Theoretical Simulation Of The Early Stage Of The Crystallization Of Open-framework Aluminophosphates

Microporous aluminophosphates(Al PO4-n) are important zeolitic materials which have great potential applications in catalysis, separation, and adsorption due to its various structures. For decades, the mechanisms of crystallization of microporous aluminophosphates have been studied by using various experimental in situ and ex situ characterization methods, however, the characterization techniques cannot unveil the phenomenon at the critical 5-10 nm length scales, and the understanding of the mechanism remains incomplete. In this work, multiple-scale theoretical and computational chemistry techniques are applied for investigating the mechanisms of zeolite formation from three perspectives – 1) on the condensation reactions occurred at the early stage of synthesis, 2) detection of fragments formed in the early stage of synthesis using mass-spectrum technique, which also provides experimental evidence to support he discover in the first task, and 3) how does structure directing agent(SDA) influence the structures of final crystal structures given the same chemical compositions.In order to study the condensation reaction occurred at the early stage of synthesis, we first carried out QM calculations to characterize unit reaction mechanisms, structures and energy changes. The data obtained provides not only new information to the literature but also baseline data for the following steps. The mechanisms of aluminophosphate oligomerization were investigated using density functional theory with the SMD solvation model. Two aluminum species, Al(OH)4- and Al(H2O)63+, and four phosphorus species, H3PO4, H2PO4-, HPO42-, and PO43- were considered as the monomers for polycondensation reactions. It was found that the most favorable pathway to dimerization was a Lewis acid-base reaction: the aprotic oxygen of phosphoric acid(P=O) performs a nucleophilic attack on the central aluminum atom of Al(OH)4-. Using this mechanism as a pattern, plausible dimerization mechanisms were investigated by varying the proticity and hydration of the phosphorus and aluminum monomers respectively. The relative reaction rates of each mechanism were estimated under different p H conditions. The chain growth of aluminophosphates to trimers, 5 tetramers, pentamers and the cyclization of a linear tetramer were also investigated. For oligomerization reactions beyond dimer formation, it is found that cluster growth favors the addition of the phosphoric monomers rather than aluminum monomers.Using the QM data, we developed reactive force field based on Reax FF functional form. force field was developed to treat the aluminophosphate system by fitting the quantum mechanics data. The new force field parameters were fitted from the data set containing charge, bonds, angles, torsions and equations of state, and chemical reactions of aluminum and phosphours monomers and oligomers.The developed reactive force field was applied to reactive molecular dynamics simulations in order to understand the condensation reactions and sol-gel formation in solution under conditions close to experimental ones. We also investigated the effect of structure directing agents(SDAs) in the early stage of the sol-gel process. Preliminary results have been obtained which shows SDAs reduce the rate of oligomerization reaction, and during the condensation reactions, SDAs help oligomers to capture monomers via hydrogen bond interaction of NH2 groups in a specific orientation, implying the SDAs have structure directing effects in the early stage of the oligomerization process.However, the simulations were severely limited by limitation of simulation time and sizes. In order to see more phenomenon during the early stage synthesis, we must overcome the time and length scale problem. We applied the temperature-based replica exchange method to molecular dynamics simulations of a model substitution reaction, AB+C ? A+BC, at high concentration and pressure to predict the equilibrium products at specified thermodynamic conditions. Using different activation and reaction energies in the model reaction, we examined the robustness of the method and calibrated the temperature set and exchange frequency. The method is efficient for modeling exothermic reactions with modest energy changes(>3.0 kcal/mol) or energy barriers(ca. >25.0 kcal/mol). A reaction with a high energy barrier and energy change has reduced sampling efficiency using this method, because of the large degree of separation in the sampled phase spaces. This difficulty is more pronounced for endothermic reactions as both temperature and potential energy change in the same direction.From experimental side, we monitored the evolution of the species in the synthesis solution of aluminophosphate Al PO4-12 as an example via ESI-MS. On the basis of several basic assumptions on the connectivity of Al and P atoms in the species and implemented by in-house codes, we developed a highly efficient, automatic, and highthroughput method to elucidate the structure of the species for a given m/z. At the final step of this method, the molecular dynamics was applied to relax the structure and the structures favorable in energy were left, which significantly reduced the quantity of candidates. The fragments obtained provide solid support for theoretical and computation studies.From the fragments obtained in the early stage to the final crystalline product is a process much more complicated that beyond the capacity of computer simulations. In this work, we tackled this problem by analyzing how different SDA changes the structure of zeolites from the same chemical composition and the same topology. A common phenomenon known as “one-structure/multiple templates” was under considered by comparing the binding free energy between organic templates and inorganic framework. Molecular mechanism was applied in four microporous system which have the same topologies and can be directed by different structure directing agents. The free energy was calculated by free energy purterbation techniques. We found that the template-framework binding free-energy level or charge transfer degree was the key to the structure directing effect. In the synthesis of zeolites and related crystalline materials with open-frameworks, the template or organic additive played a topological structure-directing role instead of a structure-directing role.