Tuning The Pore Structure And Acidity Of Hierarchical Zeolites And Its Mass Transfer-Reaction Synergistic Mechanism

Zeolites are an essential class of crystalline microporous materials with regular pore structure,adjustable acidity,and good hydrothermal stability,thus playing important roles in catalysis and separation processes.The intrinsic microporous channels of zeolite catalysts provide excellent shape selectivity for reactants and product molecules.However,the long intracrystalline diffusion path imposes severe diffusion limitations on bulky molecules.On the one hand,this diffusion limitation reduces the accessibility of internal active sites in zeolites.On the other hand,it promotes the secondary reaction of hydrocarbon products to form carbonaceous species,leading to the deactivation of zeolite catalysts.To alleviate the intracrystalline diffusion limitation of zeolites,many new structures of zeolites have been developed,including nanosized zeolites,nanosheet zeolites,epitaxially growth and core-shell structured zeolites,and hierarchical zeolites.Hierarchical zeolites have attracted much attention among the various structures due to their abundant tunable properties.However,a series of studies on the mass transfer properties of hierarchical zeolites have shown that in addition to intracrystalline diffusion limitations,surface barriers are also a key factor affecting the mass transfer performance.Surface barriers can be further divided into external and internal ones.Their effects are often coupled and need further identification to analyze their specific roles in the overall mass transfer process.In addition,a consensus about the structural nature and origins of internal and external surface barriers has yet to be formed,despite some primary exploration of their effects on catalytic reactions.Therefore,this thesis aims to establish a complete set of experimental methods to study the effects of complex pore structures and acidity distribution of hierarchical zeolites on their mass transfer-reaction mechanisms.Based on the theoretical basis of the zero-length column method,the measured apparent diffusion time constant is decoupled into the surface mass transfer time constant and the pore diffusion time constant.The structural characterization and catalytic performance evaluation elaborate the mass transfer-reaction synergistic mechanism.The main research content of this paper is as follows.(1)The distribution of internal and external acid sites in hierarchical zeolites was adjusted by changing the adding amounts of aluminum species during the synthesis and assisted with tartaric acid post-treatment.Using n-heptane catalytic cracking as a model reaction,the effects of acid site distribution on the pore diffusion and surface mass transfer properties of the reactant nheptane and one representative product,1-butene,were investigated.Combining reaction kinetics and product distribution calculations,the mass transfer-reaction mechanism of zeolite catalysts was summarized.The results show that with the decrease of acid site numbers on the internal and external surfaces,the reciprocal effective diffusion time constant of the reactant n-heptane in the hierarchical zeolites increased from 1.78 × 10-3 s-1 to 3.90 × 10-3 s-1,thus the apparent diffusion performance was improved by 119%.At the same time,the conversion rate per active site was enhanced from 0.96 s-1 to 2.81 s-1,thus increasing by 193%.After the selective removal of external acid sites by tartaric acid,the external surface barriers of olefin molecules in hierarchical zeolites significantly decrease.The degree of their secondary isomerization reaction in catalytic cracking also decreases.(2)The influence of the pore structure of hierarchical zeolites on their mass transfer and reaction performance was explored.By adjusting the adding amount of tetrapropylammonium hydroxide as a microporous template,a batch of zeolite samples with similar acid properties but varying mesopore/micropore ratios was prepared.Their mass transfer and catalytic reaction performance were investigated in n-heptane catalytic cracking reactions.Besides,the effects of two structural variables,mesopore diameter and mesopore/micropore ratio,on the diffusion coefficient of n-heptane were analyzed using molecular simulation methods.The results demonstrate that the increase in the mesopore/micropore ratio can significantly improve the pore diffusion performance of zeolites compared with mesopore diameter adjustment.The conversion rate per active site increased from 2.31 s-1 to 2.81 s-1,indicating a corresponding improvement of 22%.(3)To achieve coupled tuning of the pore structure and acid site distribution in hierarchical zeolites,the added amount of organosilane 3-aminopropyltriethoxysilane was changed during synthesizing aggregated nanocrystal hierarchical zeolites.The characterization results of pore structure and acidity reveal the dual function of organosilane during the synthesis.On the one hand,the organosilane molecules are anchored to the surface of the protozeolitic nano units,limiting the growth of nanocrystals,thereby shortening the microporous diffusion path.On the other hand,organosilanes introduce additional silicon atoms into the zeolite framework,decreasing the acid site density on both the internal and external surfaces.Based on the mass transfer and reaction performance tests,the coupled tuning strategy improved the reciprocal pore diffusional time constant from 7.84 × 10-5 s-1 to 2.16 × 10-4 s-1,with a rise of 176%.At the same time,the reciprocal surface mass transfer time constant was enhanced by 318%,from 2.17 × 10-3 s-1 to 9.06 × 10-2 s-1.These enhanced mass transfer properties are conducive to improve the conversion rate per active site and the selectivity of light olefin products in the catalytic cracking reaction.(4)The effects of the single-tuning and coupled tuning strategies are analyzed and compared.Starting from the same hierarchical zeolite,the pore structure was adjusted by improving the adding amount of microporous template,the external acidity was tuned through the surface deposition of organosilane,and a coupled tuning was achieved through introducing dualfunctional organosilane into the synthesis gel.In the n-heptane catalytic cracking reaction at 923 K,the conversion rate per active site in the hierarchical zeolites was increased from 0.96 s-1 to 1.06 s-1 and 1.04 s-1 by single tuning of the pore structure or sorely passivation of the external acid sites,with an increase of only 8%～10%.In contrast,the coupled tuning strategy increased the conversion rate to 1.36 s-1,with a rise of 42%.Further analysis shows that the coupled tuning strategy can combine the advantages of the other two single-tuning methods,thus showing the best promotion effect on the overall mass transfer performance and selectivity towards light olefins.