This ratio captures the fraction of methoxies that interact with gas
phase methanol of either kind to form acetaldehyde, with larger values
indicative of greater relative contributions from methanol fed at the
inlet versus those formed in-situ. As expected, acetaldehyde
formation:methoxy consumption ratios of 0.5 were observed when the
extraction was carried out at 373 K in the presence of 0.35 kPa water
due to the absence of co-fed methanol (Table 3). Interestingly, these
ratios were still found to lie in the vicinity of 0.5 when 0.35 kPa
methanol was co-fed with 0.35 kPa water, suggesting that fed methanol
contributes negligibly toward acetaldehyde formation, which instead
results exclusively from C-C bond formation events involving methanol
generated in-situ. A possible reason for the lack of participation of
co-fed methanol may be the significantly slower movement of methanol
through the MOF bed compared to water due to methanol outcompeting water
from the standpoint of its affinity to open metal sites. Co-fed methanol
is precluded from participating in secondary reactions due to the slower
movement of its front through the MIL-100(Cr) bed, which results in fed
methanol accessing only those regions of the bed that have been already
been depleted of methoxies through interactions with the more rapidly
progressing water concentration front. Such displacement of water by
methanol is consistent with the rollover of water to flow rates
exceeding those at the inlet (Figure 8b)- flow rates that likely
accelerate the progress of methanol and acetaldehyde fronts generated
in-situ. Reducing the water concentration to 0.12 kPa in the absence of
co-fed methanol, on the other hand, reduces methanol and acetaldehyde
formation rates to values below the detection limit of the mass
spectrometer (Figure 8a). Introduction of equimolar water-methanol feeds
at these pressures (0.12 kPa each) result in approximately the same
number of moles of acetaldehyde formed as methoxies consumed, consistent
with the lack of water methoxy interactions at these low water pressures
(Table 1). The reaction of methoxies exclusively with methanol (but not
water) at identical pressures of each reactant captures the propensity
of MIL-100(Cr) to form C-C bonds, and the resulting prevalence of C-C
bond formation steps at water pressures lower than those required for
methoxy-water interactions. These interactions are significantly more
challenging to deconvolute under conditions where C-C bond formation can
also occur between methoxies and methanol molecules that result from
water-methoxy interactions. The data presented in Table 3 suggest that
exercising precise control over the relative preponderance of
water-methoxy and methanol-methoxy interactions is highly non-trivial
due to the fact that water-methoxy interactions can an increase local
methanol concentrations that in turn make secondary reactions of
methanol more probable, and may constitute part of the explanation as to
why increasing water partial pressures appear to have an outsized effect
on methanol-methoxy interactions compared to water-methoxy interactions
(Figure 10).
Table 3. Comparison of the cumulative moles of
CH4 reacted and C2H4O
formed over MIL-100(Cr) when the product extraction step is conducted
under different partial pressures of H2O and
CH3OH at 373 K. C2H4O to
methoxy ratios are determined for 1 mol CH3O formed per
mol CH4 reacted. (Reaction conditions: 473 K, 2.9 kPa
N2O, 1.5 kPa CH4, 2 h, activated at 523
K in He for 12 h).