Promotion Effects of Copper and Lanthanum ... - ACS Publications

2-ethylhexanol is a very important industry material that is widely used in many ... catalyst modified by Cu and La for the hydrotreating of crude 2-e...
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J. Phys. Chem. C 2009, 113, 17787–17794

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Promotion Effects of Copper and Lanthanum Oxides on Nickel/Gamma-Alumina Catalyst in the Hydrotreating of Crude 2-Ethylhexanol Renchun Yang,† Xiaogang Li,*,† Junsheng Wu,† Xin Zhang,† and Zhihua Zhang†,‡ AdVanced Material & Technology Institute, UniVersity of Science and Technology Beijing, Beijing, 100083, P. R. China, and PetroChina Daqing Research Center of Chemical Engineering, Daqing, 163714, P. R. China ReceiVed: June 6, 2009; ReVised Manuscript ReceiVed: July 15, 2009

Nickel/gamma-alumina (Ni/γ-Al2O3) catalysts promoted with Cu and La were prepared by the impregnation method and characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). XRD and HRTEM indicated that the addition of La and Cu was helpful to decrease the size of active particles and increase the dispersion of Ni species. TPR showed that Cu can effectively decrease the reduction temperatures of its own and Ni oxides, whereas La can change the proportion of various Ni species and increase the content of easily reducible Ni species. XPS and UV-vis DRS indicated that electron shift occurred on the Ni-based sample modified by Cu and enhanced remarkably by adding La. These findings suggest that Cu and La species induce both structural and electronic promotion effects, resulting in smaller particle size and weaker interaction between active components and the support, higher dispersions, and reducibility of active phases, ultimately enhancing the crude 2-ethylhexanol hydrotreating catalytic performance. 1. Introduction 2-ethylhexanol is a very important industry material that is widely used in many fields.1-3 It has mainly been obtained by the hydrogenation of 2-ethylhexenal. However, during the process of hydrogenation, the existence of unsaturated byproduct (2-ethylhexanal and 2-ethylhexenol) and residual reactant (2ethylhexenal), including CdC and CdO double bonds, greatly affects the purity of the product. Thus, it is necessary to develop an effective hydrotreating catalyst and use it in a subsequent supplementary hydrogenation process for crude 2-ethylhexanol. It is generally acknowledged that Ni/Al2O3 is an effective catalyst due to its reasonable cost compared to noble metals, as well as its predominant property in hydrotreating, hydrogenation, and decomposition reactions.4-8 However, one major problem of Ni/Al2O3 is that stronger interaction often occurs between Ni and Al2O3, which results in a majority of Ni species diluted in the lattice of Al2O3 and producing hardly reducible Ni species (nickel-aluminate). To achieve high reducibility of Ni species, some researchers have investigated the effects of support,5,9 Ni loading,10,11 promoter,12,13 pretreatment condition,14,15 and preparation method.5,16 Among these methods, the addition of promoter seems to be an effective approach. According to literatures, the addition of Cu element to Ni-based catalyst is very effective for enhancing reaction activity by increasing dispersion and reducibility of Ni species,17 decreasing particle size and interaction between Ni and Al2O3,18,19 and changing the chemical environment of the surrounding Ni atoms,15 which are attributed to the specific roles of Cu (i.e., structural and electronic promotion effects). Moreover, another important role of Cu is its excellent catalytic activity for the CdO double * Corresponding author: Tel.: +86 10 62333931; fax: +86 10 62334005; e-mail: [email protected]. † University of Science and Technology Beijing. ‡ PetroChina Daqing Research Center of Chemical Engineering.

bond,20 which should be a good active component and promoter in our reaction system. However, the sintering of Cu species often occurs at high-temperature treatment process.21,22 To increase the reducibility of Ni and avoid the sintering of Cu, it is essential to properly introduce a modifier to the surface of Al2O3 so as to achieve a catalytic synergistic effect between active phases and the support. It is well-known that La element is often used to modify the Ni/Al2O3 and Cu/Al2O3 catalysts,22-25 which has been confirmed that La has particular potential in minimizing the formation of nickel-aluminate and increasing the dispersion of Ni species,23,24 as well as improving the thermal stability and inhibiting the sintering of Cu species.22 Although supported Ni-based catalysts modified by Cu or La have been widely reported, few studies have focused on the Ni/Al2O3 samples copromoted by Cu and La in a catalytic reaction, especially in the hydrogenation of crude 2-ethylhexanol. The objective of this work is to develop a high-performance Ni/γAl2O3 catalyst modified by Cu and La for the hydrotreating of crude 2-ethylhexanol. Furthermore, both structural and electronic promotion effects of Cu and La on Ni active sites and their roles in the hydrotreating will be further discussed. 2. Experimental Section 2.1. Catalyst Preparation. Ni/γ-Al2O3 and Cu/γ-Al2O3 catalysts were prepared by impregnating a commercial γ-Al2O3 support with Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O solutions, respectively. After impregnation, the samples were dried at 120 °C for 3 h and then calcined at 500 °C for 3 h in air by applying a 5 °C/min ramp. The resultant samples were designated as Ni/ Al and Cu/Al, and the γ-Al2O3 support was designated as Al. Ni-Cu/γ-Al2O3 catalyst was prepared by impregnation in a mixed solution of Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O with the same content as that in the preparation of Ni/Al and Cu/Al. The resultant sample was designated as Ni-Cu/Al. Ni-Cu/La/ γ-Al2O3 was prepared by a consecutive impregnation. The La-

10.1021/jp9053296 CCC: $40.75  2009 American Chemical Society Published on Web 09/21/2009

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TABLE 1: Contents of Various Ingredients ingredient

concent (mol %)

2-ethylhexanol 2-ethylhexenal 2-ethylhexanal 2-ethylhexenol 1-butanol 4-heptanol 3-methylheptanol 3-methylheptane butanoic acid, butyl ester propanoic acid, 2-methylbutyl ester butanoic acid, 2-methylpropyl ester butanoic acid, 2-ethylhexyl ester 2-ethyl-4-methyl-1-pentanol

90.50 0.54 0.13 0.02 1.93 0.78 0.31 0.25 3.98 0.12 0.31 0.22 0.91

modified γ-Al2O3 was first prepared by impregnating γ-Al2O3 in La(NO3)3 · 6H2O solution and then dried at 120 °C for 3 h and calcined at 500 °C for 3 h. After that, active phase was added to the modified support by impregnation of mixed aqueous solutions of Ni(NO3)2 · 6H2O and Cu(NO3)2 · 3H2O and then further dried and calcined at the same temperatures as the first step. The sample was designated as Ni-Cu/La/Al. 2.2. Characterization. The chemical compositions of samples were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Powder X-ray diffraction (XRD) patterns of the samples were obtained with a Rigaku diffractometer/MAX-RB by nickel-filtered Cu KR radiation (λ ) 154.056 pm). Diffraction patterns were recorded over a range of 2θ angles from 10° to 100° using a 0.02° step size. Transmission electron microscopy (TEM) experiments were preformed on a JEM-2010 high-resolution transmission electron microscope with an energy dispersive spectrum (EDS) analysis facility. A Thermo ESCALAB 250 XPS Spectrometer, with Al KR radiation, was used for obtaining XPS data. In the XPS study, a C 1s-binding energy value of 285 eV was taken as a reference level. UV-vis diffuse reflectance spectroscopy (UV-vis DRS) was performed in the range of 240-800 nm by a Hitachi U-3900H PC spectrophotometer using γ-Al2O3 as a reference. Temperature programmed reduction (TPR) measurements were carried out in a TPDRO 1100 series (Thermo electron corporation) instrument. About 50 mg catalyst was placed in a U-shaped quartz sample tube. Prior to TPR studies, the catalyst sample was pretreated in an inert gas (N2, 40 mL/min) at 200 °C for 1 h. After pretreatment, the sample was cooled to ambient temperature and the carrier gas consisting of 5% hydrogen balance argon (40 mL/min) was allowed to pass over the sample and the temperature was increased from ambient to 950 °C at a heating rate of 10 °C /min. 2.3. Activity Test. The initial concentrations of the reactants are given in Table 1. Among the various ingredients, 2-ethylhexanal, 2-ethylhexenal, and 2-ethylhexenol are main three ingredients that can affect the product quality and need to be removed. The hydrotreating of crude 2-ethylhexanol was performed in a continuous-flow system. A stainless tube reactor (50 cm long, 10 mm o.d., and 8 mm i.d.) was filled with catalyst (d ) 250-830 µm) diluted with SiC to ensure a homogeneous thermal distribution. The height of the catalyst bed is about 18 cm. To control homogeneous temperature distribution of the catalyst bed, the tubular reactor was heated by three zones and measured with three thermocouples in contact with the catalyst bed. Before any measurements, 6.0 mL catalyst was pretreated at a scheduled temperature in a 400 mL min-1 flow of hydrogen for 180 min, followed by cooling in hydrogen flow to the desired reaction temperature. The experiments were carried out at 2.5

Figure 1. XRD patterns of various samples.

MPa, 120 °C, and 3 h-1 volume space velocity. The liquid feed and hydrogen were dosed by a HPLC pump and a mass flow controller, respectively. Both reactants were mixed before they entered the reactor. The reactants and the products were rapidly collected in cold traps and analyzed by an offline gas chromatograph (SP-3420) equipped with a flame ionization detector. The hydrogenation activities of 2-ethylhexanal, 2-ethylhexenal, and 2-ethylhexenol on the catalysts were determined by the equation:

Conversion (mol %) ) 100 ×

Xin - Xout Xin

where Xin and Xout stand for the 2-ethylhexanal, 2-ethylhexenal, and 2-ethylhexenol contents in feed and product, respectively. 3. Results and Discussion 3.1. Structural Promotion Effects of Cu and La. 3.1.1. XRD. The XRD profiles of the oxide catalysts and the support are shown in Figure 1. The support exhibited three peaks at 2θ ) 37.6°, 45.8°, and 66.7°, corresponding to the planes (311), (400), and (440) of γ-Al2O3 (JCPDS 29-0063), respectively. All of the supported samples (Ni/Al, Cu/Al, Ni-Cu/Al, and Ni-Cu/La/Al) also exhibited three similar peaks with those of the support. However, no detectable XRD crystallines of NiO, CuO, or La2O3 were present in the corresponding oxide catalysts. A routine explanation for the result is that nickel, copper, and lanthanum oxidic species are either completely amorphous or composed of crystallites smaller than 4 nm based on the detection limit of XRD. The larger part of these metal oxides may migrate into the lattice of the support and then combine with the support to produce solid solutions or new compounds (i.e., spinel) after calcination. Moreover, the supported samples all exhibited more intense peaks than those of the support, which is attributed to the presence of spinel. The experiment results showed that additional calcination for alumina support at the same temperature for 3 h does not change its XRD pattern. Thus, the difference of XRD patterns between the support and the supported samples caused by additional calcination can be excluded. It is well-known that a strong interaction between γ-Al2O3 and divalent metal species can easily result in the formation of a metal aluminate phase (for example, nickel aluminate and copper aluminate in this system) on heat treatment at high temperature.18 Because the characteristic peaks of nickel-alumina and copper-alumina spinel are very similar

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TABLE 2: Physicochemical Properties of Various Samples sample

content (wt %) Ni

Al Ni/Al Cu/Al Ni-Cu/Al Ni-Cu/La/Al

Cu

average size (nm) La

XRD

1.0

5.6 5.7 5.6 5.4 4.8

5.7 5.6 5.6

4.1 3.8 3.8

TEM 15 10

to those of γ-Al2O3,19 XRD peaks representing metal aluminates cannot be clearly resolved. The only slight difference observed in these XRD patterns is their peak height depending on the types of various supported oxides. To study the effect of Cu and La on these Ni-based samples, the crystallite sizes of the peak at 2θ ) 45.8° were calculated based on Scherrer’s equation, and results are shown in Table 2. It must be mentioned that the calculated crystallite sizes are used to estimate roughly only because the peaks at 2θ ) 45.8° are overlapped peaks of the support and the metal aluminate phase. On the identical support (aluminate), the crystallite particles of the peak at 2θ ) 45.8° on the various supported catalysts are approximately considered as metal aluminate here. From the XRD results in Table 2, the estimated average crystallite size was 5.6 nm for Al, 5.7 nm for Ni/Al, 5.6 nm for Cu/Al, 5.4 nm for Ni-Cu/Al, and 4.8 nm for Ni-Cu/ La/Al. The results showed that the formation of nickel-alumina and copper-alumina spinel do not change the size of the γ-Al2O3, which may be attributed to such fact that Ni or Cu ions only migrate into the lattice of the support and do not affect other structure because of their low contents. However, the size of Ni-Cu/Al is slightly decreased relative to those of the singlemetal supported samples. This means that a solid solution or Cu-Ni oxides occur between Ni and Cu.19 Some publications have confirmed that Cu species can induce both structural and electronic promotion effects for a Ni-based sample.17,18,26 Clearly, the decrease in the size of Ni-Cu/Al supports the structural promotion effect. Certainly, we expected a higher dispersion of Ni on Ni-Cu/La/Al than on Ni/Al and Ni-Cu/ Al. Fortunately, Ni-Cu/La/Al exhibited a lower size than that of Ni/Al and Ni-Cu/Al, indicating that a higher dispersion occurred for Ni-Cu/La/Al. Some publications have also confirmed that the Ni particle size can be effectively decreased by adding La.23-25,27 3.1.2. HRTEM. HRTEM analysis has been carried out to investigate the dispersion of active components on the various samples and the results are shown in Figure 2. To further qualitatively analyze surface distribution of particles, the elemental compositions were also detected by EDS. It is clearly seen that, on the unmodified sample Ni/Al2O3, two isolated darkcolor particles are located in the middle of the image (part a of Figure 2). On the basis of the analysis of EDS, dark zones are due to Ni oxide, whereas light parts are ascribed to the support Al2O3. The two particles all exhibited an irregular geometrical shape with a short range around 3-5 nm and a long range of about 10-15 nm. The size of long range is obviously bigger than the average size evaluated by foregoing XRD, which may be due to the Ostwald ripening process.28,29 Another possible explanation is that the particles are quasi-amorphous in structure because the lattice fringes of crystal are not distinct. Thus, it is not possible for XRD to exactly estimate the size of quasiamorphous structure particles, although they can be directly observed by TEM. The relative big size and isolated distribution of the particles indicate that a poor dispersion of Ni oxides occurred on the surface of the support.

For the Ni/Al2O3 sample modified by Cu, besides a particle with obvious crystal lattice fringes, the remainder area of the whole image is filled with homogenized color of black and white (part b of Figure 2). The crystal is attributed to Cu oxides based on EDS analysis because the atomic ratio of Cu in area C is much higher than that in area D. All areas exhibiting Cu element may be due to the used copper network. The crystal shows clearer lattice fringes than those of Ni oxides in part a of Figure 2, suggesting that a perfect crystal occurred. The perfect crystalline form indicates that lower dispersion and stronger aggregation occurred for Cu oxides than for Ni oxides. However, it is important to note that the aggregation of Ni oxides did not occur as the addition of Cu, which can be observed on the uniform distribution area around the surrounding Cu oxides. This should be attributed to the promotion effect of Cu on Ni, for which it has been reported that Cu can increase the dispersion of Ni oxides.17,18 More importantly, no crystal particle can be found when the Ni/Al2O3 sample is modified simultaneously by Cu and La (part c of Figure 2). The result reveals that the modification of La on the support increases the strong metal-support interaction, which promotes the dispersion of active components. 3.1.3. TPR. The H2-TPR profiles of calcined samples are shown in Figure 3. The TPR profile of Ni/Al presented a broad reduction band around 550-950 °C, which can be deconvoluted into one reduction peak centered at 660 °C and another peak at 810 °C. The first peak can be attributed to the reduction of 2+ (Ni2+ in an octahedral geometry), and the second is due to NiOC the reduction of NiT2+(Ni2+ in a tetrahedral geometry) because 2+ is greater an acknowledged view is that the reducibility of NiOC 2+ 4 than that of NiT . Another small peak can also be observed at low temperature, which is related to the presence of a trace of crystal NiO.30,31 The Cu/Al sample shows two very broad peaks centered at 400 and 750 °C. However, no reduction contribution can be observed at such a high-temperature band (around 750 °C) in publications,18,32 which may be due to the difference in preparation conditions. The two peaks may be attributed to the reduction of Cu species weakly and strongly interacting with the support. Metal-aluminate compounds often occur through solid-state reactions during the calcination process,27,33,34 which can result in stronger interaction and higher reduction temperature. For the Ni-Cu/Al sample, peaks at low temperature (280 and 350 °C) should be attributed to the reduction of Cu species and peaks at high temperature (600 and 780 °C) would be due to that of Ni species. On the basis of the TPR profiles of Ni-Cu/ Al, Ni/Al, and Cu/Al, a clear indication is that the addition of Cu strongly promotes Ni and Cu reduction, which has also been found in some papers.18,27 The decrease of reduction temperature for Ni-Cu/Al may be attributed to a decrease of the interaction between Ni species, as well as Cu species, and the support by the addition of Cu. Another possible reason may be due to a synergistic interaction between Ni and Cu. It is well-known that the outermost electron of Cu2+ is unpaired, which can interact with Ni and result in the electron cloud deviation or structure distortion of Ni ion. The interaction may also spur Cu to modify the valence state of Ni by donating charge into the ds subband of Ni, which will be confirmed by later XPS. Moreover, a decrease of their crystallite sizes indicates that a significant promotion effect occurs for Ni or Cu species, which has been confirmed in foregoing XRD. Thus, both the structural promotion and electronic promotion effects may occur at the same time when Cu is added. Compared with the Ni-Cu/Al sample, the La-modified Ni-Cu/La/Al shows a slight shift to low temperature for the

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Figure 2. HRTEM images of Ni/Al (a), Ni-Cu/Al (b), and Ni-Cu/La/Al (c), and EDS results of areas A, B, C, D, and E.

peaks of Cu species and an opposite change trend for Ni species, indicating that the interaction among Ni, Cu, La, and the support is very complicated. Although the reduction temperatures of Ni species on Ni-Cu/La/Al are higher than those on Ni-Cu/ Al, they are still lower than those on Ni/Al, suggesting that Cu and La all produce some specific effects on Ni species. The addition of La resulting in a slight increase in the temperature of Ni species may be due to the stronger interaction between Ni and La, in accordance with the TPR results observed by other groups.24,35,36 Such stronger interaction also occurs between Cu and La because the low-temperature peak intensity of Cu species is decreased on Ni-Cu/La/Al with respect to that on Ni-Cu/

Al. Generally, the addition of La can effectively decrease the size of Ni particles,24,30,37 increase the dispersion and reducibility of Ni supported on the support,23-25,35,38 and then increase the number of active sites and reaction activity. Although the reduction temperature of Ni species on Ni-Cu/La/Al is increased (in our experiment), the peak area of the low temperature is obviously bigger than that of the high temperature compared with that of Ni-Cu/Al. The result indicates that the hardly reducible Ni species (NiT2+) may be partly replaced by 2+ ), in accordance with the the easily reducible Ni species (NiOC reports that the proportion of easily reducible Ni species, such as surface NiAl2O4 and β1-NiO,30,39 can be increased when La

Hydrotreating of Crude 2-Ethylhexanol

Figure 3. H2-TPR profiles of calcined samples.

was introduced into the Ni/Al sample. Thus, we can speculate that La act as a structural promotion element, which inhibits the formation of hardly reducible Ni species. 3.2. Electronic Promotion Effects of Cu and La. 3.2.1. XPS. To further study the effects of lanthanum and copper on these Ni-based catalysts, the Ni/Al, Ni-Cu/Al, and Ni-Cu/ La/Al samples were characterized by XPS. Ni 2p and Cu 2p spectra of these samples are shown in Figure 4. Peak areas of the recorded spectra were estimated by fitting the curves with combination of Gaussian curves of variable proportion. The corresponding characteristics of binding energy values of Ni 2p3/2 and Cu 2p3/2 along with the Ni/Al and Cu/Al atomic ratios are given in Table 3. Part a of Figure 4 shows the Ni 2p region of the X-ray photoelectron spectra. A typical Ni 2p XPS spectrum of the Ni/Al sample shows peaks at 856.5, 862.5, and 873.8 eV. The characteristic lines at 856.5 and 874.6 eV are attributed to Ni 2p3/2 and Ni 2p1/2 of nickel-alumina spinel40,41 respectively, and the peak at 862.5 eV is ascribed to a satellite peak of 856.5 eV.4,42 However, it can be seen that a new peak at 855.5 eV occurs immediately when Cu is added into the Ni/Al sample. Most importantly, the peak at 856.5 eV almost disappears, whereas peak at 855.5 eV becomes more intensive when La is added. Moreover, the corresponding bonding-energy shift also occurs in Ni 2p1/2. It is generally acknowledged that the bonding energy of a component is usually affected by its chemical environment and/or structure. The significant differences of Ni 2p3/2 among the three samples suggest that chemical environment and/or structure of the surrounding Ni ions are changed by Cu and La elements. With the addition of copper or lanthanum and copper, the peak of Ni 2p3/2 binding energy shifts negatively from 856.5 to 855.5 eV, implying that Ni in Ni-Cu/Al or Ni-Cu/La/Al sample gains electrons form Cu and thus becomes electron-rich. Because the content of La is very low, the signal of the La and its probable change in electronic state are not detected by XPS. Although the information of the La cannot be obtained, the effect of La on Ni is confirmed by the shift of Ni binding energy. Hou et al. thought that La was neither a donating electron element nor an accepting electron element.43 However, the Ni 2p3/2 spectrum in the Ni-Cu/La/Al sample exhibits a pronounced shift greater than that of in Ni-Cu/Al. This implies that La exhibits a specific effect on the chemical environment of the surrounding Ni atoms. Thus, we speculate that Cu is an effective electronic promoter element for Ni,

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17791 whereas La can enhance the promotion effect of Cu on Ni. The electronic effect of Cu on Ni may be relatively weak for a larger amount of Cu embedded into the lattice of the support when Cu was added solely. However, the addition of La on the surface of γ-Al2O3 can effectively avoid the embedding of Cu into the lattice of the support and then increase the contact chance between Cu and Ni. Part b of Figure 4 shows the Cu 2p region of the X-ray photoelectron spectra. The spectra of the two samples (Ni-Cu/ Al and Ni-Cu/La/Al) all show two parent peaks at B.E. positions of 932.3-932.5 and 934.9-935.0 eV. The peak at 934.9-935.0 eV can be easily identified as CuAl2O4, whereas the peak at 932.3-932.5 seems difficult to estimate because its value is lower than that of familiar Cu species, such as CuO, Cu2O, and CuAl2O4.26 This peak may be assigned to Al-rich copper-alumina spinel. Al-rich copper-alumina spinel is an unsaturated structure; to keep a balance of electron, electron donating may occur from Al2O42- to Cu. Thus, the Cu 2p exhibits a lower B.E. value. Moreover, the Cu 2p3/2 on the Ni-Cu/La/Al sample shows a 0.2 eV shift with respect to Ni-Cu/Al. The positive shift of Cu 2p3/2 may be attributed to Cu donating electrons to Ni when La is added, which supports the foregoing result of the negative shift of Ni 2p3/2. Thus, a probable electron shift route is Al2O42- f Cu f Ni when copper-alumina spinel is an unsaturated structure. Such speculation can be well explained by the result of adding La. Because the migration of Cu into the support and the formation of Alrich copper-alumina spinel will be inhibited when La is added, the chance of Cu gaining electrons from Al2O42- is decreased. Thus, Cu 2p exhibits a higher B.E. value on Ni-Cu/La/Al than that on Ni-Cu/Al. The binding energies and atomic ratios of Ni 2p and Cu 2p on various catalysts are given in Table 3. The surface ratios for (Ni:Al and Cu:Al) were calculated using the sensitivity factors, as determined by Scofield.44 The Ni:Al atomic ratio is found to be higher for Ni-Cu/Al than for Ni/Al, suggesting that the addition of Cu results in Ni enriched on the surface of the sample, which is attributed to the structural promotion effect of Cu. The Ni enrichment is more obvious at the Ni-Cu/La/Al sample, indicating that La produces a more effective structural promotion effect, which supports the foregoing discussion. In addition, the Cu:Al atomic ratio is also increased when La was added into the Ni-Cu/Al sample, suggesting that the structural promotion effect of La is synchronous for both elements (Ni and Cu). The role of La seems to inhibit the migration of Ni and Cu into the lattice of the support, whereas the role of Cu seems to not only inhibit Ni migration into the support but also donate electrons to Ni. Thus, it can be speculated that Cu can result in the increase of inherent activity, whereas La can increase the number of active sites of Ni, which are responsible for the higher activity of hydrogenation of crude 2-ethylhexanol. 3.2.2. Diffuse Reflectance Spectroscopy. Diffuse reflectance UV-vis spectroscopy was often used to study the symmetry and coordination of the surface species of the catalysts. Figure 5 presents the diffuse reflectance UV-vis spectra of various catalysts. To exclude the effect of experiment error on the profile of spectra, the detection of UV-vis spectra for every sample was performed three times. As can be seen in Figure 5, the results showed that three measurements of every sample overlap each other perfectly. Thus, the differences among various catalysts are attributed to their different essential features and do not concern the experimental error. Because the spectrum of the support was subtracted from the spectra of each examined catalyst, absorption observed in Figure

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Figure 4. Ni 2p (a) and Cu 2p (b) XPS spectra of various samples.

TABLE 3: XPS Characteristics of Various Catalysts catalyst Ni/Al Ni-Cu/Al Ni-Cu/La/Al

B.E. (eV)

atomic ratio

Ni 2p3/2

Cu 2p3/2

Ni:Al

Cu:Al

856.5 855.5,856.5 855.5

932.3,934.9 932.5,935.0

1.47 1.65 1.74

1.36 1.44

5 should be assigned to ligand-to-metal charge transfer. It can be seen that the Ni/Al sample shows three absorption bands with maxima at 410, 585, and 630 nm. According to the literature,45,46 the band in the vicinity of 410 nm is attributed to octahedral Ni2+ ions, whereas the two absorption bands at 585 and 630 nm are due to the d-d electronic transition of tetrahedral Ni2+ ions. The absence of an absorption band at 720 nm indicated that no NiO crystallites occur on the surface of the Ni/Al sample,45,47 which is consistent with the results in XRD, TPR, and XPS. The Cu/Al sample presents two wide absorption bands around 240-500 and 500-800 nm, which are almost outside the measuring limit of the UV-vis-DRS equipment used. These hardly identified absorption edges should be attributed to the characteristic spectra of Cu/Al.48 In a similar way for the Ni-Cu/Al sample, the result shows that the difference between it and Ni/Al or Cu/Al in the whole scanning region is obvious. Note that the absorbance of Ni-Cu/

Figure 5. UV-vis spectra of various catalysts.

Al is not a simple sum of Ni/Al and Cu/Al absorbance, indicating that ligand-to-metal charge transfer (LMCT) or Ni-Cu metal ion interaction occurs on the Ni-Cu/Al sample.28,49 As can be seen, the Ni-Cu/Al sample exhibits a slightly larger increase of absorbance at the 500-800 and 375-500 nm bands respectively with respect to that of the Cu/Al sample. Clearly, the intensity of band at 500-800 nm is obviously higher than that at 375-500 nm on the Ni/Al sample. A higher absorbance at 500-800 nm band with respect to that at the 375-500 nm band should occur on the Ni-Cu/Al sample if the absorbance of Ni-Cu/Al is a simple adding operation between Ni/Al and Cu/Al; however, the result does not occur. Thus, the result may be attributed to the electronic promotion effect between Ni and Cu. The increase in the intensity of the 500-800 nm band is no more than that of the 375-500 nm band, suggesting that a decrease in tetrahedral Ni2+ ions while an increase in octahedral Ni2+ ions may occur on Ni-Cu/Al, which is consistent with foregoing discussion in TPR and XPS. Moreover, a remarkable conclusion is that the absorbance of the Ni-Cu/Al sample is lower than that of the Cu/Al at 190-375 nm band, suggesting that there is a blue shift of the Cu absorption edge. This shift means that the electronic promotion effect is strengthened in the UV region. The decrease of absorbance in the UV region may be due to copper ions occupying the tetrahedral sites of alumina and avoiding nickel ions in an oxide environment in tetrahedral coordination. Thus, more nickel ions will occupy the octahedral sites of alumina and show a higher activity. The Ni-Cu/La/Al sample exhibits a very similar profile to that of Ni-Cu/Al. The only difference between them is that the absorbance of Ni-Cu/La/Al is higher than that of Ni-Cu/ Al. It seems that the profile of Ni-Cu/La/Al is a vertical translation curve of Ni-Cu/Al. The result indicates that La produces a whole promotion effect but not a single effect on Ni or Cu. This can be attributed to La inhibiting the migration of nickel and copper into the tetrahedral sites of alumina, giving a greater chance for copper to produce an electronic promotion effect on nickel. Thus, La can be considered a structural promoter element, which supports the foregoing discussions. 3.3. Catalytic Activity. Figure 6 shows conversions of 2-ethylhexenal, 2-ethylhexanal, and 2-ethylhexenol on various catalysts as functions of reduction temperature. Note that the Ni/Al catalyst exhibits very poor hydrogenation activities at low

Hydrotreating of Crude 2-Ethylhexanol

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Figure 6. Conversions of 2-ethylhexenal, 2-ethylhexanal, and 2-ethylhexenol as functions of reduction temperature on various catalysts: (a) Ni/Al, (b) Cu/Al, (c) Ni-Cu/Al, and (d) Ni-Cu/La/Al.

reduction temperature, even a negative conversion for 2-ethylhexanal at 400 °C, and the poor activities continue until a relative high reduction temperature (600 °C). The appearance of negative conversion reveals that the content of 2-ethylhexanal increases during the reaction process. A possible reason is that an incomplete hydrogenation reaction of 2-ethylhexenal (only CdC is hydrogenated) may occur, and 2-ethylhexenal can be hydrogenated into byproduct 2-ethylhexanal. However, the hydrogenation performance has been improved greatly on Cu/ Al at low reduction temperature (350-550 °C). It is well-known that the reduction temperature of Cu species is lower than that of Ni species.18 Thus, the distinct difference of hydrogenation between Ni/Al and Cu/Al should be due to their different reducibility. The result indicates that metallic Ni and Cu all can produce active sites for hydrogenation of 2-ethylhexenal, 2-ethylhexanal, and 2-ethylhexenol, but some differences still exist. Clearly, the hydrogenation conversion of the three unsaturated compounds is basic agreement with the following order: 2-ethylhexenal >2-ethylhexenol >2-ethylhexanal on Ni/ Al, however, the order is changed into: 2-ethylhexenal >2ethylhexanal >2-ethylhexenol on Cu/Al, which may be attributed to the difference in intrinsic activity between Ni and Cu for different organic functional groups. Generally, Ni-based catalyst can exhibit high hydrogenation activity for the CdC bond,50 whereas Cu-based sample is more effective for the CdO bond.20 A similar phenomenon has been observed on Ni/SiO2 and Co/ SiO2; Trasarti et al. thought that the significant extent of Co d-orbitals favors the hydrogenation of carbonyl group, whereas Ni has a narrower d-bandwidth compared with Co and favors the hydrogenation of the CdC bond.50 From the structural formulas of the three compounds, we know that the unsaturated functional groups are CdC and CdO for 2-ethylhexenal, CdC for 2-ethylhexenol, and CdO for 2-ethylhexanal. Thus, the conversion of 2-ethylhexenol is bigger than that of 2-ethylhexanal on Ni/Al, whereas an opposite result occurs on Cu/Al.

The hydrogenation conversion of Ni-Cu/Al is shown in part c of Figure 6. Compared with Cu/Al, Ni-Cu/Al exhibits a better hydrogenation performance not only for 2-ethylhexenal but also for 2-ethylhexanal and 2-ethylhexenol. More importantly, the decrease of the activities on the Cu/Al sample at high reduction temperature has been effectively inhibited on the Ni-Cu/Al sample. The decrease in the hydrogenation activities of Cu/Al should be due to the sintering of Cu at high temperature. Thus, a promoting effect may occur between Ni and Cu, which can increase the dispersion and decrease aggregation of Ni and Cu particles.18,19,26 Moreover, from 350 to 450 °C, 2-ethylhexanal exhibits an increasing conversion with increasing temperature; however, from 450 to 550 °C, the activity of 2-ethylhexenol decreases gradually with increasing temperature. The result should be attributed to the fact that Ni is not reduced at low temperature (350-450 °C) and Cu can be aggregated at high temperature (450-550 °C), suggesting that the Ni-Cu/Al sample does not possess a good stable hydrogenation performance at a wider reduction temperature window for CdC and CdO. However, this defect has been improved significantly when the Ni-Cu/Al sample was promoted by La. As can be seen in part d of Figure 6, from 350 to 550 °C, 2-ethylhexenal, 2-ethylhexenol, and 2-ethylhexanal all show excellent hydrogenation performance and their conversions almost always are maintained at 99%. The excellent performance displayed on the Ni-Cu/La/Al sample suggests that La plays as a promoter by changing the surface state of Ni-Cu/Al, which supports the discussion in foregoing characterization. 4. Conclusions The presence of copper in Ni/Al2O3 catalysts caused a decrease in particle size and an increase in dispersions and reducibility of Ni species and its own. When the sample was modified by La, the active phases on the Ni-Cu/La/Al sample

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showed higher dispersions and reducibility because the hardly reducible metal aluminate phase was inhibited. The combination of XPS and UV-vis DRS demonstrated that Cu species caused an important effect on the chemical environment of Ni species and acted as an electronic promoter on Ni surface active sites. As a result, the Ni-Cu/La/Al sample exhibited higher activities at a wider low reduction temperature window. Moreover, the role of added Cu species was not only as a promoter but also as a main active phase because it exhibited a higher hydrogenation performance for the CdO double bond than that of Ni active phase. Acknowledgment. The authors are grateful for the financial support of CNPC for High Technology Research and Development. The authors also wish to thank Prof. Yizhong Huang (Material Department, Oxford, UK) for language help. References and Notes (1) Arabi, M.; Mohammadpour Amini, M.; Abedini, M.; Nemati, A.; Alizadeh, M. J. Mol. Catal. A 2003, 200, 105. (2) Saha, B.; Streat, M. React. Funct. Polym. 1999, 40, 13. (3) Skrzypek, J.; Sadlowski, J. Z.; Lachowska, M.; Nowak, P. Chem. Eng. Process 1998, 37, 163. (4) Kirumakki, S. R.; Shpeizer, B. G.; Sagar, G. V.; Chary, K. V. R.; Clearfield, A. J. Catal. 2006, 242, 319. (5) Jasik, A.; Wojcieszak, R.; Monteverdi, S.; Ziolek, M.; Bettahar, M. M. J. Mol. Catal. A 2005, 242, 81. (6) Wang, X. Q.; Ozkan, U. S. J. Phys. Chem. B 2005, 109, 1882. (7) Jones, T. E.; Rekatas, A. E.; Baddeley, C. J. J. Phys. Chem. C 2007, 111, 5500. (8) Park, C.; Keane, M. A. J. Catal. 2004, 221, 386. (9) Saadi, A.; Merabti, R.; Rassoul, Z.; Bettahar, M. M. J. Mol. Catal. A 2006, 253, 79. (10) Guemini, M.; Rezgui, Y. Appl. Catal., A 2008, 345, 164. (11) Sedor, K. E.; Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 2994. (12) Dias, J. A. C.; Assaf, J. M. Appl. Catal., A 2008, 334, 243. (13) Natesakhawat, S.; Oktar, O.; Ozkan, U. S. J. Mol. Catal. A 2005, 241, 133. (14) Chang, F. W.; Tsay, M. T.; Kuo, M. S. Thermochim. Acta 2002, 386, 161. (15) Marin˜o, F.; Baronetti, G.; Jobbagy, M.; Laborde, M. Appl. Catal., A 2003, 238, 41. (16) Akande, A. J.; Idem, R. O.; Dalai, A. K. Appl. Catal., A 2005, 287, 159. (17) Vizcaı´no, A. J.; Carrero, A.; Calles, J. A. Int. J. Hydrogen Energy 2007, 32, 1450. (18) Youn, M. H.; Seo, J. G.; Kim, P.; Kim, J. J.; Lee, H. I.; Song, I. K. J. Power Sources 2006, 162, 1270. (19) Lee, J. H.; Lee, E. G.; Joo, O. S.; Jung, K. D. Appl. Catal., A 2004, 269, 1. (20) Nagaraja, B. M.; Padmasri, A. H.; Seetharamulu, P.; Hari Prasad Reddy, K.; David Raju, B.; Rama Rao, K. S. J. Mol. Catal. A 2007, 278, 29.

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