Theoretical Calculation On The Acidity And Adsorption Properties Of Zeolite P

Many kinds of hydrocarbons emitted by diesel engine pollute the environment seriously.With the continuous improvement of environmental regulations and the increasing requirements of environmental protection,how to effectively control the exhaust emissions of diesel vehicles has become one of the focus of research.Adsorption by zeolite and reburning subsequently are considered to be the most effective control measures.Materials Studio simulation software was used to simulatezeolite P doped by metal,and the situation of its falling position was obtained.The modification of alkali metal ions was used to investigate the change of the acidity in zeolite P.The simulation was also carried out to explore the controlling factors of the adsorption capacity of zeolite P for hydrocarbons.The results of the study were as follows:(1)The 4T model of zeolite P was established,the substitution energies and relative substitution energies of different T sites were obtained by substitution of different metal atoms for Si atoms at different positions.The results showed that when the metal atom of Al,Fe,or Ni was doped,the T4 position Si would be preferentially replaced,and then the T3 position was replaced.The correctness of the result was verified by the data of bond length and bond angle.(2)The Lewis acidity of zeolite P was not directly related to the selected analytical methods and the models established,and there was a strong adsorption between zeolite P and probe molecules.Using the basic probe molecule adsorption and Fukui function calculation,the order of the zeolite P acidity was as following:Li-P>Na-P>K-P>Rb-P>Cs-P.(3)The adsorption capacity of zeolite P was related to the amount of metal doped in the zeolite.Moreover,it was influenced by external factors such as temperature,pressure,molecular polarity and molecular size.The effect of temperature on the adsorption of zeolite P was greater than that of pressure.In the process of adsorption,molecular polarity occupies a more dominant position than molecular size.Zeolite P doped by Al was beneficial to improve its adsorption properties,while the doping of Fe and Ni lead to a decrease in the adsorption capacity of zeolite P.(4)During the adsorption process of the zeolite P,the cloud distribution of adsorbed particles showed that the adsorbate molecules were mainly adsorbed in the cage which was the composite structure unit of zeolite P.When the two components of ethylene and propylene molecules were adsorbed,the competitive adsorption behavior was produced.The initial adsorption of propylene molecules will reduce the subsequent adsorption of ethylene molecules.(5)Metal modification could improve the thermal stability of zeolite P in the adsorption process,it had nice adsorption properties at high temperature.However,due to the limitation of the size of the modified ions and the change of the space structure,the zeolite P modified by Li~+had better adsorption properties for hydrocarbons.