Methylacetylene cyclotrimerization

TMB to 1,2,4-TMB was obtained for the methylacetylene cyclo-trimerization. A decade ago, Zecchina et al. revisited this catalytic system and found that 1,3,5-TMB is the only product of the cyclotrimerization of methylacetylene (Zecchina et al., 2003). Recendy, we performed a theoretical investigation on the mechanism of acetylene and methylacetylene cyclotrimerization catalyzed by the PhiUips chromium catalyst (Liu et al., 2012, 2013). 

Figure 3.14 Gibbs free energy profile at 298.15 K of the triplet reaction pathway (PES-T1) for methylacetylene cyclotrimerization over the Cr ll)/Si02 cluster model. The reaction pathway via intermolecular [4 + 2] cycloaddition is depicted in black, while the stepwise pathway is in gray. The reaction to generate 1,3-dimethyle-cyclobutadiene 5Ea is in light black. Energies are in kcal/mol and relative to 1C plus the corresponding number of methylacetylenes.

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

The observation of negative apparent activation energy can most simply be interpreted in terms of the competition between the adsorption and desorption of methylacetylene on the surface. This qualitative explanation is illustrated in Figure 3, where the steady-state production of trimethylbenzene is compared with the TPD trace of methylacetylene. The fall off in steady state cyclotrimerization rate matches the tail of the desorption spectrum and illustrates the role of reactant desorption at higher temperatiu-es controlling the availability of alkyne monomers and thus the overall cyclotrimerization rate in this temperatime/pressure regime. 

The analogous reaction of methylacetylene (H CC CH) on the alloy surfaces, i.e., cyclotrimerization of methylacetylene to form trimethylbenzene (TMB), was not observed [55]. The absence of TMB and the high yield of propylene are likely due to facile dehydrogenation of methylacetylene because of the relatively weak H-CH feCH bond compared to acetylene. The desorption of several hydrocarbon products at low (<170 K) temperature indicated a minor pathway on the surface involving C-C bond formation. 

Phillips chromium catalyst has been primarily used as ethylene polymerization catalyst in a large industrial scale. However, the same catalyst could cyclotrimerize acetylene and methylacetylene into benzene and tri-methylbenzene (TMB) rather than polyacetylene and polymethylacetylene, respectively (McDaniel, 2010). The mechanism has never been studied up to now, which also own particular academic interests in this field. Hogan and coworkers first reported alkyne cyclotrimerization catalyzed by the Phillips chromium catalyst only a few years later after they invented the Phillips chromium catalyst (Clark et al., 1959). In their report, the acetylene was found primarily cyclotrimerized into benzene, while a ratio of 0.18 of... 

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