Preparation And Properties Of Porous Acid Catalysts Containing Tungstophosphoric Acid Anchored With SiO₂ And/or Graphene

With the continuous increase of energy consumption and the depletion of fossi resources,the development of biomass fuels has become one of the important ways to alleviate the energy crisis and achieve the sustainable development.As one of the biofuels,ethyl levulinate which could be used as diesel additive,can be synthesized through esterification of levulinic acid derived from cellulose hydrolysis over acid.In order to overcome the shortcomings of the conventional mineral acid catalysts?HCl,H2SO4 and H3PO4 etc.?such as corrosion equipment,un-recyclability,and polluting the environment etc,it is urgent to develop the new acid catalyst.As a solid heteropoly acid,tungstophosphoric acid(H₃PW₁₂O₄₀,HPW)exhibits strong acidity and strong oxidizing properties.This ensures it displays excellent catalytic performance in many reactions.However,the disadvantages of pure HPW,such as the poor thermal stability,the low specific surface area,and the easy dissolution in polar solvents resulting in difficulty for recovery,restrict its application in practical production.Studies have confirmed that the immobilization of HPW with suitable supports can overcome these shortcomings and increase the catalytic properties.In this thesis,three-dimensional?3D?graphene aerogel?GA?,reduced graphene oxide?rGO?,SiO₂@GA,MCM-41 and MCM-41@rGO were prepared by hydrothermal method,chemical reduction method and a near room temperature synthesis method in an open system,respectively.Then,the resulting porous materials were used as supports to prepare phosphotungstic acid-based solid acid catalysts?HPW/GA,HPW/rGO,HPW/SiO₂@GA,HPW/MCM-41,and HPW/MCM-41@rGO?with different HPW loadings by a wet impregnation method.The catalysts were characterized by various techniques,such as XRD,FT-IR,Py-IR,SEM,TEM,and N2 adsorption-desorption.Their catalytic performances in esterification of levulinic acid?LA?with ethanol were further investigated.Specific work is as follows:?1?Graphene oxide?GO?solution was prepared by a modified hummer method.3D GA and SiO₂@GA composites were prepared from GO and ethyl silicate by a directly hydrothermal reduction method combined with the freeze-drying technique.MCM-41 mesoporous molecular sieves were prepared from ethyl silicate with cetyl ammonium bromide as template by a non-hydrothermal method at near room temperature in an open system combined with the calcination process.rGO and MCM-41@rGO composites were prepared from GO by a chemical reduction method using hydrazine hydrate as reductant.Five kinds of tungstophosphoric acid-based catalysts with different loadings were prepared by a wet impregnation method using the porous materials as the supports.?2?The analytic results showed that the as-prepared solid acid catalysts not only maintained the porous structure of the corresponding supports,but also kept the Keggin structure of HPW.In addition,with the increasing of HPW loading,the amount of B acid increased and the total amount of acid increased,whereas the texture parameters?specific surface area,total pore volume,etc.?decreased.Meanwhile,the long-range orders of the supports were destroyed to a certain extent.This may be due to that the interactions between HPW and oxygen-containing functional groups in graphene or Si-OH on the surface of the supports destroy its textural structure to some extent.?3?The aforementioned catalysts were used in the synthesis of ethyl levulinate,respectively.The effects of various reaction parameters,such as reaction temperature,reaction time,ethanol-to-LA molar ratio,catalyst dosage and HPW loading on the catalytic activity were investigated.Furthermore,the as-prepared catalysts were also repeatedly used to examine their reusability.The results showed that the resulting solid acid catalysts were efficient in the synthesis of biofuel additives via esterification of levulinic acid with ethanol and the LA conversion was as high as 96.6% under the present conditions.Among these catalysts,the HPW/SiO₂@GA displayed the highest catalytic activity.Meanwhile,the HPW/MCM-41 catalysts exhibit the highest reusability and the LA conversion was kept as high as 56.8% after five cycles over them.