Modulation of the Bifunctional CrVI to CrIII Photoreduction and Adsorption Capacity in ZrIV and TiIV Benchmark Metal-Organic Frameworks

Abstract

The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.

Chitin/Metal‐Organic Framework Composites as Wide‐Range Adsorbent

“…The immobilization of hexavalent chromium anions in MOF-808, [47,64] CH/MOF-808 composites, and CH matrixes is governed by two factors: (i) adsorption at the CH/MOF-808 structure, [65][66][67] and (ii) reduction from Cr VI to Cr III due to the electron donor groups of CH matrix. [19,68] MOF-808 nanoparticles have a near 100 % capacity for the removal of Cr VI species from water; whilst CH and CH/MOF-808 show just a 40 % removal.…”

“…First, it is necessary to synergistically combine micro-, meso-, and macropores within the sorbent. [8] Second, functionalization of the porous scaffold is needed, as this enables to introduce hostguest interactions to trap neutral, anionic, and cationic compounds. [9] Encompassing these characteristics in one sorbent is not straightforward, since well-ordered microporous materials lack the capacity to trap large molecules, whilst macro-and mesoporous polymers usually fail to retain small metal ions.…”

“…To achieve broad‐scope sorption the sorbent materials should comply with at least two requirements. First, it is necessary to synergistically combine micro‐, meso‐, and macropores within the sorbent [8] . Second, functionalization of the porous scaffold is needed, as this enables to introduce host‐guest interactions to trap neutral, anionic, and cationic compounds [9] .…”

Multivariate Functionalization of UiO‐66 for Photocatalytic Water Remediation

“…[13,15] In fact, the technical limitations of current water remediation technologies to capture traces of chromate anions from water media arises from these properties. [16] The photocatalytic reduction of hazardous Cr VI to the less toxic and mobile Cr III [17][18][19][20] has stood out as a particularly promising remediation strategy due to its effectiveness, low-cost, and absence of addition and/or generation of secondary hazardous chemicals during the process. [13,21,22] However, one of the limiting issues of the photoreduction of Cr VI is the poor efficiency to capture the Cr III generated during photocataysts.…”

Visible-Light-Driven Sonophotocatalysis for the Rapid Reduction of Aqueous Cr(VI) Based on Zirconium–Porphyrin Metal–Organic Frameworks with csq Topology

“…Metal–organic frameworks (MOFs), a class of crystalline inorganic–organic hybrid materials, have gathered particular attention in the field of photocatalysis due to their intriguing structures, powerful designability, and high surface areas. − What is more exciting is that the photophysical properties of MOFs can be effectively tuned at the atomic level by virtue of abundant metal, organic secondary building units and their multiple interactions. − In recent years, MOFs as heterogeneous catalysts have made notable progress in photoreduction of Cr­(VI) to Cr­(III). − In general, MOF catalysts with high stability, large specific surface area, broad-band visible absorption and relative high density of catalytic active sites are pursued in Cr­(VI) photoreduction, where the assembly of high-valent metal ions (e.g., Ti 4+ , Zr 4+ and Fe 3+ ) and photoactive polycarboxylate ligands is the most popular strategy. − However, MOF catalysts possess high energy barriers between metal clusters and ligands, which hinders the long-range migration of photogenerated charges in the MOF structure and further limits the photocatalytic performance of MOFs. − In addition, relatively narrow apertures of MOF catalysts also limit the mass transfer rate of the substrate in channels and then restrict the promotion of catalytic efficiency. Inspired by the influence of sonochemistry for heterogeneous catalytic reactions, MOF-based photocatalytic systems with acoustic stimulation should be developed more to give scope to the advantage characteristics of MOF materials in photocatalytic reactions …”