Alkane adsorption, kinetics

M. Dogan, M. Alkan, Adsorption kinetics of methyl violet onto perlite , Chemosphere, 50, 517-528, (2003). 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

Kinetic studies of ion exchange on partially ion-exchanged type A zeolites of Mg Ca and Mn " revealed that mini-mums and maximums characterize the differential coefficients of internal diffusion for every exchange of 2 Na " ions for one divalent cation per unit cell of the zeolite. On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite. 

Qince the natural zeolites were discovered and introduced as adsorbents, numerous investigations have been devoted to the sorption behavior and ion exchange properties of zeolites 1, 2, 5, 14). Sorption behavior of zeolites can be influenced powerfully by ion exchange, partly in the uptake capacity of the zeolites and partly in sorption rate since these may undergo decisive changes. The present study undertakes, on the basis of kinetic research in ion exchange and in alkane adsorption on type A zeolites after partial ion exchange, to show what influence is exerted by cation distribution on sorption by type A zeolites. 

A. Dyer (University of Salford, Salford, Lancs., England) Were the experiments concerned with the kinetics of alkane adsorption carried out in the absence of water ... 

Alkan, M. et al.. Surface properties of bovine serum albumin-adsorbed oxides Adsorption, adsorption kinetics and electrokinetic properties, Micropor. Mesopor. Mater., 96, 331, 2006. 

As an experimental prerequisite for studies at liquid/liquid interfaces, the two liquids have to be mutually saturated. For example, even solvents like alkanes are remarkably soluble in water and a transfer of solvent molecules across the interface would influence surfactant adsorption kinetics. The table in Appendix 5E summarises the mutual solubility of some solvents with water. 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

The drop shape method is possibly the most useful one for the investigation of the adsorptive transfer, i.e. the adsorption kinetics at the interface between two liquid phases containing the surfactant from the partition equilibrium. This phenomenon is particularly significant when situations far from the partition equilibrium are considered, in systems characterised by a high solubility of the surfactant in the recipient phase or by a large solubility of the surfactant in both phases. The latter case represents a typical situation for many types of ionic surfactants in water-oil and water-alkane systems, as demonstrated by the partition coefficients measured for various solvents [52, 53, 54, 55, 56]. 

The support induced changes in hydrogenolysis reactions of alkanes can be explained to a large extent by support induced changes in the Pt-H bond strength and hydrogen adsorption site on Pt. This can easily explain the well-known compensation effect found in the kinetics of the hydrogenolysis of alkanes catalyzed by supported metal catalysts. 

The kinetics of alkane hydrogenolysis, that is, the dependence of rate on reactant concentration, have been the subject of numerous studies, and much effort has been devoted to devising rate expressions based on the Langmuir-Hinshelwood formalism to interpret them. Reactions of ethane, propane, and n-butane with H2 on EUROPT-3 and -4 have been carefully studied, with the surfaces in either as clean a state as possible, or deliberately carbided [21, 22] kinetic measurements at different temperature permitted adsorption heats and true activation energies to be obtained. There were two surprises (but like all surprises they were obvious afterwards) ... 

Related to their similar pore diameter and pore structure, unsurprisingly the Henry adsorption constants for linear alkanes are very close to each other on zeolite ZSM-22 and ZSM-23 (Table I). Somewhat higher constants are obtained for 2- and 3-methylbranched alkanes on ZSM-23 compared to zeolite ZSM-22. The adsorption constants of linear alkanes are obviously hi er than branched alkanes on the two cases. The separation power of a zeolite between a linear and a branched hydrocarbon may be given by the separation factor (a), which is the ratio of Henry consteints of linear and branched molecules at a certain temperature, a values at 523 K are given for both zeolites in Table 1. For comparison, values for ZSM-5 are also included, which is one of the most popular shape selective catalyst used in isomerization reactions. From this table it can be seen that both ZSM-22 and ZSM-23 have higher separation constants compared to ZSM-5. The zeolites can be listed in the following order with respect to their separation capacity between linear and 2- and 3-methylbranched alkanes ZSM-22 > ZSM-23 > ZSM-5. In narrow pore structures such as zeolites ZSM-22 and ZSM-23 it is very probable that linear alkanes with smaller kinetic diameters have more access to the available adsorption sites compared to the more bulky branched molecules. This may be regarded as the first... 

An excellent illustration of the LHHW theory is catalytic cracking of n-alkanes over ZSM-5 [8]. For this reaction, the observed activation energy decreases from 140 to -50 ( ) kj/mol when the carbon number increases from 3 to 20. The decrease appeared to linearly depend on the carbon number as shown in Fig. 3.11. This dependence can be interpreted from a kinetic analysis that showed that the hydrocarbons (A) are adsorbed weakly under the experimental conditions. The initial rate expression for a rate-determining surface reaction applies (3.30), which in the limiting case of weak adsorption of A reduces to Eqn. (3.52). The activation energy is then represented by equation (3.53). 

To deduce riso, the elementary rate constant of isomerization, kjso, has been assumed to be rate hmiting. The competition of protonic sites adsorption for alkane or alkene has been exphcitly included in the kinetic scheme of reactions. Using available data on the adsorption isotherms of alkane and theoretical protonation energies, the elementary rate constant parameters of A/iso can be deduced from experiment by measuring the rate of isomerization... 

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