Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review
Introduction
Methanol is a widely used and globally distributed product with number of industrial applications. It is also very important due to the current depletion of fossil fuel resources. It is considered to be an ideal alternative fuel due to fast dismissing oil and gas resources [1]. In chemical industry, commercial uses of methanol include the production of formaldehyde, aromatics, ethylene, methyl tertiary butyl ether (MTBE), acetic acid and other chemicals [2]. There is also a growing demand for methanol in fuel application such as production of dimethyl carbonate (DMC), biodiesel production, the direct blending into gasoline and it could provide conventional energy storage for fuel cell applications due to its cleaner emissions as compare to fossil fuel resources [3]. Fig. 1, Fig. 2 illustrate data regarding the worldwide methanol consumption and its industrial demand.
The commercial production of methanol is mainly from fossil fuel based syngas that generally contains CO and H2 with small traces of CO2. Meantime, the high temperature and pressure requirement for this process has a serious impact on the environment [5]. Over the past decades, researchers have been focusing on the potential of CO2 hydrogenation to produce methanol. CO2 is an important greenhouse gas that is the main causal agent for climate change and global warming [6]. In this regard, its utilization is an attractive way to reduce CO2 concentration in the atmosphere [7].
In addition, according to the Kyoto Protocol, some industrialized countries and European community are committed to reduce their greenhouse gas emissions [8]. The emission level must be reduced by 5% below their emission level in 1990 during a five year period (2008–2012). Three market-based mechanisms were offered to help in achieving the targets i.e. (i) emission trading known as “the carbon market”, (ii) clean development mechanism (CDM) and (iii) joint implementation (JI) [9]. Even though carbon dioxide is readily available, it is thermodynamically stable, coupled with its standard free energy of formation (ΔG°=−394.359 kJ/mol) [10]. Large energy source is required in reduction/splitting process. It is also well establish that the methanol can be readily produced through CO and CO2 hydrogenation [11].
Recently, the main interest in methanol synthesis is to develop highly efficient and innovative catalysts. In this regard, a number of investigations have been conducted to develop catalysts with large surface area, high active site dispersion and smaller particle size in order to increase activity and selectivity. Among these, Cu-based catalysts have been given great attention. A work by Zhang et al. [12] showed that the addition of metal ions onto Cu-based catalyst increased the activity and stability of the catalysts. Hong [13] found that the properties and performance of Cu-based catalyst could be adjusted by varying preparation parameters and methods. In the work of Robinson and Moi [14], it was proven that besides the active site on which the reaction occurred, type and structure of the support also had an influence on the methanol synthesis. Even with all these findings, there is still a need for enhancement of Cu-based catalyst and also the preparation of new catalysts to address drawbacks of currently available catalysts. Ideal catalyst properties include a primary component that shows good selectivity and activity towards the desired product, a support that not only provides good configuration and stability but also has some modulating interaction between the primary component and promoter, and a promoter that further enhances the catalyst ability. All these objectives could be achieved by using a robust preparation method.
Similarly, another environmentally friendly way to produce methanol that has been gaining more attention is photocatalytic reduction of CO2 with water in the presence of light irradiation. An attractive feature of photocatalysis is that it occurs under relatively mild conditions with readily available and relatively cheap source of reactant. By using suitable semiconductor material as a catalyst, the absorption of light energy generates electron and holes needed for the reduction reaction. In this case, CO2 can be reduced to useful chemicals such as formic acid, formaldehyde, methane and methanol [15]. To date, a number of catalysts have been investigated for photocatalysis such as ZnS, CdS, ZrO2, TiO2, MgO and ZnO [16]. Among these catalysts, TiO2 has been widely used due to its high catalytic activity [17] and comparable band energy (3.2 eV) to the reduction potential of CO2 [18].
Even though TiO2 is a good catalyst for methanol production, one of the most challenging tasks is to have enhanced efficiency in the photochemical process. Methanol yield from this process is still competitive to the yield by syngas or hydrogenation process. Looking at the thermodynamics of the reaction, 228 kJ of energy is required to convert one mole of CO2 to methanol with six electrons to convert C4+ in CO2 to C2− in methanol. In addition, with poor visible light response from TiO2, significant improvement is necessary in order to upgrade this process to an industrial scale. One of the prominent ways is by modifying TiO2 catalyst with other metals and semiconductors. Since this is still a new research area, there are only few reported studies that involve modification of TiO2 catalyst. Further innovation in TiO2 catalysts as well as the use of other semiconductor is vital in enhancing the efficiency of the process.
Section snippets
Effect of catalyst preparation method
Currently, the synthesis of methanol is rather promising due to its dramatic economic values and significance for use as alternative fuel. The chemical conversion of CO2 via catalytic reaction especially hydrogenation of CO2 is recently becoming attractive because it may lead to the production of commercial compounds like formic acid, methanol, carbon monoxide, methane, and other hydrocarbons [19]. Eq. (1) represents the hydrogenation of CO2 into methanol as below.CO2+H2↔CH3OH+H2O
Among the
Effect of catalyst preparation method
Detailed studies on Cu/ZnO/Al2O3 catalyst including the effects of precipitation parameters and aging process on the activity of catalyst have been reported [20]. Such studies mainly focused on Cu and ZnO effect in methanol synthesis. Al2O3 is known to be useful for inhibiting sintering of metal particle, accelerating the adsorption and activation of CO. It also improves the stability of the catalyst. However, extensive research works on such aspects are quite limited.
According to Bai [50], it
Catalyst for methanol synthesis from photo reduction of CO2
Photo catalytic method employs the principle of photosynthesis in plant, in which plants absorb carbon dioxide together with sunlight and water to produce carbohydrate energy for themselves and oxygen as the by-product. Photo catalysis is defined as a process where one or more reaction steps occur by the electron pairs hole created when a source of energy (light energy) is illuminated on the surfaces of semiconductors. Methanol can be synthesized from CO2 according to the reactions below, in
Analysis of catalyst system
Most of the catalyst developments carried out for CO and CO2 hydrogenation are on Cu-based catalysts. It is clear that all the parameters discussed earlier have significant effects on the catalyst activity. The issue arises as some of the works reported are not in agreement with other works. In the case of Cu active site, even though many studies have been conducted, the nature of the active site is yet to be fully understood and there are still controversies regarding the roles of active
Conclusions
In this paper, the advancement and innovations in the catalyst system and the effect on the efficiency for methanol synthesis have been discussed. Based on reported findings, it can be concluded that an effective catalyst should have a large surface area. The large surface area will give rise to better dispersion of active metal to enhance the performance of the catalyst. For that, the right selection of support is vital as there are some interactions between active metal and support that can
Acknowledgement
A Long Term Grant Scheme (LRGS) from Ministry of Education Malaysia (203/PKT/6723001) to support this work is gratefully acknowledged.
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