Figure 9. Cr2+ open-metal and Cr3+-OH- site densities (left axis) with increasing extent of activation and the corresponding quantity of acetaldehyde formed (right axis) when ethanol (0.11 kPa) is fed over MIL-100(Cr) at 373 K following thermal activation in He or vacuum (when indicated).
To further investigate the role of water in methane conversion to methanol/acetaldehyde, methoxy extraction was carried out over MIL-100(Cr) at varying water partial pressures while keeping the total water flow rate constant, and also at varying water flow rates while keeping the water partial pressure constant (Figure 10, Table S7). Regardless of water flow rates and partial pressures, water, methanol, and acetaldehyde break through simultaneously, consistent with methanol formation resulting from primary reactions between surface methoxies and water, and acetaldehyde resulting from secondary reactions between methoxies and gas phase methanol. The primary(/secondary) nature of methanol(/acetaldehyde) is consistent with the water breakthrough time- the time required for the edge of the water concentration front to reach the bottom of the MIL-100(Cr) bed- being identical to the time required for both methanol and acetaldehyde to elute through the bed. The rank of products in these stoichiometric reactions is also consistent with the relative sharpness of their concentration fronts; methanol, which breaks through with water, exhibits significantly sharper concentration profiles compared to acetaldehyde (the secondary product), and elutes from the bed exclusively during the period when water molar flow rates at the exit of the bed lie between zero and that at the inlet. In fact, acetaldehyde fronts are broad enough that acetaldehyde is produced through methanol-methoxy interactions long after methanol ceases to be detected at the outlet of the bed (Figure 10).
Water partial pressures not only enable us to tune the relative number of primary (methoxy-water) interactions to secondary (methoxy-methanol) interactions as shown in Figure 10, but also the possibility of water-methoxy interactions in the first place. The stoichiometric nature of both these interactions suggests that under conditions where acetaldehyde is the exclusive product the ratio of moles of acetaldehyde formed to methoxies consumed should lie between 0.5 and 1, with a ratio of 0.5 indicating that every molecule of acetaldehyde owes its formation to an interaction between a methoxy species with a methanol molecule that is desorbed into the gas phase subsequent to a primary interaction with water (Scheme 2b). A ratio of unity, on the other hand, indicates the exclusive participation in methanol-methoxy interactions of methanol molecules fed at the inlet of the reactor, and the lack of C-C bond formation contributions from methanol molecules generated ’in-situ’ through water-methoxy interactions (Scheme 2c).