(445d) Effects of Zeolite Structure and Composition on the Synthesis of Dimethyl Carbonate by Oxidative Carbonylation of Methanol on Cu-Exchanged Y, Zsm-5, and Mordenite

Dimethyl carbonate (DMC) is of current interest as fuel additives and building blocks for organic synthesis. The vapor-phase synthesis of DMC by oxidative carbonylation of methanol over Cu-exchanged zeolites avoids serious corrosion issues attending the use of CuCl or CuCl2, the catalyst now used. The present work was undertaken in order to establish the effects of zeolite structure on the activity and selectivity of Cu-exchanged Y (Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by solid-state ion-exchange of H-form of the zeolite with CuCl, and were characterized by XRD, FTIR and X-ray absorption spectroscopy (XAS). Infrared spectroscopy confirmed that Brønsted-acid protons were fully exchanged for Cu+ cation during high-temperature exchange with CuCl via the reaction ZH + CuCl ® ZCu + HCl. The zeolite framework was preserved and no crystalline CuCl was observed. The XANES portion of the XAS data shows that all of the copper is present as Cu+ cations, and analysis of the EXAFS portion of the data shows the Cu+ cations have a Cu-O coordination number of 2~2.7 at an average bond distance of 1.98 Å for all catalysts. The activity and selectivity of Cu-Y, Cu-ZSM-5, and Cu-MOR were measured over a wide range of reaction conditions. The principal products observed were DMC and dimethoxymethane(DMM), together with smaller amounts of dimethyl ether (DME) and methyl formate (MF). The distribution of products for each catalyst was a function of methanol conversion for a fixed feed composition and temperature. With increasing methanol conversion, the selectivity to DMC decreases, whereas the selectivities to DMM and DME increase. The selectivities to DMC and DMM are strong functions of catalyst composition. DMM and DMC are primary products, whereas DME appears to be a secondary product formed via the decomposition of DMM. At a methanol conversion of 1.8%, the selectivity of methanol to DMC for Cu-Y is 80%, whereas it is 26% and 20% on Cu-ZSM-5 and Cu-MOR, respectively. Correspondingly, the methanol selectivity to DMM is 17% on Cu-Y and 50% and 57%, on Cu-ZSM-5 and Cu-MOR, respectively. The cause for the differences in catalyst selectivities with catalyst composition was probed by in situ infrared spectroscopy and XANES. Both methods showed clear evidence for stronger CO adsorption on Cu-ZSM-5 or Cu-MOR than on Cu-Y. It was also observed that upon introduction of methanol and oxygen, the adsorption of methanol displaced much of the adsorbed CO, and that the infrared band for C-O vibrations red-shifted by ~30 cm-1. These results suggest that the residual CO is bonded to Cu+ cations that are also bonded to methoxide species. While the mechanisms for DMC and DMM formation are not known, there is evidence that DMC forms via the carbonylation of adsorbed methoxide species, whereas DMM is formed via the coupling of methanol with formaldehyde, derived from methanol partial oxidation. The zeolite structure and composition, e.g., Si/Al ratio, are hypothesized to influence the electronic properties of Cu+ cations exchanged into the zeolite. Consistent with this reasoning, the selectivity to DMC and the strength of CO adsorption are observed to correlate well with the calculation of the partial charge on the Cu+ cations.