Synthesis Of Nano SO₄^2-/ZrO₂ Solid Acid Materials And Their Catalytic Performance

Due to the excessive utilization of fossil fuel and the limitation of energy storage,increasing attention has been devoted to develop some new green resources.Biodiesel,as a renewable energy,has potential to be alternative to traditional petrochemical fuels.The product of biodiesel generally involves the transesterification or esterification reaction in the presence of liquid acid/base catalysts.But the complex processes and higher cost of the following treatment limit the practical application of biodiesel.In order to reduce its production costs and take full advantage of low-graded feedstocks(woody oil,waste oil,etc.),the recovered and recyclable solid acid catalysts are applied to the preparation process of biodiesel.Among many solid acid catalysts,sulfated zirconia exhibits good catalytic properties in the transesterification and esterification reaction because of its unique acid properties.However,low specific surface area,poor catalytic activity and stability are the major drawbacks of sulfated zirconia in the catalytic process.Therefore,it is a challenge to prepare sulfated zirconia catalyst with high catalytic activity,good stability,and solve the problem of sulfur loss and rapid inactivation of the catalyst in the transesterification reaction.Based on the previous work in our laboratory,by changing the preparation methods,we have synthesized highly active and stable sulfated zirconia catalysts: microcrystalline sulfated zirconia catalyst with high activity synthesized by steam phase hydrolysis method,and mesostructured nano sulfated zirconia catalysts with different morphologies and stability prepared by two templates in the solvent-thermal system.The results obtained in the present paper are summarized as follows:(1)The microcrystalline sulfated zirconia with high activity was prepared via steam phase hydrolysis by using ammonium persulfate as sulfur species.The obtained results of crystal structure and pore structure show that the catalyst calcinated at 500 °C has tetragonal crystal phase,which is contributed to the inhibiting effect derived from the sulfur introduced by in-situ synthesis.The catalyst also shows abundant acid centers,the acid strength and acid amount are affected by the calcination temperature.Moreover,by using the prepared catalyst,the soybean oil conversion of 90-100% is obtained in the range of 100-110 °C in the transesterification with methanol;the complete conversion of glycerol and the selectivity of 53% of diacetin and 34% of triacetin are obtained in the esterification of glycerol with acetic acid.(2)A mesoporous sulfated zirconia catalyst with nanocrystalline was synthesized by using cetyltrimethylammonium bromide as template and its catalytic performance in the transesterification reaction of soybean oil was investigated.The structure characterization results show that the catalyst obtained by the calcination at 600 °C has high specific surface area of 137 m2/g,uniform pore size of 3.6 nm and average sulfate surface density distribution of 1.6 atoms/nm2.The results of acid characterization and surface sulfate density distribution of different catalysts show that the calcination temperature affects the coverage of sulfate on the surface,and promotes the formation of acid sites in the catalyst,when the sulfur density distribution is lower than half-monolayer.In addition,chemical properties of the catalyst show that the sulfates are combined with zirconium in the form of bidentate chelating and located in two kinds of chemical environment.The study in the repetitive runs indicates that when compared with reduced specific surface,the loss of sulfur has more important effect on the deactivation of the catalyst.(3)The zirconia with multi-dimension size nanoparticles were successfully synthesized in the solvent-thermal system using double-chain quaternary ammonium salt as a template.The effects of the single/double-chain quaternary ammonium salts with the same carbon chain number on the formation process and the morphologies of mesoporous zirconia nanomaterial were systematically studied.The results show that the synthesized zirconia material with double-chain quaternary ammonium salt was composed by multistage nanoparticles and had a specific surface area of 195 m2/g and a uniform pore size of 5.0 nm.In addition,the two templates in the ethanol solution were characterized by the transmission electron microscopy and the results display that the single chain quaternary ammonium salts form dispersive spherical micelles and the double-chain quaternary ammonium salts form a network structure composed by many crosslinked rod-like micelles.The observation results of the mixture of zirconium and the templates micelles represent that lots of homogeneous nanoparticles are reunited in the single-line system,and the mixture is reunited with different size agglomerates in the double-chain system.The scanning electron microscopy results of material obtained during the synthesis process show that the synthesis conditions have little effect on the morphology of the prepared material.The prepared zirconia with single-chain quaternary ammonium salt is composed of smooth microspheres,but that with double-chain quaternary ammonium salt material consists of multi-dimension size nanoparticles,indicating that the structure of the templates plays a decisive role in the morphology of zirconia.(4)A mesoporous sulfated zirconia polycrystal composed of multi-dimension size nanoparticles was synthesized by using double-chain quaternary ammonium salt template and the structure stability and deactivation reason of the catalysts were investigated in the transesterification and esterification reactions.The prepared catalysts calcined at 550 °C and 600 °C exhibit higher catalytic activity and better stability than that prepared by the coprecipitate method.The catalyst obtained at 600 °C also shows excellent performance during the circular runs in the esterification-transesterification reaction,indicating that the catalyst has the potential to be suitable for feedstocks containing free fatty acids.In addition,the structure characterization results of the fresh and reused catalysts show that the prepared sulfated zirconia has good crystal structure and stable pore structure.After repeated high temperature regeneration processes,only a small amount of sulfur decreases,indicating that the sulfur has formed a stable structure with zirconia surface.Moreover,the comparison of physicochemical properties of the fresh and reused catalysts shows that covalent sulfates combined with lots of surface defective sites are the basis of the structural stability.With higher sulfate concentration,the ionic sulfate species absorbed on the low-index plane induce the production of abundant Br?nsted acid sites,which are responsible for the activity in the transesterification or esterification,and the loss of these active sulfate species during the reaction is the intrinsic reason for the catalyst deactivation.Furthermore,simulation calculations confirm that position of two H+ affects the configuration and the stability of SO42-of B acid sites and the most stable 3-fold coordinated configuration sulfates play a dominant role in the formation of stronger Br?nsted acid sites.