Let your light shine: the photocatalytic reduction of carbon dioxide to the formate anion under visible light irradiation is for the first time realized over a photoactive Ti-containing metal-organic framework, NH(2)-MIL-125(Ti), which is fabricated by a facile substitution of ligands in the UV-responsive MIL-125(Ti) material.

Metal-organic framework (MOF) NH2 -Uio-66(Zr) exhibits photocatalytic activity for CO2 reduction in the presence of triethanolamine as sacrificial agent under visible-light irradiation. Photoinduced electron transfer from the excited 2-aminoterephthalate (ATA) to Zr oxo clusters in NH2 -Uio-66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr(III) and its involvement in photocatalytic CO2 reduction was confirmed by ESR analysis. Moreover, NH2 -Uio-66(Zr) with mixed ATA and 2,5-diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO2 reduction due to its enhanced light adsorption and increased adsorption of CO2 . This study provides a better understanding of photocatalytic CO2 reduction over MOF-based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO2 reduction.

The capture and efficient use of CO 2 is an important issue because CO 2 released by burning fossil fuels is a primary cause of global warming. One of the best solutions is to convert CO 2 into valuable organic products by means of solar energy. Thus many research efforts have been made to develop efficient heterogeneous photocatalysts for the reduction of CO 2 . The examined photocatalysts range from semiconducting materials, such as TiO 2 , CdS, ZnGa 2 O 4 , and Zn 2 GeO 4 , to metal-incorporated zeolites including Ti-bzeolite, Ti-MCM-41, Ti-MCM-48, and ZrCu(I)-MCM-41. [1,2] The fact that the Ti-immobilized zeolites show even much higher efficiency for the reduction of CO 2 over TiO 2 indicates that the incorporation of Ti into porous materials will be a promising strategy, although almost all currently developed photocatalysts are only active in the region of UV light and their efficiency for the reduction of CO 2 is still quite low.Metal-organic frameworks (MOFs) are a class of crystalline micro-mesoporous hybrid materials with an extended 3D network which have shown a variety of potential applications. [3][4][5][6][7][8][9] Especially, MOFs are used for the development of heterogeneous catalysts. In fact, a variety of heterogeneous MOF catalysts has been realized over the past decade by introducing different types of catalytic sites into a porous MOF matrix. [9] Photocatalysis is a unique kind of heterogeneous catalysis which involves the use of a light source. Theoretically, it is also feasible to develop MOF photocatalysts by immobilizing photoactive catalytic sites in MOF materials. The effective use of solar light can be facilely achieved by modifications on the metal ions or the organic ligands in the MOFs. [10] Although theoretically established, a successful tuning of the absorption of the MOFs for their application in visible light photocatalysis has not yet been realized experimentally. Besides this matter, only a couple of MOFs have been reported to show photocatalytic activities for dye degradation and water splitting. [11] Motivated by the above-mentioned discoveries on the Ti-incorporated zeolite photocatalysts, we initiated a research program to develop photoactive MOF photocatalysts for the reduction of CO 2 after a Ti-based MOF Ti 8 O 8 (OH) 4 (bdc) 6 (MIL-125(Ti)) had been reported (BDC = benzene-1,4-dicarboxylate). [12] This type of MOF material cannot only introduce high density of the immobilized Ti sites within porous MOFs, but it can also lead to isostructural MOFs, the photocatalytic properties of which can simply be tuned by incorporation of BDC derivatives. Indeed, the introduction of NH 2 groups in BDC in the MIL-125(Ti) material to prepare isostructural NH 2 -MIL-125(Ti) for H 2 adsorption was recently reported by Zlotea et al. [13] Herein we report for the first time a targeted photoactive catalyst Ti 8 O 8 (OH) 4 (bdc-NH 2 ) 6 (NH 2 -MIL-125(Ti)), which reduces CO 2 even under visible light irradiation (BDC-NH 2 = 2-amino-benzene-1,4-dicarboxylate; ATA = 2-aminoterephthalate).NH ...

M-doped NH2-MIL-125(Ti) (M=Pt and Au) were prepared by using the wetness impregnation method followed by a treatment with H2 flow. The resultant samples were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analyses, N2-sorption BET surface area, and UV/Vis diffuse reflectance spectroscopy (DRS). The photocatalytic reaction carried out in saturated CO2 with triethanolamine (TEOA) as sacrificial agent under visible-light irradiations showed that the noble metal-doping on NH2-MIL-125(Ti) promoted the photocatalytic hydrogen evolution. Unlike that over pure NH2-MIL-125(Ti), in which only formate was produced, both hydrogen and formate were formed over Pt- and Au-loaded NH2-MIL-125(Ti). However, Pt and Au have different effects on the photocatalytic performance for formate production. Compared with pure NH2-MIL-125(Ti), Pt/NH2-MIL-125(Ti) showed an enhanced activity for photocatalytic formate formation, whereas Au has a negative effect on this reaction. To elucidate the origin of the different photocatalytic performance, electron spin resonance (ESR) analyses and density functional theory (DFT) calculations were carried out over M/NH2-MIL-125(Ti).The photocatalytic mechanisms over M/NH2-MIL-125(Ti) (M=Pt and Au) were proposed. For the first time, the hydrogen spillover from the noble metal Pt to the framework of NH2-MIL-125(Ti) and its promoting effect on the photocatalytic CO2 reduction is revealed. The elucidation of the mechanism on the photocatalysis over M/NH2-MIL-125(Ti) can provide some guidance in the development of new photocatalysts based on MOF materials. This study also demonstrates the potential of using noble metal-doped MOFs in photocatalytic reactions involving hydrogen as a reactant, like hydrogenation reactions.

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We report a highly efficient bifunctional catalyst, Pd/SO 3 H-MIL-101(Cr), consisting of Pd nanoparticles immobilized on a mesoporous sulfonic acid-functionalized metal-organic framework SO 3 H-MIL-101(Cr), which exhibits high catalytic performance in promoting biomass refining. The use of SO 3 H-MIL-101(Cr) as a support renders highly dispersed Pd nanoparticles with uniform size distribution, sufficient reactants contact in aqueous media, and rapid activation of the reactants induced by the Brønsted acid coordination sites (sulfonic acid groups from SO 3 H-MIL-101(Cr)). Thus, the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst exhibits novel synergy in the hydrodeoxygenation of vanillin (a typical model compound of lignin) at low H 2 pressure under mild conditions in aqueous media. Excellent catalytic results (100% conversion of vanillin with exclusive selectivity for the 2-methoxy-4-methylphenol product) could be achieved, and no loss of catalytic activity and selectivity were observed after seven recycles in succession.13 achieved over the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst including a 100% conversion of vanillin with a 100% selectivity for the 2-methoxy-4-methylphenol product within 120 min (entry 3 in Table 1). While in the presence of 2.0 wt.% Pd/MIL-101(Cr) catalyst, a rather low catalytic activity and selectivity were obtained when the reaction was carried out at the same conditions (entry 4). This is probably due to the lower acid strength of the MIL-101(Cr) as compared to that of the SO 3 H-MIL-101(Cr) support. Over the pure support SO 3 H-MIL-101(Cr) or MIL-101(Cr), however, no reaction took place, implying that Pd nanoparticles are inevitable for the vanillin hydrodeoxygenation (entries 1 and 2). Moreover, the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst was also compared with the commercially available Pd/C catalyst under the same conditions. The textural properties of the 2.0 wt.% Pd/C are shown in Table S1. The loading amounts of Pd within the Pd/SO 3 H-MIL-101(Cr), Pd/MIL-101(Cr) and Pd/C catalysts, determined by the ICP-AES analysis, were found to be 1.98 wt.%, 1.99 wt.% and 2.01 wt.%, respectively, very close to the nominal amount of 2.0 wt.%. The results (entry 5 in Table 1) clearly show that the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst gives significantly higher activity and 2-methoxy-4-methylphenol selectivity as compared to the Pd/C catalyst. It should be noted that the selectivity for 2-methoxy-4-methylphenol over the prepared 2.0 wt.% Pd/SO 3 H-MIL-101 catalyst is also significantly higher than that reported by Xiao et al [25] over 4.5 wt.% Pd/MSMF under the same reaction conditions with a similar conversion of vanillin (entry 6). The high selectivity over the 2.0 wt.% Pd/SO 3 H-MIL-101(Cr) catalyst is probably due to the readily accessible Brønsted acidic sites distributed throughout the framework as well as an abundance of mesoporous cages of MIL-101(Cr) thus, greatly facilitating the transfer of substrates [46,47].

Aerobic oxidation of cyclohexene over [Cu 2 (OH)(BTC)(H 2 O)] n ?2nH 2 O (Cu-MOF, BTC = 1,3,5benzenetricarboxylic acid) and [M 2 (DOBDC)(H 2 O) 2 ]?8H 2 O (Co-and Ni-MOF, DOBDC = 2,5dihydroxyterephthalic acid) in the absence of solvent under mild conditions was studied. It is observed that both Cu-MOF and Co-MOF can selectively oxidize cyclohexene to give 2-cyclohexen-1-ol and 2-cyclohexen-1-one as the main products, while Ni-MOF is totally inactive for cyclohexene oxidation. The mechanism of the catalytic oxidation of cyclohexene over Cu-MOF and Co-MOF has been proposed. These MOF-based catalysts are stable and recyclable under current reaction conditions. This study highlights the great potential of developing MOFs as highly stable, molecularly tunable, recyclable and reusable heterogeneous catalysts for alkenes oxidation.