Design of dual active site composites of HPW@MOF-199 embedded in SBA-15 functionalized with -COOH groups: Efficient catalysts for esterification reactions

Metal-organic framework (MOF)-based composites have received significant attention for catalytic applications and are believed excellent heterogeneous catalysts. In this article, we prepared a new type of material, HPW@MOF-199/SBA-15C (PMS-15C). For this material, the MOF-199 encapsulating Keggin-type phosphotungstic acid (HPW) was grown inside the pores of the SBA-15 functionalized by carboxylic acids. The N 2 adsorption-desorption isotherms and pore size distributions, X-ray powder diffraction, transmission electron microscopy, and scanning electron microscopy analyses showed that the PMS-15C composites had an ordered two-dimensional hexagonal mesoporous structure, in which HPW encapsulating by MOF-199 loaded into the SBA-15C. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that HPW was successfully loaded, and the Keggin structure of HPW in PMS-15C remained unchanged. With the reaction of acetic acid and ethanol, the catalytic performance of PMS-15C was studied. Under the optimal conditions, ethyl acetate yield reached 81.4 %. The forward reaction rate was 0.432 mL mol −1 min −1 , and the activation energy was 38.35 kJ mol −1 . And the data were found to be consistent with the second-order reversible reaction kinetic model. The catalytic activity of the PMS-15C-0.2 composites remained stable after five catalytic cycles. In addition, the esterification of n-octanol/acetic acid and ethanol/oleic acid were studied using PMS-15C-0.2 as the catalyst. The results suggested that the catalytic activity of PMS-15C-0.2 was weakened upon increasing the substrates’ alkyl chain length.