Pillared interlayer clays

Pillared interlayer clays

284 8th International Congress on Catalysis As I write, in late August, the Berlin International Congress on Catalysis is over, together with all t...

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284 8th International

Congress

on Catalysis

As I write, in late August, the Berlin International Congress on Catalysis is over, together with all the satellite symposia and meetings. The first of the reports on the associated meetings which have been promised appear in this issue. Despite the fact that a report appeared in Applied Catalysis (Vol.1, Number 1) on the Tokyo Cogress, we do not plan to publish a similar report of the Berlin Congress as the organisers themselves plan to produce an overview of the meeting. This will be published together with the collection of discussion remarks due to appear as the sixth volume of the Proceedings. As many of our readers, as participants in the Congress, will automatically receive copies of this volume, it is felt that it is unneccessary to duplicate such a report. A different approach to reporting some of the highlights of the Congress is planned. At the meeting, (I was there, even though my name did not appear on the list of participants), I invited a number of people to make reports on papers or groups of papers which were of interest to them. A number of the contributions in this issue are in this category. We hope to include further such contributions in future issues. The Congress itseJf was an enormous organisational feat.The Organising Committee and DECHEMA are to congratulated on the smooth running of the event; they can hardly be blamed for the uncharacteristically cold weather which we all experienced. The International Congress Centre provided an excellent venue for the event, there being a feeling of space and lack of over-crowding. The organisers of the next Congress, to be held in Canada in 1988, have much to live up LO; we have already been promised that the social events may include a rodeo. The Joint Chairmen of the next Congress wil be Professors R.B. Anderson and H.W. Habgood. The new President of the International Committee is Professor K. Tamaru, the President-Elect is Professor J. Haber and the Vice-President is Professor W.M.H. Sachtler; the Treasurer is Professor F.S. Stone and the Secretary is Dr.Y. Amenomiya. J.R.H.

mm-m-

Volume

12 No.

2 -

October

Ross

1984

Pillared

Interlayer

Clays

At the 8th International Congress on Catalysis in Berlin, only two posters were presented on the subject of pillared interlayered clays (PILC). However, it was clear from the amount of discussion of these papers that this greatly underestimates the level of interest in these novel materials. It is well known that layer-type claq structures can be expanded by exposure to aqueous conditions. Cations between the layers can be exchanged and large interlayer spacings can be created by the introduction of bulky organic molecules. Such materials have many of the valuable characteristics of conventional zeolites, such as high acidity, with the further potential advantage of relatively large pore dimensions. Clays intercalated with organic molecules have poor thermal stability and are therefore unsatisfactory as catalysts for many important industrial processes. Recently, methods have been developed which allow the introduction of inorganic pillars between the clay layers. When hydroxv Al oligocations or hydroxy Zr cations are used, materials stable in air to over jOO°C are obtained. These PILC have interlayer distances of 1 nm or more and there exist even larger spaces between the pillars. polycyclic organic comConsequently, pounds which would he able to pass through the pores of a tonventional zeolite can penetrate inside a PILC. The much greater adsorption capacity of a PILC compared with a conventional zeolite was demonstrated by the work of Occelli et al. Molecular transport through a PILC was se:,eral orders of magnitude faster than through 5A, Y or T zeolites. Shabtal et al. reported on the shape selectivity of PILC for the cracking of bulky polycvclic naphthenoaromatics. For example, dodecahydrotriphenylene reacted about 1000 times faster over a PILC than over a conventional zeolite cracking catalyst. Jn addition to the work presented at the Congress, there is current interest in PILC containing different intercalating ions (Al, Zr, Si, Rh phosphines, etc.). Reactions currently being investigated include: proton catalysed reactions; cracking reactions; hydroisomerisation; alcohol dehydration; and synthesis gas reactions. Although PILC prepared with Al or Zr have moderate thermal stability, surf-

285

ace area is lost rapid18 if the temperature exceeds about 600 C. One of the major challenges in this area is to improve the thermal and, particularly, the hydrothermal stability of the PILC. There are conflicting reports on the relative merits of Al and Zr for this purpose. It has been claimed that Zr pillars are much less stable. However, this has not been our own experience; Zr PILC which are stable to above 600°C have been prepared in our laboratory. Much more research on pillared interlayer clays will be required before the problem of thermal stability is overcome. However, because of the growing interest in these materials, many new developments may be anticipated. R. Burch

The Current Status of Strong Metal-Support Interactions Strong metal-support interactions (SMSI) were postulated a few years ago to account for the observation that metals supported on reducible transition metal oxides developed unusual adsorptive and catalytic properties when the temperature of reduction was increased. It has become fashionable recently to explain away a whole range of quite different effects by invoking SMSI. Active phase-support interactions were the subject of one of the sessions at the 8th International Congress on Catalysis held in West Berlin from 2-6 July 1984. It had previously been establlshed that SMSI produces a very large decrease in activity for hydrogenolysis reactions (although this may vary from about one order of magnitude for Ru to many orders of magnitude for Rh, Pt, etc.), a small decrease in activity for hydrogenation and dehydrogenation reactions, and an enhancement of activity for the CO/H reaction. In papers presented at tI! e Congress, it was further demonstrated that titaniasupported Pd catalysts had enhanced specific activities for benzene hydrogenation (Vannice and Chou) and improved selectivity for alkadiene hydrogenation (Stadler et al.). Significantly, however, it was suggested in both cases that the changes in catalytic activity or selectivity were due

ae(llbd~Ia

-Volume 12 No.2 - October 1984

catalytic reaction rather than to a modification to the properties of the metal particles involvement of Tis+sj~;l:a;l;sd;l:~~t proposed to account for the enhanced activity of titania-supported catalysts reaction, even in the in the CO/H absence of 3MS1 effects (Burch et al.). While it is well established that transition metal oxide-supported catalysts have unusual properties, there is a continuing lively debate about the origin of SMSI. Two quite distinct models have been postulated. The first model proposed was that there exists electron transfer between the reduced oxide and the metal particle. This model is similar in many ways to the much earlier electronic models of Schwab and Solymosi and accounts for the change in catalytic properties by invoking a fundamental modification of the electronic properties of the metal particles. Several papers at the Congress still favour such an interaction (Goldwasser et al., WenTha et al., Conesa et al.). There are problems, however, with this explanation (see, for example, the discussion to the paper by Anderson et al.) since it has been pointed out that, because of charging effects, only a small amount of electron transfer should occur. A localised electronic interaction between the ions of the support and the metal atoms at the metal/support interface is not precluded. it is now thought Alternatively, that many of the so-called SMSI effects may be due to contamination of the surface of a metal particle by the oxide support. Evidence is accumulating that, under reducing conditions, oxide species can diffuse from the support onto the metal. Papers presented at the Congress by Haller et al. and by Kramer and Zuegg supported this migration model. It is now clear that SMSI includes a wide range of effects which should really be separated from each other and not be collected under the title of a single type of interaction. .At the very least, researchers need to be more careful to distinguish those effects which may be due to fundamental changes in the properties of a metal particle (geometric or electronic) from those which are merely caused bv surface contamination. Research on all types of "SMSI" will undoubtedly continue. The most product-