| Piperazine?PIP?and triethylene diamine?TEDA?is important ethylene amine products and important intermediates in the fine chemicals.Homogeneous catalysis method in the production of PIP and TEDA always produces a large amount of by-products such as inorganic salts and wastewater resulting in a low economical feasibility.By contrast,synthesis of PIP and TEDA from ethanolamine catalyzed by heterogeneous catalytic technology is more environmental-friendly and can be operated under mild reaction conditions with a low-cost.The catalytic activity and lifetime of zeolite largely depend on the modification methods,and macromolecules of amine compounds are difficult to desorb from the strong acid sites in zeolite.Changing the acidic properties of the catalysts is therefore important to improve the selectivity of PIP and TEDA during the synthetic process.The synthesis of PIP and TEDA from MEA from ethanolamine was conducted in a continuous flow fixed-bed reactor?diameter 15 mm with a length of 1200 mm?under 330°C and atmospheric pressure,filled with?zeolite,mordenite?MOR?or HZSM-5.The results indicate that is the selectivity to PIP and TEDA both follows the order of?-zeolite>HZSM-5>MOR.The respective selectivity of?-zeolite,HZSM-5 and MOR to PIP is 23.8%,21.4%and 18.7%,and to TEDA is 20.1%,17.1%and 12.6%.The respective lifetime of?-zeolite,HZSM-5 and MOR is 22,18 and12h.The selectivity and lifetime of?-zeolite is therefore better than those of HZSM-5 and MOR.After?-zeolite was loaded with metal or non-metal species,its BET surface area and total pore volume decrease.For instance,the lifetime of?-Zn2%is 12h and its selectivity to PIP and TEDA is 21.4%and 24.2%,respectively.The lifetime of?-Cu2%is 20 h and its selectivity to PIP and TEDA increases to 27.8%and 26.2%,respectively.The higher selectivity to PIP and TEDA is due to the formation of the mesoporous structure and the improvement of medium-strong acid sites in the catalyst.The selectivity to PIP and TEDA over?-P2%increases to 28.7%and 30.7%respectively,and the lifetime of?-P2%is 28h,which is prolonged by 21%than that of?-zeolite.The selectivity and lifetime of?-P2%is therefore better than those?-Zn2%and?-Cu2%.However,the excessive by-products cannot be solved by loading metals or non-metals on the catalysts.In order to improve the selectivity of the catalysts to PIP and TEDA,we tried to modify the surface of?-zeolite with a silylation method??d?to decrease the content of by-products.The pore volume of?d has no change,whereas the number of both strong acid and weak acid sites in?-zeolite decreases after silylation modification.After the?d zeolite is further treated by the 500°C steam(?dw),the pore volume and BET surface area increase,with significant decreases of strong acid sites and no change of weak acid sites in the catalyst.On the other hand,the lifetime of?d zeolite decreases to 16 h and decreases by 27.3%than those of?zeolite.The lifetime of?dw zeolite increases to 30h and by 36.4%than that of?zeolite.The selectivity of by-products in the reaction system over?and?d zeolite decreases from 34.4%to 28.8%and 15.8%with a fall of 16.3%and54.1%,respectively. |
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