Investigation of the Ti active site in TS-1 for phenol hydroxylation via seed and dissolution-recrystallization methods

Abstract

TS-1 (titanium silicalite-1) zeolite has been one of the most popular catalysts for phenol hydroxylation in the industry to produce hydroquinone and catechol for its high catalytic activity. However, ideal Ti active site in TS-1 for phenol hydroxylation is controversial in academia and still remains urgent to be determined for more convincing guidance to prepare TS-1 with higher catalytic activity. Thus, controlled experiments for detailed research were conducted via seed and dissolution-recrystallization methods to explore the Ti active site for phenol hydroxylation. Through comparison of micron TS-1 and hierarchical TS-1, several factors including crystal size, specific surface area, pore volumes and anatase TiO2 were proved to have an influence on catalytic performance for phenol hydroxylation. Furthermore, hierarchical TS-1 using seeds with different molar ratios of Si/Ti were synthesized to regulate the ratio of framework Ti and hexa-coordinated Ti and exclude above variables. With binding analysis of UV–vis, UV-Raman, and XPS, it was found that TS-1 with the most framework Ti showed the best performance with TON of 1461 and dihydroxybenzenes selectivity of 97.3 %, and TS-1 with less framework Ti and more hexa-coordinated Ti showed worse performance. Hence, it was demonstrated that framework Ti was the active site for phenol hydroxylation rather than hexa-coordinated Ti, while hexa-coordinated Ti would cause excessive oxidation of phenol to benzoquinone.