4094
Ind. Eng. Chem. Res. 2002, 41, 4094-4105
Adsorption and Diffusion Properties of Butanes in ZSM-5 Zeolite Membranes Tracy Q. Gardner, Justin B. Lee, Richard D. Noble, and John L. Falconer* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Adsorption isotherms and diffusion coefficients for n-C4 and i-C4 in the transport pathways through tubular ZSM-5 zeolite membranes were determined over a range of temperatures by a transient permeation method. The permeate response to step changes in the feed were measured, and the transport was modeled as Maxwell-Stefan diffusion with single site Langmuir adsorption in the zeolite. The heats of adsorption and the Langmuir parameters were comparable to values reported for MFI powders. The similarity of the membrane and powder isotherms indicates that butanes diffuse mainly through zeolite pores. Maxwell-Stefan diffusion coefficients for n-C4 and i-C4 in the ZSM-5 membranes were similar to those measured by other macroscopic techniques for zeolite crystals and membranes, and diffusion coefficients increased with increasing feed partial pressure for a high-quality membrane. The effective membrane thickness was also estimated from these transient measurements. 1. Introduction Zeolites are crystalline aluminosilicates with pore sizes between 0.3 and 1.3 nm. Zeolite membranes are polycrystalline films of these materials grown on porous supports that provide mechanical stability. Because their pore sizes are of molecular dimensions and they have high chemical and thermal stability, zeolite membranes have the potential to perform many industrially important separations. They are also catalytically active and may be useful for catalytic membrane reactors. Most zeolite membrane research has been on synthesis, characterization, and permeation behavior of films on stainless steel or alumina supports. Films of zeolite types MFI,1-15 FER,16,17 FAU,13,18-20 LTA,13,21,22 MOR,13,21 MEL,23 ANA,21 AFI,13 β,24 and zeolite L13 have been reported, with silicalite and ZSM-5 (both of the MFI structure type) receiving the most attention. Framework substitution, where some of the Si atoms in the framework are replaced by atoms such as boron, germanium, and iron has improved membrane properties in some cases,25,26 and boron-substituted ZSM-5 membranes showed good separation behavior at high temperatures.26 Zeolite powders have been extensively characterized by techniques such as XRD, SEM, EPMA, and adsorption uptake measurements, but tubular-supported zeolite membranes cannot be characterized by these techniques without destroying the membrane. Zeolite membranes are often assumed to have the same properties as the zeolite crystals synthesized at the same time, but the adsorption properties of polycrystalline membranes and powders had not been directly compared until recently.27 To better understand the transport behavior of permeating components, and to provide a quantitative basis for comparison and separation behavior prediction, the transport pathways through zeolite membranes need to be quantitatively characterized. * To whom correspondence should be addressed. Phone: 303 492-8005. Fax: 303 492-4341. E-mail: john.falconer@ colorado.edu.
Zeolite membranes separate mixtures based on differences in the adsorption and diffusion properties of the permeating components. Intracrystalline surface diffusion generally dominates transport through highquality zeolite membranes. The Maxwell-Stefan model has been shown to quantitatively describe steady-state and transient diffusion in zeolites and zeolite membranes.4,27-32 Diffusion coefficients in zeolite crystals have been measured by microscopic (pulsed-field NMR33,34 and quasi-elastic neutron scattering35) and macroscopic techniques (frequency response,36 uptake measurements,36 and chromatography37) and diffusion has been modeled by computer simulations.38-40 Macroscopic techniques yield diffusion coefficients that are 2-3 orders of magnitude lower than those obtained by microscopic techniques or computer simulations. Diffusion in large single crystal and polycrystalline membranes has also been studied by permeation techniques. Kapteijn et al., using a steady-state Wicke-Kallenbach system, measured n-butane diffusion through a polycrystalline silicalite-1 membrane.29 Their diffusivities were comparable to those obtained for zeolite powders using macroscopic techniques. Sun et al.41 measured diffusion coefficients of CO2 and C1-C4 alkanes in a silicalite single crystal using transient measurements at low pressure where Fickian diffusion applied. Their diffusion coefficients were in the 10-10 m2/s range, which was between measurements made by microscopic and other macroscopic methods. Adsorption in zeolite crystals has been measured by gravimetric,42,43 chromatographic,44 and calorimetric45,46 methods. Zhu et al.47 measured adsorption isotherms of butane isomers in silicalite powders with a tapered element oscillating microbalance (TEOM). Langmuir type I adsorption described n-C4 and i-C4 adsorption well at temperatures of 383 K and above. At lower temperatures, a dual site Langmuir isotherm, treating the channels and intersections as adsorption sites with different sorption capacities and adsorption equilibrium constants, was necessary to describe the behavior for i-C4. These results agreed with molecular simulations39,48 that show that linear alkanes preferentially
10.1021/ie020144h CCC: $22.00 © 2002 American Chemical Society Published on Web 06/26/2002
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4095
adsorb in the zeolite channels but also adsorb in the intersections at high coverages. Branched alkanes preferentially reside in the channel intersections, and at higher coverage they also adsorb into the channels. Zhu et al.49 later reconciled TEOM adsorption isotherm measurements of butane isomers with molecular simulations, applying a dual site Langmuir adsorption model for all conditions. The experiments and the simulations agreed and indicated that eight n-C4 molecules per unit cell first adsorb in the channels and, at higher pressures, two more molecules per unit cell adsorb in the intersections. At lower pressures, i-C4 first fills four intersection sites per unit cell and, at higher pressures, i-C4 fills six channel sites per unit cell. The number of adsorption sites per unit cell and whether channels or intersections fill first depend on the adsorbing molecule and the channel and intersection dimensions.50 Gravimetric and calorimetric methods are difficult to use for measuring adsorption on zeolite membranes because the zeolite is typically less than 3% of the membrane weight, and they would also measure adsorption on material that may not be part of the transport pathway. We recently developed a transient permeation method that measured adsorption isotherms for the transport pathways through zeolite membranes and simultaneously determined effective membrane thicknesses and diffusion coefficients. Isotherms generated by this method at 295 K for N2, CH4, and CO2 in high-quality H-ZSM-5 membranes were remarkably similar to those obtained by calorimetry for H-ZSM-5 crystals.27 This suggested that light gases permeated mainly through zeolite pores and indicated that the transient method was valid for characterizing membrane properties. In the current study, the transient model was extended to include support resistance, and a faster experimental method was developed that allows an adsorption isotherm to be determined in a single experiment. Moreover, measurements were made at elevated temperatures so heats of adsorption and activation energies for diffusion could be determined. Two membranes with significantly different properties were prepared for this study. Membrane M1 (H-ZSM-5), prepared by the method of Tuan et al.,12 exhibited molecular sieving behavior, and more than 99% of the n-C4 permeation was through zeolite pores. Membrane M2 (Na-ZSM-5), prepared by the method of Lin et al.,51 separated mixtures based on preferential adsorption and had more flow through parallel nonzeolite pores than membrane M1. Isotherms and diffusion coefficients were measured for temperatures from 295 to 480 K. Maxwell-Stefan diffusion coefficients and their concentration dependence were also measured. 2. Experimental Methods 2.1. Membrane M1 Preparation. Membrane M1 was prepared by an alkali-free method12 on a porous sintered stainless steel tube (0.65-cm i.d., 2.5-cm long, 0.32-cm thick, 26% porosity; Mott Metallurgical Co.) with 500-nm diameter pores. Nonporous stainless steel tubes (1 cm in length) were welded onto the ends of the porous supports to provide smooth surfaces for sealing the membranes into the permeation module. The alkalifree H-ZSM-5 membrane was crystallized hydrothermally on the inside surface of the support. Silica sol (Ludox AS40) was the silicon source, aluminum isopropoxide (Aldrich, 98+%) was the aluminum source, and
tetrapropylammonium hydroxide (TPAOH) was the template for ZSM-5 synthesis. A clear gel with a molar composition of 438 H2O/19.5 SiO2/0.0162 Al2O3/1 TPAOH was used. One end of the support tube was wrapped in Teflon tape and covered with a Teflon cap, and the support was filled with approximately 2 mL of synthesis gel. As the gel soaked into the porous support, the tube was refilled throughout the day. The supports were left to soak overnight and were refilled 3 more times the following day. The other end was then wrapped with Teflon tape and covered with a Teflon cap before placing the tube in a Teflon-lined autoclave. The first zeolite layer was synthesized for 22 h at 458 K. The tubes were then rinsed with DI water, brushed on the inside surface to remove loose crystals, and dried for 10 min in a vacuum oven at 383 K. After synthesis of each layer, N2 at 138 kPa was used to determine if the membranes were gastight. If N2 permeation was detected, another layer was added with the membrane’s vertical orientation in the autoclave switched for each subsequent layer. The membrane required three layers before being impermeable to N2 prior to calcination. After the third synthesis, the membrane was dried in a vacuum oven at 383 K overnight and calcined in stagnant air at 758 K for 8 h (heated and cooled at 0.01 and 0.018 K/s, respectively). 2.2. Membrane M2 Preparation. Membrane M2 was prepared by the method of Lin et al.51 on an R-alumina support (0.7-cm i.d., 4.7-cm long, 500-nm diameter pores, 0.32-cm thick, 40% porosity; U.S. Filter) with glazed ends (∼1 cm on each end) to provide a smooth surface for sealing the membrane into the module. The Na-ZSM-5 membrane was hydrothermally synthesized on the inside surface of the support. The molar gel composition was 987 H2O/21 SiO2/0.105 Na2Al2O4‚3H2O/1 TPAOH/3 NaOH. The gel for membrane M2 was thicker than that for M1 and thus did not need to soak into the support before synthesis. One end of the tube was wrapped with Teflon tape and covered with a Teflon cap, and approximately 2 mL of gel was poured into the support. The other end was then wrapped with Teflon tape and capped with a Teflon cap. The membrane was synthesized, dried, and calcined by the same procedures used for membrane M1. Between synthesis layers, membrane M2 was boiled in deionized water for 1 h and dried at 443 K. Two synthesis layers (15 h for the first layer, 8 h for the second layer) were added at 443 K, with the membrane’s vertical orientation switched for the second layer. 2.3. Transient Permeation Measurements. A Wicke-Kallenbach system,4 with helium as the carrier gas and feed diluent, was used for the transient permeation measurements. The membranes were sealed in a stainless steel module with Viton O-rings on each nonporous end. The module and feed line were wrapped with heating tape controlled by a variac. Before each measurement, helium flowed on the feed (inside of the tube) and permeate sides of the membrane, and the module was heated to 473 K to remove adsorbed species. The permeate pressure was ∼82 kPa (atmospheric pressure in Boulder, CO) and the feed pressure was ∼120 kPa. Both the helium feed gas and the butane flowed at 100 cm3/min (standard temperature and pressure) to a manual switching valve that sent one inlet to a vent and the other to the membrane feed side. Initially, helium flowed to the membrane and butane flowed to the vent. When the valve was switched, helium
4096
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002
went to the vent and butane was fed to the membrane. Back pressure regulators on the retentate and bypass lines were adjusted so the feed pressures of helium and butane were each at 120 kPa and did not change when the valve was switched. The response in the permeate concentration to this step change in the feed was measured with a quadrupole mass spectrometer (Pfeiffer Vacuum Prisma) that monitored multiple mass peaks simultaneously. The total flow rate was measured with a bubble flow meter. 2.3.1. Adsorption/Desorption Measurements. For some experiments, one feed was pure helium and the other was either butane or a butane/helium mixture. After the permeate response to a butane feed reached steady-state, the feed was switched to helium and the desorption response was measured until butane was no longer detected in the permeate or retentate. When the temperature was low and desorption was slow, the module was heated to 473 K to speed up butane desorption. After all of the butane had desorbed, the next highest concentration of butane was fed to the membrane. The initial measurement was 5% butane, and it was repeated at the end to check reproducibility. This measurement method was the same as that used previously for light gases.27 2.3.2. Step Increase Measurements. For other measurements, both feeds consisted of helium/butane mixtures. Initially, helium flowed to the membrane and 5% butane flowed in the bypass line. The valve was switched so 5% butane fed the membrane, and the other feed line concentration was then changed to 20% butane in helium. When the permeate concentration reached steady-state, the valve was switched so 20% butane flowed to the membrane. The feed concentration was then increased stepwise to 50% and 100% butane and then decreased from 100% to 5% butane without desorbing butane between changes. These measurements were significantly faster than those by the previously described method. Adsorption/desorption and step increase measurements were analyzed in the same way. 2.4. System Response. To determine the system response time and how close the feed input is to a step change, transient measurements were carried out on a support without a zeolite layer because its permeance was more than 1000 times higher than that through a zeolite membrane. Two seconds after the feed valve was switched, the signal for the test gas on the permeate side was at 98% of its steady-state value. This time included the times for the feed gas to reach the support, flow through it, and for the sweep gas to carry the permeate to the mass spectrometer. 3. Modeling Transient butane permeation was modeled similarly to light gas permeation as described previously.27 In the current study, a mass transfer resistance due to the support was added and the model was modified to account for the high coverages of butane in the zeolite. Also, step increase experiments were modeled for each feed concentration using the steady-state coverage profile from the previous feed concentration as the initial condition. Modeling the transport through the membranes as Maxwell-Stefan surface diffusion with Langmuir type I adsorption leads to the following description of the transient surface occupancies:
∂θ ∂ 1 ∂θ ) ^MS ∂t ∂z 1 - θ ∂z
(
)
(1)
where ^MS is the Maxwell-Stefan diffusion coefficient and θ is the fractional coverage. The coverage for Langmuir adsorption is described by
θ)
bP q ) qsat 1 + bP
(2)
where q is the coverage (mol/kg zeolite), qsat is the saturation coverage (mol/kg zeolite), P is the partial pressure of the component of interest (kPa), and b is an adsorption equilibrium constant (kPa-1). The feed side boundary condition is determined from eq 2, where P is Po, the feed partial pressure of the permeating component. The permeate side boundary condition is also calculated from eq 2, where P is Pδ, the partial pressure of the permeating component at the zeolite/ support interface. 3.1. Support Resistance. Van de Graaf et al.52 showed that the partial pressures can drop significantly across the support during Wicke-Kallenbach measurements, and therefore, the resistance of the support to mass transfer could not be neglected in some cases. Krishna53 developed an approach based on the MaxwellStefan equations for modeling binary molecular diffusion, such as that in the pores of the support. The partial pressure gradient across the support layer of thickness, δsup (m), and porosity, , for component i in helium is described by
-
yHeJi + yiJHe 1 ∇Pi ) RT sup^iHe
(3)
where R is the gas constant, T is the temperature (K), y’s are the mole fractions in the support, Ji is the flux of the permeating component (mol/m2 s), and JHe is the counter flux of helium (mol/m2 s). The molecular diffusivity, ^iHe (m2/s), was calculated using the correlation of Fuller et al.54
10-7T1.75 ^AB ) Ptot((
(
1
+
MWA
νa ) ∑ A
1/3
1 MWB
νa) ∑ B
+(
)
1/2
(4)
1/3 2
)
where T is the temperature (K), MWi is the molecular weight (g/mol) of component i, Ptot is the total pressure (atm), and the νa’s are the diffusion volumes. Each atom in a molecule has a diffusion volume, and the sums in eq 4 are over all atoms in molecules A and B. Fuller et al.54 list atomic diffusion volumes for common atoms and simple molecules. The support pores are assumed to be filled with stagnant helium, and the partial pressure drop across the support is assumed linear. The partial pressure of the permeating component (i) at the zeolite/support interface, Pδ, is then
Pδ )
Ji
Ji + Pb; Pb ) Ptot,perm Kperm Ji + C
(5)
where Pb (kPa) is the partial pressure of the permeating component in the bulk permeate stream (with the carrier gas), Ptot,perm (kPa) is the total pressure on the
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4097
permeate side, C is the flow rate of the carrier gas converted to flux units (mol/m2 s), and Kperm (mol/m2 s kPa) is the permeate side mass transfer coefficient defined by
Kperm )
sup^iHe δsupRT
(6)
Including the resistance of the support leads to higher coverages at the support/zeolite interface, which can significantly affect the values of the parameters measured by this method, especially when the conditions are such that small changes in the permeate pressure lead to large changes in the permeate side coverage (e.g., when b is large and the membrane is not saturated). Because Kperm is in the denominator in eq 5, the effect of including the support resistance on Pδ is greater when Kperm is smaller. The support resistance affects Pδ more when the support is thicker, the support porosity is lower, the temperature is lower (Kperm R T0.75), the permeate pressure is higher, or the permeant is heavier. 3.2. Adsorption. At steady state, eq 1 with boundary conditions q(0) ) qo and q(δ) ) qδ, can be solved analytically for the coverage profile, q(z). The steadystate coverage profile integrated across the thickness of the membrane yields the total number of moles in the membrane at steady state (Qc)
(
q0 - qδ qsat Qc ) FAqsatδ 1 + qsat - q0 ln qsat - qδ
(
)
)
(7)
This equation assumes that the membrane is a single crystal of cross-sectional area A (m2), density F (kg zeolite/m3), and thickness δ (m), and the subscript c denotes that Qc (mol) was determined from the steadystate coverage profile. The total number of moles in the membrane at steady state can also be determined from the transient flux. The accumulation of molecules in the membrane per unit cross-sectional area is equal to the flux in (Jin, mol/ m2 s) minus the flux out (Jout, mol/m2 s) added up over time. Integrating this difference from the step feed change (t ) 0) until steady state (t ) tSS) and multiplying the result by the membrane area yields the total number of moles in the membrane at steady state (Qt)
∫0t
Qt ) A
(Jin - Jout)dt
SS
(8)
The subscript t indicates that this measure of the total amount of moles in the membrane at steady state is determined from the measured transient flux. To measure the flux into the membrane, both the permeate and the retentate have to be analyzed. When the butane feed concentration is high, small changes between the feed and retentate concentrations cannot be accurately measured, and for pure butane the transient flow rate rather than the concentration must be measured. In addition, a small amount of helium back-diffuses to the feed side and affects the retentate flow rate. Thus, instead of trying to measure the retentate concentration and flow rate, we calculated the integrated value of flux in minus flux out. A factor, F, relating the accumulated moles to the measured transient flux out is defined as
∫0t F) t ∫0
(Jin - Jout)dt
SS
Qt
)
(JSS - Jout)dt
∫0t
A(JSStSS -
SS
SS
(9) Jout dt)
When F is known, Qt can be determined from the transient permeate response. For Fickian diffusion, eq 9 can be solved analytically for F in terms of the Fickian diffusion coefficient (D, m2/s), mass transfer coefficient, and membrane thickness55
FFick )
3(2D + δK)(D + δK) δK(3D + δK)
(10)
The limit of eq 10 as K f ∞ (as mass transfer resistance of the support becomes negligible) is F ) 3. For Maxwell-Stefan diffusion with support resistance at the permeate boundary, eq 9 cannot be solved analytically, so F must be determined numerically from the transient model. For this study, F was determined iteratively for each feed condition as described in the following sections. By minimizing the sum of squared errors (SSE) for (Qc - Qt) for a set of three or more feed conditions, the fitting parameters δ, qsat, and b were determined simultaneously. The heat of adsorption (-∆Hads, kJ/mol) and entropy of adsorption (∆S, kJ/(mol‚K)) were calculated from the adsorption equilibrium constant measured at different temperatures using eq 11
b ) b0 exp
(
)
(
)
-∆Hads ∆S ∆Hads ) exp RT R RT
(11)
3.3. Diffusion. Once the adsorption parameters and effective membrane thickness were known, MaxwellStefan diffusion coefficients were calculated from the steady-state fluxes (JSS, mol/m2 s) by
^MS ) δ
JSS 1 + bP0 Fqsat ln 1 + bPδ
(
)
(12)
which is the result of integrating eq 1 at steady-state once over z to yield an expression for dθ/dz and evaluating that at the permeate boundary (z ) δ) to determine the flux
1 ∂θ | J ) -Fqsat^MS 1 - θ ∂z z)δ
(13)
Maxwell-Stefan diffusion is activated, and the preexponential (^MS0, m2/s) and activation energy for diffusion (Ea diff, kJ/mol) were determined from Arrhenius plots of ^MS versus 1/T following eq 14
^MS ) ^MS0 exp
(
)
-Ea diff RT
(14)
3.4. Modeling Transient Permeation. The adsorption and diffusion parameters were then used to determine the transient permeate response to step changes in the feed concentrations by numerical solution of eq 1 using the method of lines. The input parameters were Po, qsat, b, ^MS, Kperm, δ, and Ptot,perm, and the model calculated the transient flux and the coverage profiles across the membrane for each time step. The spatial derivatives were estimated by finite differences, and the Adams Gear ODE solver was used to solve the time
4098
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002
Table 1. Adsorption Isotherm Parameters for n-C4 in MFI-type Zeolites
a
temp (K)
qsat (mol/kg)
b (kPa-1)
reference
296 303a 300 295 300a 308
1.9 1.2 (c), 0.18 (i) 2.1 2.2 1.6 (c), 0.17 (i) 1.6
27 17 (c), 0.20 (i) 0.72 4.7 16 (c), 0.011 (i) 11
this study, H-ZSM-5 membrane TEOM, silicalite powder47 gravimetric, silicalite powder63 volumetric, silicalite powder64 CBMCb simulation, silicalite65 gravimetric, silicalite powder43
Isotherms were fit with a dual-site Langmuir isotherm (channels (c) and intersections (i)). b Configurational-bias Monte Carlo.
Table 2. Adsorption Isotherm Parameters for n-C4 and i-C4 in H-ZSM-5 Membrane M1 and for Silicalite Powder47(in Parentheses) n-C4 temp (K)
qsat (mol/kg)
296 338 383 406 426 469
1.9 (1.2 (c), 0.18 (i)) 1.8 (1.1 (c), 1.9 (i)) 1.6 (1.2) 1.5 (1.2) 1.4 (1.1) 1.3 (0.87)
i-C4 b
(kPa-1)
27 (17 (c), 0.20 (i)) 2.1 (1.9 (c), 0.14 (i)) 0.27 (0.24) 0.09 (0.064) 0.073 (0.024) 0.022 (0.013)
derivative. Step experiments were modeled using the modeled steady-state coverage profile from the previous feed concentration as the initial condition for the new feed concentration. 3.5. Iterative Solution Method. The factors in eq 9 and the adsorption parameters and membrane thickness were determined iteratively using the transient model via the following steps: (1) assume factor (F) for each feed condition is 3 to get Qt’s from the measured transients; (2) minimize SSE for all conditions for one temperature to get δ, qsat, and b; (3) run the transient Maxwell-Stefan numerical model to get Jin, Jout, and coverage profiles; (4) numerically integrate elements in eq 9 to get new F for each feed condition; (5) use new F’s to get new Qt’s from the measured transients; and (6) repeat steps 2-5 until factors (and thus adsorption parameters and δ) do not change.
qsat (mol/kg)
b (kPa-1)
0.70 (0.59) 0.66 (0.57) 0.65 (0.55) 0.53 (0.55)
0.45 (0.30) 0.23 (0.093) 0.09 (0.039) 0.04 (0.016)
Figure 1. Adsorption isotherms for n-C4 in H-ZSM-5 membrane M1. The filled dots (b) are the measurements from step experiments, the open squares (0) are measurements from adsorption/ desorption experiments, and the dashed lines (- -) are from the model.
4. Results Ideal selectivities (n-C4/i-C4) varied from 57 at 383 K to 10 at 473 K for membrane M1 and from 5 at 383 K to 2 at 473 K for membrane M2. Because membrane M1 had high n-C4/i-C4 ideal selectivities, the majority of its transport is through zeolite pores. Membrane M2 is of lower quality, and has a higher fraction of its flow through parallel nonzeolite pores. 4.1. Adsorption and Diffusion in Membrane M1. The n-C4 adsorption isotherms in Figure 1 were generated at six temperatures by the transient method for membrane M1. Both adsorption/desorption and step experiments were run at 469 K, and the coverages were almost identical, as shown in Figure 1. Because isotherms and diffusion parameters were the same for both methods, the faster step experiments were used for the rest of the measurements. The SSE for (Qc - Qt) was minimized for the 24 transient responses to determine the 13 parameters: qsat and b for the six temperatures and the membrane thickness. The membrane thickness was 220 µm, which is the same value determined from transient experiments for N2, CH4, and CO2 for this membrane.27 The Langmuir parameters for n-C4 in membrane M1 near room temperature are listed in Table 1 along with literature parameters for MFI-type zeolite crystals. The Langmuir parameters for n-C4 at higher temperatures are compared to parameters measured on silicalite powders by TEOM in Table 2.
Figure 2. Adsorption isotherms for i-C4 in H-ZSM-5 membrane M1. The points are the measurements from step experiments, and the dashed lines are from the model.
Isotherms generated for i-C4 at four temperatures for membrane M1 are shown in Figure 2. Note that the coverages are approximately half of those for n-C4. Below 383 K, the single site Langmuir isotherm does not adequately describe i-C4 adsorption in silicalite crystals.47 The dual site adsorption isotherm significantly complicates the model and adds more parameters, so more measurements would be needed for each isotherm. The single site model is sufficient for higher temperatures, so we did not analyze i-C4 data below 383 K. All transient i-C4 experiments were step experiments where the membrane pores were not empty between increasing feed concentrations. The membrane thick-
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4099
Figure 3. Adsorption equilibrium constants versus inverse temperature for n-C4 and i-C4 adsorption in H-ZSM-5 membrane M1. Table 3. Adsorption Equilibrium Properties for n-C4 and i-C4 in H-ZSM-5 Membrane M1 gas n-C4
i-C4
-∆Sads (J/mol K)
-∆Hads (kJ/mol)
131 ( 3.8 113(c)
46 ( 1.4 56(c)
126(i)
50(i)
89 85 110 ( 4.7 200(c)
37.5 45.9 48 40 ( 2.0 66(c)
83(i)
46(i)
144(c)
45(c)
88(i)
50(i)
82
34.1
Figure 4. Maxwell-Stefan diffusion coefficients (^MS) versus inverse temperature for n-C4 and i-C4 diffusion through H-ZSM-5 membrane M1: (b) Po ) 6 kPa, ([) Po ) 23 kPa, (2) Po ) 59 kPa, (9) Po ) 117 kPa.
reference this study TEOM, silicalite powder (channels)49 TEOM, silicalite powder (intersections)49 gravimetric, silicalite powder63 isobaric, silicalite powder64 TAP reactor, H-ZSM-5 powder66 this study TEOM, silicalite powder (channels)49 TEOM, silicalite powder (intersections)49 TPD, silicalite powder (channels)57 TPD, silicalite powder (intersections)57 isobaric, silicalite powder64
Figure 5. Adsorption isotherms for n-C4 in Na-ZSM-5 membrane M2. The points are the measurements from step experiments, and the dashed lines are from the model.
ness was assumed to be 220 µm, and for each isotherm, the SSE for (Qc - Qt) was minimized for the four transient permeate responses at each temperature to determine qsat and b. Langmuir parameters for i-C4 in membrane M1 are compared to those measured for MFI crystals in Table 2. The heats of adsorption determined from Arrhenius plots of the adsorption equilibrium constants for n-C4 and i-C4 in membrane M1 (Figure 3) were 46 ( 1.4 kJ/ mol for n-C4 and 40 ( 2.0 kJ/mol for i-C4. The heats and entropies of adsorption for butane isomers in membrane M1 are compared in Table 3 with literature values for MFI crystals. Maxwell-Stefan diffusion coefficients of the butane isomers in membrane M1 (Figure 4) were higher for
higher feed partial pressures and exhibited the Arrhenius-type dependence on temperature expected for activated diffusion. The diffusion activation energies decreased slightly with increasing feed concentration and were on average 18 ( 1 kJ/mol for n-C4 and 48 ( 4 kJ/mol for i-C4. Activation energies for each feed concentration are listed in Table 4 along with diffusion preexponential factors and activation energies for MFI crystals and membranes measured by microscopic and macroscopic methods. 4.2. Adsorption and Diffusion in Membrane M2. Adsorption isotherms for n-C4 and i-C4 measured at four temperatures for membrane M2 are shown in Figures 5 and 6. Each isotherm follows Langmuir type I behavior in the pressure range studied, but coverages are higher at higher temperatures, which is unreasonable, and some show much higher coverages than would be
Table 4. Diffusion Parameters for n-C4 and i-C4 in H-ZSM-5 Membrane M1 and Literature Values for Membrane and Crystalsa n-C4 feed % C4 5 20 50 100 silicalite membrane (steady state) transient; SCM TAP reactor; crystals69 PFG-NMR; crystals FR; crystals72 CPC; crystals ZLC; crystals
Do ×
1010
(m2/s)
320 ( 74 250 ( 77 250 ( 115 88 ( 9.1 40,64 50,30063 90
i-C4 Ea diff (kJ/mol) 20 ( 0.7 19 ( 1.0 18 ( 1.4 14 ( 0.3 13.7,64 29.8,63 2167 5.8,68 1041 7.3 7,70 8.171 21.5 45.537, 5273 41.474
Do ×
107
(m2/s)
Ea diff (kJ/mol)
33 ( 20 130 ( 90 83 ( 60 51 ( 37 1564
46 ( 3.4 50 ( 4.2 48 ( 4.4 46 ( 4.4 15.164
0.61
29
a SCM, single-crystal membrane; TAP, temporal analysis of products; PFG-NMR, pulsed field gradient nuclear magnetic resonance; FR, frequency response; CPC, concentration pulse chromatography; ZLC, zero length chromatography.
4100
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002
Figure 6. Adsorption isotherms for i-C4 in Na-ZSM-5 membrane M2. The points are the measurements from step experiments, and the dashed lines are from the model.
Figure 7. Measured (solid lines) and modeled (dashed lines) n-C4 permeate responses through H-ZSM-5 membrane M1 at 469 K for step changes in the feed. The feed partial pressure was sequentially increased from 0 to 6 kPa, then to 23, 59, and 117 kPa, and then back to 6 kPa. The transients have been shifted on the time axis so that t ) 0 represents the time of the feed step change for each step.
expected in MFI pores. The adsorption equilibrium constants did not show Arrhenius-type behavior with temperature. Thus, the model assuming MaxwellStefan diffusion through zeolite pores is not valid for membrane M2, and no further analysis was carried out. 4.3. Transient Responses and Coverage Profiles. 4.3.1. n-C4 in Membrane M1. Typical measured and modeled transient responses for the step experiments are shown in Figure 7 for n-C4 permeation in membrane M1 at 469 K. The model consistently underpredicts the slope of the rise to steady state for the higher concentrations, but overall the model accurately predicts the breakthrough times and the shapes of the transient flux responses. The desorption response, when the feed concentration was lowered from 100% to 5%, is also wellpredicted by the transient model. The partial pressures of n-C4 at the zeolite/support interface were between 0.2 kPa (5% at 294 K) and 5 kPa (100% at 383, 406, and 426 K). In all cases, because of the support resistance, this pressure was approximately double the partial pressure of n-C4 in the permeate gas stream. When the adsorption equilibrium constant is high, these small pressure changes can significantly affect the coverage profiles. The steady-state coverage profiles calculated from the model for n-C4 in membrane M1 at 383 and 469 K are shown in Figures 8 and 9, respectively. Coverage profiles calculated with the same adsorption parameters, but without support resistance in the model are shown for comparison. 4.3.2. i-C4 in Membrane M1. The transient permeate response through membrane M1 to a feed step
Figure 8. Modeled coverage profiles for steady-state n-C4 permeation in H-ZSM-5 membrane M1 at 383 K and feed partial pressures of 6, 23, 59, and 117 kPa. Solid lines are the profiles with the support resistance included in the model, and dashed lines are the profiles without the support resistance included.
Figure 9. Modeled coverage profiles for steady-state n-C4 permeation in H-ZSM-5 membrane M1 at 469 K and feed partial pressures of 6, 23, 59, and 117 kPa. Solid lines are the profiles with the support resistance included in the model, and dashed lines are the profiles without the support resistance included.
Figure 10. i-C4 permeate responses at 383 K to feed step changes from 0 to 6 kPa for H-ZSM-5 membrane M1. The insert shows a close-up of the initial breakthrough. The x- and y-axis labels and units are the same for the insert as for the figure.
change from 0% to 5% i-C4 at 383 K approached steadystate in two distinct stages (Figure 10). Burggraaf et al.56 saw a similar two-step transient for n-C4 flow through a silicalite membrane and attributed the first step to flow through parallel nonzeolite pores. Following Burggraaf’s analysis, under these conditions, approximately 4% of the i-C4 flow is through nonzeolite pores. The i-C4 transient permeate response at higher temperatures also approached steady-state in two stages, but at the higher temperatures the breakthrough time for the zeolite pores was shorter and the first step was not at steady state before the zeolite pore flow domi-
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4101
Figure 11. Measured (solid lines) and modeled (dashed lines) i-C4 permeate responses through H-ZSM-5 membrane M1 at 480 K for step changes in the feed. The feed partial pressure was sequentially increased from 0 to 8 kPa, then to 23, 58, and 117 kPa, and then back to 8 kPa. The transients have been shifted on the time axis so that t ) 0 represents the time of the feed step change for each step.
Figure 12. Modeled coverage profiles for steady-state i-C4 permeation in H-ZSM-5 membrane M1 at 383 K and feed partial pressures of 8, 23, 58, and 117 kPa. Solid lines are the profiles with the support resistance included in the model, and dashed lines are the profiles without the support resistance included.
nated. N-Butane permeation did not exhibit the twostep approach to steady state, even at 295 K and 6 kPa partial feed pressure, where the breakthrough time for n-C4 permeation was 4.5 min. Typical measured and modeled responses of i-C4 through membrane M1 are shown in Figure 11. The steady-state coverage profiles for i-C4 in membrane M1 at 383 K with and without support resistance in the model are shown in Figure 12. The measured i-C4 transients in M1 demonstrated curious behavior when the step changes were made. When the feed concentration was increased, the permeate flux initially dropped by about 5% before rising to the new steady state. When the feed concentration was decreased from 100% to 5%, the permeate flux increased to a maximum about 5-8% higher than the previous steady-state flux before dropping to the new steady-state flux. This behavior was observed at 409, 437, and 480 K for i-C4 in membrane M1 and was more pronounced at higher temperature. The model did not predict this behavior. 4.4. Factors Relating Flux In and Flux Out. The factors F relating the time integrals of (Jin - Jout) and (JSS - Jout) (eq 9) between 383 and 480 K were 2.6-3.0 for n-C4 and 1.9-2.9 for i-C4 and generally decreased with increasing average coverage in the membrane. The heat of adsorption calculated for i-C4 when the factors were set to 3 was 26 kJ/mol (Figure 13), and the calculated saturation coverages decreased from 1.2 to 0.5 mol/kg as the temperature increased (Figure 14). When the factors were determined iteratively, the heat
Figure 13. i-C4 adsorption equilibrium constants calculated by the transient method with the factor in eq 9 set equal to 3 (2), with the factors determined iteratively (b), and as measured for silicalite-1 crystals by TEOM47 (9).
Figure 14. i-C4 saturation coverages calculated by the transient method with the factor in eq 9 set equal to 3 (2), with factors determined iteratively (b), and as measured for silicalite-1 crystals by TEOM47 (9). Dashed lines are included as a guide to the eye.
of adsorption for i-C4 was 40 kJ/mol, similar to the heat of adsorption for i-C4 measured in MFI crystals (Table 3), and the saturation coverages were almost constant at approximately 0.66 mol/kg. 5. Discussion In this study, significant improvements were made to the transient characterization method that was first used for light gases.27 The step measurement technique significantly reduced the time required to measure an adsorption isotherm. It also made the assumption of constant ^MS for each step more valid because the change in coverage for each transient response was smaller. Including the support resistance was necessary for modeling butane adsorption because small changes in the permeate pressure significantly changed the modeled coverage profiles used to calculate adsorption parameters and membrane thicknesses (Figure 8). The factor F (eq 9) was lower than 3 for butane adsorption, so the iterative approach was necessary to obtain accurate values of the parameters. With these improvements, the n-C4 and i-C4 adsorption and diffusion properties in the transport pathways of an H-ZSM-5 membrane were accurately measured over a range of temperatures. The results for membranes M1 and M2 were significantly different, presumably because of the different relative extents of flow through parallel nonzeolite pores. Membrane M1 was of higher quality, with higher ideal selectivities, no measurable n-C4 flow through parallel nonzeolite pores, and adsorption and diffusion properties comparable to MFI-type zeolite crystals.
4102
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002
Membrane M2 had higher fluxes than M1 and was also a useful membrane because it separated a butane isomer mixture at elevated temperature (n-C4/i-C4 separation selectivity ) 106 at 413 K).51 Membrane M2 separates the isomers because of diffusivity differences (ideal selectivity greater than 1) and selective adsorption, because the separation selectivity was more than 20 times higher than the ideal selectivity. Membrane M2 was used to demonstrate that this transient method can distinguish differences in membranes that have different qualities (fractions of flow through nonzeolite pores). 5.1.1. Step Increase versus Adsorption/Desorption Measurements. This transient method was first used to measure N2, CH4, and CO2 adsorption and diffusion properties in zeolite membranes at 295 K. Adsorption isotherms for the light gases were remarkably similar to isotherms measured for H-ZSM-5 powders, suggesting that the Maxwell-Stefan model and the assumptions made were valid and that these light gases permeated mainly through zeolite pores. For CO2, the strongest adsorbing of the three gases, the adsorption/desorption experiments took up to 10 h to measure seven isotherm points because of the time needed for CO2 to desorb between measurements. Measuring four isotherm points for n-C4 at 383 K by the adsorption/ desorption method took over 24 h, even though the membrane was heated to 473 K between measurements to speed up butane desorption. Thus, the mathematical model was modified to start with a coverage profile as an initial condition so step increase measurements could be analyzed. Obtaining the data for the i-C4 isotherm at 383 K (the longest experiment) took 13 h by the step increase method. Speeding up the measurements also minimized errors due to atmospheric pressure and temperature changes and mass spectrometer drift that can occur over longer times. Further, the assumption of constant ^MS for each step is a better approximation than a constant ^MS for the adsorption/desorption measurements because the coverage changes over a smaller range. Both step increase and adsorption/ desorption measurements were made for n-C4 at 470 K. The isotherm points fell on top of each other (Figure 1), so both methods give the same results. The step increase method requires that the transient responses be long enough to yield many flux measurements before steady state. Transient responses are shorter with step increase measurements than with adsorption/desorption measurements because the membrane is initially partially loaded. Measurement errors are greater for shorter transients because the area between the steady-state flux and the flux out curves is smaller. Thus, for conditions where coverages remain low throughout the experiment (i.e., weaker sorbing permeants, high temperatures, low pressures), the adsorption/desorption experiments may be more accurate than step increase measurements without taking much more time. 5.1.2. Support Resistance. Van de Graaf et al.52 modeled the steady-state permeation of light gases through a silicalite-1 membrane by three measurement methods: Wicke-Kallenbach with the feed on the zeolite side, Wicke-Kallenbach with the feed on the support side, and the batch method. The membrane orientation and measurement method strongly affected the steady-state permeances by affecting the feed and permeate side partial pressures. They concluded that
the partial pressure drop across the support layer could not be neglected for proper modeling of transport through a composite zeolite membrane. The transient technique integrates the steady-state coverage profile to calculate the number of moles in the membrane at steady state (Qc). For large adsorption equilibrium constants, a small increase in the permeate pressure can significantly increase the permeate side coverage. The coverage profiles for n-C4 in Figure 8 show that the support resistance changed the coverage profile significantly. The calculated number of moles at steady state (Qc) was almost 10% lower when the support resistance was neglected. Also, the error in Qc depends on feed concentrations, so the shape of the isotherm (b) and its height (qsat) would be affected by incorrectly estimating the permeate boundary coverage. Therefore, the support resistance was necessary for accurate determination of the adsorption parameters and membrane thickness. The importance of including the support resistance can also be seen in the estimation of the membrane thickness. If the permeate boundary coverage were underestimated, the effective membrane thickness determined by the transient method would be overestimated (a thicker membrane would be required to hold the same number of molecules). When the support resistance was not included, the estimated effective thickness of membrane M1 was 330 µm when calculated from the n-C4 measurements made at 295 K and 230 µm when calculated with the n-C4 measurements made at 480 K. When the support resistance was included, the membrane thickness calculated from the butane measurements at each of the six temperatures was 220 ( 10 µm, the same as that calculated from the light gas measurements.27 At higher temperatures, adsorption equilibrium constants are lower, and small changes in the permeate pressure do not change the coverage profile as much (Figure 9). For example, the Qc estimated without including support resistance for n-C4 adsorption at 470 K was 3% lower than the Qc estimated with support resistance. The support resistance was not included in the model for transient studies of light gases.27 This did not cause significant error because the light gases adsorb weakly on ZSM-5 and, thus, the adsorption equilibrium constants were relatively low (0.002 kPa-1 for N2 to 0.024 kPa-1 for CO2). Likewise, when fluxes are low, as they are for i-C4, the partial pressure at the permeate side can be so low that doubling it (as including the support resistance does) does not affect the coverage profile significantly (Figure 12). The largest difference between Qc calculated with and without support resistance for i-C4 was only 0.5%. 5.1.3. Relation between Flux In and Flux Out. The F factors decreased with increasing coverage for both isomers but were lower for i-C4, even though it had lower coverage. For Maxwell-Stefan diffusion with the permeate boundary coverage equal to zero, F depends only on the feed coverage and decreases from 3 to 2 as θ0 increases from zero to one.55 With mass transfer resistance at the permeate boundary, the factors depend on the feed coverage, the diffusion coefficient, the membrane thickness, and the mass transfer coefficient. The F factors increase with increasing diffusion coefficients (eq 10 shows this for Fickian diffusion). Because the n-C4 diffusion coefficients are an order of magnitude
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4103
larger than those for i-C4, the F factors were higher for n-C4, even though n-C4 had higher coverages. Adsorption equilibrium constants were underestimated by assuming the factor was the Fickian value of 3. At higher temperatures, coverages are lower, and Maxwell-Stefan diffusion deviates less from Fickian diffusion. Consequently, at higher temperatures, the factors deviate less from 3, and the factor F was also calculated numerically to be 3 for the low coverages obtained for light gases at room temperature. The heat of adsorption calculated for i-C4 when the factors were set to 3 was 26 kJ/mol (Figure 13), and the calculated saturation coverages decreased from 1.2 to 0.5 mol/kg as the temperature increased (Figure 14). When the factors were determined iteratively, the heat of adsorption for i-C4 was 40 kJ/mol, similar to the heat of adsorption for i-C4 measured in MFI crystals (Table 3), and the saturation coverages were almost constant at approximately 0.66 mol/kg. According to molecular dynamics simulations,48,50,57,58 four molecules of i-C4 per unit cell (0.69 mol/kg) reside in the channel intersections for the temperatures and pressures studied here. The heat of adsorption for n-C4 was 46 kJ/mol with the iterated factors compared to 44 kJ/mol when the factors were assumed to be 3. The main difference for n-C4 was that the saturation coverage was overestimated when the factors were assumed to be 3. With the iterated factors, the saturation coverages decreased by about 25% and the new average was approximately 1.6 mol/kg. The saturation coverage predicted by molecular simulations is 10 molecules/unit cell, eight in channels and two in intersections, which corresponds to a qsat of 1.72 mol/kg. Thus, the qsat calculated from iterated factors is closer to the theoretical value than when qsat was calculated assuming the factors were 3. 5.2. Membrane M1. 5.2.1. Adsorption. The n-C4 and i-C4 adsorption isotherms measured by the transient method in membrane M1 (Figures 1 and 2, Tables 1 and 2) have adsorption parameters (qsat, b) that are comparable to those reported in the literature for MFI powders. The heats of adsorption in membrane M1 (Tables 3 and 4) were also similar to those measured for MFI crystals. The fact that the membrane M1 isotherm parameters are comparable to those measured for powders, under conditions of both low and high coverage, indicate that butane transport is mainly through zeolite pores in membrane M1. These results, coupled with the remarkable similarities between adsorption isotherms for HZSM-5 crystals and those measured by the transient method for light gases in membrane M1 previously,27 also validate the model. The heat of adsorption for i-C4 in membrane M1 was at the low end of the range of values reported for i-C4 adsorption in silicalite crystals (Table 3). This may partially to due to some i-C4 diffusing through pores with weaker adsorption sites than those inside the zeolite pores. Indeed, the two-step approach to steadystate seen for i-C4 permeation through membrane M1 (Figure 10) indicates that a small fraction (∼4% at 383 K and Po ) 6 kPa) of the i-C4 flow is through parallel nonzeolite pores. 5.2.2. Diffusion. N-Butane diffusion coefficients in membrane M1 increased with temperature as expected for activated diffusion (Figure 4), and Ea diff for n-C4 and i-C4 were comparable to those measured for diffusion in MFI crystals (Table 4). Maxwell-Stefan diffusion
coefficients for i-C4 in membrane M1 were an order of magnitude lower than those for n-C4, which is consistent with diffusion coefficients reported in the literature for butane isomers in MFI-type zeolites. The MaxwellStefan diffusion coefficients increased with increasing pressure, suggesting that the Darken approximation that ^MS is independent of coverage may not be valid. This has been suggested previously by others.59,60 Jobic et al.59 measured the self-diffusivity and the corrected diffusivity simultaneously by quasi-elastic neutron scattering and found that ^MS increased with coverage up to some loading and then leveled off. Kapteijn et al.29 used Wicke-Kallenbach permeation measurements and found that ^MS increased with coverage for light alkanes and alkenes permeating through silicalite. In contrast, Skoulidas et al.60 used molecular simulations to show that ^MS decreased with coverage for CH4 and CF4 permeation through silicalite. If molecules interact with each other in the zeolite structure differently than predicted by the Darken factor, ^MS values calculated from transport measurements could depend on coverage. Xiao and Wei experimentally investigated the concentration dependence of the Fickian diffusion coefficient and developed a model including effects of molecule-molecule interactions within the zeolite. For sufficiently high loadings, the diffusion coefficient increased with coverage more sharply than with the 1/(1 - θ) dependence assumption.61,62 Conditions permitting molecules inside the zeolite cages to interact could thus lead to ^MS values that increase with coverage. 5.2.3. Two-Stage Approach to Steady State. The first breakthrough in the two-stage approach of i-C4 to steady state (Figure 10) is attributed to flow through parallel nonzeolite pores. The two-stage approach is seen for i-C4 but not n-C4 because i-C4 diffuses slower through zeolite pores and, thus, its permeation through the zeolite pores is not seen at short times. Moreover, a larger fraction of i-C4 permeation is through nonzeolite pores, which are assumed to not be selective for n-C4. 6. Conclusions (i) A transient method was developed to rapidly measure adsorption and diffusion properties of zeolite membranes. The method included the support resistance and was valid for high coverages. (ii) Adsorption isotherms of n-C4 and i-C4 in the transport pathways of a high-quality zeolite membrane were measured. The heats of adsorption and activation energies for diffusion were similar to those measured for zeolite powders, indicating that most of the flow in a high-quality ZSM-5 membrane is through zeolite pores. (iii) The flow of i-C4 through nonzeolite pores was quantitatively measured from the transient two-stage approach to steady state. (iv) Maxwell-Stefan diffusion coefficients for butanes increased slightly with increasing feed partial pressure in zeolite pores. Acknowledgment We gratefully acknowledge support by the National Science Foundation Grant CTS-9908796. We gratefully acknowledge Dr. Kazuhiro Tanaka of Yamaguchi University, Japan, for his design and original setup of the experimental apparatus used in this research.
4104
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002
Literature Cited (1) Vroon, Z. A. E. P. Synthesis and Transport studies of thin Ceramic Supported Zeolite (MFI) Membranes. Ph.D. Thesis, Universiteit van Twente, Enschede, The Netherlands, 1995. (2) Vroon, Z. A. E. P.; Keizer, K.; Burggraaf, A. J.; Verweij, H. Preparation and characterization of thin zeolite MFI membranes on porous supports. J. Membr. Sci. 1998, 144, 65. (3) Bai, C.; Jia, M.-D.; Falconer, J. L.; Noble, R. D. Preparation and separation properties of silicalite composite membranes. J. Membr. Sci. 1995, 105, 79. (4) Bakker, W. J. W.; Kapteijn, F.; Poppe, J.; Moulijn, J. A. Permeation characteristics of a metal-supported silicalite-1 zeolite membrane. J. Membr. Sci. 1996, 117, 57. (5) Jia, M.-D.; Chen, B.; Noble, R. D.; Falconer, J. L. Ceramiczeolite composite membranes and their application for separation of vapor/gas mixtures. J. Membr. Sci. 1994, 90, 1. (6) Kapteijn, F.; Bakker, W. J. W.; van de Graaf, J. M.; Zheng, G.; Poppe, J.; Moulijn, J. A. Permeation and separation behavior of a silicalite-1 membrane. Catal. Today 1995, 25, 213. (7) Noble, R. D.; Falconer, J. L. Silicalite-1 zeolite composite membranes. Catal. Today 1995, 25, 209. (8) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of Zeolite ZSM-5 Membranes by In Situ Crystallization on Porous R-Al2O3. Ind. Eng. Chem. Res. 1995, 34, 1652. (9) Davis, M. E.; Lobo, R. F. Zeolite and Molecular Sieve Synthesis. Chem. Mater. 1992, 4, 756. (10) Matsukata, M.; Nishiyama, N.; Ueyama, K. Synthesis of zeolites under vapor atmospheresEffect of synthetic conditions on zeolite structure. Microporous Mater. 1993, 1, 219. (11) den Exter, M. J.; Jansen, J. C.; van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A.; van Bekkum, H. Zeolite-Based Membranes Preparation, Performance and Prospects. Stud. Surf. Sci. Catal. 1996, 102, 413. (12) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Alkali-free ZSM-5 Membranes: Preparation Conditions and Separation Performance. Ind. Eng. Chem. Res. 1999, 38, 3635. (13) Bein, T. Synthesis and Applications of Molecular Sieve Layers and Membranes. Chem. Mater. 1996, 8, 1636. (14) Gora, L.; Nishiyama, N.; Jansen, J. C.; Kapteijn, F.; Teplyakov, V.; Maschmeyer, T. Highly reproducible high-flux silicalite-1 membranes: Optimization of silicalite-1 membrane preparation. Sep. Purif. Technol. 2001, 22-3, 223. (15) Xomeritakis, G.; Gouzmis, A.; Nair, S.; Okubo, T.; He, M.; Overney, R. M.; Tsapatsis, M. Growth, microstructure, and permeation properties of supported zeolite (MFI) films and membranes prepared by secondary growth. Chem. Eng. Sci. 1999, 54, 3521. (16) Matsukata, M.; Nishiyama, N.; Ueyama, K. Crystallization of FER and MFI zeolites by a vapor-phase transport method. Microporous Mater. 1996, 7, 109. (17) Nishiyama, N.; Matsufuji, T.; Ueyama, K.; Matsukata, M. FER membrane synthesized by a vapor-phase transport method: its structure and separation characteristics. Microporous Mater. 1997, 12, 293. (18) Li, S.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. Separation of 1,3-Propanediol from Glycerol and Glucose using a X-Type Zeolite Membrane. Ind. Eng. Chem. Res. 2001, 40, 1952. (19) Kusakabe, K.; Kuroda, T.; Morooka, S. Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeolite membranes formed on porous support tubes. J. Membr. Sci. 1998, 148, 13. (20) Kusakabe, K.; Kuroda, T.; Uchino, K.; Hasegawa, Y.; Morooka, S. Gas Permeation Properties of Ion-Exchanged Faujasite-Type Zeolite Membranes. AIChE J. 1999, 45, 1220. (21) Matsukata, M.; Kikuchi, E. Zeolitic Membranes: Synthesis, Properties, and Prospects. Bull. Chem. Soc. Jpn. 1997, 70, 2341. (22) Hedlund, J.; Schoeman, B.; Sterte, J. Ultrathin oriented zeolite LTA films. Chem. Commun. 1997, 13, 1193. (23) Tuan, V. A.; Li, S. G.; Noble, R. D.; Falconer, J. L. Preparation and pervaporation properties of a MEL-type zeolite membrane. Chem. Commun. 2001, 6, 583. (24) Tuan, V. A.; Li, S.; Falconer, J. L.; Noble, R. D. In Situ Crystallization of β Zeolite Membranes and Their Permeation and Separation Properties. Chem. Mater. 2002, 14, 489. (25) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Isomorphous substitution of Al, Fe, B, and Ge into MFI-zeolite membranes. Microporous Mesoporous Mater. 2000, 41, 269.
(26) Tuan, V. A.; Falconer, J. L.; Noble, R. D. Boron Substituted ZSM-5 Membranes: Preparation and Separation Performance. AIChE J. 2000, 46, 1201. (27) Gardner, T. Q.; Flores, A. I.; Noble, R. D.; Falconer, J. L. Transient Measurements of Adsorption and Diffusion in H-ZSM-5 Membranes. AIChE J. 2002, 48, 1155. (28) Burggraaf, A. J. Single gas permeation of thin zeolite (MFI) membranes: theory and analysis of experimental observations. J. Membr. Sci. 1999, 155, 45. (29) Kapteijn, F.; Bakker, W. J. W.; Zheng, G.; Poppe, J.; Moulijn, J. A. Permeation and separation of light hydrocarbons through a silicalite-1 membranesApplication of the generalized Maxwell-Stefan equations. Chem. Eng. J. 1995, 57, 145. (30) Kapteijn, F.; Moulijn, J. A.; Krishna, R. The generalized Maxwell-Stefan model for diffusion in zeolites: sorbate molecules with different saturation loadings. Chem. Eng. Sci. 2000, 55, 2923. (31) Krishna, R.; van den Broeke, L. J. P. The Maxwell-Stefan description of mass transport across zeolite membranes. Chem. Eng. J. 1995, 57, 155. (32) van den Broeke, L. J. P. The Maxwell-Stefan Theory for Micropore Diffusion. Ph.D. Thesis, Universiteit van Amsterdam, Amsterdam, The Netherlands, 1994. (33) Caro, J.; Bulow, M.; Schirmer, W.; Kerger, J.; Heink, W.; Pfeifer, H.; Zdanov, S. P. Microdynamics of methane, ethane, and propane in ZSM-5-type zeolites. J. Chem. Soc., Faraday Trans. 1985, 81, 2541. (34) Datema, K. P.; den Ouden, C. J. J.; Yistra, W. D.; Kuipers, H. P. C. E.; Post, M. F. M.; Karger, J. Fourier Transform PulsedField-Gradient H-1 Nuclear-Magnetic-Resonance Investigation of the Diffusion of Light Normal-Alkanes in Zeolite ZSM-5. J. Chem. Soc., Faraday Trans. 1991, 87, 1935. (35) Jobic, H.; Bee, M.; Caro, J. Translational and RotationalDynamics of Methane in ZSM-5 ZeolitesA Quasi-Elastic NeutronScattering Study. Zeolites 1989, 9, 312. (36) Bulow, M.; Schlodder, H.; Rees, L. V. C.; Richards, R. E. Molecular mobility of hydrocarbon ZSM5/silicalite systems studied by sorption uptake and frequency response methods. 7th Proc. Int. Conf. Zeolites; Elsevier: New York, 1986; p 579. (37) Hufton, J. R.; Danner, R. P. Chromatographic Study of Alkanes in Silicalite: Transport Properties. AIChE J. 1993, 39, 962. (38) Catlow, C. R. A.; Freeman, C. M.; Vessal, B.; Tomlinson, S. M.; Leslie, M. Molecular-Dynamics Studies of Hydrocarbon Diffusion in Zeolites. J. Chem. Soc., Faraday Trans. 1991, 87, 1947. (39) Goodbody, S. J.; Watanabe, K.; MacGowan, D.; Walton, J. P. R. B.; Quirke, N. Molecular Simulation of Methane and Butane in Silicalite. J. Chem. Soc., Faraday Trans. 1991, 87, 1951. (40) Maginn, E. J.; Bell, A. T.; Theodorou, D. N. Transport Diffusivity of Methane in Silicalite from Equilibrium and Nonequilibrium Simulations. J. Phys. Chem. 1993, 97, 4173. (41) Sun, M. S.; Talu, O.; Shah, D. B. Diffusion Measurements through Embedded Zeolite Crystals. AIChE J. 1996, 42, 3001. (42) Choudhary, V. R.; Mayadevi, S. Adsorption of methane, ethane, ethylene, and carbon dioxide on silicalite-I. Zeolites 1996, 17, 501. (43) Sun, M. S.; Shah, D. B.; Xu, H. H.; Talu, O. Adsorption Equilibria of C1 to C4 Alkanes, CO2, and SF6 on Silicalite. J. Phys. Chem. 1998, 102, 1466. (44) Hufton, J. R.; Danner, R. P. Chromatographic Studies of Alkanes in Silicalite: Equilibrium Properties. AIChE J. 1993, 39, 954. (45) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Calorimetric Heats of Adsorption and Adsorption Isotherms. 1. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on Silicalite. Langmuir 1996, 12, 5888. (46) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Calorimetric Heats of Adsorption and Adsorption Isotherms. 2. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on NaX, H-ZSM-5, and NaZSM-5 Zeolites. Langmuir 1996, 12, 5896. (47) Zhu, W.; van de Graaf, J. M.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. TEOM: A Unique Technique for Measuring Adsorption Properties. Light Alkanes in Silicalite-1. Ind. Eng. Chem. Res. 1998, 37, 1934. (48) Vlugt, T. J. H.; Krishna, R.; Smit, B. Molecular Simulations of Adsorption Isotherms for Linear and Branched Alkanes and Their Mixtures in Silicalite. J. Phys. Chem. B 1999, 103, 1102.
Ind. Eng. Chem. Res., Vol. 41, No. 16, 2002 4105 (49) Zhu, W.; Kapteijn, F.; Moulijn, J. A. Adsorption of light alkanes on silicalite-1: Reconciliation of experimental data and molecular simulations. Phys. Chem. Chem. Phys. 2000, 2, 1989. (50) Richards, R. E.; Rees, L. V. C. Sorption and Packing of n-Alkane Molecules in ZSM-5. Langmuir 1987, 3, 335. (51) Lin, X.; Falconer, J. L.; Noble, R. D. Parallel Pathways for Transport in ZSM-5 Zeolite Membranes. Chem. Mater. 1998, 10, 3716. (52) van de Graaf, J. M.; Kapteijn, F.; Moulijn, J. A. Methodological and operational aspects of permation measurements on silicalite-1 membranes. J. Membr. Sci. 1998, 144, 87. (53) Krishna, R. A unified approach to the modelling of intraparticle diffusion in adsorption processes. Gas Sep. Purif. 1993, 7, 91. (54) Fuller, E. N.; Schettler, P. D.; Giddings, J. C. A new method for the prediction of gas-phase diffusion coefficients. Ind. Chem. Eng. 1966, 58, 19. (55) Gardner, T. Q.; Falconer, J. L.; Noble, R. D.; Zieverink, M. Analysis of Transient Permeation Fluxes into and out of Membranes for Adsorption Measurements. Chem. Eng. Sci. 2002, submitted for publication. (56) Burggraaf, A. J.; Vroon, Z. A. E. P.; Keizer, K.; Verweij, H. Permeation of single gases in thin zeolite MFI membranes. J. Membr. Sci. 1998, 144, 77. (57) Millot, B.; Methivier, A.; Jobic, H. Fundamentals of Adsorption, 6th ed.; Elsevier: Paris, France, 1998. (58) Takaba, H.; Koshita, R.; Mizukami, K.; Oumi, Y.; Ito, N.; Kubo, M.; Fahmi, A.; Miyamoto, A. Molecular dynamics simulation of iso- and n-butane permeations through a ZSM-5-type silicalite membrane. J. Membr. Sci. 1997, 134, 127. (59) Jobic, H.; Karger, J.; Bee, M. Simultaneous Measurement of Self- and Transport Diffusivities in Zeolites. Phys. Rev. Lett. 1999, 82, 4260. (60) Skoulidas, A. I.; Sholl, D. S. Direct Tests of the Darken Approximation for Molecular Diffusion in Zeolites Using Equilibrium Molecular Dynamics. J. Phys. Chem. B 2001, 105, 3151. (61) Xiao, J.; Wei, J. Diffusion Mechanism of Hydrocarbons in ZeolitessI. Theory. Chem. Eng. Sci. 1992, 47, 1123. (62) Xiao, J.; Wei, J. Diffusion Mechanism of Hydrocarbons in ZeolitessII. Analysis of Experimental Observations. Chem. Eng. Sci. 1992, 47, 1143. (63) Kapteijn, F.; Bakker, W. J. W.; Zheng, G.; Moulijn, J. A. Temperature- and occupancy-dependent diffusion of n-butane through a silicalite-1 membrane. Microporous Mater. 1994, 3, 227.
(64) Bakker, W. J. W.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Temperature Dependence of One-Component Permeation through a Silicalite-1 Membrane. AIChE J. 1997, 43, 2203. (65) Krishna, R.; Paschek, D. Separation of hydrocarbon mixtures using zeolite membranes: a modelling approach combining molecular simulations with the Maxwell-Stefan theory. Sep. Purif. Technol. 2000, 21, 111. (66) Keipert, O. P.; Baerns, M. Determination of the intracrystalline diffusion coefficients of alkanes in H-ZSM-5 zeolite by a transient technique using the temporal-analysis-of-products (TAP) reactor. Chem. Eng. Sci. 1998, 53, 3623. (67) van de Graaf, J. Permeation and separation properties of supported silicalite-1 membranessA modeling approach. Ph.D. Thesis, Delft University of Technology, Delft, 1999. (68) Talu, O.; Sun, M. S.; Shah, D. B. Diffusivities of n-Alkanes in Silicalite by Steady-State Single-Crystal Membrane Technique. AIChE J. 1998, 44, 681. (69) Nijhuis, T. A.; van den Broeke, L. J. P.; Linders, M. J. G.; Makkee, M.; Kapteijn, F.; Moulijn, J. A. Modeling of the transient sorption and diffusion processes in microporous materials at low pressure. Catal. Today 1999, 53, 189. (70) Heink, W.; Karger, J.; Pfeifer, H.; Datema, K. P.; Nowak, A. K. High-temperature pulsed field gradient nuclear magnetic resonance self-diffusion measurements of n-alkanes in MFI-type zeolites. J. Chem. Soc., Faraday Trans. 1992, 88, 3505. (71) Karger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; Wiley/Interscience: New York, 1992. (72) van den Begin, N. G.; Rees, L. V. C. Diffusion of hydrocarbons in silicalite using a frequency-response method; van den Begin, N. G., Rees, L. V. C., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1989; p 915. (73) Chiang, A. S.; Dixon, A. G.; Ma, Y. The determination of zeolite crystal diffusivity by gas chromatography. Chem. Eng. Sci. 1984, 39, 1461. (74) Hufton, J. R.; Ruthven, D. M. Diffusion of Light Alkanes in Silicalite Studied by the Zero Length Column Method. Ind. Eng. Chem. Res. 1993, 32, 2379.
Received for review February 20, 2002 Revised manuscript received May 13, 2002 Accepted May 17, 2002 IE020144H
