Molecular-sieve catalysts zeolites

A major step in catalyst development was the introduction of crystalline zeolitic, or molecular sieve catalysts. Their activity is very high, some of the active sites being estimated at 10,000 times the effectiveness of amorphous silica-... 

Chemical prqperties are also used in the largest field of application for the rare earth elements as catalysts. Most important are the cracking catalysts for the petroleum industry. The rare earth elements are combined into molecular sieves (Y-Zeolite) and serve in fluid bed or fixed bed reactors to increase the yield of gasoline. In addition thereto, there are the combustion catalysts for automobiles and for air pollution control. 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

Some zeolitic and non-zeolitic molecular sieve catalysts are claimed to be capable for ortho- and para-selective alkylation using olefin as alkylating agent (refs. 1,2). Zeolite catalysts are less active and selective in the methylation of aniline by methanol (refs. 3,4). Reaction is usually carried out with a large excess of methanol since a large fraction of the alcohol decomposes without participating in the alkylation. Numerous N- and C-alkylated aniline derivatives appear in the reaction product. It was found that N-alkylation requires basic sites while C-alkylation occurs mainly on acidic sites (refs. 5-7). 

Figure 2.2 Fixed bed reactor, (a) Scheme of a plug flow reactor, (b) Scheme of a flow type unit with a fixed bed reactor for studying a liquid-phase reaction on zeolite or mesoporous molecular sieve catalyst, (hi) Catalyst pretreatment F, flowmeter D, dessiccant H, oven R, pretreatment reactor K, catalyst N, inert material T, thermocouple. (b2) Reaction R, thermostated glass reactor H, oven S, syringe C, cooling system T, thermocouple and thermostat K, catalyst N, inert. Adapted from Richard et al.

Competition between reactant, solvent and product molecules for adsorption within the zeolite micropores is demonstrated directly (adsorption experiments) and indirectly (effect of the framework Si/Al ratio on the activity, kinetic studies) to occur during Fine Chemical synthesis over molecular sieve catalysts. This competition, which is specific for molecular sieves (because of confinement effects within their micropores), adds up to the competition which exists over any catalyst for the chemisorption of reactant, solvent and product molecules on the active sites. Both types of competition could affect significantly the activity, stability and selectivity of the zeolite catalysts. Although the relative contributions of these two types of competition cannot be estimated, the large change in the activity of the acidic sites (TOF) with the zeolite polarity seems to indicate that the competition for adsorption within the zeolite micropores often plays the major role. 

In general the selectivity in toluene methylation found with MFI molecular sieve catalysts is proposed to be caused either by a restricted transition state to form m- and o-xylene [10,11,12] and/or diffusional constraints of the bulkier isomers, o- and m-xylene, in the pores of zeolite ZSM5 [3,4], Recent results on the methylation of toluene based on in situ analysis of the working catalyst showed that all three isomers were primary products in toluene methylation [13]. The high p-selectivity was explained to be due to transport constraints of the bulkier... 

Molecular sieve catalysts that have been used for the conversion of methanol to hydrocarbons fall into two general classifications. Most of the initial research was done using ZSM-5 (MFI), a medium-pore size zeolite with a three dimensional pore system consisting of straight (5.6 x 5.3 A) and sinusoidal channels (5.5 x 5.1 A). While most of this work was directed at the conversion of methanol to liquid hydrocarbons for addition to gasoline, it was found that the product slate could be shifted toward light olefins by the use of low pressure and short contact times. 

The use of molecular sieve catalysts has also become more widespread in the past decade for the production and inter-conversion of olefins from feedstocks other than oxygenates. The addition of a modified ZSM-5 additive to the Y zeolite-based catalyst can substantially increase the amount of propylene produced in a conventional Fluid Catalytic Cracking (FCC) unit. This has become a very valuable modification, particularly in areas where propylene supplies are tight. More recently, a number of processes have been announced for the direct cracking of C4+ olefinic steams to propylene. These processes also use modified ZSM-5 based... 

Two sorts of catalyst have been widely applied in plastics pyrolysis [85], namely molecular sieve catalyst or reformed molecnlar sieve catalyst, such as Y-zeoUte and REY zeolite metal oxide catalyst, snch as silica-alumina, AI2O3, CuO, ZnO, Fe203, cerium oxide and Co-Mo oxide. 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

Aluminosilicates (zeolites) are widely used as acidic and bifunctional catalysts. The formation of carbocationic intermediates is generally ascribed to the protons present in the open zeolite structure. A newer class molecular-sieve catalyst is the aluminophosphates. These may contain sihcon (SAPO) or metal (MeAPO) in their AiP04 frameworks. These framework substitutions in several cases generate protonic acidity that makes SAPO and MeAPO acid catalysts. There is already an extensive literature of this subject (20, 21). 

Michels, P. and De Hcrdt, O. C. E. 1987. Molecular Sieve Catalysts. EPO Applied Technology Series - Volume 9, Pergamon Infoline Inc., Pergatnon Press, Oxford. Chen, N. Y., Degnan. Ir., Th. F.. Smith, C. M. 1994. Molecular Transport and Reaction in Zeolites, VCH Verlagsgesellschaft, Weinheim. 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

Olefins can be produced from methanol, in varying concentrations, over many catalysts. Again, the patent literature is extensive. The most promising ones seem to be zeolites and other molecular sieve catalysts. Some of the most prominent ones are listed in Table 1. Experimental data for many of these catalysts were summarized before (refs. 8-10). MTO catalysts can be divided into two major groups "small pore", such as erionite, and ZSM-34, and "medium pore", such as ZSM-5. Various combinations and modifications of catalysts are also known. A particularly interesting example is phosphorous modified ZSM-5 (ref. 11). ... 

The synthesis of olefins from methanol using aluminophosphate molecular sieve catalysts was studied [76], Process studies were conducted in a fluid-ized-bed bench-scale pilot plant unit utilizing small-pore silicaluminophosph-ate catalyst synthesized at Union Carbide. These catalysts are particularly effective in the catalytic conversion of methanol to olefins, when compared to the performance of conventional aluminosilicate zeolites. The process exhibited excellent selectivities toward ethylene and propylene, which could be varied considerably. Over 50 wt% of ethylene and 50 wt% propylene were synthesized on the same catalyst, using different combinations of temperatures and pressures. These selectivities were obtained at 100% conversion of methanol. Targeting light olefins in general, a selectivity of over 95% C2-C4 olefins was obtained. The catalyst exhibited steady performance and unaltered... 

In Chapter 1, Fyfe, Mueller, and Kokotailo describe the applications of solid-state NMR to the study of zeolite molecular sieve catalysts and related systems. Zeolites provide an apt arena in which to demonstrate the capabilities of modern techniques such as sample spinning, cross-polarization, and multidimensional correlation spectroscopy. In Chapter 2, Karger, and Pfeifer consider the question of molecular diffusion in catalyst systems. Applications of NMR techniques such as imaging, lineshape analysis, relaxation, pulsed field gradient echo spectroscopy, and NMR tracer exchange are described and compared with other, more traditional techniques such as radioactive tracing. In Chapter 3, Haw discusses the use of NMR to probe catalytic processes, showing how the combination of temperature control with novel NMR probes makes it possible to elucidate reaction mechanisms in situ. 

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