Adsorption of olefins

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

Amiridis followed that also in propene hydrogenation over Pt-Au/Ti02 a minimum ensemble of Pt atoms is necessary for a successful concomitant adsorption of olefin and hydrogen. However, beside this dilution effect of gold on the bimetallic particles, an electronic impact can not be excluded. An application of Amiridis ... 

Olex A version of the Sorbex process for separating olefins from paraffins in wide-boiling mixtures. It can be used for hydrocarbons in the range C6 - C20. Based on the selective adsorption of olefins in a zeolite and their subsequent recovery by displacement with a liquid at a different boiling point. Mainly used for extracting Cn - C14 olefins from the Pacol... 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H2 molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (7tu) and the LUMO (re ) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the... 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

The base-catalysed epoxidation of electron-deficient alkenes was also described1157,1741 and proceeded with excellent conversions and selectivities, when the surface was passivated by silylation. Their high efficiency in the epoxidation of alken-2-one results from their ability to deprotonate H202 leading to an ion pair (HOO-, MTS-TBDH+) and from their lipophilic character, which favours the adsorption of olefin which then reacts via 1 -4 addition (Scheme 9.9). 

If adsorption of olefin is competitive and equilibrium is set up between solution and surface... 

This is carried out by selective adsorption of olefins in the liquid phase on a solid. The distribution of the circuits is designed to simulate a countercurrent exchange between the liquid and solid phases, wthout any effective movement of the adsorbenn... 

Associative adsorption of the hydrocarbon and homolytic dissociation of hydrogen. At higher temperatures dissociative adsorption of olefins may also occur. Desorption over the range 323-773 K gave some CS2 and H2S, and coke remaining on the catalyst led to irreversible poisoning. ... 

There is no evidence as to whether hydrogen or olefin is adsorbed first. Further, there might be an intermediate adsorption of olefin as a tt-complex. 

If isomerization does not result from the addition of one hydrogen atom and the removal of another, it could result from the removal of a hydrogen atom and subsequent addition of hydrogen at another position. We suggest, then, that double-bond migration, process (7), results from heterolytic dissociative adsorption of olefin to a proton and adsorbed allyl. Exchange without isomerization, process (5), will not originate in dissociative adsorption to form -011=011—R since such a process should lead to ethylene-d, which is not observed. Both processes (5) and (7) could result from an allylic intermediate of one of two types. 

In many papers in which the reaction rates or adsorption coefficients of a series of substrates were obtained the effect of structure is discussed only qualitatively. Rilzicka and Cerveny (5) found that the rate of hydrogenation of 1-olefins C6-C17 in ethanol under usual conditions on platinum, palladium and rhodium catalysts decreased monotonically with the chain length. Since in this olefinic series the chain length did not affect (68) the relative adsorptivity of olefins, as demonstrated in Table II, the observed phenomenon may be explained only by the effect exerted by the chain length on the rate constants. Since, moreover, the polar effect of substituents in this series is virtually the same, an explanation can be sought in the steric hindrance of the adsorbed double bond by the free end of the olefin molecule. The trans isomers were adsorbed much more weakly than the respective cis... 

Identification of participants in electrode reactions with high chemical specificity. A knowledge of chemical participants is indispensible to achieving an understanding of electrode processes that will permit manipulation and improvement of important processes, such as the electro-oxidation of methanol or the adsorption of olefins on platinum. Among established techniques for chemical identification, vibrational spectroscopies offer the best opportunities for improvement. The current high level of effort with these techniques should be... 

The catalytic activity of certain activated clays toward hydrocarbons was observed as early as 1912, when Gurwitsch (5) in his studies of the adsorption of olefins on activated clays reported that polymerization occurred. Herbst (6) in 1926 observed that the decomposition of hydrocarbons is accelerated by kieselguhr at moderately elevated temperatures, and Kobayashi and Yamamota (7) obtained similar results with Japanese acid clays. Several patents (8) covering the use of floridin, pumice, and hydrosilicates of aluminum as cracking catalysts were issued in the period 1923-1932. 

Certain sample groups i have been excluded from the plots of Fig. 10-10 because they merit special attention. These include olefinic or aromatic carbon atoms (—CH=), halogen substituents, acids, and moderately strong bases (pKj, < 5). The preferential adsorption of olefins and aromatic hydrocarbons on magnesia has already been noted (Section 7-3B). It is of interest to compare the values of an unsaturated carbon group (—CH=) for the different adsorbents with values calculated from Fig. 

P-06 - Infrared observation of the stable carbenium ions formed by adsorption of olefins on zeolite Y at low temperatures... 

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