Efremov, Aleksandr A.Poryvaev, Artem S.Polyukhov, Daniil M.Fedin, Matvey, V...
8页
查看更多>>摘要：Shaping of metal-organic frameworks (MOFs) is one of the crucial steps toward their industrial applications. A number of methods for preparation of pellets containing MOF-micro/nanoparticles have been developed up to date; however, the uptake rates of guest molecules by such shaped MOFs and their dependence on preparation procedure were not analyzed in detail. Since optimization of the uptake rates is vital for practical use, in this work we employ spin-probe Electron Paramagnetic Resonance (EPR) to study solvent diffusion into ZIF-8 based pellets. This dedicated approach allows one to selectively monitor the impregnation of ZIF-8 particles embedded in the pellet, and corresponding diffusion coefficients can be obtained for the molecules of interest. Optimization of shaping procedure upon EPR control yields robust pellets with pore filling rates comparable to those for powdered ZIF-8. The proposed methodology yielded highly permeable ZIF-8-based pellets and is promising for future application in shaping of various MOFs.
查看更多>>摘要：This study investigates shaping and characterization of fluidizable methanol to olefins catalysts using single-binder or dual-binder. Physicochemical and mechanical properties were investigated with different methods likewise FESEM, XRD, TGA, FTIR, TPD-NH3 and BET-BJH. Furthermore, MTO process tests were accomplished to investigate the catalytic activity in the fluidized bed and fixed bed reactor and revealed that the sample shaped using dual-binder has larger surface area than the shaped one using single-binder (alumina sol or silica sol). Catalysts with silica sol and alumina-silica sol binder show relatively lower attrition resistance than the catalysts with alumina sol binder. Furthermore, implementation of alumina-silica sol provides increased acidity of the MTO catalyst which led to the increased selectivity and life time in fluidized bed reactor compared to alumina sol and silica sol binder. In addition, the MTO catalyst shaped with alumina-silica sol exhibited slightly lower coke deposition compared to the one made with single binder.
查看更多>>摘要：For four decades, the definition of "molecular traffic control (MTC effect)" in zeolite shape selective catalysis has been controversially discussed. So far, there is no clear evidence to prove the existence of MTC effect in the catalytic oxidation reaction. In this work, a series of TS-1 zeolites with homogeneous distribution of Ti atoms and free of extra-framework Ti species and/or anatase were synthesized, they were used to catalyze the oxidation of 1-hexene, 4-methyl-1-pentene and styrene. The systematic investigations exhibited that the catalytic oxidation process of TS-1/H2O2 occurred in both straight and sinusoidal channels when 1-hexene was used as substrate. When 4-methyl-1-pentene and styrene were used as substrates, the catalytic oxidation process of TS-1/H2O2 tended to occur in the straight channel, and the larger the size of the substrate, the more the oxidation reaction tends to occur in the straight channel. The above characteristics of oxidation performance are consistent with the intracrystalline diffusion characteristics of substrate molecules proposed by Derouane's group. It provides an evidence that MTC effect exists in TS-1/H2O2 catalytic oxidation reaction. The proof of MTC effect is of great significance to the development of the mechanism of heterogeneous reaction process.
Nobandegani, Mojtaba SinaeiYu, LiangHedlund, Jonas
13页
查看更多>>摘要：The adsorption and mass transport of CO2 and CH4 in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO2 permeance of 70 x 10(-7) mol/(m(2) s Pa) was observed for a feed of an equimolar CO2/CH4 mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity.
查看更多>>摘要：The strength of Bronsted acid sites (BAS) affects the properties of 2D and hierarchical zeolites, but the relative contribution of internal and external BAS remains unknown. Accordingly, this study aims to assess the acidity of external and internal BAS in nanosponge-like MFI zeolites by comparatively analyzing hydrogen-deuterium exchange kinetics between zeolitic deuteroxyl groups and C2H6 molecules monitored by in-situ FTIR spectroscopy. For this purpose, (i) a sample pre-treatment procedure was specifically developed to deuterate only internal or only external acid sites using 2,6-di-tert-butylpyridine (DTBP) as a masking agent and (ii) DFT modeling of surface BAS was performed. Theoretical models of the thin MFI layer revealed that the external surface of MFI crystals contains three types of BAS: (i) BAS positioned in 5-membered rings, either shielded by silanol nests, rendering the site inaccessible for DTBP or yielding a very low adsorption energy for ethane, (ii) BAS pointing into the pores, due to the presence of aluminol, which hinders DTBP accessibility, or the BAS undergoes transformation to a three-coordinate aluminium site, and (iii) BAS accessible to both DTBP and ethane. The results from our kinetics measurements showed that H/D exchange at external BAS of nanosponge MFI zeolites is faster than at internal BAS (rate constants at 425 degrees C: 3.8 10(-3) vs. 2.4 10(-3) s(-1) for external and internal BAS, respectively), but this cannot be attributed to the effect of diffusion. Therefore, the differences in exchange kinetics between external and internal BAS are given by mutual interplay of subtle differences in the corresponding activation barriers (113 vs. 117 kJ/mol for external and internal BAS, respectively) and pre-exponential terms (1.09 10(6) vs. 1.45 10(6) s(-1) for external and internal BAS, respectively).
查看更多>>摘要：High-performance xylene isomerization catalysts were prepared by physical mixing of two types of ZSM-5 after reasonable modification. Here, SZ/LZ catalysts were prepared by simple physical mixing of two modified Pt-3Si/SZ-5 and Mo/LZ-5 with different crystal sizes. The catalytic performance of the catalysts was evaluated in the mixed xylene isomerization reaction on a fixed bed. The catalysts were characterized by XRD, BET, SEM, TEM, XPS, Py-IR, NH3-TPD and probe molecular reactions. The results demonstrated that the deposition of silica finely adjusted the size of the pore opening and passivated the acid sites on the external surface of the SZ-5 zeolite. The loading of MoO3 reduced the distribution of acidic site density of LZ-5. The reaction sites of ethylbenzene, mxylene and o-xylene were precisely distributed by the 2SZ/8LZ catalyst. Optimizing the mixing ratio and reaction temperature of the catalysts further improved their synergistic catalysis. High EB conversion rate (74.9%), high PX yield (23.3%) and low xylene loss (1.41%) were obtained by controlling the reaction path of ethylbenzene, mxylene and o-xylene.
NSTL
Elsevier