Alcohol promoted methanol synthesis enhanced by adsorption of water and dual catalysts

Abstract

Alcohol-promoted methanol synthesis uses heterogeneous methanol synthesis catalysts in alcoholic solvents where the alcohols act as a co-catalyst. In the presence of alcohol, the reaction proceeds through alcohol formate ester as an intermediate, allowing methanol synthesis at lower temperatures than conventional gas-phase synthesis. In the present work, alcohol-promoted CO2 hydrogenation to methanol was studied experimentally using a Cu/ZnO catalyst with 1-butanol and 2-butanol as solvents. As water is known to inhibit methanol synthesis on Cu/ZnO catalysts, the alcohol-promoted process was further developed by in-situ adsorption of water using a 3Å molecular sieve. The methanol productivity significantly improved as a result of the lowered concentration of water.The concentration of water was thus identified as a key factor affecting the overall methanol productivity. As the alcohol-promoted methanol synthesis process is characterized by two separate reaction steps, the use of separate catalysts optimized for each step offers an interesting approach for the development of this process. Such a dualcatalysis concept was tested using a copper chromite catalyst together with Cu/ZnO. Promising results were obtained, as methanol productivity increased with the addition of copper chromite. Catalyst characterization was carried out using XRD and SEM-EDS and potential effects of observed changes in catalyst structure during reaction are discussed.

Cu-Based Nanocatalysts for CO2 Hydrogenation to Methanol

“…As already suggested previously, Zhao et al 105 argues that a trans-hydrocarboxyl intermediate over bulk Cu (111) is kinetically more favorable via a different route (Figure 4), as formate has a higher hydrogenation activation energy (1.24 compared to 0.43 eV). Active metals with a higher surface charge than Cu, such as Ce or Rh, 106,107 have been reported to potentially generate other intermediates, including carboxylates.…”

“…Through using liquid paraffin, the formation of H 2 CO becomes the rate-determining step (Figure ), indicating that liquid media alters surface activity from altered surface charges. , However, liquid paraffin reduces the reaction activity from altered Cu surface charges, specifically due to the d-band being further away from the Fermi level (that is, electron adsorption becomes more difficult), thereby not making this system ideal for methanol generation due to reduced CO 2 and H 2 reactant adsorption. Fan et al (and Nieminen et al) suggested that using an alcoholic solvent (e.g., 1-butanol or 2-butanol) can promote methanol production at lower reaction temperatures due to ethyl formate being formed at ∼170 °C, lower than the normal reaction temperature range at 200–300 °C. , The former research converted CO 2 and H 2 to methanol faster if using ethyl formate directly, while the latter research operated a “blank” experiment by using a hexane solvent instead of an alcoholic solvent which generated no methanol to suggest the promotional effect, although neither investigation truly confirmed the existence of a formate intermediate. Nonetheless, given the highly altered and seemingly more difficult reaction mechanisms and how operating pressures remain high (up to 60 bar), liquid media should perhaps be avoided for this reaction.…”

“…Several three-dimensional Cu bulk structures exist (Cu(xyz), i.e., x, y, and z dimensions, also known as Miller indices), all of which can affect catalytic performance differently. 89,90 The most popular atom-packed structures for CO 2 hydrogenation to methanol include Cu (111), Cu (110), and Cu(100), 91−93 while other structures such as Cu (211), 94 Cu(533), 95 and Cu(775) 96 have also been reported in the literature.…”

“…Reviewing the popular Cu structures further (Figure 2), Cu (111) has a face-centered cubic structure with evenly dispersed active sites for CO 2 and H 2 adsorption, which could reduce deactivation from having a higher surface stability, although adsorption strength would also be lower from it having a flat surface. 95,100 Figure 3 displays the activation mechanisms of the highly favored formate intermediate pathway over Cu (111), as formate is more stable than the alternate hydrocarboxyl pathway, 101 with formate hydrogenation as the rate-determining step. However, the total sum of the activation energies suggests that hydrocarboxyl is instead the favored intermediate.…”

“…Fan et al (and Nieminen et al) suggested that using an alcoholic solvent (e.g., 1-butanol or 2-butanol) can promote methanol production at lower reaction temperatures due to ethyl formate being formed at ∼170 °C, lower than the normal reaction temperature range at 200−300 °C. 111,112 The former research converted CO 2 and H 2 to methanol faster if using ethyl formate directly, while the latter research operated a "blank" experiment by using a hexane solvent instead of an alcoholic solvent which generated no methanol to suggest the promotional effect, although neither investigation truly confirmed the existence of a formate intermediate. Nonetheless, given the highly altered and seemingly more difficult reaction mechanisms and how operating pressures remain high (up to 60 bar), liquid media should perhaps be avoided for this reaction.…”