A Tunable Multivariate Metal–Organic Framework as a Platform for Designing Photocatalysts

Catalysts for photochemical reactions underlie many foundations in our lives, from natural light harvesting to modern energy storage and conversion, including processes such as water photolysis by TiO2. Recently, metal–organic frameworks (MOFs) have attracted large interest within the chemical research community, as their structural variety and tunability yield advantages in designing photocatalysts to address energy and environmental challenges. Here, we report a series of novel multivariate metal–organic frameworks (MTV-MOFs), denoted as MTV-MIL-100. They are constructed by linking aromatic carboxylates and AB2OX3 bimetallic clusters, which have ordered atomic arrangements. Synthesized through a solvent-assisted approach, these ordered and multivariate metal clusters offer an opportunity to enhance and fine-tune the electronic structures of the crystalline materials. Moreover, mass transport is improved by taking advantage of the high porosity of the MOF structure. Combining these key advantages, MTV-MIL-100(Ti,Co) exhibits a high photoactivity with a turnover frequency of 113.7 molH2 gcat.–1 min–1, a quantum efficiency of 4.25%, and a space time yield of 4.96 × 10–5 in the photocatalytic hydrolysis of ammonia borane. Bridging the fields of perovskites and MOFs, this work provides a novel platform for the design of highly active photocatalysts.

E nergy and the environment are among the most important issues of society nowadays. Solar energy, a clean and renewable source, can be efficiently harnessed by photocatalytic processes to tackle such challenges. The efficiency of photocatalysis in reactions such as hydrogen production and the degradation of organic molecules is highly dependent on the catalyst, which can create electron−hole pairs and generate free radicals for secondary reactions. Since the discovery of water photolysis by TiO 2 , 1 it has become the predominant material for photocatalysis due to its high efficiency, high stability, and abundance.
Among the various materials developed as photocatalysts, 2−6 metal−organic frameworks (MOFs) are a family of inorganic− organic hybrid materials with permanent porosity and can have extraordinarily high surface areas. 7,8 More importantly, compared to TiO 2 -based materials, MOFs provide great opportunities in the design of building units using supramolecular and reticular chemistry. 9 By a rational selection of metal nodes and organic linkers, the optical response, electronic structures, and binding energetics of MOFs can be tailored. These aspects make MOFs a promising class of photocatalysts for applications such as artificial photosynthesis, 10−14 degradation of organics, 15−17 and water oxidation. 18−21 Moreover, multiple metals can be incorporated in a MOF structure. The resulting materials then belong to the subclass of multivariate (MTV)-MOFs, which are characterized by the possibility of having synergetic effects between different components in order to achieve optimal performance. 22−29 In the context of photocatalytic properties, Ti-oxo clusters are attractive secondary building units. By acting as analogues of TiO 2 nanoparticles, high chemical stability, redox activity, and photocatalytic properties are expected. 30−33 However, the formation of Ti-oxo clusters is highly sensitive to reaction conditions, which are often incompatible with those for MOF crystallization. Consequently, less than 30 Ti-based MOFs have been reported among the more than 80 000 different MOF structures, 34 and Ti-based MTV-MOFs are even more scarce. 29 Doping with a second metal element is a widely adopted approach to prepare mixed-metal MOFs. 35 However, doping is hard to control and the resulting materials often exhibit compositional gradients, e.g., from the surface to the center.
Here, we present a solvent-assisted approach that leads to a highly tunable platform for the design and synthesis of MOFs. The core of the platform is based on trinuclear clusters with a well-defined atomic structure of AB 2 OX 3 , where the transition metal ions on the A site connect with two other ions on the B sites through a bridging μ 3 -O, while X sites (−OH or H 2 O) coordinate to the A and B sites as terminal species (Figure 1). Similar to perovskites, it therefore offers great advantages over conventional materials such as metal oxides and carbon-based materials for fine-tuning the acquired compositions and corresponding electronic structures for a wide range of catalysis. To focus on the photocatalytic applications, we present a series of trinuclear clusters featuring Ti(IV) in the A site. Different transition metals (Co, Ni, Mn, Fe) residing in the B sites are used for fine-tuning the electronic structure and enhancing catalytic cooperativity.
Specifically, the reaction was carried out in a solvent mixture of ethylene glycol (EG) and N,N-dimethylacetamide (DMA). As EG can simultaneously coordinate to A and B cations, it offers protection against moisture and prevents A and B cations from undergoing an otherwise rapid hydrolysis, which would result in dense phases. The high excess of EG (EG/B 2+ = 1800, molar ratio) also serves as a competing reagent to 1,3,5-benzenetricarboxylate (BTC) linkers to control reaction kinetics and crystal growth. In addition, due to the high EG content, which leads to an acidic environment, X sites are favored to be H 2 O instead of OH − . This promotes the ordered organization of A and B cations to minimize the charge during the formation of AB 2 OX 3 clusters (Figures S1−S5). A low amount of EG resulted in smaller crystal sizes and lower crystallinities ( Figure S6). After linking the clusters with BTCs, the resulting materials exhibit an octahedral morphology with a 2−5 μm diameter (Figures S7, S8 and Movie S1). The powder X-ray diffraction (PXRD) pattern shows sharp and intense peaks indicating a high crystallinity (Figures 1 and S9). The MOF crystallized in a large cubic unit cell with a = 73.6449(1) Å. Rietveld refinement against synchrotron PXRD data ( Figure  1d and Table S1) revealed that the crystals have a MIL-100 topology 36 (denoted as MTV-MIL-100(Ti,B)), with pore diameters of 25−30 Å (Figure 1b and c).
Elemental analysis shows a constant A:B molar ratio of 1:2 in a wide range of starting ratios (Figures 1f and S2−S5), which confirms the consistent formation of AB 2 OX 3 clusters. X-ray absorption near edge structure (XANES) and X-ray absorption spectroscopy (XAS) analyses were performed to further understand the local coordination environment of the AB 2 OX 3 clusters. As shown in Figures 1e and S10, MTV-MIL-100(Ti,Co) has a distinct XANES spectrum, and the pre-edge peaks, when compared to bulk Ti-metal, rutile, and anatase references, indicate a different coordination environment. Specifically, the asymmetric peak in the low-energy region ( Figure S10, indicated by A1) and the absence of the high-energy peak ( Figure S10, indicated by B) suggest a distorted coordination geometry, 37,38 which resulted from the removal of terminally bonded solvent X in the AB 2 OX 3 clusters. Densityfunctional theory (DFT) calculations were further applied to analyze the coordination geometry of Ti(IV). The results show that the Ti(IV) ion is closer to the central oxygen atom, causing distortions from an octahedral geometry ( Figure S11). The dihedral angle between Ti(IV) and three of the four O atoms of the formate groups is ∼15°, which is deviated from 0°, as would be expected in an ideal octahedral coordination geometry. Moreover, the continuous Cauchy wavelet transform (CCWT) of the spectra shows an asymmetric wavelet for the MTV-MIL-100(Ti,Co) local environment, which confirms the heterogeneity of the cluster in the Ti−O−Co linkage (Figures S12−S14). In addition to the XAS analysis, Raman spectroscopy also indicated that the metal−metal linkages differ significantly from those in single-component crystals, such as MIL-125(Ti) ( Figure S15).
The strong interaction between Ti(IV) and O atoms endows MTV-MIL-100(Ti,Co) with high chemical stability as its structure remains intact after being treated with highly acidic and basic aqueous solutions (Figure 2a). The permanent porosity of MTV-MIL-100(Ti,B) was confirmed by the analysis of N 2 sorption isotherms (Figures S16−S19). MTV-MIL-100(Ti,Co) shows a high Brunauer−Emmett−Teller (BET) surface area of 1695 m 2 g −1 (Figure 2b). Notably, each AB 2 OX 3 cluster in MTV-MIL-100 is accessible via their mesopores (Figure 1b), which offers a much better masstransport pathway compared to traditional photocatalysts such as TiO 2 . Furthermore, the terminal X sites can cleave from the cluster upon activation, leaving open metal sites for binding reaction species. The ordered AB 2 OX 3 cluster offers a platform to adjust the electronic structure of the resulting MOF. As calculated from ultraviolet−visible (UV−vis) spectra ( Figure  2c), MTV-MIL-100(Ti,Co) has an optical band gap of 2.7 eV, which is within the range for visible light. By further altering the transition metal ions of the B sites, the optical band gap can be tuned from 2.3 eV to 3.1 eV ( Figure S20). The  composition of MTV-MIL-100(Ti,Co) was also confirmed by X-ray photoelectron spectroscopy (XPS) analysis, which further indicated the presence of both Co(II) and Co(III) sites, together with Ti(IV) in the cluster (Figure 2d). This agrees well with the electron energy-loss spectroscopy (EELS) analysis ( Figure S21). In addition, the structure of MTV-MIL-100(Ti,Co) is stable up to 360°C (Figures S22 and S23).
To gain insight into the photocatalytic activity of the MTV-MIL-100(Ti,Co), its electronic structure was investigated by computational methods using truncated models ( Figures S11  and S24). The calculation of molecular orbital diagrams was based on an octet spin state, which was found as the most stable spin state (Table S2). The HOMO is mostly a d orbital with β spin localized on Co(II), and the energy of the HOMO was set to be the zero reference. As shown in Figure 3, the molecular orbitals diagram therefore indicates a metal-to-metal charge transfer (MMCT) from a Co(II) to a Co(III) (from the HOMO to the LUMO), producing separation of charges that can give rise to the photocatalytic activity. 39−42 The transition is located at 2.54 eV, which is in the visible light range and in good agreement with the experimental value of 2.7 eV. The MMCT from Co(II) to Ti(IV) is located at 3.51−3.68 eV. Considering the uncertainty of the method, this transition could also lie in the visible range. The ligand-to-metal charge transfer (LMCT) is located at higher energies, 3.57 eV for BTC to Co(III) and 4.54−4.71 eV for BTC to Ti(IV), which should be an unfavorable transition due to their higher spatial separation. Moreover, TDDFT calculation shows that the BTC local transition is located at 5.77 eV ( Figure S25), which is much higher than the visible light range. Illuminating MTV-MIL-100(Ti,Co) results in an intense paramagnetic signal in the paramagnetic resonance (EPR) spectra ( Figure S26), which is attributed to the MMCT between Co−Co and Co− Ti.
With the ultrahigh surface area, which provides high accessibility within the crystals, tunable electronic structures, which offers an MMCT route to control the optical response, and excellent water resistance, which prevents catalyst degradation, MTV-MIL-100(Ti,B) presents an ideal example for designing and building photocatalysts. As a proof of concept, the photochemical hydrolysis of ammonia borane (AB) was tested (Figures S27 and S28). Figure 4a (Table 1; see the Supporting Information for more details). Moreover, MTV-MIL-100(Ti,Co) represents one of the best photocatalysts in comparison with other state-of-the-art catalysts (Table S3).
We further studied the catalytic activities of MTV-MIL-100(Ti,Co) in different temperatures to investigate reaction kinetics (Figure 4c). MTV-MIL-100(Ti,Co) shows an enhanced photocatalytic activity with an increase of reaction temperature. Normalizing the relationships between ln(TOF) and 1/T, the linear slope confirms that the hydrolysis of AB over MTV-MIL-100(Ti,Co) follows the Arrhenius equation (Figure 4c). The activation energy (E a ) was calculated as 53.4 kJ mol −1 , indicating a low-energy barrier to promote the hydrolysis of AB. Meanwhile, MTV-MIL-100(Ti,Co) also displays good catalytic stability (Figure 4d). After five runs, only 10.5 min was needed for the complete hydrolysis of AB with a TOF of 64.8 mol H2 g cat. −1 min −1 . The decreased TOF is mainly due to catalyst deactivation and dilution of the reactants. 43,44 In addition, using a higher Pt loading could further improve the performance, and the TOF reaches a maximum of 136.4 mol H2 g cat.
−1 min −1 by loading at least 2.0 wt % of Pt ( Figure S29). Postcatalytic characterization indicates that the crystal structure of MTV-MIL-100(Ti,Co) remains intact, without any obvious degradation or phase transitions occurring ( Figure S30).
In conclusion, we have developed a series of photocatalysts based on the AB 2 OX 3 cluster, which have well-defined heterostructures that provide tunability of the electronic structures and synergetic cooperation for photocatalysis through MMCT. The resulting materials are among the rare examples of MTV-MOFs with ordered spatial arrangements of the multiple metal ions. 45 In addition to the structural tunability, MTV-MIL-100(Ti,Co) exhibit high porosity and water stability. Due to these advantages, it demonstrates high photocatalytic activity compared to the pure Ti-MOF MIL-125 and commercial TiO 2 . Our solvent-assisted synthetic strategy when making MTV-MIL-100 presents a facile way of  controlling the composition of the material. Considering the large compositional diversity, as well as the huge library of organic linkers available, we believe that there are many discoveries to be made by utilizing AB 2 OX 3 clusters for constructing inorganic−organic hybrid materials, especially in the development of even more efficient photocatalysts.