Olefins reversible adsorption

From these considerations we can see an outline of the kinetics and mechanism of catalyst decay. While the catalyst remains in the presence of the reactant-product stream, on each active site the processes which dominate are the "fiuitful" processes of the attached carbenium ions, involving protolysis, P-cracking, disproportionation, and the reversible adsorption-desorption of product olefins. These events, in combination, constitute the dKiin mechanism of cracking 4) and yieldthe major products of the "cracking" reactioa None of these processes results in an irreversible reduction of catalyst activity, although the various carbenium ions present will undergo various mainline reactions at different rates. 

Interestingly, reversible adsorption with isomerization is not limited to olefins with electron donating substituents. 

To obtain olefins in the pure state, intended for other uses than alkylation (oxo synthesis, alkyl sulfates), they can be separated by selective and reversible adsorption on solids. UOP employs a technique designated Olex , which is similar in principle to that nf its Molex and Parex processes. 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

Kinetic models referred to as adsorption models have been proposed, especially for olefin polymerisation with highly active supported Ziegler-Natta catalysts, e.g. MgCl2/ethyl benzoate/TiCU AIR3. These models include reversible processes of adsorption of the monomer (olefin coordination at the transition metal) and adsorption of the activator (complexation via briding bonds formation). There are a variety of kinetic models of this type, most of them considering the actual monomer and activator concentrations at the catalyst surface, m and a respectively, described by Langmuir-Hinshelwood isotherms. It is to be emphasised that M and a must not be the same as the respective bulk concentrations [M] and [A] in solution. Therefore, fractions of surface centres complexed by the monomer and the activator, but not bulk concentrations in solution, are assumed to represent the actual monomer and activator concentrations respectively. This means that the polymerisation rate equation based on the simple polymerisation model should take into account the... 

For selective acetylene adsorption from other hydrocarbons (e.g., ethylene and ethane), NiCl2 supported on alumina or silica can form reversible jr-complexation bonds with acetylene but not olefins. Pure component acetylene-ethylene ratios of up to 3 were obtained (Kodde et al., 2000). The bonding between acetylene and NiCl2 is reasonably understood (Huang and Yang, 1999). 

With these mechanistic rationales and a knowledge of how the nature of the catalyst affects the reversibility of the individual steps in the hydrogenation process as well as how steric factors can be involved in olefin adsorption, it... 

FTIR model experiments were performed to reveal the nature of catalyst deactivation in C02. The spectrum taken at 15 bar in a C02/H2 mixture is shown in Fig. 1. The bands at 2060 and 1870 cm 1 indicate considerable coverage of Pt by linearly and bridge-bonded CO [12], formed by the reduction of C02 on Pt (reverse water gas shift reaction). The three characteristic bands at 1660, 1440 and 1235 cm 1 are attributed to C02 adsorption on A1203, likely as carbonate species [13, 14], It is well known [15] that CO is a strong poison for the hydrogenation of carbonyl compounds on Pt, but can improve the selectivity of the acetylene — olefin type transformations. Based on the above FTIR experiments it cannot be excluded that there are other strongly adsorbed species on Pt formed in small amounts. It is possible that the reduction of C02 provides also -COOH and triply bonded COH, as proposed earlier [16]. 

At 80 °C, the value of the pseudo second order rate constant, k, was 2.05 X 10 mole" hr" (gram cat)" These observed kinetics provide firm quantitative support for a Rideal mechanism, which requires fast, reversible, and noncompetitive adsorption of the olefin on the catalyst acid sites see Scheme 5 above). The strictly first order dependency of toluene proves freedom from intracrystalline diffusion problems with respect to ethylene. Likewise, the product distribution, which contains only traces of disubstituted toluenes, shows absence of diffusion limitation for the product, ethyltoluene. Intracrystalline diffusion problems may, however, exist in the isopropylation experiments with extremely active catalysts. 

We have presented a mechanism for the hydrogenation of olefins and exchange of alkanes based upon heterolytic dissociative adsorption and its reverse, associative desorption (21). Ignoring the question of whether the acidic sites are Cr or Cr2+, the mechanism is represented... 

Stage I may be considered as the period during which the polymerization center is established on the surface of titanium trichloride from Al2Etg and the olefin monomer. Al2Ete adsorbs reversibly on the surface S to form a surface complex C. Assuming that the adsorption equilibrium is of the Langmuir type [cf. Eq.(P9.5.4)],... 

Propylene dissolves in two modes, a physical dissolution mode in the matrix and a binding mode resulting from a reversible chemical reaction with metal complexes. Henry s law commonly expresses the physical dissolution mode for small molecules, while the Langmuir model adequately describes the reversible binding mode, as shown in Eq. (9-2). Mathematically, a Langmuir adsorption isotherm for a small molecule in a porous media is identical to the expression of the olefin concentration bound to the metal complex. 

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