- Email: [email protected]
Liquid-phase catalytic hydrogenation using palladium alloy membranes
Applied
Catalysis A: General,
Elsevier
Science
APCAT
A2374
Publishers
96 (1993)
25
25-32
B.V., Amsterdam
liquid-phase catalytic hydrogenation using palladium alloy membranes T.S. Farris and J.N. Armor Air Products (Received
& Chemicals,
22 June
Inc., 7201 Hamilton
1992, revised manuscript
Blvd., Allentown,
received
25 August
PA 18195-1501
(USA)
1992)
Abstract The hydrogenation of several liquid alkenes was studied over Pd/Ru alloy membranes in either metal or glass membrane reactors. For cyclohexene, cycle-octadiene, and octadecene as liquids, conversions of ca. 2-5% were common in contrast to the facile conversion of acetylenic alcohols. Keywords: alkenes;
cylcohexene;
hydrogenation;
membranes;
palladium
alloys
INTRODUCTION
The use of inorganic membranes for separation and puri~cation is an old concept that is receiving renewed attention especially as applied to catalytic processes. In this capacity membranes can serve as the catalyst or a catalytic support which permits the selective addition or removal of products/reactants. Inorganic oxide membranes, hollow carbon fibers and palladium foils are currently being tested [ 11. Shu et al. [ 21 also contend this technology will become economically feasible for small and medium sized plants and find its niche in producing fine chemicals and pharmaceuticals by its ability to control selective hydrogenations. Alumina membranes are commercially available in pore sizes of 40 to 200 A [ 3,4] suitable for liquid separations. Experimental asymmetric membranes with porosities of 5 to 40 A [5,6 ] were prepared using TiO, or ZrO, coatings on alumina supports. Their stability at high temperatures is uncertain and availability is limited to small laboratory samples. Despite finer control of membrane porosity, in actual use these membranes are still unable to exclude selectively reactants from the permeate stream. Hollow carbon fibers are a potential source for super microporous membranes ( < 10 A). Certain polymers (e.g., Saran) when pyrolyzed under controlled conditions generate pores Correspondence to: Dr. J.N. Armor, Air Products town, PA 181951501, USA. Tel. (+l-215)4815792,
0926-860X/93/$06.00
0 1993 Elsevier
Science
& Chemicals, Inc., 7201 Hamilton fax. (+ l-215)4812989.
Publishers
B.V.
All rights
reserved.
Blvd.,
Allen-
26
T.S. Farris and J.N. Armor/Appl.
Catal. A 96 (1993) 23-32
between 5 and 8 A. Hollow fibers are presently available with porosity between 10 and 40 A. Until these fibers are routinely produced with less than 10 A pores, they will not achieve complete separation of reactants and products [ 11. Dense metal foils provide an alternative method for separation which differs from size exclusion offered by metal oxides and carbon membranes. In the case of palladium, hydrogen dissociates, dissolves and is transported through the metal. Infinite hydrogen selectivity can be achieved within limitations. The dissolution of hydrogen varies as a function of hydrogen partial pressure and temperature. As palladium absorbs hydrogen, the transition from the hydrogen-lean QIphase (H/Pd< 0.1) to the hydrogen-rich /? phase is accompanied by a large shift in the palladium lattice constant. Continued cycling between these phases distorts the metal and reduces its mechanical strength [ 71, Phase transitions are avoided by maintaining the process temperature above 310” C where only the Q!hydride phase exists or using alloys of palladium. Palladium alloys with other group VIII or 1B metals alter the cu//3transition temperature and make low-temperature reactions feasible [ 81. These metals, as binary or ternary alloys may improve mechanical strength often without limiting hydrogen permeability and may even enhance permeability [ 91. The minor alloy metal can also provide catalytically active sites for certain reactions. Commercial use of catalytically active palladium foils is presently limited to hydrogen purification and recovery of hydrogen from waste methanol. Earlier, Gryaznov and co-workers [lo-121 championed the use of large spiral palladium alloy tubes for various hydrogenation and dehydrogenation reactions. So far, applications are limited to producing small amounts of value added products where the ability to control selectivity by maintaining a specific concentration of hydrogen dissolved in the catalyst is critical [lo]. As Karavanov et al. [ 131 stated earlier, “a significant shortcoming of membrane hydrogenation catalysts of palladium is their low mechanical strength in a hydrogen atmosphere...“. Of the 22 alloys they tested in 1990 for the hydrogenation of the acetylenic alcohol 2,6-dimethyloct-2-en-7-yn-6-01 (dehydrolinalool) to 2,6-dimethyloctadi-2,7-en-6-01 (linalool ), Pd/Ru (6) (the number in parentheses designates the percentage ruthenium) or Pd/Rh alloys were the most promising with respect to stability, mechanical strength, rate and selectivity (to alkene) at 130°C. Our program objective was to study inorganic membranes for use as catalysts and examine the limitations of this emerging technology. To achieve this goal we designed and constructed glass and metal reactors for palladium foils and compared these reactors/catalysts with work reported in the literature using simple alkenes. Initially, we sought to study low-temperature liquid-phase hydrogenations before attempting vapor-phase hydrogenations and dehydrogenations. The limitations we encountered coupled with the scarcity of information on membrane catalysis of liquid alkenes prompts this article.
T.S. Farris and J.N. ArmorfAppl.
Catal. A 96 (1993) 23-32
27
EXPERIMENTAL
Initially, a 500 cm3 quartz reactor was designed to accommodate the Pd/ Ru(6) spiral membrane tube (69 cm long, 0.1 cm O.D.; with a calculated surface area of 0.0022 m’). The reactor was fitted with a condenser, an inert gas purge line, and a sample tube. The tubular membrane was activated at 400 ’ C in situ using an electric heating jacket. All reactions were conducted with 100 cm3 of reagent grade substrate. Later, liquid-phase reactions were run in a modified 300 cm3 Parr reactor (Fig. 1)using the coiled tubular Pd/Ru spiral (obtained on loan from Professor V. Gryaznov, A.V. Topchiev Institute of Petrochemical Synthesis, Moscow). The Parr Reactor was partially filled with cyclohexene, octadecene or cyclooctadiene as a neat solution (100 g) or diluted to 10% (w/w) in a suitable solvent. All the reagents used for the experiments were dried using 4A molec-
Fig. 1. Pd/Ru spiral membrane mounted into Parr reactor (left to right): thermocouple, inlet to spiral, sample withdrawal line, stirrer, outlet from spiral, and fritted metal gas inlet tube.
Fig. 2. Photograph of disassembled knife-edge flange membrane reactor: (left to right) inlet head, flow distributor, membrane mounted in Cu gasket; distributor; Cu gasket; exit head.
ular 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-5980 gas chromatograph (GC)by flame ionization detection (FID ) . Activations were carried out at 400’ C. Four times, air or hydrogen was passed both through and over the coils for one hour each. The reactor was cooled to the reaction temperature in nitrogen. For vapor-phase hydrogenations, palladium and palladium alloy foils (Johnson Matthey, Ltd.) were cut into 5-cm discs and washed with hexane and methanol. The air dried foil was compressed between two annealed copper gaskets held together with 2.75 inch O.D. knife edge flanges (Kurt J. Lesker, Inc. ) (See Fig. 2 ) . Vapor-phase hydrogenations, permeability, and pretreatment of these membranes are described in ref. 14. Liquid hydrocarbons were vaporized by passing nitrogen through a 7 ,um frit in a 75-cm3 stainless steel sample cylinder maintained at 30” C. Transfer lines were maintained at 50°C. Analysis of the reactant and product streams was accomplished with an HP 5890 gas chromatograph. Toluene, cyclohexene and hydrogenated analogs were analyzed using a 30 meter DB-WAX megabore column (1.0 pm film) with a helium carrier gas. Hydrocarbon vapors carried in a constant flow of nitrogen were trapped on a known weight of activated carbon. The weight gain over 30-60 min was recorded and used to calculate the amount of hydrocarbon in the vapor phase and equated to the GC area counts. RESULTS
AND DISCUSSION
Initial experiments conducted with cyclohexene in a static batch mode in a quartz reactor used a tubular Pd/Ru foil (S.A. ~0.0022 m”) at 70°C. Activation in air followed by reduction in hydrogen at 400’ C preceded each reaction. Results of the five initial runs showed no visible sign of hydrogen permeation
T.S. Farris and J.N. ArmorjAppl.
29
Catal. A 96 (1993) 23-32
(gas bubbles) and conversions averaged below 5%. We first suspected 70°C was too low and thermal mixing did not provide sufficient agitation at the alloy/liquid interface. In addition, we became uncomfortable heating this glass system containing hydrogen and a combustible material and conducted the remaining experiments in a stirred 300 cm3 autoclave reactor (Fig. 1). Table 1 presents the experimental results showing the effect of various hydrogenations in which either the temperature or reactant was varied using the autoclave reactor. Palladium supported on high surface area oxides certainly are more effective catalysts. (Separate experiments indicated cyclohexene conversion did not improve with agitation.) Continuing work with cyclohexene at 70’ C in the Parr reactor, we obtained 4.4% conversion after the tubular spiral was treated in boiling carbon tetrachloride for 2 h followed by two, 1 M HCl washes of 50 cm3 each and an extensive de-ionized water wash. This procedure removes ruthenium from the surface which hinders catalytic activity [ 151. We found it improved catalytic activity in one instance (run 12) which could not be sustained in a subsequent run 13, nor could it be duplicated (runs l&l6 and 17). Doubling the activation time (run 14) did not improve membrane performance at 70’ C. (In a recent private communication with V. GryTABLE
1
Temperature Run
and reactant effect on hydrogenation
Reactant
Temperature
TOS”
(“Cl 10 11 12
Cyclohexene Cyclohexene Cyclohexene
13 14
Cyclohexene Cyclohexene
15 16 17
Cyclohexene Cyclohexene Cyclohexene
6 7 8 9
70 70
Octadecene Octadecene
Conversion
Productivityb
(%o) 360 190 210
3.0 1.7 4.4
180 185
1.3 1.4
70 70
150 120
1.5 0.9
70
185
0.8
200 200 200 200
120
1.5
120 300
1.8
240
70 70 70
Octadecene Octadecene
of liquid alkenes
1.82 3.27 7.75 2.64 2.16 3.68 2.62 1.70 0.15 1.75 Not talc. Not talc.
18
Cyclooctadiene
70
120
1.4
1.4
18a
Cyclooctadiene
130
198
0.9
0.4
LITime-on-stream
(min ).
* Moles converted/m’/h.
30
T.S. Far&
and J.N. Ar~~r~A~p~. Catal. A 96 (1993) 23-32
aznov, he confirmed our results with liquid cyclohexene were comparable to his. ) Since hydrogen permeability increases dramatically with temperature, we sought to improve hydrogen permeation and conversion by conducting the reaction at 200°C using octadecene. Despite more favorable permeation conditions conversion rates remained below 2% (runs 6-9). In the final two runs (18 and 18a) cycle-octadiene (COD) replaced the monoalkenes. In the gas phase COD easily converts to cyclooctene and cyclooctane between 350-475 *C over Pd/Ru membranes [ 161. From our work, clearly liquid-phase hydrogenations of alkenes over a similar foil are unacceptably slow at lower temperatures. Our results indicate Pd/Ru foils have limited practical applicability for liquid-phase hydrogenation of alkenes. Although low surface areas of the foils contribute to lower reaction rates, dilute feeds (10% alkene in alkane) also had comparable conversion rates as undiluted alkene. Furthermore, even when the reactor was pressurized with hydrogen to 25 psig (1 psig=6.895 kPa), little improvement in conversion occurs. Lower hydrogen permeability at 70 ‘C limits reaction rates; however, conversion at higher temperatures remains unchanged despite greater hydrogen permeation. Our reactor scheme did not permit hydrogen analysis; however, an observed pressure increase (ca. 11 psi) during reactions at 200’ C was a sign of considerable hydrogen permeation. Gryaznov [lo] cites the use of catalytic Pd/Ru membranes for hydrogenation of acetylenic alcohols between 60 and 180°C. He fed technical grade reactants into a flow reactor as a neat feed or 30% aqueous solution at rates of 3 to 145 ml/h. Feed rates were based on the available surface area. Conversions greater than 95% are reported but it remains unclear if this was obtained in a single pass. Our inability to obtain comparable conversion in our model system may reflect our choice of these alkenes as reactants. We did not test any acetylenic compounds nor did we study the effect of higher stir rates on conversion. The tubular foils used in these experiments were coiled spirals, with the ends silver soldered to 316 stainless steel. The soldered joints frequently failed. Most likely phase changes in the metal play a major role in fracturing these joints however vibrational stresses, induced by the stirring action, may be an ancillary factor in their failure. (In one patent, Gryaznov et al. [ 171 claim these stresses can be avoided by using a compressed bank of flat spirals. ) In contrast to early attempts at hydrogenating liquid cyclohexene with hydrogen (0.5 atm) flowing through Pd/Ru(G) spiral tubes, operating in the vapor phase produced conversions of 35% at 150°C (over an activated 100 pm thick Pd/Ru (5 ) foil membrane ). The selectivity to cyclohexane was 93% and 7% to benzene [ 141. At 70°C we obtained cyclohexene conversions of 7 to 9% in the vapor phase (35% at lSO°C), in contrast to the 3 to 4% found in the liquid-phase experiments. At two atmospheres of hydrogen a nonsustainable maximum conversion of 23% was obtained in the vapor phase. Activity fell to
T.S. Farris and J.N. ArmorjAppl.
31
Catal. A 96 (1993) 23-32
16% over 47 min but selectivity to cyclohexane was maintained. The ineffectiveness of Pd/Ru alloys for liquid alkenes vs. acetylenics evaluated is probably related to the greater degree of adsorption of acetylenics, the temperature, the pressure, and/or the alloy composition. CONCLUSION
Liquid-phase hydrogenations over Pd/Ru membranes are feasible but the results reported here suggest the choice of substrate is crucial. Using cyclohexene, cycle-octadiene and octadecene conversions of 2% were common but never exceeded 5% with a single spiral membrane. In contrast, facile liquidphase hydrogenations with palladium membranes are known for the conversion of acetylenic alcohols. Our limited success in converting these compounds may be the consequence of the foil composition (i.e., the Pd/Ru foil is not the appropriate composition), the use of alkenes vs. acetylenics, the temperature, the pressure of hydrogen, the surface area of membrane, sorption of reactants/ products, and /or sufficient level of active metal on the surface of the activated membrane. We chose to report the work at this stage to alert others to the difficulty and complexity of liquid-phase hydrogenations of these simple alkenes over palladium alloy membranes. Given improved membrane compositions, selected substrates, etc. additional utility may yet emerge. ACKNOWLEDGEMENTS
We wish to thank Professor V. Gryaznov for arranging the loan of Pd/Ru spiral membranes. We wish to thank Professor Hank Foley for his collaboration on this program. We also acknowledge Air Products and Chemicals, Inc. for support and permission to publish this work at this time. Thanks are expressed to Professor David Trimm for agreeing to serve as editor for this manuscript. REFERENCES 1 2
J.N. Armor, Appl. Catal., 49 (1989) l-25. J. Shu, B.P.A. Grandjean, A. Van Neste and S. Kaliagulne,
3
1036-1060. R.R. Bhave, Inorganic Membranes,
4 5 6 7 8 9 10
Van Nostrand
Reinhold,
Can. J. Chem. Eng., 69 (1991) N.Y., 1991, pp. 64-67.
B.Z. Egan, Using Inorganic Membranes to Separate Gases, Report of Oak Ridge National Labs, 1989, Order No. DE90004386. Q. Xu and M.A. Anderson, J. Mater. Res., 6 (1991) 1073. Q. Xu and M.A. Anderson, US Patent 5 006 248 (1991). J. Philpott and D.R. Copeland, Chem. Ind., 31 (1988) 679-694. J.N. Armor, Chemtech, (1992) 557-563. R. Goto, Kagaku Kogaku, Chem. Engineering, 34 (1970) 35-36 (381-2). V.M. Gryaznov, A.N. Karavanov, T.M. Belosljudova, A.P. Ermolaev, A.P. Maganjuk and I.K. Sarycheva, US Patent 4 388 479 (1983).
32
T.S. Farris and J.N. Armor/Appl. Catal. A 96 (1993) 23-32
11 12 13
V.M. Gryaznov, Platinum Met. Rev., 30 (1986) 68. A.N. Karavanov and V.M. Gryaznov, Kinet. Katal., 25 (1984) 74-76. A.N. Karavanov, V.M. Gryaznov, N.R. Roshan and I.G. Batyrev, Kinet. Katal., 32 (1991) 1162-1168. T.S. Farris and J.N. Armor, Proceedings of 10th International Congress on Catalysis, Budapest, Hungary, July 1992, Institute of Isotopes of the Hungarian Academy of Sciences, Budapest, 1992. N.R. Roshan, A.P. Mishchenko, V.P. Polyakova, N.I. Parfenova, E.M. Savitsky, E.A. Voitekhova, V.M. Gryaznov and M.E. Sarylova, J. Less Common Met., 89 (1983) 423. V.M. Gryaznov and G. Slin’ko, Discuss. Faraday Sot., 72 (1981) 73-93. V.M. Gryaznov, A.P. Mischenko, A.P. Maganjuk, V.N. Kulakov, N.D. Fomin, V.P. Polyakova, N.R. Roshan, E.M. Savitsky and J.V. Saxonov, UK Pat. Appl. GB 2 056 043 (1981).
14
15 16 17
Catalysis A: General,
Elsevier
Science
APCAT
A2374
Publishers
96 (1993)
25
25-32
B.V., Amsterdam
liquid-phase catalytic hydrogenation using palladium alloy membranes T.S. Farris and J.N. Armor Air Products (Received
& Chemicals,
22 June
Inc., 7201 Hamilton
1992, revised manuscript
Blvd., Allentown,
received
25 August
PA 18195-1501
(USA)
1992)
Abstract The hydrogenation of several liquid alkenes was studied over Pd/Ru alloy membranes in either metal or glass membrane reactors. For cyclohexene, cycle-octadiene, and octadecene as liquids, conversions of ca. 2-5% were common in contrast to the facile conversion of acetylenic alcohols. Keywords: alkenes;
cylcohexene;
hydrogenation;
membranes;
palladium
alloys
INTRODUCTION
The use of inorganic membranes for separation and puri~cation is an old concept that is receiving renewed attention especially as applied to catalytic processes. In this capacity membranes can serve as the catalyst or a catalytic support which permits the selective addition or removal of products/reactants. Inorganic oxide membranes, hollow carbon fibers and palladium foils are currently being tested [ 11. Shu et al. [ 21 also contend this technology will become economically feasible for small and medium sized plants and find its niche in producing fine chemicals and pharmaceuticals by its ability to control selective hydrogenations. Alumina membranes are commercially available in pore sizes of 40 to 200 A [ 3,4] suitable for liquid separations. Experimental asymmetric membranes with porosities of 5 to 40 A [5,6 ] were prepared using TiO, or ZrO, coatings on alumina supports. Their stability at high temperatures is uncertain and availability is limited to small laboratory samples. Despite finer control of membrane porosity, in actual use these membranes are still unable to exclude selectively reactants from the permeate stream. Hollow carbon fibers are a potential source for super microporous membranes ( < 10 A). Certain polymers (e.g., Saran) when pyrolyzed under controlled conditions generate pores Correspondence to: Dr. J.N. Armor, Air Products town, PA 181951501, USA. Tel. (+l-215)4815792,
0926-860X/93/$06.00
0 1993 Elsevier
Science
& Chemicals, Inc., 7201 Hamilton fax. (+ l-215)4812989.
Publishers
B.V.
All rights
reserved.
Blvd.,
Allen-
26
T.S. Farris and J.N. Armor/Appl.
Catal. A 96 (1993) 23-32
between 5 and 8 A. Hollow fibers are presently available with porosity between 10 and 40 A. Until these fibers are routinely produced with less than 10 A pores, they will not achieve complete separation of reactants and products [ 11. Dense metal foils provide an alternative method for separation which differs from size exclusion offered by metal oxides and carbon membranes. In the case of palladium, hydrogen dissociates, dissolves and is transported through the metal. Infinite hydrogen selectivity can be achieved within limitations. The dissolution of hydrogen varies as a function of hydrogen partial pressure and temperature. As palladium absorbs hydrogen, the transition from the hydrogen-lean QIphase (H/Pd< 0.1) to the hydrogen-rich /? phase is accompanied by a large shift in the palladium lattice constant. Continued cycling between these phases distorts the metal and reduces its mechanical strength [ 71, Phase transitions are avoided by maintaining the process temperature above 310” C where only the Q!hydride phase exists or using alloys of palladium. Palladium alloys with other group VIII or 1B metals alter the cu//3transition temperature and make low-temperature reactions feasible [ 81. These metals, as binary or ternary alloys may improve mechanical strength often without limiting hydrogen permeability and may even enhance permeability [ 91. The minor alloy metal can also provide catalytically active sites for certain reactions. Commercial use of catalytically active palladium foils is presently limited to hydrogen purification and recovery of hydrogen from waste methanol. Earlier, Gryaznov and co-workers [lo-121 championed the use of large spiral palladium alloy tubes for various hydrogenation and dehydrogenation reactions. So far, applications are limited to producing small amounts of value added products where the ability to control selectivity by maintaining a specific concentration of hydrogen dissolved in the catalyst is critical [lo]. As Karavanov et al. [ 131 stated earlier, “a significant shortcoming of membrane hydrogenation catalysts of palladium is their low mechanical strength in a hydrogen atmosphere...“. Of the 22 alloys they tested in 1990 for the hydrogenation of the acetylenic alcohol 2,6-dimethyloct-2-en-7-yn-6-01 (dehydrolinalool) to 2,6-dimethyloctadi-2,7-en-6-01 (linalool ), Pd/Ru (6) (the number in parentheses designates the percentage ruthenium) or Pd/Rh alloys were the most promising with respect to stability, mechanical strength, rate and selectivity (to alkene) at 130°C. Our program objective was to study inorganic membranes for use as catalysts and examine the limitations of this emerging technology. To achieve this goal we designed and constructed glass and metal reactors for palladium foils and compared these reactors/catalysts with work reported in the literature using simple alkenes. Initially, we sought to study low-temperature liquid-phase hydrogenations before attempting vapor-phase hydrogenations and dehydrogenations. The limitations we encountered coupled with the scarcity of information on membrane catalysis of liquid alkenes prompts this article.
T.S. Farris and J.N. ArmorfAppl.
Catal. A 96 (1993) 23-32
27
EXPERIMENTAL
Initially, a 500 cm3 quartz reactor was designed to accommodate the Pd/ Ru(6) spiral membrane tube (69 cm long, 0.1 cm O.D.; with a calculated surface area of 0.0022 m’). The reactor was fitted with a condenser, an inert gas purge line, and a sample tube. The tubular membrane was activated at 400 ’ C in situ using an electric heating jacket. All reactions were conducted with 100 cm3 of reagent grade substrate. Later, liquid-phase reactions were run in a modified 300 cm3 Parr reactor (Fig. 1)using the coiled tubular Pd/Ru spiral (obtained on loan from Professor V. Gryaznov, A.V. Topchiev Institute of Petrochemical Synthesis, Moscow). The Parr Reactor was partially filled with cyclohexene, octadecene or cyclooctadiene as a neat solution (100 g) or diluted to 10% (w/w) in a suitable solvent. All the reagents used for the experiments were dried using 4A molec-
Fig. 1. Pd/Ru spiral membrane mounted into Parr reactor (left to right): thermocouple, inlet to spiral, sample withdrawal line, stirrer, outlet from spiral, and fritted metal gas inlet tube.
Fig. 2. Photograph of disassembled knife-edge flange membrane reactor: (left to right) inlet head, flow distributor, membrane mounted in Cu gasket; distributor; Cu gasket; exit head.
ular 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-5980 gas chromatograph (GC)by flame ionization detection (FID ) . Activations were carried out at 400’ C. Four times, air or hydrogen was passed both through and over the coils for one hour each. The reactor was cooled to the reaction temperature in nitrogen. For vapor-phase hydrogenations, palladium and palladium alloy foils (Johnson Matthey, Ltd.) were cut into 5-cm discs and washed with hexane and methanol. The air dried foil was compressed between two annealed copper gaskets held together with 2.75 inch O.D. knife edge flanges (Kurt J. Lesker, Inc. ) (See Fig. 2 ) . Vapor-phase hydrogenations, permeability, and pretreatment of these membranes are described in ref. 14. Liquid hydrocarbons were vaporized by passing nitrogen through a 7 ,um frit in a 75-cm3 stainless steel sample cylinder maintained at 30” C. Transfer lines were maintained at 50°C. Analysis of the reactant and product streams was accomplished with an HP 5890 gas chromatograph. Toluene, cyclohexene and hydrogenated analogs were analyzed using a 30 meter DB-WAX megabore column (1.0 pm film) with a helium carrier gas. Hydrocarbon vapors carried in a constant flow of nitrogen were trapped on a known weight of activated carbon. The weight gain over 30-60 min was recorded and used to calculate the amount of hydrocarbon in the vapor phase and equated to the GC area counts. RESULTS
AND DISCUSSION
Initial experiments conducted with cyclohexene in a static batch mode in a quartz reactor used a tubular Pd/Ru foil (S.A. ~0.0022 m”) at 70°C. Activation in air followed by reduction in hydrogen at 400’ C preceded each reaction. Results of the five initial runs showed no visible sign of hydrogen permeation
T.S. Farris and J.N. ArmorjAppl.
29
Catal. A 96 (1993) 23-32
(gas bubbles) and conversions averaged below 5%. We first suspected 70°C was too low and thermal mixing did not provide sufficient agitation at the alloy/liquid interface. In addition, we became uncomfortable heating this glass system containing hydrogen and a combustible material and conducted the remaining experiments in a stirred 300 cm3 autoclave reactor (Fig. 1). Table 1 presents the experimental results showing the effect of various hydrogenations in which either the temperature or reactant was varied using the autoclave reactor. Palladium supported on high surface area oxides certainly are more effective catalysts. (Separate experiments indicated cyclohexene conversion did not improve with agitation.) Continuing work with cyclohexene at 70’ C in the Parr reactor, we obtained 4.4% conversion after the tubular spiral was treated in boiling carbon tetrachloride for 2 h followed by two, 1 M HCl washes of 50 cm3 each and an extensive de-ionized water wash. This procedure removes ruthenium from the surface which hinders catalytic activity [ 151. We found it improved catalytic activity in one instance (run 12) which could not be sustained in a subsequent run 13, nor could it be duplicated (runs l&l6 and 17). Doubling the activation time (run 14) did not improve membrane performance at 70’ C. (In a recent private communication with V. GryTABLE
1
Temperature Run
and reactant effect on hydrogenation
Reactant
Temperature
TOS”
(“Cl 10 11 12
Cyclohexene Cyclohexene Cyclohexene
13 14
Cyclohexene Cyclohexene
15 16 17
Cyclohexene Cyclohexene Cyclohexene
6 7 8 9
70 70
Octadecene Octadecene
Conversion
Productivityb
(%o) 360 190 210
3.0 1.7 4.4
180 185
1.3 1.4
70 70
150 120
1.5 0.9
70
185
0.8
200 200 200 200
120
1.5
120 300
1.8
240
70 70 70
Octadecene Octadecene
of liquid alkenes
1.82 3.27 7.75 2.64 2.16 3.68 2.62 1.70 0.15 1.75 Not talc. Not talc.
18
Cyclooctadiene
70
120
1.4
1.4
18a
Cyclooctadiene
130
198
0.9
0.4
LITime-on-stream
(min ).
* Moles converted/m’/h.
30
T.S. Far&
and J.N. Ar~~r~A~p~. Catal. A 96 (1993) 23-32
aznov, he confirmed our results with liquid cyclohexene were comparable to his. ) Since hydrogen permeability increases dramatically with temperature, we sought to improve hydrogen permeation and conversion by conducting the reaction at 200°C using octadecene. Despite more favorable permeation conditions conversion rates remained below 2% (runs 6-9). In the final two runs (18 and 18a) cycle-octadiene (COD) replaced the monoalkenes. In the gas phase COD easily converts to cyclooctene and cyclooctane between 350-475 *C over Pd/Ru membranes [ 161. From our work, clearly liquid-phase hydrogenations of alkenes over a similar foil are unacceptably slow at lower temperatures. Our results indicate Pd/Ru foils have limited practical applicability for liquid-phase hydrogenation of alkenes. Although low surface areas of the foils contribute to lower reaction rates, dilute feeds (10% alkene in alkane) also had comparable conversion rates as undiluted alkene. Furthermore, even when the reactor was pressurized with hydrogen to 25 psig (1 psig=6.895 kPa), little improvement in conversion occurs. Lower hydrogen permeability at 70 ‘C limits reaction rates; however, conversion at higher temperatures remains unchanged despite greater hydrogen permeation. Our reactor scheme did not permit hydrogen analysis; however, an observed pressure increase (ca. 11 psi) during reactions at 200’ C was a sign of considerable hydrogen permeation. Gryaznov [lo] cites the use of catalytic Pd/Ru membranes for hydrogenation of acetylenic alcohols between 60 and 180°C. He fed technical grade reactants into a flow reactor as a neat feed or 30% aqueous solution at rates of 3 to 145 ml/h. Feed rates were based on the available surface area. Conversions greater than 95% are reported but it remains unclear if this was obtained in a single pass. Our inability to obtain comparable conversion in our model system may reflect our choice of these alkenes as reactants. We did not test any acetylenic compounds nor did we study the effect of higher stir rates on conversion. The tubular foils used in these experiments were coiled spirals, with the ends silver soldered to 316 stainless steel. The soldered joints frequently failed. Most likely phase changes in the metal play a major role in fracturing these joints however vibrational stresses, induced by the stirring action, may be an ancillary factor in their failure. (In one patent, Gryaznov et al. [ 171 claim these stresses can be avoided by using a compressed bank of flat spirals. ) In contrast to early attempts at hydrogenating liquid cyclohexene with hydrogen (0.5 atm) flowing through Pd/Ru(G) spiral tubes, operating in the vapor phase produced conversions of 35% at 150°C (over an activated 100 pm thick Pd/Ru (5 ) foil membrane ). The selectivity to cyclohexane was 93% and 7% to benzene [ 141. At 70°C we obtained cyclohexene conversions of 7 to 9% in the vapor phase (35% at lSO°C), in contrast to the 3 to 4% found in the liquid-phase experiments. At two atmospheres of hydrogen a nonsustainable maximum conversion of 23% was obtained in the vapor phase. Activity fell to
T.S. Farris and J.N. ArmorjAppl.
31
Catal. A 96 (1993) 23-32
16% over 47 min but selectivity to cyclohexane was maintained. The ineffectiveness of Pd/Ru alloys for liquid alkenes vs. acetylenics evaluated is probably related to the greater degree of adsorption of acetylenics, the temperature, the pressure, and/or the alloy composition. CONCLUSION
Liquid-phase hydrogenations over Pd/Ru membranes are feasible but the results reported here suggest the choice of substrate is crucial. Using cyclohexene, cycle-octadiene and octadecene conversions of 2% were common but never exceeded 5% with a single spiral membrane. In contrast, facile liquidphase hydrogenations with palladium membranes are known for the conversion of acetylenic alcohols. Our limited success in converting these compounds may be the consequence of the foil composition (i.e., the Pd/Ru foil is not the appropriate composition), the use of alkenes vs. acetylenics, the temperature, the pressure of hydrogen, the surface area of membrane, sorption of reactants/ products, and /or sufficient level of active metal on the surface of the activated membrane. We chose to report the work at this stage to alert others to the difficulty and complexity of liquid-phase hydrogenations of these simple alkenes over palladium alloy membranes. Given improved membrane compositions, selected substrates, etc. additional utility may yet emerge. ACKNOWLEDGEMENTS
We wish to thank Professor V. Gryaznov for arranging the loan of Pd/Ru spiral membranes. We wish to thank Professor Hank Foley for his collaboration on this program. We also acknowledge Air Products and Chemicals, Inc. for support and permission to publish this work at this time. Thanks are expressed to Professor David Trimm for agreeing to serve as editor for this manuscript. REFERENCES 1 2
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