Hierarchical Flower-like Bimetallic NiCu catalysts for Catalytic Transfer

May 29, 2019 - (54) With regard to bimetallic NiCu-x samples, a broad absorption appears at about ..... 2015, 17, 3075– 3083, DOI: 10.1039/C5GC00421...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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Hierarchical Flower-like Bimetallic NiCu catalysts for Catalytic Transfer Hydrogenation of Ethyl Levulinate into γ‑Valerolactone Mengran Liu, Siqi Li, Guoli Fan, Lan Yang, and Feng Li* State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, No.15, Beisanhuan East Road, 100029, Beijing, China

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ABSTRACT: Due to fast exploitation and consumption of fossil resources, efficient transformation of renewable biomass would be of vital significance for the production of biofuels and value-added chemicals. Here, we developed new alumina microsphere (AMS) supported bimetallic NiCu catalysts with a hierarchical flower-like architecture for highly efficient catalytic transfer hydrogenation of biomass-derived ethyl levulinate (EL) to γ-valerolactone (GVL), which were derived from flower-like core−shell structured AMS@Ni−Cu−Al layered double hydroxide precursors (AMS@NiCuAl-LDH). Various characterizations demonstrated that the reduction of NiCuAl-LDH precursors in situ grown on the AMS could generate bimetallic NiCu alloy nanoparticles embedded into surface standing and intercrossed LDH-derived alumina nanoplatelets (ANPs), thereby forming a novel hierarchical multilevel AMS@NiCu@ANPs superstructure. As-fabricated bimetallic NiCu catalyst with 0.5 Cu/ Ni molar ratio exhibited enhanced activity for catalytic transfer hydrogenation, in comparison with monometallic and other bimetallic ones, owing to the synergy between Ni−Cu species in highly dispersed bimetallic NiCu nanoparticles, i.e., the electronic effect, favorable surface acid−base property, and highly porous architecture. Moreover, the catalyst possessed good stability and recyclability, due to strong interactions between ANPs and AMS support, as well as between NiCu NPs and ANPs matrix.

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INTRODUCTION Currently, efficient transformation of renewable and abundant biomass resources to important chemicals and biofuels is a promising solution to increasing energy issues and resulting environmental pollution.1 Especially, many researchers are making great efforts toward the utilization of lignocellulose biomass for manufacturing a broad wide of chemicals (e.g., aldehydes, polyols, and acids) and liquid fuels in fine chemical industries and biorefinery processes.2−9 Typically, as an important platform molecule, versatile and viable levulinic acid (LA), which is easily and economically produced through the decomposition of lignocellulosic biomass, may be converted into other useful compounds including γ-valerolactone (GVL), 1,4-pentanediol, 2-methyltetrahydrofuran (MTHF), valerate esters, and so on.10−15 Among them, GVL bearing unique properties including high boiling point, good stability, low toxicity, and high energy density can be widely applied as a fuel additive, a green solvent, an alkene-based © 2019 American Chemical Society

transportation fuel precursor, and a versatile building block for producing other chemicals. To date, a wide range of heterogeneous noble metal and non-noble metal catalysts, such as Cu/ZrO2,16 Ni/ZrO2− Al2O3,17 Au/ZrO2,18 Pd/SiO2,19 Ru/C,20 Ru/graphene,21 Ru− carbon nanofiber,22 and Zr-based catalysts,23 have been explored for the LA hydrogenation to produce GVL. Meanwhile, bimetallic catalysts, such as Cu−Fe,24 Pd−Au,25 Ru−Ni,26 Ru−Sn,27 and Ni−Fe,28 also exhibited good activities. With regard to non-noble metal catalysts, however, hydrogenations are commonly performed under relatively high hydrogen pressures and/or high reaction temperatures (>250 °C), despite high GVL yields. Recently, due to better chemical Received: Revised: Accepted: Published: 10317

April 2, 2019 May 21, 2019 May 29, 2019 May 29, 2019 DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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Industrial & Engineering Chemistry Research Scheme 1. Synthetic Procedure for AMS@NiCu@ANPs through Transformation of AMS@NiCuAl-LDH Precursor

cursor methodology. It was demonstrated that the reduction of NiCuAl-LDH precursors in situ grown on the AMS generated bimetallic NiCu alloy NPs embedded into surface standing and intercrossed LDH-derived alumina nanoplatelets (ANPs), thereby constructing a unique hierarchical multilevel AMS@ NiCu@ANPs superstructure (Scheme 1). This unique architecture with abundant space between ANPs not only can facilitate the exposure of more metallic sites, but also is beneficial to the diffusion and transfer of reactants. Systemic characterizations and experiments disclosed that both the electronic effect and the acid−base property of as-fabricated bimetallic NiCu catalysts, as well as the novel highly porous architecture, were responsible for enhanced performance in the liquid phase CTH of EL with the help of 2-propanol as hydrogen donor and solvent, compared with their monometallic counterparts. So far, this is the first research report on in situ synthesis of supported bimetallic NiCu catalysts with a hierarchical flower-like architecture for the CTH of EL.

and physical properties, levulinate esters as starting substrates also have been utilized for GVL hydrogenation.29 Nevertheless, the use of molecular hydrogen always brings about several drawbacks including hydrogen flammability and the high cost of hydrogen transportation and storage. In this aspect, catalytic transfer hydrogenation (CTH) of LA or its esters utilizing cheap alcohols as both hydrogen honors and solvents is particularly emerging as an efficient and safe hydrogenation process.30−33 Meanwhile, carbonyl compounds produced by the dehydrogenation of alcohols can be further used circularly during the synthesis. In this regard, recently, bimetallic NiCu catalyst system has been explored in a variety of catalytic transformations of biomass. Especially bimetallic NiCu catalysts were applied widely in the CTH of biomass-derived furfural and 5-(hydroxymethyl)furfural.34−36 Meanwhile, the CTH processes of ethyl levulinate (EL), glycerol, and p-cresol have also been reported.33,37,38 In the above reactions, usually, isopropanol is used as a hydrogen donor,35,37,38 besides formic acid.34,36 These bimetallic NiCu catalysts, which can be prepared by traditional impregnation,33,34,36,38 coprecipitation,35 and sol−gel method,37 exhibit good CTH activity, mainly due to a unique synergistic effect between nickel and copper atoms in alloys. Despite great progress made in the above reaction, designing and developing new low-cost and stable heterogeneous non-noble-metal-based catalysts is still quite desirable for practical applications. As a family of two-dimensional functional materials, layered double hydroxides (denoted as LDHs) may be homogeneously converted into mixed metal oxides (MMOs) with highly dispersive and thermally stable characters, as well as metal nanoparticles (NPs), over oxide matrix through the topological transformation,17,39 thanks to their structural versatility and compositional adjustability. Therefore, LDHs-derived bi- or multimetallic catalysts are attracting more and more attention owing to their excellent componential and electronic properties.40,41 Recently, it was reported that magnetically recoverable Ni-based catalyst derived from NiCuFeAl-LDHs was utilized to catalyze the LA hydrogenation to produce GVL using molecular hydrogen.42 In some cases, surface energy of LDHs-derived metal NPs is extremely high, thus resulting in the easy agglomeration of metal NPs formed and even the deactivation of catalysts during reactions. In this contribution, to combine the stabilizing property of alumina support with the synergistic effect between bimetallic species, we developed low-cost hierarchical alumina microsphere (AMS) supported bimetallic NiCu alloy catalysts via a flower-like core−shell structured AMS@NiCuAl-LDH pre-

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EXPERIMENTAL SECTION Preparation of Samples. Alumina microsphere (AMS) was prepared by our previously reported method.40 In the synthesis of AMS@NiCuAl-LDH precursors, first, 280 mL of aqueous salt solution containing Cu(NO3)2·3H2O, Ni(NO3)2· 6H2O, NH4NO3 (0.0375 mol), and NH4F (0.005 mol) was prepared. Then, AMS (0.4 g) and urea (0.017 mmol) were added into the salt solution and the obtained slurry was aged at 100 °C for 7 h. The precipitate obtained was washed with deionized water and dried at 70 °C in air overnight to obtain AMS@LDH-x precursors (x means the [Ni2+]/([Cu2+] + [Ni2+]) molar ratio; x = 0, 0.33, 0.50, 0.67, and 1) . Subsequently, AMS@LDH-x samples were heated at 500 °C for 5 h at air to get calcined AMS@MMO-x samples. Last, AMS@MMO-x samples were reduced under H2/Ar (10/90, v/ v) flow at 600 °C for 3 h to get reduced NiCu-x samples. For comparison, AMS-supported bimetallic NiCu catalyst with the same metal loadings as those for NiCu-0.67 was also prepared by conventional impregnation and denoted as NiCu/ AMS-0.67. Characterization. A powder X-ray diffractometer (Rigaku Co., Ltd.) with a graphite-filtered Cu Kα source (λ = 0.15418 nm) was applied to collect X-ray diffraction (XRD) patterns for samples. The content of metal was determined using a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectrometer (ICP-AES). Before measurement, samples were treated by nitrohydrochloric acid. 10318

DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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samples present characteristic diffractions for nitrate-type LDH phase, as evidenced by a gradual increase in the basal spacing of the (003) plane. However, as for [email protected] and [email protected] samples, the (003) diffraction for carbonatetype LDH phase also can be observed at about 12.2°. Meanwhile, the diffraction intensities for the LDH phase in Nicontaining samples are reduced, indicative of the decreased integrity of the layered structure. For Cu-free AMS@LDH-1, only pure carbonate-type LDH phase with the broadened diffractions is detected, due to the formation of small-sized LDH crystallites. The above results confirm the successful formation of LDH phase on the AMS. Figure 1B presents XRD patterns of reduced NiCu-x samples. In all cases, three intense (110), (200), and (220) diffractions related to monometallic Cu, Ni, or bimetallic Ni− Cu alloy phases are observed. Noticeably, the above characteristic lines present a gradual shift to higher 2θ values with the increasing Ni/(Cu + Ni) molar ratio, confirming the formation of bimetallic Ni−Cu alloy phase. Additionally, the diffractions assigned to crystalline alumina can be found in each case, due to the topological transformation of Al-containing LDH precursors. As for NiCu/AMS-0.67 sample prepared by the impregnation method, the indexed crystalline phase mainly is crystalline Ni−Cu alloy phase (Figure S1). Figure 2 shows SEM images of two representative AMS@ LDH-0.67 and NiCu-0.67 samples. Notably, after in situ direct

N2 adsorption−desorption isotherms of samples were obtained using a Micromeritics ASAP 2020 sorptometer. The total specific surface area was calculated by the Brunauer− Emmett−Teller (BET) method. Field emission scanning electron microscopy (FE-SEM) (Zeiss, model Supra-55) was applied to investigate the surface morphology of samples, while transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained from a JEOL JEM-2100 microscope. High-angle annular dark-field scanning TEM−energy-dispersive X-ray spectroscopy (HAADF-STEM−EDX) was performed on a JEOL 2010F instrument. X-ray photoelectron spectra (XPS) were obtained with a Thermo VG ESCALAB250 X-ray photoelectron spectrometer combined with Al Kα X-ray radiation. The graphite C 1s peak (284.6 eV) was referred to, to calibrate binding energy values. Hydrogen temperature-programmed-reduction (H2-TPR) and temperature-programmed-desorption (CO2-TPD and NH3-TPD) experiments were performed using Micromeritics ChemiSorb 2920 with a thermal conductivity detector. In situ infrared (IR) spectra of adsorbed EL were obtained on a Thermo Nicolet 380 FT-IR spectrometer. The selfsupporting wafer sample (0.3 g) was evacuated at 300 °C for 1 h. Then, when the temperature was cooled to room temperature, EL was introduced and held for 1 h. CTH Tests of EL. The activity and selectivity of catalysts were tested for the CTH of EL in the presence of isopropanol. Typically, EL (4.2 mmol) and the catalyst (100 mg) were added into a batch autoclave reactor, followed by isopropanol (10 mL). under N2 atmosphere, the CTH was carried out with a magnetic stirring speed of 900 rpm at a certain temperature. After the reaction, liquid products were analyzed using a gas chromatograph (Agilent 7890B) with an HP-5 column and flame ionization detector. In repeated tests, the spent catalyst was washed with ethanol and acetone, and then dried at 80 °C for 12 h.

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RESULTS AND DISCUSSION Structural Analysis of Samples. Figure 1A presents XRD patterns of different AMS@LDH-x precursors. Noticeably, NiFigure 2. SEM images of AMS (a), [email protected] (b and c), and NiCu-0.67 (d) samples.

growth of LDH crystallites on the surface of spherical AMS, large quantities of closely packed and interlaced LDH nanoplatelets with visible edges and thickness of about 10− 20 nm are homogeneously distributed over the surface of microspheres, thus forming a perfect hierarchical flower-like microstructure, while LDH nanoplatelets formed are almost perpendicular to the substrate in their ab-direction, thus generating a porous superstructure. Such growth of LDH crystallites may obey a homogeneous nucleation mechanism,43 thereby avoiding collisions between growing LDH crystallites. After calcination and reduction, it is interesting to note that the hierarchical morphology remains relatively intact, and no obvious agglomeration or sintering of adjacent nanoplatelets is found. Similarly, other reduced NiCu-x samples also retain a similar morphology (Figure S2). Inspiringly, these abundant spacings formed between surface thin “petal-like” nanoplatelets can facilitate the transformation of reactants and products in heterogeneous reactions and the exposure of surface active

Figure 1. XRD patterns of AMS@LDH-x (A) and NiCu-x (B) samples with different Ni/(Cu + Ni) molar ratios of 0 (a), 0.33 (b), 0.5 (c), 0.67 (d), and 1.0 (e).

free AMS@LDH-0 sample mainly presents (003), (006), and (012) characteristic diffractions for carbonate-type LDH phase,17 in addition to an impurity, malachite (Cu2(OH)2CO3, PDF No. 41-1390). When Ni is gradually introduced into the brucite-like layers of LDH structure, three Ni-containing 10319

DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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TEM images (Figure 4) presents a slight change in the average particle size of NPs with the increasing Ni/(Cu + Ni) molar ratio (Table 1). In comparison, there are some large irregular NPs of ∼18.5 nm in size randomly distributed on the support in the case of NiCu/AMS-0.67 (Figure S4). To further identify the microstructure of bimetallic NiCu NPs formed, HAADF-STEM analysis of NiCu-0.67 sample was carried out. Based on the EDX mapping from HAADF-STEM images (Figure 5a,b1−b4), Ni, Cu and Al elements are found to be uniformly distributed on the surface. Further, STEM−EDX line scan spectra reveal very similar distributions of Ni and Cu elements over NPs (Figure 5c,d), indicating that Ni and Cu species obviously appear at the same positions. The chemical compositions determined by ICP-AES indicate that the samples possess a similar total loading of Ni and Cu (about 29.0 wt %). As listed in Table 1, the specific surface area of NiCu-0.67 is higher than those of other samples, possibly owing to more a homogeneous distribution of surface nanoplatelets. As we know, alumina is widely used as a catalyst support for immobilizing catalytically active metal species. However, effective techniques for enhancing the metal dispersion on the surface of Al2O3 are rarely reported. The present applied synthesis strategy for supported metal catalysts lies in the fabrication of highly dispersed bimetallic NiCu NPs embedded into surface intercrossed LDH-derived Al2O3 nanoplatelets (ANPs) via a two-step process including in situ direct growth of anisotropic LDH crystallites perpendicular to the substrate of easily available AMS and following calcination−reduction processes. Herein, AMS acts as both the Al source and the stabilizer to govern the LDH particle growth. Correspondingly, uniform bimetallic NiCu NPs can be embedded and stabilized within petal-like ANPs, thereby constructing a novel hierarchical multilevel AMS@NiCu@ANPs superstructure. Surface Properties of Samples. To determine the reducibility of calcined AMS@MMO samples, H2-TPR experiments were carried out. As displayed in Figure 6, with regard to Ni-free AMS@MMO-0 or Cu-free AMS@MMO-1,

sites. In contrast, for the NiCu/AMS-0.67 sample, some irregular aggregates of particles of 60−70 nm in size are present on the AMS surface (Figure S3), and no surface flower-like morphology is developed. TEM and high-magnification TEM images of representative NiCu-0.67 sample (Figure 3) depicts that numerous well-

Figure 3. TEM (a and b) and HRTEM (c) images of NiCu-0.67 sample and size distribution of NPs over NiCu-0.67.

dispersed black NPs with a uniform size of ∼9.0 nm are embedded into surface petal-like nanoplatelets. Further, an HRTEM image of a single particle on the nanoplatelet shows a clear lattice interplanar spacing of 0.203 nm, which is indexed to the (111) plane of fcc bimetallic Ni−Cu alloy phase. The histogram of the size distribution of NPs obtained based on

Figure 4. TEM images and particle size distributions of NiCu-0 (a and e), NiCu-0.33 (b and f), NiCu-0.5 (c and g), and NiCu-1 (d and h) samples. 10320

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Industrial & Engineering Chemistry Research Table 1. Compositional and Structural Properties of Different Samples contenta (wt %) sample

Ni

Cu

RBb

SBETc (m2/g)

DTEMd (nm)

NiCu-0 NiCu-0.33 NiCu-0.5 NiCu-0.67 NiCu-1 NiCu/AMS-0.67

0 9.7 14.4 19.8 29.2 21.4

28.5 18.9 13.5 9.4 0 10.0

0 0.33 0.47 0.68 1.0 0.70

76 144 141 163 149 71

11.6 11.0 10.6 8.6 9.9 18.5

Dbasee (mmol/g) 0.355 0.532 0.548 0.668 0.439 0.303

(0.347)f (0.486) (0.500) (0.660) (0.412) (0.227)

Dacidg (mmol/g) 0.35 0.61 0.67 0.69 0.51 0.67

a

Determined by ICP-AES analysis. bNi/(Cu + Ni) molar ratio. cBET specific surface area. dParticle size of NPs estimated through more than 300 NPs counted. eSurface density of total basic sites based on CO2-TPD results. fData in parentheses is the density of moderate and strong basic sites. g Density of total acidic sites based on NH3-TPD results.

other AMS@MMO samples, two broad peaks are observed in the ranges 255−270 and 394−408 °C, respectively, which are ascribed to the reduction of cationic Cu2+ and Ni2+ species. The low-temperature-reduction peak (α) with an obvious right shoulder (β) is associated with the reduction of highly dispersed Cu2+ species followed by the reduction of bulk CuO. Compared with those for AMS@MMO-0 and AMS@ MMO-1, the reduction for Ni- and Cu-containing samples shifts to relatively lower temperatures. This illustrates that the introduction of Cu and Ni elements can promote their simultaneous reduction, because of the synergistic interactions between Cu−Ni species, thereby easily forming highly dispersive Cu−Ni alloy NPs. Especially, [email protected] presents lower reduction temperatures, suggestive of the formation of stronger interactions between Cu−Ni species in this sample. XPS analysis was performed to obtain chemical states of metal species in reduced samples. Inthe fine Ni 2p spectrum for NiCu-1 sample (Figure 7A), one fitted Ni 2p3/2 peak is located around 853.7 eV, which is associated with metallic Ni0 species. Meanwhile, the sample presents another Ni 2p3/2 peak about 855.5 eV, along with its satellite about 860.7 eV, which is characteristic of Ni2+ species due to surface oxidation of metallic Ni at air. Obviously, the binding energy (BE) of Ni 2p3/2 core level decreases gradually with the introduction of Cu. For the NiCu-0.33 sample, Ni 2p3/2 peaks related to Ni0 and Ni2+ species appear around 853.0 and 854.8 eV,44,45 respectively. It is interestingly noted that the present BE value of metallic Ni0 in samples are larger than that reported in the literature (about 852.6 eV), indicating the formation of the strong metal−support interactions. Moreover, based on the integrated areas of two kinds of Ni species, surface Ni0 species account for about 32.0% over NiCu-0.33, 45% over NiCu-0.5, 38.0% over NiCu-0.67, and 28.0% over NiCu-1 in the total surface Ni component. Specially, higher proportions of Ni0 species on NiCu-0.5 and NiCu-0.67 samples imply that the formation of bimetallic Ni−Cu phase would be beneficial to the stability of metallic species. In the fine Cu 2p spectrum of NiCu-0 (Figure 7B), there are two obvious Cu 2p3/2 and 2p1/2 peaks at 932.6 and 953.7 eV, respectively, indicating that Cu species exist in the form of zero valence state. Interestingly, the change in the BE values of Cu 2p3/2 core level presents an opposite trend. For bimetallic samples, the positive shift of BEs can be mainly attributed to either a change of particle size or electronic interactions.46,47 Since the particle sizes of metal NPs in different samples are almost similar, the BE shift is most likely due to electronic interactions between Ni−Cu atoms in NiCu NPs, with a net electron transfer from Cu to Ni species.

Figure 5. HAADF-STEM images (a and c) of NiCu-0.67 sample with EDX mapping of Ni K (b1), Cu K (b2), Al K (b3), and O K (b4). EDX line scan spectra (d) of Ni K and Cu K along the red line in (c).

Figure 6. H2-TPR profiles of calcined AMS@MMO-0 (a), AMS@ MMO-0.33 (b), [email protected] (c), [email protected] (d), and AMS@MMO-1 (e) samples.

only one broad reduction peak appears at about 286 or 420 °C, which represents the reduction of Cu2+ or Ni2+ species. For 10321

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Figure 7. XPS of Ni 2p (A) and Cu 2p (B) regions of NiCu-0 (a), NiCu-0.33 (b), NiCu-0.5 (c), NiCu-0.67 (d), and NiCu-1 (e).

Figure 8. NH3-TPD (A) and CO2-TPD (B) profiles for NiCu-0 (a), NiCu-0.33 (b), NiCu-0.5 (c), NiCu-0.67 (d), and NiCu-1 (e).

Figure 9. Influence of reaction time on EL conversion (A) and GVL selectivity (B) over NiCu-0 (a), NiCu-0.33 (b), NiCu-0.5 (c), NiCu-0.67 (d), NiCu-1 (e), and NiCu/AMS-0.67 (f) samples in CTH of EL. Reaction conditions: EL 4.2 mmol, catalyst 0.1 g, isopropanol 10 mL, 220 °C, and N2 atmosphere.

50−700 °C is associated with weak basic sites (WB) below 200 °C, moderate basic sites (MB) between 200 and 500 °C, and strong basic sites (SB) above 500 °C, respectively. In the present catalyst system, basic sites mainly originate from O2− species in Al3+−O pairs from both AMS support and ANPs matrix. Based on desorption areas, the bimetallic NiCu-0.67 sample possesses the highest density of total basic sites, as well as the highest density of strong basic sites. In contrast, monometallic NiCu-0 and NiCu-1 samples present relatively weak surface basicity (Table 1). Commonly, high surface area

Figure 8A presents NH3-TPD curves for NiCu-x samples. It can be clearly seen that a broad desorption appears in the range 60−400 °C in each case, indicative of the presence of weak and moderate acidic sites. These acidic sites should mainly originate from surface cationic Al3+ species. In contrast (Table 1), the NiCu-0.67 sample possesses the highest density of acidic sites among all samples, because of its higher surface area beneficial to the exposure of acidic sites. Further, CO2TPD experiments was conducted to determine surface basicity (Figure 8B). Noticeably, a broad CO2 desorption in the range 10322

DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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Industrial & Engineering Chemistry Research can favor the exposure of acidic and basic sites, thus accounting for the increased densities of acidic and basic sites over the NiCu-0.67 sample to a certain extent. Meanwhile, as seen in Table 1, although the surface area of NiCu-0.67 is about only 9.4% higher than that of NiCu-1, the density of surface basic or acidic sites is greatly increased by about 52.2 or 35.3%, indicating that the surface area of NiCu-x samples is not the only reason for the enhanced surface acid−base property. It is speculated that for the NiCu-0.67 sample, besides higher surface area, the presence of more abundant metal−support interface in bimetallic AMS@NiCu@ANPs superstructure may greatly increase the densities of acidic and basic sites. CTH Performance of NiCu-x Catalysts. The catalytic performance of NiCu-x catalysts for the CTH of EL was evaluated. Figure 9 shows the change in the EL conversion and GVL selectivity with the reaction time. It is seen that the EL conversion increases progressively with the reaction time during the CTH reaction. Compared with monometallic NiCu-0 and NiCu-1 catalysts, bimetallic NiCu catalysts exhibit much higher catalytic activity. After a reaction for 6 h, the EL conversion over NiCu-0.67 reaches 91.6%, which is much higher than those over monometallic NiCu-0 (31.5%) and NiCu-1 (68.1%). Meanwhile, among these tested catalysts, NiCu-0.67 also yields the highest GVL selectivity of 89.3% along with a 10% selectivity to isopropyl levulinate as the main byproduct. When the reaction is prolonged to 12 h, a complete EL conversion can be achieved over NiCu-0.67, with an almost constant GVL selectivity of 89.0%. In contrast, the NiCu/ AMS-0.67 comparison sample only gains a much lower EL conversion (27.7%) in 6 h, with a low GVL selectivity of about 60.4%, probably due to the poor metal dispersion originating from the formation of large bimetallic NiCu particles (Figure S4). To confirm that our catalytic reaction is indeed heterogeneous in nature, a hot filtration test also was conducted. Typically, after a reaction of 1 h, NiCu-0.67 catalyst was quickly removed from the reactants. Then, the filtrate was resubmitted for further reaction under the same reaction conditions. After another reaction of 3 h, no obvious increase in the LA conversion was observed. The results undoubtedly verify that active NiCu NPs can be tightly anchored on the support surface, thus guaranteeing the nature of heterogeneous reaction. Given the better CTH performance of NiCu-0.67 catalyst, the effect of reaction temperature on the activity was evaluated (Figure 10). Clearly, both the EL conversion and the GVL selectivity increase gradually with the elevated reaction temperature from 160 to 220 °C. At 240 °C, a slightly decreased GVL selectivity is observed, with a small increase in the conversion. Therefore, 220 °C is the most appropriate reaction temperature for the CTH of EL. In addition, the productivity of GVL (mmol·g−1metal·h−1) over different catalysts was calculated according to the moles of produced GVL per unit mass of active metal species per time (Table S1). Notably, the productivity of GVL over NiCu-0.67 is higher than those of Cu-, Ni-, and Zr-based catalysts previously reported,32,33,48−53 despite different levulinates and alcohols employed as substrates, as well as different hydrogen sources and reaction conditions. The above results indicate the importance of the bimetallic NiCu NPs in catalysis and that the appropriate Cu/Ni ratio would be beneficial to the improvement of catalytic activity, reflecting the role of synergy between Ni−Cu species in the present NiFe-x catalyst system.

Figure 10. Influence of reaction temperature on conversion of EL and selectivity to GVL over NiCu-0.67. Reaction conditions: EL 4.2 mmol, catalyst 0.1 g, isopropanol 10 mL, 3 h, and N2 atmosphere.

To examine the stability of NiCu-0.67 catalyst, the recycling CTH of EL was further conducted at lower EL conversion. As shown in Figure 11, the GVL yield is only decreased by about

Figure 11. Recycling test of NiCu-0.67 catalyst. Conditions: EL 4.2 mmol, catalyst 0.1 g, isopropanol 10 mL, 220 °C, 6 h, and N2 atmosphere.

3.6% after five consecutive cycles, indicating the high stability of the catalyst. Further characterizations of ICP-AES and TEM analyses disclose that the leaching of Ni and Cu species is negligible, and particle agglomeration is not observed (Figure S5). The good stability of NiCu-0.67 catalyst is correlated with the strong interactions between ANPs and AMS support and between NiCu NPs and ANPs matrix. Last, to test the catalytic capacity of as-formed NiCu-0.67, the CTH reactions of several biomass-derived carbonyl compounds were conducted. As shown in Table S2, NiCu0.67 can significantly catalyze the transfer hydrogenation of other substrates, such as LA, methyl levulinate, butyl levulinate, and cyclopentanone to corresponding GVL and cyclopentanol, along with high selectivities to alcohols (>85−95%) at high conversions (88−95%), despite different reaction conditions. This illustrates that NiCu-0.67 has a promising application to produce important chemicals through the CTH processes. Mechanism on CTH over NiCu-x Catalysts. Commonly, the catalytic activity of supported metal catalysts is closely correlated with their surface microstructure, which can remarkably affect the adsorption and activation of substrate 10323

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Industrial & Engineering Chemistry Research

electron-rich Ni0 species may fix carbonyl groups in EL, thereby promoting the chemical adsorption of EL. In addition, as electron acceptors, surface Lewis acidic sites can interact with the carbonyl group of EL through the lone electron pair of an oxygen atom,27,56 thus favoring CO hydrogenation through the nucleophilic attack of active H species. Therefore, in a way, more acidic sites on NiCu-0.67 also can further promote CO activation. (ii) It was reported that the CTH of EL into GVL over various Zr-based catalysts obeyed a Meerwein−Ponndorf− Verley (MPV) reduction route.30,32,50,57 Moreover, the synergetic effect of Lewis acid and base can promote the MPV reduction,58−60 because the increased acid−base property is in favor of the lactonization in the synthesis of GVL. Correspondingly, the present hierarchical flower-like bimetallic NiCu catalysts bearing the increased densities of surface acid and base sites also enable the enhanced catalytic performance for the producibility of GVL from EL through the MVP route. Especially the NiCu-0.67 sample with higher surface acid and base densities exhibited better catalytic performance than other catalysts. The above results emphasize an optimal interface between bimetallic NPs and ANPs for maximum performance. (iii) Although the synthesis of supported CuNi NPs over Al2O3 support by conventional impregnation for CTH of methyl levulinate was reported previously,33 this work did not explore the effects of surface morphologies and acid−base properties of catalysts on the activity. In the present MS@ NiCu@ANPs catalyst system with novel hierarchical superstructure, it is worth mentioning that the AMS acting as a rigid support can provide Al source for the growth of LDH precursors, thus effectively immobilizing active components and improving the dispersion of metallic sites. As a result, a possible reaction mechanism for the synergistic hydrogenation of EL over bimetallic NiCu catalysts was tentatively proposed (Scheme 2). On the one hand, first, the CO group of EL can strongly interact with NiCu NPs. In addition, the adsorbed CO group can be activated by surface Lewis acid (LA) sites (Scheme 2, route 1). Subsequently, the CO group was efficiently transfer hydrogenated by active hydrogen species released through the dehydrogenation of isopropanol over bimetallic catalysts acting as both active dehydrogenation sites and hydrogenation active sites. Finally, EL is converted into GVL through an intramolecular transesterification. For bimetallic NiCu-0.67 catalyst, both abundant surface activation sites for EL substrate and an appropriate amount of active dehydrogenation−hydrogenation

molecules. As we know, the EL substrate contains two carbonyl groups, the hydrogenation of which is a key step in the synthesis of GVL. To gain insight into the activation of carbonyl group in EL, in situ IR spectra of EL adsorbed over different samples were analyzed (Figure 12). For monometallic

Figure 12. In situ IR spectra of adsorbed EL over different catalysts at 25 °C.

samples, a broad band appears at approximately 1731 cm−1 for NiCu-0 or 1719 cm−1 for NiCu-1, due to the stretching vibration of carbonyl group in physiosorbed EL.54 With regard to bimetallic NiCu-x samples, a broad absorption appears at about 1668 cm−1 for NiCu-0.33, 1666 cm−1 for NiCu-0.5, and 1665 cm−1 for NiCu-0.67, respectively. Such a negative absorption shift is assigned to a weakening of the CO bond.55 The above results illustrate the presence of stronger interaction of carbonyl group in EL with the catalyst surface in bimetallic samples, thereby facilitating the EL activation. Particularly, compared with other bimetallic ones, NiCu-0.67 catalyst more easily activates the substrate according to the slightly increased negative shift of the stretching vibration of CO bond. Based on the above various characterization and catalytic results, the enhanced catalytic performance of NiCu-0.67 may be explained by the following three factors: (i) As for bimetallic NiCu catalysts, strong interactions between Ni−Cu species, as identified by the XPS results, result in the formation of electron-rich Ni0 species. The kind of

Scheme 2. Proposed Possible Mechanisms for CTH of EL to GVL over NiCu-x Catalyst

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DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

Industrial & Engineering Chemistry Research metallic sites are beneficial to the activation, dehydrogenation, and hydrogenation of substrate molecules, which all contribute to its enhanced catalytic performance. On the other side, abundant basic and acidic sites in NiCu-0.67 also is responsible for its best catalytic activity via MPV reduction route (Scheme 2, route 2). Meanwhile, the as-fabricated hierarchical multilevel flower-like superstructure of AMS@NiCu@ANPs can facilitate substrates to easily get access to more fully surface adsorption/ reaction sites, thus greatly improving the catalytic performance, compared with the bimetallic NiCu catalyst obtained by impregnation. In a word, NiCu-0.67 catalyst effectively combines the above favorable structural and compositional features, leading to improved catalytic performance.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In summary, we have developed new hierarchical multilevel supported bimetallic Ni−Cu catalysts via a flower-like Al2O3@ NiCuAl-LDH precursor route and employed them for CTH of EL to produce GVL. It was demonstrated that an appropriate Cu/Ni ratio was beneficial to enhance surface acid−base properties of bimetallic catalysts, as well as increased surface area. As-fabricated bimetallic NiCu catalyst with the Ni/(Ni + Cu) molar ratio of 0.67 was found to exhibit the best catalytic property. High catalytic activity of the catalyst could be attributed to several positive factors including the synergistic effect between Ni−Cu species in bimetallic NiCu NPs, which was beneficial for the adsorption and activation of EL substrate, abundant surface acidic and basic sites promoting the MPV reduction process, and a novel highly porous superstructure. Moreover, the catalyst could be successively reused at least five times without significant loss in the catalytic performance. From the view of taking full advantage of catalytically active ingredients and high structural stability, these findings provide new routes for the synthesis of costeffective and stable bimetallic catalysts, as well as new possibilities for applications in the production of biofuels. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01774.

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This study was funded through the National Natural Science Foundation of China (21776017; 21521005).

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Article

XRD patterns of NiCu/AMS-0.67, SEM images of NiCu-x samples, SEM image and TEM images of NiCu/ AMS-0.67, TEM image of used NiCu-0.67, comparable results of CTH of levulinates over different catalysts, and transfer hydrogenation of different carbonyl compounds over NiCu-0.67 catalyst (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +8610-64451226. Fax: +8610-64425385. E-mail: [email protected]. ORCID

Feng Li: 0000-0002-1094-7455 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 10325

DOI: 10.1021/acs.iecr.9b01774 Ind. Eng. Chem. Res. 2019, 58, 10317−10327

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