Thermally And Mechanically Stable Catalysts For Steam Reforming And Methanation. A New Concept In Catalyst Design

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam";" Printed in The Netberlands

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THERMALLY AND MECHANICALLY STABLE CATALYSTS FOR STEAM REFORMING AND METHANATION. A NEW CONCEPT IN CATALYST DESIGN K. B. MOKa , J. R. H. ROSS a* and R. M. SAMBROOKb aSchool of Chemistry, University of Bradford, Bradford BD7 lOP (U.K.) and bDyson Refractories Ltd., Owler Bar, Sheffield S17 3BJ (U.K.)

ABSTRACT The various catalysts for use in the steam reforming and methanation process produced by existing techniques (such as the impregnation of a relatively high surface area preformed carrier or the forming into pellets of coprecipitated materials) represent a compromise between strength, activity and stability. A process has now been developed to give catalysts which have relatively high catalytic activities but which also have the high mechanical strength of a ceramic matrix. It involves the homogeneous precipitation of precursors of the active component together with promoters and spacers within the pores of a preformed matrix of a low surface area. For example, for use in the steam reforming and methanation processes, the active phase may be derived by calcination and reduction from a coprecipitate consisting of nickel, lanthanum and aluminium species.

1. INTRODUCTION Industrial catalysts for the continuous high-pressure steam reforming of hydrocarbons to produce hydrogen and synthesis gas:

and for the methanation of residual traces of CO in ammonia or hydrogen plants:

have in common the fact that they have nickel as their active constituents.

* Present address: Department of Chemical Technology, Twente University of Technology, P. O. Box 217, 7500 AE Enschede (Netherlands).

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For the former process, high thermal and mechanical stability are essential and there is little need for very high specific activity; in hydrogen production, exit temperatures in excess of 700 0C are considered normal. For the latter process, ease of reducibility and high generally carried out at temperatures below ~3500C, activity are the most important parameters and stability is of secondary importance. However, with the recent great interest in the methanation of high concentrations of carbon monoxide (from the gasification of coal or from the steam reforming of methane in the NFE project (ref. 1», there is a need for methanation catalysts which have relatively high activity at low temperatures as well as high stability at hjgh temperatures in the presence of steam. Previous work, reported in outline at the last symposium (ref. 2), has shown that various parameters in the preparation of coprecipitated Ni-A1 203 catalysts have a marked effect on the properties of the final catalysts (refs. 3.4) and that these materials can have high activity coupled'with reasonable thermal stability (ref. 5): this stability was attributed to the high degree of interaction between the Ni and Al of the catalyst. However, the coprecipitated materials must be formed into suitable pellets and these pellets have relatively low mechanical strength under the reaction conditions encountered in typical reactors. The main aim of the present project was to find a method of incorporating coprecipitates of this type into the pores of a preformed ceramic matrix of high mechanical strength (typically used for the preparation by impregnation techniques of catalysts for the cyclic steam reforming process (ref. 6)) in such a way that the high and stable activities characteristic of the reduced materials derived from the coprecipitates are maintained in the ceramic-based samples. The paper first discusses in outline the general method of preparation (which can also be applied to other types of catalyst) and then discusses the properties of some of the resultant materials, outlining the development of samples for commercial use. 2. EXPERIMENTAL 2.1 Catalyst Characterisation The methods by which the catalysts were prepared are described in section 3. They were characterised in a number of ways, including x-ray fluorescence (Te1sec Instruments Ltd.) for Ni content. X-ray line broadening (Phillips diffractometer) for Ni particle size. Kr (-1950C) and HZ (20oC) adsorption (pyrex adsorption system) for total and metallic areas and differential scanning calorimetry (DuPont 910 DSC) for the measurement of the activities for the methanation of CO (see ref. 7). The activities were determined in a standard reaction mixture of CO (lZ%), HZ(36%) and Ar (52%). The calcination and reduction of the catalysts were examined, using thermogravimetry (DuPont 951 Thermobalance). in a flowing atmospheres of Ar and of Ar+H2 (l:l).The extent of ageing of the catalysts in hydro-

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thermal reducing atmospheres was estimated by examining the catalysts after exposure to a flow of H2 and H20 (4:1) at BOOoC for 4h. The performance of the samples under steam reforming conditions and, in particular, the resistance of the catalysts to carbon deposition were examined using an atmospheric-pressure continuous steam-reforming system in which the feedstock was heptane, the exit temperature of the bed was 650 0C and the steam: carbon ratio was 4:1. 2.2 Materials Analar and reagent grade chemicals, from various sources, were used throughout. Solutio~s were made up using de-ionised water and the vessels used were made of stainless steel, Pyrex or quartz as appropriate. The gases (Kr and H2) used for total and metallic areas were supplied in sealed ampoules (Grade X) and the remainder in cylinders (BOC Ltd.). The preformed ceramic matrice~ used for the samples described in this paper were composed of pure a-A1 203 with an apparent porosity in the range 50-60% and a mean pore diameter in the rangeO.5-2.0~m; they had been fired at >1400 oC. Other materials, such as silicon carbide, alumino-silicates or silica could also be used. The matrices were formed as Raschig rings of various sizes; rings of approximately 1 cm height and diameter were normally used. 3. RESULTS AND DISCUSSION The aim of this and parallel wor~ (ref. 8) was to prepare mechanically stable catalysts derived from coprecipitates of composition such as Ni6 A12(OH)16 C0 3· 4H20 (ref. 3). Preliminary experiments showed that attempts to incorporate the precipitate by normal precipitation techniques using Na2C03 or (NH4)2C03 gave deposits on the surfaces of the rings and in the bulk of the solutions but not in significant amounts in the pores of the a-A1203' When the rings were immersed and heated in a solution to which had been added urea or some other easily hydrolysable material (the homogeneous precipitation technique pioneered by Geus and coworkers (ref. 9) and used since by others (see, for example, ref. 10», significant precipitation occurre~ within the pores. A further improvement was realised when the rings were vacuum impregnated with the urea-containing deposition solution and excess solution was drained off prior to heating the impregnated rings at a temperature up to 1100C in an oven; deposition now occurred almost exclusively within the pores of the matrix. By suitable adjustment of the concentrations and compositions of the urea-containing solutions and by repeated depositions, relatively high loadings of the matrices (up to 20 wt% Ni) could be achieved. Between depositions, it is advantageous to heat the rings to approximately 310 0C for 2h to bring about partial or complete decomposition of the precipitate formed, thus increasing the capacity of the pores for further deposition solution. Washing with alkali (see section 3.3) can also improve the subsequent

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uptake. After the deposition stages have been completed, the catalyst orecursor is "calcined" at 4500C in a stream of air for 'V4h and is then reduced in flowing H2 (preferably with a gradual increase in temperature to 600oC) prior to use. A variety of different formulations have been prepared and tested and the results of some of these tests are described below together with more detail of the compositions of the solutions used in the preparations, etc. 3.1 NiAl, NiMgAl and NiMgAl Ba Formulations Some typical formulations prepared in this work and selected results obtained with them are presented in the table. Table: Results for Selected Samples. Catalyst

A* B C D

E

F G

H I

Composition

Ni/a- A1203 Ni/a- A1203 Ni/a-A1203 Ni2.3Al/a-A1203 Ni 2.3A1/a-A1203 Ni3.5A12.5Mg/a-A1203 NilO.OA13.3La/a-A1203 Ni 7A1 5La/a-A1 203 Ni14A14.7la/a-A1203

Ni Content /wt %

Total A,ea /m 2g-

5.0

1.0

4.5 4.3 7.7 6.5 4.2 6.5 7.6

2.5 7.6 17.8 6.5 13.5

Ni Area /m 2g-1

0.47 0.42 0.63 0.56 0.98 1. 91

Ni partiile Size / Fresh Aged 300 250 257 198 235 180 117 180 107

1000

750 467 530 340 331

* Sample A was prepared by impregnation, all the others were prepared by the urea hydrolysis me thodj- signifies not determined.

Sample A was prepared by a normal impregnation technique and is similar to catalysts used in cyclic steam-reforming plants (ref. 6). Sample B had a similar Ni content ('V5wt %) but was prepared by precipitation at 900C from a urea-containing "deposition" solution (350g of Ni(N03)26H20 and 217g of urea in 150g of water). Sample B had rather smaller Ni particles and there was a more marked difference between the samples aged in H2/H20 at 8000 e for 4h although both sintered significantly. Sample e is similar to B. Its total area is considerably higher than that of the untreated matrix (2.5 m2g-1 compared with 0.5m 2g-1) and the nickel area is about a quarter of the total; we infer that the nickel must have interacted to some extent with the alumina of the matrix to give

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roughening of the interior of the pores. When an appropriate amount of aluminium nitrate (depending on the desired Ni/Al ratio) was added to the above deposition solution, a further considerable improvement was found in the particle sizes of the calcined and reduced catalysts both before and after the ageing test (samples D and E); the total area of the samples is increased considerably although the Ni areas are unchanged. Tests of the methanation activities of such samples show that they have activities which compare very closely with normal coprecipitates (ref. 11) when the relative weights of nickel are considered. Fig. 1 shows differential thermogravimetric (DTG) results for the reduction of samples C and D. While the sample without added Al (C) is G almost completely reduced at 4000C, D the presence of Al decreases the ease of reduction, temperatures III ...., 0C >600 being required for complete reduction, a result typical of coprecipitates (ref. 2). Samples C and E were tested for the steam reforming of heptane. The C former had good initial activity but deactivated relatively rapidly and 300 disintegrated, presumably due to the Fig. 1. growth of carbon in the voids of the matrix. Sample E had a better activity but, although it did not disintegrate, a back-pressure built up, again presumably because of C deposition in the matrix. A further improvement in the particle sizes of the freshly reduced and aged samples was achieved by incorporating magnesium nitrate in the deposition solution (sample F). A further sample (F~ was prepared by impregnating F with barium acetate solution; the ~ddition of barium to urania-containing catalysts (the NUA series of catalysts made by Dyson Refractories Ltd; see ref. 12) is known to improve the carbon gasification activities of such catalysts and a similar property was sought here. Samples F and Fl were each tested for steam reforming activity and both gave 100% gasification; F was gradually deactivated and there was a bUild-up of back-pressure, again due presumably to carbon deposition, but sample Fl operated well with no apparent change.

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3.2 Ni La Al Formulations Lanthanum was then tested as a possible promoter. This was done for two reasons: (a) The NUA samples referred to above are thought to owe their success

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to the ability of urania to dissociate molecular water and lanthana is likely to have a similar ability; (b) Wallace and coworkers (see, for example, ref. 14) have shown that catalysts derived from rare-earth intermetallics (e.g. La NiS) have high methanation activities. Sample G was therefore made as described above for Sample B but with the addition of a suitable weight of La(N03)3 to the deposition solution. Sample H was made in a similar way, but has a slightly higher metal content and was made by several deposition sequences with partial decomposition of the precipitates at 310 0C between each. Sample I had an even higher Ni content but was washed with alkali (see section 3.3) between depositions. Fig. 1 shows the DTG-trace obtained for the reduction of sample G, from which it is seen that La decreases the reducibility of the sample still further compared with the addition of only Al (Sample E). Another sample, not shown in the table, was prepared without the addition of Al to the deposition solution; it can be seen that La now has little effect on the reducibility (compare sample C, with no additives). The addition of La appears to reduce the particle size of the freshly reduced sample and also to stabilise the particles in the ageing test. Samples G and H were tested for activity and resistance to carbon deposition in the steam reforming of heptane, as discussed above.They gave 100% conversion over the period of the test and there was no evidence for C deposition. We therefore conclude that La imparts a resistance to C deposition similar to that imparted by urania. 3.3 Effect of Washing During Preparation Although the amount of urea added to the deposition solution was sufficient. if completely hydrolysed on heating, to bring about complete precipitation of the metallic species present (Ni, La, Mg, Al) the hydrolysis was not complete in a reasonable time at 9So~ or even at 110o~ as is shown by results such as those of Fig. 2. This presents DTG data for the heating in Ar of samples derived from the uncalcined precursor of sample G after washing in water and solutions of NaOH, Na2C03 and (NH4)2C03 (0.1 mol dm- 3). For the precursor washed in the carbonate solutions (and .0 s, Itl for the unwashed precursor, not shown for reasons of KOH clarity), there was a NaOH large peak in the DTG trace at 300 0C which was shown 200 300 T/0~00 500 600 to be due to unhydrolysed DTG of Calcination of Sample G after various washing treatments.

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nitrate. Washing with water or with NaOH or KOH caused the disappearance of these peaks and the decomposition behaviour was very similar to that of pure coprecipitates (ref. 3). Washing with water reduced the Ni content, presumably by the removal of unreacted nitrate, while the hydroxides caused complete precipitation of any free nitrate. Fig. 3 shows DTG results for the reduction of these samples after decomposition. The carbonate-washed samples are reduced much more easily than the others, presumably due to the presence of discrete NiO particles formed by the decomposition of nickel nitrate. The reduction behaviour of the other samples is similar to that of the pure co~-\;"'-~-.d=---'--..,!-,.-----L-----L -l---.-'.'.....--'-----'----;~:-150 250 350 Tlot50 550 precipitates. The methanFig. 3. DTG of Reduction of samples of Fig. 2. ation activities of the samples were also determined using the DSC. The water, NaOH and KOH samples had activities which were a factor of approximately three times those of the unwashed sample and of those washed in carbonate solutions. We conclude from these and other results that the most highly dispersed and active catalysts result from those materials which are reduced with the greatest difficulty. 3.4 Use of Samples in Commercial Plants. Materials similar to Sample H have been prepared in commercial quantities and have been used successfully since mid-1981 in two plants, the first reforming a naphtha feed by the cyclic process to give a town gas and the second reforming a butane feed to produce hydrogen. Both plants have worked with high efficiency since the catalysts were installed. In the first case, the gas produced is very much leaner than that produced with conventional cyclic catalysts and the efficiency of the plant is consequently improved considerably. In the second, the catalyst is able to reform more feedstock than the design capacity of the plant; it has also survived,without apparent damag~ a major plant breakdown during which the steam-carbon ratio was very low while a competitive catalyst in a parallel plant using the same feedstock was destroyed. 4. CONCLUSIONS Mechanically and thermally stable catalysts suitable for high-temperature steam reforming, and probably also for methanation of CO-rich gases, have been

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successfully prepared by homogeneous deposition within the pores of a-A1203 rings. Formulations containing NiMgAl Ba and also NiLaAl have been shown to have considerable advantages over NiAl alone. ACKNOWLEDGEMENTS Thanks are due to Dyson Refractories Ltd. for their sponsorship of this work and for permission to publish the paper, to C. Whitehurst for X-ray results, to D. Salt for steam reforming results and to Mr. J. Laming and Dr. B. Jackson for continued encouragement. We also thank Professor L. L. van Reijen and Mr. E. B. M. Doesburg for useful discussions. REFERENCES 1 B. H~hlein, R. Menzer and J. Range, Appl. Catal., 1 (1981), 125. 2 E.C. Kruissink, L.E. Alzamora, S.Orr, E.B.M. Doesburg, L.L.van Reijen, J.R. H. Ross and G. van Veen, Preparation of Catalysts II, Ed. B. Delmon et al Elsevier (1979), p; 143. 3 E.C.Kruissink, L.L. van Reijen and J.R.H. Ross, J. Chem. Soc. , Faraday Trans. I, 77 (1981) 649. 4 L.E. Al zamora , J. R. H. Ross, E.C. Kruissink and L.L. van Reijen, J. Chem. Soc., Faraday Trans. I, 77 (1981) 665. 5 G.van Veen, E.C. Kruissink, E.B.M. Doesburg, J.R.H. Ross and L.L. van Reijen, Rn, Kinet. Catal. Lett., 9 (1978) 143. 6 Gas Making and Natural Gas, 1972 B.P. Trading Ltd. ,Chapter 8. 7 T. Beecroft, A.W. Miller and J.R.H. Ross, J. Catal., 40 (1975) 281. 8 H. Schaper, E.B.M. Doesburg and L.L. van Reijen, Paper C.4, this symposium. 9 See L.A.M. Hermans and J.W. Geus, Preparation of Catalysts II, Ed. B. Delmon et al., Elsevier (1979), p. 113. 10 J.T. Richardson, R.J. Dubus, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale, Preparation of Catalysts II, Ed. B. Delmon et al ., Elsevier (1979}, p. 131. 11 M.R. Gelsthorpe and J.R.H. Ross, to be published. 12 T. Nicklin, F. Farrington, R.J. Whittaker, Inst. od Gas Engineers J., (1970), 151. 13 V.T. Coon, T. Takeshita, W.E. Wallace and R.S. Craig, J. Phys. Chem. ,80 (1976) 1878.

299 DISCUSSION J.W.E. COENEN: It is somewhat surpr1s1ng to find deposition-impregnation appearing at this Conference as a relatively new method. About 10 years ago Unilever applied for patents for this method. Last week the Dutch patent was granted. J.R.H. ROSS: Dyson Refractories are fully aware of the existing patent literature. Indeed, a urea hydrolysis technique was first patented in 1942. The novelty of the Dyson Refractories technique lies in the deposition of a complex multicomponent active phase within the pores of a preformed low surface area ceramic matrix. B. NIELSEN : Do you have any analyses of alkali metals in the La-promoted catalysts? It is known that alkali is able to prevent carbon formation which could be the explanation for the better resistance against carbon formation. Many of the preparations include washing with alkali metals. J.R.H. ROSS: The alkali metals content of the lanthanum catalysts is negligible, e.g. potassium less than 0.05 wt.%. ZHAO JIUSHENG : Have you been doing any research on coke deposition on catalyst with La? Is it any differentce between your catalyst and the usual commercial catalyst ? J.R.H. ROSS: Laboratory and plant data show the presence of a lanthanum species in a steam reforming catalyst confers carbon gasification activity to that catalyst. However, the level of carbon gasification activity depends greatly on the method of catalyst preparation. M.S. SCURRELL : Could you comment on the possible importance of the chemical nature of the ceramic material used? You mention that the nickel interacts at least to some extent with the a-alumina support. Is this of special significance or could any other low-area supports such as silica or silica-alumina be considered for use in this application? J.R.H. ROSS: Dyson Refractories use a pure a-alumina carrier of high stable mechanical strength in the manufacture of steam reforming catalysts. Chemical purity is important as silica and alkali metals are known to migrate under high pressure steam reforming conditions. F. GADALLAH: The role of La-Elaborate. on reactivity and stability.

When and how it is added.

Its effect

We cannot comment on when and how the lanthanum is added in the J.R.H. ROSS manufacture of these cat~lysts. The lanthanum species has a dramatic beneficial effect on the thermal and hydrothermal stability of the nickel catalyst. M. BHASIN: D'i.d-you compare the activity of your coprecipitated catalysts with those prepared by impregnation/decomposition of aluminium nitrate followed by impregnation/decomposition of the nitrate of the nitrate of nickel and lanthanum ? J.R.H. ROSS Many aspects of the preparation of the initial lanthanum catalysts have been studied including sequential impregnation deposition or decomposition of the components of the active phase. J.W. GEUS You did not completely precipitate the nickel using the urea hydrolysis. I therefore wonder what the urea (nickel and aluminium) ratio was you have used ? J.R.H. ROSS catalysts.

We cannot comment on matters concerning the manufacture of the