Re-Modified M(Pt, Ir) Catalysts For Glycerol Hydrogenolysis:Structure Manipulation And Structure-Performance Relationship

Glycerol is an important building block platform for biomass conversion. Currtently, selective hydrogenolysis of glycerol to valuable propanediols, especially 1,3-propanediol, becomes a hot issue because it is one of the most potential and profitable routes for industrial bio-glycerol transformation as well as a model reaction for the hydrogenolysis of more complecated bio-related molecules (e.g., sugars and sugar alcohols). Since the most efficient catalysts for glycerol hydrogenolysis are mainly composed of noble metals (e.g. Pt, Ir), higher activity and stability in addition to high selectivity to 1,3-propanediol are required. In this work, aiming at highly efficient Re-modified M(Pt, Ir) catalysts, we focused on the study of size sensitivity of Pt-Re bimetallic catalyst, structure and performance manipulation of Ir-Re bimetallic catalyst and the vital role of Re in reaction and structure manipulation. In addition, the effects of catalyst preparation method, thermal treatment conditions and nature of supports on Ir-Re structure and catalytic performance were investigated in order to establish the structure-performance relationship.(1) Size effects of Pt-Re bimetallic catalysts. A volcanic curve relationship between activity and particle size is established. Higher activity is observed for smaller particle size due to the enrichment of Re on the particle surface, while too much Re on the surface of small sized Pt-Re catalysts disrupt the Pt-Re synergy, resulting in decreased activity. Characteristics of glycerol and propanediols hydrogenolysis over different sized Pt-Re catalysts indicate that the scission of the primary C-O bond of glycerol and 1,2-propanediol as well as the scission of C-C bond is favored over smaller sized Pt-Re catalysts.(2) Remarkable influence of thermal processing on Ir-Re structure. The thermal processing procedure and calcination condition changes the exisitence form, reducibility and mobility of Re species, and hence affects the Ir-Re structure. Active components exist as Ir4 and ReO4- ions in the liquid layer of the support surface after impregnation and drying, which show high mobility and reducibility and favors the formation of highly alloyed Ir-Re catalyst by direct activation of the impregnated samples. However, even though low temperature calcination (350 ℃) retains the reducibility of Re2O7, the mobility is greatly reduced, resulting in low extent of Ir-Re alloy during reduction. High temperature calcination (≥500 ℃ remarkably increases the metal-support interaction, resuting in decreased mobility and reducibility of Re species, thereby leading to the formation of Ir-ReOx core-shell structure during reduction.(3) Effect of support nature on the extent of Ir-Re alloying. Re species can strongly interact with Si-Al-OH groups on the Al-containing supports. As a result, the mobility of Re species as well as the efficient Ir-Re interaction is inhibited. The hindered Ir-Re interaction leads to the formation of monometallic Ir and Re nanoparticles, resulting in restrained Ir-Re synergy and lowered activity. Instead, inert siliceous materials such as KIT-6 and G-6 are more appropriate for supporting Ir-Re alloy catalysts.(4) Highly active Ir-Re alloy catalyst prepared by direct reduction method. With the assitance of amberlyst-15, the formation rate of 1,3-propanediol over Ir-Re alloy catalyst reaches 25.6 mol1,3-PDmolIr-1h-1,the highest value so far, under 120 ℃,8 MPa H2 and 20 g 20 wt% glycerol aq. reaction conditions. And the highest selectivity and yield of 1,3-propanediol reach 50.5% and 22.0%, respectively. In addition, the bifunctional reaction mechanism of Ir-Re alloy catalyst is evidenced by the linear relationship between acid amount and activity.(5) Stability, reusability and deactivation mechanism. The Ir-Re alloy catalyst is highly stable during reaction. However, its reusability is not satisfactory when it is reused as recovered. The activity gradually decreases during reuse, while the selectivity to 1,3-propanediol is almost unchanged. The decrease of activity during reuse is mainly attributed to the segregation of Re, which leads to changed Ir-Re alloy structure and reduced Ir dispersion.(6) Promoting mechanism of solid acids on activity and stability. During reaction and reuse, the solid acids can play a role in (i) promoting catalytic reaction through providing proton for protonation and regeneration of Re-O- to active Re-OH groups and (ii) stabilizing Ir-Re structure through preventing the over-oxidation of Re species and hence segregation of Re.