The influence of Si/Al and Si/Ti molar ratios of different [Ti, Al]-beta catalysts in the partial oxidation of cyclohexene with hydrogen peroxide

The catalytic activity, epoxide selectivity and hydrogen peroxide conversion and efficiency using [Ti, Al]-beta catalysts with different Ti and Al contents have been evaluated in the epoxidation of cyclohexene at 70 °C with hydrogen peroxide and acetonitrile as the oxidant and the solvent, respectively. The catalysts tested were obtained by two methods: (1) varying the alkalinity in the initial gel (Method I), and (2) substituting part of the template agent with equivalent quantities of fluoride ions in the mother gel (Method II). Characterizations were used to examine the chemical composition, crystallinity, framework Ti and Brønsted acidity in the catalysts. The catalysts crystallized by Method II hold both less Ti and Al and showed better performances in the cyclohexene epoxidation. Catalysts obtained from more alkaline gels and crystallized for shorter times by Method I contained less Ti but more Al. These catalysts showed both lower epoxide selectivity and hydrogen peroxide efficiency. For catalysts with similar Al, but different Ti contents, those richer in Ti showed higher activity and epoxide selectivity. Catalysts with both higher Ti (Si/Ti: 50–60) and Al (Si/Al: 36–56) contents, obtained from more alkaline gels for more prolonged crystallization times by Method I showed the lowest activities, epoxide selectivities and peroxide efficiencies. The dramatic influence of the Al and Ti contents in catalysts on the epoxide yields and peroxide efficiencies were explained through the intrinsic properties of the Ti-hydroperoxo complex formed, responsible for the oxygen transfer to olefin on Ti centers having different Lewis acid strengths.

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► The catalysts TB042132F and TB042180F, obtained from gels containing F- ions (Method II) with higher Si/Al and Si/Ti molar ratios, showed higher activity (TOF), epoxide selectivity and oxidant efficiency.
► For two catalysts with similar Ti contents, (Si/Ti> 40), but different Si/Al ratios, those with lower Si/Al ratio showed both lower epoxide selectivity and peroxide efficiency. For two catalysts with comparable and relatively higher Si/Al ratios, but different Ti contents, those with less Ti showed higher activity and epoxide selectivity.
► Catalyst TB040108, with low Si/Ti and relatively low Si/Al ratios, showed the lowest activity, epoxide selectivity and H2O2 efficiency. Also, catalyst TB040156 containing extra-framework Ti, showed poor activity in the epoxidation reaction after 2 hrs.
► The catalysts obtained from more alkaline gels (OH-/TO2 = 0.45) using Method I more probably had higher concentration of Ti per unit of outer catalyst surface. According to this, the highly acidic nature of a Ti center within a larger Ti cluster also requires a higher number of neighbors Al to form a Ti-OOH ligand capable to facilitate the oxygen transfer to the olefin.