Zn5.6−xMgxAl2O8.6 catalysts: The role of basicity and hydrogen spillover

Journal of Catalysis 296 (2012) 1–11

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Hydrogenolysis of glycerol over Cu0.4/Zn5.6xMgxAl2O8.6 catalysts: The role of basicity and hydrogen spillover Shuixin Xia a, Renfeng Nie a, Xiuyang Lu b, Lina Wang c, Ping Chen a, Zhaoyin Hou a,⇑ a

Institute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou 310028, China Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China c Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China b

a r t i c l e

i n f o

Article history: Received 26 May 2012 Revised 31 July 2012 Accepted 14 August 2012 Available online 15 October 2012 Keywords: Hydrogenolysis Glycerol Solid base Hydrogen spillover

a b s t r a c t A series of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts with different Zn/Mg ratios were prepared and used in hydrogenolysis of glycerol in aqueous solution. This reaction proceeded more easily and efficiently over Cu0.4/Zn0.6Mg5.0Al2O8.6 than Cu0.4/Mg5.6Al2O8.6 and Cu0.4/Zn5.6Al2O8.6. The selectivity of 1,2-propanediol is higher than 98.6% in all experiments over Cu0.4/Zn0.6Mg5.0Al2O8.6, and the activity of surface Cu atoms reached 26.6 h1 at 200 °C. The structure, morphology, acidity (basicity), and adsorption ability of glycerol/hydrogen for these catalysts were characterized and discussed. It was concluded that the conversion of glycerol depended strongly on the basicity of these Cu-based catalysts and hydrogen spillover also enhanced its performance. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Biodiesel, an emerging new green fuel, is produced via transesterification of vegetable oil with methanol or ethanol, with glycerol as a major byproduct (10% in weight). In recent years, with expanding demand for biodiesel, large amounts of glycerol are available and even regarded as waste [1]. Current reported technologies in glycerol utilization include oxidation to glyceric acid [1,2], dehydration to acrolein [3,4], and hydrogenolysis to propanediols (PDOs) [5–44]. Hydrogenolysis of glycerol to PDOs has attracted more and more attention because it provides an economical and environmentally friendly alternative process in place of dwindling petroleum [28]. Significant efforts have been made and a great number of heterogeneous catalysts are reported for the production of 1,2-PDO from glycerol. These reported catalysts can be classified into two groups. One is noble-metal-based catalysts, such as Ru [14–22,26], Pt [16,17,19,23,24,26], Pd [16,19], Rh [16,19], and Ag [25]. Besides these noble metals, Cu [29–41], Ni [42,43], and Co [44] are also reported as catalysts for the hydrogenolysis of glycerol. The activities of these non-noble metals are generally lower than those of noble metals, but lower prices and higher resistance to poisoning by trace impurities make these catalysts more competitive [28]. Among these non-noble metals, it is widely accepted that Cu is

⇑ Corresponding author. Fax: +86 571 88273283. E-mail address: [email protected] (Z. Hou). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.08.007

superior to Ni and Co in terms of selectivity toward PDOs, due to its lower activity for C–C bond cleavage, and copper chromite has been selected in current industrial projects [29,30]. Cu/Al2O3 [31], Cu/SiO2 [32,33], Cu/ZnO [34–36], and Cu/MgO [37] catalysts have also been reported by several groups. But the role of the acidity and/or basicity of Cu-based catalysts in the hydrogenolysis reaction is explained in different ways in published works. Mane et al. supposed that the activity of copper chromite catalysts increased with their acidity [30], while Guo et al. said that the activities of strong solid acid HZSM-5-, HY-, and Hb- supported Cu catalysts were lower than that of Cu/Al2O3 and the formation of byproduct (acrolein) increased with their acidity [31]. Huang et al. found that the performance of Cu/SiO2 decreased with increasing sodium content [32,33], but Wang and Liu reported that the performance of Cu/ZnO catalyst was enhanced when NaOH was added to the reaction mixture [34]. Previous work in our group found that the activity of Cu/MgO, hydrotalcites supported Cu catalysts increased with their basicity [37]. Solid acid or solid base, which is better as a support (or co-catalyst) of Cu for the hydrogenolysis of glycerol, is unknown. Several groups have published their evidences, and their suggestions fit well with experimental data. But these experiments and discussion were performed separately on solid acids or solid bases; more fundamental studies are still needed. Layered double hydroxide (LDH) compounds, also known as ‘‘anionic clays,’’ have attracted more and more attention in recent years because of their potential applications as solid-base catalysts [45–48], and catalyst supports and precursors [40,41,49,50]. LDH

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S. Xia et al. / Journal of Catalysis 296 (2012) 1–11

compounds possess two-dimensional layered structures with alternating positively charged mixed metal hydroxide sheets and negatively charged interlayer anions, along with water molecules. xþ n x The empirical formula is ½M II1x M III  mH2 O, where x ðOHÞ2  ½Ax=n  MII is a divalent metal such as Mg, Zn, Ni, Co, or Cu, and MIII is a trivalent metal such as Al, Cr, Fe, or Ga. An is an anion with charge  2 n, such as NO 3 ; Cl , or CO3 , and m is the molar amount of cointercalated water [45–48]. The structure of LDH compounds can be interpreted by comparing it to a brucite [Mg(OH)2] lattice, in which magnesium cations (Mg2+ ion) occupy the centers of hydroxide octahedral. In LDH compounds, partial Mg2+ions (MII) are partially replaced with Al3+ ions or other isomorphous trivalent cations, and the resulting net positive charge in the cationic sheet   is compensated for by anions such as CO2 3 ; Cl , or OH that are present in the gallery between brucite-like sheets [37,45–48]. During controlled thermal decomposition, LDH progressively suffers the loss of physisorbed and interlayer water, decomposition of interlayer anions, and dehydroxylation of brucite-like sheets. Calcined LDHs convert to mixed oxides with high specific surface areas (>200 m2/g) and strong Lewis basic sites (O2 species). Interestingly, the LDH structure recovers easily when calcined LDHs are exposed to water, incorporating OH as charge-balancing anions. The OH groups are capable of acting as Brösted basic sites in the catalytic reactions [46,47]. These adjustable basic sites would be favorable for the hydrogenolysis of glycerol. In this work, a series of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts were prepared via thermal decomposition of Cu0.4Zn5.6xMgxAl2(OH)16CO3. The acidity (and basicity) of these catalysts was manipulated by adjusting the Zn/Mg ratio. It was found that Cu0.4/Zn0.6Mg5.0Al2O8.6 showed the highest performance, and its activity was comparable with that of Pd- or Rh-promoted Cu0.4/Mg5.6Al2O8.6 catalysts for the hydrogenolysis of glycerol in aqueous solution. The structure, morphology, acidity (basicity), and adsorption (and activation) ability of these catalysts for glycerol/hydrogen were characterized and are discussed with their catalytic performance.

2. Experimental 2.1. Catalyst preparation A series of Cu0.4Zn5.6xMgxAl2(OH)16CO3 LDHs were prepared by co-precipitation. Cu(NO3)23H2O (0.01 mol), Al(NO3)39H2O (0.05 mol), Zn(NO3)26H2O, and Mg(NO3)26H2O (AR, Sinopharm Chemical Reagent Co. Ltd., China) were dissolved together in 400 mL deionized water. The total amount of Zn(NO3)26H2O and Mg(NO3)26H2O was controlled to 0.14 mol, and this mixed solution is referred to as A. Solution B was a mixture of Na2CO3 and NaOH (AR, Sinopharm Chemical Reagent Co. Ltd., China) with concentrations of 0.25 and 0.8 mol/L, respectively. Solutions A and B were simultaneously added into a glass reactor under vigorous stirring at room temperature and of a pH value of 9.5. The slurry was transferred into a custom-designed stainless autoclave (1000 mL) equipped with a Teflon inner layer, put into an oil bath, and aged at 120 °C for 20 h under stirring, filtered off, and washed thoroughly with distilled water. The precipitate was then dried overnight at 80 °C and identified as Cu0.4Zn5.6xMgxAl2(OH)16CO3, in which x refers to the amount of Mg (0 < x < 5.6). The layered double structure of prepared Cu0.4Zn5.6xMgxAl2(OH)16CO3 samples was confirmed by X-ray diffraction (XRD) (see Fig. 1). Second, these LDHs were heated slowly (2 °C/min) to 400 °C and calcined at 400 °C in a stationary air for 4 h. The calcined product catalysts were identified as Cu0.4Zn5.6xMgxAl2O9. Last, the calcined catalysts were further reduced in hydrogen flow (30 mL/min) at 350 °C for 1 h before the hydrogenolysis reaction, these reduced catalysts were identified as Cu0.4/Zn5.6xMgxAl2O8.6.

Fig. 1. XRD patterns of fresh Cu0.4Zn5.6xMgxAl2(OH)16CO3 LDHs. (a) Cu0.4Zn5.6Al2(OH)16CO3; (b) Cu0.4Zn3.7Mg1.9Al2(OH)16CO3; (c) Cu0.4Zn2.8Mg2.8Al2(OH)16CO3; (d) Cu0.4Zn1.9Mg3.7Al2(OH)16CO3; (e) Cu0.4Zn0.6Mg5.0Al2(OH)16CO3; (f) Cu0.4Zn0.3Mg5.3Al2(OH)16CO3; and (g) Cu0.4Mg5.6Al2(OH)16CO3.

2.2. Hydrogenolysis of glycerol Hydrogenolysis of glycerol was performed in a custom-designed stainless steel autoclave equipped with a Teflon inner layer (68 mL). Before reaction, the catalyst was pretreated in a flow of H2 at 350 °C for 1 h. The reaction mixture included 8.0 g aqueous solution of glycerol (75 wt.%) and 1.0 g reduced catalyst. Then the autoclave was purged with H2 to 2.0 MPa, placed in an oil bath preheated to the required temperature and maintained at that temperature under vigorous stirring (MAG-NEO, RV-06M, Japan). After reaction, the reactor was cooled to room temperature, and the vapor phase was collected by a gas bag and analyzed with a gas chromatograph (Shimadzu, 8A) with a thermal conductivity detector (TCD). Solid catalyst powder in liquid phase was separated via centrifugation and washed with deionized water (5  5 mL). The eluent and reaction mixture were collected and charged into a 50-mL volumetric flask and analyzed using a flame ionization detector gas chromatograph (Shimadzu, 14B) equipped with a 30-m capillary column (DB-WAX 52 CB, USA). All products detected in the liquid were verified by a gas chromatography and mass spectrometry system (GC–MS, Agilent 6890) and quantified via external calibration. The selectivity of the product was calculated on a carbon basis. 2.3. Catalyst characterization The elemental compositions of Cu, Zn, Mg, and Al in prepared Cu0.4Zn5.6xMgxAl2(OH)16CO3 were detected on inductively coupled plasma atomic emission spectroscopy equipment (ICP, plasma-Spec-II spectrometer, Perkin–Elmer Optima 2000 instrument). N2 adsorption was measured at its normal boiling point using an ASAP 2010 analyzer (Micromeritics) after pretreatment at 250 °C for 4 h under vacuum. Surface area and pore size distribution were calculated using the adsorption isotherms. XRD patterns of Cu0.4Zn5.6xMgxAl2(OH)16CO3 samples were detected at room temperature on a Rigaku D/WAX-2500 diffractometer using CuKa radiation (k = 1.5406 Å) with a 2h step of 0.02°. Before XRD analysis of the reduced Cu0.4/Zn5.6xMgxAl2O8.6 catalysts, they were pretreated in hydrogen flow (30 mL/min) at 350 °C for 1 h, cooled to room temperature in Ar flow (30 mL/min), and poured into ethanol under Ar. Scanning electron microscopic (SEM) images were detected on a Leo Evo Series SEM (VP 1430, Germany). Samples were coated with gold using sputter coating to avoid charging. Analysis was carried out at an accelerating voltage of 15 kV.

S. Xia et al. / Journal of Catalysis 296 (2012) 1–11

Temperature-programmed reduction (TPR) studies were carried out in a quartz reactor. Samples were first pretreated at 400 °C for 1 h under N2 at a flow rate of 30 mL/min and cooled to room temperature. A reduction agent (10% H2/N2 mixture, 30 mL/min) was shifted and the reactor was heated to 450 °C at a ramp of 10 °C/ min. Effluent gas was dried by powder KOH and the consumption of hydrogen was recorded by TCD. The amount of reduced Cu in these samples was calibrated with pure CuO (AR, Sinopharm Chemical Reagent Co., Ltd., China) of known amount. It was confirmed that all Cu species in these samples could be reduced in the temperature range from 50 to 450 °C, and the reduction of Zn species and MgO could be excluded. The number of surface metallic copper atoms was determined by N2O oxidation and followed H2 titration using the procedure described by Van Der Grift et al. [51]. Catalysts were first reduced in the procedure described in the TPR experiment in a 10% H2/N2 mixture at a flow rate of 30 mL/min until 450 °C. The amount of hydrogen consumption in the first TPR was denoted as X. Then the reactor was purged with He to 50 °C. A flow of 20% N2O/N2 (30 mL/min) was used to oxidize surface copper atoms to Cu2O at 50 °C for 0.5 h. The reactor was flushed with He to remove the oxidant. Finally, another TPR experiment was performed in 10% H2/N2 at a flow rate of 30 mL/min. Hydrogen consumption in the second TPR was denoted as Y. The dispersion and the area of surface Cu were calculated according to equations reported by Van Der Grift et al. [51], which are shown below: Reduction of all copper atoms:

CuO þ H2 ! Cu þ H2 O; hydrogen consumption ¼ X Reduction of surface copper atoms only:

Cu2 O þ H2 ! 2Cu þ H2 O; hydrogen consumption ¼ Y The dispersion and the area of surface Cu were calculated as

D ¼ ð2  Y=XÞ  100% S ¼ 2  Y  Nav =ðX  M Cu  1:4  1019 Þ ¼ 1353  Y=Xðm2  Cu=g  CuÞ: In these equations, Nav is Avogadro’s constant, MCu is the relative atomic mass of copper (63.46 g/mol), and 1.4  1019 is the number of copper atom of per square meter, because the average surface area of copper atom is assigned as 7.11  102 nm2. Average volume–surface diameter can be expressed as a function:

dv:s ¼ 6=ðS  qCu Þ  0:5  X=YðnmÞ:

qCu in this equation is the density of copper (8.92 g/cm3). The acidity, basicity, and H2 activation activity of the reduced catalysts were found via temperature-programmed desorption (TPD) of NH3, CO2, and H2 (NH3 TPD, CO2 TPD, and H2 TPD). The sample was first reduced at 350 °C in a H2 flow of 30 mL/min for 1 h, purged in purified Ar, and further treated at 450 °C for 0.5 h in Ar. Then the reactor was cooled to 50 °C in Ar, exposed to 20% NH3/Ar for 30 min, and purged with by Ar for 5 h at 50 °C in order to eliminate the physically adsorbed NH3. NH3-TPD was conducted by ramping to 550 °C at 10 °C/min and NH3 (m/e = 16) in effluent was detected and recorded as a function of temperature on a quadrupole mass spectrometer (OmniStarTM, GSD301, Switzerland). CO2 TPD and H2 TPD were carried out by the same procedure. Raman spectra of adsorbed glycerol were obtained with an ultraviolet high-resolution (UV-HR) Raman spectrometer. The exciting wavelength from a He–Gd laser was 514 nm. Spectra consisted of two accumulations of 30 s with a resolution of 2 cm1. Adsorption of glycerol was performed in a three-necked flask (100 mL). A 50 mL aqueous solution of glycerol (0.1 g/mL) and

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0.5 g of catalyst were added into the flask and stirred 1 h at 60 °C in N2 flow. Then the catalyst was filtered off and washed with water 10 times to remove the physically adsorbed glycerol before Raman detection. 3. Results 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of Cu0.4Zn5.6xMgxAl2(OH)16CO3 dried at 80 °C. Peaks at 11.7, 23.6, 35.0, 39.7, 47.1, 60.9, and 62.4° were observed in these catalysts, which were assigned to the (0 0 3), (0 0 6), (0 0 9), (1 0 5), (1 0 8), (1 1 0), and (1 1 3) diffractions of LDHs [45–48]. However, the intensity of characteristic reflections decreased with increasing amounts of Mg, which could be ascribed to decreased thickness of solid lamellar. This suggestion was confirmed by the following SEM images. ZnO phase (JCPDS 65-3411) was detected in Cu0.4Zn5.6Al2(OH)16CO3, Cu0.4Zn3.7Mg1.9Al2(OH)16CO3, and Cu0.4Zn2.8Mg2.8Al2(OH)16CO3. These results indicated that partial Zn2+ cannot enter the brucite-like layers in these three samples because the ionic radius of Zn2+ (0.74 Å) is larger than that of Mg2+ (0.72 Å) [45–48]. Fig. 2 shows the SEM images of fresh Cu0.4Zn5.6xMgxAl2 (OH)16CO3. A Well-ordered layered structure of solid lamellar can be identified under high resolution. The average thickness of the solid lamellar in Cu0.4Zn5.6Al2(OH)16CO3 is around 52 nm (Fig. 2A). The thickness of the lamellar decreased to 34 nm in Cu0.4Zn3.7Mg1.9Al2(OH)16CO3, 30 nm in Cu0.4Zn2.8Mg2.8Al2(OH)16CO3, and 24 nm in Cu0.4Zn1.9Mg3.7Al2(OH)16CO3 and further decreased to 12 nm in Cu0.4Zn0.6Mg5.0Al2(OH)16CO3 and 13 nm in Cu0.4Zn0.3Mg5.3Al2(OH)16CO3. On the other hand, the average thickness of the solid lamellar of Cu0.4Mg5.6Al2(OH)16CO3 increased slightly to 20 nm. These results fit well with XRD analysis, which could be attributed to the ionic radii of Zn2+ being larger than that of Cu2+ and Mg2+. The ordered arrangement of these layered solid lamellars brought the interlayer space and slit pores between lamellars, which can be regarded as the source of mesopores. These mesopores could enhance the access of reactants to active sites. After calcination at 400°C, the characteristic reflections of LDH disappeared. Separated ZnO (JCPDS 65-3411) was detected in Cu0.4Zn5.6Al2O9, Cu0.4Zn3.7Mg1.9Al2O9, and Cu0.4Zn2.8Mg2.8Al2O9 (see Fig. 3). The diffraction peaks of ZnO became faint with decreasing Zn/Mg ratio. No diffraction peak of ZnO, MgO, or CuO was detected in Cu0.4Zn1.9Mg3.7Al2O9, Cu0.4Zn0.6Mg5.0Al2O9, Cu0.4Zn0.3Mg5.3Al2O9, or Cu0.4Mg5.6Al2O9. These results indicate that MgO was present in an XRD amorphous state and ZnO (at high concentration) was crystalline. Cu, Zn, and Mg were highly dispersed in Cu0.4Zn1.9Mg3.7Al2O9, Cu0.4Zn0.6Mg5.0Al2O9, Cu0.4Zn0.3Mg5.3Al2O9, and Cu0.4Mg5.6Al2O9. XRD spectra of reduced Cu0.4/Zn5.6xMgxAl2O8.6 catalysts are shown in Fig. 4. Apart from the obvious diffraction peaks of ZnO, a small peak at 43.7° was detected in Cu0.4/Zn5.6Al2O8.6 and Cu0.4/ Zn2.8Mg2.8Al2O8.6, which could be assigned to the formation of separated Cu (JCPDS 04-0836). This peak, along with another peak at 63.1°, became obvious in Cu0.4/Zn0.6Mg5.0Al2O8.6, Cu0.4/Zn0.3 Mg5.3Al2O8.6, and Cu0.4/Mg5.6Al2O8.6 because of the formation of MgO (JCPDS 65-0476). These results confirmed that magnesium and zinc still existed in the oxidation state and the reduction of Zn species and MgO could be excluded. 3.2. N2 adsorption Nitrogen adsorption–desorption isotherms of calcined Cu0.4Zn5.6xMgxAl2O9 are shown in Fig. 5. All of them are type IV patterns according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The closure points of hysteresis

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Fig. 2. SEM patterns of fresh Cu0.4Zn5.6xMgxAl2(OH)16CO3 LDHs. (A) Cu0.4Zn5.6Al2(OH)16CO3; (B) Cu0.4Zn3.7Mg1.9Al2(OH)16CO3; (C) Cu0.4Zn2.8Mg2.8Al2(OH)16CO3; (D) Cu0.4Zn1.9Mg3.7Al2(OH)16CO3; (E) Cu0.4Zn0.6Mg5.0Al2(OH)16CO3; (F) Cu0.4Zn0.3Mg5.3Al2(OH)16CO3; and (G) Cu0.4Mg5.6Al2(OH)16CO3.

loops of Cu0.4Mg5.6Al2O9 are located at a relative pressure of 0.6. However, the hysteresis loops of Cu0.4Zn0.6Mg5.0Al2O9 and Cu0.4Zn0.3Mg5.3Al2O9 closed at 0.5, which suggested that the pore diameter decreased with the addition of Zn. The calculated surface area of Cu0.4Mg5.6Al2O9 was 204.6 m2/g, decreased continuously to 185.1 (of Cu0.4Zn0.3Mg5.3Al2O9), 170.2 (of Cu0.4Zn0.6Mg5.0Al2O9), and 77.4 m2/g (of Cu0.4Zn1.9Mg3.7Al2O9), and then changed slightly. The lower surface area of Cu0.4Zn5.6Al2O9 could be attributed to its

thicker solid lamellar in Cu0.4Zn5.6Al2(OH)16CO3 (Figs. 1a and 2A) and separated ZnO formed in calcined Cu0.4Zn5.6Al2O9 (Fig. 3a). At the same time, it was found that the adsorption volumes of N2 of Cu0.4Zn5.6Al2O9, Cu0.4Zn2.8Mg2.8Al2O9, Cu0.4Zn0.6Mg5.0Al2O9, Cu0.4Zn0.3Mg5.3Al2O9, and Cu0.4Mg5.6Al2O9 at low pressure (p/po < 0.057) are 15.4, 7.7, 15.8, 19.7, and 30.5 cm3/g, respectively. These data indicated that these lamellar-structured catalysts would be micropore-free materials.

S. Xia et al. / Journal of Catalysis 296 (2012) 1–11

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3.3. H2 temperature programmed reduction

Fig. 3. XRD patterns of calcined Cu0.4Zn5.6xMgxAl2O9. (a) Cu0.4Zn5.6Al2O9; (b) Cu0.4Zn3.7Mg1.9Al2O9; (c) Cu0.4Zn2.8Mg2.8Al2O9; (d) Cu0.4Zn1.9Mg3.7Al2O9; (e) Cu0.4Zn0.6Mg5.0Al2O9; (f) Cu0.4Zn0.3Mg5.3Al2O9; and (g) Cu0.4Mg5.6Al2O9.

Fig. 6 shows the H2 TPR profiles of calcined Cu0.4Zn5.6xMgx Al2O9. Only one broader peak from 250 to 340 °C was detected in Cu0.4Mg5.6Al2O9. This hydrogen consumption could be assigned to the reduction of Cu2+ ions in the lamellar structure of Cu0.4Mg5.6Al2O9 [37,40]. It was found that this hydrogen consumption peak became sharp and narrow with increasing Zn in Cu0.4Zn5.6x MgxAl2O9. A shoulder peak at around 210–260 °C was detected in Cu0.4Zn1.9Mg3.7Al2O9 (see Fig. 6d). This peak became more pronounced and converted into a separated peak in Cu0.4Zn2.8Mg2.8 Al2O9, Cu0.4Zn3.7Mg1.9Al2O9, and Cu0.4Zn5.6Al2O9. These results indicated that another reducible CuO species formed with the addition of Zn, and this species would contact mainly with ZnO [52]. That is, ZnO enhanced the reducibility of CuO and this improvement would be ascribed to hydrogen spillover between Cu and ZnO [52]. Copper dispersion and average diameter detected in N2O oxidation and following H2 titration are summarized in Table 1. The dispersion and average diameter of copper in reduced Cu0.4/ Mg5.6Al2O8.6 (calcined at 400 °C) are 43.2% and 2.3 nm. The addition of a small amount of Zn improved the dispersion of copper slightly to 49.4% (in Cu0.4/Zn0.6Mg5.0Al2O8.6), but the addition of a large amount of Zn depressed the dispersion of copper. The lower dispersion of Cu in Cu0.4/Zn5.6Al2O8.6 might be attributed to its lower surface area. 3.4. H2 temperature-programmed reduction on reduced catalysts

Fig. 4. XRD patterns of the reduced Cu0.4/Zn5.6xMgxAl2O8.6. (a) Cu0.4/Zn5.6Al2O8.6; (b) Cu0.4/Zn2.8Mg2.8Al2O8.6; (c) Cu0.4/Zn0.6Mg5.0Al2O8.6; (d) Cu0.4/Zn0.3Mg5.3Al2O8.6; and (e) Cu0.4/Mg5.6Al2O8.6.

Fig. 5. N2 adsorption isotherms at –196 °C. (a) Cu0.4Zn5.6Al2O9; (b) Cu0.4Zn2.8Mg2.8Al2O9; (c) Cu0.4Zn0.6Mg5.0Al2O9; (d) Cu0.4Zn0.3Mg5.3Al2O9; and (e) Cu0.4Mg5.6Al2O9.

Hydrogen activation and its spillover ability over Cu0.4/ Zn5.6xMgxAl2O8.6 catalysts were detected by H2 TPD and are shown in Fig. 7. No desorption peak was observed in the whole T range over Cu0.4/Mg5.6Al2O8.6. However, obvious hydrogen desorption peaks were observed when Zn was added, and these peaks increased continuously with increasing amount of Zn. Two separated peaks in the range of 80–170 and 240–360 °C were detected on Cu0.4/Zn5.6Al2O8.6 catalyst. These two peaks could be ascribed to the desorbed hydrogen that adsorbed onto ZnO and the desorbed hydrogen from split H–H on the surface of Cu particles [53,54], respectively. Another desorption peak in the range 170–280 °C was detected on Cu0.4/Zn1.9Mg3.7Al2O8.6, Cu0.4/Zn0.6Mg5.0Al2O8.6, and Cu0.4/Zn0.3Mg5.3Al2O8.6. This peak was assigned to the desorbed hydrogen from split H–H on the surface of mixed Zn– Mg–Al–O oxides with defects [55,56].

Fig. 6. H2 TPR profiles of calcined samples. (a) Cu0.4Zn5.6Al2O9; (b) Cu0.4Zn3.7Mg1.9Al2O9; (c) Cu0.4Zn2.8Mg2.8Al2O9; (d) Cu0.4Zn1.9Mg3.7Al2O9; (e) Cu0.4Zn0.6Mg5.0Al2O9; (f) Cu0.4Zn0.3Mg5.3Al2O9; and (g) Cu0.4Mg5.6Al2O9.

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Table 1 The structure of calcined catalysts and Cu dispersion in the reduced catalysts. Catalysts

Structurea 2

Cu0.4Mg5.6Al2O9 Cu0.4Zn0.3Mg5.3Al2O9 Cu0.4Zn0.6Mg5.0Al2O9 Cu0.4Zn1.9Mg3.7Al2O9 Cu0.4Zn2.8Mg2.8Al2O9 Cu0.4Zn3.7Mg1.9Al2O9 Cu0.4Zn5.6Al2O9 a b c

Cu dispersionb 3

SBET (m /g)

Pore size (nm)

Pore volume (cm /g)

Dispersionb (%)c

Cu particle sizeb (nm)

204.6 185.1 170.2 77.4 73.6 74.4 76.0

16.8 13.9 12.6 n.d. n.d. n.d. 5.1

0.86 0.64 0.54 n.d. n.d. n.d. 0.10

43.2 44.4 49.4 49.1 43.4 41.7 28.7

2.31 2.25 2.02 2.04 2.30 2.40 3.48

(±6%) (±6%) (±6%) (±6%) (±6%) (±6%) (±6%)

All of the catalysts are calcined at 400 °C, 4 h. Calculated from N2O chemical adsorption after the catalysts were reduced in H2 at 400 °C. Data in parentheses are estimated standard deviations that were calculated on the deviation of H2 consumption in repeated TPR experiments.

Fig. 7. H2 TPD profile of the reduced catalysts. (a) Cu0.4/Zn5.6Al2O8.6; (b) Cu0.4/ Zn3.7Mg1.9Al2O8.6; (c) Cu0.4/Zn2.8Mg2.8Al2O8.6; (d) Cu0.4/Zn1.9Mg3.7Al2O8.6; (e) Cu0.4/ Zn0.6Mg5.0Al2O8.6; (f) Cu0.4/Zn0.3Mg5.3Al2O8.6; and (g) Cu0.4/Mg5.6Al2O8.6.

Hydrogen spillover on the surface of Cu/ZnO catalysts was thoroughly reviewed by Prins [56]. The occurrence of hydrogen spillover over Cu/ZnO has been demonstrated. It was concluded that the H atoms on ZnO were formed by spillover from Cu to ZnO surface; such spillover could increase the hydrogenation capability of a catalyst. But hydrogen spillover from a metal surface to the surface of a defect-free support such as Al2O3 and MgO is energetically impossible [56]. These conclusions could make the spillover ability of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts shown in Fig. 7 clearly understood. These results indicate that ZnO enhanced the adsorption and activation of H2. Hydrogen spillover phenomena driven by added ZnO bring about the formation of active H–H species on the surface of Cu0.4/Zn1.9Mg3.7Al2O8.6, Cu0.4/Zn0.6Mg5.0Al2O8.6, and Cu0.4/Zn0.3Mg5.3Al2O8.6. These enhanced H2 activation ability would improve its activity for the hydrogenolysis of glycerol. 3.5. The acidity and basicity of reduced catalysts The acidity of reduced Cu0.4/Zn5.6xMgxAl2O8.6 catalysts was detected via NH3 TPD and is shown in Fig. 8a. Only one peak in the range 140–240 °C was observed in Cu0.4/Mg5.6Al2O8.6, and this peak could be assigned to the desorbed NH3 that adsorbed weakly on the surface of Cu [57]. A shoulder peak in the range 240–340 °C was detected when Zn was added in Cu0.4/Zn5.6xMgxAl2O8.6, and it increased continuously with increasing Zn/Mg molar ratio. This peak could be ascribed to the desorbed NH3 that adsorbed strongly onto the acid sites of Zn–Al–O support [58–60]. These results

indicated that the acidity of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts increased with increasing Zn/Mg molar ratio. CO2 TPD profiles of reduced Cu0.4/Zn5.6xMgxAl2O8.6 catalysts are shown in Fig. 8b. It was found that the total amount of desorbed CO2 increased with decreasing Zn/Mg molar ratio. The desorption profile of CO2 in effluent can be deconvoluted into a main peak (at 120 °C) and a shoulder peak. The first peak could be assigned to the desorbed CO2 that adsorbed onto the surface of catalysts and the second peak could be ascribed to the desorbed CO2 that adsorbed onto the Lewis basic sites of MgO [23,37,45– 48,61]. The intensity of the shoulder peak enhanced with increasing amount of Mg. These results indicated that the acidity (and basicity) of these catalysts could be manipulated by adjusting the molar ratio of Zn/Mg in Cu0.4/Zn5.6xMgxAl2O8.6. The acidity of Cu0.4/Zn5.6Al2O8.6 was stronger than that of Cu0.4/Mg5.6Al2O8.6, while the amount of basic sites in Cu0.4/Mg5.6Al2O8.6 was several times higher than that of Cu0.4/Zn5.6Al2O8.6. The acidity/basicity of Zn–Mg–Al spinels was discussed in detail by Vargas-Tah et al. [60]. It was concluded that (1) mainly Lewis acid sites formed in Zn–Mg–Al spinels, (2) these acid sites are due to those aluminum sites, and (3) they increase with the Zn content [60]. The basic sites in Cu0.4/Zn5.6xMgxAl2O8.6 catalysts originated from the O2 species that connected with Mg2+ [45–50]. 3.6. Raman spectra of adsorbed glycerol Fig. 9 shows the Raman spectra of adsorbed glycerol on Cu0.4/ Zn5.6Al2O8.6 and Cu0.4/Zn0.6Mg5.0Al2O8.6 in 800–1200 and 2500– 4000 cm1. The peaks at 1053 and 1086 cm1 come from the asymmetric stretch vibrations of primary C–OH groups and of secondary C–OH groups, respectively [62–64]. The peaks at 2882 and 2942 cm1 are assigned to the symmetric and asymmetric vibrations of the –CH2 group in glycerol. And the broad Raman peaks from 3150 to 3770 cm1 region are attributed to the symmetric and asymmetric vibrations of CO–H groups in glycerol. Raman peaks of both primary and secondary C–OH groups of glycerol adsorbed onto Cu0.4/Zn0.6Mg5.0Al2O8.6 shifted to red compared with those on Cu0.4/Zn5.6Al2O8.6. However, the broad Raman peaks from the 3150 to 3770 cm1 region shifted to blue, and the total amount of adsorbed glycerol on Cu0.4/Zn0.6Mg5.0Al2O8.6 is higher than that on Cu0.4/Zn5.6Al2O8.6. These results indicated that the primary C–OH group in glycerol was efficiently activated on the surface of Cu0.4/Zn0.6Mg5.0Al2O8.6 [62–64]. 3.7. Hydrogenolysis of glycerol Table 2 summarizes the activity of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts for hydrogenolysis of glycerol at 180 °C in aqueous solution. At low hydrogen pressure (2.0 MPa) and short time (10 h), the

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Fig. 8. NH3 TPD (A) and CO2 TPD (B) profiles of the reduced catalysts: (a) Cu0.4/Zn5.6Al2O8.6; (b) Cu0.4/Zn3.7Mg1.9Al2O8.6; (c) Cu0.4/Zn2.8Mg2.8Al2O8.6; (d) Cu0.4/Zn1.9Mg3.7Al2O8.6; (e) Cu0.4/Zn0.6Mg5.0Al2O8.6; (f) Cu0.4/Zn0.3Mg5.3Al2O8.6; and (g) Cu0.4/Mg5.6Al2O8.6.

indicated that the addition of small amount of Zn enhanced the activity of Cu and the conversion of glycerol over Cu0.4/Zn5.6xMgxAl2O8.6 catalysts depends strongly on the basicity of the catalysts. 3.8. Hydrogenolysis of glycerol with different amounts of catalyst Table 3 summarizes the conversion of glycerol over different amounts of added Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst for the hydrogenolysis of glycerol at 180 °C and 2.0 MPa H2. The conversion of glycerol increased from 22.6% to 78.2% when the amount of catalyst increased from 0.2 to 1.0 g. The selectivity of 1,2-PDO remained higher than 99%. The highest activity of surface Cu atoms reached 14.1 h1. These results confirmed that glycerol hydrogenolysis could occur on Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst in a wide range of catalyst amounts.

Fig. 9. Raman spectra of adsorbed glycerol (a) on Cu0.4/Zn5.6Al2O8.6 and (b) on Cu0.4/ Zn0.6Mg5.0Al2O8.6.

conversion of glycerol and the selectivity to 1,2-PDO on Cu0.4/ Mg5.6Al2O8.6 were 56.7% and 97.1%, respectively. It is interesting to note that the conversion of glycerol increased first with the addition of Zn. The highest conversion of glycerol (78.2%), with a 99.3% selectivity of 1,2-PDO, was detected on Cu0.4/Zn0.6Mg5.0 Al2O8.6. However, the conversion of glycerol decreased when larger amounts of Zn were added. The conversion of glycerol on Cu0.4/ Zn5.6Al2O8.6 was only 14.1%. The activity of surface Cu atoms was also calculated ((mol of converted glycerol)/(mol of surface Cu atom)(reaction time, h)) and summarized in Table 2. These data

3.9. Hydrogenolysis of glycerol with different water content Water content in glycerol has a significant impact on the hydrogenolysis reaction. As summarized in Table 4, the conversion of glycerol increased with decreasing water content in glycerol solution and the highest glycerol conversion was achieved in an aqueous solution of glycerol with 25% water content. According to the reaction mechanism proposed by Gandarias et al. [24], this phenomenon could be ascribed to water obstructing the adsorption of glycerol and hydrogen. At the same time, water is a byproduct in the hydrogenolysis reaction and excessive amounts of water will tend to shift the reaction equilibrium to the reactant side [25]. However, a decrease in conversion was also observed when the water content was lower than 15%, which could be due to the

Table 2 Hydrogenolysis of glycerol in aqueous solutiona. Catalyst (%)

Cu0.4/Mg5.6Al2O8.6 Cu0.4/Zn0.3Mg5.3Al2O8.6 Cu0.4/Zn0.6Mg5.0Al2O8.6 Cu0.4/Zn1.9Mg3.7Al2O8.6 Cu0.4/Zn2.8Mg2.8Al2O8.6 Cu0.4/Zn3.7Mg1.9Al2O8.6 Cu0.4/Zn5.6Al2O8.6 a b c

Conversion (%)

56.7 60.3 78.2 64.3 29.6 18.9 14.1

Activity of surface Cu (h1)b

7.5 8.1 9.7 8.0 5.2 3.7 4.7

Selectivity (%) 1,2-PDO

Othersc

97.1 99.4 99.3 99.2 98.1 99.4 98.9

2.9 0.6 0.7 0.8 1.9 0.6 1.1

Reaction conditions:75 wt.% aqueous solutions of glycerol 8.0 g, 1.0 g catalyst, 2.0 MPa H2, 180 °C, 10 h. The catalysts were reduced under H2 for 1 h. Defined as (mol of converted glycerol)/(mol of surface Cu atom).(reaction time, h). Ethylene glycol, methanol, ethanol, and 1-propanol.

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3.11. Kinetics of glycerol hydrogenolysis in aqueous solution

Table 3 Hydrogenolysis of glycerol over different amounts of Cu0.4/Zn0.6Mg5.0Al2O8.6a. Catalyst weight (g)

Conversion (%)

Activity of surface Cu (h1)

0.2 0.4 0.6 0.8 1.0

22.6 39.7 55.7 65.7 78.2

14.1 12.4 11.6 10.2 9.7

Selectivity (%) b

1,2-PDO

Others

99.5 99.6 99.3 99.2 99.3

0.5 0.4 0.7 0.8 0.7

c

a Reaction conditions: 75 wt.% aqueous solutions of glycerol 8.0 g, 2.0 MPa H2, 180 °C, 10 h. The catalysts were reduced under H2 for 1 h. b Defined as (mol of converted glycerol)/(mol of surface Cu atom).(reaction time, h). c Ethylene glycol, methanol, ethanol, and 1-propanol.

Fig. 10 shows the time courses of hydrogenolysis of glycerol over Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst in the temperature range from 150 to 180 °C. It was observed that the conversion of glycerol increased quickly in the first 7 h at 180 and 170 °C and the selectivity of 1,2-PDO remained higher than 98.6% during the reaction process. The conversion of glycerol increased continuously and slowly with prolonged reaction time at 160 and 150 °C. The calculated concentrations of glycerol in the form of ln (cglycerol) in these time courses decreased linearly with the reaction time t at these temperatures (see Fig. 11). These results indicate that hydrogenolysis of glycerol in the temperature range of 150– 180 °C over Cu0.4/Zn0.6Mg5.0Al2O8.6 might be a first-order reaction of glycerol. The rate constants of this reaction at different temperatures and the activation energy were calculated and summarized

Table 4 Hydrogenolysis of glycerol with different water contenta. Water content (wt.%)

Conversion (%)

Activity of surface Cu (h1)

50 40 25 15

11.5 24.6 39.7 29.4

3.6 7.7 12.4 9.2

Selectivity (%) b

1,2-PDO

Others

99.4 99.5 99.6 98.6

0.6 0.5 0.4 1.4

c

a Reaction conditions: 6.0 g glycerol with different amounts of water, 2.0 MPa H2, 10 h, 0.4 g reduced catalyst. b Defined as (mol of converted glycerol)/(mol of surface Cu atom).(reaction time, h). c Ethylene glycol, methanol, ethanol, and 1-propanol.

available number of catalytic sites of the catalyst being constant [22, 29]. The selectivity of 1,2-PDO remained practically constant (>98.6%).

3.10. Hydrogenolysis of glycerol at different temperatures Table 5 summarizes the activity of Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst for hydrogenolysis of glycerol at different temperatures. It can be found that glycerol hydrogenolysis accelerated with increasing the temperature. The conversion of glycerol increased quickly from 12.8% (at 160 °C) to 85.5% (at 200 °C), but the selectivity of 1,2-PDO decreased slightly from 99.6% (at 160 °C) to 98.6% (at 200 °C). The calculated activity of surface Cu atoms reached 26.6 h1 at 200 °C. These results indicated that this catalyst was efficient and selective for the hydrogenolysis of glycerol to 1,2-PDO and no obvious cleavage of C–C bonds was observed even at higher temperatures. The high selectivity of 1,2-PDO could be attributed to the reaction temperature being low (below 200 °C) and the C–C bond cleavage reaction is suppressed over Cu catalysts [28].

Fig. 10. Time courses of glycerol hydrogenolysis over Cu0.4/Zn0.6Mg5.0Al2O8.6 at different temperatures: (a) 150 °C; (b) 160 °C; (c) 170 °C; and (d) 180 °C. (Reaction conditions: aqueous solution of glycerol 8.0 g (75 wt.%), 1.0 g reduced catalyst, 2.0 MPa H2).

Table 5 Hydrogenolysis of glycerol on Cu0.4/Zn0.6Mg5.0Al2O8.6 at different temperaturesa. Temperature (°C)

Conversion (%)

Activity of surface Cu (h

160 180 200

12.8 39.7 85.5

4.0 12.4 26.6

Selectivity (%) -1

)

b

1,2-PDO

Others

99.6 99.4 98.6

0.4 0.6 1.4

c

a Reaction conditions: 75 wt.% aqueous solutions of glycerol 8.0 g, 2.0 MPa H2, 10 h, 0.4 g reduced catalyst. b Defined as (mol of converted glycerol)/(mol of surface Cu atom).(reaction time, h). c Ethylene glycol, methanol, ethanol, and 1-propanol.

Fig. 11. Concentration–time curves for glycerol hydrogenation on Cu0.4/Zn0.6Mg5.0Al2O8.6 at different temperatures: (a) 150 °C; (b) 160 °C; (c) 170 °C; and (d) 180 °C. (Reaction conditions: aqueous solution of glycerol 8.0 g (75 wt.%), 1.0 g reduced catalyst, 2.0 MPa H2).

S. Xia et al. / Journal of Catalysis 296 (2012) 1–11 Table 6 Kinetics of glycerol hydrogenolysis over Cu0.4/Zn0.6Mg5.0Al2O8.6a. k (h1)

T (°C) 150 160 170 180

A (h1)

Ea (kJ mol1) 6

0.0475 0.0684 0.0998 0.1652

5.6  10

65.5

a Reaction conditions: 75 wt.% aqueous solutions of glycerol 8.0 g, 2.0 MPa H2, 1.0 g reduced catalyst. k: Rate constant and Ea: apparent activation energy.

Table 7 Hydrogenolysis of glycerol on recycled Cu0.4/Zn0.6Mg5.0Al2O8.6a. Recycles

1 2 3 4 5

Conversion (%)b

Activity of surface Cu (h1)

Selectivity (%) 1,2-PDO

Othersc

39.7 (0.40) 34.2 25.6 25.3 24.6 (0.27)

12.4 – – – 11.0

99.6 99.7 99.7 99.6 99.7

0.4 0.3 0.3 0.4 0.3

a Reaction conditions: glycerol aqueous solution (75 wt.%) 8.0 g, 2.0 MPa H2, 0.4 g catalyst, 10 h. b Data in parentheses are the weights of catalyst (g). c Ethylene glycol, methanol, ethanol, and 1-propanol.

in Table 6. The calculated activation energy of glycerol hydrogenation over Cu0.4/Zn0.6Mg5.0Al2O8.6 is 65.5 kJ mol1 (see Table 6). 3.12. Recycled usage of Cu0.4/Zn0.6Mg5.0Al2O8.6 Table 7 summarizes the activity of recycled Cu0.4/Zn0.6Mg5.0Al2O8.6. It can be found that the detected conversion of glycerol decreased slightly from 39.7% (of the fresh catalyst) to 34.2% (in the second recycle), and then decreased obviously to 25.6% (in the third recycle), but then it remained stable. The decreased activity might be ascribed to the weight loss of catalyst associated with separation and transfer of the catalyst in recycled process (from 0.4 to 0.28 g in the fifth cycle). The calculated activity of surface Cu atoms decreased slightly from 12.4 to 11.0 h1. The actual compositions of fresh catalyst and five-times-recycled catalyst were detected and compared (see Table 8). No leaching of Cu and Zn was observed after five recycles. 4. Discussion 4.1. Hydrogenolysis of glycerol over Cu-based catalyst In the past few years, hydrogenolysis of glycerol has received more and more attentions because of the rapid production of biodiesel. More than 220 papers have been published since 2006, and these achievements were reviewed by Nakagawa and Tomishige [28].

Table 8 Compositions of fresh and recycled Cu0.4/Zn0.6Mg5.0Al2O8.6 catalystsa. Sample

Fresh Used b a b

Composition (mol %) Zn

Cu

Mg

Al

7.6 (7.5) 7.1 (7.5)

5.21 (5) 5.44 (5)

61.75 (62.5) 62.13 (62.5)

25.44 (25) 25.33 (25)

The values in parentheses are controlled values. The used Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst has been recycled five times.

9

Many noble-metal-based catalysts, such as Ru [14–22,26], Pt [16,17,19,23,24,26], Pd [16,19], Rh [16,19], and Ag [25], have been investigated in detail. However, the activity of noble metals was limited when they were used alone, and most experiments were carried out at high temperature (180–220 °C) and high pressure (4.0–10.0 MPa) [16,17,19,20]. In published work, acids [16,19,20] or bases [17,26] were added as co-catalysts to improve the activity of these noble metals. On mixed Rh/C + H2WO4 catalysts, Chaminand et al. found that the selectivity towards 1,3-PDO was two times higher than that toward 1,2-PDO in sulfolane at a 32% conversion of glycerol [5]. Balaraju et al. found that solid acid as a co-catalyst could improve the conversion of glycerol over Ru/C catalyst and increasing the amount of Ru/C and solid acid led to a substantial increase in both the glycerol conversion and the selectivity to propylene glycol [22]. Maris and Davis disclosed that both NaOH and CaO enhanced the rate of glycerol hydrogenolysis over Pt and Ru catalysts [26]. Lahr and Shanks also found that the conversion rate of glycerol per gram of catalyst increased quickly with increasing concentration of OH in the reaction mixture [27]. But the addition of acid or NaOH inevitably resulted in other drawbacks, such as the reaction mixture needing further neutralization and environmental pollution. Recently, the performance of Cu-based catalysts for hydrogenolysis of glycerol has attracted attentions of several groups [28– 41]. Previous work in our laboratory found that highly dispersed Cu on a solid base was extremely effective for the hydrogenolysis of glycerol in aqueous solution. The detected conversion of glycerol reached 80.0% with 98.2% selectivity of 1,2-PDO at 180 °C, 3.0 MPa H2, and 20 h [37]. High yields of 1,2-PDO were also reported over Al2O3 [31]-, SiO2 [32,33]-, and ZnO [34–36]-supported Cu catalysts. Under optimized conditions (220 °C, 1.5 MPa initial H2 pressure, 10 h, Cu/glycerol molar ratio 3:100), the conversion of glycerol over Cu/Al2O3 reached 49.6% (with 96.8% selectivity to 1,2-PDO) [31]. 4.2. The influence of acidity/basicity In published work [29–41], the role of the acidity and/or basicity of Cu-based catalysts is explained in different ways. Some scientists assumed that the activity of Cu catalyst increased with its acidity [30,31]; opposite suggestions said the performance of Cu/ZnO catalyst could be enhanced when NaOH was added in the reaction mixture [34]. In this work, a series of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts with different Zn/Mg ratios were prepared. It was found that the acidity (and basicity) of these catalysts could be manipulated by adjusting the molar ratio of Zn/Mg in Cu0.4/Zn5.6xMgxAl2O8.6. The acidity of these catalysts increased with the addition of Zn (Fig. 8a), while the amount of basic sites increased with the amount of Mg (Fig. 8b). Cu dispersed highly on the surface of Cu0.4/Zn5.6xMgxAl2O8.6, except Cu0.4/Zn5.6Al2O8.6 (see Table 1). These catalysts were tested under the same conditions. It is quite interesting to note that the activities of these catalysts for hydrogenolysis of glycerol increased obviously with their basicity. It was found that Cu dispersed equally in Cu0.4/Zn2.8Mg2.8Al2O8.6 (43.4%) and Cu0.4/Mg5.6Al2O8.6 (43.2%) (see Table 1), but the conversion of glycerol over these catalysts was 29.6% and 56.7%, and the activity of surface Cu atoms was 5.2 and 7.5 h1, respectively (see Table 2). The higher activity of Cu0.4/Mg5.6Al2O8.6 could be attributed to its stronger basicity (see Fig. 8b). Similar conclusions can be drawn when we compare the activity and basicity of Cu0.4/Zn0.6Mg5.0Al2O8.6 with that of Cu0.4/Zn1.9Mg3.7Al2O8.6. That is, basic sites in Cu0.4/Zn5.6xMgxAl2O8.6 promoted the hydrogenolysis of glycerol. The activity of these supported Cu catalysts depends strongly on their basicity.

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The role of acid sites combined with Ru/C catalysts and Ir–ReOx/ SiO2 catalysts in the hydrogenolysis of glycerol was reported by Miyazawa et al. [16,20], Balaraju et al. [22], and Nakagawa et al. [65]. The combination of Ru/C with solid acid (Amberlyst) enhanced the TOF of 1,2-PDO formation drastically; in contrast, it did not affect the TOF of 1,3-PDO + 1-propanol + 2-propanol. This indicates that (1) the dehydration of glycerol to acetol is catalyzed by the acid catalysts and the subsequent hydrogenation of acetol on the metal catalysts gives 1,2-PDO [16] and (2) 1,2-PDO can be formed mainly via dehydration of glycerol to acetol, catalyzed by Amberlyst, and subsequent hydrogenation of acetol to 1,2-PDO, catalyzed by Ru/C. In contrast, 1-propanol and 2-propanol can be formed via 1,3-PDO, catalyzed by Ru/C [20]. Gandarias et al. also suggested that acid sites in Pt-based catalysts are responsible for glycerol dehydration to acetol, while Pt metal sites catalyze acetol hydrogenation to 1,2-PDO. However, Pt also catalyzes C–C bond cleavage, and its presence reduces the formation of coke, as Pt is responsible for hydrogen spillover, which moderates coke formation [24]. The role of basic sites in the hydrogenolysis reactions was well summarized by Maris and Davis [26] and Lahr and Shanks [27]. It was found that glycerol was first dehydrogenated to aldehyde or ketone, which undergoes either a C–C or a C–O cleavage. The overall reaction sequence leading to C–C cleavage, known as the retro aldol mechanism, or C–O cleavage, which occurs by dehydration, is catalyzed by base. The formed aldehyde or ketone intermediates are subsequently degraded by hydroxyl attack. The overall rate of hydrogenolysis of glycerol depended strongly on the concentration of OH in the reaction mixture [26,27]. The Raman spectra of glycerol adsorbed on solid acid (Cu0.4/ Zn5.6Al2O8.6; see curve a in Fig. 9) and solid base (Cu0.4/Zn0.6Mg5.0Al2O8.6; see curve b in Fig. 9) in this work disclosed that solid base is also efficient for the first dehydrogenation step in the hydrogenolysis of glycerol. Because dehydrogenation of glycerol is proposed to be the rate-determining step in the glycerol hydrogenolysis reactions, the hypothesis that the activity of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts increased with their basicity is reasonable and fits well with the reported reaction mechanism [26,27].

4.3. Hydrogen spillover Previous works [23,37] and the preceding discussion concluded that the activity of Cu0.4/Zn5.6xMgxAl2O8.6 catalysts increased obviously with their basicity. The lowest activity of Cu0.4/Zn5.6Al2O8.6 (see Table 2) could be attributed to its poor basicity (and/or low surface area and/or big Cu particles). It is quite interesting to note that Cu0.4/Zn0.6Mg5.0Al2O8.6 showed the highest activity, with 78.2% conversion of glycerol, but its basicity (and surface area) is lower than that of Cu0.4/Mg5.6Al2O8.6 and Cu0.4/Zn0.3Mg5.3Al2O8.6 (see Fig. 8b). The calculated activity of surface Cu atoms in Cu0.4/Zn0.6Mg5.0Al2O8.6 reached 9.7 h1, which is also higher than that in Cu0.4/Mg5.6Al2O8.6 and Cu0.4/Zn0.3Mg5.3Al2O8.6 (see Table 2). We think that the high activity of Cu0.4/Zn0.6Mg5.0Al2O8.6 could be attributed to its higher hydrogen spillover ability than Cu0.4/Mg5.6Al2O8.6 and Cu0.4/Zn0.3Mg5.3Al2O8.6. H2 TPD experiments disclosed that the addition of Zn enhanced the adsorption and activation of H2 and might improve the spillover of split H–H on the surface of Mg–Al–O (Fig. 7). And this enhanced H2 activation ability would improve the activity for glycerol hydrogenolysis. Similar results were reported over bimetallic Ru–Cu [38,39], Pd–Cu [40], and Rh–Cu [41], and this promotion was attributed to hydrogen spillover from noble metals to Cu. This discussion further concluded that the hydrogen spillover ability of Cu0.4/Zn0.6Mg5.0Al2O8.6 also contributed to its higher activity [53–56].

4.4. The predominant performance of Cu0.4/Zn0.6Mg5.0Al2O8.6 In this work, the detected conversion of glycerol over Cu0.4/ Zn0.6Mg5.0Al2O8.6 at 180 °C reached 78.2% and the calculated high activity of surface Cu atoms reached 14.1 h1 (see Table 3), which is similar to that for Pd–Cu and Rh–Cu [40,41]. These results indicate that the promotion effect of small amounts of Zn is comparable with Pd and Rh. Kinetics experiments also disclosed that the activation energy of glycerol hydrogenolysis over Cu0.4/Zn0.6Mg5.0Al2O8.6 catalyst of 150–180 °C is 65.5 kJ mol1. On the basis of these characterizations and discussion, the higher activity of Cu0.4/Zn0.6Mg5.0Al2O8.6 than those of Cu0.4/Zn1.9Mg3.7Al2O8.6, Cu0.4/Zn2.8Mg2.8Al2O8.6, Cu0.4/Zn3.7Mg1.9Al2O8.6, and Cu0.4/ Zn5.6Al2O8.6 is attributed to its stronger basicity. At the same time, the predominant performance of Cu0.4/Zn0.6Mg5.0Al2O8.6 over Cu0.4/Mg5.6Al2O8.6 and Cu0.4/Zn0.3Mg5.3Al2O8.6 would be attributed to the hydrogen spillover. That is, the conversion of glycerol depends strongly on the basicity of catalysts, and hydrogen spillover from Cu to ZnO also enhanced its performance. 5. Conclusion Cu0.4/Zn5.6xMgxAl2O8.6 catalysts with different Zn/Mg ratios were prepared via thermal decomposition of Cu0.4Zn5.6xMgxAl2(OH)16CO3 layered double hydroxides. The acidity (and basicity) of these catalysts could be manipulated by adjusting the molar ratio of Zn/Mg. Cu0.4/Zn0.6Mg5.0Al2O8.6 exhibited the best performance for hydrogenolysis of glycerol in aqueous solution at 180 °C. The calculated highest activity of surface Cu atom reached 14.1 h1, which was comparable to small amount of Pd- and Rhpromoted Cu/solid base catalysts. Characterizations and discussion concluded that the conversion of glycerol over Cu0.4/Zn5.6xMgxAl2O8.6 catalysts depends strongly on the basicity of catalysts, and hydrogen spillover from Cu to ZnO also enhanced its performance. Kinetics experiments found that activation energy of glycerol hydrogenation over Cu0.4/Zn0.6Mg5.0Al2O8.6 is 65.5 kJ mol1. Acknowledgments This research work was supported by the National Natural Science Foundation of China (Contracts 21273198, 21073159), the Zhejiang Provincial Natural Science Foundation (Grant No. LZ12B030001) and the Pre-research Ocean Foundation of Zhejiang University (2012HY025B). References [1] B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Science 330 (2010) 74. [2] A. Villa, G.M. Veith, L. Prati, Angew. Chem. Int. Ed. 49 (2010) 4499. [3] F. Cavani, S. Guidetti, L. Marinelli, M. Piccinini, E. Ghedini, M. Signoretto, Appl. Catal. B Environ. 100 (2010) 197. [4] F. Wang, J.L. Dubois, W. Ueda, J. Catal. 268 (2009) 260. [5] J. Chaminand, L. Djakovitch, P. Gallezot, P. Marion, C. Pinel, C. Rosier, Green Chem. 6 (2004) 359. [6] A. Alhanash, E.F. Kozhevnikova, I.V. Kozhevnikov, Catal. Lett. 120 (2008) 307. [7] L. Huang, Y.L. Zhu, H.Y. Zheng, G.Q. Ding, Y.W. Li, Catal. Lett. 131 (2009) 312. [8] L.Z. Qin, M.J. Song, C.L. Chen, Green Chem. 12 (2010) 1466. [9] L.F. Gong, Y. Lua, Y.J. Ding, R.H. Lin, J.W. Li, W.D. Dong, T. Wang, W.M. Chen, Appl. Catal. A Gen. 390 (2010) 119. [10] J. Oh, S. Dash, H. Lee, Green Chem. 13 (2011) 2004. [11] Y. Shinmi, S. Koso, T. Kubota, Y. Nakagawa, K. Tomishige, Appl. Catal. B Environ. 94 (2010) 318. [12] Y. Nakagawa, Y. Shinmi, S. Koso, K. Tomishige, J. Catal. 272 (2010) 191. [13] M. Chia, Y.J. Pagan-Torres, D. Hibbitts, Q. Tan, H.N. Pham, A.K. Datye, M. Neurock, R.J. Davis, J.A. Dumesic, J. Am. Chem. Soc. 133 (2011) 12675. [14] L. Ma, D. He, Z. Li, Catal. Commun. 9 (2008) 2489. [15] L. Ma, D. He, Catal. Today 149 (2010) 148. [16] T. Miyazawa, Y. Kusunoki, K. Kunimori, K. Tomishige, J. Catal. 240 (2006) 213. [17] E.P. Maris, W.C. Ketchie, M. Murayama, R.J. Davis, J. Catal. 251 (2007) 281. [18] E.S. Vasiliadou, E. Heracleous, I.A. Vasalos, A.A. Lemonidou, Appl. Catal. B Environ. 92 (2009) 90.

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