| With the decrease of fossil resources,an alternate resource should be greatly developed.Biomass resource is a good choice for its advatages such as huge reserves,sustaibable available,less waste production,and low carbon emission.The conversion of biomass materials are through(i)the pyrolysis of biomass raw materials to pyrolysis oil and syn-gas and the sequential utilisation or(ii)degradation of bimass materials to platform molecules and the transformation of platform molecules to fine chemicals and bio-fuels.Among these routes,the transformation of platform molecules to fine chemicals have attracted much attention as the products are of high values.Nevertheless,a great challenge for conversion of biomass platform molecules is the development of highly efficient catalysts.Typically,the platform molecules are of high oxygen content,and many active functional groups,therefore efficient catalysts for biomass conversion are very different from those used in traditional chemical industries.Nowadays,the major pratical catalysts for are enzyme and homogeneus catalysts,where the enzyme systems are cost and molecular acid/base/salt catalysts in the homogeneous process are corrosive to the equipments and difficult to separate the products and recycle the catalysts.Therefore,it is necessary to develop novel efficient heterogeneous catalysts for biomass conversion.Currently,many heterogeneous catalysts have been developed in the laboratory,but they still suffer from low activity,poor selectivity,and weak stability.Herein,the thesis is devoted to improving the catalytic performances of heterogeneous catalysts in the conversion of biomass platform molecules through designed strategies.These strategies include to(1)improve its catalytic performance in dehydration reaction by preparation of a superhydrophobic porous solid acid catalyst;(2)enhance the hydrogenation-amination of levulinic acid to pyrrilidones by precisely adjusting the synergistic effect between the nanoparticles and the Lewis acid support;(3)synthesize a hierarchical Sn-Beta catalyst for the conversion of saccharides to alkyl lactates;(4)develop zeolite encapsulated metal nanoparticle catalysts for improving the stability of the supported metal catalysts.Dehydration reaction is important for reduction of the oxygen content in biomass conversion,and conventional heterogeneous catalysts are acidic ion-exchange resins.These hydrophilic catalysts have relatively low surface areas,giving poor catalytic properties.Therefore,it is designed that we might increase the properties significantly by hydrophobic acidic catalysts with large surface areas.As expected,a superhydrophobic mesoporous solid acid catalytst(P-SO3H)via solvothermal route with abundant mesoporosity gives much higher activity than the conventional sulfonated resin(Amberlyst-15)in catalytic dehydration of sorbitol.More importantly,the P-SO3H catalyst is more stable than conventional Amberlyst-15 resin.By N2 sorption isotherms and acidity determination,it is observed that the structure of Amberlyst-15 catalyst was destroyed and most part of acid sites were leached,while our superhydrophobic catalyst was very stable.These results have been summarized in Chapter 2.Most of the biomass platform molecules have multiple functional groups,which are highly active and easy to occur side reactions.These issues are particularly pronounced when transforming platform molecules to the fine chemical products.For example,in the hydrogenation-amination of levulinic acid to pyrrolidone,in order to reduce the direct hydrogenation of the substrate,less active Au and Pt based catalysts are used as the catalysts,and highly active Pd based catalysts are rarely used because of the high hydrogenation capacity of Pd species.Considering that Pd has a lower price and higher activity than Au and Pt,it is strongly desirable to develop Pd catalysts with both excellent activity and high selectivity.The hydrogenation-amination reaction process was studied in Chapter 3.The amination reaction occurred firstly in the hydrogenation-amination reaction process,followed by the hydrogenation of the amine intermediate to pyrrolidone product.The main side reaction is the direct hydrogenation of levulinic acid to ?-valerolactone.If the rate of amination reaction rate can be enhanced,the selectivity to pyrrolidine product would be improved.Therefore,it is designed Lewis acid support Pd nanoparticles(Pd/ZrO2)catalyst to catalyze the hydrogenation-amination of levulinic acid.The catalyst showed both excellent activity of Pd catalyst and very high selectivities to the pyrrolidine products.This is mainly due to that the strong Lewis acidity of the ZrO2 support can catalyze the amination of levulinic acid with organic amines,inhibiting the direct hydrogenation of levulinic acid.In addition,Pd/ZrO2 catalyst also showed very high stability,the activity did not decrease even after recycling for six times.The excellent catalytic performance of the Pd/ZrO2 catalyst offer a good choice for catalyzing the hydrogenation-amination in the future.Zeolites are widely used in petrochemical industry because of their unique pore structures,large surface areas,and variable acid sites.Side reactions such as polymerization and carbonization always occurred during the process of biomass conversion,blocking the micropore of zeolites.Therefore,hierarchical zeolites are often used for biomass transformation.In chapter four,the Sn-Beta catalyst with abundant mesoporosity was synthesized through a post-synthesis strategy.Compared with the conventional microporous one,the hierarchical zeolite catalyst exhibits very high activity and selectivity in the catalytic conversion of glucose to methyl lactate.By kinetic studies,it is observed that the hierarchical Sn-Beta accelerate the conversion of glucose to triose,which is the key step of the reaction.The transition state of this step reaction is a six-membered ring structure that formed by glucose and Sn sites,which is of very large molecular size.The hierarchical structure facilitate its formation and transformation.On the contrary,the traditional microporous zeolite pore is small,hindering the fast conversion of glucose to triose.In addition,the tests of the catalyst stability showed that the activity was only slightly reduced after six recycles,and the initial reaction rate was almost maintained in each run To improve catalytic activities of supported metal catalysts,a sea of strategies have been developed to reduce the size of metal nanoparticles.However,smaller metal nanoparticles with higher surface energy are easier to aggregate into larger metal particles,leading to the deactivation of the catalyst.Therefore,the preparation of catalysts with both high activity and selectivity as well as excellent stability is always challengeable in the preparation of heterogeneous catalysts.Biomass conversion processes often require high reaction temperature,accompanied by a large amount of water,which is regarded as harsh conditions.Conventional supported metal catalysts are easy to lose the activity under such reaction conditions.Therefore,the development of highly stable metal nanocatalysts for biomass conversion is really necessary.To solve this problem,it is developed a zeolite-encapsulated metal nanoparticle catalyst.After encapsulated into zeolite single crystals,the metal nanoparticles were separated from each other,which gives the catalyst very high resistance to sintering,as summarized in Chapter 5.The zeolite single crystal-encapsulated metal nanoparticles(Au-Pd@S-1)was prepared by solvent-free synthesis strategy.After calcination at high temperature,the size distribution of the metal nanoparticles was almost unchanged.This kind of catalysts were used for catalytic aerobic oxidation of bioethanol.Compared with traditional supported metal nanoparticles,the catalyst exhibits very high activity and excellent stability.Furthermore,the synthesis strategy was also extended to other systems through seed directed crystalization strategy.All these catalysts exhibited extraordinary stability at high temperatures. |
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