Recent progress for direct synthesis of dimethyl ether from syngas on the heterogeneous bifunctional hybrid catalysts

Highlights

• Dimethyl ether (DME) is a multi-purpose alternative synthetic fuel for diesel and liquefied petroleum gas.

• Recent direct DME synthesis from syngas was overviewed over hybridized bifunctional catalysts.

• Surface acidity and metallic sites for CO or CO₂ hydrogenation to DME are crucial factors.

Abstract

The recent rising demand of renewable energies and climate changes has been driving intensive academic researches into new chemical routes to sustainable and clean fuel productions in order to meet the demands of industrial evolution by solving energy crisis due to limited fossil fuel reservoirs and increasing environmental pollutants. Dimethyl ether (DME) is a multi-purpose synthetic fuel and chemical that can be used as an excellent alternative to diesel fuel and liquefied petroleum gas (LPG). The present review paper briefly provides an overview of the recent developments for a direct synthesis of DME from synthesis gas (syngas, CO + H₂) over some hybridized bifunctional heterogeneous catalysts composed of copper-based hydrogenation catalysts with solid acid components such as alumina or zeolites mainly, where the catalytic activities significantly depend on its properties influenced by synthesis protocols, porosities, surface areas, interactions of active metals with supports, distributions of metal particles on the supports and so on. We have also briefly covered the hydrogenation of CO₂, a model reaction for the utilization of CO₂ containing in syngas, to produce DME and thereby significantly mitigate its environmental impacts. Furthermore, the catalytic performances of the direct synthesis of DME by hydrogenation of carbon oxides were explained in terms of the acid sites of the solid acid catalysts and surface area of metallic copper nanoparticles in the hybridized bifunctional catalysts with their preparation protocols.

Introduction

The energy utilization cycle generally consists of three stages such as an energy generation, distribution and consumption, all of which must be closely balanced for an ideal energy infrastructure [1]. Meanwhile, the world energy consumption is steadily increasing, and thus rapidly depleting energy resources owing to an industrial evolution by generating significant environmental pollutants from an increasing population and globalization [2]. The conventional fossil resources such as crude oil, coal and natural (or shale) gases are the major sources of world’s primary energies (Fig. 1a) that are used mostly as fuels (Fig. 1b) [3]. However, when fossil fuels are burnt they predominantly produce CO₂ that has been identified as a major source contributing to climate changes. Besides they generate other harmful gases such as SOx and NOx which need to be removed to meet the environmentally acceptable fuel requirements. To limit these unwanted harmful gas increases, some supplements of fossil fuels are prerequisite for a sustainable society [1], [4]. Furthermore, the present use of natural resources does not secure the ability of future generations to meet their own energy needs. Although the exploitation of unconventional fossil fuel resources such as shale oil and shale gas could significantly increase the availability of affordable fossil fuels, the impacts of their production on the environment are also raising numerous concerns [5]. The utilization of these fossil and unconventional resources also impair the problems associated with the greenhouse gas emissions (especially, CO₂) and thus it is vital for mankind to find renewable, sustainable and environmentally friendly alternative chemicals for heat, power and transportation and so on.

A great deal of researches has been conducted to meet the rising demands for energy and to mitigate CO₂ emission by developing more sustainable technologies that use the available raw materials like coal, natural gas and biomass [4], [6]. Synthesis gas (syngas, mainly CO + H₂ mixtures), the raw materials for clean fuels and platform chemicals, can be produced from coal, natural gas, biomass and other waste resources. Although, for economic reasons, syngas is now exclusively produced from natural gas and coal, it could be made from any carbon containing feedstock including biomass. Biomass, CO₂ neutral resource, extensively distributed in the world and it is considered as one of the alternative feedstock for the production of fuels and chemicals [7], [8].

Two main chemical transformation routes were reported in the literatures for the conversion of syngas into fuels. (1) The production of linear aliphatic hydrocarbons including methane by methanation has been well known by Fischer-Tropsch synthesis (FTS) reaction which can be catalyzed by the supported transition metals such as Ru, Fe and Co. (2) Syngas to methanol which gives dimethyl ether (DME) by dehydration. Both the above routes have been successfully implemented in industry for the production of synthetic fuels [9]. However the former method should require CO₂-free syngas, whereas methanol/DME synthesis route can be conducted in the presence of CO₂-rich syngas and thus it has been considered as a promising method to get synthetic clean fuels and to mitigate CO₂ emission. Both methanol and DME can be used as synthetic fuels. Nevertheless, DME provides a high H/C ratio with relatively harmless. Therefore, DME is more preferable and often plays an alternative role to methanol [10]. DME, also called as methoxymethane (CH₃OCH₃), the smallest aliphatic ether, is a non-toxic, non-carcinogenic and non-corrosive compound. DME can be used as an excellent alternative to diesel fuel due to its high cetane number (55–60) and a low emission of CO, NOx in the exhaust gases from a diesel engine as it has no CC bond structures. It also has similar physical properties as that of liquefied petroleum gas (LPG) and hence can be used as an alternative fuel for cooking and heating [11a]. Furthermore, the well-developed infrastructures of LPG can be adapted for DME and this makes DME outstanding for practical uses [10]. As multisource, multi-purpose clean fuels, it is also projected as a chemical feedstock of the 21st century for the production of hydrocarbon, oxygenates and higher ethers [7]. For instance, there is a huge market value for acetic acid, gasoline and olefins which can be possibly derived from DME (Fig. 2). Since DME having a high H/C ratio and intense energy density, it can be used for hydrogen and dimethoxyethane (DMET) production by steam reforming (or partial oxidation) and steam plasma, respectively [11]. DME is an excellent solvent and also degrades rubber materials [12]. It has also been increasingly used as an aerosol propellant to replace conventional chlorofluorocarbons which are found to destroy ozone layers of the atmosphere [13]. The DME aerosol propellant has been used in a wide range of personal care products including shaving creams, hairspray, foams, and antiperspirants because of its higher water solubility relative to other propellants. Besides, DME has been used in a limited amount to freeze meat and fish by direct immersion [12].

DME was firstly appeared in 1867 as an anesthetic agent that tested in rabbits and pigeons [12]. In 1963, Akzo Nobel Corporation firstly used DME as an aerosol propellant. In 1970s, oil prices were increased, and significant efforts have been made to produce cleaner diesel fuels and other alternative fuels [14]. From the late 1970s, a global oil company called Amoco has started work on the production of liquid fuels from syngas. Majunke and Mueller filed a German patent in 1984 where they pointed out “methanol based fuel consisting of DME” [15]. Following this patent, a US patent published on “method of operating a diesel engine with a fuel consisting of >95% DME” in 1990 [16]. In the late 1990s, Amoco jointly collaborated with the General Electric Co. (GE) and the Electric Power Development Corporation (EPDC) of Japan tried DME as a “gas turbine fuel”. DME production was efficiently performed in this test and it showed low emissions of particulate matter as well [12]. As shown in Fig. 3, the history of DME production and its established milestones was summarized into early and developing stages from 1963 to 1990 and active stage after 1990s. In the subsequent years, several publications on DME synthesis were reported using single-step method over homogeneous and heterogeneous catalysis (Fig. 4). Similarly, a lot of citations on DME synthesis are exponentially increasing every year as summarized in Fig. 4.

This topic was first reviewed in 2006 by Semelsberger et al. [18] on the viability of DME as an alternative fuel. Two reviews (2008 and 2014) were described the potential usage of DME in compression-ignition engines [19]. Yoon and Han pronounced some technological and economic aspects of DME production and its future uses [20]. In 2010, two short-reviews were published on catalyst development for the production of DME [7], [21]. Fleisch et al.[22] described the history and status of DME as an alternative fuel. Recently, Bhattacharya et al. [23] reviewed on the current and potential future applications of Victorian brown coal for DME synthesis. Very recently, two reviews were summarized in 2014 on the catalysts and reactors used for the production of DME [24], [25]. Among them, the last two review papers were covered the technologies in reactor design and randomly chosen hybrid/bifunctional catalysts for syngas to DME synthesis [24], [25] that are inadequate in a catalysis point of view, while others just mention DME as potential alternative fuel [18], [22], but this area has been progressing very fast, and over 200 articles have been published in scientific journals from 2006 to 2016 (Fig. 4). In this review paper, we try to provide an overview of the recent developments in the direct synthesis of DME from syngas (CO (or CO₂) + H₂) over typical heterogeneous hybrid/bifunctional catalysts, where syngas to DME performance was well correlated with the acid sites of the solid-acid catalysts as well as the metal particle surface areas in the hybridized bifunctional catalysts.