Abstract
The deactivation of a composite catalyst based on HZSM-5 zeolite (agglomerated in a matrix using boehmite as a binder) has been studied during the transformation of dimethyl ether into light olefins. The location of the trapped/retained species (on the zeolite or on the matrix) has been analyzed by comparing the properties of the fresh and deactivated catalyst after runs at different temperatures, while the nature of those species has been studied using different spectroscopic and thermogravimetric techniques. The reaction occurs on the strongest acid sites of the zeolite micropores through olefins and alkyl-benzenes as intermediates. These species also condensate into bulkier structures (polyaromatics named as coke), particularly at higher temperatures and within the meso- and macropores of the matrix. The critical roles of the matrix and water in the reaction medium have been proved: both attenuating the effect of coke deposition.
Highlights
The chemical industry needs non-conventional processes to be developed, that are environmentally friendly and based on renewable sources, to meet the growing demand for fuels and chemicals
The current production of light olefins is based on naphtha steam cracking, the high energy requirement of this process and its reduced selectivity of propylene explain the interest of other processes [3]
The analysis of the deactivated catalysts used in dimethyl ether (DME) conversion evidences that the intermediates involved in the dual cycle mechanism act at the same time as coke precursors; evolving sequentially towards polyalkyl-benzenes and polyaromatic structures
Summary
The chemical industry needs non-conventional processes to be developed, that are environmentally friendly and based on renewable sources, to meet the growing demand for fuels and chemicals. The current production of light olefins is based on naphtha steam cracking, the high energy requirement of this process (with high CO2 emissions) and its reduced selectivity of propylene explain the interest of other processes [3] In this way, the increasing industrial implantation of the MTO (methanol to olefins) process [4] offers the possibility of obtaining methanol from different fossil sources alternative to petroleum (carbon, natural gas) and from biomass (via gasification). The synthesis of dimethyl ether (DME) is thermodynamically more favorable than that of methanol [5], which allows it to operate at higher temperature, lower pressure, and using a lower H2 /CO ratio (facilitating the use of biomass syngas). Reactor operability is simpler in the DTO process than in the MTO due to the elimination of the exothermic step of methanol dehydration [7]
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