Methanol activation barriers

The reason for the lack of methanol decomposition on Pt(lll) has been explained using DFT results. On a 1/4 ML covered surface, the activation barriers for the formation of hydroxymethyl (CH2OH) and methoxy (CH30) have been calculated to be 0.96 eV and 1.47 eV, respectively.92 For a 1/9 ML covered surface, the barriers have been calculated as 0.63 eV and 0.81 eV, respectively.96 These barriers are significantly larger than the desorption barriers. Thus, adsorbed methanol will desorb prior to decomposition in UHV. 

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

As expected, the solvent effect of propane on Reaction 7.1 is negligible. On the other hand, both methanol and water strongly stabilize the TS. In methanol, the TS is stabilized by 2.4 kcal mol while in water, its stabilization is 4.2 kcal mol This is similar to the experimental value (3.8 kcal moL ) for the lowering of the activation barrier in water relative to isooctane. ... 

In our laboratory, we have studied the coupling of two methanol molecules at the acid site of chabazite (see Fig. 1) (Giurumescu and Trout, 2001). This is hypothesized to be an important elementary step in the formation of the first carbon-carbon bond in methanol-to-olefins processes. Because this step has a significant activation barrier, we have chosen to use the method of constraints, with the constraint being the carbon-carbon distance. Simulations were performed at 673 K for 1.5 ps at each point. Our free energy profile is shown in Fig. 15. 

The similar C-H o-bond activation by platinum(II) sulfonate complex was theoretically investigated [36-38]. This is a model of the methane-to-methanol conversion by the platinum(II) complex in dry sulfuric acid experimentally reported [3]. There are two possible reaction courses in the C-H o-bond activation one is the oxidative addition of the C-H o-bond to the platinum(II) complex and the other is the metathesis of the C-H o-bond with the Pt-OSOjH moiety. Hush et al. proposed that the C-H o-bond activation took place through the oxidative addition to the platinum(II) complex [36]. This is not surprising because the oxidative addition of methane to the coordinatively unsaturated platinum(II) complex is not very difficult, as discussed above. However, Ziegler et al. reported that the metathesis could take place with the similar activation barrier to that of the oxidative addition [37]. Recently, Goddard et al. clearly concluded that the metathesis more easily proceeded than the oxidative addition in sulfuric acid [38]. They investigated this reac-... 

Okamoto et al. [47] examined the homolytic dehydrogenation of methanol in an aqueous media and found that only the C-H activation step was favorable. They had not, however, considered the heterolytic path. We calculate that the heterolytic activation of methanol is exothermic at —43 kJ/mol [24,25] (Eq. 19.8) and is expected to have a lower activation barrier as shown for heterolytic vs. homolytic water activation [40] (vide supra). 

There are also indications that the adsorption of small molecules on coal, such as methanol, occurs by a site-specific mechanism (Ramesh et al., 1992). In such cases, it appears that the adsorption occurs first at high-energy sites but with increasing adsorption the (methanol) adsorbate continues to bind to the surface rather than to other (polar) methanol molecules and there is evidence for both physical and chemical adsorption. In addition, at coverages below a monolayer, there appears to be an activation barrier to the adsorption process. Whether or not such findings have consequences for surface area and pore distribution studies remains to be seen. But there is the very interesting phenomenon of the activation barrier which may also have consequences for the interpretation of surface effects during coal combustion (Chapters 14 and 15). As an aside, adsorption studies of small molecules on coal has been claimed to confirm the copolymeric structure of coal (Milewska-Duda, 1991). 

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