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Controlling the location and microenvironment of active centers in the zeolite framework is critical for understanding the in-depth structure–performance relationships of catalytic systems and constructing highly efficient catalysts. Herein, we have developed an MOR-type titanosilicate (denoted as 6M-Ti-M360) with an extremely low framework Ti content (Si/Ti = 300), exhibiting not only ultrahigh catalyst weight-based conversion (81%) but also a record-breaking turnover number (TON = 5845) per Ti site in batchwise ammoximation of cyclohexanone. Its highly isolated and active Ti species took the specific position of defective T3 sites within the eight-member ring side pockets of the MOR topology, evidenced by molecular dimension-dependent shape-selective experiments and theoretical evaluation of the catalytic activation ability of the different crystallographic Ti sites at the molecular level. Despite an extremely low Ti content but with the most active Ti on the defective T3 sites, the 6M-Ti-M360 catalyst maintained the cyclohexanone conversion and cyclohexanone oxime selectivity both as high as 99% for a long lifetime (314 h) in a continuous slurry bed reactor, capable of producing 1100 kg of oxime per gram of Ti. The clarification of the location and local microenvironment of Ti active sites may provide new insights into the exploration and construction of highly active sites in zeolitic catalysts.
Wan et al. (Thu,) studied this question.