Thermal desorption of trimethylphosphine (TMP) on the HY zeolite followed by FT-IR and 31P MAS NMR

Abstract

Characterization of the acidic properties of solid acid catalysts is essential to understanding their catalytic performance with respect to activity, deactivation rate and product selectivity. In this work, trimethylphosphine (TMP) was used as a probe molecule for the study of acidity in a HY zeolite activated at 773 ​K. The variations in type, concentration and acid strength with the desorption temperature of the probe molecule were followed by infrared spectroscopy (FT-IR) and solid-state Nuclear Magnetic Resonance (ss-NMR). Oxidation of TMP to trimethylphosphine oxide (TMPO) provided additional information about Brønsted (BAS) and Lewis (LAS) acid sites with different acid strengths. The variation in acid strength of certain species determined by calculating the concentration percentage of the desorption and reabsorption of the majority species by increasing the desorption temperature. The 31P ss-NMR chemical shifts at δ=-62, −32 to −58, 45 and 51 ​ppm correspond to weak acid sites (<423 ​K). The peaks at δ= 54 and 55 ​ppm arose from medium acid sites (423–623 ​K). The strong acid sites at δ= 58, 61, 63, 64, 65, 67, 70, 73, 76 and 80 ​ppm correspond to sites with different acid strengths (>623 ​K). Our results demonstrated differences in the concentrations of acid and the distribution of acid strengths of extra-framework (EFAl) aluminum species in different cavities by varying the desorption temperature of the probe molecule. The developed methodology provides more detailed information about acidic properties and can be used for solid acid catalysts using alkylphosphines and their oxides as probe molecules.