(25j) Synthesis of Dimethyl Carbonate Via Vapor-Phase Carbonylation Catalyzed By Pd-Doped Zeolites: Interaction of Lewis Acidic Sites and Pd Species

Dialkyl carbonates have obtained widespread attention due to their excellent physical and chemical properties. Vapor-phase carbonylation of alkyl nitrite over Pd-doped zeolites catalysts has been considered to be a more promising route to produce dimethyl carbonate (DMC). However, the acidic nature of zeolite support and Pd species are not fully understood.  In order to investigate the acidic nature of zeolite, Pd-doped FAU zeolites with varied SiO₂/Al₂O₃ ratio were evaluated for vapor-phase carbonylation of alkyl nitrite. They exhibited different catalytic activities, owing to the diverse acidic properties of FAU zeolite. The Brønsted sites could be converted into Lewis sites by treatment with Na₂EDTA solution, leading to an increase in catalytic performances of Pd-doped FAU zeolite with high SiO₂/Al₂O₃ ratio. Passivation with tetraethylorthosilicate, we further determined the the quantitative relationship between amount of Lewis acid sites and catalytic performances. In addition, XPS results showed that Pd sites became electron deficient due to the Lewis acidity, which in turn promote the dialkyl carbonates synthesis. For vapor-phase carbonylation reaction, Pd (II) species gradually reduced and agglomerated, resulting in the target product DMC excessive carbonylation to form dimethyl oxalate. Therefore, the effects of Pd clusters size on the activity and selectivity of Pd-doped zeolite were studied. By controlling the Pd-loading, the calcination and reduction temperature, a series of Pd clusters with good dispersity were prepared. Combined with TEM results, we found DMC has a high selectivity for the smaller Pd particles. CO-adsorption FTIR results showed the CO activating abilities of Pd particles differed in size, to generate the different catalytic activities.