Effect of precursor on the performance of phosphate-modified γ-Al2O3 catalysts for the dehydration of methanol

Abstract

A series of PO₄³⁻/γ-Al₂O₃ monolith catalysts were prepared by incipient wetness impregnation of phosphate with different additives, including ammonia, triethanolamine (TEA), hexamethylenetetramine (HMT) and diethylenetriamine (MSDS). These catalysts were compared for their catalytic properties in a fixed-bed reactor at 1 atm, 250–350 °C and liquid hourly space velocity (LHSV) of 4 h⁻¹. The catalysts were characterized by nitrogen adsorption, X-ray diffraction (XRD) and temperature-programmed desorption of ammonia (NH₃-TPD). The results revealed that the catalytic activity of PO₄³⁻/γ-Al₂O₃ for methanol dehydration increased significantly as the content of phosphate increased to 7 wt%. However, when the phosphate content of PO₄³⁻/γ-Al₂O₃ was further increased to 10 wt%, the activity for methanol dehydration decreased. The catalytic activity of PO₄³⁻/γ-Al₂O₃ is found to be determined mainly by the amounts of weak and medium acid sites. At 7 wt% PO₄³⁻ loading, the proportion of weak and medium acid sites can be improved by addition of additives with high basicity to the impregnation process. The catalyst containing 7 wt% phosphate with added diethylenetriamine demonstrated the highest activity.