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Ind. Eng. Chem. Res. 2009, 48, 1154–1162
Characterization and Hydrogenation Activity of Ni/Si(Al)- MCM-41 Catalysts Prepared by Deposition-Precipitation Rube´n Nares,† Jorge Ramı´rez,†,‡ Aı´da Gutie´rrez-Alejandre,‡ and Rogelio Cuevas*,‡ Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas, 152, Me´xico D. F., 07730 Me´xico, and UNICAT, Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, UniVersidad Nacional Auto´noma de Me´xico (UNAM), Cd. UniVersitaria, Me´xico D.F., 04510, Me´xico
The deposition-precipitation (DP) method was used for the preparation of Ni/SiMCM-41 and Ni/AlMCM41 catalysts with different DP times (1-4 h), and the catalysts were characterized by BET, XRD, TPR, FTIR spectroscopy, and TEM. On both types of catalysts, the Ni(II) phases formed are a mixture of 1:1 nickel phyllosilicate and Ni(OH)2. The quantity of each phase at the end of the deposition process determines the final Ni particle size; i.e., greater amounts of Ni(OH)2 lead to larger Ni particles. In addition, it was found that the presence of aluminum in the framework of MCM-41 delays the formation of the nickel hydrosilicate phase. As a result of the consumption of the siliceous pore walls during DP, the long-range order of the MCM-41 hexagonal pore structure is gradually lost. Catalytic activity in the hydrogenation of naphthalene was higher for Ni/SiMCM-41 catalysts and was a linear function of the exposed area of Ni particles. 1. Introduction For Ni-supported catalysts, several strategies and novel methods have been designed to prepare materials with high dispersion and high metal loading.1-10 For the case of Ni/SiO2 catalysts, such features can be achieved using the depositionprecipitation (DP) method developed by Geus and co-workers11-13 and then extensively studied by Burattin et al.14-17 This method is based on the precipitation of a nickel(II) phase onto silica by the slow and homogeneous pH increment to basic conditions, followed by urea hydrolysis at 363 K of a solution containing the metal precursor and the support. The nature of the deposited Ni(II) phase depends on the DP time and silica surface area.15,16 After reduction in hydrogen, Ni/SiO2 catalysts prepared by the DP method lead to highly dispersed Ni particles with average sizes from 27 to 79 Å, depending on the DP time and support surface area,16,17 with the small particle size being caused by the strong interaction of the Ni with the siliceous support.18-21 Thus, the precipitation of the Ni(II) phase and final Ni metallic particle (after reduction) depend on some of the properties of the support. Recently, the DP method was extended to deposit Ni onto a high-Si/Al-ratio Hβ-zeolite (Si/Al ) 75), a crystalline Si-Al system with a high surface area.22 It was found that the Ni species deposited on the surface of the zeolite support were similar to those found during the deposition of Ni onto lowsurface-area SiO2 and that a greater proportion of Ni was deposited on the external zeolite surface. The use of MCM-41-type materials opens new possibilities for the development of better Ni catalysts supported on siliceous materials. These types of materials have high surface areas (∼1000 m2 g-1), large pore volumes, and ordered hexagonal pore arrays with pore diameters between 20 and 100 Å. Moreover, the partial isomorphous substitution of Si4+ ions in the MCM-41 framework structure by Al3+ ions leads to a significant increase in the ion-exchange capacity and to the formation of Bro¨nsted acid sites,23-25 making these materials of great industrial interest.26,27 The incorporation of Ni(II) ions * To whom correspondence should be addressed. E-mail: cuevas@ servidor.unam.mx. † Instituto Mexicano del Petro´leo. ‡ Universidad Nacional Auto´noma de Me´xico (UNAM).
into purely siliceous (SiMCM-41) or aluminum-containing (AlMCM-41) MCM-41 materials can be performed by ion exchange, which produces low loadings of well-dispersed nickel nanoparticles, or by impregnation, which leads to higher metal loadings but with limited dispersion. For example, Ni has been deposited by impregnation with an excess of Ni(II) solution onto MCM-41 to obtain hydrogenation catalysts.28 This method of preparation yielded Ni particle sizes of about 25 nm, which are greater than those obtained previously for Ni/SiO2 and Ni/HBEA using the DP method.14 It is appealing to determine whether the DP method can be extended to obtain highly dispersed Ni/ MCM-41 catalysts and the extent to which the long-range order of the MCM-41 hexagonal pore structure is preserved as increasing amounts of Ni are deposited on the surface. This work aims to combine the interesting textural properties of Si- and Al-MCM-41 with the advantages of the Ni deposition-precipitation method to obtain highly active Ni/Si(or Al-) MCM-41 hydrogenating catalysts. During the study, special attention was paid to the following key points: (i) determining how the textural and structural properties of Siand Al- MCM-41 are affected as increasing amounts of Ni are deposited-precipitated, (ii) determining which Ni(II) phase is mainly formed during DP on the surfaces of SiMCM-41 and AlMCM-41, (iii) investigating how the presence of Al in the siliceous framework of MCM-41 affects the deposition of Ni(II), and (iv) examining how metal dispersion changes with Ni loading. To this end, different amounts of Ni were deposited-precipitated onto Si- and Al-MCM-41. Different physicochemical techniques including X-ray diffraction (XRD), nitrogen physisorption, Fourier transform infrared (FTIR) spectrocopy, temperature-programmed reduction (TPR), and transmission electron microscopy (TEM) were used to characterize the changes occurring to supports and Ni phases during the different stages of catalyst preparation. The performance of the Ni/MCM-41 catalysts with various amounts of Ni was evaluated in the naphthalene hydrogenation reaction.
10.1021/ie800569j CCC: $40.75 2009 American Chemical Society Published on Web 09/27/2008
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2. Experimental Section 2.1. Preparation of Supports and Catalysts. SiMCM-41 was prepared by mixing 18.5 g of a 25% aqueous solution of tetraethyl ammonium hydroxide (TEAOH) with 18.5 g of Ludox 40% and 5.5 g of C16H33(CH3)NBr (cetyltrimethylammonium bromide, CTAB) dissolved in 16.5 g of demineralized water. The mixture was stirred for 30 min and transferred into a Teflonlined autoclave, which was kept at 373 K for 24 h. The resulting gel, which had a molar composition of SiO2/0.33 TEAOH/0.12 CTAB/20 H2O, was filtered. After filtration, the solid was refluxed for 3 h in a solution of 2.6 mL of H2SO4 in 1 L of ethanol. The products were filtered and dried at 373 K for 24 h, after which the solid was heated at 473 K under a flow of nitrogen and later calcined in flowing air for 6 h at 773 K. AlMCM-41 was prepared in the following way: First, three solutions were prepared: solution A, containing 0.126 g of NaOH, 0.4 mL of H2O, and 0.130 g of Al(OH)3; solution B, containing 9.26 g of tetraethyl ammonium hydroxide (TEAOH, 25%) and 9.26 g of Ludox (40%); and solution C, made by stirring 16.5 g of demineralized water and 11 g of CTAB 25% for 20 min. Solutions A and B were thoroughly mixed; then, solution C was added, and stirring was maintained for 30 min. The product mixture was then transferred to a Teflon-lined autoclave and kept without agitation at 373 K for 24 h. The resulting gel mixture had a molar composition of SiO2/0.013 Al2O3/0.026 Na2O/0.12 CTAB/20 H2O. The elimination of CTAB and calcination of the catalysts were performed as described above for SiMCM-41. 2.2. Deposition-Precipitation of Ni. Ni/SiMCM-41 and Ni/ AlMCM-41 were prepared according to the following procedure: First, 250 mL of an aqueous solution containing nickel nitrate (0.14 M) and nitric acid (0.02 M) was prepared. This solution was divided into two parts: 40 mL was used to dissolve 6.3 g of urea at room temperature, and the other 210 mL was used to make a suspension with 1.9 g of SiMCM-41 or AlMCM-41 in a thermostatted vessel. The suspension was heated to 313 K and mixed with the urea solution. The mixture was heated to 363 K to start the deposition-precipitation of nickel onto SiMCM-41 or AlMCM-41. After a chosen DP time (1-4 h), the suspension was cooled to 288-293 K and filtered, and the solid was washed three times with 20 mL of distilled hot water (∼323-333 K). Finally, the samples were dried at 383 K for 24 h. Hereafter, the catalyst samples are referred to as Ni/ SiMCM-41-x and Ni/AlMCM-41-x, where x is the DP time expressed in hours. 2.3. Sample Characterization. The BET nitrogen adsorptiondesorption isotherms were obtained in an ASAP 2000 instrument using N2 at 78 K. Prior to the adsorption, the samples were pretreated under vacuum at 543 K for 3 h. XRD patterns were recorded with a Philips 1050/25 diffractometer using Cu KR radiation (λ ) 1.5418 Å) with goniometer speeds of 1° (2θ) min-1 and 0.5° (2θ) min-1 for high- and lowangle diffractograms, respectively. FTIR spectra of the dried samples were obtained at room temperature in a Nicolet Magna IR 760 instrument (100 scans, 4 cm-1 resolution). For the study of the fundamental region, catalyst samples were finely ground and dispersed in KBr using a 1:100 ratio and then pressed into 20-mg wafers. To study the hydroxyl region, self-supported pressed wafers of catalyst powder were analyzed in an IR cell after activation by heating under high vacuum at 573 K for 1 h. Temperature-programmed reduction (TPR) of dried samples was performed in an ISRI-RIG-100 automated catalyst characterization apparatus. Prior to reduction, the samples were
treated at 423 K under a flow of argon (40 mL min-1). The TPR conditions were as follows: 125 mg of sample was reduced at atmospheric pressure under a stream of 5% v/v H2 in argon (total flow rate of 25 mL min-1), using a linear temperature program from room temperature to 1273 K, with a heating rate of 10 K/min. The Ni contents Ni (wt %) )
( g of NiOg+ofgNiof support ) × 100
of the different Ni/MCM-41 samples were determined with a JEOL JSM-5900LV microscope equipped with an analytical energy dispersion X-ray (EDX) accessory and confirmed by titrating of a previously dissolved sample with EDTA using 5,5′nitrilodibarbituric acid monoammonium salt (“murexide”, ACS reagent, Aldrich 22246-1) as the indicator. For the TEM observations of the metallic Ni particles, the catalyst samples were reduced in a quartz gas flow reactor at atmospheric pressure for 6 h at 723 K under a H2 flow (50 mL min-1). The observations were made with a JEOL JEM 2010 high-resolution electron microscope operating at 200 kV. Histograms of the metal particle size distributions were made from the measurement of at least 300 particles of each sample. The average metal particle size was calculated according to the equation d¯)
∑nd ⁄∑n i i
i
where ni is the number of particles of size di. 2.4. Catalytic Activity. The catalysts were tested in the hydrogenation of naphthalene. Prior to the catalytic activity tests, the catalysts were activated as described in the last paragraph of the preceding section. The hydrogenation of naphthalene was performed in a 300mL batch reactor, where 34 g of n-decane was mixed with 6 g of naphthalene (99.4%). The amount of catalyst used in each test was varied according to the Ni concentration in the catalyst in order to maintain a fixed concentration of metal in all catalytic tests. The experiments were performed at 473 and 60 kg cm-2 of pressure. The products were identified and quantified with an HP 6890 chromatograph equipped with a 50-m HP-1 capillary column. 3. Results and Discussion 3.1. pH Change during DP. The shape of the pH evolution with DP time and the proposed reaction mechanism for the deposition-precipitation of Ni onto SiO2 have both been explained in detail in the literature.12,15,29 Figure 1 shows the variations of pH with DP time during the deposition-precipitation of Ni onto SiMCM-41 and AlMCM-41 at 363 K. Because of the similarity of these curves to those previously reported for SiO2,15,29-31 we assume that, for the Ni/MCM-41 system studied here, the same main reactions take place. A careful analysis of the pH vs DP time curves shows that the pH values for the aluminum-containing sample are always slightly higher than those obtained for SiMCM-41, which means that the consumption of OH ions is slower for AlMCM-41 than for SiMCM-41. This result points to the possibility that the presence of aluminum in the framework structure delays the formation of the nickel hydrosilicate phase. In line with this, the chemical analysis of the samples indicates that, at a given DP time, the amount of Ni deposited on the pure SiO2 support is always slightly greater than that for the corresponding aluminumcontaining sample.
1156 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 1. Variations of pH with DP time caused by the hydrolysis of urea during the deposition-precipitation of Ni onto SiMCM-41 and AlMCM41 at 363 K.
Figure 2. Changes in Ni concentration as a function of DP time for the deposition of Ni onto Si- and Al-MCM-41.
Figure 2 shows the changes in Ni concentration as a function of DP time for the deposition of Ni onto Si- and Al-MCM-41. From repeated experiments, it was found that the Ni loading increases with DP time in a reproducible fashion and that it is possible to establish fairly accurately the change in Ni loading as a function of DP time. 3.2. Changes in Textural Properties with Ni DepositionPrecipitation. The surface areas, average pore diameters, and average pore volumes of the catalysts and supports at the different DP times are presented in Table 1. Surface area values per gram of catalyst show an extensive decrease with DP time. However, these values are affected by the large amount of Ni deposited on the support, which, at DP time of 4 h, is as high as 22.6 wt %. More realistic information can be extracted from the analysis of the results per gram of support. Seeing that significant amounts of Ni(II) phases [Ni(OH)2 and 1:1 nickel phyllosilicate] are deposited on the support, the amount of each phase was estimated from a deconvolution of the TPR profiles (see below) and used in the calculation of the weight of the support in each case. The surface area per gram of support with DP time shows a slight drop (Table 1); however, for high DP times, a small increase in this variable can be observed, most likely due to the contribution of the newly formed Ni phases. Clearly, the surface areas and porosities of the different samples are affected by the partial dissolution of the pore walls occurring during DP and by the deposition of the Ni(II) phases formed during DP, as the Ni phases block part of the support surface area and contribute themselves to the total surface area of the catalyst. This can explain the observed behavior for the
aluminum-containing catalysts: the average pore diameter drops slightly at low DP times (DP ≈ 1 h) as a result of the deposition of Ni phases into the pore walls and then increases most probably because of the contributions to the porosity of the newly formed nickel hydroxide and nickel phyllosilicate phases. For the Ni/SiMCM-41 samples, the pore diameter increases from the start of the DP process, indicating a greater consumption of the siliceous pore walls. Both observations are in line with the idea that the presence of Al in the framework of the MCM-41 delays the dissolution of the pore walls. Figure 3 shows the N2 adsorption-desorption isotherms of Ni catalysts supported on Si- and Al-MCM-41; the isotherms of the corresponding supports are included for reference. Some changes are evident from the adsorption-desorption isotherms as the DP time increases. The hysteresis loop exhibits the following features: (i) It extends from 0.4 to 1.0 P/P0. (ii) It widens as the DP time increases. (iii) Its shape can be assigned to a B- or H3-type isotherm, characteristic of lamellar structures.32,33 These results suggest the formation of Nicontaining lamellar structures (most probably nickel phyllosilicate and/or nickel hydroxide), which, as explained previously, contribute to the surface area and average pore diameter values. The inflection point at P/P0 ) 0.3 for the pure AlMCM-41 sample, characteristic of mesoporous materials, moves to higher P/P0 values for Ni/AlMCM-41 samples at DP times of 2 h or higher, indicating a partial loss of long-range order of the porous structure. This phenomenon could be ascribed to the partial destruction of the pore system associated to both events: first, the formation of the new Ni(II) phases and, second, the dissolution of the support pore walls, a function of the time, pH, and support surface area. 3.3. X-ray Diffraction. 3.3.1. Low-Angle Region (0° < 2θ < 6°). The XRD patterns of SiMCM-41 and AlMCM-41 show three peaks, which can be indexed to (100), (110), and (200) characteristic reflections from the well-ordered hexagonal structure of MCM-41 (Figure 4). The structural properties of Ni/SiMCM-41 and Ni/AlMCM-41 are gradually lost with increasing Ni loading. The intensity of the Ni/AlMCM-41 peaks is higher than that of the Ni/SiMCM-41 peaks. This result signals that the ordered structure is better preserved in the Ni/ AlMCM-41 samples, in agreement with the proposal that the incorporation of aluminum into the MCM-41 framework delays the dissolution of silica during the deposition-precipitation process. 3.3.2. Region 5 < 2θ < 70°. Figure 5 shows the XRD patterns of Ni/SiMCM-41 and Ni/AlMCM-41. In addition to the support reflections, for both samples, one observes two new asymmetric reflections at 2θ ) 33.2° and 59.2° with d spacings of 2.66 and 1.54 Å, respectively. A broad peak at 2θ ) 23.2° with a d spacing of ∼4.0 Å characteristic of amorphous silica is also observed. According to previous work,14 the two new reflections can be assigned to the (10) and (11) reflections of nickel hydroxide, or to the (201) and (132j0), and the (063j3) reflections of 1:1 nickel phyllosilicate. The increase in the intensity of these reflections with DP time, observed in Figure 5, is consistent with the increasing amount of Ni deposited on the SiMCM-41 and AlMCM-41 supports. These reflections appear at smaller DP times in Ni/SiMCM-41 than in Ni/ AlMCM-41, suggesting that the formation of 1:1 nickel phyllosilicate is delayed when Al is present in the MCM-41 structure. Because the characteristic reflections of Ni(OH)2 and 1:1 nickel phyllosilicate appear at similar 2θ positions, it is not possible from the XRD results to rule out the presence of any of the two phases.
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1157 Table 1. Textural Properties of Supports and Catalysts sample AlMCM-41 Ni/AlMCM-41
SiMCM-41 Ni/SiMCM-41
DP time (h)
Ni wt %
1 2 3 4
4.8 12.3 19.1 22.6
1 2
6.6 12.8
surface area (m2/gcat)a
surface area (m2/gsup)a
avg pore diameter (Å)
avg pore volume (cm3 g-1)
1012 788 651 557 537 1050 637 538
1012 911 964 943 1072 1050 987 984
32 31 42 44 47 27 35 39
1.2 0.9 0.8 0.7 0.7 0.9 0.62 0.58
a Estimated considering a proportion of Ni(OH)2 and 1:1 nickel phyllosilicate obtained from the deconvolution of TPR experiments. gcat ) gram of catalyst. gsup ) gram of support.
Figure 3. N2 adsorption-desorption isotherms for (a) SiMCM-41, (b) Ni/ SiMCM-41-1, (c) Ni/SiMCM-41-2, (d) AlMCM-41, (e) Ni/AlMCM-41-1, (f) Ni/AlMCM-41-2, (g) Ni/AlMCM-41-3, (h) Ni/AlMCM-41-4.
Figure 4. Powder X-ray diffraction spectra (0 < 2θ < 6°) of (a) SiMCM41, (b) Ni/SiMCM-41-1, (c) Ni/SiMCM-41-2, (d) AlMCM-41, (e) Ni/ AlMCM-41-1, (f) Ni/AlMCM-41-2.
3.4. Temperature-Programmed Reduction. The TPR profiles of dried Ni/SiMCM-41 and Ni/AlMCM-41 at DP times of 1 and 2 h are shown in Figure 6 a-d. All samples present three main reduction peaks: the low-temperature peak at 655-681 K is assigned to the reduction of Ni(OH)2, whereas the two high-temperature peaks are attributed to the reduction of 1:1 nickel phyllosilicate.4,14,16,17 The positions of these peaks shift to higher temperatures as the DP time increases. As the XRD results show, this is most probably due to a greater degree of crystallization of the nickel phyllosilicate phases. For Ni/SiMCM-41, the contribution of the low-temperature peak assigned to Ni(OH)2 is not too significant (Figure 6a and
Figure 5. Powder X-ray diffraction spectra (5 < 2θ < 70°) of (a) SiMCM41, (b) Ni/SiMCM-41-1, (c) Ni/SiMCM-41-2, (d) AlMCM-41, (e) Ni/ AlMCM-41-1, (f) Ni/AlMCM-41-2.
c). A deconvolution exercise shows that it contributes only 6% to the total area for DP times of 2 h; in contrast, for Ni/AlMCM41, the contribution of Ni(OH)2 is 27% at the same DP time. Moreover, the intensity of the Ni(OH)2 peak increases faster for Ni/AlMCM-41. This result can be explained as a consequence of the greater difficulty for silica dissolution when Al is present in the MCM-41 framework. The production of Ni(OH)2 and 1:1 nickel phyllosilicate is governed by the relative rates of precipitation of Ni from solution on one hand and the dissolution of silica followed by reaction with the Ni complexes in solution to form nickel hydrosilicates on the other. If the dissolution of silica is retarded by the presence of Al, less 1:1 nickel phyllosilicate will be formed, and the -OH ions generated by the urea hydrolysis will be consumed through the formation of Ni(OH)2. 3.5. FTIR Spectroscopy. The IR spectra in the hydroxyl region of Ni/SiMCM-41 and Ni/AlMCM-41, after they had been outgassed at 573 K, are shown in Figure 7. The spectrum of AlMCM-41 shows an intense band at 3742 cm-1 corresponding to external silanol groups (sites isolated or terminating the particles). For Ni/AlMCM-41, as the DP time increases, the 3742 cm-1 band decreases in intensity, and two new bands appear: a sharp one at 3732 cm-1 and a small one at 3702 cm-1. The former could be assigned to silanol groups located inside the channels of the mesoporous structure (silanols interacting through hydrogen bonds), whereas the latter, according to literature reports, could be due to silanol groups located at defects or in mesopores of small diameter.34 These defects could result from the partial dissolution of the siliceous pore walls, causing the breakage of SisOsAl or SisOsSi bonds during Ni deposition. We also observed the growth of an intense, broad band at ∼3637 cm-1, which, at high DP times (DP g 3),
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Figure 6. TPR profiles of (a) Ni/SiMCM-41-1, (b) Ni/AlMCM-41-1, (c) Ni/SiMCM-41-2, (d) Ni/AlMCM-41-2.
Figure 7. OH-region FTIR spectra of (a) SiMCM-41, (b) Ni/SiMCM-411, (c) Ni/SiMCM-41-2, (d) AlMCM-41, (e) Ni/AlMCM-41-1, (f) Ni/ AlMCM-41-2, (g) Ni/AlMCM-41-3, (h) Ni/AlMCM-41-4. Samples outgassed at 573 K.
develops into a band with two maxima localized at 3647 and 3630 cm-1. The bands associated with these maxima correspond to the presence of hydroxyl groups of poorly crystallized nickel hydroxide and/or nickel phyllosilicate, respectively.7 Figure 8 shows the IR spectra of the Ni/AlMCM-41 samples in the 450-4000 cm-1 region. At 1-h DP time, the sample exhibits an IR spectrum similar to that of the pure support. At 2-h DP time, some new bands appear. In the 600-750 cm-1 range, there is a small, broad band with two maxima at 635 and 671 cm-1. These maxima correspond to ill-crystallized R-Ni(OH)2 and 1:1 Ni-phyllosilicate respectively.7 As DP time increases these two bands develop into a single broad band centered at ∼650 cm-1. Additionally, the shoulder at ∼1000 cm-1, which becomes more evident after 2-h DP time, is typical of the presence of 1:1 nickel phyllosilicate. In Figure 8, the weak band at 2217 cm-1 is due to isocyanate ions adsorbed in the interlayer space of Ni(OH)2 or 1:1 nickel phyllosilicate.14 3.6. Electron Microscopy of Dried and Reduced Samples. Figure 9 presents the micrographs of Ni/AlMCM-41 at DP times of 2 and 3 h before and after reduction at 723 K. In the dried samples, without reduction, the micrographs show
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1159
Figure 8. FTIR spectra of (a) Ni(OH)2, (b) 1:1 nickel phyllosilicate synthesized at 298 K, (c) 1:1 nickel phyllosilicate synthesized at 423 K, (d) SiMCM-41, (e) Ni/SiMCM-41-1, (f) Ni/SiMCM-41-2, (g) AlMCM41, (h) Ni/AlMCM-41-1, (i) Ni/AlMCM-41-2.
Figure 9. TEM micrographs of Ni/AlMCM-41 at DP times of 1 and 2 h before and after reduction at 693 K: (a) Ni/AlMCM-41-1, dried; (b) Ni/ AlMCM-41-1, reduced; (c) Ni/AlMCM-41-2, dried; (d) Ni/AlMCM-41-2, reduced.
clearly the formation of lamellar or foil-like structures with random orientations. Similar structures have been observed on dried Ni/SiO2 and Ni/HBEA samples prepared by deposition-precipitation12,18,35,36 and identified as 1:1 nickel phyllosilicate. According to the other characterization techniques, the populations of these lamellar structures are observed to increase with DP time. After reduction at 723 K, the formation of small, homogeneously distributed nickel crystallites is evident. However, it is still possible to observe the presence of some 1:1 nickel phyllosilicate, indicating that, at 723 K, the reduction of Ni is not complete. For each DP time, more than 300 particles were measured to determine the particle size distribution for Ni/AlMCM-41 and Ni/ SiMCM-41 (Figures 10 and 11). Comparison of the Ni/
AlMCM-41 and Ni/SiMCM-41 samples at the same DP time indicates that the average Ni particle size is greater for the aluminum-containing samples. Moreover, for the Ni/AlMCM41 case, the average particle size increases more rapidly with DP (from 34 to 44 Å); in contrast, for the Ni/SiMCM-41 samples, the increase in particle size is only marginal (from 32 to 33 Å) with DP time and can be attributed to the higher proportion of nickel phyllosilicate (see Figure 11). It has been found that Ni metallic particles formed from the reduction of 1:1 nickel phyllosilicate tend to preserve a high degree of dispersion with DP time, despite the increasing amounts of Ni deposited on the support.29,35-37 Therefore, the larger Ni particle size in Ni/AlMCM-41 compared to Ni/SiMCM-41 can be related to a higher amount of Ni(OH)2, which is weakly bound to the support,32 in the former sample. 3.7. Activity of Ni/MCM-41 Catalysts. The hydrogenation of naphthalene is a consecutive reaction; naphthalene is hydrogenated to tetraline, and then tetraline is saturated to cisor trans-decaline (Scheme 1). Considering pseudo-first-order kinetics, the reaction scheme in a simplified form can be represented by the following set of ordinary differential equations, in which the hydrogenation of naphthalene (N) to tetraline (T) is taken as a reversible reaction dCN ) k2CT - k1CN dt dCT ) k1CN - (k2 + k3 + k4)CT dt dCCD ) k3CT dt dCTD ) k4CT dt Estimation of the rate constant values (ki) from the experimental results was done using Micromath Scientist software. The set of equations was integrated using a Runge-Kutta fourthorder method, and the minimization was solved by the Levenderg-Marquard method. The fitting of the concentration (Ci/C0) data was concluded when a deviation of
