Research Article pubs.acs.org/journal/ascecg
Inclusion of Zn into Metallic Ni Enables Selective and Effective Synthesis of 2,5-Dimethylfuran from Bioderived 5‑Hydroxymethylfurfural Xiao Kong,† Yifeng Zhu,‡ Hongyan Zheng,§ Yulei Zhu,*,§,∥ and Zhen Fang*,† †
College of Engineering, Nanjing Agricultural University, Nanjing 210031, P. R. China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China § Synfuels China Co. Ltd, Beijing, 101407, P. R. China ∥ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China ‡
S Supporting Information *
ABSTRACT: 5-Hydroxymethylfurfural (HMF) is an important platform chemical derived from C6 sugars and cellulose. The formed 2,5-dimethylfuran (DMF) via hydrogenolysis of HMF was denoted as a promising and sustainable fuel candidate. The further improvement of DMF yield is the central premise of this work, which is based on the elucidation of the relationship between catalytic performance and active sites. Herein, a NiZn alloy catalyst was formed through Zn inclusion to Ni by controllable reduction of the NiZnAl hydrotalcite-derived NiO−ZnO−Al2O3 mixed oxide. The combination of temperature-programmed reduction (TPR), in situ X-ray diffraction (XRD), CO-adsorbed infrared spectroscopy (CO−IR), and X-ray photoelectron spectroscopy (XPS) revealed that the surface of the NiZn catalyst was composed of β1-NiZn while the bulk was composed of α-NiZn. Moreover, the surface Ni atoms were geometrically isolated by Zn atoms and modulated to be electron-rich. Finally, the rate of CO/CO hydrogenolysis over CC/CC hydrogenation for NiZn alloy catalyst was approximately three times higher than that of monometallic Ni catalyst. A 93.6% yield of DMF was obtained over NiZn alloy catalyst. The greatly improved DMF yield was thus attributed to the electron modification and isolation of Ni atoms due to the formation of NiZn alloy. KEYWORDS: Biomass, 5-Hydroxymethylfurfural, Hydrogenation, Ni, Intermetallic
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furan) are easily formed over some of them (Scheme 1).12,16,17 Development of a non-noble catalyst with superior CO/C O hydrogenolysis ability and suppressed CC/CC session ability is important for achieving high selectivity, which may also have implications for selective hydrogenation of unsaturated aldehydes. Alloying and/or fabrication of intermetallic compounds are good choices for catalysts with such an ability.18−20 For example, compared with Ni/Al2O3 catalyst, the Ni−In intermetallic compound was highly selective for furfural hydrogenation to furfuryl alcohol, because of electron transfer and active-site isolation in the Ni−In intermetallic compound.21 The Ni−Ga intermetallic compound was proposed for semihydrogenation of phenylacetylene with a suppressed ability for complete hydrogenation.20 The Ni−Sn-based alloy catalyst exhibited an efficient hydrogenation ability of CO rather
INTRODUCTION
The rapid depletion of fossil resources and the growing concerns about global warming stimulated the research on clean biofuel production from renewable resources. 5Hydroxymethylfurfural (HMF) is a platform chemical derived from cellulose and C6 sugar. The selective hydrogenolysis of HMF could form 2,5-dimethylfuran (DMF; Scheme 1), which is an alternative and sustainable fuel with high energy density (30 kJ/cm3) and octane number (RON = 119).1 In addition, the unsaturated furanic ring of DMF facilitates the production of various chemicals (e.g., p-xylene) by Diels−Alder reactions, which is also an important process.2,3 Various noble metals (e.g., Pt, Ru, and Pd) have been studied for DMF synthesis, while the high price hindered their further application.1,4−10 Design of non-noble catalysts attracts great attention. In particular, bifunctional Ni-based catalysts (e.g., Ni−W2C/AC, NiSi−PS, Ni−Al2O3, Ni/Co3O4, and Ni−Co/C) were demonstrated with high efficiency.11−15 For this process, 2,5-dimethyltetrahydrofuran (DMTHF) and CC cracking products (e.g., 2-methylfuran, C1−C5 alkane and tetrahydro© 2017 American Chemical Society
Received: June 6, 2017 Revised: September 22, 2017 Published: October 24, 2017 11280
DOI: 10.1021/acssuschemeng.7b01813 ACS Sustainable Chem. Eng. 2017, 5, 11280−11289
Research Article
ACS Sustainable Chemistry & Engineering
interactions, and then to the NiZn species with an alloy bulk (α-NiZn) and intermetallic overlayer (β1-NiZn). The superior performance of the NiZnAl-derived catalyst was thus attributed to the modulated electron density and geometric properties of Ni due to NiZn alloy formation. This work also opens up a novel avenue for developing efficient Ni-based catalysts for selective hydrogenation of CO/CO versus CC/CC bonds.
Scheme 1. (A) CO/CO Hydrogenolysis versus CC Saturation and CC Cracking for Selective Conversiona of HMF, and (B) Modulation of Metallic Ni by Forming NiZn Alloy
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a
EXPERIMENTAL DETAILS
Catalyst Preparation. All the samples were prepared by constantpH coprecipitation method employing NaOH/Na2CO3 as precipitating agent.29 For NiZnAl catalyst, an aqueous solution containing the Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, and Al(NO3)3·9H2O and a solution containing Na2CO3 and NaOH were simultaneously added to a beaker under vigorous stirring at 63 °C. The Ni/Zn/Al molar ratio in solution was 1:1:1. The precipitate was aged at 63 °C for 18 h. Then, the precipitate was filtered, washed, and dried at 63 °C for 24 h and then calcined at 600 °C in air for 4 h. The catalysts with different Ni/Zn ratios were also prepared as references by the constant-pH coprecipitation method as described for the NiZnAl catalyst, including NiZnAl(1:3) and NiZnAl(3:1). The NiAl (Ni loading, 28.5 wt %) and ZnAl (Zn loading, 31.4 wt %) references were prepared in a similar way. Catalyst Characterization. The BET surface area was tested via N2 physical adsorption at −196 °C by a Micromeritics ASAP 2420 instrument. Before tests, the samples were degassed at 90 °C for 1 h and 350 °C for 8 h. The XRD pattern of the precursor and calcined NiZnAl was recorded by an X-ray diffractometer (MiniFlex II, Rigaku) with Cu Kα radiation. In situ XRD patterns of reduced samples were recorded by a Bruker D8 Advance diffractometer with Cu Kα radiation at a rate of 10 °C/min under 40 mL/min of 30 vol % H2/Ar. XRD patterns of used catalyst were analyzed over a Bruker D8 Advance diffractometer with Cu Kα radiation. The ICP experiments were conducted on a PerkinElmer Optima2100DV instrument. Temperature-programmed reduction (TPR) and H2 temperature-programmed desorption (H2−TPD) were conducted on a Tianjin XQ TP-5080 instrument equipped with a TCD. For the TPR experiment, 20 mg of the catalyst was loaded into a quartz tube and heated in 30 mL/min of 10 vol % H2/N2 at a rate of 10 °C/min. For the H2−TPD experiment, the catalysts (100 mg) were first reduced at 500 °C for 2 h. The catalysts were then purged with N2 to remove the physisorbed H2 at 50 °C for 1 h until the baseline became smooth. Subsequently, the catalysts were heated to 750 °C with a rate of 10 °C/min. The surface Ni amounts were estimated from the areas. High-resolution transmission electron microscopy (HRTEM) measurements were carried out on a JEM-2100F instrument operated at 200 keV. X-ray photoelectron spectroscopy (XPS) spectra of reduced catalysts were recorded on a Thermo XPS ESCALAB 250Xi spectrometer equipped with a monochromatic Al Kα (1486.8 eV) source after an in situ reduction at 500 °C under H2. Temperature-programmed desorption of NH3 (NH3−TPD) experiments were conducted on the AutoChem II 2920 instrument (Micromeritics) equipped with a mass spectrum detector. The catalysts (100 mg) were reduced in H2 at 500 °C for 2 h, saturated with NH3, and then purged with He to remove the physisorbed NH3 at 100 °C for 30 min. Subsequently, the catalysts were heated to 700 °C with a rate of 10 °C/min. For CO−IR experiments, the catalysts were first reduced in H2 at 500 °C for 2 h and then cooled to room temperature under He to remove the physisorbed H2. After that, the catalysts were adsorbed with CO for 0.5 h at room temperature. Subsequently, the CO was closed, and the catalysts were blown under He flow. The data was recorded every minute, and that used in the paper was obtained after blowing for 12 min in He at room temperature. Catalyst Tests. The tests were performed in a 100 mL tank reactor. Prior to the test, the catalysts were reduced in a quartz tube under H2 flow at 500 °C for 2 h. The reduced catalysts were then protected by 1,4-dioxane and transferred to the reactor. For a typical procedure, the reactor was fed with HMF (1.5 g), 1,4-dioxane (35
Red dotted line: undesired reactions.
than CC in the hydrogenation of unsaturated carbonyl compounds.22 The bimetallic Ni−Fe/CNT catalyst was reported with a 91.3% yield of DMF due to the NiFe alloy formation.23 The Zn-modified metal catalysts including PtZn and RuZn were also frequently investigated for selective hydrogenation of α,β-unsaturated aldehydes.19,24 The PtZn alloy formation for the Pt/ZnO catalyst could change the adsorption mode of crotonaldehyde and exhibited high selectivity for crotyl alcohol.19 The RuZn alloy was reported with a changed electron structure to enhance the cyclohexene selectivity from benzene hydrogenation.24 The unique catalytic properties of alloys/intermetallic compounds could be rooted in the “electronic effect”, an electron transfer between metals resulting in modified adsorption/desorption properties, a “geometric effect” leading to weakly π-bonded reactants on top of isolated metal atoms, and a “kinetic effect” due to the decreased availability of hydrogen.25 However, a fabrication strategy and potential application of the base NiZn alloy were merely reported. Nørskov et al.26 first predicted NiZn alloy as a good candidate for acetylene removal with avoidance of further CC saturation, using density functional theory calculations. This result demonstrated that NiZn alloy is promising for reservation of CC bonds. The Rioux group showed that the decrease of adsorption energy of acetylene was the main reason for the enhanced selectivity to ethylene for bulk NiZn.27,28 Moreover, they reported that strong interaction between Ni and ZnO support could contribute to the formation of Ni1−xZnx alloy after reduction while decreasing the ability of Ni/ZnO to dissociate H2.28 Despite the modulated catalytic properties, the NiZn alloy catalyst has never been reported for selective hydrogenation of CO/CO bonds. Herein, NiZn alloy was determined via reduction of mixed oxide from NiZnAl hydrotalcite because of Zn inclusion to metallic Ni. The NiZn alloy was highly selective for DMF synthesis from HMF with a yield of 93.6%, which was greatly higher than that of monometallic Ni catalyst (Scheme 1). The XRD, TPR, XPS, and CO−IR (CO-adsorbed infrared spectroscopy) characterizations demonstrated the structural evolution from hydrotalcite precursor to mixed oxides with strong 11281
DOI: 10.1021/acssuschemeng.7b01813 ACS Sustainable Chem. Eng. 2017, 5, 11280−11289
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Main Physiochemical Properties of the Samples catalyst
Nia (wt %)
Zna (wt %)
SBETb (m2/g)
dNic/dNid (nm)
He (mmol/g)
Sacidf (μmolNH3/g)
NiZnAl NiAl ZnAl
25.0 27.8
29.2
143.7 188.3 97.9
5.8/4.4 7.3/4.8
0.36 0.19 0
0.09 0.10
30.6
a
Determined by ICP experiments. bDetermined by N2 physical adsorption. cCalculated by Ni (200) reflection (2θ = 51.8°) based on the Scherrer equation after reduction at 500 °C for 2 h. dCalculated by HRTEM images. eActual amount of H consumed over 20 mg of the catalyst by H2−TPR analysis. fDetermined by NH3−TPD results. mL), and a certain amount of prereduced catalyst, then sealed and purged by H2 (5 times). After that, the reactor was filled with 1.5 MPa H2 and heated up to 180 °C. The timing started the moment the temperature reached the desired value. After the test, the reactor was quenched in ice−water, and then, the products were analyzed by a GC instrument with an FID detector and a capillary column (J&W DBWAX). The stability of the NiZnAl catalyst for DMF synthesis was also investigated. The catalyst was reused directly after separation by centrifugation, without further washing or other treatment. For the product analysis, the detector and inlet temperatures were set at 250 °C. The product was analyzed by the following oven-heat procedure: starting at 40 °C for 6 min, increasing the temperature with a rate of 8 °C/min until 220 °C, and holding at 220 °C for 20 min. The products were identified by GC−MS and comparison of retention times of pure chemicals. The pure chemicals including DMF and DMTHF were purchased from TCI Corporation and used as standard samples. The conversion and selectivity were determined by calibrated area normalization.
Figure 1. XRD patterns of calcined catalysts.
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RESULTS AND DISCUSSION Catalyst Characterization. NiZnAl hydrotalcite was prepared by coprecipitation method with a Ni/Zn/Al nominal molar ratio of 1:1:1 (theoretical Ni loading, 28.5 wt %; Zn loading, 31.4 wt %). The hydrotalcite precursor could facilitate the formation of highly dispersed and adjacent Ni and Zn metal oxides, which would benefit the reduction of Zn by spill-over of H from Ni.24 The catalysts with different Ni/Zn molar ratios were also investigated, including the ratio of 1:3 and 3:1. The effect of Ni/Zn ratios on the catalytic performance will be discussed in the following section. The NiAl (Ni loading, 28.5 wt %) and ZnAl (Zn loading, 31.4 wt %) references were prepared in a similar way. ICP results showed that the metal loadings were similar to nominal values (Table 1). XRD patterns of the precursor are shown in Figure S1. The XRD pattern of the NiZnAl precursor confirmed the hydrotalcite structure, while ZnAl and NiAl samples exhibited an Al(OH)3 phase other than the pure hydrotalcite structure. XRD patterns of calcined samples are shown in Figure 1. After calcination, diffraction peaks at 37.2°, 43.3°, and 62.9° were observed for the NiAl catalyst, which were assigned to NiO. Compared with the NiAl catalyst, the NiZnAl displayed peaks that shifted to lower angles, which was characteristic of a larger unit cell. The enlargement of the lattice is rooted in the doping of Zn into NiO due to the strong interaction.28 The ZnAl sample exhibited obvious peaks assigned to ZnAl2O4. N2 adsorption results of calcined samples revealed that NiZnAl exhibited a slightly lower surface area than NiAl (Table 1). The amount of acid sites was also calculated by NH3−TPD and is shown in Table 1. No obvious change of acid amounts was found for NiAl and NiZnAl catalysts. The reducibility of catalysts was investigated using temperature-programmed reduction (TPR; Figure 2). There were 20 mg portions of catalysts that were used in our case. The theoretical amount of H to reduce NiO was 0.19 mmol for a catalyst of 20 mg. Calibration of the TPR area of the NiAl
Figure 2. TPR results of calcined NiZnAl, NiAl, and ZnAl catalysts.
catalyst was 0.19 mmol (see the Supporting Information), revealing the total reduction of NiO species (Table 1). NiZnAl showed ∼1.9 times H consumption (0.36 mmol) while ZnAl exhibited almost no H consumption peak. The promoted H consumption of NiZnAl indicated the reduction of Zn2+ species. Actually, it is hard for Zn2+ species to be reduced to metallic Zn by H2, as indicated by the ZnAl catalyst, despite the fact that H2 treatment could generate the oxygen vacancies. The Zn2+ reduction to Zn0 over the NiZnAl catalyst might arise from the metallic Ni. It is documented that hydrogen is dissociatively adsorbed on metallic Ni to form a hydrogen atom, which may spill over to reduce adjacent Zn species, as reported in many catalyst systems (e.g., PtZn, RuZn, and PdZn).19,24,30 The excessive H consumption of NiZnAl was thus ascribed to the reduction of ZnO by spill-over H atoms from Ni.31 In addition, the NiZnAl catalyst exhibited a higher reduction temperature than NiAl, indicating the strong interaction between Ni and the support (ZnO, Al2O3), which hindered the reducibility of Ni species.32 The strong interaction would also benefit the stability of Ni. In all, ZnO was reduced with the help of Ni and further decorated the Ni sites, which will be discussed in the following sections. 11282
DOI: 10.1021/acssuschemeng.7b01813 ACS Sustainable Chem. Eng. 2017, 5, 11280−11289
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ACS Sustainable Chemistry & Engineering For determination of the structure of metallic species over the reduced catalysts, XRD experiments for NiAl and NiZnAl catalysts reduced at different temperatures were carried out (Figure 3). The NiAl-500 catalyst exhibited diffraction peaks at
Figure 3. In situ XRD patterns of reduced catalysts (the number indicates the reduction temperature). Figure 4. In situ XRD patterns of NiZnAl reduced under different reaction temperatures. R1, the XRD of the catalyst when the reduction temperature was just elevated at the object temperature; R2, the XRD of the catalyst when the reduction was just finished; R3, the XRD of the catalyst when the temperature was cooled down to room temperature. The pink lines at the bottom indicate the characteristic peaks of Ni.
44.5°, 51.8°, and 76.4° which correspond to the characteristic metallic Ni (dotted lines). In contrast, the peaks of NiZnAl-500 all shifted to lower angles, which might be caused by the enlargement of the unit cell, i.e., inclusion of Zn into the crystal lattice and formation of a Ni−Zn structure (Ni 1−x Zn x compound), or overlap of NiO. The characteristic peak at 43.3° may overlap that of metallic Ni diffractions at 44.5°. However, the diffraction peaks of Ni species at 51.8° and 76.4° assigned to Ni also shifted (more clearly in Figure 4). Thus, the shift of diffraction peaks of the NiZnAl-500 catalyst is induced by the inclusion of Zn atoms into the crystal lattice. The XRD results of NiZnAl catalysts reduced at higher temperatures (NiZnAl-600 and NiZnAl-700) supported the inclusion of Zn atoms. NiO peaks disappeared for both catalysts while the shift of metallic Ni existed and intensified, confirming the formation of the Ni1−xZnx alloy. This is consistent with the literature reports. Zn-doped NiO could be formed by calcination of nickel nitrate on a nanoparticulate ZnO support. Subsequently, The Ni1−xZnx/ZnO alloy was formed after reduction, with x increasing with reduction temperature.28 In detail, the Zn-doped NiO transformed first to the Ni-rich NiZn alloy (α-NiZn, e.g., Ni4Zn, Ni0.87Zn0.13, Ni7Zn3) around 400 °C followed by a transformation to intermetallic β1-NiZn at about 525 °C as detected by X-ray absorption near-edge structure at a rate of 1.3 °C/min under a 50 mL/min flow of 5% H2/He. Different reduction procedures influenced the specific temperature, as a rate of 10 °C/min under a 40 mL/min flow of 30% H2/N2 was used in our case. Nevertheless, the gradual shift of peaks for the NiZnAl catalyst also indicated a transformation process, especially for the NiZnAl-700 catalyst. The characteristic peaks based on reported works for Ni4Zn (a kind of α-NiZn) and NiZn (i.e., β1-NiZn) are shown in Figure 3, which is in good agreement with the formation of the NiZn alloy over the NiZnAl catalyst.28,33 To determine the evolution during NiZn alloy formation, in situ XRD experiments of the catalysts treated under different temperatures (500, 600, and 700 °C) were performed (Figure 4). The XRD spectra of different temperatures were recorded at
3 stages (R1, R2, and R3): R1, the spectra collected when temperature was just elevated at the object reduction temperature; R2, the spectra collected when reduction was just finished; R3, spectra collected when the temperature was dropped to room temperature. The pink lines at the bottom indicate the characteristic peaks of metallic Ni. Compared with metallic Ni, all catalysts exhibited a shift to lower angles due to Ni1−xZnx formation. In addition, for NiZnAl-500 and NiZnAl600 catalysts, no other shift of the NiZn alloy peaks was observed during the reduction, due to the good stability of the NiZn alloy for both catalysts. Instead, for the NiZnAl-700 catalyst, the peaks shifted to higher angles gradually during the process, despite the more pronounced NiZn alloy peaks than those for NiZnAl-500 and NiZnAl-600 when the temperature was just elevated at 700 °C (NiZnAl-700, R1). The change of the NiZnAl-700 catalyst indicated the change of the NiZn alloy. A decomposition behavior of the β1-NiZn alloy to α-NiZn alloy in methanol steam reforming at 600 °C was once demonstrated by Friedrich et al., likely following the reaction ZnNi + H2O → Ni70Zn30 + ZnO + H2.34 In our case, the high-temperature reduction might contribute to the β1-NiZn formation for NiZnAl-700, which undergoes decomposition to α-NiZn gradually. That is, the intermetallic NiZn was oxidized to the NiZn alloy with low Zn content. In conclusion, NiZnAl catalysts reduced at 500 and 600 °C mainly consist of stable αNiZn, while the higher reduction temperature contributes to the formation of unstable β1-NiZn. However, the β1-NiZn decomposed during the reduction process, which transformed into the NiZn alloy with lower Zn content. The conclusions are also in good consistency with Rioux’s and Friedrich’s research.28,34 11283
DOI: 10.1021/acssuschemeng.7b01813 ACS Sustainable Chem. Eng. 2017, 5, 11280−11289
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ACS Sustainable Chemistry & Engineering Moreover, NiAl-500 and NiZnAl-500 exhibited a similar particle size with 7.3 and 5.8 nm, respectively, as calculated by Ni (111) reflection based on the Scherrer equation (Table 1). HRTEM was used to confirm the mean particle size of both catalysts. NiAl-500 and NiZnAl-500 exhibited mean sizes of 4.8 and 4.4 nm, respectively, after counting from the HRTEM images, supporting the formation of highly dispersed particles with similar size. However, for NiZnAl-600 and NiZnAl-700 catalysts, larger particle sizes were observed despite of the formation of the NiZn alloy with more Zn content. Thus, for the effect of NiZn alloy formation, NiAl and NiZnAl catalysts reduced at 500 °C were further compared and tested for HMF hydrogenation by minimizing the size effect. The Zn−L3M45M45 Auger spectra were also used to investigate the reduction of Zn species for NiZnAl-500 (Figure 5). The ZnAl catalyst exhibited a peak at 987.3 eV, which was
Figure 6. CO−IR images of reduced NiZnAl, NiAl, and ZnAl catalysts at 500 °C for 2 h. The spectra were obtained after blowing for 12 min in He at room temperature.
bridged configuration of CO on Ni, respectively, which were associated with the atoms located on the terrace and steps.39,40 Compared with spectra for the NiAl catalyst, the band at 1800 cm−1 associated with the bridged CO adsorption diminished for NiZnAl, revealing that Zn sufficiently diluted Ni atoms and decreased Ni steps, which supports the “active-site isolation” effect.21 This further indicated that the surface of the NiZnAl catalyst is composed of β1-NiZn species, since the α-NiZn is composed of random arranged and adjacent Ni atoms. Notably, a 40 cm−1 red shift of CO linear adsorption was observed for NiZnAl, indicating the amount decrease of backbonding of the Ni d orbitals.38 This could be induced by electron transfer from Zn to the Ni, according to the electronegativity values of Zn (1.65) and Ni (1.9). This is also supported by the reports of Duan et al., that the addition of electropositive species (In, Ga) metals could enhance the charge density of Ni metals.20,21 Combined with in situ XRD and XPS results, it could be concluded that both the Ni-rich NiZn alloy and surface intermetallic NiZn (a typical type of alloy) were present for the NiZnAl hydrotalcite-derived catalyst after the reduction at 500 °C, which can be classified to α-NiZn and β1-NiZn phases, respectively. The clarification of active species for the traditional NiZnAl hydrotalcite-derived catalyst merits substantial work, and further optimizations are also needed. In addition, the nanosized NiZn alloy compounds with geometricisolated and electron-efficient Ni properties could greatly alter the adsorption of reactants and catalytic performance for hydrogenolysis of CO bonds. The formation of the Ni-rich alloy and intermetallic NiZn by introducing Zn species could attenuate the adsorption of H atoms, thereby influencing the hydrogenation ability of Ni.28 H2−TPD experiments provide insight into the H2 activation ability of Ni over NiZnAl and NiAl catalysts (Figure S2). Compared with NiAl results, a smaller area of the H2−TPD peak was observed over NiZnAl. This could be due to the dilution of the Ni sites by Zn inclusion. Moreover, an additional broad desorption peak at high temperature was observed for the NiZnAl catalyst, which might be attributed to the spill-over H2 and/or H2 located in a new surface. In all, the H2−TPD results also supported that the surface structures of NiZnAl and NiAl are different, as discussed with the CO−IR results. NH3−TPD experiments were conducted to reveal the effect of Zn addition on catalyst acidity properties (Figure S3). The peaks around 200 and 267 °C were observed for NiAl, corresponding to the weak acid sites, which could be associated with different Al3+ species. Compared with the NiAl catalyst,
Figure 5. XPS/XAES spectra from Zn−L3M45M45 spectra of reduced NiZnAl and ZnAl samples under 500 °C.
assigned to the Zn species in ZnAl2O4. The NiZnAl catalyst showed two peaks at 988 and 991 eV, which were attributed to the ZnO and metallic Zn species, respectively.35 The above XPS results confirmed the reduction of Zn species over the NiZnAl catalyst under 500 °C, which was in good agreement with XRD results and supported the formation of the NiZn alloy. Moreover, we quantitatively analyzed the XPS results of reduced NiZnAl catalysts. The surface Zn/Ni atomic ratio is determined to be 1.16. The enrichment of Zn over the surface is also in agreement with the previous reports on strong metal− support interactions (SMSIs) proposed first by Tauster et al., whereby the partially reduced Zn species prefer to migrate over the metallic Ni surface.36,37 The ratio was inconsistent with the composition of Ni-rich NiZn alloy formation (e.g., Ni4Zn, Ni7Zn3, Ni0.87Zn0.13) while it was similar to that of the intermetallic NiZn (β1-NiZn, 1:1). As Bokhoven et al.30 reported, the intermetallic formation started at the surface, and the crystallinity increased with reduction temperature. Thus, the surface atomic ratio supported the existence of intermetallic β1-NiZn, in spite of the undetectable formation in XRD. The intermetallic β1-NiZn was confirmed by XRD at 700 °C. In situ CO-adsorbed infrared spectroscopy (CO−IR) of reduced catalysts was used to reveal the electron density of the surface Ni and the binding geometry of CO (Figure 6). The bonding of CO to metal atoms is described in terms of the Ni− C d-donation and Ni−C π-backbonding.38 The backbonding is related to the occupancy of the Ni d orbitals. The NiAl catalyst exhibited a band at 2083 cm−1 and a broad band around 1800 cm−1. The two bands could be assigned to the linear and 11284
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Figure 7. Catalytic performance of HMF hydrogenation over (a) NiAl and (b) NiZnAl catalysts. Reaction conditions: 1.5 MPa, 180 °C, HMF 1.5 g, catalyst 0.15 g. C−C cracking products included the 2-methyltetrahydrofuran, 2-methylfuran, C1−C5 alkane, furan, and tetrahydrofuran. (c) Product evolutions for HMF conversion.
atom, would decrease the hydrogenation activity. The wellorganized metal distributions in the NiZnAl precursor would facilitate the formation of highly dispersed Ni sites and benefit the activity. The slight decrease of acid sites over NiZnAl might also be one reason for the slow hydrogenation process, due to the fact that acid sites can benefit the CO/CO activation and conversion.12 A control experiment was carried out over the ZnAl catalyst at the same condition (Table S1). The ZnAl catalyst only exhibited a ∼10.1% conversion of HMF, indicating the low hydrogenation ability of the ZnAl catalyst. Further, the NiZnAl catalyst exhibited a high DMF yield of 93.6% at 15 h, which was greatly higher than that of the NiAl catalyst (63.5%) at optimized conditions. Because of the similar particle size and surface acid amounts, the high DMF yield could be attributed to a comprehensive result of the electronic and geometric effect, rooted in the Zn inclusion. The enhanced DMF yield could be attributed to the suppressed side-reactions. First, almost no CC cracking alkanes were obtained over the NiZnAl catalyst whereas it increased gradually to ∼8% over NiAl after HMF was totally converted. Nørskov et al.43 reported that the Ni step-edge is active for CC breaking, and blocking the Ni steps by alloying with Ag is an effective way to suppress CC scission. Thus, the decreased concentration of Ni steps due to Zn inclusion might be responsible for the suppressed CC cracking reactions, as illustrated by CO−IR results.32 Second, NiZnAl has a low propensity for overhydrogenation of CC bonds, producing very little DMTHF. Actually, pure Ni has 12 nearest Ni neighbors, while the Ni in intermetallic β1-NiZn is coordinated by 8 nearest Zn neighbors, with 6 Ni atoms
only a slight decrease of desorption area was observed over the NiZnAl catalyst. The amounts of acid sites were determined through NH3−TPD results (Table 1). NiAl adsorbed an amount of 0.10 μmolNH3/g, while NiZnAl adsorbed an amount of 0.09 μmolNH3/g. The slight decrease of acid sites might decrease the activity of HMF conversion, because of the fact that Lewis acids are known as active sites benefiting CO/ CO activation.12 Little work has reported the effect of Lewis acid sites for CC/CC transformation. Tseng et al.41 experimentally found that the homogeneous Lewis acids remain inactive for CC activation by attachment of boron Lewis acids to a Ru catalyst, making a superior catalyst for alkyne hydrogenation. Thus, the change of products involving CC/ CC hydrogenation (e.g., DMTHF and CC cracking products) cannot be explained by the change of acid sites, and the effect of acid sites will not be discussed in the following sections. Catalytic Results. Catalytic Performance of NiZnAl and NiAl for DMF Synthesis. NiZnAl and NiAl catalysts upon reduction at 500 °C were tested for HMF hydrogenation and exhibited different catalytic performances (Figure 7). A gradual transformation of HMF and the intermediates can be observed. The O-containing intermediates mainly include 2,5-dihydromethylfuran (DHMF), 5-methylfurfuryl alcohol (MFA), and 5methylfurfural (MFR). A 100% conversion of HMF was observed over the NiAl catalyst at 7 h, while HMF was totally converted at 9 h over NiZnAl indicating a slight decrease of activity. As reported in previous work,42 the dilution of the Ni site at the surface and electron transfer from Zn to Ni, which could occupy the partially vacant d-electron orbital of the Ni 11285
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ACS Sustainable Chemistry & Engineering slightly further away.27 The active-site isolation increases Ni− Ni distances on the surface and leads to only a weakly πbonded furan ring on the Ni atom.25 Gorte et al.17,44 also concluded that the high DMF yield over the bimetallic surface (e.g., PtCo, PtNi, PtZn) could be attributed to the weakly interacted furan ring. In addition, the hydrogenation of the furan ring and CO follow the nucleophilic and electrophilic addition mechanism, respectively.21 In this work, the electronegative Ni atom prefers to attack CO, instead of the furan ring. Thus, a greatly enhanced DMF yield was obtained over the NiZnAl catalyst. Catalytic Performance of the NiZnAl Catalyst Prepared by Different Methods. The NiZn/Al and Ni/ZnAl catalysts with the same preset metal loadings were prepared by impregnation method for a comparison. NiZn/Al was prepared by the coimpregnation process of the solution of nickel and zinc nitrates onto a commercial Al2O3 support. Ni/ZnAl was prepared by the impregnation process of the solution of nickel nitrate onto a ZnAl support. The ZnAl support was prepared by a coprecipitation method similar to the preparation of the NiZnAl catalyst. The XRD patterns of the calcined samples are shown in Figure S4. The crystalline sizes of NiZn/Al and Ni/ZnAl calculated on the basis of the Scherrer equation were 15.6 and 9.9 nm, respectively, which were larger than that of the NiZnAl catalyst prepared by the coprecipitation method. The catalytic performance of the three catalysts for HMF hydrogenation is shown in Table S2. All catalysts exhibited a total conversion of HMF after 9 h. The NiZnAl catalyst prepared by coprecipitation method exhibited the highest DMF selectivity of 59.4%. MFA and DHMF are the main byproducts. NiZn/Al and Ni/ZnAl catalysts prepared by the impregnation method showed much lower DMF selectivity due to the insufficient hydrogenolysis of oxygen-containing intermediates of MFA and DHMF. Importantly, all three catalysts exhibited a low yield for the CC/CC hydrogenation products. However, the byproducts for the NiAl catalyst mainly include DMTHF and CC cracking products (Figure 7), because of the strong hydrogenation ability for CC/CC bonds. In conclusion, the above results demonstrated that NiZnbased catalysts are more selective for CO/CO hydrogenation compared with the monometallic NiAl catalyst. Moreover, the catalyst preparation method has a great effect on the catalytic performance. The highly dispersed NiZnAl catalyst prepared by coprecipitation method exhibited superior reactivity. Catalytic Performance of the NiZnAl Catalyst with Different Ni/Zn Ratios. The effect of Ni/Zn ratios on the catalytic performance was also investigated and is shown in Table S3. All catalysts exhibited a total conversion of HMF after 9 h. The DMF selectivity increased with increasing Ni/Zn ratio. NiZnAl(3:1) exhibited the highest DMF yield of 68.0%. However, the NiZnAl(3:1) catalyst also showed high selectivity of DMTHF due to the strong hydrogenation ability. For the NiZnAl(1:3) catalyst, a yield of 36.2% was obtained because of the insufficient hydrogenolysis of MFA and DHMF. The low hydrogenation ability of NiZnAl(1:3) can be assigned to the low Ni contents. Among the three catalysts, the NiZnAl catalyst exhibited the best results with a 59.4% yield of DMF, while the MFA and DHMF can be further converted into DMF. The final 93.6% yield of DMF can be achieved over the NiZnAl catalyst as demonstrated in Figure 7.
In summary, the Ni/Zn ratio has a great effect on the catalytic performance of the NiZnAl catalyst. The hydrogenolysis ability increased with increasing Ni/Zn ratio. However, too high of a Ni/Zn ratio caused the formation of DMTHF and other byproducts. Finally, the NiZnAl catalyst with a Ni/Zn ratio of 1:1 exhibited the best performance. Further Applications of the NiZnAl Catalyst. The reactivity of the NiZnAl catalyst for HMF hydrogenation was tested at 100 °C for DHMF synthesis. As shown in Table 2, a high Table 2. Catalytic Performance of the NiZnAl Catalyst for HMF and Furfural Conversion feedstock
conversion (%)
main products [selectivity (%)]
HMFa furfuralb furfuralc
100 100 100
DHMF (98.2), DHMTHF (1.1) 2-MF (90.8), furfuryl alcohol (6.2) furfuryl alcohol (89.1), 2-MF (4.4)
1.5 MPa, 100 °C, 10 h, HMF 1.5 g, catalyst 0.15 g, 1,4-dioxane 35 g. DHMTHF indicates 2,5-dihydroxymethyltetrahydrofuran. b1.5 MPa, 220 °C, 6 h, furfural 1 g, catalyst 0.15 g, 1,4-dioxane 35 g. 2-MF indicates 2-methylfuran. Others mainly include 2-methyltetrahydrofuran. c1.5 MPa, 180 °C, 2 h, furfural 1 g, catalyst 0.15 g, 1,4-dioxane 35 g. Others mainly include tetrahydrofurfuryl alcohol. a
DHMF yield was obtained over the NiZnAl catalyst, demonstrating the high selectivity for the CO bond with low hydrogenation ability for the furan ring. We also investigated the catalytic performance of NiZnAl for furfural hydrogenation. The products for furfural hydrogenation mainly focused on 2-methylfuran (2-MF) and furfuryl alcohol, which are a promising fuel candidate and chemical, respectively. Furfuryl alcohol is the intermediate for 2-MF synthesis, in which a deep hydrogenolysis of a side hydroxymethyl group can form 2-MF. High yields of 2-MF and furfuryl alcohol were obtained by using furfural as the feedstock. On the basis of the above results, NiZnAl exhibited superior hydrogenolysis ability for CO/CO over CC/CC bonds, supporting the mechanisms of Zn inclusion and showing great potential for upgrading of biomass via hydrodeoxygenation. Reusability of the NiZnAl Catalyst. The stability of the NiZnAl catalyst for DMF synthesis was also investigated (Figure 8). To track the slight changes in catalyst activity over time, a middle conversion of HMF was chosen. HMF
Figure 8. Reusability of the NiZnAl catalyst for HMF hydrogenation to DMF. Reaction conditions: 1.5 MPa, 180 °C, 3 h, 1,4-dioxane 35 g, catalyst 0.15 g, HMF 1.5 g. 11286
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NiZn). Moreover, the surface of the NiZnAl catalyst was composed of isolated and electron-density-increased Ni sites, as revealed by CO−IR results. This can be explained by the geometry of isolated Ni sites over the surface, the formation of intermetallic β1-NiZn, and the altered electronic structure of Ni sites by electron transfer from Zn atoms. Moreover, bridge adsorption of CO in the IR spectrum for the NiZnAl catalyst was not observed, which required a configuration with linked Ni atoms over the surface and can be formed over the Ni-rich NiZn alloy (α-NiZn). β-NiZn could not be obtained in our work while a reduction temperature of ∼700 °C is needed on the basis of the Ni−Zn bulk phase diagram.47 The structure evolutions can be interpreted by the following steps: (1) NiO− ZnO−Al2O3 oxide formation with strong metal support interaction from NiZnAl hydrotalcite; and (2) NiZn alloy formation by Zn inclusion to the Ni lattice via reduction treatment (Figure 9A). We steadily reached the conclusion that
conversion was around 44.3% with the DMF yield of 21.8% in the first run. The O-containing intermediates DHMF and 5methylfurfuryl alcohol were the main byproducts. After 4 runs, HMF conversion and DMF yield exhibited a great decrease, indicating the deactivation of the catalyst. XRD experiments were carried out to investigate the status of the catalyst after reaction. As shown in Figure S5, no obvious change was observed for the used NiZnAl catalyst, including the particle size and the Zn inclusion (peak shift), demonstrating the good stability of the NiZn alloy catalyst. The good stability of the NiZn alloy could be attributed to the strong interactions between the Ni and Zn species, as indicated by TPR results. For the catalyst used for HMF hydrogenation to DMF, the deposition of carbonaceous species easily occurred and was the major deactivation problem, which was also proven by our previous reports.11 Moreover, there is a lot of work which has reported the removal of carbonaceous species to regenerate the deactivated catalyst on the basis of the calcination or reduction processes. For example, Jae et al.6 investigated the Ru/C catalyst for DMF synthesis at 190 °C and found that initial activity of the catalyst was almost completely regained after regeneration by heating at 573 K under H2 flow for 2 h. Perret et al.45 studied a Ni/Al2O3 catalyst derived from Ni−Al hydrotalcite-like materials for HMF hydrogenation. They also found that the organic impurities chemisorbed on the surface were the main reason for the deactivation, while the activity can be restored after a calcination and reduction at 773 K. However, the regeneration of the catalyst was not investigated, which was beyond the scope of this work.
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DISCUSSION Active Phase. Determination of the active phase for NiZnAl is important for a further rational design of selective catalysts for CO/CO hydrogenation reactions. As observed from the TPR and XPS results, Zn2+ species were reduced over the NiZnAl catalyst. For the reduced NiZnAl catalyst, XRD results showed a shift of Ni peaks to low angles, indicating the increase of lattice spacing and formation of Ni− Zn structures. Moreover, a pronounced shift was detected over NiZnAl-700. As reported by previous works,28,46 different Ni1−xZnx alloys could be formed by changing the x value (Zn content). Metallic Ni exhibited an fcc crystal structure over the NiAl catalyst. Zn inclusion with low content would cause the random substitution of Ni atoms and form the α-NiZn phase (e.g., Ni4Zn, Ni0.87Zn0.13, Ni7Zn3), with retaining the Ni fcc crystal structure but changing the lattice spacing due to the different atom size. At high inclusion content of Zn, a highly ordered intermetallic β1-NiZn (Ni0.5Zn0.5) structure could be formed, constituting an fc tetragonal super lattice. However, the β1-NiZn was unstable, which could decompose into α-NiZn as demonstrated by our XRD results. A further increase of Zn content induced the β-NiZn phase (e.g., Ni5Zn21) formation. The reduction temperature could modulate the content of Zn inclusion and NiZn alloy compositions, in which the Zn content increased with the reduction temperature. Herein, 500 °C was used to reduce the NiZnAl-derived catalyst, and to test for HMF hydrogenation. Because of the low reduction temperature, a low amount of Zn was reduced and included in the Ni1−xZnx alloy. Thus, α-NiZn (Ni-rich NiZn alloy) was mainly formed, as confirmed by XRD. On the other hand, the surface Zn/Ni atomic ratio is determined to be 1.16, which is similar to that of the intermetallic NiZn (β1-
Figure 9. (A) Structure evolution of the NiZnAl catalyst and (B) comparisons of rate for C−O conversion to C−C conversion under similar conversions.
the NiZnAl catalyst was composed of the nanoparticles with an intermetallic NiZn (β1-NiZn) overlayer and Ni-rich NiZn alloy (α-NiZn) bulk phase, which was different from the NiAl catalyst. Structure−Performance Relationship. The inclusion of Zn also had a great effect on the catalytic performance. Under similar HMF conversions, the NiZnAl catalyst gave a greatly improved DMF selectivity than the monometallic NiAl catalyst. The modulated selectivity toward selective CO hydrogenation is related to the structural alternation by the addition of Zn, due to the similar particle size and acidity amounts. The rates of CO/CO hydrogenolysis over CC/CC hydrogenation at similar HMF conversions were calculated for better clarification. The calculation equation was based on the method shown in the Supporting Information. As shown in Figure 9B, the rate for NiZnAl is 41.2, approximately 3 times higher than that of NiAl. As reported by Sitthisa et al.,48 the planar configurations of furfural on Ni(111) is the optimized geometry, in which the carbonyl group (CO) is adsorbed on a bridge site, and the furanyl ring plane sits parallel to the surface across two 3-fold hollow sites. The interaction of the furan ring with the Ni surface is stronger, even stronger than that of Pd, which can facilitate the hydrogenation reactions.49 Thus, the diluted Ni sites would cause a weakly bonded furan ring due to the increased NiNi distance. On the other hand, the Ni step-edge was greatly decreased by the introduction of 11287
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ACS Sustainable Chemistry & Engineering Zn, which is active for CC cracking reactions. Finally, the CC/CC hydrogenation reactions were suppressed. The electronic structure was also changed with the formation of the NiZn alloy (α-NiZn + β1-NiZn), which was different with Ni. As reported for the PdZn alloy catalyst, an electronic structure similar to Cu was obtained for PdZn, while Cu was known as being highly selective for CO hydrogenation.30 In our case, the NiZn alloy might also exhibit a new electronic structure. From the other point, the electron transfer induced an oxophilic and electron-dense nature in Zn and Ni, respectively, which could provide an enhanced interaction with the CO bond.48 The stable η2-(C,O) adsorption over the NiZn alloy would benefit the selective hydrogenation of CO bonds, while the η2-(C,O) species over the Ni metal would be converted into a surface acyl species and subsequently decompose. In conclusion, a greatly improved selectivity for DMF was obtained over the NiZnAl-derived catalyst, because of the formation of surface intermetallic NiZn and Ni-rich NiZn alloy species (combination of α-NiZn and β1-NiZn). Further experiments are needed for the elucidation of the NiZnAl catalyst and for a rational design of the NiZn alloy catalyst.
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CONCLUSIONS
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ASSOCIATED CONTENT
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ACKNOWLEDGMENTS
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REFERENCES
This project was supported by the Major State Basic Research Development Program of China (2012CB215305) and National Science Foundation in Jiangsu Province (BK20170707).
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This work demonstrated that the surface intermetallic β1-NiZn and bulk α-NiZn alloy could be obtained by reduction of NiZnAl hydrotalcite. The characterizations combining XRD, TPR, XPS, and CO−IR show the systematical structural evolution from hydrotalcite precursor to Zn-doped NiO, and then to the NiZn species with an alloy bulk (α-NiZn) and intermetallic surface (β1-NiZn). The altered electronic and geometric properties of Ni atoms by the intermetallic NiZn formation were further proven by CO−IR and H2−TPD, which influenced the adsorption of reactants and catalytic performance. The final NiZnAl catalyst showed a greatly enhanced DMF yield (93.6%), in comparison with that of the monometallic Ni catalyst (63.5%). This finding also opens up the application of the NiZn alloy catalyst for selective hydrogenation reactions of unsaturated aldehyde.
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01813.
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Research Article
Calculation methods, H2−TPD and NH3−TPD results, XRD results, and catalytic performances (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yifeng Zhu: 0000-0001-8965-5051 Yulei Zhu: 0000-0002-0694-3881 Zhen Fang: 0000-0002-7391-372X Notes
The authors declare no competing financial interest. 11288
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