Applied Catalysis A: General, 96 (1993) Elsevier
Science
APCAT
A2354
Publishers
15
15-23
B.V., Amsterdam
Preparation and catalysis over palladium composite membranes
V.M. Gryaznov, Russian Peoples’
O.S. Serebryannikova
Friendship
and Yu.M. Serov
University,
6 Miklukho-Maklaya,
A.N. Karavanov,
A.P. Mischenko
Moscow
117198 (Russian
Federation)
and M.M. Ermilova, A. V. Topchiev Prosp., Moscow
Institute
of Petrochemical
Synthesis,
and N.V. Orekhova
Russian Academy
of Sciences,
29 Leninsky
117912 (Russian Federation)
(Received 18 May 1992, revised manuscript received 24 July 1992)
Abstract Binary and ternary palladium alloys with manganese, cobalt, ruthenium, tin and lead were obtained in the form of continuous thin films on asymmetric polymer membranes, porous metal sheets and oxide plates. All of these compositions proved to be permeable for hydrogen only. Compositions on polyarilyde (polydiphenylenephtalide) were stable in air up to 473 K and did not swell in hydrocarbons. A metallised polyarilyde membrane had a hydrogen permeability of 13.8 ms/m’ h at 473 K and pressure drop of 1 MPa. Productivity and selectivity for pentadiene vapour phase hydrogenation to pentenes of these compositions are the same as for the palladium alloy foil but contained 180 times less palladium than the foil. The selectivity of palladium-ruthenium foil for ethene and propene synthesis by carbon monoxide hydrogenation increased drastically after coverage of this foil with islands of cobalt. Hydrogen permeability of a stainless steel porous sheet deposited with a 10 pm thick film of palladium-ruthenium alloy by magnetron sputtering decreased twofold after a 1000 h test at 1073 K. Introduction of a 0.8 pm thick intermediate layer of tungsten into this composition maintained its hydrogen permeability at high temperatures. Magnesia and zirconia intermediate layers were effective as well. A porous plate made of magnesia and 15% yttria covered with a 10 pm film of palladium alloy with 6% of ruthenium had a hydrogen permeability of 108 m”/m” h (at 973 K and a pressure drop of 2 MPa) which was constant for 1000 h. Palladium-lead clusters implanted in the pores of a stainless steel sheet had 96% selectivity for the liquid phase hydrogenation of dehydrolinalool to linalool at 423 K with a productivity of 9 mol/m* h. A membrane catalyst for acetone condensation to 2-methylpentanone-4 was prepared by introduction of active alumina particles into the porous surface of palladium alloy foil. Keywords: alloys, carbon monoxide/hydrogen; coatings; composites; membranes; palladium-cobalt; palladium-indium-ruthenium; palladium-ruthenium; pentadiene-1,3 hydrogenation Correspondence to: Dr. V.M. Gryaznov, Russian Peoples’ Friendship Maklaya, Moscow 117198, Russian Federation. Tel. (+7-095)9520745,
0926-860X/93/$06.00
0 1993 Elsevier
Science
Publishers
B.V.
University, 6 Miklukhofax. (+7-095)2302454.
All rights reserved.
16
V.M. Gryaznov et al./Appl. Catal. A 96 (1993) 15-23
INTRODUCTION
The concept of the membrane catalyst as a permselective membrane which is covered by a catalyst layer on one or both surfaces [l] was first demonstrated by surface modification of hydrogen permeable palladium-based alloys. The use of palladium-rhodium foils as a catalyst for cyclohexane dehydrogenation results in an increase in rhodium concentration in the surface layer. This was accompanied by an increase in catalytic activity [ 21. Similar data were obtained [ 31 for palladium-~thenium foil. In these cases the membrane was hydrogen permeable and catalytically active and became more active due a redistribution of alloy components between the bulk and the surface layer. The next step in the preparation of a catalyst-membrane system was deposition of a chemically active metal on the surface of the palladium alloy membrane followed by heat treatment and acid removal of the additional metal [ 4,5]. The formed porous layer of palladium alloy is strongly bound to the bulk of the foil and is not dispersed during the catalytic reactions, unlike Raney catalysts. The chemically active metal remains partly in palladium alloy after the acid treatment. Thus the membrane catalyst can be modified on one or on both surfaces. The examples of this modification were given in ref. 6. The following generation of the membrane catalysts has been produced by thin palladium alloy film deposition on porous polymeric [ 7-111 or inorganic [ 12) supports to increase the hydrogen permeability and to diminish the precious metals content. The properties of palla~um-based membrane catalysts were discussed at five previous International Congresses on Catalysis [ 13-171. The main results concerning monolithic and supported membrane catalysts can be found in various reviews [ 18-231. In this paper we present several methods of catalyst fixation at different loading levels on the surface or inside the pores of membranes. EXPERIMENTAL
The composite membrane catalysts were prepared by two methods: sputtering of metal or alloy on the surface of an inorganic or polymer membrane or insertion of catalyst particles into the membrane pores. Commercially available (Metallurgical plant, Vyksa, Russia) porous metal sheets 0.2 mm thick such as nickel and stainless chromium-nickel steel were used as membranes after heating at 920-970 K for 10 min at a residual pressure less than 1.3.lo-” Pa (just prior to palladium alloy deposition). Ma~etron sputtering was performed at argon pressures from 0.1 to 1 Pa. The target disk had a diameter of 130 mm. The rate of alloy deposition was about 1 pm per minute. The membrane was maintained at the same temperature to provide a continuous film.
V.M. Gryaznov et al.fAppl. Catal. A 96 (1993) 15-23
17
Ceramic membranes of magnesia and up to 15% of yttria were prepared in the D.I. Mendeleev Institute of Chemical Technology (Moscow) in the form of discs 60 mm in diameter and 3 mm thick. Mean pore diameter was 0.3 ,um, with a total porosity of ca. 50%. These membranes were coated with palladium alloys after treatment at 1070 K. Polymer membranes were 30 pm thick sheets of polyarilyde (polyphenylenephthalide) prepared using the method described in ref. 24. The sheets were covered by metals on a water-cooled drum at an argon pressure of 0.2 Pa after argon ion treatment. Polymetallic layers were obtained from several targets. The ratio of target areas determined the layer composition. The polymer membranes were asymmetric in structure with porous, microporous and sealing layers of polyarilyde, which is resistant in air up to 623 K and does not swell in hydrocarbons. Asymmetric polyarilyde membranes have been synthesized at the A.V. Topchiev Institute of Petrochemical Synthesis and at the Chemical Machine Building Institute (Moscow). A scanning transmission electron microscope SCAN 60A was used to study film thickness and structures before and after the hydrogen permeability and catalytic activity investigations. Thickness of the deposited layer was measured in several points of fracture on the micrographs. X-ray photoelectron spectroscopy (XPS ) spectra of the obtained layers were recorded using a Kratos XSAM 800 photoelectron spectrometer equipped with a DS 300X datahandling system [ 11. The pressure inside the photoelectron chamber was less than 1.3*10-5 Pa. Membrane catalysts with mono- and bimetallic clusters of palladium, palladium-manganese and palladium-lead inserted into pores of stainless chromium-nickel steel sheets 0.15 mm thick, total porosity 45% were obtained through cryochemical synthesis [ 251. These metals were evaporated by heating with an electric current in the stationary reactor of a cryochemical unit [26]. Toluene was used for organic matrix formation. The product obtained after the matrix melting of a sol of metals in toluene was introduced into the pores of a membrane (0.2 mm thickness) under ultrasonic treatment. The effective pore diameter of the initial membrane was 2-3 ,umand did not change noticeably by insertion of the sol. The imbedded metals content in these catalysts was about 1.5%. The laboratory reactor with the membrane catalyst in the form of a foil or thin film on the metallic support consisted of two stainless steel plates [ 1,3]. Each plate had two slots which were connected with the holes in the butt-ends. Two tubes, with an outer diameter of 6 mm, were inserted into the holes in the butt-ends of the plates. These tubes were used for introduction and discharge of the reagents in two chambers of the reactor separated by the membrane catalyst. The membrane catalyst in the form of foil was pressed between two copper gaskets inside the hollow of the lower plate and the ledge of the upper plate by ten bolts and nuts. The gaskets were shaped into a frame 2 mm wide and 0.4 to 0.5 mm thick. These gaskets were milled from a pile of annealed
18
V.M. Gryaznov et al./Appl. Catal. A 96 (1993) 15-23
copper sheets with a steel template. Before use the gaskets were washed in carbon tetrachloride, acetone, pure ethanol and distilled water. The polymer membrane covered by palladium alloy film was pressed between two teflon gaskets. Ceramic membranes were sintered under pressure jointly with two rings of stainless steel having the same coefficient of thermal expansion as the ceramic. These rings were attached between two halves of a cylindrical reactor after palladium alloy film deposition on the ceramic membrane and steel ring. RESULTS AND DISCUSSION
A foil of palladium with 10 wt.-% of ruthenium alloy was treated with oxygen at 1073 K for 10 min, then it was reduced with hydrogen at 673 K for 30 min and cooled slowly to room temperature in an argon stream. A cobalt layer 100 nm thick was deposited by magnetron sputtering at an argon pressure of 0.8 Pa. After heat treatments at 673 K for 1 h in hydrogen the initially continuous cobalt film became spotty. Cobalt surface content decreased from 100 to 33%. The hydrogen permeability and its activation energy for this system was the same as for the initial palladium-~thenium foil. Carbon monoxide hydrogenation at 523 K and atmospheric pressure on cobalt modified foil gave 7% of C&-C3alkenes. Traces of C,-C, were produced, but oxygenated species were not found. The partial removal of hydrogen from the reaction volume through the membrane catalyst increased the alkene concentration to 40% of the converted carbon monoxide. This result differs drastically from that observed on the foils of palladium-cobalt and palladiumruthenium alloys. In the first case the formed alkenes converted into alkanes almost completely. On palladium-~thenium alloys containing from 5 to 10% of ruthenium, propene was not formed and the ethene yield was smaller than on palladium-ruthenium foil modified by cobalt. Palladium alloy catalysts on polymer membranes Unlike most polymers, polyarilyde can be used at temperatures up to 623 K. The asymmetric polyarilyde membranes were coated with the following alloy films: palladium with 4% tin and palladium with 6% ruthenium. The latter alloy was sputtered also on the intermediate layers of nickel and cobalt. The total thickness of the alloy metal layer was 0.4 pm. After heating repeatedly to 473 K and cooling to 313 K in hydrogen the composites remained permeable for hydrogen only. XPS spectra showed that diffusion of nickel or cobalt into the outer surface of palladium alloy does not take place after thermocycling. Hydrogen permeability at 473 K at a pressure drop of 1 MPa was 13.8 m”/m” h.
V.M. Gryaznov et al./Appl. Catal. A 96 (1993) 15-23
19
These catalysts were tested for vapour-phase hydrogenation of 1,3-pentadiene. Pentenes production on polyarilyde/Co/Pd-Ru composite at 423 K and atmospheric pressure was 8 mol/m2 h with pentadiene conversion equal to 99.8% and the selectivity towards pentenes equal to 92.7%. Conventional hydrogenation of 1,3-pentadiene gives lower conversion (90%) at the same selectivity [ 31. Catalysts with a film of palladium-tin alloy on polyarilyde exhibited nearly the same pentenes productivity as a foil of this alloy but contained 180 times less palladium than the foil. Palladium alloy thin films on porous inorganic membranes A continuous 10 pm thick film of palladium alloy with 6% ruthenium was obtained on a stainless steel porous sheet. Hydrogen permeability of this composite at 1073 K and a pressure drop of 2 MPa was 120 m”/m” h, decreasing twofold after 1000 h. XPS spectra revealed the diffusion of steel components to the outer surface of the palladium alloy film. This did not take place for the composite with a molybdenum layer thicker than 0.08 pm and the above mentioned film of palladium-ruthenium alloy. The hydrogen permeability of this composite with an intermediate layer diminished 30% after 1000 h test at 1073 K. The influence of the nature of the intermediate layer on the stability of the membrane’s hydrogen permeability was studied. Table 1 summarizes the ratios of permeabilities before and after the test for the membranes with different intermediate layers. Strong bonding of palladium to tungsten at low coverage was observed [ 271 in palladium desorption experiments on W(110) substrates. Recently the thermal desorption spectra of palladium from clean silica were investigated [ 281. The results suggest that at low coverage palladium interacted with the polarizable oxygen of the substrate. The influence of temperature and rate of palladium deposition on film growth was not considered [ 281. TABLE 1 Influence of the intermediate layer (IL) nature on the ratio of hydrogen permeability (P) after 1000 h test at 1073 K and initial permeability (PI) of stainless steel/IL/Pd-Ru 6 composites IL
None Molybdenum Tungsten Tantalum oxide Magnesia Alumina Zirconia
Thickness of IL (wn)
0.08 0.8 0.1 0.5 1.2 1.0
PIP1
(So) 50 70 100 100 100 92 100
V.M. Gryaznov et al./Appl.
20
Catal. A 96 (1993) 15-23
A porous sheet of stainless steel coated with a 1.5 pm thick layer of palladium alloy with 6% indium and 0.5% ruthenium was tested with pure hydrogen obtained from the reforming gases. The hydrogen permeability proved to be stable and equal 4 m”/m” h at a temperature of 645 K and a pressure drop of 0.1 MPa. A bench scale reactor with this membrane worked at a refinery for obtaining pure hydrogen from reforming gases over a period of one month. In the XPS spectra an increase in indium content in the membrane surface layer was detected after this test. Oxide supported palladium alloy films were prepared by magnetron sputtering without an intermediate layer. A membrane made from magnesia and 15% yttria covered with a lo-pm thick film of palladium alloy with 6% ruthenium was permeable only to hydrogen. Hydrogen permeability at 973 K and a pressure drop of 1 MPa was 108 m”/m” h and remained stable during a 1000-h test. Scanning electron microscopy of the tested membrane did not detect defect formation on the film outer surface or on its interface with the support. Porous membrane catalysts with metal clusters The membrane catalysts with clusters of palladium, palladium-manganese and palladium-lead inside the pores of stainless steel sheets were investigated in hydrogenation of dehydrolinalool (3,7-dimethyloctadiene-1,6-01-3). The catalyst sheet divided the flow reactor into two chambers. A constant stream of liquid dehydrolilool was introduced into one chamber, and hydrogen under atmospheric pressure was passed into another chamber. Hydrogen diffused through the membrane catalyst to the hydrogenation chamber and reacted with the acetylenic alcohol. After finishing the hydrogenation process, the catalyst was regenerated without taking it out of the reactor. First, it was washed with ethanol and then heated for 1 h at 673 K in a flow of air, and finally treated with hydrogen. The productivity of the palladium cluster catalyst at 423 K was 13 mol me2 membrane hh’. It is equal to the productivity per unit area of the TABLE
2
Characteristics
of dehydrolinalool
hydrogenation
to linalool at 423 K and atmospheric
pressure on the catalysts with mixed metal clusters inserted into porous steel sheets Catalyst
Pd/Me
Selectivity (%o)
Productivity (mol/m’h)
Pd Pd+Pb
1:2
80 95
13 2
Pd+Mn
2:l
96
9
hydrogen
V. M. Gryaznov et al. fApp1. Catal. A 96 (1993) 15-23
21
membrane catalyst in the form of a 50-pm thick foil of palladium-6% ruthenium alloy [ 291, but it was 20 times more productive based upon the weight of palladium. The selectivity for hydrogenation of dehydrolinalool into linalool on the palladium cluster catalyst is close to 80% and can be improved by using a mixed metal cluster catalyst. The corresponding data for the catalysts with cluster content of 1.5% of palladium-lead and palladium-manganese are shown in Table 2. Bifunctional membrane catalyst The bifunctional membrane catalyst was prepared by imbedding active alumina into the porous surface layer of the foil made from the palladium alloy with 6% indium and 0.5% ruthenium. The method of porous layer formation was outlined previously [4,5]. Sample no. 1 contained 0.1 mg alumina per 1 cm2 membrane surface area and sample no. 2 contained twice as much alumina. Just before the experiments catalysts were treated in turn by air and by hydrogen at 773 K. The membrane catalyst with a surface area of 24 cm2 divided the reactor volume into two chambers. The catalysts were tested in model reactions of acetone condensation to 2methylpentene-2-one-4 and hydrogenation into 2-methylpentanone-4. Over the alumina containing surface of the catalyst, acetone vapour (at a partial pressure of 0.026 MPa) was introduced in argon at a space velocity 720 h-l. Hydrogen was passed at atmospheric pressure along the other side of the catalyst. The activity was stable for 40 h. An electron microscopy study showed that the size of the alumina particles did not change after a long series of catalytic runs and catalyst regeneration cycles (by air at 723 K for 1 h and by hydrogen at 673 K for 1 h). The reaction products were 2-methylpentene-2one-4 and 2-methyl-pentanone-4. The yield of the first substance on catalyst no. 1 went through a maximum value 24% on increasing temperature from 320 to 420 K. Catalyst no. 2 showed higher activity at 500 K and produced 37.5% of 2-methylpentene-2-one-4 and 26.4% of 2-methylpentanone-4. (2-Methylpentene-2-one-4 is used as a solvent and 2-methylpentanone-4 as a curing agent of lacquers. ) CONCLUSIONS
This investigation demonstrated that high-temperature resistant hydrogen permeable membranes and membrane catalysts can be prepared by magnetron sputtering of palladium alloys on porous supports. Crack-free, pinhole-free thin alloy films proved to be stable during 1000 h testing at 1073 K. The intermediate layer of high-melting metal or oxide increased the life-time of these compositions. The hydrogen permeability of such a composition is one hundred times larger than that of dense metal membranes. Selectivity and catalytic
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V.M. Gryaznov et al./Appl.
Catal. A 96 (1993) 15-23
activity of compositions are close to these known for the dense membranes. The amount of precious metals on the surface of the composition is up to 180 times less than for alloy foil. A technique is also reported for the attachment of the composite membrane catalyst in the reactor. The other method for forming porous membrane catalysts is embedding bimetal clusters or oxide particles into the pores of an inorganic support. In this case the composition of the catalyst can be tuned without the limits of solid solution formation.
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