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).