Construction of strong Lewis acidity through pre-calcining octahedral Zr-MOF to exhibit high activity for the selective isomerization of α-epoxypinane

Abstract

This work has for the first time reported the construction of strong Lewis acidity through one route of pre-calcining octahedral Zr-MOF material, which was synthesized by the rotating hydrothermal crystallization method with regular octahedral structure, large specific surface area and very uniform particle size. After low-temperature calcination, the resulting material Zr-MOF-R-X-Cal did retain basically an intact structure of Zr-MOF-R precursor and became more hydrophobic, as characterized by XRD patterns, SEM/TEM images and WCA angles. More importantly, the surface Lewis acidity and Lewis acid ratio on the calcined materials were significantly enhanced. Thus-calcined materials have exhibited highly catalytic activity for selective isomerization of α-epoxypinane to campholenic aldehyde. As we know, campholenic aldehyde (CA) was an important intermediate for the production of sandalwood and drugs, and generally prepared by selective isomerization of α-epoxypinane on strong Lewis acid sites. Among the tested materials, Zr-MOF-R-300-Cal presented the best catalytic effect on this isomerization to obtain 95.9% conversion of α-epoxypinane and 82.4% selectivity of CA, much superior to the Zr-MOF-R precursor and those catalytic materials reported previously in the literature. This could be assigned with strong Lewis acidity and high Lewis acid content on the Zr-MOF-R-300-Cal material, as proven by NH3-TPD and pyridine-FTIR tests. In addition, the increase of surface hydrophobicity promoted surface diffuse, adsorption and mass transfer of substrate molecules. Notably, catalytic activity of the Zr-MOF-R-300-Cal material was not decreased a lot after repeated-use six times, indicative of its excellent stability.