SiO2 Catalysts

Article pubs.acs.org/IECR

Hydrogenation of Dimethyl Oxalate Using Extruded Cu/SiO2 Catalysts: Mechanical Strength and Catalytic Performance Li Zhao, Yujun Zhao, Shengping Wang, Hairong Yue, Bo Wang, Jing Lv, and Xinbin Ma* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: In this work, the extrusion process of Cu/SiO2 catalysts prepared by the ammonia-evaporation (AE) method has been investigated and optimized in order to obtain materials with convenient catalytic and mechanical properties for their application in the gas-phase hydrogenation reaction of dimethyl oxalate (DMO) to ethylene glycol (EG). Thereby, a variety of Cu/SiO2 extrudates were prepared with different Cu loading. It has been observed that a special microstructure including defects, flaws, and discontinuities plays a significant role on the mechanical strength. Larger CuO grains may act as fracture origins, which negatively affect the mechanical strength. The 20Cu/SiO2 extrudate with high Cu dispersion and high porosity is optimized for hydrogenation of DMO to EG, achieving a 98% conversion of DMO and 85% selectivity of EG under the liquid hourly space velocity (LHSV) of 1.0 h−1.

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INTRODUCTION Ethylene glycol (EG) is an important chemical widely used as antifreeze and in polyester manufacture.1 At present, ethylene oxidation is a universal industrial approach to produce EG. However, as crude oil resources shrink, and the demand for EG constantly increases, the synthesis of EG from syngas attracts increased interest.2 This indirect synthesis process includes two steps: the coupling of CO with methanol to oxalates, and the subsequent hydrogenation of oxalates to EG.3,4 The first step has been scaled up to commercial production with a capacity of 10 000 tons per year in 2010.5 The second step has been investigated in both homogeneous and heterogeneous systems, using noble-metal catalysts such as Ru6 and Ag.7 Considering the catalyst−product separation for homogeneous catalysts and expensive noble metal, Cu-based heterogeneous catalysts have been recently investigated in the vapor-phase hydrogenation of oxalates to EG.8 Different carriers (e.g., SiO2, Al2O3, ZnO, and La2O3) for Cu-based catalysts were studied,9−11 among which Cu/SiO2 catalysts afforded the highest yield of EG in the hydrogenation of dimethyl oxalate (DMO) or diethyl oxalate (DEO), because of the neutral properties of SiO2. From an industrial point of view, Cu/SiO2 powdered catalysts should be shaped into assemblies such as granules, spheres, and extrudates in order to decrease the pressure drop and achieve a desirable mechanical strength.12,13 Extrusion is the most important shaping technique applied in the manufacturing of fixed-bed catalysts. In general, an extruded material is manufactured from a paste, which is first obtained by blending the catalyst powder with a binder to provide the mechanical strength and a liquid phase to provide effective lubrication during the extrusion.14,15 Molding catalysts are normally agglomerates of mixed metal oxides, or supported metal, metal oxides, and sulfide with special binders.16−19 In recent years, the extrusion of zeolites is also applied.20−22 Solid catalysts are normally fragile.23 Their mechanical properties are similar to those of green ceramics. The failure of this type of materials is often due to brittle fracture.24,25 For the fracture strength of brittle material, a general relationship © 2012 American Chemical Society

between the tensile fracture strength of brittle materials and its physical properties has been described roughly by the Griffith equation:26

σ=

2Eγ πc

(1)

where E is the Young’s modulus, γ a specific surface energy (i.e., the energy required to create a unit area of new crack surface), and c a factor that characterizes the size and state of the defects existing in the material. Clearly, the fracture strength of brittle materials such as solid catalysts has a close relationship with the essence of the materials and flaw properties or morphology. For the extrusion of catalyst, the early literature consists of mainly one-parameter experimental reports for the purpose of increasing the mechanical strength.27−29 There were some studies discussing the stress state in the pellets leading to strength failure.30 The statistical properties of the strength data in general also have been the topics of a series of publications.31−33 Some authors have tried to analyze the nature of the catalyst mechanical strength,34 However, there has been a lack of the literature on establishing a correlation between mechanic strength and physicochemical properties and the explanation on the fracture of brittle materials. Despite many reports on the synthesis, properties, and catalytic applications of the Cu/SiO2 catalyst,2,8,35,36 little information can be found in the open literature about its agglomeration. Therefore, this papers attempts to investigate the extrusion of Cu/SiO2 catalysts for the vapor-phase hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). We hope to establish a correlation between the mechanic strength and the intrinsic properties of Cu/SiO 2 materials. Cu/SiO 2 extrudates prepared from different Cu loadings have been Received: Revised: Accepted: Published: 13935

March 23, 2012 June 29, 2012 October 3, 2012 October 3, 2012 dx.doi.org/10.1021/ie300779a | Ind. Eng. Chem. Res. 2012, 51, 13935−13943

Industrial & Engineering Chemistry Research

Article

performed using a Philips XL-30 microscope equipped with an EDAX detector for EDS analysis. The Cu/SiO2 catalyst was coated with gold particles before SEM measurements. Textual properties of the catalysts were determined by a nitrogen adsorption method following ASTM Standard 4365 standard using a Micromeritics Tristar II 3000 at 77 K. Before analysis, the samples were degassed at 573 K for 4 h under vacuum. Pore size distribution was estimated by the Barrett− Joyner−Halienda (BJH) method from the desorption branches of the adsorption isotherms, and the specific surface areas were calculated from the isotherms using the BET method. Temperature-programmed reduction (TPR) was carried out on a Micromeritics Model ASAP 2010 chemisorption apparatus. The Cu/SiO2 catalyst of 100 mg was loaded into a quartz tube and dried in an argon stream at 393 K for 1 h before the reduction. The catalyst was then heated in 30 mL/ min of 10% H2 in argon at a heating rate of 10 K/min up to 1073 K. A thermal conductivity detector (TCD) was employed to determine the amount of hydrogen consumption during the run. X-ray diffraction (XRD) measurements were performed using a Rigaku Model C/max-2500 diffractometer, employing the graphite-filtered Cu Kα radiation (λ = 1.5406 Å) at room temperature. The particle size of copper was calculated by X-ray broadening technique using the Scherrer equation. Data points were acquired by step scanning with a rate of 12°/min from 2θ = 10° to 2θ = 90°. Copper loadings are determined by inductively coupled plasma−atomic emission spectroscopy (ICP-AES) (VISTAMPX) operated at high-frequency emission power of 1.5 kW and plasma airflow of 15.0 L/min. The sample was dissolved in the mixture of HNO3, HF, and HBO3, and then diluted with water. Specific surface area of metallic copper was measured by the adsorption and decomposition of N2O using a pulse titration method. Briefly, 100 mg of catalyst sample was reduced in 5% H2−Ar at 623 K for 4 h and cooled to 363 K. 15% N2O−Ar was then introduced at a rate of 30 mL/min for 2 h, ensuring that surface Cu atoms were completely oxidized. The copper surface areas were calculated from the consumed amount of nitrous oxide, according to the method described by Chinchen et al.37 Infrared (IR) spectra were recorded on a Nicolet EDX spectrometer equipped with a DTGS detector. The samples were finely grounded, dispersed in KBr, and pelletized. The spectral resolution was 4 cm−1, and 32 scans were recorded for each spectrum. The mechanical strength of the extrudates over individual extrudate was measured by a ZQJ-II pellet strength meter. Prior to all of the measurements, the catalyst extrudates were heated in air at 413 K for 8 h in order to eliminate the effect of water and gas adsorbed. According to elastic mechanics,12 there is an approximate relationship between the maximum load F at fracture and the maximum tensile stress σ inside the pellet leading to fracture induced during the measurement of extrudates. For cylindrical pellet in crushing, the relationship is shown in eq 2.12 The number of pellets measured for the regression is dependent on the scattering range of the strength data. More than 50 tablets were measured for the extrudates. This measurement of the mechanical strength over one particle is shown in Scheme 1.

explored to evaluate the effects such as micromorphology, pore distribution, and grain size on the mechanical strength, according to the fracture theory of brittle materials. Meanwhile, the catalytic activity of the Cu/SiO2 extrudates with different Cu loading has been evaluated.

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EXPERIMENTAL SECTION Catalyst Preparation. Cu/SiO2 powder prepared by the ammonia-evaporation (AE) method is described briefly as follows. A certain amount of Cu(NO3)2·3H2O and 25 wt % ammonia aqueous solution dissolved in deionized (DI) water were mixed and stirred for 10 min. Silica sol was then added to a copper ammonia complex solution and stirred for 6 h. The initial pH of the suspension was 11−12. All the above operations were performed at room temperature. The suspension was heated to 353 K in a water bath to allow for the evaporation of ammonia and the decrease of pH and, consequently, the deposition of copper species on silica. When the pH value of the suspension decreased to 6−7, the evaporation process was terminated. The filtrate was washed with deionized water three times and dried at 393 K for 4 h to obtain the catalyst powder. For comparison, the reference Cu/ SiO2 catalyst with 40 wt % copper loading denoted as 40Cu/ SiO2−IM was prepared by impregnation of silica gel powder with an aqueous solution of Cu(NO3)2; thereafter, the deionized water was evaporated away under reduced pressure. Finally, the catalyst powder was dried at 393 K for 4 h. The various steps involved in the preparation of extrudates are powder mixing, paste preparation, extrusion, drying and calcination. The dry mixing was carried out by mixing a certain amount Cu/SiO2 catalyst powder and binder silica gel powder, and then the powder was mixed with 150% m/m deionized water in a kneader for 4 h at a rotor speed of 40 min−1. The paste was transferred to a piston extruder and formed to cylindrical green bodies with a diameter of 3 mm. The green bodies were dried at 353 K for 1 h and calcined in air at 673 K for 2 h. Finally, the extrudates were cut into pieces ∼3 mm in length for the characterization and examination in the hydrogenation of DMO to EG. Catalytic Activity Tests. Catalytic tests were carried out in a continuous flow unit equipped with a stainless-steel fixed-bed tubular reactor. The catalyst bed had a diameter of 15 mm and a length of ∼65 mm. The reaction testing was carried out upon the reduction of catalyst in a hydrogen atmosphere at 623 K for 4 h. The reactant (20 wt % DMO (99.9% purity) in methanol (analytical reagent (AR) purity) solution) was injected to the gasification chamber through a high-pressure pump (Lab Alliance Series II Pump) at a system pressure of 2.5 MPa and 523 K. Subsequently, the mixture blended with the hydrogen was injected from the top of the reactor. The reaction was performed at a system pressure of 2.5 MPa and temperature of 453−493 K. The room-temperature space velocity (LHSV) of DMO was varied from 0.50 h−1 to 1.0 h−1. The reaction products were condensed and analyzed on an Agilent Micro GC 6820 with an HP-INNOWAX capillary column (Hewlett− Packard Company, 30 m × 0.32 mm × 0.50 μm) equipped with a flame ionization detector (FID). Main byproducts include ethanol, methyl glycollate (MG), and 1,2-butanediol (BDO). Four to eight separate GC samples were taken, and the results were averaged for each experimental data point, and uncertainties were typically within 3%. Catalyst Characterization. Scanning electron microscopy/ enerfgy-dispersive X-ray spectroscopy (SEM/EDS) analysis was

σ= 13936

2F πdl

(2) dx.doi.org/10.1021/ie300779a | Ind. Eng. Chem. Res. 2012, 51, 13935−13943


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Scheme 1. Mechanical Strength Evaluation over an Individual Particle

where σ is the tensile strength (expressed in terms of N/cm2), F the applied load (given in units of N), d the diameter of the extrudates (given in units of cm), and l the length of the extrudates (also expressed in units of cm). The mean strength (in units of N/cm) mentioned in this study is the applied load (F) per unit length at fracture, which is calculated as F/l.

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RESULTS Morphology of the Extrudates. The extrusion of the catalyst pastes to obtain stable and smooth extrudates is a key technology for an industrial catalyst production, in which the rheological behavior of the paste plays an important role. Generally, water is used as the film adhesive in the extrusion paste, which is the critical factor. Very small variation in the water content (±1%) will result in a dramatic change in the rheological properties. An insufficient amount of water leads to a dry paste with low plasticity, which is difficult to press through the nozzle of the extruder. Finally, the extrudates will be fissured and scaly. On the other hand, when the amount of water is too high, the paste is wet and sticky, which results in deliquescent extrudates, and the mechanical strength of the calcined extrudates is low. The most beneficial pastes for extrusion should be plastic in nature and does not display viscous flow in the nozzle but, instead, undergo deformation. The extruded strings after calcinations are shown in Figure 1a. After the extrudates were calcined in air for 2 h at 673 K and cut into pieces ∼3 mm in length. We investigated the load− displacement curve of the extrudates with the help of universal testing machine. As illustrated in Figure 2, the load− displacement curve during the crushing strength test of the catalyst tablets shows that the tablets experience very little plastic deformation before failure occurs. It can be deduced that the Cu/SiO2 catalyst is a type of material with a brittle failure mode. Therefore, the mechanical failure phenomenon of Cu/ SiO2 extrudates can be analyzed via the fracture mechanics mechanism of brittle materials. The morphological and surface properties of the final extrudates with 15%−30% Cu loading were determined by SEM (Figure 1). 40Cu/SiO2−IM catalyst powder prepared via the impregnation method cannot be extruded. It is clearly observed that the extrudates corresponding to 25Cu/SiO2 and 30Cu/SiO2 present rougher and more irregular surfaces with many protuberances and flaws, compared to 15Cu/SiO2 and 20Cu/SiO2 extrudates. More microcracking can be also detected in 30Cu/SiO2 extrudates than in 15Cu/SiO2 and 20Cu/SiO2 (see Figure 1f), which agrees with the results of the mercury intrusion method to be discussed later. The elemental concentration differences between the surface and the small protuberances of the 25Cu/SiO2 extrudates were analyzed by the energy-dispersive spectroscopy (EDS). As shown in Figure 3, the elements concentration for Cu, Si, O at the marked areas are listed in the inset table. We can observe that the proportion of elements significantly changes between region 1 and region 2, and this discontinuity structure or

Figure 1. Images of (a) extruded strings after calcinations, (b) extruded tablet, (c) 15Cu/SiO2, (d) 20Cu/SiO2, (e) 25Cu/SiO2, and (f) 30Cu/SiO2.

Figure 2. Load−displacement curves during the crushing strength tests of 20Cu/SiO2 extrudates.

segregated phases present denser in the catalyst bulk when the copper loading is up to 30%. However, the surface of the 20Cu/SiO2 extrudates is smoother, with a higher degree of homogeneity, and no phase segregation is perceptible. As will be discussed later, this microstructure (e.g., defects, flaws, and discontinuity) plays an important role in the mechanical strength of Cu/SiO2 extrudates. Textural Properties and Mechanical Strength. Copper contents in the Cu/SiO2 extrudates are determined by ICPAES measurements (see Table 1). When the copper loading exceeds 30%, the filtrate appears with a dark blue color, indicating a partial loss of the Cu species. To better understand the role of the copper component and compare with the Cu/ SiO2 extrudates by the AE method, the 40Cu/SiO2−IM material was prepared. Pore size distribution curves of the calcined Cu/SiO2 extrudates are illustrated in Figure 4. The Brunauer− Emmett−Teller (BET) surface area, pore volume, and average 13937

dx.doi.org/10.1021/ie300779a | Ind. Eng. Chem. Res. 2012, 51, 13935−13943


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Figure 4. Pore size distribution of calcined catalysts of different copper loadings: (a) blank, (b) 15Cu/SiO2, (c) 20Cu/SiO2, (d) 25Cu/SiO2, and (e) 30Cu/SiO2.

Figure 3. SEM images of the 25Cu/SiO2 extrudates. Values (concentrations in mol %) in the inset table indicate crossed points where elemental analysis was measured.

where E is the elastic modulus of the porous polycrystalline specimen, E0 the elastic modulus of a nonporous polycrystalline specimen, e the Napierian number (e = 2.71828), b an empirical constant, and P the volume fraction porosity. Phani et al. proposed a widely used semiempirical equation, which shows excellent agreement with the data on α- and βalumina over a wide range of porosity.39

pore diameter are summarized in Table 1. As can be seen, the BET surface area increased from 330 m2 g−1 to 472 m2 g−1 and the pore volume increased from 0.65 cm3 g−1 to 0.84 cm3 g−1 when the Cu contents were varied from 15% to 30%, while the average pore diameter seemed nearly unchanged. The pore size distribution curves indicate that pores at ca. 3.0 nm and 10.0 nm were the major contributors to the total pore volume. It is interesting that the pure silica extrudates, indicated by the blank sample, exhibited pores 3.0 nm in size and did not display pores 10.0 nm in size. The absence of pores 10.0 nm in size suggests that the presence of copper species is responsible for the pores appearing at ∼10 nm. Furthermore, we also found the pores at 10.0 nm shifted to the lower values upon increasing copper loading, which could be caused by the aggregation of copper species in the catalyst with higher copper loading. The porosity and the Young’s modulus (E) are summarized in Table 1. The Young’s modulus is also known as the tensile modulus. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke’s Law holds. It can be experimentally determined from the slope of a stress−strain curve created during tensile tests conducted on a sample of the material. According to eq 1, E has great influence on the tensile fracture strength of brittle material and also is related to the porosity of the material.38−40 Spriggs has proposed an empirical equation to predict the elastic modulus of porous brittle solids. This equation is purely empirical in nature and not based on theory, but generally it fails to satisfy the boundary condition that E = 0 for P = 1.38 E = E0exp( −bP)

E = E0(1 − aP)n

(4)

Here, E is the elastic modulus of the porous polycrystalline specimen, E0 the elastic modulus of a nonporous polycrystalline specimen, and a and n are material constants. The material constant a in the equation may be defined as the “packing geometry factor”, the value of which lies between 1 and 3.85. According to eq 4, if material constants n are kept consistent, the Young’s modulus (E) and porosity should show the reverse changes, but we found that variation tendency between E and the porosity disagree with the aforementioned theory. Phani suggested that the material constant n is dependent on grain morphology and pore geometry of the material.39 Combined with the presence of the microstructure, as shown in the SEM images during increasing Cu loading, it is deduced that the material constant n may have a close connection with the morphology. The pore size distribution of Cu/SiO2 extrudates by mercury intrusion method is illustrated in Figure 5. It is found that macropores ranging from 100 nm to 1000 nm are obviously present when Cu loading is up to 30%, which is consistent with the SEM results. The mean strength and standard deviation of Cu/SiO2 extrudates are also illustrated in Table 1. The fracture load of

(3)

Table 1. Results of ICP-AES Analysesa sample 15Cu/ SiO2 20Cu/ SiO2 25Cu/ SiO2 30Cu/ SiO2

porosityc (%)

fracture load (N/cm)

56.0

52.0

4.0

51.9

115.8

63.5

0.52

31.5

7.1

42.6

96.9

40.3

0.51

29.7

8.9

38.1

96.0

27.6

0.44

shape

Cu loading (%)

ABET (m2/g)

pore volume (cm3 g−1)

dp (Å)

Scu (m2 g−1)

bar

14.9

330

0.65

65

bar

18.5

389

0.70

60

27.7

bar

22.0

415

0.79

64

bar

28.3

472

0.84

56

dcub (nm)

standard deviation

Ed (MPa) 0.42

a

Data taken from Zhao et al.15 bCu crystallite size, calculated using the Scherrer formula. cPorosity of the material, calculated by mercury intrusion method. dYoung’s modulus, calculated by universal testing machine. 13938



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Figure 5. Pore distribution of the calcined extrudates using the mercury intrusion method.

solid catalysts is always maldistributed, even if a set of nominally identical specimens, taken from a batch of catalysts, are tested under the same conditions.12,32 Hence, 50 pieces of Cu/SiO2 extrudates were measured in the process. As can be seen, the fracture load increases as the Cu loading increases, while the fracture load decreases when the Cu loading is up to 25%. Surprisingly, 40Cu/SiO2-IM cannot be extruded using the identical extrusion shaping process. Thus, we deduce that increasing the Cu loading is beneficial to the mechanical strength of Cu/SiO2 extrudates until the presence of the microstructure (e.g., defects, flaws, and discontinuity), the formation of which is detrimental. The low mechanical strength of 15Cu/SiO2 could be due to the variation of surface energy (γ), to which we will devote more research in the near future. On the other hand, the standard deviation of 30Cu/SiO2 extrudates is obvious lower than that of 20Cu/SiO2 and 25Cu/SiO2. The Griffith equation (i.e., eq 1) shows that variations of size, shape, and orientation of these flaws result in a large scatter of the strength data of solid catalysts.26 The decrease of the standard deviation with increasing copper loading may result from the uniform distribution of flaws in the bulk of the catalyst. The reason that 40Cu/SiO2 cannot be extrudated may be due to the IM method, which may lead to larger CuO crystalline sizes and decrease the mechanical strength, which will be analyzed below. In this preliminary study, we suggested that the ideal mechanical strength for industrial application is ∼100 N/cm. Hence, we choose 20Cu/ SiO2, 25Cu/SiO2, and 30Cu/SiO2 extrudates to analyze the correlation between the chemical property and the catalytic performance in the following section. Crystalline Phase and Reduction Behavior. XRD patterns of the extrudates are presented in Figure 6. The feature at ∼22° is ascribed to amorphous silica. The characteristic peeks of CuO (tenorite) at 2θ = 35.6° and 38.7° (JCPDS File Card No. 05-0661) are found in the 40Cu/ SiO2-IM extrudate,2 which are dramatically weakened in 20%− 30% Cu/SiO2 samples. Several equations relating the tensile strengths of brittle polycrystalline materials to their grain size have been presented by Orowan41,42 and Knudsen.42 They found that the strength increases as the grain size decreases,

Figure 6. XRD patterns of the calcined Cu/SiO2 samples with different Cu loadings: (a) 20Cu/SiO2, (b) 25Cu/SiO2, (c) 30Cu/ SiO2, and (d) 40Cu/SiO2-IM.

because the brittle fracture of polycrystalline metals is a coherent crack propagation process. Assuming this information to be true, the brittle fracture will begin in one grain and spreads to others in the surroundings.42 This may be the reason why 40Cu/SiO2-IM cannot be extruded. The XRD patterns of the reduced Cu/SiO2 extrudates (Figure 7) show a strong diffraction peak at 2θ = 43.3°, along with two weak ones at 2θ = 50.4° and 74.1°, which can be ascribed to metallic copper (JCPDS File Card No. 04-0836). A strong diffraction peak at ∼35.8° from the (111) plane of Cu2O (JCPDS File Card No. 05-0667) indicates that a portion of copper exists as Cu+ after reduction. The absent of CuO species in the samples suggests that CuO was completely reduced to Cu2O or metallic Cu0. The particle size of metallic Cu obviously increases as the copper loading increases, as listed in Table 1. TPR measurements were carried out to investigate the reducibility of the copper species in various Cu-loaded extrudates. As shown in Figure 8, a high reduction peak for 13939



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loading was sufficiently low, the reduction temperature was generally lower than that with a higher copper loading.43,44 The peak of the 30Cu/SiO2 extrudate (ca. 600 K) decreased significantly, which is further attenuated for the 25Cu/SiO2 extrudate, and vanished in the 20Cu/SiO2 extrudate.45 This can also be confirmed by the XRD results in Figure 6. The main reduction peak at a lower temperature of ∼534 K is assigned to the reduction of highly dispersed Cu species with small particle size, but not confined to copper phyllosilicate2 formed during the catalyst preparation. For copper phyllosilicate, the reduction ceases at Cu+ under the present reduction conditions, because of the strong interaction between Cu ions and SiO2, which agrees with the XRD results. Further reduction of Cu+ to Cu0 requires a temperature of >873 K.2,46 It is worthwhile noting that main reduction peaks for 20Cu/SiO2, 25Cu/SiO2, and 30Cu/SiO2 are 515, 525, and 534 K, respectively, which suggests the copper oxide in the catalyst with lower copper loading is easily reduced. The IR technique has been adopted to determine the filandrous compounds of phyllosilicates.47−49 As shown in Figure 9a, the formation of copper phyllosilicate is supported by the appearance of the δOH band at 663 cm−1 and the νSiO shoulder peak at 1040 cm−1 on the low-frequency side of the νSiO asymmetric stretching band of SiO2 at 1110 cm−1.47 The relative amount of copper phyllosilicate in calcined Cu/SiO2 samples is calculated by considering the integrated intensity of the δOH band at 663 cm−1 normalized to the integrated intensity of the νSiO symmetric stretching band of SiO2 at 800 cm−1, which is termed as I663/I800.2 Figure 9b clearly shows that the relative amount of copper phyllosilicate increased as the copper loading increased, which means that the amount of Cu+ after reduction increased as the Cu loading increased. The result may have an influence on catalytic activities. Combined with the SEM results, it is also induced that the presence of the microstructure may result from the increasing amount of copper phyllosilicate. Catalytic Activities. The catalytic performances of the Cu/ SiO2 extrudates with various Cu loading were investigated in gas-phase hydrogenation of DMO. This reaction comprises several steps, including DMO hydrogenation to MG, MG hydrogenation to EG, and deep hydrogenation of EG to ethanol.36 Moreover, byproducts of 1,2-butanediol (1,2-BDO) and 1,2-propanediol (1,2-PDO) could also be generated from

Figure 7. XRD patterns of the reduced Cu/SiO2 with different Cu loading: (a) 20Cu/SiO2, (b) 25Cu/SiO2, and (c) 30Cu/SiO2.

Figure 8. H2-TPR profiles of the calcined Cu/SiO2 extrudates: (a) 20Cu/SiO2, (b) 25Cu/SiO2, (c) 30Cu/SiO2, and (d) 40Cu/SiO2-IM.

the 40Cu/SiO2-IM was found at ∼600 K, which can be assigned to the reduction of large CuO particles to metallic Cu, as has been proved by XRD result. The 40Cu/SiO2-IM exhibited a higher TPR temperature, which should be due to the larger particle size of copper species and the super high loading of copper on the silica. Moreover, when the copper

Figure 9. (a) IR spectra of the calcined Cu/SiO2 extrudates. (b) The I663/I800 intensity ratio, representing the relative amount of incorporated copper species. 13940



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Figure 10. (A) Effect of copper loading on DMO conversion and EG, MG, ethanol, and 1,2-BDO selectivities in DMO hydrogenation reaction. Conditions: P = 2.5 MPa, T = 473 K, H2/DMO = 50 (mol/mol), and DMO LHSV = 1.0 h−1. (B) Copper metal surface area measured by N2O titration and yield of EG (%), as a function of copper loading.

Several equations relating the tensile strengths of brittle polycrystalline materials to their grain size have been put forward by Orowan41,42 and Knudsen.42 They found that the strength increases as the grain size decreases, because the brittle fracture of polycrystalline metals is a coherent crack propagation process. Assuming this fact to be true, the brittle fracture will begin in one grain and spread to others in the surroundings.42 This may be the reason why 40Cu/SiO2-IM cannot be extruded. Moreover, the mechanical strength of the extrudates prepared via the AE method may be also due to the formation of the copper hydrosilicate (e.g., behaving as a binder), while the copper hydrosilicate was not present in the extrudates prepared via the impregnation method, resulting in a lower mechanical strength. Therefore, the preparation method can also be a factor that affects the mechanical strength of the catalyst. Physicochemical Properties of the Catalysts on the Catalytic Performance. Generally, the surface area of the copper metal should account for the reactivity of the reaction. In the above section, it was revealed that the surface area of the copper metal changed little with increases in copper loading (see Figure 10), while the Cu particle size increased dramatically (see Table 1). Moreover, the porosity of the extrudates showed an obvious decrease when the copper loading increased from 20% to 30%. Apparently, our previous work suggested that high porosity of the catalyst will benefit the diffusion of the reactant, thus resulting in a higher activity.4 On the other hand, we can tentatively hypothesized that the grain size has little effect on catalytic performance. Combined with the IR result, although the amount of Cu+ after reduction obviously increased, the catalytic activity changed little. Yin et al. suggested that the surface area of the copper metal contributes mainly to the catalytic performance in the hydrogenation of DMO to EG.52 The proper ratio of Cu0/ Cu+ can greatly improve the catalytic performance.52 Since the formation of the copper valence state will be partially related to the structure of the catalyst precursor, the structure control during the catalyst preparation procedure is necessary, especially the appropriate amount of copper phyllosilicate that should be manufactured. Finally, the extrudated 20Cu/ SiO2 with higher Cu species dispersion and porosity were considered as an ideal catalyst for the DMO hydrogenation in industrial application.

the dehydrogenation reaction between EG and ethanol or methanol.4,35,50 The activity and selectivity, as a function of Cu loading, are shown in Figure 10. The 20Cu/SiO2 extrudates possess the highest activity (97.9% conversion) for DMO hydrogenation with 86.8% of EG selectivity. However, both the conversion of DMO and selectivity of EG slightly decreased during increasing the Cu loading, and the selectivity of MG also increased.

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DISCUSSION Physicochemical Properties of the Catalysts on the Mechanical Strength. From the results of tensile test, it can be deduced that the Cu/SiO2 catalyst is a material with a brittle failure mode. As a force is loaded on a brittle material, a tensile stress field is induced inside the material bulk.51 Brittle fracture of the material originates from tensile stress concentration at the tips of an existing critical microcrack (flaw) inside the material bulk, which leads to a sudden catastrophic growth of the critical flaw under tensile stress. Hence, the fracture strength of brittle materials such as Cu/SiO2 catalysts has a close relationship with the flaw properties or morphology in the materials. From the result of SEM, a special microstructure including defects, flaws is present during increasing the Cu loading, which has a negative influence on the mechanical strength. The elemental concentration variation between the smooth surface and the small protuberances can be identified as a type of discontinuity in the catalyst bulk. Li et al. suggested that solid catalysts are porous and full of defects, crystal edges, dislocations or nonidentical materials enclosed.28 Any discontinuity that appears in the catalyst bulk may be treated as a flaw and, hence, as the origin of tensile stress concentration, and the heterogeneity of the catalyst strength is an intrinsic property inherited from the brittle fracture nature of solid catalyst materials.51 Hence, the special microstructures, including defects, flaws, and discontinuities that are present during the increase in Cu loading, result in the dramatic change in mechanical strength. The Young’s modulus (E) is related to the porosity of the material.38−40 In our work, it can be induced that the variation of the microstructure has an obvious influence on E, further changing the mechanical strength. The mechanical strength increases as the Cu loading increases until the special microstructure appears. The decrease in the standard deviation with increasing copper loading may result from the uniform distribution of flaws in the bulk of the catalyst. 13941



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CONCLUSIONS We have presented an investigation regarding the effect of Cu loading on the mechanical strength and the catalytic activities of extrudates. The microstructures including defects, flaws, and discontinuities are present during increasing the Cu loading, which are treated as the origin of tensile stress concentration and have a negative influence on the mechanical strength. On the other hand, the presence of special microstructure also altered the porosity and grain morphology, further changed the Young’s modulus and mechanical strength. In addition, the mechanical strength is particularly low when CuO grain size is large. On the basis of suitable strength (∼100 N/cm) of extrudates (20%−30%), we evaluated the catalytic performance in gas-phase hydrogenation of DMO to EG. The higher Cu species dispersion and the higher porosity of the 20Cu/SiO2 extrudated catalyst were considered as an ideal catalyst for DMO hydrogenation in industrial application.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-22-87401818. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from the Program of Introducing Talents of Discipline to Universities (No. B06006), the National Science Foundation of China (No. 21276186), and the China Postdoctoral Science Foundation (No. 20090450090).

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GREEK SYMBOLS E = modulus of elasticity, also called Young’s modulus, MPa c = crack half-length F = applied load at fracture (N) l = length of the extrudates (cm) E0 = elastic modulus of nonporous polycrystalline specimen (MPa) e = Napierian number; e = 2.71828 b = an empirical constant P = volume fraction porosity a, n = material constants

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