-propanol, and ● methanol/ethanol mixture). Appl. Microbiol. Biotechnol. 2007, 77, 23−35.
(2) Dürre, P. New Insights and Novel Developments in Clostridial
Acetone/Butanol/Isopropanol Fermentation. Appl. Microbiol. Biotech-
nol. 1998, 49, 639−648.
molecules rather than a full exclusion of the longer molecules (3) Qureshi, N.; Hughes, S.; Maddox, I.; Cotta, M. Energy-Efficient
Recovery of Butanol from Model Solutions and Fermentation Broth
like that proposed by previous studies for adsorption of by Adsorption. Bioprocess. Biosyst. Eng. 2005, 27, 215−222.
mixtures of 1-alcohols on chabazite-type adsorbents.12−14 In (4) Oudshoorn, A.; van der Wielen, L.; Straathof, A. Assessment of
SAPO-34, the 1-alcohols tend to maximize the adsorbent− Options for Selective 1-Butanol Recovery from Aqueous Solution. Ind.
adsorbate and adsorbate−adsorbate interactions by adopting a Eng. Chem. Res. 2009, 48, 7325−7336.
complex packing inside the cages. Consequently the separation (5) Nielsen, L.; Larsson, M.; Holst, O.; Mattiasson, B. Adsorbents for
of 1-alcohols with SAPO-34 should be performed on a kinetic- Extractive Bioconversion Applied to the Acetone-Butanol Fermenta-
tion. Appl. Microbiol. Biotechnol. 1988, 28, 335−339.
based procedure. (6) Bowen, T.; Vane, L. Ethanol, Acetic Acid, and Water Adsorption
■ CONCLUSIONS
We have demonstrated that the liquid phase diffusion of 1-
from Binary and Ternary Liquid Mixtures on High-Silica Zeolites.
Langmuir 2006, 22, 3721−3737.
(7) Rowsell, J.; Yaghi, O. Metal−Organic Frameworks: a New Class
of Porous Materials. Microporous Mesoporous Mater. 2004, 73, 3−14.
alcohols into SAPO-34 is a very slow process exhibiting a (8) Wu, H.; Gong, Q.; Olson, D.; Li, J. Commensurate Adsorption of
nonuniform chain-length-dependent behavior. A clear cutoff in Hydrocarbons and Alcohols in Microporous Metal Organic Frame-
uptake rate occurs for 1-alcohols longer than ethanol. The works. Chem. Rev. 2012, 112, 836−868.
variation of the external concentration did not affect the uptake (9) Kärger, J.; Ruthven, D.; Theodorou, D. Diffusion in Nanoporous
Materials; Wiley-VCH Verlag: Weinheim, Germany, 2012.
of 1-alcohols into SAPO-34. Activation energies showed that
(10) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.;
the diffusion into SAPO-34 is a highly activated process due to O’Keffe, M.; Yaghi, O. High-Throughput Synthesis of Zeolitic
the dimensions of the SAPO-34 framework being similar to Imidazolate Frameworks and Application to CO2 Capture. Science
those of the 1-alcohols. The uptake of binary mixtures 2008, 319, 939−943.
evidenced the presence of codiffusion. Although the results (11) Cousin Saint Remi, J.; Remy, T.; van de Perre, S.; Duerinck, T.;
suggest an enhancement of the uptake and mutual slowdown Maes, M.; De Vos, D.; Gobechiya, E.; Kirschhock, C.; Baron, G.;
Denayer, J.; et al. Biobutanol Separation with the Metal−Organic
effects, a more complex data-analysis should be carried out to
Framework ZIF-8. ChemSusChem 2011, 4, 1074−1077.
evidence this. Finally, this work indicates that the small (12) Daems, I.; Singh, R.; Baron, G.; Denayer, J. Length Exclusion in
dimensions of the SAPO-34 framework result into very efficient the Adsorption of Chain Molecules on Chabazite Type Zeolites. Chem.
kinetically based separations. Commun. 2007, 1316−1318.
9764 dx.doi.org/10.1021/jp312287k | J. Phys. Chem. C 2013, 117, 9758−9765The Journal of Physical Chemistry C Article
(13) Remy, T.; Cousin Saint Remi, J.; Singh, R.; Webley, P.; Baron, (36) ter Horst, J.; Bromley, S.; Rosmalen, G.; Jansen, J. Molecular
G.; Denayer, J. Adsorption and Separation of C1-C8 Alcohols on Modelling of the Transport Behaviour of C3 and C4 Gases Through
SAPO-34. J. Phys. Chem. C 2011, 115, 8117−8125. the Zeolite DD3R. Microporous Mesoporous Mater. 2002, 53, 45−57.
(14) Krishna, R.; van Baten, J. Entropy-Based Separation of Linear (37) Ruthven, D. Principles of Adsorption and Adsorption Processes;
Chain Molecules by Exploiting Differences in the Saturation John Wiley & Sons: New York, 1984.
Capacities in Cage-Type Zeolites. Sep. Purif. Technol. 2011, 76, (38) Krishna, R.; Baur, R. Modelling Issues in Zeolite Based
325−330. Separation Processes. Sep. Purif. Technol. 2003, 33, 213−254.
(15) Cousin Saint Remi, J.; Baron, G.; Denayer, J. Adsorptive (39) Krishna, R.; van Baten, J. M. Mutual Slowing-Down Effects in
Separations for the Recovery and Purification of Biobutanol. Mixture Diffusion in Zeolites. J. Phys. Chem. C 2010, 114, 13154−
Adsorption 2012, 18, 367−373. 13156.
(16) Ackley, M.; Rege, S.; Saxena, H. Application of Natural Zeolites (40) Krishna, R.; van Baten, J. M. Investigating the Influence of
Diffusional Coupling on Mixture Permeation across Porous Mem-
in the Purification and Separation of Gases. Microporous Mesoporous
branes. J. Membr. Sci. 2013, 430, 113−128.
Mater. 2003, 61, 25−42.
(41) Choudhary, V.; Akolekar, D.; Singh, P. Single- and Multi-
(17) Ruthven, D.; Reyes, S. Adsorptive Separation of Light Olefins
Component Sorption/Diffusion of Hydrocarbons from their Iso-
from Paraffins. Microporous Mesoporous Mater. 2007, 104, 59−66. Octane Solution in H-ZSM-5 Zeolite. Chem. Eng. Sci. 1989, 44, 1047−
(18) Stocker, M. Methanol-to-Hydrocarbons: Catalytic Materials and 1060.
Their Behavior. Microporous Mesoporous Mater. 1999, 29, 3−48. (42) Karge, H. Infrared Spectroscopic Investigation of Diffusion, Co-
(19) Vora, B.; Chen, J.; Bozzano, A.; Glover, B.; Barger, P. Various Diffusion and Counter-Diffusion of Hydrocarbon Molecules in
Routes to Methane Utilization - SAPO-34 Catalysis Offers the Best Zeolites. C. R. Chim. 2005, 8, 303−319.
Option. Catal. Today 2009, 141, 77−83. (43) Lettat, K.; Jolimaitre, E.; Tayakout, M.; Methivier, A.; Tondeur,
(20) de Boer, J.; Lippens, B.; Linsen, B.; Broekhoff, J.; van den D. Co-Diffusion with a Slow Species in Zeolites: Cyclic Exper-
Heuvel, A.; Osinga, T. The t-Curve of Multi-Molecular N2-Adsorption. imentation and Advanced Modeling. Adsorption 2008, 14, 475−484.
J. Colloid Interface Sci. 1966, 21, 405−414. (44) Krishna, R.; van Baten, J. M. Hydrogen Bonding Effects in
(21) Lagergren, S. Zur Theorie der Sogenannten Adsorption Adsorption of Water-Alcohol Mixtures in Zeolites and the
Gelöster Stoffe. Kungliga Svenska Vetenskapsakademiesn 1898, 4, 1−39. Consequences for the Characteristics of Maxwell-Stefan Diffusivities.
(22) Kärger, J.; Ruthven, D. Diffusion in Zeolites and Other Langmuir 2010, 26, 10854−10867.
Microporous Solids; John Wiley & Sons: New York, 1992.
(23) Dyer, A.; Amin, S. Self-Diffusion of Simple Alcohols in
Heteroionic Forms of LTA Zeolites. Microporous Mesoporous Mater.
2001, 46, 163−176.
(24) Van Assche, T.; Remy, T.; Desmet, G.; Baron, G.; Denayer, J.
Adsorptive Separation of Liquid Water/Acetonitrile Mixtures. Sep.
Purif. Technol. 2011, 82, 76−86.
(25) Krishna, R. Describing the Diffusion of Guest Molecules Inside
Porous Structures. J. Phys. Chem. C 2009, 113, 19756−19781.
(26) Gorring, R. Diffusion of Normal Paraffins in Zeolite T.
Occurrence of Window Effect. J. Catal. 1973, 31, 13−26.
(27) Ruthven, D. The Window Effect in Zeolitic Diffusion.
Microporous Mesoporous Mater. 2006, 96, 262−269.
(28) Dubbeldam, D.; Smit, B. Computer Simulation of Incom-
mensurate Diffusion in Zeolites: Understanding Window Effects. J.
Phys. Chem. B 2003, 107, 12138−12152.
(29) Jobic, H.; Methivier, A.; Ehlers, G.; Farago, B.; Haeussler, W.
Accelerated Diffusion of Long-Chain Alkanes between Nanosized
Cavities. Angew. Chem., Int. Ed. 2004, 43, 364−366.
(30) Choudhary, V.; Nayak, V.; Choudhary, T. Single-Component
Sorption/Diffusion of Cyclic Compounds from Their Bulk Liquid
Phase in H-ZSM-5 Zeolite. Ind. Eng. Chem. Res. 1997, 36, 1812−1818.
(31) Denayer, J.; Devriese, L.; Couck, S.; Martens, J.; Singh, R.;
Webley, P.; Baron, G. Cage and Window Effects in the Adsorption of
n-Alkanes on Chabazite and SAPO-34. J. Phys. Chem. C 2008, 112,
16593−16599.
(32) Bär, N.; Kärger, J.; Pfeifer, H.; Schaefer, H.; Schmitz, W.
Diffusion Anisotropy in Natural Chabazite. Microporous Mesoporous
Mater. 1998, 22, 289−295.
(33) Karwacki, L.; van der Bij, H.; Kornatowski, J.; Cubillas, P.;
Drury, M.; de Winter, M.; Anderson, M.; Weckhuysen, B. Unified
Internal Architecture and Surface Barriers for Molecular Diffusion of
Microporous Crystalline Aluminophosphates. Angew. Chem., Int. Ed.
2010, 49, 6790−6794.
(34) Dai, W.; Scheibe, M.; Li, L.; Guan, N.; Hunger, M. Effect of the
Methanol-to-Olefin Conversion on the PFG NMR Self-Diffusivities of
Ethane and Ethene in Large-Crystalline SAPO-34. J. Phys. Chem. C
2012, 116, 2469−2476.
(35) Agarwal, K.; John, M.; Pai, S.; Newalkar, B.; Bhargava, R.;
Choudary, N. SAPO-34 Assisted C3 Separation: Modeling and
Simulation. Microporous Mesoporous Mater. 2010, 132, 311−318.
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