Hydrogenolysis of biomass-derived platform chemicals to glycols over non-noble metal catalysts
Wang, Xinde; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2020
The conversion of biomass to glycols has been regarded as a promising direction for the valorization of biomass given the broad applications of glycols in industry. Since components of the raw biomass are inherently interconnected and complicated, its derived rich-oxygen-contained platform chemical sorbitol is seen as a proper platform molecule paving the way to approach the ultimate transformation. So far, noble-metal catalysts (e.g. Ru, Pd, and Pt) have been extensively studied in this reaction. Even though superior activity was obtained over noble metal catalysts, the selectivity to glycols was not favored due to the excessive hydrogenolysis, resulting in the production of gaseous products such as CO, CO2, and CH4, thus lowering the selectivity to glycols. Besides, the low abundance of noble metals on the planet makes the corresponding catalyst increasingly expensive for future research. Therefore, more abundant non-noble metal-based catalysts with moderate hydrogenolysis activity can be potentially suitable alternatives to catalyze sorbitol to produce glycols. In this doctoral thesis, therefore, a series of catalysts with different copper loadings supported on activated carbon was prepared. The results of N2 physisorption, X-ray diffraction, N2O chemisorption, and TEM confirmed a clear trend between the Cu loading and the average particle size. These catalysts were tested in the presence of Ca(OH)2. Activity measurements revealed a distinct correlation between the TOF and the particle size, which increased until 14 nm (N2O chemisorption based) after which a plateau was reached. However, the variation of particle size had no apparent effect on the selectivity for C3/C3 and C2/C4 cleavage. In contrast, the various Ca(OH)2 equivalences had a profound impact on selectivity. The ratio of C3/(C2+C4) products reached an optimum at 0.45 eq. of Ca(OH)2. Notably, the time course of the reaction demonstrated that the byproducts lactic acid (LA) and glycerol (GLY) were converted in-situ to the desired product 1,2-PDO, contributing to the unprecedented combined glycols selectivity of 84.5% at 513 K, 5 MPa H2 and 0.3 eq. of Ca(OH)2. Furthermore, recycling tests revealed a rapid deactivation which was attributed to the aggregation of Cu observed in TEM. For the first time, the non-noble metal catalyst was observed to be active in the conversion of LA to 1,2-PDO in aqueous solution. In order to better study this reaction, different metals (e.g. Ni, Co, and Zn) and supports (e.g. SiO2, La2O, and CeO2), as well as preparation methods (i.e. incipient wetness impregnation and ammonia evaporation-precipitation), were thoroughly investigated. Catalytic results demonstrated that the Cu/SiO2 catalyst prepared by the ammonia evaporation-precipitation method outperformed the others in terms of selectivity to 1,2-PDO. The influence of base on the conversion was also comprehensively investigated. Among the screened bases (e.g. NaOH, Ca(OH)2, Mg(OH)2 and La2(OH)3), Mg(OH)2 was observed to be the most active. A conversion of 90% with a selectivity of 98% to 1,2-PDO was achieved over the Cu/SiO2 catalyst in the presence of Mg(OH)2 at 513 K and 5 MPa H2. In the conversion of Mg(LA)2 over Cu/SiO2, an accelerated activity was observed. And the generation of Mg(OH)2 was detected after the reaction, indicating that the Mg(LA)2 is an important intermediate in the conversion of LA in the presence of Mg(OH)2. DFT calculations were also employed to obtain deeper insights into the reaction mechanism. A decreased energetic span of the reaction from 46.6 kcal/mol with no cation to 43.6 kcal/mol with [Mg(OH)]+ was revealed, confirming the facilitating effect of Mg(OH)2.In addition to Cu giving the good catalytic performance, another non-noble metal Co was also found to be highly active in the conversion of LA. Whereas the selectivity to 1,2-PDO was very low due to the excessive hydrodeoxygenation leading to the production of 1-propanol and 2-propanol. Notably, supports (e.g. SiO2, TiO2, and Al2O3) were observed to significantly affect the catalytic performance of Co-based catalyst. In order to investigate the effect of the acidity of support on Co catalysts in the conversion of LA, a series of HZSM-5 with different Si/Al ratios (i.e. 15, 50, 90 and 200) were screened. Based on the catalytic results, we propose that the acidity of the support presents a key factor affecting the activity of Co catalysts, which needs to be further proven by characterizations such as TEM, XRD, and XPS. Among the synthesized catalysts, Co/HZSM-5 (90) was observed to be the best catalyst enabling 74.7% conversion and 100% selectivity to 1,2-PDO at 513 K and 5 MPa. All in all, this thesis contributes to the valorization of biomass-derived platform chemical to value-added glycols over non-noble metal-based catalysts. The observations are conducive to realize an economic and sustainable process for chemical production in the future.