Ziheng Cui has published a new paper on Chemical Engineering Science.
(Z Cui, X Feng, H Li*, T Tan*, Interconversion of Lewis Acid and Brønsted Acid Catalysts in Biomass-Derived Paraxylene Synthesis, Chem. Engi. Sci., 2020, https://doi.org/10.1016/j.ces.2020.115942 )
The change of acidity of catalyst site during the catalytic reaction may enforce great influence on the efficiency of catalyst. Here, the WOx catalyst highly dispersed on surface of SiO2 molecular sieve is synthesized, which is found to possess strong Lewis acidity and great catalytic ability for producing paraxylene through the Diels-Alder cycloaddition reaction of 2,5-dimethylfuran and ethylene. Based on a comprehensive density functional theory study, we find that the water generated during the reaction can convert the Lewis acid sites into Brønsted acid sites, thereby, turn the Lewis acid catalytic reaction into the Brønsted acid assisted reaction. The kinetic Monte Carlo (kMC) simulations reveal that the selectivity of PX relative to the by-product 2,5-hexanedione increases with reaction temperature, which is consistent with the experimental observation. The unravelling of the interconversion of Lewis acid and Brønsted acid gives us new insight of the mechanism of acid-catalytic reactions.
利用DFT计算，我们研究了在WOx催化剂表面发生的2,5-二甲基呋喃与乙烯的Diels-Alder环加成反应制备对二甲苯(PX)的反应过程水会导致催化位点的酸性的转化。动力学蒙特卡洛(kinetic Monte Carlo, KMC)模拟表明，随着反应温度的升高，PX对副产物2,5-己二酮的选择性增加，这与实验结果一致。路易斯酸与Brønsted酸的相互转化过程的揭示，使我们对酸催化反应机理有了新的认识。
Figure. 1 Reaction details of DMF and ethylene to PX.
Figure. 2 (a) Atomic structure of [WO4]2- catalyst dispersed on a SiO2 molecular sieve; (b) The TEM image of a synthesized 0.20-WO3/SBA-15-700 sample; (c) The py-FTIR spectra of the 0.20-WO3/SBA-15-700 catalyst before and after water treatment; (d) Experimentally measured product selectivities at 0.5 h under various temperatures; (e) Experimentally measured product selectivities at 6 h under various temperatures; (f) Lattice structure of kMC and AS of a single site.
Figure. 3 Energy profiles of (a) main reaction catalyzed by Lewis acid; (b) water decomposition and hydrolysis of DMF; (c) main reaction catalyzed by Brønsted acid; (d) Atomic structures of the [WO4]2- Lewis acid site, water adsorption, and water decomposition (the Brønsted acid site).
Figure. 4 Reaction networks of total four reactions evolved in PX synthesis.
Figure. 5 Evolution of surface occupation over time at (a) 450 K and (b) 600 K; (c) TOF of PX and HDO under different temperatures; (d) The selectivities of PX and HDO under different temperatures obtained from the kMC simulations. (e) Experimental TOF of PX and HDO under different temperatures; (f) Arrhenius plot of experimental TOF.