Multi-Scale Modeling Of Molecular Adsorption And Surface Diffusion In Zeolites

Zeolites are microporous crystalline materials widely used for a lot of zeolite-based separation and catalysis processes in chemical and petroleum industries,in particular those of silicalite,mordenite,ZSM-5,β,and Y zeolites. In such applications,the adsorption and diffusion behaviors of the guest molecules in a zeolite represent the thermodynamic and dynamic(transport) properties of the system,respectively.For an effective design of such zeolite-based processes,it is evitable for a basic understanding and thereafter a proper description of both the thermodynamic and dynamic properties.To this end,with a particular focus on the zeolite-based separation and catalysis processes,in the present thesis we explore the macroscopic phenomenologies and the microscopic mechanisms underlying the adsorption and diffusion of hydrocarbon molecules in zeolites by adopting a multi-scale modeling strategy.Firstly,on the bottom scales,we completed the construction of the zeolite-guest systems to be investigated in this work and obtained the corresponding parameters.Secondly,molecular simulation methods were adopted to study the adsorption of the host-guest systems.The major observations and conclusions are as follows.·The adsorption of C₁-C₇ linear alkanes in silcalite,β,and mordenite zeolites was investigated by the Configuration-bias Monte Carlo(CBMC) method at 300K.The C₁-C₇ linear alkanes follow a two steps process in adsorption in zeolites,which is controlled by enrtopy effects,including size and configurational entropy effects.The entropy effects decide the inflection in the isotherms and the differences in adsorption ability for different host-guest systems.Quantitatively,the two step adsorption behavior was found to be well described by the dual site Langmuir equation.·The adsorption of C₂-C₄ alkenes in silicalite and H/La/ZSM-5 at different Si/Al ratios was investigated by the Grand Canonical Monte Carlo(GCMC) method at 303K.For pure component adsorption,H and La cations do not significantly affect the adsorption properties of molecules at high Si/Al ratios, while adsorption heats and amounts increase because of the existence of the cations at low Si/Al ratios.Another effect of the ion exchange is the suppression of the adsorption capacity due to lowered surface area and internal pore volume available.For binary component adsorption,the size effects favour the components with a smaller number of C atoms at high molecular loadings.Thirdly,combining with theoretical method,molecular simulations were carried out to study the diffusion of the host-guest systems.The main observations and conclusions are as follows.·The diffusion of xenon,tetrafluoromethane,methane,n-butane, isobutene and 2-methylhexane in silicalite was investigated by the Kinetic Monte Carlo(KMC) method at 300K.For pure component diffusion,if the size entropy effects control the host-guest systems or the adsorption of guest molecules stop at the first step,corresponding diffusivities decrease with increasing in loading.If the configurational entropy effects control the host-guest systems and the second step of the adsorption of guest molecules occures,the corresponding occupancy dependence of diffusivities exhibits a maxima.Furthermore,based on Maxwell-Stefan(MS) equation which describes surface diffusion in porous matrix,a new occupancy dependent equation for MS diffusivity in MFI zeolite was developed,which yields better results than the previous equations.·A new group-contribution based model was developed for predicting the activation energy of molecule diffusion in zeolites.Once the activation energies of the basic groups are correlated based on a data-base of a diffusant-zeolite system,it is valid to use them to predict the activation energy of other analogues diffusing in the same kind of zeolite by calculating the sums of the corresponding increments of the groups after accounting for weight factors.Fourthly,following a multi-scale modeling strategy,the GCMC,KMC, MD methods and finite volume method were used comprehensively to study the adsorption and diffusion properties of the NaY zeolite-benzene molecule system.The important observations and conclusions are as follows.·A realistic representation of the structure of the adsorbate-adsorbent system of interest was obtained based on GCMC simulation.The simulation clearly shows the characteristics of the adsorption sites of the benzene-NaY system,from which two kinds of preferably adsorbing sites for benzene molecules,called S_â…¡and W sites,were identified.The structure thus obtained is then used as a basis for KMC simulation.Considering three different mechanisms underlying jump of benzene molecules,the corresponding dependence of the self-diffusivity on loadings and temperature was investigated by the KMC method.And a comparative study by the KMC and MD methods shows that for benzene diffusion in NaY,the S_â…¡-W-S_â…¡jumps of benzene molecules are dominated,while the W-W jumps do not exist in the process.Finally,based on our work,two relations for predicting the self-and MS diffusivities have been derived and found to be in fair agreement with the KMC and MD simulation.·The multi-scale modeling strategy has been implemented to investigate the diffusion of benzene molecules in a NaY zeolite membrane module. Comparing the calculated diffusing flux with experimental results in the literature,it is further demonstrated that when diffusing in NaY zeolite,the S_â…¡-W-S_â…¡jumps of benzene molecules are dominated.The study indicates that our multi-scale modeling strategy is an effective and convenient method to design zeolite-based processes.