Ind. Eng. Chem. Res. 2008, 47, 3943–3948
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Characterizing Nonzeolitic Pores in MFI Membranes Miao Yu, John L. Falconer,* and Richard D. Noble Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424
Methods that use n-hexane (n-hexane permporosimetry and n-hexane/2,2-dimethylybutane (DMB) separation) are shown to not be effective for characterizing MFI zeolite membranes because n-hexane adsorption swells MFI crystals and shrinks the size of nonzeolitic pores. Measurements on a membrane in which 30% of its helium flux at 300 K was through nonzeolitic pores demonstrate that benzene permporosimetry and isooctane vapor permeation as a function of feed activity provide better characterizations. Isooctane condensed in nonzeolitic pores at high activities, and this was used to estimate the sizes of those pores. The average nonzeolitic pore size in this membrane decreased from approximately 3.0 to 1.5 nm as the temperature increased from 300 to 348 K, apparently due to thermal expansion of MFI crystals. Benzene permporosimetry yielded dramatically different results from n-hexane permporosimetry because benzene does not swell the MFI crystals significantly. Single-component pervaporation fluxes as a function of molecular kinetic diameter verified the results from benzene permporosimetry. Larger molecules had higher fluxes than n-hexane because they diffused through nonzeolitic pores that were shrunk by n-hexane adsorption. Nonzeolitic pores were estimated to account for only 0.5% of the membrane permeation area, but 30% of the helium flux, because these pores were significantly larger than MFI pores. Introduction 1–6
Zeolite membranes have been studied for separations, chemical sensors,7–9 and corrosion protective coatings10–12 because of their molecular-sized pores, hydrophobic/organophilic adsorption properties, and high thermal and chemical stability. The most widely studied zeolite membranes are composed of MFI zeolite, which has a pore diameter of approximately 0.6 nm. These zeolite membranes have been shown to separate hydrocarbon mixtures, including isomers,1,13,14 because the MFI pore size is similar to the sizes of many industrially important organic molecules. Their ability to separate mixtures depends on the size and number of nonzeolitic pores, which are intercrystalline pathways in these polycrystalline membranes that are larger than the MFI pores and thus typically not selective. Separations measurements may not always indicate membrane quality. A membrane that is selective at low pressures may not be selective at high pressures,15 and a membrane with a high separation selectivity for C6 isomers may have a low selectivity for xylene isomers16 because the MFI structure is not rigid but changes with adsorption.15 Xylene and C6 isomer separations have been used to characterize MFI membranes because the larger isomers are almost excluded from the MFI pores. Butane isomer separation was also used because n-butane has a higher diffusivity than i-butane, although both isomers adsorb in MFI pores.1,17 Differences in the size distributions of nonzeolitic pores may significantly affect transport of molecules with different sizes. Nonzeolitic pores have also been characterized by hydrogen/ n-butane separation because n-butane adsorbs in zeolite pores and inhibits H2 permeation. This method is similar to permporosimetry, in which the flux of helium is measured as a function of the activity of hydrocarbon, such as n-hexane, in the feed.18 As the n-hexane activity increases, its occupancy in the zeolite pores increases, and it effectively blocks helium permeation through the zeolite pores. Thus, the remaining flux * To whom correspondence should be addressed. E-mail: john.falconer@ colorado.edu. Phone: (303) 492-8005.
is through nonzeolitic pores. Membranes have also been characterized by single-gas permeation.14,19 Many of these separation and single-gas permeation measurements were made at low concentrations. As the feed concentration increases, the selectivity can decrease significantly15,20 because proportionally more transport is through nonzeolitic pores. Also, we showed recently that the characterization method may change the membrane properties.21,15 We found that n-hexane adsorption in MFI membranes decreased the flux through nonzeolitic pores. Thus, a method that uses n-hexane, such as permporosimetry or C6 isomer separation, may not provide an accurate characterization of membrane properties.15 The effect of n-hexane on MFI membranes was demonstrated in previous studies by single-component pervaporation, mixture pervaporation, and vapor permeation. For single-component pervaporation, the fluxes of molecules that were larger than the MFI pores were many times higher than the n-hexane flux. In mixtures, n-hexane effectively decreased the fluxes of these larger molecules, which diffused through nonzeolitic pores. These results were explained by concluding that when n-hexane adsorbed, the MFI crystals expanded slightly, but enough to shrink the nonzeolitic pores. X-ray diffraction measurements22,23,41 showed directly that MFI crystals expanded when n-hexane adsorbed. In this paper, a combination of permporosimetry, singlecomponent and mixture pervaporation, and vapor permeation as a function of pressure were used to characterize nonzeolitic pores in a MFI membrane (boron-substituted ZSM-5) that had significant flow through defects. Because benzene expands MFI crystals only 10% as much as n-hexane,24 benzene and n-hexane permporosimetry were compared. To determine the validity of the permporosimetry measurements, fluxes of molecules that are larger than the MFI pores were measured on the same membrane. Benzene and n-hexane permporosimetry yielded dramatically different results. Permporosimetry of 2,2-dimethylbutane (DMB), which cannot adsorb in the MFI pores at room temperature for the experimental time scale,25,26 was also carried out. The DMB only blocks helium permeation in nonzeolitic pores. Vapor permeation of isooctane as a function of pressure,
10.1021/ie071577t CCC: $40.75 2008 American Chemical Society Published on Web 04/17/2008
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up to high activities, was shown to characterize the nonzeolitic pores by determining the activity where capillary condensation takes place. Isooctane was chosen because it is too large to adsorb in the MFI pores at the conditions used,27,28 and it has vapor pressures at the temperatures of interest that make measurements practical. These measurements provide an indication of both the size and the relative flux through nonzeolitic pores. The membrane was also used for separations at high feed concentrations (pervaporation) to compare to the characterization methods. Experimental Methods Membrane Preparation. The MFI zeolite membrane was synthesized by in situ crystallization onto the inside of a tubular R-alumina support (5 cm long, 1.0 cm o.d., 0.7 cm i.d., 0.2 µm pores, Pall Corp.). The synthesis gel had a molar composition of 4.44 TPAOH:19.5 SiO2:1.55 B(OH)3:500 H2O. The resulting membrane contained boron isomorphously substituted in the framework. The synthesis was similar to that described previously,29 but the solutions were prepared with slower addition of the components in order to obtain more homogeneous solutions. The resulting gel was aged at room temperature for at least 6 h. One end of the support tube was wrapped with Teflon tape and plugged with a Teflon cap, and the inside of the support was filled with about 2 mL of the synthesis gel. The other end was then plugged with a Teflon cap and left overnight at room temperature while the porous support soaked up most of the gel. The tube was again filled with synthesis gel, plugged with a Teflon cap, and put into an autoclave for zeolite synthesis at 458 K for 24 h. The inside of the tube was then brushed, washed with DI water, and dried. The same synthesis procedure was repeated, except that the tube was not soaked overnight, and the membrane’s vertical orientation in the autoclave was reversed. A membrane synthesized with two zeolite layers was impermeable to N2 at room temperature for a 138 kPa pressure drop. It was calcined at 700 K for 8 h, with a heating and cooling rate of 0.6 and 0.9 K/min, respectively, to remove the TPAOH template. An XRD pattern measured for crystals collected from the bottom of the autoclave confirmed the MFI crystal structure. Scanning electron microscopy images on a similar membrane15 showed crystals 15 µm × 15 µm × 0.5 µm, and the membrane was approximately 15 µm thick. Permporosimetry. Adsorption branch porosimetry16,18 was used in an effort to measure the percentage of flow through nonzeolitic pores. The membrane was sealed in a stainless steel module using Viton o-rings. The helium flux was measured as a function of the activity of a condensable hydrocarbon in the helium stream. Helium flow through the membrane was progressively blocked as the condensable hydrocarbon activity increased. A pure helium stream was mixed with a helium stream saturated with n-hexane (>99.5%, Fluka), benzene (99+%, Sigma-Aldrich), or 2,2-dimethylbutane (>99%, Fluka). The flow rates were controlled by mass flow controllers. The hydrocarbon activity was changed by adjusting the temperature of the liquid hydrocarbon bath and the ratio of the two helium streams. Two hydrocarbon liquid reservoirs in series were used in order to completely saturate the helium flow. The feed hydrocarbon activity was calculated from its vapor pressure at the temperature of the liquid hydrocarbon bath, the flow rates of the two helium streams, and the total pressure. This calculated activity was checked by GC analysis, and activities from these two methods were within 5% of each other. The permeate pressure was 84 kPa, and a backpressure regulator controlled the feed pressure at 185 kPa. A bubble flow meter was used to
Figure 1. Single-component pervaporation flux at 300 K vs kinetic diameter: DMB 2,2-dimethylbutane, TMB 1,3,5-trimethylbenzene, i-C8 isooctane.
measure the helium flow rate. An activated-carbon adsorber on the permeate line removed the hydrocarbon before the bubble flow meter. The membrane temperature was controlled by a programmable oven. Pervaporation and Vapor Permeation. The pervaporation system has been described previously.19 The membrane was sealed in a stainless steel module using Viton o-rings. A centrifugal pump circulated liquid feed through the inside of the membrane tube at approximately 1 L/min to minimize concentration polarization. The feed and membrane were insulated and heated with heating tape. A thermocouple measured the feed temperature, which was maintained by a temperature controller. The permeate side pressure was kept below 20 Pa using a LN2 trap and a mechanical pump. Steadystate fluxes were measured by condensing the permeate in the LN2 trap for 2-4 h, and permeate concentrations were measured with a GC with a flame ionization detector. After each measurement with a given component, the membrane was calcined at 673 K for 4 h with heating and cooling rate of 0.9 K/min, respectively. The membrane permeation properties were not changed by repeated calcinations. Vapor permeation was performed in a continuous flow system, as described in detail elsewhere.30 The membrane was sealed in a stainless steel module using Viton o-rings. A syringe pump injected a liquid hydrocarbon into a preheated helium carrier stream, which then passed through a heated zone at 337-480 K. A helium sweep stream was used on the permeate side. Both the feed and permeate streams were analyzed online by a GC with a flame ionization detector. A feed bypass line allowed analysis of the feed before entering the module. Bubble flow meters were used to measure flow rates. Results and Discussion Single Component Pervaporation. Similar to what we observed previously on other MFI membranes,15,21 benzene, DMB, isooctane, and TMB fluxes were all higher than the n-hexane flux in this membrane (Figure 1). In addition, the water flux was 4.6 times the benzene flux. As shown in Figure 1, except for n-hexane, the fluxes decreased approximately exponentially as the kinetic diameter increased, although benzene, DMB, and isooctane had similar fluxes. Because the kinetic diameters of DMB (0.63 nm), isooctane (0.70 nm), and TMB (0.75 nm) are larger than the MFI pores (∼0.6 nm), they cannot enter MFI pores at 300 K on our experimental time scale.25,27,31 Thus, their fluxes are due to permeation through the nonzeolitic pores. The benzene flux is also mainly through nonzeolitic pores. Benzene diffuses approximately 3 orders of magnitude slower than n-hexane in MFI pores at 300 K.32,33 The n-hexane flux was only 4.8 mol/m2 · h, and even if all the n-hexane diffused
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through the MFI pores, the benzene flux through the MFI pores would be insignificant relative to its flux through the nonzeolitic pores. The benzene, DMB, isooctane, and TMB fluxes are higher than the n-hexane flux because they diffuse through nonzeolitic pores, but these pores are not available for n-hexane transport because crystal expansion by n-hexane shrinks them. That is, n-hexane diffuses through the much smaller nonzeolitic pores that remain after MFI crystal expansion, and also through MFI pores.15,21 On the basis of the approximately exponential dependence of flux on kinetic diameter, the n-hexane flux might be expected to be 15 times its measured value. The n-hexane pervaporation flux was also lower than the fluxes of larger molecules for other MFI membranes prepared in our laboratory.15,21 This unusual behavior was attributed to the swelling of MFI crystals by n-hexane. Apparently benzene and water did not expand the MFI crystals enough to significantly change the nonzeolitic pore size. Morell et al.22,34 observed that each axis of the unit cell of the MFI crystal structure increased by 0.7% upon n-hexane adsorption at 180 K. In recent studies, we found that n-hexane increased each axes of the MFI unit cell by approximately 0.4% at room temperature, and the unit cell volume increased by 1.2%.41 This increase in crystal size would close nanometer-sized nonzeolitic pores for crystals that are approximately one micron in diameter.15,21 In contrast, XRD measurements indicate that benzene only expands the MFI unit cell axis by 0.08%24 and
