Aromatic hydrocarbon-selective adsorbents

Typical nonsieve, polar adsorbents are siUca gel and activated alumina. Kquilihrium data have been pubUshed on many systems (11—16,46,47). The order of affinity for various chemical species is saturated hydrocarbons < aromatic hydrocarbons = halogenated hydrocarbons < ethers = esters = ketones < amines = alcohols < carboxylic acids. In general, the selectivities are parallel to those obtained by the use of selective polar solvents in hydrocarbon systems, even the magnitudes are similar. Consequendy, the commercial use of these adsorbents must compete with solvent-extraction techniques. 

The principal nonpolar-type adsorbent is activated carbon. Kquilihrium data have been reported on hydrocarbon systems, various organic compounds in water, and mixtures of organic compounds (11,15,16,46,47). With some exceptions, the least polar component of a mixture is selectively adsorbed eg, paraffins are adsorbed selectively relative to olefins of the same carbon number, but dicycUc aromatics are adsorbed selectively relative to monocyclic aromatics of the same carbon number (see Carbon, activated carbon). 

Certain highly porous solid materials selectively adsorb certain molecules. Examples are silica gel for separation of aromatics from other hydrocarbons, and activated charcoal for removing liquid components from gases. Adsorption is analogous to absorption, but the principles are different. Layers of adsorbed material, only a few molecules thick, are formed on the extensive interior area of the adsorbent - possibly as large as 50,000 sq. ft./lb of material. 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

The Sumitomo-BF PSA process uses carbon molecular sieves (CMS) as the selective adsorbent, CMS has a higher capacity of adsorption than zeolites for methane and oxygen, and it is considered to be advantageous for hydrogen purification. If dirty raw gases are fed to this process, minor amounts of heavy hydrocarbon components such as aromatics are likely to cause deterioration of the adsorbents. To remove the heavy hydrocarbons, prefilter columns that contain activated carbon are placed upstream of the main CMS adsorbent beds4. 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

The reaction of n-butane at different WHSV (1.5 h to 5.5 h ) over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts resulted in the same type of products as those formed at different temperatures, indicating that the reaction products formed at longer contact time are not adsorbed on the surface of zeolite or trapped in the zeolite channel system. The n-butane conversion and selectivity to aromatics over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts decreased with increase in the space velocity. The effect of space velocity on n-butane conversion and selectivity to aromatic hydrocarbons is given in Figures 6 and 7. 

In a study of selective adsorption of sulfur compounds and aromatic compounds in a hexadecane on commercial zeolites, NaY, USy, HY, and 13X by adsorption at 55 °C and flow calorimetry techniques at 30 °C, Ng et al. found that a linear correlation between the heat of adsorption and the amount of S adsorbed for NaY.162 Competitive adsorption using a mixture of anthracene, DBT, and quinoline indicates that NaY selectively adsorbs quinoline, while anthracene and DBT have similar affinity to NaY, indicating that NaY is difficult to adsorptively separate sulfur compounds from aromatic hydrocarbons with the same number of the aromatic rings. 

Very recently , we have carried out an intercomperison study of hquid-hquid extraction (LLE) and sorption on polyurethane foam (PUF) and Amberlite XAD-2 for the analysis of aliphatic, aromatic and chlorinated hydrocarbons dissolved in seawater. The application of these methods, sampling in parallel the same body of water, has shown significant differences in the recovery of higher molecular weight components in the complex mixtures of both aliphatic and aromatic hydrocarbons. These are attributed to selective associations of these hydrophobic species with macromolecular organic matter such as fulvic and humic acids and to the effects of the dissolved organic molecules on adsorbent properties. 

Selective absorption of olefins and aromatic hydrocarbons was carried out in an absorber (diameter 6 mm) containing 2 ml of concentrated sulphuric acid on a layer of fibre-glass. The length of the packing layer was 43 cm. At the outlet of the absorber a layer of molecular sieve 4A was placed this sieve adsorbs methane, water, ethane, acetylene and propylene, but not propane and hi -molecular-weight compounds. A few granules of Ascarite were also added. The reactor absorbs olefins, toluene and higher aromatic hydrocarbons at 54°C benzene is absorbed incompletely. 

Some interesting secondary solvent effects have been noted for adsorption of the polycyclic aromatic hydrocarbons and certain of their derivatives on alumina 14). The more nearly linear isomers (e.g., anthracene relative to phenanthrene, 2-bromonaphthalene relative to 1-bromonaph-thalene) are preferentially adsorbed from most solvents, owing to the apparent weak localization of these compounds on linear site complexes see Section 11-2B, There is also a tendency for the preferential adsorption of strong solvent molecules on these same linear site complexes, with the result that strong solvents (or their solutions in weaker solvents) behave as selectively stronger solvents toward the preferentially adsorbed linear aromatics, relative to less linear isomers. As a consequence the ratio of values for two such isomers varies sharply with the solvent used, despite the fact that Eq. (8-3) predicts that this ratio should remain constant for all solvents i.e., A. is generally constant for two or more isomers. Jn extreme cases the ratio of K" values for two isomers of this type can be varied by a factor of 10 or more, depending upon the solvent used (see Table 11-4). 

The LAB-grade product from the recovery process is further processed in an optional purification section, where residual aromatics and other impurities are further reduced to below 100 ppmwt. Purification is accomplished in a liquid-phase, fixed-bed adsorption system. The impurities are selectively adsorbed on a molecular sieve, and subsequently removed with a hydrocarbon desorbent. 

Similarly, product dijfusion selectivity may act on the products of a reaction that are formed within the catalyst, so that the fastest diffusing products are able to leave the catalyst preferentially whereas the bulkier remain behind, where they can react further. The direct observation of the difference in relative abundance of aromatic molecules in adsorbed and gas phases during the MTG process over H-ZSM-5 by MAS NMR (Section 8.5) is a good example of this. The effect of product diffusion control on product selectivity has been well demonstrated by the use of in situ C NMR under reaction conditions," " " where the adsorbed hydrocarbon mix of the methanol-to-gasoline reaction over H-ZSM-5 is demonstrated to have a distribution much closer to thermodynamic equilibrium than the product, due to strongly different diffusion rates out of the pores. Where equilibria exist within the pores, the effect of diffusion selectivity is to enrich the product mixture in the faster-diffusing product to levels that can be far above equilibrium concentrations. A well-studied example of this is the alkylation of aromatic molecules within zeolites, when the produet stream is enriched to well above equilibrium levels in the faster diffusing para-substituted isomer, which is the most valuable one. 

Micropacked columns are available with most of the bonded-phase packings used in HPLC. Porous and non-porous silica particles are optionally functionalized with covalently ound silanes or other strongly adsorbed materials. ALkyl-bonded silicas produce separations, generally based on solute volatility, but with the potential for selectivity differences based on interaction with silanol groups. Underivatized silica is popular for petroleum separations of aliphatic and aromatic hydrocarbons. Silver-ion-containing silica columns are selective for olefin separations. Huoroalkyl-bonded... 

Some ionic adsorbents show unusu ll properties (see, for example, [15, 58, 68-70]). Belyakova and co-workers [68-70] have proposed barium sulfate as a selective adsorbent They successfully used barium sulfate modified with sodium chloride for separation of some isomers of unsaturated, aromatic hydrocarbons, and oxygen- and nitrogen-containing heterocyclic compounds [68]. Barium sulfate was prepared by interaction of s< ium sulfate and barium chloride solutions of various concentration present in equimolar proportions. The specific surface areas varied from 2.5 to 8 m /g. To investigate this ionic adsorbent, glass Ccipillary columns (1 mm i.d.) were packed with barium sulfate particles (0.16-0.20 mm). The maximum value of separation selectivity for all xylene isomers was observed on barium sulfate samples modified with 15% sodium chloride solution [69]. According to electron spectroscopy for chemical analysis these samples contained on the surface about 2% of... 

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