Study On Preparation And Catalytic Performances Of Catalysts For Conversion Of Glycetol To Aromatics

With development of biodiesel industry and exploration of biomass energy,numerous crude glycerol by-product has been produced.It is meaningful for conversion of the cheap glycerol into high value-added aromatics,which not only realize the utilization of surplus glycerol,but also can improve the economic profits of biodiesel industry.However,there are some serious issues such as low aromatic yield and rapid deactivation in the aromatization reaction over the currently reported catalysts.Therefore,we have deigned and prepared series of catalysts with high catalytic activity and stability from the perspectives of activated metals,mesopore and crystallite size,respectively.Meanwhile,the relationship between the structures of catalyst and the catalytic performances was studied,and the possible catalysis mechanism and deactivated mechanism of the prepared catalyst were also explored.The main contents and results are as follows:(1)Four Zn-modified HZSM-5 catalysts were prepared by impregnation(IM),physical mixing(PM),ion exchange(IE)and isomorphous substitution(IS).The introduction method and addition amount of Zn have important influences on the existence state and distribution of Zn species,textural properties,acidic properties,reaction routes,and catalytic performances in glycerol to aromatics reaction(GTA).The H[Zn,Al]ZSM-5 prepared by ismorphous substitution possessed suitable acidic strength and acidic distribution,which resulted in a coordination of liquid route and gas route in GTA.In addition,some inter-crystalline mesopore was generated in H[Zn,Al]ZSM-5,which could enhance the diffusion of bulky molecules in the zeolite channel.Consequently,H[Zn,Al]ZSM-5 exhibited the optimal catalytic performances among the four Zn-modified catalysts.(2)A Zn-modified hierarchical HZSM-5 catalyst was prepared by the addition of Zn-loaded porous carbon template derived from ZIF-8 in the zeolite synthesis procedure.The introduced Zn species in Zn@C/HZSM-5 converted partial Br?nsted acid sites into Zn-Lewis acid sites(ZnOH~+),and this could facilitate the aldol condensation and dehydrogenation-aromatization steps in GTA,which further resulted in an improved catalytic activity.Additionally,the coinstantaneous formed intra-crystalline mesopore not only promoted the diffusion of bulky molecules and thus slowed down the coke formation rate,but also provided more space to accommodate coke in hierarchical zeolite,which finally slowed down the deactivation process and increased the catalyst lifetime.Finally,34.6%BTX yield and 54.7%aromatic yield was obtained over the stable Zn@C/HZSM-5 with an 8.5 h lifetime due to the synergistic effect among the Zn-Lewis acid sites,intra-crystalline mesopore and HZSM-5 substrate.(3)ZF/HZSM-5 catalyst was prepared by directly addition of ZIF-8 template in the zeolite synthesis procedure.The ammonia dosage,ZIF-8 preparation method and ZIF dosage have important influences on the existence state and distribution of Zn species,textural properties,acidic properties,reaction routes and catalytic performances in GTA.A catalyst with optimal catalytic performances was obtained by the absence of ammonia due to the generation of more inter-crystalline mesopore and acid sites.Compared with ZF(R)/HZSM-5,ZF(F)/HZSM-5 and ZF(G)/HZSM-5possessed smaller crystalline size,more inter-crystalline mesopore and more acid sites,leading to superior catalytic performances over ZF(F)/HZSM-5 and ZF(G)/HZSM-5.The ZF/HZSM-5 catalyst with 0.45 ZIF-8 dosage was composed of nano-crystalline zeolite and possessed appropriate acidity,leading to the optimal synergistic effect among the Zn-Lewis acid sites,inter-crystalline mesopore and HZSM-5 substrate,thus36.7%BTX yield,59.5%aromatic yield and 11 h lifetime was obtained over 0.45-ZF/HZSM-5 catalyst.(4)Me-Zn/ZF/HZSM-5 catalyst was prepared by addition of bimetal ZIF-8template containing Zn and hybrid metal in the zeolite synthesis procedure.Among all the catalysts prepared by bimetal template,only La-Zn/ZF/HZSM-5 exhibited superior catalytic activity as well as higher catalytic stability at the same time.Compared with ZF/HZSM-5,La-Zn/ZF/HZSM-5 possessed more strong acid sites and less medium strong acid sites due to the combined interaction of La and Zn,which resulted in a higher BTX yield and lower C9+aromatic yield over La-Zn/ZF/HZSM-5.In addition,the La species in La-Zn/ZF/HZSM-5 could promote the reaction between the formed coke and water,and thus removed the original coke,which further improved the catalytic stability.As a result,La-Zn/ZF/HZSM-5 still exhibited a 64.5%aromatic yield after reaction for 7 h,which was higher than the corresponding ZF/HZSM-5 with only54.6%aromatic yield.(5)A series of hierarchical HZSM-5 catalysts with tunable mesopore were prepared by addition of imidazolium based ionic liquids template in the zeolite synthesis.The textural properties,acidity,catalytic performances and coke deposition of the prepared catalyst were obviously influenced by the alkyl chain group of the template.Hierarchical HZSM-5 zeolites could been synthesized by addition of template with hexyl group,dodecyl group and hexadecyl group,whereas amorphous material or silicalite was obtained by addition of template with methyl group.The SIM-12-HZSM-5 with dodecyl group possessed the maximal medium strong acid sites and intra-crystalline mesopore,which enhanced the liquid route in GTA and facilitated the diffusion of bulky molecules,and thus showed a maximal aromatic yield of 45.2% and 8.5h lifetime.