Acid catalysis, with zeolites

Martens, J.A. and Jacobs, P.A. (2001) Introduction to acid catalysis with zeolites in hydrocarbon reactions, in Introduction to Zeolite Science and Practice, 2nd edn (eds H. Van Bekkum, E.M. Flanigen, P.A. Jacobs, and J.C. Jensen), Stud. Surf Sci. Catal., vol. 137, Elsevier, Amsterdam, pp. 525-577. 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

While zeolites are mostly used in acid catalysts, there are various procedures to introduce basic sites with variable strength into these materials. Depending on the nature of the active site, one is able to selectively catalyze reactions with different basicity requirements, and this is probably the main virtue of base catalysis with zeolites. For instance, in a classical Knoevenagel condensation, the reaction selectivity can be decreased by a consecutive Michael reaction, since the Knoevenagel product can serve itself as a Michael receptor -. 

The catalytic addition of H2S to olefins follows the Markovnikov rule, with the -SH moeity attaching itself to the carbon atom connected to the least number of hydrogen atoms. With linear olefins, having terminal or internal double bonds, acid catalysis using zeolites, cation-exchange resins, and silica-alumina yields secondary mercaptans. The preferred route for secondary mercaptans is the H2S addition to olefinic compounds. 

We have seen that it is possible to design active sites with acidic or basic strength better adapted to the needs of a particular reaction. In comparison with progress made in acidic or basic catalysis with zeolites, redox catalysis with zeolites is in its infancy. We are just now understanding that properties such as zeolite hydrophobicity, pore dimensions, and active site location are determinant factors for this type of catalyst. Several reviews are recommended for those interested in organic chemistry [111-113]. 

With respect to applications, there will certainly be more and more investigations where ordered mesoporous materials are used as catalysts or catalyst supports. However, the more skeptical note of the section on catalysis shall be repeated here In many cases, much cheaper and simpler alternatives exist, and the properties of ordered mesoporous materials are not so much superior to justify the higher effort of their synthesis. On a longer time scale, non-siliceous compositions will probably be used more frequently in catalysis. If one analyzes the catalytic processes implemented today, the majority is not based on silica as catalyst or support, and the single most important area of aluminumsilicates, acid catalysis by zeolites, seems to be less attractive for ordered mesoporous aluminumsilicates, unless a crystallization of the walls to zeolitic structures or the assembly of such materials from colloidal zeolites to enhance the acid strenght becomes possible. 

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

Fejes, P. Kiricsi, I. Hannus, I. Tihanyi, T. Kiss, A., Poisoning of acidic centres in zeolites with sodium azide, Imelik, B. Naccache, C. Taarit, Y. Ben Vedrine, J.C. Coudrier, G Praliaud, H., Eds., Catalysis by Zeolites, Studies in Surface Science and Catalysis 5, Elsevier Sci. Publ. Co. Amsterdam, 1980, pp. 135-140. 

As was shown here in some examples, the field of catalysis over zeolites, although marnre, is still very much alive. The chemists who work with the synthesis zeolites continue to be very creative, the focus now being placed on the synthesis of materials that can catalyze reactions other than the acidic ones and/or reactions of bulkier molecules, that is, synthesis of zeolites with larger micropores or with a very large external surface, such as nanosize and delaminated zeolites. New concepts related to the mode of action of zeolite catalysts continue to emerge, as shown here with the shape selectivity of the external surface. These concepts are particularly useful to scientifically design selective and stable catalysts. 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

By avoiding the acid catalysis mechanism of the conventional FCC zeolite catalyst (optimized over the years for high octane gasoline), the novel MAB catalyst will produce substantially lower aromatics in the liquid products than is possible by less extreme FCC catalyst adaptations. By changing the FCC reaction system, it is possible to overcome the MAB catalyst low activity drawback and achieve slurry yields compatible with those observed in maximum distillate operation in today s FCC units. 

There has been a phenomenal growth of interest in theoretical simulations over the past decade. The concomitant advances made in computing power and software development have changed the way that computational chemistry research is undertaken. No longer is it the exclusive realm of specialized theoreticians and supercomputers rather, computational chemistry is now accessible via user-friendly programs on moderately priced workstations. State-of-the-art calculations on the fastest, massively parallel machines are continually enlarging the scope of what is possible with these methods. These reasons, coupled with the continuing importance of solid acid catalysis within the world s petrochemical and petroleum industries, make it timely to review recent work on the theoretical study of zeolite catalysis. 

As an example of hybridization of zeolites with cellulose derivatives, self-supporting zeolite membranes with a sponge-like architecture and zeolite microtubes were prepared by using CA filter membranes as a template [154]. The hierarchical structure with sub-nanometer- to micrometer-sized pores is a characteristic of great promise for a wide range of applications such as catalysis, adsorption, and separation. There was also an attempt to prepare alginate membranes incorporated with zeolites, e.g., for pervaporation separation of water/acetic acid mixtures [155]. 

A widely studied example of this kind is the synthesis of methyl isobutyl ketone (MIBK, used as a solvent for inks and lacquers) from acetone. The former was previously prepared from the latter through a catalytic three-step process base-catalysed production of 4-hydroxy-4-methylpentan-2-one, acid dehydration into mesityloxide (MO), then hydrogenation of MO on a Pd catalyst. Since acetone aldolization occurs through acid catalysis as shown over a H-MFI zeolite at 433 K (MO is the main reaction product, the aldolization product being rapidly dehy-drated[5]), it is possible, by associating with the acid catalyst a hydrogenation phase,... 

Obviously, it is very desirable to substitute these modes of benzylic ether preparation by an heterogeneous catalysis process. Clays (50) and resins (51, 52) which were the first solid acid catalysts used have given low or moderate yields. The first experiments with zeolites were carried out by Rhodia (53, 54) on the etherification of vanillic alcohol (A) in a batch reactor over a HBEA zeolite with a Si/Al ratio of 12.5 ... 

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