Using kinetics to study intermediates in acid catalyzed reactions on solids

Abstract

Careful observation and assiduous data collection in kinetic studies can point to the nature of the surface intermediates in a reaction. It can also result in a kinetic "titration" of the active sites, as demonstrated in the study of 1-hexene and cumene kinetics. Furthermore, information on the location of active sites inside the pore structure of a zeolite can be obtained by this means as was shown in paraffin cracking on HZSM-5. These are important and interesting properties which cannot be ascertained by alternative means. Methodology for this kind of work is simple but requires a laborious experimental program. On the other hand, no other experimental technique will yield data which can lead to a quantitative description of reaction kinetics and of selectivity under practical reaction conditions while offering an opportunity to formulate a mechanistic interpretation of the underlying phenonmena. Work to date confirms that carbocations are the dominant species in reactions on acid catalysts. Free radical reactions may be important in coke dehydrogenation, but very little systematic work has been done in this area. Carbocations are formed on Bronsted sites by proton addition to unsaturated bonds in olefins or aromatics. They may also form by two less well appreciated processes: proton addition to form carbonium ions at Bronsted sites and hydride abstraction to form carbenium ions at Lewis sites. Evidence from the cracking of olefins, paraffins and cycloparaffins indicates that, although all involve the formation of carbocations, the ions are different in each case and lead to initial products which are characteristic of the parent molecular species. This suggests that each parent molecule reacts in a specific manner on specific sites. Evidence for the existance of specialized sites and hence specific intermediates comes also from 1-hexene isomerization and from cumene cracking. At this point in the development of our understanding of acid catalysis and its intermediates we are faced with a rather complex picture of catalyst interaction with hydrocarbons. Both Bronsted and Lewis sites seem to be active. Furthermore, site acid strength governs the initiation specificity and conversion selectivity. The picture is further complicated by the steric configuration surrounding a site and by the steric restrictions of the pore structure of the solid catalyst. Taking only two, three or four alternatives in each of the above categories, it is easy to calculate that there may be as many as one hundred different site specific combinations possible! Rather than giving up systematization and attempting to write a catalogue of all possible observations it seems wise to formulate a list of important site characteristics. For example: 1. How many types of acids are present on the catalyst as shown by kinetic phenomenona ? 2. What are their acid strength distributions as shown by acid-base titration and kinetics ? 3. What are their relative abundances as shown by acid-base titration and kinetics? 4. What are the steric restrictions of the catalyst regarding feed molecule entry into the interior of the pore system as shown by mechanistic studies? 5. What is the "shape" of the interior surface as reflected by cis/trans isomerization of olefins or other pertinent reactions? 6. What is the site density on the catalyst as reflected by sterically demanding reactions such as cyclization? 7. What is the hydrogen transfer potential of the catalyst as reflected in coke make, aromatization, etc.? 8. What are the decay characteristics of the catalyst? Is the decay selective for one type of site? All these questions and more can be answered by studying reaction intermediates by techniques described above. Even then the task of fully characterizing catalytic properties is very large and will require that each piece of the puzzle be carefully fitted to the rest. "Quick and dirty" experimentation must be avoided in making a contribution to this enterprise.