Surface Acidity And Mechanism Of Pd/NaY Catalysts In The Synthesis Of Dimethyl Carbonate By Carbonylation Reaction

Dimethyl carbonate(DMC), being an environment-friendly chemical, has wide application prospects. It can be used as intermediate in organic synthesis, lithium battery electrolyte and oil additives. Therefore, the production of DMC has been paid more and more attention in recent years. The carbonylation of methyl nitrite(MN) to DMC is a more economical synthetic route. The chloride-free Pd-based zeolite catalyst has gained more popularity due to its high catalytic activities and stability. However, some problems are unclear, such as the nature of Pd species, zeolite support and catalyst and the role of additives for MN carbonylation reaction. This dissertation studied the chloride-free Pd/NaY catalyst from the foresaid three aspects in detail.To illustrate the position of Pd active species in NaY zeolite, TP-MS, CO-TPR and other characterization techniques were employed. The results indicated that Pd species located in the supercage near the outer surface of Y-type zeolite. According to the formation and growth mechanism of nanoparticles, a controllable preparation of Pd clusters with different sizes and uniform distribution was achieved through modulation of catalysts preparation conditions. The Pd clusters catalysts were evaluated in a continuous fixed-bed microreactor and the results showed that the smaller size Pd clusters possessed, the process of MN carbonylation to DMC could conduct more smoothly. Combined the results of pyridine adsorption infrared spectroscopy with catalytic activities of Pd clusters catalysts, it was found that the large amount of Br?nsted acid sites on catalysts surface played an important role in the performance of carbonylation.Through modification of catalyst preparation conditions and post-treatment of NaY zeolite, the amount of Br?nsted and Lewis acid sites of catalysts can be quantitatively regulated, respectively. The analyses of IR spectra of pyridine adsorption and catalytic activities indicated that the carbonylation performance of MN monotonously increase with the amount of Lewis acid sites, while decrease with the amount of Br?nsted acid sites on catalyst surface. In other words, Lewis acid sites facilitate the production of DMC, but Br?nsted acid sites lead to the decline in space time yield(STY) and selectivity of DMC caused by MN excessive decomposition.In order to further increase the amount of Lewis acid sites and consequently improve catalytic activity, alkali metal additives(Li, K, Rb and Cs) were selected to decorate Pd/NaY catalyst. MN carbonylation activity test showed that the potassium-doped catalyst performed best. The influences of potassium precursor, doping sequence and method on the carbonylation performances of MN were investigated. The results indicated that the improvement in activity was closely related to the precursor and doping method of potassium, but was independent on the doping sequence. The observations of experimental characterization and density functional theory(DFT) proved that the potassium enhanced the electron density of Pd species resulting in a stronger interaction between Pd species and CO molecule, meanwhile the potassium ions as Lewis acid sites interacted with O atom of CO. The synergy between Pd active species and Lewis acid sites(Pd-CO···Lewis acid) make CO molecule a more active participant role in carbonylation reaction, further facilitating the synthesis of DMC.