Glycerol On Nickel Catalyst Reforming Hydrogen - 1, 2 - Propylene Glycol Solution System Research

The transformation of glycerol to value-added chemicals has caught extensive attentions in recent years owing to its tremendous availability in biodiesel production. Specifically, the production of1,2-PDO from glycerol is of great prospect in terms of its widespread applications and its current petroleum-dependent production way. To date, researchers in catalysis community have developed various routes to convert glycerol to1,2-PDO, and atom economy and energy-saving production strategies are attracting special interests. The combined aqueous-phase reforming and hydrogenolysis (APR-hydrogenolysis) strategy is regarded as advantageous in atom economy and energy efficiency, thus being considered as very promising. Non-precious Ni-based catalysts can fulfill both APR and hydrogenolysis requirements, thus being believed as potential catalysts for this reaction.In the first part of our work, we found that Mo-modified rapidly quenched Ni (RQ Ni50-xMox) catalysts can efficiently covert glycerol to1,2-PDO under inert atmosphere. The catalytic activity and1,2-PDO selectivity increased with the amount of Mo. A1,2-PDO yield of32.0%was obtained over the RQ Ni40Mo10catalyst, which is much higher than that acquired over precious Pt/Al2O3catalyst under similar reaction conditions. According to X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) characterizations, we disclosed that:ⅰ) the crystallite size of Ni decreased with Mo promotion, thus promoting the catalytic activity; ⅱ) the Mo species over the catalyst was in oxidized state, which served as Lewis acid sites beneficial for the dehydration of glycerol and hydrogenation of intermediate acetol. In addition, the detailed reaction pathway of APR-hydrogenolysis of glycerol was proposed, which can explain the origin of all detected products in the reaction process. This reaction pathway is instructive for us to develop more effective catalytic systems for APR-hydrogenolysis of glycerol in later work.Enlightened by certain literature works that physically mixed, co-catalytic system consisted of physically mixed oxides and RQ Ni40Mo10was employed in APR-hydrogenolysis of glycerol. Among the oxides investigated, ZnO was unexpectedly found to show unique promotion effect in suppressing the APR process and facilitating the hydrogenolysis process, thus leading to a remarkable increase of1,2-PDO selectivity. Control experiments showed that the promotion effect of ZnO only occurred when it was homogeneously mixed with RQ Ni40Mo10, evidencing the synergetic effect between the two catalytic components. According to the literature and characterization results, we attributed the promotion effect of ZnO to the interaction between CO2and ZnO. Namely, CO2chemisorbed on the non-polar facets of ZnO to form a stable tridentate carbonates. This adsorption mode took place on every second Zn cation, resulting in both occupied and unoccupied surface zinc sites. The adsorbed CO2had a charge-accepting effect and made the unoccupied Zn cation more positive, thus greatly enhancing the Lewis acidity of the surface unoccupied zinc cations.Then, we prepared a series of ZnO with different morphologies. The phase composition, crystal habits and optical properties of these ZnO samples were adequately revealed by XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman and photoluminescence (PL). It was found that when phycially mixed with RQ Ni40Mo10ZnO with larger proportion of non-polar facets showed better catalytic performance. This observation provided experimental verification for the above-mentioned conjecture in understanding the ZnO promotion effect in the co-catalytic system. Besides, according to XPS, Raman and PL characterizations, the effect of surface areas, crystal defects and electronic interaction between ZnO and the metal catalyst in the reaction process were ruled out..In the last part, various Ni/ZnO catalysts prepared by different methods were employed for APR-hydrgenolysis of glycerol. These catalysts were systematically characterized by N2-adsorption (BET), temperature-programmed reduction (TPR), XRD and TEM. These catalysts can convert glycerol to1,2-PDO under inert atmosphere with catalytic performance comparable to that over precious catalysts. The effect of weight-hourly space velocity (WHSV) on the product distribution was investigated in detail. Results showed that1,2-PDO tended to undergo degradation reactions to form ethanol and gas phase products at relatively low WHSV. While the hydrogenation of acetol may severely affect the selectivity to1,2-PDO at relatively high WHSV. The Ni/ZnO-CT catalyst with the highest Ni dispersion degree exhibited the best catalytic performance.