Preparation, Characterization, Application Of Nitrogen-Incorporated ZSM-5 Zeolites And Its Density Functional Theory Study

Recently, Nitrogen/carbon-incorporated zeolites (ZOL, zeolite with organic group as lattice), have been drawing lots of attentions as a new family of hybrid zeolite. And, most of investigations focus on the nitrogen-incorporated zeolites as they own stronger surface basicity than carbon-incorporated zeolites. In this work, nitrogen-incorporated ZSM-5 zeolites, including its preparation, physicochemical properties, application and substituted mechanism, were investigated by combined experiment method and density function theory (DFT) calculations.Characterized by XRD, BET, N₂ adsorption, SEM, and TEM, Nitrogen-incorporated ZSM-5 zeolites prepared by temperature-programmed nitndation at certain conditions were found to maintain good pore structures and framework comparing with the precursor zeolites. The surface acidity and basicity were characterized qualitatively and quantitatively by pyridine-adsorbed IR and CO₂/NH₃-TPD. New weak acid sites and basic sites with some strength were formed on the surface of nitrogen-incorporated ZSM-5 zeolites. According to the DFT calculated results, the acid sites were attributed to Si-NH₂-Al groups which were formed by substitutions of 0 atoms in AlO₄ tetrahedron by N atoms and basic sites were attributed to Si-NH-Al groups which were formed by the proton H migration out from Si-NH₂-Al groups or Si-NH-Si groups formed by substitution of O atoms in the group of Si-O-Si groups by N atoms.The nitrogen content was increased by introducing metal with ammonolysis on the surface of precursor zeolites before nitridation. The results showed that load of metal Ni and Co can increase the nitrogen content of zeolite when nitridated at high temperatures and the same nitrogen content could be achieved at low temperatures by loading Ru on the surface of zeolite, which means load of Ru can lower nitridation temperatures.Characteristic peak at -92 ppm corresponding to SiO₃N species was observed in the spectra of ²⁹Si MAS NMR. Bands at 1151 and 985 cm^-1, attributed to bend vibration of NH species from Si-NH-Si group and vibration of Si-N-Si group respectively, were firstly found in the IR fingerprint region of nitridated microporous zeolites. The IR simulation of nitrogen-incorporated ZSM-5 zeolite verified the attribution of experimental results. Moreover, concluding from experimental results, bridging -NH-groups would be formed when nitridation temperature is up to 800℃, which suggests that high temperature can promote the nitridation reaction.Nano ZSM-5 zeolites were firstly used as precursor for nitridation investigations. The activity of atoms in the framework of zeolite was increased as the stability of Nano ZSM-5 zeolite was decreased, which thus improved the interaction between framework and NH₃ molecule and consequently promoted the nitridation reaction. So characterized by FTIR and ²⁹Si MAS NMR, N atom was demonstrated to be introduced into the framework of zeolite and then not only bridging -NH- groups were formed, but further nitridated species SiO_4-xN_x (x>2) were formed. To the best of our knowledge, this is the first time to report further nitridated species in the framework of nitrogen-incorporated microporous zeolite. Meanwhile, the nitrogen content of Nano ZSM-5 zeolite was strongly increased. The sample nitridated at 900℃can reach up to as high as 2.60 wt%, which is 70% more than general ZSM-5 zeolite and consequently weaken the surface acidity.The total system energies before and after N atoms substitution were calculated by DFT method. The results implied that there is no difference among 26 O atoms before substitution, which indicates each O atom has equal probability to react with NH₃, while N atom after substituting O₁₁ atom has the most stability after substitution. Thus, O₁₁ site, which would be in the same tetrahedron with Al atom, is considered as the most preferred site for N atom substitution.Combined the experimental results and the optimized equilibrium configurations of NH₃ molecule adsorbed on the clusters before and after substitution, a rough mechanism for nitridation of zeolite was proposed. Hydrogen bond was found to play an important role to stabilize the adsorbed state, activated state, and transition state/intermediate during nitridation and substitution is mainly depended on the nitridation temperature. Subsequently, the catalytic performances of nitrogen-incorporated zeolites were evaluated by Knoeveangal basic probe reaction, ethylation of ethylbenzene with ethanol to para-diethylbenzene and alkylation of toluene with methanol to para-xylene. The results showed that nitrogen-incorporated ZSM-5 zeolites present high catalytic activities in Knoevenagal concentration reaction, which indicated that basic sites with certain strength existed on the surface of nitrogen-incorporated zeolite. In ethylation of ethylbenzene with ethanol to para-diethylbenzene, a relatively stable catalytic activities and constant selectivities to para-diethylbenzene (as high as 90%), were achieved on nitrogen-incorporated ZSM-5 zeolites. It suggests that the surface acidity and basicity of nitrogen-incorporated zeolites were modified and N species in the framework of nitrogen-incorporated ZSM-5 zeolites can exist stably during reaction. Obviously, nitridation can be used as a new method to modify the surface acidity and basicity of zeolite. However, in alkylation of toluene with methanol to para-xylene, nitrogen-incorporated nano ZSM-5 zeolites present lower catalytic activity, which is also decreased with increases of nitrogen content. Further more, the nitrogen-incorporated nano ZSM-5 zeolites almost lost activity in ethylation of ethylbenzene with ethanol to para-diethylbenzene when nitridation temperature is up to 800℃. This indicated that the surface basicity of nitrogen-incorporated nano ZSM-5 zeolite increased with nitrogen content and unambiguously confirmed that the surface acidity and basicity of zeolite can be improved by modifying the nitrogen content of nitrogen-incorporated zeolites.