Methanol dehydration catalysts in direct and indirect dimethyl ether (DME) production and the beneficial role of DME in energy supply and environmental pollution

Abstract

Environmental pollution and global warming are pressing worldwide issues. Providing renewable and sustainable energy resources can reduce carbon extraction and release. Dimethyl ether (DME) has similar chemical and physical characteristics to gasoline, can be derived from syngas and CO2, and is an appropriate alternative for fossil fuel. DME is conventionally produced in two steps (indirect process) from syngas by methanol synthesis and dehydration in separate reactors. However, many researchers have worked on bi-functional catalyst formulations to synthesize DME directly and overcome methanol synthesis thermodynamic limitations that limit the indirect process efficiency. Different reactors and solid acid catalysts that have been investigated for methanol dehydration in direct and indirect processes are discussed in this review. Process simulation results demonstrated that fluidized bed reactors are more economical than common fixed-bed reactors. The performance and stability of γ-Al2O3, zeolites, ion exchange resins, heteropoly acids (HPAs), and metal organic framework (MOF) catalysts have been reviewed. Even though γ-Al2O3 catalysts have been employed industrially, some zeolites (ZSM-5, FER, Rho, and KFI) have superior activity at moderate conditions (<240 °C). The effects of particle size, pore structures, promoters, and preparation methods on mentioned catalysts have been reviewed. The DME selectivity and methanol conversion are influenced not only by the catalyst acid strength, but also positively influenced by smaller particle sizes and mesoporosity. Although the direct route is thermodynamically preferable, bi-functional catalysts should be investigated and modified to mitigate the negative impact of water and understand the effect of various operational conditions compared to the indirect method.