Cluster assisted water dissociation mechanism in MOF-74 and controlling it using helium

Abstract

We show that the water dissociation reaction H2O → OH+H in the confined environment of MOF-74 channels can be precisely controlled by the addition of the noble gas He. Elucidating the entire reaction process with ab initio methods and infrared (IR) spectroscopy, we prove that the interaction between water molecules is critical to the formation of water clusters, which reduce the dissociation barrier by up to 37% and thus influence the reaction significantly. Our time-resolved IR measurements confirm that the formation of these clusters can be suppressed by introducing He gas, providing unprecedented control over water dissociation rates. Since the water dissociation reaction is the cause of the structural instability of MOF-74 in the presence of water, our finding of the reaction mechanism lays the groundwork for designing water stable versions of MOF-74 as well as understanding water related phenomena in MOFs in general.

Controlling Chemical Reactions in Confined Environments: Water Dissociation in MOF-74

“…The alcohol/D 2 O mixture was then introduced into the main chamber and allowed to stabilize for 10 min, after which the temperature was raised to 180 • C. Spectra were recorded as a function of time until stabilization of the 970 cm −1 peak was reached (~8 h). For completeness, the results are compared to data previously reported by our group using He and pure D 2 O [14]. Measurements with pure MeOH and its deuterated analog (MeOD) are also presented as reference in the Supplementary Information (See Figure S2).…”

“…The IR spectra were recorded until the intensity of the 970 cm −1 peak stabilized. In this manner, the kinetics of methanol/D 2 O and isopropanol/D 2 O mixtures could be compared for different alcohol pressures with previously reported data with either pure D 2 O or inert gas/D 2 O mixtures [14], using the integrated area of the 970 cm −1 peak as a quantification of D 2 O dissociation. Results with pure methanol and pure deuterated methanol are also presented for comparison in the supplementary information (see Figure S2).…”

“…The reaction with water (H 2 O) itself follows of course the same dissociation pathway, but the spectroscopic signature is impossible to detect with H 2 O as hydrogen blue shifts the peak outside the phonon gap of the MOF, where it strongly couples to many other modes. It was also demonstrated that the formation of water networks within the MOF pores was crucial to further lower the corresponding reaction barrier via a proton-exchange mechanism and that physical obstruction of this network with inert molecules (He, Ar) hinders the reaction [14].…”

“…Although understanding the mechanism of water dissociation at surfaces is of paramount interest, its study on flat oxide surfaces is unfortunately highly non-trivial-mostly due to experimental challenges in structural characterization and high pressure required (on the order of 20 atm) for in-situ studies on flat surfaces [1,7,[9][10][11][12]. In this respect, metal organic frameworks (MOFs) are attractive systems to study reactions since they are well-controlled crystalline environments with well-defined metal oxide centers [3,4,13,14], in which high gas densities can be achieved at relatively low external pressures. MOFs are networks of metal ions linked by organic ligands, forming a well-characterized cross-linked structure.…”

“…In particular, Zn-MOF-74 exhibits a strong affinity to water, making it ideal for examining various water reaction mechanisms [30]. We have already studied the reaction of water molecules alone in Zn-MOF-74 [2,3,14,31], finding direct evidence for water dissociation at only 150 • C-this was achieved through the use of D 2 O instead of H 2 O, observing a clear fingerprint peak at 970 cm −1 that corresponds to a O-D bending vibration of OD groups formed upon deuteration of the organic linker [3]. The concerted action between open metal sites and linker phenolate group plays an essential role in breaking up water molecules since water molecule establish strong coordinative bond with metal center via its oxygen and hydrogen bonding interaction with nearby -C-O-moiety from organic linker.…”