Research Article pubs.acs.org/journal/ascecg
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 Zhi Gao,† Renfeng Xie,†,‡ 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, Beijing 100029, China ‡ Patent Examination Cooperation Sichuan Center of the Patent Office, SIPO, No. 2032, South Tianfu Avenue, Chengdu, 610213, China S Supporting Information *
ABSTRACT: As a promising renewable alternative to the production of petroleum-derived chemicals and energy, biomass transformation is attracting increasing attention in terms of green chemical processes and sustainable development. Specifically, selective aerobic oxidation of cellulose-derived 5-hydroxymethylfurfural (HMF) into high value-added 2,5-furandicarboxylic acid (FDCA) is regarded as one of the most attractive biomass transformations due to a wide range of its application prospects. Herein, we report the synthesis of a highly efficient and stable bimetallic AuPd nanocatalyst over the La-doped Ca−Mg−Al layered double hydroxide (La-CaMgAl-LDH) support for base-free aerobic oxidation of HMF to FDCA in water, which makes the biomass-based chemical process green and cost effective. Under optimized reaction conditions, the yield of FDCA can reach above 99%. Such encouraging performance of the catalyst is believed to be correlated with both the higher surface basicity of La-CaMgAl-LDH support and the synergy between Au−Pd atoms in the bimetallic AuPd nanoparticles, which can greatly favor the activation of reactants and reaction intermediates in the course of tandem oxidation reactions. The present work provides an effective strategy for developing highly efficient bimetallic catalysts with the enhanced stability by adjusting surface structures and compositions of supports for a wide range of base-free aerobic oxidation of other biomass-derived compounds in water. KEYWORDS: 5-Hydroxymethylfurfural, Base-free aerobic oxidation, Bimetallic nanoparticles, Surface basicity, Layered double hydroxide
■
INTRODUCTION With the rapid development of the economy and human society, the continuous need and consumption of energy and chemicals from nonrenewable fossil feedstocks, as well as the resulting serious environmental problems, people are forced to seek new and sustainable sources of energies and resources.1 In this regard, as an ideal renewable route for the manufacture of important chemicals, polymer precursors, and petroleumderived commodities, biomass transformation is attracting increasing attention in terms of green chemical processes and sustainable development.2−4 Currently, cellulose, as the main component in renewable and inedible lignocellulosic resources, can be selectively converted into various platform compounds and other high value-added chemicals.4−10 For example, aerobic oxidation of cellulose-derived 5-hydroxymethylfurfural (HMF), which is one of the most important platform compounds,11−13 into 2,5-furandicarboxylic acid (FDCA) has been specifically targeted in recent years14 because FDCA can be used as a versatile building-block chemical for the replacement of petroleum-derived commercially important terephthalic acid for the production of polyethylene terephthalate plastics.15,16 © 2017 American Chemical Society
Due to a wide range of its application prospects, FDCA is identified as one of 12 potentially useful building blocks for fine chemicals from biomass by the U.S. Department of Energy. As is well known, aerobic oxidation of HMF to produce FDCA goes through several tandem steps involving both the oxidation of the alcohol hydroxyl group and the oxidation of the aldehyde group in HMF, which puts forward high requirements for the design and performance of catalysts. Recently, the explorations of efficient heterogeneous catalysts for the oxidation of HMF to FDCA have been widely carried out. In particular, supported Au nanoparticles (NPs) over CeO2, TiO2, and carbon materials were found to be active for the production of high yields of FDCA.17−23 In most cases, however, the stability of catalysts is not satisfactory, and an excess amount of base additives such as NaOH are often added into reaction systems to improve the catalytic activity of catalysts. In addition, it was reported that Pt NPs stabilized by Received: February 23, 2017 Revised: May 9, 2017 Published: May 26, 2017 5852
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering
dissolved in 100 mL of decarbonated deionized water, and the obtained mixed salt solution with a [M2+]/[M3+] molar ratio of 3.0 was transferred into a 500 mL flask under a N2 atmosphere. Subsequently, H2PdCl4 (0.29 mmol) and HAuCl4 (0.25 mmol) were dissolved in 10 mL of decarbonated deionized water, and the obtained solution with an Au/(Au+Pd) mass ratio of about 0.6 was added into the above mixed salt solution under vigorous stirring. Afterward, a NaOH solution (0.96 M) was dropwise added into the solution until the pH value reached 10. After the suspension was aged at 60 °C overnight, 50 mL of NaBH4 solution (0.16 M) was added under an ice water bath. After reduction for 6 h, the solution was centrifuged and washed with decarbonated deionized water until the pH value was about 7.0. The obtained supported sample (denoted as AuPd/LaCaMgAl-LDH) was dried at 70 °C for 12 h under vacuum. In addition, other supported bimetallic AuPd catalysts with an Au/(Au+Pd) mass ratio of 0.6 in the synthesis mixture over CaMgAl-LDH, MgAl-LDH and CaAl-LDH supports were also synthesized according to the same procedure as that for AuPd/La-CaMgAl-LDH in the absence of La(NO3)3·6H2O. For comparison, supported monometallic catalysts, Au/CaMgAl-LDH and Pd/CaMgAl-LDH with a noble metal loading of about 3.0 wt %, were synthesized according to the above identical procedure. Characterization. Powder X-ray diffraction (XRD) measurements of samples were performed at room temperature using a Shimadzu XRD-6000 diffractometer with a graphite-filtered Cu Kα source (λ= 0.15418 nm) at a scanning rate of 5° min−1. Elemental analysis was carried out using a Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectroscope (ICP-AES). The samples were dissolved in nitrohydrochloric acid before measurement. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of samples were obtained on a JEOL JEM-2100 microscope operated at an accelerating voltage of 200 kV. The size distributions of nanoparticles were determined from more than 200 particles. Fourier transform infrared (FT-IR) spectra of samples were collected on a Bruker Vector-22 spectrometer. High-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (HAADFSTEM-EDX) measurements of samples was performed by a JEOL2010F instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed using Thermo VG ESCALAB250 X-ray photoelectron spectrometer with Al Kα X-ray radiation. The binding energy calibration of all spectra was referenced to the C 1s signal at 284.6 eV. Temperature-programmed desorption of CO2 (CO2-TPD) of samples was conducted on Micromeritics ChemiSorb 2920 with a thermal conductivity detector. First, the sample (0.1 g) was heated to 100 °C under a He flow (40 mL/min) and held for 1 h. At room temperature under the He flow, the sample was exposed to pure CO2 (20 mL/min) for 1 h. Subsequently, the sample was purged with a He flow (40 mL/min) for 1 h in order to remove physically adsorbed CO2 and then heated to 700 °C to detect the CO2 signal timely. The amount of total basic sites can be assessed assuming that one CO2 molecule will be adsorbed at one basic site. Surface base properties of solids were determined through the adsorption isotherms of phenol dissolved in cyclohexane at 25 °C. Before measurement, all samples were degassed at 120 °C with air in a muffle furnace. No acid or base was used to adjust the pH value of organic cyclohexane solution. The amount of phenol adsorbed by samples was measured using a Shimadzu UV-2700 instrument (λ = 271.6 nm). Base-Free Oxidation of HMF. The base-free aerobic oxidation of HMF in water was conducted with a 100 mL Teflon-lined stainlesssteel autoclave under a magnetic stirring. Typically, before the reaction, the catalyst (0.05 g) and HMF (0.5 mmol) were added into the reactor precharged with 40 mL of deionized water. Before reaction, pH values of both initial reaction solutions and pure deionized water were measured to be 6.92 due to the dissolution of a small amount of CO2. Then, the reactor was sealed and the air was flushed out using 2
an ionic polymer also were active for HMF oxidation without the use of base additives.24 Although the yield of FDCA exceeded 99%, the applied HMF/Pt molar ratio was only 20. Therefore, the above disadvantages make this process less cost effective and environmentally friendly. Presently, bimetallic NPs have been investigated intensively in a variety of heterogeneous catalytic reactions25−29 because they can combine the advantages of two metal components and construct a unique synergistic effect between them at the atomic level, thereby offering enhanced catalytic performance compared to their monometallic counterparts. As for the aerobic oxidation of HMF to FDCA, some researchers envisioned that alloying Au with other active metals (e.g., Au−Pd,30,31 Au−Cu19) led to more efficient catalyst systems than monometallic catalysts. However, in most cases, the addition of base additives is still necessary in order to promote this tandem reaction.31 On the other side, layered double hydroxides (LDHs, [M1−x2+Mx3+(OH)2]x+ [Ax/n]n−·mH2O), known as a class of ordered layered materials,32 have been employed widely as catalyst supports because of their structural flexibility and compositional versatility.33,34 Specifically, LDHs with regular hydroxyl arrays on brucite-like layers have generated special interest as solid base catalysts for various organic reactions including condensation,35−37 isomerization,38,39 and transesterification.40 Specifically, LDHs as base additives or supports have been demonstrated to be effective for the aerobic oxidation of HMF to FDCA in water without the addition of a liquid base additive.41 However, the stability of such catalysts is very poor due to easy leaching of magnesium in the MgAlLDH as the support under acidic reaction environments.31 Therefore, designing and developing stable LDH-type supports for the aerobic oxidation of HMF to produce FDCA under base-free reaction conditions is highly desirable and remains challenging. For catalytic reactions involving multiple pathways, modifying the surface structure of catalysts to achieve desired catalytic performance is of vital importance. As we know, rare-earthdoped metal oxides may create basic sites and stabilize catalytically active species.42−45 In the present work, we reported the synthesis of La-doped CaMgAl-LDH (LaCaMgAl-LDH) support and a corresponding supported bimetallic AuPd nanocatalyst. It was found that La-CaMgAlLDH possessed more surface basic sites than undoped CaMgAl-LDH and MgAl-LDH supports, and as-formed AuPd/La-CaMgAl-LDH was a more active and stable heterogeneous catalyst for the aerobic oxidation of HMF to produce FDCA under base-free conditions in water than other LDHs-supported catalysts. Under optimized reaction conditions, the yield of FDCA reached above 99%. Moreover, compared with the MgAl-LDH supported one, AuPd/LaCaMgAl-LDH possessed greatly enhanced stability, which was superior to previously reported LDHs-supported catalysts. This is the first report about the base-free aerobic oxidation of HMF to FDCA catalyzed by a highly efficient and stable LDHsupported bimetallic AuPd catalyst.
■
EXPERIMENTAL SECTION
Preparation of Catalysts. LDHs-supported bimetallic AuPd catalysts with a noble metal loading of about 3.0 wt % were synthesized by the coprecipitation method. In a typical synthesis, first, Ca(NO3)2·4H2O (18 mmol), Mg(NO3)2·6H2O (18 mmol), Al(NO3)3·9H2O (11.4 mmol), and La(NO3)3·6H2O (0.6 mmol) were 5853
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering MPa N2 at least 10 times. Finally, after a certain O2 pressure (typically 0.5 MPa) was purged and introduced into the reactor, the reactor was heated to the specified temperature (typically 100 °C) to initiate an oxidation reaction under vigorous stirring at a stirring speed of 900 rpm. After a fixed time (typically 6 h), the reactor was cooled to the room temperature in an ice bath and then depressurized carefully. The liquid product was analyzed using liquid chromatography (Shimadzu LC-20A) equipped with UV detector and HPX-87H chromatographic column (column temperature: 70 °C) using a dilute H2SO4 aqueous solution (10 mM) as a mobile phase (flow rate: 0.6 mL/min). On the basis of external standard curves constructed with authentic standards, the HMF conversion was defined as the molar percentage of HMF converted in the reaction, while the selectivity of each product was defined as the molar percentage of each product in HMF converted. In each case, the mass balance was above 97%. Experimental errors for the conversion and selectivity were less than 3% obtained according to at least three parallel experiments.
system, lattice parameters a (= 2d110) and c (= 3d003) may be calculated. It is noted that the partial replacement of Mg with Ca leads to a small increase in lattice parameter a from 0.3036 nm for AuPd/MgAl-LDH to 0.3052 nm for AuPd/CaMgAlLDH due to the difference in the ionic radii for Mg2+ and Ca2+ ions. Similarly, the partial replacement of Mg with Ca also leads to a small increase in lattice parameter c from 2.3114 nm for AuPd/MgAl-LDH to 2.3195 nm for AuPd/CaMgAl-LDH because the decrease in the charge density on the brucite-like layers with the introduction of Ca2+ ions having a larger ion radius can result in a decrease in Coulombic attractive force between interlayer anions and brucite-like layers. The above results confirm that Ca2+ ions should enter the brucite-like layers of the CaMgAl-LDH support. However, as for AuPd/LaCaMgAl-LDH, no characteristic diffractions related to crystalline La-containing oxide or hydroxide phases are observed. To determine the existence and chemical state of La species in AuPd/La-CaMgAl-LDH, the fine La 3d spectrum was further analyzed (Figure S1). It is found that two intensive peaks at about 834.8 and 851.5 eV are associated with La 3d 5/2 and La 3d 3/2, respectively, indicating that La species probably exist in the form of the La2O3 phase.48,49 It demonstrates that La3+ ions do not enter into the brucite-like layers of the LDH structure. Correspondingly, a small amount of La2O3 formed may uniformly and highly disperse on the surface of the CaMgAlLDH support at the atomic level. For the Au/CaMgAl-LDH sample, typical (111) and (200) diffractions at 38.2° and 44.4° can be indexed to the crystalline metallic Au phase. In addition, weak diffractions of the crystalline metallic Pd phase are observed in the Pd/CaMgAlLDH sample. In the case of all supported bimetallic AuPd samples, no obvious characteristic diffractions related to the metallic Pd phase can be found. Interestingly, two weak diffractions are observed at 2 θ between Au (111) and Pd (111) planes and between Au (200) and Pd (200) planes, respectively, as further demonstrated by the detailed XRD patterns in the 2 θ range of 36−42° (Figure 1B). The above results suggest the successful formation of a bimetallic AuPd phase. Elemental analysis by ICP-AES reveals the actual contents of Au, Pd, Ca, Mg, Al, and La elements in different samples (Table 1). It is seen that the Au/(Au+Pd) mass ratios in bimetallic samples are close to 0.6:1, well consistent with those in the synthesis mixture. The surface microstructure of the representative AuPd/LaCaMgAl-LDH sample was investigated by TEM and HRTEM analysis (Figure 2). It can be found that great quantities of small NPs of about 3−4 nm in particle size are well-dispersed on the LDH support. Further, a typical HRTEM image of a single NP reveals clear lattice fringes with an interplanar spacing of about 0.231 nm, which is slightly smaller than the (111) lattice spacing of metallic Au (0.235 nm) but larger than
■
RESULTS AND DISCUSSION Structural Characterization. Figure 1A depicts XRD patterns of a series of LDHs-supported monometallic and
Figure 1. XRD patterns (A) of different supported samples: (a) Pd/ CaMgAl-LDH, (b) Au/CaMgAl-LDH, (c) AuPd/CaMgAl-LDH, (d) AuPd/La-CaMgAl-LDH, (e) AuPd/CaAl-LDH, and (f) AuPd/MgAlLDH. Detailed XRD patterns (B) of different supported samples in the 2θ range of 36°−42°.
bimetallic samples. All samples exhibit the typical characteristic diffractions indexed to (003), (006), (012), and (110) crystalline planes of hydrotalcite-like materials. Specifically, for the binary CaAl-LDH-supported AuPd sample, the existence of characteristic diffractions for the LDH phase demonstrates that Ca2+ ions with larger ion radius (0.1 nm) than Mg2+ ions (0.072 nm) also enter into the brucite-like layers of LDH structure, in good agreement with the literature.46,47 In addition, assuming a 3R stacking of the brucite-like layers of a hexagonal LDH
Table 1. Textual and Structural Data of Different LDHs-Supported Samples
a
sample
Pda (wt %)
Aua (wt %)
Pd+Aua (wt %)
Caa (wt %)
Mga (wt %)
Ala (wt %)
Laa (wt %)
SBETb (m2/g)
DTEMc (nm)
Pd/CaMgAl-LDH Au/CaMgAl-LDH AuPd/CaMgAl-LDH AuPd/La-CaMgAl-LDH AuPd/CaAl-LDH AuPd/MgAl-LDH
2.75 0 1.12 1.18 1.20 1.15
0 2.51 1.55 1.53 1.59 1.63
2.75 2.51 2.67 2.71 2.79 2.78
26.62 27.19 28.38 27.42 46.49 0
18.39 17.49 17.63 16.54 0 40.25
16.20 15.91 15.31 14.78 13.83 16.54
0 0 0 3.42 0 0
108 101 105 112 115 96
3.5 3.4 3.3 3.3 3.4 3.1
Determined by ICP-AES. bBET specific surface area. cExtracted surface area weighted mean size of NPs calculated by TEM analysis. 5854
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. TEM (a and b) and HRTEM (c) images and particle size distribution (d) of representative AuPd/La-CaMgAl-LDH sample.
the (111) lattice spacing of metallic Pd (0.225 nm). These indications strongly suggest the formation of bimetallic AuPd NPs with the preferentially exposed (111) plane, which is accordance with the XRD results. Moreover, as shown in Figure 2d, the size distribution of AuPd NPs is very narrow, and the average crystallite size of AuPd NPs is about 3.32 nm. The average size of monometallic or bimetallic NPs in all of the samples is in the range of 3.1−3.5 nm (Table 1), indicating a slight difference in the particle size in spite of different metallic compositions. In order to demonstrate the formation of bimetallic AuPd NPs on the La-CaMgAl-LDH support, STEM characterization of the AuPd/La-CaMgAl-LDH sample was further performed. As shown in Figure 3, the uniform distributions of Au, Pd, Ca, Mg, Al, and La elements can be observed based on the EDX mapping from HAADF-STEM images. Obviously, Au and Pd elements appear in the same positions, indicative of the uniform distribution of Au and Pd elements over NPs. Further, the very similar distributions of Au and Pd elements over NPs also are found from the STEM-EDX line scan spectra. As a result, the above XRD, TEM, and STEM results strongly demonstrate the successful formation of bimetallic AuPd NPs on the LaCaMgAl-LDH support. Surface Chemical State and Basicity of Samples. Electronic structures of Au species in two representative supported samples (Au/CaMgAl-LDH and AuPd/CaMgAlLDH) were examined by XPS (Figure 4). Since the Mg 2s region may partially overlap the Au 4f region, fine Au 4f spectra are fitted by four contributions. For Au/CaMgAl-LDH, two contributions at about 83.6 and 87.4 eV are assigned to spin− orbit Au 4f 7/2 and Au 4f 5/2 core levels, respectively, indicative of the character of metallic Au0 species. Meanwhile, the appearance of another two contributions at about 84.4 and 88.3 eV indicates the presence of cationic Auδ+ species in samples. The formation of Auδ+ species is associated with the strong interaction between Au species and the support.50,51 Notably, the proportion of Auδ+ species in the total Au species for AuPd/CaMgAl-LDH (0.41) is much higher than that for Au/CaMgAl-LDH (0.29). In addition, it is demonstrated that when Au is combined with Pd to form bimetallic AuPd NPs, the binding energy values for Au 4f regions in AuPd/CaMgAl-
Figure 3. HAADF-STEM images (a,c) of representative AuPd/LaCaMgAl-LDH sample with EDX mapping of Ca−K(b1), Mg−K(b2), Al−K(b3), La−K(b4), Au-L(b5), and Pd-L(b6). EDX line scan spectra (d) of Au-L and Pd-L along the red line in panel (c).
LDH all slightly shift toward lower values by about 0.3 eV. We speculate that the electronic states of Au species can be affected by the unique bimetallic AuPd structure, thus leading to the partial charge transfer from Pd to Au due to higher electronegativity of Au (2.4) than that of Pd (2.2), as well as the formation of more Auδ+ species at the interface due to the strong interactions between AuPd NPs and the CaMgAl-LDH support. As shown in Figure 4, the signals centered at about 335.6 and 340.5 eV for supported Pd/CaMgAl-LDH and AuPd/CaMgAlLDH samples are assigned to Pd 3d 5/2 and Pd 3d 3/2 regions, respectively, indicative of the existence of metallic Pd0 species.52 Besides, another two components at about 337.3 and 342.2 eV are associated with unreduced Pd(II) species. On the basis of the integral areas for different Pd species, the proportion of Pd(II) species in total Pd species for Pd/CaMgAl-LDH (0.17) is much smaller than that for AuPd/CaMgAl-LDH (0.63). It is found that the binding energy value of the Pd 3d 5/2 region for Pd0 species is higher than the previously reported value (335.0 eV).53,54 The above results are due to the electronic interaction between Pd and Au in the bimetallic sample and the strong interaction between AuPd NPs and the LDH support in the case of AuPd/CaMgAl-LDH, thereby leading to positively polarized palladium atoms, well consistent with the XPS results of Au 4f regions. To determine surface basicity of supported samples, CO2TPD characterization was conducted. As shown in Figure 5, different CO2 desorption peaks are assigned to weak basic sites (WB) below 200 °C and different medium-strength and strong basic sites (MSB) above 200 °C in all of samples, respectively. As for LDH supports, surface basic sites mainly originate from hydroxyl groups on the lattices of LDH crystals. On the basis of the areas of CO2 desorption peaks, the density of total basic sites as well as the density of medium-strength and strong basic sites gradually increases in the following order: AuPd/MgAlLDH < AuPd/CaAl-LDH < AuPd/CaMgAl-LDH < AuPd/La5855
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. XPS of Au 4f and Pd 3d regions for different samples.
Table 2. Surface Basicity of Supported Samples sample
basicitya (mmol/g)
basicityb (mmol/g)
basicityc (mmol/g)
AuPd/MgAl-LDH AuPd/CaAl-LDH AuPd/CaMgAl-LDH AuPd/La-CaMgAl-LDH
1.70 1.90 2.34 3.38
1.30 1.45 1.75 2.70
0.46 0.63 0.67 1.02
a
Density of total basic sites determined by CO2-TPD. bDensity of medium-strength and strong basic sites determined by CO2-TPD. c Density of surface basic sites determined by phenol adsorption.
weak acidity were performed.34,55,56 Figure 6 presents adsorption isotherms for phenol from cyclohexane solution at
Figure 5. CO2-TPD profiles of AuPd/MgAl-LDH (a), AuPd/CaAlLDH (b), AuPd/CaMgAl-LDH (c), and AuPd/La-CaMgAl-LDH (d).
CaMgAl-LDH (Table 2). Specifically, the density of surface basic sites for AuPd/La-CaMgAl-LDH is much larger than those for other samples. The above results demonstrate that the simultaneous introduction of Ca, Mg, and La into LDH is beneficial to the generation of more medium-strength and strong basic sites probably due to the formation of La3+−O2+ pairs in the amorphous La-containing oxide phase on LDH and more crystal defects at the edges of LDH platelets. Further, in order to get more accurate information about surface medium-strength and strong basic sites of different supported samples, adsorption experiments for phenol with
Figure 6. Adsorption isotherms for phenol on AuPd/MgAl-LDH (a), AuPd/CaAl-LDH (b), AuPd/CaMgAl-LDH (c), and AuPd/LaCaMgAl-LDH (d). 5856
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering 25 °C. It is found that the chemical interaction between the phenol and catalyst surface is in good accordance with the Langmuir adsorption isotherm equation, suggestive of a surface monolayer adsorption process for phenol. In this case, the adsorption capacity of supported samples follows the order of AuPd/La-CaMgAl-LDH > AuPd/CaMgAl-LDH > AuPd/CaAlLDH > AuPd/MgAl-LDH. As shown in Table 2, the density of surface basic sites on AuPd/La-CaMgAl-LDH, which is determined based on the saturation adsorption capacity of phenol, is much higher than those on other supported samples. This result is well consistent with the change in the density of medium-strength and strong basic sites determined by CO2TPD, regardless of different adsorption modes for CO2 and phenol on basic sites. As a result, the introduction of La inevitably increases the amount of surface basic sites, which is probably assigned to the generation of new La3+−O2+ pairs and the increased number of exposed hydroxyl groups on the LaCaMgAl-LDH. Base-Free Aerobic Oxidation of HMF. Scheme 1 shows main reaction pathways that take place in the course of HMF
FDCA, along with a low HMF conversion of 52.1%. When CaAl-LDH is used as the support, the HMF conversion and the selectivity to FDCA increase to 65.8% and 57.4%, respectively. In the case of CaMgAl-LDH as the support, most HMF (94.2% conversion) is converted into FDCA with a 84.5% selectivity over AuPd/CaMgAl-LDH. However, CaMgAl-LDH-supported monometallic Au and Pd catalysts, which are less active and selective for the HMF oxidation to FDCA than bimetallic AuPd/CaMgAl-LDH, only give low FDCA yields of ∼59% under the identical reaction conditions. On the basis of the distributions of all products, it can be found that the oxidation of the HMFCA intermediate is easily performed over the AuPd/CaMgAl-LDH catalyst in comparison with Au/CaMgAlLDH and Pd/CaMgAl-LDH, indicative of the promotional effect of AuPd NPs on further oxidation of the alcohol hydroxyl group in HMFCA. In order to further determine the existence of a synergistic effect in bimetallic AuPd NPs, the oxidation of the HMFCA intermediate as the initial substrate was performed using different supported monometallic and bimetallic catalysts (Table S1). Obviously, the oxidation of the alcohol hydroxyl group in HMFCA over AuPd/CaMgAl-LDH is actually faster than those over Au/CaMgAl-LDH and Pd/CaMgAl-LDH, leading to a higher FDCA yield. The above XPS results also reveal the formation of more Auδ+ species on the surface of AuPd/CaMgAl-LDH. Such cationic Auδ+ species can promote C−H bond dissociation and the activation of oxygen molecule,57,58 thereby improving the reaction activity. It demonstrates that a significant synergy between Au and Pd in bimetallic AuPd NPs greatly enhances both the HMF conversion and the FDCA selectivity. In addition, in the presence of pure CaMgAl-LDH support, the conversion of HMF is only 15.9%, along with an almost negligible formation of FDCA, indicating that metallic NPs are essential for the aerobic oxidation of HMF. AuPd NPs loaded on the LaCaMgAl-LDH support yield higher HMF conversion (96.1%) and FDCA selectivity (89.4%) than those loaded on La-free CaMgAl-LDH, reflecting that La-CaMgAl-LDH is an excellent support for HMF oxidation to FDCA. On the other side, it is noted that with the increased density of surface basic sites on catalysts, both the HMF conversion and the FDCA selectivity increase sharply, with a simultaneously reduced selectivity to FFCA. The above results demonstrate the critical role of surface basicity of supports in improving the catalytic activity and clearly highlight the importance of the corporation between basic LDH supports and bimetallic AuPd NPs for base-free aerobic oxidation of HMF to FDCA in water. In order to identify the heterogeneity of the present catalyst system, the reaction was conducted over AuPd/La-CaMgAl-
Scheme 1. Schematic Illustration of Main Reaction Pathways for Aerobic Oxidation of HMF
oxidation. First, HMF may be oxidized to 5-hydroxymethyl-2furancarboxylic acid (HMFCA) or 2,5-diformylfuran (DFF) in the presence of molecular oxygen. The two intermediates may be further oxidized to 5-formyl-2-furancarboxylic acid (FFCA). Finally, FFCA is finally oxidized to generate FDCA product. As presented in Table 3, catalytic studies indicate that the LDH support can play a crucial role in the HMF oxidation over supported bimetallic AuPd catalysts. In the present catalyst system, the specific surface area of catalysts is not a key factor in affecting the catalytic performances due to the slight difference in the surface area (Table 1). The employment of MgAl-LDH as the support mainly results in the formation of FFCA and
Table 3. Catalytic Performance for Base-Free Aerobic Oxidation of HMF over Different Catalystsa selectivity (%)
a
entry
catalysts
conv. (%)
FDCA
HMFCA
FFCA
DFF
1 2 3 4 5 6 7
AuPd/MgAl-LDH AuPd/CaAl-LDH AuPd/CaMgAl-LDH Au/CaMgAl-LDH Pd/CaMgAl-LDH CaMgAl-LDH AuPd/La-CaMgAl-LDH
52.1 65.8 94.2 87.4 83.2 15.9 96.1
35.2 57.4 84.5 67.7 72.5 0.1 89.4
6.2 5.0 1.5 19.4 9.2 43.2 0.3
55.3 33.3 13.2 12.2 17.3 30.5 9.5
3.3 4.3 0.8 0.7 1.0 26.2 0.8
Reaction conditions: catalyst, 0.05g; HMF, 0.5 mmol; O2 pressure, 0.5 MPa; reaction time, 6 h; reaction temperature, 100 °C. 5857
DOI: 10.1021/acssuschemeng.7b00573 ACS Sustainable Chem. Eng. 2017, 5, 5852−5861
Research Article
ACS Sustainable Chemistry & Engineering
(
