Novel Heteropolyacid Nanoporous Carbon Reactive Barriers for

Jul 12, 2005 - Supra-Equilibrium Conversion and In Situ Component Separation. Michael S. Strano. Department of Chemical Engineering, University of ...
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Novel Heteropolyacid Nanoporous Carbon Reactive Barriers for Supra-Equilibrium Conversion and In Situ Component Separation Michael S. Strano Department of Chemical Engineering, University of Illinois Urbana-Champaign, 118 RAL, MC-712, Box C-3, Urbana, Illinois 61801

John Wyre DuPont Company, Central Research and Development, Experimental Station, P.O. Box 80228, Wilmington, Delaware 19880-0228

Henry C. Foley* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802

A novel type of thin-film catalyst has been developed that combines high activity with selective transport across a micron-scale membrane film. Nanoporous carbon was used as a support for 12-tungstophosphoric acid with composite films synthesized by two distinct approaches: insitu polymerization and adsorption from solution. Ion sputtering and XPS were used in combination to characterize the high dispersion of the catalyst within the film in the former case and a gradient in catalyst concentration in the latter. Both methods produce intact Keggin structures as demonstrated by FTIR spectroscopy. Gas permeation experiments indicate a preservation of separation capacity with O2/N2 permeability ratios as high as 6.5. The catalytic films were benchmarked using the decomposition of methyl tert-butyl ether at 55 °C and demonstrate remarkable increases in both conversion and simultaneous separation of decomposition products. Integrated conversions in a semi-batch reactor configuration far exceed the thermodynamic limit of 67.4% at 55 °C and 0.25 atm initial MTBE pressure to conversions approaching 99.9%. At the same time, methanol and isobutylene are separated on either side of the membrane with methanol selectivities as high as 99.8% on the permeate side. These results indicate that this novel catalyst design and geometry have significant potential to increase reactor efficiency and simultaneously to reduce or eliminate subsequent separation processes in selected applications. Introduction The next generation of catalytic materials will combine advanced material properties with micron- to nanoscale processing techniques to arrive at highly integrated catalytic reactors.1-3 In particular, nanoporous films, such as supported zeolite4,5 or molecular sieving carbon membranes,6,7 have significant potential as they have been shown to possess highly selective transport properties. Despite this, there have been few examples of these systems applied as reactive films or other types of novel catalysts. Studies using zeolite films as inert (noncatalytic) boundary membrane reactors4,5,8 have shown only marginal increases in reactor conversion or separation efficiency of reactants and products. Nanoporous carbon membranes, however, have recently been used as thin film reactors utilizing highly dispersed Pt within the carbon substrate.9 Formed from the pyrolysis of nongraphitizing natural or synthetic polymeric precursors, nanoporous carbon is a disordered material having a mode in pore size below 1 nm and has been demonstrated to discriminate in the adsorption of gases based roughly on molecular size.10,11 This material is promising from the perspective of a high * To whom all correspondence should be addressed. Phone: (814)863-9580. Fax: (814)863-9659. [email protected].

selectivity reactive film in that it is chemically inert under most nonoxidative conditions and thermally stable at temperatures well above 200 °C where many industrially relevant reactions occur.12 Poly(furfuryl alcohol) (PFA) derived nanoporous carbons have a pore size mode of 0.5 nm as measured from N2 and methyl chloride adsorption isotherms.13 Carbon membranes of this type have been fabricated that demonstrate remarkable efficiency in the separation of low molecular weight gases.6,7,14 As catalysts, heteropolyacids have both redox and acidic properties and have been used in several commercialized processes such as propylene and isobutylene hydration.15,16 Porous carbons in particular have been demonstrated as remarkably efficient supports for heteropolyacid catalysts, although the exact nature of irreversible adsorption into the carbon pores is not yet understood.24 Schwegler and co-workers20 have explored the synthesis of activated carbon-supported phosphotungstic and silicotunstic acid catalysts and have suggested that chemical adsorption of the catalyst involves proton transfer to the carbon surface. Alternatively, Lee and co-workers17,18 have used silica-supported H3PW12O40 packed externally over various polymer membranes and have demonstrated increases as high as 10% in conversion at 100 °C over a conventional plug-flow

10.1021/ie0496582 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005

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Figure 1. Two synthetic routes to HPW catalytic nanoporous carbon membranes: (a) adsorption from solution; (b) polymerization/ carbonization.

reactor as well as a 97% methanol purity in the permeate stream for the decomposition of methyl tertbutyl either (MTBE.) In this study, synthetic routes to novel reactive films for in situ reaction and separation are explored. Catalytic membranes are characterized using low molecular weight gas permeation as well as X-ray photoelectron spectroscopy (XPS.) Ion sputtering of membrane external surfaces followed by intermediate XPS analysis allows for a comparison of relative catalytic distributions. The films are also characterized by XRD, FTIR, and NH3 chemisorption and benchmarked using the decomposition of methyl tert-butyl ether to methanol and isobutylene. Experimental Section Two synthesis routes are explored for fabricating catalytic nanoporous carbon membranes with varying distributions of 12 tungstophosphoric acid, H3PW12O40 (HPW), within the film. These routes are described in Figure 1. In one approach, the heteropolyacid was deposited from solution upon prefabricated carbon membranes. The second approach utilizes the acidity of a methanol/HPW solution to polymerize furfuryl alcohol monomer during carbonization to create a highly dispersed catalyst distribution within the film. Adsorption from Solution. Carbon membranes were synthesized by spray deposition followed by pyrolysis of poly(furfuryl alcohol) (Monomer Polymer & Dajac Laboratories Inc., Lot A-1-143) on 316 sintered metal stainless steel supports (0.2 µm pore size, 11.4 cm2 area) obtained from Mott Metallurgical Co. Detailed descriptions of the synthesis methodology and membrane characteristics appear elsewhere.6,10,14 Each film possessed two coats of poly(furfuryl alcohol) with approximately 100 mg of carbon remaining on the support. The temperature protocol of the pyrolysis was a 5 °C/ min ramp to 600 °C and hold for 2 h. The carrier gas utilized was dry, de-oxygenated Ar at 50 standard cm3.

Carbon membranes were placed individually into a 500 mL Pyrex dish with the carbon layer facing upward. A quantity of 50 mL of a 10 mg/mL methanol solution of HPW (Sigma Aldrich) used without further purification was added to the dish and stirred using an enclosed magnetic stir bar. The solvent was allowed to evaporate at 298 K under stirring after which time the membrane was removed and was washed in a steady stream of methanol and dried in a vacuum furnace at 100 °C at 4 Torr pressure for 12 h. Membranes labeled AD1 and AD2 were synthesized using the re-precipitation method. Sample AD3 was similarly prepared, except that the prefabricated carbon membrane was synthesized using a 50/50% (wt/wt) mixture of a poly(furfuryl alcohol) and 3400 MW poly(ethylene glycol), (Sigma-Aldrich). Membranes fabricated from PFA alone were entirely nanoporous while the addition of PEG has been found to introduce significant mesoporosity into the resulting carbon.19 For comparison, a low selectivity, low gas flux “standard membrane” consisting of two circular graphite disks of 1 cm thickness was machined from readily available graphite sheet material (McMaster-Carr). A 0.3 mm diameter wire was used to produce 10 off-center holes through each disk that were then rendered catalytic using the adsorption method in the absence of the washing step. The disks were subsequently pressed together in a stainless steel reactor module described below. Polymerization/Carbonization Method. A solution of 3 wt % of HPW used without purification in methanol was added to an equal amount by weight of furfuryl alcohol monomer and spray-coated on the surface of the same type of 11.4 cm2 area circular stainless steel supports used above. The 100 mg wet coat layer was allowed to cross link at ambient temperature for 30 min in a stream of dry Ar (50 standard cm3). After this time, a heating rate of 5 °C/min brought the sample to the carbonization temperature (between 400 and 600

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Table 1. Summary of Catalytic Membranes and Synthesis Conditions

membrane label AD1 AD2 AD3 PC1 PC2 PC3 porous graphite

carbon film precursor poly(furfuryl alcohol) poly(furfuryl alcohol) 50% poly(furfuyl alcohol)/ poly(ethylene glycol) furfuryl alcohol/HPW/methanol furfuryl alcohol/HPW/methanol furfuryl alcohol/HPW/methanol graphite

pyrolysis temp (°C)

carbon film (g/m2)

600 600 600

26.4 24.5 9.0

500 400 600 none

66.3 86.9 83.9 2631.6

catalytic method

HPW loading (g/g)a

adsorption/impregnation adsorption/impregnation adsorption/impregnation

0.020 {0.22}b 0.022 0.058

polymerization/carbonization polymerization/carbonization polymerization/carbonization adsorption/impregnation

0.09 {0.014} 0.09 0.09 0.017

a Estimated from precursor mass balance. Quantity in brackets from elemental W analysis. b Loading of the HPW/C active layer from sputtering model.

°C as specified) and held at this temperature for 2 h. For each sample prepared in this way, this coating procedure was repeated 3 times yielding 76-100 mg of carbon total per membrane (final carbonized weight.) Membranes PC1, PC2, and PC3 were carbonized at 400, 500, and 600 °C, respectively. Table 1 summarizes the properties of the HPW/NPC films described in this work. Characterization Powder X-ray Diffraction. Carbon material was removed from the stainless steel support using a surgical scalpel and mounted for X-ray diffraction using an Phillips X’Pert model auto-sampling diffractometer with a Cu KR radiation source at 45 keV and 40 mA. X-ray Photoelectron Spectroscopy. XPS was performed using a PHI Quantum 2000 scanning ESCA microprobe (Physical Electronics). All scans were performed using 100 W, 100 µm beam scanned over 1500 × 200 µm areas on the external surface of membranes with a take-off angle of 40° and 29.35 eV pass energy. External membrane surfaces were subjected to ion sputtering using Ar at 2 keV over a raster area of 16 mm2 for 3-min intervals with XPS analysis performed between each bombardment in order to obtain a concentration profile with depth. FTIR Spectroscopy. Keggin-type heteropolyacids have a particularly distinct FTIR signature corresponding to their primary and secondary structure.20 Carbonaceous material was removed from the stainless steel support and ground with KBr to make a 1% mixture. These samples were then pressed into translucent pellets and mounted for adsorption spectral analysis using an Infinity series ATI Mattson FTIR spectrometer. Molecular probe chemisorption using NH3 was performed by placing granular membrane material in a tube furnace and heating to 100 °C for 4 h in 50 standard cm3 of dry Ar. Subsequently, a 5% NH3 in Ar stream was passed over the sample at 100 °C for 4 h after which the pure Ar purge was restored, and the sample was allowed to desorb excess NH3 overnight at 100 °C. Samples were fashioned into KBr pellets as described above, and FTIR spectroscopy was used to quantify chemisorbed NH3. Single-Component Gas Permeability. Permeation of He, N2, O2, Ar, CO2, and SF6 was used to characterize the selective porosity and integrity of each catalytic carbon membrane. The disk-shaped membranes were sealed into a stainless steel module using Viton gaskets coated with a small amount of silicone. The apparatus and method of measuring gas transport are described elsewhere.11 The flux of each gas was measured as a function of driving force pressure applied to each of the catalytic membranes.

Decomposition of Methyl tert-Butyl Ether. Catalytic membranes were tested for the decomposition of MTBE using a semi-batch equilibrium membrane reactor detailed in Figure 2. The catalytic membrane was sealed into a stainless steel module using Viton gaskets (MDC Vacuum products) with the catalytic surface facing in toward the enclosed chamber. This chamber was purged with dry, de-oxygenated Ar before reaction. The membrane reactor setup was held isothermal at 55 °C by enclosing the entire module into a 2000 mL heating mantle (Ace Glass Inc.) with an insulated cover. At a time t ) 0, a volume of liquid MTBE metered using a microsyringe (0-100 µL) was rapidly injected into the enclosed chamber above the surface of the membrane. The pressure was allowed to reach a maximum, and the product fluxes permeating through the opposing face of the membrane were carried from the reactor using an Ar sweep gas flowing at 10 standard cm3. This stream was analyzed every 15 min using a Varian 3700 gas chromatograph with an FID detector and a Porapak Q column (0.25 in. o.d., 4 ft long, held at 160 °C isothermal, 50 standard cm3 Ar carrier, 40 psig total pressure). A Porapak N column was also used separately to affect greater DME/methanol peak resolution. Results and Discussion FTIR Spectroscopy. Figure 3 presents the FTIR spectra of catalytic membranes synthesized using the polymerization method with subsequent carbonization at 600 (a), 500 (b), and 400 °C (c), respectively. In the 400 and 500 °C samples, the characteristic bands at 812 and 893 correspond to the νW-O-W groups with 982 and 1080 each representing νWdO and νP-O, respectively. These bands are indicative of a partially intact Keggin structure after carbonization. Intensities diminish upon heat treatment at 600 °C as the temperature approaches the decomposition temperature of H3PW12O40 of about 590 °C.21 The broad band from 950 to 1080 cm-1 is characteristic of the carbon support, as seen in the spectrum for the 600 °C PFA carbon inert sample (Figure 3d). The FTIR spectrum of pure HPW hydrate (Figure 4a) and physical mixture of HPW hydrate and PFA-derived carbon (Figure 4b) look virtually identical. Although lower in intensity and slightly broadened, the spectrum of the material derived from HPW adsorption onto the carbon membrane (Figure 4c) is quite similar to that of the physical mixture. For comparison, the spectrum of the carbon alone in a pristine state (Figure 4d) is quite dissimilar from that of the material containing HPW hydrate. It is noteworthy that neither synthesis method leads to shifting of adsorption bands as reported by

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Figure 2. Diagram of semi-batch membrane reactor configuration used to benchmark reaction and separation.

Figure 3. FTIR spectra of carbon membranes synthesized by polymerization/carbonization (PC) method (a) carbonized at 600 °C; (b) 500 °C; (c) 400 °C; (d) inert PFA carbon membrane 600 °C.

Figure 4. FTIR spectra of carbon membranes synthesized by reprecipitation (AD) method: (a) HPW hydrate; (b) HPW/carbon 50% physical mixture; (c) AD carbon membrane after synthesis; (d) before HPW adsorption.

Schwegler and co-workers.20 This is consistent with adsorption on an essentially noninteracting support as in the case of SiO2 impregnation.22 The broadening of the band at 595 cm-1 (corresponding to δ O-P-O) for the catalytic membrane sample arises from the overlap with a similarly positioned band from the carbon film as shown. Powder X-ray Diffraction. Figure 5 presents the XRD patterns of membranes synthesized by adsorption before and after impregnation as well as HPW hydrate and a 50% HPW/carbon physical mixture for comparison. After mixing with carbon (Figure 5b) or heating to 100 °C (Figure 5c), the XRD patterns of the HPW (Figure 5a) are left intact. However, the catalytic membranes (Figure 5d,e) display XRD patterns dominated by the amorphous carbon, although weak reflections due to HPW are barely visible above the carbon background signal for the adsorbed sample (Figure 5c). The presence of these lines may indicate that catalyst

dispersion is significantly lower than that produced on larger pore carbons, which characteristically demonstrate no diffraction pattern corresponding to HPW structure as in (Figure 5d) even at loading as high as 25%.23 Upon exposure to NH3 at 100 °C, these peaks markedly intensify and shift slightly to higher 2θ values (Figure 5f) due to the incorporation of the NH4+ ion and formation of (NH4)3-xHxPW12O40.22 Nanoporous carbons derived from poly(furfuryl alcohol) typically have a broad diffraction peak at 23° 2θ corresponding to a nominal 002 d spacing normally associated with a graphitic (but defective) interlayer spacing.24 Smaller peaks at 44° and 48° corresponding to the 100 and 101 planes of graphite are convoluted. The dispersion of the catalyst on mesoporous carbon (AD3) is significantly higher, and no diffraction peaks corresponding to HPW are observed. Having a significantly higher internal pore surface area, the PFA/PEG

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Figure 5. XRD spectra of AD carbon membranes: (a) HPW hydrate; (b) HPW/carbon 50% physical mixture; (c) HPW hydrate heated to 100 °C; (d) AD PFA/PEG carbon membrane; (e) AD membrane; (f) AD membrane after NH3 chemisorption forming (NH4)3-xHxPW12O40

Figure 6. XRD spectra of PC membranes: (a) inert PFA carbon 450 °C; (b) PC carbonized at 400 °C; (c) 500 °C; (d) 600 °C.

carbon adsorbs the catalyst to a greater extent within the mesopore volume. Heat treatment temperatures between 400 and 600 °C have little effect on the diffraction patterns of polymerized membranes as shown in Figure 6. The peak mode at 25°-26° corresponds to a smaller interlayer spacing of the carbon as compared to that formed in the absence of HPW.24 For nanoporous carbons synthesized in the absence of HPW, the peak mode at 23° (d spacing of ∼0.5 nm) is believed to correlate with the pore size mode of 0.5 nm. This implies that the polymerized/ carbonized films have a decreased mean pore size in the nanopore region. The broad diffraction peak at about 5°-11° 2θ appearing in the carbon samples derived from HPW adsorption and polymerization arise from superimposed effects of both the HPW catalyst and the nanoporous carbon structure. Poly(furfuryl alcohol)-derived carbons typically show a shoulder on the 002 peak in this location (Figure 6a). This low angle feature has been shown to correspond to the formation of a discotic mesophase that develops during carbonization. For catalytic membranes formed by polymerization, an increase in intensity of this peak may be explained as

the discotic domains being constrained from further graphitization due to acid-catalyzed cross-linking. This cross-linking may also constrain the interlayer spacing from increasing to the defective 0.5 nm mode found for inert nanoporous carbons, and this is consistent with the location of the 002 peak for polymerized carbons. The peak in this region can also be attributed to an amorphous halo due to the secondary structure of the HPW. This effect has been observed to a lesser extent for HPW impregnation into other porous supports.25 Single-Component Gas Permeability. Figure 7a-d shows the fluxes versus pressure of a series of probe molecules at 298 K through the catalytic membranes. Figure 7a reports the fluxes for membrane AD1 and demonstrates linearity with driving force pressure for these gases as observed with their inert analogues. The adsorption synthesis route produces O2/N2 permeability ratios of 3.4 and 6.5 for AD1 (Figure 7a) and AD2, respectively. By contrast, Figure 7b corresponds to the porous graphite disk showing evidence of viscous and Knudsen transport. Figure 7c is the analogous graph for a PFA/3400 g/mol PEG (AD3) membrane after deposition and shows a similar quadratic trend. The inclusion of the PEG creates a mesoporous membrane with decreased gas selectivity but significantly increased throughput. Figure 7d presents the permeation results through a polymerized/carbonized membrane (PC1). The characteristics of the flux versus pressure appear to be between that of AD1 and AD3 samples: the selectivities observed are greater than Knudsen values, but there is a slight pressure dependence to the flux that is not observed for other nanoporous membranes. This evidence suggests that larger, viscous flow pathways exist in parallel with the nanoporosity as the O2/N2 ratio of 1.2 is higher than the Knudsen value and yet there is a slight pressure dependence that is experimentally detectable. From the XRD data in Figure 6, it is clear that polymerization/carbonization yields a less graphitic and more glass-like carbon. This results in decreased mechanical stability of the polymerized poly(furfuryl alcohol) film and an increase in cracking with subsequent loss of selectivity and increase in viscous flow character. It is noteworthy that both single-component membrane permeabilities and individual permselectivities decrease after deposition for membrane AD1. The N2 permeance of 5.5 × 10-11 mol m-2 s-1 Pa-1 falls to 3.53 × 10-11 mol m-2 s-1 Pa-1 after deposition. Similarly the He value of 8.5 × 10-10 decreases to 3.5 × 10-10 mol m-2 s-1 Pa-1. This decrease in flux after catalytic deposition is likely due to blockage of larger pores by adsorption of the catalyst. X-ray Photoelectron Spectroscopy. Figure 8 is a broad scan of the binding energy spectrum from 0 to 1400 eV for both AD and PC synthesized carbon membranes. Visible in the spectrum for the sample prepared by adsorption are peaks corresponding to Fe as Fe2O3. The source of this Fe is evidently from the slight oxidation of the stainless steel support in the acidic methanol/HPW solution during adsorption. The low energy region of the spectrum reveals a disparity in surface HPW concentration of the catalyst between AD and PC membranes as anticipated. Table 2 reports the differences between surface concentrations before sputtering. Membranes synthesized using the polymerized/carbonized method show two prominent peaks corresponding to Si evidently from silicone used on the gaskets to

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Figure 7. Gas fluxes vs applied pressure for (a) AD1; (b) porous graphite; (c) AD3; (d) PC1.

Figure 8. XPS surface scan of (a) AD and (b) PC membranes.

seal the membrane under vacuum. Membranes synthesized by adsorption have a nonexistent uptake as the increased HPW at the surface reduces the uptake of the nonpolar silicone.26 Table 2 summarizes the analysis of the surface compositions as determined from integration of peak areas in Figure 8. The Fe composition detected as Fe2O3 on the representative AD sample can be used to subtract the molar amount of O attributed to this impurity leaving compositions of P, O, and W attributed to the adsorbed catalyst. The ratios of O:W:P are close to the values expected for anhydrous HPW of 40:12:1. The same calculation performed for the polymerized/carbonized membrane reveals ratios that deviate from this expected composition. The ratio of O/W of 2.5 (compared

with 3.33 for HPW) indicates the partial decomposition of the HPW at carbonization temperatures and subsequent formation of oxides of tungsten, which then decrease O relative to W. The membranes were depth profiled using bombardment of Ar ions followed by subsequent XPS scans as described above. The profiles show differences in catalyst distribution between these methods with the adsorption route yielding a strong gradient in HPW material at the membrane external surface. The polymerization/carbonization method yields a concentration profile that varies little with depth from the membrane external surface. Extrapolation of this average loading in the sample to the whole membrane thickness yields a loading of about 1.4%, which is less than that calculated from the 0.25 g of 3% PW/furfuryl alcohol mixture that yields a calculated 9% catalytic loading (assuming 33.3% conversion of the PFA to carbon). Losses are likely due to phase separation and volatilization during pyrolysis. Such behavior was observed by Lafyatis27 for the case of carbonizing H2Cl6Pt acid in PFA where Pt loading of the resulting carbon was nearly 50% less than that calculated by precursor mass balance. In contrast, the membrane synthesized by the AD method demonstrates a distinct gradient at the membrane surface. The nature of irreversible adsorption of the heteroploy acid within the pore structure of the carbon is still unclear. Izumi and Urabe28 attributed the apparently strong, irreversible adsorption of HPW from the solution phase on microporous carbons to the adsorption potential of the narrowest pores. More recently, Schwegler and co-workers20 have argued that an electrostatic interaction via proton transfer to the carbon is consistent with experimental observations. During the sputtering process, a time dependence in the reduction of W was observed for the adsorption sample and not for the polymerized/carbonized sample.

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Table 2. Comparison of Initial Catalytic Surface Concentrations for Absorption Prepared (AD) and Polymerized/ Carbonized Membranes (PC) peak binding carbon 1s oxygen 1s tungsten 4f phosphorus 2p iron 2p1 a

surface comp.

energy (eV)

AD (%)

PCa (%)

284.80 531.07 36.06 134.57 712.02

38.28 45.31 12.14 1.07 3.19

96.56 2.04 0.82 0.04 0.00

Fe2O3

HPW12O40

atomic ratios

AD (%)

AD (%)

PC (%)

AD (%)

PC (%)

4.79

40.53 12.14 1.07

2.04 0.82 0.04

37.87 11.35 1.00

51.00 20.50 1.00

3.19

Surface composition after 3 min of initial ion sputtering to remove impurities.

Figure 9. FTIR spectra before and after NH3 chemisorption: (a) PC1 before chemisorption; (b) PC1 after chemisorption; (c) AD1 before chemisorption; (d) AD1 after chemisorption.

The time dependence in the reduction can be attributed to larger crystallite sizes relatively unshielded by carbon material, which has a sputter yield about 3-4 times less than W for 2 keV Ar using the current theory of ion sputtering.29,30 Fitted sputtering rate constants, ksi, also reflect this disparity in catalyst dispersion as values for 0.85 and 0.006 (min-1) for W and C, respectively, were obtained for the adsorption method while the polymerized/carbonized sample yielded 0.035 and 0.061 (min-1) for W and C, respectively. The sputtering of the latter sample is evidently controlled by that of the shielding carbon, which has an intrinsically lower sputter yield. NH3 Molecular Probe Chemisorption. Figure 9 presents the FTIR spectra of both adsorption and polymerization/carbonization membranes before and after exposure to NH3 at 100 °C. Membranes from polymerization show no detectable peak in the 1420 cm-1 region corresponding to NH4+ and, hence, to the presence of Bro¨nsted acidity while the impregnation samples show a prominent peak in this region. The result is consistent with those of XRD in that the decreased pore volume (inferred from the low angle feature in the diffraction pattern) significantly restricts the available catalyst surface area and active sites. The result is also consistent with gas permeation data presented above in that samples prepared via the polymerization route demonstrated lower gas selectivity and, hence, decreased flow though nanoporous pathways. MTBE Decomposition. Figure 10a-d are the permeate fluxes from membranes AD1 through AD3. For AD1 and AD2 the membrane responses are characterized by a rapid increase in the methanol flux to a maximum as the membrane attains steady-state transport and reaction followed by a broad decay as the

reactant is consumed within the catalytic membrane and the driving force decreases. For AD1 (Figure 10a), the maximum in methanol flux is directly proportional to the injected reactant volume with 1.0 × 10-5, 5.7 × 10-6, 3.6 × 10-6, and 6 × 10-7 mol m-2 s-1 for 100, 50, 30, and 5 µL injections, respectively. The membrane suppresses transport of the MTBE and to a lesser extent isobutylene in favor of the faster permeating methanol. Integral reactor conversions based on MTBE are quite high with 84.4%, 93.3%, 96.5%, and 98.3% each corresponding 5, 30, 50, and 100 µL, respectively. Shikata et al.26 observed a decrease in activity over HPW with methanol pressure above 0.03 atm for the MTBE synthesis reaction at 323 K. The authors attributed this decrease to the onset of a “low activity” state of the catalyst due to absorption of excess methanol at a ratio of 3.5 per proton as compared with a 1:1 ratio corresponding to the “high activity” state. The catalytic membrane (AD1) of Figure 11, by suppressing the partial pressure of methanol in the enclosed portion of the reactor by rapid transport, has an additional advantage in that it can operate in this high activity state. Figure 10b is the flux distribution from AD2 prepared in a similar manner but using a starting NPC membrane with a higher gas selectivity (O2/N2 ) 6.5). The overall throughput is decreased, with the maximum in methanol flux occurring at a factor of 5 less (1.1 × 10-6 mol m-2 s-1) than that of AD1 at the same injection volume. The time of permeation is also significantly increased as the maximum occurs at 600 min. This decrease in throughput accompanies a significant increase in membrane selectivity. The isobutylene and MTBE fluxes are less than 1.0 × 10-7 and 2 × 10-8 mol m-2 s-1, respectively. Conversion is nearly complete based upon the MTBE balance and an extrapolation of the results to t ) ∞. The analogous plot of product fluxes from membrane AD3 (50% PFA/PEG carbon film) at 50 µL of injected MTBE is shown in Figure 10c. The reactor behavior is strongly influenced by the mesoporous pathways created by the PEG template. The time of permeation is significantly decreased compared with those above apparently due to the substantially higher rate of permeation. The maximum in the MTBE fluxes does not show the same linearity at 7.5 × 10-5, 1.6 × 10-4, and 1 × 10-3 mol m-2 s-1 for 5, 10, and 50 µL injections, respectively. Each occurs at about 2 min from the injection time. The nonlinear increase in permeation rate also explains the decrease in membrane conversions, which are 17.8%, 17.5%, and 9.5% for 5, 10, and 50 µL based on MTBE mass balance. Methanol and isobutylene fluxes appear in nearly equal amounts and tert-butyl alcohol (formed from methanol coupling to yield water and subsequent hydration of the isobutylene) was detected at less than 0.9% of the product flux

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Figure 10. Permeate fluxes for catalytic membranes at 55 °C with 50 µL injection: (a) AD1; (b) AD2; (c) AD3; (d) machined graphite.

decomposition of MTBE. The maximum in flux occurs at 15 min at a value of 3.3 × 10-5 mol m-2 s-1. The conversion is nearly undetectable with less than 0.1% of reactor effluent consisting of decomposition products. While FTIR confirms that a detectable Keggin-type structure exists within the polymerized/carbonized film, XRD reveals no measurable crystalline cross section indicating a high degree of catalyst dispersion within the film. The XRD patterns for these types of membranes do reveal a highly amorphous, carbonized resin that is predicted to have significantly less porosity than conventional carbons. This decrease in accessible porosity limits the activity of the film and lowers the effectiveness factor, despite the higher catalyst dispersion. The lack of accessibility to the active sites is also confirmed by the results of NH3 chemisorption. Figure 11. Comparison of integral conversions with calculated equilibrium value for a nonselective semi-batch membrane reactor.

composition. Despite the fact that the XRD pattern suggests increased dispersion of the catalyst and therefore an increase in accessible protons, the conversion is significantly decreased compared with AD1 and AD2 because of the decreased residence time of the reactor. Figure 10d describes the reactor results using a nonselective membrane (porous graphite), prepared as described above. The membrane flux consists of appreciable quantities of all three products. The transient behavior is complex with an initial escape of unreacted MTBE reaching a maximum of 1.85 × 10-5 mol m-2 s-1 at 20 min followed by a significant decrease as methanol and isobutylene fluxes increase. The integral conversion based upon the MTBE balance is 64.1%. The product flux distribution from PC1 after a 50 µL injection shows essentially no catalytic activity in the

The equilibrium constant as a function of temperature for MTBE formation from isobutylene and methanol has been tabulated by Lisal et al.,31 and a value of 18.2 bar-1 is obtained for 55 °C. Integrating the calculated equilibrium flux from the injection pressure to zero yields MTBE conversions of 74.3%, 67.1%, 57.6%, and 90.2% for 30, 50, 100, and 5 µL MTBE injections assuming that the dilute MTBE/Ar mixture behaves ideally and using an estimated reactor volume of 43.2 cm3. The nonselective membrane sample conversion of 64.1% is close to the equilibrium value calculated at 67.1% at 50 µL. The nanoporous membranes synthesized by adsorption yield conversions that are significantly higher than these thermodynamic limiting values. Figure 11 plots the observed integral conversions as a function of initial injected MTBE pressure for each membrane as compared with the predicted thermodynamic limited semibatch conversion.

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Conclusions Nanoporous carbon membranes rendered catalytic in this way with O2/N2 ideal permselectivities greater than the Knudsen value can convert MTBE to products in yields well above equilibrium. As the O2/N2 permselectivity increases, the separation of isobutylene and methanol increases to near complete rejection of the MTBE and isobutylene but at the expense of membrane throughput. In contrast, membranes with Knudsen-like selectivities do not adequately separate reactants and products and conversions are limited by thermodynamic equilibrium. Heteropolyacid catalyst prepared within the nanoporous carbon is relatively inaccessible to MTBE, and consequently, the effectiveness factor for the reaction is low. This low activity of these membranes is also consistent with the observation that MTBE transport within nanoporous carbon is hindered. Also, the acidic environment created during carbonization of the furfuryl alcohol/heteropolyacid mixture has been shown to lead to the development of viscous flow and Knudsen defect pathways and hence decreases nanopore surface area and membrane selectivity. Alternatively, the catalyst adsorbed on the carbon surface of the membrane is highly accessible for reaction and is optimally placed. For the first time, it has been shown that a nanoporous material can be used to drive reaction conversions to exceed thermodynamic equilibrium and simultaneously completely segragate products with relatively high purity. Hence, this next generation of catalytic composite materials yields enormous potential to revolutionize chemical processes by reducing downstream separations, lowering energy demands, increasing per pass conversions, and minimizing mixed waste streams. Acknowledgment This work was supported by the Department of Energy, Office of Basic Energy Sciences; the Delaware Research Partnership; and the DuPont Company. The authors wish to acknowledge Dennis Redmond at the Dupont Co. for his help in conducting the XRD experiments. Literature Cited (1) Armor, J. N. Challenges in membrane catalysis. CHEMTECH 1992, 22, 557-563. (2) Armor, J. N. Membrane catalysissWhere is it now, what needs to be done. Catal. Today 1995, 25, 199-207. (3) Saracco, G.; Neomagus, H.; Versteeg, G. F.; van Swaaij, W. P. M. High-temperature membrane reactors: potential and problems. Chem. Eng. Sci. 1999, 54, 1997-2017. (4) van de Graaf, J. M.; Zwiep, M.; Kapteijn, F.; Moulijn, J. A. Application of a zeolite membrane reactor in metathesis of propene. Chem. Eng. Sci. 1999, 54, 1441-1445. (5) van de Graaf, J. M.; Zwiep, M.; Kapteijn, F.; Moulijn, J. A. Application of silicalite-1 membrane reactor in metathesis reactions. Appl. Catal. A 1999, 178, 225-241. (6) Acharya, M.; Raich, B. A.; Foley, H. C.; Harold, M. P.; Lerou, J. J. Metal-supported carbogenic molecular sieve membranes: synthesis and applications. Ind. Eng. Chem. Res. 1997, 36, 29242930. (7) Shiflett, M. B.; Foley, H. C. Ultrasonic deposition of highselectvity nanoporous carbon membranes. Science 1999, 285, 1902-1905.

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Received for review April 27, 2004 Revised manuscript received June 8, 2005 Accepted June 8, 2005 IE0496582