Ketones & Alcohols From Lignocellulose Hydrogenolysis Over Carbon Encapsulated Metal Catalysts

With the depletion of fossil resources and the environmental problems caused by the combustion of fossil energy,the exploration and utilization of renewable energy has attracted increasing interests.Lignocellulose,as a primary biomass resources,has the merits of large natural reserves and renewability.The catalytic conversion of lignocellulose into high-value chemicals through chemical approaches is currently a hotspot in scientific research areas.Lignocellulose mainly includes cellulose,hemi-cellulose and lignin.Herein,we focus on the catalytic conversion of cellulose and xylose(constructive components of hemicellulose).In this research,we prepared a carbon-encapsulated metal catalyst,and by introducing heteroatoms and controlling calcination temperature,the hydrogenation and isomerization capacities of the catalyst were tuned to achieve the catalytic conversion of cellulose to ketone compounds and xylose to glycol compounds.Encapsulating the transition metals with graphene layer can not only enhance the robustness of the internal metals under harsh conditions like high temperature and high pressure,but also guarantee the catalytic activity of the internal metal benefiting from graphene’s superior electron transmission capacity.The research mainly includes the following two aspects:The reaction of cellulose to ketones was studied.A nitrogen-doped graphitic carbon-coated metal Ni catalyst,named as Ni@NC,was prepared by the bottom-up method.The physical and chemical properties of the Ni@NC prepared at different precursor ratios and calcination temperatures were studied.The direct conversion of cellulose was conducted in phosphoric acid aqueous solution,through the screening of reaction conditions,34.1%yield of 2,5-hexanedione and 24.5%yield of 1-hydroxy-2-hexanone were obtained under optimal reaction conditions.The yield of 2,5-hexanedione exceeded the previous literature reporting non-noble metal catalyzed cellulose,and to our best knowledge,the production of 1-hydroxy-2-hexanone from cellulose has yet been reported.Then we investigated the reaction route.It was found that furan compounds were the key intermediates for producing 2,5-hexanedione,and 1-hydroxy-2-hexanone was derived from the hydrodeoxygenation and ketone-aldehyde isomerization of glucose.The mechanism of synergistic catalysis between H3PO4 and Ni@NC was discussed.The role of H3PO4 included catalyzing the hydrolysis of cellulose to glucose,promoting the dehydration of sugars to 5-hydroxymethylfurfural,the hydrolysis/ring-opening reaction of furan intermediates;the N doping in Ni@NC provided the basic sites to facilitate the isomerization of glucose to fructose.Comparing Ni@NC with different base content as well as Ni@C without N doping,it was found that moderate basic amounts had an pivotal effect on the product distribution:when the base was strong,the product distribution shifted to levulinic acid,and the isomerization reaction was suppressed while the base was scarce,under which case the ketones cannot be obtained.The confinement of the graphene shell led to the mild hydrogenation activity of Ni@NC,which was conducive to the retention of C=O bonds.The conversion of xylose to diols species was studied.Different transition metals and their alloys were screened,and finally Co@NC was selected as the catalyst.Using xylose as raw material in the water phase,a total diols yield of 70.1%was obtained at 200℃,mainly ethylene glycol,1,2-propanediol and 1,2-pentanediol.The doping of N provided basic sites and promoted the isomerization of xylose to xylulose.Xylulose underwent retro-aldol reaction to obtain glycolaldehyde and acetol,and then went through hydrogenation to obtain ethylene glycol and 1,2-propanediol,while 1,2-pentanediol came from the hydrodeoxygenation of xylose.