Simulation Application In The Synthesis And Modification Of Zeolite Molecular Sieves

Zeolite is a kind of inorganic crystal materials with regular framework and structure consisting of three-dimensional open networks of TO4(T=Si, Al, P, etc.) tetrahedron. The materials are widely used in some industry processes, such as heterogeneous phase catalysis, gas/liquid phase adsorption and separation. It’s very important for us to study its synthesis, structure and adsorption property. Early in1950, artificial zeolites have been paid a great importance to the fields of industry, science and technology because of their high thermal stability, ion-exchange, surface acid-base properties and shape selective property, and found tremendous applications in many fields of industry, science and technology.Compared to the traditional modification with ammonia/zeolite gas/solid reaction at high temperature condition, the advantage for preparing a novel methylamine (nitrogen) organic/inorganic framework hybrid-zeolite is more moderate modification conditions and lower cost. The hybrid zeolite possesses unique acid-base bi-functional catalysis properties, which can be used as green catalysts in biomass-derivate energy source and fine chemicals system. A few reports, which related to the preparation, microcosmic-structures, acid-base nature and application for the hybrid-zeolite, have been published. Therefore, the studies for possible mechanism in modification reaction, and the microcosmic-structures which bring in by-function acid-base properties are very important and interesting.In chapter V, the possible fine-structure unit (geometries) for the hybrid-zeolites was studied by DFT simulation. In the way, which calculate the system error firstly by comparing the data of experimental13C MAS NMR peaks with corresponding simulation data and then gradually correct the simulation data, the complex resonance peaks for the hybrid-zeolites were finally identified. Based on the identification, the main probable geometries for the acid-base functional groups are recognized as follows:1) In the case of high-silica zeolites, the main fine structural unit is branched hybrid-geometry (≡Si-NHCH3). The acid-group is Si-OH generated in process of amine (nitrogen)-hybrid reaction. The base is induced from the nitrogen atom of the branched amine-hybrid-geometry, oxygen-defect-site(Si-O-) generated occasionally in hybrid reaction or branched ammonia-hybrid-geometry(=Si-NH2) generated by removal of-CH3in heating process for actual application.2) In case of low-silica zeolites, the main unit is bridged hybrid-geometry (≡Al-NHCH3-Si≡) which yield multi-ply acid-base active sites(Alδ+,Nδ-,Hδ+,Siδ+). The Al-N and Si-N bonds at the unit can be attacked by acid or base, then easily break into the units such as≡Si-NHCH3,≡Al and≡Si-OH), which also have acid/base activities.A simple potential-energy calculation with DFT simulation is used for calculating the energy changes of the optimized-geometries in hybrid-reaction. Some phenomena are understood based on the analysis for reaction energies. This phenomena are as follows:1) the methyl amine-hybrid modification reaction with can be performed at moderate conditions while the amonnia-hybrid reaction must be in high temperature;2) the main hybrid-geometry in low-silica zeolites, such as bridged hybrid-geometry=Al-NHCH3-Si=, can be break into moieties later in the continuous heating process,.Possible adsorbed-geometries are investigated to compare the adsorption strength. The energy in amine-hybrid reaction for low-silica zeolite is lower than that in adsorption. Therefore, the latter may provides the energy needed for the amine hybrid-reaction. In the case of high silica MFI zeolite, the near-adsorption-geometry at some sites may be an important intermediate for the hybrid reaction, due to the structural similarity of the part framework units in the zeolite with the near-adsorption-geometry (Si…N).In chapter VI, the characterizations with TG/DTG/DTA and NH3-TPD/CO2-TPD are used to confirm the unique acid-base bi-functional properties of the hybrid zeolites. The characterization with XRD、29Si and13C MAS NMR are accepted to understand the nature of the unique acid-base properties and possible applications for the hybrid zeolite. Simulating serial possible hybrid-geometry with Gaussian program, and comparing the experiment data, the resonance peaks and the related main hybrid-geometries in the acid-base bi-functional hybrid-zeolite have been identified as follows:silica-aether (=Si-OCH3), POM-like hybrid-geometries (=Si-O[-CH2O-]nH) or (=Si-O[-CH2O-]nSi=), PEG-like hybrid-geometries (=Si-O[-CH2CH2O-]nH) or (=Si-O[-CH2CH2O-]nSi), and minor hybrid-geometries, such as carboxylic geometries, silica-methanol et al.There identifications combined with the calculation for the reaction energies of all possible hybrid-geometries indicate two possible hybrid-reaction mechanism:A) two precursors are formed firstly by "zeolite-catalyzed" Cannizzaro reaction in the adsorption process of formaldehyde, and then the precursors continues reaction with each to generate some intermediate hybrid-geometries with higher potential energies, which will turn into the final ether-like hybrid-geometries in hydrotherm condition; B) in both adsorption process and hydrotherm-hybrid process, the framework of the zeolite can open the circle of POM to obtain POM-like hybrid-geometries directly.The Cannizzaro reaction provides more energy enough for the carbon-hybrid-zeolite reaction. The mechanism suggests a new way for the preparation of the organic/inorganic hybrid materials.In chapter VII, all silica b-axis preferentially oriented single crystals of high silica MFI type zeolite are hydrothermally synthesized in R1·R2·Na2O·SiO2·H2SO4·NaF·H2O reactant system with certain molar ratios for each component of the composition. The first template (R1)-n-PrNH2(propylamine) and the second template (R2)-EtHMTA+Br-(N-ethyl-hexamethylenetetramonium bromide) are used. MD simulation combined with NMR investigation is used to modify the template role and its action mechanism.An index of CPO is accepted to characterize the crystallographic preferred orientation of the zeolite. The influence of the relative content for R1and R2in the reactant with various compositions on the CPO values of the obtained MFI type zeolite single crystals is investigated. Obviously, CPO value is closely related to the ratio of R1/R2in the reactant. The obtained zeolite crystals, crystallized in the reactant with R1/R2=7/3at the temperature of160～180℃, possess the CPO value up to0.8-0.9. The zeolite crystals, obtained in the reactant with R1as the only one template, lose preferential growth. The reactant with the R2/SiO2>=0.5keeps amorphous for long time, and finally crystallizes in the form of MTN type zeolite with a framework of clathrate topography after reaction for14days.The13C MAS NMR spectra for samples of b-axis preferentially oriented single crystals after washing with1,3,5-trimethylbenzene or without washing indicate that more adsorptive organics is impossible to inter into the zeolite channels. These organics must belong to R2molecular which locate at the sites near the inner close to the channel window of the Zeolite.The adsorption sites for R2at the openings of the zeolite channels are studied by MD simulation. Site Ⅰ/Ⅲ is close to the adsorption sites for R2in inner intra-channel-cage. At site Ⅰ along a-axis, R2may back up the channel-wall to facilitate the generation of new crystal-face floor. On the other hand, R2adsorbed along b-axis or far from site Ⅲ occupies the positions which help the growth of the crystal surface in the axis. Consequently, R2along b-axis push Si atoms around and interference their exact lattice-site, and block the translation and deposition for the silicons. In this way, the crystal growth along b-axis is inhibited.MSD (mean square displacement) analyses for two type of template in inner intra-channel-cage of MFI zeolite were performed. Based on the MSD analyses, the template-directing role for R1is confirmed by the strong interaction between R1and the zeolite framework. R2is restricted at the intra-channel-cage and blocks the translation along the channels.The co-template mechanism was proposed as:R1induces the nucleation and the growth for the zeolite crystals, while R2play a molecule-size effect, backs up channel-wall to facilitate the crystal-growth along a axis and push silicons away from their exact lattic-sites to stunt the crystal-growth along b-axis. The mechanism provides a new way for controlling the crystal morphology in zeolite synthesis process.