Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review

Renewable and Sustainable Energy Reviews 44 (2015) 508–518

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

Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review Khozema Ahmed Ali, Ahmad Zuhairi Abdullah n, Abdul Rahman Mohamed Low Carbon Economy Group, School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 27 January 2014 Received in revised form 4 December 2014 Accepted 3 January 2015

In the current era of energy crises, alternative feedstock such as methanol are commonly used as fuels and solvents in various industries. Methanol is commonly produced from non-renewable sources. Recently, sustainable methanol synthesis via innovative and efficient catalytic processes has drawn a lot of attention and research is currently aimed at finding a suitable catalyst for optimized production at commercial scale. Nowadays, one of the main interests is catalytic synthesis of methanol from CO2. This work presents a critical review on innovative catalysts for methanol synthesis, research progress for their development and their use in the catalytic process. It also provides an overview on recent development in methanol synthesis from syngas, CO2 hydrogenation and photo-catalytic reduction of CO2. The use of various reactors, the influence of preparation method, support, promoter, different type of catalysts used, their properties and performance during methanol synthesis are also thoroughly reviewed. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Catalytic methanol synthesis Renewable energy Carbon dioxide Hydrogenation Syngas Photoreduction

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol synthesis through CO2 hydrogenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Effect of catalyst preparation method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Effect of promoter on the catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Effect of support on catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Other catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Catalyst for methanol synthesis from syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Effect of catalyst preparation method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Effect of promoter on catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Other catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Catalyst for methanol synthesis from photo reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Effect of promoter on catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Effect of catalyst preparation method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Other catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Analysis of catalyst system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

508 509 509 510 511 511 512 512 512 513 513 513 514 514 515 516 516 516

1. Introduction n

Corresponding author. E-mail address: [email protected] (A.Z. Abdullah).

http://dx.doi.org/10.1016/j.rser.2015.01.010 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

Methanol is a widely used and globally distributed product with number of industrial applications. It is also very important

K.A. Ali et al. / Renewable and Sustainable Energy Reviews 44 (2015) 508–518

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]. Figs. 1 and 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

Fig. 1. Methanol consuming industries [4].

509

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 (ΔG1¼  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 þ 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.

2. Methanol synthesis through CO2 hydrogenation 2.1. Effect of catalyst preparation method

Fig. 2. Trend in methanol demand for a 10 year period [3].

Currently, the synthesis of methanol is rather promising due to its dramatic economic values and significance for use as alternative

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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 þH22CH3OH þH2O

(1)

Among the studied catalysts, Cu-based catalyst is considered one of the most popular choices for methanol synthesis due to its improved and better activity [20]. It is also reported that due to the sensitive structure of the catalyst, even a slight change in the preparation method may cause considerable effects on the catalytic performance and yield [21]. Conventional catalyst preparation methods include co-precipitation, impregnation and sol gel which are sensitive to pH, time consuming and are restricted by deviation from stoichiometry [22]. In order to overcome such drawbacks and to enhance the performance of synthesized catalysts, novel preparation methods are recommended e.g. the preparation of Cu/ZnO/Al2O3 catalysts through decomposition of M (Cu,Zn)-ammonia complexes (DMAC) under sub-atmospheric pressure at various temperatures [23]. In this regard, XRD patterns indicated that precursor prepared at medium temperature (343 K) contained more aurichalate phase than those synthesized at any other temperature. Rich aurichalate phase could improve Cu dispersion and also formed small and homogenous CuO and ZnO crystalline for higher activity [24]. The catalyst with this phase also showed high stability and activity after calcination [25]. The BET surface area and particle size of the catalyst were smaller compared to the other catalyst which is consistent with the view that aurichalate phase led to smaller Cu particle size. The high temperature CO2 release peak based on the EGA was ascribed to secondary decomposition of hydroxyl carbonate that could restrain the Cu particle growth, thus maintaining the activity of the catalyst [26]. Guo [27] on the other hand reported the effect of glycine content in combustion synthesis method on CuO/ZnO/ZrO2 (CZZ) catalyst properties and catalytic performance for methanol synthesis. Similarly, combustion synthesis method is an attractive technique to synthesize metal oxide as it offers precise stoichiometric ratio, low cost, and short reaction time [28]. Glycine, a molecule with zwitterionic character can be an ideal complexion agent for various types of metal. Such characteristics can complex metal ions with varying ionic size that helps in preventing selective precipitation and maintain the compositional homogeneity [29]. However, SEM images were used to study changes in morphology as the glycine contents during the combustion were varied from 50% to 150% of stoichiometry [29]. This observation was consistent with XRD pattern and BET surface area. The data regarding effect of glycine content on the physicochemical properties and catalytic performance of the catalyst are presented in Table 1. It is clearly seen that particle size, methanol selectivity and yield show volcanic trends with the increase in glycine content while the surface area shows an inverse trend. As for ZrO2 support, variation in glycine content effected its phase formation. For low Table 1 Physicochemical properties and catalytic performance of CZZ catalysts [30]. Catalysts CuO crystallite size (nm)

SBET (m2/g)

n(m-ZrO2)/n (t-ZrO2)

CH3OH selectivity (%)

CH3OH yield (%)

50 CZZ 75 CZZ 100 CZZ 125 CZZ 150 CZZ

17.8 4.8 3.3 4.8 5.5

0.85 3.30 10.30 0.49 0.07

71.1 80.0 82.1 79.1 78.9

8.5 3.9 2.9 4.0 4.7

16.0 19.7 25.3 23.0 19.5

glycine content (50%), both monoclinic, m-ZrO2 and tetragonal t-ZrO2 phase were observed. A maximum m-ZrO2 was observed when the stoichiometry content (100%) was used. In contrast, the intensity for t-ZrO2 was higher for 125% and 150% CZZ. Based on these results, methanol selectivity did not only depend on Cu surface area but also on the ZrO2 phase. According to Wang [23], the introduction of CO2 into the mother liquid during the aging process in co-precipitation method for Cu/ZnO/Al2O3 catalyst preparation could improve the activity and physiochemical properties of the catalyst. During the aging process, the reaction (2) occurs by releasing OH  ion that increases the pH value [24,30]. The introduction of CO2 according to reactions (3) and (4) can avoid the pH change. Constant pH is found to improve physiochemical properties and catalytic performance of the catalyst [26]. CuðNO3 Þ2 þ 3CuðOHÞ2 ðsÞ þ 2CO23  -2Cu2 ðOHÞ2 CO3 ðsÞ þ 2OH  þ 2NO3

ð2Þ H2 O þ CO2 -2H þ þ CO23 

ð3Þ

OH  þH þ -H2O

(4)

The introduction of CO2 in the aging process can increase the surface area, pore volume. At the same time, it avoids the agglomeration of smaller particle into larger aggregates and also inhibits the crystalline growth. In the absence of CO2, the XRD pattern of the precursor showed low intensity of stable hydroxycarbonate phase which indicates the decomposition of this phase during aging process. In addition, the use of CO2 promotes the formation of stable hydroxycarbonate phase. According to Zhuang [31], fractional precipitation (FP), solidstate reaction (SS), impregnation precipitation (IP)) not only affected the interaction between Cu and ZrO2 in Cu/ZrO2 catalyst but they also affected the reducibility and physical structure of the catalyst. The preparation method mostly affects the surface area and the pore structure of the catalyst. The catalyst prepared using FP method catalyst has generally larger surface area and wider pore distribution where as catalyst prepared via impregnation (IP) method has smaller surface area and more centralized pore distribution. The interaction between ZrO2 and copper oxide causes the change in microscopic state of Cu surface. In IP method, the catalyst exhibited better CO2 conversion and methanol yield. Furthermore, it is noted that the catalyst performance here is mainly determined by the strong interaction between Cu and ZrO2 rather than the surface area. 2.2. Effect of promoter on the catalyst system It is well noted in literature that the catalytic performance of bimetallic catalyst is considered better than monometallic catalyst and therefore, it has attracted the research attention for further development. The purpose of introducing metal/metal compound to the catalyst is to better tune the selectivity and to enhance the activity of the catalyst [32]. Wang [11] reported the effect of promoter on Cu based catalyst activity. The study focused on the key difference between Cu based catalyst with different promoters and its effect on copper crystalline size. It is well established that crystalline size has a direct correlation with the catalyst activity [33]. Among the catalysts prepared, the CuZnAlZr catalyst showed the highest space time yield (STY) for methanol synthesis over the whole temperature range (463–513 K). The presence of Al and Zr reduced the crystallinity of copper which increased the catalytic activity. Meanwhile, the introduction of Mn increased the copper crystalline and reduced the catalyst activity. Based on the TEM

K.A. Ali et al. / Renewable and Sustainable Energy Reviews 44 (2015) 508–518

images, ring like structures were observed with metallic copper forming the rings and ZnO particles occupying the middle area. This structure provided more active sites for methanol synthesis and improves the catalyst stability by making it harder to sinter and it was attributed to the metallic copper coating by ZnO particle [11]. Other than that, the addition of aurum has also been found to affect the performance of Cu-based catalyst. In the work by Mierczynski [34], 20% Cu/Cr2O3/3Al2O3 gave the highest methanol yield and 5% Au–20% Cu/Cr2O3/3Al2O3 showed the highest selectivity for methanol synthesis. High selectivity could be due to the formation of Au–Cu alloy and that Au particle in the catalyst that served as nucleation center for Cu crystallization. Low yield of methanol for Au–Cu catalyst might be ascribed by low dispersion of Au particle obtained from using classical impregnation method [35]. To further improve the Cu/ZrO2 catalyst performance, the catalyst was doped with various amounts of La [27]. The surface area of the catalyst increased with the increase in La loading due to the formation of surface defects that hinders the crystallization and grain growth of CuO and ZrO2. As for the Cu surface area SCu, there was a decrease with increasing La loading. This is because with high La loading, the position of Cu was occupied by La. The catalyst activity was found to have correlation with SCu. Meanwhile, the selectivity for methanol was related to the distribution of basic site on the catalyst surface. In another investigation, Zhang [36] reported the effect of TiO2, SiO2, and TiO2–SiO2 promoters on the performance of CuO/ZnO/ Al2O3 catalyst for methanol synthesis (Table 2). According to him, TiO2, SiO2, and TiO2–SiO2 promoted CuO/ZnO/Al2O3 catalysts exhibited better catalytic activity than the one without promoter. Among these, TiO2–SiO2 promoted catalyst had the highest CO2 conversion and methanol yield. The XRD results indicated that the promoters enhanced the dispersion of CuO and ZnO based on larger BET and Cu specific surface area. This well dispersion of ZnO increased the interaction between CuO and ZnO, leading to better activity [37]. The H2-TPR showed that TiO2–SiO2 promoted catalyst not only enhanced the dispersion, but also increased the CuO reduction temperature which led to higher catalytic performance [38]. The addition of CeO2 to Cu/ZrO2 and Cu/ZnO catalyst reported by Bonura [39] showed a negative effect on activity but enhanced the selectivity of the catalyst. The introduction of CeO2 might cause a decline in the surface area of the catalyst and increase in pore volume and average pore diameter. These properties might have been contributed by the poor thermal and chemical stability of CeO2 and strong interaction between CeO2 and support [40]. In the work done by Samei [41], the addition of colloidal silica and metal oxides (Mn, Ga and Zr oxides) was found to increase the CuO/ZnO/Al2O3 catalyst activity and stability. Based on the temperature-programmed reduction (TPR) curves, the addition of SiO2 increased the reduction temperature of the catalyst. Further increase in reduction temperature was observed with the introduction of metal oxide together with SiO2. The increase in reduction Table 2 CuO/ZnO/Al2O3 BET surface area and performance for methanol synthesis [36]. Catalysts

SBET (m2/g)

CO2 CH3OH conversion (%) selectivity (%)

CH3OH yield (%)

CuO/ZnO/Al2O3 TiO2/CuO/ZnO/ Al2O3 SiO2/CuO/ZnO/ Al2O3 TiO2–SiO2/CuO/ ZnO/Al2O3

26.32 33.69

15.81 20.24

23.31 27.15

3.69 5.50

37.77

16.10

25.29

4.07

42.72

40.70

41.17

16.76

511

temperature resulted in better Cu dispersion or smaller Cu particles. The BET surface area and pore volume was also increased with the addition of SiO2. The methanol yield was greater with the presence of SiO2 due to its ability to reduce catalyst deactivation by suppressing the crystallization of the constituents during the reaction. Catalyst deactivation is normally caused by the water formation along with methanol during methanol synthesis [42]. The stability and activity were even better when ZrO2 and Ga2O3 were added as they improved the specific surface area and specific activity per unit Cu surface area. 2.3. Effect of support on catalyst system It is reported that other than the metallic active site, support may also plays a vital role to modulate and enhance the catalyst performance [43]. The work of Maniecki [44] showed the effect of various support (CrAl3O6, FeAlO3, and ZnAl2O4) on the catalytic activity of Cu based catalyst for methanol synthesis. Comparison of catalytic activity indicated that the 20% Cu/ZnAl2O4 catalyst had the highest selectivity and activity, with 20% Cu/FeAlO3 catalyst showed the lowest selectivity and activity. In addition, FeAlO3 support had higher BET surface area compared to ZnAlO3 even at higher calcination temperature. The better catalyst activity of Cu/ ZnAlO3 was due to the synergetic effect of support and Cu composition which is shown in the XRD pattern. Similarly, Mierczynski [44] investigated the influence of support composition of copper supported catalysts containing 20 wt% Cu over (Al2O3, ZnAl2O4, ZnAlO2.5, Zn2AlO3.5 and ZnO) on the structure of the catalysts and their catalytic activity in methanol synthesis. The high yield and selectivity of methanol obtained from Cu/ZnAl2O4 catalyst suggested that Cu ¼Zn sites were the active site for methanol synthesis. Higher Cu/ZnO ratio increased the catalyst activity. The optimum ratio for this catalyst was reported as 0.72 for Cu/ZnO and 0.5 for Al/Zn. These results were further confirmed by the TPR profile for 20% Cu/ZnAl2O4. The IR spectrum indicated that bidentate formate was directly hydrogenated to methanol through methoxy synthesis. The main intermediate species during methanol synthesis are b-HCOO-groups and hydrogenation of this species is the rate-limiting step of the reaction [45]. However the work by Jia [46] showed that copper content had an influence on the structural properties and catalytic performance of pre-reduced LaMn1  xCuxO3 (0 r xr1) when used for methanol synthesis. Methanol selectivity and CO2 conversion were the highest at 50% copper content (x ¼0.5). For this composition, an interaction between Cu þ and Mn was observed. This interaction provided fine dispersion of active site (Cu þ ) to absorb more CO2 and inhibit further reduction of Cu þ to Cu0. Catalyst with higher copper content showed lower activity due to lack of interaction between Cu þ and Mn and the failure of perovskite structure to form completely. 2.4. Other catalyst system Conventional Cu-based catalysts have been extensively studied for futher improvement in methanol synthesis. The use of alternative catalysts containing active sites other than Cu is rarely investigated due to low selectivity and activity. However, there are few studies that indicate the opposite [46]. In the work done by Grabowski [47], Ag/ZrO2 catalyst showed good performance in methanol synthesis. It was found that both selectivity and activity did not have any correlation with Ag0 dispersion and content. Instead, it showed dependency toward t-ZrO2 phase content and Ag þ /Zr ratio. This revealed that methanol synthesis did not occur on the metallic Ag surface but on Ag þ sites. The rate of methanol formation increased with the increase in t-ZrO2 phase content and

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Ag þ concentration. The Ag þ active sites were stabilized by the oxygen vacancies in the t-ZrO2 phase. It was also noted that the introduction of ZnO had no significant effect on the methanol yield which differed from the behavior of Cu-based catalyst. Kong [48] on the other hand reported the development of coprecipitate type of Pd-decorated multiwalled carbon nanotubes (MWCNT)-promoted Pd-Ga2O3 catalyst. Combining metal catalyst with MWCNT could increase the catalyst selectivity and activity due to its advantagoues features such as excellent performance for H2 adsorption, high thermal conductiviy, large surface area, and highly graphitized tube wall and nano sized channels [49]. High methanol yield and selectivity was observed for the doped Pd– Ga2O3 compared to the undoped catalyst. This result could be related to the CNTs excellent adsorption/activation of H2 [13]. The presence of CNTs also increased the stability of the catalyst as there was no catalyst deactivation after about 200 h of reaction. Ahmed [7] investigated methanol synthesis using ordered layered double hydroxides (LDH) consisting Zn and/or Cu. The catalyst was further enhanced by the addition of Al or Ga. The advantages of using LDH are the sorption capacity for CO2 in the layered space and tunable semiconductor properties based on the choice of metal cations. The main product form is CO and methanol. The highest selectivity and yield for methanol were achieved using [Zn1.5Cu1.5Al(OH)8]2þ (CO3)2  mH2O catalyst. Comparison with other catalysts indicated that Cu was the active site for methanol formation. The addition of Ga increased methanol formation whereas the introduction of Al favored CO formation.

3. Catalyst for methanol synthesis from syngas 3.1. 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 was found that the preparation of ZrO2 support had considerable effects on the methanol synthesis. The catalyst prepared using alcogel/thermal treated with nitrogen method (CuO/ZrO2-AN) showed better catalyst performance than the one obtained through conventional precipitation method (CuO/ZrO2-CP). In this regard, the good performance of CuO/ ZrO2-AN catalyst was attributed to the larger surface area, which allowed better dispersion of the active Cu species. Other than that, calcination temperature of the support and Cu/ZrO2 also affected the catalyst activity. Higher calcination temperature might increase the growth of ZrO2 particle and reduce the surface area. These are undesirable because of the low dispersion of active species leading to reductions in the conversion of CO and selectivity towards methanol. Effects of aluminum emulsion and its preparation method on the performance of the copper-based catalyst modified with aluminum precursor has been investigated by Wang [23]. In this investigation, the activity of these catalysts was compared with that of traditional catalyst prepared using pseudo boehmite (AB-3) and commercial catalyst (GA-4). It was found that, the catalyst prepared using aluminum emulsion (AF-1) showed the highest CO and CO2 conversion as well as the highest selectivity for methanol. The precursor of (AF-1) catalyst had high rosasite phase (Cu, Zn)2CO3(OH)2 which could promote catalyst activity. Subsequent calcinations caused the generation of sosoloid of CuO and ZnO by

Table 3 Effect of calcination time and temperature on methanol synthesis [55]. Calcination temperature (K)

Calcination time (h)

Deactivation rate (%)

CH3OH Highest STY selectivity (%) (g/kgcat h)

573 623 623 623 623 623 673

4.0 0.5 1.0 2.0 3.0 4.0 4.0

6.33 2.09 0.48 0.43 0.48 1.01 1.83

99.3 99.3 99.5 99.4 99.4 99.3 99.2

157.4 161.5 165.4 172.2 159.8 169.1 116.9

the decomposition of rosasite phase [51]. As for the aluminum emulsion preparation, significant effect on the morphology of the catalyst could be observed. Emulsion prepared using normal precipitation method showed uniform size distribution, lower crystallization degree and high Cu þ /Cu0 ratio. The ratio of Cu þ / Cu0 was responsible for good catalytic activity as both Cu metal and Cu2O were required for an active catalyst [52]. This high synergy between CuO and ZnO resulted in higher catalytic activity. The aluminum precursor also strengthened the CuO–ZnO and CuO–alumina interaction and increased the reduction temperature of Cu species. Meanwhile, co-precipitation method is generally used to synthesize Cu/ZnO catalyst for low temperature methanol synthesis from syngas with alcohol in which solvent and promoter usually have some problems. It was found that the precursor experienced significant sintering during drying [38]. To overcome this problem, Meng [53] proposed the use of supercritical CO2 (SCCO2) to extract the solvent from the catalyst precursor to avoid agglomeration of the catalyst structure. The Cu/ZnO catalyst prepared by SC-CO2 drying method showed excellent performance for methanol synthesis from syngas compared to the conventional Cu/ ZnO catalyst. The presence of water in the catalyst precursor could enlarge the pore size and destroy the structure of the catalyst during calcination. With SC-CO2 drying method, water can be extracted by maintaining the precursor structure, thus improving the dispersion of Cu active site [54]. Based on such findings, drying temperature and pressure were deemed the main parameters that influence the catalyst performance. An optimum temperature and pressure of 308 K and 8 MP gave maximum BET and Cu specific surface areas. In the liquid phase methanol synthesis, the use of gas phase Cu/ZnO/Al2O4 catalyst has shown effective activity but it lacks in stability. Thus far, not much attention has been given on the effect of preparation parameter on the stability of liquid phase Cu/ ZnO/Al2O4 catalyst. Zhang [55] reported that both calcinations temperature and time could influence the catalyst activity and stability. As shown in Table 3, the best calcinations temperature and time were found to be 623 K and 2 h, respectively. The XRD, DTG and TPR data showed that the catalyst prepared at lower calcinations temperature and shorter calcinations time did not fully decompose [56]. When the calcinations temperature was sufficiently high and time was sufficiently long, weaker interaction between CuO and ZnO species was caused by sintering of CuO species [57]. Stronger interaction between CuO and ZnO led to better performance of the catalyst. 3.2. Effect of promoter on catalyst system Previously, many of the catalysts developed are strong base catalysts which have low industrial application as the feedstock for industrial methanol synthesis such as gas from methane reformer or coal gasifier generally contains traces of CO2. A study conducted by Tian et al. [58] showed that combination of solid catalyst (copper– magnesia) and homogeneous catalyst (potassium formate) enhanced

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Fig. 3. Reaction pathway for methanol formation [58].

low temperature methanol synthesis from syngas containing traces of CO2. When the catalysts were used separately, low CO conversion and high selectivity of methanol were found for Cu/MgO catalyst, whereas potassium formate catalyst showed high CO2 conversion but no methanol formation. Higher CO and CO2 conversions were achieved when Cu/MgO was combined with HCOOK catalyst. According to the reaction pathway postulated (Fig. 3), the synergy between HCOOK for the esterification process and Cu/MgO for hydrogenolysis process was crucial for high methanol yield. In a similar work, Na compounds such HCOONa, Na2CO3, NaHCO3, and NaOH have been found to have an effect on methanol synthesis using Cu/MgO catalyst [59]. Among the promoters, Na2CO3 showed the highest selectivity and yield of methanol. The chemical interaction between alkali and copper–magnesium surface improved the hydrogenolysis activity of the catalyst and formed active site more easily. As the loading of Na2CO3 was increased, the efficiency of the catalyst increased up to a peak point of 9 wt% Na2CO3 and then decreased. Increasing loading of Na2CO3 resulted in increasing concentration of methyl formate which is desirable for methanol synthesis. However, with higher loading, the methanol yield was reduced due to the excessive alkali that could cover some part of hydrogenolysis site on the catalyst surface. Even with good activity towards methanol yield, it is noteworthy that the catalyst activity dropped significantly after a short period of time. For further enhancement in catalytic activity and stability, exploration works to modify the catalysts could be performed with the addition of metal and metal oxides. Meshkini [60] carried out a study on the effect of metal oxides (Mn, Mg, Zr, Ce, Ba, Cr, and W) on the Cu/ZnO/Al2O4 catalyst. Using design of experiment, the effect of each promoter and their interaction could be obtained. It was found that the addition of Ce reduced the activity and stability of the catalyst, whereas the addition of Mn and Zr showed desirable effects. When only Mn was added, there was only improvement in the activity. In the case of Zr addition, the effect was more towards the stability of the catalyst. Thus, the addition of both promoters created a synergy that enhanced both catalytic activity and stability. Mn and Zr are also structural promoters as they could improve the dispersion of Cu particles on the surface of the catalyst.

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stability of the catalyst was also good as no deactivation was observed after few hours. As catalyst pallet and particle packed beds are sensitive to temperature and has low single pass conversion, Phan [63] conducted a study on methanol synthesis using Cu-based monoliths. Monolithic catalysts give higher mechanical strength, low pressure drop and also enhanced heat and mass transfer due to straight channel and thin walls [64]. The work was focused on the effects coating technique (CuO/ZnO/Al2O3 slurry coating, Al2O3 sol–gel coating with impregnated CuO–ZnO, Al2O3, colloidal coating with impregnated CuO–ZnO, Al2O3, colloidal coating and deposition–precipitation) for Cu-based catalyst onto steel monolith for methanol synthesis. The result indicated that, the Cu-based monolith prepared by slurry coating via two-stage co-precipitation showed better Cu dispersion which translated into higher catalytic activity as compared to the other coating methods. It even showed superior activity compared to the powder catalyst used in a fixed bed reactor. This could be due to the good thermal properties and low pressure drop displayed by the monolith. Other than monoliths, the use of long carbon nano tubes (CNTs) has been reported to enhance the methanol yield [65]. The addition of long CNTs to Cu/Zn/Al/Zr catalyst created a catalyst with large surface area that provides more dispersion of the Cu/Zn active site. Other than that, CNTs could also reversibly adsorb and store considerable amount of H2, which can later be spillover to the active sites to increase the hydrogenation reaction [66].

4. 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 which water serves as hydrogen source [7]. CO2 þ H2O-CH3OH þ3/2O2 þ

H2Oþ 2p -1/2O2 þ2H  ; H2 O-eaq

n n

þ

þ

H ; OH; H ; H2 ; H2 O

(5) (6) ð7Þ

4.1. Effect of promoter on catalyst system 3.3. Other catalyst system The catalyst developed for methanol synthesis has been tested in packed bed reactor. Researchers have found that fluidized bed reactor is not only better in terms of good heat transfer and less expensive to manufacture, it can also limit diffusion limitation as usually smaller sized catalyst particles are used. Since common fixed bed reactor catalyst is not suitable to be used in fluidized bed reactor [61], a series of Cu–Zn–Zr catalysts with different compositions have been developed to have good catalytic activity and mechanical strength [11]. For methanol synthesis reaction, the catalyst with a composition of Cu:Zn:Zr ¼4.5:1.5:3 had the highest activity and attrition resistance. The SEM images of the catalyst showed that the particles were nearly spherical which was in agreement with Pham [62] that reported spherical particles were more attrition resistant than non-spherical particles. Overall

Even though TiO2 catalyst generally shows good selectivity and product yield as compared to the other catalysts, the yield is still considered to be low. The goal of increasing the yield has driven studies to be conducted for further improvement in the catalyst activity. One of the prominent ways is by doping TiO2 catalyst with other metals and semiconductors. Tseng et al. [67] studied the effect of copper loading on TiO2. The yield of methanol was greatly increased and was much higher compared to that of the sol gel derived TiO2 and commercial Degussa P25. The photo catalytic activity of Cu/TiO2 was improved due to the lowering of recombination probability for hole–electron pairs. The supported Cu facilitated in the redistribution of electric charge and the Schottky barrier of Cu and TiO2. Slamet et al. [68] reported the doping of TiO2 with CuO. CuO was found to be the most active dopant compared to other copper species because it has the highest potential redox. High Cu loading (optimum loading of 3 wt% Cu)

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increased the efficiency of the catalyst as Cu served as active site and good electron trapper. Catalyst with more than 3 wt% Cu reduced the methanol yield due to its shading effect by which the photo excitation capacity of TiO2 was inhibited. Wang [11] investigated the visible light (λ 4420 nm) photoreduction of CO2 using CdSe quantum dots (QD) sensitized Pt/TiO2 heterostructured catalysts. It was found that the QD’s uniform dispersion and close/direct contact with TiO2 was important for efficient separation across the CdSe and TiO2 heterojunction. The distribution can be observed from SEM image and EDS analysis. The Quantum confinement of the dots shifts the conduction band of CdSe to higher energies, which facilitates charge injection into TiO2. According to the redox potential, this shift is also favorable for the injected electron to initiate the reduction of CO2 with water, thus increasing the catalyst activity. Other than that, the product composition was also dependent on the metal co-catalyst (in this case Pt) as H2 was the main product when Fe was used. Luo [69] reported the effect of Cu and Ce doping on methanol synthesis. Three types of TiO2 co-doped catalyst, Ce–Cu/TiO2, Cu– Ce/TiO2, and (Ce–Cu)/TiO2 were synthesized. It was observed that the sequence in which the metals were impregnated significantly affected the surface area of the catalyst, the structure of the catalyst and the phases formed on the surface of the catalyst. Of the three types of catalyst, Cu–Ce/TiO2 showed the highest yield of methanol. This was due to the well-proportioned Cu and Ce on the surface of the catalyst [67] as compared to the other two. Similar increase in methanol yield was observed when TiO2 was doped with Ag [17]. The introduction of Ag increased the specific surface area of Ag-modified catalyst as compared to pure TiO2 catalyst. However, the major factor that influenced the activity of the catalyst was the Ag concentration. The catalytic activity of the Ag/TiO2 catalyst increased with the increase in the Ag dopant concentration. The highest yield for methanol was obtained with 7 wt% Ag/TiO2 catalyst. For catalysts having Ag concentrations up to 5%, more effective electron hole pair was generated due to the decrease of absorption edge created by Ag impurity inside TiO2. As for catalysts having Ag concentrations of more than 5%, the formation of metal cluster inside TiO2 formed a Schottky barrier at the metal–semiconductor interface that decreased the electron– hole recombination rate [70]. TiO2 nanotubes (TNTs) are less preferred over TiO2 powders for photoreaction due to low quantum efficiency and low energy absorption properties [71]. However, TNTs performance can be improved with the addition of CdS or Bi2S3 [72]. The visible light absorption of TNTs is greatly enhanced through the modification with CdS and Bi2S3. It is theoretically due to smaller band gap of CdS and Bi2S3 as compared to TNTs. On the other hand, the surface area and the CO2 absorption capacity of the catalyst experienced reductions when CdS and Bi2S3 were added. Despite these drawback, TNTs–Bi2S3 showed the highest methanol yield followed by TNTs–CdS and TNTs. This high yield was attributed to the formation of heterojunction structure which improved the separation of electron and holes. This prevented charge recombination and prolonged the lifetime of the photo carrier [73]. Narrow band semiconductor compounds FeTiO3 was coupled with TiO2 to enhance visible light response and at the same time improve electron–hole separation [74]. For both UV–vis and visible light irradiation, FeTiO3/TiO2 (FTC) catalyst exhibited better photocatalytic activity than the bare TiO2 and Degussa P25 catalyst. The improvement in catalyst activity may be attributed to junction effect between two semiconductors and the narrow band gap of FeTiO3 [75]. It was also reported that increasing loading of FeTiO3 caused a reduction in methanol yield. The high concentration of metal content might form recombination centers that could lead to reduction in photocatalytic efficiency.

4.2. Effect of catalyst preparation method Cobalt-phthalocyanine (CoPc) has been used as a sensitizer to enhance the TiO2 catalyst performance under visible light irradiation but the product yield was still low. Further enhancement in catalytic activity could be achieved by in-situ CoPc–TiO2 synthesis method [76]. Through the in-situ synthesis process, the reactants (Co salt and 1,2-dicyanobenzene) were isolated homogenously in pores of TiO2, that limited long distance movement and collision between molecules. The distribution of CoPc molecules into the pores of TiO2 matrix created a “cage effect” that prevented the CoPc molecules from migrating to adjacent pores, which could successfully avoid dimerization and aggregation of the molecules [77]. This contact enhanced the catalytic activity as the efficiency of electron–hole produced by CoPc and electron trapping by TiO2 increased. Other than that, this method also ensured that the particle sizes obtained were uniform and had optimum diameter that gave maximum catalytic efficiency [65]. In a work by Koci [17], TiO2 particles were prepared using sol gel and precipitation methods. Anatase TiO2 particles with crystallite sizes ranging from 4.5 nm to 29 nm were synthesized using both methods. There was a change observed in band gap energy with the variation of TiO2 particle sizes with the largest band gap energy (  3.14 eV) for catalyst having smaller particle size prepared by precipitation method. The smallest band gap energy ( 3 eV) was recorded by the catalyst having larger particle size prepared using sol gel method [78]. As for methanol yield, the highest yield was obtained with the catalyst prepared using precipitation method with particle size of 14 nm. Usually, the catalyst activity depends on surface area and band gap energy [79]. However in this case, the particle aggregation [80], size quantization effect [55] and increased surface electron–hole recombination [79] could overshadow the major effects. The competing effect of charge carrier dynamic, specific surface area and light absorption efficiency of the catalyst prepared by precipitation method determined the better catalyst.

4.3. Other catalyst system Besides TiO2, other semiconductors with narrow band gap that can give good selectivity for CO2 photoreduction is Cu2O. Li et al. [72] investigated photocatalytic reduction of CO2 to methanol using Cu2O/SiC catalyst under visible light radiation. SiC is well known for its strong reduction performance [81]. Thus, it was postulated that the combined effect of Cu2O and SiC could increase the catalyst performance. The result showed that Cu2O/SiC had the best photocatalytic performance as compared to Cu2O and SiC. This good performance was attributed by the potentials of conduction bands of Cu2O and SiC that are more negative than those of methanol yield in water and also the large average pore diameter and small particle size of SiC. The narrow band gap and much more negative conduction bands potentials has made CdS and Bi2S3 ideal for methanol synthesis under visible light irradiation [82]. In the work by Li et al. [72], Bi2S3 modified CdS catalyst showed higher methanol yield compared to CdS and Bi2S3 alone. Based on the UV–vis response, the enhanced visible light response of the catalyst could be attributed to the presence of Bi2S3. The introduction of Bi2S3 also increased the specific surface area and the average pore volume of CdS. Fig. 4 shows the mechanism for the catalyst. Since the conduction band potentials of CdS and Bi2S3 are more negative than those of methanol yielded from CO2, CO23  , and H2CO3, both could serve as CO2 reducing agent. As for the oxidation of H2O, only CdS served as the oxidation agent because it has a more positive valence band potential.

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Fig. 4. Mechanism of photoreduction of CO2 to methanol [72].

lnNbO2 has also shown good catalytic activity under visible light radiation. Better activity has been achieved with the addition of NiO and Co3O4 co-catalysts [83]. The effect of pretreatment has also been investigated. Without any pretreatment, 1 wt% NiO– lnNbO3 had the highest methanol yield while with pretreatment, 0.5 wt% NiO–lnNbO3 showed the highest methanol yield. The pretreatment causes ultra fine NiO particles to aggregate to form large particles on lnNbO3 surface. This induced the formation of positive electron–hole which is undesirable in CO2 photoreduction process. As for 0.5% lnNbO3, the presence of shell–core structure with NiO thin shell and metallic Ni core acted as bias for electron transfer from lnNbO3 to NiO layer. Comparing NiO with Co3O4, higher methanol yield was obtained with NiO co-catalyst without pretreatment. The difference in activity might be due to the redox property of Co cations that served as good recombination centers [84]. NiO–lnNbO3 showed high activity because of the presence of tiny NiO particles and small amount of Nb2O5.

5. 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 sites. Some researcher suggests that the catalytic activity is proportional to the surface area of metallic Cu (Cu0) [85]. This is based on the fact that only Cu0 was observed on the surface of Cu(1 0 0) [86] and that the catalyst activity increased as the Cu0 increased up to Cu/ZnO ratio of 8 and then decreased [87]. Works by Chinchen [88] on the other hand concluded that large fraction of Cu0 surface was covered by oxygen containing species and that the activity of the catalyst was independent of Cu0 surface area. This result was supported when Cu þ ion was found dissolved in ZnO matrix and the rod-like catalyst structure with more Cu þ had higher catalytic activity than that of the platelets [89]. When comparing methanol synthesis on oxidized Cu(1 0 0) with clean Cu(1 0 0), Szanyi and Goodman [90] concluded that Cu þ was the active site that governed the catalyst activity. Even with all this disagreement, it is well established that the activation of CO/CO2 and homogenous splitting of H2 occur on Cu þ or Cu0. Knowing this information is important so that when support and promoters are added to the catalyst, the improvements

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expected from them are clear. This is vital for better understanding and further enhancement of the catalyst. A good supports must be able to provide large surface area for better dispersion of active site [91]. It also helps in terms of improving the interaction between active site and promoter, modulating the adsorption strength between reactant molecules and catalyst surface, which influences catalyst selectivity [38] and also avoiding catalyst poisoning and sintering [92]. From the work described above, the supports used have provided one or more criteria for a good support but none could fulfill all the necessary criteria. Among the supports, Zn-based and Zr-based supports have shown the best improvement on the stability and activity of the catalyst. It has been observed that heterogeneous splitting of hydrogen occurs on ZnO surface and it serves as hydrogen reservoir which enhances the reaction by reverse spillover of hydrogen from ZnO [93]. The adsorption of hydrogen is greater when ZnO is combined together with Cu metal [94]. Few studies have also suggested that ZnO support is an active component in methanol synthesis [95]. ZnO support provides Zn–Cu active site by mitigating to the Cu surface [96]. Other than that, ZnO can also absorb poisonous species available in the feed gas such as H2S. As for Zr-based support, the interaction between Cu active site and ZrO2 changes the morphology of the catalyst from monoclinic crystal phase to tetragonal crystal phase [97]. This special structure and can improve the stability of the catalyst and it is the main reason for increasing interest in Zr support even tough methanol yield is lower than Zn-support. Good thermal resistance of Zr further increases its stability that is essential under reducing and oxidizing atmospheres [98]. Since methanol synthesis catalysts are structurally sensitive, they are often modified with the addition of promoter to further enhance the selectivity, activity and stability. Most of the metal and metal oxides described above show positive improvement but there are few that do not show any enhancement in the catalyst properties. It may be inaccurate to say that these promoters are unfavorable as combination of different parameters could also affect the final result. Preparation method, support [99], and promoter precursor [100] are few parameters that could interfere and hinder the promotional effect of the promoter. Work by Rynkowski and Gomez [101] showed that classical impregnation method led to low dispersion of Au particle on Cu-based catalyst which resulted in low catalytic activity. The interaction between promoter and support was reported by Boujana [102]. Different FT-IR spectra were obtained when Pt was added on different support which indicated that the catalyst had different properties. Metals such as Zn, Al, Zr, Mg and metal oxides such as SiO2, TiO2, Al2O3, can be regarded as promoters that have shown good improvement for Cu catalyst. Even so, researchers are still engaged in the search for a new promoter that can produce a better methanol synthesis catalyst. In order to achieve that, problems related to active site, effect of support and interaction of support, promoter and active site must be overcome first [100]. In order to produce an optimum catalyst, the influence of preparation method and preparation conditions must also be fully understood. Reports on various improvements for conventional preparation method and preparation condition as well as novel preparation method are available in literature. However, issues such as sensitivity towards pH, catalyst preparation time and cost, and effect of calcination temperature and time for co-precipitation method have been unsatisfactorily addressed. Many of the works have resulted in significant enhancement in stability and activity of the catalyst. Varying the pH value in catalyst preparation can affect the phase composition of the precursor. Li and Inui [103] found that

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constant pH was important to have very fine inter-dispersion of CuO and ZnO after calcination. In addition, novel preparation method for low temperature methanol synthesis catalyst and improvement of liquid phase methanol synthesis catalyst showed promising prospect to overcome thermodynamic limitation of the synthesis process. Since the reaction is exothermic, reaction heat should be remove to avoid reduction in catalytic activity [104]. Methanol synthesis at low temperature can overcome this problem by offering high heat transfer efficiency, high conversion per pass and low operating cost [105]. Another way to overcome this limitation is by using membrane dual-type reactor [106]. This type of reactor can also enhance kinetic limited reaction. It also ensures lower catalyst deactivation and better control of stoichiometrical feed. Higher reactant conversion can be achieved by removing products from reaction zone and reactants permeation to the reaction zone, while sintering can be minimized by removing water using permselective membrane [40]. The availability of H2 gas and the poisonous nature of CO are the main drawbacks for methanol synthesis from CO and CO2 hydrogenation. From the environmental point of view, these processes are undesirable because of the energy requirement to produce H2 and syngas. This requirement is usually met by fossil fuel burning which increases environmental problems such as emission of greenhouse gasses. On the other hand, methanol synthesis from CO2 and H2O using light irradiation at ambient temperature and pressure is a good approach environmentally. Much of the work done so for is focused on semiconductors with narrow band gap energy. Semiconductors are chosen based on their band gap position and electron acceptor species and electron donor species redox potentials [107]. Reaction using visible light irradiation is made possible by the introduction of dopant and variation in synthesis method for the semiconductors. These improvements have also lowered the electron and hole recombination rate that can affect methanol yield. In order to have high catalytic activity, low recombination rate is required. This means that more electron and hole can migrate to the surface for reaction to occur. A good photocatalyst should be able to improve charge separation to increase the life time of charge carrier. It should also enhance the efficiency of charge carrier and prevent charge carrier recombination [72]. Thus far, most of the researches have focused on improving the catalyst activity by introducing metals or other semiconductor materials. However, the methanol yield is not as high as yield from CO and CO2 hydrogenation. Therefore, other factors that could affect the catalyst performance should be explored and investigated. Factors such as the influence of TiO2 facet on the catalyst activity and its effect and interaction with the metal introduced can be investigated [45]. The current available preparation method can be further modified so that TiO2 with specific facet that show good methanol selectivity can be synthesized. Other than that, most of the work done so far has neglected the due mention on stability and deactivation of the catalyst. This is important to ensure that the catalyst has good reusability and can be used for long and continuous process. Therefore, it clear that even tough methanol can be synthesized from CO2 and H2O, there still much to be done before it can be applied in industry.

6. 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 affect the physicochemical properties of the catalyst. The same can be said in the case of promoter selection. Thus a proper understanding between these interactions must be developed to obtain a tailor made catalyst for this application. Other than that, the catalyst developed must also have good stability. This feature is rarely discussed in literature so far. A catalyst that has high stability is important when it comes to industrial application. Therefore, these aspects together with the molecular interaction mechanism should be the focus of future studies.

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