Magnetically Recyclable Core-Shell Structured Pd-Based Catalysts for

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Magnetically Recyclable Core-Shell Structured Pd-Based Catalysts for Semi-Hydrogenation of Phenylacetylene Lei Yang, Yuzhuo Jin, Xiangchen Fang, Zhen-Min Cheng, and Zhiming Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03016 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Industrial & Engineering Chemistry Research

Magnetically Recyclable Core-Shell Structured Pd-Based Catalysts for Semi-Hydrogenation of Phenylacetylene Lei Yang,a Yuzhuo Jin,a Xiangchen Fang,*,b Zhenmin Cheng,a and Zhiming Zhou*,a

a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, China b

Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun 113001, China

* Corresponding Author Phone: +86 21 6425 2230; Fax: +86 21 6425 3528 Email: [email protected] (Z. Zhou); [email protected] (X. Fang)

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ABSTRACT Magnetic core-shell structured Pd-based catalysts (Fe3O4@M/Pd, M = SiO2, AlOOH, TiO2, Cu3(BTC)2 and ZIF-8) were prepared and applied to semi-hydrogenation of phenylacetylene. The catalysts were characterized by various techniques and it was found that the predominant chemical state of Pd species on the catalyst surface was PdO2, which was stable under the reaction condition (40 ˚C and 0.1 MPa H2). Among all the catalysts studied, Fe3O4@ZIF-8/Pd showed the highest selectivity to styrene, which was also superior to conventional catalysts such as Pd/C, Pd/Al2O3 and Lindlar catalyst. The high selectivity of Fe3O4@ZIF-8/Pd was mainly ascribed to the electronic effect between PdO2 and ZIF-8. Further investigation revealed that the phenylacetylene hydrogenation on Fe3O4@ZIF-8/Pd was solvent-dependent with the activity increasing with the solvent polarity. Among eight solvents tested, methanol was the most effective, with high and stable selectivity to styrene (around 92%) at complete conversion of phenylacetylene (≥ 99.5%) during five consecutive cycles.

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1. INTRODUCTION Semi-hydrogenation of phenylacetylene plays an important role in styrene polymerization via the removal of a trace amount of phenylacetylene from styrene because even several dozens of ppm of phenylacetylene can poison the catalyst and decrease the degree of polymerization of polystyrene.1-4 Palladium is one of the most widely used metals in semi-hydrogenation of phenylacetylene owing to its excellent activity under mild conditions.2,5-7 However, the further hydrogenation of styrene to ethylbenzene often occurs over Pd-based catalysts once the conversion of phenylacetylene is above 95%.8-10 How to achieve high selectivity to styrene at complete conversion of phenylacetylene is still a challenge facing the research community. One of the effective strategies is to employ appropriate supports to modify the metal dispersion, metal-support interaction or electronic effect. The commonly used supports for Pd-based catalysts in the semi-hydrogenation of phenylacetylene are Al2O3,2,9,10 SiO2,8,11 MCM-41,12,13 TiO2,14,15 and carbon material.16,17 Note that all these supports belong to inorganic materials. Recently, Metal-organic frameworks (MOFs) characterized by large surface areas, tunable topologies and designable surface properties have found many applications in heterogeneous catalysis,18,19 but very few studies have been published specifically on the semi-hydrogenation of phenylacetylene.20-22 Although most researchers claimed that good activity and selectivity were achieved using their catalysts in the phenylacetylene hydrogenation, it is very difficult to compare results from different studies because of different reaction conditions (temperature, pressure, concentration, solvent, catalyst loading, etc.) used for evaluating the catalysts. In this respect, it is necessary to test the Pd catalysts supported on different materials under the same conditions, based on which a reliable conclusion could be drawn on which material is better.

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Most catalysts reported on the semi-hydrogenation of phenylacetylene were studied in a stirred tank reactor,7-17,20-22 which has the advantages such as uniform temperature and concentration as well as enhanced interphase mass transfer. However, the efficient and prompt recovery of catalyst from the reaction media remains an intractable problem. Although the common-used techniques such as filtration and centrifugation can separate the catalyst from the liquid phase, they are timeconsuming and most importantly cause the catalyst loss during cyclic operation. Recently, magnetic catalysts offer an alternative to traditional catalysts in that they are readily recovered by an external magnetic field.23-24 Moreover, some investigations25-27 have demonstrated that the synergistic effect between Pd and Fe3O4 can improve the catalytic performance. If magnetic core-shell structured catalysts, namely Fe3O4@M/Pd (M = Al2O3, SiO2, TiO2, MOFs, etc.), can be prepared and applied to the phenylacetylene hydrogenation, one would expect good results. Indeed, our preliminary study26 showed that magnetic Fe3O4@SiO2/Pd had a higher activity and selectivity to styrene than non-magnetic Pd/SiO2. Unfortunately, only SiO2 was taken into account in the previous work. To date, few studies have been carried out using Fe3O4@M as supports of Pd-based catalysts for hydrogention reactions, let alone for the semi-hydrogenation of phenylacetylene. Among the aforementioned factors affecting the catalyst performance during laboratory-scale catalyst screening, the solvent is a very important but often neglected factor. Only few researchers7,11,27 have investigated the solvent effect on the catalyst activity and selectivity in the hydrogenation of phenylacetylene. Lee et al.11 studied this reaction on SiO2@CuFe2O4-Pd using 5 solvents (methanol, chloroform, pentane, hexane and cyclohexane) and found that hexane gave rise to the highest activity and selectivity to styrene, but the reason was not explained. Deng et. al.7 reported that ethanol was the best among 8 non-polar and polar solvents using Pd/mpg-C3N4 as the catalyst, and they ascribed the difference in the activity primarily to the interaction of the catalyst 4


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hydrophilicity with the solvent polarity, which might influence the dispersion of catalyst in the solution and the resulting mass transfer process. However, no systematic correlation was present between the catalyst activity and solvent polarity. In addition, the explanation in terms of catalyst dispersion in the reaction media seems to contradict the data of Lee et al.11 Recently, Yang et al.27 tested 11 solvents and acetonitrile was found to the best one, but unfortunately no explanation was provided. Therefore, it is necessary to probe the relationship between the catalyst performance and the solvent-related properties such as solvent polarity, which will contribute to the development of high-performance catalysts for the semi-hydrogenation of phenylacetylene. In this work, to the best of our knowledge, we for the first time prepared a series of magnetic core-shell structured catalysts (Fe3O4@M/Pd, M = SiO2, AlOOH, TiO2, Cu3(BTC)2 and ZIF-8) for the semi-hydrogenation of phenylacetylene. The physicochemical properties of the catalysts were determined, and the catalytic activity and selectivity were evaluated and compared. Additionally, the solvent effect on the activity and selectivity was explored and correlated with the solvent polarity and H2 solubility. Finally, the catalyst reusability was examined. The major achievements of this work lie in two aspects: first, a magnetic core-shell structured catalyst with high activity, selectivity and reusability in the semi-hydrogenation of phenylacetylene is developed, i.e., Fe3O4@ZIF-8/Pd; second, the solvent effect in the phenylacetylene hydrogenation is clarified.

2. EXPERIMENTAL SECTION 2.1. Materials. Ferric chloride hexahydrate (FeCl3·6H2O, > 99%), sodium hydroxide (NaOH, > 99% ), ethylene glycol (C2H6O2 (EG), > 99%), ammonium acetate (CH3COONH4 (NH4Ac), > 99%), sodium citrate dihydrate (C6H5Na3O7·2H2O, > 99%), ammonia solution (NH3·H2O, 25-28%), acetonitrile (C2H3N, > 99%), tetraethyl orthosilicate (Si(OC2H5)4 (TEOS), SiO2 ≥ 28.4%), 5



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aluminum isopropoxide (C9H21AlO3, Al2O3 ≥ 24.7%), tetrabutyl titanate (C16H36O4Ti (TBOT), > 98%), cupric acetate monohydrate (Cu(CH3COO)2·H2O, > 99%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥ 99%), 1,3,5-benzenetricarboxylic acid (C9H6O6 (H3BTC), > 98%), 2-methylimidazole (C4H6N2, > 98%), palladium(II) nitrate dihydrate (Pd(NO3)2·2H2O, Pd ≥ 39.5%), absolute ethanol (C2H5OH, ≥ 99.7%), methanol (CH3OH, > 99.5%), N,N-dimethylformamide (C3H7NO (DMF), > 97%), acetone (C3H6O, ≥ 99.5%), toluene (C7H8, > 99.5%), cyclohexane (C6H12, > 99.5%), n-hexane (C6H14, > 99%) and n-octane (C8H18, ≥ 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Phenylacetylene (> 98%) was obtained from Alfa Aesar. All chemicals were used as received without further purification. 2.2. Preparation of Magnetic Core-Shell Materials. Preparation of Fe3O4: Fe3O4 was synthesized by solvothermal method.28 FeCl3·6H2O (1.35 g), NH4Ac (3.85 g) and C6H5Na3O7·2H2O (0.4 g) were first dissolved in EG (70 mL). Then, the mixture was stirred vigorously at 170 ˚C for 1 h to form a homogeneous black solution followed by transferring into a Teflon-lined stainless-steel autoclave. The autoclave was heated at 200 ˚C and maintained for 16 h. Finally, the autoclave was cooled to room temperature and the black product was washed several times with C2H5OH and dried at 60 ˚C in a vacuum oven for 6 h. Preparation of Fe3O4@SiO2: Silica was coated on Fe3O4 by a modified Stöber method.29,30 Fe3O4 (0.06 g) was first dispersed in a mixture of C2H5OH (40 mL) and H2O (10 mL) under ultrasonic for 10 min. Next, NH3·H2O (1.5 mL) was added and ultrasonically mixed for another 10 min, and then TEOS (0.2 mL) was introduced and the hydrolysis-condensation reaction was allowed to proceed at room temperature under ultrasonic for 1 h. Finally, the product was separated magnetically, washed successively with ethanol and water several times and dried at 60 ˚C for 10 h under vacuum. Preparation of Fe3O4@AlOOH: The coating of AlOOH on Fe3O4 was performed with a modified 6


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method originally developed by Zheng et al.31 First, Fe3O4 (0.06 g) was dispersed in C2H5OH (30 mL) containing C9H21AlO3 (0.2 g) with the aid of ultrasonic vibration for 10 min. Next, a mixture of C2H5OH (50 mL) and H2O (10 mL) was added slowly and stirred vigorously for 1 h. Then, the above liquid was transferred into a Teflon-lined stainless-steel autoclave, which was heated up to 160 ˚C for 20 h. Finally, the powder was separated magnetically, washed with ethanol several times and dried at 60 ˚C under vacuum for 10 h. Preparation of Fe3O4@TiO2: TiO2 was coated on Fe3O4 by a modification technique.32 First, Fe3O4 (0.06 g) was dispersed ultrasonically in a mixed solution containing C2H5OH (90 mL) and C2H3N (30 mL) for 10 min, and then NH3·H2O (0.5 mL) was added and mechanically mixed for another 30 min. Next, TBOT (0.2 mL) was added and the solution was strongly stirred for 1 h. Then, the powder was washed successively with C2H5OH and C2H3N several times, followed by redispersing in a mixture of C2H5OH (40 mL), H2O (20 mL) and NH3·H2O (3 mL), which was transferred into a Teflon-lined stainless-steel autoclave and heated up to 160 ˚C for 20 h. Finally, the sample was washed with C2H5OH and dried at 60 ˚C in vacuum for 10 h. Preparation of Fe3O4@Cu3(BTC)2: This material was prepared by a step-by-step assembly strategy.33 The as-prepared Fe3O4 NPs (0.06 g) were first dispersed in an ethanol solution of Cu(CH3COO)2·H2O (4 mL, 10 mmol·L-1) under ultrasonic for 5 min and then transferred to a water bath at 70 °C for 15 min. Next, the suspension was magnetically separated, washed with C2H5OH, and redispersed in an ethanol solution of H3BTC (4 mL, 10 mmol·L-1), followed by transferring to the water bath at 70 °C for 30 min. After repeating the above steps for 10 times, the sample was finally dried under vacuum at 60 °C for 10 h. Preparation of Fe3O4@ZIF-8: Fe3O4@ZIF-8 was prepared by a method developed by Zheng et al.34 Fe3O4 NPs (0.06 g) was dispersed in a methanol solution (70 mL) containing Zn(NO3)2·6H2O 7



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(0.25 g) and C4H6N2 (0.15 g) under ultrasonic for 10 min and then stirred at 70 °C for 30 min, after which the suspension was magnetically separated and washed with CH3OH several times. After repeating the above steps for 3 times, the sample was dried under vacuum at 60 °C for 10 h. Preparation of Fe3O4@M/Pd: The Pd species was introduced onto Fe3O4@M (M = SiO2, TiO2, AlOOH, Cu3(BTC)2 and ZIF-8) by precipitation method.35 The nominal Pd loading of each catalyst was kept at 0.5 wt%. First, a Pd(NO3)2 solution (0.8 mL, 29.5 mmol·L-1) was mixed with CH3OH (20 mL), and then the magnetic Fe3O4@M (0.5 g) were added and stirred for 1 h. Next, a NaOH solution (1.0 mol·L-1) was dropwise added into the above solution to adjust the pH value to around 12, after which the solution was stirred for 20 h. Finally, the suspension was magnetically separated and washed with CH3OH several times, followed by drying under vacuum at 60°C for 10 h. 2.3. Catalyst Characterization. The Brunauer-Emmett-Teller (BET) surface area, pore volume and average pore diameter of samples were acquired from N2 adsorption-desorption data collected at -196 ˚C by using a Micrometrics ASAP 2010 instrument. Before measurement the sample was degassed at 150 °C and 133 Pa for 6 h. The crystalline structure of samples was identified by X-ray diffraction (XRD, Rigaku D/Max 2550 VB/PC diffractometer, Cu Kα radiation) in the range of 2θ = 5-80˚. The functional groups and chemical bonds of samples were investigated by Fourier transform infrared spectroscopy (FTIR) over a range of 4000-400 cm-1 with a resolution of 4 cm-1 using KBr pellets on a Bruker Equinox-55 spectrometer. The actual Pd loading of catalyst was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Varian 710-ES) after aqua regia procedure. The micromorphology of samples was observed by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100) and scanning transmission electron microscopy (STEM, Philips Tecnai F20 FEG-TEM). The sample was first dispersed in ethanol under ultrasonic for 30 min and then the suspension was dropped onto a carbon-coated copper grid. The surface 8


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phases of samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250 Xi) using the Al Kα radiation (hv = 1486.6 eV) and a pass energy of 40 eV, and the binding energy was calibrated by C 1s (284.8 eV). 2.4. Catalyst Test. The semi-hydrogenation of phenylacetylene over various catalysts was conducted in a semi-batch stirred tank reactor (300 ml, Parr 5100). In each test, 5 g of phenylacetylene, 5 g of n-octane acting as the internal standard, 90 g of ethanol and 0.15 g of catalyst were first put into the reactor and heated to 40 ˚C under N2. Then, the reactor was purged with H2 five times, after which the reaction proceeded to completion at 40 ˚C under 0.1 MPa with vigorous stirring (about 1000 rpm). Unless otherwise specified this reaction condition was followed in this study. The liquid samples were withdrawn at regular interval and analyzed by gas chromatography (Hewlett Packard 6890) equipped with a capillary column (PEG-20M, 30 m × 0.32 mm × 0.50 µm) and a flame ionization detector. Next, the best catalyst with good activity and selectivity was employed to study the solvent effect on the phenylacetylene hydrogenation. Eight solvents with different polarities and H2 solubilities were used: n-hexane, cyclohexane, toluene, methanol, ethanol, acetone, acetonitrile and DMF. Finally, the reusability of the best catalyst was tested in the best solvent for five cycles. After each cycle the catalyst was recovered by a magnet and washed with the solvent to remove any residue.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of Core-Shell Supports and Catalysts. Table 1 lists the BET surface area, pore volume and average pore diameter of the core-shell supports and catalysts. As a result of the loading of Pd species into the core-shell support, the BET surface area and pore volume of each catalyst are somewhat lower than those of the support. Figure 1 shows the N2 sorption 9



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isotherms for different catalysts. The isotherm of Fe3O4@ZIF-8/Pd belongs to a mixture of type I isotherm, which is characteristic of microporous materials, and type IV isotherm, which indicates the presence of mesopores.34,36 Such a porous structure leads to the highest BET surface area and the smallest average pore diameter of Fe3O4@ZIF-8/Pd among all catalysts. For Fe3O4@TiO2/Pd and Fe3O4@AlOOH/Pd, type IV isotherms (type H3 hysteresis loop for the former and H4 for the latter) are observed, but the hysteresis loop of the former occurs at a higher relative pressure, implying a larger average pore diameter, which is in agreement with Table 1. Fe3O4@Cu3(BTC)2/Pd also displays a type IV isotherm but with a small H4 hysteresis loop, and its surface area and pore volume are lower than those of Fe3O4@TiO2/Pd and Fe3O4@AlOOH/Pd. As for Fe3O4@SiO2/Pd, a type II isotherm is present and its surface area and pore volume are greatly decreased compared to other catalysts, indicating fewer pores in the catalyst. [Table 1] [Figure 1] [Figure 2] [Table 2] Figure 2 shows the HRTEM images of magnetic core-shell supports (A-E) and catalysts (a-e). It is clear that all materials display a core-shell structure, where the core ranges from 100 to 300 nm, while the shell thickness varies from 20 to 70 nm depending on the type of material employed. For Fe3O4@SiO2/Pd, small Pd particles with an average size of (1.5 ± 0.7) nm are observed (inset in Figure 2c), but for all other catalysts the Pd species is invisible, even under high-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM) mode (Fe3O4@ZIF-8/Pd as an example in Figure 2f). It is probably because the small Pd particles are encapsulated in the porous support (TiO2, AlOOH, Cu3(BTC)2 or ZIF-8) that is too thick to be transmitted by the 10


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electron beam, while SiO2 is almost nonporous. The presence of Pd species is confirmed by EDS analysis (Figure 2f). The ICP-OES analysis also reveals the presence of Pd species in the catalysts. As listed in Table 2, the measured Pd loadings of all catalysts are close to the nominal loading of 0.5 wt%, showing the effectiveness of the precipitation method used to prepare Fe3O4@M/Pd catalysts. [Figure 3] Figure 3 presents the FTIR spectra of the core and core-shell structured supports. The band at 570 cm-1 ascribed to the Fe-O bond is present for all samples, but its intensity becomes lower in the core-shell supports (Fe3O4@M) in comparison with the core (Fe3O4) itself. The bands at 1100 cm-1 (C-O bond), 1630 and 1410 cm-1 (C=O), and 3400 cm-1 (O-H) are associated with carboxyl groups, which are intentionally introduced through addition of C6H5Na3O7·2H2O during preparation of Fe3O4 and act as a surface modification agent to initiate the shell growth.34 For Fe3O4@TiO2, bands assigned to the Ti-O bond vibration occur in the range of 500-640 cm-1,37 which overlaps the Fe-O band. For Fe3O4@AlOOH, the bands at 480, 620 and 730 cm-1 correspond to the stretching vibration of the Al-O bond,37 and the band at 1070 cm-1 is connected with the Al-OH bending mode of boehmite,38 implying the boehmite phase of the shell. For Fe3O4@SiO2, the bands at 800 and 1085 cm-1 are due to the asymmetric and symmetric stretching vibrations of the Si-O-Si bond, respectively, while the band at 470 cm-1 is attributed to the Si-O bond.26 For Fe3O4@Cu3(BTC)2, the bands at 1370, 1445 and 1570 cm-1 are assigned to the stretching vibration of the C=C bond of the benzene ring,39,40 and the bands at 730 and 760 cm-1 are ascribed to metal Cu substitution on benzene groups, which are the characteristic bands of Cu3(BTC)2.39 In the case of Fe3O4@ZIF-8, the bands in the range of 650-1330 cm-1 and 1330-1500 cm-1 are related to the plane bending and stretching of imidazole ring, respectively,34,41,42 and the band at 422 cm-1 is attributed to the Zn-N stretch mode of ZIF-8.41 The above FTIR analysis confirms the coating of different materials on the 11



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Fe3O4 particles. [Figure 4] Figure 4 displays the crystalline phases of the core, core-shell structured supports and catalysts. The core exhibits diffraction peaks belonging to Fe3O4 (2θ = 30.1, 35.5, 43.1, 57.2 and 62.7˚, JCPDS 19-0629), which are well preserved in the supports and catalysts, indicating that the crystal structure of the magnetite core is not damaged by coating with different shells. With regard to support and catalyst, characteristic diffraction peaks assigned to anatase TiO2 (2θ = 25.3, 37.9, 48.0, 53.9 and 55.0˚, JCPDS 21-1272) and ZIF-8 (2θ = 7.3, 10.3, 12.7 and 18.1˚)34 are detected for Fe3O4@TiO2 (Fe3O4@TiO2/Pd) and Fe3O4@ZIF-8 (Fe3O4@ZIF-8/Pd), respectively, while no peaks indexed to crystalline SiO2 and AlOOH are observed for SiO2- and AlOOH-coating samples, respectively, indicating that the SiO2 and AlOOH are amorphous. It is worth noting that SiO2, TiO2, AlOOH and ZIF-8 are maintained during catalyst preparation, with the exception of Cu3(BTC)2. The characteristic diffraction peaks of Cu3(BTC)2 (2θ = 9.4, 11.5 and 13.3˚)39 presented in Fe3O4@Cu3(BTC)2 disappear in Fe3O4@Cu3(BTC)2/Pd, implying that the structure of Cu3(BTC)2 is damaged during the introduction of Pd species into the support. As for the Pd species, no diffraction peaks are observed in all catalysts, in accordance with the HRTEM results. [Figure 5] Figure 5 shows the XPS spectra of the Pd 3d core level of the magnetic core-shell catalysts except Fe3O4@Cu3(BTC)2/Pd whose structure is damaged. The devolution of the Pd 3d of all catalysts gives a prominent peak at a binding energy (BE) of 338.0 (or 338.3) eV, which is assigned to PdO2.43-45 For Fe3O4@SiO2/Pd, another two small BE peaks at 335.5 and 336.6 eV are ascribed to metallic Pd0 and PdO,10,35 respectively, indicating the presence of three different chemical states of Pd in the catalyst although PdO2 accounts for a large proportion of Pd species. Similarly, the Pd 12


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particles in Fe3O4@TiO2/Pd are mainly in the form of PdO2 with a minor presence of PdO (337.0 eV). The presence of highly oxidized PdO2 is probably associated with the very small Pd particles. Bai et al.46 found that the oxidative state of Pt nanoparticles on SiO2@RF/Pt (RF: resorcinolformaldehyde resin) increased continuously from Pt0 to PtO and further to PtO2 with decreasing the particle size from 5.3 to 0.7 nm. In another study made by Rathi et al.35 who used a precipitation method similar to this study to prepare maghemite supported Pd catalyst, the average size of Pd particles consisting of Pd0 (around 80%) and PdO (20%) is 2.5 nm. They attributed the formation of Pd0 and PdO to the dehydration of Pd(OH)2 (formed by precipitation of PdCl2 with NaOH) during drying. As a comparison, it is noteworthy that the average Pd particle size of Fe3O4@SiO2/Pd is 1.5 nm and the predominant Pd species is PdO2. It appears that the size effect of Pt particles on the chemical state of Pt is applicable to Pd, i.e., the Pd chemical state on the catalyst surface is shifted towards higher valence with decreasing the Pd particle size. [Figure 6] Figure 6 shows the XPS spectra of C 1s and N 1s for Fe3O4@ZIF-8/Pd, which further confirms the existence of ZIF-8 in the catalyst. The core level C 1s spectrum can be deconvoluted into three peaks located at 284.8, 285.7 and 291.7 eV, which are assigned to C-C, C=N and the delocalized π conjugation (π → π*) in the ZIF-8 framework, respectively.42 Deconvolution of the N 1s XPS spectrum results in two peaks at 398.8 and 400.5 eV, corresponding to the pyridinic N and the defect-induced pyrrolic and quaternary N, respectively.42 3.2. Semi-Hydrogenation of Phenylacetylene over Core-Shell Catalysts. The activity and selectivity to styrene of the core-shell catalysts in semi-hydrogenation of phenylacetylene are shown in Figure 7. The selectivity is defined as the ratio of styrene with respect to the converted phenylacetylene. As is evident from the conversion vs time curves, the catalyst activity follows the 13



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order Fe3O4@SiO2/Pd > Fe3O4@TiO2/Pd >

Fe3O4@ZIF-8/Pd

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Fe3O4@AlOOH/Pd >

Fe3O4@Cu3(BTC)2/Pd, which is related to the textural property of catalyst. Indeed, the order in the activity is reverse to that in the specific surface area, except for Fe3O4@ZIF-8/Pd and Fe3O4@Cu3(BTC)2/Pd. The low surface area and almost nonporous structure of Fe3O4@SiO2/Pd make most of the hydrogenation reaction occur at the outer surface of catalyst, which can remarkably decrease the internal diffusion limitation and thus improve the catalyst activity.3,26 However, although the surface area of Fe3O4@Cu3(BTC)2/Pd is not high compared to other catalysts, some of active Pd species are probably covered by other species derived from the damaged Cu3(BTC)2, and consequently Fe3O4@Cu3(BTC)2/Pd exhibits a low activity, with a conversion of only about 40% even after 540 min of reaction. As for Fe3O4@ZIF-8/Pd, because Pd species can be encapsulated in ZIF-8,47 a well dispersion of PdO2 particles without aggregation is achieved, which is in favor of the activity. Thus, despite the fact that Fe3O4@ZIF-8/Pd has the largest surface area, it can still catalyze the phenylacetylene hydrogenation at a moderate rate. [Figure 7] [Figure 8] Note that all catalysts are used without H2 reduction pretreatment, meaning that at the beginning of the hydrogenation reaction, PdO2 is the predominant palladium species in the catalysts. The PdO2 state remains unchanged after the reaction at 40 ˚C and 0.1 MPa of H2, as evidenced by the XPS analysis of the used Fe3O4@ZIF-8/Pd (Figure 8(a)). In order to clarify the chemical state of Pd species during the reaction, Fe3O4@ZIF-8/Pd is intentionally reduced in a flow of H2 for 6 h at either 40 or 180 ˚C, after which the catalyst is tested in the semi-hydrogenation of phenylacetylene. As presented in Figure 8(b), the Pd 3d XPS spectrum of the 40 ˚C-reduced Fe3O4@ZIF-8/Pd is almost identical to that of the unreduced catalyst, indicating that PdO2 is quite stable in 0.1 MPa H2 14


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at 40 ˚C. Therefore, under the experimental condition used in this work, the predominant chemical state of Pd species on Fe3O4@ZIF-8/Pd during reaction is PdO2. However, for the 180 ˚C-reduced Fe3O4@ZIF-8/Pd, the XPS analysis reveals that PdO2 is absent and the predominant Pd species is metallic Pd0 together with a minority of PdO, implying that PdO2 is reduced. Similar results were reported by Otto et al.43 and Bi and Lu.48 Otto et al.43 pointed out that the palladium oxide was stable in H2 at ambient temperature when the Pd loading was no higher than 0.5 wt%, while Bi and Lu48 found that PdO2 was converted to PdO and Pd when reduced at 135 °C for 6 h. The 40 ˚C- and 180 ˚C-reduced Fe3O4@ZIF-8/Pd present different activity and selectivity. As shown in Figure S1, the conversion vs time and selectivity curves of the 40 ˚C-reduced Fe3O4@ZIF-8/Pd are, as expected, very similar to those of the unreduced catalyst, but quite different from those of the 180 ˚C-reduced catalyst. Compared to the unreduced Fe3O4@ZIF-8/Pd, the 180 ˚C-reduced catalyst possesses a higher activity (105 min vs 270 min for about 100% conversion) but a lower selectivity (79.6% vs 90.5% at about 100% conversion), which is probably associated with the different active Pd species in each catalyst. In short, the above analysis demonstrates that PdO2 is the active species if the unreduced catalyst is applied to the hydrogenation of phenylacetylene at 40 ˚C and 0.1 MPa H2. This result is different from most previous studies,5-17,20-22 where metallic Pd0 is normally regarded as the active species, although PdO2 and PtO2 have proven to be active in the hydrogenation of indole derivatives49 and quinoline.46 Further investigation are needed to explain the relationship between the chemical state of the active Pd species and the Pd particle size. As far as the selectivity is concerned, Fe3O4@ZIF-8/Pd has the highest selectivity to styrene, being 90.5% at 99.5% conversion. In contrast, at about the same conversion, lower selectivities of 52.2, 60.2 and 70.3% are obtained for Fe3O4@SiO2/Pd, Fe3O4@TiO2/Pd and Fe3O4@AlOOH/Pd, respectively. As for Fe3O4@Cu3(BTC)2/Pd, although the reaction is stopped due to the very low 15



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activity of the catalyst, the selectivity on this catalyst at low conversion levels is still lower compared to other four catalysts. The observed highest selectivity on Fe3O4@ZIF-8/Pd is mainly attributed to the electronic effect between PdO2 and ZIF-8. As presented in Figure 5, the Pd 3d5/2 BE for PdO2 on Fe3O4@ZIF-8/Pd (338.0 eV) is shifted by -0.3 eV relative to that on other catalysts (338.3 eV), reflecting a higher electron density of PdO2 on Fe3O4@ZIF-8/Pd. This is caused by the charge transfer from ZIF-8 to PdO2, as the BE of the C=N bonding (285.7 eV) on Fe3O4@ZIF-8/Pd (Figure 6) is higher than that of the standard C=N bonding on ZIF-8 (285.4 eV).41 In addition, this charge transfer is determined by comparison of the FTIR spectra of Fe3O4@ZIF-8/Pd and Fe3O4@ZIF-8. As shown in Figure 9, the C=N and Zn-N stretches on Fe3O4@ZIF-8/Pd display a red shift relative to Fe3O4@ZIF-8, implying the weakening of the C=N and Zn-N bonds and the electron donation from the aryl ring to PdO2.19 Previous studies7,22 have demonstrated that increasing the electron density of Pd particles can decrease the adsorption energy of styrene and make its desorption easier, which is in favor of the selectivity to styrene. Therefore, it is the increased electron density of PdO2 that gives rise to the high selectivity to styrene on Fe3O4@ZIF-8/Pd. [Figure 9] 3.3 Comparison with Conventional Catalysts. Table 3 summarizes the results of phenylacetylene hydrogenation over several conventional catalysts such as Pd/Al2O3, Pd/C, and Lindlar catalyst (Pd/CaCO3 poisoned by a lead complex). Although Pd/C exhibits the highest activity, with only 15 min of reaction required for complete conversion of phenylacetylene, its selectivity to styrene is very low (< 10% selectivity at 99.7 % conversion). Likewise, Pd/Al2O3 has a low selectivity (about 35%). In contrast, Lindlar catalyst shows much higher selectivity (87.5%), but its activity is very poor, requiring a reaction time as long as 1400 min for complete conversion of phenylacetylene. 16


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Compared with these conventional catalysts, Fe3O4@ZIF-8/Pd displays the highest selectivity (90.5%) with a moderate activity (270 min needed for 99.5% conversion), indicating good performance in the semi-hydrogenation of phenylacetylene. In addition, all the core-shell structured catalysts except Fe3O4@Cu3(BTC)2/Pd have higher activity than Pd/Fe3O4, as the time required for complete conversion of phenylacetylene over the former is lower than 350 min (Figure 7) but above 400 min over the latter (Table 3). Detailed conversion vs time and selectivity curves for the conventional catalysts are shown in Figures S2-S5. [Table 3] 3.4. Solvent Effect. Fe3O4@ZIF-8/Pd is used to study the solvent effect on the catalyst activity and selectivity in semi-hydrogenation of phenylacetylene. Table 4 summarizes the activity and selectivity with different solvents. In addition, the solvent polarity and H2 solubility are listed. It can be clearly seen that varying the solvent has a noticeable influence on the catalyst activity and selectivity. For example, for methanol as the solvent, the reaction time required for complete conversion of phenylacetylene is about 170 min, while for n-hexane or cyclohexane, even after 540 min the conversion (around 50%) is far from complete, suggesting that methanol is superior to both n-hexane and cyclohexane in increasing the catalyst activity. Indeed, the initial rate in methanol is about 4 times that in n-hexane or cyclohexane. For another instance, at complete conversion of phenylacetylene, the selectivity to styrene is lower in DMF (74.1%) than in methanol (92.9%). From these results it can be concluded that Fe3O4@ZIF-8/Pd is highly solvent-dependent and methanol is the most effective solvent, which yields both high activity and selectivity. [Table 4] [Figure 10] Figure 10 is a plot of the initial rate vs. the solvent polarity and the H2 solubility. It appears that 17



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the catalyst activity in terms of initial rate has a positive correlation with the solvent polarity, but no clear relationship with the H2 solubility, implying a major role of solvent polarity in promoting the catalyst activity. This is probably due to the competitive adsorption between solvent and reactants on the active sites that decreases with increasing the solvent polarity. It is well known that ZIF-8 is highly hydrophobic,53 which results in a strong adsorption of non-polar or less polar solvents (e.g., n-hexane and cyclohexane) on the catalyst surface and in turn hinders adsorption and subsequent hydrogenation of phenylacetylene. On the other hand, for polar solvents such as methanol and DMF, the solvent adsorption is weakened on Fe3O4@ZIF-8/Pd and, as a result, the adsorption and reaction of phenylacetylene is promoted. Supplementary experiments on the adsorption of phenylacetylene on Fe3O4@ZIF-8/Pd in various solvents (Figure S6) confirm that the adsorption amount of phenylacetylene depends greatly on the solvent polarity: the higher the solvent polarity, the larger the adsorption amount of phenylacetylene on Fe3O4@ZIF-8/Pd. The solvent-dependence of the activity is also observed by Bramwell et al.54 for the hydrogenation of diphenylacetylene. Moreover, by analyzing the activity data reported by Lee et al.11 for the phenylacetylene hydrogenation on SiO2@CuFe2O4-Pd with different solvents, we find that the catalyst activity in their study is in general negatively correlated with the solvent polarity, which is indeed in accordance with our analysis because SiO2 is hydrophilic. As far as the selectivity to styrene is concerned, no clear correlation can be established between the selectivity and the solvent polarity or the H2 solubility, implying that the variation of selectivity with solvent is not predominantly affected by a single factor and a combined effect is possible. 3.5. Reusability of Fe3O4@ZIF-8/Pd. Five consecutive cycles of phenylacetylene hydrogenation were carried out on Fe3O4@ZIF-8/Pd using methanol as the solvent. In each cycle the reaction was continued for 180 min. As shown in Figure 11, Fe3O4@ZIF-8/Pd has good reusability and can be 18


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used for 5 cycles without loss of activity and selectivity. After 5 cycles the conversion and selectivity to styrene are maintained at 99.5 % and 92.6%, respectively. The good reusability of Fe3O4@ZIF-8/Pd originates from the well-preserved structure: (1) no obvious change in the micromorphology of the catalyst could be observed after 5-cycle reaction (Figure 11); (2) the Pd loading of the used Fe3O4@ZIF-8/Pd measured by ICP-OES is 0.48 wt%, almost the same as the initial value of 0.49 wt%, and no Pd leaching is detected in the liquid; and (3) the chemical state of Pd species (PdO2) on the catalyst is also preserved after 5 cycles (Figure 8). Therefore, Fe3O4@ZIF-8/Pd is a highly recoverable, reusable and active catalyst for the semi-hydrogenation of phenylacetylene. [Figure 11]

4. CONCLUSIONS Magnetic core-shell structured Pd-based catalysts (Fe3O4@M/Pd) with different shell materials (M = SiO2, AlOOH, TiO2, Cu3(BTC)2 and ZIF-8) and a low Pd loading of 0.5 wt% were successfully prepared by three steps: preparation of Fe3O4, coating of Fe3O4 particles with different materials to synthesize core-shell supports (Fe3O4@M) and introduction of Pd species onto support surface. These catalysts were evaluated for semi-hydrogenation of phenylacetylene to styrene. The core-shell structure of the catalysts was evidenced by HRTEM and FTIR analysis. The XPS studies on fresh and used catalysts revealed that the predominant active Pd species on the catalyst surface was PdO2 instead of metallic Pd0 or PdO, which was probably associated with the very small Pd particles. The catalyst activity generally varied inversely with its specific surface area, which was mainly attributed to the internal diffusion effects. Fe3O4@SiO2/Pd showed the highest activity owing to its low surface area and almost nonporous structure that can greatly decrease the internal 19



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diffusion limitation. However, Fe3O4@SiO2/Pd had the lowest selectivity to styrene, only 52% at complete conversion of phenylacetylene. Among all the catalysts investigated, Fe3O4@ZIF-8/Pd was the most efficient, with the highest selectivity to styrene (90.5% at conversion of 99.5% in ethanol as solvent) and meanwhile good activity. In addition, compared to conventional catalysts such as Pd/C, Pd/Al2O3 and Lindlar catalyst, Fe3O4@ZIF-8/Pd had a higher selectivity. The high selectivity of Fe3O4@ZIF-8/Pd was mainly ascribed to the electronic effect resulting from electron donation to PdO2 by ZIF-8, which in turn facilitated desorption of styrene from the active sites and thus inhibited its further hydrogenation. The phenylacetylene hydrogenation on Fe3O4@ZIF-8/Pd was found to be solvent-dependent, whose activity increased with an increase in the solvent polarity. Nevertheless, there was no systematic correlation between the selectivity to styrene and the solvent polarity or the H2 solubility. The Fe3O4@ZIF-8/Pd catalyst proved to be recyclable up to 5 cycles, whose selectivity remained unchanged (about 92% in methanol as solvent) at complete conversion of phenylacetylene.

ASSOCIATED CONTENT Supporting Information Comparison of reduced and unreduced Fe3O4@ZIF-8/Pd (Figure S1); results of phenylacetylene hydrogenation over conventional catalysts (Figures S2-S5); adsorption experiments (Figure S6)

ACKNOWLEDGMENTS Financial supports from the National Natural Science Foundation of China (21776088) and the Fundamental Research Funds for the Central Universities (222201718003) are gratefully acknowledged. 20


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hydrogenation of indole derivatives. Selective access to tetrahydroindoles or cis-fused octahydroindoles. Org. Biomol. Chem. 2012, 10, 6587–6594. (50) Barwick, V. J. Strategies for solvent selection-a literature review. Trends Anal. Chem. 1997, 16, 293–309. (51) Brunner, E. Solubility of hydrogen in 10 organic solvents at 298.15, 323.15, and 373.15 K. J. Chem. Eng. Data 1985, 30, 269–273. (52) Young, C. L. Solubility data series: hydrogen and deuterium. IUPAC; Pergamon: Exeter, vol. 5/6, 1981. (53) Zhang, K.; Lively, R. P.; Zhang, C.; Chance, R. R.; Koros, W. J.; Sholl, D. S.; Nair, S. Exploring the framework hydrophobicity and flexibility of ZIF-8: from biofuel recovery to hydrocarbon separations. J. Phys. Chem. Lett. 2013, 4, 3618−3622. (54) Bramwell, P. L.; Gao, J.; Waal, B.; Jong, K. P.; Gebbink, R. J. M. K.; de Jongh, P. E. A transition-metal-free hydrogenation catalyst: pore-confined sodium alanate for the hydrogenation of alkynes and alkenes. J. Catal. 2016, 344, 129–135.

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Figure captions Figure 1. N2 sorption-desorption isotherms of magnetic core-shell catalysts. Figure 2. HRTEM images of magnetic core-shell supports (A-E) and catalysts (a-e): (A,a) Fe3O4@AlOOH and Fe3O4@AlOOH/Pd; (B,b) Fe3O4@TiO2 and Fe3O4@TiO2/Pd, (C,c) Fe3O4@SiO2 and Fe3O4@SiO2/Pd; (D,d) Fe3O4@Cu3(BTC)2 and Fe3O4@Cu3(BTC)2/Pd; (E,e) Fe3O4@ZIF-8 and Fe3O4@ZIF-8/Pd; (f) HADDF-STEM and HADDF-EDS of Fe3O4@ZIF-8/Pd. Figure 3. FTIR spectra of magnetic core-shell supports. Figure 4. XRD patterns of magnetic core-shell supports and catalysts. Figure 5. Pd 3d XPS spectra of magnetic core-shell catalysts. Figure 6. C 1s (a) and N 1s (b) XPS spectra of Fe3O4@ZIF-8/Pd. Figure 7. Conversion and selectivity of magnetic core-shell catalysts in the hydrogenation of phenylacetylene (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in ethanol). Figure 8. Pd 3d XPS spectra of (a) unreduced and (b) reduced Fe3O4@ZIF-8/Pd. Figure 9. FTIR spectra of Fe3O4@ZIF-8 and Fe3O4@ZIF-8/Pd. Figure 10. Variation of the initial rate over Fe3O4@ZIF-8/Pd with the solvent polarity and H2 solubility (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in different solvent). Figure 11. Reusability of Fe3O4@ZIF-8/Pd in 5 consecutive cycles of reaction (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in methanol).

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Table 1 Textual Properties of Magnetic Core-Shell Supports and Catalysts BET surface area (m2·g-1)

pore volumea (cm3·g-1)

average pore diametera (nm)

Fe3O4@AlOOH

125.9

0.233

6.7

Fe3O4@TiO2

79.3

0.326

14.1

Fe3O4@SiO2

5.2

0.032

78.9

Fe3O4@Cu3(BTC)2

47.3

0.098

8.3

Fe3O4@ZIF-8

937.4

0.391b

3.0b

Fe3O4@AlOOH/Pd

103.3

0.173

7.1

Fe3O4@TiO2/Pd

65.9

0.276

14.8

Fe3O4@SiO2/Pd

3.8

0.031

83.1

Fe3O4@Cu3(BTC)2/Pd

40.7

0.055

8.4

Fe3O4@ZIF-8/Pd

740.1

0.277b

3.8b

sample

a b

Determined from the adsorption branch of the isotherm by the BJH method. Determined from micropore program by t-plot method.

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Table 2 Composition of Magnetic Core-Shell Catalysts composition (wt%) catalyst Pda

shell

Fe3O4

Fe3O4@AlOOH/Pd

0.41 (0.5)

11.32 (AlOOH)

81.32

Fe3O4@TiO2/Pd

0.49 (0.5)

28.45 (TiO2)

62.27

Fe3O4@SiO2/Pd

0.47 (0.5)

26.81 (SiO2)

65.79

Fe3O4@Cu3(BTC)2/Pd

0.50 (0.5)

13.73 (Cu)

64.34

Fe3O4@ZIF-8/Pd

0.49 (0.5)

10.21 (Zn)

58.86

a

Actual and theoretical metal loadings are given, respectively, by the values outside and inside the brackets.

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Table 3 Comparison of Conventional Catalysts with Fe3O4@ZIF-8/Pd in Phenylacetylene Hydrogenationa. catalyst

reaction time (min)

conversion (%)

selectivity

1

Pd/C

15

99.7

9.6

2c

Lindlar catalyst

1400

99.7

87.5

3d

Pd/Al2O3

60

99.6

34.7

4e

Pd/Fe3O4

410

99.5

56.2

5

Fe3O4@ZIF-8/Pd

270

99.5

90.5

entry b

(%)

a

Reaction condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in ethanol.

b

Purchased from Aladdin Reagent Int., Pd (5 wt%).

c

Purchased from Aladdin Reagent Int., Pd (5 wt%) poisoned by a lead complex.

d

Data taken from our previous work (ref. 10), Pd (0.5 wt%).

e

Prepared by the method described in this study, Pd (0.5 wt%).

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Table 4 Activity and Selectivity of Fe3O4@ZIF-8/Pd in Various Solvents with Different Polarities and H2 Solubilities solvent

reaction time (min)

conversion/selectivity (%)

initial ratea -1 (mmol·g ିଵ ୔ୢ ·s )

polarityb

H2 solubilityc (mol·m-3)

methanol

170

99.5/92.9

4.47

6.6

3.69

DMF

225

99.6/74.1

3.16

6.4

1.07

acetonitrile

255

99.8/81.2

2.73

6.2

3.10

acetone

260

99.6/82.6

2.31

5.4

3.86

ethanol

270

99.5/90.5

2.53

5.2

3.39

toluene

290

99.5/83.5

1.77

2.3

2.89

cyclohexane

540

53.6/95.4

1.16

0.2

3.76

n-hexane

540

49.2/95.1

1.14

0.1

5.07

a

Calculated based on the conversion lower than 20%. Excerpted from ref. 50. c Calculated from the data reported in the literature (DMF and acetonitrile from ref. 51, and others from ref. 52). b

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700 Volume adsorbed / cm3⋅g-1

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600 500

Fe3O4@ZIF-8/Pd Fe3O4@TiO2/Pd Fe3O4@AlOOH/Pd Fe3O4@Cu3(BTC)2/Pd Fe3O4@SiO2/Pd

400 300 200 100 0 0.0

0.2

0.4 0.6 0.8 Relative pressure

1.0

Figure 1. N2 sorption-desorption isotherms of magnetic core-shell catalysts.

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Figure 2. HRTEM images of magnetic core-shell supports (A-E) and catalysts (a-e): (A,a) Fe3O4@AlOOH and Fe3O4@AlOOH/Pd; (B,b) Fe3O4@TiO2 and Fe3O4@TiO2/Pd; (C,c) Fe3O4@SiO2 and Fe3O4@SiO2/Pd; (D,d) Fe3O4@Cu3(BTC)2 and Fe3O4@Cu3(BTC)2/Pd; (E,e) Fe3O4@ZIF-8 and Fe3O4@ZIF-8/Pd; (f) HADDF-STEM and HADDF-EDS of Fe3O4@ZIF-8/Pd.

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core (Fe3O4) Fe3O4@TiO2

Transmittance / a.u.

Page 35 of 44

Fe3O4@AlOOH

Fe3O4@SiO2

Fe3O4@Cu3(BTC)2 Fe3O4@ZIF-8 570

4000

3200

2400

1600

Wavenumbers / cm

800

-1

Figure 3. FTIR spectra of magnetic core-shell supports.

35



3

1 - Fe3O4; 2 - TiO2; 3 - ZIF-8; 4 - Cu3(BTC)2

3 3

Fe3O4@ZIF-8; Fe3O4@AlOOH; Fe3O4@SiO2;

3 33

33

3 1

1

Fe3O4@Cu3(BTC)2 Fe3O4@TiO2 Fe3O4

1

1

1

4 4 4

Intensity / a.u.

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

2

3

3 33

333

1

20

30

22

Fe3O4@Cu3(BTC)2/Pd Fe3O4@TiO2/Pd

1

1

2

2

10

2

Fe3O4@ZIF-8/Pd; Fe3O4@AlOOH/Pd; Fe3O4@SiO2/Pd

3 3

2

1

2

40 50 2θ / degree

1

22

60

70

80

Figure 4. XRD patterns of magnetic core-shell supports and catalysts.

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338.0 eV

Pd 3d

Fe3O4@ZIF-8/Pd 338.3 eV

Intensity / a.u.

Page 37 of 44

337.0 eV

Fe3O4@TiO2/Pd

Fe3O4@AlOOH/Pd

336.6 eV 335.5 eV

Fe3O4@SiO2/Pd

350

345 340 335 Binding energy / eV

330

Figure 5. Pd 3d XPS spectra of magnetic core-shell catalysts.

37



398.8 eV

C 1s

N 1s

Intensity / a.u.

284.8 eV

Intensity / a.u.

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

285.7 eV

400.5 eV

291.7 eV

296

292 288 284 280 Binding energy / eV

(a)

404 402 400 398 396 394 Binding energy / eV

(b)

Figure 6. C 1s (a) and N 1s (b) XPS spectra of Fe3O4@ZIF-8/Pd.

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100 Conversion of phenylacetylene / %

Page 39 of 44

80

Fe3O4@SiO2/Pd Fe3O4@TiO2/Pd Fe3O4@ZIF-8/Pd Fe3O4@AlOOH/Pd Fe3O4@Cu3(BTC)2/Pd

60

40

20

0

0

150 300 450 50 60 70 80 90 100 Reaction time / min Selectivity to styrene / %

Figure 7. Conversion and selectivity of magnetic core-shell catalysts in the hydrogenation of phenylacetylene (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in ethanol).

39



(a) unreduced Fe3O4@ZIF-8/Pd

Pd 3d

338.0 eV fresh

Intensity / a.u.

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

5-cycle used

(b) reduced Fe3O4@ZIF-8/Pd

40 oC-reduced 335.2 eV 336.6 eV

180 oC-reduced

350

345

340

335

330

Binding energy / eV

Figure 8. Pd 3d XPS spectra of (a) unreduced and (b) reduced Fe3O4@ZIF-8/Pd.

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Transmittance / a.u.

Page 41 of 44

C=N

Fe3O4@ZIF-8/Pd

Zn-N

Fe3O4@ZIF-8

1800 1600 1400 1200 1000 800 Wavenumber / cm-1

600

400

Figure 9. FTIR spectra of Fe3O4@ZIF-8 and Fe3O4@ZIF-8/Pd.

41



5

-1 Initial rate / mmol⋅g-1 Pd⋅s

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

4

3

2

1

0

0

2 4 6 Solvent polarity

0

1 2 3 4 5 6 H2 solubility / mol⋅m-3

Figure 10. Variation of the initial rate over Fe3O4@ZIF-8/Pd with the solvent polarity and H2 solubility (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in different solvent).

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Figure 11. Reusability of Fe3O4@ZIF-8/Pd in 5 consecutive cycles of reaction (condition: 40 ˚C, 0.1 MPa, 5 wt% phenylacetylene in methanol).

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For Table of Contents Only

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Magnetically Recyclable Core-Shell Structured Pd-Based Catalysts for

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