The concentration of carbon dioxide in the atmosphere is at alarming level at 416 ppm with a prediction of 570 ppm by the end of this century [1, 2]. The rise in concentration level of carbon dioxide in the atmosphere is due to combustion of fossil fuel leading to massive emissions of anthropogenic CO2 in the atmosphere. The increasing CO2 concentrations resulted in some widespread concerns pertaining to environmental degradations such as global warming, ocean acidification and over all climate change [3, 4]. The increasing concentrations of CO2 is exhausting waste by keeping in mind its environmental consequences. However, the conversion of this emitted CO2 has been under investigations among the research community to utilize it as an alternative carbon source by producing fuels and other chemical commodities. Up to date, extensive research papers have been documented to convert CO2 to some valuable products. Amongst other strategies, catalytic hydrogenation of CO2 to methanol has gained a lot of interest in the research community as a viable, sustainable and economical route to combat CO2 concentration in the environment [5, 6].
On industrial basis, methanol has been produced by catalytic hydrogenation of syngas (mixture of CO and CO2) over alumina based Cu/ZnO catalysts [7, 8]. However, the performances of this traditional catalysts were not enthusiastic when applied to pure CO2 hydrogenation. Many studies have been reported by applying variety of catalytic systems to find a suitable catalytic system for pure CO2 hydrogenation to methanol. A variety of catalytic systems having different metals like Cu, Pt, Au, Co, Fe,Ni, Zn etc. have been applied to develop CO2 hydrogenation . Similarly, catalytic systems with different support such as alumina, silica, zeolite, carbon nanofibers etc. have been utilized to carry out the pure CO2 hydrogenation to methanol . In terms of active metals, the role of Cu has been recognized as one of the active metals for CO2 hydrogenation to methanol. Likewise, the promoting role of Co metal has also been observed in methanol synthesis catalysts. An extensive research has been reported for single metal catalyst with good reactivity profile for the CO2 hydrogenation to methanol [11, 12]. However, the combination of two or more metals have been resulted with further improvement of catalyst properties of methanol synthesis catalysts. This can be further justified by the fact that application of bimetallic systems led to significant improvement in catalytic activity as compared to their monometallic counterparts . In addition, the combination of active metals resulted in creating a synergic effect by increasing the reducibility of metals as well as their dispersion on the surface of the support.
Different methods of preparations have been adopted to synthesize metal supported catalysts. Co-precipitation method, is one of the commonly used methods to synthesize metal supported catalysts. However, catalysts prepared by this method generally require number of washing procedures to get rid off residual precursors in the form of nitrates, potassium, sodium etc. in order to avoid agglomeration and sintering of active metal particles. Although, an alternate option in the form of chloride or sulfate based metal precursors is always there to be utilized however, sulfate and chloride have been identified as poison in methanol synthesis catalysts. To circumvent this problem, application of metal acetate based metal precursors provides a greener approach by avoiding production of undesired nitrates contaminated wastewater on one hand and avoid possible leaching of active metals during the washing process in case of traditional protocol [13, 14].
To investigate the combination effect of Cu/CO molar ratio for the CO2 hydrogenation to methanol over zeolite supported Cu/Co bimetallic catalysts, herein, both Cu and Co were deposited on the surface of zeolite. Physicochemical parameters were assessed by employing different analytical techniques. Similarly, the reactivity profile of Cu/Co bimetallic catalysts were evaluated for methanol synthesis by CO2 hydrogenation in three phase slurry reactor.