Zeolites acid catalysis

Crystalline aluminosilicates (zeolites) have pores with diameters of the order of 1 nm. The smallness and regularity of these pores account for shape-selectivity and many of the important applications of zeolites in acid catalysis. Zeolite frameworks consist of linked TO4 tetrahedra (T = Si, Al). The zeolites that have been most often investigated as supports for metal clusters are faujasites (zeolites X and Y), which have three-dimensional pore structures incorporating nearly spherical cages with diameters of about 1.2 nm connected by apertures that are 12-membered oxygen rings, with diameters of about 0.75 nm. Zeolite LTL, which is used as the support for industrial aromatization catalysts, has a two-dimensional pore structure consisting... 

In the area of heterogeneous acid catalysis, zeolites (microporous, crystalline aluminosilicates) have had a massive impact on the conversion of small molecules, usually at high temperatures, and their role in the conversion of crude oil to smaller, typically unsaturated molecules has been a cornerstone of chemistry for almost half a century. This relies on a combination of tunable (and potentially very strong) acidity, shape selectivity due to small and very uniform pores, and exceptional thermal stability, which allows regeneration to be carried out by burning off heavy byproducts. 

Some work has also been achieved with heterogeneous catalysis. These catalysts include Amberlyst-15, Nafion-H, montmorillonite KSF clay, ferrihydrite silica gel aerogels containing 11-13% iron, silica sulfuric acid, and zeolites. ... 

Zeolite and Lewis-acid catalysis in Diels-Alder reactions of isoprene [20b]... 

Catalysis zeolites possess acid sites that are catalytically active in many hydrocarbon reactions, as we shall discuss in Chapter 9. The pore system only allo vs molecules that are small enough to enter, hence it affects the selectivity of reactions by excluding both the participation and formation of molecules that are too large for the pores. 

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. 

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Most of the commercial zeolite catalyzed processes occur either through acid catalysis fluid catalytic cracking (FCC), aromatic alkylation, methanol to olefins (MTO),... 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

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. 

Boronat, M., Virrruela, M., and Corma, A. (2004) Reaction intermediates in acid catalysis by zeolites prediction of the relative tendency to form alkoxides or carbocations as a function of hydrocarbon nature and active site structure. 

Bhan, A. and Iglesia, E.A. (2008) link between reactivity and local structure in acid catalysis on zeolites. Acc. Chem. Res., 41 (4), 559-567. 

The efficient and selective catalysis of some Diels-Alder reactions by lanthanide P-diketonate complexes has been known since 1975 [226, 227]. The fluorinated p-diketonate complexes Ln(fod)3 (cf. Scheme 12.5) selectively catalyze the Danishefsky transformation (Scheme 12.23) as a consequence of their mild Lewis acidity. Importantly, zeolites and Lewis acid modified silica or alumina also catalyze Diels-Alder reactions [228-232]. 

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. 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

Microcrystalline solids such as zeolites and zeolite like structures have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as base catalysts. It has been shown [ref. 1,2] that zeolites have basic sites which are able to catalyze reactions needing weak and medium basic strengths. Moreover, a correlation between the basicity and the Sanderson s average electronegativity Df the framework has been observed [ref. 3], Then, their activity as base catalysts can be modified by changing the countercation [ref. 4], the framework Si/Al ratio, or by introducing atoms other than Si and Al in the framework [ref. 5],... 

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,... 

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. 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

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