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Synthesis of Robust MOF-derived Cu/SiO2 Catalyst with Low Copper Loading via Sol-gel Method for the Dimethyl Oxalate Hydrogenation Reaction Run-Ping Ye, Ling Lin, Chong-Chong Chen, Jin–Xia Yang, Fei Li, Xin Zhang, De-Jing Li, Ye-Yan Qin, Zhangfeng Zhou, and Yuan-Gen Yao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00501 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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ACS Catalysis

Synthesis of Robust MOF-derived Cu/SiO2 Catalyst with Low Copper Loading via Sol-gel Method for the Dimethyl Oxalate Hydrogenation Reaction Run-Ping Ye,†,‡,§# Ling Lin,†# Chong-Chong Chen,†,‡ Jin-Xia Yang,† Fei Li,†,‡ Xin Zhang,† De-Jing Li,†,∥ Ye-Yan Qin,† Zhangfeng Zhou,† and Yuan-Gen Yao* ,† †

Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of

Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P.R. China ‡

§

University of Chinese Academy of Sciences, Beijing 100049, P.R. China Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY

82071, USA ∥

College of Chemistry, Fuzhou University, Fuzhou, Fujian 350116, P.R. China

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ABSTRACT: The insufficient catalytic stability of Cu-based catalysts is still its bottleneck in industrial application, and it is widely studied how to synthesize highly stable Cu-based catalysts.

Herein,

we

have

developed a MOF-derived preparation of robust Cu/SiO2 catalyst for the efficient hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). The sol-gel method was used to in situ load the HKUST-1 into SiO2 support with only a copper loading of 7.83%. The advantages of MOF-derived Cu/SiO2 catalyst are that copper species could escape from severe coating by silica matrix and expose more active sites with small particles sizes (4.2 nm). Furthermore, it is inclined to generate more stable Cu2O reduced from Cu-O-Si units by using MOF as precursor. As a result, more than 95.0% selectivity of EG with a long lifetime of 220 h was achieved over the Cu/SiO2-MOF catalyst. On the contrary, when using the copper nitrate as the copper precursor, the prepared Cu/SiO2-CN catalyst with similar copper loading exhibited much lower catalytic activity because that copper species were encapsulated into SiO2 network and difficult to be available for reactants. Therefore, using MOF as copper precursor solves the problem of high-dispersed copper species but inactive by coating with SiO2 matrix.

KEYWORDS: HKUST-1; Ethylene Glycol; Cu/SiO2 Catalyst; Sol-gel Method; Vapor-phase Hydrogenation

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ACS Catalysis

1. INTRODUCTION Cu-based catalysts are of paramount significance in the hydrogenation of C-O bonds from esters,1, 2 carboxylic acids,3 ethers,4 furfural,5, 6 and CO2.7, 8 For example, the silica-supported copper catalysts is a promising candidate in the hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) (DMO-to-EG reaction) owing to its high catalytic activities and low cost, achieving much attention during the past decade.9-13 However, an inherent problem, namely poor stability severely restricts its industrial application. Currently, extensive efforts have been devoted to resolving the poor stability of Cu/SiO2 catalysts.12, 14-19 In industry, Cu-Cr catalysts have been largely used for the hydrogenation of DMO due to its highly efficient activity and long-term lifespan.19 Nonetheless, the toxic chromium is dangerous to humans and environment. Thus it is still urgent to design stable and environmental Cu/SiO2 catalysts for the DMO hydrogenation reaction. Besides the typical strategies of changing Cu loading 20 and doping additives including Pd,21 Au,22 Ag,23 Co,17 Ni,9 La,12 B,24 etc., nowadays some novel and efficient approaches have been introduced for the controlled preparation of Cu/SiO2 catalysts with high performance and long-term stabilities. A highly efficient Cu/SiO2 catalyst for DMO hydrogenation reaction was synthesized via a novel hydrolysis precipitation method using tetraethyl orthosilicate (TEOS) as silicon source by Zhao et al.10 Yue et al.25 presented more than 300 h steady performance of DMO hydrogenation to ethanol with a core (copper)-sheath (copper phyllosilicate) nanoreactor, which possesses confinement effects, intrinsically high surface area and unique tunable tubular morphology. In our previous work, we also fabricated some thermally stable Cu/SiO2 catalysts with a copper loading of ~20% by doping with dextrin15 or β-cyclodextrin26 to improve the morphologies of the supported copper particles. However, how to prepare the robust Cu-based catalysts with low copper loading for DMO hydrogenation reaction has been rarely 3 / 37


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investigated. The sol-gel method has also been widely employed to synthesize stable Cu-based catalysts for the advantageous of effective incorporation of the copper species into silica network with a higher stability.27-29 Meanwhile, a difficult issue occurred when using the copper salts and silicon alkoxide to directly prepare Cu-Si based catalysts. On the one hand, we found that the copper species were highly dispersed and seriously enveloped into the silica matrix if the copper loading were less than 10%, resulting in weakly accessible to reactants for the copper species with lower activities. On the other hand, although the catalyst could expose more active sites on its surface by increasing copper loading to more than 10%, the agglomeration among the copper species became more serious.28, 30 Thus, we have been trying to synthesize an efficient Cu/SiO2 catalyst where the copper species can not only escape from silica’s encapsulation but also keep its dispersion. Fortunately, this contradictory question can be addressed by using the metal-organic frameworks (MOFs) as copper precursors. But then a fundamental question arises: why use expensive MOFs as precursors to make simple Cu/SiO2 catalyst? (1) Due to their special structures: it is well known that the MOFs with crystalline regular structures offer potential applications in ion exchange,31 gas storage,32 luminescence, and catalysis.33-35 Their regular and large frameworks would not be easy coated by the silica. (2) Due to the fact that HKUST-1 is a special MOF: a copper 1,3,5-benzenetricarboxylate (BTC) MOF [Cu3(BTC)2], which has been wildly employed as the precursor to synthesize metal oxides,36 nanoparticles@carbon,37 carbon nanodots38 and films39 because of its facile preparation method, inexpensive reactants, large ordered pores and large surface area. (3) Due to the pure Cu/SiO2 catalysts possessing poor catalytic activities and stabilities for DMO hydrogenation reaction, especially with low copper loading (< 10%).22, 4 / 37


26

The catalytic performance of

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ACS Catalysis

MOF-derived Cu/SiO2 catalyst can be enhanced. (4) Due to the green preparation and unnecessary tabletting process by sol-gel method using TEOS as silica source. The ammonia-evaporation method (AEM) to prepare conventional Cu/SiO2 catalysts using SiO2 sol or SBA-15 generates fumes with high concentration of ammonia and should be tabletted to small particles in industry, which would produce a lot of poisonous dusts. In fact, the systems of MOFs and SiO2 have been extensively investigated in the past five years, because that most of the MOFs have limited thermal and chemical stabilities and should be loaded on stable silica supports.37 Stein’s group has successfully assembled well-dispersed single site catalytic clusters from NU-1000 on nanocast silica supports for high-temperature reactions.40, 41Most of reported HKUST-1@silica composite focused on directly working in chromatographic separation,42-44 gas sorption,45, 46 and catalysis.47-49 However, their derived Cu-Si systems have been less reported. Although the copper-based catalysts for the DMO-to-EG reaction have been reported a lot,21, 22, 50-52

the direct synthesis of Cu/SiO2 catalyst with MOF as copper precursor has never been reported

for DMO hydrogenation reaction. Herein the HKUST-1 was synthesized first and then further in situ immobilized into silica support with one-pot sol-gel method (Scheme 1). As a result, the copper species of the dried catalyst precursor were ultrasmall and uniform on the silica support. After calcination in air and reduction in H2 atmosphere, the fabricated Cu/SiO2 catalyst exhibited significantly catalytic activity and excellent durability for the DMO-to-EG reaction. This unique synthetic approach led to obtaining low copper loading with high-performance catalysts suitable for high-temperature reactions.

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Scheme 1. Schematic illustration of the preparation of Cu/SiO2-MOF with photographs via sol-gel method.

2. EXPERIMENTAL SECTION 2.1. Materials All the reagents and solvents were commercially available and used for the synthesis without further purification. Cupric acetate (Cu(CH3COO)2·H2O, AR, Tianjin Fuchen Chemical Reagent Factory), copper nitrate trihydrate (Cu(NO3)2·3H2O, AR, Sinopharm Chemical Reagent Co., Ltd), 1,3,5-benzenetricarboxylic acid (BTC, 98%, Aladdin), ethanol (EtOH, AR, Sinopharm Chemical Reagent Co., Ltd) and tetraethyl orthosilicate (TEOS, AR, Sinopharm Chemical Reagent Co., Ltd) were used as received.

2.2. Catalyst preparation Synthesis of HKUST-1 powder. The HKUST-1 powder was synthesized by the solvothermal method according to previously reported.38, 39 Briefly, Cu(CH3COO)2·H2O (7.5 mmol, 1.5 g) and 6 / 37


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ACS Catalysis

BTC (5 mmol, 1.05 g) were mixed in 8 ml pure ethanol in a sealed glass bottle and dissolved in an ultrasonic bath. After 30 min, the mixture was heated at 65 °C for 48 h and cooling to room temperature. The resulting HKUST-1 powder was washed with pure ethanol and dried at 50 °C for 5 h.

Synthesis of Cu/SiO2-MOF catalyst. The Cu/SiO2-MOF catalyst was prepared by the sol-gel method. HKUST-1 powder (5.0 g) was dispersed in distilled water (22.0 g) and then added pure ethanol (37.0 g) and TEOS (28.5 g). The above precursor solution was stirred at 65 °C for about 1~2 h in a water bath and aged at room temperature for 24 h. After drying in oven at 65 °C for 7 h, 70 °C for 7 h, 100 °C for 40 h and 120 °C for 4 h, the solid product was sent to calcination in static air at 350 °C for 5 h. The sample was named as Cu/SiO2-MOF catalyst.

Synthesis of Cu/SiO2-CN catalyst. For comparison, the Cu/SiO2-CN catalyst was also synthesized by the sol-gel method similar to Cu/SiO2-MOF catalyst except that HKUST-1 powder (5.0 g) was replaced by copper nitrate trihydrate (3.0 g) as copper precursor. Besides, the amount of distilled water, pure ethanol, and TEOS was changed to 21.6, 41.4 and 31.8 g, respectively. The sample was named as Cu/SiO2-CN catalyst. In addition, pure SiO2 was prepared similarly to Cu/SiO2-MOF catalyst, only except that HKUST-1 powder was not added.

2.3 Catalyst characterization The copper content was detected by inductively coupled plasma optical emission spectrometer (ICP-OES) on a Jobin Yvon Ultima2. The copper dispersion and copper surface area of the catalyst were measured on the on a Micromeritics Autochem II 2920 instrument connected with a Hidden Qic-20 mass spectrometry (MS) using the automatic pulse titration method based on H2 reduction 7 / 37


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and N2O chemisorption. N2 adsorption-desorption measurement was performed at 77 K using a Micromeritics ASAP 2020 instrument. Thermogravimetric analysis (TGA) data was recorded on a NETSCHZ STA-449C thermal analyzer from 30 to 900 °C (heating rate: 10 °C·min-1) under N2 atmosphere. Fourier-transform infrared (FT-IR) spectra were collected on a Bruker Vertex 70 FT-IR spectrometer (spectral resolution: 2 cm-1). The temperature-programmed reduction (H2-TPR) and temperature-programmed desorption profiles (NH3-TPD) were obtained on a Micromeritics Autochem II 2920 instrument and TP-5080 instrument, respectively. The Powder X-ray diffraction (PXRD) analysis was performed on a Rigaku MiniFlex II diffractometer using Cu-Kα radiation with a scanning angle (2θ) range of 5-80°. Scanning electron microscope (SEM) images were collected using an FEI Quanta FEG 450 apparatus. Transmission electron microscope (TEM) images of the dried and calcined samples were collected using an FEI Titan 80-300 S-Twin (S)TEM equipped with a spherical aberration image corrector at 300 kV, then the reduced and used samples were obtained using a Tecnai F20 apparatus at 200 kV. The size distribution of copper species was calculated from TEM images by the software of Nano Measurer 1.2. The UV-Vis spectra were measured at room temperature with a Shimadzu UV-2600 UV-vis spectrophotometer. The X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (XAES) were performed using an ESCALAB 250Xi equipped with an Al Kα X-ray radiation source (hν = 1486.6 eV). The binding energies (BEs) were calibrated on the basis of Si 2p peak (103.7 eV).

2.4. Catalytic performance test The catalytic performance test was done in a fixed-bed reactor. The Cu/SiO2 catalyst (2.5 g, 10-20 meshes) was first reduced and activated in a pure H2 (99.99% purity) atmosphere at 300 °C for 5 h. Furthermore, the system was set to reaction temperature and pressured to 2.0 MPa. Then a 20 wt% 8 / 37


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ACS Catalysis

DMO (99% purity) in CH3OH was continuously pumped into the reactor with feeding H2 (H2/DMO molar ratio = 50). The weight liquid hour space velocity (WLHSVDMO) was ranged from 0.38 to 2.02 h-1. The products were cooled down and detected by a gas chromatography (Agilent 7820A) with a flame ionization detector (FID). The main products of DMO hydrogenation reaction were EG and methyl glycolate (MG), others include EtOH, 1,2-butanediol and 1,2-propanediol. The DMO conversion and product selectivity were calculated as follows:

moles of DMO (input) - moles of DMO (output)

× 100

Conversion (%) =

moles of DMO (input)

moles of one product

× 100

Selectivity (%) = moles of DMO (input) - moles of DMO (output)

3. Results 3.1. Textural properties The HKUST-1 powder was prepared by one-pot solvothermal reaction of Cu(OAc)2 and BTC in pure ethanol at 65 °C for two days. As shown in Table 1 and Figure S1, the HKUST-1 exhibited a surface area of 292.8 m2 g-1 and a copper concentration of 21.21%. When it was introduced to silica support by the sol-gel method, the SBET, pore volume (Vp), and actual copper loading of Cu/SiO2-MOF sample were changed to 455.3 m2 g-1, 0.44 m3 g-1 and 7.83 %, respectively. Compared with pure SiO2 (SBET=635.9 m2 g-1, Vp=0.85 m3 g-1), the slightly decreased SBET and pore volume of Cu/SiO2-MOF catalyst suggested that some of the copper species were occupied the 9 / 37


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silica cavities while most of them were dispersed on the surface. However, after the copper precursor was changed to Cu(NO3)2 with similar Cu loading of 7.32%, the SBET and Vp of Cu/SiO2-CN were dramatically decreased to 263.8 m2 g-1 and 0.08 m3 g-1, implying that introduction of copper species seriously affected the pore shape of SiO2 support. Furthermore, Cu dispersion and surface area of Cu0 (SCu0) for Cu/SiO2-MOF were 10.0% and 5.1 m2 g-1, respectively, which were much higher than those in the Cu/SiO2-CN sample (1.4% and 0.7 m2 g-1). On the basis of H2 reduction and N2O chemisorption [2Cu(s) + N2O

N2 + Cu2O (s)], the low surface copper

dispersion indicated that copper species were incorporated into SiO2. The colour of the calcined Cu/SiO2-CN sample is clear blue, suggesting that Cu2+ are dispersed into silica gel matrix in atomic level. While the calcined Cu/SiO2-MOF sample is dark gray, indicating that it possesses CuO phase (Figure S2).

Table 1. Physicochemical features of Cu/SiO2 catalysts with different copper precursors. Cu

Surface

Pore

Cu Catalysts

Cu

Pore SCu

loading

Area

Volume

dispersion

Diameter

Colour

(m2 g-1)b

precursor (wt%)a

(m2 g-1)

(cm3 g-1)

(nm)

/

/

635.9

0.85

5.1

/

/

White

HKUST-1

Cu(OAc)2

21.21

292.8

0.06

2.2

/

/

Dark blue

Cu/SiO2-CN

Cu(NO3)2

7.32

263.8

0.08

2.1

0.7

1.4

Clear blue

Cu/SiO2-MOF

HKUST-1

7.83

455.3

0.44

3.6

5.1

10.0

Dark grey

SiO2

a

Cu loading obtained from ICP-OES.

b

Cu metallic surface area and dispersion collected by N2O titration.

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(%) b

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ACS Catalysis

The FT-IR spectra and data of pure HKUST-1, SiO2, Cu/SiO2-MOF and Cu/SiO2-CN samples can be seen in Figure 1. The absorption band of carboxyl groups at around 1710 cm-1 for HKUST-1 were observable, as well as the symmetric and asymmetric stretching vibrations of –C=O appearing at 1645/1590 cm-1 and 1450/1372 cm-1, respectively (Figure 1A).53 For the amorphous silica spectra, band at 1080 cm-1 is attributed to stretching vibrations of Si-O bonds; that at 970 cm-1 corresponds to the stretching vibration of Si–OH bonds; and that at 800, 470 cm-1 belong to bending vibrations of Si-O bonds, -O-Si-O- bonds, respectively.54 When the HKUST-1 was loaded on the silica support, the characteristic FT-IR peaks of HKUST-1 could be observed over the Cu/SiO2-MOF-Dried sample. After calcined at 350 °C in air, the absorption band of BTC disappeared, suggesting that the organic linkers were removed completely and left two main copper species on the SiO2 support. One is CuO, which can be seen from the absorption bands at 580, 500, 460 cm-1.30 The other is Cu-O-Si units, which can be indicated from the band at 970 cm-1 for pure SiO2 shifts to 960 cm-1 and the I960/I800 intensity ratio raises from 0.86 (SiO2) to 2.13 (Cu/SiO2-MOF-Calcined). Kong et al. also observed similar bands shift for the 10Cu-MCM-41 sample and attributed to forming Cu-O-Si bonds, because that the Cu-O bond length is greater than that of Si-O and result in a decrease in the force constant of the bond.55 Furthermore, the Cu2+ sites of HKUST-1 are coordinatively unsaturated and can interact with SiO2 support.56-58 As shown in Figure 1B, in the case of dried Cu/SiO2-CN sample, the Cu2(OH)3NO3 was distinguished by the characteristics of NO3- bands at 1422, 1383, 1339 cm-1,59, 60 showing that some Cu2+ and NO3- ions were coated with the SiO2 matrix to form copper nitrate hydroxide instead of copper phyllosilicate. In addition, the band of stretching vibration of Cu-O-Si bonds has shifted to 950 cm-1 and also has a weak adsorption band of CuO at 580 cm-1. The two Cu/SiO2 catalysts reported here do not show the phase of copper phyllosilicate, which often exists in the catalysts prepared by AEM and has the characteristic 11 / 37


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absorption bands at around 1033 and 673 cm-1.13, 15, 61

Figure 1. FT-IR spectra of HKUST-1, pure SiO2, and Cu/SiO2-MOF samples (A), as well as Cu/SiO2-CN samples (B) and characteristics of different species (C). Cu/SiO2-AEM: prepared by ammonia-evaporation method (AEM).

Figure 2 shows the UV-vis spectra of HKUST-1 and Cu/SiO2 catalysts. The adsorption bands at around 258 nm are attributed to a ligand to metal charge transfer transition from oxygens to copper ions and the other absorption bands (~ 700 nm) are due to a d-d transition of octahedral Cu2+ ions.49, 62 Two points are worth mentioning: one is that the absorption intensity at 258 nm increases as well as the peak redshifts from 676 nm of the pure HKUST-1 to 718 nm after loading to silica support. This is due to the interaction between HKUST-1 and SiO2 and consistent with the result of 12 / 37


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ACS Catalysis

one-pot synthesis of Cu3(BTC)2@SiO2 core-shell nanoparticles reported by Li et al.49 The second point is that the Cu/SiO2-MOF-Calcined sample exhibits a wide adsorption between 350 and 800 nm that can be attributed to the characteristic absorption of CuO nanoparticles.63 After calcination at 350 °C for 5 h, the HKUST-1 or Cu2(OH)3NO3 was mostly decomposed, as evidenced by the thermogravimetric analysis curves (Figure S3). Both the dried HKUST-1 and Cu/SiO2-MOF displayed a sharp weight loss during the temperature of 290-360 °C, which was attributed to BTC decomposing. The Cu2(OH)3NO3 was decomposed to CuO from 158 to 290 °C over the dried Cu/SiO2-CN sample.

Figure 2. UV-vis spectra of different samples.

3.2. Evolution of copper species and morphology Because of a different copper precursor, the copper species in the dried or calcined precursors of Cu/SiO2 samples are different from each other, and the PXRD profiles of them are shown in Figures 13 / 37


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3-4. In Figure 3A, the PXRD profile of the obtained HKUST-1 matched well with the simulated ones, indicating that HKUST-1 was successfully synthesized. Apart from the diffraction pattern of amorphous SiO2 at 2θ of ~ 22°, some diffraction peaks with changed relative intensities could still be observed over the dried Cu/SiO2-MOF catalyst precursor, demonstrating that the HKUST-1 framework structure was not influenced seriously through the sol-gel procedure. The dried catalyst precursor was transformed to highly dispersed CuO after calcination, which was demonstrated by the diffraction peaks at 2θ of 35.5° and 38.7° (Figure 3A, JCPDS 05-0661).13 Furthermore, the diffraction peaks of both Cu (2θ= 43.3° and 50.4°, JCPDS 04-0836) and Cu2O (2θ= 36.4°, JCPDS 05-0667) can be observed over the reduced Cu/SiO2-MOF catalyst (Figure 3B). It was reported that CuO was reduced facilely to Cu0 while the Cu-O-Si species were ceased at Cu+ under the same reduction condition due to the strong interaction between copper ions and SiO2.11, 13, 20 Figure 4 shows almost different PXRD profiles of Cu/SiO2-CN samples from Cu/SiO2-MOF. The Cu/SiO2-CN-Dried sample exhibited a phase of copper nitrate hydroxide as demonstrated by diffraction peak at 2θ of 12.8°, 25.8° and 33.5° (JCPDS 75-1779), corresponding with the FT-IR results (Figure 1B). After calcination, the diffraction peak of copper nitrate hydroxide disappeared and no diffraction peaks of copper species were found, indicating Cu2(OH)3NO3 in dried precursor were converted into amorphous or very small copper species. The distinct diffraction peak of Cu while no obvious diffraction peak of Cu2O was observed in the Cu/SiO2-CN-Reduced sample, demonstrating that copper species in the Cu/SiO2-CN catalyst were reduced to Cu0 and highly dispersed Cu+, which was different from the Cu/SiO2-MOF catalyst.

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ACS Catalysis

Figure 3. PXRD patterns of pure HKUST-1, SiO2, dried Cu/SiO2-MOF (A), calcined and reduced Cu/SiO2-MOF samples (B).

Figure 4. PXRD patterns of Cu/SiO2-CN samples.

The dispersion of copper species in Cu/SiO2 catalysts were observed by TEM. As shown in Figure 5A, the TEM image of the dried Cu/SiO2-MOF sample clearly displayed that the ultrafine 15 / 37


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nanoparticles with approximately 2.3 nm (3.3 nm for the dried Cu/SiO2-CN, Figure 5D) were highly dispersed on the SiO2 support. After the calcination and reduction process, the copper nanoparticles grew larger, but the average size (4.2 nm) was still less than 5 nm (Figure 5B-C). Furthermore, the average sizes of Cu nanoparticles over the calcined and reduced Cu/SiO2-CN samples (Figure 5E-F) are similar with the Cu/SiO2-MOF. It was reasonable to consider that during the prolonged heating of Cu/SiO2-MOF catalyst at 350 °C for 5 h and 300 °C for 5 h, the frameworks of HKUST-1 were destroyed and converted into copper nanoparticles and other volatile products. Fortunately, it was also believed that the SiO2 support around the copper nanoparticles depressed their sintering during the thermal treatment, resulting in the high distribution of tiny copper nanoparticles in the SiO2 support, as evidenced by the TEM images. Niu et al.

64

reported

that pure HKUST-1 calcined at a high temperature of 600 °C resulted in much larger Cu/Cu2O nanoparticles with about 40 nm. Malonzo et al.40 estimated that when the NU-1000@SiO2 was heated to 600 °C, the crystallites were still single isolated clusters. However, when the bare NU-1000 was heated to only 500 °C, the average size of ZrO2 nanoparticles was about 18 nm. Hence, the silica layer provides anchoring sites for the copper species. The structures of the solid precursors of Cu/SiO2-Dried were investigated by SEM and TEM. The HKUST-1 and dried Cu/SiO2 samples’ morphologies were shown in Figure S4. After HKUST-1 was loaded into silica, more spherical particles can be seen in the Cu/SiO2-MOF-Dried samples. As shown in Figures S5-6 of EDX mapping images from TEM, the HKUST-1 were highly deposited on silica support while copper species over the Cu/SiO2-CN were separated and coated by silica matrix. It is more clear over the calcined Cu/SiO2 samples in Figure S7. Some dispersed CuO particles show on the surface of Cu/SiO2-MOF sample while the copper species cannot be seen over the Cu/SiO2-CN sample. 16 / 37


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ACS Catalysis

Figure 5. TEM images with size distributions of dried, calcined, and reduced Cu/SiO2-MOF (A-C, respectively); dried, calcined, and reduced Cu/SiO2-CN (D-F, respectively).

3.3. Reducibility and surface acid properties To determine the reducibility and surface acid properties of the Cu/SiO2 catalysts, the H2-TPR and NH3-TPD test of the Cu/SiO2 catalysts were conducted and displayed in Figures 6-7. As can be seen in Figure 6, the HKUST-1 exhibited two big reduction peaks before and after 300 °C, corresponding to decomposition of HKUST-1 and reduction of CuO to Cu0.13, 59 However, only a main reduction peak at 214 °C was found for the calcined Cu/SiO2-MOF catalyst, indicating that copper species were dispersed uniformly on SiO2 (Figure 6). From this point of view, the HKUST-1 should be loaded on the carriers, otherwise, it would be seriously aggregated during the thermal treatment. 17 / 37


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The H2 consumption of Cu/SiO2-MOF was much less than pure HKUST-1 owing to only small amount of HKUST-1 loaded on silica support. The Cu/SiO2-CN catalyst exhibited a much weakly broad reduction peak from 150 to 350 °C, suggesting that most copper species were protected by silica, thus hard to be decomposed and reduced. In addition, the NH3-TPD was performed to evaluate the strength and quantity of acidic sites on the reduced Cu/SiO2 catalysts and the obtained profiles were shown in Figure 7 and Figure S8. Pure SiO2 and Cu/SiO2-CN catalyst showed two desorption peaks: around 100 and 500 °C, respectively. However, for the Cu/SiO2-MOF catalyst, the HKUST-1 precursor weakened these two desorption peaks and appeared another large desorption peak at 245 °C, as well as a small desorption peak at 383 °C, suggesting that moderate acidic sites dominated the Cu/SiO2-MOF catalyst surface and BTC precursor was responsible for the acidity changes.

Figure 6. H2-TPR profiles of dried HKUST-1, calcined Cu/SiO2-MOF and Cu/SiO2-CN samples. 18 / 37


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Figure 7. NH3-TPD profiles of Pure SiO2, Cu/SiO2-MOF, and Cu/SiO2-CN samples.

3.4. Surface chemical states of the catalysts The XPS and XAES results of the Cu/SiO2 catalysts were illustrated in Figure 8 and Table 2. Only two peaks of Cu 2p1/2 (953.1 eV) and Cu 2p3/2 (933.6 eV) were observed in the reduced Cu/SiO2 catalysts (Figure 8A), with the disappearance of the Cu 2p satellite peak centered at around 943.2 eV, indicating that all the surface Cu2+ species over the calcined catalysts had been reduced to Cu0 or Cu+ at 300 °C. The signal intensities of Cu/SiO2-MOF samples are obvious strong than those of Cu/SiO2-CN samples, indicating that more surface copper species over the Cu/SiO2-MOF catalyst.20, 65

Furthermore, in the Figure 8B of Cu LMM XAES spectra, broad and asymmetry Auger peaks

were displayed and deconvoluted into two symmetrical peaks, suggesting that both Cu0 and Cu+ exhibited over the Cu/SiO2-Reduced samples. As summarized in Table 2, the Auger parameter value 19 / 37


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for Cu0 and Cu+ over the Cu/SiO2 catalysts were 1849.0±0.3 and 1846.0±0.4 eV, respectively, which were almost 2~3 eV lower than the normal bulk compounds (1851.0 eV for Cu and 1848.8 eV for Cu2O).66, 67 This was because that copper species were highly dispersed and intimately contacted with the silica support.20, 68 Besides, the ratio of Cu+/(Cu+ + Cu0) was much enhanced from 52.4 to 73.3% with using HKUST-1 as copper precursor, which was presumed to more Cu-O-Si units and Cu2O in the Cu/SiO2-MOF catalyst as also evidenced above.

Figure 8. Cu 2p XPS spectra (A) and Cu LMM XAE spectra (B) of Cu/SiO2 samples. (a)

Cu/SiO2-CN-Calcined,

(b)

Cu/SiO2-MOF-Calcined,

Cu/SiO2-MOF-Reduced.

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(c)

Cu/SiO2-CN-Reduced,

(d)

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Table 2. Copper species in the reduced Cu/SiO2 samples based on Cu LMM XAES spectra. K.E. (eV)a Catalysts

A.P. (eV)b

B.E. of

XCu+ (%)c

Cu+

Cu0

Cu+

Cu0

Cu 2p3/2 (eV)

Cu/SiO2-CN

912.1

915.1

1845.7

1848.7

933.6

52.4

Cu/SiO2-MOF

913.1

916.0

1846.4

1849.3

933.3

73.3

916.3

-

1848.8

-

932.5

-

-

918.3

-

1851.0

932.7

-

Cu2O

d

d

Cu a

Kinetic energy from Cu LMM peak.

b

Auger parameter.

c

Intensity ratio between Cu+ and (Cu+ + Cu0) by deconvolution of Cu LMM XAES.

d

The XPS data of bulk Cu2O and Cu were also collected by the ESCALAB250 equipment from ref.66

3.5. Cu/SiO2 catalysts performance It is well-known that EG is a significant intermediate and widely used in polyester and plasticizer manufacture.69 The Cu/SiO2 catalysts prepared from two different copper precursors were applied in the synthesis of EG via vapor-phase hydrogenation of DMO and the results were presented in Figure 9 and Table 3. The conversion of DMO (Conv.DMO) and the selectivity of EG (Selec.EG) over the Cu/SiO2-MOF sample at 170 °C were only 27.8% and 7.2%, respectively. With increasing the reaction temperature from 170 to 240 °C, both the Conv.DMO and Selec.EG increased greatly, reaching a maximum EG selectivity of 98.6% at 200 °C (Figure 9A). However, it is surprising that the Cu/SiO2-CN catalyst displayed nearly no catalytic activity at 200 °C (Table 3). Even when the reaction temperature was increased to 240 °C, Conv.DMO and Selec.EG were only 21.6% and 3.9%, respectively, producing 95.7% of MG. Compared with two other Cu/SiO2 catalysts reported before, 15, 26

catalytic performance of the lower copper loading Cu/SiO2 sample derived from MOFs was

much better than Cu/SiO2 catalyst prepared by sol-gel method with more copper loading and comparable with Cu/SiO2 catalyst synthesized by AEM (Table 3). The results showed that the 21 / 37


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Cu/SiO2-MOF sample exhibited the highest yield of EG at 200 °C, and much intermediate product of MG still in the sample Cu/SiO2b while much deep hydrogenation products were in the sample Cu/SiO2c at 240 °C. Even compared with SBA-15, MCM-41, or HMS supported copper catalysts,20, 65, 70

the EG yield of Cu/SiO2-MOF sample was still higher than theirs at 200 °C (Table 3). Yuan’s

group indicated that pure Cu/SBA-15 with lower copper loading (< 10%) possessed poor catalytic performance for DMO hydrogenation reaction and should be enhanced by doping with Au or Pt.22, 71

Figure 9B shows the catalytic performance of Cu/SiO2-MOF as a function of WLHSVDMO ranged from 0.38 to 2.02 h-1. Under a low WLHSVDMO of 1.01 h-1, the catalyst displayed high Conv.DMO of more than 99.0% and high Selec.EG of more than 90.0%. Further increasing the WLHSVDMO, the Conv.DMO and Selec.EG decreased while Selec.MG increased dramatically. When the WLHSVDMO was 2.02 h-1, the Conv.DMO and Selec.EG were still high up to 80.2% and 41.9%, respectively. The results indicated that the catalytic performance of Cu/SiO2-MOF catalyst was still better than 20Cu/SBA-15 and 10Cu-MCM-41catalysts under high WLHSVDMO.65, 70 Besides, a negligible changing of Conv.DMO and Selec.EG with the EG selectivity of more than 95.0% was observed over the Cu/SiO2-MOF catalyst after performance for 220 h (Figure 10). This indicated that the MOF-derived Cu/SiO2 catalyst exhibited not only good activity but also superior stability.

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Figure 9. Catalytic performance of Cu/SiO2-MOF sample in different reaction temperatures (170-240 °C) with WLHSVDMO= 0.82 h-1(A) and different WLHSVDMO (0.38-2.02 h-1) at 210 °C (B).

Figure 10. Long-term stability test of Cu/SiO2-MOF sample at reaction temperature of 210 °C and WLHSVDMO of 0.82 h-1.

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Table 3. Comparison of Cu/SiO2 catalysts performance for DMO hydrogenation at 200 °C and 240 °C. Reaction T= 200 °C WLHSVDMO

Conv.DMO

Selec.EG

Selec.MG

Selec.others

STYEGa

/ h-1

/%

/%

/%

/%

/ h-1

Cu/SiO2-CN

0.41

16.9

6.9

92.7

0.4

0.003

Cu/SiO2-MOF

Catalysts

0.82

99.9

98.6

0.8

0.6

0.424

Cu/SiO2

b

0.82

58.6

34.2

65.5

0.3

0.086

Cu/SiO2

c

0.82

99.2

89.8

7.0

3.2

0.384

0.85

~ 20.0

~ 20.0

-

-

~0.018

0.75

98.0

70.0

~ 28.0

~ 2.0

0.270

0.45

~ 75.0

~77.0

~ 15.0

~ 8.0

0.137

20Cu/SBA-15d e

10Cu-MCM-41 10Cu-HMS

f

Reaction T= 240 °C Cu/SiO2-CN

0.82

21.6

3.9

95.7

0.4

0.004

Cu/SiO2-MOF

0.82

99.6

81.1

0.9

18

0.348

Cu/SiO2

b

0.82

99.1

84.2

11.8

4.0

0.360

Cu/SiO2

c

0.82

99.9

62.0

1.0

37.0

0.267

a

STYEG is the abbreviation of space-time yield of EG, and the unit is short of g

g-catal-1

-1

h .

b

26

Cu/SiO2: prepared by sol-gel method with Cu loading of 17.14%.

c

Cu/SiO2: prepared by AEM with Cu loading of 16.19%.15

d

20Cu/SBA-15: prepared by the ammonia-driving deposition-precipitation method with Cu loading of 20%.70

e

10Cu-MCM-41: prepared by AEM with Cu loading of 11.7%.65

f

10Cu-HMS: prepared by a one-pot synthesis method with Cu loading of 8.6%.20

4. DISCUSSION 4.1. The role of copper precursors in structural evolution and catalytic performance Yin et al.72 demonstrated that different inorganic copper precursors have an obvious effect on copper loading, texture properties and surface composition of the Cu-HMS catalyst prepared by the ion-exchanged method, thus resulting in diverse DMO hydrogenation activity. In this work, Cu(NO3)2·3H2O and HKUST-1 were used as the precursors to synthesize Cu/SiO2 catalysts via the sol-gel method (Scheme 1). From the data summarized in Table 1, it illustrates that the physicochemical properties of Cu/SiO2-MOF catalyst were much different from Cu/SiO2-CN 24 / 37


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catalyst. Most of the copper species were coated in the silica matrix over the Cu/SiO2-CN sample while the Cu/SiO2-MOF sample exposed more active sites for the DMO hydrogenation reaction. A lot of previous studies showed that the performance of Cu-based catalysts were significantly affected by the Cu dispersion, synergistic effect of Cu+ and Cu0, SBET and Cu particle’s size.36, 73 These factors can probably account for the much different catalytic performance between the two Cu/SiO2 catalysts in one way. On the other hand, different copper species can also be obtained when using HKUST-1 as precursors. From the FT-IR and PXRD results (Figure 1 and Figure 3B), some well-dispersed CuO and Cu-O-Si units were on the calcined Cu/SiO2-MOF sample. Furthermore, only one reduction peak in H2-TPR (Figure 6) demonstrated that the copper species were prevented from aggregating to bulk CuO by supporting with SiO2 after the BTC linkers were removed at high temperature. After reduction, strong diffraction peaks of Cu2O reduced from Cu-O-Si units and Cu0 reduced from CuO were shown over the reduced Cu/SiO2-MOF catalyst (Figure 3B). Furthermore, the Cu0 and Cu+ species were supposed to be well distributed on the interface between the Cu and SiO2 surface (Scheme 2). For the Cu/SiO2-CN sample, some copper species of Cu2(OH)3NO3 formed in its dried precursor, then decomposed to well dispersed CuO and finally reduced Cu0. Other copper species were coated by the silica matrix and hard to be reduced. Owing to the different physicochemical properties and copper species, the ratio of Cu+/(Cu+ + Cu0) was only 52.4% over Cu/SiO2-CN catalyst, which was lower than that of Cu/SiO2-MOF catalyst (73.3%, Table 2). Both Gong et al.52 and Chen et al.13 confirmed that the remarkable stability and efficiency of Cu/SiO2 catalysts can be ascribed to the cooperative effect of Cu0 and Cu+, which dissociates H2 and activates the C=O bond in DMO molecules, respectively. In addition, with the HKUST-1 as precursor, moderate acidic sites dominated the Cu/SiO2-MOF catalyst while Cu/SiO2-CN sample exhibited only some weak acidic sites (Figure 7). Zhu et al.74 reported that suitable acid sites over 25 / 37


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the Al2O3 surface were good for the DMO hydrogenation reaction. In short, the catalytic performance is closely related with their structures, especially the copper species (Scheme 2).

Scheme 2. The schematic of structure evolution over Cu/SiO2-CN and Cu/SiO2-MOF catalysts with different copper precursors.

4.2. Comparison of Cu/SiO2-MOF catalyst with other Cu-based catalysts for DMO hydrogenation reaction The present work found that the Cu/SiO2-MOF catalyst existed a superior activity and stability for the DMO-to-EG reaction. As shown in Table 3 and Table S1, many conventional Cu/SiO2 catalysts prepared by sol-gel method or ammonia-evaporation method, though with more copper loading, showed obvious deactivation within 100 h. However, it was interesting to note that the Conv.DMO and the Select.EG were stable over the Cu/SiO2-MOF catalyst during 220 h test (Figure 10). The high TOF value (19.3 h-1) of the Cu/SiO2-MOF catalyst indicated that the performance of 26 / 37


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Cu/SiO2-MOF sample enhanced compared with other pure Cu/SiO2 catalysts and even comparable with La or Ag modified Cu/SiO2 catalysts. Generally, the deactivation of the Cu-based catalysts was mainly because of the deterioration of Cu dispersion by the Cu sintering, structural collapse or copper’s chemical state change. The remarkable activity and stability of the robust Cu/SiO2-MOF catalyst were attributed to two main causes. One was the high SBET, as well as moderate surface acidic sites. Another cause was that the ultrafine copper nanoparticles (4.2 nm) with copper species of Cu0 and Cu2O over the reduced Cu/SiO2-MOF catalyst, with a high amount of surface Cu+ species. Ding et al.11 reported that Cu+ species in the Cu/SiO2 catalyst prepared by AEM were unstable and easily oxidized to copper phyllosilicate during the hydrogenation reaction, resulting in the deactivation within 100 h. However, the Cu2O reduced from Cu-O-Si units in the Cu/SiO2 catalyst prepared by the deposition-precipitation method based on (NH4)2CO3 was more stable. Furthermore, the PXRD patterns and TEM images (Figure S9) of the used catalyst indicated that no obvious aggregation of the Cu nanoparticles after the long-term catalytic test over the Cu/SiO2-MOF catalyst. After the HKUST-1 was in situ immobilized into silica support and further removed the organic linker, the copper species have a strong interaction with the support, which played significant roles in preventing sintering of Cu nanoparticles during the performance test. Although the synthetic procedures were a bit complicated, the preparation method of HKUST-1 was simple and environmentally relative to CNTs or AEM, and the final catalyst was green and without tableting process as well as no precious additives such as Cr, Ag, Au, La, and Pd. Therefore, Cu/SiO2-MOF catalyst could be a good alternative for the DMO-to-EG reaction compared with other Cu-based catalysts.

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5. CONCLUSIONS In conclusion, the utilization of MOF as copper precursor for the loading of Cu nanoparticles on SiO2 has been achieved by sol-gel method, successfully addressing the contradiction between copper loading and copper dispersion. It is attributed to the large framework of MOF precursor that it is difficult to be completely encapsulated by the silica. After pyrolysis in an air atmosphere at 350 °C, the Cu-BTC MOF was transformed into a mixture of ultrafine copper species (3.3 nm) highly dispersed on the porous SiO2 matrix. As the copper loading was only 7.83%, the copper species could keep its dispersion with the silica anchoring them. The reduction treatment of the sample yielded well-dispersed Cu nanoparticles with Cu0 and Cu+ species on the interface between the Cu and SiO2 surface and a high ratio of Cu+/(Cu+ + Cu0), which exhibited extraordinary catalytic activity and good durability for DMO-to-EG reaction over the Cu/SiO2-MOF catalyst. However, the Cu/SiO2-CN catalyst using copper nitrate as precursor with similar copper loading showed very poor catalytic activity, owing to the copper species were seriously coated by the silica networks. The synthesis of highly dispersed and efficient Cu/SiO2 catalyst derived from MOF precursor may offer its potential applications in the preparation of other precious metal-based catalysis by the sol-gel method.

ASSOCIATED CONTENT Supporting Information Additional N2 adsorption-desorption curves, TGA, TEM, SEM, NH3-TPD, PXRD, and Table S1 files. This information is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Yuan-Gen Yao); Tel: +86-591-63173138 ORCID Yuan-Gen Yao: 0000-0002-1064-1974 Run-Ping Ye: 0000-0001-7508-2694 Ling Lin: 0000-0003-0082-1389 Chong-Chong Chen: 0000-0001-7572-7819 Jin-Xia Yang: 0000-0002-3796-1513 Fei Li: 0000-0003-0110-1826 Xin Zhang: 0000-0002-0659-1804 De-Jing Li: 0000-0001-8785-1235 Ye-Yan Qin: 0000-0002-3672-0857 Zhangfeng Zhou: 0000-0002-8453-4960

Author Contributions # Run-Ping Ye and Ling Lin contributed equally to this work and should be considered co-first authors. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT 29 / 37


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This work was supported by the China Scholarship Council (File No. 201704910592); the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA07070200, XDA09030102); the Science Foundation of Fujian Province (2006l2005) and Fujian industrial guide project (2015H0053, 2016H0048). The authors also thank Mr. Zhao Sun, Department of Chemical and Petroleum Engineering of University of Wyoming, for the support of SEM measurements; Mr. Joshua Razink, Director of Transmission Electron Microscopy at University of Oregon, for the support of TEM measurements; And all the researchers at the test center, FJIRSM, CAS.

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TOC 194x94mm (150 x 150 DPI)


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Figure 1 196x137mm (300 x 300 DPI)


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Figure 2 185x151mm (300 x 300 DPI)


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Figure 3 97x36mm (300 x 300 DPI)


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Figure 5 189x126mm (300 x 300 DPI)


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Figure 6 176x147mm (300 x 300 DPI)


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Figure 7 167x130mm (300 x 300 DPI)


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Figure 9 91x33mm (300 x 300 DPI)


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Scheme 1 138x67mm (300 x 300 DPI)


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