Overall Insights into Sustainable Utilization of Methane and Carbon Dioxide in Heterogeneous Catalysis

Abstract

The development of society is dependent on commodities such as fuels and chemical feedstock. Most of these commodities are obtained from oil as raw material. Although the need to find a friendly solution to couple an economically viable energy model with a greener solution, it is known that technologies applying renewable sources are in an early stage of development. The conversion of methane into clean fuels or chemical feedstock with high commercial value, such as hydrogen, ethylene, or methanol is interesting from the energetic and economic point-of-view. Among the methods of methane conversion, the industrially used is the steam reforming (MSR), in which methane reacts with water to produce syngas, a mixture of CO and H2. Nevertheless, this reaction is highly endothermic and responsible for a large volume of CO2 emitted by the reactor burners that provide energy to the reactors. An interesting alternative process for methane conversion is the dry reforming of methane (DRM), which consists of the reaction of methane with CO2, also yielding syngas. The advantage of this reaction is the utilization of two harmful gases to the atmosphere. The disadvantage of this reaction is due to the catalyst deactivation by carbon deposition. In heterogeneous catalysis, there is a strong relationship between catalytic performance and surface and textural properties, that are outlined by the number and distribution of available active sites. In this way, different synthetic routes may be used to design these properties and obtain products of commercial interest, such as ethylene. The commercial production of ethylene occurs by the recuperation of refinery gases, thermal cracking of light hydrocarbons, mainly ethane and propane, or a combination of both processes. An alternative process of ethylene production may be from natural gas and or biogas. This process can be performed by the syngas route or by oxidative coupling of methane (OCM). The first process is an indirect conversion of methane and involves several steps, which increases the process costs. The second one is a direct conversion, where methane is directly converted into C2 (ethane and ethylene) hydrocarbons. Although the OCM is not yet a reaction on an industrial scale, several efforts are being made to design a catalytic system to achieve C2 hydrocarbons yields above 30%, the minimum required. Besides that, another technology has been studied to directly produce ethylene via the oxidative coupling of methane using CO2 as a mild oxidant (CO2-OCM). These technological routes for the valorization of methane and CO2 will be addressed in this review.