Tailor and Control of Acidic Strength in Ordered Mesoporous Aluminosilicates by Using Preformed Zeolite Precursors

The acidic strength of ordered mesoporous aluminosilicates of MAS-2, MAS-3, MAS-7 and MAS-9 and mi-croporous crystals of Y, L, beta, and ZSM-5 zeolites was systemically investigated by temperature-programmed desorption of ammonia (NH3-TPD). Due to the use of preformed zeolite precursors of Y, L, beta and ZSM-5, the ordered mesoporous aluminosilicates with distinguished acidic strength were obtained, being dependent on the type of preformed zeolite precursors. Therefore, the acidic strength of these mesoporous aluminosilicates could be tai-lored and controlled.

构筑开放锡位点实现酸性增强的二甲醚羰基化丝光沸石催化剂设计

Due to their tunable acidity,shape selectivity,and excellent stability,zeolites are of great importance as solid acid materials in industrial catalysis.Tuning the properties of the acid sites in zeolites allows for the rational design and fabrication of catalysts for target reactions.Dimethyl ether(DME)carbonylation,a critical chain-growth reaction for C1 resource utilization,is selectively catalyzed by the Brønsted acid sites within the eight-membered rings(8-MRs)of mordenite(MOR).It is anticipated that strengthening the Brønsted acidity—particularly in 8-MRs—will improve the catalytic performance of MOR.In this work,density functional theory(DFT)calculations are first employed and the results used to design a modified MOR with stannum(Sn)and to predict the corresponding changes in acidity.Guided by the theoretical studies,a series of Sn-modified MOR are synthesized via a defect-engineering and subsequent heteroatom-substitution strategy.After partial desilication,isolated tetrahedral Sn species in an open configuration are successfully synthesized for the first time,within which tetrahedrally coordinated Al sites are preserved.An acidic characterization is used to confirm that the acidity of the Brønsted acid sites is enhanced by the introduction of the Sn species;as a result,the sample exhibits excellent activity in DME carbonylation reaction.Kinetic and DFT studies reveal that this strengthened acidity facilitates the adsorption of DME and reduces the activation barriers of DME dissociation and acetyl formation,accounting for the improved activity.The work demonstrates mechanistic insights into the promoting effects of strong acidity on DME carbonylation and offers a promising strategy to precisely control the acidic strength of zeolites.

Extend ethylene aromatization single-event kinetic modeling with physical and chemical descriptor based on ZSM-5 catalyst

The ethylene aromatization is critical for the methanol to aromatics and light alkane dehydroaromatization process.The single-event microkinetic(SEMK)model combining the linear free energy theory and solid acid distribution concept were established and extend for the ethylene aromatization process,which can reduce the kinetic parameters and simplify the reaction network by comparison with the SEMK model including subtype elementary steps based on the type of carbenium ions.Further introducing deactivation parametersφinto the model and applying the linear free energy model to the deactivation experimental data,the obtained deactivation parametersφindicate that the carbon deposition precursors have the greatest impact on reducing the reaction rate of single-molecular reactions and the smallest impact on the hydrogen transfer reaction.Meanwhile,according to the change of reaction enthalpy,effect of carbenium ion structure on methylation,ethylation,cyclization and endo-βscission was investigated by introducing linear free energy concept into the SEMK model.The effect of different acid strengths on elementary steps was investigated based on the acid strength distribution model,it was found that the methylation and oligomerization reactions,the ali-βscission reaction,endo-βscission reaction and the cyclization reaction were more sensitive to strong acidity sites.The physisorption and chemisorption heat are separated from the protonation heat in the linear free energy kinetic model and the acid strength distribution kinetic model,and the absolute values of the obtained physisorption and chemisorption heat increase with the carbon number of carbenium ions.Furthermore,the parameters of the acid strength distribution kinetic model were applied to propane dehydroaromatization on H-ZSM-5 and the ethane dehydroaromatization on Zn/ZSM-5 to confirm the independence of parameters in the SEMK model with the similar reaction network.

Conversion of CO_(2) and H_(2) into propane over InZrO_(x) and SSZ-13 composite catalyst

Direct converting carbon dioxide into hydrocarbon fuels and value-added chemicals would offer a very attractive approach for efficient utilization of CO_(2) as a carbon resource.Although,olefins,aromatics and gasoline have been successfully synthesized by CO_(2) hydrogenation,highly selective conversion of CO_(2) and H_(2) into C_(2+)hydrocarbon is still challenging due to a high C-C coupling barrier and inhibiting the production of other long-chain hydrocarbons.Here,we report a composite catalyst made of InZrO_(x) and SSZ-13 molecular sieve(InZrO_(x)+SSZ-13),which exhibits 74.5% propane selectivity at 623 K.The 8-MR micropores and the higher strength of the acid for SSZ-13 benefit the formation of propane.Compared with pure InO_(x) and m-ZrO_(2) the composite oxide InZrO_(x) containing more oxygen vacancies,exhibits to be more readily reduced by H_(2) and easier to adsorb and desorb CO_(2) within the reaction temperature.All those could be beneficial to the activation and conversion of H_(2) and CO_(2).The catalytic performance of InZrO_(x)+SSZ-13 in CO_(2) hydrogenation provides a potential for production of propane.

Ethene and butene oligomerization over isostructural H-SAPO-5 and H-SSZ-24: Kinetics and mechanism

BrФnsted-acidic zeolite and zeotype materials are potential catalysts for the conversion of ethene to higher alkenes. In this study, two materials with AFI structure but different acid strength, H-SAPO-5 and H-SSZ-24, were subject to studies of ethene, cis-2-butene and ethene-butene mixture conversion under conditions where C3-C5 alkene formation is thermodynamically favoured over higher hydrocarbons(673-823 K, 1 atm). Ethene and cis-2-butene partial pressures were varied in the range 9-60 and 0.9-8.1 kPa, respectively, and contact times were varied in the range 3.78-756 and 0.573-76.4 s.μmol H+/cm^3 over H-SAPO-5 and H-SSZ-24, respectively. Less than 1% conversion of ethene and less than 10% conversion of butene was obtained in the range of conditions used for elucidation of rate parameters. The ethene conversion rates were more than an order of magnitude higher over the more acidic H-SSZ-24 than over H-SAPO-5(6.5 vs. 0.3 mmol/mol H+.s at 748 K, Pethene = 33 kP a), with corresponding lower reaction order in ethene(1.5 vs. 2.0 at 673 K) and lower apparent activation energy(52 vs. 80 kJ/mol at 698-823 K). Propene selectivity was substantially higher over H-SSZ-24 than over H-SAPO-5(68% vs. 36% at 0.5% ethene conversion). A similar difference in apparent reaction rates was observed for cis-2-butene conversion over the two catalysts, and for co-feeds of ethene and cis-2-butene. However, the cis-2-butene conversion to C3-C5 alkenes was found to be severely influenced by thermodynamic limitations, impeding a detailed kinetic analysis, and leading predominantly to isobutene formation at the highest temperatures.