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Room-temperature interaction of n-hexane with ZSM-5 zeolites

Microcalorimetric and temperature-programmed desorption studies

Journal of Thermal Analysis and Calorimetry
Authors: V. Rac, Vesna Rakić, S Gajinov, Vera Dondur, and Aline Auroux

Abstract  

In this work, room temperature interaction of n-hexane with HZSM-5 (Si/Al=20) and ion-exchanged samples containing one (CuZSM-5, FeZSM-5 and MnZSM-5) or two transition-metal cations (Fe,CuZSM-5; Cu,MnZSM-5 and Fe,MnZSM-5) was studied by microcalorimetry and TPD methods. Both differential heats and the amounts of n-hexane adsorbed per one unit cell were quantitatively determined. Higher heats of adsorption and higher amounts of adsorbed gas were found for ion-exchanged samples than for HZSM-5. The experiments of n-hexane adsorption on hydrated samples were also performed. The amounts of n-hexane adsorbed on hydrated ZSM-5 were lower in comparison with dehydrated samples, while the energies of interaction were similar.

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-22 are less explored [ 17 ]. Previously, we reported preliminary studies regarding the potentialities of MCM-22 zeolite in n -hexane transformation [ 7 , 18 ]. This work presents a more detailed study regarding n -hexane transformation used as a

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Bianca V. Sousa, Karoline D. Brito, José J. N. Alves, Meiry G. F. Rodrigues, Carlos M. N. Yoshioka, and Dilson Cardoso

isomerization of n -paraffin fractions. As a result, the need to maintain high octane number fuel has recently attracted much interest in the isomerization processes involving alkanes, particularly linear alkanes such as n -hexane and n -heptane with octane

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]. Accordingly, there is a pressing need for new, inexpensive and highly selective reusable catalysts for the oxidation of alkanes. Recently bimetallic Co–Mg oxide silica supported catalysts were found to promote complete oxidation of n -hexane to CO 2

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Abstract  

Temperature-programmed desorption coupled with mass spectrometer as a detector (TPD), IR and 13C NMR measurements are used to study the adsorption of n-hexane on hydrated HZSM-5 and NH4ZSM-5 zeolites. The 13C NMR measurements show that n-hexane can access the pore structure of ZSM-5 zeolites previously saturated with water. TPD spectra of n-hexane are monitored in the temperature region 50–300C, in the case of fully or partially hydrated samples; two-stage desorption of n-hexane is found. Simultaneous desorption of water and n-hexane in the same temperature region are found, in all investigated samples.

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Abstract  

The origin of kinks (steps) on sorption isotherms was examined for the sorption of benzene and n-hexane on silicalite-1. In both cases sorption revealed the existence of two different binding sites. There was no equilibrium (or a very slow one), between molecules bound at different sites. Sorption energies within particular centres display more or less wide overlapping distributions leading to a single resultant isotherm. Depending on differences in binding energies and degree of overlapping, the resulting isotherms exhibit steps (benzene) or no steps (n-hexane). In fact, the sorption isotherm of benzene being a sum of two elemental isotherms (Ω1 and Ω2) of different shapes is characterized by a ‘kink’, in contrast to n-hexane the elemental isotherms of which are of the same shape.

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Abstract  

Hydroisomerization of n-hexane simultaneously proceeding with benzene saturation over platinum impregnated nano-crystalline zeolite beta (Si/Al = 25) at 1.0 MPa and 573–673 K was studied. The feed contained 30 wt% benzene. The synthesized catalyst was characterized by various techniques, e.g., powder X-ray diffraction, IR spectroscopy, thermal analysis, scanning electron microscopy and nitrogen adsorption. At the operating conditions used, total saturation of benzene with 95% selectivity to cyclohexane (CH) and methylcyclopentane (MCP) was observed. n-Hexane conversion increased with temperature whether benzene was present or not during the reaction. However, in the presence of benzene, n-hexane conversions as well as isomer selectivities were observed to be lower. This was attributed to reduced availability of acid sites by predominating benzene adsorption. Formation of carbenium ion on an acid site is questioned and plausible mechanism has been suggested.

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Abstract  

The isotherms of adsorption-desorption of the vapour of benzene, cyclohexene and n-hexane on different porous silica gels were measured gravimetrically. For all the adsorbates, the amount of vapour condensed in the adsorbent pores is significantly greater than the amount adsorbed. The isotherms of all the adsorbates are linear in a wide range of relative pressures.

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Abstract  

Equilibrated thermodesorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) were employed as methods for studying the influence of different extraframework cations (Na+, K+, Li+, Cu2+, Zn2+, or Mg2+) on adsorption of n-hexane on ZSM-5 zeolite with high Al content (Si/Al = 15). Considerable influence of the cations on both initial adsorption in the micropores and ordering of the adsorbed molecules, occurring at high coverages, has been observed. This influence is reflected by the values of the adsorption enthalpy and entropy, determined by fitting the dual site Langmuir (DSL) adsorption function to the equilibrated thermodesorption profiles. However, no clear correlation between the determined parameters and properties of the extraframework cations could be found.

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Abstract  

The synergistic solvent extraction of Eu(III) and some other rare earth elements from nitrate solutions (HNO3+LiNO3) by a mixture of (TBP+D2EHPA) in n-hexane and cyclohexane has been investigated at 22 °C. Antagonism found in europium extraction from 0.1M HNO3 transforms into a synergistic effect. The synergistic effects existing for all investigated metals in extraction from 0.1M HNO3+3M LiNO3 were caused by formation of mixed complexes of the type Ln(D2EHPA)2nH2n–3+1(NO3)1TBPm, where 1=1 or 2. The selectivity of the extraction in a synergistic system is lower for the La–Yb pair than in the case of D2EHPA extraction under the same conditions. On the other hand, the application of the synergistic mixture is more suitable for Eu–Ho separation. Thus the synergistic effect can be used for the separation or refining of some lanthanides.

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