Molecular-sieve catalysts

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-... 

A well-known example of the latter type is titanium silicalite-1 (TS-1), a redox molecular sieve catalyst [7]. 

Kumar, R., Garces, L.J., Son, Y., Suib, S.L. and Malz, R.E. (2005) Manganese oxide octahedral molecular sieve catalysts for synthesis of 2-aminodiphenylamine. Journal of Catalysis, 236, 387-391. Opembe, N.N., Son, Y., Sriskandakumar, T. and Suib, S.L. (2008) Kinetics and mechanism of 9H-fluorene oxidation catalyzed by manganese oxide octahedral molecular sieves. ChemSusChem, 1, 182-185. 

K. Lourvanij and G. L. Rorrer, Reaction rates for the partial dehydration of glucose to organic acids in solid-acid, molecular-sieving catalyst powders,... 

MTO [Methanol to olefins] A catalytic process for converting methanol to olefins, mainly propylenes and butenes. Developed by Mobil Research Development Corporation and first demonstrated in 1985. Another version of this process was developed by UOP and Norsk Hydro and has been ran at a demonstration unit at Porsgrunn, Norway, since June 1995. It is based on fluidized bed technology using a SAPO molecular sieve catalyst. It converts 80 percent of the carbon in the feed to ethylene and propylene. 

J. M. Thomas, R. Raja, G. Sankar, and R. G. Bell, Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen. Nature 398,227 (1999) J. M. Thomas, Designing a molecular sieve catalyst for the aerial oxidation of n-hexane to adipic acid, Angew. Chem. Int. 

Trimm, D. L. and B. J. Cooper. 1970. The preparation of selective carbon molecular sieve catalysts. Chem. Commun. No, 8 477-78. 

Chang, Y Martens, L.R., and Vaughn, S.N. (2005) Molecular sieve catalyst composition, its making and use in conversion processes. US Patent 7,122,500. 

Klinowski, J. (1991) Solid-state NMR studies of molecular sieve catalysts. Chem. Rev., 91,1459-1479. 

Other than a few fine chemicals reactions, cis-trans isomerization is seldom practiced using molecular sieve catalysts. 

Lewis, J.M.O. (1988) Methanol to olefins process using silicoalumino-phosphate molecular sieve catalysts, in Catalysis 1987 (ed. J.W. Ward), Elsevier, Amsterdam, p. 199. 

Rabo, J.A., Pickert, P.E., Stamires, D.N., and Boyle, J.E. (1960) Molecular sieve catalysts in hydrocarbon reactions, in Proceedings of the Second International Congress on Catalysis, Ed. Tech, Paris, p. 2055. 

SCHEME 157. Synthesis of heterocyclic A-oxides over molecular sieve catalysts... 

The shape-selectivity of ZSM-5 is particularly remarkable. Active centres at the inner walls of the catalyst readily release protons to organic reactant molecules forming carbonium ions, which in turn, through loss of water and a succession of C—C forming steps, yield a mixture of hydrocarbons that is similar to gasoline. The feedstock can be methanol, ethanol, corn oil or jojoba oil. The shape-selectivity of this catalyst is particularly striking, as can be seen from the product distribution obtained for the dehydration of three different alcohols (Table 8.2). The product distribution can be understood in terms of the intermediate pore size of ZSM-5, which can accommodate linear alkanes and isoalkanes as well as monocyclic aromatic hydrocarbons smaller than 1, 3, 5-trimethyl benzene. In Table 8.3, we list some of the recent innovations in catalysis, to highlight the important place occupied by molecular-sieve catalysts. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

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). 

Zamaraev and Thomas provide a concise summary of work done with a family of classic catalytic test reactions—dehydration of butyl alcohols—to probe the workings of acidic molecular sieve catalysts. This chapter echoes some of the themes stated by Pines and Manassen, who wrote about alcohol dehydration reactions catalyzed by solid acids in the 1966 volume of Advances in Catalysis. 

Li Y, X-q Zhao, Y-j Wang (2005) Synthesis of dimethyl carbonate from methanol, propylene oxide and carbon dioxide over KOH/4A molecular sieve catalyst. Appl Catal A-Gen 279(1-2) 205-208... 

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... 

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