Theoretical Investigations On The Hydrolysis-Oligomerization Mechanisms Of Silicon And Aluminum Alkoxides

Aerogels made from alkoxides are representative low-density and high porosity materials, and possess desirable physical properties such as extremely low thermal conductivity, high acoustic impedance, large specific surface area, and low relative dielectric constants. As a reults, aerogels, the lightest solid materials in the world with the density lower than0.002g/cm3, are also known as frozen smoke. They are ideal candidates for various applications, such as thermal super-insulators, adsorbents, sensors, catalyst carriers and inorganic fillers, and have broad application prospects in launch vehicles and manned spacecrafts as well as in the industrial manufacture. To produce ideal aerogels, it is crucial to control the hydrolysis and oligomerization of alkoxides. However, the formation and microstructrural evolution of aerogels comprise intricate multistep chemical and physical processes, which can be represented as:1) the formation of colloidal particles is rather complicated. Currently most colloidal particles are prepared by chemical methods, viz. by the hydrolysis and oligomerization of alkoxide precursors, and the nelceation is interwoven with hydrolysis and oligomerization, so the formation mechanisms with high randomicity are very complex;2) The internal and surface structures are unmeasurable. Colloidal particles with the size between1-100nm belong to neither the representative macroscopical system nor the representative microscopical system. To experimental measurements, the particles are too small;3) The properties of colloidal particles are unique. They lie in the transition region between atomic clusters and macromolecules, so have strong surface effect, small-size effect and macroscopic quantum tunneling effect, which bring puzzledom to experimental studies;4) Gelation processes involve the condensation of colloidal particles, and are also chemically and physically complicated. Because the multitude of reactions occur simultaneously in solution, it is difficult to extract information from experimental data. Therefore, this dissertation will clarify the formation mechanisms of typical aerogels at the atomic and molecular level, unpuzzle the hydrolysis-condensation reactions and the sol-gel behavious of alkoxides by using the bottom-up method, and provide necessary theoretical guidance for the preparation of aerogels with unique properties.Quantum chemistry is an important branch of theoretical chemistry, is an elementary natural science to solve chemical problems with the basic principles and methods of quantum mechanics, and is the fundamental theoretical foundation of modern structural chemistry and computational chemistry. Since1960s, based on the development of computer technology, quantum chemistry has acquired significant successes and enormous achievements. Some computational results have already achieved the level to supersede experimental ones, and among them there are cases that the computation exceeds and rectifies experiments. The rapid development of quantum chemistry has made it as important as experiments. As a result, computer and computer software become a kind of "chemical experimental instruments" and a strong assistant of teaching and scientific research, so quantum chemistry evolves from a new peripheral discipline to the theoretical tool of chemistry. Now density functional theory (DFT) is the fastest growing and the most popularly accepted theoretical method. Since1990s DFT was greatly refined to better model the exchange and correlation interactions, and its calculation accuracy is comparable with the Hartree-Fock (HF) perturbation method, but the computational time is greatly reduced. Density function theory is currently the exclusive method that may be applied to the first-principle calculation of macromolecular systems. Now more than90%calculations are accomplished with the DFT methods.Silicon and aluminum aerogels are typical non-crystalline and crystalline aerogels prepared from the hydrolysis and condensation of silicon and aluminum alkoxides, and their miscrostructure evolution are representative. This work will employ the B3LYP quantum chemistry approaches with the conductor-like polarizable continuum model (CPCM) approximation to systematically elucidate the hydrolysis and oligomerization mechanisms as well as the condensation mechanisms of oligomers; to investigate their hydrolysis and oligomerization processes with different substituents in existence in neutral, acidic and alkaline solutions to further understant the influence of various conditions on the morphology of oligomers. The main roles of this dissertation are stated as follows:(1) Mechanisms of hydrolysis-oligomerization of Al(OC3H7)3As one of the representative super insulating materials, the aluminum trioxypropyl Al(OC3H7)3aerogel may be applied in launch vehicles and manned spacecrafts. In this study, the structures and hydrolysis mechanisms of the monomer, dimers and trimers of Al(OC3H7)3in neutral and alkaline environments were studied at B3LYP/6-31G(d,p) level combined with the CPCM solvation model to understand the fundamental chemistry of Al(OC3H7)3hydrolysis and oligomerization. Calculations show that the first-order hydrolyses of the monomer and oligomers are energetically favorable in both alkaline and neutral solutions. In alkaline solutions, they are apt to oligomerize than to hydrolyze due to the large binding energies in the formation of anionic species.Aluminum alkoxides exist at least in the form of dimmers, and the condensation does not need barriers. For the oligomers under neutral condition,1)Al(OC3H7)3is linked by four-membered Al-O rings with penta-coordinated bridging and tetra-coordinated Al atoms;2) the hydrolyzed propoxy groups will be expelled by solvent molecules;3) partly hydrolyzed species can condense to oligomers with bridging OH groups or O atoms.(2) DFT investigation on the mechanisms of silicon alkoxides Si(OR)4hydrolysis-oligomerization ReactionsSilica aerogels possess a variety of unique and remarkable properties, but the mechanisms of the hydrolysis and oligomerization of silicon alkoxides Si(OR)4in the initial stage of sol-gel processes are still not well understood. On the basis of density functional theory full optimizations at B3LYP/6-31G(d,p) level, considering the computational time requirement, the CPCM single point energy (SPE) calculations with the gas-phase equilibrium geometries were carried out at the more rigorous B3LYP/6-311++G(d,p) basis set level of theory. The SPE energies were scaled by the zero-point energies in gas phase. The CPCM SPE calculations were also performed with G09package at M06-2X/6-311++G(d,p) level to compare the reactivity of methyl-and ethyl-substituted species because of the good performance of the M06-2X functional in the main-group thermochemistry. The M06-2X CPCM energies were scaled with thermal corrections to Gibbs free energies in gas phase. The M06-2X results show that both the free energy barriers and total barriers at M06-2X/6-311++G(d,p) level are unfeatured, and B3LYP may be better in this reaction system. We will employ the B3LYP method to systematically investigate the hydrolysis-oligomerization mechanisms of Si(OR)4in neutral, acidic and alkaline solutions.Calculations show that, in acidic solutions, the precursor Si(OCH3)4is inclined to hydrolyze than to condense, and the hydrolysis processes is energetically more favorable than the neutral ones. Moreover, hydrolyses under alkaline and acidic conditions are much easier than those in neutral solutions. In acidic solutions:1) the precursor Si(OCH3)4is inclined to hydrolyze than to condense;2) the precursor does not hydrolyze completely;3) proton blocks the cationic dimers to cyclize.In alkaline solutions, the hydrolysis products oligomerize via a SN1dimerization mechanism, and the condensation rates will be fast to form denser colloidal aerogels. This theoretical model also testifies that the succedent cyclization reactions are energetically unfavorable.(3) Mechanistic investigations on Al(OH)3oligomerizationThe aluminum alkoxide precursors are electron-deficient compounds, and inclined to coordinate with water and alcohols. If we use aluminum alkoxides as precursors to manufacture aerogels, the hydrolysis and oligomerization reactions wil! be fast, and the reaction mechanisms will be unique. Commonly, the oligomerization of aluminum alkoxide or Al(OH)3precursor bears no energy barrier, and release a large energy. As a result, it will be much more valuable to regard the oligomerization of Al(OH)3as the prototype reaction for the evolution of Al2O3aerogels. In the present work, all species were fully optimized at B3LYP/6-311++G**basis set level followed by frequency calculations to obtain the zero-point energies. Single-point energy calculations using the CPCM solvation model were performed at the same basis set level of theory to model the liquid environment, and the SPE total free energies were corrected with the zero-point energies in gas phase.Under neutral conditions,1-6water molecules were placed explicitly around Al(OH)3in order to ascertain its coordination modes and existing forms. Along with the increase of water molecules, the free energy reductions decrease remarkably, so Al atoms are mostly tetra-coordinated and penta-coordinated. Similarly, the CPCM SPE calculations were also performed with G09package at M06-2X/6-311++G(d,p) level to testify the reliability of the B3LYP method, and the The M06-2X CPCM energies were scaled with thermal corrections to Gibbs free energies in gas phase. The SPE calculations suggest that the B3LYP results are better than the M06-2X ones. In alkaline solutions, the complexation of OH-and Al(OH)3to form [Al(OH)4]-decreases the free energy significantly by179.7kJ/mol. The hydrogen-transfer barrier of [Al(OH)4]-is large, suggesting that the monomer does not exist in the form of [AlO(OH)2]-.The dimerization of Al(OH)3in weak and strong alkaline solutions was also investigated, and the computational results show that Al(OH)3is apt to condensed into more soluble polyhydroxy compounds. The neutral dimerization of Al(OH)3and the shift of bridging hydroxyl hydrogen are energetically favorable. But the most stable geometry is an Al-O four-membered ring structure linked by two bridging hydroxyls, decreasing the free energy by-233.3kJ/mol, where the hydrogen atoms are too far to dehydrate via hydrogen transfers.The trimerization of the most stable dimer and Al(OH)3in neutral solutions is very intricate. Here the dehydration processes of3trimers are investigated. The theoretical model shows that the first step, viz. the formation of Al-O tetraatomic rings is all easy to take place, but the proceses leading to cage-like structures bears much higher barrers; further dehydration to the second bridging hydroxyl hydrogen is also energetically unfavorable. In a word, Al(OH)3is inclined to form tetra-coordinated oligomers spontaneously, and then develops into three-dimensional cage-like structures connected together with Al-O tetraatomic rings.(4) DFT investigation on the decarboxylation of pyrrole-2-carboxylic acidDecarboxylation is normally a dissociative process, commonly catalyzed by proton or enzymes. Structural optimization was performed at B3LYP/6-311++G**basis set level, followed by single point energy calculation with the CPCM solvation model. Then the CPCM energies were corrected with the zero-point energes in gas phase.The direct decarboxylation and decarboxylation aided with water were investigated to consider the functions of proton and water. Without any catalysts, the carboxyl hydrogen of pyrrole-2-carboxylic acid shifts to the α-carbon to expel CO2. With the aid of water, the carboxyl group was hydrated by H2O, and then hydrogen transfer occurs to form pyrrole and carbonic acid. The potential energy of this step decreases slightly to49.74kcal/mol, but the hydration of the carboxyl group also bears a relatively high potential energy of47.48kcal/mol. The calculations with GaussianO3package show that, with the assistance of H3O, the decarboxylation mechanism of pyrrole-2-carboxylic acid involves the addition of water to the carboxyl group, and the C-C bond cleavage leading to the protonated carbonic acid. The potential energy of the C-C rupture decreases significantly to9.77kcal/mol, and the total energy barrier decreases to33.99kcal/mol.Because of the competition of the alkoxide hydrolyses and oligomerizations, none of them can be isolated experimentally, this dissertation will carry out a systematical comparative study on the hydrolysis and oligomerization of silicon and aluminum alkoxides as well as their microstructure evolution. The research highlights and innovations are listed as follows:1) By using quantum chemistry method, the hydrolysis and oligomerization mechanism were investigated systematically to ascertain the chain, reticular, cage-like or ring structures of oligomers, which will help us to understand the evolution rules of alkoxides, and the liquid environment was modeled with the CPCM solvation model;2) This dissertation probed into the hydrolysis and condensation reactions of silicon and aluminum alkoxides as well as the nucleation mechanisms to discuss the formation of dimers, trimers to ascertain the evolution rules of larger orderly congeries; 3) For the hydrolysis-oligomerization mechanism of silicon alkoxides, the influence of different alkoxy groups and solution conditions (neutral, alkaline or acidic) on the hydrolysis rates as well as the hydrolysis-condensation mechanisms of precursors was clarified.(4) For the aluminum alkoxide systems, although the hydrolyses are energetically very favorable, the precursors are apt to condense rapidly via non-barrier processes; for the silicon alkoxide systems, the hydrolyses under acidic and alkaline conditions are much easier, but hydrolysis rates are all faster than the oligomeration. An additional coordinated water molecule can favor and accelerate the hydrolyses of aluminum alkoxides, but does not help to the Si(OCH3)4hydrolysis. In neutral solutions, Al(OH)3is inclined to form tetra-coordinated oligomers, then develops into three-dimensional structures connected together with Al-O tetra-atomic rings, but is apt to condense into more soluble polyhydroxy compounds under alkaline conditions.5) Using the advantages of quantum chemistry, this dissertation clarified the formation mechanisms of silicon and aluminum aerogels at the atomic and molecular level, unpuzzled the hydrolysis-condensation reactions and the sol-gel behavious of alkoxides, and provided necessary theoretical guidance for the preparation of aerogels with unique properties.