Study on gas phase reaction mechanism of HCN and H₂/H₂O based on density functional theory

HCN is a highly toxic substance that can enter the human body through the skin and respiratory system, and in severe cases, cause death. HCN can achieve partial conversion under high-temperature gasification conditions, mainly by reacting with H₂ and H₂O. In order to further explore the micro reaction mechanism of HCN with H₂ and H₂O during gasification, and to investigate the effects of temperature and pressure changes on the reaction, this paper uses quantum chemical simulation methods to study the reaction path, reaction thermodynamics, and kinetics of the above reactions, and quantitatively analyzes the changes in thermodynamic parameters and reaction rate constants with temperature, fitting the Arrhenius equation related to the reaction. Calculate the distribution of Fukui functions for various reactants and intermediates in the reaction process of HCN with H₂ and H₂O using Multiwfn, and speculate on possible reaction pathways. The transition state search and single point energy calculation of the reaction process between HCN and H₂ and H₂O were carried out using Gaussian & Gaussian View. Similarly, using the wave function program Multiwfn to calculate the Mayer bond order. The analysis of the bond order curve during the reaction process can reveal the changes in the strength of chemical bonds and the situation of bond formation and breaking. Use the Shermo program to calculate the thermodynamic parameters of each stagnation point at different temperatures, including enthalpy, entropy, Gibbs free energy, and partition function. Finally, the KiSThelP program was used to calculate the reaction rate constants for each step of the reaction based on classical transition state theory at different temperatures. The results show that the relatively optimal path for the reaction between HCN and H₂ is as follows: three H₂ molecules are added in three steps on C≡N to obtain the product CH₄+NH₃; The relatively optimal path for the reaction between HCN and H₂O is as follows: H₂O molecules attack C atoms, and the H of O and C atoms are transferred to N atoms to obtain the product CO+NH₃. The first step of reaction between HCN and H₂ is R1→IM1 which is below 534 K with ΔG<0. After exceeding this temperature, ΔG>0 becomes a reverse spontaneous reaction. It can be considered that an increase in temperature is not conducive to the electrophilic addition reaction of the first H₂ on C≡N. The second step is IM1→IM2, with ΔG below 1103 K less than 0 and above greater than 0, indicates that the spontaneity of the second step H₂ addition reaction is inhibited as the temperature gradually increases. The third step is IM2→P1. Within the set temperature range, its ΔG is always less than 0, and the reaction can always proceed spontaneously. The ΔG of the first step reaction R2→IM5 in Path 3 is only less than 0 at room temperature, indicating that this step is difficult to occur spontaneously at high temperatures. Path 3 second step reaction IM5→IM6 ΔG is always less than 0 within the set temperature range. The third step of the reaction is IM6→P2, and the temperature of ΔG below 958 K is greater than 0, making it difficult to occur spontaneously. The research results on changes in pressure and free energy show that pressure can increase the upper temperature limit for spontaneous reaction.The reaction rates of HCN with H₂ and HCN with H₂O are relatively fast at high temperatures. The rate determining steps for Path 1 and Path 3 at high temperatures are R1 → IM1, R2 → IM5, respectively. The rate constants for the two reaction steps above 1473 K are 9.57×10⁻⁴ and 1.71 mol/(L·s), respectively. The pre-exponential factors for these two reactions were calculated to be 4.45×10⁹ and 4.68×10⁸ s⁻¹, and the activation energies were 357.62 and 239.30 kJ/mol, respectively.