Methane faujasites

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

The diffusion coefficient was estimated to be 4 x 10 9 m2/s. Experimental values for benzene in faujasites range from 10 10 to 10"13 m2/s, depending on the measurement technique (24, 97). PFG-NMR measurements are the closest to the MD value, which was admitted by the authors to be a crude estimate (mainly on grounds of a short simulation and inflexible molecules). The simulation time was too short to permit a calculation of the residence times of the benzene at either the cation or the window site or inside a particular cage. The cage residence times were estimated to be at least an order of magnitude longer than those for methane in NaY zeolite (43). 

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. 

Fig. 7. Predicted minimum energy sites of methane in faujasite from Monte Carlo calculations. The methane molecules are shown as large dark circles within the faujasite cavity. Reprinted with permission from Nature, Ref. 46. Copyright 1988 Macmillan Magazines Limited.

Experiments which have been carried out indicate that the results discussed above which have all been obtained on H-ZSM-5 zeolites are actually valid for all protonated zeolites. Experiments have so far been carried out on protonated ZSM-11, erionite, mordenite, faujasite (Y-zeolite), offretite, ferrierite and L-zeolite (ref. 4). Till now, all the the investigated zeolites have given fully concordant results. While they all exhibit the same behaviour as regards activation, their resistance to deactivation and their product spectrum are in many cases dramatically different. Offretite for instance deactivates almost completely after a couple of hours with high activity, and the dominant hydrocarbon, by far, is methane (ref. 4). 

Li,Na,K,Mg,Ca) faujasites, examining the dipole moment of the interacting molecule and the charge density within the cluster. Sauer et al. have performed ab initio cluster calculations for simple models of interaction sites for water, methane, ethylene and the ammonium ion, obtaining interaction energies which appear to be... 

Several authors suggested using the rate of H/D exchange between solid acids and methane as an indicator of the relative acidity of these catalysts.The first theoretical and experimental work in this field was published by van Santen and coworkers in Nature.For the theoretical part, the zeolite structure was represented by a small H3Si-OH-Al(OH)2-SiHs cluster, and for the experimental part, proton forms of Faujasite and MFI were chosen. The study concluded that there was a concerted exchange mechanism, in which no positive charge developed in the transition state (Fig. 3) with an activation energy of 150 20 kJ mol ... 

In Fig. 4.21 one notes the large difference in the intensity of the vibrational excitation aroimd 2800 cm in methane adsorbed on Zn + in ZSM-5 and zeolite Y. This excitation is not observed in the gas phase, where by symmetry it is vibrationally forbidden. The effective charge of the Zn + cation polarizes the molecule, which results in symmetry breaking, thus allowing for the excitation of this symmetric CH4 mode. The lower effective charge of Zn " " in the faujasite structure zeolite Y with higher A1 concentration results in a smaller polarization and hence a decreased relative intensity. 

Microcalorimetry measurements are combined with Grand Canonical Monte Carlo simulations in order to understand more deeply the interactions between methane and two types of faujasite systems. The modelling study, based on newly derived force fields for describing the adsorbate/adsorbate and adsoibate/adsorbent interactions, provide isotherms and evolutions of the differential enthalpy of adsorption as a fimction of coverage for DAY and NaX which are in very good accordance with those obtained experimentally. The influence of the location of the extra-framework cations within the supercages on these thermodynamics properties is also pointed out. Furthermore, the microscopic mechanisms of CH4 adsorption is then carefully analysed in each faujasite system which are consistent with the trend observed for the differential enthalpies of adsorption. 

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