Zeolite/adsorbate systems

The previous two sections of this review deal with classical simulation methods. A description of the activation of adsorbates by acidic sites, together with any bond breaking or bond formation that may take place, is the realm of quantum mechanical (QM) simulations. These types of calculations are particularly well-suited to zeolite-adsorbate systems when the cluster approximation is used. The active acidic site in the zeolite is modeled by a molecular cluster, formed by cutting out a small portion of... 

Subsequently, it is possible to consider that the adsorbate-adsorbent interaction field inside these structures is characterized by the presence of sites of minimum potential energy for the interaction of adsorbed molecules with the zeolite framework and charge-compensating cations. A simple model of the zeolite-adsorbate system is that of the periodic array of interconnected adsorption sites, where molecular migration at adsorbed molecules through the array is assumed to proceed by thermally activated jumps from one site to an adjacent site, and can be envisaged as a sort of lattice-gas. 

The statistical calculation of thermodynamic constants for the zeolite-adsorbate system should be made. A comparison of these constants with the experimental values obtained in Stages 2 and 3 would allow the introduction of some correction factors for the constants describing the potential functions of the interaction. As a result, a more accurate general scheme of molecular-statistical calculation of the molecule-zeolite system would be obtained. 

IR investigations of zeolite/adsorbate systems may have various aspects They may... 

With the advent of sohd-state NMR, another powerfiil tool for the characterization of zeohtes and related materials emerged. Similarly and, in many respects, complementarily to infrared spectroscopy, solid-state NMR spectroscopy enabled investigations to be carried out of the zeolite framework, extra-frame-work cations, hydroxyl groups in zeolites, pore structure, and zeolite/adsorbate systems. The contributions of solid-state NMR to molecular sieves research is reviewed by M. Hunger and E. Brunner in Chapter 2. 

The information that is presently available on the heat of adsorption of gases and vapors by zeoUtes is very extensive, but only a small part of the experimental data are sufficiently reliable to be regarded as thermodynamic constants characterizing the equihbrium in zeolite/adsorbate systems. 

Gopa et al. [23] reported about open Zeolite/water systems for the storage of solar heat. In their work they were presenting stability tests, methods for the definition of the adsorption enthalpy in dependence on the adsorbed amount of water and the possible heating power during the discharging process. 

Vibrational dynamics of small molecules adsorbed on cation sites in zeolite channel systems IR and DFT investigation... 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

First experimental results on dye-loaded zeolite L systems modified with commercial stopcock molecules on the external surface show that electronic excitation energy can be transferred from molecules inside the channels to the stopcocks and vice versa and that the stopcocks prefer to adsorb on the cylinder base instead of the coat [42]. 

If the recovery of the adsorbed VOCs is highly desirable, then instead of steam, a vacuum regeneration system may be used. According to this method, the VOCs are forced to volatilize not by temperature, but by means of pressure. Specifically, a vacuum pump is employed to decrease the pressure in the carbon below the vapor pressure of the VOCs, which leads to then1 boiling at ambient temperature. This method is generally used with carbons, polymers, and zeolite adsorbents (EPA, 1999). 

Contrary to catalytic applications, zeolite adsorbents are mostly applied in a fixed-bed operation. A number of columns packed with zeolite adsorbent(s) are interconnected with an automatic valve system to facilitate a continuous flow of the industrial stream being processed. Each bed, however, goes through a stepwise cyclic operation, and during each cycle the adsorbed molecules in the zeolite bed are desorbed by raising the bed temperature, lowering the bed pressure, displacing the adsorbate with another adsorbate, or combination. 

Hydrocarbon Trap System. The concept of a hydrocarbon trap or adsorber system is based on molecular sieve hydrocarbon adsorber systems. The temperatures at which hydrocarbon adsorption takes place exist in the auto engine exhaust system during the period of cold start of an automobile when the catalytic control system has not yet reached functional temperature. Zeolites have been reportedly useful for hydrocarbon adsorption (53,169). Zeolites desorb hydrocarbons at temperatures of 400°C, ie, once the catalytic control system is functional. Therefore, hydrocarbons adsorbed by the zeolite can also be desorbed then oxidized by a catalyst. Methods to accomplish cold start hydrocarbon adsorption, heatup of the main catalyst, and desorption have been identified. Some of these systems use exhaust pipe valves to divert the exhaust gases to the hydrocarbon trap for the low temperature portion, and by-pass the gases around the trap after the main catalyst has heated up. One device that uses a heat exchanger is shown in Figure 15 (44). The Si—Al ratio in the zeolite is important, and by lowering the alumina content, the zeolite is rendered more hydrophobic and more able to adsorb... 

A nonlinear increase of Ss upon increasing 0 is evident for all the adsorbate/adsorbent systems (Figure 3a) the curvatures reflect many-body interactions between a single xenon atom with benzene molecules within the zeolite supercage. Regressional fitting by a third-degree polynomial to the experimental values produces ... 

Today the melt crystallization can be advantageously replaced by a more challenging separation method known as simulated moving bed (SMB) technology. The method exploits the differences in affinity of zeolitic adsorbents for p-xylene with respect to other A8 components. Despite the name, the adsorbent phase is stationary and only fluid phase is distributed in a cyclic manner by a multivalve system. Operation parameters are temperatures of 125 to 200 °C and pressures up to 15 bar. Lighter (toluene) or heavier solvents (p-diethylbenzene) may be used as a desorbent. The Parex process working on this principle today has many applications. 

McCabe-Thiele diagrams for nonlinear and more practical systems with pertinent inequality constraints are illustrated in Figures 11 and 12. The convex isotherms are generally observed for zeolitic adsorbents, particularly in hydrocarbon separation systems, whereas the concave isotherms are observed for ion-exchange resins used in sugar separations. 

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