Thermal Stability Improvement of Cu-Based Catalyst by Hydrophobic Modification in Methanol Synthesis

Water can cause the growth and oxidation of Cu nanoparticles on the surface of Cu-based catalysts, leading to their deactivation. However, during methanol synthesis process from syngas on Cu-based catalysts, water is inevitably produced as a by-product due to the presence of CO2. Therefore, enhancing the stability of Cu-based catalysts during the reaction, particularly in the presence of water, is crucial. In this study, Cu/ZnO/Al2O3 was first subjected to wet etching and then hydrophobically modified using the sol-gel method with methyltrimethoxysilane (MTMS) and the grafting method with 1H,1H,2H,2H-perfluoroalkyltriethoxysilanes (PFOTES) as modifiers. These modifications aimed to mitigate the impact of water on the catalyst and improve its stability. After modification, the catalysts exhibited excellent hydrophobicity and enhanced catalytic activity in the methanol synthesis process. The surface physical properties, composition, and thermal stability of the catalysts before and after hydrophobic modification were characterized by SEM, FT-IR, BET, XRD and TGA. Additionally, molecular dynamics simulations were employed to compare the diffusion behavior of water molecules on the catalyst surfaces before and after hydrophobic modification. The results indicated that the modified catalyst surface formed a micro/nano structure composed of nanosheets and nanosheet clusters, while the hydrophobic modification did not alter the structure of the catalyst. According to the results of simulations, the hydrophobic layers on the modified catalysts were able to expel water quickly from the surfaces and reduce the relative concentration of water molecules at the active sites, thereby improving the stability of the catalyst. Notably, the thermal stability and hydrophobicity of the PFOTES-modified catalyst were superior to those of the MTMS-modified catalyst, resulting in a more significant enhancement in catalyst stability, which aligned with the experimental results.