One Step Backward Is Two Steps Forward: Enhancing the Hydrolysis Rate of UiO-66 by Decreasing [OH^–]

Abstract

The rapid destruction of chemical threats such as phosphate-based nerve agents is of considerable current interest. The hydrolysis of the nerve-agent simulant methylparaoxon, as catalysed by UiO-66 and UiO-67, was examined as a function of pH. Surprisingly, even though typical phosphate-ester hydrolysis mechanisms entail nucleophilic attack of the simulant by aqueous hydroxide, the rate of hydrolysis accelerates as the solution pH is lowered. The unexpected behavior is attributed to a pH-dependent composition change followed by ligand substitution at the Zr 6 -based node.

Screening for Improved Nerve Agent Simulants and Insights into Organophosphate Hydrolysis Reactions from DFT and QSAR Modeling

“…All simulants studied are indeed less toxic than the nerve agents, where rat oral LD 50 −log 10 values for GD and GB are 5.66 and 5.41 mol kg −1 , respectively. For context, rat oral LD 50 −log 10 =4.88 mol kg −1 for methyl‐paraoxon (DMNP), which is one of the more common simulants used when testing catalysts for detoxification . For precautionary measures, we set the upper bound toxicity criterion at rat oral LD 50 −log 10 <2.50 mol kg −1 .…”

“…All simulants studied are indeed lesstoxic than the nerve agents, where rat oral LD 50 Àlog 10 values for GD [65] and GB [66] are 5.66 and 5.41 mol kg À1 , respectively.F or context,r at oral LD 50 Àlog 10 = 4.88 mol kg À1 for methyl-paraoxon (DMNP), [62] which is one of the more commons imulants used when testing catalysts for detoxification. [67][68][69][70][71] For precautionary measures, we set the upper bound toxicityc riterion at rat oral LD 50 Àlog 10 < 2.50 mol kg À1 .T his upper bound on the toxicity endpoint was chosen to err on the side of caution, given any possible inaccuracies in the T. E.S.T.p redictions.T oo ur knowledge, DMNP,w ith its slightly higher toxicitye ndpoint, has been used safely without incident in many previous studies. One attractive feature of DMNP is its low vapor pressure (3.3 10 À5 mmHg at 25 8C), [62] which reduces the inhalation hazard.…”

When defects turn into virtues: The curious case of zirconium-based metal-organic frameworks

“…prepared with the same procedure can in fact be distinguished and quantified by acid-base titration [120]. Other works from the same group assumed the same type of model, justified by experimental observations [115,121]. A definitive molecular-level characterization of the coordination environment around the defective clusters from SCXRD data was claimed to have been achieved [94].…”

“…In the case of NU-1000, based on eight-connected clusters where one water molecule and one hydroxide group fill each remaining defective site, a significant increase of the catalytic activity results from formation of Lewis acid sites upon dehydration of the clusters. The authors exploited a similar approach on UiO-66 and UiO-67, synthesizing defective samples containing water and hydroxide as compensating groups, finding that also in this case dehydration leads to enhanced catalytic activity towards the hydrolysis of methylparaoxon [115]. The dependence of the catalytic activity on pH was investigated, finding that it decreases at pH > 8.8.…”

“…To explain this evidence, the authors proposed a mechanism (Figure 17 [Reprinted with permission from ref. [115]. Copyright 2015, American Chemical Society.…”