Synthesis Of Novel Solid Acid And Its Catalytic Activity In Esterification

The esterification of carboxylic acid with alcohol is a typical reaction in organicsynthesis, esters are used widely in drugs, food preservatives, solvents, cosmetics and biofuelsfields and so on. Nowadays, industrial esterification reactions are commonly catalyzed usingliquid inorganic acids, the catalytic activity of liquid acid catalysts is high, however, theysuffer from several drawbacks, such as difficult separation from products, their toxic and riskynature, corrosive to the equipment and environmental pollution. Compared with thetraditional liquid acid catalysts, solid acid catalysts have some advantages such as highcatalytic activity, easy detached and reused, non-toxic and safe, and environmentally friendly.In this paper, a variety of nanosized solid acid catalysts were produced by new synthesismethods, the structure and texture of the catalysts were investigated by XRD, FT-IR, Raman,TG-DTG, TEM, XPS and so on, and the catalytic activity of the catalyst was investigated bythe esterification of acetic acid with n-butanol.A series of nanosized SO₄^2-/TiO₂catalysts were synthesized by a novel hydrothermalmethod, the influence of the structure of the support, hydrothermal temperature, concentrationof sulfuric acid and calcination temperature were studied. The anatase titania synthesized bythe hydrothermal method has good thermal stability, high surface area and mesoporousstructure, it was a feasible support preparation of SO₄^2-/TiO₂catalyst. The SO₄^2-/TiO₂catalystprepared under120℃hydrothermal temperature got bigger surface area, higher content ofstrong and very strong acid sites. The SO₄^2-/TiO₂catalyst was small nanoparticle, its averageparticles size was8-11nm, the structure of SO₄^2-/TiO₂catalyst was bridge bidentate oftitanium atoms to sulfur center. As the concentration of sulfur acid increase, the sulfur contentand acidity content of the catalyst present a first increase, then decrease trend, two types ofsulfur species appeared at the catalyst when the concentration of sulfur acid after more than1mol/L, they were sulphate species and bisulphate species, respectively. With increase of thecalcination temperature, the surface area and acidity content of the catalyst present adecreases trend, the optimum calcination temperature was400℃. The optimum conditions ofthe esterification were as follows: reaction temperature120℃, molar ratio acetic acid:n-butanol=1:2,1wt%catalyst, when use HST-120-400as the catalyst, the conversion ofacetic acid reached98.09%even with a reaction time of120min. SO₄^2-/TiO₂catalyst can bereused as there is not big loss in catalytic activity during the four cycles.WO_x/TiO₂catalysts were prepared by a one-step hydrothermal method and a two-stepincipient-wetness impregnation method with phosphotungstic acid as W source, the effects ofpreparation method, tungsten oxide loading and calcination temperature were examined. Astungsten oxide loading increase, the surface area present a first increase, then decrease trend, The optimum tungsten oxide loading was10%, when the tungsten oxide loading was morethan10%, WO₃crystal was formed. With increase of the calcination temperature, the surfacearea decrease, however, the number of polytungsten species increase. The WO_x/TiO₂catalystsynthesized by the one-step method exhibited a better catalytic acitivity than the WO_x/TiO₂catalyst synthesized by the two-step method. The catalyst prepared by one-step method can bereused as there is no loss in catalytic activity during the five cycles. Selective poisoning of thepotential catalytic centers of WO_x/TiO₂catalyst revealed that Br nsted site play the major rolein the esterification.