Dimethylether (DME) is the simplest ether, which has outstanding physical and chemical properties in many areas including oxygenated fuels and industrial chemicals. Since DME only contains C, H, and O elements, no NOX and SOX emission is generated during the combustion of DME. The high oxygen content of DME also alleviates the emission of CO as well, which is a common product of incomplete combustion. It is also reported that DME has a higher cetane number (CN) than the conventional petrodiesel. Therefore, DME is regarded as a promising substitute for petrodiesel as well as a cetane enhancer. Besides the advantages of using DME as fuel, the recently developed processes of producing lower olefins like ethylene and propylene [1], gasoline range boiling hydrocarbons [2], industrial chemicals such as oxygenates [3], methyl acetate [4], and ethylene glycol intermediate like dimethoxyethane (DMET) [5] make a strong case for DME to become a promising building block chemical for the synthesis chemistry. Co-production of methanol and DME under a dual catalyst system of Cu/ZnO/Al2O3 and γ-Al2O3 has been developed for years. However, deactivation of the methanol synthesis catalyst is nontrivial, thus rendering a practical applicability problem of the system. This problem is at least partially caused by the growth of the copper crystallite size of the catalyst during the synthesis reaction. It is reported that the crystallite size of Cu could increase from an average of 37 angstroms to 109 angstroms after thermal aging under CO-free or CO2-free syngas only for short periods of time-on-stream [6]. The aged catalyst only provides 78% of the original reactivity [7]. Besides the accelerated growth of the catalyst’s copper crystallite size, deactivation of the catalyst can also be triggered by accumulation (i.e., build-up) of water in the catalyst pores, which could promote the mechanical breakdown of the catalyst while reducing the overall reaction conversion. Therefore, understanding of the detailed catalytic mechanism and developing a more robust catalyst are the key challenges for improving the efficiency of the DME production. In this study, supercritical fluid treatment is performed as a pre-treatment step for the unreduced catalyst of CuO/ZnO/Al2O3 to eventually improve the catalytic reactivity of the reduced (activated) catalyst. The supercritical treatment is carried out in a stainless steel autoclave-based reactor system. Chemically pure CO2 is compressed by a booster pump and stored in a CO2 storage vessel. A certain metered amount of CO2 will be fed into the reactor from the bottom (Figure 1). Prior to the supercritical treatment, the catalyst loaded reactor will be purged and rinsed by gaseous CO2 three times to remove the residual moisture and air. Then, the system will be pressurized to a desired supercritical condition gradually, according to the estimated P-V-T values using the Peng-Robinson equation of state. Different temperatures, pressures, and treatment times are compared to find the most effective and beneficial combinations for this treatment step.
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