Hydrocarbons, adsorption from vapour

Figure 7 shows an example of type IV isotherms for adsorption of trace water from toluene and p-xylene mixtures on Alcoa H-152 alumina at 22° C [22], The abcissa of the plot represents relative saturation of water (xi/x ), where x is the molar fraction of water at the solubility limit in the hydrocarbon liquid. Equilibrium isotherm models analogous to those used for pure water vapour adsorption can be derived for describing trace water adsorption from liquid mixtures [20-22],... 

Reactions for the synthesis of fine chemicals differ in many aspects from the hydrocarbon reactions that constitute today the major application of zeolites and other micro- or mesoporous catalysts, as they often involve the transformation of molecules with several functional groups. Chemoselectivity is therefore of prime importance. These reactions are generally operated in rather mild conditions and condensed media (rather than vapour phase) to avoid undesired secondary reactions. The use of solvents can have major impacts on the activity and selectivity of these catalysts as they may affect the adsorption and desorption of reactants and products on these catalysts. 

An early normalizing procedure, proposed by Kiselev (1957) to compare adsorption isotherms of hydrocarbons, water vapour, etc. on a series of different adsorbents, was simply to plot the surface excess concentration F (=n/A), obtained from a knowledge of the BET-nitrogen surface area, A (BET), versus p/p°. It is also possible to plot, instead of f, the reduced adsorption , n/nm, which still relies on the BET method to determine the monolayer capacity nm but does not require knowledge of the molecular cross-sectional area a. 

Direct pressure measurements may be made with conventional manometers using mercury, silicone oils, hydrocarbon oils, butyl phthalate and other fluids. Apart from the danger of their affecting the reaction studied directly, by chemical reaction, vapour adsorption or modifying the surface of the RV, they produce a varying dead space . It is important to keep the dead space between 1 and 4 %... 

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