Theoretical Studies On Mechanisms Of Molecular Reaction And Particle Aggregation In Zeolite Synthesis

The early stages of zeolite growth have great relevance in controlling the microscope properties as well as macroscope properties of the final zeolite crystalline. Many uncertainties in the process arise from its brevity and the small size scale, which prevents a prediction of when and where it will appear. Only very few experimental techniques are available to catch the process and analyze typical small clusters, and no technique is able to capture the total mechanism completely on its own. If the zeolite growth in solution was considered a series of chemical reaction steps, each one of which may have its own activation energy. With this knowledge, it should be possible to study the mechanism of zeolite growth on a molecular level even to a larger scale.In the present work, the mechanism of zeolite growth involving the events occurring from molecule to nanoparticle size scale, is investigated by the quantum chemical calculation and Monte Carlo simulation. The detailed formation mechanisms of aluminosilicate clusters with various sizes and structures in solution are determined, and subsequently the aggregation growth of sol particles is approached.The condensations of silicic acid with aluminate in alkaline environment are studied using the density-functional theory, at the B3LYP /6-31+G(d,p) and 6-311++G(d,p) levels. The Si(OH)₄ monomer, Al(OH)₄^-and Si(OH)₃O^- anion are used as the most elementary reactant models to study the condensation pathways in basic solution. The solvent effect is included by the COSMO-RS model. The study includes the complete geometry optimization and frequency calculation of reactants, products, reaction intermediates and transition states, as well as the calculation of the activation energy of the different pathways involved. The intrinsic reaction coordinate method is used to verify the reactant and product corresponding to the transition state. The calculation shows that the formation of Si-O-Al linkage can proceed via two possible reaction pathways. The first is a single-step process, in which the formation of SiO...Al bond and removal of water are synchronous, with the activation energy for formation of dimer of 83.7 kJ/mol. The second is a stepwise route, in which the AlO...Si bond is first formed to give a 5-coordianted Si intermediate, and then water is removed to yield a dimer aluminosilicate, with the barriers for two steps of 62.7 and 69.3 kJ/mol, respectively. The overall potential barrier is calculated as 122.0 kJ/mol. Thus, the SiO...Al bond formation is preferred over the A10...Si bond formation pathway. The formation of Si-O-Si linkage can proceed via ionic stepwise route and neutral single-step route, respectively. The ionic pathway involves the formation of 5-fold Si intermediate and subsequent removal of water, with the activation energies of 21.3 and 85.2 kJ/mol, respectively; the neutral pathway proceeds in one step, with the activation energies of 138.6 kJ/mol. The Si(OH)₄ monomer can substitute hydrogen atom of terminal hydroxyl group of q²Al species and obtain q³Al species via the SiO...Al bond formation mechanism, and further replacement can give the q⁴Al species, the calculated activation energies are 75.2, 48.8 kJ/mol, respectively.The formations of 3-ring and 4-ring from linear trimer and tetramer aluminosilicates proceed via the SiO...Al bond formation mechanism, with activation energy of 82-86 kJ/mol. The formation of cage species containing double-rings, such as, the prismatic hexamer Q₆³ and cubic octamer Q₈³, can proceed via three and four reaction steps, respectively. The activation barriers of formation Si-O-Al and Si-O-Si bridging bonds are in ranges of 58-79 kJ/mol and 127-147 kJ/mol, respectively, thus, the latter is rate-determinating step.The study of the aggregation of silica sol particles is presented by using Monte Carlo simulation including reaction parameters: activation energy, temperature, and particle concentration. In this simulation, all of the system particles are classified as active and inactive types; only the activated particles can aggregate into clusters, and consequently the aggregation exhibits selectivity. The influences of the above reaction parameters on structural properties and growth kinetics of aggregates are investigated and compared with the existing relevant results. The evolutions of the fractal dimension and size distribution of aggregates over the entire process suggest that the sol system exhibits the behaviors of diffusion-limited cluster aggregation at earlier times and crosses over to those of diffusion-limited aggregation at later times. In the some range of particle concentration, the fractal dimension of the resulting aggregates about 2.0 is comparative well with that of the reaction-limited cluster aggregation. As the temperature becomes higher and activation energy becomes lower, the aggregate transits from the open and highly-branched to compact and weak-branched morphology, corresponding to the variation of fractal dimension from about 1.5 to 2.5, which is consistent with the published experiment result. The study of aggregation kinetic shows that the aggregation goes through three regimes: flocculation, compactification, and gelation, which are separated by two crossover time t1 and t2. This result is agreement with the experimental observations.