Catalytic Performance of Zeolites on Urea Thermolysis and Isocyanic Acid Hydrolysis

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• 作者：Weijuan Yang ; Zhenchao Chen ; Junhu Zhou ; Zhenyu Huang ; Kefa Cen
• 刊名：Industrial & Engineering Chemistry Research
• 出版年：2011
• 出版时间：July 6, 2011
• 年：2011
• 卷：50
• 期：13
• 页码：7990-7997
• 全文大小：1107K
• 年卷期：v.50,no.13(July 6, 2011)
• ISSN：1520-5045

文摘

Urea thermolysis and isocyanic acid hydrolysis over three kinds of zeolites are investigated experimentally and the main decomposition products, NH₃ and HNCO, are focused on. The results gained using thermogravimetric analysis with heating rates of 2 掳C/min from 50鈥?00 掳C show NH₃ releases mainly in 133鈥?50 掳C and the main thermal product above 250 掳C is HNCO. NH₃ release process appears double-peaked while HNCO triple-peaked. Zeolites shift urea decomposition to lower temperature and shorten the process. The experiments of urea thermolysis over zeolites were done in the fixed reactor at programmed temperature and constant temperature. Although zeolites enhance the production of both NH₃ and HNCO, the yield ratio of NH₃ increases to 1.1 from 0.9 when adding zeolites but the yield ratio of HNCO is always below 0.8. Under zeolites鈥檚 catalytic effect, the peak of NH₃ release and the second peak of HNCO release become stronger. Moreover, zeolites can result in the HNCO peaks integrating into a stronger peak at 500 掳C. The total yield of NH₃ and HNCO increases about 0.1鈥?.2 with zeolites and the catalytic effect is more obvious at low temperature. In the experiment of thermolysis with a urea-water spray, over 96% urea could decompose to NH₃ and HNCO when the temperature is over 550 掳C and the residence time is more than 1.0 s. Zeolites show good catalytic performance on HNCO hydrolysis to NH₃ and the conversion of HNCO to NH₃ increases with increasing temperature and reaches above 80% at 250 掳C and can touch 100%. The catalytic effect on urea thermolysis and HNCO hydrolysis decreases in the order H鈥揧 > H-尾 > H-ZSM5, which might be due to the amount of acidic sites on the catalysts. The apparent activation energy of the hydrolysis reaction is so low that the overall hydrolysis reaction rate on catalysts is mainly determined by external and internal mass-transfer limitations.