Dispersion interactions, zeolite

The standard adsorbent contactor is a randomly packed bed of pelletized or particulate adsorbents. The commonly used adsorbents include activated carbons, which separate mostly based on dispersive interactions zeolites that separate based on polarity and size carbon molecular sieves, which use the relative differences in intra-particle diffusion rates or silica gel and alumina, generally hydrophilic. The particle size and shape should provide a suitable compromise between pressure drop (AP) and mass transfer resistance. Note that AP is rarely a dominant economic problem, except for the largest systems. In order to minimize it, the cross-sectional area must be increased, leading to a small length-to-diameter ratio LID). In small systems, pressure drop is relatively less important than performance in terms of separation efficiency. 

The decrease in the heat of adsorption as the pore size is increased beyond this size is not surprising dispersive interactions with the zeolite pore decrease. The behavior at lower pore dimensions is explained by considering the location of the sorbed molecules. In the cases of zeolites rho and A, the alkanes were found to adopt highly coiled conformations in the centers of the a-cages that form these structures. The alkanes thus are located in pores with a larger diameter than that usually used to characterize the zeolite (namely, the diameter of the windows that connect the cages). If the heat of adsorption as a function of pore diameter is replotted to reflect the locations of the sorbed molecules, a more straightforward inverse relationship is obtained. 

The distinguishing feature of dehydrated zeolites as microporous aluminosilicate adsorbents lies in the presence in their voids—i.e., micropores—of cations. These cations compensate excess negative charges of their aluminosilicate skeletons. The cations form, in the zeolite micropores, centers for the adsorption of molecules with a nonuniform distribution of the electron density (dipole, quadrupole, or multiple-bond molecules) or of polarizable molecules. These interactions, which will be called, somewhat conventionally, electrostatic interactions, combine with dispersion interactions and cause a considerable increase in the adsorption energy. As a result, the adsorption isotherms of vapors on zeolites, as a rule, become much steeper in the initial regions of equilibrium pressures as compared with isotherms for active carbons. 

If we neglect the difference between electrostatic and dispersion interaction energies, then, in accordance with the concept of volume filling of micropores described previously, the equation of adsorption of methane on zeolite L will be expressed by Ref. 3 as... 

Bonding within the zeolite framework is mainly covalent, and ionic bonding contributes only 10% [52-54], Tlie attractive interaction between the siliceous zeolite framework and hydrocarbons can best be described as a van der Waals dispersive interaction due to the attraction of fluctuating dipole moments of electronic motion on the oxygen atoms and hydrocarbons [55-59], The van der Waals interaction... 

Despite all the advantages of the DFT method one should be aware of well known failure of this method. Exchange-correlation functionals currently available are not capable to account for dispersion interaction, e. g., interaction between zeolite channel wall and hydrocarbons cannot be properly described at the DFT level (sec section 4.3). 

Localization of stationary points along the reaction path for reactions taking place inside the zeolite pores is one of the greatest challenges in zeolite modeling. The reactions of hydrocarbons are particularly difficult to model since the hydrocarbon...zeolite interaction can be dominated by the dispersion interaction that is not properly accounted at the DFT level. Only one example is presented here. Clark et al. investigated the role of benzenium-lype carbenium ion in the bimolecular w-xylene disproportionation reaction in zeolite faujasite.163] The benzenium-type carbenium ion 1 was identified in zeolite catalyst for the... 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

The methods of computer simulation of adsorption (and diffusion) in micro-porous solids were described in Chapter 4 a summary is given in Table 4.1. These techniques are now sufficiently well developed for physisorption that thermodynamic properties can be predicted routinely for relatively simple adsorbates, once the structural details of the host are known. Molecular mechanics using standard forcelields are very successful for zeolitic systems, which take into account dispersive interactions satisfactorily, but it is also possible to use higher level calculations. 

We note that more sophisticated schemes for deducing parameters for dispersive interactions from atomic data (e.g., for adsorbate/adsorbent interactions in zeolites) have been developed and applied in recent years. s- i... 

This is caused by dispersion interaction between the carbon atoms of the reactant and the oxygen atoms of the zeolite framework. Interaction of the reactants with the Brpnsted acid site further reduces the potential. The intrinsic energy barrier for the forward reaction is the difference between the bottom of the well for the coadsorption of A and B at the active site and the top of the transition state. First, the... 

Sorption affinity of fluid components with molecular sieves is governed by the type of their mutual interactions, i.e., fluid-solid interactions, on the microphysical level. For the sorption systems given, besides van der Waals-type atom-atom dispersion interactions of all types of molecules involved, with the solid, zeolitic ion-dipole interactions for H2O, H2S and COS as well as zeolitic ion-quadrupole interaction for CO2 have to be taken into account. A measure of the overall strength of interaction energy is the isosteric heat of sorption, qisosteric-This quantity differs from the differential heat of sorption or sorption enthalpy, - AH, usually measured calorimetrically, by the mechanical work term, RT qisosteric AH -t RT. 

Af/ads is the heat of adsorption from the gas phase, which takes into account the dispersion interaction of hexene with the oxygen atoms in the wall of the zeolite pores. This energy depends both on the size of the reactant (hexene in this case) and the size o .the pores in the zeolite (Figure 8a and 8b) and is estimated with the configurational-bias Monte Carlo method (CB-MC). - The CB-MC method differs from conventional Monte Carlo (see Monte Carlo Simulations for Polymers) in so far as.ti guest species is grown atom by atom inside the host rather than inserted as a complete molecule. ... 

Following Kiselev and co-workers [24], the zeolite is modeled as a rigid crystal [140]. This allows the use of interpolation techniques to determine the interaction of an alkeme atom with the zeolite and avoids having to consider all zeolite atoms [107,141]. The interactions of the alkane atoms with the zeolite atoms are dominated by the dispersive interactions with the oxygen atoms [24], these interactions cire described with a Lennard-Jones potential. 

Dynamic Structural Change of Pd Induced by Interaction with Zeolites Studied by Means of Dispersive and Quick XAFS 427... 

To improve the adhesion and interaction in the zeolite/polymer interface, the surface of the zeolites can also be sized (or primed ) by coating the zeolite with an ultrathin layer of the matrix polymer or a different polymer. Sizing of the zeolite particles prior to dispersion in the polymer matrix reduced the stress at the... 

The zeolite provides the environment for shape selective chemistry and is also a high surface area support on which to disperse platinum in a relatively confined environment. The small platinum crystals within the zeolite channels and the orientation effect of the channel window are responsible for the high efficiency of the Pt-KL catalyst to convert linear paraffin to aromatics. Zeolite KL also provides an electron rich environment to enhance stronger platinum-substrate interaction via stronger platinum-support interaction. A review on the subject can be found in the article written by Meriasdeau and Naccache [85]. 

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