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The Characterization Of Acidity/Basicitv And Determination Of Reaction Mechanism Inside Confined Zeolite Channels

Posted on:2018-07-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:X F YiFull Text:PDF
GTID:1311330512499401Subject:Analytical Chemistry
Abstract/Summary:PDF Full Text Request
Zeolite catalysts have been widely utilized in modern chemical and petrochemical industries due to their high catalytic activities,selectivity,and environmentally friendly properties.Both the acidity/basicity and pore confinement effect have been demonstrated to play crucial roles in governing the catalytic reactivity.Herein,we devoted to solving some urgent issues in zeolite-catalyzed systems from both the perspectives of acidity/basicity and confinement effect by using a combined NMR experiment and theoretical calculation approach.And some interesting results have been obtained:(1)Due to its small molecule size,deuterated acetonitrile(CD3CN)was selected as NMR probe molecule to systematically investigate the acidic properties of 12-MR main channels and 8-MR side pockets of MOR zeolite by a combined experimental and theoretical approach.The DFT theoretical calculations demonstrated that there is a linear correlation between the 1H chemical shift of adsorbed CD3CN and the intrinsic acid strength(DPE)of solid catalysts.A larger 1H chemical shift of adsorbed CD3CN corresponds to a stronger acidity.Therefore,CD3CN could be used as a probe to quantitatively characterize the intrinsic acidity of solid acid catalysts.The follow-up CD3CN adsorption experiments,accompained by the co-adsorption of 2,6-dimethylpyridine,which was used to selectively block the 12-MR main channel without influence on the 8-MR side pockets,clearly distinguish the acid strength of 12-MR main channels and 8-MR side pockets.Thus,we provide here the direct experimental evidence that the acid strength of 8-MR side pockets is stronger than that of 12-MR main channels.Simultaneously,the NMR probe method present here was also demonstrated to be capable of precisely determining the acid strength and distribution of various porous acidic materials,including the large-pore and medium-pore,especially the small-pore zeolite catalysts.(2)Pyrrole was employed as NMR probe molecule to quantitatively determine the base strength of solid catalytic materials by using a combined NMR experiment and theoretical calculation approach.The solid-state NMR experimental results indicated that a larger 1H chemical shift of adsorbed pyrrole corresponds to a stronger basicity,while the DFT theoretical calculations revealed that there is a linear correlation between the 1H chemical shift of adsorbed pyrrole and the proton affinity of solid bases,which is independent of the basic central atoms(O/N)involved in the basic catalysts.In combination with the available 1H MAS NMR experimental data,it is conclusive that pyrrole could be used as a probe to quantitatively characterize the intrinsic basicity of various solid catalysts,and the 1H chemical shift threshold for superbasicity is 10.0 ppm.In addition,the influence of confinement effect on the adsorption structures and 1H chemical shifts of pyrrole over basic catalysts was investigated as well.The results presented here are of great importance for comprehensively understanding the detailed basic properties of solid catalysts,which should be beneficial for the design and/or modification of solid base catalysts for practical applications.(3)Theoretical calculations based on molecular dynamics(MD)simulations and density functional theory(DFT)calculations have been performed to explore the reaction pathways of propene H/D exchange over deuterated acidic ZSM-5 zeolite(D-ZSM-5).The deuterated propene-D5 is confirmed to be readily formed through the route involving an isopropyl intermediate.With regard to the formation of completely deuterated propene-D6,the propene loading is found to play a crucial role in governing the reaction pathway.The dimerization route(through the dimerization of propene,the intramolecular hydride transfer and then the cracking process)is demonstrated to be predominant with a relatively lower activation energy barrier at higher propene loading,while the n-propoxy pathway is preferred at lower propene loading.Furthermore,the influence of acid strength on the reactivity of propene H/D exchange over zeolites has been explored as well.It's demonstrated that stronger acid strength can largely reduce the activation barrier and improve the H/D exchange reaction activity.Our theoretical results also reveal that the confinement effect can effectively enhance the H/D exchange reaction activity due to the electrostatic stabilizations of the zeolite framework.Apparently,the results presented here are helpful for understanding the activation of alkanes and olefins by zeolite catalysts,and provide a theoretical guide for the modification,design,and application of novel zeolite catalysts in the petrochemical industry.(4)The adsorbed structures and strain energies of the reaction intermediates during ethylbenzene disproportionation have been theoretically investigated to elucidate the intermediate shape selectivity.The theoretical study provides clear-cut evidence that the formations of the reaction intermediates are strongly controlled by the dimension and size of zeolite void spaces.Our theoretical data are obviously in good agreement with the experimental GC-MS relative intensities.These lead us to conclude that the zeolite void spaces play a crucial role in governing the formations of reaction intermediates in the EB disproportionation.Our work also shows that the strain energies of various reaction intermediates confined within zeolites with different pore topologies could be regarded as a useful quantitative means in better understanding the shape-selective nature of this important class of microporous crystalline catalysts.
Keywords/Search Tags:zeolite catalysts, acidity/basicity, pore confinement effect, reaction mechanism, solid-state NMR, theoretical calculation
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