Study Of Adsorption And Diffusion Of N And I-alkanes And Alkene Over Pt/ZSM-22 Catalysts

Hydroisomerization technology is to convert long straight-chain n-alkanes into branched isomeric alkanes with the same carbon number under the action of catalysts.This technology can transform low-value Fischer-Tropsch waxes into high-value-added lubricant base oils,and can effectively solve the problem of consistent dependence on imported lubricant base oils in China.The n-alkane hydroisomerization catalysts are bifunctional catalysts,one is responsible for the metal sites of hydrodehydrogenation reaction,mainly Pt,Pd and other noble metals;the other is responsible for providing zeolite molecular sieves of isomerization acid sites,mainly ZSM-5,Beta,SAPO-11,ZSM-22 and ZSM-48 zeolite molecular sieves.The focus of the research is mainly on the preparation and modification of catalysts,but as the research continues,it is found that the adsorption and diffusion behavior of reactants,intermediates and products involved in the reaction process within the catalyst have a great influence on the catalytic reaction activity and selectivity,yet little basic research has been reported in this area.In order to investigate the effects of the pore structure,composition and acidity of zeolites and the dispersion of metals on the surface of the carrier on the catalytic performance,and to reveal the nature of the catalyst performance,this paper selected n-hexane,2-methylpentane and 3-methylpentane and the reaction intermediate 1-butene as probe molecules for the product distribution characteristics of the n-alkane hydroisomerization reaction,and investigated their adsorption and diffusion behaviors in ZSM-22 The adsorption and diffusion behaviors of these molecules in ZSM-22 zeolite and its Pt-loaded catalyst were investigated,and the analysis and comparison were carried out to provide basic data and ideas for further improvement of the catalyst performance.The study and results include:In this paper,three adsorption isotherms at 308 K,323 K and 338 K were measured using an intelligent gravimetric analyzer(IGA)for adsorption thermodynamic studies;a zero-length column(ZLC)device was used for diffusion studies;and 1-butene-TPD(gravimetric method)was used for adsorption studies.Ⅰ.Study on the adsorption and diffusion properties of hexane in ZSM-22 zeolite and Pt/ZSM-22 catalysts1.Study on the adsorption of hexane in ZSM-22 zeoliteThe adsorption isotherms of n-hexane in ZSM-22 zeolite at three temperatures were measured by an intelligent gravimetric analyzer(IGA),and the adsorption isotherm was a composite isotherm of type Ⅰ+Ⅳ,which was in accordance with the Dual Toth model.The smaller the pore volume of zeolite,the stronger the acidity,and the larger the interaction force b between hexane and zeolite;and the initial heat of adsorption Qst was found to be positively correlated with the interaction force b1,0 between hexane and micropores.2.n-Hexane diffusion in ZSM-22 zeolite and Pt/ZSM-22 catalystsThe higher the diffusion temperature,the faster the diffusion;the diffusion rate is influenced by the pore structure and acidity,the stronger the acidity,the smaller the pore volume,the smaller the effective diffusion time constant,and the slower the diffusion;the Pt/ZSM-22 catalyst generates a larger electrostatic force between hexane and catalyst due to the influence of Pt loading,which leads to a smaller effective diffusion time constant and the diffusion becomes slower.Ⅱ.Study on the adsorption and diffusion properties of 2-methylpentane 3-methylpentane within ZSM-22 zeolite and Pt/ZSM-22 catalysts1.adsorption study of 2-methylpentane and 3-methylpentane in ZSM-22 zeoliteThe measured adsorption isotherms are still type Ⅰ+Ⅳ isotherms,and the adsorption amount is mainly related to the pore structure and B acidity,the larger the pore volume,the stronger the acidity,and the more the adsorption amount;the smaller the micropore volume of the adsorbent sample,the smaller the interaction force within the micropore,but the 1-2 nm pore size increases the interaction force;the smaller the mesopore volume,the stronger the acidity,and the larger the interaction force within the mesopore.The adsorption of n-hexane,2-methylpentane and 3-methylpentane in the same adsorbent was compared.As the molecular dynamics size of the adsorbent sample increased,the adsorption amount of the adsorbent in the same adsorbent decreased,the interaction force in the micropore decreased and the interaction force in the mesopore increased.2.Diffusion studies of 2-methylpentane 3-methylpentane within ZSM-22 zeolite and Pt/ZSM-22 catalystsDiffusion studies revealed that the effect of micropore structure on the speed of diffusion should not be underestimated,the smaller the micropore volume and the more acidic,the slower the diffusion in different samples.After loading Pt,the catalyst still shows a slower diffusion rate.The diffusion analysis study comparing three different adsorbents showed that the diffusion became faster as the size of the adsorbent molecules increased.Ⅲ.1-Butene TPD study with Pt/ZSM-22 catalyst1-butene was selected as the probe molecule,and its chemisorption behavior and the distribution of adsorption sites on ZSM-22 zeolite and Pt/ZSM-22 catalyst were investigated for 1-butene-TPD(weight method).The adsorption amount of 1-butene was found to be related to the specific surface area and acidity;the larger the specific surface,the stronger the acidity and the more adsorption;the smaller the specific surface area of micropores,the stronger the acidity,the higher the desorption resistance and the more undesorbed amount.4 adsorption sites(120℃,210℃,300℃ and 340℃,respectively)were found for 1butene-TPD of ZSM-22 zeolite,and it is presumed that the 120℃ adsorption site is an L acid adsorption site and 340℃ is a strong B acid adsorption site,and 3 adsorption sites(120℃,210℃ and 310℃,respectively)were found for 1-butene-TPD of Pt/ZSM-22 zeolite.The more acidic the sample,the higher the desorption temperature required,the higher the amount of desorption at the higher temperature desorption peak.After loading Pt,the total adsorbed and undesorbed amount of 1-butene became more,and the adsorption sites disappeared at 340℃and the adsorption sites were more uniformly regular.