Positron emission profiling study of the diffusion of 3-methylpentane in silicalite-1

The influence of the loading on the self-diffusion of 3-methylpentane within silicalite-1 was studied with positron emission profiling technique. A monotonous decrease in the self-diffusivities of alkane was obsd. with zeolite loading, which could be approximated by a stretched exponential. As a consequence, the apparent activation energy increases with partial pressure

Positron emission profiling study of the diffusion of 3-methylpentane in silicalite-1

The influence of the loading on the self-diffusion of 3-methylpentane within silicalite-1 was studied with positron emission profiling technique. A monotonous decrease in the self-diffusivities of alkane was obsd. with zeolite loading, which could be approximated by a stretched exponential. As a consequence, the apparent activation energy increases with partial pressure

Concentration and temperature dependence of the diffusivity of n-hexane in MFI-zeolites

The concentration dependence of the self-diffusivity of n-hexane in large crystals of silicalite-1 and HZSM-5 zeolites has been investigated with positron emission profiling (PEP) in the temperature range of 393–483 K. A monotonic increase in the diffusivity was observed with increasing alkane loading for both zeolites up to 4 molecules per unit cell. The diffusion coefficient in HZSM-5 was approximately half of that in silicalite-1 due to the interaction of n-hexane with the Brønsted acid sites. The increase in the diffusivities with the loading is assumed to be caused by the repulsive interactions between the molecules adsorbed in the channel intersections and the adjacent straight channel. The apparent activation energy of diffusion was measured to be independent of the partial pressure of alkane, while the jump frequency increased. In HZSM-5 zeolite the apparent activation energy decreased with the partial pressure due to stronger interactions between n-hexane and the acid sites at low pressures

Adsorption and diffusion of n-hexane/2-methylpentane mixtures in zeolite silicalite : experiments and modeling

With the tracer-exchange positron emission profiling (TEX-PEP) technique, the reexchange process of radioactively labeled molecules with a steady-state feed stream can be measured inside a zeolite-packed bed reactor. When the experimental tracer-exchange curves are modeled, values for the micropore diffusion and adsorption constant can be obtained. As one can choose which component to label, this technique is ideally suited for studying multicomponent diffusion. In the present study, this technique has been used to measure the diffusive and adsorptive properties of an n-hexane/2-methylpentane mixture in zeolite silicalite. The measurements were performed at different ratios of n-hexane and 2-methylpentane in the gas phase at a constant total hydrocarbon pressure of 6.6 kPa and a temperature of 433 K. A slight preference for the adsorption of n-hexane was found because it is entropically more favorable to adsorb these molecules as they have no preferential siting in the zeolite pores; The diffusivity of the slowly moving 2- methylpentane is not strongly affected by the presence of the fast moving n-hexane. The mobility of the linear alkane however strongly decreases with increasing 2-methylpentane ratio and suddenly drops at a loading of approximately three 2- methylpentane molecules per unit cell. This is caused by the fact that the branched alkanes are preferentially sited in the intersections between the straight and zigzag channels of silicalite and therefore effectively block the zeolite pore network. These results show that the adsorptive properties of the components and the structure of the zeolite network play an important role in the behavior of multicomponent mixtures in zeolite

Influence of the acid sites on the intracrystalline diffusion of hexanes and their mixtures within MFI-zeolites

The intracrystalline diffusivities and component loadings of mixtures of n-hexane and 2-methylpentane within silicalite-1 (crystal size 150 mum) and HZSM-5 (140 mum) have been measured at 433 K as a function of the n-hexane/2-methylpentane ratio in the gas phase (at constant total hydrocarbon pressure of 6.6 kPa) using the Positron Emission Profiling technique. Strong preferential adsorption of the linear alkane over the branched one has been observed in HZSM-5, while in silicalite only a slightly higher adsorption of n-hexane was observed. The diffusivities of both components in the mixtures decrease with increasing 2-methylpentane content and have been found to be approximately two times lower in HZSM-5 than those in silicalite. When the concentration of the branched hexane in the adsorbed phase becomes close to 3 molecules per unit cell, the influence of the acid sites on the diffusion of linear hexane diminishes compared to the influence of the blockage of the zeolite network with the slowly moving 2-methylpentane. The apparent activation energies of the diffusion for n-hexane and 2-methylpentane have been measured at temperatures between 393 K and 533 K under similar conditions. For n-hexane, these values were found to be very close in silicalite and ZSM-5: 18.5 +/- 1.5 kJ/mol and 22 +/- 2 kJ/mol, respectively. For 2-methylpentane the activation energies were found to be very high: 66 +/- 6 kJ/mol and 72 +/- 3 kJ/mol, respectivel

Adsorption and diffusion of alkanes and their mixtures in silicalite studied with positron emission profiling technique

The Tracer-Exchange Positron Emission Profiling (TEX-PEP) technique has been applied for in-situ measurement of the adsorption and diffusion of hexane and 2-methylpentane mixts. in large crystallite (150mm) of silicalite-1 at 160°C under steady-state conditions. N-hexane adsorption is slightly preferred. Mobility of n-hexane strongly decreases with increasing fraction of 2-methylpentane, and it suddenly drops when the pore occupation with 2-methylpentane is much bigger then with n-hexane. A small decrease in the diffusivity of 2-methylpentane is also obsd. Pulse expts. were performed to measure isotherms of adsorption of 3-methylpentane on silicalite-1 at temps. 130-210°C. Obtained values of enthalpy and entropy of adsorption are in a good agreement with theor. calcns

The application of non-hydrothermally prepared stevensites as support for hydrodesulfurization catalysts

The stevensite-like materials containing Mg, Mg-Ni, and Mg-Co in the octahedral sheets were synthesized under non-hydrothermal conditions. The nature of divalent octahedral cations influences the surface area and pore volume of these materials. The thiophene HDS activities of stevensite-supported catalysts prepared by addition of Mo or W to the Ni-containing supports, especially to the Ni2+-exchanged stevensite, were superior in relation to those of their counterparts made by conventional co-impregnation of the Mg-stevensite with NiMo and NiW.The use of chelating agents, NTA and EN, affects beneficially the HDS activity of the catalyst which, however, is less obvious in the case of the CoMo catalysts. The positive role of the chelating agents is explained by weakening of the Ni(Co)-support interaction and by a change in the sulfidation sequence of Ni(Co) and Mo(W).The Ni2+- and Co2+-exchanged Mg-stevensite or that isomorphously substituted with these cations are promising candidates for application as supports for hydro-processing catalysts. The thiophene HDS activity of the NiMo and NiW stevensite-supported catalysts is comparable with that of the corresponding gamma-Al2O3 commercial one