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Selective oxidative esterification of alcohols over Au-Pd/graphene Ruiyi Wanga, Huan Liua,b, Chaoyang Fana, Jie Gaoa, Chengmeng Chenc, Zhanfeng Zhenga,d,* a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi, 030001, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, PR China d Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China b c
ARTICLE INFO
ABSTRACT
Keywords: Graphene Au-Pd alloy Alcohols Selective oxidation DFT
Direct selective oxidative esterification of readily available alcohols under mild conditions is an attractive approach to synthesis valuable esters. Developing high performance catalyst is the key factor to the efficient esters synthesis. We report graphene supported Au-Pd alloy catalyst exhibits excellent catalytic performance in the synthesis of methyl benzoate from benzyl alcohol and methanol, a turnover frequency (TOF) of 230 h−1 and selectivity of 100% to methyl benzoate were achieved under 1 atm O2 at 25 °C, which is superior to the majority of the state-of-the-art catalysts. Experimentally observed volcano-like reactivity trends and DFT calculations prove the outstanding performance was mainly ascribed to unique electronic structures of AuPd alloy catalyst for the adsorption and activation of reactant molecules. The catalytic reaction mechanism for interpretation of the structure-activity relationships of various catalysts at molecular level was investigated. The present study could help to unravel the synergistic effect of Au-Pd catalyst and provides a mild and efficient route for synthesis highvalue esters in terms of green and sustainable chemistry.
1. Introduction Esters are a kind of important chemicals for both industrial manufacture and laboratorial basic research, they are extensively utilized in pharmaceuticals, solvents, condiments, as well as chemical intermediates in synthetic organic chemistry [1,2]. Traditionally, esters production is accomplished by the concentrated acid catalyzed reaction of alcohols with carboxylic acids or activated acid derivatives such as acyl chlorides and anhydrides, which accompanied with large amount of undesired by-products. Another alternate is transition-metal catalyzed carbonylation reaction of aryl halides with alcohol and CO; however, the high reaction temperature and CO pressure are not favorable [3]. Besides, much effort has been devoted to the direct conversion of aldehydes into the corresponding esters, generally require stoichiometric amount of heavy metals or homogenous metal complexes [4]. All above esters synthetic routes are high energy consumption and not environmentally friendly. Accordingly, the direct synthesis of esters form alcohols may be the most desirable process from both economic and environmental points of view, since alcohols are readily available, low cost and less toxic compared with aldehydes and carboxylic acids [5]. Hitherto, much effort has been devoted to developing catalytic
systems for synthesis of esters from alcohols. Homogeneous systems based on Au, Ru, Pd and other catalyst have been developed successfully with satisfied catalytic activity [6–11]. For example, Beller et al. applied Pd(OAc)2 catalyst for cross-esterification of benzyl alcohols with various aliphatic alcohols, high esters yield was achieved under mild conditions. Nevertheless, special ligands and additive are always necessary and make the products separation difficult [12]. Accordingly, alternative heterogeneous catalysts is highly desirable for practical application. It is worth to note that nitrogen-doped carbon material supported non-noble metal cobalt catalyst is highly desirable with multiple advantages including cost effectiveness, environmentally benign and facial separation; however, their catalytic performance, in terms of turnover frequency (TOF) of desired esters, is still somewhat unsatisfactory [13–15]. Supported gold catalysts have shown a remarkable catalytic activity in this reaction, Au catalysts with various support such as Al2O3, ZrO2, SiO2, MOF-5 and SBA-16 have been extensively investigated since it is well known that the nature of the support determines the catalytic efficiency of the material [16–21]. Graphene has attracted wide attention for its unique properties [22–24]. Our previous work also proved graphene based carbon materials to be excellent support for methanol oxidation esterification reactions [25,26]. Besides, it is also well-established that alloying metal
⁎ Corresponding author at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi, 030001, PR China. E-mail addresses: [email protected] (R. Wang), [email protected] (Z. Zheng).
https://doi.org/10.1016/j.mcat.2019.110687 Received 31 August 2019; Received in revised form 18 October 2019; Accepted 19 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ruiyi Wang, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110687
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with palladium can significantly enhance the catalytic activity relative to monometallic metal catalysts [27–30]. However, to the best of our knowledge, very few researches have been reported on the Pd modified Au catalyst and catalytic activity relationship for alcohol oxidation esterification reaction so far. Although various compositions of Au-Pd catalysts have been intensively studied in a wide array of reactions [31–34], the nature of the optimal Au-Pd molar ratio for a given model reaction is still unclear. In this work, graphene supported Au-Pd alloy catalysts are employed in alcohol direct aerobic oxidative esterification reaction, a remarkable increase of catalytic activity was observed when adding a small amount of palladium to gold. The effect of the Au-Pd molar ratio, support material, light irradiation and various base on the reactant conversion and product distribution were studied. Furthermore, a detailed DFT investigation on the reaction mechanism and the synergy between Au and Pd is also presented. Finally, the tolerance of various substituted alcohols, the catalyst reusability and scale-up synthesis of methyl esters were also investigated.
coupled plasma atomic emission spectroscopy (ICP-AES). To prepare the solution for elemental analysis, 3 ml of concentrated HCl and 1 mL of concentrated HNO3 was used to dissolve the catalyst sample, the solution was then diluted to 100 ml with de-ionized water. 2.3. Computational modeling and methods All first principles density functional theory (DFT) calculations were performed with the Vienna Ab Initio Simulation Package (VASP) with the projector-augmented wave (PAW) pseudo-potentials [35–37]. Perdew, Burke and Ernzerhof (PBE) parameterized generalized gradient approximation (GGA)-type functional [38,39] was applied to describe the exchange and correlation energy. The spinpolarized potentials were calculated in all situations. The kinetic energy cutoff energy for the wave basis was set to 400 eV. All the model structures were rectified until the total energy was converged less than 10−5 eV and the atomic forces were lower than 0.02 eV Å-1. Density of states (DOS) was used for quantitatively analyze the electronic structure of the system. The five catalysts was studied by carrying out the reaction on the Au (111), Pd(111), Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111) NPs surfaces [40]. The modeling of metal NPs surfaces were accomplished by four layers of metal atoms with a vacuum region of 18 Å to avoid the influence of next neighbor slabs. The adsorption species were simulated by a cubic cell of 15☓15☓15 Å3, where the Brillouin zone integration was applied for the Γ point only. The adsorption energy (Eads) was calculated by the following equation,
2. Experimental and DFT calculations 2.1. Catalyst preparation Graphene used in this work was prepared by fast pyrolysis of graphene oxide (GO) under high vacuum, which GO was fabricated by a modified Hummers’ method. The obtained GO power was placed into quartz tube, then evacuated until the pressure lower than 2.0 Pa and then heated to 200 °C with a heating rate of 30 °C/min, the fluffy graphene sample was then obtained. The bimetallic catalysts were prepared through a deposition-precipitation method. Typically, graphene sheet (200 mg) was dispersed in 200 mL deionized water, and placed in a water bath of 60 °C, a certain amount of sodium carbonate was added. The pH value was measured to be 10. After stirring for 20 min, a mixed aqueous solution of desired amount HAuCl4·4H2O and PdCl2 was added dropwise. The mixture was then stirred rigorously for another 2.5 h; the resultant solid catalysts were filtered and washed with deionized water until no Cl– was detected by the solution of AgNO3. The samples were dried at 120 °C in vacuum for 3 h and then calcined at 200 °C in a muffle furnace for 3 h. The resultant materials are denoted as AumPdn/Gr, where m and n are the weight percentage loadings of Au and Pd, respectively. Au-Pd catalysts with other supports (active carbon, carbon nanotube, and graphite) or single metal (Au and Pd) catalyst were also obtained through the similar process, denoted as AumPdn/AC, AumPdn/CNTs, AumPdn/Gi, Aum/Gr and Pdn/Gr, respectively.
Eads = Eadsorbate/substrate - Esubstrate - Eadsorbate where Eadsorbate/substrate is the total energy of the composite system, Esubstrate is the energy of the clean surface of various model systems and Eadsorbate is the energy of isolated substrate. The negative value of Eads means exothermicity, demonstrated strong adsorption of the adsorbate on the substrate. For searching the minimum energy pathway (MEP) for the benzyl alcohol oxidation, a climbing-image nudged elastic band (CINEB) method [41–43] was applied in the calculation. The activation barrier of Ea was defined as the energy difference between the transition states (TS) and the initial state (IS). The reaction energy of ΔE referred to the energy difference between the final states (FS) and the IS. 2.4. Catalyst tests and analytic procedures The catalyst evaluation was performed in a 100 mL stainless steel autoclave lined with teflon and a round quartz window. Typically, a mixture of the Aum-Pdn/Gr catalyst (50 mg, 3 wt% of total metal loading), benzyl alcohol (2 mmol), potassium carbonate (1 mmol), and 5 mL methanol were loaded into the autoclave, purged with O2 for 2 min, then the reaction mixture was kept at 25 °C for 1 h under vigorous stirring of 300 rpm. After reaction, 3 mL residual mixture were collected and filtered through a Millipore filter to remove the catalyst. The products after reaction, including aldehyde, ester and unreacted reactants, were analyzed by a Shimadzu 2014C gas chromatography (GC) with a WondaCap 5 column, the conversions and selectivity were calculated from the product formed and the reactant converted, as measured by GC. The turn‐over frequency (TOF) was calculated based on the following equation:
2.2. Catalyst characterization X-ray diffraction (XRD) measurement was performed using a desktop X-ray diffractometer (Bruker AXS D8, Germany, Cu Kα radiation source, = 0.15406 nm). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the samples were measured on a JEM-2010 system electron microscope operated at 200 kV. The sample was suspended in ethanol for ultrasonic dispersion 30 min, and then deposited on copper grids coated with carbon foil. The X-ray photoelectron spectroscopy (XPS) spectra were taken on a Thermo ESCALAB 250 instrument, with an Al Kα monochromator Xrays source (hγ =1486.6 eV). Extended X-ray absorption fine structure spectroscopy (EXAFS) measurements were tested at the beam line BL14W1 of Shanghai Synchrotron Radiation Facility, China. The storage ring energy was 3.5 GeV, and the ring current was 300 mA. The Au L3 edge data were recorded in a fluorescence mode and the EXAFS data were processed by using the IFEFFIT package. The actual content of Au and Pd was determined by inductively
TOF=
2
amount of benzyl alcohol (mol) ×Conversion (%) ×Selectivity (%) Mass of Catalyst (g) ×
{
Au loading(wt%) Mr Au
+
Pd loading(wt%) Mr Pd
} ×reaction time (h)
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the Auδ+ species [25]. For the Au-Pd alloy catalysts, an obviously negative shift of BEs was observed with the increase of palladium molar percentage. For example, compared with that of Au3.0/Gr, the Au0 4f7/2 peaks of Au2.75Pd0.25/Gr, Au2.5Pd0.5/Gr, Au2.0Pd1.0/Gr, Au1.5Pd1.5/Gr and Au1.0Pd2.0/Gr shift to lower BEs, i.e. from 84.29 eV to 84.23, 84.05, 84.07, 84.07, and 84.02 eV, respectively. Similarly, in the case of Pd 3d XPS spectrum, the peaks at 335.36 and 340.80 eV are attributed to metallic palladium species Pd0, while the peaks at 337.67 and 343.11 eV are assigned to Pd2+ species respectively. The BEs of Pd also shift to lower values after alloying gold with palladium as illustrated in Fig. 4b, compared with that of Pd3.0/Gr, the Pd2+ 3d5/2 peaks of Au1.0Pd2.0/Gr, Au1.5Pd1.5/Gr, Au2.0Pd1.0/Gr, Au2.5Pd0.5/Gr, and Au2.75Pd0.25/Gr shift to lower BEs, i.e. from 337.67 eV to 337.50, 337.53, 337.50, 337.36, and 337.24 eV, respectively. This phenomenon has been reported by various researchers in Au-Pd alloy and core-shell structures [44,45]. The negative shift BEs of both Au and Pd are due to the electron exchange between Au and Pd, i.e. a synergism between Au and Pd nanoparticles, indicating the change of electronic environment for the Au-Pd alloy. The scope of the negative shift of BEs represents the extant synergism between Au and Pd. In this work, a relative larger shift of 0.24 eV for Au 4f5/2 and 0.46 eV for the Pd 3d3/2 peak in Au2.5Pd0.5/ Gr is observed, which indicates an especially strong synergism between Au and Pd in the Au2.5Pd0.5/Gr catalysts. The actual loadings of Au and Pd on the graphene supported Au-Pd catalysts with various Au-Pd ratio were also determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). As given in Table 1, it proves that the actual loadings of Au and Pd are very close to the designed values and the loss of Au and Pd is negligible upon the aqueous rinsing during the catalyst preparation procedure.
Fig. 1. XRD patterns of graphene supported Au, Pd and Au-Pd alloy NPs catalyst.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD results The X-ray diffraction (XRD) patterns of various graphene supported Au-Pd catalysts are shown in Fig. 1. Graphene displayed a typical broaden peak at about 25° and 43.1° corresponding to C(002) and C(100), respectively, indicating the smaller crystalline size of graphene sheet in a layer structure. [26] Accordingly, after loaded the Au-Pd nanoparticles on the graphene sheets, the characteristic peaks at 38.4°, 43.9° and 65.1° corresponding to Au(111), Au(200) and Au(220), respectively, are clearly observed. No obvious diffraction peaks of palladium were detected, which might due to its relatively low content or high dispersion.
3.1.4. EXAFS results The charge redistribution of different graphene supported Au-Pd NPs is also verified by the XANES Au L3-edge spectra as shown in Fig. 5. The Au L3-edge white line located at 11,925 eV is corresponding to the electron transition from 2p to unoccupied 5d states. Theoretically, a more intense white line is corresponding to more unoccupied 5d states or less 5d-electron. [46–48] After normalizing the Au L3-edge jump, Au nanoparticles (Au3.0/Gr) have a weaker white line shoulder as compared to the bulk Au metal (Au foil) due to the lessening of s-p-d hybridization in small Au nanoparticles compared to bulk gold. Upon alloying with palladium, the white line intensity is decreased in the order of Au3.0/Gr＞Au2.75Pd0.25/Gr＞Au2.5Pd0.5/Gr＞Au2.0Pd1.0/Gr ＞Au1.5Pd1.5/Gr＞Au1.0Pd2.0/Gr, indicating the Au 5d states gaining more electrons from Pd with the increasing Pd content. The XANES analysis of Au is well consist with the XPS results, all suggesting the charge redistribution of Au occurs when alloying with Pd, and this feature will affect the catalytic performance of the catalysts as discussed follow. Based on the above catalyst characterizations, it can be seen that Au, Pd and Au-Pd alloy nanoparticles are uniformly dispersed on the two-dimensional laminar graphene support, charge redistribution occurs on the Au-Pd alloy surface which evidenced by the XPS and EXAFS results.
3.1.2. TEM and HRTEM results The Au-Pd nanoparticles are uniformly dispersed on the graphene support in a spherical shape as illustrated in TEM image (Fig. 2a) and HAADF-STEM image (Fig. 2c). The particle size estimated by randomly selecting 150 typical particles is in the range of 1–15 nm with an average of 7.7 nm (Fig. 2b). The high-resolution TEM (HRTEM) image in Fig. 2d illustrates the Au-Pd nanoparticle with a d-spacing of 0.226 nm, which is larger than the spacing of fcc Pd (0.221 nm) and lower than that of fcc Au (0.235 nm). EDS line scanning profile was recorded to identify the elemental distribution in Au-Pd/Gr, indicating that the Au element is concentrated in the core of the nanoparticle, since the content of Pd is relative low (0.5 wt%). All those suggested the formation of alloy structure. TEM images and particle size distribution of (a) Au3.0/Gr, (b)Au2.75Pd0.25/Gr, (c)Au2.5Pd0.5/Gr, (d)Au2.0Pd1.0/Gr, (e)Au1.5Pd1.5/ Gr, (f)Au1.0Pd2.0/Gr and (g) Pd3.0/Gr catalysts are illustrated in Fig. 3, which indicate that the morphology and size distribution of the Au-Pd alloy nanoparticles are related to the Au-Pd molar ratio. In all cases, the Au, Pd, and various Au-Pd alloy particles are highly dispersed on the graphene sheets, whereas the particle size decreases gradually from 8.9 nm to 8.2, 7.7, 7.1, 6.6, 5.9 and 5.4 nm, respectively, with the increase of Pd content.
3.2. Catalytic performance in the esters synthesis 3.2.1. Effect of Au-Pd ratio Benzyl alcohol and methanol were chosen as model molecules and molecular oxygen was used as oxidant initially. As shown in Table 1, a blank experiment was run for completeness (Table 2, entry 1), none reaction proceed without oxygen prove the esterification reaction is an aerobic oxidative process rather than dehydrogenative coupling reaction (Table 2, entry 2). The catalytic performance of the direct esterification reaction is tremendously affected by the Au/Pd molar ratio. The monometallic Pd catalyst show the lowest benzyl conversion of 3.6% and no target product methyl benzoate was detected with a 100%
3.1.3. XPS and ICP-AES results The valence composition and variation of Au and Pd were studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4a and b. Au 4f spectra of Au3.0/Gr can be fitted into four peaks, the two peaks located at 84.29 and 87.96 eV are attributed to Au0 4f7/2 and 4f5/2, respectively, while the other two peaks at 85.40 and 88.37 eV belong to 3
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Fig. 2. (a) TEM image, (b) Au-Pd alloy particle dimeter distribution, (c) HAADF-STEM image of Au2.5Pd0.5/Gr catalyst and (d) HRTEM image of Au-Pd alloy NPs and EDX line scan.
benzaldehyde selectivity (Table 2, entry 3). The monometallic Au catalyst, however, exhibited a moderate benzyl conversion (34.6%) and a high methyl benzoate selectivity (96.6%) (Table 2, entry 4). The performance of bimetallic Au-Pd catalysts were strongly relay on the molar ratio of palladium added, for instance, excellent catalytic performance with a sharp increase of benzyl alcohol conversion to 100% and methyl benzoate selectivity of 100% was achieved by Au2.5Pd0.5/Gr catalyst with a Pd molar percentage of 27% (Table 2, entry 6); However, further increase Pd content resulted a decrease for both benzyl alcohol conversion and methyl benzoate selectivity, when the Pd molar percentage increased to 48%, 65%, and 78%, the conversion of benzyl alcohol conversion decreased to 89.8%, 35.0% and 15.7%, respectively, meanwhile the methyl benzoate selectivity decreased to 56.9%, 21.2% and 11.6% as well, with benzaldehyde became the dominant product (Table 2, entries 7–9). A volcano-like curve of TOF values is observed as illustrated in Fig. S3. The highest TOF value of 230.0 h−1 was achieved by adding 27 mol% Pd, to the best of our knowledge, this is the highest TOF value in the reported literature. To understand the process of the alcohol direct esterification reaction, a kinetic study was conducted as illustrated in Fig. 6. Time course for benzyl alcohol conversion and products selectivity shows that the benzyl alcohol conversion reached 57.9% within 10 min, and increased with the reaction proceeded, and reached 100% at 60 min. The benzaldehyde selectivity decrease and methyl benzoate selectivity increased during the whole reaction, which indicate benzaldehyde was an intermediate during the reaction. Benzyl alcohol was first oxidized to benzaldehyde, then benzaldehyde further reacted with methanol to form the target product methyl benzoate. To reveal the remarkable effect of various Au-Pd molar ratio on the catalytic performance in the direct alcohol esterification reaction, we divided the whole reaction into 2 steps, step 1: benzyl alcohol aerobic oxidized to benzaldehyde; step 2: benzaldehyde reacted with methanol to methyl benzoate. Then we evaluated the catalytic performance of the
graphene supported various Au-Pd catalyst in the two step individually. In the benzyl alcohol oxidation reaction of step 1, as illustrated in Fig. 7a, the selectivity of benzaldehyde is 100% for all different catalysts. However, in terms of benzyl alcohol conversion, it exhibited almost the same tendency curve as the integrated benzyl alcohol and methanol esterification reaction, adding a small amount of palladium can drastically promote the benzyl alcohol conversion. The benzyl alcohol conversion increased form 5.2% of monometallic Au3.0/Gr catalyst to 70% of Au2.5Pd0.5/Gr catalyst when 27 mol% Pd added, further increase the Pd content lead a rapid decrease of benzyl alcohol conversion. In step 2 under the given reaction conditions, totally different tendency from step 1 as illustrated in Fig. 7b, monometallic Au3.0/Gr catalyst show the best catalytic performance with 100% benzaldehyde conversion and 96.2% selectivity to methyl benzoate, the by-product is benzaldehyde dimethyl acetal, which is come from nucleophilic addition reaction of one molecule benzaldehyde and two molecules methanol. With the increase of molar percentage of Pd, a linear gradually decrease for both benzaldehyde conversion and methyl benzoate selectivity was observed, meanwhile the benzaldehyde dimethyl acetal turns out to be the primary product in those cases. To further unerstand the volcano-like behaviour of the graphene supported catalyst with various Au-Pd ratio, The structure-activity relationship of Au(111), Pd(111), Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111) NPs surfaces in the selective oxidation of benzyl alcohol were further investigated by DFT calculations with geometric structure, adsorption energy, PDOS and activation energy. The geometric structure of model surfaces with various Au-Pd ratios are illustrated in Fig. 8a. From the PDOS (Fig. 8b, c) of various catalysts, we can see the difference of their electronic structure exist near Fermi level. Compared with the electronic structure of pure Pd(111) and Au(111), it is confirmed that some electrons in Pd atoms transfer to Au atoms. This suggests that the electronic effect might be the determinant affecting the catalytic activity of various catalysts. Compared with Au(111), the 4
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Fig. 3. TEM images and particle size distribution of graphene supported Au, Pd and Au-Pd alloy NPs catalyst.
co-adsorption energy of O2 and benzyl alcohol increased with the increase of Pd content in the other alloys catalysts of Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111). The Ph-CH2OH-assisted activation of O2
plays a major role since the other elementary steps are all obviously more energetically favorable than this step. According to catalysts tests, Au5Pd1(111) catalysts exhibits much higher catalytic activity than Au 5
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Fig. 4. Au 1 s (a) and Pd 3d XPS spectra (b) of the Au3.0/Gr (1), Au2.75Pd0.25/Gr (2), Au2.5Pd0.5/Gr (3), Au2.0Pd1.0/Gr (4), Au1.5Pd1.5/Gr (5), Au1.0Pd2.0/Gr (6), and Pd3.0/Gr (7) catalysts.
Au1Pd1(111) NPs surfaces are -0.15, -0.86, -1.00, -1.19 and -2.47 eV, respectively (Fig. 8d). The volcano-like profile with optimal catalytic activity achieved at Au5Pd1(111), which prove the medium interaction between the Au-Pd NPs and the reactants is crucial. In addition, some electrons in Pd transfer to Au leading the fast Ph-CH2OH activation by Au atom on the surface of Au5Pd1(111) (Fig. 8e). Meanwhile, when the reaction is conducted on the surface of Au5Pd1(111), the Pd-Pd site is much more difficult than the Pd-Au site due to a far higher energy barrier (1.10 eV vs 0.75 eV). On the base of above results, the higher catalytic activity of Au5Pd1(111) is due to its unique electronic structures for the adsorption and activation of reactant molecules. Alloying Au with Pd can modify the physical chemical properties of the Au-Pd alloy particles, such as surface energy, electronic state and redox potential, thus leading huge difference in reactant adsorption and activation on the surface of metal active centres [49,50]. The DFT calculation and experiental observed reaction activity trend outline what Balandin termed a volcano curve with an optimal activity at an intermediate bond strength. In the rate-determining step of benzyl alcohol aerobic oxidation to benzaldehyde, adding a small amount of Pd can promote the benzyl alcohol adsorption, resulted the increased benzyl alcohol conversion; However further increased Pd content leading high benzyl alcohol desorption energy, benzyl alcohol molecules failed to find active centers at the saturated catalyst to proceed the oxidation reaction to produce benzaldehyde, thus the conversion of benzyl alcohol decreased gradually [51,52]. The electron transfer between Au and Pd can also promote the rapid Ph-CH2OH activation by Au atom on the surface of Au-Pd alloy, which result in the high catalytic performance.
Table 1 Actual loadings of Au and Pd on different Au–Pd catalysts determined by ICPAES. Entry
Catalyst
Au loading (wt %)
Pd loading (wt %)
1 2 3 4 5 6 7
Au3.0/Gr Au2.75Pd0.25/Gr Au2.5Pd0.5/Gr Au2.0Pd1.0/Gr Au1.5Pd1.5/Gr Au1.0Pd2.0/Gr Pd3.0/Gr
2.89 2.65 2.49 2.05 1.45 0.93 –
– 0.32 0.47 0.92 1.53 1.98 2.92
3.2.2. Effect of support It is well established that support play an important role in the heterogeneous catalysis, as the physicochemical properties of support materials may greatly influence the particle size and electronic structure. In recent years, graphene has attracted extensive attention as a promising catalyst support owing to its large surface area, special structure and unique conductive properties. In this work, Au-Pd nanoparticles with various ratios were successfully decorated on graphene via a facile deposition-precipitation method, the only additive Na2CO3 can be easily washed away by deionized water, a well-defined particle size distribution can be achieved without any stabilizing agent like poly (vinyl alcohol) (PVA) or poly(vinylpyrrolidone) (PVP) during the
Fig. 5. Au L3-edge XANES spectra of Au foil (1), Au3.0/Gr (2), Au2.75Pd0.25/Gr (3), Au2.5Pd0.5/Gr (4), Au2.0Pd1.0/Gr (5), Au1.5Pd1.5/Gr (6), and Au1.0Pd2.0/Gr (7).
(111), Pd(111), Au2Pd1(111) and Au1Pd1(111), as medium and strong adsorption activity is the best. There is no doubt that the active centers on Au(111) surface are Au atoms, whereas both the Au and Pd atoms serve as catalytic sites on the Au5Pd1(111) surface. The energy of co-adsorption of O2 and benzyl alcohol on Au(111), Pd(111), Au5Pd1(111), Au2Pd1(111) and 6
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Table 2 Catalytic performance of various graphene supported Au-Pd catalysts in the synthesis of methyl benzoate from benzyl alcohol and methanola. Entry
1 2c 3 4 5 6 7 8 9 a b c
Catalyst
Blank Au2.5Pd0.5/Gr Pd3.0/Gr Au3.0/Gr Au2.75Pd0.25/Gr Au2.5Pd0.5/Gr Au2.0Pd1.0/Gr Au1.5Pd1.5/Gr Au1.0Pd2.0/Gr
Pd (mol%)
Conversion (%)
0 27 100 0 14 27 48 65 78
0.9 3.5 3.6 34.6 53.7 100.0 89.8 35.0 15.7
TOF (h−1)b
Selectivity (%) Benzaldehyde
Methyl benzoate
100.0 63.3 100.0 3.4 1.3 0.0 43.1 78.8 88.4
0.0 36.7 0.0 96.6 98.7 100.0 56.9 21.2 11.6
0.0 2.9 0.0 89.9 130.1 230.0 104.6 13.7 3.1
Reaction conditions: Catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 mL; K2CO3, 1 mmol; O2, 1 atm; reaction temperature, 25 °C; reaction time, 1 h. The turnover frequency (TOF) values were calculated based on the number of benzyl alcohol molecules converted to methyl benzoate per active site and hour. The reaction atmosphere is 1 atm Ar instead of O2.
Fig. 6. Time course for benzyl alcohol conversion and product selectivity. Reaction conditions: Au2.5Pd0.5/Graphene, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C.
catalyst preparation, which assure the majority of Au-Pd active sites being exposed [53]. The graphene support is curial to stabilize the AuPd alloy nanoparticle, the unique layer structure and the abundant oxygen-containing functional groups can provide numerous anchoring sites for the gold and palladium ions precursor, since the Au-Pd alloy particles aggregate to 38.1, 46.6 and 35.2 nm, respectively, when using other carbon materials such as active carbon, carbon nanotubes and graphite, as shown in Fig. 9. Comparatively, the optimized molar ratio of bimetallic Au2.5Pd0.5 catalysts supported on graphene exhibit much higher activity than other carbon support as shown in Fig. 9d, which suggested that the high dispersion of Au-Pd nanoparticle is one of the key factors for the excellent catalytic performance.
Fig. 7. (a) Benzyl alcohol conversion and benzaldehyde selectivity of graphene Supported Au-Pd catalyst with various Pd molar percentage in benzyl alcohol oxidation to benzaldehyde. Reaction conditions: catalyst, 50 mg; benzyl alcohol, 2 mmol; H2O, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C; 1 h. (b) Benzaldehyde conversion and Products selectivity (black: methyl benzoate, red: benzaldehyde dimethyl acetal) of graphene supported Au-Pd catalyst with various Pd molar percentage in the reaction of benzaldehyde and methanol. Reaction conditions: catalysts, 50 mg; benzaldehyde, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2.3. Effect of base Base is an important factor for the synthesis of methyl benzoate form alcohols [43,44]. K2CO3 perform better than other inorganic bases like NaOH, KOH, Na2CO3 and organic base diethylamine, as shown in Table 3, entries 1-5. Next, the base usage was examined (Table 3, entries 6–11), and it was found that 0.5 equivalent of potassium carbonate was appropriate for the reaction mixture under the optimal reaction conditions. The effect of base in separated steps of benzyl alcohol oxidation to benzaldehyde and reaction of methanol and benzaldehyde to methyl benzoate was investigated. Fig. 10a shows the benzyl alcohol conversion and benzaldehyde selectivity in the benzyl alcohol oxidation reaction without K2CO3. The product was benzaldehyde alone despite catalysts with different Au-Pd ratios; the benzyl alcohol conversion with various Pd contents showed the same volcano-shaped tendency as the
reaction conducted with K2CO3 in Fig. 7a. However, it is much lower than that with base. For instance, the highest benzyl alcohol conversion without K2CO3 for Au2.5Pd0.5/Gr is 21.3% compared with that of 70.0% with K2CO3. This result clearly prove that base play an important role in removing the α-H in the C–H bond of benzyl alcohol to form 7
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Fig. 8. Adsorption energy of benzyl alcohol (a) and benzyldehyde (b) on various adsorption models of Au(111), Au-Pd(5-1), Au-Pd(2-1), Au-Pd (1-1) and Pd(111). Au, Pd, C, O and H atoms are represented as yellow, cyan, gray, red and white spheres, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
benzaldehyde. As to the second step of methanol and benzaldehyde to methyl benzoate, almost a full benzaldehyde conversion was achieved for all different catalysts, however, the main product was benzaldehyde dimethyl acetal (> 99%). This result indicate that the hemiacteal species (formed by benzaldehyde and methanol) was unable to proceed the oxidative dehydrogenation to give the corresponding ester without base.
light excited electrons [54,55]. In other words, when the esterification reaction of benzyl alcohol and methanol is conducted under light irradiation with Au-Pd/Gr catalysts, a much better catalytic performance is supposed to be realized. However, when the reaction proceeded under the irradiation of 300 W Xe lamp with a light intensity of 1.0 W/ cm2 and wavelength of 300–800 nm, no obvious promote effect was observed. As shown in Fig. 11, benzyl alcohol conversion increased slightly and the methyl benzoate selectivity remain unchanged for the majority of the various Au-Pd ratio catalysts. For instance, benzyl conversion increased from 15.7% of Au1.0Pd2.0/Gr in the dark to 21.4% under light irradiation, while the selectivity is kept at 11.0%. Light irradiation adsorbed by Au-Pd nanoparticle can facilitate the reactant molecules which adsorbed on its surface, however, it is unable
3.2.4. Effect of light irradiation It is well known that there is a strong viable-light adsorption of Au nanoparticles due to the localized surface plasmon resonance (LSPR) effect, light irradiation can also promote the reaction process of reactants adsorbed on the non-plasmonic metal nanoparticles like Pd via 8
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Fig. 9. TEM images and size distribution of Au2.5Pd0.5/AC (a), Au2.5Pd0.5/CNTs (b), Au2.5Pd0.5/Gi (c) and catalytic performance of Au2.5Pd0.5/Gr (d). Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h.
to enhance the adsorption capacity of the reactant molecules. Thus, the light irradiation control experiment clearly proves the absorption performance of the reactant on various Au-Pd ratio alloy surface play an important role in this catalytic process, which is consistent with the DFT calculation and the volcano-like behavior of the Au-Pd alloy catalysts.
2-methyl formate and methyl cinnamate reach 96.2% and 100%, respectively (Table 4, entries 8–9). However, when benzyl alcohol reacted with other long chain aliphatic alcohol like ethanol, n-propyl alcohol and n-butyl alcohol, moderate benzyl alcohol conversion and corresponding ester selectivity was observed, which might due to the steri CeH indrance effect or the different adsorption-desorption capacity and stability on the active centers. The satisfactory ester yield can be optimized via a longer reaction time (3 h) as shown in Table 4, entries 1012. It is worth to point that the synthesis of methyl benzoate can performed at gram-scale with this catalyst (Table 4, entry 13), which hold great potential for industrial applications.
3.2.5. General applicability Various substituted benzylic alcohols were also employed in the aerobic oxidative esterification reaction with aliphatic alcohols under the optimized reaction conditions. As summarized in Table 4, apart from a relative unsatisfactory result of furfuryl alcohol (Table 4, entry 7), the substrates with different electron withdrawing or electron donating substituted groups can be converted to corresponding methyl esters efficiently (Table 4, entries 2–6). It is worth noting that hiophene2-methanol and cinnamyl alcohol also achieved excellent conversion of 98.6% and 100%, respectively and meanwhile selectivity to thiophene-
3.2.6. Catalyst recyclability The high catalyst stability is an important indicator in terms of practical application and sustainable chemistry. To test the catalytic stability and reusability of graphene supported Au-Pd alloy catalyst in ester synthesis, the catalyst was separated from the mixture after the
Table 3 Effect of base on catalytic performance of Au2.5Pd0.5/Gr in the direct esterification of benzyl alcohol and methanolaa. Entry
1 2 3 4 5 6 7 8 9 10 11
Base
NaOH (1 mmol) KOH (1 mmol) NaCO3(1 mmol) K3PO4(1 mmol) (C2H5)2NH (1 mmol) K2CO3 (0.1 mmol) K2CO3 (0.25 mmol) K2CO3 (0.5 mmol) K2CO3 (0.75 mmol) K2CO3 (1 mmol) K2CO3 (1.25 mmol)
Conversion (%)
TOF(h−1)
Selectivity (%)
99.82 88.50 72.92 78.86 3.40 55.60 79.90 80.59 98.27 100.00 100.00
Benzaldehyde
Methyl benzoate
0.26 3.11 17.39 5.67 73.90 28.23 7.43 7.34 5.04 0.00 0.00
99.74 96.89 82.61 94.33 26.10 71.77 92.57 92.66 94.96 100.00 100.00
228.99 197.22 138.55 171.09 2.04 91.78 170.12 171.75 214.63 230.00 230.00
a Reaction conditions: Catalyst Au2.5Pd0.5/Graphene, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 mL; K2CO3, 1 mmol; O2, 1 atm; reaction temperature, 25 °C; reaction time.
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Table 4 Oxidative esterification of different benzylic alcohols with aliphatic alcohols over Au2.5Pd0.5/Gr.a. Yield (%)
TOFb (h−1)
1
100
230
2
94
215
3
93
213
4
95
216
5
96
220
6
88
202
7
72
167
8
95
218
9
100
230
10c
77
59
11c
34
26
12c
30
23
13d
90
115
Entry
Substrate
Product
a Reaction conditions: catalysts, 50 mg; benzylic alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 1 h. b The turnover frequency (TOF) values were calculated based on the number of benzyl alcohol molecules converted to methyl benzoate per active site and hour. c Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; aliphatic alcohol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 3 h. d Reaction conditions: catalysts, 75 mg; benzyl alcohol, 1.08 g; methanol, 10 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 6 h.
Fig. 10. (a) Benzyl alcohol conversion and benzaldehyde selectivity of graphene supported Au-Pd catalyst with various Pd molar percentage in benzyl alcohol oxidation to benzaldehyde without K2CO3. Reaction conditions: catalyst, 50 mg; benzyl alcohol, 2 mmol; H2O, 5 ml; 1 atm of O2; 25 °C; 1 h. (b) Benzaldehyde conversion and Products selectivity (black: methyl benzoate, red: benzaldehyde dimethyl acetal) of graphene supported Au-Pd catalyst with various Pd molar percentage in the reaction of benzaldehyde and methanol without K2CO3. Reaction conditions: catalysts, 50 mg; benzaldehyde, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C; 1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reaction using filtration and used for the next run without any further treatment. As shown in Fig. 12c, the Au2.5Pd0.5/Gr catalyst exhibit excellent stability throughout the test, the benzyl alcohol conversion and methyl benzoate selectivity slightly decreased to 97.4% and 96.1% respectively after 6 cycles. A leaching test was taken to ascertain whether the metal leaching form graphene support happened, the Au2.5Pd0.5/Gr catalyst was filtered from the reaction mixture after the reaction was carried out under the optimal conditions for 10 min, leaving the remaining solution to continue the reaction under stirring at 25 °C for another 50 min. As illustrated in Fig. 12d, no further increase of methyl benzoate yield was observed in the absence of catalyst, which can exclude the homogeneous catalysis contribution. A comparison of the TEM image of the catalyst before and after reaction in Fig. 12 indicates that the particle size is only increased slightly from 7.7 to 7.9 nm, suggesting that the Au-Pd bimetallic nanoparticles are stable during the reaction. 3.2.7. Reaction mechanism A possible reaction mechanism for direct aerobic oxidative esterification of benzyl alcohol and methanol to methyl benzoate is proposed on the basis of experimental results and previous literature. As depicted in Fig. 13, the oxidation esterification reaction may proceed as follows: benzyl alcohol is first adsorbed at the surface of Au-Pd alloy nanoparticle, then selective oxidized to benzaldehyde under O2 atmosphere, the base additive (K2CO3) can take the hydrogen atom away and benefit for the cleavage of OeH bond, facilitate the formation of metal alkoxide
Fig. 11. Benzyl alcohol conversion and product selectivity over Au2.5Pd0.5/ Graphene catalysts of various Pd ratios with or without irradiation (1.0 W/ cm2). Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h.
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Fig. 12. TEM images and size distribution of Au2.5Pd0.5/Graphene (a) before reaction, and (b) after reaction; (c) Reusability of Au2.5Pd0.5/ Graphene (black: benzyl alcohol conversion, red: methyl benzoate selectivity) and (d) Leaching test of Au2.5Pd0.5/Graphene in the methyl benzoate synthesis, reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
performance. A TOF as high as 230 h−1 was achieved under 1 atm O2 at 25 °C. The excellent performance of Au2.5Pd0.25/Gr is mainly ascribed to due to its unique electronic structures for the adsorption and activation of reactant molecules. This catalytic system tolerated a variety of substituted benzyl alcohols and aliphatic alcohols and exhibited excellent catalytic stability and reusability. The protocol provides a mild and efficient route for synthesis high-value esters in terms of green and sustainable chemistry. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank Dr. S. Zhang and Prof. Y. Huang et al. at Shanghai Synchrotron Radiation Facility for their assistance of XAFS measurements and the financial support of Natural Science Foundation of China (21703276, 21773284), the Shanxi Science and Technology Department (201801D221093), and the Hundred Talents Programs of the Chinese Academy of Sciences and Shanxi Province.
Fig. 13. Proposed mechanism for direct aerobic oxidative esterification of benzyl alcohol and methanol over Au-Pd/Gr catalyst.
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Selective oxidative esterification of alcohols over Au-Pd/graphene Ruiyi Wanga, Huan Liua,b, Chaoyang Fana, Jie Gaoa, Chengmeng Chenc, Zhanfeng Zhenga,d,* a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi, 030001, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, PR China d Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China b c
ARTICLE INFO
ABSTRACT
Keywords: Graphene Au-Pd alloy Alcohols Selective oxidation DFT
Direct selective oxidative esterification of readily available alcohols under mild conditions is an attractive approach to synthesis valuable esters. Developing high performance catalyst is the key factor to the efficient esters synthesis. We report graphene supported Au-Pd alloy catalyst exhibits excellent catalytic performance in the synthesis of methyl benzoate from benzyl alcohol and methanol, a turnover frequency (TOF) of 230 h−1 and selectivity of 100% to methyl benzoate were achieved under 1 atm O2 at 25 °C, which is superior to the majority of the state-of-the-art catalysts. Experimentally observed volcano-like reactivity trends and DFT calculations prove the outstanding performance was mainly ascribed to unique electronic structures of AuPd alloy catalyst for the adsorption and activation of reactant molecules. The catalytic reaction mechanism for interpretation of the structure-activity relationships of various catalysts at molecular level was investigated. The present study could help to unravel the synergistic effect of Au-Pd catalyst and provides a mild and efficient route for synthesis highvalue esters in terms of green and sustainable chemistry.
1. Introduction Esters are a kind of important chemicals for both industrial manufacture and laboratorial basic research, they are extensively utilized in pharmaceuticals, solvents, condiments, as well as chemical intermediates in synthetic organic chemistry [1,2]. Traditionally, esters production is accomplished by the concentrated acid catalyzed reaction of alcohols with carboxylic acids or activated acid derivatives such as acyl chlorides and anhydrides, which accompanied with large amount of undesired by-products. Another alternate is transition-metal catalyzed carbonylation reaction of aryl halides with alcohol and CO; however, the high reaction temperature and CO pressure are not favorable [3]. Besides, much effort has been devoted to the direct conversion of aldehydes into the corresponding esters, generally require stoichiometric amount of heavy metals or homogenous metal complexes [4]. All above esters synthetic routes are high energy consumption and not environmentally friendly. Accordingly, the direct synthesis of esters form alcohols may be the most desirable process from both economic and environmental points of view, since alcohols are readily available, low cost and less toxic compared with aldehydes and carboxylic acids [5]. Hitherto, much effort has been devoted to developing catalytic
systems for synthesis of esters from alcohols. Homogeneous systems based on Au, Ru, Pd and other catalyst have been developed successfully with satisfied catalytic activity [6–11]. For example, Beller et al. applied Pd(OAc)2 catalyst for cross-esterification of benzyl alcohols with various aliphatic alcohols, high esters yield was achieved under mild conditions. Nevertheless, special ligands and additive are always necessary and make the products separation difficult [12]. Accordingly, alternative heterogeneous catalysts is highly desirable for practical application. It is worth to note that nitrogen-doped carbon material supported non-noble metal cobalt catalyst is highly desirable with multiple advantages including cost effectiveness, environmentally benign and facial separation; however, their catalytic performance, in terms of turnover frequency (TOF) of desired esters, is still somewhat unsatisfactory [13–15]. Supported gold catalysts have shown a remarkable catalytic activity in this reaction, Au catalysts with various support such as Al2O3, ZrO2, SiO2, MOF-5 and SBA-16 have been extensively investigated since it is well known that the nature of the support determines the catalytic efficiency of the material [16–21]. Graphene has attracted wide attention for its unique properties [22–24]. Our previous work also proved graphene based carbon materials to be excellent support for methanol oxidation esterification reactions [25,26]. Besides, it is also well-established that alloying metal
⁎ Corresponding author at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi, 030001, PR China. E-mail addresses: [email protected] (R. Wang), [email protected] (Z. Zheng).
https://doi.org/10.1016/j.mcat.2019.110687 Received 31 August 2019; Received in revised form 18 October 2019; Accepted 19 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ruiyi Wang, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110687
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with palladium can significantly enhance the catalytic activity relative to monometallic metal catalysts [27–30]. However, to the best of our knowledge, very few researches have been reported on the Pd modified Au catalyst and catalytic activity relationship for alcohol oxidation esterification reaction so far. Although various compositions of Au-Pd catalysts have been intensively studied in a wide array of reactions [31–34], the nature of the optimal Au-Pd molar ratio for a given model reaction is still unclear. In this work, graphene supported Au-Pd alloy catalysts are employed in alcohol direct aerobic oxidative esterification reaction, a remarkable increase of catalytic activity was observed when adding a small amount of palladium to gold. The effect of the Au-Pd molar ratio, support material, light irradiation and various base on the reactant conversion and product distribution were studied. Furthermore, a detailed DFT investigation on the reaction mechanism and the synergy between Au and Pd is also presented. Finally, the tolerance of various substituted alcohols, the catalyst reusability and scale-up synthesis of methyl esters were also investigated.
coupled plasma atomic emission spectroscopy (ICP-AES). To prepare the solution for elemental analysis, 3 ml of concentrated HCl and 1 mL of concentrated HNO3 was used to dissolve the catalyst sample, the solution was then diluted to 100 ml with de-ionized water. 2.3. Computational modeling and methods All first principles density functional theory (DFT) calculations were performed with the Vienna Ab Initio Simulation Package (VASP) with the projector-augmented wave (PAW) pseudo-potentials [35–37]. Perdew, Burke and Ernzerhof (PBE) parameterized generalized gradient approximation (GGA)-type functional [38,39] was applied to describe the exchange and correlation energy. The spinpolarized potentials were calculated in all situations. The kinetic energy cutoff energy for the wave basis was set to 400 eV. All the model structures were rectified until the total energy was converged less than 10−5 eV and the atomic forces were lower than 0.02 eV Å-1. Density of states (DOS) was used for quantitatively analyze the electronic structure of the system. The five catalysts was studied by carrying out the reaction on the Au (111), Pd(111), Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111) NPs surfaces [40]. The modeling of metal NPs surfaces were accomplished by four layers of metal atoms with a vacuum region of 18 Å to avoid the influence of next neighbor slabs. The adsorption species were simulated by a cubic cell of 15☓15☓15 Å3, where the Brillouin zone integration was applied for the Γ point only. The adsorption energy (Eads) was calculated by the following equation,
2. Experimental and DFT calculations 2.1. Catalyst preparation Graphene used in this work was prepared by fast pyrolysis of graphene oxide (GO) under high vacuum, which GO was fabricated by a modified Hummers’ method. The obtained GO power was placed into quartz tube, then evacuated until the pressure lower than 2.0 Pa and then heated to 200 °C with a heating rate of 30 °C/min, the fluffy graphene sample was then obtained. The bimetallic catalysts were prepared through a deposition-precipitation method. Typically, graphene sheet (200 mg) was dispersed in 200 mL deionized water, and placed in a water bath of 60 °C, a certain amount of sodium carbonate was added. The pH value was measured to be 10. After stirring for 20 min, a mixed aqueous solution of desired amount HAuCl4·4H2O and PdCl2 was added dropwise. The mixture was then stirred rigorously for another 2.5 h; the resultant solid catalysts were filtered and washed with deionized water until no Cl– was detected by the solution of AgNO3. The samples were dried at 120 °C in vacuum for 3 h and then calcined at 200 °C in a muffle furnace for 3 h. The resultant materials are denoted as AumPdn/Gr, where m and n are the weight percentage loadings of Au and Pd, respectively. Au-Pd catalysts with other supports (active carbon, carbon nanotube, and graphite) or single metal (Au and Pd) catalyst were also obtained through the similar process, denoted as AumPdn/AC, AumPdn/CNTs, AumPdn/Gi, Aum/Gr and Pdn/Gr, respectively.
Eads = Eadsorbate/substrate - Esubstrate - Eadsorbate where Eadsorbate/substrate is the total energy of the composite system, Esubstrate is the energy of the clean surface of various model systems and Eadsorbate is the energy of isolated substrate. The negative value of Eads means exothermicity, demonstrated strong adsorption of the adsorbate on the substrate. For searching the minimum energy pathway (MEP) for the benzyl alcohol oxidation, a climbing-image nudged elastic band (CINEB) method [41–43] was applied in the calculation. The activation barrier of Ea was defined as the energy difference between the transition states (TS) and the initial state (IS). The reaction energy of ΔE referred to the energy difference between the final states (FS) and the IS. 2.4. Catalyst tests and analytic procedures The catalyst evaluation was performed in a 100 mL stainless steel autoclave lined with teflon and a round quartz window. Typically, a mixture of the Aum-Pdn/Gr catalyst (50 mg, 3 wt% of total metal loading), benzyl alcohol (2 mmol), potassium carbonate (1 mmol), and 5 mL methanol were loaded into the autoclave, purged with O2 for 2 min, then the reaction mixture was kept at 25 °C for 1 h under vigorous stirring of 300 rpm. After reaction, 3 mL residual mixture were collected and filtered through a Millipore filter to remove the catalyst. The products after reaction, including aldehyde, ester and unreacted reactants, were analyzed by a Shimadzu 2014C gas chromatography (GC) with a WondaCap 5 column, the conversions and selectivity were calculated from the product formed and the reactant converted, as measured by GC. The turn‐over frequency (TOF) was calculated based on the following equation:
2.2. Catalyst characterization X-ray diffraction (XRD) measurement was performed using a desktop X-ray diffractometer (Bruker AXS D8, Germany, Cu Kα radiation source, = 0.15406 nm). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the samples were measured on a JEM-2010 system electron microscope operated at 200 kV. The sample was suspended in ethanol for ultrasonic dispersion 30 min, and then deposited on copper grids coated with carbon foil. The X-ray photoelectron spectroscopy (XPS) spectra were taken on a Thermo ESCALAB 250 instrument, with an Al Kα monochromator Xrays source (hγ =1486.6 eV). Extended X-ray absorption fine structure spectroscopy (EXAFS) measurements were tested at the beam line BL14W1 of Shanghai Synchrotron Radiation Facility, China. The storage ring energy was 3.5 GeV, and the ring current was 300 mA. The Au L3 edge data were recorded in a fluorescence mode and the EXAFS data were processed by using the IFEFFIT package. The actual content of Au and Pd was determined by inductively
TOF=
2
amount of benzyl alcohol (mol) ×Conversion (%) ×Selectivity (%) Mass of Catalyst (g) ×
{
Au loading(wt%) Mr Au
+
Pd loading(wt%) Mr Pd
} ×reaction time (h)
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the Auδ+ species [25]. For the Au-Pd alloy catalysts, an obviously negative shift of BEs was observed with the increase of palladium molar percentage. For example, compared with that of Au3.0/Gr, the Au0 4f7/2 peaks of Au2.75Pd0.25/Gr, Au2.5Pd0.5/Gr, Au2.0Pd1.0/Gr, Au1.5Pd1.5/Gr and Au1.0Pd2.0/Gr shift to lower BEs, i.e. from 84.29 eV to 84.23, 84.05, 84.07, 84.07, and 84.02 eV, respectively. Similarly, in the case of Pd 3d XPS spectrum, the peaks at 335.36 and 340.80 eV are attributed to metallic palladium species Pd0, while the peaks at 337.67 and 343.11 eV are assigned to Pd2+ species respectively. The BEs of Pd also shift to lower values after alloying gold with palladium as illustrated in Fig. 4b, compared with that of Pd3.0/Gr, the Pd2+ 3d5/2 peaks of Au1.0Pd2.0/Gr, Au1.5Pd1.5/Gr, Au2.0Pd1.0/Gr, Au2.5Pd0.5/Gr, and Au2.75Pd0.25/Gr shift to lower BEs, i.e. from 337.67 eV to 337.50, 337.53, 337.50, 337.36, and 337.24 eV, respectively. This phenomenon has been reported by various researchers in Au-Pd alloy and core-shell structures [44,45]. The negative shift BEs of both Au and Pd are due to the electron exchange between Au and Pd, i.e. a synergism between Au and Pd nanoparticles, indicating the change of electronic environment for the Au-Pd alloy. The scope of the negative shift of BEs represents the extant synergism between Au and Pd. In this work, a relative larger shift of 0.24 eV for Au 4f5/2 and 0.46 eV for the Pd 3d3/2 peak in Au2.5Pd0.5/ Gr is observed, which indicates an especially strong synergism between Au and Pd in the Au2.5Pd0.5/Gr catalysts. The actual loadings of Au and Pd on the graphene supported Au-Pd catalysts with various Au-Pd ratio were also determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). As given in Table 1, it proves that the actual loadings of Au and Pd are very close to the designed values and the loss of Au and Pd is negligible upon the aqueous rinsing during the catalyst preparation procedure.
Fig. 1. XRD patterns of graphene supported Au, Pd and Au-Pd alloy NPs catalyst.
3. Results and discussion 3.1. Catalyst characterization 3.1.1. XRD results The X-ray diffraction (XRD) patterns of various graphene supported Au-Pd catalysts are shown in Fig. 1. Graphene displayed a typical broaden peak at about 25° and 43.1° corresponding to C(002) and C(100), respectively, indicating the smaller crystalline size of graphene sheet in a layer structure. [26] Accordingly, after loaded the Au-Pd nanoparticles on the graphene sheets, the characteristic peaks at 38.4°, 43.9° and 65.1° corresponding to Au(111), Au(200) and Au(220), respectively, are clearly observed. No obvious diffraction peaks of palladium were detected, which might due to its relatively low content or high dispersion.
3.1.4. EXAFS results The charge redistribution of different graphene supported Au-Pd NPs is also verified by the XANES Au L3-edge spectra as shown in Fig. 5. The Au L3-edge white line located at 11,925 eV is corresponding to the electron transition from 2p to unoccupied 5d states. Theoretically, a more intense white line is corresponding to more unoccupied 5d states or less 5d-electron. [46–48] After normalizing the Au L3-edge jump, Au nanoparticles (Au3.0/Gr) have a weaker white line shoulder as compared to the bulk Au metal (Au foil) due to the lessening of s-p-d hybridization in small Au nanoparticles compared to bulk gold. Upon alloying with palladium, the white line intensity is decreased in the order of Au3.0/Gr＞Au2.75Pd0.25/Gr＞Au2.5Pd0.5/Gr＞Au2.0Pd1.0/Gr ＞Au1.5Pd1.5/Gr＞Au1.0Pd2.0/Gr, indicating the Au 5d states gaining more electrons from Pd with the increasing Pd content. The XANES analysis of Au is well consist with the XPS results, all suggesting the charge redistribution of Au occurs when alloying with Pd, and this feature will affect the catalytic performance of the catalysts as discussed follow. Based on the above catalyst characterizations, it can be seen that Au, Pd and Au-Pd alloy nanoparticles are uniformly dispersed on the two-dimensional laminar graphene support, charge redistribution occurs on the Au-Pd alloy surface which evidenced by the XPS and EXAFS results.
3.1.2. TEM and HRTEM results The Au-Pd nanoparticles are uniformly dispersed on the graphene support in a spherical shape as illustrated in TEM image (Fig. 2a) and HAADF-STEM image (Fig. 2c). The particle size estimated by randomly selecting 150 typical particles is in the range of 1–15 nm with an average of 7.7 nm (Fig. 2b). The high-resolution TEM (HRTEM) image in Fig. 2d illustrates the Au-Pd nanoparticle with a d-spacing of 0.226 nm, which is larger than the spacing of fcc Pd (0.221 nm) and lower than that of fcc Au (0.235 nm). EDS line scanning profile was recorded to identify the elemental distribution in Au-Pd/Gr, indicating that the Au element is concentrated in the core of the nanoparticle, since the content of Pd is relative low (0.5 wt%). All those suggested the formation of alloy structure. TEM images and particle size distribution of (a) Au3.0/Gr, (b)Au2.75Pd0.25/Gr, (c)Au2.5Pd0.5/Gr, (d)Au2.0Pd1.0/Gr, (e)Au1.5Pd1.5/ Gr, (f)Au1.0Pd2.0/Gr and (g) Pd3.0/Gr catalysts are illustrated in Fig. 3, which indicate that the morphology and size distribution of the Au-Pd alloy nanoparticles are related to the Au-Pd molar ratio. In all cases, the Au, Pd, and various Au-Pd alloy particles are highly dispersed on the graphene sheets, whereas the particle size decreases gradually from 8.9 nm to 8.2, 7.7, 7.1, 6.6, 5.9 and 5.4 nm, respectively, with the increase of Pd content.
3.2. Catalytic performance in the esters synthesis 3.2.1. Effect of Au-Pd ratio Benzyl alcohol and methanol were chosen as model molecules and molecular oxygen was used as oxidant initially. As shown in Table 1, a blank experiment was run for completeness (Table 2, entry 1), none reaction proceed without oxygen prove the esterification reaction is an aerobic oxidative process rather than dehydrogenative coupling reaction (Table 2, entry 2). The catalytic performance of the direct esterification reaction is tremendously affected by the Au/Pd molar ratio. The monometallic Pd catalyst show the lowest benzyl conversion of 3.6% and no target product methyl benzoate was detected with a 100%
3.1.3. XPS and ICP-AES results The valence composition and variation of Au and Pd were studied by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4a and b. Au 4f spectra of Au3.0/Gr can be fitted into four peaks, the two peaks located at 84.29 and 87.96 eV are attributed to Au0 4f7/2 and 4f5/2, respectively, while the other two peaks at 85.40 and 88.37 eV belong to 3
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Fig. 2. (a) TEM image, (b) Au-Pd alloy particle dimeter distribution, (c) HAADF-STEM image of Au2.5Pd0.5/Gr catalyst and (d) HRTEM image of Au-Pd alloy NPs and EDX line scan.
benzaldehyde selectivity (Table 2, entry 3). The monometallic Au catalyst, however, exhibited a moderate benzyl conversion (34.6%) and a high methyl benzoate selectivity (96.6%) (Table 2, entry 4). The performance of bimetallic Au-Pd catalysts were strongly relay on the molar ratio of palladium added, for instance, excellent catalytic performance with a sharp increase of benzyl alcohol conversion to 100% and methyl benzoate selectivity of 100% was achieved by Au2.5Pd0.5/Gr catalyst with a Pd molar percentage of 27% (Table 2, entry 6); However, further increase Pd content resulted a decrease for both benzyl alcohol conversion and methyl benzoate selectivity, when the Pd molar percentage increased to 48%, 65%, and 78%, the conversion of benzyl alcohol conversion decreased to 89.8%, 35.0% and 15.7%, respectively, meanwhile the methyl benzoate selectivity decreased to 56.9%, 21.2% and 11.6% as well, with benzaldehyde became the dominant product (Table 2, entries 7–9). A volcano-like curve of TOF values is observed as illustrated in Fig. S3. The highest TOF value of 230.0 h−1 was achieved by adding 27 mol% Pd, to the best of our knowledge, this is the highest TOF value in the reported literature. To understand the process of the alcohol direct esterification reaction, a kinetic study was conducted as illustrated in Fig. 6. Time course for benzyl alcohol conversion and products selectivity shows that the benzyl alcohol conversion reached 57.9% within 10 min, and increased with the reaction proceeded, and reached 100% at 60 min. The benzaldehyde selectivity decrease and methyl benzoate selectivity increased during the whole reaction, which indicate benzaldehyde was an intermediate during the reaction. Benzyl alcohol was first oxidized to benzaldehyde, then benzaldehyde further reacted with methanol to form the target product methyl benzoate. To reveal the remarkable effect of various Au-Pd molar ratio on the catalytic performance in the direct alcohol esterification reaction, we divided the whole reaction into 2 steps, step 1: benzyl alcohol aerobic oxidized to benzaldehyde; step 2: benzaldehyde reacted with methanol to methyl benzoate. Then we evaluated the catalytic performance of the
graphene supported various Au-Pd catalyst in the two step individually. In the benzyl alcohol oxidation reaction of step 1, as illustrated in Fig. 7a, the selectivity of benzaldehyde is 100% for all different catalysts. However, in terms of benzyl alcohol conversion, it exhibited almost the same tendency curve as the integrated benzyl alcohol and methanol esterification reaction, adding a small amount of palladium can drastically promote the benzyl alcohol conversion. The benzyl alcohol conversion increased form 5.2% of monometallic Au3.0/Gr catalyst to 70% of Au2.5Pd0.5/Gr catalyst when 27 mol% Pd added, further increase the Pd content lead a rapid decrease of benzyl alcohol conversion. In step 2 under the given reaction conditions, totally different tendency from step 1 as illustrated in Fig. 7b, monometallic Au3.0/Gr catalyst show the best catalytic performance with 100% benzaldehyde conversion and 96.2% selectivity to methyl benzoate, the by-product is benzaldehyde dimethyl acetal, which is come from nucleophilic addition reaction of one molecule benzaldehyde and two molecules methanol. With the increase of molar percentage of Pd, a linear gradually decrease for both benzaldehyde conversion and methyl benzoate selectivity was observed, meanwhile the benzaldehyde dimethyl acetal turns out to be the primary product in those cases. To further unerstand the volcano-like behaviour of the graphene supported catalyst with various Au-Pd ratio, The structure-activity relationship of Au(111), Pd(111), Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111) NPs surfaces in the selective oxidation of benzyl alcohol were further investigated by DFT calculations with geometric structure, adsorption energy, PDOS and activation energy. The geometric structure of model surfaces with various Au-Pd ratios are illustrated in Fig. 8a. From the PDOS (Fig. 8b, c) of various catalysts, we can see the difference of their electronic structure exist near Fermi level. Compared with the electronic structure of pure Pd(111) and Au(111), it is confirmed that some electrons in Pd atoms transfer to Au atoms. This suggests that the electronic effect might be the determinant affecting the catalytic activity of various catalysts. Compared with Au(111), the 4
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Fig. 3. TEM images and particle size distribution of graphene supported Au, Pd and Au-Pd alloy NPs catalyst.
co-adsorption energy of O2 and benzyl alcohol increased with the increase of Pd content in the other alloys catalysts of Au5Pd1(111), Au2Pd1(111) and Au1Pd1(111). The Ph-CH2OH-assisted activation of O2
plays a major role since the other elementary steps are all obviously more energetically favorable than this step. According to catalysts tests, Au5Pd1(111) catalysts exhibits much higher catalytic activity than Au 5
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Fig. 4. Au 1 s (a) and Pd 3d XPS spectra (b) of the Au3.0/Gr (1), Au2.75Pd0.25/Gr (2), Au2.5Pd0.5/Gr (3), Au2.0Pd1.0/Gr (4), Au1.5Pd1.5/Gr (5), Au1.0Pd2.0/Gr (6), and Pd3.0/Gr (7) catalysts.
Au1Pd1(111) NPs surfaces are -0.15, -0.86, -1.00, -1.19 and -2.47 eV, respectively (Fig. 8d). The volcano-like profile with optimal catalytic activity achieved at Au5Pd1(111), which prove the medium interaction between the Au-Pd NPs and the reactants is crucial. In addition, some electrons in Pd transfer to Au leading the fast Ph-CH2OH activation by Au atom on the surface of Au5Pd1(111) (Fig. 8e). Meanwhile, when the reaction is conducted on the surface of Au5Pd1(111), the Pd-Pd site is much more difficult than the Pd-Au site due to a far higher energy barrier (1.10 eV vs 0.75 eV). On the base of above results, the higher catalytic activity of Au5Pd1(111) is due to its unique electronic structures for the adsorption and activation of reactant molecules. Alloying Au with Pd can modify the physical chemical properties of the Au-Pd alloy particles, such as surface energy, electronic state and redox potential, thus leading huge difference in reactant adsorption and activation on the surface of metal active centres [49,50]. The DFT calculation and experiental observed reaction activity trend outline what Balandin termed a volcano curve with an optimal activity at an intermediate bond strength. In the rate-determining step of benzyl alcohol aerobic oxidation to benzaldehyde, adding a small amount of Pd can promote the benzyl alcohol adsorption, resulted the increased benzyl alcohol conversion; However further increased Pd content leading high benzyl alcohol desorption energy, benzyl alcohol molecules failed to find active centers at the saturated catalyst to proceed the oxidation reaction to produce benzaldehyde, thus the conversion of benzyl alcohol decreased gradually [51,52]. The electron transfer between Au and Pd can also promote the rapid Ph-CH2OH activation by Au atom on the surface of Au-Pd alloy, which result in the high catalytic performance.
Table 1 Actual loadings of Au and Pd on different Au–Pd catalysts determined by ICPAES. Entry
Catalyst
Au loading (wt %)
Pd loading (wt %)
1 2 3 4 5 6 7
Au3.0/Gr Au2.75Pd0.25/Gr Au2.5Pd0.5/Gr Au2.0Pd1.0/Gr Au1.5Pd1.5/Gr Au1.0Pd2.0/Gr Pd3.0/Gr
2.89 2.65 2.49 2.05 1.45 0.93 –
– 0.32 0.47 0.92 1.53 1.98 2.92
3.2.2. Effect of support It is well established that support play an important role in the heterogeneous catalysis, as the physicochemical properties of support materials may greatly influence the particle size and electronic structure. In recent years, graphene has attracted extensive attention as a promising catalyst support owing to its large surface area, special structure and unique conductive properties. In this work, Au-Pd nanoparticles with various ratios were successfully decorated on graphene via a facile deposition-precipitation method, the only additive Na2CO3 can be easily washed away by deionized water, a well-defined particle size distribution can be achieved without any stabilizing agent like poly (vinyl alcohol) (PVA) or poly(vinylpyrrolidone) (PVP) during the
Fig. 5. Au L3-edge XANES spectra of Au foil (1), Au3.0/Gr (2), Au2.75Pd0.25/Gr (3), Au2.5Pd0.5/Gr (4), Au2.0Pd1.0/Gr (5), Au1.5Pd1.5/Gr (6), and Au1.0Pd2.0/Gr (7).
(111), Pd(111), Au2Pd1(111) and Au1Pd1(111), as medium and strong adsorption activity is the best. There is no doubt that the active centers on Au(111) surface are Au atoms, whereas both the Au and Pd atoms serve as catalytic sites on the Au5Pd1(111) surface. The energy of co-adsorption of O2 and benzyl alcohol on Au(111), Pd(111), Au5Pd1(111), Au2Pd1(111) and 6
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Table 2 Catalytic performance of various graphene supported Au-Pd catalysts in the synthesis of methyl benzoate from benzyl alcohol and methanola. Entry
1 2c 3 4 5 6 7 8 9 a b c
Catalyst
Blank Au2.5Pd0.5/Gr Pd3.0/Gr Au3.0/Gr Au2.75Pd0.25/Gr Au2.5Pd0.5/Gr Au2.0Pd1.0/Gr Au1.5Pd1.5/Gr Au1.0Pd2.0/Gr
Pd (mol%)
Conversion (%)
0 27 100 0 14 27 48 65 78
0.9 3.5 3.6 34.6 53.7 100.0 89.8 35.0 15.7
TOF (h−1)b
Selectivity (%) Benzaldehyde
Methyl benzoate
100.0 63.3 100.0 3.4 1.3 0.0 43.1 78.8 88.4
0.0 36.7 0.0 96.6 98.7 100.0 56.9 21.2 11.6
0.0 2.9 0.0 89.9 130.1 230.0 104.6 13.7 3.1
Reaction conditions: Catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 mL; K2CO3, 1 mmol; O2, 1 atm; reaction temperature, 25 °C; reaction time, 1 h. The turnover frequency (TOF) values were calculated based on the number of benzyl alcohol molecules converted to methyl benzoate per active site and hour. The reaction atmosphere is 1 atm Ar instead of O2.
Fig. 6. Time course for benzyl alcohol conversion and product selectivity. Reaction conditions: Au2.5Pd0.5/Graphene, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C.
catalyst preparation, which assure the majority of Au-Pd active sites being exposed [53]. The graphene support is curial to stabilize the AuPd alloy nanoparticle, the unique layer structure and the abundant oxygen-containing functional groups can provide numerous anchoring sites for the gold and palladium ions precursor, since the Au-Pd alloy particles aggregate to 38.1, 46.6 and 35.2 nm, respectively, when using other carbon materials such as active carbon, carbon nanotubes and graphite, as shown in Fig. 9. Comparatively, the optimized molar ratio of bimetallic Au2.5Pd0.5 catalysts supported on graphene exhibit much higher activity than other carbon support as shown in Fig. 9d, which suggested that the high dispersion of Au-Pd nanoparticle is one of the key factors for the excellent catalytic performance.
Fig. 7. (a) Benzyl alcohol conversion and benzaldehyde selectivity of graphene Supported Au-Pd catalyst with various Pd molar percentage in benzyl alcohol oxidation to benzaldehyde. Reaction conditions: catalyst, 50 mg; benzyl alcohol, 2 mmol; H2O, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C; 1 h. (b) Benzaldehyde conversion and Products selectivity (black: methyl benzoate, red: benzaldehyde dimethyl acetal) of graphene supported Au-Pd catalyst with various Pd molar percentage in the reaction of benzaldehyde and methanol. Reaction conditions: catalysts, 50 mg; benzaldehyde, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2.3. Effect of base Base is an important factor for the synthesis of methyl benzoate form alcohols [43,44]. K2CO3 perform better than other inorganic bases like NaOH, KOH, Na2CO3 and organic base diethylamine, as shown in Table 3, entries 1-5. Next, the base usage was examined (Table 3, entries 6–11), and it was found that 0.5 equivalent of potassium carbonate was appropriate for the reaction mixture under the optimal reaction conditions. The effect of base in separated steps of benzyl alcohol oxidation to benzaldehyde and reaction of methanol and benzaldehyde to methyl benzoate was investigated. Fig. 10a shows the benzyl alcohol conversion and benzaldehyde selectivity in the benzyl alcohol oxidation reaction without K2CO3. The product was benzaldehyde alone despite catalysts with different Au-Pd ratios; the benzyl alcohol conversion with various Pd contents showed the same volcano-shaped tendency as the
reaction conducted with K2CO3 in Fig. 7a. However, it is much lower than that with base. For instance, the highest benzyl alcohol conversion without K2CO3 for Au2.5Pd0.5/Gr is 21.3% compared with that of 70.0% with K2CO3. This result clearly prove that base play an important role in removing the α-H in the C–H bond of benzyl alcohol to form 7
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Fig. 8. Adsorption energy of benzyl alcohol (a) and benzyldehyde (b) on various adsorption models of Au(111), Au-Pd(5-1), Au-Pd(2-1), Au-Pd (1-1) and Pd(111). Au, Pd, C, O and H atoms are represented as yellow, cyan, gray, red and white spheres, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
benzaldehyde. As to the second step of methanol and benzaldehyde to methyl benzoate, almost a full benzaldehyde conversion was achieved for all different catalysts, however, the main product was benzaldehyde dimethyl acetal (> 99%). This result indicate that the hemiacteal species (formed by benzaldehyde and methanol) was unable to proceed the oxidative dehydrogenation to give the corresponding ester without base.
light excited electrons [54,55]. In other words, when the esterification reaction of benzyl alcohol and methanol is conducted under light irradiation with Au-Pd/Gr catalysts, a much better catalytic performance is supposed to be realized. However, when the reaction proceeded under the irradiation of 300 W Xe lamp with a light intensity of 1.0 W/ cm2 and wavelength of 300–800 nm, no obvious promote effect was observed. As shown in Fig. 11, benzyl alcohol conversion increased slightly and the methyl benzoate selectivity remain unchanged for the majority of the various Au-Pd ratio catalysts. For instance, benzyl conversion increased from 15.7% of Au1.0Pd2.0/Gr in the dark to 21.4% under light irradiation, while the selectivity is kept at 11.0%. Light irradiation adsorbed by Au-Pd nanoparticle can facilitate the reactant molecules which adsorbed on its surface, however, it is unable
3.2.4. Effect of light irradiation It is well known that there is a strong viable-light adsorption of Au nanoparticles due to the localized surface plasmon resonance (LSPR) effect, light irradiation can also promote the reaction process of reactants adsorbed on the non-plasmonic metal nanoparticles like Pd via 8
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Fig. 9. TEM images and size distribution of Au2.5Pd0.5/AC (a), Au2.5Pd0.5/CNTs (b), Au2.5Pd0.5/Gi (c) and catalytic performance of Au2.5Pd0.5/Gr (d). Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h.
to enhance the adsorption capacity of the reactant molecules. Thus, the light irradiation control experiment clearly proves the absorption performance of the reactant on various Au-Pd ratio alloy surface play an important role in this catalytic process, which is consistent with the DFT calculation and the volcano-like behavior of the Au-Pd alloy catalysts.
2-methyl formate and methyl cinnamate reach 96.2% and 100%, respectively (Table 4, entries 8–9). However, when benzyl alcohol reacted with other long chain aliphatic alcohol like ethanol, n-propyl alcohol and n-butyl alcohol, moderate benzyl alcohol conversion and corresponding ester selectivity was observed, which might due to the steri CeH indrance effect or the different adsorption-desorption capacity and stability on the active centers. The satisfactory ester yield can be optimized via a longer reaction time (3 h) as shown in Table 4, entries 1012. It is worth to point that the synthesis of methyl benzoate can performed at gram-scale with this catalyst (Table 4, entry 13), which hold great potential for industrial applications.
3.2.5. General applicability Various substituted benzylic alcohols were also employed in the aerobic oxidative esterification reaction with aliphatic alcohols under the optimized reaction conditions. As summarized in Table 4, apart from a relative unsatisfactory result of furfuryl alcohol (Table 4, entry 7), the substrates with different electron withdrawing or electron donating substituted groups can be converted to corresponding methyl esters efficiently (Table 4, entries 2–6). It is worth noting that hiophene2-methanol and cinnamyl alcohol also achieved excellent conversion of 98.6% and 100%, respectively and meanwhile selectivity to thiophene-
3.2.6. Catalyst recyclability The high catalyst stability is an important indicator in terms of practical application and sustainable chemistry. To test the catalytic stability and reusability of graphene supported Au-Pd alloy catalyst in ester synthesis, the catalyst was separated from the mixture after the
Table 3 Effect of base on catalytic performance of Au2.5Pd0.5/Gr in the direct esterification of benzyl alcohol and methanolaa. Entry
1 2 3 4 5 6 7 8 9 10 11
Base
NaOH (1 mmol) KOH (1 mmol) NaCO3(1 mmol) K3PO4(1 mmol) (C2H5)2NH (1 mmol) K2CO3 (0.1 mmol) K2CO3 (0.25 mmol) K2CO3 (0.5 mmol) K2CO3 (0.75 mmol) K2CO3 (1 mmol) K2CO3 (1.25 mmol)
Conversion (%)
TOF(h−1)
Selectivity (%)
99.82 88.50 72.92 78.86 3.40 55.60 79.90 80.59 98.27 100.00 100.00
Benzaldehyde
Methyl benzoate
0.26 3.11 17.39 5.67 73.90 28.23 7.43 7.34 5.04 0.00 0.00
99.74 96.89 82.61 94.33 26.10 71.77 92.57 92.66 94.96 100.00 100.00
228.99 197.22 138.55 171.09 2.04 91.78 170.12 171.75 214.63 230.00 230.00
a Reaction conditions: Catalyst Au2.5Pd0.5/Graphene, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 mL; K2CO3, 1 mmol; O2, 1 atm; reaction temperature, 25 °C; reaction time.
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Table 4 Oxidative esterification of different benzylic alcohols with aliphatic alcohols over Au2.5Pd0.5/Gr.a. Yield (%)
TOFb (h−1)
1
100
230
2
94
215
3
93
213
4
95
216
5
96
220
6
88
202
7
72
167
8
95
218
9
100
230
10c
77
59
11c
34
26
12c
30
23
13d
90
115
Entry
Substrate
Product
a Reaction conditions: catalysts, 50 mg; benzylic alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 1 h. b The turnover frequency (TOF) values were calculated based on the number of benzyl alcohol molecules converted to methyl benzoate per active site and hour. c Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; aliphatic alcohol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 3 h. d Reaction conditions: catalysts, 75 mg; benzyl alcohol, 1.08 g; methanol, 10 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C, 6 h.
Fig. 10. (a) Benzyl alcohol conversion and benzaldehyde selectivity of graphene supported Au-Pd catalyst with various Pd molar percentage in benzyl alcohol oxidation to benzaldehyde without K2CO3. Reaction conditions: catalyst, 50 mg; benzyl alcohol, 2 mmol; H2O, 5 ml; 1 atm of O2; 25 °C; 1 h. (b) Benzaldehyde conversion and Products selectivity (black: methyl benzoate, red: benzaldehyde dimethyl acetal) of graphene supported Au-Pd catalyst with various Pd molar percentage in the reaction of benzaldehyde and methanol without K2CO3. Reaction conditions: catalysts, 50 mg; benzaldehyde, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C; 1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reaction using filtration and used for the next run without any further treatment. As shown in Fig. 12c, the Au2.5Pd0.5/Gr catalyst exhibit excellent stability throughout the test, the benzyl alcohol conversion and methyl benzoate selectivity slightly decreased to 97.4% and 96.1% respectively after 6 cycles. A leaching test was taken to ascertain whether the metal leaching form graphene support happened, the Au2.5Pd0.5/Gr catalyst was filtered from the reaction mixture after the reaction was carried out under the optimal conditions for 10 min, leaving the remaining solution to continue the reaction under stirring at 25 °C for another 50 min. As illustrated in Fig. 12d, no further increase of methyl benzoate yield was observed in the absence of catalyst, which can exclude the homogeneous catalysis contribution. A comparison of the TEM image of the catalyst before and after reaction in Fig. 12 indicates that the particle size is only increased slightly from 7.7 to 7.9 nm, suggesting that the Au-Pd bimetallic nanoparticles are stable during the reaction. 3.2.7. Reaction mechanism A possible reaction mechanism for direct aerobic oxidative esterification of benzyl alcohol and methanol to methyl benzoate is proposed on the basis of experimental results and previous literature. As depicted in Fig. 13, the oxidation esterification reaction may proceed as follows: benzyl alcohol is first adsorbed at the surface of Au-Pd alloy nanoparticle, then selective oxidized to benzaldehyde under O2 atmosphere, the base additive (K2CO3) can take the hydrogen atom away and benefit for the cleavage of OeH bond, facilitate the formation of metal alkoxide
Fig. 11. Benzyl alcohol conversion and product selectivity over Au2.5Pd0.5/ Graphene catalysts of various Pd ratios with or without irradiation (1.0 W/ cm2). Reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C;1 h.
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Fig. 12. TEM images and size distribution of Au2.5Pd0.5/Graphene (a) before reaction, and (b) after reaction; (c) Reusability of Au2.5Pd0.5/ Graphene (black: benzyl alcohol conversion, red: methyl benzoate selectivity) and (d) Leaching test of Au2.5Pd0.5/Graphene in the methyl benzoate synthesis, reaction conditions: catalysts, 50 mg; benzyl alcohol, 2 mmol; methanol, 5 ml; 1 atm of O2; K2CO3, 1 mmol; 25 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
performance. A TOF as high as 230 h−1 was achieved under 1 atm O2 at 25 °C. The excellent performance of Au2.5Pd0.25/Gr is mainly ascribed to due to its unique electronic structures for the adsorption and activation of reactant molecules. This catalytic system tolerated a variety of substituted benzyl alcohols and aliphatic alcohols and exhibited excellent catalytic stability and reusability. The protocol provides a mild and efficient route for synthesis high-value esters in terms of green and sustainable chemistry. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank Dr. S. Zhang and Prof. Y. Huang et al. at Shanghai Synchrotron Radiation Facility for their assistance of XAFS measurements and the financial support of Natural Science Foundation of China (21703276, 21773284), the Shanxi Science and Technology Department (201801D221093), and the Hundred Talents Programs of the Chinese Academy of Sciences and Shanxi Province.
Fig. 13. Proposed mechanism for direct aerobic oxidative esterification of benzyl alcohol and methanol over Au-Pd/Gr catalyst.
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