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Membrane Catalysis Over Palladium and its Alloys
Guczi, L a al. (Editom), New Frontiers In Catalysis P m d i n g s of thc 10th I n t c ~ ~ ~Congress t i ~ ~ on l Catalysis, 19-24July, 1992, Budapest, Hungary Q 1993 E l d e r Science Publishers B.V.All rights mewed
MEMBRANE CATALYSIS OVER PALLADIUM AND ITS ALLOYS
J. N.Armor and T.S.Farris Air Products and Chemicals, Inc., Corporate Science and Technology Center, 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195, USA
Abstract Dense palladium metal membranes offer one approach to developing permselective membrane catalysts which offer a long term opportunity for reducing unit operations while enhancing product selectivities. We report the use of palladium and palladium alloys in the form of spirals or foils as both membranes and catalysts for several model hydrogenation reactions. The substrates investigated include: ethylene, butadiene, toluene, and cyclohexene with respect to critical process issues such as membrane life, reactor stability and productivity, optimum reactor temperatures, and membrane composition. Two reactor designs were used to evaluate hydrogenations performed in the liquid or vapor phase. The performance of these reactors as they relate to current difficulties in this field will be discussed.
Introduction Membrane catalysis over permselecave inorganic membranes offers promise for performing selective reactions while simultaneously separating some products from the reactants [ 11. For producing certain chemicals this could permit a sizable cost reduction by eliminating downstream processinglseparation costs. V. Gryaznov and coworkers have published [2-71 a great deal of work on the use of Pd alloy membranes in the form of large spirals for the selective hydrogenation of a variety of olefinic substrates. Palladium's remarkable capability to dissolve €b allows one to continually remove II2 (in a dehydrogenation reaction) thus allowing an increase in the conversion of the substrate. Further, the addition of a reactive molecule (such as a)to the permeate side of the membrane further enhances conversion [8] by continually consuming permeated II2. Here we describe the use of Pd and/or Pd alloy foils for the permselective hydrogenation of ethylene, butadiene, toluene, and cyclohexene as a function of several critical process parameters.
Experimental The palladium and palladium alloy foils (Johnson Matthey, Ltd.)were cut into 5 cm discs and washed with hexane then methanol. The air dried foils were compressed between two annealed copper gaskets held together by 2.75 inch OD knife edge flanges (Kurt J. Lesker, Inc., Clairton, PA, USA) (Figure 1). The foil was activated by heating to 265°C at lO"C/rnin.
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and purging both sides of the foil with zero grade air. After one hour, a nitrogen purge is initiated for 10 minutes before starting a flow of 50% hydrogen in Nz. Metal reduction continued for one hour followed by a purge with Nz.This procedure was repeated twice. The reactor was cooled under a reducing atmosphere after the final reduction. Activated foils were cooled to 100°C under hydrogen then purged with nitrogen for 30 minutes. The various gases were delivered at 30 to 60 cc/min to each side of the foil. Hydrogen permeation was determined over activated and non-activated foils at 50°C intervals between 100 and 250°C. Non-activated foils were heated to 100°C at 10"Umin under a nitrogen purge then allowed to equilibrate for 30 minutes. A blend of 50% hydrogen in Nz at
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GASKETS
Figure 1: Schematic of knife-edge flange membrane reactor
Figure 2: Photograph of disassembled knife-edge flange membrane reactor: (1 to r) inlet head, flow distributor, membrane mounted in Cu gasket; disaibutor, Cu gasket; exit head.
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60 cc/min was fed to the retentate side of the foil and a nitrogen purge at 20 cc/min was maintained on the permeate side. After 30 minutes, sampling of the permeate stream started. New temperature setpoints were established with the foil under a nitrogen purge. Ethylene or butadiene at 20 cc/min were fed to the reaction side of the foil as dilute feeds of up to 16 mole percent in nitrogen or as an undiluted gas. Liquid hydrocarbons were vaporized at 30°C within a stainless steel sample cylinder by bubbling N2 through a 7 micron frit immersed in the olefin. Transfer lines after the vaporizer were maintained at 50°C to prevent condensation. Analysis of the reactant and product streams was accomplished with an HP 5890 gas chromatograph. Hydrogen, ethylene, and ethane were quantified using a thermal conductivity detector with a 8' x 1/8" Haysep Q column, with nitrogen as the carrier gas. Toluene, butadiene, cyclohexene and hydrogenated analogs were analyzed using a 30 meter DB-WAX megabore column (1.0 pm film) with a helium carrier gas. The sample size was 0.25 cc for all experiments. Calibration curves for hydrogen, ethylene and ethane were generated from known standards (Scott Specialty Gases, Plumsteadville, PA). Butadiene and its hydrogenated products were calibrated by diluting a constant flow of each pure component with known volumes of nitrogen. Chromatographic responses for the vaporized hydrocarbons were obtained in a similar fashion. Hydrocarbon vapors carried in a constant flow of Nz were trapped on a known weight of activated carbon. The weight gain over 30-60 minutes was recorded and used to calculate the amount of hydrocarbon in the vapor phase and equated to the GC area counts. The calibration curves show a linear GC response for these compounds across the concentration range. Daily accuracy was +3% of the calibrated values. Liquid phase reactions were run in the modified Parr reactor using coiled tubular Pd/Ru foils (obtained on loan from Professor V. Gryaznov, Moscow). The Parr Reactor (Figure 3) was partially filled with cyclohexene, octadecene or cyclooctadiene as a neat solution (1OOg) or diluted to 10% wt/wt in a suitable solvent. All the reagents used for the experiments were dried using 4A molecular sieve and de-oxygenated prior to use. The contents were stirred at 400 rpm during the experiment, A 0.5 ml aliquot was withdrawn at fixed intervals during the experiment and analyzed on a HP-5980gas chromatograph by FID. Activations were carried out at 400°C. Four times, air and hydrogen were passed both through and over the coils for one hour each. The reactor was cooled to the reaction temperature in Nz.
Results and Discussion Palladium membrane catalyzed hydrogenations of several different substrates were evaluated. These included hydrogenation of ethylene, butadiene, toluene, and cyclohexene.
Ethylene Hydrogenation: Most of our experiments were done with 0. lmm Pd or 0.025mm Pd/Ru(5) foils (11.4 cm2 of metal surface). Use of the thinner 0.025mm Pd foil was difficult due to repeated fracturing of the foil under our operating conditions.
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As demonstrated earlier 191 the permeability of Hz through the Pd foil varies considerably with temperature. Tables 1 and 2 illustrates this for Pd, Pd/Ag(3O)/Ru(2) and Pd/Ru(S) alloys. 'Since the activation of the foil in air followed by Hz provided a significant increase below 152°C (Table 2) in I-L permeability, we always activated any membrane in this manner.
Figure 3: Pdmu spiral membrane mounted into Parr reactor (1 to r): thermocouple, inlet to spiral, sample withdrawal line, stirrer, outlet from spiral, and fritted metal gas inlet tube. The beneficial effects of activation appear to become mvial above 150°C. The performance of the alloy for ethylene hydrogenation was not altered by a pretreatment of the foil with CC14 and 1N HCl; hence this latter procedure was not used. As expected the permeability of Hz was a factor of four faster with a 0.025mm Pd/Ru(5) membrane at the lower temperature. However this difference decreased with increasing temperature. Hydrogen permeability of the activated Pd foil is 30-50% greater than through an activated Pd/Ru(S) alloy (Table 2). The addition of ethylene to the permeate side actually increased the permeability of hydrogen (22 to 56 pmole/cm2/min at 205"C, Table 1). As expected increasing the volumetric flow of N2 to the permeate side, also enhanced the )I3 permeability. The work of Goto [lo] suggests that Pd/Ag/Ru alloys have higher permeability for H2 than pure Pd. The addition of silver to the Pd/Ru alloy certainly enhanced its permeability (Table
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2) to Hz. The comparison to a 0.025mm Pd foil is difficult because this thin of a pure Pd foil ruptures quite easily. Unfortunately a 0.025mm thick, 10 x lOcm sheet of Pd/Ag(3O)/Ru(2) costs $1700! Table 1. HZ Diffusion over Pda
I% Diffusion in pmole/cnP/rnin Temperature, "C
Not Activated
107 157 206 257
8.6 17 21 24
Activatedb 4.7 17 22 25
Activated + C a d C
52
46
a O.lmm Pd foil; 50% H z / N 2 retentate feed = 64 cc/min; permeate sweep = 20 cc/min N2 b activated in air at 265°C for 1 hr, then 50% HZin N2 at 265°C for 1 h c 50% H d V 2 to retentate side; W C & = 1.0; CzHJN2 to permeate side; CzH4 = 10 cc/min; total flow = 20 cc/min
Table 2. HZDiffusion over Pd alloysa
Hz Diffusion in pmole/cm*/min
Activated + c2H4 c
Temperature, "C 101 152 205
255
3.9 8.6 13 17
(3.4)b (16)b (25)b (29)b
15
22 28 33
(2)b (14)b (33)b (36)b
13 34 40 42
28 59
72 63
a Pd/Ru(5) or Pd/Ag(3O)/Ru(2) activated in air at 265"C, then in 50% W retentate feed = 61 cc/min; permeate sweep = 20 cc/min N2 b unactivated foil c WC2H4 = 0.5 (molar ratio); C2H4 = 20 cc/min as permeate sweep
z at 265°C;
1368 A number of experiments were performed over the Pd/Ru(S) foil using a variety of different flow configurations. With H&H1 = 0.4 and cofeeding reactants to the same side of the membrane, the conversion of CzH4 was only 39% (Table 3). With the reactants separated and H2 permeating the foil, the conversion jumped to 66%. Initially the WCzH4 was 0.44-0.50 for all these runs; however the effective partial pressure of Hz is less with the Pd membrane used to control the diffusion of Hz (since not all the Hz permeates the membrane). The use of a stainless steel foil with Hz and C2H* on the same side gave no conversion, but inserting the Pd/Ru(S) foil into the reactor as a catalyst with both reactants present gave 43% conversion.
Table 3. Ethylene Hydrogenation over Pd/Ru(5) at 15OOC a % CzH4 Conversion
same sideb opposite sidec no membrane, use of foild
a b c d
39 66
43
wczH4
(molar ratio)-
.44 .41 SO
T = 150°C;0.025mmPd/Ru(S); foil activated at 265°C before each use total flow = 42-44cc/min; Hz = 21 cc/min (8.8mmoleskc) retentate flow = 61 cc/min; I% = 20 mmoles/cc; permeate flow = 20 cc/min total flow = 22 cc/min; & = 1 1 mmoles/cc
Foley et al. [9] report changes in the surface morphology of Pd foils as a function of reaction time and temperature. Extensive cracking and surface pitting of Pd foils occur at 150 and 200°C with ethylene. The amount of surface xestructuring can be attributed to the pressure of I% across the foil and can be attenuated somewhat by preceding any reactions with an activation at a temperature greater than the reaction temperature. Fig 4 shows two Pd foils after being cycled from 100 to 250°C and back to 100°C. The foil on the left had been exposed to 100% HZand ethylene at various temperatures over 3 days and exhibits severe cracking and surface restructuring. The foil on the right was exposed to ethylene and a 50% H2 stream over a four day period, and gross morphological changes are less sevem. The morphological changes are probably caused by the dl3 phase transition [3, 11-13]. Despite the surface restructuring the integrity of the Pd foil was maintained for both samples. Because HZpermeation through Pd/Ru alloys is less and the transition temperature between the Q and B phase is lowered from 310°C to -77°C these gross changes are absent in the Pd alloys.
Butadiene: Earlier Nagamato and Inone 1141 studied the hydrogenation of butadiene over a Pd membrane at 100°C. They demonstrated that the membrane was affected by competitive adsorption between the diene and Hz. The reaction with permeated hydrogen was faster than
1369 with gaseous Hz [15], but the study was complicated by the presence of the a and I3 phases of Pd. We observed that at 100°C butadiene was completely hydrogenated to a mixture of butane (57%) and butenes [l-butene (3%), cis-2-butene (12%), trans-2-butene (28%)] at 100°C over an activated 0.025mm Pd/Ru(5) foil;50% Hz = 64 cc/min; 10%G& = 25 cc/min; WGfi = 4.0. The above selectivity values were measured after 40 min. The yield of butane continued to decline (39% after 2h) while the level of cis and trans-Zbutenes rose accordingly.
Toluene: Surprisingly, with -13% toluene in the vapor phase on one side' of a 0.025mm Pd/Ru(5) membrane and 50% Hz on the other side, no reduction was detected up to 300°C. Following exposure to toluene the membrane was used to perform the hydrogenation of cyclohexene successfully,indicating no irreversible poisoning of the membrane had occurred. '
Cyclohexene: Early experiments to hydrogenate cyclohexene with Hz flowing through a Pd/Ru(S) spiral (20 cm2 of Pd surface) were quite disappointing [with the foil immersed in liquid cyclohexene (containing inhibitor)]. Using the reactor shown in Fig. 3, <3% of the cyclohexene was converted at 70°C. With octadecene (200°C) or cyclo-octadiene (13OoC), ~ 2 conversion % was detected.
Figure 4. Left: O.lOmm Pd foil exposed to 100% Hz on one side and ethylene on the other for 3 days. Right: Use of 50% Hz instead of 100%for 4 days. = 3.0) using the knife-edge Operating with cyclohexene in the vapor phase at 150°C (WG~I-IIO flange membrane reactor (Figures 1 and 2), the conversion was 35% [over a O.lmm foil of Pd/Ru(5)]. The selectivity (S) to cyclohexene was 94% with 7% to benzene! At 250°C and WCd41o = 3.2, cyclohexene conversion fell from 44% to ~ 2 1 %over 2h with decreasing yields of benzene (Table 4). Reactivating the foil in air then HZ at 265°C restored the activity. Although little activity was seen at 70°C, it could be increased by raising the pressure of Hz. Use of a fresh foil (with the standard 265°C activation) gave 80% cyclohexene conversions at 150°C (83% S to cyclohexane, 17%S to benzene). Using cyclohexene as a feed at 150°C (no HZ)over the foil gave 45% conversion (52% S to cyclohexane and 48% to benzene) indicating that cyclohexene disproportionates over the alloy. Some Hz is also detected. Some reduction in conversion occurs with time (1 hr) suggestive of carbon fouling the membrane. This
1370 suggests that the foil of the previous runs had deteriorated. This unpredictable behavior points to another undesirable feature of these membranes [16]. Of the olefins studied, cyclohexene was the only one which exhibited this unpredictable behavior. Table 4. CyclohexeneHydrogenationa
T, "C
Time on Stream (min)
MID conv.,%
Selectivity 8c 6 H l l 96CaHs
150
20
40
36,80 e 33
93 94
7
25 lb
5
44
46
54
55
45
10 112 155
42 33 21 20
26 20
7.6 21
40
7oc 7od
48
60
62
6 52
40 38
100 100
a W&IB (molar ratio)= 3.0,O.lmm Pd/Ru(S) foil b reactivated in air/& on another day c 0.5 a m Ib d 2.0am & e repeated with a new piece of activated foil
Conclusion We evaluated the permeability of Hz (at c 25OOC) through of Pd, Pd/Ag(3O)/Ru(2), and Pd/Ru(5) alloy foils and the use of these metals for the hydrogenation of a variety of olefins. 5 varies dramatically with the alloy composition: Pd/Ag(3O)/Ru(2) > Pd The permeability of J > Pd/Ru(S)t and the permeability of hydrogen is significantly enhanced by adding C2H4 to the permeate side. Membrane activation in zI2 increases H2 permeability below 152°C. The use of a membrane to mnsport HZto the C3I4 side of the reactor results in increased C2H4 conversion vs co-feeding the substrates to the same side of the foil catalyst. Hydrogenation of butadiene with excess hydrogen over the membrane gave unselective hydrogenation products including butane, which continually decreases as a function of time. Surprisingly toluene is not hydrogenated at all over these Pd membranes.
1371 Liquid phase reactions over Pd/Ru spirals gave poor conversion. However vapor phase hydrogenation of cyclohexene gave -35% conversion at 150°C with 94% selectivity. Surprisingly, benzene (7% selectivity) was also p r o d u d . In the absence of hydrogen, cyclohexene disproportionates to benzene and cyclohexane with drifting yields over time. Attempts to duplicate these latter runs with a fresh membrane gave similar product distributions but with different conversions. Clearly this work points to several limitations exhibited by Pd and W alloy membranes as catalysts. These membranes do not provide a universal means to hydrogenate olefinic substrates. Instead one must balance reactor conditions, with alloy composition, reactor configuration, and catalysis. Much more fundamental research is needed prior to any future attempt at developing these membranes in commercial membrane mactors. Because of the cost of the Pd and its alloys, and since only a thin, dense layer of Pd is necessary, coatings of Pd alloys on mesoporous inorganic membranes is probably a more appropriate, realistic membrane for any future commercial use.
Acknowledgements We wish to thank both Professor Henry Foley of the University of Delaware and his former post doctoral student Dr. Andrew Wang for sharing their early experimental efforts with us in this area which allowed us to build upon their p r o p s s . We also wish to acknowledge Air Products and Chemicals Inc for their support of this exploratory mearch and permission to publish this partion of our early work in this field.
References 1 J. N. Armor, Appl. Catal., 49 (1989)1-25. 2 V. M. Gryaznov, Platinum Metals Rev., 30 (1986)68-72. 3 V. M. Gryaznov and N.V. Orekhova, Catalysis of Precious Metals, Nauka, Moscow, USSR, 1989. 4 V. G. Dobrokhotov, V. M. Gryaznov, and L. F. Pavlova, J. Less Common Metals, 89 (1983)585-86. 5 V.M. Gryaznov, M. M. Ermilova, L. S. M m m v a , N. V. Orekhova, V. P. Polyakova, N. R. Roshan, E. M. Savitskii, and N. I. ParFenova, J. Less Common Metals, 89 (1983)52935. 6 A. P. Mischenko, V. M. Gryaznov, V.S. Smimov, E. P. Senina, I. L. Parbuzina, N. R. Roshan, V. P. Polyakova, E. M. Savitsky, U.S. Patent 4,179,470(1979). 7 V. M. Gryaznov, A. N.Karavanov, T. M. Belosljudova, A. V. Ennolaev, A. P. Maganjuk and I. K. Sarycheva, U.S.Patent 4,388,479(1983). 8 N. Itoh, K.Miura, Y.Shindo, K.Haraya, K.Obata,and K.Wakabayashi, Sekiyu Gakkaishi 1989,32,47;N. Itoh, "Simultaneous Operation of Reaction and Separation by a Membrane Reactor", in Future Opportunities in Catalytic Separation and Technology, M. Misono, Y.Moro-oka,and S.Kimura.eds., Elsevier, New Yo&, 1990,p.268-83.
1372 9 H. C. Foley, A.W. Wang, B. Johnson,and J.N. Amor, "Effect of a Model
Hydrogenation on a Catalytic Palladium Membrane", in Proceedings of the ACS Symposium on "Selectivity in Catalysis", August, 1991, paper #:Cad. 11. 101 R.Goto, Kagaku Kogaku 34 (1970) 35-42. 11 J. Philpott and D. R, Coupland, Chem. Ind., 31 (1988) 679-94. 12 AS. Darling, Platinum Metals Rev., 2 (1958) 16-22. 13 A.G. Knapton, Platinum Metals Rev., 21 (1977) 44-50. 14 H. Nagamato and H. Inone, Bull. Chem. Soc. Jpn. 59 (1986) 3935-39. 15 H. Nagamato and H. Inone, Chem. Eng. Commun. 34 (1985) 315-23. 16 J. N. Armor, Chemtech, 1992, submitted for publication.
1373 DISCUSSION Q: P.L Silveston (Canada) Although this paper claims membrane reactors offer the advantage of separating products concurrently with their creation by reaction, it uses examples which do not involve product separations but rather separation of reactants. Table 3 and the discussion of butadiene hydrogenation show or suggest that segregating reactants improves performance. My comment is that segregation reactants to obtain the benefits attributed to the membrane reactor can be achieved by composition modulations (switching between hydrocarbon and hydrogen in the time dimension) much more easily and more cheaply than by the use of membranes. Unfortunately, composition modulation is not widely practiced in catalyst science, probably because it is not widely known. In the reactor engineering area composition modulation is well known. The literature contains well over 100 publications.
A: J. Armor We did not mean to imply that we were going to discuss commercial applications of palladium membranes. We began this work as a means to build a fundamental understanding about palladium membranes for transporting hydrogen with regard to membrane catalysis. We thought we would choose rather simple s stems (C*H&ydrogen, benzenehydrogen, toluenehydrogen, butadhe/ hydrogen) and t en go on to more attractive commercial opportunities incorporating reactions such as dehydrogenation. We soon began to appreciate that even simple hydrogenations were not readily achievable with these membranes. The point of this paper is to indicate that one can carry out selective hydrogenations, but this is very substrate dependent. Olefins themselves do not undergo facile hydrogenation in general and, further, these are very dependent upon the membrane composition. The literature already suggests that dehydrogenations are further complicated by carbon build-up and resulting deactivation. We are attempting to resolve these problems and circumvent them by building up a sufficient knowledge base with regard to the simpler systems before tackling these more complex systems.
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Q: J. W. Hightower (USA) My question concerns the hydrogenation of 1,3-butadiene. Apparent1 the roducts are mainly butane and some butanes as we know from the classic work of ucwe 1, Bond, Wells, and others. Pd catalysts are remarkably selective in making first n-butenes and that n-butane after most of the butadiene has been consumed. As this selectivity observed with the membranes, even at low conversions of the diene ? If it is not, does this not detract somewhat from the selectivity advantages of the membrane configuration ?
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A: J. Armor Yes,we were surprised at the high yield of butane. Interestingly, while some butenes were made in the beginning, the yield of butane continued decline while the level of cishans 2-butenes rose accordingly with time. At this time we are not able to explain this without some additional experiments. This does int to a complication observed with these membranes which may be due to the a - F p h a s e s of palladium and its alloys, carbon buildup, or as yet unexplained effects occurring with time.
Q: W. 0. Haag (USA)
Would you please clarify the interesting effect of ethylene supplied to the permeate side of the membrane in enhancing the hydrogen permeation rate ‘1 Does the ethylene merely reduce the hydrogen partial pressure and hence increase the H pressure gradient pressure drop is across the membrane, or does ethylene have an effect even when the the same ?
8,
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A: J. Armor We did monitor total flows in and out of both the retentate and permeate sides of the membranes. There was an increase in the total flow on the permeate side which we could attribute td a greater flux of h drogen through the membrane. We did examine this to a limited extent as a function o partial pressure of hydrogen without seeing a comparable effect for the expected DP of hydrogen across he membrane. Nevertheless, we might expect some rate enhancement due to this H2 flux giving rise to a change in H partial pressure Gryaznov has pointed out in a number of publications that pzladium membranes exhibit better conversions with a membrane operatin with permeating hydrogen as opposed to the simple foil being used as a catal tic sur ace. he argues that the dissociation of hydrogen provides for an activated form o hydrogen on the permeate side for which the substrate can react. In addition, he points out that this microscopic picture of the surface is very different from that of a traditional catalyst which is often saturated with the olefinic substrate; the use of hydrogen permeating through a palladium foil assures that the surface has a much higher level of hydrogen atoms and hence, perhaps is a better hydrogenation catalyst.
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r
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0:J. R. H. Ross (Ireland) ' Do you think that the fluxes obtainable with membrane systems will ever be high enough to allow the reactions to compete with conventional catalytic processes ?
A: J. Armor With regard to sufficient fluxes in membrane systems, I believe that one can optimize this to some extent by varying temperature, pressure or alloy composition. We demonstrated in this manuscript that the composition of the foil is crucial with respect to optimizing hydrogen rmeability. By choosing the right alloy composition, one can actually enhance the iffusion of hydrogen through the alloy versus pure palladium. In addition, by going to very thin layers of metal or alloy on a meso porous support, the permeability of hydrogen will be much greater. Thus, I do believe that with the right system (metal, thickness of coating, composition of alloy, etc.) it should be possible to come up with particularly attractive opportunities for membrane catalysis.
r
Q: S.Kaliaguine (Canada) In your oral presentation you have mostly discussed the use of Pd catalytic membranes in hydrogenation reactions is also extremely promising because in this case the separation of hydrogen from the reaction medium may allow one to displace the thermodynamic equilibrium and reach higher conversions at lower temperatures. Such an effect may be significant in several industrial porouses studies for example the stream reforming of methane. In this case the membrane reactor would also produces very pure hydrogen in one step.
A: J. Armor Yes, I believe that membranes, indeed, do offer the possibility of facilitating a number of dehydrogenation reactions. The difficulty with these dehydrogenations currently is that they often run at very high temperatures and invariably some carbon buildup occurs. The high temperatures in the ast have limited the application with regard to metal membranes because of the a@ e fect that we describe in the manuscript. The high temperature required for dehydroeenations often limits the utility of the% memfbranes because of sealing or fabrication ~ssues.However, running reactions such 8s oxidative dehydrogenation (where one passes oxygen on the permeate side to react with any h drogen) allows one to perhaps lower the temperature and at the same time enerate heat or the dehydrogenation process. Alternatively, the coupled reactions (invo ving Htransport) which Gryaznov has described provide another opportunity. These sorts of
P
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k
1375 systems, I believe, have good promise but we need to do more research in this area. We have some work underway in these areas as well. The work is much too preliminary and is confused by other issues such as described in Table 4 of our manuscript where we see both cyclohexane and benzene on hydrogenation of cyclohexene. Apparently, the benzene is being produced from the dehydrogenation of cyclohexane which must be a competitive reaction with the hydrogenation of the olefin! Clearly, more work needs to be done in this system.
Q: A. Srk6ny (Hungary) You have spoken about limitations. and challenges. What are the main advantages of this technique ? Can you really inhibit the poisoning in such a way ? A. J. Armor I believe the main advantages of the membrane catalytic reactors lie with the opportunity to provide simultaneous separation as well as catalysis in one unit operation. There appears to be some enhanced activation of hydrogen by using these palladium alloys and that deserves further exploration and consideration. Further, with palladium, one has perfect permselectivity. That is, only hydrogen permeates without any other gases, thus, this permits the production of very pure hydrogen and should also offer the opportunity to do very selective hydrogenations.
Q: J. W. Geus (The Netherlands) With the application of membranes, it is highly important to combine the favorable properties of metals and ceramics. Metals provide mechanical strength and resistance against thermal shocks, while ceramic materials can be processed to thermostable porous structures containing pores of dimensions down to 0.5 pm. To achieve the combination of metals and ceramics, we use metal bodies (tubes, plates) having course pores (diameter about 1 to 4 vm). Starting from metal powders, bodies of a desired shape and dimension can be produced readily on an industrial scale. Three different procedures have been developed to apply ceramic layers containing fine pores into or onto porous metal bodies. Permeability measurements have shown that all three above procedures lead to leak-tight membranes. A: J. Armor I think the approach of building composite membranes does make a lot of sense. In my review which shall appear in Chemtech this year, I focus on two attractive opportunities in this area: coating palladium onto mesoporous ceramic membranes and the deposition of catalyst into microporous ceramic membranes. Your suggestion falls into the latter category and a number of groups, as you probably are well aware are working in this area. The difficulty with this approach is to assure that one has a perfectly flawless membrane and that no cracks or voids appear as the reaction continues for days or moths of operation. Most systems have not been tested to this degree. It is known that one can prepare intermediate layers with y-alumina as a bridging composition, only to find that the alumina undergoes some sintering with time to undergo a change in pore size and distribution. If your support layer is undergoin chemistry while your trying to carryout catalysis on the surface, this can be disastrous. bviously, one has to provide the right match of the active catalytic layer with the interphase support and the meso porous support structure (whether it be metal or ceramic). Our early work here with palladium foil and its alloys is just a beginning. We have learned a lot and are beginning to extend this knowledge into other compositions, forms, and configurations.
%
MEMBRANE CATALYSIS OVER PALLADIUM AND ITS ALLOYS
J. N.Armor and T.S.Farris Air Products and Chemicals, Inc., Corporate Science and Technology Center, 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195, USA
Abstract Dense palladium metal membranes offer one approach to developing permselective membrane catalysts which offer a long term opportunity for reducing unit operations while enhancing product selectivities. We report the use of palladium and palladium alloys in the form of spirals or foils as both membranes and catalysts for several model hydrogenation reactions. The substrates investigated include: ethylene, butadiene, toluene, and cyclohexene with respect to critical process issues such as membrane life, reactor stability and productivity, optimum reactor temperatures, and membrane composition. Two reactor designs were used to evaluate hydrogenations performed in the liquid or vapor phase. The performance of these reactors as they relate to current difficulties in this field will be discussed.
Introduction Membrane catalysis over permselecave inorganic membranes offers promise for performing selective reactions while simultaneously separating some products from the reactants [ 11. For producing certain chemicals this could permit a sizable cost reduction by eliminating downstream processinglseparation costs. V. Gryaznov and coworkers have published [2-71 a great deal of work on the use of Pd alloy membranes in the form of large spirals for the selective hydrogenation of a variety of olefinic substrates. Palladium's remarkable capability to dissolve €b allows one to continually remove II2 (in a dehydrogenation reaction) thus allowing an increase in the conversion of the substrate. Further, the addition of a reactive molecule (such as a)to the permeate side of the membrane further enhances conversion [8] by continually consuming permeated II2. Here we describe the use of Pd and/or Pd alloy foils for the permselective hydrogenation of ethylene, butadiene, toluene, and cyclohexene as a function of several critical process parameters.
Experimental The palladium and palladium alloy foils (Johnson Matthey, Ltd.)were cut into 5 cm discs and washed with hexane then methanol. The air dried foils were compressed between two annealed copper gaskets held together by 2.75 inch OD knife edge flanges (Kurt J. Lesker, Inc., Clairton, PA, USA) (Figure 1). The foil was activated by heating to 265°C at lO"C/rnin.
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and purging both sides of the foil with zero grade air. After one hour, a nitrogen purge is initiated for 10 minutes before starting a flow of 50% hydrogen in Nz. Metal reduction continued for one hour followed by a purge with Nz.This procedure was repeated twice. The reactor was cooled under a reducing atmosphere after the final reduction. Activated foils were cooled to 100°C under hydrogen then purged with nitrogen for 30 minutes. The various gases were delivered at 30 to 60 cc/min to each side of the foil. Hydrogen permeation was determined over activated and non-activated foils at 50°C intervals between 100 and 250°C. Non-activated foils were heated to 100°C at 10"Umin under a nitrogen purge then allowed to equilibrate for 30 minutes. A blend of 50% hydrogen in Nz at
FLOW DISTRIBUTOR
+
I
ROW OW
ROWW
FLOW DISTRIBUTOR
0 COPPER
GASKETS
Figure 1: Schematic of knife-edge flange membrane reactor
Figure 2: Photograph of disassembled knife-edge flange membrane reactor: (1 to r) inlet head, flow distributor, membrane mounted in Cu gasket; disaibutor, Cu gasket; exit head.
1365
60 cc/min was fed to the retentate side of the foil and a nitrogen purge at 20 cc/min was maintained on the permeate side. After 30 minutes, sampling of the permeate stream started. New temperature setpoints were established with the foil under a nitrogen purge. Ethylene or butadiene at 20 cc/min were fed to the reaction side of the foil as dilute feeds of up to 16 mole percent in nitrogen or as an undiluted gas. Liquid hydrocarbons were vaporized at 30°C within a stainless steel sample cylinder by bubbling N2 through a 7 micron frit immersed in the olefin. Transfer lines after the vaporizer were maintained at 50°C to prevent condensation. Analysis of the reactant and product streams was accomplished with an HP 5890 gas chromatograph. Hydrogen, ethylene, and ethane were quantified using a thermal conductivity detector with a 8' x 1/8" Haysep Q column, with nitrogen as the carrier gas. Toluene, butadiene, cyclohexene and hydrogenated analogs were analyzed using a 30 meter DB-WAX megabore column (1.0 pm film) with a helium carrier gas. The sample size was 0.25 cc for all experiments. Calibration curves for hydrogen, ethylene and ethane were generated from known standards (Scott Specialty Gases, Plumsteadville, PA). Butadiene and its hydrogenated products were calibrated by diluting a constant flow of each pure component with known volumes of nitrogen. Chromatographic responses for the vaporized hydrocarbons were obtained in a similar fashion. Hydrocarbon vapors carried in a constant flow of Nz were trapped on a known weight of activated carbon. The weight gain over 30-60 minutes was recorded and used to calculate the amount of hydrocarbon in the vapor phase and equated to the GC area counts. The calibration curves show a linear GC response for these compounds across the concentration range. Daily accuracy was +3% of the calibrated values. Liquid phase reactions were run in the modified Parr reactor using coiled tubular Pd/Ru foils (obtained on loan from Professor V. Gryaznov, Moscow). The Parr Reactor (Figure 3) was partially filled with cyclohexene, octadecene or cyclooctadiene as a neat solution (1OOg) or diluted to 10% wt/wt in a suitable solvent. All the reagents used for the experiments were dried using 4A molecular sieve and de-oxygenated prior to use. The contents were stirred at 400 rpm during the experiment, A 0.5 ml aliquot was withdrawn at fixed intervals during the experiment and analyzed on a HP-5980gas chromatograph by FID. Activations were carried out at 400°C. Four times, air and hydrogen were passed both through and over the coils for one hour each. The reactor was cooled to the reaction temperature in Nz.
Results and Discussion Palladium membrane catalyzed hydrogenations of several different substrates were evaluated. These included hydrogenation of ethylene, butadiene, toluene, and cyclohexene.
Ethylene Hydrogenation: Most of our experiments were done with 0. lmm Pd or 0.025mm Pd/Ru(5) foils (11.4 cm2 of metal surface). Use of the thinner 0.025mm Pd foil was difficult due to repeated fracturing of the foil under our operating conditions.
1366
As demonstrated earlier 191 the permeability of Hz through the Pd foil varies considerably with temperature. Tables 1 and 2 illustrates this for Pd, Pd/Ag(3O)/Ru(2) and Pd/Ru(S) alloys. 'Since the activation of the foil in air followed by Hz provided a significant increase below 152°C (Table 2) in I-L permeability, we always activated any membrane in this manner.
Figure 3: Pdmu spiral membrane mounted into Parr reactor (1 to r): thermocouple, inlet to spiral, sample withdrawal line, stirrer, outlet from spiral, and fritted metal gas inlet tube. The beneficial effects of activation appear to become mvial above 150°C. The performance of the alloy for ethylene hydrogenation was not altered by a pretreatment of the foil with CC14 and 1N HCl; hence this latter procedure was not used. As expected the permeability of Hz was a factor of four faster with a 0.025mm Pd/Ru(5) membrane at the lower temperature. However this difference decreased with increasing temperature. Hydrogen permeability of the activated Pd foil is 30-50% greater than through an activated Pd/Ru(S) alloy (Table 2). The addition of ethylene to the permeate side actually increased the permeability of hydrogen (22 to 56 pmole/cm2/min at 205"C, Table 1). As expected increasing the volumetric flow of N2 to the permeate side, also enhanced the )I3 permeability. The work of Goto [lo] suggests that Pd/Ag/Ru alloys have higher permeability for H2 than pure Pd. The addition of silver to the Pd/Ru alloy certainly enhanced its permeability (Table
1367
2) to Hz. The comparison to a 0.025mm Pd foil is difficult because this thin of a pure Pd foil ruptures quite easily. Unfortunately a 0.025mm thick, 10 x lOcm sheet of Pd/Ag(3O)/Ru(2) costs $1700! Table 1. HZ Diffusion over Pda
I% Diffusion in pmole/cnP/rnin Temperature, "C
Not Activated
107 157 206 257
8.6 17 21 24
Activatedb 4.7 17 22 25
Activated + C a d C
52
46
a O.lmm Pd foil; 50% H z / N 2 retentate feed = 64 cc/min; permeate sweep = 20 cc/min N2 b activated in air at 265°C for 1 hr, then 50% HZin N2 at 265°C for 1 h c 50% H d V 2 to retentate side; W C & = 1.0; CzHJN2 to permeate side; CzH4 = 10 cc/min; total flow = 20 cc/min
Table 2. HZDiffusion over Pd alloysa
Hz Diffusion in pmole/cm*/min
Activated + c2H4 c
Temperature, "C 101 152 205
255
3.9 8.6 13 17
(3.4)b (16)b (25)b (29)b
15
22 28 33
(2)b (14)b (33)b (36)b
13 34 40 42
28 59
72 63
a Pd/Ru(5) or Pd/Ag(3O)/Ru(2) activated in air at 265"C, then in 50% W retentate feed = 61 cc/min; permeate sweep = 20 cc/min N2 b unactivated foil c WC2H4 = 0.5 (molar ratio); C2H4 = 20 cc/min as permeate sweep
z at 265°C;
1368 A number of experiments were performed over the Pd/Ru(S) foil using a variety of different flow configurations. With H&H1 = 0.4 and cofeeding reactants to the same side of the membrane, the conversion of CzH4 was only 39% (Table 3). With the reactants separated and H2 permeating the foil, the conversion jumped to 66%. Initially the WCzH4 was 0.44-0.50 for all these runs; however the effective partial pressure of Hz is less with the Pd membrane used to control the diffusion of Hz (since not all the Hz permeates the membrane). The use of a stainless steel foil with Hz and C2H* on the same side gave no conversion, but inserting the Pd/Ru(S) foil into the reactor as a catalyst with both reactants present gave 43% conversion.
Table 3. Ethylene Hydrogenation over Pd/Ru(5) at 15OOC a % CzH4 Conversion
same sideb opposite sidec no membrane, use of foild
a b c d
39 66
43
wczH4
(molar ratio)-
.44 .41 SO
T = 150°C;0.025mmPd/Ru(S); foil activated at 265°C before each use total flow = 42-44cc/min; Hz = 21 cc/min (8.8mmoleskc) retentate flow = 61 cc/min; I% = 20 mmoles/cc; permeate flow = 20 cc/min total flow = 22 cc/min; & = 1 1 mmoles/cc
Foley et al. [9] report changes in the surface morphology of Pd foils as a function of reaction time and temperature. Extensive cracking and surface pitting of Pd foils occur at 150 and 200°C with ethylene. The amount of surface xestructuring can be attributed to the pressure of I% across the foil and can be attenuated somewhat by preceding any reactions with an activation at a temperature greater than the reaction temperature. Fig 4 shows two Pd foils after being cycled from 100 to 250°C and back to 100°C. The foil on the left had been exposed to 100% HZand ethylene at various temperatures over 3 days and exhibits severe cracking and surface restructuring. The foil on the right was exposed to ethylene and a 50% H2 stream over a four day period, and gross morphological changes are less sevem. The morphological changes are probably caused by the dl3 phase transition [3, 11-13]. Despite the surface restructuring the integrity of the Pd foil was maintained for both samples. Because HZpermeation through Pd/Ru alloys is less and the transition temperature between the Q and B phase is lowered from 310°C to -77°C these gross changes are absent in the Pd alloys.
Butadiene: Earlier Nagamato and Inone 1141 studied the hydrogenation of butadiene over a Pd membrane at 100°C. They demonstrated that the membrane was affected by competitive adsorption between the diene and Hz. The reaction with permeated hydrogen was faster than
1369 with gaseous Hz [15], but the study was complicated by the presence of the a and I3 phases of Pd. We observed that at 100°C butadiene was completely hydrogenated to a mixture of butane (57%) and butenes [l-butene (3%), cis-2-butene (12%), trans-2-butene (28%)] at 100°C over an activated 0.025mm Pd/Ru(5) foil;50% Hz = 64 cc/min; 10%G& = 25 cc/min; WGfi = 4.0. The above selectivity values were measured after 40 min. The yield of butane continued to decline (39% after 2h) while the level of cis and trans-Zbutenes rose accordingly.
Toluene: Surprisingly, with -13% toluene in the vapor phase on one side' of a 0.025mm Pd/Ru(5) membrane and 50% Hz on the other side, no reduction was detected up to 300°C. Following exposure to toluene the membrane was used to perform the hydrogenation of cyclohexene successfully,indicating no irreversible poisoning of the membrane had occurred. '
Cyclohexene: Early experiments to hydrogenate cyclohexene with Hz flowing through a Pd/Ru(S) spiral (20 cm2 of Pd surface) were quite disappointing [with the foil immersed in liquid cyclohexene (containing inhibitor)]. Using the reactor shown in Fig. 3, <3% of the cyclohexene was converted at 70°C. With octadecene (200°C) or cyclo-octadiene (13OoC), ~ 2 conversion % was detected.
Figure 4. Left: O.lOmm Pd foil exposed to 100% Hz on one side and ethylene on the other for 3 days. Right: Use of 50% Hz instead of 100%for 4 days. = 3.0) using the knife-edge Operating with cyclohexene in the vapor phase at 150°C (WG~I-IIO flange membrane reactor (Figures 1 and 2), the conversion was 35% [over a O.lmm foil of Pd/Ru(5)]. The selectivity (S) to cyclohexene was 94% with 7% to benzene! At 250°C and WCd41o = 3.2, cyclohexene conversion fell from 44% to ~ 2 1 %over 2h with decreasing yields of benzene (Table 4). Reactivating the foil in air then HZ at 265°C restored the activity. Although little activity was seen at 70°C, it could be increased by raising the pressure of Hz. Use of a fresh foil (with the standard 265°C activation) gave 80% cyclohexene conversions at 150°C (83% S to cyclohexane, 17%S to benzene). Using cyclohexene as a feed at 150°C (no HZ)over the foil gave 45% conversion (52% S to cyclohexane and 48% to benzene) indicating that cyclohexene disproportionates over the alloy. Some Hz is also detected. Some reduction in conversion occurs with time (1 hr) suggestive of carbon fouling the membrane. This
1370 suggests that the foil of the previous runs had deteriorated. This unpredictable behavior points to another undesirable feature of these membranes [16]. Of the olefins studied, cyclohexene was the only one which exhibited this unpredictable behavior. Table 4. CyclohexeneHydrogenationa
T, "C
Time on Stream (min)
MID conv.,%
Selectivity 8c 6 H l l 96CaHs
150
20
40
36,80 e 33
93 94
7
25 lb
5
44
46
54
55
45
10 112 155
42 33 21 20
26 20
7.6 21
40
7oc 7od
48
60
62
6 52
40 38
100 100
a W&IB (molar ratio)= 3.0,O.lmm Pd/Ru(S) foil b reactivated in air/& on another day c 0.5 a m Ib d 2.0am & e repeated with a new piece of activated foil
Conclusion We evaluated the permeability of Hz (at c 25OOC) through of Pd, Pd/Ag(3O)/Ru(2), and Pd/Ru(5) alloy foils and the use of these metals for the hydrogenation of a variety of olefins. 5 varies dramatically with the alloy composition: Pd/Ag(3O)/Ru(2) > Pd The permeability of J > Pd/Ru(S)t and the permeability of hydrogen is significantly enhanced by adding C2H4 to the permeate side. Membrane activation in zI2 increases H2 permeability below 152°C. The use of a membrane to mnsport HZto the C3I4 side of the reactor results in increased C2H4 conversion vs co-feeding the substrates to the same side of the foil catalyst. Hydrogenation of butadiene with excess hydrogen over the membrane gave unselective hydrogenation products including butane, which continually decreases as a function of time. Surprisingly toluene is not hydrogenated at all over these Pd membranes.
1371 Liquid phase reactions over Pd/Ru spirals gave poor conversion. However vapor phase hydrogenation of cyclohexene gave -35% conversion at 150°C with 94% selectivity. Surprisingly, benzene (7% selectivity) was also p r o d u d . In the absence of hydrogen, cyclohexene disproportionates to benzene and cyclohexane with drifting yields over time. Attempts to duplicate these latter runs with a fresh membrane gave similar product distributions but with different conversions. Clearly this work points to several limitations exhibited by Pd and W alloy membranes as catalysts. These membranes do not provide a universal means to hydrogenate olefinic substrates. Instead one must balance reactor conditions, with alloy composition, reactor configuration, and catalysis. Much more fundamental research is needed prior to any future attempt at developing these membranes in commercial membrane mactors. Because of the cost of the Pd and its alloys, and since only a thin, dense layer of Pd is necessary, coatings of Pd alloys on mesoporous inorganic membranes is probably a more appropriate, realistic membrane for any future commercial use.
Acknowledgements We wish to thank both Professor Henry Foley of the University of Delaware and his former post doctoral student Dr. Andrew Wang for sharing their early experimental efforts with us in this area which allowed us to build upon their p r o p s s . We also wish to acknowledge Air Products and Chemicals Inc for their support of this exploratory mearch and permission to publish this partion of our early work in this field.
References 1 J. N. Armor, Appl. Catal., 49 (1989)1-25. 2 V. M. Gryaznov, Platinum Metals Rev., 30 (1986)68-72. 3 V. M. Gryaznov and N.V. Orekhova, Catalysis of Precious Metals, Nauka, Moscow, USSR, 1989. 4 V. G. Dobrokhotov, V. M. Gryaznov, and L. F. Pavlova, J. Less Common Metals, 89 (1983)585-86. 5 V.M. Gryaznov, M. M. Ermilova, L. S. M m m v a , N. V. Orekhova, V. P. Polyakova, N. R. Roshan, E. M. Savitskii, and N. I. ParFenova, J. Less Common Metals, 89 (1983)52935. 6 A. P. Mischenko, V. M. Gryaznov, V.S. Smimov, E. P. Senina, I. L. Parbuzina, N. R. Roshan, V. P. Polyakova, E. M. Savitsky, U.S. Patent 4,179,470(1979). 7 V. M. Gryaznov, A. N.Karavanov, T. M. Belosljudova, A. V. Ennolaev, A. P. Maganjuk and I. K. Sarycheva, U.S.Patent 4,388,479(1983). 8 N. Itoh, K.Miura, Y.Shindo, K.Haraya, K.Obata,and K.Wakabayashi, Sekiyu Gakkaishi 1989,32,47;N. Itoh, "Simultaneous Operation of Reaction and Separation by a Membrane Reactor", in Future Opportunities in Catalytic Separation and Technology, M. Misono, Y.Moro-oka,and S.Kimura.eds., Elsevier, New Yo&, 1990,p.268-83.
1372 9 H. C. Foley, A.W. Wang, B. Johnson,and J.N. Amor, "Effect of a Model
Hydrogenation on a Catalytic Palladium Membrane", in Proceedings of the ACS Symposium on "Selectivity in Catalysis", August, 1991, paper #:Cad. 11. 101 R.Goto, Kagaku Kogaku 34 (1970) 35-42. 11 J. Philpott and D. R, Coupland, Chem. Ind., 31 (1988) 679-94. 12 AS. Darling, Platinum Metals Rev., 2 (1958) 16-22. 13 A.G. Knapton, Platinum Metals Rev., 21 (1977) 44-50. 14 H. Nagamato and H. Inone, Bull. Chem. Soc. Jpn. 59 (1986) 3935-39. 15 H. Nagamato and H. Inone, Chem. Eng. Commun. 34 (1985) 315-23. 16 J. N. Armor, Chemtech, 1992, submitted for publication.
1373 DISCUSSION Q: P.L Silveston (Canada) Although this paper claims membrane reactors offer the advantage of separating products concurrently with their creation by reaction, it uses examples which do not involve product separations but rather separation of reactants. Table 3 and the discussion of butadiene hydrogenation show or suggest that segregating reactants improves performance. My comment is that segregation reactants to obtain the benefits attributed to the membrane reactor can be achieved by composition modulations (switching between hydrocarbon and hydrogen in the time dimension) much more easily and more cheaply than by the use of membranes. Unfortunately, composition modulation is not widely practiced in catalyst science, probably because it is not widely known. In the reactor engineering area composition modulation is well known. The literature contains well over 100 publications.
A: J. Armor We did not mean to imply that we were going to discuss commercial applications of palladium membranes. We began this work as a means to build a fundamental understanding about palladium membranes for transporting hydrogen with regard to membrane catalysis. We thought we would choose rather simple s stems (C*H&ydrogen, benzenehydrogen, toluenehydrogen, butadhe/ hydrogen) and t en go on to more attractive commercial opportunities incorporating reactions such as dehydrogenation. We soon began to appreciate that even simple hydrogenations were not readily achievable with these membranes. The point of this paper is to indicate that one can carry out selective hydrogenations, but this is very substrate dependent. Olefins themselves do not undergo facile hydrogenation in general and, further, these are very dependent upon the membrane composition. The literature already suggests that dehydrogenations are further complicated by carbon build-up and resulting deactivation. We are attempting to resolve these problems and circumvent them by building up a sufficient knowledge base with regard to the simpler systems before tackling these more complex systems.
i+I
Q: J. W. Hightower (USA) My question concerns the hydrogenation of 1,3-butadiene. Apparent1 the roducts are mainly butane and some butanes as we know from the classic work of ucwe 1, Bond, Wells, and others. Pd catalysts are remarkably selective in making first n-butenes and that n-butane after most of the butadiene has been consumed. As this selectivity observed with the membranes, even at low conversions of the diene ? If it is not, does this not detract somewhat from the selectivity advantages of the membrane configuration ?
B f
A: J. Armor Yes,we were surprised at the high yield of butane. Interestingly, while some butenes were made in the beginning, the yield of butane continued decline while the level of cishans 2-butenes rose accordingly with time. At this time we are not able to explain this without some additional experiments. This does int to a complication observed with these membranes which may be due to the a - F p h a s e s of palladium and its alloys, carbon buildup, or as yet unexplained effects occurring with time.
Q: W. 0. Haag (USA)
Would you please clarify the interesting effect of ethylene supplied to the permeate side of the membrane in enhancing the hydrogen permeation rate ‘1 Does the ethylene merely reduce the hydrogen partial pressure and hence increase the H pressure gradient pressure drop is across the membrane, or does ethylene have an effect even when the the same ?
8,
1374
A: J. Armor We did monitor total flows in and out of both the retentate and permeate sides of the membranes. There was an increase in the total flow on the permeate side which we could attribute td a greater flux of h drogen through the membrane. We did examine this to a limited extent as a function o partial pressure of hydrogen without seeing a comparable effect for the expected DP of hydrogen across he membrane. Nevertheless, we might expect some rate enhancement due to this H2 flux giving rise to a change in H partial pressure Gryaznov has pointed out in a number of publications that pzladium membranes exhibit better conversions with a membrane operatin with permeating hydrogen as opposed to the simple foil being used as a catal tic sur ace. he argues that the dissociation of hydrogen provides for an activated form o hydrogen on the permeate side for which the substrate can react. In addition, he points out that this microscopic picture of the surface is very different from that of a traditional catalyst which is often saturated with the olefinic substrate; the use of hydrogen permeating through a palladium foil assures that the surface has a much higher level of hydrogen atoms and hence, perhaps is a better hydrogenation catalyst.
r
r
f
0:J. R. H. Ross (Ireland) ' Do you think that the fluxes obtainable with membrane systems will ever be high enough to allow the reactions to compete with conventional catalytic processes ?
A: J. Armor With regard to sufficient fluxes in membrane systems, I believe that one can optimize this to some extent by varying temperature, pressure or alloy composition. We demonstrated in this manuscript that the composition of the foil is crucial with respect to optimizing hydrogen rmeability. By choosing the right alloy composition, one can actually enhance the iffusion of hydrogen through the alloy versus pure palladium. In addition, by going to very thin layers of metal or alloy on a meso porous support, the permeability of hydrogen will be much greater. Thus, I do believe that with the right system (metal, thickness of coating, composition of alloy, etc.) it should be possible to come up with particularly attractive opportunities for membrane catalysis.
r
Q: S.Kaliaguine (Canada) In your oral presentation you have mostly discussed the use of Pd catalytic membranes in hydrogenation reactions is also extremely promising because in this case the separation of hydrogen from the reaction medium may allow one to displace the thermodynamic equilibrium and reach higher conversions at lower temperatures. Such an effect may be significant in several industrial porouses studies for example the stream reforming of methane. In this case the membrane reactor would also produces very pure hydrogen in one step.
A: J. Armor Yes, I believe that membranes, indeed, do offer the possibility of facilitating a number of dehydrogenation reactions. The difficulty with these dehydrogenations currently is that they often run at very high temperatures and invariably some carbon buildup occurs. The high temperatures in the ast have limited the application with regard to metal membranes because of the a@ e fect that we describe in the manuscript. The high temperature required for dehydroeenations often limits the utility of the% memfbranes because of sealing or fabrication ~ssues.However, running reactions such 8s oxidative dehydrogenation (where one passes oxygen on the permeate side to react with any h drogen) allows one to perhaps lower the temperature and at the same time enerate heat or the dehydrogenation process. Alternatively, the coupled reactions (invo ving Htransport) which Gryaznov has described provide another opportunity. These sorts of
P
r
k
1375 systems, I believe, have good promise but we need to do more research in this area. We have some work underway in these areas as well. The work is much too preliminary and is confused by other issues such as described in Table 4 of our manuscript where we see both cyclohexane and benzene on hydrogenation of cyclohexene. Apparently, the benzene is being produced from the dehydrogenation of cyclohexane which must be a competitive reaction with the hydrogenation of the olefin! Clearly, more work needs to be done in this system.
Q: A. Srk6ny (Hungary) You have spoken about limitations. and challenges. What are the main advantages of this technique ? Can you really inhibit the poisoning in such a way ? A. J. Armor I believe the main advantages of the membrane catalytic reactors lie with the opportunity to provide simultaneous separation as well as catalysis in one unit operation. There appears to be some enhanced activation of hydrogen by using these palladium alloys and that deserves further exploration and consideration. Further, with palladium, one has perfect permselectivity. That is, only hydrogen permeates without any other gases, thus, this permits the production of very pure hydrogen and should also offer the opportunity to do very selective hydrogenations.
Q: J. W. Geus (The Netherlands) With the application of membranes, it is highly important to combine the favorable properties of metals and ceramics. Metals provide mechanical strength and resistance against thermal shocks, while ceramic materials can be processed to thermostable porous structures containing pores of dimensions down to 0.5 pm. To achieve the combination of metals and ceramics, we use metal bodies (tubes, plates) having course pores (diameter about 1 to 4 vm). Starting from metal powders, bodies of a desired shape and dimension can be produced readily on an industrial scale. Three different procedures have been developed to apply ceramic layers containing fine pores into or onto porous metal bodies. Permeability measurements have shown that all three above procedures lead to leak-tight membranes. A: J. Armor I think the approach of building composite membranes does make a lot of sense. In my review which shall appear in Chemtech this year, I focus on two attractive opportunities in this area: coating palladium onto mesoporous ceramic membranes and the deposition of catalyst into microporous ceramic membranes. Your suggestion falls into the latter category and a number of groups, as you probably are well aware are working in this area. The difficulty with this approach is to assure that one has a perfectly flawless membrane and that no cracks or voids appear as the reaction continues for days or moths of operation. Most systems have not been tested to this degree. It is known that one can prepare intermediate layers with y-alumina as a bridging composition, only to find that the alumina undergoes some sintering with time to undergo a change in pore size and distribution. If your support layer is undergoin chemistry while your trying to carryout catalysis on the surface, this can be disastrous. bviously, one has to provide the right match of the active catalytic layer with the interphase support and the meso porous support structure (whether it be metal or ceramic). Our early work here with palladium foil and its alloys is just a beginning. We have learned a lot and are beginning to extend this knowledge into other compositions, forms, and configurations.
%