Cyclohexane Dehydrogenation over Wet-Impregnated Ni on Al2O3

Ind. Eng. Chem. Res. 2006, 45, 5693-5700

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Cyclohexane Dehydrogenation over Wet-Impregnated Ni on Al2O3-TiO2 Sol-Gel Oxides Jose´ Escobar,*,† Jose´ A. De Los Reyes,‡ Toma´ s Viveros,*,‡ and Marı´a C. Barrera‡ Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan, GustaVo A. Madero, D.F., 07730, Me´ xico, and Area de Ingenierı´a Quı´mica, UniVersidad Auto´ noma Metropolitana-Iztapalapa, AV. San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, Me´ xico, D.F., 09340, Me´ xico

The effect of support composition, metal loading (∼10 and ∼20 wt %), and reduction temperature (573, 673, and 773 K) on the cyclohexane dehydrogenation activity of a series of nickel catalysts supported on Al2O3 and the corresponding mixed oxides with TiO2 at two compositions (Al/Ti ) 2 and 25) were investigated. Under the studied conditions, benzene was the only product detected. TiO2 at low concentration (∼6 wt %, AT25) in the material of lower Ni loading (∼10 wt %) reduced at 773 K improved dehydrogenating properties. In contrast, a strong metal-support interaction (SMSI) effect was apparent after reduction at that temperature for the catalyst of similar Ni concentration supported on the equimolar binary oxide (AT2). However, this support promoted enhanced activity in samples of increased nickel loading (∼20 wt %) reduced at moderate conditions (673 K). Introduction Supported Ni catalysts find wide application in processes such as CO methanation, steam reforming, and hydrocarbons hydrogenation. Impregnation with Ni(NO3)2 as metal precursor has been the most common method to obtain supported formulations of low and medium loading.1-3 For Ni/Al2O3 solids metal loading plays a major role in the catalytic properties considering that at low nickel concentration high amounts of hardly reducible spinel-type species could be originated by solid-state reaction with the support.4 Conversely, easily reducible NiO is preferably formed during calcination of highly loaded samples.5 The reducibility of oxidic nickel species is determined by support properties and type and extent of its interaction with the impregnated phase. Some authors1 reported that nickel reducibility (at ∼10 wt %) on different supports decreased in the following order: Ni/SiO2 > Ni/TiO2 (rutile) ≈ Ni/TiO2 (anatase) ≈ Ni/ZrO2 > Ni/γ-Al2O3 . Ni/MgO. Huang and Schwarz6-9 compared properties and catalytic activity of Ni/Al2O3 prepared by incipient wetness to those of wet-impregnated materials, pointing out that the latter had increased Ni dispersion and higher CO methanation activity. Furthermore, detrimental carbon deposition was diminished. The endothermal (∆H ) 49.3 kcal mol-1) cyclohexane dehydrogenation to benzene has been studied over Ni/Al2O3 catalysts in either sulfided or metallic form.10,11 In recent years, cycloalkane dehydrogenation reactions have found important technological applications in storage and transportation of hydrogen at room temperature. In this regard, some of the main advantages of cycloalkanes are related to their high hydrogen content and boiling point high enough to allow their longdistance transportation without the need of special infrastructure.12 According to Boudart classification,13 cyclohexane dehydrogenation is generally considered a structure-insensitive14 reaction. This means that no special surface atoms arrangements (catalytic sites) are required to promote aromatization. Desai * To whom correspondence should be addressed. J. Escobar: tel., (52) 5591758389; fax, (52) 5591756380; e-mail, [email protected]. T-Viveros: email, [email protected]. † Instituto Mexicano del Petro´leo. ‡ Universidad Auto´noma Metropolitana-Iztapalapa.

and Richardson10 found that activity of Ni/SiO2 catalysts in that reaction (on a per active surface area basis) did not show appreciable variations with nickel particle size. They proposed that transition species could be adsorbed on plane faces (111) or (100) of cubo-octahedral supported metallic particles. Haller and Resasco15 showed that structure-insensitive reactions are not significantly affected (∼50% activity decrease) over 2 wt % Rh/TiO2 in a strong metal-support interaction (SMSI) state where partially reduced TiOx (x < 2) moieties could cover supported metallic particles. Under the same conditions, however, the structure-sensitive ethane hydrogenolysis suffered a 3 orders of magnitude diminution in reaction rate. In previous studies,16-18 metallic catalysts supported on TiO2containing mixed oxides have shown the SMSI state after hightemperature reduction. When the active surface area available to reactants adsorption is decreased, catalytic properties of the supported metal could be severely affected. The extent of this phenomenon depended on several factors, notably TiO2 content, method of titania integration into the second oxide matrix, supported metal loading, and reduction temperature. On the other hand, titania addition to alumina supports could produce beneficial results by impeding the formation of refractory spineltype, consequentially increasing the proportion of more reducible oxidic nickel species.19,20 In this work, a study on the cyclohexane dehydrogenation activity of nickel supported on Al2O3 and the corresponding mixed oxides with TiO2 at two compositions was addressed. The effect of several synthesis parameters (support composition, metal loading, and reduction temperature) was investigated. Materials characterization was carried out by N2 physisorption, X-ray diffraction, atomic absorption spectroscopy, UV-vis DRS, and H2 chemisorption. Experimental Section Catalysts Synthesis. Al2O3 (A) and the corresponding mixed oxides at 2 compositions (molar ratio Al/Ti ) 25 and 2, AT25 and AT2, respectively) were used as supports during catalysts preparation. Those materials were prepared by a low-temperature sol-gel technique as described elsewhere.20 Some properties of these materials are shown in Table 1. Nickel was deposited

10.1021/ie050844x CCC: $33.50 © 2006 American Chemical Society Published on Web 07/06/2006

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Table 1. Properties of Supports Used during Ni Catalysts Preparation (from ref 20) support A AT25 AT2

Tca (K) 973 973 973

phase microcrystalline γ-Al2O3 amorphous amorphous

Sgb (m2 g-1) 283 355 216

Vpc (cm3 g-1) 0.70 1.10 0.70

φpd (nm) 9.9 12.4 13.0

a Calcination temperature. b Surface area. c Pore volume. d Average pore diameter from 4 × Vp/Sg.

on the supports at various loadings by equilibrium wet impregnation. Ni2+ solutions of different concentrations (∼15000170000 ppmw) were prepared by dissolving Ni(NO)3‚6H2O (Baker) in deionized water. One gram of support was immersed in 50 mL of solution under vigorous stirring. Immediately after slurry formation, the pH was controlled at 5 (Conductronic pH 20) by adding basic (NH4OH, Baker) or acidic (HNO3, Baker) solutions. After 1 h contact time equilibrium adsorption was assumed.21 The resulting impregnated precursors with different nickel loading were vacuum-filtered, oven-dried at 373 K (3 h), and finally calcined at 673 K (10 h) under dry-air flow (Praxair). Materials Characterization. Some of the materials characterization data included in this paper have already been presented elsewhere.20 However, we consider relevant to revisit them to try to correlate that infomation with the activity trends found for various catalysts studied in the cyclohexane dehydrogenation. Textural properties of supports were determined by N2 physisorption (75 K) with an AUTOSORB-1 (QUANTACHROME) apparatus. Atomic absorption spectrophotometry (Varian SpectrAA 20) was used to determine the nickel content of the calcined impregnated samples. Crystallographic phases identification was carried out by X-ray diffraction (Siemens D-500, Cu KR radiation, λ ) 0.15406 nm). Ni2+ coordination in the calcined impregnated precursors was studied by diffuse reflectance spectroscopy (Varian Cary 5E UV-Vis-NIR spectrophotometer) using the praying mantis accessory. Samples studied were previously oven-dried and sieved at 80-100 Tyler mesh (0.165 mm average particle diameter). The supported metallic nickel (Ni0) was characterized by H2 chemisorption (Praxair) at room temperature (298 K) in a Micromeritics Accusorb 2100E sorptometer. The reduced samples were previously degassed at 673 K (2 h) and high vacuum (133.32 × 10-5 Pa). Moisture and oxygen traps (Alltech) were used to eliminate impurities in the probe gas. To discard the influence of physisorbed H2, differential isotherms were obtained. First, the “total adsorption isotherm” was measured. Then, high vacuum was applied (at room temperature, 1 h) to the sample under analysis to eliminate loosely bound H2. After that, the physisorbed hydrogen isotherm was registered. Finally, chemisorbed H2 was calculated from the difference between the isotherms previously recorded. Metallic dispersion was defined by the superficial metallic nickel atoms (Nis)/total number of Ni atoms ratio of the sample. The following assumptions were made:22 exposed surface area of 1 nickel atom ) 6.77 × 10-2 nm2; spherical model for the metallic particles and H/Ni adsorption stoichiometry ) 1. Metallic phase properties of the characterized samples are shown in Table 2. Cyclohexane Dehydrogenation. Reduced Ni catalysts (∼10 and ∼20 wt % Ni, 10/ and 20/ series, respectively) were tested in the gas-phase cyclohexane deydrogenation. Fifty milligram portions of calcined impregnated precursor were sieved at 80100 Tyler mesh (0.165 mm average particle diameter) as previous experiments showed the absence of intraparticle diffusional limitations at that particle size. A 12.7 mm internal

Table 2. Effect of ReductionTemperature on the Metallic Parameters of the 10/ and 20/ (∼10 and ∼20 wt % Ni, Respectively) Catalysts Supported on Different Oxides (from ref 20) catalyst

Trb SMf Lpart.d XNia SMe (102, gNi gcat.-1) (K) %Dc (nm) (m2 gcat.-1) (m2 gNi.-1)

10/A

11.4 11.4

673 773

1.6 6.4

59.3 15.1

1.3 5.1

11.4 44.7

20/A

19.3 19.3

673 773

1.0 3.5

98.9 28.0

1.3 4.7

6.7 24.4

673 8.6 773 17.8

11.3 5.4

5.6 11.7

59.6 124.5

10/AT25

9.4 9.4

20/AT25

22.9 22.9

673 773

3.6 3.7

26.7 26.5

5.8 5.8

25.3 25.3

10/AT2

13.6 13.6

673 773

1.7 0.6

57.2 172.8

1.6 0.5

11.8 3.7

20/AT2

24.8 24.8

673 773

4.9 7.4

19.7 13.0

8.5 12.8

34.3 51.6

a Ni fraction in catalyst. b Reduction temperature. c Metallic dispersion. Particle diameter. e Metallic area (per gram of catalyst). f Metallic area (per gram of Ni).

d

diameter Pyrex glass reactor was used. The supported Ni0 catalysts were obtained by in situ reduction under H2 flow (Praxair, 20 mL min-1) of the corresponding oxidic precursors. The activation process was carried out in two steps according to Bartholomew and Farrauto.23 First, the temperature was raised from 298 to 503 K (3 K min-1) and maintained for 1 h. Then, the sample was brought to the final reduction temperature (Tr ) 573-773 K) and kept under those conditions for 12 h. During the dehydrogenation tests, the feed was composed of a H2 stream (Praxair) saturated with cyclohexane (Aldrich) at room temperature (298 K). This flow was varied according to catalyst activity in order to operate the plug-flow reaction system at a differential regime (conversion e 10%). Previous experiments (where reactants mixture flow was varied at constant contact time11) were carried out in order to rule out control by external diffusion. The reaction tests were conducted at atmospheric pressure in the 523-603 K temperature range. Thermodynamic limitations to endothermal benzene formation due to excess H2 concentration could be conveniently avoided by carefully choosing these appropriate reaction conditions (high temperature and low operating pressure).24,25 As the most active sites of metallic catalysts could be rapidly deactivated by coke deposition during the first stages of the reaction, we focused on comparing the long-term stable dehydrogenating activity. Thus, steady-state data after ∼4 h on-stream (where no appreciable variations in conversion could be detected) were considered. To verify catalyst stability, tests were carried out by raising the reaction temperature until the most severe conditions studied were reached, and then the reactor was cooled to the initial value. From the corresponding product analysis, reasonable agreement between conversion values determined at a given temperature by going upward and then downward was obtained. Identification and quantification of reactants and products were performed by gas chromatography by using a Hewlett-Packard 5790 A apparatus with a thermal conductivity detector and provided with a stainless steel (3.175 mm) packed column (Carbowax 20 M 10%/Chromosorb W-HP, 80-100). To calculate intrinsic rates of benzene formation, plug-flow behavior was assumed. At differential regime, the resulting equation was

RM )

Fx WcXNi

(1)

where RM is the benzene formation intrinsic rate (mol gNi-1 s-1),

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Figure 1. UV-Vis DRS spectra of Al2O3-supported oxidic Ni precursors at various loadings (wt %): (a) 11.4; (b) 19.3; (c) 30.5; (d) 44.7.

F is the cyclohexane feed rate (mol s-1), x is the cyclohexane conversion, Wc is the mass of the catalyst (g), and XNi is the Ni fraction in the catalyst (gNi gcat-1). Areal reaction rates (RS, per m2Ni) were calculated by using the metallic phase properties determined by H2 chemisorption shown in Table 2.

Figure 2. UV-Vis DRS spectra of oxidic Ni precursors supported on titania-poor Al2O3-TiO2 (AT25) oxides. Materials at various nickel loadings (wt %): (a) 7.1; (b) 12.9; (c) 19.6; (d) 22.9; (e) 27.8.

Results Materials Characterization. UV-Vis DRS spectra of alumina-supported oxidic nickel precursors at various loadings are shown in Figure 1. The main feature observed was an intense signal in the UV region that shifted from ∼211 to ∼289 nm as the nickel loading increased from ∼11 to ∼45 wt %. The much more evident NiO charge-transfer band (CTB)26 found for samples of higher Ni loading could be considered as evidence of increased amounts of easily reducible nickel in those solids. A shoulder at ∼394 nm related to Ni2+ in octahedral (Oh) coordination27 and a doublet at 565-642 nm attributable to the corresponding tetrahedral (Td) cations28 were also identified. At 670 nm another signal due to Ni2+ (Oh) could be observed.29 The intensity of all these signals increased with Ni concentration in the ∼11 to ∼19 wt % range but the strong absorption registered for the dark gray-black solids of higher loading impeded to observe well-defined peaks. Similar strong absorptions found over the whole wavelength range have been attributed in the past to intervalence absorption in nonstoichiometric NiO, which could contain some Ni3+ cations.30 It was clear, however, that the proportion of easily reducible species increased with nickel concentration. Zielin´ski4 prepared Ni/ Al2O3 catalysts at various loadings by wet impregnation on an alumina support obtained from aluminum isopropoxide hydrolysis. In the 20 wt % Ni/Al2O3 prepared by this author, the proportion of species of high interaction with the support (stoichiometric and nonstoichiometric aluminate) represented about 80% of the total impregnated nickel, meanwhile, in catalysts of lower loading almost all the supported phase was constituted by hardly reducible species where Ni2+ ions could be occupying octahedral (Oh) and/or tetrahedral (Td) positions of the alumina matrix. As the methodologies used in the present work to synthesize alumina carrier and to impregnate it are very similar to those of Zielin´ski,4 the properties of our supported catalysts must be similar to materials of comparable Ni loading prepared by that author. Although the NiO CTB was definite in samples with aluminatitania carrier (AT25) of low Ni loading (∼7 wt %), it broadened and red-shifted by increasing nickel concentration, Figure 2. The rest of the signals had similar behavior to that found in the case of alumina-supported materials. An additional band that appeared at ∼720 nm for a highly loaded precursor (∼23 wt %) could be related to Ni2+ (Oh) in bulk NiO.31 The CTB broadening observed for the AT25-supported samples of higher

Figure 3. UV-Vis DRS spectra of oxidic Ni precursors supported on equimolar Al2O3-TiO2 (AT2) oxides. Materials at various nickel loadings (wt %): (a) 13.6; (b) 21.0; (c) 24.2; (d) 24.8.

nickel loading could be originated in two opposite effects: on one hand, the proportion of more reducible species of lower interaction with the support could be augmented4 (originating with a red-shifted onset). On the other hand, the low pH of the original Ni solutions used during impregnation of these highly loaded precursors could provoke partial dissolution of the mixed oxide support, as suggested by textural analyses data of the corresponding samples.20 The higher nickel-support interaction provoked by that effect could result in the formation of lessdefined NiO domains (thus originating with a blue-shifted signal). Also, the existence of refractory species could be inferred from the shoulder at ∼410 nm (Oh Ni2+ in NiAl2O4),32 more intense in materials with Ni loading of ∼20 wt % or superior. In the case of Ni impregnated precursors with AT2 carrier (Figure 3), the NiO CTB band position (∼326-329 nm) remained practically unaltered by increasing Ni loading (from ∼14% to ∼25 wt %), suggesting that even at the lowest content a considerable amount of easily reducible nickel was present. Also, the broadening of the band to higher energies was less than that observed for the AT25-supported solids (Figure 2), indicating that formation of surface spinel was delayed by TiO2 addition in the support.20 The almost imperceptible signal at about ∼407-410 nm for the samples of lower nickel content was in agreement with the aforementioned fact. Nevertheless, refractory species could exist in small amounts in the case of highly loaded samples because their formation could be promoted during impregnation with originally acidic Ni solutions.20 As mentioned in our previous report,20 NiO (bunsenite phase) crystals were identified just in the highly loaded precursors supported on amorphous (see Table 1) alumina-titania mixed oxides, Figure 4. The sample with AT2 carrier showed more intense and narrower peaks, indicating lower impregnate phase dispersion, as to that of the AT25-supported material. In the case of alumina-supported samples even the solids of high Ni

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Figure 6. Intrinsic cyclohexane dehydrogenation activity (per m2Ni) of the 10/ series Ni catalysts reduced at different temperatures (673 or 773 K) and supported on various oxides. Flow reactor operating at 573 K and atmospheric pressure.

Figure 4. Diffractograms of ∼20 wt % Ni calcined precursors supported on Al2O3-TiO2 of different compositions: (a) /AT2; (b) /AT25. *: NiO (bunsenite phase) (from ref 20).

Figure 5. Intrinsic cyclohexane dehydrogenation activity (per gNi) of the 10/ series Ni catalysts reduced at different temperatures (673 or 773 K) and supported on various oxides. Flow reactor operating at 573 K and atmospheric pressure.

concentration (not shown) were amorphous. No nickel phases could be detected by XRD in the rest of the prepared samples, probably due to either the formation of well-dispersed spinellike compounds or the existence of undetectable small (∼3-4 nm) NiO particles. Cyclohexane Dehydrogenation. For all the studied catalysts, 100% selectivity to benzene was found while methane was observed in trace amounts just for the highest reaction temperatures, in agreement with previously reported tests carried out over nickel catalysts supported in other mixed oxides systems.33 In Figure 5 the intrinsic reaction rate (per gram of Ni) of 10/ series catalysts reduced at different temperature (573-773 K) and supported on various oxides is shown. After activation at 573 and 673 K the alumina-supported catalyst remained dark green, typical color of oxidic nickel phases. The presence of this unreduced phase could explain the absence of catalytic activity. After more severe reduction (773 K) the solid became black, suggesting the presence of metallic nickel (Ni0) that could effectively promote cyclohexane dehydrogenation (Figure 5). The 10/AT25 catalyst activated at 573 K showed very low dehydrogenating activity but just at the very beginning of the reaction test. After a short period of time, it was completely deactivated, probably by coke deposition. The catalytic properties of 10/AT25 were clearly improved by raising the reduction temperature to 673 K. Considering that 10/A was inert after reduction at 673 K, it seemed that TiO2 addition in the AT25 carrier increased reducibility of the supported phase. With

operation at 573 K (Figure 5), the dehydrogenating activity of 10/AT25 was ∼85% higher than that of 10/A (samples reduced at 773 K). The 10/ series sample supported on the AT2 mixed oxide was the only one that had a slight dehydrogenating activity after reduction at lower temperature (573 K); meanwhile, the rest of the catalysts were completely inert (Figure 5). This catalyst had an intrinsic reaction rate about 1 order of magnitude lower than those registered for the rest of the tested solids. The slight activity of 10/AT2 reduced at 573 K was increased about three times by raising the activation temperature to 673 K. However, after more severe treatment (at 773 K), decreased dehydrogenating properties suggested a SMSI state. Similar results have been previously observed for nickel catalysts supported in TiO2rich mixed oxides.33 The activity of 10/AT2 was more than 5 times lower (materials tested at 573 K) than that of the solid supported in alumina-rich mixed oxide 10/AT25 (both samples reduced at 673 K). Although this suggested that decreased catalytic properties due to SMSI state could already be noticed after reduction at moderate temperature (673 K), that effect was much more pronounced for the catalyst activated at more severe conditions (773 K). In this case, the intrinsic rate to benzene formation (at similar reaction temperature) for 10/AT2 was ∼44 times less than that of the equivalent AT25-supported catalyst. In Figure 6 the cyclohexane dehydrogenation areal reaction rate observed for various 10/ series catalysts is presented. On this basis, the alumina-supported sample reduced at 773 K had the highest activity. The areal reaction rate progressively decreased by TiO2 addition, although this effect was not directly related to titania content in the mixed supports. 10/AT2 reduced at 773 K (in an apparent SMSI state) had sites just half as active as those of the alumina-supported sample (10/A). In the same line, Haller and Resasco15 reported diminished activity (by a factor of ∼0.5) in the cyclohexane dehydrogenation (at 573 K) of 2 wt % Rh/TiO2 after increasing activation temperature from 473 to773 K. For samples activated at 673 K, however, the binary oxides apparently promoted higher reducibility of the supported phase as to that on the 10/A sample. Again, the areal dehydrogenating activity slightly diminished by increasing titania concentration in the mixed carrier. Effect of Increasing Ni Loading. The intrinsic reaction rate (per mass of nickel) promoted by 20/A (tested at 573 K) was 5-fold increased by raising the reduction temperature from 673 to 773 K, Figure 7. With reduction at 773 K, the dehydrogenating activity of 20/A was about 2 times higher than that of the corresponding 10/A sample (Figure 5). For catalysts reduced at 673 K, the dehydrogenating capacity clearly improved with titania addition. When tested at 573 K, the 20/ series materials with binary carrier showed activity increased by a factor of ∼2.3 (/AT25) and ∼3.5 (/AT2), as to

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Figure 7. Intrinsic cyclohexane dehydrogenation activity (per gNi) of the 20/ series Ni catalysts reduced at different temperatures (673 or 773 K) and supported on various oxides. Flow reactor operating at 573 K and atmospheric pressure.

Figure 8. Intrinsic cyclohexane dehydrogenation activity (per m2Ni) of the 20/ series Ni catalysts reduced at different temperatures (673 or 773 K) and supported on various oxides. Flow reactor operating at 573 K and atmospheric pressure.

that of the alumina-supported sample (20/A). Surprisingly, the catalysts with mixed oxide support carrier were negatively affected by increasing reduction temperature (from 673 to 773 K). This diminution was of ∼55 and 43% for the catalysts impregnated on AT25 and AT2, respectively. On an areal basis and similarly to that found for the 10/ series samples (Figure 6), the alumina-supported material reduced at 773 K was the one of the highest activity, Figure 8. Opposite to that registered in Figure 6, titania addition was not beneficial in the case of solids reduced at 673 K. Similar behavior was determined for catalysts activated at 773 K. For catalysts with alumina-titania carrier, no correlation was observed between TiO2 concentration and decrease in dehydrogenating properties. A clear trend was evident, showing areal dehydrogenating activity values for the 20/ series solids much higher than those of the corresponding samples of lower Ni loading (see Figures 6 and 8). Discussion Effect of Support Composition. According to the present results, the effect of TiO2 addition on the dehydrogenating activity of supported nickel catalysts depended on various factors, namely, carrier composition, metal loading, and reduction temperature. The 100% selectivity to benzene without measurable amounts of hydrogenolysis products observed over all catalysts could be attributed to both reaction conditions and properties of the tested catalysts. In this line, it has been reported34 that at temperatures lower than 573 K the production of light alkanes as byproducts during cyclohexane dehydrogenation is negligible. Other authors10 have pointed out that the hydrogenolysis of that cycloalkane is a structure-demanding reaction that is carried out on the edge sites of cubooctahedrallike supported nickel particles. Due to that, that reaction could be favored on catalysts containing small nickel crystallites (particle diameter < 3.5 nm) where the edge/planar sites ratio

is increased. In our case, however, for all tested catalysts that metallic particle size was clearly surpassed (Table 2). The equilibrium wet impregnation plays a major role in the magnitude of the interaction between the supported phase and the carrier. As determined by UV-Vis DRS (Figures 1 and 2), partial alumina matrix dissolution due to the contact with originally acidic Ni2+ impregnating solutions promoted formation of hardly reducible spinel-type species especially in catalysts of high Al2O3 content. The inertness of 10/A reduced at 673 (Figure 5) indicated that the impregnated phase-support interaction was strong enough to impede proper nickel activation. Thus, Ni2+ reduction at moderate temperature did not take place in an appreciable degree. This was confirmed by H2 chemisorption (Table 2) where a very low metallic area (SM) for this formulation was evident. When the severity of the treatment was increased (to 773 K), the necessary energy to activate refractory oxidic nickel species reduction could be provided. In our case, when the reduction temperature of 10/A was raised from 673 to 773 K, the amount of surface Ni0 was increased by a factor of ∼4 (Table 2). From H2 chemisorption data for the 10/ series catalysts supported on mixed oxides (Table 2), TiO2 addition in the AT25 oxide considerably improved metallic phase properties, as compared to those of 10/A. The 10/AT25 catalyst reduced at 773 K was the one with the best dehydrogenating properties (Figure 5). This seemed to be related to both increased metallic dispersion and inhibited formation of spinel-type species.20 The intrinsic activity improvement achieved by the addition of a small TiO2 amount (∼6 wt % in AT25) was remarkable. Even though the metallic area (per gram of Ni) of 10/AT25 was ∼2.8 times higher than that of 10/A (catalysts reduced at 773 K, Table 2), the increase in dehydrogenating activity was just ∼85%, resulting in lower areal reaction rate to that of the alumina-supported material (Figure 6). Partially reduced titania (TiOx, x < 2) migration seemed to provoke covering of metallic Ni0 in 10/AT2 that could result in decreased dehydrogenating activity, Figure 5. In this line, Mori et al.35 suggested that in 2 wt % Pd/TiO2 surface migration of TiOx species started during reduction at mild conditions (673 K). Me´riaudeau et al.36 have proposed that electron transfer from TiOx phase to empty d orbitals of the supported metal causes H2 adsorption supression in catalysts in the SMSI state. Taking into account the Ni0 particle size of the 10/ series catalysts supported on alumina or AT25 mixed oxide and reduced at 773 K (5.1 and 11.7 nm, respectively), the metallic nickel crystal diameter for 10/AT2 seemed to be unrealistically large (173 nm, see Table 2). This particle size, 1 order of magnitude larger than that of other samples of similar Ni loading, could be provoked by overestimation due to the spherical model supposed (see Experimental Section) in crystallites partially covered by reduced TiOx patches. Thus, we considered this fact as sound evidence of the SMSI state. Other investigators18 have observed SMSI state in metallic catalysts supported on Al2O3-TiO2 mixed oxides. The magnitude of that effect was a function of titania content, support preparation method, and reduction temperature. For instance, in 1 wt % Ir/Al2O3-TiO2 (6.6 wt % titania, where the binary oxide was obtained by impregnating alumina with the precursor of the second oxide) Foger and Anderson18 registered a SMSI state that was absent in samples with coprecipitated aluminatitania carrier of higher TiO2 concentration (16.4 wt %). Based on the proposal of those authors, at low titania content Ti and Al atoms are bonded through “oxygen bridges” (Ti-O-Al bonds). This could stabilize Ti4+ to reduction to Ti3+ and,

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consequently, could hinder TiOx migration. Then, covering of supported metallic particles by those species could be less probable or could not take place to an appreciable extent. In agreement with this stabilizing effect that alumina could exert on titania, when the reduction temperature of 10/AT25 was raised from 673 to 773 K, no decrease in dehydrogenating activity was registered (Figure 5). Conversely, for catalysts with equimolar support the number of reducible Ti-O-Ti ensembles could be much higher, facilitating a SMSI state after reduction at severe conditions.37 Interestingly, Lu et al.17 reported that in 1 wt % Pt supported on TiO2-ZrO2 equimolar carrier the SMSI state was not evident after high-temperature reduction (773 K). In contrast, they observed that the catalyst retained its chemisorptive properties and catalytic activity in the naphthalene hydrogenation. These apparently contradictory results could be rationalized considering that by forming a solid solution with titania,38 ZrO2 could exert a more effective stabilizing effect than Al2O3. Although the main influence of Ni0 particles covered by TiOx patches appeared to be geometrical in nature (lower metallic surface for reactants adsorption), an electronic effect that could affect quimisorptive properties of Ni0 crystals could not be ruled out. As recently reported,40 decoration of Ni0 particles could decrease their ability to adsorb O2, CO, and H2, the magnitude of this phenomenon depending on the nature of the dopant. If the chemisorptive ability of the metallic phase were diminished, the areal activity could be negatively affected, as indeed observed in Figure 6. It is worth mentioning, however, that despite the aforementioned facts, some authors have reported41 high turnover frecuency values (in the CO hydrogenation) for Pd and Pt supported on titania for catalysts in the SMSI state. Effect of Increasing Metal Loading. Several investigations indicated that nickel present in calcined alumina-supported precursors could be forming two4 or even three different species.42 One of them could be the NiO phase, the rest being hardly reducible spinel-type compounds. The concentration of the former (easily reducible nickel) increased with the metal loading whereas the nickel occupying cationic vacancies of the alumina lattice (specially those in tetrahedral positions) could be very refractory to hydrogen reduction. According to Kester et al.,43 the most active sites in hydrogenation-dehydrogenation reactions could be those present in Ni0 crystallites directly originating from NiO clusters reduction. Conversely, sites of the lowest activity could be associated to nickel atoms surrounded by oxygen atoms from the alumina matrix. These sites corresponding to species in high interaction with the alumina support could be in progressively increased proportion as the nickel loading decreased. Based on the characterization results from UV-vis DRS of our alumina-suported samples at various Ni loadings (Figure 1), in the oxidic precursors of both catalyst series tested in cyclohexane dehydrogenation, spinel-type species were present although in the precursor with 19.3 wt % Ni (the one that originated with the 20/A catalyst) the formation of NiO clusters appeared incipient. Thus, reduction of this sample could result in a catalyst of higher number of active sites of increased dehydrogenating activity,43 as compared to the corresponding 10/ series material (see Figures 6 and 8). Even though the metallic area (per gram of catalyst) of 10/ AT25 was similar to that of 20/AT25 (both reduced at 673 K, Table 2), the areal dehydrogenation rate promoted by the latter (at 573 K) was about 4 times higher than that of the former. For catalysts with alumina carrier a similar trend was observed

where the 20/ series solid reduced at 773 K, of metallic area slightly lower than that of the corresponding 10/A material (Table 2), had areal dehydrogenating activity increased by a factor of ∼3.9 (Figures 6 and 8) compared to that of the latter. This fact corroborated that the 20/ series catalysts had sites that could promote much higher dehydrogenating activity than those of the 10/ series solids. We could speculate that the decreased dehydrogenating activity of the 20/ series catalyst supported in the binary oxides reduced at 773 K (Figure 7) might be caused by extensive coking due to an increased proportion of sites of enhanced initial activity. It is worth reminding that our reaction rate data corresponded to steady state activity reached after some hours on stream (see Experimental Section). If we assume that these highly loaded solids originally contained very active dehydrogenating sites, it is probable that they could promote accelerated coke deposition that eventually deactivated the catalysts. Sintering of the metallic phase could be discarded because the operating temperature was not high enough to produce that phenomenon. In any case, that deactivation precluded to relate the metallic properties of the catalysts (as determined by H2 chemisorption) to their dehydrogenating activity. It is clear that more detailed studies are needed to shed light on the observed behavior. For highly loaded (∼20 wt %) samples reduced at 673 K, the nickel catalyst supported on eitheir mixed oxides (AT25 and AT2) showed lower areal dehydrogenating rate than that of 20/A (Figure 8), suggesting electronic modification of the active sites due to titania addition. This behavior must be dictated by the extent of the nickel-support interaction which apparently diminished in samples of increased metal loading20 (than of larger supported particle size). The Ni0 particle size of our highly loaded catalysts supported in the equimolar oxide seemed to be large enough (see Table 2) to avoid their covering to a significant extent by partially reduced titania even after reduction at severe conditions (773 K). This limited coverage of supported crystallites could avoid a SMSI state. In this regard, it has been reported39 that titaniasupported Ni0 particles of diameter >5 nm retained a high proportion of surface available to reactants adsorption after reduction at 673 K. However, electronic effects from Nisupport interaction could still play an important role, decreasing the chemisorptive properties of the catalysts with binary oxides carrier, originating then decreased areal activity (see Figure 8). For the 20/ series materials titania addition was reflected in improved metallic phase properties (Table 2). The corresponding catalyst with AT2 carrier had metallic area much higher than that of the equivalent alumina-supported sample. Nevertheless, for catalysts supported on either mixed oxides and reduced at 773 K, that beneficial effect was limited by their susceptibility to deactivation (Figure 7), presumably by accelerated coke deposition on the most active sites. Regarding catalysts of high nickel concentration, the alumina-supported sample (reduced at 773 K) was the material of the highest stable dehydrogenating activity. Conclusions TiO2 addition clearly influenced dehydrogenating properties of alumina-supported nickel catalysts as a function of several parameters, namely, support composition, metal loading, and reduction temperature. Among the 10/ series solids (∼10 wt % Ni) activated at 773 K, that supported on the alumina-rich mixed oxide (Al/Ti )

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25) was the one with the highest catalytic activity for conversion of cyclohexane to benzene. According to H2 chemisorption data, this could be related to improved nickel dispersion promoted by titania addition at low concentration (∼6 wt %). Nevertheless, this beneficial effect was limited by an apparent SMSI state that suppressed dehydrogenating activity in samples supported on the equimolar oxide (Al/Ti ) 2). This detrimental phenomenon appeared to be geometrical and electronic in nature. In the catalysts tested, the proportion of metallic sites of higher activity increased with nickel loading. However, the higher dispersion of the ∼10 wt % Ni catalysts offset their lower areal activity. For the highly loaded catalyst (∼20 wt % Ni) supported on the equimolar binary oxide the larger Ni0 particle size seemed to contribute to decreased covering of metallic particles by partially reduced TiOx moieties. However, the electronic effect of the nickel-support interaction was still evident and being reflected in decreased areal dehydrogenating rate. Although in general dehydrogenating activity increased with the reduction temperature, activation at 773 K was detrimental to catalysts of high nickel concentration supported on either mixed oxides where deactivation was registered. This could probably be provoked by rapid coke deposition due to the high initial activity of the existing sites. Acknowledgment The authors gratefully acknowledge financial support from CONACyT (Mexico) through the 400200-5-4472A grant. J. Escobar is also indebted to IMP (Mexico). Literature Cited (1) Narayanan, S.; Sreekanth, G. Influence of Support on the Availability of Nickel in Supported Catalysts for Hydrogen Chemisorption and Hydrogenation of Benzene. J. Chem. Soc., Faraday Trans. 1. 1989, 11, 3785. (2) Grzechowiak, J. R.; Szyszka, I.; Rynkowsky, J.; Rajski, D. Preparation, Characterisation and Activity of Nickel Supported on Silica. Appl. Catal. A 2003, 247, 193. (3) Wojcieszak, R.; Monteverdi, S.; Mercy, M.; Nowak, I.; Ziolek, M.; Bettahar, M. M. Nickel Containing MCM-41 and AlMCM-41 Mesoporous Molecular Sieves: Characteristics and Activity in the Hydrogenation of Benzene. Appl. Catal., A 2004, 268, 241. (4) Zielin´ski, J. Morphology of Nickel/Alumina Catalysts. J. Catal. 1982, 76, 157. (5) Rynkowski, J. M.; Paryjczak, T.; Lenik, M. On the Nature of Oxidic Nickel Phases in Ni/Al2O3 Catalysts. Appl. Catal., A 1993, 106, 73. (6) Huang, Y. J.; Schwarz, J. A. The Effect of Catalyst Preparation on Catalytic Activity: I. The Catalytic Activity of Ni/Al2O3 Catalysts Prepared by Wet Impregnation. Appl. Catal. 1987, 30, 239. (7) Huang, Y. J.; Schwarz, J. A. The Effect of Catalyst Preparation on Catalytic Activity: II. The Design of Ni/Al2O3 Catalysts Prepared by Wet Impregnation. Appl. Catal. 1987, 30, 255. (8) Huang, Y. J.; Schwarz, J. A. The Effect of Catalyst Preparation on Catalytic Activity: The Catalytic Activity of Ni/Al2O3 Catalysts Prepared by Incipient Wetness. Appl. Catal. 1987, 32, 45. (9) Huang, Y. J.; Schwarz, J. A. The Effect of Catalyst Preparation on Catalytic Activity: IV. The Design of Ni/Al2O3 Catalysts Prepared by Incipient Wetness. Appl. Catal. 1987, 32, 59. (10) Desai, P. H.; Richardson, J. T. Crystallite Size Effects in Nickel Catalysts: Cyclohexane Dehydrogenation and Hydrogenolysis. J. Catal. 1986, 98, 392. (11) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill Book Company: New York, 1980; p 148. (12) Biniwale, R. V.; Yamashiro, H.; Ichikawa, M. In-situ Infrared Thermographic Analysis During Dehydrogenation of Cyclohexane Over Carbon-supported Pt Catalysts Using Spray-pulsed Reactor. Catal. Lett. 2005, 102, 23. (13) Boudart, M. AdVances in Catalysis; Academic Press: New York, 1969; Vol. 20, p 153. (14) Sinfelt, J. H.; Carter, H. L.; Yates, D. J. C. Catalytic Hydrogenolysis and Dehydrogenation over Copper-Nickel. J. Catal. 1972, 24, 283.

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ReceiVed for reView July 19, 2005 ReVised manuscript receiVed May 22, 2006 Accepted May 23, 2006 IE050844X