Chemical Engineering of Photoactivity in Heterometallic Titanium–Organic Frameworks by Metal Doping

Abstract

We report an ew family of titanium-organic frameworks that enlarges the limited number of crystalline,p orous materials available for this metal. They are chemically robust and can be prepared as single crystals at multi-gram scale from multiple precursors.T heir heterometallic structure enables engineering of their photoactivity by metal doping rather than by linker functionalization. Compared to other methodologies based on the post-synthetic metallation of MOFs,our approach is well-fitted for controlling the positioning of dopants at an atomic level to gain more precise control over the band-gap and electronic properties of the porous solid. Changes in the band-gap are also rationalized with computational modelling and experimentally confirmed by photocatalytic H 2 production.Metal-organic frameworks (MOFs) are crystalline,molecular solids built from the linking of organic and inorganic components with coordinative bonds.M OFs feature incomparable chemical diversity and sizeable three-dimensional porosity ideal for applications like gas storage,s eparation or catalysis. [1,2] However,t hey often suffer from poor chemical stability-particularly in humid conditions-limiting their performance and preventing large-scale application. [3] Chemically robust MOFs can be produced by using basic nitrogenated linkers [4,5] or highly charged metals like Zr or Hf IV , [6][7][8] for endowing the framework with stronger metal-linker bonds less prone to hydrolysis.Compared to these metals,t itanium is naturally more abundant and features advantageous properties like low toxicity,redox versatility and potential photocatalytic activity. However,t he synthesis of carboxylate-bridged Ti IV ,c rystalline,p orous materials remains still as ynthetic challenge. [9] This is arguably due to the high reactivity of Ti precursors, which are prone to hydrolysis in the solvothermal conditions conventional to MOF synthesis to form amorphous TiO 2 .As result, only af ew Ti IV -MOFs like MIL-125, [10] NTU-9 [11] and COK-69 [12] synthesised from simple Ti precursors or PCN-22 [13] and -by using preformed clusters-have been prepared by direct reaction with polycarboxylate linkers.Applications of Ti-MOFs in photocatalysis are continuously expanding due to the unique properties that can arise from the combination of high surface area, crystallinity good photostability and photoactivity. [15,16] Moreover,compared to traditional inorganic semiconductors like TiO 2 ,photocatalytic activity can be finely tuned by direct modification of the organic linker to enhance light absorption in the visible region. This can be attained by functionalization of the linker with substituents to shift light absorption as exemplified by NH 2 -MIL-125. [17][18][19] Still, other routes often used with oxide semiconductors remain underexplored. Here we present an ew family of chemically robust, photoactive titaniumorganic frameworks coined MUV-10 (MUV = Materials of Universidad de Va lencia), that can be prepared as single crystals at multi-gram scale.C ompared to other...

Sodium-coupled electron transfer reactivity of metal–organic frameworks containing titanium clusters: the importance of cations in redox chemistry

“…MIL-125 and NH 2 -MIL-125 have previously been shown to be redox-active, like other titanium-oxo containing MOFs. 23,30,37,38 UV irradiation (or visible for the amino derivative) in the presence of an alcohol causes reduction of the MOF, as indicated by the blue colour of the reduced material. EPR spectroscopy suggests that the reduction is Ti-based.…”

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

“…MUV-10 (Ca) and MUV-10 (Mn) were synthesized according to a literature method. 53 ( Briefly, a solvothermal reaction of calcium chloride dihydrate (0.0176 g, 0.12 mmol) or manganese (II) chloride tetrahyrate (0.0238 g, 0.12 mmol), titanium (IV) isopropoxide (36 μL, 0.12 mmol) and trimesic acid (0.125 g, 0.595 mmol) in DMF (12 mL), using glacial acetic acid (3.5 mL) as a modulator for crystal growth, is conducted at 120 ºC for 48 hours in a 50-mL Pyrex Schott bottle.…”

“…Here, we report that the Ti-MOF known as MUV-10(M) 53 can be expanded into an isostructural family of materials with clusters that incorporate a range of redox-inactive metal ions and Ti centers that support OMSs. Measurement of band gap (HOMO-LUMO) energies reveals that while the identity of the redox-inactive ions electrostatically shifts the Ti-based orbital energies, their dominant effect is to introduce structural flexibility that strongly impacts optical absorption profiles, especially in the presence of solvent.…”

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

“…Following a procedure adapted from Castells-Gil et. al., 53 trimesic acid (0.125 g, 0.60 mmol) and either barium chloride dihydrate (0.044 g, 0.18 mmol), strontium chloride hexahydrate (0.048 g, 0.18 mmol), or cadmium chloride hemi(pentahydrate) (0.041 g, 0.18 mmol) were added to a 50-mL Schott bottle dissolved in dry DMF (12 mL). The solution was then transferred to a benchtop N2 glovebox, where glacial acetic acid (3.5 mL for Cd/Sr, and 5.0 mL for Ba) and titanium (IV) isopropoxide (36 μL, 0.12 mmol) were added, and the bottle was resealed.…”