Energetics, hydrocarbon adsorption

The study of zeolites as adsorbent materials began in 1938 when Professor Barrer published a series of papers on the adsorptive properties of zeolites [28], In the last 50 years, zeolites, natural and synthetic, have turned out to be one of the most significant materials in modem technology [27-37], Zeolites have been shown to be good adsorbents for H20, NH3, H2S, NO, N02, S02, C02, linear and branched hydrocarbons, aromatic hydrocarbons, alcohols, ketones, and other molecules [2,31,34], Adsorption is not only an industrial application of zeolites but also a powerful means of characterizing these materials [1-11], since the adsorption of a specific molecule gives information about the microporous volume, the mesoporous area and volume, the size of the pores, the energetics of adsorption, and molecular transport. 

The determination of the position of adsorbed molecules by XRD and/or NMR is only feasible at low temperatures (room temperature and below), where the effects of translational motion are negligible, and limited to pure silica zeolites, with T-sites resolved in the Si MAS NMR, but it does establish a deeper understanding of the energetics of hydrocarbon adsorption as a function of structure and loading. 

The different toxicities found for 1-butene, 1,3-butadiene, and l-butyne hydrogenation can be explained by assuming that the energetic adsorption of unsaturated hydrocarbons destabilizes the metal-sulfur bond producing a real desulfurization with l-butyne. The destabilization exists also with the butadiene, as has been shown on platinum (71). 

In the TSA and substrate-based inhibitor complexes, the sn-1 and 5m-2 hydrocarbon chains lie in a hydrophobic channel that extends from the active site to the surface of the enzyme. If the lengths of the hydrocarbon moieties were increased from the 7 to 8 carbons present in the model compounds to that of natural substrates (commonly 14 to 18 carbons long), they would protrude from the enzyme surface. It is probably safe to assume that these hydrophobic tails remain imbedded in the membrane during interfacial catalysis in order to minimize the energetic costs of substrate transfer. Thus the adsorption surface chosen by Scott et al. (1990a), which envelops the hydrophobic channel, is the most likely surface to contact the bulk phase lipid directly. 

Although nitrogen has been most often used to study the energetic heterogeneity of silica [113,114,163,168-172], other adsorbates such as argon [169], benzene [113], cyclohexane and cyclohexene [138,141,142,146,173-175], n-hexane and n-hexene [135,137], n-heptane [134], chlorinated hydrocarbons [132,134,175,176], diethyl ether [132,134], methanol [132,134], ethanol [134] and pyridine [134] were nsed to probe various types of adsorption sites on the silica surface. 

Besides, it was assumed that only half of the adsorption centers on silica gel surface can be occupied by n-octadecanol molecules and that all these centers are energetically equivalent. Thus, we can consider a surface lattice of sites be composed of two interpenetrating sublattices. At low surface concentration the adsorbed molecules are distributed randomly over the centers belonging to both sublattices. However, when the film density exceeds a certain value of A /Aj (Aj is the surface area occupied by a single molecule) a preferential adsorption on one of the sublattices must begin. This leads to the formation of a highly ordered structure, e.g. the monolayer in two-dimensional solid condensed state. If the interaction between only neighbouring hydrocarbon chains, perpendicularly oriented to the solid surface are taken into account, we can derive the equation of state for the adsorbed film [39]... 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

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