Cr hydrotalcite-like compounds and their highly efficient application in catalytic synthesis of benzoin methyl ether

Accepted Manuscript Synthesis and characterization of Cu/Cr hydrotalcite-like compounds and their highly efficient application in catalytic synthesis of benzoin methyl ether Jia-Wei Kou, Shu-Yan Cheng, Jia-Wei Wang, Xian-Mei Xie PII: DOI: Reference:

S1385-8947(17)30670-8 http://dx.doi.org/10.1016/j.cej.2017.04.125 CEJ 16879

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

9 December 2016 25 April 2017 26 April 2017

Please cite this article as: J-W. Kou, S-Y. Cheng, J-W. Wang, X-M. Xie, Synthesis and characterization of Cu/Cr hydrotalcite-like compounds and their highly efficient application in catalytic synthesis of benzoin methyl ether, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.04.125

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of Cu/Cr hydrotalcite-like compounds and their highly efficient application in catalytic synthesis of benzoin methyl ether Jia-Wei Koua, Shu-Yan Chenga,b*, Jia-Wei Wangb , and Xian-Mei Xiec a

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of

Technology, Taiyuan 030024, PR China b

c

Department of Environmental Engineering, Shanxi University, Taiyuan, 030013, PR China College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China

Abstract Benzoin methyl ether (BME) is an important photosensitizer for coatings and printing technologies, and Cu/Cr hydrotalcite-like compounds (CuCr-HTLcs) is promising to be used as pollution-free catalyst for BME synthesis. However, little is known about effect of CuCr-HTLcs structure on catalytic synthesis of BME. In this work, a series of CuCr-HTLcs with different ratios of Cu to Cr (nCu/nCr) was successfully prepared by a co-precipitation method. The structure of CuCr-HTLcs were systematically characterized. The copper and chromium in CuCr-HTLcs mainly exist in the form of Cu2+ and Cr3+, respectively. The nCu/nCr has significant effects on crystallinity, acidity, and textural properties of CuCr-HTLcs. Subsequently, catalytic performance of CuCr-HTLcs was tested for BME synthesis. We found that BME synthesis is mainly influenced by combined effects of acid-site concentration and pore size. Moderate acid-site concentration and pore size (mesopores) are the critical factors for improving catalytic activity of CuCr-HTLcs and BME selectivity. Keywords: hydrotalcite-like compound; benzaldehyde; benzoin methyl ether; catalytic synthesis

________________________________ * Corresponding author: Tel: +86-351-2646321, Fax: +86-351-2646321 E-mail address: [email protected] (S.Y. Cheng) 1

1. Introduction Benzoin methyl ether (BME) is widely used as an important photosensitizer in coatings and printing industry [1–3]. In early days, benzoin ethers were prepared by etherization of benzoin and alcohols under the catalysis of hydrogen chloride. In 1832, Wöhler and Liebig [4] first used cyanide as pre-catalyst to promote benzoin condensation of benzaldehyde instead of directly adding expensive benzoin. In 1958, Breslow [5] first found that N-heterocyclic carbenes can play the roles similar to cyanide during benzoin condensation and that N-heterocyclic carbenes are better nucleophiles and leaving groups than cyanide. In the 1970s, reaction of benzoin and tert-butyl acetate was applied to synthesis of benzoin ethers in the presence of acidic catalysts such as phosphorus trichloride, thionyl dichloride, phosgene, and trichloromethyl chloroformate [6–8]. However, the high toxicity of the catalysts mentioned above is against requirement of environmental protection and thus greatly limits their industrial application [9]. Accordingly, it is necessary to develop a new catalyst that is not only efficient but also environment-friendly. Hydrotalcite-like compounds (HTLcs) may have potential to meet the requirements, because HTLcs can be used as pollution-free acid-base catalysts that catalyze BME synthesis (Scheme 1). The HTLCs have network of double-layer structure with micropores and/or mesopores, and cations in layers and anions between layers are exchangeable [10]. Their composition is generally represented by the following formula [11]: [M2+1-x M3+x (OH)2]x+ [An-x/n] nH2O, where divalent cation (M2+) may be Mg2+, Cu2+, Ni2+, Mn2+ or Zn2+; trivalent cation (M3+) Al3+, Fe3+ or Cr3+; anion (An-) CO32–, Cl–, SO42–,OH– or NO3–. There is also crystallization water (nH2O) into interlayers. The composition of cations and preparation methods will eventually determine properties of the material [12–17]. The HTLcs are widely used as catalysts, ion exchanger, carrier, adsorbents, pigment,

photoelectric material, and precursor of composite materials [18–22]. In recent years, the HTLcs catalysts are widely applied to alkali-catalyzed reactions, catalytic redox reaction, and hydrogenation [23–27]. In the previous work, the efficient catalysis of Cu/Cr hydrotalcite-like compounds (CuCr-HTLcs) has been found for BME synthesis [28]. However, there is some lack of knowledge about the influencing factors for the catalysis of CuCr-HTLcs. In this work, CuCr-HTLcs with various mole ratios of Cu to Cr (nCu/nCr) were prepared by a co-precipitation method, and their activity for BME synthesis was evaluated. Subsequently, the CuCr-HTLcs were systematically characterized to understand the relation between the catalytic performance and structure of the CuCr-HTLcs. The aim of this work was to investigate the effect of elemental molar ratios on the properties and catalysis of CuCr-HTLcs during BME synthesis. 2. Experimental 2.1. Sample preparation CuCr-HTLcs with various nCu/nCr (1.0, 2.0, and 3.0) were synthesized by a co-precipitation method. About 10 mL Cu(NO3)2 (1.0 mol·L-1) and 20 mL Cr(NO3)3 (0.5 mol·L-1) were fully mixed to synthesize CuCr-HTLcs with nCu/nCr of 1.0 (CuCr-HTLcs-1). Similarly, about 10 ml Cu(NO3)2 and 10 ml Cr(NO3)3 were used to synthesize CuCr-HTLcs with nCu/nCr of 2.0 (CuCr-HTLcs-2), and 30 ml Cu(NO3)2 and 20 ml Cr(NO3)3 to CuCr-HTLcs with nCu/nCr of 3.0 (CuCr-HTLcs-3). Subsequently, NaOH aqueous solution (1.0 mol·L-1) were added into the mixture at a constant rate of 1.0 ml·min-1 to maintain pH at a value of 4.3 ± 0.2. The mixture was stirred for 0.5 h and then heated in a stainless steel reactor at 110 °C for 3 h. The precipitate was separated by centrifugation at 3000 rpm and washed with deionized water until pH of the filtrate reached to 7.0, and then the precipitate was dried at 80 °C for 24 h to obtain CuCr-HTLcs.

2.2. Sample characterization The powder X-ray diffraction (XRD) patterns of the samples were recorded in the 2θ range of 5–85° at a speed of 8°/min on a Rigaku D/max–2500 X-ray diffractaneter (4kV, 100mA) with Cu Kα radiation (λ = 1.54 Å). The Brunauer–Emmett–Teller (BET) surface area and Barret–Joyner–Halenda (BJH) mesopore and Horvath–Kawazoe (HK) micropore size distribution were measured by N2 adsorption-desorption on an ASAP 2020 physisorption instrument (Micromeritics Co. Ltd, USA) at 77 K after the sample was degassed below 10 µmHg for 5 h in vacuum. X-ray photoelectron spectroscopy (XPS) analyses were carried out on a physical electronics ESCALAB 250 spectrometer (Thermo Fisher Scientific Co. Ltd, USA) with a non-monochromatic Al Kα radiation (1486.6 eV) as the excitation source. High-resolution spectra were recorded in the constant pass energy mode at 20 eV in an analysis area of 720 mm diameter. Ammonia–temperature

programmed

desorption

(NH3–TPD)

was

conducted

on

a

ChemBET–3000 instrument (Quantachrome Co. Ltd, USA). The samples (0.05 g) were added into a quartz reactor and heated at 100 °C for 0.5 h. After adsorption on surface of the samples was fully saturated in ammonia at 50 °C, the temperature increased from 50 to 500 °C at a rate of 10 °C/min. A least-squares curve-fitting was performed to resolve the overlapping peaks in the obtained TPD profile Transmission electron microscopy (TEM) images were obtained on a JEM-2100F microscope field-emission transmission electron microscope (Japan Electron Optics Laboratory Co. Ltd., Japan). Scanning electron microscopy (SEM) characterizations were performed on a JSM-6010plus/LV scanning electron microscope (Japan Electron Optics Laboratory Co. Ltd., Japan) to observe the

morphology and microstructure of the samples pretreated by sputtered Au. 2.3 Catalytic performance tests The CuCr-HTLcs catalyst was pretreated at 80 °C under N2 puffing for 3 h. About 3 mL benzaldehyde, 40 mL methanol, and 0.1 g CuCr-HTLcs catalyst were added into a 500 mL three-neck flask. The mixture was vigorously stirred at atmospheric pressure and 50 °C for 3 h under N2 protection. The sampling was conducted at intervals of 10 min. The components of the products were analyzed by an Agilent 7890B gas chromatograph (Agilent Technologies Co. Ltd, USA) with an OV–101 capillary column (0.2 mm×50 m) and flame ionization detector (FID). The column was heated from 50 to 210 °C at a rate of 5 °C /min. The temperatures of injector and FID were 240 and 270 °C, respectively. The sampling volume was 0.2 µL. 3. Results and Discussion 3.1. XRD analysis As shown in Fig. 1, characteristic reflection peaks of CuCr-HTLcs occurred at 2θ values of 11, 22, 34, and 61°, which were attributed to (003), (006), (009), and (110) crystal planes of CuCr-HTLcs, respectively. For CuCr-HTLcs-2, the symmetry of the peaks was better than that for other samples. Moreover, the characteristic reflection peaks of (012) crystal plane appeared at 2θ value of 35° only for CuCr-HTLcs-2. The XRD patterns of the CuCr-HTLcs suggest that crystallization degree of the CuCr-HTLcs and dispersity of Cu and Cr are optimal when nCu/nCr reaches to 2.0. This finding was concluded in our previous paper [28]. The lattice parameters of the samples are included in Table 1. The parameter (a) was calculated as twice interplannar spacing of (110) crystal plane (d110) [11]. The parameter a relates to the distance between metal ions in the layers of HTLcs. The parameter a was nearly identical for all the samples,

suggesting that the difference in elemental molar ratios has little effect on layer structure of CuCr-HTLcs. Assuming that CuCr-HTLcs crystal were formed by close-packed hexagonal lattice, the parameter c was calculated as triple interplannar spacing of (003) crystal plane (d003) [29]. The parameter c is a measurement of distance between the layers [30]. The CuCr-HTLcs-2 show a bigger value (c = 26.69) than other two samples, indicating that the distance between layer and interlayer of CuCr-HTLcs increases and then declines with increasing nCu/nCr. This result suggests that the increase in concentration of Cu 2+ can enhance crystallization of CuCr-HTLcs to some extent. However, further addition of Cu 2+ will go against crystallization of CuCr-HTLcs when nCu/nCr reaches to 2.0, because the increase in Jahn-Teller effect of Cu 2+ will lead to deformation and distortion of brucite-type layers [31] and thus decrease the layer thickness. 3.2. The textural properties of catalysts Specific surface area, average pore diameter, and pore volume of the samples are summarized in Table 2. As shown in Table 2, specific surface area and pore volume decrease with increasing nCu/nCr, but average pore diameter increase. This result suggests that addition of Cu2+ enlarges pore and channel of CuCr-HTLcs and thus leads to decrease in specific surface area and pore volume. Fig. 2 shows the N2 adsorption-desorption curves and pore-size distribution of the CuCr-HTLcs. There existed a typical IV adsorption isotherm and an H3-type hysteresis loop for all the samples (P/P0>0.4), indicating that the CuCr-HTLcs is a flake-like material with narrow slit. The pore-size distribution shows that pore-size is mainly in the range of 0.3–0.5 (Fig. 2a), 3.2–4.1 (Fig. 2b), and 60–80 nm (Fig. 2c) for CuCr-HTLcs-1, CuCr-HTLcs-2, and CuCr-HTLcs-3, respectively. Therefore, the major pore structures of CuCr-HTLcs-1, CuCr-HTLcs-2, and CuCr-HTLcs-3 are micropores, mesopores, and macropores, respectively.

3.3. XPS analysis XPS was used to provide information about composition and chemical state of Cu and Cr on the surfaces of CuCr-HTLcs. Fig. 3 shows the core level Cu 2p spectra of CuCr-HTLcs-1, CuCr-HTLcs-2, and CuCr-HTLcs-3. The positions of the peaks at 933.4 (P1) and 954.5 eV (P2) were close to binding energy of Cu 2p3/2 (933.6 eV) and Cu 2p1/2 (953.6 eV) for normalization Cu2+ in CuO, respectively. Moreover, the area ratio of P1 to P2 (2.04–2.07) was close to the theoretical ratio of Cu 2p3/2 to Cu 2p1/2 (2.00). Therefore, the peaks at 933.4 and 954.5 eV were assigned to the Cu 2p 3/2 to Cu 2p1/2, respectively, indicating the presence of the Cu 2+ on the surface of all the samples [32]. Additionally, the two extra shake-up satellite peaks were detected at 942.6 and 962.4 eV for Cu 2p 3/2 and Cu 2p1/2, respectively, suggesting the existence of an unfilled Cu 3d9 shell and thus further confirming the existence of Cu2+ on the surface of the samples [33]. Fig. 4 shows the core level Cr 2p spectra of the samples. The samples present the peaks at 587.3 (P3) and 577.0 eV (P4), which are close to binding energy of Cr 2p3/2 (586.3 eV) and Cr 2p1/2 (576.6 eV) for normalization Cr3+ in Cr2O3, respectively [29]. Similar to the case of Cu, the area ratio of P3 to P4 (2.03–2.08) was close to the theoretical ratio. Therefore, the peaks at 587.3 and 577.0 eV were ascribe to the Cr 2p3/2 to Cr 2p 1/2, respectively, indicating the presence of the Cr3+ on the surface of the samples. 3.4. NH3-TPD analysis The NH3-TPD analysis was used to estimate acidity and concentration of acid sites in the catalysts. Fig. 5 shows NH3-TPD curves of the three samples. The peaks near 210 and 260 °C were assigned to weak and medium strong acid sites, respectively. Higher the desorption temperature of

NH3, stronger the acidity of the acid sites. The desorption temperatures of weak acid sites were both near 215 °C for CuCr-HTLcs-1 and CuCr-HTLcs-2 but near 203 °C for CuCr-HTLcs-3. Moreover, the desorption temperature of medium strong acid sites for CuCr-HTLcs-3 (280 °C) was also higher than that for CuCr-HTLcs-1 and CuCr-HTLcs-2 (249 °C). The results indicate that further addition of Cu 2+ increases acidity of the acid sites when nCu/nCr reaches to 2.0. The peak area is a measurement of acid-site concentration. According to the variation in peak area, the concentrations of the acid sites changed in the following sequences: CuCr-HTLcs-1 > CuCr-HTLcs-2 > CuCr-HTLcs-3 (weak acid sites); CuCr-HTLcs-3 > CuCr-HTLcs-2 > CuCr-HTLcs-1 (medium strong acid sites). Accordingly, the concentration of medium strong acid sites increases with growth of Cu 2+ content, and the concentration of weak acid sites decline correspondingly. This is because the increasing content of Cu2+ enhances concentration of medium strong acid sites. However, the surface area of CuCr-HTLcs particles is limited, and thus expose of more medium strong acid sites leads to decrease in content of weak acid sites on the surface of CuCr-HTLcs. 3.5. Morphology and microstructure of catalysts Fig. 6 shows the typical morphology of CuCr-HTLcs. The foliate sheets of CuCr-HTLcs was clearly observed in Fig. 6. The lamellar morphology of irregularly shaped particles was formed by the stacked sheets, indicating that successful preparation of CuCr-HTLcs with structural characteristics of hydrotalcites. Fig. 7 shows the typical structure of CuCr-HTLcs, which well coincides with the morphology exhibited in the SEM image. The dark spots resulted from Cu species, and Cr species located in the transparent areas. The TEM image indicates that the Cu and Cr species are well dispersed on the whole.

3.6. Catalytic performance Catalytic activity of all the samples were evaluated during the BME synthesis, and benzaldehyde conversion is shown in Fig. 8. In initial stage of the reaction (0–20 min), the benzaldehyde conversion followed the sequence: CuCr-HTLcs-2 > CuCr-HTLcs-3 > CuCr-HTLcs-1. The conversion of CuCr-HTLcs-1 rapidly increased and exceeded that of CuCr-HTLcs-3 in the subsequent stage (20–120 min). Interestingly, CuCr-HTLcs-2 did not have prominent advantage compared with other two samples, such as high content of acid sites and large specific surface area, but it always showed the highest benzaldehyde conversion during the synthesis. This may be ascribed to two characteristics of CuCr-HTLcs-2. On the one hand, CuCr-HTLcs-2 had moderate content of acid sites (Section 3.4). As shown in Scheme 1, acetalization of benzaldehyde with methanol (step 1) and etherification of benzoin with methanol (step 6) are catalyzed by acid sites, and formation of carbanion (step 2) is catalyzed by alkaline sites during benzoin condensation. Therefore, the BME synthesis is influenced by combined effects of acid and alkaline sites. The over high content of acid sites may restrain catalysis of alkaline sites. Accordingly, moderate content of acid sites is beneficial to the synthesis. On the other hand, CuCr-HTLcs-2 had appropriate distribution of pore diameter. Micropores of CuCr-HTLcs-1 inhibit diffusion of the reactant into particles of the catalysts and desorption of the products compared with mesopores of CuCr-HTLcs-2 (Section 3.2). Consequently, catalytic efficiency of the active sites on the inner surface of CuCr-HTLcs-1 is far lower than that of CuCr-HTLcs-2. The overabundant macropores of CuCr-HTLcs-3 lead to great decrease in specific surface area, which is against dispersion of active sites and adsorption of the reactant on the surface of the catalysts. Hence, mesopores are the optimal

for the BME synthesis. In addition, the benzaldehyde conversion of CuCr-HTLcs-1 was lower than that of CuCr-HTLcs-3 in the initial stage. This result confirms the significant hindering effect of micropores for diffusion of reactants and products in the initial stage, and thus the conversion of CuCr-HTLcs-1 rapidly increased in the later stage due to the release of substantial products delayed by the micropores. In summary, the catalytic activity of CuCr-HTLcs is controlled by combined effect of content of acid sites, specific surface area and pore-size. As shown in Table 3, BME selectivity increases and then declines with the increase in nCu/nCr and reaches to a maximum at nCu/nCr = 2.0, indicating that mesopores can promote the formation of BME. Xie et al. [34] reported that benzaldehyde conversion and BME selectivity reached to 76.31 and 99.82% respectively for MgCoAl-HTLcs catalyst at 423 K. For CuCr-HTLcs-2, the conversion and the selectivity reached to 89.56 and 99.80%, respectively. The selectivity for the CuCr-HTLcs-2 and MgCoAl-HTLcs was close to each other, but the benzaldehyde conversion for CuCr-HTLcs-2 was significantly higher than that for MgCoAl-HTLcs, indicating the high efficiency of the CuCr-HTLcs for BME synthesis. 4. Conclusion In this work, Cu/Cr hydrotalcite-like compounds (CuCr-HTLcs) with different molar ratios of Cu2+ to Cr3+ (nCu/nCr) were prepared by a co-precipitation method. The samples were characterized by XRD, NH3-TPD, XPS, TEM, SEM, and N2 adsorption-desorption. The CuCr-HTLcs have morphology characteristics of hydrotalcites, such as foliate sheets and uniform lamellar structure. The copper and chromium in CuCr-HTLcs mainly exist in the form of Cu2+ and Cr3+, respectively. Addition of excessive Cu2+ leads to decrease in distance between the layers, enlargement of pore size, reduction of specific surface area, and increase in content of acid sites. The BME synthesis is mainly

influenced by combined effects of acid-sites content and textural properties. The CuCr-HTLcs with nCu/nCr of 2.0 have moderate content of acid sites and appropriate pore size (mesopores), which are optimal structural characteristics for BME synthesis. Acknowledgments This work was funded by the Project Supported by Youth Foundation of Taiyuan University of Technology (No. 1205-04020202), National Science Foundation of China (No. 50872086), and National Key Technology R&D Program (No. 2013BAC14B04). References [1] L. Angiolini, D. Caretti, C. Carlini, E. Corelli, E.Salatelli, Polymeric photoinitiators having benzoin methylether moieties connected to the main chain through the benzyl aromatic ring and their activity for ultraviolet-curable coatings, Polym. 40 (1999) 7197–7207. [2] R. Kuhlmann, W. Schnabel, Flash photolysis investigation on primary processes of the sensitized polymerization of vinyl monomers: 2. Experiments with benzoin and benzoin derivatives, Polym. 18 (1977) 1163–1168. [3] J.P. Fouassier, D. Ruhlmann, K. Zahouily, L. Angiolini, C. Carlini, N. Lell, Polymeric photoinitiators bearing side-chain benzoin methyl ether moieties: reactivity and excited-state processes, Polym. 33 (1992) 3569–3573. [4] F. Wöhler, J. Liebig, J. Ann, Untersuchungen über das Radikal der Benzoesäure, Pharm. 3 (1832) 249–282. [5] R. Breslow, On the mechanism of thiamine action: Ⅳ Evidence from studies on model system, J. Am. Chem. Soc. 80 (1958) 3719–3726. [6] W.J. Ranus, L.D. McClure, Bisbenzoin ethers and method of producing benzoin ethers: US, 4101584, 1978. [7] H. Mueller, H.J. Traenckner, H. Habetz, Verfahren zur herstellung von benzoinaethern: German, DE2420474, 1975. [8] Y. Suenobu, T. Ike, Preparation of benzoin ether: Japan, JPS5424860, 1979. [9] Y. Boyjoo, H. Sun, J. Liu, V.K. Pareek, S. Wang, A review on photocatalysis for air treatment:

From catalyst development to reactor design, Chem. Eng. J. 310 (2017) 537–559. [10] T. Wang, Z. Cheng, B. Wang, W. Ma, The influence of vanadate in calcined Mg/Al hydrotalcite synthesis on adsorption of vanadium (V) from aqueous solution, Chem. Eng. J. 181 (2012) 182–188. [11] Q. Li, H. Yi, X. Tang, S. Zhao, B. Zhao, D. Liu, F. Gao, Preparation and characterization of Cu/Ni/Fe hydrotalcite-derived compounds as catalysts for the hydrolysis of carbon disulfide, Chem. Eng. J. 284 (2016) 103–111. [12] S.B. Ghorbel, F. Medinab, A. Ghorbela, Phosphoric acid intercalated Mg-Al hydrotalcite-like compounds for catalytic carboxylation reaction of methanol in a continuous system, Appl. Catal. A: Gen. 493 (2015) 142–148. [13] T. Yang, H. Ling, J.F. Lamonier, M. Jaroniec, J. Huang, M.J. Monteiro, J. Liu, A synthetic strategy for carbon nanospheres impregnated with highly monodispersed metal nanoparticles, NPG Asia Mater. 8 (2016) e240. [14] D. Wan, H. Liu, R. Liu, J. Qu, S. Li, J. Zhang, Adsorption of nitrate and nitrite from aqueous solution onto calcined (Mg–Al) hydrotalcite of different Mg/Al ratio, Chem. Eng. J. 195 (2012) 241–247. [15] R. Trujillano, M.J. Holgado, F. Pigazo, Preparation, physicochemical characterisation and magnetic properties of Cu-Al layered double hydroxides with CO32− and anionic surfactants with different alkyl chains in the interlayer, Phys. B 373 (2006) 267–273. [16] M. Jabłońska, A.E. Palomares, L. Chmielarz, NOx storage/reduction catalysts based on Mg/Zn/Al/Fe hydrotalcite-like materials, Chem. Eng. J. 231 (2013) 273–280. [17] N.T. Thao, L.T.K. Huyen, Catalytic oxidation of styrene over Cu-doped hydrotalcites, Chem. Eng. J. 279 (2015) 840–850. [18] Q. Li, M. Meng, F. Dai, Y. Zha, Y. Xie, T. Hu, J. Zhang, Multifunctional hydrotalcite-derived K/MnMgAlO catalysts used for soot combustion, NOx storage and simultaneous soot–NOx removal, Chem. Eng. J. 184 (2012) 106–112. [19] J. Liu, N.P. Wickramaratne, S.Z. Qiao, M. Jaroniec, Molecular-based design and emerging applications of nanoporous carbon spheres, Nat. Mater. 14 (2015) 763–774. [20] Y. Boyjoo, M. Wang, V.K. Pareek, J. Liu, M. Jaroniec, Synthesis and applications of porous non-silica metal oxide submicrospheres, Chem. Soc. Rev. 45 (2016) 6013–6047. [21] S. Zhao, H. Yi, X. Tang, C. Song, Low temperature hydrolysis of carbonyl sulfide using Zn–Al

hydrotalcite-derived catalysts, Chem. Eng. J. 226 (2013) 161–165. [22] J. Gao, X. Zhang, Y. Lu, S. Liu, J. Liu, Selective functionalization of hollow nanospheres with acid and base groups for cascade reactions, Chem. Eur. J. 21 (2015) 7403–7407. [23] K.J. You, C.T. Chang, B.J. Liaw, C.T. Huang, Y.Z. Chen, Selective hydrogenation of α, β-unsaturated aldehydes over Au/MgxAlO hydrotalcite catalysts, Appl. Catal. A: Gen. 361 (2009) 65–71. [24] A.E. Palomares, A. Uzcátegui, NOx storage/reduction catalysts based in cobalt/copper hydrotalcites, Catal. Today 137 (2008) 261–266. [25] S. Vijaikumar, A. Dhakshinamoorthy, K. Pitchumani, L-Proline anchored hydrotalcite clays: An efficient catalyst for asymmetric Michael addition, Appl. Catal. A: Gen. 340 (2008) 25–32. [26] S. Zhao, H. Yi, X. Tang, Characterization of Zn-Ni-Fe hydrotalcite-derived oxides and their application in the hydrolysis of carbonyl sulfide, Appl. Clay Sci. 56 (2012) 84–89. [27] S.B. Ghorbel, F. Medina, A. Ghorbel, M. Segarra, Phosphoric acid intercalated Mg-Al hydrotalcite-like compounds for catalytic carboxylation reaction of methanol in a continuous system, Appl. Catal. A: Gen. 493 (2015) 142–148. [28] S.Y. Cheng, X.M. Xie, J.Y. Liao, K. Yan, X. An, X. Wu, Synthesis and catalytic application of copper chroIllic-hydrotalcite-like compounds, Chinese J. Inorg. Chem. 25 (2009) 2220–2224. [29] M. Crivello, C. Pérez, J. Fernández, G. Eimer, E. Herrero, S. Casuscelli, E. Rodríguez-Castellón, Synthesis and characterization of Cr/Cu/Mg mixed oxides obtained from hydrotalcite-type compounds and their application in the dehydrogenation of isoamylic alcohol, Appl. Catal. A: Gen. 317 (2007) 11–19. [30] S. Kannan, V. Rives, H. Knözinger, High-temperature transformations of Cu-rich hydrotalcites, J. Solid State Chem. 177 (2004) 319–311. [31] Q. Li, H. Yi, X. Tang, Preparation and characterization of Cu/Ni/Fe hydrotalcite-derived compounds as catalysts for the hydrolysis of carbon disulfide, Chem. Eng. J. 284 (2016) 103–111. [32] R.T. Figueiredo, A. Martínez-Arias, M. López Granados, J.L.G. Fierro, Spectroscopic evidence of Cu–Al interactions in Cu–Zn–Al mixed oxide catalysts used in CO hydrogenation, J. Catal. 178 (1998) 146–152. [33] H. Ma, Y. Liu, Y. Fu, C. Yu, X. Dong, X. Zhang, X. Zhang, W. Xue, Improved photocatalytic activity of copper heterostructure composites (Cu–Cu2O–CuO/AC) prepared by simple carbothermal

reduction, Aust. J. Chem. 67 (2014) 749–756. [34] X.M. Xie, K. Yan, J. P. Li, Z.Z. Wang, Efficient synthesis of benzoin methyl ether catalyzed by hydrotalcite containing cobalt, Catal. Commun. 9 (2008) 1128–1131.

Table 1. Lattice parameters of all the samples Sample

a (nm)

c (nm)

d003 (nm)

d110 (nm)

CuCr-HTLcs-1

3.04

25.86

8.62

1.52

CuCr-HTLcs-2

3.08

26.69

8.90

1.54

CuCr-HTLcs-3

3.06

26.54

8.88

1.53

Note: d110—distance between (110) lattice planes; d003—distance between (003) lattice planes; a=2d 110; c=3d 003

Table 2. Pore-structure parameters of CuCr-HTLcs Sample

SBET (m2·g-1)

d (nm)

Vp (mL·g-1)

CuCr-HTLcs-1

25.6

18.81

0.073

CuCr-HTLcs-2

16.6

20.82

0.054

CuCr-HTLcs-3

6.3

28.22

0.003

Note: SBET—specific surface area; d—average pore diameter; Vp—pore volume

Table 3. Changes in benzaldehyde conversion and selectivity of benzoin methyl ether Sample

Benzaldehyde conversion (%)

Selectivity of benzoin methyl ether (%)

CuCr-HTLcs-1

84.89

98.66

CuCr-HTLcs-2

89.56

99.80

CuCr-HTLcs-3

67.99

98.90

(1)

(2)

(3)

(4)

(5) Benzoin

(6) Benzoin methyl ether

Scheme 1. Proposed synthesis mechanism of benzoin methyl ether and benzoin

Fig. 1. XRD patterns for CuCr-HTLcs-1 (a), CuCr-HTLcs-2 (b), and CuCr-HTLcs-3 (c)

Fig. 2. N2 adsorption-desorption curves for CuCr-HTLcs-1(a), CuCr-HTLcs-2 (b), and CuCr-HTLcs-3 (c)

Fig. 3. Core level Cu 2p spectra of CuCr-HTLcs-1 (a), CuCr-HTLcs-2 (b), and CuCr-HTLcs-3 (c)

Fig. 4. Core level Cr 2p spectra of CuCr-HTLcs-1 (a), CuCr-HTLcs-2 (b), and CuCr-HTLcs-3 (c)

Fig. 5. Plots of TPD signal versus temperature for CuCr-HTLcs-1 (a), CuCr-HTLcs-2 (b), and CuCr-HTLcs-3 (c)

Fig. 6. SEM images of CuCr-HTLcs-2

Fig. 7. TEM images of CuCr-HTLcs-2

Fig. 8. Conversion of benzaldehyde for all the samples

OHCu2+ Cr3+ NO3-

CuCr-HTLcs

+

Highlights




Molar ratios of Cu to Cr have significant influence on structure and properties of CuCr-HTLcs Pore size is a determinant in initial stage of the BME synthesis Moderate acid-site content is a critical factor for improving catalytic activity of CuCr-HTLcs