The Reactions of Hydrocarbons on Multi metal lie Catalysts

BY D. A. DOWDEN

1 Introduction

Elemental metals have been used since the Industrial Revolution as active phases contributing their characteristic catalytic properties to both simple and complex catalysts. Until the late fifties most such catalysts contained only a single base metal, as for example nickel extended upon kieselguhr, copper stabilized with chromia or unsupported cobalt (for the selective hydrogenation of various functional groups), iron in the Haber synthesis of ammonia and the Fischer-Tropsch process, etc. Individual precious metals, notably platinum and palladium, were widely used in small quantities for specific hydrogenations on a small scale, and to a larger extent in processes involving oxidation, e.g. platinum in earlier sulphuric, nitric, and hydrocyanic acid plants, and silver for formaldehyde and ethylene oxide production. Indeed the best examples of the application of well-defined binary alloys in industrial catalysis were and are platinum-rhodium solid-solutions (wire gauzes for ammonia oxidation but also supported crystallites for the modern Andrussov HCN process).

The petroleum refining industry is currently a major user of heterogeneous catalysts and the energy crisis has speeded the search for catalysts and processes leading to greater efficiency and economy. Catalysis in petroleum refining developed rapidly in the period 1930 — 1945 initially because of the attempts to devise a viable process for the production of petrol by the hydrogenation of coal, and later because of the exigences of the Second World War. The large amounts of compounds of sulphur, nitrogen, and oxygen in the heavy oils from the liquefaction of coal were removed by hydrocracking over sulphide catalysts but a final stage, similar to the present day reforming of naphtha, involved a multifunctional catalyst comprising a base metal (e.g. iron) supported on an acidified natural alumino-silicate (e.g. fluorided Fullers earth). Petroleum fractions were also upgraded to higher octane numbers by dehydroaromatization over oxides of the metals of Group VI. However, the need to upgrade the greater yields of naphthas coming from catalytic cracking led to the emergence of new reforming catalysts in which active metals were combined with oxides of various acid strengths; typical examples were nickel on silica-aluminas and platinum on halogenated γ-alumina, among which those containing precious metals soon became dominant, as in the 'Platforming' process which emerged in the late fifties. The potential of alloys to affect the activity and the selectivity of the metallic components of catalysts had been known since 1950, but economic pressures were insufficient to compel the adoption of even more complicated catalysts until the end of the past decade when platinum reforming-catalysts began to be displaced by 'bimetallic' catalysts especially by the Pt-Re pair ('Rheniforming'). Subsequently the empirical exploration of multimetallic catalysts by industry and the cognate fundamental research in the establishments of higher education increased in all the major industrialized countries, more especially in connection with the catalytic reforming of hydrocarbons.

The first generation of bimetallic reforming catalysts possessed increased stability and enabled operating pressures to be decreased while maintaining times on line comparable to those for Pt catalysts at higher temperatures. The second generation of bimetallic and multimetallic catalysts gives further gains in selectivity while improving still further stability at low pressures. Current publications show that much relevant research continues world-wide, aimed at improving the activity, selectivity, and life of catalysts, and the efficiency of the processes as reflected in longer periods of use before re-activation (cycle times), less expensive recycling of hydrogen and hydrocarbons, and more effective re- activation of spent catalysts.

This Report will treat only those catalysts that contain at least two metallic elements in zero-valency states and their properties in the catalysis of reactions of hydrocarbons, with emphasis on industrial findings; other material is introduced only insofar as it is essential to an understanding of the main theme. The earlier sections outline the background information both to provide a framework and to minimize later interpolations. Readers are referred to an earlier review in this series.

2 The Metals

The large number of patents making claims for enhanced activity and selectivity in catalysis by combinations of two or more metals stems from empirical investigations, which seldom advance unequivocal evidence about the nature of the states in which the metals exist in situ. Current research shows clearly that such information is elusive where the concentrations of the metals in the catalyst and the sizes of their aggregates are small; the usual situation when at least one component is a precious metal. In this Report therefore a somewhat arbitrary selection has been made of catalysts which, in use, can reasonably be supposed to contain atoms of not less than two metals in their zero-valency state dispersed, alloyed, or juxtaposed in clusters.

Metals with Reducible Oxides. — The ubiquity of small concentrations of oxidants, especially water, adventitious or intrinsic, requires that the metals, if they are to be formed and to remain unoxidized throughout the life of the catalyst, must have oxides which are reducible in operation. Metals having oxides which are more or less readily reduced are shown in Table 1; the intrinsic semiconductors ('semi-metals') of Groups VB and VIB are also included. Reducibility increases with atomic number across the A-section but decreases in the B-sections of the long periods. Other oxides cannot be reduced to elemental metal under the conditions usually prevailing during the reactions of hydrocarbons, although it must be noted that some degree of reduction of even the very stable aluminium oxide (in the form of γ-alumina) has been claimed. The elements marked with an asterisk have a relatively volatile higher oxide. The table provides only a rough indication; detailed statements about the phases persisting under steady-state conditions require direct observation. The complications introduced into investigations of multicomponent catalysts formed...