Catalysis with permselective inorganic membranes

Catalysis with permselective inorganic membranes

Applied Catalysis, 49 (1989) l-25 Elsevier Science Publishers B.V., Amsterdam - 1 Printed in The Netherlands Review Catalysis with Permselective I...

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Applied Catalysis, 49 (1989) l-25 Elsevier Science Publishers B.V., Amsterdam -

1

Printed in The Netherlands

Review

Catalysis with Permselective Inorganic Membranes J.N. ARMOR Corporate Science and Technology Center, Air Products and Chemicals, Inc., Allentown, PA 18195 (U.S.A.) (Received 19 August 1988) CONTENTS

1 Abstract ............................................................................................................... 2 1. Introduction .................................................................................................... 3 2. Noble metal membrane catalysts .................................................................. 3 2.1. Hydrogenation .......................................................................................... 5 2.2. Dehydrogenation ...................................................................................... 7 ........................................................................ 2.3. Porous metal membranes ..................................................................................... 8 2.4. Coupled reactions 9 2.5. Mechanisms for hydrogenation .............................................................. 3. Oxygen transfer inorganic membranes ......................................................... 9 4. Membranes as catalyst supports .................................................................... 11 4.1. Polymer membranes ................................................................................. 11 4.2. Inorganic membranes as supports .......................................................... 12 4.3. Supported palladium metal ..................................................................... 16 5. Additional examples of membrane catalysts with inorganic membranes .. 18 20 6. Summary .......................................................................................................... 7. Acknowledgements .......................................................................................... 22 8. Appendix: definitions ...................................................................................... 22 Note added in proof ............................................................................................. 22 23 References ............................................................................................................

ABSTRACT This review provides a survey of key patents and publications in the emerging area of catalysis with high-temperature inorganic membranes of potential to the chemical process industry. Specific topics include dense and porous metal membranes for catalytic hydrogenation and dehydrogenation, inorganic membranes as catalyst supports, oxygen transfer inorganic membranes and examples of catalysis at elevated temperatures ( > lOO-300°C) over inorganic membranes.

0166-9834/69/$03.50

0 1989 Elsevier Science Publishers B.V.

2 1. INTRODUCTION

A perselective membrane is a thin film or layer of material that can selecpass (or permeate) one component of a mixture. This review provides a survey of an emerging new generation of perselective, inorganic membranes that are catalytic. It is possible to select inorganic membranes which can serve either as the catalyst (e.g. palladium foil membranes) or as a support for a catalytic species (e.g. microporous alumina), while separating reactants from products. Although the production of inorganic (or ceramic) membranes is still at an early stage of development, a number of high-temperature ( > 100-300 oC ) applications of these materials as catalytic membranes have recently appeared. The use of thin, organic polymer films as membranes for the separation of water from salt solutions (reverse osmosis) and, more recently, the recovery of gases from mixtures has also attracted much attention. However, the incorporation of catalysts into membranes offers the opportunity not only to perform selective catalytic synthesis but also to separate simultaneously reactants and/or products. In conventional (non-membrane) liquid-phase hydrogenation, usually metalsupported solid catalysts are highly dispersed and ultimately need to be separated from the products. With membranes, this is not a problem. In addition, catalytic noble metal membranes resist corrosion. For hydrogenation reactions, palladium membranes are often more selective than non-membrane catalysts, because side reactions are repressed owing to the effectively lower hydrogen pressures. Further, membranes can impart greater product selectivity in a reaction by shifting the steady-state concentration of reactants and products away from an otherwise unfavorable equilibrium over the working catalyst. This review identifies key articles on membrane catalysis of potential interest to the chemical process industry. Although the bulk of the text summarizes the work of others, some commentary and limitations of previous work will be provided where appropriate. This review begins with the early, pioneering work of Gryaznov [ 11 on pure metal membranes (principally palladium-ruthenium alloys) [ 2 ] and seeks to summarize all key contributions in the area of permselective, catalytic inorganic membranes. The availability of thin, inorganic membranous materials with porosity on the molecular level will be discussed, and current commercial inorganic membranes and their use for several specific reactions will be described. This review covers the following specific topics: noble metal membranes, including examples of hydrogenation and dehydrogenation; porous metal membranes; coupled reactions over dense metal membranes; mechanisms of hydrogenation over Pd” membranes; oxygen transfer membranes; polymer membranes as supports for noble metal membranes; inorganic membranes as supports for catalytic materials; supported palladium membranes; tively

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and new examples of inorganic, catalytic membranes. These are topical areas that are rapidly expanding. Examples of electrocatalytic membranes [ 31, liquid membranes [ 41, Nafion membranes [ 5,6] and phase-transfer membranes [ 7,8] and extensive discussions on reactor engineering issues are not included. Likewise, organic polymer membranes, which are limited by poor heat and chemical resistance and durability, are not discussed at length. For the novice in the field, an Appendix is provided with definitions of terms frequently used in membrane technology. A quick search of Chemical Abstracts yielded 615 citations (up to May 1988) of articles with both the words “membrane” and “catalysis” in either the title or abstract. Many of these references refer to biochemical separations or processes utilizing enzyme-supported membranes [ 91, which are not covered in this review. Other citations deal with gas separation, ion-exchange or facilitated transport membranes [lo]. This review focuses on a smaller subset of membrane catalysis, namely inorganic membranes that serve as catalysts with demonstrated or potential utility for the synthesis of a number of attractive commercial products above 100°C. The review covers publications available up to June 1988. 2. NOBLE METAL MEMBRANE CATALYSTS

2.1. Hydrogenation Gryaznov pioneered much of the early work on inorganic, catalytic membranes. Most of his work is summarized in a recent review stressing selective and continuous methods of hydrogenation with hydrogen permeable, thinwalled (1 mm) dense, palladium metal membranes [ 11. Gryaznov developed and tested a number of reactor designs (foil, thin-walled tubes, or double spiral) . An experimental/commercial device was produced with 200 spirals of a palladium alloy and tested for the production of linalool via the hydrogenation of dehydrolinalool (reaction 1). Linalool is a valuable intermediate in the production of fragrance chemicals [ 111 and is particularly used in the perfume industry and for the synthesis of cyzerol. The A.V. Topchiev Institute reported a yield of 90% at half the cost of producing the material from coriander seed oil.

(1)

Other examples [ 121 of hydrogenation using palladium alloy membranes

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include cyclopentadiene to cyclopentene (92% selectivity), naphthalene to tetralin (99% ), furan to tetrahydrofuran (100% at 140” C) and furfural to furfury1 alcohol (72% selectivity). In related work, Ermilova et al. [ 131 described the selective hydrogenation of cyclic polyolefins on palladium-ruthenium alloys. They pointed out that, for the conventional selective hydrogenation of cyclic polyene hydrocarbons to monoenes, it is usually necessary to use nitrogen- or sulfur-containing compounds as deactivators and also high hydrogen pressures over transition metal-supported catalysts. With a 0.1 mm Pd-Ru (90.2:9.8) foil (21.7~ 10e4 m* area), they demonstrated the hydrogenation of 1,3-and I$-cyclooctadiene and cyclooctatetraene at different hydrogen pressures (0.19-16.2 MPa) and temperatures (626-746 “C) at a feed rate of 0.83 cm”/s. With 1,3- cyclooctadiene, they obtained yields of 83% cyclooctene with 94% selectivity. For many of these membrane reactors, the reactor (Fig. 1) is separated into two chambers by the heated membrane. Reactants are pumped onto one side of the membrane as the hydrogenated product leaves. Hydrogen is added to the other chamber, where it diffuses through the membrane to react with the substrate. For the hydrogenation of 1,3-cyclooctadiene, the level of hydrogen in the reactor zone is 1.7 times greater than that for hydrogen diffusing into a stream of argon, suggesting that this reaction is limited by the rate at which hydrogen atoms recombine. In a 1979 U.S. Patent Mischenko et al. [ 141 described a process for the production of aniline by the catalytic hydrogenation of nitrobenzene using a Pd-Ru (92-97:8-3% ) hydrogen permselective membrane. In one example, a lOO-pm foil (18 cm* area) separates two chambers. Hydrogen (0.5 cm”/s) is continuously fed to one chamber at 1 atm (1 atm= 101.325 kPa), while nitrobenzene vapor (39 cm”/min/m”) is fed to the other. The nitrobenzene vapor reacts with active hydrogen formed by diffusion of hydrogen through the membrane. The reactor is maintained at 170” C for 3 h, during which production of aniline is complete with no loss in membrane-catalyst activity. The reaction was evaluated between 30 and 260” C. They claimed that such a process avoids some purification of end products and loss of catalyst and uses milder hydrogenation conditions than conventional catalysts. Also, they claimed that the

Fig. 1. Diagram of typical bench-scale flow-through, two-chamber membrane reactor cell [ 311.1, Upstream side; 2, reaction side; 3, palladium membrane; 4, thermocouple; 5, inlet; 6, outlet.

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participation of atomic hydrogen (as opposed to the dissolution of molecular hydrogen in a liquid medium) provides a lOO-fold higher productivity with the palladium catalyst [ 121. Gryaznov and Slin’ko [ 151 followed the hydrogenation of acetylene to ethylene over a Pd-Ni (94.1:5.9) membrane at 180°C with the aid of a mass spectrometer. Ethylene production began immediately after hydrogen emerged through the palladium membrane and continued (even after the hydrogen feed was stopped) until hydrogen ceased to emerge from the membrane. They concluded that hydrogen atoms from the subsurface of the membrane participate in the hydrogenation of acetylene. One can also use a membrane to reduce the number of process steps. Gryaznov and Karavanov [ 121 point out that vitamin K, (2-methyl-1,4-diacetoxynaphthalene) is conventionally produced in several stages (80% yield) by Raney nickel hydrogenation of 2-methyl-1,4_naphthoquinone followed by separation of products and further treatment with acetic anhydride using sulphuric acid as a catalyst. In contrast, the use of a Pd-Ni alloy (in tubular form) to hydrogenate a mixture of the quinone and acetic anhydride at 132°C produces vitamin K, in one step with 95% yield at 1 atm external hydrogen pressure. 2.2. Dehydrogenation On can envision using a palladium alloy membrane for endoergic reactions such as dehydrogenation. As early as 1966, Pfefferle [ 161 patented a process for the dehydrogenation of hydrocarbons using palladium alloy membrane reactors. Ethane (125 psig ) was fed to Pd/Ag diffusion tubes with a small amount of oxygen on the downstream side of the tubes (to remove the hydrogen as it permeated the metal membrane). Small amounts (0.7% ) of ethylene were detected. The dehydrogenation of hydrocarbons is often performed at very high temperatures and with low yields. The use of a membrane to remove hydrogen continuously from the products can drive a dehydrogenation reaction towards more product. Another example is the dehydrogenation of cyclohexanediol to pyrocatechol over Pd-Rh foil with 95% yield and no phenol formation [ 171. Since the organic reactants and products remain on one side of the membrane, very pure hydrogen is produced on the other side of the membrane. Mikahlenko et al. [ 181 also studied the influence of hydrogen on the dehydrogenation of isopropanol over a Pd-Ni (94.1:5.9) alloy membrane at 702 GC. They observed the need for some adsorbed hydrogen for dehydrogenation to proceed. Conversion of isopropanol increased with the ratio of hydrogen partial pressure to isopropanol, with a maximum conversion of 68% when this ratio equalled 4 (PHZ z 0.5 kPa). Additional changes in reaction conditions led to a conversion of 83%. Smirnov et al. [ 191 demonstrated the dehydrogenation or dehydrocyclization of alkanes to olefins. Using a Pd-W-Ru (94:5:1) membrane, they dehy-

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drogenated 2methylbutene-1 to isoprene (28% yield at 44O”C), cyclohexane to benzene (51% yield at 492 aC ) and heptane to benzene (55% benzene yield at 590’ C with a methane yield of 31% ). They also reported the hydrodealkylation of toluene (321-671°C) to benzene (22% yield, 671°C) and methane (40%, 671’C). In another U.S. Patent [20], they reported yields of 91% (w/ w) of benzene from the dehydrogenation of cyclohexane over 70-pm Pd-Ru (9O:lO) foil at 340°C (eqn. 2).

(2) In a different approach, Itoh [ 211 described the use of palladium membrane (200pm thick) containing 0.5% (w/w) Pt/Al,O, (Fig. 2) for the dehydrogenation of cyclohexane to benzene. In this set-up, the palladium tube serves as a container for the catalyst and as a membrane, shifting the reaction to the right by allowing the hydrogen to diffuse away from the catalyst. The reactor was maintained at 746’ C at 1 atm pressure with argon as a purge gas to remove the hydrogen permeating the palladium membrane. Essentially complete conversion of cyclohexane was achieved (2.0. 10m7mol/s of cyclohexane, 11.8. 10m5 mol/s of argon purge), which is markedly better than the equilibrium value of 18.7% without a membrane reactor. For a related system, Clayson and Howard [22] described the use of a PdAg membrane for the dehydrocyclodimerization of alkanes to aromatics. An activated, acidic zeolite catalyst is added to the reactant side of the membrane and the hydrogen which permeates through the membrane is swept away with a flow of argon. With a 0.1 mm Pd-Ag (76:24) alloy at 55O”C, they obtained 87% conversion of propane with 64% selectivity to aromatics. In a control reaction with the zeolite but without a membrane, they obtained 89% conversion and 53% selectivity. As an alternative, the palladium can be coated with a O.Ol-O-l-pm film of a transition metal known to form a stable hydride (e.g. titanium or vanadium) [ 231. In one example, titanium was deposited by vapor deposition, and the coated membrane was found to be more effective for the diffusion of hydrogen. Eventually the membrane deactivates, probably owing to the deposition of a carbonaceous film. Most research with dense metal membranes incorporates the use of palladium or its alloys. In a review on dense metal membranes, Gryaznov [2] rerl,rye +

gas

(Ar)

pollodtum

+ feed

membrane

p%uct [mm1

Hz + Ar

Fig. 2. Membrane tube reactor containing a catalyst [21]. (Reproduced by permission of the American Institute of Chemical Engineers.)

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ported that pure palladium (which becomes embrittled by repeated hydrogen cycling) is not as effective as binary palladium membranes. Alloys containing elements from Groups VI to VIII of the Periodic Table give higher activity than pure palladium [ 21. Other metals such as titanium, tantalum and vanadium are capable of sorbing and easily permeating hydrogen. In addition, silver is known to permeate oxygen. Therefore, opportunities may exist for other membrane compositions beyond those mentioned here. Sokol’skii et al. [ 241 succeeded in further regulating the selectivity of a hydrogenation process with an electrically polarized palladium membrane. They polarized one side of a membrane while simultaneously performing hydrogenation on the other side. They investigated the hydrogenation of dimethylethynylcarbinol (DMEC ) to dimethylvinylcarbinol (DMVC ) and ultimately to tert.-amyl alcohol (TAA). At low current densities (ea. 100 A/m’), the selectivity to DMVC reaches 88%. At a high current density (ca. 440 A/m’), the hydrogenation selectivity is very high with respect to TAA (98% ). The effect of current density is explained by the relative amounts of palladium hydride. At low current densities, the surface is almost completely covered by the unsaturated DMEC, and the hydrogenation consumes the limited amount of dissolved hydrogen in the membrane to produce the intermediate product, DMVC. At higher current densities where the palladium is supersaturated with hydrogen, DMEC is completely hydrogenated to TAA. Little progress has been made in commercializing hydrogenation and dehydrogenation over dense metal membranes on a large scale, even though proof of the concept has been demonstrated. Cost, fabrication, durability and catalyst poisoning are issues with palladium. The utility of palladium metal membranes may be limited by the insuffient hydrogen flux through such dense membranes. Nevertheless, Gryaznov, has built a pilot plant using these membranes [ 21 and reported the production of fine chemicals (in one case at 1 ton/ year) [25]. 2.3. Porous Metal Membranes In a 1987 patent application, Mischenko et al. [26] described a technique for increasing the permeability of palladium membrane 15-fold at 100°C (130fold at 25 oC ) by using a porous metal overlayer on the membrane. They prepared the membrane by electrolytically depositing a lo-pm film of zinc onto the surface of the palladium foil, heating the foil at 250°C for 2 h, cooling the foil and removing the zinc with boiling 20% hydrochloric acid. At lOO”C, the hydrogenation of 1,3_cyclopentadiene proceeds completely (C= 100% ) and more selectively to pentane (S = 95% ) [when compared with a foil not treated with zinc, C=50%, S=l% (pentane), 13% (1-pentene), 36% (2-pentene)]. Additional examples are given of the selective hydrogenation of styrene to ethylbenzene, p-carboxybenzaldehyde (254”C, 5.4 MPa) to p-toluic acid

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(99% + conversion), acetaldehyde to ethane (13% conversion at 25O”C), nitrobenzene to aniline (250’ C, 100% conversion) and nitroethane to ethylamine ( 120 oC, 100% yield). Another related patent application described a similar membrane derived by leaching deposited copper or mercury from the surface of the palladium foil [ 271. A recent U.S. Patent [ 281 described the preparation of microporous amorphous metal membranes. An amorphous metal alloy (e.g., Ag-Au, Cu-Zr, NiPd, P-Pd, Sb-Au or Cu-Au) was prepared by rapid quench techniques. Phase separation resulted after high-temperature heat treatment. Removal of one component was achieved by leaching with an alkaline or acidic solution to produce a porous, sometimes spongy membrane with < 1 pm pores. The inventors suggest that this material might be useful as a catalyst. In view of Mischenko et al.‘s work [ 261, this seems plausible. 2.4. Coupled Reactions Noble metal hydrogen-transfer membranes can be used to effect a dehydrogenation reaction on one side of the membrane coupled with a hydrogen-consuming reaction on the opposite side. An example of this was given by Basov and Gryaznov [ 291 for the dehydrogenation of cyclohexanol coupled with the hydrogenation of phenol to cyclohexanone (reaction 3). The hydrogen for the reduction of phenol on one side of a Pd-Ru membrane was derived from the

&+.qy()

(3)

dehydrogenation of cyclohexanol to cyclohexanone. At 683 ’ C, they obtained 39% conversion of phenol with 95% selectivity to cyclohexanone. Added hydrogen is necessary to limit the extent of the dehydrogenation reaction. By controlling the hydrogen pressure and feed rates, they were able to achieve a maximum output of 92% for the one-step hydrogenation of phenol by cyclohexanol to cyclohexanone. As Gryaznov pointed out, catalysis with hydrogen with these commercial prototype palladium reactors does not suffer from problems associated with other catalyst forms (crushed pellets, non-homogeneous catalyst preparation, dust build-up and clogged valves ). A disadvantage of precious metal catalytic membranes lies in the availability of only a small fraction of palladium atoms on the surface compared with the bulk of the palladium used. Apparently this surface palladium is much more active for hydrogen-transfer reactions. The expense of palladium may be minimized by reducing the amount per unit surface area of the membrane. For this purpose, metallo-ceramic sheets of O.l-pm palladium have been developed [ 301. These will be discussed (see Section 4.3. ),

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but first let us discuss how hydrogen may be transferred across these noble metal membranes. 2.5. Mechanisms for Hydrogenation For a conventional (non-membrane) catalyst, the selectivity of the hydrogenation material depends on the changing ratio of hydrogen to the hydrogenated product over the catalyst surface along the entire catalyst bed. For catalytic hydrogenation with a membrane catalyst, hydrogen and the substrate are absorbed on opposite sides of the membrane. Hence the surface concentration of both reactants may be independently controlled and hydrogen is supplied along the entire membrane. The mechanism of hydrogenation of ethylene through a palladium membrane was studied by Nagamoto and Inoue [31,32]. Hydrogen transport was divided into three processes: hydrogen adsorption or desorption on the membrane surface; transfer into or out of the membrane via the interface; and/or diffusion through the membrane. Experiments were performed with hydrogen premixed with ethylene (mixture system) and then introduced to one side of the membrane reactor (nitrogen on the other side). In other experiments, diluted hydrogen was introduced to the membrane and allowed to permeate to the other side containing only ethylene. Two ethylene pressure domains were observed. The hydrogenation rate for the ethylene-independent terms was controlled by diffusion of hydrogen through the palladium membrane. The other domain corresponded to hydrogen adsorption or desorption on the membrane surface. In a comparison of the reactivity of gas-phase hydrogen vs. permeate hydrogen, the same authors later studied the hydrogenation of other olefins, including butadiene [33]. In contrast to the hydrogenation of ethylene, the hydrogenation of butadiene with permeated hydrogen was much greater than that with a hydrogen-olefm mixture on the same side of the membrane. With butadiene, hydrogen transfer in the mixture system is apparently in competition with butadiene adsorption and/or reaction. 3. OXYGEN TRANSFER INORGANIC MEMBRANES

As we have seen, most of the membrane catalysis work has focused around hydrogen transfer. However, there has been a limited amount of work on oxygen transfer through ceramic or metal membranes. Oxygen transfer (via O*- ) through a ceramic membrane of calcium-stabilized zirconia (2 mm I.D.) was demonstrated by Nigara and Cales [ 341. They studied the thermal decomposition of carbon dioxide: co,=co

+ l/2 02

(4)

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At 2227°C the resulting Pop indicated 21.5 mol-% of carbon dioxide was converted to carbon monoxide. An equilibrium value of only 1.2% carbon monoxide is expected at this temperature. Owing to the dynamics of their system, only a fraction of the carbon dioxide is converted to carbon monoxide. While a thinner membrane is more desirable, the efficiency of the process is limited by the low conductivity of the stabilized zirconia wall. The use of an inorganic membrane for the thermal decomposition of water was described by Cales and Baumard [ 351. Using a closed end tube of calciastabilized zirconia, water vapor is decomposed at temperatures between 1500 and 1700’ C with oxygen diffusing through the membrane. Oxygen transport is controlled by the flow of oxide ions through the zirconia membrane (an ionic conductor). Whereas significant amounts of hydrogen are produced, oxygen transport is limited by the low permeability of oxygen through the stabilized zirconia. This limitation will continue to be a problem until one can devise an alternative membrane. In a fresh approach, the Sohio group [36] demonstrated that certain benzylic and allylic compounds were selectively and catalytically oxidatively dehydrodimerized. Discs consisting of a mixture of lanthanum and bismuth oxides (2-3 mm) were used as membranes between two high-pressure chambers. The organic substrate was fed to one side of the membrane (600 oC ) and air to the other side. With a propylene-helium (2O:SO) feed, they obtained conversions of 3.2% with the following products (selectivities): 1,5-hexadiene (53% ), benzene (25% ), carbon dioxide (20% ), methane (2% ) and carbon monoxide (1% ). The reaction was run continuously for 24 h. In a comparative experiment without continuous reoxidation of the membrane, the conversion and selectivity decreased within the first hour and the reaction eventually stopped. Additional oxidative dehydrodimerization reactions were demonstrated for toluene, isobutene, methacrylonitrile (to 2,5-dicyano-1,5-hexadiene and 1,4dicyanobenzene), 2-methylstyrene and p-toluonitrile. Although the conversions are low, the potential exists for additional improvements in selectivity or conversion with improved oxygen transfer membranes. In another approach to oxygen transfer through a membrane, Gryaznov et al. [ 371 studied the oxidation of hydrocarbons, alcohols and ammonia with a silver membrane. In their system, a silver membrane (100 pm) contained oxygen at 0.001-0.1 MPa. Reactions with oxygen (diffused through the membrane) were followed at 796-946°C. At 926”C, ammonia was oxidized to nitrogen (50% yield), nitrous oxide and nitric oxide. With the silver membrane, the oxidation of ethanol to acetaldehyde proceeded with 83% yield, whereas the oxidation of an ethanol-oxygen mixture over a silver foil under the same conditions produced only 56% acetaldehyde.

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4. MEMBRANES

AS CATALYST

SUPPORTS

4.1. Polymer Membranes Porous polymer resins can be used as membrane catalysts [ 7,381. Organic polymer membranes are limited by poor heat resistance and durability; however, most polymer membranes can be made very thin. Thinness is important, because the gas flux through the membrane is directly proportional to its thickness. Also, polymers can be easily fabricated into a number of forms (hollow fiber, spiral wound, etc.). The functionalization of such resins with sulfonic acid groups provides an acidic catalyst for ethanol dehydration. Unfortunately, some of these catalysts can suffer from deactivation [ 381. Ultimately, with all organic polymer-based membranes, catalysis is limited to lower process temperatures (e.g. < 100 ’ C ) . A few, selected examples using polymer composite membranes for catalytic reactions are given below. In one creative approach, Hershman and co-workers [39,40] described a novel selective, permeable membrane configuration consisting of a noble metal catalyst (e.g. palladium) on a carbon support coated with a polymer of polydimethylsiloxane. This membrane-coated catalyst is used to effect the oxidation of formaldehyde in the presence of amines. The polymer layer allows the formaldehyde to permeate the polymer while preventing passage of the amine, which poisons the palladium catalyst. Apart from rejecting catalyst poisons, such permselective catalysts could be used to control catalyst activity (e.g. the reaction of enantiomers ). Other recent work described the impregnation of zeolite into polymer membranes. Demertzis and Evmiridis [ 411 prepared A and X zeolites embedded in epoxy resins that were selective to cations. However, difficulties arose in pelletizing the composite and with adhesion between the two phases. In a related system, te Hennepe et al. [ 421 embedded silicalite-1 at 50% (w/w) into silicone rubber (polydimethylsiloxane ) and were able to cast a flat membrane (28.3 cm2 area). This composite membrane was used to enhance the separation of alcohol-water mixtures. They reported that the zeolite pores participate in the total transport of alcohol through the membranes. Breslau [43] described the preparation of high concentrations of immobilized enzymes into the outer spongy layer of anisotropic hollow-fiber membranes. In one example, he also dispersed a water-insoluble nickel catalyst (in the presence of a surfactant) into the spongy layer of the fiber. Acetone was produced by the nickel-catalyzed dehydrogenation of isopropanol. The catalysis was limited by the characteristics of the catalyst, which ultimately blocked some of the micropores necessary for the flow of the permeate. Certainly, one ought to be able to deposit other catalysts in the same manner.

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4.2. Inorganic Membranes as Supports The use of organic polymers as supports for catalytic membranes is limited by their decomposition or failure above lOO-300°C. There are now several types of inorganic (or ceramic) membranes, some commercially available, which offer advantages of greater thermal and mechanical stability and resistance to chemicals. With further modification, they offer promise for a size-selective separation of products on the molecular level. In a non-catalytic mode, ceramic membranes are used in thermal decomposition reactions to separate products; however, an intent of this review is to focus on their use to build composite, catalytic membranes. Some of the recent work on non-catalytic inorganic membranes will be included as part of this review. Future developments in catalytic membrane reactors for the process chemical industry will be linked to the eventual commercial use of high-temperature inorganic membranes as supports. In a recent review, Hsieh [44] provided a technical overview of inorganic membranes, focusing on porous, pressure-driven membranes. Also, he provided a detailed study of commercially available, porous, inorganic membranes. Another review [45] on inorganic membranes focused on inorganic polymers (such as polyphosphazenes) and metallic membranes. Phosphazene membranes were stable to 180’ C for up to 5 h with high permeability to simple alcohols, but without any selectivity (for the membranes tested). A number of recent technical advances have produced a few commercial inorganic membranes. In a feature article [ 461, Zanetti stated that industrial observers estimate that the present world market for ceramic membranes is ca. $200 million per year with a growth rate of 30% per year. Many of these membranes are used as filters in food processing (and biotechnology) and for gases, liquids or molten metals. Extensive use of inorganic membranes appears in liquid-phase separations, and a specialized application exists with gaseous diffusion for uranium recovery. These materials have the ability to withstand temperatures as high as 1000°C. A number of alumina membranes are now commercially available, but they are inappropriate for extended use in the presence of strong acids or bases. In addition, these membranes, or the catalytic layer, must be kept very thin in order to maintain a high permeability through the membrane. Currently, there are a limited number of sizes, shapes and porosities available. Anotec Separations (New York, U.S.A.; part of Alcan International) sell high-purity, unsupported, narrow pore (0.02-0.2 pm) Anotec alumina membranes, which are still fragile. These are manufactured by anodizing aluminum and then dissolving the remaining aluminum metal below the porous alumina membrane layer. Fig. 3 shows the non-intersecting, cylindrical, homogeneous 0.2-pm pore structure, and another membrane with asymmetric 0.02-pm pores within the 0.2-pm pores. Although the pore size distribution is very narrow,

Fig. 3. Anotec alumina membranes. (a) Top view , AnoporeTM inorganic membrane (b ) crosssection of a 0.2-pm membrane with 0.23 pm latex beads on the surface; (c) homogeneous pores of 0%pm membrane; (d) 0.02-pm asymmetric pores. (Photographs from Anotec Separations brochure, 1988).

Anotec claim they can control the pore size. In addition, metal particles of varying size can be deposited in these pores. Moskovits [47] demonstrated that these membranes can serve as supports by depositing an active metal into the micropores. He deposited a nickel sulfate solution onto the porous alumina membrane, and on electrolysis produced a nickel metal catalyst. In similar manner, iron-, palladium- and silver-supported catalysts were prepared. Reactions of carbon monoxide and hydrogen to produce methane and carbon dioxide, the dehydrogenation of cyclohexane to benzene, and the hydrogenation of propene to propane were studied. For the reduction of nitric oxide by hydrogen at 300 “C, the nickel-containing catalyst performed as well as a commercial noble metal catalyst. Yamada et al. [ 481 recently described similar alumina membranes prepared by anodic oxidation of aluminum. The catalytic activity of the material was

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assessed by following the isomerization of l-butene at 130-150°C. Although the material did not actually function as a separation device, the catalytic activity of this porous alumina was higher than that of powdered anodized alumina. Alcoa/SCT [ 44,49,50] have also commercialized a family of alumina membranes, named Membralox@. These monoliths contain as many as nineteen tubular channels (4 or 6 mm diameter), which are coated with a thin, selective layer with pore diameter from 40 A to 5 pm. A cross-flow velocity is recommended to control fouling [ 441. A cross-cut through the membrane is depicted in Fig. 4. In the brochure, Alcoa also describe an ultra-filter, y-alumina membrane with pore diameter of 40-1000 A. Other commercial inorganic membranes are produced by Norton (alumina) (Worcester, MA, U.S.A.), Corning Glass Works (Corning, NY, U.S.A.), Ceraver (Tarbes, France ), Societe de Fabrication d’Elements Catalytiques (SFEC ) (a’carbon tube coated with zirconia) and NGK Insulators (Nagoya, Japan) ]461. Scientists at Twente University have published extensively [ 51-591 on their porous alumina membranes. Using sol-gel techniques, (e.g. ref. 54), they developed y-alumina membrane films on porous supports. By dipping the support [54-561 into a boehmite sol and calcining to 4OO”C, an B-pm y-alumina layer can be prepared with pore size of 32 A, which withstands sintering up to 900” C.

Fig. 4. Cross-cut through SCT alumina ceramic membrane with 0.2-m surface. (Photographs from SCT brochure, April 1986).

diameter pores at top

15

They also prepared films (at 50% porosity) of > lo-20 pm thickness, which suffered from microcrack formation. Also, they developed three-layer membranes, some of which contained pinholes [ 571. They reported that the slitshaped micropores withstand prolonged heating (850 h) without collapse. Recently, they reported that defects in the top layer can be repaired such that this layer is suitable for gas and vapor transport as well as separation [ 581. They have used supported y-alumina membranes as catalysts for the dehydrogenation of methanol [ 591. Compared with powdered alumina, the alumina membrane exhibits about ten times higher activity. With the membrane reactor, the main product was dimethyl ether (85% selectivity at 200°C). Above 450’ C, the main reaction was the decomposition of methanol to carbon monoxide and hydrogen (ca. 70% conversion). When oxygen (5-6 vol.-% of methanol) is supplied to the permeate side of the reactor, carbon deposition can be minimized. Suzuki et al. [ 601 described a transparent alumina membrane prepared from aluminum isopropoxide via the sol-gel method. The pore size distribution of the membrane shifted dramatically with final treatment temperature (e.g. at 600” C, pores of 20 A where obtained). Another recent Japanese publication [ 61,621 reported the formation of a ceramic membrane with < 10 A pores prepared by impregnating a macroporous ceramic substrate with an alumina sol, calcining and repeating this operation several times. A recent U.S. patent to Suzuki [ 631 described the formation of a composite membrane with a continuous, ultra-thin film of non-granular zeolite on a variety of porous substrates. Specific examples were given for substrates composed of stainless steel, porous nickel, Vycor 7930 and porous alumina. After the surface of the support was activated, 20-100 A films of Na or Ca X or A type zeolites were prepared. Suzuki described this zeolite-impregnated Vycor membrane at the 1988 meeting of the Chemical Congress of North America in Toronto, Canada [ 641. The membrane is synthesized by hydrothermal in situ crystallization at the entrance of 40 or 20 A pores of a Vycor glass [ Vycor 7930, 40 A nominal pore size: Corning Glass Works]. Several applications of this zeolite membrane were described. In particular a zeolite 4A-Vycor membrane effectively removed traces of water in hydrocarbons. The sensitivity of the untreated Vycor to water and alkali was improved with a zirconia coating; however, extended use of Vycor at high temperatures caused some shrinkage of the support. While a zeolite 5A-Vycor membrane could pass reagent-grade n-pentane, n-hexane or n-heptane, small amounts of water plugged the membrane to hydrocarbon permeation. A commercial membrane (1 m x 10.4 mm O.D., 8.2 mm I.D. ), fitted with Swaglok connections at either end is available from Suzuki Laboratories (East Tokyo, Japan). Catelas [65] described a method for manufacturing membranes of titania and silica mixed oxides. In one example, Si ( OCHB )4 was partially hydrolyzed and treated with Ti( isopropoxide), to produce the mixed alkoxide, Ti

16

[OSi(OCH3)4. This inorganic polymer was hydrolyzed at pH 11.5 and then treated with 2-methyl-2,4-pentanediol and a binder (polyethylene glycol). The solution was applied to a porous support and dried in stages to 700°C to produce a 5-ym layer. Catelas suggested that such a layer can be used as a reverse osmosis membrane. Although the porosity and uniformity of micropores in this material are unclear, the material may serve as another example of a potentially useful route to an inorganic membrane for use as a support for a catalytic species. Soffer and Koresh [ 66,671 have patented both flat and hollow-fiber carbon membranes for use in gas separation. Bird and Trimm [ 681 also described the preparation of unsupported and supported carbon molecular sieve membranes. Considerable shrinkage of the membrane occurs on carbonization, however, leading to cracking and deformation of the membranes. Also, the production of c,ontinuous, reproducible membranes is difficult. If the technology can be improved to produce larger, stronger membranes, these materials could serve as supports for catalytic chemical processes. The production of porous, hollow, silica-rich fibers with good alkaline resistance was reported in a recent European Patent Application by Beaver [69], who described the production of hollow fibers (l-250 pm diameter) ranging from 2.5 to 64 cm in length. Pores (8-20 A) are generated by acid leaching hollow borosilicate glass fibers. Perhaps these can be further modified to incorporate a catalyst. Some interesting conceptual approaches to inorganic precipitation membranes were reviewed by Woermann [ 701. These include membranes consisting of copper hexacyanoferrate, slightly soluble salts (barium sulfate or lead chromate) on supports, glass membranes and aluminate layers and gels via hydrolysable metal salts. Although most of these inorganic membranes have been used for filtration or gas separation, the opportunity certainly exists to take them a step further by incorporating catalytic centers into the membrane. However, this must occur while reducing the thickness of the inorganic support in order to promote permeability, without the formation of pinholes or cracks in the final membrane. In addition, one must consider new issues of uniform temperature control and heat transfer which may be more serious in ceramic (vs. metal) membranes. Please, refer to Note Added in Proof at the end of the text. 4.3. Supported

Palladium

Metal

In efforts to reduce the amount of noble metal required in the production of palladium membranes, Langley et al. [71], suggested the use of palladium coated ceramics for separating hydrogen from a mixtures of gases. Gryaznov et al. demonstrated a number of palladium coated membranes as catalysts. In a 1979 U.S. Patent [ 301, they described the preparation of a metallo-ceramic,

17

hydrogen-permeable palladium membrane catalyst on a base of porous copper, nickel or stainless steel with an interlayer of an organosilicon polymer. Specifically, a mixture of a polydimethylsiloxane polymer, an aminosilane and zinc oxide (a filler) was deposited on a base of copper and cured to provide a O.&mm polymer film. A layer of palladium (0.1 pm) was vapor deposited on this support. At 151 aC, the hydrogenation of cyclopentadiene over the membrane with 89% conversion and 93% selectivity to cyclopentene. When Pd-Ru alloys with and without the metallo-ceramic support (at the same conversion levels) are compared, the required level of Pd-Ru in the composite membrane is 100 times lower than that for the pure foil. In a later U.S. Patent [ 721, the same group described the use of a composite membrane catalyst on a cermet substrate. Background information in their patent indicates that the use of the membrane described in their earlier patent [ 291 was limited by a non-uniform layer of palladium over a large area of the catalyst and the size of the composite (restricted by the size of vacuum chambers for palladium deposition ). In the later work, they described a membrane prepared by depositing palladium (via aqueous palladium chloride solution ) onto a siloxane-treated silica gel. This supported palladium catalyst ( -c 0.1 mm particle size) was then mixed with a polydimethylsiloxane-cu,w-diol, zinc oxide and methyltriacetoxysilane. This matrix was applied on a sheet of porous copper (119 x 22 x 0.1 mm). The composite was vulcanized in air at room temperature and the membrane mounted in a flow-type membrane reactor with cyclopentadiene-argon and hydrogen supplied to opposite sides of the membrane. At 161°C they observed conversions of 90% with 95% selectivity to cyclopentene. Abe [73] described the separation of hydrogen-nitrogen mixtures over a thin palladium membrane supported on porous alumina. The a-alumina support was coated with a boehmite gel to produce a lo-pm layer with a mean pore diameter of 3000 A. A 5-lo-pm layer of palladium was deposited either from a palladium salt or physical vapor deposition of Pd-Ag alloy. Palladium diffused into the inner walls of the pores, and the ability of PdOto adsorb and selectively permeate hydrogen produced a synergistic effect in combination with the hydrogen gas separation effect of the pores alone. Abe claimed that one could separate high concentrations of hydrogen without the catalyst serving as an obstacle to the permeation of hydrogen through the membrane. He also suggested that membrane strength and the formation of cracks and pinholes are not a problem. The question remains as to whether this membrane would be effective for selective, catalytic hydrogenation or dehydrogenation reactions on a commercial scale.

18 5. ADDITIONAL EXAMPLES

OF MEMBRANE

CATALYSIS WITH INORGANIC

MEMBRANES

In Section 4.2, some membranes were described with combined catalytic and separation applications (e.g. refs. 59, 63, 72 and 73). This section provides some additional examples. In an extension of their work with microporous alumina membranes, researchers at ALCAN prepared a catalyst-impregnated alumina membrane [ 741. Using membranes produced by anodizing aluminum, they deposited a catalytically active material within the pores. Examples are given for carbon monoxide oxidation over a platinum-impregnated alumina membrane. A separation factor of 1.31 for carbon dioxide, which increased considerably with increased flow of argon over the membrane surface, was obtained. With an osmium-impregnated membrane, they demonstrated the dehydrogenation of ethane and the hydrogenolysis of ethylene and the hydrogenation of ethylene. By impregnating Cr,O, onto the support, they demonstrated the dehydrogenation of ethane at 200°C. By depositing platinum on the wide capillary pore side of their alumina membrane, they generated a bifunctional (platinum metal and acid sites of the support) catalyst. While feeding n-hexane vapor to this Pt/Al,O, membrane, they identified products consistent with the cracking of benzene. Although palladium metal membranes are useful for permeation, they may be inappropriate if undesirable side-reactions occur or when poisons are present. Shinji et al. [75] demonstrated the dehydrogenation of cyclohexane to benzene using a porous glass membrane. A 0.38% (w/w) Pt/A1203 catalyst was packed into a Vycor glass tube (Corning Code 7930). Here, the Vycor tube serves as a container for the catalyst and transports hydrogen away from the catalyst. Nitrogen was passed over the outside of the membrane (Fig. 5 ), while

Non-porousPyrex

Fig. 5. Schematic diagram of Vycor membrane tube packed with Pt/AI,O, catalyst [ 751.

19

nitrogen-cyclohexane was admitted to the inner tube at 215°C (total gas pressure ca. 1 atm with a material balance of 90%). With a non-porous tube the conversion was 35% compared with 80% using in the Vycor tube. However, they found that a low inner flow-rate and a high outer flow-rate were necessary to achieve high conversions. This creates problems with respect to recovery of the hydrogen from a dilute product stream. Recently, Mohan and Gorvind [ 761 provided an analysis of a cocurrent membrane reactor for the dehydrogenation of cyclohexane using a permeable wall membrane reactor. They offered several conclusions with respect to optimum conversions. Kameyama et al. [77-791 described the use of a porous Vycor glass membrane and a microporous alumina membrane for the separation of hydrogen sulfide from hydrogen via the decomposition of hydrogen sulfide at 800°C. Later Abe [80] studied the decomposition of hydrogen sulfide (reaction 5) over molybdenum sulfide contained in a porous glass pipe. The porous membrane was prepared from a H,S=H,+

S

(5)

boehmite sol (derived from the hydrolysis of aluminum alkoxides ) and mixed with l-pm particles of molybdenum sulfide. The mixture was ground, spray dried, mixed with an organic binder to form an extruded pipe and heated to 800” C. Alternatively, a porous support (e.g. alumina or cordierite) was dipped into the boehmite-molybdenum sulfide slip. With a multiple membrane reactor at 800°C and an inlet pressure of 3.8 atm of hydrogen sulfide, they obtained 14 vol.-% hydrogen through the membrane. In comparison, a control run produced only 3.5 vol.-% hydrogen using pellets of molybdenum sulfide contained within a porous cordierite pipe. Dobo and Graham [ 811 of Monsanto described the preparation of an essentially inorganic, monolithic hollow fiber. They claimed that inorganic and metallic hollow fibers that are radially anisotropic can be prepared by blending large amounts of a metal or non-metal oxide (e.g. Fe304, NiO, Fe203, silicates or Al,O,) or a metal [e.g. aluminum or nickel (via reduction of the oxide at 1100 3C in hydrogen ) ] with acrylonitrile copolymer. The mixture can be spun into fibers, and the fibers treated at 1100°C under a reducing atmosphere. Most of their examples described the use of these fibers for the separation of hydrogen from a gas mixture, but they also suggested that these hollow fibers can be filled with catalyst particles or the walls of such fibers can serve as a catalyst for selected reactions. Kim [82] described two novel types of membrane reactors: a porous-walled catalytic reactor-separator for heterogeneous catalysis and a membrane reactor-separator utilizing a homogeneous catalyst. A mathematical model describing the concept of a porous-walled reactor-separator was developed. For the second reactor, two permselective polymer membranes encased a porous steel sheet impregnated with a solution of RhH(C0) (P( C6H5) )3 which was used to catalyze the hydroformylation of ethylene to propionaldehyde at 110°C.

20

The rhodium complex was dissolved in a mixture of dioctyl phthalate and acetone, which was absorbed into a porous (90 pm ), sintered, stainless-steel disk (76 mm diameter, 5 mm thick). (The acetone was removed by allowing the disk to stand in fume-hood.) Two types of membranes were used to contain the disk: a UF Type F 150-pm thick cellulose ester membrane from Ultra/Par or a 200-pm thick Filinert PTFE membrane from Nucleopore. The pressure over the top of the membrane was 65-75 p.s.i.g., and the lower compartment was purged with helium at 20-40 p.s.i.g. (O-100 cpm). Unfortunately, the membrane degraded after 2 h of use, even with a high ratio of P& to rhodium complex. This type of reactor does avoid catalyst recovery and recycle while producing a partially enriched product at a relatively low temperature. Kim suggested that further improvements might be achieved with metallic or ceramic membranes. Recently, Ziembecki [83] demonstrated the use of a palladium membrane for the dissociated chemisorption of carbon-containing molecules. Using a palladium foil membrane, ethylene was fed to one side of the membrane at 873 0C. Carbon was formed, which diffused through the palladium and reacted with water vapor on the other side of the membrane to produce carbon monoxide and hydrogen. Once carbon had been formed, the selectivity for the reaction with water to hydrogen and CO was better than 80%. The process continued for several hours with no indication of decreasing activity. It was calculated that the total number of carbon atoms consumed during the steady-state reaction exceeds the amount of carbon available in the volume of the membrane by a factor of about 6. 6. SUMMARY

This review suggests that there may be a real opportunity for a dramatic growth in membrane catalysis. The substantial, pioneering work of Gryaznov certainly demonstrates that one can catalytically synthesize and separate a number of potentially attractive commercial products. The commercial applications of palladium membranes may be limited to those who can readily afford large amounts of precious metals and possess the ability to fabricate these metals. Costs could be reduced by forming thin metallic membranes on a support, but these may still prove unsatisfactory. Ideally supported membranes should consist of an ultra-thin, unbroken layer of metal on the support, but achieving a crack-free, pinhole-free ultra-thin layer that remains flawless with continued use is technically very difficult. An acceptable alternative may be to distribute a highly dispersed catalyst through the inorganic membrane support. Over the last few years, remarkable progress has been made in the synthesis of microporous inorganic substrates that can support highly dispersed metals. Here is an opportunity for membrane catalysis to grow. Some patents and a few publications in this direction are beginning to appear; this is one approach

22

ability of inorganic membranes to catalyze a reaction with high selectivity while simultaneously achieving a chemical separation remains a tantalizing, but challenging objective. 7. ACKNOWLEDGEMENTS

I thank both Guido Pez and Jim Roth for many helpful discussions and acknowledge Air Products and Chemicals for permission to publish this work. 8. APPENDIX

The following definitions are summarized from ref. 84. Flux (J), the flow-rate of t.he permeating species per unit cross-sectional area of the membrane. Where Fick’s law of diffusion is applicable, the flux is related to the effective diffusion coefficient, D, and the concentration gradient, dC/dl, between the two interfaces of the membrane (dEis the thickness of the membrane, dp is the pressure difference across the membrane and P is the permeability, defined below ) : Flux= -D(dC/dZ) = -P(dp/dZ) Permeability (P), in Barrer units, the rate at which a gas flows through a membrane of thickness 1 cm with a cross-sectional area of 1 cm2 and a transmembrane partial pressure differential of 1 cmHg: 1 Barrer= 10V1’cm3 (STP) cm thickness cm2 area*s*cmHg Permselectiue, permeable to different extents to different molecular species under an equal driving force. Membrane selectivity (a) or permselectiuity, the measure of a membrane’s ability to separate two gases (A and B): a= PA/PB. In this review, seZectiuity (S ) refers to chemical selectivity which is (yield/conversion ) *100. NOTE ADDED

IN PROOF

An entire, recent issue of the Journal of Membrane Science, 39 (1988) 197314, is devoted to Inorganic Membranes. Several articles on ceramic membranes contrast advantages (thermal stability, configuration, and chemical stability) vs. disadvantages [cost ($X0-2OO/sq. ft. vs. $82O/sq. ft. for polymeric membranes), fragility, thickness, and current diffusional limitations]. A few articles (Goldsmith, p, 197; Fleming et al., p. 221; and Keizer et al., p. 285) discuss the operational separation mechanisms occurring within current

23

inorganic membranes. These articles offer a fresh, critical view about where the problems are and what limitations exist.

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