Highly efficient selective oxidation of 2-methylnaphthalene to vitamin K3 over mesoporous Al/Ti-SBA-15 catalysts: The effect of acid sites and textural property

Abstract

Catalytic oxidation of 2-methylnaphthalene to 2-methyl-1,4-naphthoquinone (vitamin K3) is a significant pathway to produce vitamin groups K. High pollution and low-productivity of the traditional method require a greener catalyst and process. In this work, Al/Ti-SBA-15 with different nSi/nAl and nSi/nTi ratios were hydrothermally prepared in mild acidic condition and tested in partial oxidation of 2-methylnaphthalene. Commercial zeolites NaY, β and ZSM-5 were applied to investigate the influence of mass transfer resistance and acidity on catalytic performance. Catalysts were characterized by XRD, N2 adsorption-desorption, TEM, NH3-TPD, FTIR and Py-FTIR. We have found that the 2-methylnaphthalene conversion and vitamin K3 selectivity depend strongly on the mesoporous structure as well as Brönsted acid concentration and strength. Irregular micropores and strong acidity will accelerate the cleavage of C = C in aromatic rings, leading to the production of phthalic anhydride. DFT calculation reveals that incorporation of aluminum in SBA-15 can reduce the energy gap, facilitating the electron transfer. And as a consequence of the substitution of Al in tetrahedral positions in SBA-15, Al-SBA-15 (10) gives the best result with 72% yield to vitamin K3. This catalyst markedly reduces the activation energy due to the modified activated center. Repeated experiments recommend that Al-SBA-15 is a steady and high-efficiency catalyst for the production of vitamin K3.