Molecular sieve as catalyst

The aluminophosphate molecular sieves have an interesting property for potential use as catalyst supports, due to their excellent thermal stabilities and unique structures. AIPO4-5 is known to retain its structure after calcination at 1000°C and have uni-directional channels with pore size of 8 A bounded by 12-membered rings [2]. To utilize molecular sieves as catalyst support, chemical interactions between the molecular sieve and active component, chemical stabilities, and surface structures must be determined. However, iittle attempt has been made to clarify the surface structures or properties of catalytically active components supported on the aluminophosphate molecular sieves. 

IRON-PHTHALLOCYANINES ENCAGED IN ZEOLITE Y AND VPI-5 MOLECULAR SIEVE AS CATALYSTS FOR THE OXYFUNCTIONALIZATION OF n-ALKANES... 

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

Carvalho WA, Wallau M, Schuchardt U. Iron and copper immobilised on mesoporous MCM-41 molecular sieves as catalysts for the oxidation of cyclohexane. Journal of Molecular Catalysis A Chemical. 1999 144(l) 91-99. 

Recently, the use of medium-pore molecular sieves as catalysts for the isomerization of n-butenes has aUowed the development of new famihes of catalysts that are highly efficient for this reaction. Several patents claiming the use of zeolitic and nonzeohtic molecular sieves for the skeletal isomerization of C4—C5 olefins have been published the catalysts are ferrierite (5, 6) and the nonzeolitic molecular sieves MeAPO and MeAPSO (7). Since the appearance of the patents, important scientific contributions have appeared in the literature. 

The use of molecular sieves as catalysts or catalyst components for synthesis of intermediates and fine chemical has increased impressively over the last two decades. A large number of reactions has been explored over a growing number of microporous materials. Also the level of imderstanding of the catalytic chemistry and the structure-activity relationships has greatly improved. Since the first review of Venuto and Landis in 1968 [1] and the one of Venuto in 1994 [2] the discovery of medium pore zeolites such as ZSM-5 [3] and of phosphate based molecular sieves [4] had the largest impact on the field. 

The growth in the use of molecular sieves as catalysts as compared with macro- and mesoporous oxides was stimulated by several factors (i) The high concentration of active sites (in comparison with oxides) results in very active catalysts, (ii) The defined pore structure allows to exclude reactants from being converted and/or products to be formed or transported out of the pores due to a too large size, (iii) The active site and the environment of that site can be designed on an atomic level for exEimple by ion exchange [9] or chemical functionalization of the framework [10]. (iv) It is possible to tailor the chemical properties of molecular sieves better than those of conventional macro and mesoporous oxides. 

The examples discussed demonstrate that the utilization of molecular sieves as catalysts in fine chemical and intermediates synthesis has been advanced dramatically over the last decades. The community has reached a sound level of understanding of the catalytic chemistry and the options to manipulate it While also the limitations of molecular sieves are now well understood there is still a large number of molecular sieves to be explored Some trends observed over the last years are noted below. 

Metal-substituted Molecular Sieves as Catalysts for Allylic and Benzylic Oxidations... 

Gunnewegh, E.A., Gopie, S. S., and van Bekkum, H. 1996. MCM-41 type molecular sieves as catalysts for the Friedel-Crafts acylation of 2-methoxy-naphthalene. /. Mol. Catal. A Chem. 106 151-158. 

The use of molecular sieves as catalysts for the production of chemicals and fine chemicals has increased over the last two decades (Holderich and van Bekkum 1991, Venuto 1994, 1997, Feast and Lercher 1996, Corma and Garcia 1997). In this way, rare-earth-containing microporous materials appear to be interesting catalysts in fine chemical reactions. This is the case for the synthesis of ketone acetals. Thus, Ce-Y, La-Y and R-Y zeolites are very effective catalysts in the synthesis of cyclohexanone diethyl acetal (Wortel et al. 1977). However, the results obtained in acylation and carbonyl condensation reactions are particularly relevant. 

P-12 - Nb- and Ti-containing silica-based mesoporous molecular sieves as catalysts for photocatalytic oxidation of methane... 

Camblor, M.A., A. Corma, P. Esteve, A. Martinez, and S. Valencia, Epoxidation of Unsaturated Fatty Esters Over Large-Pore Ti-Containing Molecular Sieves as Catalysts Important Role of the Hydrophobic-Hydrophilic Properties of the Molecular Sieve, J. Chem. Soc. Chem. Commun., 795-796 (1997). 

Reaction of indoles with glycosyl halides in the presence of silver oxide and molecular sieve as catalysts gave orthoamides of the type shown in (35) and indole nucleosides (see Chapter 19). ... 

Cheng, S., Tsai, T., Chou, B., et al. (2002). Synthesis of TMBQ with Transition Metal-containing Molecular Sieve as Catalyst, US Patent US 143198. 

Held., A. and Florczak, B. (2009). Vanadium, niobium and tantalum modified mesoporous molecular sieves as catalysts for propene epoxidation, Catal. Today, 142, pp. 329-334. 

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