Diffusion Behavior Of Hydrocarbons In Zeolite Sieves

The catalytic reaction of hydrocarbons involves diffusion,adsorption and reaction steps.At present,most of the research focus on the reactivity of the catalysts,but lack of the relationships between hydrocarbon molecules and its diffusion properties.In this paper,the diffusion behavior of iso-butane,toluene,1,3,5-triisopropylbenzene through ZSM-5 and USY sieves were investigated by using the Zero Length Column method.With direct sampling of adsorbed gas,the diffusion results of iso-butane in con-ZSM-5,nano-ZSM-5 and nanosheet ZSM-5 sieves showed that the diffusivities of iso-butane in the two latter sieves were much larger than that in con-ZSM-5,and the diffusion activation energy in nanosheet ZSM-5 was the lowest,which was mainly due to the existence of mesoporous greatly decreasing the resistance of diffusivity,making the diffusion time constant increased.With carrier gas getting through liquid phase adsorbate and sampling,in the area of kinetic control,the diffusion of n-heptane,toluene,octane and xylene in USY sieves was studied,the results showed that the diffusivity of n-heptane was about 2-8 times of toluene,and the diffusivity of octane was slightly larger than that of xylene,indicating that the molecular configuration had a certain impact on their diffusion in the sieves channel,and the diffusion performance of chain alkane were better than aromatics.In order to promote the diffusivity,the improvement of size of pore and orifice is necessary.The diffusion of 1,3,5-tripropylbenzene,which has a critical diameter slightly larger than that of pore size of USY sieves,were also studied by using three different USY sieves.The results showed that the diffusivity in USY_Mg was significantly higher than that in the mesoporous USY sieves and the commercial USY sieves,and the USY_Mg had the optimal diffusion performance.