Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic

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Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on AmineFunctionalized Iron-Based Metal-Organic Frameworks Shenghong Dong, Zhen Liu, Ruihan Liu, Limin Chen, Jinzhu Chen, and Yisheng Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01039 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Applied Nano Materials

Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal-Organic Frameworks

Shenghong Dong,†,#,‡ Zhen Liu,†,‡ Ruihan Liu,§ Limin Chen,§ Jinzhu Chen,*,# and Yisheng Xu*,†

†State

Key Laboratory of Chemical Engineering, International Joint Research Center of Green

Energy Chemical Engineering, East China University of Science and Technology. 130 Meilong Road, Shanghai 200237, China. #College

of Chemistry and Materials Science, Jinan University. No. 601 Huangpu Avenue West,

Tianhe District, Guangzhou 510632, China. §Guangdong

Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of

Environment and Energy, South China University of Technology. 382 Zhonghuan Road East, Guangzhou Higher Education Mega Centre, Panyu District, Guangzhou 510006, China. ‡S.

Dong and Z. Liu contributed equally to this work.

*Corresponding author. E-mail address: [email protected] (J. Chen), [email protected] (Y. Xu); Tel./Fax: (+86)-20-8522-0223.

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Abstract: Visible-light-induced selective transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was achieved by using Pd nano-catalyst supported on amine-functionalized iron-based Metal–Organic Frameworks [Pd/MIL-101(Fe)-NH2] with triethylamine (TEA) as an electron donor and HCOOH as a proton source. The Pd/MIL-101(Fe)-NH2, obtained by an in-situ photodeposition method, showed homogeneously and highly dispersed Pd nanoparticles (NPs) with a uniform size throughout the MIL-101(Fe)-NH2 support due to an effective stabilization role of amine groups on the backbone linkage of MIL-101(Fe)-NH2. The resulting Pd/MIL-101(Fe)-NH2 exhibited excellent catalytic performance towards a visible-light-induced transfer hydrogenation of benzaldehyde by producing a benzyl alcohol yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH. In addition to benzaldehyde, biomass-based renewable platform molecules such as furfural and 5-hydroxymethylfurfural (HMF) were successively converted into the corresponding alcohols with the yields of 29% for furfuryl alcohol and 27% for 2,5dihydroxymethylfuran (DHMF), respectively, which are the highest yields reported so far by visiblelight-induced transfer hydrogenation method. Our experimental investigation reveals that a preliminary photo-irradiation promotes a in situ photodeposition of Pd salt [MIL-101(Fe)NH3]+∙1/2[PdCl4]2− to form Pd catalyst Pd/MIL-101(Fe)-NH2 in the presence of TEA-HCOOH, a further

photo-irradiation

successively

triggers

Pd/MIL-101(Fe)-NH2-promoted

transfer

hydrogenation of aldehyde, again, with the help of TEA-HCOOH. While, our theoretical research based on density functional theory (DFT) further confirms a dual function of amine group in the Pd/MIL-101(Fe)-NH2 for Pd NPs stabilization as well as for enhancement of the electron density of the Pd center upon light adsorption. The photo-catalytic system of Pd nano-catalyst and TEAHCOOH thus demonstrates an environmental-friendly and efficient strategy for aldehyde hydrogenation by using a renewable solar energy as a driven force.

Keywords: Biomass, Metal–organic frameworks, Palladium, Photo-catalysis, Transfer hydrogenation

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1. Introduction It is well known that selective hydrogenation of primary aldehydes to alcohols is one of classical reactions to produce fine chemicals and intermediates for pharmaceuticals, fragrances, food flavorings, fuels and fuel additives.1-5 Typically, selective hydrogenation of aldehydes such as benzaldehyde, methylbenzaldehydes and ethylbenzaldehydes have received a growing interest for huge demands of the corresponding alcohol products in recent years.6-11 Currently, molecular hydrogen was widely used as reducing agent for these catalytic hydrogenations. However, the hydrogen-based hydrogenation generally required effective catalyst, high reaction temperature and high reaction pressure with a low space time yield (STY). Alternatively, catalytic transfer hydrogenations (CTH) were systematically investigated by using hydrogen sources of triethylamineformic acid (TEA-HCOOH), iso-propanol and methanol.12-14 The CTH system can significantly reduce the reaction pressure. On the other hand, the in situ generated hydrogen from the hydrogen sources in CTH system can effectively couple with the subsequent hydrogenation reaction. It is suggested that in situ formed hydrogen from CTH system can well contact and therefore efficiently react with hydrogenation substrates such as aldehydes and ketones, 14 thus leading to a remarkably enhanced hydrogenation rate. In order to improve the sustainability of the reaction, photo-catalysis has recently been adopted because of its great merits for environmental and green energy considerations.15-17 Such photoinduced reactions are generally related to the conversions of renewable solar energy into chemical energy under environmental benign and mild conditions.18-19 Notably, both TEA and HCOOH are used in the photo-catalytic reaction as sacrificial reagents. For example, a supramolecular photocatalyst of a heterodinuclear Ru-Pd complex was examined for a hydrogen production and selective

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hydrogenation of tolane by using TEA as electron donor.20 Pt-loaded silicon was reported for a photocatalytic hydrogen evolution with HCOOH as the most efficient sacrificial reagent.21 Nitrobenzene reduction was investigated under visible-light irradiation with a Pd-loaded silicon semiconductor as catalyst by using HCOOH as a sacrificial reagent.22 Graphitic carbon nitride (g-C3N4)-supported Pt catalyst was reported for a photo-promoted hydrogenation of 5-hydroxymethylfurfural (HMF).23 A Ru (0) complex was developed for a photo-chemical carbonylation of benzene, hydrogenation and hydrosilation of benzaldehyde.24 Tokunaga and his co-workers investigated corrole-promoted photoreduction of benzaldehyde with benzenethiol as proton source.25 Therefore, the key issue of the photoinduced reduction reaction is the construction of highly efficient catalytic system. Metal–Organic Frameworks (MOFs), also named as porous coordination polymers (PCPs), are one of the most attractive porous organic-inorganic crystalline materials with wide applications in catalysis, membranes, H2 storage, carbon dioxide capture, and optical materials.26 Moreover, MOFs were proposed as potential photo-catalysts since the late 1990’s and the early 2000’s.27 Currently, considerable attentions were paid to the development of these inorganic-organic hybrid materials as novel photo-catalysts. Compared with classical semiconductors, MOFs possessed an outstanding light absorption ability, which can be more easily adjusted by both of the metal ions and the organic linkers. In fact, a series of MOFs were investigated as photo-catalysts for photo-induced CO2 reduction and water splitting;28 photo-promoted oxidation of aromatic aldehydes,29 aromatic alcohols and sulfides;30 photo-catalytic reduction of Cr(VI);31-32 as well as photo-degradation of organic pollutants such as rhodamine and methylene blue.33 Previous studies revealed that iron was a unique metal in promoting electrical conductivity in MOFs, which endowed Fe-based MOFs unparalleled advantages as photo-catalysts.34 For instance,

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MIL-101(Fe)-NH2 was examined for photo-induced one-pot reaction between active methylene compounds and aromatic alcohols, and photo-catalytic CO2 reduction.37,43 MIL-68(Fe) and MIL100(Fe) were reported for photo-promoted tandem oxidative coupling of o-aminothiophenols and alcohols, and photo-enhanced selective hydroxylation of benzene.35-36 In addition to the pristine MOFs, incorporation of transition metal nanoparticles (NPs) into porous matrices of MOFs to form a hierarchical assembly NPs/MOF significantly broadened the photo-catalytic performance.32,37 The narrow micropore size distribution inside MOFs leads to the formation of uniform and monodisperse photo-active species anchored on MOFs. Additionally, the organic linkers in MOFs can not only function as antennas to absorb light upon irradiation for subsequent activation of the metal clusters, but also stabilize the supported transition metal NPs. 38-39 Several studies indicated that noble-metal NPs showed a dual role in photo-catalysis including both electron traps and active sites. For instance, highly dispersed Pd NPs on UiO-66(Zr)-NH2 was investigated as a visible-light-driven photo-catalyst for Cr(VI) reduction.32 A Pd nanocubes@ZIF-8 composite with core–shell structure was reported for selective reduction of small olefins under light irradiation.40 Pt NPs imbedded in amine-functionalized Cr-based MIL-101(Cr) was developed as a durable photo-catalyst for hydrogen production from water.41 Pt/PCN-224(M) was investigated for the photo-induced selective oxidation of aromatic alcohols into the corresponding aldehydes. 29 On the basis of the aforementioned investigations, photo-induced selective transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was investigated over Pd nanocatalyst supported on amine-functionalized iron-based MOFs [Pd/MIL-101(Fe)-NH2, Figure 1a] with TEA as the sacrificial electron donor and HCOOH as the proton source. The Pd/MIL-101(Fe)-NH2 exhibits excellent catalytic performance towards a visible-light-induced transfer hydrogenation of

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benzaldehyde by producing a benzyl alcohol yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH (Figure 1a). In addition to benzaldehyde and its derivatives, biomassbased renewable platform molecules such as furfural and HMF were successively converted into the corresponding alcohols with the yields of 29% for furfuryl alcohol and 27% for 2,5dihydroxymethylfuran (DHMF), respectively, which are the highest yields reported so far by visiblelight-induced transfer hydrogenation method.

Figure 1. (a) Photo-induced transfer hydrogenation of aromatic aldehydes by using Pd/MIL-101(Fe)NH2, (b) synthesis of Pd/MIL-101(Fe)-NH2.

2. Experiment section 2.1. Materials and characterization Materials and characterization were provided in the Supporting Information. 2.2. Catalyst preparation Preparation of catalyst supports

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MIL-101(Fe)-NH2,28,42 MIL-53(Fe)-NH2,42 MIL-101(Fe),28 MIL-53(Fe),28 UiO-66(Zr)-NH2,43 UiO-66(Zr),43 MIL-125(Ti)-NH2,44 MIL-101(Al)-NH2,45 MIL-101(Cr)-NH2,46 nano cerium(IV) oxide (CeO2) 47 and titanate nanotube (TNT)48 were prepared via a hydrothermal treatment according to the literature methods. Synthesis of catalyst precursors In a typical synthesis of [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2−, the activated MIL-101(Fe)-NH2 (200 mg) in deionized water (150 mL) was sonicated for 20 min. An aqueous solution containing H2PdCl4 (PdCl2 16.7 mg dissolved in 2-3 drops of hydrochloric acid, 1×10−3 M, 100 mL) was slowly dropwised into the above sample under vigorous stirring for 8-10 min. The resulting mixture was vigorously stirred for 12 h and the slurry was subsequently filtered and thoroughly washed with deionized water for 3-5 times. The resulting samples were collected and further dried at 80 C for 24 h under vacuum before use. [MIL-53(Fe)-NH3]+∙1/2[PdCl4]2−, [MIL-101(Al)-NH3]+∙1/2[PdCl4]2−, [MIL-101(Cr)-NH3]+∙1/2[PdCl4]2−, NH3]+∙1/2[PdCl4]2− were

[MIL-125(Ti)-NH3]+∙1/2[PdCl4]2−,

obtained with the

same

procedure

and

[UiO-66(Zr)-

used for

[MIL-101(Fe)-

NH3]+∙1/2[PdCl4]2− except that the support MIL-101(Fe)-NH2 was replaced by the corresponding MOFs support. [MIL-101(Fe)-NH3]+∙1/2[PtCl6]2− were obtained with the same procedure used for [MIL101(Fe)-NH3]+∙1/2[PdCl4]2− except that H2PdCl4 (1×10−3 M, 100 mL) was replaced by H2PtCl6 (1×10−3 M, 100 mL). Synthesis of bare MOFs-derived and classical semi-conducts-derived Pd catalyst precursors In a typical procedure for H2PdCl4/MIL-101(Fe) preparation, the activated MIL-101(Fe) (200 mg) were dispersed in deionized water (150 mL) and sonicated for 20 min. An aqueous solution

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containing H2PdCl4 (PdCl2 16.7 mg dissolved in 2-3 drops of hydrochloric acid, 1×10−3 M, 100 mL) was prepared and dropwised into the above sample under vigorous agitation for 8-10 min. The mixture was vigorously stirred for 12 h. Then the solvent was removed by rotary evaporator. The collected samples were further dried at 80 C for 24 h under vacuum before use. H2PdCl4/MIL-53(Fe), H2PdCl4/UiO-66(Zr), H2PdCl4/TNT, H2PdCl4/CeO2 and H2PdCl4/P25 were obtained with the same procedure used for H2PdCl4/MIL-101(Fe) except that the support MIL101(Fe) was replaced by the corresponding support. 2.3. Catalytic performance In a typical experiment, a Pyrex test tube (10 mL) was continuously loaded with a magnetic stir bar, [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− (30 mg, Pd 3.2 wt.%), acetonitrile (4.0 mL), formic acid (0.4 mL), triethylamine (0.6 mL), benzaldehyde (15 mg, 0.14 mmol). The mixture was stirred for 6 h under an irradiation of a visible-light source by using Xenon lamp (210 W) equipped with a 420-nm cut-off filter. The sealed test tube was positioned in a quartz bath for cooling. After the reaction was halted, the sample was quickly filtered with an organic membrane filter (0.45 μm) to remove the insoluble powder. The obtained filtrate was analyzed by Gas Chromatography (GC Fuli 9790II) equipped with a KB-5 capillary column (0.32 mm ×30 m) and a FID detector with nitrogen as carrier gas using biphenyl as the internal standard material. 2.4. DFT calculations All the DFT calculations were performed using Gaussian 09 program. 49 The structures were fully optimized to the ground state using the B3LYP functional in combination with the def2-SVP basis set on all atoms. Next, a singlet point energy of the vertical excitation was computed at the ground-state structures using the TDDFT approach and the cam-B3LYP functional as recommend for

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calculation of the excited state. The Grimme’s dispersion correction was incorporated into the geometry optimizations as well as the single point energy calculations. The electron density difference (threshold: 0.002 au) was obtained by subtraction of the electron density of the excited state from the electron density of the ground state. The thermal correction to Gibbs free energy was computed through vibrational frequency calculations.

3. Results and discussions The precursor of Pd photo-catalyst was prepared by an initial neutralization of the amine tags on the MIL-101(Fe)-NH2 followed by an anionic exchange reaction to give Pd salt [MIL-101(Fe)NH3]+∙1/2[PdCl4]2− (Figure 1b). While, the Pd catalyst Pd/MIL-101(Fe)-NH2 was obtained by a direct photodeposition of the Pd salt in the presence of TEA-HCOOH. The in situ formed Pd catalyst, without any separations, successively promoted the subsequent photo-induced hydrogenation of aldehyde. The obtained Pd/MIL-101(Fe)-NH2 was systematically investigated with nitrogen adsorption desorption, powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), room-temperature UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Figure S1 (Supporting Information) shows the nitrogen adsorption desorption curves of MIL101(Fe)-NH2, Pd/MIL-101(Fe)-NH2 with various Pd load levels and the recovered Pd/MIL-101(Fe)NH2. While, the resulting Brunauer–Emmett–Teller (BET) surface areas, pore volumes are presented in Table S1 (Supporting Information). The bare MIL-101(Fe)-NH2 exhibits large BET specific

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surface area of 994.0 m2·g−1 and pore volume of 1.34 cm3·g−1, respectively (Table S1, entry 1). As expected, both the surface area and pore volume of Pd/MIL-101(Fe)-NH2 significantly decrease upon the loading of the Pd NPs over MIL-101(Fe)-NH2 (Table S1, entries 1−4). Moreover, an increase of Pd loading level in the Pd/MIL-101(Fe)-NH2 leads to a decreased surface area (Table S1, entries 1−4). Notably, the recovered Pd/MIL-101(Fe)-NH2 after five-time recycling shows a decreased surface area if compared with the fresh sample (Table S1, entries 3 and 5). Bare MIL-101(Fe)-NH2, [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− and Pd/MIL-101(Fe)-NH2 with various Pd load level were further investigated by wide range PXRD (Figure S2, Supporting Information). The wide range PXRD pattern of bare MIL-101(Fe)-NH2 is in good accordance with the published literature result (Figure S2),32 confirming the formation of MIL-101(Fe)-NH2. After loading Pd salt, the [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− with various Pd loading levels shows similar crystallinity

with

MIL-101(Fe)-NH2.

However,

the

Bragg

density

of

[MIL-101(Fe)-

NH3]+∙1/2[PdCl4]2− decreases with the increase of Pd salt loading, indicating a successful immobilization of Pd salt. In the case of Pd/MIL-101(Fe)-NH2 sample, its Bragg density is extremely low if compared with pristine MIL-101(Fe)-NH2. Notably, diffraction peaks corresponding to the Pd (111) at 40.1 were unobserved, which suggests that the Pd NPs size is small without any obvious aggregations in the Pd/MIL-101(Fe)-NH2. The FT-IR spectrum of both MIL-101(Fe)-NH2 and Pd/MIL-101(Fe)-NH2 show two strong absorption bands at the wavelengths of 3475 and 3365 cm−1, corresponding to the symmetrical and asymmetrical stretching vibration of amine groups in the 2-aminoterephthalic acid ligand, respectively (Figure S3, Supporting Information).50 On the lower frequency region, both the two samples show strong absorption bands at the wavelengths of 1230, 1350 and 1580 cm−1, which can

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be respectively indexed to the stretching vibration peaks of C−N bond, C−O bond and benzene ring.45,51-52 The TGA was further examined to probe the thermal stability of MIL-101(Fe)-NH2 and Pd/MIL-101(Fe)-NH2 (Figure S4, Supporting Information). The TGA curves of weight loss versus temperature for the two samples show no significant weight loss before 170 C, which can be attributed to the moisture and N,N-dimethylformamide solvent molecules in the sample cage. A significant weight loss of the two samples starts from 200 C, corresponding to framework degradation.42 The SEM images of MIL-101-(Fe)-NH2, [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− and Pd/MIL101(Fe)-NH2 samples show no evident differences on the MOFs morphology (Figure S5, Supporting Information). Figure 2a-d show the TEM images of Pd/MIL-101(Fe)-NH2, the Pd NPs with a uniform size are homogeneously and highly dispersed throughout the MIL-101(Fe)-NH2 support. The Pd/MIL-101(Fe)-NH2 exhibits an average Pd NPs size around 1.8 nm without any evident agglomerations due to an effective stabilization role of amine groups on the backbone linkage of MIL101(Fe)-NH2.70 Additionally, HAADF-STEM imaging of Pd/MIL-101(Fe)-NH2 shows the distribution of the Fe, N and Pd elements as blue, red and light cyan spots, respectively (Figure 2i-l), indicating a homogeneous distribution of the Pd over Pd/MIL-101(Fe)-NH2 and excellent stabilization effect of amine groups on Pd NPs. As further revealed by HRTEM of Pd/MIL-101(Fe)NH2 (Figure 2c), the Pd NPs in MIL-101(Fe)-NH2 have clear lattice fringes with an average lattice fringe of d = 0.19 nm which was assigned to the (200) facet of Pd, indicating decent crystallinity of Pd NPs. In addition to Pd/MIL-101(Fe)-NH2, various Pd NPs immobilized on the MOFs with amine groups on the frameworks (Pd/MOFs-NH2) were analyzed by TEM, which involved Pd/MIL-53(Fe)NH2 (Figure 2g-h), Pd/UiO-66(Zr)-NH2, Pd/MIL-125(Ti)-NH2 and Pd/MIL-101(Cr)-NH2 (Figures S

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Figure 2. TEM images and nanoparticle size distribution of Pd/MIL-101(Fe)-NH2 prepared by (a-d) photodeposition method and (e-f) H2 reduction, (g-h) TEM images and nanoparticle size distribution of Pd/MIL-53(Fe)-NH2 obtained by photodeposition, (i) HAADF-STEM of Pd/MIL-101(Fe)-NH2 and elemental mapping of the Fe (j), N (k) and Pd (l) elements present in the Pd/MIL-101(Fe)-NH2.

6d-f, Supporting Information). Generally, the Pd/MOFs-NH2 samples show similar TEM images with Pd/MIL-101(Fe)-NH2, possessing highly dispersed Pd NPs and uniform sizes ranging from 1.9 to 3.0 12


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nm. To prove the stabilization effect of MOFs support on the catalyst, we further prepared and investigated the Pd NPs samples supported on the MOFs without amine tags on the backbone linkage (Pd/MOFs), involving Pd/MIL-101(Fe), Pd/MIL-53(Fe) and Pd/UiO-66(Zr) (Figures S6a-c, Supporting Information). In contrast, the Pd/MOFs samples demonstrate readily aggregated Pd NPs over the MOFs support surface with mean NP sizes from 5.5 to 6.7 nm. In addition to the photodeposition method for Pd/MIL-101(Fe)-NH2 preparation, the [MIL101(Fe)-NH3]+∙1/2[PdCl4]2− was treated with hydrogen (0.5 MPa) at 80 ºC for 12 h as a control experiment. The resulting Pd/MIL-101(Fe)-NH2 sample also shows uniform and highly dispersed Pd NPs throughout the MIL-101(Fe)-NH2 with, however, an average Pd NPs size around 2.8 nm (Figure 2e-f) which is larger than that obtained by the photodeposition method. For comparison, a series of classic and commercially available semi-conductor such as titanate nanotubes (TNT), P25 and CeO2 were investigated as the supports for Pd catalyst. The Pd/TNT, Pd/P25 (Figure S6g-h, Supporting Information) samples immobilized Pd NPs with average sizes ranging from 4.9 to 7.9 nm on the support surface. In the case of Pd/CeO2, an obvious agglomeration of the Pd NPs on CeO2 was observed (Figure S6i, Supporting Information). The surface composition of Pd/MIL-101(Fe)-NH2 and the oxidation states of surface element of Pd were investigated by XPS. As shown in Figure 3a, the signals corresponding to carbon, nitrogen, palladium, oxygen and iron were detected in the survey scans. The high-resolution spectra of Pd 3d in Pd/MIL-101(Fe)-NH2 were further investigated and deconvoluted into two major doublet peaks (Figure 3b). The larger doublet peaks at 334.8 eV and 340.1 eV were indexed to Pd 3d 5/2 and Pd 3d3/2, respectively, suggesting the presence of metallic Pd(0). 53 The binding energy difference around 5.3 eV between Pd 3d5/2 and Pd 3d3/2 is the characteristic feature of metallic Pd 3d states.32 While, the

13


b Pd/MIL-101(Fe)-NH Pd 3d XPS

2

Pd/MIL-53(Fe)

Pd/MIL-101(Fe) Pd/MIL-101(Fe)-NH2

Pd

Intensity

Pd/MIL-53(Fe)-NH2

0

H2 reduction

Pd

2+

Pd

2+

0

Pd

e 399.3 eV

399.0 eV

710 720 730 Binding Energy / eV

Fe 2p1/2

Pd/MIL-53(Fe) Pd/MIL-101(Fe)

Intensity

Intensity

shaked up

Fe 2p XPS Fe 2p3/2

Pd/MIL-101(Fe)-NH2

Fe 2p1/2

0

401.3 eV

Pd/MIL-53(Fe)-NH2 Pd/MIL-101(Fe)-NH2 MIL-101(Fe)-NH2

MIL-101(Fe)-NH2

396

Fe 2p3/2


N 1s XPS

2

Fe 2p XPS

Pd = 86% 2+

Pd

c Pd/MIL-101(Fe)-NH

0

0

Pd

332


d

Pd 2+

Pd = 20% 0 Pd

MIL-101(Fe)-NH2

Photodeposition

0

Page 14 of 42

Intensity

Fe 2p

C 1s Pd 3d N 1s

a Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O 1s



705

720 735 Binding Energy / eV

Figure 3. (a) XPS scan surveys of various MOFs-supported Pd samples and MIL-101(Fe)-NH2, (b) Pd 3d XPS spectra and (c) Fe 2p XPS spectra of Pd/MIL-101(Fe)-NH2, (d) N 1s XPS of MIL-101(Fe)NH2 and Pd/MIL-101(Fe)-NH2, (e) Fe 2p XPS of various MOFs-supported Pd samples and MIL101(Fe)-NH2.

other two signals at the binding energies of 337.2 eV and 342.4 eV with very low intensity were assigned to Pd 3d5/2 and Pd 3d3/2 (Figure 3b), respectively, demonstrating the existence of Pd(II). 53 The content of metallic Pd(0) is 86% based on the relative areas of these signals, thus showing a successful reduction of Pd2+ in [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− to give a predominant Pd(0) in Pd/MIL-101(Fe)-NH2 via photodeposition method in the presence of TEA-HCOOH. As a comparison, [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− was treated with hydrogen as a control experiment. The XPS analysis of the resulting hydrogen reduction sample reveals the presence of both metallic Pd(0) and

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Pd(II) species on the MIL-101(Fe)-NH2 surface (Figure 3b). However, Pd(II) species are prevailing with the percentage of 80%, further suggesting a higher reduction efficiency of TEA-HCOOH under visible-light irradiation. For the Fe 2p XPS, the binding energy peak at 711.4 eV and 724.6 eV were indexed to the Fe 2p3/2 and Fe 2p1/2, respectively (Figure 3c).54 Additionally, the peak separation of 13.2 eV (2p 1/2 2p3/2) is the characteristic feature of α-Fe2O3.55 The above results evidently indicate the presence of Fe3+ in the Pd/MIL-101(Fe)-NH2.56 In the case of N 1s XPS of Pd/MIL-101(Fe)-NH2, the highresolution spectrum was fitted by two peaks at binding energies of 399.3 and 401.3 eV. The binding energy around 399.3 eV is attributed to the pristine amine group in the H2ATA linkers, which can be apparently observed in the support sample MIL-101(Fe)-NH2 (Figure 3d).57 While, the binding energy of 401.3 eV is related to the amine group in the H2ATA linkers interacting with Pd NPs.58 Figure 3e exhibits the high-resolution XPS spectra regions of the Fe 2p, taken from the surface of MIL-101(Fe)-NH2, Pd/MIL-101(Fe)-NH2, Pd/MIL-53(Fe)-NH2, Pd/MIL-101(Fe) and Pd/MIL53(Fe). The comparison of Pd/MIL-101(Fe)-NH2 and its corresponding support MIL-101(Fe)-NH2 reveals a negative shift of 0.5 eV for the binding energy of both Fe 2p 1/2 and Fe 2p3/2 of Pd/MIL101(Fe)-NH2 (Table S2, entries 1 and 2, Supporting Information), indicating an increase in electron density on the surface iron atoms upon Pd loading. 59 For the comparison of Pd/MOFs-NH2 and Pd/MOFs samples, Pd/MIL-101(Fe) show a negative shift of 0.3 eV for both Fe 2p1/2 and Fe 2p3/2 relative to the Pd/MIL-101(Fe)-NH2 (Table S2, entries 2 and 4). Similarly, Pd/MIL-53(Fe) also reveals a negative shift of Fe 2p relative to the corresponding Pd/MIL-53(Fe)-NH2 (Table S2, entries 3 and 5). The observed binding energy shift can be attributed to the chemical interaction between Pd, Fe and N.60 In addition to XPS analysis, the interactions between Pd nano-catalyst and various MOFs

15



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support were systematically analyzed with density functional theory (DFT) calculations (will be discussed below). The optical and electronic properties of these iron-based MOFs are further investigated by UVvisible absorption spectrum (Figure S7, Supporting Information). MIL-101(Fe)-NH2 shows absorption edges at the wavelength of 550 nm. In contrast, MIL-101(Fe), MIL-53(Fe), MIL-53(Fe)NH2 and Pd/MIL-101(Fe)-NH2 samples, prepared under a higher temperature or doped with transition metal NPs, exhibited a red-shift wavelength in the absorption edges. The corresponding band gap value of MIL-101(Fe)-NH2 (2.3 eV) was then obtained based on the relationship of Eg = 1240/λ.61 Evidently, a new absorption band formed in the optical spectrum of MIL-53(Fe), which is associated with the chromophore in H2BDC.53 Additionally, a weak peak centered at 440 nm was observed for these MOFs, which is attributed to the transition of [6A1g => 4A1g + 4Eg (G)] between Fe(III) and H2BDC.62 The above phenomenon further suggested that MIL-101(Fe)-NH2 can be excited in visible light region. Therefore, it is feasible to conduct a photo-induced transfer hydrogenation with Pd on the iron-based MOFs under visible light irradiation (λ > 420 nm). Both MIL-101(Fe)-NH2 and Pd/MIL-101(Fe)-NH2 exhibit the strongest absorption bands with a large volatility around the wavelength of 350 nm, which is usually caused “π-π” transitions between the organic ligands. While, researches have shown that the strong absorption responses in the visible light region are presumably related to the type of Ligand-to-Metal Charge Transfer (LMCT) optical transition63 or a direct excitation from the Fe-O clusters,36 suggesting the bonding interactions between carbonyl oxygen and iron. Generally, the Surface Plasmon Resonance (SPR) peak of Pd NPs is always located at the wavelength around 300 nm.48 Therefore, the SPR peak of Pd here was shaded by the strong absorption of MIL-101(Fe)-NH2. Evidently, the Pd/MIL-101(Fe)-NH2 has a better absorption capability than

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MIL-101(Fe)-NH2 in the visible light region, as confirmed by the absorption shift from brick red to black. Therefore, the strengthened light absorbance here was expected to improve the photo-activity for the transfer hydrogenation.

Table 1. Reaction conditions screened for the Pd/MIL-101(Fe)-NH2-promoted transfer hydrogenation of benzaldehyde to benzyl alcohol under visible-light irradiation.

aReaction

Entry

Catalyst (mg)

Pd load level Conversion Yield (wt.%) (%) (%)

1

Pd/MIL-101(Fe)-NH2 (30)

3.2

100

77

2b 3 4 5

Pd/MIL-101(Fe)-NH2 (30) Pd/MIL-101(Fe)-NH2 (30) Pd/MIL-101(Fe)-NH2 (30) Pd/MIL-101(Fe)-NH2 (30)

3.2 0.7 1.2 2.1

37 26 43 89

9 17 30 60

6 7 8

Pd/MIL-101(Fe)-NH2 (30) Pd/MIL-101(Fe)-NH2 (5) Pd/MIL-101(Fe)-NH2 (10)

4.9 3.2 3.2

100 55 62

70 50 54

9 10

Pd/MIL-101(Fe)-NH2 (15) Pd/MIL-101(Fe)-NH2 (20)

3.2 3.2

72 88

60 67

11 12

Pd/MIL-101(Fe)-NH2 (25) Pd/MIL-101(Fe)-NH2 (35)

3.2 3.2

92 100

62 71

13

Pd/MIL-101(Fe)-NH2 (40)

3.2

100

69

14

Pt/MIL-101(Fe)-NH2 (30)

3.6

22

0

conditions: benzaldehyde (15 mg), TEA (0.6 mL), HCOOH (0.4 mL), CH3CN (4.0 mL)

under photo-irradiation of a xenon lamp equipped with a 420-nm cut-off filter at room temperature with reaction time of 6 h. The benzyl alcohol yield and benzaldehyde conversion were determined by GC-FID based on benzaldehyde using biphenyl as the internal standard material. bWithout any visible light irradiation.

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Photo-induced catalytic transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was then investigated over various MOF-supported Pd nano-catalysts (Pd/MOF) by using TEA as a sacrificial electron donor and HCOOH as a proton source. Initially, a mixture of benzaldehyde, Pd salt [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2−, and TEA-HCOOH was irradiated under visible-light source from a xenon lamp (300-W) equipped with a filter (420-nm cut-off). Surprisingly, a benzyl alcohol yield of 77% with a full conversion of benzaldehyde was examined under the reaction conditions (Table 1, entry 1). In comparison, a control experiment in the presence of Pd salt and TEA-HCOOH, but in the absence of photo-irradiation shows a negligible benzyl alcohol yield of 9% (Table 1, entry 2), which indicates a remarkable promotion effect of the photo-irradiation. In addition, a blank irradiation experiment indicates that benzyl alcohol was not observed in the absence of Pd salt or TEA-HCOOH base on GC analysis, reflecting a key role for the transfer hydrogenation reaction by the combination of Pd salt and TEA-HCOOH. In fact, the photo-irradiation of [MIL101(Fe)-NH3]+∙1/2[PdCl4]2− with TEA-HCOOH leads to a smooth photodeposition to give Pd/MIL101(Fe)-NH2, resulting a corresponding Pd2+ reduction to metallic Pd NPs immobilized on MIL101(Fe)-NH2 support. This assumption is supported by XPS analysis (Figure 3b) and an evident color change from orange Pd salt to black Pd/MIL-101(Fe)-NH2 after the photo-irradiation (Figure S8, Supporting Information). Therefore, the above control experiments for photo-induced benzaldehyde hydrogenation reveal that a preliminary photo-irradiation promotes a in situ photodeposition of Pd salt [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− in the presence of TEA-HCOOH to form Pd catalyst Pd/MIL101(Fe)-NH2. Further photo-irradiation successively triggers Pd/MIL-101(Fe)-NH2-promoted benzaldehyde hydrogenation to give benzyl alcohol, again, with the help of TEA-HCOOH. Notably,

18


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a direct separation of the in-situ formed Pd/MIL-101(Fe)-NH2 from the photodeposition system leads to an immediately spontaneous combustion of the catalyst upon removal of reaction solvent, further demonstrating high activity of the catalyst. Previously, Sonoda reported photo-induced benzaldehyde hydrogenation with benzyl alcohol yield of 92% by using toxic H2 Se as hydrogen sources.64 Tokunaga and his co-workers investigated photo-induced corrole-promoted benzaldehyde reduction by using benzenethiol as the proton source with a benzyl alcohol yield of 50%. 25 Wade and co-workers developed a Pd catalyst [Zr6O4(OH)4(L-PdX)3] by incorporating Pd pincer complex ([L-PdX]4− = [(2,6-(OPAr2)2C6H3)PdX]4−, Ar = p-C6H4CO2−, X = Cl, I) into a MOFs.65 The resulting Zr6O4(OH)4(L-PdX)3 sample showed excellent catalytic performance on a thermo-induced transfer hydrogenation of benzaldehyde by producing benzyl alcohol yield of 84%. In our case, a readily available Pd catalyst [Pd/MIL-101(Fe)-NH2] was obtained by a photodeposition Pd NPs on the MIL101(Fe)-NH2 support. The in situ generated Pd/MIL-101(Fe)-NH2 can successfully promote visiblelight-induced transfer hydrogenation of benzaldehyde. Notably, visible-light-induced transfer hydrogenation reactions were very limited, especially, under the environment-friendly conditions. The influence of the Pd loading amount in Pd/MIL-101(Fe)-NH2 on its catalytic performance indicates that the benzyl alcohol yield is well improved to a maximum of 77% with an optimum Pd loading level of 3.2 wt.% (Table 1, entries 1, 3-6), suggesting a key role of active Pd species. In the case of the effect of catalyst loading amount on the hydrogenation, as expected, both benzaldehyde conversion and benzyl alcohol yield are significantly increased with the catalyst loading up to 30 mg under the investigated conditions (Table 1, entries 1, 7-11), reflecting an enhancement of availability and accessibility of catalytic Pd species. However, further increase of the catalyst loading amount leads to a slightly reduced benzyl alcohol yield (Table 1, entries 12-13). In addition to Pd/MIL-

19



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101(Fe)-NH2, an in-situ generated Pt/MIL-101(Fe)-NH2 from [MIL-101(Fe)-NH3]+∙1/2[PtCl6]2− was investigated for the photo-induced hydrogenation. However, benzyl alcohol product was undetected, further revealing a unique promotion effect of Pd on the hydrogenation To understand the effect of catalyst support on the catalytic performance, the Pd catalysts supported on amine-functionalized MOFs, bare MOFs and classical semi-conducts were systematically investigated for the photo-catalytic benzaldehyde hydrogenation. When the threevalent metal-based and amine-functionalized MOFs, including MIL-101(Fe)-NH2, MIL-53(Fe)-NH2, MIL-101(Al)-NH2, and MIL-101(Cr)-NH2, were subjected to the support of Pd catalysts, an excellent to moderate benzyl alcohol yields were observed (Table S3, entries 1-4, Supporting Information). In addition to benzyl alcohol yield, the catalytic activity of the above Pd/MOF-NH2 samples was quantitatively compared with turnover frequency (TOF). Herein, the TOF for the hydrogenation was measured under a low benzaldehyde conversion level around 10%-20%, given as the amount of benzyl alcohol yield per amount of active Pd site per hour for the catalyst, the dispersion of Pd in Pd/MIL-101(Fe)-NH2 was 0.15 based on CO chemisorption.66 The obtained TOFs are 75 h−1 for Pd/MIL-101(Fe)-NH2, 54 h−1 for Pd/MIL-101(Cr)-NH2, 41 h−1 for Pd/MIL-101(Al)-NH2 and 30 h−1 for Pd/MIL-53(Fe)-NH2 (Table S3, entries 1-4), further proving a super catalytic performance of Pd/MIL-101(Fe)-NH2 among the investigated samples. However, the Pd catalysts of Pd/MIL125(Ti)-NH2 and Pd/UiO-66(Zr)-NH2, with four-valent metal-based and amine-functionalized MOFs as the supports, show no catalytic performance towards benzyl alcohol formation (Table S3, entries 5 and 6). As compared, bare MOFs-derived Pd catalysts such as Pd/MIL-101(Fe), Pd/MIL-53(Fe) and Pd/UiO-66(Zr) (Table S3, entries 7-9), classical semi-conducts-derived Pd catalysts including Pd/TNT, Pd/CeO2 and Pd/P25 (Table S3, entries 10-12), were ineffective to photo-induced

20


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benzaldehyde hydrogenation. Therefore, the amine group on the catalysts of Pd/MIL-101(Fe)-NH2, Pd/MIL-53(Fe)-NH2, Pd/MIL-101(Al)-NH2 and Pd/MIL-101(Cr)-NH2 plays a key role on their photo-catalytic performance. To understand the promotion effect of the amine group on the Pd/MIL-101(Fe)-NH2 upon light absorption, we built two cluster models of Pd/MIL-101(Fe) and Pd/MIL-101(Fe)-NH2 as shown in Figure 4 for computational studies. Each cluster model contains three Fe centers and was constructed using benzoate and amino benzoate as simplified ligands to represent the terephthalate and aminoterephthalate ligands in the MIL-101 topology, respectively. In the absence of the amine group, Pd has a strong interaction with the top Fe center as indicated by a short Pd−Fe distance (2.803 Å) in the Pd/MIL-101(Fe). The interaction between Pd and amine group (2.340 Å) is stronger than that between Pd and Fe (3.364 Å) in the case of Pd/MIL-101(Fe)-NH2. DFT calculations thus showed that the adsorption of Pd on the top Fe center (∆G = -22.4 kcal·mol−1) as well as the adoption of two trimethylamine molecules on the other two Fe centers (∆G = -33.6 kcal·mol−1) are thermodynamically favorable.

Figure 4. The μ3-O-centered cluster models of Pd/MIL-101(Fe) and Pd/MIL-101(Fe)-NH2.

Besides stabilizing the Pd particle, the amine group can also enhance the electron density of the 21



Page 22 of 42

Pd center upon light adsorption, which is of crucial importance in increasing the catalytic activity. This enhancement can be readily understood through comparison of the electron density difference (EDD) between the ground state and the excited state of these two cluster models derived from the TDDFT calculations as depicted in Figure 5. Upon light absorption, clearly, the EDD plots for Pd/MIL-101(Fe)2[N(CH3)3] (Figure 5a) show the electron density loss from the Fe center and the electron density gain to the Fe center in the absence of the amine group. Interestingly, the EDD plots for Pd/MIL-101(Fe)-NH22[N(CH3)3] (Figure 5b) show a clear electron density loss from the Fe center and the electron density gain to the Pd center stabilized by the amino benzoate ligand. The increase of the electron density of the Pd center upon photon adsorption could explain the promoting effect of the amine group on the Pd/MIL-101(Fe)-NH2.

Figure 5. Electron density difference (EDD) plots for (a) Pd/MIL-101(Fe)2[N(CH3)3] and (b) Pd/MIL-101(Fe)-NH22[N(CH3)3]. The regions in magenta (cyan) correspond to increase (decrease) of the electron density upon photon absorption. The contour threshold is 0.002 au in all cases.

To probe the scope and limitations of the Pd/MIL-101(Fe)-NH2, we further investigated photoinduced transfer hydrogenation of various aldehydes with TEA-HCOOH. Table 2 lists the hydrogenation results. Methyl-substituted benzaldehydes were hydrogenated to the corresponding 22


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Table 2. Photo-induced transfer hydrogenation of various aldehydes by using Pd/MIL-101(Fe)-NH2 catalyst.a Entry

Aldehyde Conversion

Product Yield

(%)

(%)

1 97

51

64

42

39

26

32

16

95

33

47

29

35

27

2

3

4

5 12

8

6

7 aReaction

conditions: aldehyde (15 mg), TEA (0.6 mL), HCOOH (0.4 mL), catalyst (30 mg), CH3CN

(4.0 mL) under photo-irradiation of a xenon lamp equipped with a 420-nm cut-off filter at room temperature with reaction time of 6 h. The alcohol yield and aldehyde conversion were determined by GC-FID based on aldehyde using biphenyl as the internal standard material.

methyl-substituted benzyl alcohols with a decreased yield in the order of p-methyl- > m-methyl- > omethyl-benzyl alcohol (Table 2, entries 1-3), indicating a steric hindrance effect. In addition, a 23



Page 24 of 42

comparison of the hydrogenation of benzaldehyde, p-methyl benzaldehyde and p-ethyl benzaldehyde shows a reduced alcohol yield in the order of benzyl alcohol > p-methyl-benzyl alcohol > p-ethylbenzyl alcohol (Table 1, entry 1; Table 2, entries 1 and 4). In the cases of p-chlorobenzaldehyde, dechlorination and hydrogenation occur competitively yielding a mixture benzaldehyde, benzyl alcohol and p-chlorobenzyl alcohol (Table 2, entry 5). In addition to benzaldehyde derivatives, furan ring-containing aldehydes such as furfural and HMF were successively converted into the corresponding furan ring-containing alcohols under the investigated conditions (Table 2, entries 6 and 7). Notably, both furfural and HMF were biomass-based renewable platform molecules which are formed and transferred by the renewable solar energy in this work.

Scheme 1. Thermo-induced hydrogenation and transfer hydrogenation of benzaldehyde to benzyl alcohol by using Pd/MIL-101(Fe)-NH2.

In addition to photo-induced transfer hydrogenation of benzaldehyde, thermo-induced hydrogenation and transfer hydrogenation of benzaldehyde to benzyl alcohol were further investigated over Pd/MIL-101(Fe)-NH2. Previously, Pinna and co-workers investigated Pd/Cpromoted benzaldehyde hydrogenation with H2 as hydrogen source by producing benzyl alcohol

24


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selectivity of 83% with a TOF of 0.29 s−1.67 In our case, a full conversion of benzaldehyde with benzyl alcohol yield up to 94% was observed over Pd/MIL-101(Fe)-NH2 (Scheme 1 and Supporting Information). The TOF of the thermo-induced hydrogenation was 969 h−1 at 120 C for Pd/MIL101(Fe)-NH2 under the investigated conditions. For thermo-induced transfer hydrogenation reactions, Fan et. al reported Pd/C-catalyzed transfer hydrogenation of benzaldehyde with a benzyl alcohol yield of 63% by using iso-propanol as the hydrogen donor.68 Zhang and co-workers examined Pd-Fe/Al2O3 catalyst for transfer hydrogenation of benzaldehyde with benzyl alcohol yield of 12.9% by using methanol as the hydrogen source.69 Herein, both iso-propanol and methanol were respectively investigated as hydrogen source for transfer hydrogenation of benzaldehyde with Pd/MIL-101(Fe)NH2 as catalyst. For the hydrogen source of iso-propanol, a full benzaldehyde conversion and the benzyl alcohol yield of 96% was observed over Pd/MIL-101(Fe)-NH2 with a TOF value up to 880 h−1 (Scheme 1 and Supporting Information). In the case of the hydrogen source with methanol, only a 16% yield of benzyl alcohol was detected with a TOF value of 132 h−1 (Scheme 1).

Benzaldehyde Conv.

100

Benzyl alcohol Yield

100

99

96

91

80 69

60

91

80

67

60 54

51

40

51

20

Yield / %

100

Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 20

0

0 1

2

3

Cycle

4

5

Figure 6. Pd/MIL-101(Fe)-NH2 recycling on photo-induced transfer hydrogenation of benzaldehyde.

Finally, to further examine the stability and reusability of Pd/MIL-101(Fe)-NH2, recycling 25



Page 26 of 42

experiment on the photo-induced transfer hydrogenation of benzaldehyde was carried out with Pd/MIL-101(Fe)-NH2 under the optimized conditions as shown in Table 1. In a typical cycle, the Pd/MIL-101(Fe)-NH2 was separated from the reaction medium by centrifuge, and then thoroughly washed by CH3CN and deionized water for 3-5 times before the next run. As shown in Figure 6, a slightly reduced activity of Pd/MIL-101(Fe)-NH2 was observed during a five-recycling experiment. The benzaldehyde conversion decreased from 100% to 91%; while, the benzyl alcohol yields accordingly reduced from 69% to 51%.

4. Conclusions In conclusion, we demonstrated a photo-induced transfer hydrogenation of aromatic aldehyde to the corresponding alcohol under a visible-light irradiation by using Pd/MIL-101(Fe)-NH2 in the presence of TEA-HCOOH. The Pd/MIL-101(Fe)-NH2 is formed by an in-situ photodeposition of [MIL-101(Fe)-NH3]+∙1/2[PdCl4]2− in TEA-HCOOH and shows highly dispersed and uniform Pd NPs with a mean size around 1.8 nm. The Pd/MIL-101(Fe)-NH2 efficiently promotes a visible-lightinduced transfer hydrogenation of benzaldehyde by producing benzyl alcohol yield of 77% with the TOF of 75 h−1 in the presence of TEA-HCOOH. Prominently, biomass-based furfural and HMF were successively converted into the corresponding alcohols with the highest yields reported so far by the visible-light-induced transfer hydrogenation method. Our theoretical research based on DFT further confirms a dual function of amine group in the Pd/MIL-101(Fe)-NH2 for Pd NPs stabilization as well as for enhancement of the electron density of the Pd center upon light adsorption. This work demonstrates an environmental-friendly and efficient strategy for aldehyde hydrogenation by using a renewable solar energy as a driven force.

5. ASSOCIATED CONTENT

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxxx. Materials, characterization, reaction conditions, textural properties and XPS binding energies of the catalysts, effect of catalyst support on the hydrogenation, N2 adsorptiondesorption curves, PXRD patterns, FT-IR, TGA pattern, SEM images, TEM images and nanoparticle size distributions, UV-visible absorption spectra, photography of catalyst (PDF).

6. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected]. Tel.: (+86)-20-8522-0223, Fax: (+86)-20-8522-0223 (J. Chen), *E-mail address: [email protected] (Y. Xu). S. Dong and Z. Liu contributed equally to this work. ORCID Jinzhu Chen: 0000-0002-6475-1431 Yisheng Xu: 0000-0001-8245-9787

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by National Natural Science Foundation of China (21676089, 21472189 and 91645119), Shanghai Talent Development Fund (2017038), the Fundamental Research Funds for the Central Universities (21617431, 222201717013), and Natural Science Foundation of Guangdong Province, China (2018B030311010).

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 38


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Figure 1. (a) Photo-induced transfer hydrogenation of aromatic aldehydes by using Pd/MIL-101(Fe)-NH2 499x451mm (150 x 150 DPI)



Figure 2. TEM images and nanoparticle size distribution of Pd/MIL-101(Fe)-NH2 prepared by (a-d) 792x1517mm (96 x 96 DPI)


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Figure 5. Electron density difference (EDD) plots for (a) Pd/MIL-101(Fe) 390x219mm (150 x 150 DPI)



TOC 483x259mm (150 x 150 DPI)


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Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic

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