Studies On Novel SO₄^2- Promoted Oxide Solid Superacids

SO₄^2-/M_xO_y type solid superacids,especially SO₄^2-/ZrO₂,have attracted much attention in the last 30 years,because they are noncorrosive,environmentally friendly and reusable at high temperatures.Compared with traditional solid acid catalysts,the typical feature of these solid superacids is that they are highly acidic.Thus,they make up the deficiency in acid strength for the former acid catalysts,and meet the requirement for the strong acid-catalyzed reactions.Mover,they are easy to activate C-H and C-C bonds of the substrates,and catalyze the reaction at relatively low temperatures.As a result,energy can be saved and side reactions are decreased.Also branched hydrocarbons with high octane number are favored to produce.Therefore, they are recognized as a class of novel catalytic materials which are green and have potential application.Recently,with the increasing of environmental constrains,the environmental unfriendly catalysts used in petrochemical industry,such as HF,H₂SO₄,H₃PO₄ and AlCl₃,are urgent to be substituted by new clean catalysts.Solid superacids are capable of activating the covalent C-H and C-C bonds at lower temperatures,and reveal all the advantages of heterogeneous catalysts such as regenerability coupled with the benefits of liquid superacids such as high activity and selectivity.They are possible to become the substitute of those environmentally unfriendly catalysts,and thus a number of environmental friendly processes may be established.In these solid superacids,the acidity of SO₄^2-/ZrO₂ is strongest,and the research of this kind of solid superacid catalysts is also most broad.However,for the SO₄^2-/ZrO₂ type solid superacid catalysts,the relatively small surface area and pore volume and non-uniform pore size limits their potential applications.In the present dissertation,we prepared mesoporous sulfated Al₂O₃-ZrO₂ catalysts with higher hydrothermal stability,and the catalysts are more active than the conventional SO₄^2-/ZrO₂ for various acid-catalyzed reactions.We put forward for the first time to prepare a novel mesoporous solid superacid system by loading sulfated zirconia into mesostructuredγ-Al₂O₃.The novel mesoporous solid superacids exhibit superior catalytic performances to mesostructured SiO₂ supported sulfated zirconia that were often studied in the literature as well as traditional sulfated zirconia catalysts, affording a new pathway for the preparation of mesoporous solid superacids.We advanced a method of hydrothermal assistant synthesis for preparing non-transition metal oxide Al₂O₃ and transition metal oxide Fe₂O₃ promoted SO₄^2-/ZrO₂ solid superacid catalysts.Their catalytic activities are distinctly higher than the corresponding catalysts without hydrothermal assistant treatment,supplying an effective modification method for SO₄^2-/ZrO₂ solid superacid catalysts.At the same time,we discover for the first time the promoting effect of Al₂O₃ on SO₄^2-/SnO₂ solid superacid.In the third chapter of this thesis,mesoporous sulfated Al₂O₃-ZrO₂(MSAZ) catalysts with large surface areas and pore volumes after calcination at high temperature(650℃) and with higher Al₂O₃ content than 20wt%were successfully prepared from a template of block copolymer(P84).Zirconia in MSAZ catalysts is tetragonal crystalline.The pore diameters are narrow in distribution.The surface area of MSAZ catalysts is 165-262m²/g,the pore volume is 0.156-0.340cm³/g and surface SO₄^2- content is 7.0-8.4%,which are much higher than those of conventional sulfated zirconia(labeled as CSZ).The nature of acid sites present on the MSAZ catalysts is Lewis and Brφnsted type,and the strength of Lewis acid sites is stronger than that of Brφnsted ones.MSAZ catalysts have greater number of strong and very strong acid sites than CSZ.Catalytic tests show that mesoporous sulfated Al₂O₃-ZrO₂ catalysts exhibit higher activities than conventional sulfated zirconia for conversion of n-pentane at low temperature,Friedel-Crafts benzoylation of toluene with benzoyl chloride and dealkylation of 1,3,5-tri-tert-butyl-benzene.Catalytic evaluation for the dealkylation of 1,3,5-tri-tert-butyl-benzene shows that mesostructured MSAZ catalysts elucidate the advantages evidently over microporous zeolites and provide potential application for catalyzing bulky molecules such as those encountered in the production of pharmaceuticals and fine chemicals.In the fourth chapter,we have shown that novel mesoporous SO₄^2-/ZrO₂/γ-Al₂O₃ catalysts were successfully synthesized via chemical liquid deposition method using the carrier of mesostructuredγ-Al₂O₃.The pore diameters of the catalysts are narrow in distribution,the surface area is 217-258m²/g,the pore volume is 0.296-0.352 cm³/g,and the surface sulfate content is 6.8-7.7%,which are evidently higher than those of conventional sulfated zirconia(labeled as CSZ).Zirconia in the catalysts exhibits only the tetragonal phase.The acidity and catalytic activity can be adjusted via changing the loading of ZrO₂.Both Lewis and Brφnsted acid sites are present on the catalysts.Mesostructuredγ-Al₂O₃ supported sulfated zirconia catalysts have the equivalent acid strength to sulfated zirconia,but have stronger acid strength than MCM-41 supported sulfated zirconia.Moreover,the number of acid sites is higher on the former catalysts than on the latter one and CSZ.Hence,the novel mesoporous solid superacids are more active than MCM-41 supported sulfated zirconia and sulfated zirconia for various acid-catalyzed reactions.The different sulfating agents lead to different sulfate contents,which explains the differences in acidity and catalytic reactivity,the order is(NH₄)₂S₂O₈＞H₂SO₄＞(NH₄)₂SO₄.The catalytic activity of mesoporous solid superacids is evidently higher than microporous zeolites and conventional sulfated zirconia in catalyzing bulky molecules.In the fifth chapter,non-transition metal oxide Al₂O₃ promoted SO₄^2-/ZrO₂ solid superacid catalysts were synthesized through hydrothermal assistant treatment method. The results indicate that hydrothermal assistant synthesis is helpful for increasing the surface areas of Al₂O₃ promoted SO₄^2-/ZrO₂ solid superacid catalysts,stabilizing the sulfate species on surface of the catalysts,and enhancing the acid amount of catalysts, but the coordinate circumstance of Al remains unchanged.The nature of acid sites present on the catalysts is Lewis and Brφnsted type,and after hydrothermal treatment, the number of Lewis acid sites increases and that of Brφnsted ones decreases. Hydrothermal assistant synthesis can enhance the catalytic activity ofα-pinene isomerization reaction.The acid amount and catalytic acidity forα-pinene isomerization is highest after 1 day hydrothermal treatment.The conversion is 80.8%, increased by 54%in comparison with the corresponding untreated sample(52.3%).In the sixth chapter,transition metal oxide Fe₂O₃ promoted SO₄^2-/ZrO₂ solid superacid catalysts were synthesized through hydrothermal assistant treatment method. The results clearly indicate that hydrothermal assistant synthesis is also helpful for increasing the surface area of Fe₂O₃ promoted SO₄^2-/ZrO₂ solid superacid catalysts, stabilizing the surface sulfate species,restraining the growth of ZrO₂ crystallites, enhancing the catalytic activity ofα-pinene isomerization reaction.Incorporation of small amounts of Fe₂O₃ into SO₄^2-/ZrO₂ results in higher surface areas,more surface SO₄^2- content and smaller crystallite size of ZrO₂.Hence,the catalytic activity ofα-pinene isomerization reaction is improved.The activity of the catalyst containing 2wt%Fe is highest.The conversion reaches 71.3%,increased by 56%compared with the corresponding sample without hydrothermal assistant treatment(45.8%),and is 2.2 times as high as SO₄^2-/ZrO₂. In the seventh chapter,we prepared small amounts of Al₂O₃ doped SO₄^2-/SnO₂ through co-precipitation method.SO₄^2-/SnO₂-Al₂O₃ catalysts have distinctively larger surface area(ranging from 139 to 150m²/g) than SO₄^2-/SnO₂(118m²/g).The content of SO₄^2- on the surface of SO₄^2-/SnO₂-Al₂O₃ catalysts is 5.8-8.7wt%),obviously higher than SO₄^2-/SnO₂(5.0wt%),showing that incorporation of small amounts of Al₂O₃ can stabilize more sulfur species on the surface of SO₄^2-/SnO₂.SnO₂ in the catalysts is tetragonal crystalline after calcination at 500℃.As the Al₂O₃ content is increased,the intensity of characteristic diffractive peaks of SnO₂ decreases progressively.This result demonstrates that the growth of SnO₂ crystallites is inhibited in the presence of Al₂O₃.There is only six-coordinate aluminum species in the SO₄^2-/SnO₂-Al₂O₃ catalysts.Only Lewis acid sites are present on the catalysts,and the number of acid sites is increased after incorporation of Al₂O₃ into SO₄^2-/SnO₂.Benzoylation of 2-Methoxynaphthalene with acetic anhydride was used as a test reaction to investigate the activities of the catalysts.The results indicate that incorporation of Al₂O₃ into SO₄^2-/SnO₂ can enhance the catalytic activity.The activity of SO₄^2-/SnO₂ increases most after doping 0.2-0.5 mol%Al₂O₃,increased by about 60%(from 36.1%to 58%). The reason for activity enhancement is that the surface SO₄^2- content and number of acid sites of SO₄^2-/SnO₂ catalysts are increased after incorporation of small amounts of Al₂O₃.