Olefinic compounds, adsorption, sulfur

This increasing adsorption energy of olefinic compounds by sulfurization of metals can induce a partial desorption of sulfur or else a surface reaction giving thiols, as was pointed out by Oudar et al. (71) in the case of butadiene chemisorption at 100°C on Pt(110) and Pt(lll) faces. 

Highly pure / -hexane can be produced by adsorption on molecular sieves (qv) (see Adsorption, liquid separation) (43). The pores admit normal paraffins but exclude isoparaffins, cycloparaffins, and aromatics. The normal paraffins are recovered by changing the temperature and/or pressure of the system or by elution with a Hquid that can be easily separated from / -hexane by distillation. Other than ben2ene, commercial hexanes also may contain small concentrations of olefins (qv) and compounds of sulfur, oxygen, and chlorine. These compounds caimot be tolerated in some chemical and solvent appHcations. In such cases, the commercial hexanes must be purified by hydrogenation. 

In conclusion, coadsorption of sulfur with different molecules induces a decrease of the adsorption capacity (by a geometrical effect) and a change in the binding energy (by a ligand effect). This change can be a decrease (H2, CO, or saturated hydrocarbon) or an increase (olefinic compounds with high electronic densities on the double bond). 

The IRVAD process by Black Veatch Pritchard Inc. and Alcoa Industrial Chemicals is claimed to be a low-cost process for low-sulflir gasoline . The process uses an alumina-based selective adsorbent to counter-currently contact liquid hydrocarbon in a multistage adsorber. The adsorbent is regenerated in a continuous cross-flow reactivator using heated reactivation gas. The process operates at lower pressure, and does not consume hydrogen or saturate olefins. The adsorption mechanism is based on the polarity of sulfur compounds. It is not very selective for gasoline sulfur, and no information is available on diesel sulfur. 

It is believe that the HDS sites (rim sites and edge sites) are different than the olefin hydrogenation sites (rim sites) opening an opportunity for the development of selective HDS catalysts [45 171. Another concept to exploit in catalyst development is the competitive adsorption, by which the sulfur compounds inhibit olefins hydrogenation [48]. 

However, the sulfurization of the surface palladium has a detrimental effect on the consecutive hydrogenation (Fig. 17) the olefin yield is lower on sulfided palladium. Such a result is not in accordance with the literature cited above and this difference can be explained by differences in experimental conditions the improved consecutive selectivities were obtained with the sulfur present in the feedstock, i.e., in adsorption competition, when the detrimental effect on the consecutive hydrogenation was pointed out on partly sulfided palladium without sulfur compounds in the feedstock. The selectivity for olefin production in the hydrogenation of iso-prene is increased by the presence of sulfur in the substrate (Fig. 18). 

Because sites that are active for adsorption also tend to be active catalytic sites, compounds in the streams that would not normally cause deactivation may react to form nondesorbables. Olefins, diolefins, and other unsaturated hydrocarbons are especially difficult since they easily polymerize to long-chain species in the presence of high-surface-atea solids. Hydrocarbons in the presence of oxygen can form oxygenated species such as aldehydes and ketones, which can further react by aldol condensation to finrn heavier components. The presence of oxygen with sulfur compounds can create elemental sulfur. 

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