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).