Dispersion and Reactivity of Copper Catalysts Supported on Al2O3

Dispersion and Reactivity of Copper Catalysts Supported on Al2O3-ZrO2. Guggilla Vidya Sagar, Pendyala Venkat Ramana Rao, Chakravartula S. Srikanth, an...
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J. Phys. Chem. B 2006, 110, 13881-13888

13881

Dispersion and Reactivity of Copper Catalysts Supported on Al2O3-ZrO2 Guggilla Vidya Sagar, Pendyala Venkat Ramana Rao, Chakravartula S. Srikanth, and Komandur V. R. Chary* Catalysis DiVision, Indian Institute of Chemical Technology, Hyderabad - 500 007, India ReceiVed: December 26, 2005; In Final Form: May 16, 2006

A series of CuO/Al2O3-ZrO2 catalysts with Cu loadings varying from 1.0 to 20 wt % were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of CO2 and NH3, electron spin resonance (ESR), and Brunauer-Emmett-Teller surface area measurements. The dispersion and metal area of copper were determined by the N2O decomposition method. XRD results suggest that the copper oxide is present in a highly dispersed amorphous state at copper loadings < 10 wt % and as a crystalline CuO phase at higher Cu loadings. ESR results suggest the presence of two types of copper species on the Al2O3-ZrO2 support. TPR results suggest well-dispersed copper oxide species at low Cu loadings and crystalline copper oxide species at high Cu loadings. Well-dispersed copper oxide species were reduced more easily than large copper oxide species by H2. The results of CO2 TPD suggest that the basicity of the catalysts was found to increase with an increase of copper loading up to 5.0 wt % and decreases with a further increase of copper loading. The results of NH3 TPD suggest that the acidity of the catalysts was found to decrease with an increase of copper loading up to 5.0 wt % and increases with a further increase of copper loading. The catalytic properties were evaluated for the vapor-phase dehydrogenation of cyclohexanol to cyclohexanone and correlated with the results of CO2 TPD measurements and the dispersion of Cu on the Al2O3-ZrO2 support.

1. Introduction Supported copper catalysts have attracted considerable attention because of their recent practical applications in promoting the steam reformation of methanol to produce hydrogen for fuel-cell operation,1-3 the synthesis of methanol,4 CO oxidation and the synthesis of glyoxal from glycol,5 the oxychlorination of ethylene,6 the synthesis of cyclohexanone from cyclohexanol,7,8 and the selective catalytic reduction of nitrogen oxides by hydrocarbons in an oxygen-rich atmosphere,9 and more recently, these catalysts were employed in CO hydrogenation.10 The catalytic properties of the active copper phase can be greatly influenced by the nature of the supported oxide and the dispersion of the active component.11 However, the nature of the active species of these catalysts is still the subject of extensive investigation by many researchers. For example, in methanol synthesis, it is suggested that the active component is not only Cu+ but also Cu0 and that the support plays a major role in controlling the Cu+/Cu0 ratio, which further influences the catalytic activity. It is well-known that the catalytic performance of supported catalysts depends to a certain extent on the kind of support used. The kind of support influences the catalyst properties revealed by (i) an improvement of the dispersion of the active phase, (ii) a decrease of the formation of the inactive spinel phases, (iii) modification of the reducibility of the oxide precursors through a change of the interaction between the active phase and support, and (iv) controlling the deactivation due to coke formation. Al2O3- and ZrO2-supported copper catalysts are reported to be active for the dehydrogenation of alcohols.12-14 The inherent favorable properties of both alumina and zirconia * Corresponding author. Tel.: +91-40-27193162. Fax: +91-4027160921. E-mail: [email protected].

supports can be explored by a combination of both the supports in a mixed oxide. The Al2O3-ZrO2-supported catalysts were found to exhibit better catalytic properties than the catalyst supported on either Al2O3 or ZrO2 and are attracting considerable interest because of their potential use as catalyst supports. The advantages of using Al2O3-ZrO2 as a catalyst support include a high surface area, basicity, and thermal stability. In recent years, Al2O3-ZrO2-based materials have been employed as catalysts in numerous catalytic applications.15-19 Hence, the characterization of Al2O3-ZrO2-supported catalysts is very important to discuss for the catalytic functionalities of copper catalysts during the dehydrogenation of cyclohexanol. The catalytic dehydrogenation of cyclohexanol to cyclohexanone is an important reaction in view of its industrial applications. Cyclohexanone is used in the manufacture of adipic acid and caprolactam, which are the main raw materials for the production of nylon-6,6 and nylon-6, respectively. Copper-based catalysts are used in the dehydrogenation of cyclohexanol to cyclohexanone,20 which is also produced either by the hydrogenation of phenol or by the air oxidation of cyclohexane. The catalytic dehydrogenation of cyclohexanol has gained much importance in the recent past.14,21 Jeon and Chung22 reported the dehydrogenation of cyclohexanol over various supported metal oxide catalysts and concluded that Cu/SiO2 and Cu/MgO catalysts showed relatively high catalytic performances in conversion and selectivity. The activity of supported copper catalysts depends mainly on the method of preparation, the nature of the support, and the dispersion of the active component. The study of the determination of dispersion of the active phase in supported Cu catalysts has been an interesting topic of research in recent years for understanding the role of the

10.1021/jp0575153 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/24/2006

13882 J. Phys. Chem. B, Vol. 110, No. 28, 2006 active phase in catalytic properties. A fundamental understanding of the structure-activity relationships observed in heterogeneous catalytic dehydrogenation is of basic importance for the development of new catalytic materials and for improving the performance of existing catalysts. For this, the N2O decomposition method was studied to find active-phase dispersion in supported copper catalysts.23 In the present investigation, we report the characterization of CuO/Al2O3-ZrO2 catalysts by powder X-ray diffraction (XRD), electron spin resonance (ESR), temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD) of CO2 and NH3. The catalytic properties have been evaluated for the dehydrogenation of cyclohexanol to cyclohexanone. We also report the relation between the dispersion of copper and the catalytic properties of the catalysts during the vapor-phase dehydrogenation of cyclohexanol to cyclohexanone. The purpose of this work is to estimate the dispersion of copper supported on Al2O3-ZrO2 as a function of copper loading and to identify the changes in structure of the copper phase with increased active-phase loading and also to understand the relation between activity/selectivity and Cu metal area. 2. Experimental Section The Al2O3-ZrO2 mixed oxide (1:1 wt %) support was prepared by a coprecipitation method. The requisite quantity of aqueous solutions containing aluminum nitrate hydrate (Fluka), zirconium nitrate hydrate (Fluka), and NH4OH were continuously stirred for 6 h at 303 K. The precipitation was completed after 5 h of stirring, at which the pH of the solution was 9. The precipitate thus obtained was filtered, washed several times with deionized water until it was free from the base, dried overnight at 393 K, and finally calcined at 773 K for 6 h. The resulting mixed oxide had a Brunauer-Emmett-Teller (BET) surface area 207 m2/g. For comparison, the pure Al2O3 and ZrO2 supports were also synthesized following the above precipitation method using the same precursors. A series of copper catalysts with Cu loadings varying from 1 to 20 wt % were prepared by the wet impregnation of Cu(NO3)2‚3H2O (Fluka) on the Al2O3-ZrO2 support. The samples were dried at 383 K for 16 h and subsequently calcined at 773 K for 5 h in the air. X-ray powder diffraction patterns were obtained with a Rigaku Miniflex diffractometer, using Cu KR radiation (1.5406 Å) at 30 kV and 150 mA. The measurements were recorded in steps of 2° with a count time of 1 min in the 2θ range of 1065°. The BET surface area of the samples was determined from multipoint BET isotherms (Quantachrome Autosorb-1) using nitrogen as an adsorbate at 77 K. Before measurement, the samples were degassed at 423 K for 1 h. ESR spectra of the precalcined samples were recorded on a Bruker EMX-X-band spectrometer at the X-band frequency 9.7667 GHz at 293 K. The spectra were calibrated with an ER 035M NMR Gauss meter. Temperature-programmed reduction studies were carried out on an Auto Chem 2910 (Micromeritics, USA) instrument to estimate the copper dispersion and reducibility. In a typical experiment, 250 mg of the oven-dried sample (dried at 373 K for 15 h) was placed in a U-shaped quartz sample tube. The catalyst was mounted on a quartz wool plug. Prior to TPR studies, argon gas was passed with a flow rate of 50 mL/min at 393 K for 2 h through the catalyst sample. After pretreatment, the sample was cooled to ambient temperature, and a TPR

Sagar et al. analysis was carried out in a flow of 10% H2-Ar (50 mL/min) from ambient temperature to 673 K at a heating rate of 10 K/min. H2 consumption and Tmax positions are calculated using GRAMS/32 software. Copper surface area and dispersion were calculated by N2O decomposition conducted on an Auto Chem 2910 (Micromeritics, USA). This method consists of two steps: (i) the oxidation of Cu to Cu2O by N2O and (ii) H2 temperature-programmed reduction of the formed Cu2O surface species (s-TPR).24 Before analysis, an in situ prereduction of the CuO phase to Cu (0) was performed in flow mixture of 10% H2-Ar, raising the temperature at a heating rate of 10 K/min up to 623 K. The catalyst was purged and cooled to 353 K in Ar, and the Cuto-Cu2O oxidation by the adsorptive decomposition of N2O was performed at 333 K for 45 min with flowing N2O (50 mL/min). In sequence, the catalyst was again purged in an Ar flow and cooled to room temperature. After this, s-TPR was carried out similarly to TPR, raising the temperature up to 723 K on the freshly oxidized Cu2O surface in order to reduce Cu2O to Cu. H2 uptake during s-TPR was calculated using GRAMS/32 software. Temperature-programmed desorption of CO2 studies were also conducted on an Auto Chem 2910 (Micromeritics, USA) instrument. In a typical experiment for TPD studies, about 200 mg of the oven-dried sample (dried at 383 K overnight) was placed in a U-shaped quartz sample tube. Prior to TPD studies, the catalyst sample was pretreated at 473 K for 30 min by passing very pure helium (99.999%, 50 mL/min) through it. After pretreatment of the sample, it was reduced at 523 K for 2 h by passing very pure hydrogen (99.99%, 50 mL/min) through it and subsequently flushed with pure helium (50 mL/ min) for 30 min to remove the excess hydrogen. After the sample was reduced, it was saturated with CO2 in a flow of a 10% CO2-He mixture at 303 K with a flow rate of 50 mL/min and was subsequently flushed at 378 K for 1 h to remove physisorbed CO2. A TPD analysis was carried out from ambient temperature to 1073 K at a heating rate of 10 K/min. The amount of CO2 desorbed was calculated using GRAMS/32 software. The temperature-programmed desorption of NH3 was carried out using the same instrument employed in CO2 TPD measurements. The sample, about 200 mg, was exactly weighted, pretreated, and reduced by the same procedure as that described for CO2 TPD. After reducing the sample, it was saturated with NH3 in a flow of a 10% NH3-He mixture at 353 K at a flow rate of 75 mL/min and was subsequently flushed at 378 K for 2 h to remove physisorbed NH3. A TPD analysis was carried out from ambient temperature to 1073 K at a heating rate of 10 K/min. The amount of NH3 desorbed was calculated using GRAMS/32 software. A downflow fixed-bed reactor operating at atmospheric pressure and made of Pyrex glass was used to test the catalysts during the dehydrogenation of cyclohexanol to cyclohexanone. About 500 mg of the catalyst diluted with an equal amount of quartz grains was charged into the reactor and was supported on a glass wool bed. Prior to introducing the reactant cyclohexanol with a syringe pump, the catalyst was reduced at 523 K for 3 h, in a purified hydrogen flow. After the prereduction, the reactor was fed with cyclohexanol (6 mL/min) at 523 K in N2 (flow rate 50 mL/min), which is used as a carrier gas. The liquid products, mainly cyclohexanone and cyclohexene, were analyzed by a Hewlett-Packard 6890 gas chromatograph equipped with a flame ionization detector using an HP-5 capillary column. The products were also identified using an HP 5973 quadruple GC-MSD system using an HP-1MS capillary column.

Dispersion and Reactivity of Copper Catalysts

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Figure 1. X-ray diffraction pattern of various CuO/Al2O3-ZrO2 catalysts (• means peak due to CuO).

TABLE 1: BET Surface Area and Pore Size Distribution of Various CuO/Al2O3-ZrO2 Catalysts s. no.

Cu loading wt %

BET surfacea area (m2/g)

total poreb volume (ml/g)

total poreb area (m2/g)

1 2 3. 3 4 5 6

0.0 1.0 2.5 5.0 10 15 20

207 194 181 168 155 133 121

0.6906 0.6433

216 201

Figure 2. Electron spin resonance spectra of various CuO/Al2O3ZrO2 catalysts.

0.5738 0.5355 0.4666

182 168 151

TABLE 2: ESR Parameters of Various CuO/Al2O3-ZrO2 Catalysts

a

Measured from nitrogen physisorption. b Measured by mercury porosimetry.

3. Results and Discussion The powder X-ray diffraction patterns of CuO catalysts supported on Al2O3-ZrO2 calcined at 773 K are shown in Figure 1. In all of the samples, no diffraction lines due to Al2O3 or ZrO2 were observed in the XRD pattern of CuO/Al2O3ZrO2 (1:1) catalysts, which confirms that the Al2O3-ZrO2 phase is in an amorphous or poorly crystalline material. The XRD patterns suggest that there are no detectable diffraction peaks representing crystalline CuO present in the sample with Cu loading less than 10 wt %, which clearly indicates that the copper oxide species is present in a highly dispersed amorphous state. However, the presence of a highly dispersed surface CuO phase cannot be ruled out, because such particles would be highly dispersed on the Al2O3-ZrO2 support surface and these might be present as smaller crystallites of 700 K, respectively, are reported in Table 5. In the case of supported copper catalysts, the total basicity increases with copper loading up to 5.0 wt % and decreases with a further increase of Cu loading. However, moderate basic sites increase with copper loading up to 5.0 wt % of Cu and decrease at higher loadings of copper. The decrease in basicity at higher copper loadings might be due to the formation of copper oxide crystallites. These findings are also supported by the dehydrogenation of cyclohexanol, wherein the conversion is found to increase up to 5.0 wt % Cu loading and decrease at higher copper loadings. This further illustrates that moderate basic sites are responsible for cyclohexanol dehydrogenation activity. The moderate basicity of the catalysts is mainly due to the copper phase because CO2 uptake increases with an increase in copper loading. The TPD results further suggest that the pure Al2O3-ZrO2 support is more basic than the supported copper catalysts.

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Sagar et al.

TABLE 6: Temperature-Programmed Desorption of NH3 Results of Various Cu/Al2O3-ZrO2 Catalysts NH3 uptakea (µmol/g)

s. no.

Cu loading, wt %

A

B

C

D

1 2 3 4 5 6

0.0 1.0 2.5 5.0 10 15

407 359 339 295 262 254

637 744 731 505 562 508

693

831 843 624 780 794

a Calculated from the temperature-programmed desorption of NH3. A ) result due to physisorption of ammonia; B ) result due to weak acidic sites; C ) result due to moderate acidic sites; D ) result due to strong acidic sites

The TPD of ammonia and pyridine is a versatile and popular method for the determination of the acidity of solid catalysts as well as acid strength. In the present investigation, the acidity of the samples was measured by NH3 TPD. The TPD results of Cu/Al2O3-ZrO2 catalysts are reported in Table 6. Tanabe38 and Kung39 have proposed different models to predict the formation of acid sites when two oxides are combined to form a mixed oxide. Dumesic and co-workers40,41 have discussed the generation of acid sites when one oxide is deposited onto another to form a surface-phase oxide. Thus, it is interesting to study the acidity of the copper supported on the Al2O3-ZrO2 support. Recently, Dominguez et al.15 showed that a Al2O3-ZrO2 support prepared by the sol-gel method was found to be acidic and active in the 2-propanol dehydration reaction. The acidic strength distribution by NH3 TPD is described in three regions depending on their strength. The numbers of weak, medium, and strong acidic sites, expressed in micromoles per gram of NH3 desorbed over the range of temperatures 423-523, 523-623, and >623 K, respectively, are reported in Table 6. From the TPD results, it was found that the total acidity decreases with copper loading up to 5.0 wt % and levels off at higher Cu loadings. However, moderate acidic sites decrease with copper loading up to 5.0 wt % of Cu and increase at higher loadings of copper. The increase in acidity at higher copper loadings might be due to the formation of copper oxide crystallites, as evidenced from XRD results. These findings are also supported by the dehydrogenation of cyclohexanol, wherein the conversion is found to increase up to 5.0 wt % Cu loading and levels off at higher copper loadings. This further illustrates that moderate and weak acidic sites are responsible for cyclohexanol dehydration and dehydrogenation activity, respectively. Figure 5 shows the dependence of activity and selectivity on copper loading during the dehydrogenation of cyclohexanol to cyclohexanone at 523 K. As can be seen from Figure 5, the cyclohexanol conversion increases with an increase of Cu loading up to 5.0 wt % and decreases with a further increase of copper loading on Al2O3-ZrO2. The decrease in the catalytic activity of the catalysts beyond 5.0 wt % of Cu is due to an increase in the crystallinity of copper on the Al2O3-ZrO2. The conversion for the 1.0 wt % Cu loading catalyst was 30% and increases to 57% as the copper loading is increased to 5.0 wt %. The basic sites with medium strength were also found to increase with copper loading up to 5.0 wt % and level off at higher copper loadings, suggesting that the catalytic properties are in good agreement with basicity measurements. Thus, the findings of XRD, N2O decomposition, and CO2 TPD further support the catalytic properties exhibited by various Cu/Al2O3ZrO2 catalysts. The selectivity toward cyclohexanone was found to be very low for 1.0 wt % catalysts because of the acidic sites of the bare support. However, a further increase of copper

Figure 5. Dehydrogenation of cyclohexanol over various Cu/Al2O3ZrO2 catalysts.

loading on Al2O3-ZrO2 increases the selectivity drastically toward cyclohexanone formation to 81% from 5.0 wt % of Cu. The selectivity was increased with a further increase of the copper loading, except a slight decrease is observed for the 20 wt % Cu catalyst. This might be due to an increase in the crystallinity that makes more of the support surface available for dehydration activity, which results in the formation of cyclohexene because of the acidic sites of the catalysts at 523 K. The bare support (pure Al2O3-ZrO2) was also found to be less active for cyclohexanone formation under experimental conditions similar to those employed and corrected for the catalyst samples. The conversion of cyclohexanol by pure Al2O3-ZrO2 was found to be 5%, and the products formed were cyclohexanone and cyclohexene with selectivities 2% and 98%, respectively. The contribution of pure Al2O3-ZrO2 toward dehydrogenation was subtracted from the conversion of the Cu/ Al2O3-ZrO2 catalysts. A comparison of the surface characterization and catalytic activity results of 5.0 wt % Cu supported on alumina and zirconia and with different Al/Zr ratio catalysts is reported in Table 7. The catalytic experiments were carried out under conditions similar to those employed for all of the samples. The aim of this study is to see the effect of the support on the catalytic properties in relation to surface properties such as dispersion and metal area. The results of Table 7 clearly show that copper is well-dispersed on the Al2O3-ZrO2 support with an Al/Zr ratio of 50:50 and also has a higher copper metal area. The conversion of cyclohexanol is also found to be higher in the case of the Cu/Al2O3-ZrO2 catalysts with an Al/Zr ratio of 50:50 compared to those of Cu/Al2O3, Cu/ZrO2, and Cu supported on Al2O3-ZrO2 with different ratios of Al/Zr catalysts. However, the selectivity during cyclohexanone formation is found to be less than that of the Cu/ZrO2 catalyst. This is probably due to the redox nature of the support and also the presence of stronger basic sites on ZrO2 as compared to Al2O3ZrO2 and Al2O3, leading to an increase of dehydrogenation functionality during the vapor-phase dehydrogenation of cyclohexanol. The basic and acidic sites of these samples also have been measured by CO2 TPD and NH3 TPD, respectively, and are reported in Table 7. Cyclohexene is the only byproduct formed during the vapor-phase dehydrogenation of cyclohexanol, and it is formed because of the weak basic sites and strong acidic sites of supported Cu catalysts. Carline et al.42 reported similar observations in their study of the dehydrogenation/hydrogenation sites of various Cu/MgAl catalysts for the condensation of methanol with n-propanol

Dispersion and Reactivity of Copper Catalysts

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TABLE 7: Dispersion, Copper Metal Area, CO2 TPD, NH3 TPD, and Dehydrogenation Activity Results of Various Supported Copper Catalystsa NH3 uptake (µmol/g)

CO2 uptake (µmol/g)

5 wt % Cu on Al-Zr

%D*

MA*

PA

WA

MA

SA

WB

MB

100-0 75-25 50-50 25-75 0-100

55 57 83 76 60

361 368 536 490 386

99 140 295 134 73

366 238 624 331 246

1067 708 505 538 275

309 688

393 478 243 408 225

800 723 827 608 260

486

SB

% conversion

% selectivity of cyclohexanone

77

43 49 57 52 50

51 36 81 41 100

%D* ) % dispersion of Cu; MA* ) metal area of Cu (m /g); PA ) physisorbed ammonia (350 °C); WB ) weak basic sites (150-250 °C); MB ) moderate basic sites (250-350°); SB ) strong basic sites (>350 °C); * means calculated from N2O decomposition. a

2

Acknowledgment. G.V.S. thanks the Council of Scientific and Industrial Research (CSIR) for a Senior Research Fellowship (SRF). References and Notes

Figure 6. Relation between turnover frequency and copper loading.

to isobutyl alcohol and also compared the total amount of basic sites and percentages of medium and strong basic sites of various metal-supported catalysts. Manriquez et al.43 have reported that strong acidic sites and weak basic sites are responsible for the dehydration product; the dehydrogenation product required moderate acidic sites and basic sites. The data given in Table 5-7 are consistent with this assertion. To find the relation between the dehydrogenation activity of cyclohexanol and the copper loading, a plot of turnover frequency (TOF) versus copper loading on Al2O3-ZrO2 is shown in Figure 6, where TOF is defined as the number of cyclohexanol molecules converted per second per site of copper. The TOF was found to be constant for all of the catalysts except for the 1.0 and 2.5 wt % of Cu catalysts. This might be due to the presence of amorphous bulk copper or isolated copper at 1.0 and 2.5 wt % catalysts, as shown from the ESR and TPR results. 4. Conclusions XRD results reveal the presence of crystalline CuO at high copper loadings (>5.0 wt %). N2O decomposition is found to be a valuable method for measuring the dispersion of Cu on Al2O3-ZrO2. The results of N2O decomposition suggest that copper oxide is found to be highly dispersed on the Al2O3ZrO2 support. The information obtained by ESR and TPR reveals the presence of two types of copper species on the Al2O3-ZrO2 support. CO2 TPD indicates that the basicity falls into two regions, and the basicity of the catalysts was found to increase with an increase in copper loading and decrease at higher loadings. NH3 TPD indicates that the acidity falls into three regions, and the acidity of the catalysts was found to decrease with an increase in copper loading and level off at higher Cu loadings. Cu supported on Al2O3-ZrO2 is highly active for the vapor-phase dehydrogenation of cyclohexanol. The catalytic activity during cyclohexanol dehydrogenation is related to the dispersion of copper.

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