Copper Supported on MgAlOx and ZnAlOx Porous Mixed-Oxides for Conversion of Bioethanol via Guerbet Coupling Reaction

Abstract

The direct conversion of biomass-derived ethanol to high-valued-added chemicals has attracted widespread attention recently due to the great economic and environmental advantages. In the present study, the conversion of bioethanol through the Guerbet coupling process was studied in a fixed-bed reactor for MgAlOx and ZnAlOx mixed-oxides supported Cu catalysts. From the results, Cu adding into the system greatly enhance the dehydrogenation of ethanol and increase the H-transfer in the course of Guerbet coupling process. Simultaneously, the porous mixed-oxides provide the acid-base property of the catalysts for intermediate transformation. Notably, for Cu/MgAlOx, the main product of ethanol conversion is butanol, but for Cu/ZnAlOx, the primary product is ethyl acetate. Characterizations such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and CO2 temperature programmed desorption (TPD) were carried out to evaluate the structure and property of the catalysts. In combination with the catalytic performances with the characterization results, the synergistic catalytic effect between metal sites and acid-base sites were elaborated.

Industrial crude bioethanol dehydration to ethylene: Doping ZSM-5 to enhance selectivity and stability

“…In Zone II (Figure 6) associated with the formation of ─CH3 and ─CH2 groups, a higher intensity (2900─2600 cm -1 region) is observed for the zeolite with a lower Si/Al ratio (i.e., Z26, Figure 6a) than for zeolites with a higher Si/Al ratio (Z280 and Z371 in Figures 5b and c, respectively) [48]. The above suggests that, in addition to facilitating ethanol adsorption, zeolite Z26 promotes the formation of C2H4, which is confirmed by the bands at ⁓2880 and ⁓1645 cm -1 [49]. However, desorption of ethoxy groups is slower in zeolite Z26…”

“…Furthermore, in Zone II it is observed that the dopant also influences the formation of ethoxy species involved in the formation of C2H4 [49]. Both dopants (i.e., Ce and Cu) show less intense bands in the 2900─2600 cm -1 region compared to the undoped samples (Figure 6), which would be associated with their lower selectivity to C2H4 (Figure 3).…”

“…In general, zeolite Z26 favors higher formate and carbonate formation over Si─O─Al groups (1250 cm -1 ) [50,51], these are intermediaries in the dehydration of ethanol to form olefins such as C2H4 [52], which explains its higher activity compared to H-ZSM-5 samples with higher Si/Al ratios. However, the presence of Brønsted acid sites favors not only the ethanol dehydration but also the olefin polymerization and the formation of carbonaceous compounds and aromatic deposits [17], including the 1645 cm -1 peak corresponding to the C=C group [49]. Also, during desorption, the Z26 catalyst appears to form strong bonds with formates and carbonates as it releases them more slowly compared to the other undoped samples (i.e., Z280 and Z371).…”

Ni-Based Hydrotalcite (HT)-Derived Cu Catalysts for Catalytic Conversion of Bioethanol to Butanol

“…However, when the support of NiAlO x was replaced by ZnAlO x , the product distribution changed greatly, with a large amount of ethyl acetate (Sel. of 42%) appearing in the system [34]. Previously, Boscolo et al also found that the ethanol conversion, product selectivity and carbon deposition would be greatly affected by the support component, as the Mg 2+ ion was partially substituted by Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ions [21].…”

“…For the NiFeO x , the largest amount of weak acidic sites (131.6 µmol/g) is shown when compared with those of NiAlO x (42.0 µmol/g) and NiCoO x (37.3 µmol/g). This might decrease the selectivity of the Guerbet coupling process, as the acid sites are often reported to be beneficial for the dehydration reaction in producing ether and ethyl acetate [19,34].…”