Synthesis And Application Of Solid Superacids So₄^2-/tio₂-hzsm-5 In The Preparation Of Allantoin

Chemical synthesis of Allantoin has attracted extensive attention due to its wide application as one of important fine chemicals in medicine, agriculture, biological engineering and so on. Generally, Allantoin is prepared by the conventional routine of glyoxylic acid reacting with urea. Liquid acids (H₂SO₄, HCl, HNO3, et al.) are conventionally used as catalysts. But the problems of corrosion, pollution and low selectivity strongly limit their industrial applications. For the purpose of overcoming these drawbacks, solid superacids have been selected as catalysts in the process of synthesis allantoin from glyoxylic acid and urea. However, these sulfated metal oxides possess poor structural properties such as low surface areas and little number of acidic sites and acid strength. This thesis aims to develop a new catalyst for synthesis of Allantion by combining organically the merits of solid superacid and HZSM-5 zeolite.Firstly, effects of the various factors in preparation of catalysts such as the methods, different Si/Al ratios of HZSM-5, different contents of HZSM-5, concentrations of H₂SO₄, baking temperatures, pH value for Ti(OH)4 deposition on the structures and activities of catalysts were investigated. The results show that the catalyst made by sol-gel method has better performance. The optimum process conditions are the weight ratio of TiO2 to HZSM-5 (Si/Al = 25) 2 to 1, the concentration of H₂SO₄ 0.5mol/L, the temperature for calcinations 500℃, and the pH for Ti(OH)4 deposition 8 .Secondly, the crystal stucture, structure of acid center and specific surface area for self-made catalyst were characterized by XRD, FT-IR, Py-IR, BET and SEM. The results prove that the catalysts made by sol-gel method have higher specific surface area and stronger acid capability.Thirdly, using the allantoin reaction as probes, effects of the kind and amount of catalyst, the feed ratio, the reaction temperature and time on the yield of allantoin were studied. Also, the recycle test of catalyst and the indiscriminate application test of the mother liquor after isolation were conducted. The results indicate that the optimal process conditions for allantoin synthesis are the molar ratio of glyoxylic acid to urea 1:4, the amount of catalyst 3w% of the reaction substrates, and the temperature and time 75℃and 4 h respectively. Compared with conventional liquid superacids and one-component solid acids (SO₄^2-/TiO₂ and SO₄^2-/HZSM-5), the self-developed catalyst SO₄^2-/TiO₂-HZSM-5 in this thesis displayed markedly higher activity. Under the optimal conditions, allantoin was obtained in a yield of more than 61%. Besides, the catalyst can be recycled several times, and the isolated mother liquor can be reused indiscriminately in the synthesis of allantoin.Finally, the mechanism was proposed for this reaction in the presence of solid superacid SO₄^2-/ TiO₂-HZSM-5.