Highly efficient transfer hydrodeoxygenation of vanillin over Sn4+-induced highly dispersed Cu-based catalyst

Applied Surface Science 480 (2019) 548–556

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Highly efficient transfer hydrodeoxygenation of vanillin over Sn4+-induced highly dispersed Cu-based catalyst

T

Zhi Gao , Fengqing Liu, Li Wang, Feng Luo ⁎

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State Key Laboratory of Nuclear Resources and Environment, School of Chemistry, Biology and Materials Science, East China University of Technology, Nanchang 330013, PR China

ARTICLE INFO

ABSTRACT

Keywords: Lattice confinement Layered double hydroxide Cu-based catalyst Catalytic transfer hydrodeoxygenation Vanillin

Developing highly efficient non-noble-metal catalysts for the upgrade of abundant and low-cost renewable raw biomass into high-quality biofuels and important chemicals is especially desirable, but still remains huge challenges. Herein, Sn4+-induced highly dispersed Cu-based catalyst, Cu/Zn-Al-Sn layered double hydroxide (Cu/ZnAlSn-LDH), is delicately constructed for catalytic transfer hydrodeoxygenation of vanillin to promising 2methoxy-4-methylphenol (MMP) biofuel using 2-propanol as hydrogen source and solvent without any external hydrogen supply. Nearly total MMP yield is achieved under moderate reaction conditions (180 °C, 4 h) and the turnover number (TON) value calculated in Cu/ZnAlSn-LDH is about 3 times higher than that in Sn-free catalyst (Cu/ZnAl-LDH). Characterizations results reveal that the Sn4+ species confined in the lattice of brucite-like layer of ZnAlSn-LDH are existed in electron-rich state, which can promote the formation of smaller Cu nanoparticles (1.95 nm) in Cu/ZnAlSn-LDH compared to those in Sn-free Cu/ZnAl-LDH catalyst (6.08 nm), as well as stronger metal-support interaction, thus leading to the higher catalytic performance and stability. The present findings offer a new avenue to strategically fabricate highly dispersed non-noble metal catalysts with enhanced catalytic performance by adjusting surface structures and compositions of supports for a wide range of hydrodeoxygenation of other biomass-derived compounds without any external hydrogen.

1. Introduction Upgrading abundant renewable lignin and lignin-related phenols to high-quality biofuels and essential chemicals is one of the most promising solutions to address the current energy crisis [1,2]. Vanillin as a common component of lignin-derived pyrolysis oil is generally considered to be the crucial platform molecule, which can be hydrodeoxygenated to promising 2-methoxy-4-methylphenol (MMP) liquid biofuel [3]. However, the hydrodeoxygenation of vanillin to desired MMP is more difficult and challenging compared to cellulose-derived pyrolysis oil because of its highly complex structure [4]. Up to now, tremendous efforts have been devoted to enhance the catalytic performance of vanillin hydrodeoxygenation to MMP. Various noble metal (e.g., Pd [5–10], Ru [11–14] and Au [15]) catalysts have been widely investigated and excellent catalytic performance has already been achieved. However, the high cost, low earth-abundance and easy deactivation hinder their large-scale application. Thus, developing highly active, cost-effective and stable non-noble metal catalysts is imperative and informative. At present, several non-noble metal catalysts have been reported for hydrodeoxygenation of vanillin to generate

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MMP [16–20]. But, the long reaction time, high H2 pressure and reaction temperature are usually necessary because of the intrinsic low efficiency of non-noble metal catalysts. Moreover, external H2 is not ideal hydrogen donor because of its expensive cost for storage and transformation. Recently, formic acid was used as hydrogen donor to replace H2 for hydrodeoxygenation of vanillin to MMP [21]. However, formic acid is highly acidic, which will inevitably corrode the reactors and thus increase the cost of production. Compared with formic acid, noncorrosive alcohols are more suitable hydrogen sources, which can be continuously obtained from renewable biomass, and the dehydrogenation products (aldehydes or ketones) can be easily separated from reaction system [22–24]. Consequently, efficient hydrodeoxygenation of vanillin to MMP using alcohols as hydrogen donors without any external hydrogen over non-noble metal catalysts is a very meaningful and challenging work. In the case of supported catalysts, the higher active metal dispersion will result in more excellent catalytic activity. In recent years, tremendous efforts were made to improve the dispersion of noble-metal catalysts inspired by the unparalleled catalytic activity of single-atom catalysts. For example, different single-atom noble metal catalysts were

Corresponding authors. E-mail addresses: [email protected] (Z. Gao), [email protected] (F. Luo).

https://doi.org/10.1016/j.apsusc.2019.02.219 Received 25 October 2018; Received in revised form 16 February 2019; Accepted 25 February 2019 Available online 01 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Scheme 1. The synthetic strategy for highly dispersed Cu-based catalyst (Cu/ZnAlSn) by the inducement of Sn4+ species confined in brucite-like layer of ZnAlSnLDH.

prepared by the anchoring role of the defect sites on support surface, which presented extraordinary catalytic performance [25–28]. The design and fabrication of highly dispersed non-noble metal catalysts by simple and feasible method is more important due to its lower cost, which have not obtained enough attention. Layered double hydroxides (LDHs) own a wide range of metal M2+ and M3+ or M4+ cations in the brucite-like layers, where different cations are highly distributed owing to the confinement effect of lattices [29–32]. It have been reported by numerous literatures that mixed metal oxides produced through topological transformation of LDHs can be reduced to form uniformly metal catalysts under reduction atmosphere [33–36]. However, the fabrication of highly dispersed non-noble metal catalysts induced by the layer element confined in the layer of pristine LDHs has not been reported. In this work, a highly dispersed and low-cost Cu-based catalyst is strategically designed by the inducement of Sn4+ species confined in brucite-like layer of ZnAlSn-LDH (Scheme 1) and applied for the transfer hydrodeoxygenation of vanillin to produce MMP biofuel under atmospheric N2 pressure. It is found that Sn4+ species are electron-rich state owing to the lattice confinement of LDH layer, which play a key role in the formation of well-dispersed Cu nanoparticles (NPs) and strong metal-support interactions. The developed Cu/Zn15Al4Sn1-LDH catalyst possesses smaller Cu nanoparticles (1.95 nm) compared to Snfree Cu/ZnAl-LDH catalyst (6.08 nm), thus leading to nearly total MMP yield without any external hydrogen at 180 °C for 4 h, which is much higher than Sn-free catalyst (Cu/Zn15Al5-LDH). Moreover, compared with Cu/Zn15Al5-LDH, Cu/Zn15Al4Sn1-LDH exhibits greatly enhanced stability. This is the first report about the transfer hydrodeoxygenation of vanillin to MMP catalyzed by a highly efficient and stable LDHsupported non-noble Cu catalyst.

comparison, Zn15Al5-LDH support with Zn/Al molar ratio of 15:5 was prepared according to the same procedure as for Zn15Al4Sn1-LDH without the addition of SnCl4·5H2O. In addition, Sn(OH)4 was loaded on Zn15Al4-LDH to obtain Sn(OH)4/Zn15Al4-LDH using a co-precipitation method. Firstly, Zn15Al4-LDH with Zn/Al molar ratio of 15:4 was synthesized using the above identical procedure for Zn15Al5-LDH. Then, SnCl4·5H2O and Zn15Al4-LDH were added into 100 mL of distilled water under ultrasonication for 0.5 h. Then, NaOH aqueous solution was dropwise added until the pH = 10.0 under vigorous stirring and then aged at room temperature for 2 h. Afterwards, the obtained suspension was centrifuged, washed thoroughly until the pH = 7.0 and dried in a vacuum oven at 60 °C for 12 h. Cu was deposited on the Zn15Al4Sn1-LDH, Zn15Al5-LDH and Sn (OH)4/Zn15Al4-LDH to obtain Cu/Zn15Al4Sn1-LDH, Cu/Zn15Al5-LDH and Cu/Sn(OH)4/Zn15Al4-LDH, respectively. First, 0.5 g support was added into 100 mL of Cu(NO3)2·3H2O aqueous solution (7.8 mM) and stirred at room temperature for 6 h. Then, NaBH4 solution (50 mL, 312 mM) was added into the above suspension under ice water bath and maintained for 6 h under nitrogen atmosphere. Finally, the as-formed products was centrifuged, washed with deionized water and dried at 70 °C overnight under vacuum. Moreover, Zn15Al4Sn1-LDH-B sample was also prepared using the same procedure as that for Cu/Zn15Al4Sn1LDH without the addition of Cu using 15 mL NaBH4 solution (312 mM) as reducing agent. 2.2. Characterization Powder X-ray diffraction (XRD) tests of samples were performed on a Shimadzu XRD-6000 diffractometer (Cu Kα radiation, k = 0.154 nm) at a 2ɵ ranging from 3° to 70° with a scanning rate of 10° min−1. Elemental analysis of metal in samples was obtained using a Shimadzu ICPS-7500 inductively coupled plasma emission spectrometer (ICP-AES). Before measurements, the samples were totally dissolved in nitrohydrochloric acid. The low-temperature N2 adsorption-desorption tests were carried out at 77 K on a Micromeritics ASAP 2020 sorptometer apparatus. The specific surface area was estimated using multipoint Brunauer-EmmettTeller (BET) method based on the isotherms. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were made on a JEOL JEM-2100 transmission electron microscope at 200 kV. The sample was firstly sonicated in ethanol for 30 min and then placed on a Cu grid covered with a thin film of carbon. The molar ratios of Cu+/(Cu0 + Cu+) in different samples were obtained through H2-N2O titration using a Micromeritics ChemiSorb 2920 instrument. Prior to the tests, the sample (0.1 g) was pretreated in an Ar flow (40 mL/min) at 200 °C for 1 h. After cooling down the

2. Experimental section 2.1. Synthesis of catalysts Zn15Al4Sn1-LDH support was synthesized by co-precipitation method. Typically, Zn(NO3)2·6 H2O (3 mmol), Al(NO3)3·9H2O (0.8 mmol) and SnCl4·5H2O (0.2 mmol) with Zn/Al/Sn molar ratio of 15:4:1 were firstly dissolved in 100 mL deionized water to form a mixed salt solution, while Na2CO3 and NaOH ([CO32−] = 2([Al3+] + [Sn4+]); [OH−] = 1.6([Zn2+] + [Al3+] + [Sn4+])) were also dissolved in 100 mL deionized water to form a mixed base solution. Then, the base solution was dropwise added into the salt solution under vigorous agitation at room temperature until the pH = 10.0. After that, the suspension was aged at 70 °C for 10 h and then centrifuged and washed with deionized water for several times until the pH reached 7.0. Finally, the as-obtained Zn15Al4Sn1-LDH precipitation was dried at 70 °C for 12 h. For 549

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temperature to 60 °C, the 10% N2O/N2 was injected to oxide the surface Cu0 atoms to Cu2O, followed by Ar purging and cooling the sample to room temperature. Finally, Hydrogen temperature-programmed reduction (H2-TPR) was performed under 10% H2/Ar. According to the consumed amount of H2, Cu+/(Cu0 + Cu+) ratio was calculated assuming that all Cu atoms are loaded on the surface of support. X-ray photoelectron spectroscopy (XPS) and X-ray-induced Auger electron spectroscopy (XAES) spectra were obtained using a Thermo VG ESCALAB250 at a base pressure of 2 × 10−9 Pa with Al Ka radiation. The samples were stored under vacuum condition to prevent oxidation by air. Moreover, the Cu 2p region and Cu LMM was tested by a singlescan analysis and the time is less than 20 s and 1 min, respectively, thus minimizing the influence of radiation used on the chemical states of copper species. The binding energy of all spectra was calibrated referenced to the C1s signal at 284.6 eV. The total number of basic sites was determined by irreversible adsorption of acrylic acid referenced to the literature [37].

Table 1 The structural and chemical compositions for different catalysts. Sample

Cu/Zn15Al4Sn1-LDH Cu/Zn15Al5-LDH Cu/Sn(OH)4/ Zn15Al4-LDH a b

Weight content (wt%)a Cu

Zn

Al

Sn

9.1 9.6 9.2

38.2 38.8 37.9

6.8 7.3 6.6

4.3 0 4.5

SBET (m2 g−1)

Cu+/(Cu0 + Cu+) ratiob

135 109 117

0.58 0.38 0.40

Determined by ICP-AES characterization. Determined by XAES analysis.

radius of Sn4+ compared to that of Al3+, demonstrating that a partial of Al3+ species confined in the LDH layer are replaced by Sn4+ [40]. In the case of Zn15Al4-LDH supported Sn(OH)4 sample (Sn(OH)4/Zn15Al4LDH), no typical diffraction peaks indexed to Sn(OH)4 phase are found due to the low Sn content. For Cu/Zn15Al5-LDH, Cu/Zn15Al4Sn1-LDH and Cu/Sn(OH)4/Zn15Al4-LDH catalysts, Cu2O and LDH phases are obviously detected (Fig. 1B). Moreover, the typical diffractions related to metallic Cu phase are observed in Cu/Zn15Al5-LDH and Cu/Sn(OH)4/ Zn15Al4-LDH, but it is not found in Cu/Zn15Al4Sn1-LDH, which strongly proves that Sn4+ species confined on the LDH layer can promote the dispersion of Cu NPs. Moreover, the metal element contents in different catalysts were determined by ICP-AES characterization. As shown in Table 1, the Cu loading in all samples is about 9.5 wt% and the Sn content is controlled as about 4.3 wt%, very close to the nominal amount. Noticeably, the BET surface area of Cu/Zn15Al4Sn1-LDH (135 m2 g−1) is higher than those of Cu/Zn15Al5-LDH (109 m2 g−1) and Cu/Sn(OH)4/Zn15Al4-LDH (117 m2 g−1), which should be attributed to the smaller LDH crystal size. TEM characterization was carried out to investigate Cu NPs size in different samples. For Cu/Zn15Al4Sn1-LDH sample, the highly dispersed Cu NPs are clearly observed and the average size is only 1.95 nm (Fig. 2a, b), which is much smaller than those over Cu/Sn(OH)4/Zn15Al4-LDH (6.03 nm, Fig. 2c, d) and Cu/Zn15Al5-LDH (6.08 nm, Fig. 2e, f), well consistent with the results of XRD characterizations, further indicating that the lattice-confined Sn4+ is good for the dispersion of Cu NPs. Moreover, the illustration in Fig. 2a reveals that one single particle processes clear lattice fringes with the interplanar spacings of 0.209 nm, which is assigned to the (111) plane of metallic Cu phase [41].

2.3. Catalyst test The hydrodeoxygenation of vanillin was performed in a stainlesssteel autoclave (100 mL) equipped with a magnetic stirrer. In a typical experiment, vanillin (1 mmol), catalyst (20 mg) and 2-propanol (10 mL) were put into the autoclave. Afterwards, the autoclave was sealed tightly and the air was purged out using 2 MPa N2 for 10 times, and subsequently initiated under N2 at atmospheric pressure at the required temperature for a desired time with magnetic stirring (800 rpm). After the completion of reaction, the autoclave was carefully cooled down to room temperature and then depressurized. Finally, a gas chromatograph (Agilent GC7890 B) equipped with DB-wax capillary column and flame ionization detector was used to analyze the liquid products, which is identified as compared with known standards with toluene as an internal standard. In all cases, the carbon balances were above 95%. 3. Results and discussion 3.1. Structural analysis of samples As shown in Fig. 1A, XRD patterns of the catalysts precursors all exhibit the characteristic diffractions indexed to (003), (006), (012), (015), (110) and (113) crystalline planes of hydrotalcite-like materials [38,39]. Sn-containing LDH (Zn15Al4Sn1-LDH) sample shows weaker diffraction intensity relative to Zn15Al5-LDH, suggesting that the introduction of Sn species decreases the crystallinity of LDH phase. The lattice parameter of a calculated in Zn15Al4Sn1-LDH (0.3087 nm) is larger than that in Zn15Al5-LDH (0.3082 nm), owing to the larger ionic

3.2. Surface properties of samples XPS characterization is an efficient tool to investigate metal-support interaction [42]. For Cu 2p spectra (Fig. 3A), the satellite peak at

Fig. 1. XRD patterns of A) the catalysts precursors: a) Zn15Al5-LDH, b) Sn(OH)4/Zn15Al4-LDH and c) Zn15Al4Sn1-LDH and B) reduced catalysts: a) Cu/Zn15Al5-LDH, b) Cu/Sn(OH)4/Zn15Al4-LDH and c) Cu/Zn15Al4Sn1-LDH. 550

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Fig. 2. HRTEM images and histograms of particle size distribution of Cu NPs for Cu/Zn15Al4Sn1-LDH (a, b), Cu/Sn(OH)4/Zn15Al4-LDH (c, d) and Cu/Zn15Al5-LDH (e, f).

942–944 eV relative to Cu2+ species in three samples is not detected, indicating the total reduction of Cu2+ species. The binding energy of Cu0 species in Cu/Zn15Al4Sn1-LDH (932.3 eV) is lower than that in Cu/ Zn15Al5-LDH (932.9 eV), which reflects that the lattice-confined Sn4+ can enhance the metal-support interactions, thus promoting the transfer of electron from support to metallic Cu. Interestingly, the difference of binding energy of Cu0 species in Cu/Sn(OH)4/Zn15Al4-LDH (932.8 eV)

and Cu/Zn15Al5-LDH (932.9 eV) samples is negligible, which can deduce that the Sn4+ loaded on the surface of Zn15Al4-LDH is hardly to influence the electronic interaction between Cu0 and support. In order to further investigate the chemical states of Cu species, Cu XAES spectra was analyzed. As shown in Fig. 3B, the broad peak within 912.5–922 eV in three samples was deconvoluted to two components by Gaussian peak fitting method. The peaks at 918.1 eV and 916.8 eV correspond to 551

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Fig. 3. XPS (A) of Cu 2p region and Cu LMM XAES (B) for Cu/Zn15Al5-LDH (a), Cu/Sn(OH)4/Zn15Al4-LDH (b) and Cu/Zn15Al4Sn1-LDH (c) samples.

Cu0 and Cu+ species, respectively [43,44]. Noticeably, the Cu+/ (Cu0 + Cu+) molar ratio determined by Cu XAES spectra (Table 1) and H2-N2O titration (Table S1) in Cu/Zn15Al4Sn1-LDH is higher than those in Cu/Sn(OH)4/Zn15Al4-LDH and Cu/Zn15Al5-LDH, suggesting the stronger metal-support interactions in Cu/Zn15Al4Sn1-LDH sample induced by the Sn4+ species confined in the layer. For the Sn 3d5/2 XPS spectra, the binding energy centered at about 486.3 and 485.7 eV is assigned to Sn4+ and Sn0 species, respectively [45,46]. As presented in Fig. 4, the binding energy of Sn4+ in Zn15Al4Sn1-LDH (486.3 eV) is lower than that in Sn(OH)4/Zn15Al5-LDH (486.6 eV), revealing that the lattice-confined Sn4+ in LDH layer has a higher electronic density. The positively charged Cu2+ will be better anchored on Zn15Al4Sn1-LDH support owing to the electrostatic interaction between Cu2+ and electron-rich Sn4+ sites. Moreover, Sn0 species is not found in Zn15Al4Sn1-LDH and Sn(OH)4/Zn15Al4-LDH support. In the case of Cu-containing catalysts (Cu/Zn15Al4Sn1-LDH and Cu/Sn (OH)4/Zn15Al4-LDH), the Sn0 is obviously observed. However, the molar ratio of Sn0/(Sn4+ + Sn0) calculated by the deconvolution of XPS spectra in Cu/Zn15Al4Sn1-LDH (10.7%) is much lower than that in Cu/Sn(OH)4/Zn15Al4-LDH (19.5%). This result demonstrates that the lattice confinement of brucite-like layer can well stabilize the Sn4+ species, retarding the reduction of confined Sn4+ to Sn0. The Sn0 and Sn4+ species are also detected in Zn15Al4Sn1-LDH-B sample prepared

using NaBH4 as reduction agent with a similar Sn0/(Sn4+ + Sn0) ratio to that in Cu/Zn15Al4Sn1-LDH (Fig. S1). Furthermore, the binding energy of Sn4+ species increase by about 0.5 eV in Cu/Zn15Al4Sn1-LDH compared to that in Zn15Al4Sn1-LDH, indicative of the electron transfer from Sn4+ to Cu species, well consistent with the result of Cu 2p spectra. Noticeably, the binding energy of Sn4+ species in Cu/Sn(OH)4/ Zn15Al4-LDH is only 0.1 eV higher than that in Sn(OH)4/Zn15Al5-LDH. Above results prove stronger electron interactions formed between confined Sn4+ sites and supported Cu species in Cu/Zn15Al4Sn1-LDH sample. It has been reported that the hydroxyl group as basic sites can promote the transfer hydrogenation of biomass-derived carbonyl compounds via Meerwein–Ponndorf–Verley (MPV) reduction pathway [47–49]. Vanillin, including a carbonyl group, should be easily converted to intermediate 4-hydroxymethyl-2-methoxyphenol (HMP) with the help of surface basic sites that originates from the hydroxyl group of LDHs. Temperature-programmed desorption of CO2 is an efficient tool to determine the basic sites of catalysts, but it will destroy the structure of LDHs at high temperature. Thus, the amount of surface basic sites in different samples was investigated by acrylic acid absorption. As shown in Fig. 5, the total amounts of surface basic sites in Cu/Zn15Al4Sn1-LDH is only slightly higher than those in Cu/Zn15Al5-LDH and Cu/Sn(OH)4/Zn15Al4-LDH, suggesting that the

Fig. 4. XPS spectra of Sn 3d5/2 region for Zn15Al4Sn1-LDH (A) and Sn(OH)4/Zn15Al4-LDH (B) without (a) or with (b) Cu. 552

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Fig. 5. The total amounts of surface basic sites in different samples. Fig. 6. The yield of acetone in different catalysts. Reaction conditions: catalyst, 20 mg; vanillin, 1 mmol; 2-propanol, 10 mL; reaction time, 4 h; reaction temperature, 180 °C; N2 atmosphere.

introduction of Sn species has negligible effect on the number of surface basic sites.

and TON value in Cu/Zn15Al4Sn1-LDH is about 3 times higher than those in Cu/Zn15Al5-LDH and Cu/Sn(OH)4/Zn15Al4-LDH. Noticeably, the reaction in this work is performed without any external H2 under atmospheric N2 pressure, which shows a promising industrial application for the transformation of vanillin to MMP without the high requirements for instrument. Catalytic transfer hydrodeoxygenation of vanillin is composed of two processes including dehydrogenation of hydrogen donor 2-propanol and subsequent hydrodeoxygenation of vanillin. The easy dehydrogenation of 2-propanol can facilitate the hydrodeoxygenation of vanillin. Thus, the dehydrogenation ability of different catalysts was explored to find out the intrinsic origin of catalytic activity using 2propanol as reactant without the addition of vanillin, which could exclude the influence of hydrodeoxygenation process. As shown in Fig. 6, the yield of dehydrogenation product acetone follows the order of Cu/ Zn15Al5-LDH < Cu/Sn(OH)4/Zn15Al4-LDH < Cu/Zn15Al4Sn1-LDH, in accordance with the order of the amounts of surface basic sites. However, the difference of acetone yield in different catalysts is very slight, which may be due to the very close amounts of surface basic sites. Therefore, it can be reasonably concluded that the significant difference of catalytic performance in three catalysts is not related to the surface basic sites. The influence of reaction time on the hydrodeoxygenation of vanillin was studied over Cu/Zn15Al4Sn1-LDH to explore the reaction routes at 180 °C under N2 atmosphere. As shown in Fig. 7, the catalytic efficiency of Cu/Zn15Al4Sn1-LDH is found to be obviously affected by the reaction time. With the increase of reaction time, the conversion of vanillin and the selectivity of MMP are gradually improved. At the reaction time of 10 min, vanillin is mainly transformed to intermediate HMP with the higher selectivity of 64.2% relative to that of targeted MMP (35.8%). When the reaction time is prolonged to 120 min, the selectivity of HMP dramatically decreases to 10.3%, while the selectivity of MMP can reach 89.7%. Simultaneously, the conversion of vanillin increases from 33.6% at 10 min to 82.5% at 120 min. The increase of vanillin conversion and the decrease of HMP selectivity reveal the facts that vanillin is firstly hydrogenation to form HMP and then hydrodeoxygenation to generate MMP. Completed conversion of vanillin and high MMP selectivity (98.5%) are achieved after 240 min. More importantly, other byproducts are not detected in the reaction system because of the mild reaction conditions, which can decrease the cost of separation and thus promote the achievement of industrial application.

3.3. Catalytic transfer hydrodeoxygenation of vanillin The catalytic efficiency of different catalysts for liquid-phase transfer hydrodeoxygenation of vanillin was tested and the results are listed in Table 2. It can be seen that the catalytic performance is obviously different over different samples. For Cu/Zn15Al4Sn1-LDH sample, the conversion of vanillin and the selectivity of desired MMP can reach as high as 100% and 98.5%, respectively, at 180 °C for 4 h, indicative of extremely high catalytic performance. In contrast, the vanillin conversion in Cu/Zn15Al5-LDH and Cu/Sn(OH)4/Zn15Al4-LDH is only 52.4% and 53.7% at the same reaction conditions, respectively. The poor catalytic activity should be ascribed to the larger Cu NPs in them. Moreover, the MMP selectivity in Cu/Zn15Al4Sn1-LDH (98.5%) is also much higher than those in Cu/Zn15Al5-LDH (66.3%) and Cu/Sn (OH)4/Zn15Al4-LDH (68.5%). As reported by our precious work [34], the Cu+ species can promote the absorption and subsequent activation of carbonyl and hydroxyl groups in biomass-derived platform molecules. Thus the excellent MMP selectivity should be related to higher Cu+ content in Cu/Zn15Al4Sn1-LDH. The only byproduct of HMP is detected in this system owing to the mild reaction conditions. Noticeably, for Cu-free Zn15Al4Sn1-LDH and Zn15Al4Sn1-LDH-B samples, vanillin only can be transformed to intermediate HMP without the formation of targeted MMP under same reaction conditions, demonstrating that the Cu species are key active sites for the hydrodeoxygenation of hydroxyl group in HMP. Furthermore, the MMP yield Table 2 The hydrodeoxygenation of vanillin over different catalysts.a Catalyst

Cu/Zn15Al4Sn1-LDH Cu/Zn15Al5-LDH Cu/Sn(OH)4/ Zn15Al4-LDH Zn15Al4Sn1-LDH Zn15Al4Sn1-LDH-B

Con. (%)

Selectivity (%)

MMP yield (%)

TON (mol mol−1)b

MMP

HMP

100 52.4 53.7

98.5 66.3 68.5

1.5 33.7 31.5

98.5 34.7 36.8

34.6 11.6 12.8

35.6 32.1

0 0

100 100

0 0

– –

a Reaction conditions: catalyst, 20 mg; vanillin, 1 mmol; 2-propanol, 10 mL; reaction time, 4 h; reaction temperature, 180 °C; N2 atmosphere. b The TON value was calculated based on the moles of MMP generated per mole of Cu.

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relatively low temperature. The reaction rate of vanillin decreases significantly with the further increase of temperature. The selectivity of products is also obviously influenced by the temperature. The main product is intermediate HMP at 120 °C and 140 °C. Along with the rise of temperature to 160 °C, the selectivity of MMP exceeds that of HMP. At 180 °C, the vanillin conversion and MMP selectivity can reach as high as 100% and 98.5%, respectively. These results indicate that the hydrogenation of vanillin to HMP is easier than the hydrodeoxygenation of HMP to MMP and the high temperature can accelerate the hydrodeoxygenation of hydroxyl group in HMP. The catalytic performance for transfer hydrodeoxygenation of vanillin over Cu/Zn15Al4Sn1-LDH was also investigated in different hydrogen sources including primary alcohol and secondary alcohol. It was found that the conversion of vanillin is higher in secondary alcohols because of lower reduction potential compared to primary alcohol. Although the conversion of vanillin is affected by hydrogen sources, the selectivity of MMP always can keep high value (Table S2), implying that the MMP selectivity associated only with the reaction system cannot be changed by the properties of hydrogen sources. Moreover, furfural and 5-hydroxymethylfurfural including aldehyde group were used as reactant to investigate the feasibility of Cu/Zn15Al4Sn1-LDH. As listed in Table S3, the high yield of targeted 2-methylfuran and 2,5dimethylfuran are obtained, which offers a huge potential to realize large-scale application in the field of catalytic transfer hydrodeoxygenation of biomass-derived compounds. The reusability of catalysts for transfer hydrodeoxygenation of vanillin was tested under low conversion to accurately reveal the change of catalytic performance. After reaction, the generated reaction mixture was centrifuged and washed with deionized water and ethyl alcohol for several times to obtain used catalyst for next test. As shown in Fig. 9A, Cu/Zn15Al4Sn1-LDH catalyst still can keep almost unchanged catalytic performance even after five runs and the yield of MMP only decreases by about 3.4%, demonstrating the outstanding stability. But, the yield of MMP decreases by as high as 18.7% in Cu/Zn15Al5-LDH. To elaborate the deactivation mechanism, the filtrate after five consecutive cycles was analyzed by ICP-AES characterization. The Cu leaching loss in Cu/ Zn15Al4Sn1-LDH is only 0.8 wt% of the total Cu content, which is much lower than that in Cu/Zn15Al5-LDH (8.9 wt%). Furthermore, the electronic state of Cu species of spent Cu/Zn15Al4Sn1-LDH catalyst was determined by Cu XAES characterization (Fig. 9B) and the result reflects that the molar ratio of Cu+/(Cu0 + Cu+) (0.57) almost keeps unchanged compared to that in fresh Cu/Zn15Al4Sn1-LDH (0.58). Additionally, the Cu/Sn(OH)4/Zn15Al4-LDH exhibits poor reusability, with the obvious decrease of MMP yield (18.2%) after five consecutive runs (Fig. S2). And, negligible loss of vanillin conversion and MMP yield were detected in Zn15Al4Sn1-LDH and Zn15Al4Sn1-LDH-B. Above results undoubtedly confirm that the Sn4+ species are confined in the lattices of the brucite-like layers of ZnAlSn-LDH, which can enhance the interaction between Cu NPs and ZnAlSn-LDH support, thus retarding the leaching of Cu particles and the change of electronic state of Cu species. Based on above catalytic performance and characterization results, the enhanced catalytic activity of Cu/Zn15Al4Sn1-LDH should be related to several key factors. In the case of dehydrogenation process, the surface basic sites that originates from surface hydroxyl group of LDHs can promote the dehydrogenation of hydrogen source 2-propanol to form active hydrogen. For the hydrodeoxygenation of vanillin, the high content of electrophilic Cu+ species that originate from the strong interaction between Cu species and Sn-containing Zn15Al4Sn1-LDH support can absorb and activate the carbonyl and hydroxyl groups in vanillin or HMP [34]. Then, the high yield of MMP will be obtained with the help of the highly dispersed Cu NPs induced by the lattice-confined Sn4+ in the ZnAlSn-LDH layer. In summary, three key actors, surface basic sites, highly dispersed Cu NPs and electrophilic Cu+ species, synergistically drive the transfer hydrodeoxygenation of vanillin to produce MMP without any external hydrogen, leading to the extremely high catalytic efficiency.

Fig. 7. The effect of reaction time on the hydrodeoxygenation of vanillin. Reaction conditions: Cu/Zn15Al4Sn1-LDH, 20 mg; vanillin, 1 mmol; 2-propanol, 10 mL; reaction temperature, 180 °C; N2 atmosphere.

Further, the hydrodeoxygenation of vanillin was carried out at different temperatures to find out the relationship between reaction temperature and catalytic performance of Cu/Zn15Al4Sn1-LDH. As depicted in Fig. 8, the reaction temperature has significant effect on the conversion of vanillin and selectivity of products. At the low reaction temperature of 120 °C, the conversion of vanillin is only 12.9%. Upon increasing the reaction temperature to 160 °C, the conversion can be remarkably improved to 85.1%, indicating that the transfer hydrodeoxygenation of vanillin is a very temperature-sensitive reaction at

Fig. 8. The effect of reaction temperature on the hydrodeoxygenation of vanillin. Reaction conditions: Cu/Zn15Al4Sn1-LDH, 20 mg; vanillin, 1 mmol; 2propanol, 10 mL; reaction time, 4 h; N2 atmosphere. 554

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Fig. 9. The reusability (A) of Cu/Zn15Al4Sn1-LDH (dark cyan) and Cu/Zn15Al5-LDH (blue) in the transfer hydrodeoxygenation of vanillin and Cu LMM XAES (B) of Cu/Zn15Al4Sn1-LDH before (a) and after reaction (b). Reaction conditions: catalyst, 20 mg; vanillin, 1 mmol; 2-propanol, 10 mL; reaction time, 0.5 h for Cu/Zn15Al4Sn1-LDH and 4 h for Cu/Zn15Al5LDH; reaction temperature, 180 °C; N2 atmosphere.

4. Conclusions

2900–2908. [7] L. Wang, B.S. Zhang, X.J. Meng, D.S. Su, F.S. Xiao, Hydrogenation of biofuels with formic acid over a palladium-based ternary catalyst with two types of active sites, ChemSusChem 7 (2014) 1537–1541. [8] S. Verma, R.B.N. Baig, M.N. Nadagouda, R.S. Varma, Visible light mediated upgrading of biomass to biofuel, Green Chem. 18 (2016) 1327–1331. [9] X. Xu, Y. Li, Y.T. Gong, P.F. Zhang, H.R. Li, Y. Wang, Synthesis of palladium nanoparticles supported on mesoporous N-doped carbon and their catalytic ability for biofuel upgrade, J. Am. Chem. Soc. 134 (2012) 16987–16990. [10] A.A. Ibrahim, A. Lin, F.M. Zhang, K.M. AbouZeid, M.S. El-Shall, Palladium nanoparticles supported on hybrid MOF-PRGO for catalytic hydrodeoxygenation of vanillin as a model for biofuel upgrade reactions, ChemCatChem 9 (2016) 469–480. [11] F. Huang, W. Li, Q. Lu, X. Zhu, Homogeneous catalytic hydrogenation of bio-oil and related model aldehydes with RuCl2(PPh3)3, Chem. Eng. Technol. 33 (2010) 2082–2088. [12] J.L. Santos, M. Alda-Onggar, V. Fedorov, M. Peurla, K. Eränen, P. Mäki-Arvela, M.Á. Centeno, D.Y. Murzin, Hydrodeoxygenation of vanillin over carbon supported metal catalysts, Appl. Catal. A Gen. 561 (2018) 137–149. [13] L.H. Zhu, Y.Y. Jiang, J.B. Zheng, N.W. Zhang, C.L. Yu, Y.H. Li, C.-W. Pao, J.-L. Chen, C.H. Jin, J.-F. Lee, C.-J. Zhong, B.H. Chen, Ultrafine nanoparticle-supported Ru nanoclusters with ultrahigh catalytic activity, Small 11 (2015) 4385–4393. [14] X.M. Yang, Y. Liang, Y.Y. Cheng, W. Song, X.F. Wang, Z.C. Wang, J. Qiu, Hydrodeoxygenation of vanillin over carbon nanotube-supported Ru catalysts assembled at the interfaces of emulsion droplets, Catal. Commun. 47 (2014) 28–31. [15] X.M. Yang, Y. Liang, X. Zhao, Y.F. Song, L.H. Hu, X.F. Wang, Z.C. Wang, J.S. Qiu, Au/CNTs catalyst for highly selective hydrodeoxygenation of vanillin at the water/ oil interface, RSC Adv. 4 (2014) 31932–31936. [16] R.F. Nie, H.H. Yang, H.F. Zhang, X.L. Yu, X.H. Lu, D. Zhou, Q.H. Xia, Mild-temperature hydrodeoxygenation of vanillin over porous nitrogen-doped carbon black supported nickel nanoparticles, Green Chem. 19 (2017) 3126–3134. [17] L.L. He, Y. Qin, H. Lou, P. Chen, Highly dispersed molybdenum carbide nanoparticles supported on activated carbon as an efficient catalyst for the hydrodeoxygenation of vanillin, RSC Adv. 5 (2015) 43141–43147. [18] L. Jiang, P. Zhou, C.J. Liao, Z.H. Zhang, S.W. Jin, Cobalt nanoparticles supported on nitrogen-doped carbon: An effective non-noble metal catalyst for the upgrade of biofuels, ChemSusChem 11 (2018) 959–964. [19] D. Verma, R. Insyani, H.S. Cahyadi, J.-Y. Park, S.M. Kim, J.M. Chao, J.W. Bae, J. Kim, Ga-doped Cu/H-nanozeolite-Y catalyst for selective hydrogenation and hydrodeoxygenation of lignin-derived chemicals, Green Chem. 20 (2018) 3253–3270. [20] A.L. Jongerius, R. Jastrzebski, P.C.A. Bruijnincx, B.M. Weckhuysen, CoMo sulfidecatalyzed hydrodeoxygenation of lignin model compounds: An extended reaction network for the conversion of monomeric and dimeric substrates, J. Catal. 285 (2012) 315–323. [21] H.H. Yang, R.F. Nie, W. Xia, X.L. Yu, D.F. Jin, X.H. Lu, D. Zhou, Q.H. Xia, Co embedded within biomass-derived mesoporous N-doped carbon as an acid-resistant and chemoselective catalyst for transfer hydrodeoxygenation of biomass with formic acid, Green Chem. 19 (2017) 5714–5722. [22] L. Bui, H. Luo, W.R. Gunther, Y. Roman-Leshkov, Domino reaction catalyzed by zeolites with Brønsted and Lewis acid sites for the production of γ–valerolactone from furfural, Angew. Chem. Int. Ed. 125 (2013) 8180–8183. [23] J. Jae, W.Q. Zheng, R.F. Lobo, D.G. Vlachos, Production of dimethylfuran from hydroxymethylfurfural through catalytic transfer hydrogenation with ruthenium supported on carbon, ChemSusChem 6 (2013) 1158–1162. [24] Z. Gao, L. Yang, G.L. Fan, F. Li, Promotional role of surface defects on carbonsupported ruthenium-based catalysts in the transfer hydrogenation of furfural, ChemCatChem 8 (2016) 3769–3779. [25] B.T. Qiao, A.Q. Wang, X.F. Yang, L.F. Allard, Z. Jiang, Y.T. Cui, J.Y. Liu, J. Li, T. Zhang, Single-atom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634–641.

In summary, a facile and effective method was developed for the fabrication of high dispersed Cu-based catalyst induced by the Sn4+ species confined in the lattice of brucite-like layer of ZnAlSn-LDH. Without any external H2, under N2 at atmospheric pressure, the asobtained Cu/Zn15Al4Sn1-LDH catalyst can deliver excellent catalytic performance in the hydrodeoxygenation of vanillin to generate MMP. Total conversion of vanillin and MMP yield of as high as 98.5% were achieved at 180 °C after reaction of 4 h. The high catalytic performance should be ascribed to the cooperation of three key factors (i.e. surface basic sites, highly dispersed Cu0 NPs and Cu+ species), which efficiently and cooperatively dominated the dehydrogenation of 2-propanol and hydrodeoxygenation of vanillin to MMP. Noticeably, Cu/ Zn15Al4Sn1-LDH exhibited higher stability and recyclability compared to Cu/Zn15Al5-LDH due to the strong interaction between Cu NPs and Zn15Al4Sn1-LDH support, exhibiting promising large-scale industrial application. The distinct features of Cu/Zn15Al4Sn1-LDH catalyst shows huge prospect for the hydrodeoxygenation of other lignin-derived phenolic compounds without any external hydrogen. Acknowledgments We gratefully thank the financial support from National Natural Science Foundation of China (21871047; 21661001) and Natural Science Foundation of Jiangxi Province of China (20181ACB20003). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.02.219. References [1] P. Gallezot, Conversion of biomass to selected chemical products, Chem. Soc. Rev. 41 (2012) 1538–1558. [2] Y.P. Li, Z.M. Liu, W.H. Xue, S.P. Crossley, F.C. Jentoft, S.W. Wang, Hydrogenation of o-cresol on platinum catalyst: catalytic experiments and first-principles calculations, Appl. Surf. Sci. 393 (2017) 212–220. [3] S. Crossley, J. Faria, M. Shen, D.E. Resasco, Solid nanoparticles that catalyze biofuel upgrade reactions at the water/oil interface, Science 327 (2010) 68–72. [4] J.Y. He, C. Zhao, J.A. Lercher, Ni-catalyzed cleavage of aryl ethers in the aqueous phase, J. Am. Chem. Soc. 134 (2012) 20768–20775. [5] Z.B. Zhu, H.Y. Tan, J. Wang, S.Z. Yu, K.B. Zhou, Hydrodeoxygenation of vanillin as a bio-oil model over carbonaceous microspheres-supported Pd catalysts in the aqueous phase and Pickering emulsions, Green Chem. 16 (2014) 2636–2643. [6] F.M. Zhang, S. Zheng, Q. Xiao, Y.J. Zhong, W.D. Zhu, A. Lin, M.S. El-Shall, Synergetic catalysis of palladium nanoparticles encaged within amine-functionalized UiO-66 in hydrodeoxygenation of vanillin in water, Green Chem. 18 (2016)

555

Applied Surface Science 480 (2019) 548–556

Z. Gao, et al. [26] M. Moses-DeBusk, M. Yoon, L.F. Allard, D.R. Mullins, Z.L. Wu, X.F. Yang, G. Veith, G.M. Stocks, C.K. Narula, CO oxidation on supported single Pt atoms: experimental and ab initio density functional studies of CO interaction with Pt atom on θAl2O3(010) surface, J. Am. Chem. Soc. 135 (2013) 12634–12645. [27] M. Yang, L.F. Allard, M. Flytzani-Stephanopoulos, Atomically dispersed Au-(OH)x species bound on titania catalyze the low-temperature water-gas shift reaction, J. Am. Chem. Soc. 135 (2013) 3768–3771. [28] J. Lin, A.Q. Wang, B.T. Qiao, X.Y. Liu, X.F. Yang, X.D. Wang, J.X. Liang, J. Li, J.Y. Liu, T. Zhang, Remarkable performance of Ir1/FeOx single-atom catalyst in water gas shift reaction, J. Am. Chem. Soc. 135 (2013) 15314–15317. [29] P.J. Sideris, U.G. Nielsen, Z.H. Gan, C.P. Grey, Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy, Science 321 (2008) 113–117. [30] Y.L. Shen, K.J. Yin, C.H. An, Z.H. Xiao, Design of a difunctional Zn-Ti LDHs supported PdAu catalyst for selective hydrogenation of phenylacetylene, Appl. Surf. Sci. 456 (2018) 1–6. [31] A.M.P. Sakita, E. Vallés, R. Della Noce, A.V. Benedetti, Novel NiFe/NiFe-LDH composites as competitive catalysts for clean energy purposes, Appl. Surf. Sci. 447 (2018) 107–116. [32] Z. Gao, R.F. Xie, G.L. Fan, L. Yang, F. Li, Highly efficient and stable bimetallic AuPd over La-doped Ca-Mg-Al layered double hydroxide for base-free aerobic oxidation of 5-Hydroxymethylfurfural in water, ACS Sustain. Chem. Eng. 5 (2017) 5852–5861. [33] Q. Hu, L. Yang, G.L. Fan, F. Li, Hydrogenation of biomass-derived compounds containing a carbonyl group over a copper-based nanocatalyst: insight into the origin and influence of surface oxygen vacancies, J. Catal. 340 (2016) 184–195. [34] Z. Gao, C.Y. Li, G.F. Fan, L. Yang, F. Li, Nitrogen-doped carbon-decorated copper catalyst for highly efficient transfer hydrogenolysis of 5-hydroxymethylfurfural to convertibly produce 2,5-dimethylfuran or 2,5-dimethyltetrahydrofuran, Appl. Catal., B 226 (2018) 523–533. [35] W. Li, G.L. Fan, L. Yang, F. Li, Highly efficient synchronized production of phenol and 2,5-dimethylfuran through a bimetallic Ni-Cu catalyzed dehydrogenation-hydrogenation coupling process without any external hydrogen and oxygen supply, Green Chem. 19 (2017) 4353–4363. [36] S.Y. Zhang, G.L. Fan, F. Li, Lewis-base-promoted copper-based catalyst for highly efficient hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate, Green Chem. 15 (2013) 2389–2393. [37] R. Ionescu, O.D. Pavel, R. Bîrjega, R. Zavoianu, E. Angelescu, Epoxidation of cyclohexene with H2O2 and acetonitrile catalyzed by Mg-Al hydrotalcite and cobalt

modified hydrotalcites, Catal. Lett. 134 (2010) 309–317. [38] S. Sankaranarayanan, A. Sharma, K. Srinivasan, CoCuAl layered double hydroxidesefficient solid catalysts for the preparation of industrially important fatty epoxides, Catal. Sci. Technol. 5 (2015) 1187–1197. [39] C. Rudolf, B. Dragoi, A. Ungureanu, A. Chirieac, S. Royer, A. Nastro, E. Dumitriu, NiAl and CoAl materials derived from takovite-like LDHs and related structures as efficient chemoselective hydrogenation catalysts, Catal. Sci. Technol. 4 (2014) 179–189. [40] Y.R. Zhu, Z. An, J. He, Single-atom and small-cluster Pt induced by Sn (IV) sites confined in an LDH lattice for catalytic reforming, J. Catal. 341 (2016) 44–54. [41] Q. Hu, G.L. Fan, L. Yang, F. Li, Aluminum-doped zirconia-supported copper nanocatalysts: surface synergistic catalytic effects in the gas-phase hydrogenation of esters, ChemCatChem 6 (2014) 3501–3510. [42] Z. Gao, G.L. Fan, M.R. Liu, L. Yang, F. Li, Dandelion-like cobalt oxide microspheresupported RuCo bimetallic catalyst for highly efficient hydrogenolysis of 5-hydroxymethylfurfural, Appl. Catal., B 237 (2018) 649–659. [43] Q. Hu, G.L. Fan, S.Y. Zhang, L. Yang, F. Li, Gas phase hydrogenation of dimethyl1,4-cyclohexane dicarboxylate over highly dispersed and stable supported copperbased catalysts, J. Mol. Catal. A Chem. 397 (2015) 134–141. [44] Z. He, H.Q. Lin, P. He, Y.Z. Yuan, Effect of boric oxide doping on the stability and activity of a Cu-SiO2 catalyst for vapor-phase hydrogenation of dimethyl oxalate to ethylene glycol, J. Catal. 277 (2011) 54–63. [45] X. Liu, W.-Z. Lang, L.-L. Long, C.-L. Hu, L.-F. Chu, Y.-J. Guo, Improved catalytic performance in propane dehydrogenation of PtSn/γ-Al2O3 catalysts by doping indium, Chem. Eng. J. 247 (2014) 183–192. [46] B.K. Vu, M.B. Song, I.Y. Ahn, Y.-W. Suh, D.J. Suh, W.-I. Kim, H.-L. Koh, Y.G. Choi, E.W. Shin, Pt–Sn alloy phases and coke mobility over Pt-Sn/Al2O3 and Pt-Sn/ ZnAl2O4 catalysts for propane dehydrogenation, Appl. Catal. A Gen. 400 (2011) 25–33. [47] X. Tang, H.W. Chen, L. Hu, W.W. Hao, Y. Sun, X.H. Zeng, L. Lin, S.J. Liu, Conversion of biomass to γ-valerolactone by catalytic transfer hydrogenation of ethyl levulinate over metal hydroxides, Appl. Catal., B 147 (2014) 827–834. [48] W.W. Hao, W.F. Li, X. Tang, X.H. Zeng, Y. Sun, S.J. Liu, L. Lin, Catalytic transfer hydrogenation of biomass-derived 5-hydroxymethyl furfural to the building block 2,5-bishydroxymethyl furan, Green Chem. 18 (2016) 1080–1088. [49] Z. Gao, G.L. Fan, L. Yang, F. Li, Double-active sites cooperatively catalyzed transfer hydrogenation of ethyl levulinate over a ruthenium-based catalyst, Mol. Catal. 442 (2017) 181–190.

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