Conversion of CO2 into methanol

Formate has been reported over ZnO from CO2 and H2 during the water-gas shift reaction (7,8) and following exposure of Cu/ZnO to CO2/H2 (9). Several groups (5,10-13) have reported the direct conversion of CO2 into methanol. Chinchen et al. (11) proposed that CO2 adsorbed on Cu/Zn0/Al203 and reacted with hydrogen atoms to form a... 

In an attempt to mimic Nature, a biotechnological approach to the conversion of CO2 into methanol has been investigated based on the use of enzymes. In fact, it has been shown that the enzymes FateDH, FaidDH, and ADH are able to reduce CO2 in water at room temperature [7]. A minimum of 3 mol of NADH are consumed per mol of CH3OH produced (Scheme 9.18). 

Chapter introduces the high-temperature processes of CO2 conversion Dry Reforming of Methane (DRM) and the relevant general use of CO2 as oxidant or dehydrogenating (DH) agent. The conversion of CO2 into methanol is discussed here as it has similarities to the other processes. Such applications deal with the conversion of large volumes of CO2 into fuels or energy-rich molecules. 

Schafer A, Saak W, Haase D, Muller T (2012) Silyl cation mediated conversion of CO2 into benzoic acid, formic acid, and methanol. Angew Chem Int Ed 51 2981... 

The reasons why CO2 reduction by water is so inefficient are both thermodynamic and kinetic. From the thermodynamic point of view, CO2 reduction is highly endothermic and consequently requires more than one photon to occur. It should be noted that in natural photosynthesis the transfer of one electron requires the absorption of two photons, one in each of the two photosynthetic centers (Scheme 1.5) [18], Also from the kinetic point of view, the process of CO2 reduction is very complex since, depending on the products formed, several steps of reduction and protonatiOTi have to take place. For instance, conversion of CO2 into methane is a process that requires eight electrons and eight protons, and most probably this transformation has to occur stepwise, each step consisting in the transfer of one electron followed more or less synchronously by protonation. For the reaction of Eq. 1.1, formation of methanol requires the reduction of six electrons and six protons. 

Multiple-enzyme conversion of CO2 to formate, then formaldehyde, then methanol by FateDH, FaldDH, and ADH Enzymes loaded in CaC03 followed by LbL deposition and dissolution of core, then encapsulation into a gel bead... 

We compared our novel catalysts with a commercial methanol synthesis catalyst Cu-Zn-A1 (Cu-Zn-Al=42 45 13, in atomic ratio). As shown in Fig.la, although the conversion of CO2 for the Cu-Zn-Al/HY composite catalyst was the highest (30.5%), the selectivity of hydrocarbons was the lowest (6.3%) in our study. The decomposition of methanol to CO at high temperatures accounts for the remarkable decrease in the selectivity of hydrocarbons. Moreover, because olefins are easily hydrogenated into paraffins over Cu-based catalysts [7], no olefins and only a trace of iso-butane appeared in the products. The Fe-Zn-Zr (1 1 1)... 

In CO2 hydrogenation over Cu/ZrOj based catalysts, the methanol formation activity could be correlated with copper dispersion. The reaction intermediates of methanol synthesis were carbonate, formate, formaldehyde and/or methoxy, and the rate determining step for methanol synthesis seems to be the conversion of formate into formaldehyde or methoxy. 

The first of these new cobalt catalysts were made in 1986 by coprecipitation techniques using aqueous solutions with ammonium bicarbonate as the precipitant in a similar way to the methods used for methanol synthesis catalysts. The new catalysts were immediately found to be very active and selective catalysts for the conversion of syngas into hydrocarbons. A particularly attractive feature was their low methane make and tolerance of CO2 The CO2 tolerance was ascribed to the interplay between the support and the cobalt phase both in the oxidized and reduced forms. The general belief is that the support stabilizes the cobalt phase such that the catalyst can be operated at the higher temperatures, required to maintain activity despite competitive adsorption by CO2, without any loss in stability. Other investigators e.g. Shell have used similar strategies [2]. 

DME hydrolysis is an equilibrium-limited reaction and is considered as the rate-limiting step of overall DME steam reforming. The equilibrium conversion of hydration of DME is low at low temperatures (e.g. about 20% at 275 °C). However, when methanol formed in the first step is rapidly converted into H2 and CO2 by methanol steam reforming catalysts, high DME conversion is expected. Therefore, enhancement of DME hydrolysis is an important factor to obtain high reforming conversion. 

Thermodynamics of partial reactions. On the basis of thermodynamic data (Table 2) the energetics of the reduction of CO2 to CH4 can be divided into three parts, the energy coupling of which can be tested experimentally with cell suspensions the reduction of CO2 to methylene-H4MPT (CH2=H4MPT, formaldehyde level), the conversion of methylene-H4MPT to methyl-coenzyme M (CH3-S-C0M, methanol level), and the reduction of CH3-S-C0M to CH4 (Fig. 4). 

Catalytic hydrogenation of CO2 to hydrocarbons is classified into two categories. The one is direct hydrogenation fix)m H2/CO2 to hydrocarbons. The other is indirect process which includes methanol sjmthesis fix>m H2/CO2, followed by in situ methanol conversion to hydrocarbons using sohd acid catalyst in H2/CO2 feed. Study on indirect hydrocarbon synthesis is now popular. 

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