Direct conversion technologies of methane to methanol: An overview

Renewable and Sustainable Energy Reviews 65 (2016) 250–261

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Direct conversion technologies of methane to methanol: An overview Z. Zakaria a, S.K. Kamarudin a,b,n a b

Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 23 September 2014 Received in revised form 31 March 2016 Accepted 24 May 2016

The emission of greenhouse gases (GHGs) is a major air pollution issue that affects climate change across the globe. Methane (CH4), behind carbon dioxide (CO2), is the second most abundant GHGs that negatively impact the atmosphere layer. Many studies have been conducted to identify a method for reducing the concentration of methane in the atmosphere. Converting methane to alternative forms source of energy, such as methanol, is a preferred method for methane reduction. This review aims to present an overview of recent literature that focuses on conversion of methane to methanol, with a focus primarily on the manufacturing systems and processes used in this conversion. Basic descriptions are given of several relevant technologies for converting methane to methanol and their characteristics, including conventional catalytic processes, plasma technology, photo-catalysts, supercritical water processes, biological processes and other processes. All of these options are feasible for use in the conversion process of methane to methanol. & 2016 Published by Elsevier Ltd.

Keywords: GHGs Methane Methanol Conversion of methane to methanol

Contents 1. 2.

3.

4. 5. 6.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Manufacturing and processing of methanol from methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 2.1. Direct conversion of methane to methanol via conventional catalytic processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 2.1.1. A high-temperature route based on homogeneous radical gas-phase reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 2.1.2. A low temperature catalytic route involving heterogeneous catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 2.1.3. Homogeneous catalysis in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 2.1.4. Bio-catalysis based on enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 2.2. Conversion methane to methanol via plasma technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 2.3. Conversion methane to methanol via photo-catalysts technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 2.4. Conversion methane to methanol via supercritical water technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 2.5. Conversion methane to methanol via biological process (Membrane technologies) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Challenges and advancements in technologies for direct conversion of methane to methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 3.1. Single-step oxidation of methane to methanol via conversional catalytic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 3.1.1. Homogeneous gas-phase partial oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 3.1.2. Heterogeneous catalytic partial oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3.1.3. Homogeneous catalysis in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3.1.4. Biocatalysis based on enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3.2. Plasma technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3.3. Photocatalyst technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 3.4. Supercritical water technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 3.5. Biological process (Membrane technology) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Potential for expanding direct conversion of methane to methanol in industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Other possible technologies for conversion of methane to methanol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

Corresponding author. Tel.: þ 60 389216422; fax: þ60 389216148. E-mail address: [email protected] (S.K. Kamarudin).

http://dx.doi.org/10.1016/j.rser.2016.05.082 1364-0321/& 2016 Published by Elsevier Ltd.

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Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

1. Introduction Since the 1960s, many different human activities have required the use of energy, especially from the burning of fossil fuels, which produces car exhaust and industrial smoke emissions, to produce electricity, power automobiles, heat houses, power factories, and provide for cooking. These human activities listed above contribute to the production of GHGs [1–3]. Fig. 1 shows the composition of GHGs produced by consumers worldwide since 1978. All of these gases represent greenhouse gas emissions that potentially contribute to global warming, which has a negative impact on human life. The emission of GHGs such as carbon dioxide, nitrous oxide, methane and chlorofluorocarbons (CFCs) has resulted in additional heat being trapped in the lower atmosphere, ozone depletion, and an increase in global temperature. According to a World Health Organization (WHO) report, the Earth's temperature has risen approximately 0.75 °C in the last one hundred years. In effect, every quarter-century is experiencing an increase in temperature of about 0.18 °C [5], and this high rate of global warming is adversely affecting the world's ecosystems. A notable portion of the increase in GHG emissions is due to methane. Methane is the simplest alkane and contains only one carbon atom; it is a building block of most bulk organic compounds on earth [6]. An advantage of methane as a fuel source is that it is high calorific. As a main component of natural gas, methane is used for residential heating and cooking and as a fuel in gas turbines for generating electricity [6,7]. Due to its desirable properties, methane is a widely consumed fuel in the chemical industry and households alike. In general, methane emission sources can be classified in two categories: natural sources of

methane and anthropogenic sources of methane. Fig. 2 presents the anthropogenic methane emissions from all different source sectors while Fig. 3 shows the contribution of individual sources to total natural methane emissions [2,7–11]. Aside from all the benefits of methane to daily human activities, it is nonetheless classified as GHGs [2]. As shown in Fig. 1, there has been a significant increase in GHGs emissions into the atmosphere since 1978, which creates an imbalance air composition resulting in climate change. Although the emission of methane is smaller than carbon dioxide, methane has a tendency to absorb and release infrared radiation (IR). The combined effect of heat trapping and IR absorption will increase world temperature on a continuous basis due to the formation of a thermal layer in the atmosphere [8]. Therefore, methane has a 25 times larger potential impact to trap heat than carbon dioxide on increasing global warming and thus affecting the climate [7,8]. Another detrimental effect of methane in the environment is that as it sinks in the atmosphere, it reacts with hydroxyl radicals in the troposphere and chloride radicals in the stratosphere leading to an imbalance in composition of methane [8]. Many researchers today are addressing the issue on recycling the GHGs by converting into value added product [1,2,7–11]. Reddy et al. [8] presented a review on the conversion of methane to value added product while Alvarez-Galvan et al. [12] presented work in direct conversion of methane to fuel and chemicals. This paper will present the direct conversion technologies of methane to particularly to methanol. From another angle, the worldwide energy demand is increasing daily due to worldwide population increases. The human population in 1750 was approximately 800 million. In

Fig. 1. Global averages of the concentrations of the greenhouse gases carbon dioxide, methane, nitrous oxide, CFCs-12 and CFCs-11 [4].

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Fig. 2. Anthropogenic methane emissions from all different source sectors [8].

Fig. 4. Theoretical energy densities [15].

Fig. 3. Contribution of individual sources to total natural methane emissions [10].

1900, it increases to 1.6 billion. In 2005, the global population achieve to 6.5 billion [1,13]. A survey from the US Department of Energy in 2009 revealed that the consumption of electricity increases significantly every year and is projected to increase by 44% from 2006 to 2030 [14]. Therefore, to finding alternative energy sources are necessary to ensure that the advancement of modernity is sustainable for future generations and, simultaneously, to solve the environmental crisis involving emissions of methane, which has become a formidable challenge. Additionally, methanol is a clean and renewable fuel source that contains a large amount of useful energy and is in great demand as an intermediate source of green energy to provide electric energy generation via fuel cell (FC) technology applications [14]. FCs act as alternative sustainable power sources and offer high potential for distributed power generation in portable and stationary applications as well as transportation [19]. Zakaria et al. [20] described the FC as an electrochemical device that directly converts fuel such as methanol into electrical energy via zeroemission fuel combustion. Direct methanol fuel cells (DMFCs) have emerged as a one type of green technology for alternative energy sources and are gaining attention among researchers. The efficiency of DMFC electrical energy conversion reaches 60%; this compares favorably with the conventional methods of generating electrical energy, which require many conversions [21]. Compared with hydrogen, which is the pioneer fuel used in FCs, methanol offers promising characteristics as a simple organic liquid with reactivity at low temperatures, efficient energy storage, and handling that does not require a cryogenic container. In addition, the volumetric energy density of methanol is 6.09 kW h kg  1, which is higher than the value of 3.08 kW h kg  1 for liquid hydrogen [1,20]. Kamarudin et al. [18] reported that the DMFC has demonstrated sustained cell performance for over 200 h of operation and did not significantly degrade in performance until after 1002 h of operation. DMFCs could potentially replace conventional batteries due to their high energy density compared with other energy

sources such as the lithium rechargeable battery. Fig. 4 clearly illustrates the advantages of the DMFC energy density compared with that of batteries. Kamarudin et al. [18] published an excellent overview of several applications of DMFCs as energy sources in portable electronic devices such as cellular phones, laptop computers, and portable cameras. Recently, industrial companies such as Panasonic, Toshiba, Hitachi, Sanyo and NEC have developed DMFC products. Panasonic plans to manufacture a portable power generator that is more compact than engine-based generators and produces an output of 100 W [22]. Samsung's Advanced Institute of Technology and Motorola Labs have created portable power sources with power densities of 23 mW cm  2 and 12– 27 mW cm  2, respectively [23,24]. Hence, DMFCs offer the advantages of high energy density, easy of charging, low weight, simplicity, rapid start-up, small-size storage and low environmental impact as a green energy source that is safe for consumer use [21,24]. This study includes the different technologies for the direct conversion of methane to methanol, including conventional catalytic processes, photocatalysis technologies, plasma technologies, supercritical water oxidation technologies, membrane technologies and other methods. We also summarize the advantages, disadvantage, challenges and advancements of each technology. Based on the expected potential of this technology, this review addresses two critical global issues involved in the reduction of methane emission as a GHG via conversion to methanol, which is a promising alternative source of green energy for meeting world demand and consumption. Fig. 5 illustrates the diversity of direct conversion technologies for conversion of methane to methanol.

2. Manufacturing and processing of methanol from methane 2.1. Direct conversion of methane to methanol via conventional catalytic processes The conventional mechanism of methane to methanol direct conversion goes directly through a catalytic oxidation processes. Methane can be converted to methanol in two ways: either via synthesis gas and steam reforming or directly into methanol. The fundamental conversion takes place by partial oxidation of methane, as shown in Eq. (1): CH4 þ 0.5O2-CH3OH ΔH298 ¼  30.4 kcal/mol

(1)

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Methane processing method: Oxidation process

Conversion technologies:

Yielding production: Methanol

Conventional catalytic processes

Homogeneous radical gasphase reactions

Plasma technologies

Low temperature heterogeneous catalysis

Photo-catalysis technologies

Homogeneous catalysis in solution

Supercritical water oxidation technologies

Bio-catalysis based on enzymes

Membrane technologies

Fuel Cells

Other technologies

Electro-synthesis

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Fig. 5. Diagram of diverse conversion pathways of methane to methanol.

The reaction can occur in both gas and liquid phases. In gas phase oxidation, the parameters are typically as follows: 30–200 bars and 200–500 °C with catalysts. In the liquid phase, these reactions are implemented with the oxidization of methane by super acids. This mechanism process was occurred by the activation of C–H bonding in the methane and subsequently it is splitting the covalent bonding through electrophilic attack to yield e þ –CH3. Then, oxidation reaction yields the oxidized products of methane [2,25]. Currently, conventional catalytic processes for conversion methane to methanol exist according to: 2.1.1. A high-temperature route based on homogeneous radical gasphase reactions The gas-phase reactions occur due to a free radical mechanism at high temperature and pressure [12]. Thermodynamic and kinetic analysis revealed that the partial oxidation of methane is the rate-limiting step due to the formation of methyl radicals. The initiators and sensitizers have been combined into the reaction mixture to decrease the energy barrier of H-abstraction. Babero et al. [26] introduced nitrogen oxide as a novel initiator in order to promote the gas-phase reaction with methane, whereas, Tabata et al. [27] make the comparison between the oxygen and nitrogen oxide in order to promote gas-phase reaction with methane. Meanwhile, the Fujimoto [28] reported that the consumption of small quantities of hydrocarbons, such as ethane, was indicative of a lower initiation temperature and increased selectivity and yield of methanol. One of importance factor that pronounced effect on the selectivity of methane oxidation is control the pressure process. In the Huels process via cold-flame burners using a pressure of 60 bars, the conversion of methane is achieved to a selectivity of 71% methanol and 14% formaldehyde [29]. While, Zhang et al. [31] claimed indicate that under best conditions, i.e., temperatures of 723–773 K and pressures of 30–60 bars. Holmen [25] concluded that the best conditions for the gas-phase reaction are a temperature of 450–500 °C and a pressure of 30–60 bars, with a methanol yield reaching 30–40% at 5–10% methane conversion.

Besides, there are several works presented on design and modification of reactor with different wall materials to enhance the conversion for minimize reactions on a metal surface, which could diminish the methanol selectivity such as Arutyunov [30] investigated on fast-flow gas-phase partial oxidation in reactors with different wall materials (stainless steel and quartz) and examined four parameters: pressure, methane/oxygen ratio, temperature, and residence time. Zhang et al. [31] was provided a comparatively high yield of methanol (7–8%, methanol and 63% selectivity at 13% methane conversion) which set up the parameters over a 40 °C temperature range (430–470 °C) at 5.0 MPa and CH4/O2/N2 ¼ 100/10/10 (ml/min, STP). They also designed a new reactor based on the quartz-lined and stainless-steel-lined tubular reactors and introduced the ringed gap by using a Viton Oring pressed as a locking nut to avoid the feed gas flow through the ringed gap, as shown in Fig. 6. Another effort to enhance the conversion via developing models of multiple beds gas–solid–solid reactor, such a containing of a catalyst fixed bed and a second solid that flowing gas in counter-current. This reactor would tolerate continuing lower temperatures in the adsorbent and higher levels of adsorption because the temperature excess is removed after each stage [32]. The high conversion of methane into methanol also was obtained by working under non-catalytic reaction conditions. The specific operation begins at 350 °C, and the temperature is increased to 500 °C under fuel-rich mixtures with the oxidant to minimize the extent of combustion reactions. Unfortunately, this reaction is detrimental to controlling methanol selectivity [12]. There are also researchers examined that conversion through under the non-catalytic gas-phase reaction accomplishment converts methane from 10% to 80% [33,34]. 2.1.2. A low temperature catalytic route involving heterogeneous catalysis The development of an active and selective catalyst is another optional for the partial oxidation conversion of methane to methanol because the gas-phase operation is unfavorable in terms of controlling the selectivity of the desired yield and requiring a

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impregnated on copper iron pyrophosphate catalysts show high reactivity for or selective oxidation of methane to methanol among the heterogeneous catalysts. The highest single-pass yield of useful oxygenates (1.8%) was achieved with N2O over the Cu/Fe ¼ 1:2 pyrophosphate catalyst which consists mainly of the crystalline FeIIFe III 2 (P2O7)2 and copper-containing nano-domains [41]. Zocony Structure Mobil Oil-5 (ZSM-5) zeolite catalysts have been exposed in this conversion. This catalyst was modified with iron impregnation (Fe-ZSM5) by α-oxygen pre-deposited from nitrous oxide was studied at room temperature and placed in a specially designed one-piece quartz IR reactor cell [42]. Another modification of ZSM catalyst was impregnation with cobalt (Co-ZSM-5) for the conversion of methane until achieved 42% and product yield methanol of 7.56% [43]. Another approach, Co-ZSM-5 was tried to modified with Fe and Cu via a chemical vapor impregnation method and presented high selectivity to methanol (492% selectivity, 0.5% conversion) [44].

Fig. 6. Reactor designed with Viton O-ring [31].

high operation pressure. Nevertheless, at lower pressure, i.e., 1 atm, the selection of catalyst is very important for the yield. Several of catalysts have concerning effort through the researcher, such as, optimizing the catalytic performance with a slightly reduced state by storing the isolated metal oxide on a silica substrate [12]. Tabata et al. [35] proposed that the selection of Oinsertion into CH3 from the first H-abstraction molecules of methane by reduction of molybdenum and vanadium to their oxides (MoO3 and V2O3). Aoki et al. [36] founded the formation a silicomolybdic acid (SMA)-like structure on a MoO3/SiO2 catalyst seems to drive the successive oxidation of methanol and increase the amount of oxygenates produced during the partial oxidation of methane due to the high catalytic activity. Reaction temperature was kept at 873 K in an excess amount of water vapor. Vafajoo et al. [37] optimized the catalyst through the CFD mathematical model for the stimulated direct conversion of methane to methanol over a V2O5/SiO2 catalyst in a Fixed-Bed Reactor with a temperature from 450 to 500 °C, a pressure range of 20–120 bars and a time on stream of 3 s. The feed composition consisted of 95% methane and 5% oxygen as the oxidizing agent. Zhang et al. [38] prepared the lanthanum cobalt oxide impregnated with different amounts of a (NH4)6Mo7O24 aqueous solution. This perovskite-type oxide has a strong ability to activate and diffuse oxygen. Therefore, the catalyst showed good activity in the partial oxidation of methane. The catalytic test was carried out in a continuous vertical-flow fixed-bed reactor. Zhang et al. [39] reported that as high as 7–8% methanol production was achieved in the absence of catalysts. The reactor inertness is suitable for obtaining good selectivity to methanol and the feed gas should be barrier by using quartz and Pyrex glass-lined reactor. The utilization of phthalocyanine complexes of Fe and Cu encapsulated in zeolites X and Y as catalysts and O2/tert-butyl hydro-peroxide as oxidants in ambient conditions successfully produces high-yield activity (turnover number, TON above 100) and selectivity (CO2 less than 5%). A 4.9 mol% of methane conversion was achieved with 107.2 t yield, of which 52.9 t is a methanol. These values are among the highest for the catalytic oxidation of methane at ambient temperature, as reported by Raja et al. [40]. Another catalyst was recognized favorable in selective oxidation of methane to methanol is catalyst based on iron. Fe3 þ species

2.1.3. Homogeneous catalysis in solution The conversion of methane to methanol also recognized through the homogeneous catalysts under low temperature in solution. Under this conditions, the catalyst activation of C–H bonds do not involve radicals, which has the potential to lead to more selective reactions than those promoted by heterogeneous catalysts operating at high temperatures [12]. The homogeneous reaction kinetics of methane to methanol can achieve higher conversion by controlling the operating parameters, such as temperature, CH4/O2 ratio, OH concentration, and residence time, to their optimum values. In the lower temperature range under consideration, i.e., from 35 to 700 °C, a temperature of 100 °C was found to be most suitable for this maximum conversion [16]. Several of catalysts have been introduced by the researcher for this method. For examples, Shilov et al. [45] used of Pt (II) and Pt (IV) for this conversion and they contributed in this field since the 1970s. Since then Periana et al. [46,47] have developed several oxidation catalysts based on Pt (II), Pd (II), and Hg (II) salts, such as bipyrimidyl platinum (II) complex. They are proven to functionalize C–H bonds, which obtain a good yield of partially oxidized (Eq. (2)). CH4 þ 2H2SO4-CH3OSO3 þ SO2 þ2H2O

(2)

PtCl2 catalyzes the selective oxidation of methane in fuming sulfuric acid to give methyl bisulfate in a 72% one-pass yield at 81% selectivity based on methane. Then, methyl bisulfate is hydrolyzed to methanol (Eq. (3)). CH3OSO3 þH2O-CH3OHþ H2SO4

(3)

Unfortunately, using H2SO4 as a solvent makes it difficult to separate methanol from the solvent. It is also necessary to use expensive corrosion-resistant materials, and regeneration of spent H2SO4 also proves difficult. In addition, Pd (II) salts have also been analyzed as a catalyst for the conversion of methane to methanol products in sulfuric acid as a solvent. However, due to the reduction of Pd (II) to Pd (0) species and the slow re-oxidation of Pd (0), this system was not as suitable as the ligated Pt (II) complexes. Rahman et al. [48] increased the performance of the catalyst using strong and environmental unfriendly oxidizing agents like SO3, K2S2O8 and NaIO4. Meanwhile, Li et al. [49] reported methane conversion is 24.9%, selectivity being up to 71.5% and methanol yield being 17.8% by binding nano-particle gold [Au/SiO2] catalyst and in [Bmim] Cl a ionic liquids (IL) as the solvent, trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA) as the acidic reagents, and K2S2O8 as the oxidant. Besides, Pd(OAc)2-p-benzoquinone-CO catalyst also has explored with acetic acid as a solvent to increase the selectivity oxidation of methane. The catalytic performance of Pd(OAc)2 catalyst obviously improved by adding p-

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benzoquinone [50]. The porous structure of zeolite catalysts based on aluminosilicate minerals previously introduced for this technique like Fe-ZSM-5 catalysts, which are metal-containing zeolites was investigated the kinetics of the process of partial methane oxidation and produced the highest methanol and formaldehyde yield until 3.16% and 4.52%, respectively [51]. Beznis et al. [52] reported that the Co-ZSM-5 also can be used as a catalyst for direct partial oxidation of methane due to the activity and selectivity of cobalt. Cobalt in the ion-exchange position results in a selectivity towards methanol by impregnating larger Co-oxide species (CoO and Co3O4). Besides, the cuprum-containing zeolite, Cu-ZSM-5, was applied as a catalyst for this conversion at the temperature of 100 °C with more than 98% selectivity when oxygen used as oxidizing agent [53]. 2.1.4. Bio-catalysis based on enzymes Methane monooxygenase (MMO) enzymes are natural catalysts were recognized has the opportunities to accompany the direct conversion of methane to methanol under ambient or physiological conditions [42]. Lunsford [54] provided a major advance in methane conversion technology through the discovery of the formation of oxygenates such as methanol through methane MMO enzymes. The enzyme activates O2 at iron centers with the aid of a reductant known as NADH. Besides, Michalkiewicz [55] studied partial oxidation of methane to formaldehyde and methanol with molecular oxygen by using zeolite catalysts, i.e., Fe-ZSM-5, FeNaZSM-5 catalysts and MMO enzymes. The activation of methane over Fe-ZSM-5 is similar to methane activation in MMO inside methanotrophic bacteria catalysis. The reaction temperature was carried out at the atmospheric pressure at 350–650 °C. Higher values of methane conversion to methanol were observed when using Fe-HZSM-5 (31.51% for Si/Fe¼22) and Fe-NaZSM-5 (74% for Si/Fe¼45). Furthermore, these enzymes classifiable are according two types of enzymes whether they recover methane from solution for use as an energy source or they synthesize the methanol. First is soluble methane monooxygenase (sMMO), which is a complex of iron found in the cytosol of some methanemetabolizing bacteria, and second are particulate methane monooxygenase (pMMO), which is a methanotrophic integral protein and a complex of Cu [56]. Besides, Razumovsky et al. [57] explored another biocatalyst based on the cell of the bacteria Methylosinus sporium B-2121 for conversion methane to methanol for biochemical formation. 2.2. Conversion methane to methanol via plasma technologies The oxidation of methane to methanol can also be performed under plasma conditions. Generally, plasma technology is used under atmospheric gas pressure. Plasma is often referred to as the fourth state of matter, and it includes several components: positive ions, negative ions, electrons, and neutral species. Reddy et al. [8] reported that plasma has found widespread use in many applications, including oxidative decomposition of methane. Generally, plasma technology can be classified into thermal plasma and nonthermal plasma. Roth [58] elaborately discusses both conditions. Thermal plasma can be described as a gas consisting of electrons, highly excited atoms, ions, radicals, photons, and neutral particles. Meanwhile, non-thermal plasma (non-equilibrium plasma), also called low-supplied power plasma, is populated by electrons that have much higher energy than other surrounding particles. Okazaki et al. [3] reported that the conversion of methane to methanol was achieved using non-equilibrium plasma chemical reactions under atmospheric pressure by ultra-short pulsed barrier discharge in an extremely thin glass tube reactor. Direct synthesis of methanol from methane and water vapor mixtures has been successfully realized with exergy regeneration. Methanol yield

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reached the order of 1% at the water-vapor concentration of approximately 50%, and it has been proposed that the yields of methanol and other products may be increased due to the effective dissociation of source gas molecules by adding rare gases such as Kr or Ar to the source gas. Various designs for plasma reactors for methane and oxygen conversion to methanol have been proposed in order to enhance the conversion of methane to methanol. For examples in thermal plasma reactors, Larkin et al. [59] used the dielectric barrier discharge (DBD) reactor for synthesis of methanol from methane. It could be considered analogous to the catalytic reactor because the DBD reactor was able to reduce the required temperature and pressure needed for reactions to occur as well as its ability to control the products selectivity. Nozaki et al. [60] reported a single-step, non-catalytic, direct and selective synthesis of methanol via methane partial oxidation at room temperature using a new non-thermal discharge micro-reactor. This method enabled onepass methane conversion of 40% with a selectivity for useful oxygenates (including methanol) of 30–50%, given that microplasma produces a relatively large amount of syngas with 40% selectivity and H2/CO¼ 1. Assuming one-step catalytic DME synthesis as a post-discharge reaction, an overall liquid yield of 30% with 80% selectivity is feasible. Meanwhile, Okumoto et al. [61] expands researched this conversion using pulsed DBD plasma under room temperature and atmospheric pressure. These experiments pay special attention to the effect of the specific input energy (SIE), also known as the electrical input energy per unit mass of the material gas. The experiment shows that the highest methanol production ability and reaction selectivity were achieved with a relatively low specific input energy of 360 J/l, based on the feed gas. Under this optimum condition, a maximum production ability of approximately 0.65 mmol J  1 and a selectivity of 64% were obtained. Another approach, Wang et al. [62] conducted non-catalyzed reactions in an argon (Ar) environment with a 50 W radiofrequency (RF) plasma system for conversion of methane to methanol. Several parameters were studied, including the effects of various feed compositions, CH4/O2 ratio, and plasma discharge areas. Among the various feed compositions, including CH4/O2, CH4/CO, CO/H2 and CH4/H2/O2 plasma systems, a higher composition of methanol was found for the CH4/O2 plasma system, where 19.1% of methane was converted with a 1.12% yield of CH3OH; the thin plasma discharge area is suitable for large-scale production of CH3OH. In another paper, Wang et al. [63] investigated several parameters that affect the production of methanol from methane. Various specific energies (power or flow variation) and CH4 and O2 feeding concentrations were examined. Tsuchiya et al. [64] studied a low-pressure discharge without using catalysis in low-pressure steam plasma. The experiment was conducted under different discharge parameters, such as voltage, gas flow rate, and gasmixing ratio, where methane was mixed with steam at a total gas pressure of 1–10 Torr. Another interesting aspect is non-thermal plasma was integrated the reactor with addition of catalyst to improve the activity and selectivity. Recent studied developed the Cu doped Ni supported on the CeO2, the presence of catalysts enhance the selectivity of methanol until 36% [65]. Besides, there are efforts to introduce multicomponent catalyst was combined with plasma in two different configurations, i.e., in-plasma catalysis (IPC) and post-plasma catalysis (PPC) for achieving high levels in both methane conversion and aimed methanol selectivity through the synergetic effect of Fe2O3–CuO/γ–Al2O3 catalyst [66]. The combination of catalyst with plasma is recommending the efficiency of methane conversion [81].

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Fig. 7. Schematic representation of band gap formation and photo catalytic processes [67].

Fig. 8. Graphical represent reaction of conversion methane to methanol via photocatalysts [72].

2.3. Conversion methane to methanol via photo-catalysts technologies The fundamental principal of the photo-catalytic process is a photo-chemical reaction that is carried out with external energy provided by ultraviolet light radiation that has energy equal to or greater than the energy band gap (EBG) of a semiconductor. Radiation strikes the semiconductor surface and generates electron (e  )–Hole (h þ ) pairs. Several of oxidation and reduction processes are involved in the photo-generated electron and hole. Fig. 7 shows the general reaction of photo-catalytic schematic reaction when using titanium oxide (TiO2) as the catalyst. TiO2 catalysts are extensively used as semiconductor photo-catalysts and have been applied to a wide array of environmental applications [67]. Numerous photo-catalysts were deliberated via several works. Among the candidate materials have been studied, tungsten oxide (WO3) highest talented as photo-catalysts due to high chemical stability in aqueous solution under acidic conditions, non-toxicity,

and moderate oxidizing power [68]. For examples, Taylor et al. founded that the WO3 photo-catalyst doped to an electron transfer molecule will produce a hydroxyl radicals which can react with a methane molecule to produce a methyl radical, which eventually produces methanol [69]. Gondal et al. [70] reported on the photocatalytic conversion of methane into methanol using a visible laser which WO3 was used as the semiconductor photo-catalyst. The investigation included different experimental parameters, such as catalyst concentration, laser power, laser exposure time, effects of free the radical generator (H2O2) and electron capture agent (Fe3 þ ), using visible laser light. Gondal et al. [71] expended the examined of WO3 for conversion of methane to methanol through comparison with TiO2 (rutile) and NiO as semiconductor photo-catalysts. The WO3 conversion showed the largest conversion of methane to methanol. On the other hand, the light source of intense monochromatic light with 355 nm laser photons, facilitated the study of complex processes conversion via WO3 catalyst in a short time span. The characterization techniques confirmed that Ag þ ions share surface oxygen with WO3 to give Ag2O, which enhanced the absorption of photons and the lifetime of excited states. Hence, an increase in the yield of methanol was observed, the reaction of this conversion clearly shown in Fig. 8 [72]. Recently, Villa et al. [68] introduce the electron scavengers such as (Fe3 þ , Cu2 þ , and Ag þ ) and H2O2 species to the WO3 catalyst to improve the production of methanol. They successfully enhance the methanol yield until 58.5% selectivity via WO3/Fe3 þ catalyst. Furthermore, Lanthanum as transection element was doped in mesoporous WO3 in order to increase the selectivity and yield of methanol. Yield of methanol generated over WO3/La sample is around double higher than that of neat WO3 [73]. Another catalyst, an imp V-Containing MCM-41 catalyst would be suitable for the photo-catalytic oxidation of methane into methanol. The catalyst was prepared with a vanadium oxide support on mesoporous molecular sieves (MCM-41) with impregnation of an aqueous solution of NH4VO3 into MCM41. Nitric oxide (NO) was used as an oxidant for the oxidation of methane under UV irradiation at 295 [74].

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2.4. Conversion methane to methanol via supercritical water technologies

3. Challenges and advancements in technologies for direct conversion of methane to methanol

The supercritical water oxidation (SCWO) method is one of the most promising options for the production of methanol. SCWO is a process that occurs in water at elevated temperatures and pressures above the thermodynamic critical point of the mixture. Under these conditions, water becomes a fluid with unique properties that aid in the destruction of hazardous wastes like methane. Under the supercritical fluid conditions, the reaction environment can be adjusted because the properties of water such as viscosity and dielectric constant can be readily manipulated between high gas-like diffusion rates and high liquid-like collision rates by varying pressure and temperature [8]. Dixon et al. [75] reported that methanol production can be accomplished by catalytic partial oxidation over Cr2O3 in SCWO. A high concentration of water inhibits the methane conversion and promotes the yield of methanol because methanol oxidation correlates positively with oxygen concentration, and the presence of a Cr2O3 catalyst can enhance the efficiency of the SCWO process by increasing the methane conversion rate. Lee et al. [76] reported that isothermal conditions with a laminar reactor in SCWO could be feasible for the direct partial oxidation of methane to methanol. The highest production of methanol was approximately 35%, with a methane conversion of 1–3% at temperatures of 400–410 °C. Phillip et al. [77] described that the type of reactor used along with other factors influence the production of methanol in SCWO. Two types of reactors have been examined: glass-lined reactors and stainlesssteel reactors. Both of these reactors have been investigated experimentally with several congruent parameters, and the glasslined reactor showed higher conversion of methane to methanol.

In industry, practical production of methanol from conversion of methane is executed via a two-step process. Initially, methane steam reforming produces synthesis gas, followed by catalytic conversion at high pressure of synthesis gas to methanol. Because this method incurs high capital and maintenance costs for steam reforming plants, this technology is not applied on a smaller scale. Energy intensive synthesis gas manufacturing consumption occurs in operational plants at pressures between 200 and 600 psi with process outlet temperatures in the range of 700–1000 °C [82]. Although the two-step process is implemented and operated in a stable manner, it is not feasible for high efficiency operation in the new proposed system due to the strongly endothermic reaction in the reforming process that necessities a large external energy supply [3,37]. Therefore, the conversion of methane to methanol via an alternative process is highly desirable to overcome this problem. Several processes have been reported in the academic sphere but are not yet practical for wide application in industry.

2.5. Conversion methane to methanol via biological process (Membrane technologies)

3.1.1. Homogeneous gas-phase partial oxidation Direct partial oxidation of methane to methanol is highly appealing because of its high potential for effective industrial utilization of abundant natural gas resources. As an exothermic reaction, the direct conversion of methane to methanol via homogeneous gas-phase partial oxidation is a potential option for replacement of the conventional industrial process via syngas production by steam reforming of methane. A technical/economic evaluation has demonstrated over 70% methanol selectivity at 8– 15% methane conversion [31,32]. Two well-known drawbacks of homogeneous gas-phase partial oxidation are the low conversion of methane per pass and relatively low selectivity of methanol formation [30,40]. The problems with this method arise from both thermodynamics and kinetics, such as complex chain-branched kinetic processes [30]. This method requires high pressures that exceed to 10 atm and high temperatures of up to 1000 °C to favor and activate greater methane conversion for formation of methanol selectivity. In reality, the methane conversion rate is still less than remarkable [31,32,40]. The strength of the C–H bond in methane is stronger than that of methanol, which means that methanol tends to be more reactive than methane (the C–H bond strength in methanol is 389 kJ/mol, whereas that in methane is 440 kJ/mol). For these reasons, the foremost challenge in methane conversion is related to selectivity rather than reactivity [25,85]. Experimental results achieved approximately 10% conversion of methane, but the limitation of methanol selectivity was quite poor because the required product of methanol is more active than methane [32,83]. Moreover, this method operates under a free radical mechanism, which is highly difficult to control and requires recycling of unconverted methane, which limits industrial applications [25,31,65]. Therefore, the proposed participation of a catalyst could improve the methanol selectivity and yield. However, until

Another method that allows the conversion of methane to methanol via biological processes is based on membrane reactor technology. Reddy et al. [8] reviewed the chemical reactor that operates on the membrane. This membrane reactor is able to perform dual functions at once. During the conversion process, the reaction occurs inside the separation membrane of the physical device. Thus, the membrane serves as the separator and reactor. Membrane reactors are classified based on the type of membrane materials, either inorganic or polymeric, and on the porosity of the membranes, such as micro, macro, and dense. Commonly, an inorganic membrane reactor is used instead of a polymeric membrane due to a low tolerance to chemical and temperature effects. Additionally, an inorganic membrane is mainly composed of metallic or ceramic materials and has greater physiochemical stability. Lee et al. [78] used Methylosinus trichosporium OB3b with a high concentration of Cu2 þ to produce methanol by an oxidation process. Takeguchi et al. [79] also investigated Methylosinus trichosporium OB3b for methanol production. Several parameters play an important role in the optimization of the conversion rate, including changes in cell density, temperature, pH away from 7.0 and concentrations of sodium formate, phosphate buffer and cyclopropanol. Duan et al. [80] reported a methane conversion rate as high as 60%. An approximate yield of methanol of 0.95 g/L was produced using Methylosinus trichosporium OB3b in a biotransformation process carried out in a membrane aerated reactor into which methane and oxygen were introduced via two separate dense silicone tubing. The high yield of methanol production reached over 1.12 g/L, which is 4.5-fold higher than the highest value (0.25 g/L) previously reported.

3.1. Single-step oxidation of methane to methanol via conversional catalytic processes Partial oxidation has attracted a sweeping increase in attention due to the attractive feature of methane-to-methanol conversion via direct conversion in a single-step reaction. This technology is simple, leading to a more economical process with efficient conversion of the internal energy of methane and to the possibility of direct formation compared with the two-step process. However, the major challenge is that the yield of organic oxygenates is quite inferior in this process [83].

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now, catalytic reactions have not produced yields of methanol plus formaldehyde that are better than those of the reported gas phase process, except for a few excellent reports [31]. Hence, research and development are urgently needed in the area of direct conversion to methanol in a single exothermic step with high yield [40]. 3.1.2. Heterogeneous catalytic partial oxidation The direct methane to methanol oxidation process at low temperature via heterogeneous catalytic partial oxidation is an interesting alternative to the current process. If this method can successfully convert methane, the new process could lead to a decrease in the current market price of methanol [37]. The conversion is conducted with lower energy consumption and at lower temperature, practically at ambient conditions. Selective oxidations at low temperature give the impression that this route might avoid excessive formation of CO2 [38,40]. One important factor is the need for high conversion with highly selective catalysts and the use of silent discharge, which selectively converts methane to methanol directly. Unfortunately, in practice, methane conversion and methanol selectivity in the proposed new process are less effective than those of the homogeneous gas-phase partial oxidation method [83]. However, the conversion ratio of methanol via catalysis is not sufficiently high to provide a new energy system and to induce silent discharge consumed only as triggered through the input energy for partial oxidation of methane due to an exothermic reaction. Although numerous studies have been reported, they were unable to satisfy the highly sophisticated energy utilization system without recovery of heat generated efficiently in the exothermic reactions [3]. 3.1.3. Homogeneous catalysis in solution A low-temperature homogeneous catalyst in solution for activation of C–H bonds is unnecessary in the free radical mechanism, which is difficult to control, and the reuse of unconverted methane could possibly promote more selective reactions than those promoted by the heterogeneous catalyst method of partial oxidation at high temperatures [12]. In addition, this green chemical process conversion is performed under mild conditions such as silica [49]. The key problem in this method is the introduction of a catalyst system with suitable reactivity and selectivity that also tolerates oxidizing and protic conditions [12]. Another consideration in catalyst identification is a suitable yield of methanol and other byproducts, such as formaldehyde [51]. Additionally, in finding the correct solvent system for catalysts, it is quite difficult to separate the yielded methanol and the solvent. Sulfuric acid (H2SO4) was commonly applied as the solvent system in several studies. The main disadvantage of H2SO4 as a solvent system is the difficulty in separating methanol product from the H2SO4. Moreover, as a strong acid, H2SO4 has the potential to corrode applied catalysts such as Pd(II). Practical solutions to this issue must overcome the obstacles of corrosion-resistant materials and periodic regeneration, which lead to costs that contribute to the overall cost of production [12]. Another approach to finding suitable solvents uses K2S2O8 and KMnO4 for conversion of methane, specifically to oxidize methane into methanol under the appropriate conditions. Nevertheless, molecular oxygen is undoubtedly the ideal oxidant from the viewpoints of environmental safety and raw material cost [50]. The consumption of H2SO4 or heavy metals as catalyst systems could lead to problems of strong corrosion and serious pollution. As a result, the exploration of new of green processes for methane conversion has attracted further significant attention [49].

3.1.4. Biocatalysis based on enzymes This seemingly simple reaction has remained one of the major unanswered challenges in catalytic chemistry, the area of research that has proposed new green processes. Using methane monooxygenases (MMOs) that are classified as ammonia-oxidizing bacteria (AOB), this conversion method is able to oxidize methane to methanol at room temperature [42], and thus the conversion of methane occurs. An additional advantage of using AOB to oxidize methane to methanol via the nonspecific action of the enzyme ammonia mono-oxygenase co-metabolism in AOB without any net energy synthesis is that contaminants such as moisture and CO2 do not create a limitation for biological conversion and consumption of CO2 in gas mixtures for cell synthesis. Although biocatalysts are a potential alternative for conversion of methane to methanol, microorganisms are limited for industrial consumption. Inhibition of cell growth by H2S was observed when methane in biogas was used. In addition to the high-cost electron donors required for conversions, the limitation of mass transfer from gas to liquid and NH3 might inhibit the growth of microorganisms, including methanotrophs [86,87]. 3.2. Plasma technologies Oxidation of methane to methanol was also achieved under plasma conditions, which offer clean destruction of hazardous waste and no harmful emission of toxic waste such as CO2 and CO [88]. Recently, the development of micro-reactor plasma offered the benefits of removal of heat generated by efficient methane partial oxidation. A low temperature reaction is essential in operation temperature because the oxygenates are condensed on the micro-reactor wall, which realizes product separation from O2rich reactive plasma. Additionally, the micro-plasma reactor design is benign, simple, and does not involve hazardous or costly materials such as palladium, platinum, ionic liquid, or mercury, thus rendering the process more economically attractive [60]. Unfortunately, the productivity for this conversion is lower and is restricted by the quantity of methane that can be dissolved in water. This approach requires the dissolution of methane in an aqueous solution, but the solubility of methane at room temperature in aqueous solution is notably low. To enhance the conversion, the concentration of methane in water was cultivated using methane hydrates. This process has also been studied experimentally but did not result in breakthroughs. The processes involved in plasma conversion are still less selective than catalytic processes, leading to greatly complex product distributions. As a possible approach to overcoming this problem, certain researchers have attempted to exploit the inherent synergetic potential between plasma-mediated reactions and heterogeneous catalysts [25,66]. 3.3. Photocatalyst technologies Two decades later, researchers are still working on photocatalysis technologies for environmental applications. One recognized process holds promise as a tool for allowing methane conversion to methanol under mild conditions using an appropriate catalyst and light. In this sense, this technique can be classified as a green alternative for selective oxidation in a wide range of applications. The basic requirements for this method are the use of three relatively abundant and inexpensive reactants of light, water, and methane to produce methanol. Therefore, this process option is highly attractive [68,70]. The performances of various photocatalysts have been investigated but have still not achieved the level of selectivity required for methanol yield. The addition of chemical additives such as H2O2 in photocatalytics has also been reported as a source of hydroxyl radicals, which are an important

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intermediate that contributes to the methanol formation reaction and improves the generation of methanol yield [68]. This potential approach to conversion of methane to methanol could not be extended to the industrial sector because of the limited investigation of photocatalysts and suitable reactors for this method, but this method has gained attention from academicians for CO2 conversion [89,90]. 3.4. Supercritical water technologies Supercritical water oxidation (SCWO), also known as hydrothermal oxidation (HTO), provides a high-efficiency conversion and thermal oxidation process to treat a wide variety of hazardous and non-hazardous wastes. However, no research papers have reported applications of SCWO in methanol processing via methane conversion, and progress is limited. Among the possible factors that restrict the use of this method are a highly complicated SWCO reactor engineering design, the high temperature environment of the SWCO reactor and a high corrosion rate when treating wastes containing halogen, such as chlorine, that do not provide adequate protection against chloride attack under the oxidizing conditions found in SCWO systems [8,91]. 3.5. Biological process (Membrane technology) In the pursuit of an alternative method for methane to methanol conversion, membrane technologies are also feasible for applications due to their advantage of consumption of the membrane, including membrane processes that can separate the methane and methanol. This alternative implies that a number of bulky separation essentials could be replaced by a membrane process, which does not involve a phase change to produce separation. Therefore, the energy requirements for conversion will decrease, unless large amounts of energy are required to increase the pressure of the methane feed stream to drive the permeating methane across the membrane, and the yield production will display notably high selectivities for the components to be separated. Because low priced polymer based membranes have a low tolerance to, e.g., chemicals and temperature, the membrane method has relatively limited applicability [8,92]. However, the limitations of these technologies involve incompatibilities of the membrane applied in conversion of methane. The high conversion of methane will produce a high concentration of various organic compounds. Therefore, the permeability of methane might cause dissolution, swelling or breakage of the membrane, which lead to membrane degradation. Moreover, membranes are usually suitable for operation at room temperature. Therefore, reactions that occur at higher temperatures might create a problem for membranes that cannot maintain their physical integrity under these conditions [8,92].

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Table 1 Summary of conversion methane to methanol production. Type of conversion process

Conversion of methane (%)

Selectivity of methanol (%)

Refs.

Gas-phase reaction Gas-phase reaction Gas-phase reaction Gas-phase reaction Gas-phase reaction Gas-phase reaction Heterogeneous catalyst Heterogeneous catalyst Heterogeneous catalyst Heterogeneous catalyst Heterogeneous catalyst Heterogeneous catalyst Homogeneous catalyst Homogeneous catalyst Homogeneous catalyst Bio-catalysis Bio-catalysis Plasma technology Plasma technology Plasma technology Plasma technology Plasma technology Plasma technology Plasma technology Photo-catalysts technology Photo-catalysts technology Membrane technology Membrane technology

30–40 5–10 13 13 10 10 8.2 0.66–1.52 11.6 – 1.3 0.5 8 24.9 1 0.6 31.51 50 40 – 19.1 25 23 46.9 –

5–10 30–40 60 60 80 80 11 93.4–91.9 33.2 50.5 10.1 92 72 17.8 3.16 23 74 0.8–7.5 30–50 64 1.12 20 36 1.6 58.5

[25] [29] [31] [32] [33] [34] [36] [37] [38] [40] [41] [44] [47] [49] [51] [53] [55] [3] [60] [61] [62] [64] [65] [66] [68]

–

50

[73]

6 20

0.7 60

[78] [80]

efficiencies, slow reaction rates, and lack of economic competitiveness because these methods are typically highly energy intensive. Oxidation of methane still suffers from poor conservation and low methanol product selectivity while high selectivity and yield remain desirable [70,85]. In addition, the consumption of oxidants such as oxygen is a key aspect for commercialization of conversion technologies. The limitations of an oxidation reaction that occurs via oxygen are based on the spin-forbidden reaction, i.e., molecules of methanol and methane have a singlet ground state. Thus, adequate catalysts are necessary to activate this reaction. However, no previous reports have noted catalysts suitable for the industrial scale. Therefore, research and development on this conversion are still urgently needed for the improvement of methanol production, which remains a costly alternative fuel compared with other fuels [85]. Table 1 shows a summary of the results of methane conversion and methanol selectivity and all conversions that have still not satisfied the requirement for industrial scale use.

4. Potential for expanding direct conversion of methane to methanol in industry

5. Other possible technologies for conversion of methane to methanol

Complete conversion of methane to methanol at the industrial scale was discussed previously, and generally, several requirements must be fulfilled for these academic discussions to apply. Most importantly, the direct conversion of methane to methanol should lead to an economic advantage over the indirect method [70,85]. The key to single-step direct conversion is the feasibility of replacing the existing industrial methanol production methods if the conversion of methane can achieve at least 5.5% with 80% methanol yield production, as described in the literature [84]. Unfortunately, direct conversion methods have yet to successfully reach the commercialization stage due to poor conversion

A newly developed fuel-cell-type reactor enables single-step oxidation of methane and selective synthesis of methanol yield. The reactor consists of a metal anode and cathode or metal oxide catalysts. Various catalysts have been analyzed, such as various metals (Pd, Ru, Au, and Ag) or metal oxides (V2O5, Fe2O3, CoO, Mn2O3, MoO3, and CrO) and were impregnated on the surface of the optimized support catalysts. The investigated operating temperatures ranged from 50 °C to 250 °C. Among the various catalysts, the consumption of the V2O5/SnO2 catalyst anode was efficient during methanol production, and the selectivity reached as high as 61.4% and 88.4% at a temperature of 100 °C [93].

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Another type of technology for conversion of methane to methanol is electrosynthesis via gas diffusion electrodes (GDEs). Electrosynthesis is the synthesis of chemical compounds in an electrochemical cell. The key advantages of this method are avoidance of possibly wasteful half-reactions and the ability to precisely tune the required potential. The reaction that occurs at the TiO2/RuO2/V2O5/PTFE GDE with a methane fuel feed was designed to potentially enhance the selectivity for methanol at low values of current density. However, one of the obstacles in this reaction is the formation of by-products such as formic acid and formaldehyde during oxidation of methane [94].

6. Conclusion In this article, we discussed the potential of methane as one of the GHGs that can be converted to methanol for use as a valuable fuel. The increasing demands of consumption in DMFCs as an alternative source of energy make the production of methanol highly attractive. Therefore, we listed and discussed the various alternative conversion methods that are feasible for conversion of methane to methanol, including recent progress, challenges and advances in each of the current conversion technologies and the latest conversion technologies. Advancement in all of these technologies for conversion is eventually expected to resolve global environmental issues and offer alternative sources of energy for world consumption. Hence, we conclude that single-step oxidation methane to methanol is a topic that will continue to attract attention from academicians and researchers in the environmental and energy sectors worldwide.

Acknowledgment The authors gratefully acknowledge the financial support given for this work by the Universiti Kebangsaan Malaysia (UKM) under UKM-GUP-2013-031 and FRGS/2/2013/TK06/UKM/01/1.

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