Chabazite propylene

Rozanska et al. compared plane wave LDA and GGA calculations for the chemisorption of propylene in chabazite.282 They concluded that allowing zeolite atoms to relax upon chemisorption made a significant impact on the computed results, but relaxation of the zeolite s unit cell shape and volume had considerably less effect. The same group performed plane wave GGA calculations for the chemisorption of isobutene in three different zeolites, chabazite, ZSM-22 (TON), and mordenite.283... 

Fig. 13. Changes in energy of adsorption of a protonated molecule with respect to the gas phase for protonation of propylene in chabazite. Fads is the adsorption energy of propene in chabazite.

In small pore zeolite systems, the polymerization of propylene (118), isobutylene (120), vinyl chloride (121), and styrene (121) over Linde 5A, the polymerization of propylene and isobutylene over chabazite (19), and the double bond isomerization of 2-methyl-l-pentene over Linde 5A (122) have been reported. Since most of these reactants and the products derived from them cannot pass through the 4-5 A entry pores, it is assumed that these reactions occurred on the external surface of the zeolite. 

The small pore SAPO-34 having a crystal structure analogous to chabazite was also ineffective for propylene oligomerization. 

Anthony and Singh concluded from a kinetic analysis of the methanol conversion to low molecular weight olefins on chabazite that propylene, methane, and propane are produced by primary reactions and do not participate in any secondary reactions, whereas dimethylether, carbon monoxide, and ethane do. Ethylene and carbon dioxide appear to be produced by secondary reactions. It was also shown that the product selectivities could be correlated to the methanol conversion even though the selectivity and the conversion changed with increasing time on stream due to deactivation by coke formation. 

The window dimensions and hence the diffusivity and the diffusivity ratio are correlated with the imit cell size. SiUcon chabazite, which has the smallest cell size, has the highest kinetic selectivity but the diffusion of propylene is rather slow, thus restricting the cycle time. The choice between a high selectivity with slow uptake of propylene and a lower selectivity with faster uptake thus represents an interesting optimization problem. 

More recently, the conversion of methanol to C2 C olefins has been also reported using aluminophosphate-based molecular sieves. Surprisin y, in contrast to zeolite catalysts where best results were obtained with the medium pore ZSM-5, the best results with aluminophosphate catalysts have been described with the small pore SAPO-34 as catalyst. This molecular sieve, with a crystal structure belonging to the chabazite family, produces ethylene, propylene, and butenes with 90% or even higher selectivity. According to the data, methanol can be converted with emphasis to ethylene or to propylene as principal products by using an appropriate choice of reaction conditions (Table 6). Practical process development efforts for the conversion of methanol to C2-C4 olefins have been reported using SAPO-34 catalyst in a fluid-bed configuration. 

Light olefins especially ethylene ( 2 ) and propylene ( 3 ) can be formed from methanol in the MTO process (Chang et al., 1979) using catalyst SAPO-34. Several other catalysts like ZSM-5 (Marchi and Froment, 1991), and Chabazite (Liu et al.. 1984) have been tested. Physical and chemical properties of the catalyst influence its selectivity to hydrocarbons. The physical factors that affect the selectivity of the catalyst are temperature, pressure of the fixed bed reactor, and space velocity of the feed. Other physical characteristics that influence selectivity are crystal size, crystal size distribution, pore size and pore size arrangement. The chemical characteristics that influence the selectivity are acid site density, strength of acid sites, and type of surface acid groups. 

Figure 4.5. Comparison of the DFT-calculated structures of the reactant, transition and product states of the protonation reaction of propylene by a zeolitic proton, (a) Results of calculations using zeolite clusters. (b)Results from periodic DFT calculations on the structure and the resulting energy for the protonation of propylene by the protonated form of chabazite. A1 values are in kJ/mol.

Figure 4.5 shows the energies of the initial weak hydrogen-bonded adsorbed state of propylene, the proton-activated transition state and the final alkoxy product state of the protonated propylene. The structures and energies are established from DFT cluster calculations using the model structure shown in Fig. 4.5a and periodic DFT calculations using the unit cell of chabazite and the zeolitic protons (Fig. 4.5b). The cluster used in Fig.

The declining oil reserves have stimulated considerable efforts towards the exploration of alternative sources of energy and organic chemicals. One solution is to use the abundant supply of coal as a source of synthesis gas (CO + H2) which is readily converted to methanol (MeOH). MeOH can then be transformed into higher molecular weight hydrocarbons (olefins, aliphatics and aromatics) over shape-selective zeolite catalysts, the most successful of which in this respect is H-ZSM-5, capable of converting MeOH to hydrocarbons up to Cio- The selective synthesis of ethylene and propylene, the key intermediates for the production of detergents, plasticizers, lubricants and a variety of chemicals, proceeds over smaller pore zeolites such as chabazite and erionite. 

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