During the thirty years of research that we have dedicated to catalysis, we have been working on a concept whose validity seems intuitively evident, but one which only solid scientific proof could confirm as fact :
Initially our sole objective was to prove a connection between two sciences, which seem quite distinct: homogeneous catalysis (mostly concerned with molecular organometallic chemistry) and heterogeneous catalysis (mostly concerned with surfaces science).
Over the years, we have been able to measure the considerable progress, which has been made in homogenous catalysis as a result of our ever-increasing level of understanding of molecular chemistry, notably organometallic chemistry. At the same time, however, we were forced to admit that our knowledge of heterogeneous catalysis was growing at a much slower pace. Clearly, Physico-chemical uses of surface science were increasing, but they were intrinsically unable to account for either a divided heterogeneous surface with weakly concentrated “active sites” (a concept introduced in 1927 by Sir H.S.Taylor), nor – especially – the quasi-molecular character of the catalytic process. This means that few conventional heterogeneous catalytic reactions are currently actually included at the mechanical level (oxidation of CO, ammonia synthesis).
In order to fill this kind of gap between homogeneous and heterogeneous catalysis, we have developed a new field of investigation that we called “Surface organometallic chemistry”.
This new discipline in chemistry involves studying the reactivity of all the organometallic molecules (main group metals, transitional metals, lanthanides and actinides) with surfaces of all materials produced from solid-state chemistry, especially of oxides and metals. Before us, there has not been much exploration of this branch of chemistry because of the lack of experimental tools which avoided us writing cartoons for the description of this new chemistry.
Soon, it has been possible to shed light on an important number of new surface structures. Yet however much interest it might generate, developing a new chemistry has not been the only objective of our work.
At the beginning our research had several objectives, aiming to address several questions:
It soon became apparent that the reply to this question was positive. The atoms or functional groups that can be found on the surface of metals and indeed oxides have a reactivity approaching that of the model molecules of molecular chemistry. Of course we needed to include a certain number of parameters linked to the presence of a crystalline network.
In order to answer this question we had to develop the specific experimental tools adapted to air sensitive compounds on surfaces (in situ EXAFS, in situ IR, Raman, XPS, modeling and above all in situ solid State NMR). As soon as the tools could be adapted to our goals, it appeared that the answer to this question has also been positive. The surface plays the role of a ligand with the grafted metal in such a way that certain rules of the basis of molecular chemistry such as electron count, electronic configuration etc can be applied to the surface organometallic fragments. Nevertheless, surface rigidity (one of the particular characteristics of this chemistry) compelled the inclusion of several new parameters. Unlike the case of molecular chemistry, this rigidity permits the stabilization of highly unsaturated electronic entities – for example some types of highly reactive 8-electron metal hydrides, stable in certain cases up to 400C.
Here too, it has become very quickly clear that the mode of action of heterogeneous catalysts can be modeled by organometallic surface chemistry and rationalized using certain steps from molecular chemistry. Most elementary steps of molecular chemistry are valid in heterogeneous catalysis: alpha-H, beta-H, gamma-H, alpha- alkyl, beta-alkyl, insertion, oxative addition, reductive elimination etc…
The ultimate goal of such a strategy has been to use novel concepts to construct active sites of the uniform size, distribution and composition necessary for heterogeneous catalysis using true molecular architecture. We call these sites “single sites”.
Reaching these objectives has been a two-part process:
First of all, it was possible to construct generally much better-defined active sites on oxide or metallic surfaces (single site). This resulted in new generations of oxides (metathesis of olefins, polymerization of olefins etc..) or metals (nanoparticles of alloys for alkane dehydrogenation, ester hydrogenolysis). The first well identified metallocarbene on a surface (ºSi-O)Re(CH2tBu)(=CHtBu)(ºCtBu) was able to achieve the metathesis of olefins. Here also we find a highly active and selective catalyst.
Secondly, it has been possible to find, with these single site catalysts, new catalytic reactions, which used not to exist in either homogeneous catalysis or heterogeneous catalysis. We have thus discovered:
The “Metathesis of alkanes” that transforms most acyclic paraffins into their higher and lower homologues, a new reaction that results from the cleavage and selective re-formation of a bond which is hard to activate in chemistry: the C-C bond of paraffins. The great electron deficiency of surface sites (ºSi-O)2Ta-H (an 8 electron surface structure which is far from the stable 18 electron species of coordination chemistry) allowed this very new cleavage of C-C bonds of paraffins. Metathesis of olefins was known for almost 40 years (!) but the challenge of achieving a similar kind of cleavage with paraffins was unexpected. One could apply to this reaction the same “dance approach” of paraffins around the metal as that involved in olefin metathesis.
The “Cleavage of alkanes by methane” a novel reaction linked to the activation of a C-H bond which is even more difficult to activate – that of methane – followed by a series of concerted pathways. This catalytic reaction, a recent discovery, opens up some interesting perspectives in the area of the use of natural gas. If this reaction can be improved in terms of activity, it may open the possibility of incorporating natural gas in petroleum.
The “Transformation of polyethylene to diesel range gasoline” that we also called “Ziegler-Natta Depolymerization” with (ºSi-O)3Zr-H (which transforms polyethylene or polypropylene in car fuel under in very mild conditions). This reaction obeys just the reverse elementary step of that of “Ziegler Natta” polymerization: the beta- alkyl transfer that constitutes the microscopic reverse of the insertion of an olefin into a metal-alkyl bond. The impact of such a reaction on certain environmental problems is potentially significant.
The “Non oxidative coupling of methane to ethane and hydrogen” which allows forming a C-C bond from methane alone. Again this new reaction, if optimized in terms of turnover numbers, could be of great interest in the field of energy and the better use of fossil fuels.
The “Direct transformation of light alkanes to gasoline” which represents another facet of the metathesis of alkanes when the right experimental conditions.
The “Direct transformation of ethylene to propylene” a very simple new reaction in which a tungsten tris-hydride on alumina can transform catalytically 3 moles of ethylene into 2 moles of propylene. This new reaction has revealed to us that a surface organomatellic fragment can achieve 3 catalytic reactions at the same time: in his particular case it may achieve 3 consecutive reactions of dimerisation, isomerisation amd metathesis. A new concept of tris-functional single sites was proposed.
The “Direct activation of dinitrogen on a single metal atom”. The concept of activation of dinitrogen on defect sites of surface iron was nicely demonstrated by G. Ertl. However this concept could be revisited on highly electron deficient (ºSi-O)2Ta-H. In fact we found possible to activate dinitrogen on a single metal atom in the presence of hydrogen, showing that it is not necessary to use surface to accommodate the fragments derived from di-nitrogen in fact (ºSi-O)2Ta(=NH)(NH2).
The “metathesis of Imines on well-defined supported Zr imido complexes”
The “iso-metathesis of cycloalkanes with supported W pentamethyl “
The “Surface Organometallic Chemistry” on metals constitutes another part of this discipline. On this occasion it means studying the way in which an organometallic molecule reacts with the surface of a zero-valent metal, whether supported or not. When a metallic particle of a zero-valent metal is deposited on an oxide support, the organometallic complex reacts selectively with the metal particle and not with the oxide support. Initially, we studied the basic rules that govern the reactivity of organometallics with metal surfaces.These studies showed that hydrogenolysis happens in stages and that it was possible to stabilize organometallic fragments of tin on metal surfaces with one lone alkyl group before these change into “adatoms” and then surface alloys.
These studies have been generalized to other metals such as As, Ge, In, Cd. The concept has been applied immediately to some environmental problems. But probably the most interesting is the selective removal of Arsenic from water which is a real problem in some countries like Bengladesh. With the concept of surface organometallic chemistry on metals like Nickel, we can now lower the level of Arsenic in water to the ppb level.
This discovery of the exceptional reactivity of organometallics with metal surfaces has also opened up new perspectives on heterogeneous catalysis on metals as it allows the adjustment of certain problems of chemo-,regio- or enantio-selectivity.
Long ignored, but fundamental in essence, Surface Organometallic Chemistry, responds to a deep need for the rationalization of heterogeneous catalysis. This discipline today arouses great interest in the industrial world confronted to huge problems of energy, environment and sustainability along with strong economic developments.
Since we are in Saudi Arabia, beside the construction of a new catalytic lab, extermly performing we have been able to move from Surface Organo Metallic Chemistry to Design of Single Site Heterogeneous Catalysts and we have now the key to use the concept of Surface Organometallic Fragments to discover new reaction or to rationalize heterogeneous catalysis via our concepts.
The sketch below indicates how we can rationalize or predict catalytic performances via our new concept of SOMF as a way to enter a catalytic cycle.
But we have also extended the ability of our concepts to design new catalysts for CH bond activation and functionalization or CO2 functionalisation as described below: