Theoretical Study On The Condensation Reaction Of Oligosilicic Acid In The Initial Stage Of Zeolite Nucleation

The study on nucleation and growth of zeolites has always been an important challenge in microporous materials.The investigation on the formation of zeolite can not only deepen the understanding of the synthesis law,but also guide the design of new zeolite more effectively.However,due to the complexity of the crystallization,the basic understanding of its generation process is still not unified.The difficulty lies in the understanding of the chemical reactions occurring in the initial stage of zeolite synthesis.On the one hand,small species generated by silicic acid condensation are usually several Si?OH?₄ molecules in size,with extremely short existence time and fast movement,which are difficult to be successfully captured by detection technology.On the other hand,if inorganic or organic reagents are added in this process,the bonding and non-bonding interactions in the solution will become more complicated,and these interactions are bound to have an impact on the structure,types and properties of the generated oligomer.In this paper,the chemical reactions between the raw materials providing framework elements during the initial stage of zeolite nucleation,including the types and structures of polysilicate species generated therefrom,and the changes in the stability of small structural units in the solution when the reaction conditions are changed are studied in detail.The main conclusions are as follows:Firstly,we investigated 18 kinds of oligomers containing five silicon atoms by first principles based calculations,compared the relative stability of different conformations,calculated the Gibbs free energy change of chain reaction and cyclization reaction at 298 K and 450 K,and discussed the factors affecting condensation of silicate oligomers:solvent and temperature.We found that most polymerization reactions can be successfully happened at room temperature.In vacuum,chain reaction and cyclization reaction compete with each other.In solution,the solvent has a promoting effect on the occurrence of cyclization.When the temperature increased from 298 K to 450 K,the difference in?G between cyclization and chain reaction is greater than 50 kJ/mol.In this case,cyclization reaction can be considered to occupy the dominant position of silicic acid condensation.These results not only show that the structure and stability of oligomeric silicic acid species formed in the early nucleation stage of zeolite are strongly dependent on the existing reaction conditions,but also confirm that the solvent and temperature play a positive role in promoting the cyclization,which provides guidance for further revealing the formation process of zeolite.Secondly,we studied the formation mechanism of a series of silicate oligomers that may exist in the early nucleation of zeolite.The results show that silicic acid condensation can be carried out according to two reaction mechanisms.One is that the reaction molecules undergo hydrogen transfer first,then oxygen atoms attack,the energy barrier are between 124 kJ/mol and 145 kJ/mol.The other is the simultaneous occurrence of hydrogen atom transfer and oxygen atom attack.The energy barrier is 133¹59 kJ/mol.We found that the reaction energy barrier with silicate as the attack group is lower,presumably because the electrostatic interaction between bare O and Si strongly stabilizes the unstable complex structure caused by proton transfer,so the activation energy decreases.For chain reaction,we have compared the reaction between monomer and linear molecules with different lengths,the reaction between monomer and monocyclic ring with different silicon atoms,and the reaction between monomer and monocyclic ring with neighboring chains.As for cyclization reaction,we have made a detailed exploration for the potential contributions of ring distortions and neighboring linear branches.The results show that there is no direct relationship between the degree of difficulty of silicic acid condensation and the molecular structure of reactants.Finally,we compared the physical interactions between different species in the initial stage of zeolite nucleation,and analyzed the reaction mechanism of silicic acid condensation in the presence of C₂H₇N and Na⁺.We found that the hydrogen bond interaction between silicic acid and C₂H₇N is very strong.C₂H₇N can capture H atom in silicic acid molecule and generate the corresponding silicate and ethylamine positive ion.This not only changed the existing state of oligomers,but also affected the subsequent reaction mechanism.Under neutral conditions,silicic acid condensation follows a one-step reaction mechanism.When C₂H₇N is added,it is converted into a two-step reaction with the forming Si-O-Si bonds and removing H₂O.C₂H₇N significantly reduced the reaction activation energy of oligomer formation.We also explored the role of Na⁺in silicic acid condensation.The results showed that the electrostatic interaction between Na⁺and oligomeric silicate will reduce the barrier of forming Si-O-Si bond.This indicated that the addition of organic and inorganic reagents in the solution was beneficial to the aggregation and nucleation of oligomeric silicic acid.