Esterification And Transesterification On Sulfated Tin Oxide Solid Superacid Catalysts Doped With Metal Oxides

The type solid superacids of SO42-/MxOy are recognized as a class of novel catalytic materials which are green and have potential application. They have attracted much attention in recent thirty years, because they are noncorrosive, environmentally friendly, easily seperative and well reusable. The study indicates that SO42-/SnO2 is one of the strongest surface acidic catalysts, or at least the acid strength is as high as that of SO42-/ZrO2, and it could exhibit higher catalytic activities in many acid catalyzing reactions. Howerver, there have been few papers concerning SO42-/SnO2 catalyst for difficulty in preparation, compared with the relative ease of preparation for SO42-/ZrO2. Tin oxide gels were usually obtained as fine particles in conventional method, and a large part of the precipitates were passed through a conventional filter paper, resulting in their diminished yields.As a clean and regenerable energy source, biofuel is of wide application. Esterification and transesterification are the two kinds of major methods to synthesis biodiesel. This method has simple procedure, low dispense and stable product. The main catalysts are concentrated sulfuric acid and alkaline metal hydroxides. Furthermore, solid superacid catalysts in the reaction can overcome the base-catalyzed saponification and the catalyst poisoning problem, and they also have strong acidity and high activity. So it will become good catalysts in biodiesel preparation.Sulfated tin oxide catalysts promoted with Fe2O3 (0.2-3.0 mol%) have been successfully prepared via co-condensation method and characterized by isothermal nitrogen adsorption/desorption, powder x-ray diffraction (XRD), thermal gravimetric analysis (TG), Raman spectra, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ESR spectroscopy. The number of acid sites on the catalysts was measured by means of potentiometric titration of n-butylamine. Triacetin with methanol and lauric acid with methanol were used as the representative compounds for the investigation of the catalytic activity for transesterification and esterification. The results indicate the growth of SnO2 crystallites is inhibited in the presence of small amounts of Fe2O3, further enhance the surface of the catalysts and could stabilize more sulfur species on the surface of SO42-/SnO2. Acidity is also enhanced incomparison with conventional sulfated tin oxide. All irons exist in the tetrahedral SnO2 network when the content of Fe2O3 below 0.2 mol%. But the cluster of iron oxides will be formed increasing with the content of Fe2O3. As a result, the sulfated tin oxides promoted with appropriate amount of Fe2O3 have been manifested higher catalytic activity than conventional sulfated tin oxide in transesterification and esterification. At 1.0 mol% promoter content, the maximum conversion in lauric acid with methanol (6 h) and triacetin with methanol (8 h) reactions were about 88.9% and 92.1%, respectively. Moreover, the catalytic activity of SO42-/SnO2 is apparently higher than that of SO42-/ZrO2.A series of Al2O3-doped (0.5-3.0 mol%) sulfated tin oxide catalysts have been prepared by a co-precipitation method. The structures and textural properties of these catalysts were characterized using N2 adsorption, thermogravimetric analysis (TG), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Raman spectroscopy, and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) techniques. The number of acid sites on the catalysts was measured by means of potentiometric titration of n-butylamine. Their catalytic performances for the esterification of lauric acid with methanol and transesterification of triacetin with methanol were also investigated. The results showed that the addition of Al2O3 to sulfated tin oxide inhibite the growth of SnO2 crystallites, enhance the surface of the samples, stabilize more sulfur species on the surface of SO42-/SnO2 and improve the catalytic activity markedly. The SO42-/SnO2 catalyst doped with molar fraction Al2O3 of 1.0 mol% exhibited the highest activity. The lauric acid conversion was 92.7% after the esterification for 6 h and the triacetin conversion was 91.1% after the transesterification for 8 h on this catalyst. The remarkable activities of the Al2O3-doped catalysts are caused by an enhanced number of acid sites. Moreover, catalyst reusability and regeneration were studied in esterification and transesterification using 1.0 mol% Al2O3-SO42-/SnO2. The catalyst was calcinated to regenerate after each reaction. The results showed that the main reason for catalyst deactivation was due to reactant molecules covering the activity of the catalyst acid sites, through washing and calcination approach, the cover molecules could be removed to restore most effective acid sites, so that the catalytic activity was almost restored.A series of Ga2O3-doped (0.5-5.0 mol%) sulfated tin oxide catalysts have been prepared in the same method. The structures of these samples were characterized using X-ray powder diffraction (XRD), N2 adsorption, thermogravimetric analysis (TG), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Raman spectroscopy (Raman). The number of acid sites on the catalysts was measured by means of potentiometric titration of n-butylamine. Be similar with the catalysts doping Al2O3, SnO2 diffraction peak intensity was increased with the content of Ga2O3, mainly because of the presence of Ga2O3 inhibiting SnO2 grain growth. The results showed that adding a small amount of Ga2O3 could significantly increase the catalyst's surface acidity in SO42-/SnO2. And the catalytic activity could be also enhanced remarkablely. Similarly, the 1.0mol% Ga2O3-SO42-/SnO2 catalyst exhibited the highest activity. The lauric acid conversion was 91.3% after the esterification for 6 h, and the triacetin conversion was 89.2% after the transesterification for 8 h on this catalyst.