Study On Catalyst Performance And Kinetics Of Catalytic Transfer Hydrogenolysis Of Xylitol

Xylitol,one of the key platform chemicals derived from carbohydrates,can be converted into high value-added alcohols including ethylene glycol,1,2-propylene glycol and glycerol.They are widely used as antifreezes,cosmetics and pharmaceuticals.In the conversion of xylitol,high temperature（T>200 ^oC）and high H₂ pressure（4～8 MPa）were required to reach high activity and selectivity.However,these harsh conditions caused considerable technological issues.Excessive temperature leads to the formation of gas by-products and rapid catalyst deactivation.And industrial hydrogen brings additional costs and energy consumption.Therefore,we proposed a process of catalytic transfer hydrogenolysis（CTH）in-situ formed H₂,which can be an alternative method for facile transformation of xylitol to value-added glycols.In this paper,we explored the effect of synergistic bimetallic Pd-Pt/TiO₂ catalyst on the catalytic transfer hydrogenolysis of xylitol in batch reactor.Pd/TiO₂ and Pt/TiO₂ catalysts show poor activity and selectivity.Pd metal is beneficial to the dehydrogenation reaction（C-H bond cleavage）.Pt metal is beneficial to the deoxidation reaction（C-O bond cleavage）.But the bimetallic Pd-Pt/TiO₂ catalysts showed synergetic performances for tandem H₂ generation and hydrogenolysis of xylitol.A combined yield of 42.4%to propylene glycol and ethylene glycol was thus obtained on Pd-Pt/TiO₂ catalyst at 220 ^oC and 1 MPa N₂.A series of catalysts with different particle diameters（2.4-5.2 nm）were obtained by changing the preparation conditions.The catalyst showed the highest activity at 3.6 nm(731.8 h^-1).We found that the xylitol catalytic transfer hydrogenolysis reaction has particle size sensitivity to the Pd-Pt/TiO₂ catalyst under an inert atmosphere excluding the effect of pore size,pore volume and electronic structure of the catalyst.Detailed structure-dependency studies on Pd Pt particle size revealed that,C-H,C-O and C-C bond cleavage display strong size-determining trends.Therefore,conversion of xylitol displays an optimal selectivity towards glycols and alcohols with Pd Pt particle size of approximately 4.4 nm.In addition,influence of experimental parameters,including temperature（200-230 ^oC）,N₂ pressure（0-3 MPa）and alkali/xylitol molar ratio（0-0.25）were also studied with respect to conversion and production distribution.According to the experimental results and the aqueous hydrogenolysis pathway of xylitol in the literature,we use Athena Visual Studio software to simulate the proposed pathway.According to the simulated rate constant and activation energy,we analyzed the possibility of the reaction pathway and proposed the final reaction network of xylitol catalytic transfer hydrogenolysis.According to the simulated rate constant and activation energy,the reaction pathway is analyzed and the final reaction network of xylitol catalytic transfer hydrogenolysis is proposed.According to the simulated rate constant and activation energy,we analyze the reaction pathway and proposed the final reaction network of xylitol catalytic transfer hydrogenolysis.We found that the reaction pathway of xylitol catalyzed transfer hydrogenolysis is different from the aqueous phase hydrogenolysis.The result of this work provides an effective method for converting oxygen-containing compound atoms from biological sources into high value-added renewable chemicals.