Heterogeneous process methanol formation selectivity

At complete oxygen conversion, the selectivity of methanol formation in these experiments was influenced neither by changing the residence time of the mixture in the reactor nor by quenching the products at the reactor outlet. In contrast to [90], in [43], there were no signs of any oscillations as claimed by the authors, the process was stable and reproducible. In general, the heterogeneous nature of the process in this study remains the most likely explanation of its high characteristics, substantially different from the results of most other studies. 

In the experiments performed in [43], increasing the total pressure of the methane mixture with 6.5% O2 from 10 to 50 atm by diluting it with CO2 decreased the temperature of complete oxygen conversion and significantly increased the selectivity of methanol formation however, these effects may be associated with a general increase in the pressure. In addition, in these experiments, heterogeneous processes could play a significant role, as discussed above. 

On the other hand, studying the influence of the surface material on the DMTM process in one [140] of a series of works, where a very high selectivity of methanol formation (P = 30 atm, T = 350 °C, tj- > 100 s) was observed, revealed no significant differences in the selectivity and yield of methanol in reactors with different surfaces, such as Pyrex, Teflon, stainless steel, silver, and copper. In these experiments, the selectivity of methanol formation on all surfaces reached values close to 90% or more, whereas the methanol yield was as high as 10.7%. However, the authors do not exclude the possibility of influence of the surface material on the temperature and reaction time. In particular, it was found that the maximum selectivity in reactors with metal surfaces was achieved at temperatures higher by nearly 50 °C. This is probably due to the fact that, at lower temperatures in the presence of a metal surface, the oxidation occurs mostly in the heterogeneous mode with the formation of mainly deep-oxidation products. 

A different type of chemistry has been realized in the process commercialized by EniChem in 1986, which is based on employment of and the titanium-silicalite TS-1 as heterogeneous catalyst [106, 111]. The hydroxy lation mechanism involves activation of hydrogen peroxide via the formation of a titanium hydroperoxo complex, TiOOH, followed by electrophilic oxygen atom transfer to phenol. Methanol and acetone are the solvents of choice to achieve high selectivity. The nature of solvent, phenol concentration, reaction time, and size of catalyst particles affect the CAT/ HQ ratio. The TS-l-based process offers clear advantages in terms of conversion, selectivity, efficiency (see Table 14.1), catalyst separation/recycling, and, hence, environmental impact. 

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