A review of the process of ethylene production from ethanol

The increasing world worries about clean energy provision and expanding environmental pollution have turned into serious problems in recent decades. Owing to the shortage of fossil feedstock and enhancing requests for energy, renewable and replacement energy resources are highly favorable and attract wide interest. Biomass-derived ethanol can be an outstanding candidate, because of its wide potential and availability for further production of value-added fuels. The catalytic dehydration of ethanol reforming to the other chemicals like aldehydes, ethylene, 1-butanol olefins, and hydrogen, is one of the procedures widely investigated, as it needs lower temperatures and attains higher product yields. Moreover, ethanol can be gained from renewable sources by fermentation of natural carbohydrates such as sugar-industry residues, lignocellulosic waste, and vegetable biomass. Of particular achievement from catalytic dehydration of ethanol is the production of valuable ethylene, which is the key chemical to produce widely used compounds and plastics like polyvinyl chloride, polyethylene, styrene, ethylene oxide etc [1]. Currently ethylene is attained from petroleum by thermal cracking, an endothermic reaction needing great temperatures. The shortage of natural resources, besides to the exponential increase in the crude oil prices have encouraged the search of new sustainable ways of ethylene production. In the catalytic dehydration of ethanol to form ethylene, an acid catalyst first protonates the hydroxyl group, which leaves as a water molecule. The conjugate base of the catalyst then deprotonates the methyl group, and the hydrocarbon rearranges into ethylene. The reaction is endothermic, and because of this, the optimal reaction temperature is fairly high, ranging from 180 °C to 500 °C. Maintaining the reaction temperature constitutes much of the energy cost in industrial application of the reaction, since competing reactions into diethyl ether or acetaldehyde are favoured outside of the temperature range and so decrease ethylene yield. To promote ethanol dehydration for ethylene, diverse catalysts have been developed such as transition metal oxides, heteropolyacids, zeolites, and alumina. The important catalysts utilized for the dehydration of ethanol are based on HZSM-5 zeolite and γ-Al2O3. The γ-Al2O3 usually needs higher reaction temperatures and yields lower ethylene selectivities. The HZSM-5 zeolite allows lower reaction temperatures and produces high ethylene yields. However, because of the strong acidity of HZSM-5 catalysts, the deactivation by coke deposition is easily produced [2]. The presence of water in the reaction decreases the deactivation by coke, but produces a significant zeolite dealumination at high temperatures. Generally, to achieve high ethylene selectivity, severe conditions of high pressure/temperature are necessary for these reported catalysts. Thus, it is still a great challenge to explore a preferable catalyst for ethanol dehydration in mild condition. Thus, future researchers should conduct their research in the context of economical factors and with respect to the other steps in the process of producing ethylene from ethanol.

Keywords:

Energy resources , Catalytic dehydration of ethanol , Ethylene , Catalyst