Guanidine Modified Silica Particles For Pickering Interfacial Catalytic Preparation Of Biodiesel

Pickering interfacial catalysis?PIC4?system has huge potential applications in the field of catalysis due to its large phase interfacial area,easy recovery of catalyst,and reusability.In this work,a series of guanidine-based surface-modified fumed silica particles were designed and prepared to study the emulsification efficiency and catalytic performance of soybean oil/methanol PIC4 system.The research contents and conclusions are as follows:Two kinds of monofunctionalized solid particles,SiO₂-TMG and SiO₂-G,were prepared by chemically grafting tetramethylguanidine groups with different linking chain lengths onto the surface of SiO₂ nanoparticles.Both particles could stabilize the soybean oil/methanol Pickering emulsion.And the increase of SiO₂-TMG particle concentration is beneficial to the stability of the emulsion.Both particles could catalyze the transesterification of soybean oil with methanol to produce biodiesel.It was found that the increasing of both the particle concentration and the reaction time would gradually increase the conversion of soybean oil.The SiO₂-G particles modified with shorter guanidine chain has better emulsifying and catalytic properties than that of SiO₂-TMG particles modified with longer guanidine chain.However,PIC4 system stabilized by SiO₂-G particles requires an energy-consuming ultrasonic emulsification and that by SiO₂-TMG particles can quickly form by hand shaking.Therefore,both particles have their own advantages.On the basis of monoguanidine-modified SiO₂ particles,a series of bifunctional SiO₂ nanoparticles were prepared by combining a guanidine group and n-alkyl chains with n-butyl,n-octyl,n-dodecyl,or n-hexadecyl chains?C_n-SiO₂-TMG,n=4,8,12,16?.The infrared spectroscopy,X-ray photoelectron spectroscopy,and BET analysis were used to characterize the bifunctional solid particles.The results showed that the guanidine and alkyl groups were successfully grafted onto the surfaces of SiO₂ particles without changing the particle morphologies.Although the specific surface area showed a slight decrease,the pore volume and pore size remained basically unchanged.The C_4n-SiO₂-TMG particles can stabilized a stable soybean oil/methanol Pickering emulsion.With the increase of particle concentration,the droplet diameters of the emulsion decreased gradually,and the volume fraction of the emulsion increased.Except for the poor emulsification performance of C₄-SiO₂-TMG particles,C_4n-SiO₂-TMG?n=8,12,16?particles showed significantly good emulsification performance.The C_4n-SiO₂-TMG particles can catalyze the transesterification of soybean oil and methanol to prepare biodiesel.Using C₈-SiO₂-TMG particles as the catalyst,the conversion increased first and then decreased as the particle concentration increased as high as 9 wt%to the total weight of reactants,giving a best catalytic concentration at 7 wt%.This was attributed to the balance between the decreased mass transfer by the increased viscosity of the system and the increased catalytic active sites.The conversion steadily increased with the increase of reaction time.The increase of reaction temperature is beneficial to the improvement of the conversion rate.The activation energy E'a of the C₈-SiO₂-TMG system is about 26.8 kJ·mol-1,indicating a chemically controlled reaction.It was found that the normalized conversion of C₈-SiO₂-TMG and C₁2-SiO₂-TMG catalytic systems were much higher than those of C₄-SiO₂-TMG and C₁6-SiO₂-TMG systems,indicating the evidently better catalytic performance.This is because the butyl chain is too short to serve C₄-SiO₂-TMG to be a good emulsifier,and the hexadecyl chain is too long to shield the catalytic active sites on the guanidyl chain.Both situations decreased the mass transfer or catalytic site and thus decreased the catalytic efficiency.When the chain length was equal to the chain length of the catalytic functional group,the best emulsifying performance and the catalytic performance would be achieved.