Saturated hydrocarbons, adsorption

As stated earlier, these hydrocarbons are difficult to quantify with accuracy. The FIA method, which is a chromatographic adsorption on silica, gives volume percentages of saturated hydrocarbons, olefins and aromatics. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

Saturable dye absorber, 14 677 Saturated aqueous salt solution, 9 34 Saturated calomel electrode (SCE), 9 571 Saturated fatty acids, 10 829, 830 Saturated hydrocarbons adsorbent affinity, 1 674 adsorption by zeolites, 1 624 fluorine reactivity with, 11 831 isomerization of, 12 172—173 Saturated polyester resins, based on trimethylpentanediol, 12 673 Saturated polyesters, 10 7 Saturated synthetic rubber, 10 705 Saturation and coating processes, 10 12-13 Saturation bonding, 17 509-510 Saturation color, 19 262 Saturation concentration, 15 677 Saturation index... 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Heats of adsorption measurements do not lead to very specific interpretation since the isosteric heat of adsorption (AH) arises from both nonspecific interactions, which occur in all cases of adsorption, and from specific interactions with the hydroxy groups nevertheless, valuable conclusions about the binding forces can be deduced. Saturated hydrocarbons, e.g., n-pentane, have a value of — AH of 8.0 kcal/mole, while saturated ethers have values of around 16 kcal/mole.14 Probably dispersion forces only are involved in the former case and additional specific interaction with the silanol-OH occurs in the second case. On graphite, where there is no specific interaction, the heats of adsorption of hydrocarbons and ethers are very similar.17 The heat of adsorption of furan (11 kcal/mole) is 5 kcal/mole less than that of tetrahydrofuran this again indicates the effect that delocalization of electrons by the double bonds has on the binding forces.14... 

The values of — AH for benzene are in the range 10-12 kcal/mole,15,19-20 being intermediate between values attributed to pure dispersion forces for saturated hydrocarbons and those in which more specific forces are involved. Furthermore, Ron and coworkers calculated the entropies of adsorption for benzene and concluded that the mobile gas model of adsorption was applicable, and Whalen18 found no simple relationship between the hydroxy site content and benzene adsorption. These results confirm the conclusions reached from the infrared data that benzene adsorption is essentially due to dispersion forces which should be greater than with saturated compounds, and that no hydrogen bonding is involved. 

At room temperature, ethylene is adsorbed, without fragmentation, by opening of the double bond and formation of two Ni—C bonds similarly, benzene appears to be adsorbed by six-site (Ni—C) attachment. Adsorption of saturated hydrocarbons involves fragmentation with the formation of Ni—H, as well as Ni—C, bonds. At 100° ethylene also undergoes fragmentation. 

In most cases the slow step of the reaction is not simply the activation or chemisorption of hydrogen, but involves other chemisorbed species. Thus, the exchange of deuterium with methane and with other saturated hydrocarbons is much slower than with hydrogen and probably proceeds through dissociative adsorption of the hydrocarbon. 

The adsorption of saturated hydrocarbons on metallic substrates is typically considered as an example of a weak physical interaction, which is dominated by van der Waals forces. The classification of this type of interaction, denoted physisorption where no direct chemical bonds are formed between the adsorbate and substrate, has been based on the heat of adsorption. A physisorbed state is considered to be one in which the heat of adsorption is comparable to the heat of vaporization or... 

Adsorption of saturated hydrocarbons on a Cu substrate provides a good model system for investigating the electronic structure since the rf-band interaction appears entirely in the occupied states, making the effects more clearly visible and the analysis of the electronic structure easier. There is an advantage to use the (110) surface with a two-fold symmetry if the molecule adsorbs with preferential alignment allowing projection of the electronic structure in three directions as discussed in the previous sections. 

At an early stage, Eischens and Pliskin (3) reported that no absorption bands were observed when ethane was in contact with hydrogen-covered Ni/Si02, but when the catalyst was hydrogen-free, essentially the same absorptions were obtained as from the adsorption of ethene (Fig. 13, Part I). The latter spectrum implied the presence of both unsaturated and saturated hydrocarbon groups. 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

The magnitude of the work function change associated with the adsorption of unsaturated hydrocarbons where 7i-electrons make major contributions to the bonding is in the range —1.3-2.0 V. Saturated hydrocarbons that were studied produce much smaller work function changes, in the range... 

The compounds studied were divided into two groups, based on their adsorption activity, as highly reactive and relatively unreactive compounds. Organic compounds adsorb through electron donation to the metal surface (Fischer et al., 1977 Stair, 1982). The two saturated hydrocarbons, n-hexane and... 

Adsorption Using data from Table 5.1 Adsorption of organic compounds on the metal surface under static conditions and on the nascent steel surfaces , discuss the adsorption activity for each of the following (a) saturated hydrocarbons (n-hexane, cyclohexane), (b) the compounds which have rr-electrons (benzene, 1-hexene), (c) compounds with functional polar groups (propylamine, propionic acid). 

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 strength of adsorption of unsaturated hydrocarbons by a polar adsorbent (zeolite) is much greater than for saturated hydrocarbons, and increases with increasing carbon number (Fig. 3) (5). This observation maybe understood as a consequence of the increasing polarizability of molecules with increasing numbers of bonds and the presence of dipole and stronger quadrupole moments in the unsaturated hydrocarbons compared to the saturated hydrocarbons. 

Paramagnetic ions are now being used quite extensively to study adsorption phenomena. Mn ions have been used as probes for studying molecular motion in synthetic zeolites, (350) Co and Ni ions have been used for studying the complexation of molecular hydrogen on the surface of zeolites, (351) and these same ions have been used in a variety of studies of adsorption on Aerosil surfaces. (352-358) Adsorbed molecules studied include olefins, saturated hydrocarbons, alcohols, and benzene. From the measured line-shifts the number of active surface sites can be deduced in favourable cases. (357, 358)... 

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