Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Brønsted Acidity: The Role of the Hemilabile Linker.

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.

Insights into electrolytic stabilization with weak polarization as treatment for archaeological copper objects.

Immersion of corroded copper artefacts in dilute sodium sesquicarbonate solution is a well-recognized stabilization technique--especially in the conservation of objects recovered from marine environments and therefore saturated with chlorides. Here we describe three linked experiments performed to investigate a variation on this treatment, involving the application of a low potential to the artefact in order to drive the chloride extraction process. This includes a new spectroelectrochemical approach which allows 2-D pseudorandom X-ray reflection diffraction patterns to be obtained without interrupting the reaction in solution. Experiments were carried out on synthetically produced chloride layers on copper (nantokite and atacamite). We show that a thick chloride layer is, in general, replaced by a thin cuprite layer through a mechanism which involves detachment of the chloride crystallites from the surface prior to dissolution.

Evaluation of corrosion potential measurements as a means to monitor the storage and stabilization processes of archaeological copper-based artifacts.

The focus of this study consists of examining how corrosion potential measurements can contribute in providing information on the effectiveness of storage and stabilization treatments of copper alloys in aqueous solutions. We report on the electrochemical behavior of artificial copper alloy coupons (covered or not with corrosion layers), simulating the behavior of real artifacts, immersed in sodium sesquicarbonate solutions. Particular attention is given to the transformation of the corrosion layer as a function of time. In addition, synchrotron radiation X-ray diffraction measurements are performed before and after the treatment in order to understand the reactions that take place during the immersion processes.