Chapter 18. Titanium Silicalite-1
- Prof. S. David Jackson and
- Dr. Justin S. J. Hargreaves
Published Online: 26 MAR 2009
Copyright © 2009 Wiley-VCH Verlag GmbH & Co. KGaA
Metal Oxide Catalysis
How to Cite
Clerici, M. G. (2008) Titanium Silicalite-1, in Metal Oxide Catalysis (eds S. D. Jackson and J. S. J. Hargreaves), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. doi: 10.1002/9783527626113.ch18
University of Glasgow, Department of Chemistry, WestCHEM, Joseph Black Building, Glasgow, G12 8QQ, United Kingdom
- Published Online: 26 MAR 2009
- Published Print: 15 OCT 2008
Print ISBN: 9783527318155
Online ISBN: 9783527626113
- Alkane hydroxylation;
- Olefin oxidation;
- Aromatic hydroxylation;
- Propene oxide
Chapter 18 illustrates the catalytic properties of Titanium Silicalite-1 (TS-1), with a comprehensive picture of the oxidations performed and of industrial applications, a critical analysis of the problems implied by the use of hydrogen peroxide in petrochemical processes and of possible solutions, an in depth discussion of mechanistic aspects. The related properties of titanium silicalite-2 (TS-2), Ti-beta zeolites (Ti,Al-β, Ti-β), Ti-MWW (Ti-MCM-22) and Ti-mordenite (Ti-MOR) are sometimes considered for a comparison.
The variety of reactions comprises the oxidation of alkanes to sec- and t-alcohols, of arenes to phenols, of olefins to epoxides and of alcohols to carbonyl compounds. Unsaturated alcohols produce, through the oxidation of the double bond or of the alcohol group, various products depending on the substrate and reaction conditions. Different products can be obtained also by the oxidation of aliphatic and aromatic amines. The oxidation of ammonia in the presence of carbonyl compounds yields corresponding oximes. Sulfoxides and sulfones are the products of the oxidation of sulfides. Dilute solutions of hydrogen peroxide, mild temperatures, protic and polar solvents characterize all these reactions, with yields and selectivity often very high. Shape selectivity effects lead to unusual reactivity orders, particularly in the epoxidation of olefins.
Three industrial processes are currently commercial: the oxidation of phenol to hydroquinone and catechol (10000 ta−1), the ammoximation of cyclohexanone to cyclohexanone oxime (60000 ta−1), the epoxidation of propene or HPPO (300000 ta−1). Different solutions have been envisaged to circumvent the problem of the relatively high cost of hydrogen peroxide: direct synthesis from the elements, in situ generation, process integration. Direct synthesis in methanol is increasingly studied in view of the production of a dilute solution of hydrogen peroxide, to be directly used in the HPPO process.
A Ti-hydroperoxide, TiOOH, is formed by the chemisorption of hydrogen peroxide on Ti-sites. This can be the active species in a heterolytic mechanism, as in the oxidation of olefins and, possibly, of alcohols, or the precursor of a Ti-centred radical species, responsible of homolytic oxidations, as in the hydroxylation of alkanes and in the decomposition of hydrogen peroxide. The adsorption of a protic component of the solution affects the activity of active species. Heterolytic double bond epoxidation, homolytic CH hydroxylation and H2O2 decomposition, alcohol solvent oxidation are competitive processes in oxidations with Ti-zeolites. The direction of the oxidation path is determined by a combination of different factors: the nature of catalyst surface, the polarity of the solvent, the reactivity of the substrate, the temperature. On these grounds, a general oxidation mechanism is proposed for TS-1 and other Ti-zeolites.