Synthesis And Catalytic Performance Of KFI Zeolites

KFI is a small pore zeolite with three-dimensional eight-membered ring（8MR）channels,with a pore size of 3.9×3.9?.Because of the unique shape selectivity in molecular size,large specific surface area,high thermal/hydrothermal stability,and other characteristics,it is widely used in gas adsorption and separation,methanol amination reaction,ammonia selective reduction reaction（NH₃-SCR）,methanol to olefins（MTO）reaction,and methanol dehydration to dimethyl ether reaction.Especially,it shows excellent catalytic performance in methanol amination and NH₃-SCR reactions.However,the crystallization time for KFI zeolite is quite long,especially for high-silica KFI zeolite（framework Si/Al=5.0）,the crystallization time needs at least 10 days,which not only greatly increases the energy consumption in the synthesis of KFI zeolites,but also significantly reduces the space-time yield of KFI zeolites.In addition,the Si/Al ratio of KFI zeolites has been reported to be 1.23～5.0,so the synthesis of KFI zeolites with higher Si/Al ratio is still a challenge.Methanol amination is the main reaction of industrial synthesis of important chemical raw materials of methylamine and dimethylamine in which the ammonia reacts with methanol in the presence of catalyst in a continuous gas phase,and methylamine,dimethylamine,and trimethylamine are obtained by a series of distillation processes.The small pore zeolitic catalyst exhibits high shape selectivity to monomethylamine and dimethylamine in the methanol amination reaction,which inhibits the formation of trimethylamine and has received extensive attention.However,the selectivity and yield of methylamine and dimethylamine of the reported small-pore zeolitic catalysts are still low,which limits their practical application.Therefore,it is of great significance to develop small pore zeolitic catalysts with high selectivity and high yield of monomethylamine and dimethylamine.The emission of atmospheric pollutant nitrogen oxides（NO_x）will lead to environmental problems such as photochemical smog,acid rain,and haze.Diesel vehicle exhaust is one of the main sources of NO_x.NH₃-SCR is an effective technology to reduce NO_x emissions from diesel vehicles.At present,the commercial Cu-SSZ-13zeolitic catalyst shows good SCR activity,hydrothermal stability,and N₂ selectivity in NH₃-SCR reaction,but BASF controls its intellectual property rights.Therefore,it is of great significance to try to develop other zeolitic catalysts with high SCR activity,hydrothermal stability,and N₂ selectivity.Therefore,this thesis focuses on the efficient synthesis and catalytic performance of KFI zeolites.Low-silica KFI zeolite with a Si/Al ratio of 3.2 was rapidly synthesized using silicoaluminophosphate SAPO-34 as seed and KFI zeolite with a Si/Al ratio of5.4 was rapidly synthesized by using 18-crown-6/K⁺complex as organic structure-directing agent（OSDA）.The effects of SAPO-34 seed and OSDA on the synthesis of low-silica and high-silica KFI zeolites were systematically investigated by solid-state NMR,mass spectrometry,UV-Vis,and control experiments,respectively.H-KFI and Cu-KFI catalysts with different Si/Al ratios were prepared by ion-exchange and calcination,and their catalytic performances in methanol amination and NH₃-SCR reaction were investigated,respectively.The main results are as follows:1.The low-silica KFI-3.2（Si/Al=3.2）zeolite was rapidly synthesized by using as-synthesized SAPO-34 as seed at 160°C for 24 h.Further study revealed that SAPO-34 seed was completely dissolved in the strong alkaline initial gel,generating Si（3Al）species,phosphorus-containing species,and morpholine.By designing and implementing control experiments,it was found that Si（3Al）species rather than phosphorus-containing species directed the formation of KFI-3.2.In addition,the experimental results show that the morpholine in SAPO-34 seed contributes to the formation of Si（3Al）species and cooperates with Si（3Al）species to guide the formation of KFI-3.2.In the methanol amination reaction,the selectivity of monomethylamine and dimethylamine in the three methylamines of H-KFI-3.2 catalyst is 94.4%,which is higher than that of H-KFI（K,Cs）synthesized by traditional method and other reported small pore zeolites（SAPO-34,DNL-6,H-rho,and H-PST-29）.It is a promising catalyst for methylamine synthesis.2.The 18-crown-6/K⁺complex with a molar ratio of 18-crown-6 to K⁺of 2.85 was prepared from 18-crown-6 and KOH.The high-silica KFI-5.4 zeolite with a framework Si/Al ratio of 5.4 was successfully synthesized by hydrothermal crystallization for 5days using the complex as an OSDA.The crystallization time of the reported high-silica KFI（framework Si/Al ratio of 5.0）zeolite takes at least 10 days.Further study indicated that K⁺and OH^-play an important role in the formation of KFI-5.4.In the reaction of methanol amination reaction,the total yield of monomethylamine and dimethylamine of H-KFI-5.4 is 72.4%,and the total yield and total selectivity of monomethylamine and dimethylamine are well balanced,which is better than H-KFI-3.2 and other reported small pore zeolites（SAPO-34,DNL-6,H-rho,and H-PST-29）.In addition,metal modification can improve the catalytic performance of KFI-5.4 in methanol amination reaction,which provides an experimental basis for further improving the catalytic performance of zeolitic catalysts.3.The rapidly synthesized high-silica KFI-5.4 was prepared into Cu-KFI-5.4catalyst and its NH₃-SCR activity was investigated.The results show that high-silica Cu-KFI-5.4 has high NO_x conversion rate before and after hydrothermal aging at750°C or 800°C,showing high hydrothermal stability and high reactivity.After hydrothermal aging under the same conditions,the catalytic activity of low-silica Cu-KFI-3.2 is lower than that of high-silica Cu-KFI-5.4.The results also show that compared with low-silica Cu-KFI-3.2,high-silica Cu-KFI-5.4 has a more stable framework and more active copper species after hydrothermal aging,which is the main reason why high-silica Cu-KFI-5.4 has higher NH₃-SCR reactivity.