Adsorbed cyclohexadiene

At a pressure of 1 x 10 5 Torr 1,4-cyclohexadiene, the surface shows ordered domains of hexagonal rings approximately 18 A in diameter made up of six adsorbed molecules with an intermolecular distance of approximately 10 A (Figure 7.22a). A ( 43 x y/43)i 7.6° structure was proposed as schematically shown in Figure 7.22b. 1,4-Cyclohexadiene adsorbs in a boat configuration on bridge or hollow sites on the Pt (1 1 1) surface with comparable adsorption energies of 145.6 and 141.6kj mol-1, respectively [39]. 

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

If various feeds give the same TPR spectrum for their end product, a common rate determining step can be assumed. This was the situation when TPR spectra of benzene formed over Pt-AljOj from adsorbed n-hexane, 1-hexene, and 1,5-hexadiene were studied. This re-confirms the hexane-hexene-hexadiene stepwise mechanism since cyclohexane, cyclohexene, and cyclohexadiene gave another type of TPR spectrum (62b). 

The VEEL spectrum of 1,3-cyclohexadiene on Pt(lll) at 95 K has already been discussed in relation to the thermal evolution of cyclohexene adsorbed on that surface (Section VI.A.l and ref 240). Palazov and Shopov et al. (241, 242) reported that cyclohexadiene adsorbed on Ni/SiOz at room temperature decomposes into benzene on evacuation. SER spectra of 1,3-and 1,4-cyclohexadiene have been obtained indicating 77-complexes on a gold electrode (83), and the general results have been discussed earlier in Section VI.A.2. 

In zeolites the mobility of hydrocarbon molecules with double bonds is specifically restricted because of a specific interaction between the 7r-elec-trons and the zeolite (2). As expected, proton spin relaxation of benzene, cyclohexadiene, cyclohexene, and cyclohexane adsorbed on NaY reveals an increasing restriction of mobility with increasing number of -electrons (8, 4, 8). This is shown in Figure 1, where the longitudinal (7 ) and transverse (T2) proton relaxation times are plotted. 

Adsorption of ethylene [71], cyclopentene and cyclohexane [72], and cyclohexadiene [73] on Ge(100)-(2 x 1) has been observed, but the resulting monolayers are disordered. FTIR spectra of adsorbed cyclopentene [72] imply the formation of what would be the [2 + 2] cycloaddition product, but while on Si the sticking coefficient of cyclopentene is close to unity, on Ge it is only about 0.1. Conjugated polyenes react with Ge(100) to form predominantly what appear to be [4 + 2] cycloaddition products [73,74], but, in contrast to reactions on Si(100), the chemisorption is weak and reversible [74]. All of these observations of much weaker chemisorption can be rationalized by noting that the Ge-C bond is less stable than the Si-C bond by lOkcalmol-1 [75],... 

Benzene is reduced in 95% current yield to a mixture of 23% cyclohexadiene, 10% cyclohexene and 67% cyclohexane. HMPTA as a solvent additive seems to play a dual role. Firstly it is selectively adsorbed at the cathode surface, thereby preventing hydrogen evolution from the protic solvent. Thus it permits the attainment of a potential sufficiently cathodic for the generation of the solvated electron. It secondly stabilizes the solvated electron, thus suppressing its reaction with protic solvents (eq. (130) ). With decreasing HMPTA concentration in the electrolyte the current efficiency for reduction decreases and hydrogen evolution dominates. In pure ethanol the current efficiency is less than 0,4%. 

A priori, the mechanism of hydrogenation of benzene may be represented as a series of hydrogen transfers from the catalyst to the adsorbed benzene and the adsorbed intermediates (Scheme 5)." The often observed first order reaction in H2 suggests that the addition of the second hydrogen atom is the more difficult step, indicating that the largest energy barrier lies between adsorbed arene and adsorbed diene. However, no cyclohexadienes have been detected as intermediates. 

The last example quoted deals with dehydrogenation, this time of larger molecules, studied by UPS for cyclohexane over Pd(lll) and Pd films. Rubloff et did not study catalysis but chemisorption. At r< 120K adsorption was without decomposition, but at 300 K dehydrogenation gave benzene as the surface species. This was suggested from comparative experiments in which benzene itself was adsorbed. It appears that the 7r-orbitals of benzene are involved in bonding. They found no evidence for the formation of cyclohexene or cyclohexadiene. Their work should be compared with that of Tetenyi et al., who identified such intermediates by radiochemical methods. 

According to the sextet model, the flatly adsorbed cyclohexane is transformed into benzene by means of detaching all six atoms of hydrogen in such a way that the six-raembered ring does not leave the active center until it turns into a benzene ring. Therefore, there is no cyclohexane and cyclohexadiene in the gaseous phase. 

Initially, the MCP adsorbs on the metal and dehydrogenates up to methylcy-clopentadiene, which then isomerize on an acid site to cyclohexadiene. Cyclohexa-diene migrates to the metal and finally gets dehydrogenated to benzene. Neither silica-alumina nor platinum supported on silica is active for this reaction, but a physical mixture of them is active to form benzene from MCP, which show the bifunctional character of this reaction. The metal content has no effect on the reaction rate, what indicate that the acid function is the one that controls the reaction rate. 

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