Epoxidation of 1-octene under harsh tail-end conditions in a flow reactor I: a comparative study of crystalline vs. amorphous catalysts

Amorphous silica versus crystalline delaminated-zeolite catalysts consisting of grafted Ti(IV) Lewis-acid active sites were investigated from the perspective of 1-octene olefin epoxidation with ethylbenzene hydroperoxide (EBHP) as oxidant. Reactions were performed at conditions of temperature and concentrations of organic hydroperoxide and inhibitors (epoxide product and alcohol co-product) that mimic the harsh conditions found at the tail-end of the flow reactor for industrial propylene-oxide (PO) synthesis, where there is a current need to improve activity and selectivity, because of deactivation. Catalyst synthesis was performed by grafting a Ti-alkoxide precursor onto framework vacancies ("silanol nests") of the delaminated zeolite UCB-4, as well as onto amorphous SiO2. Both catalysts were characterized by powder X-ray diffraction (PXRD), nitrogen physisorption at 77 K, and UV-visible spectroscopy before and after catalysis. Experiments at different conversions were performed, and show that crystalline Ti-UCB-4 exhibits a similar to 9% higher average selectivity (73% versus 64%) and greater conversion, stability, and robustness upon increasing time on stream relative to amorphous Ti-SiO2. UV-vis spectra are discussed for fresh, spent, and spent/calcined materials and demonstrate that Ti sites in Ti-UCB-4 exist as isolated grafted complexes with four-fold coordination to the zeolite framework, whereas Ti-SiO2 consists of grafted Ti-sites on the silica surface, some of which are isolated but a dominant proportion of which are TiO2 oligomers. The observed increased stability of the crystalline catalyst under tail-end reactor conditions is attributed to the surface pockets of the crystalline material, in which Ti is grafted.