| The biorefinery technology of renewable resources replacing petrochemical resources is the key technology of sustainable development in the future.Among the many renewable resources,biomass resources have the advantages of extensive supply sources,renewable and high yield,and effective conversion into platform compounds is the key to its utilization.Compared with saccharides such as sugar and starch,the conversion of cellulose has more research value:(1)the sources of raw materials are extensive,and there is no dispute with people;(2)abundant ?-O-4 bonds and hydrogen bonds in the cellulose molecular structure can provide references for the transformation of monosaccharides and disaccharides;(3)the stability of the molecules provides a variety of choices for designing the catalytic activity centers;(4)the products varied with different reaction conditions.Therefore,the design of catalysts for cellulose directional transformation is an important research topic.The Polyoxometalates(POMs)as acidic catalysts play important roles in the biomass conversion due to its strong Br(?)nsted acid and Lewis acid forms.Up to now,there are few reports about the effects of POMs Br(?)nsted acidity,Lewis acidity or base site on the cellulose cascade reaction catalytic performance and intermediate products distribution.Moreover,there are also no deep quantitative research about the effects of POMs Bronsted/Lewis on the cellulose conversion and products selectivity.In this paper,the design and synthesis of multi-center POMs and their catalytic performance in directional conversion of cellulose were systematically studied.Therefore,the specific research contents of this paper are as follows:(1)The synthesis of temperature-responsive and strong Bronsted acid POMs:a series of heteropolyacid catalysts(HOCH2CH2N(CH3)3)xH3-xPW12O40(abbreviated as ChxH3-xPWi2O40,x = 1,2 and 3)had been synthesized using choline chloride(abbreviated as ChCl)and H3PW12O40 as the raw materials,which were used as heterogeneous catalysts in cellulose hydrolysis in double solvent system containing methyl isobutyl ketone(abbreviated as MIBK)and H2O.Research showed that a remarkable hydroxymethylfurfural(abbreviated as HMF)yield of 75.0%and cellulose conversion of 87.0%was achieved by ChH2PWi2O40 within 8 h at 140 ?.The results showed that the conversion and products selectivity was improved by its higher Br(?)nsted acidity,thermoregulate property comparable to its homogeneous form H3PW12O40.Also,the hydrophobic chain of ChXH3-xPWi2O40 could obsob cellulose molecule and then improve cellulose hydrolysis and products selectivity.The temperature-responsive properties of ChXH3-xPWi2O40 not only could overcome the mass transfer barrier between catalyst and cellulose but also solve the problem of difficult separation of catalyst and product.Moreover,such heteropolyacid catalysts could be recycled under simply lowering the reaction temperature to room temperature without loss of its weight,which were reused for more than 10 times without loss of activity.The good catalytic activity was obtained from different substrates including monosaccharide,polysaccharide and raw lignocellulosic biomass straw using our catalyst.Such as the corn straw gave HMF yields of 27.6%.(2)The synthesis of double acid centers POMs including strong Br(?)nsted acid and Lewis acid:a series of Lewis metal-substituent phosphotungstic acids HnPW11LO39(HPWL,L = Cu?,Sn?,Cr?,Zn?,Fe?;L = Ti?,Zr?,oxygen is 40)were synthesized and studied the effects on the acid nature,acid distribution and total acidity of POMs after indroducing different Lewis acid metal ions.It was found that the total acidity of the catalysts was in the order of HPWTi(2.15 mmol/g)>HPWCu(1.93 mmol/g)>HPWSn(1.54 mmol/g)>HPWZr(1.39 mmol/g)>HPWZn(1.33 mmol/g)>HPWFe(1.15 mmol/g)>HPWCr(0.94 mmol/g)and the B/L ratios were in order of HPWZn(5.33)>HPWZr(4.56)>HPWCu(3.81)>HPWSn(3.71)>HPWTi(2.84)>HPWCr(1.35)>HPWFe(0.98)with the introduction of different Lewis acid ions.Then,the reaction activity,pathway and mechanism of HnPW11LO39 in cellulose hydrolysis and alcoholysis reactions were investigated.It was found that the cellulose conversion was in order of HPWTi>HPWCu>HPWSn>HPWZr>HPWZn>HPWFe>HPWCr.However,the selectivity of levulinic acid(LA)was related to the introduction of Lewis metal in HPW,and the order was HPWTi>HPWCu>HPWSn>HPWZr>HPWZn>HPWCr?HPWFe.From the mechanism study:the hydrolysis of cellulose to glucose was mainly dependent on the synergism of Br(?)nsted acid and Lewis acid;the isomerization of glucose to fructose was mainly dependent on Lewis acidity;the dehydration of fructose to HMF was mainly dependent on Br(?)nsted acidity;the hydration of HMF to LA was mainly dependent on Br(?)nsted acid.Thus,the strong Lewis acid could enhance the selectivity of LA.Therefore,the conversion of cellulose was mainly dependent on the POMs total acidity and the appropriate Lewis acid could synergistically promote the hydrolysis of cellulose.It could be concluded that the highest cellulose conversion was obtained by the POMs including both Br(?)nsted acid and Lewis acid.Meanwhile,the LA selectivity could be controlled by adjusting the ratio of the Br(?)nsted acid and Lewis acid(B/L ratio).When the B/L ratio is 2.84:1,the highest cellulose conversion(90.5%)and the LA yield(65.6%)were obtained under the optimum reaction conditions of 130 0C,8 h.Meanwhile,the effects of the introduction of different substituted Lewis metal in HPWL on the alcoholysis of cellulose were also investigated.The results showed that the effect of HPWL on cellulose hydrolysis was similar to that on cellulose hydrolysis:cellulose to methyl glucoside(MG)was mainly dependent on the synergism of strong Br(?)nsted acid and Lewis acid;the isomerization of MG into methyl fructoside(MF)was mainly dependent on Lewis acid;methyl levulinate(ML)from MG or MF was was mainly dependent on the synergism of Br(?)nsted acid and Lewis acid.The 94.7%cellulose conversion and 51.3%ML yield were obtained under the optimum reaction condition of 160 ? and 7 h.Moreover,the catalyst can be reused for 12 times without any significant decrease in activity.The higher activity was obtained from ChH4PWTi than HPWTi due to the temperature-responsive property of ChH4PWTi.The 93.8%cellulose conversion and 76.1%LA yield were obtained under the optimum reaction condition of 130 ? and 8 h.The organic groups have the enrichment function and could adsorb cellulose molecules.The catalyst was resused 12 times without significant loss of activity and still maintained its original composition and structure.The good catalytic activity was obtained from different substrates including monosaccharide,polysaccharide and raw lignocellulosic biomass straw using our catalyst.Such as the husk of xanthoceras gave 68.0%conversion and 33.0%HMF yield under 130 ? and 12 h.(3)The synthesis of nanoparticles containing double acid centers with strong Br(?)nsted acidity and strong Lewis acidity:a series of ultrathin silver-exchanged polyoxometalate nanoparticles(AgxH5-xPWi11TiO40,x = 1,2,3,4 and 5,abbreviated as AgXH5-xPWTi)had been prepared using AgNO3 and H5PWi11TiO40 as raw materials through solid-state chemical reaction at room temperature,which were characterized by elemental analysis,FT-IR,Raman,XRD,SEM,TEM,potentiometric titration,NH3-TPD and contact angle.The results proved that the AgxH5-xPWTi catalyst was the nanoparticles including both strong B acid and L acid and the size of the nanoparticle can be controlled by the reaction time.Then the AgxH5-xPWTi nanoparticles were evaluated in cascade reaction of cellulose in methanol and methyl isobutyl ketone(abbreviated as MIBK)/H2O system,respectively.AgxH5-xPWTi nanoparticles showed the different total acidity and B/L ratio with the different Ag content and the total acidity and B/L ratio were in order of AgH4PWTi(2.06 mmol/g,B/L = 1.90:1)>Ag2H3PWTi(1.93 mmol/g,1.47:1)>Ag3H2PWTi(1.83 mmol/g,1.18:1)>Ag4HPWTi(1.69 mmol/g,0.90:1)>Ag5PWTi(1.06 mmol/g,0.12:1).The cellulose conversion was in the order of AgH4PWTi>Ag5H3PWTi>Ag4HPWTi>Ag5PWTi and the results showed that the AgH4PWTi(200 nm)gave the highest cellulose conversion.It was found that the cellulose conversion was mainly dependent on the AgxH5-xPWTi nanoparticles total acidity and Br(?)nsted acidity;strong Br(?)nsted acidity could promote the cellulose hydrolysis and alcoholysis;Lewis acidity could promote the isomerization of intermediate products to target product;the nanoparticles of catalyst were easier access to substrate and could improve the reactivity.Moreover,the introduction of Ag+changed the redox property of the catalyst and improved the selectivity of the product.The results showed that the AgH4PWTi nanoparticle with size of 200 nm presented the highest methyl levulinate(ML)yield of 72.8%and cellulose conversion of 95.1%in methanol system at 160 ? for 5 h,and levulinic acid(LA)yield of 75.2%and cellulose conversion of 93.0%in MIBK/H2O at 130 ? for 6 h.The good catalytic activity was obtained from different substrates including monosaccharide,polysaccharide and raw lignocellulosic biomass straw using our catalyst.Such as the corn straw gave ML yield of 37.8%under 160 ? and 7 h,and husk of xanthoceras gave LA yield of 31.9%under 130 ? and 12 h.AgH4PWTi showed long duration and high stability,which can be reused several times without loss of reactivity.(4)The synthesis of triple-functional POMs with strong Br(?)nsted acidity,Lewis acidity and base:Csio.6[H2.4GeNbi3O4i](abbreviated as CsGeNb)had been evaluated for its catalytic activity in production of methyl levulinate(ML)from cellulose in methanol.It was found that CsGeNb exhibited certain B acidity(0.63 mmol/g),L acidity(0.12 mmol/g)and base(1.69 mmol/g).Three active sites of POMs act on different reaction pathways of cellulose transformation:Lewis acid and base sites could promote the methyl glucoside(MG)isomerization to methyl fructoside(MF)and strong Br(?)nsted acid sites could promote MF transformation to ML products.The improvement of cellulose alcoholysis activity due to the synergistic effect of strong Br(?)nsted acid sites,Lewis acid sites,and base sites of CsGeNb.The results showed that higher conversion of cellulose(86.0%)and higher yield of ML(53.0%)were obtained under optimized reaction conditions:0.1 g of cellulose,0.04 mmol of catalyst,6 mL of methanol,at 165 ? for 10 h.Assisted by microwave irradiation,the efficiency could be improved to 55.4%yield of ML and 85.2%conversion of cellulose under 165 ? within 3 h.Meanwhile,CsGeNb acted as a heterogeneous catalyst in methanol for 6 recycles without loss of activity. |
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