Preparation, Characterization, and Performance of HMS-Supported Ni

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Ind. Eng. Chem. Res. 2009, 48, 3802–3811

Preparation, Characterization, and Performance of HMS-Supported Ni Catalysts for Hydrodechlorination of Chorobenzene Jixiang Chen,* Jianjun Zhou, Rijie Wang, and Jiyan Zhang Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China

HMS-supported Ni catalysts were prepared by the direct synthesis and the impregnation method. In the direct synthesis, the effect of nickel content and pH value of the preparation system on the catalyst structure and hydrodechlorination performance was systematically investigated. The physicochemical properties of the catalysts were characterized by means of N2 adsorption, hydrogen temperature-programmed reduction, lowand wide-angle X-ray diffraction, hydrogen chemisorption, hydrogen temperature-programmed desorption, transmission electron microscope, and atomic absorption spectroscopy. The catalyst activity in the hydrodechlorination of chlorobenzene was evaluated in a fixed-bed reactor at atmospheric pressure. For the n%Ni(m%)-HMS samples prepared by the direct synthesis method, BET surface area, pore volume, and the pore (2∼5 nm) diameter decrease with increasing Ni content, and the mesostructures becomes worse. When the nickel content exceeds 7.0 wt%, the sample with mesostructures cannot be prepared. This is attributed to the decrease of pH value in the preparation system and the embedment of Ni2+ in the SiO2 matrix. Ni2+ ions highly disperse in the n%Ni(m%)-HMS samples and mainly exist as nickel silicate. After reduction at 450∼650 °C, the metallic nickel particles in n%Ni(m%)-HMS uniformly distribute at about 3 nm. However, for the im-4.1%Ni/HMS sample prepared by the impregnation method, the metallic nickel particles are much larger than those of n%Ni(m%)-HMS. In the hydrodechlorination of chlorobenzene, the n%Ni(m%)-HMS samples show higher activities than im-4.1%Ni/HMS, which can be attributed to the strong interaction between small metallic nickel particles and the support, a greater amount of spilt-over hydrogen, and the acidity of nickel silicate. When the nickel content exceeds 5.9 wt % and the reduction temperature is above 450 °C, there is no remarkable difference in chlorobenzene conversion for n%Ni(m%)-HMS samples. This is perhaps related to the intraparticle mass transfer limitation. 1. Introduction Silica-based mesoporous materials have attracted a great attention since they possess high thermal stability, large surface areas, and uniform-sized pores. Moreover, the pore size of the materials can be tuned by controlling the chain length of the template or by using auxiliary solvent cosurfactant to swell the micelles. HMS, a hexagonal molecular sieve, is prepared through hydrogen bonding and self-assembly between neutral primary amine surfactant (S0) and neutral inorganic precursor (I0).1,2 Unlike MCM-41 and SBA-15, HMS-based materials lack the long-range hexagonal framework structures and have been shown to possess wormhole-like or sponge-like framework structures.3 However, HMS can be easily synthesized via a sol-gel process in the presence of primary alkylamine as a template at low temperature, even room temperature, and it possesses thick framework walls, small particle size of primary particles, and complementary textural porosity.3,4 The porosity favors the mass/heat transfer in the catalytic reaction. Moreover, many transition metal ions, such as Ti4+,2,4,5 Al3+,3,6 Co2+,7,8 V5+,9 and W6+,10 can be uniformly incorporated in the HMS framework or supported on HMS, which can induce the formation of active sites. HMS-based materials have been used as catalysts or supports for oxidation reaction,5-7,10 photocatalysis,9 hydrotreating,11,12 Fischer-Tropsch synthesis,8 and acid-catalyzed reaction.13 Supported nickel catalysts have been widely studied and applied because of their very high activities in hydrogenation, hydrotreating, and stream-reforming reactions. Because of the * To whom correspondence should be addressed. E-mail: jxchen@ tju.edu.cn. Tel.: +86-22-27890865. Fax: +86-22-87894301.

advantages of mesoporous materials, mesoporous silica (such as MCM-41,14-20 SBA21-24) supported nickel catalysts are of great interest nowadays and have been tested in the hydrogenation of benzene,17,18 hydrodechlorination,19,21-23 adsorptive desulfurization,24 selective oxidation,20 and so on. However, there are few literatures on the preparation and the application of the nickel-modified mesoporous HMS materials.25 It is well-known that catalytic hydrodechlorination (HDC) is a promising method for disposing of chlorinated organic compounds that are produced as wastes or byproduct,26 and the nickel catalysts have been widely used in HDC.19,21-23,27-30 However, there have been no reports that concern HMSsupported Ni catalysts. Because of the advantages of HMS base materials as above-mentioned, HMS-supported Ni catalysts were tested for HDC in the present work. Mesoporous silica supported nickel catalysts can be prepared by direct synthesis14-16,21,22 and the impregnation method.17-19,24 The impregnation method is commonly used to prepare supported metal catalysts. However, this method usually leads to the loss of the surface area and/or the blockage of mesopores with large Ni particles. In addition, the metal dispersion is often low because of weak interaction between nickel and support. When the direct synthesis method is adopted, the nickel species can be incorporated in the matrix of the support, and the catalyst with high nickel dispersion can be obtained. In the present work, in order to prepare HMSsupported nickel catalysts with high dispersion, the direct synthesis method was adopted, and the effect of nickel content and pH value of preparation system on the physicochemical properties of the catalysts was investigated. As a reference, the HMS-supported nickel catalyst was also prepared by the

10.1021/ie801792h CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3803 Table 1. Ni Contents and Change of pH Value during Preparation Ni and SiO2 mass ratio (%)

pH value of preparation system

sample

actual

calculated

actual Si/Ni ratio

B solution

after addition of A to B

before aging

after aging

HMS 4.2%Ni(5.0%)-HMS 5.9%Ni(10%)-HMS 7.5%Ni (15%) -HMS 7.8%Ni(20%)-HMS

4.2 5.9 7.5 7.8

5.0 10 15 20

22.3 15.6 12.1 11.6

10.0 10.0 10.0 10.0 10.0

9.8 8.2 7.4 6.5 6.1

9.7 8.1 7.3 5.8 5.3

9.6 8.0 7.2 5.4 5.0

impregnation method. The hydrodechlorination of chlorobenzene was chosen as a model reaction for evaluating the activities of the concerned catalysts. 2. Experimental Details 2.1. Catalyst Preparation. HMS was synthesized by the neutral S0 I0 templating route, proposed by Tanev and co-workers.1,2,31 Dodecylamine (C12H25NH2, DDA) and tetraethyl orthosilicate (TEOS) were used as the neutral structure director and the neutral silica precursor, respectively. A 44.7 mL portion of TEOS was added into 40.0 mL of ethanol to obtain solution A, and 12.5 mL of DDA, 426.0 mL of H2O, and 106.0 mL of ethanol were mixed to form solution B. Under stirring, solution A was dropwise added to solution B at 55 °C. Afterward, the mixture was stirred for 4 h and then held at 55 °C for 14 h. The obtained solid was washed with ethanol and water. Subsequently, the solid was dried at 115 °C for 4 h and calcined at 550 °C for 5 h. When Ni(NO3)2 · 6H2O was added to solution A, the Nimodified HMS sample was prepared according to the similar procedure to HMS. Ni-modified HMS samples are labeled as n%Ni(m%)-HMS, where n% and m% denote the actual and the calculated mass ratios of nickel and SiO2, respectively. HMS-supported nickel catalyst was also prepared by the impregnation method. The prepared HMS was impregnated with the aqueous solution of Ni(NO3)2, followed by drying at 120 °C for 12 h and calcination at 550 °C for 3 h. The sample is labeled as im-n%Ni/HMS, where n% denotes the actual nickel and SiO2 ratio. 2.2. Catalyst Characterization. N2 adsorption-desorption isotherms were obtained on a Micromeritics TriStar 3000 apparatus. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area, SBET. Total pore volumes were estimated at a relative pressure of 0.99. The Barrett-Joyner-Halenda (BJH) method applied to desorption isotherms was used to determine the mesopore size distribution and average pore diameter. The reducibility of the samples was characterized by hydrogen temperature-programmed reduction (H2-TPR) in a quartz U-tube reactor, into which 50 mg of sample was loaded. The reduction was conducted in a 10% H2/N2 flow of 60 mL/min at a heating rate of 10 °C/min. The hydrogen consumption signal was determined using a thermal conductivity detector (TCD). Taking CuO as the external standard, the actual H2 consumption amount was calculated. The reduction degree of nickel was also measured with the same apparatus. First, in a 10% H2/N2 flow of 60 mL/min and at a rate of 10 °C/min, the sample was heated to a designated temperature that was maintained until the TCD signal returned to the baseline. Second, the H2 consumption amount at this temperature was calculated. The reduction degree of nickel at this temperature is the ratio of H2 consumption amount at the temperature and total H2 consumption amount. X-ray diffraction (XRD) patterns of the samples were obtained on a Rigaku D/max 2500 V/PC powder diffractometer using Cu KR radiation (40 kV, 200 mA). The average size of

metallic nickel crystallites was calculated using the Scherrer equation, d ) 0.9λ/(β cos θ), where d is the crystallite size, λ is the wavelength of the radiation, β is the width of the peak at half-maximum intensity, and θ is the Bragg angle. Hydrogen chemisorption amount and temperature-programmed desorption (H2-TPD) were determined using a TPD/R/O 1100 SERIES (Finnigan) unit. A 0.4 g portion of the sample was loaded in a quartz reactor and reduced at the designated temperature in an H2 flow for 1 h. After the reduction, the hydrogen on the nickel surface was removed with 20 mL/ min of He (99.999%) for 30 min. The catalyst sample was subsequently cooled to 30 °C under He stream, and then He stream was switched to N2. After the TCD was stable under N2 stream, H2 pulses (100 µL/pulse) were injected until the effluent areas of consecutive pulses were constant. Afterword, H2-TPD was performed in a 50 mL/min N2 flow at a heating rate of 10 °C/min. The water derived from the dehydroxylation of the supports was removed by soda-lime, and the desorbed H2 was detected with a TCD. Transmission electron microscope (TEM) analysis was carried out on a Philips Tecnai G2 F20 microscope operating at 200 kV. The powder sample was ultrasonically dispersed in ethanol and then deposited on a holey carbon film supported on a copper grid. The Ni contents in the catalysts were measured on a HITACHI 180-80 Polarized Zeeman atomic absorption spectrophotometer. 2.3. Catalyst Reactivity Evaluation. The catalytic HDC reaction was performed in an atmospheric fixed-bed quartz reactor (i.d. ) 12 mm); 0.8 g of catalyst precursor (0.15∼0.25 mm in diameter) was supported on quartz cotton and a layer of ceramic beads was placed on the catalyst bed. After it was reduced in situ in a H2 flow at the designated temperature for 1 h, the catalyst bed was adjusted to 300 °C. The flow rate of H2 was then designated as 4 × 103 mL/h, and 3 mL/h of chlorobenzene was delivered by a pump and vaporized before passing the catalyst bed. Total space velocity was 0.27 mol · gcat-1 · h-1 and the molar ratio of H2 and CB was 6.4. To avoid the loss due to volatilization, the reaction products were absorbed with anhydrous ethanol in the ice-water bath, and they were subsequently analyzed by a gas chromatograph equipped with a hydrogen flame ionization detector and an OV101 capillary column. 3. Results and Discussion 3.1. Effect of Preparation Conditions on Nickel Content of Catalyst. Table 1 shows the nickel contents in the n%Ni(m%)HMS samples. When m% is between 5% and 15%, n% increases linearly with increasing m%. n% is always lower than m%, indicating that nickel in the preparation system was not completely incorporated into the silica framework. Moreover, the higher m% is, the more remarkable is the difference between n% and m%. When m% exceeds 15%, there is no obvious difference in n% between 7.5%Ni(15%)-HMS and 7.8%Ni(20%)HMS. This phenomenon is related to the pH value of the

3804 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 2. Textural Parameters of Different Samples sample

SBET (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

HMS 4.2%Ni(5.0%)-HMS 5.9%Ni(10%)-HMS 7.5%Ni(15%)-HMS 7.8%Ni(20%)-HMS 5.9%Ni(10%)-HMSa im-4.1%Ni/HMS

887 888 688 505 416 682 695

1.38 0.87 0.68 0.52 0.45 0.67 0.69

4.1 3.4 4.5 8.6 8.8 4.5 3.8

a

Reduced at 550 °C.

Figure 1. N2 adsorption-desorption isotherms of samples.

Figure 3. Low-angle XRD patterns of samples: (a) HMS; (b) 4.2%Ni(5.0%)HMS; (c) 5.9%Ni(10%)-HMS; (d) 7.5%Ni(15%)-HMS.

Figure 2. Pore size distributions of samples.

preparation system. As shown in Table 1, the pH value of the preparation system decreases with increasing m%, which is due to the increase of Ni(NO3)2 amount. It is well-known that the aqueous solution of Ni(NO3)2 is acidic. Since the lowest pH value for the precipitation of Ni2+ is about 6.7, a lower pH value than 6.7 does not favor the precipitation of Ni2+. 3.2. Textural Properties. N2 adsorption-desorption isotherms and pore size distribution curves of the samples are shown in Figure 1 and Figure 2, respectively. The corresponding textural parameters are listed in Table 2. Isotherms of HMS, 4.2%Ni(5.0%)-HMS, 5.9%Ni(10%)-HMS, and im-4.1%Ni/ HMS are characteristics of the IV isotherm according to IUPAC classification32 with a sharp step at intermediate relative pressures. The appreciable type H1 hysteresis loops32 indicate the presence of textural mesopores and cylindrical pores. According to refs 33 and 34, four well-defined stages can be identified: (1) an increase in nitrogen uptake at low relative pressures, corresponding to monolayer-multilayer adsorption

on the pore walls; (2) an inflection (the first hysteresis) at intermediate relative pressure (about p/p0 ) 0.3) indicative of capillary condensation in the “framework-confined mesoporous” that is contained within the uniform channels, and the porous structure belongs to mesostructures; (3) a relative plateau at high relative pressures associated with multilayer adsorption on the external surface; and (4) a well-defined hysteresis loop in the p/p0 region from 0.8 to 1.0, the indication of the “textural mesoporous” arising from noncrystalline intra-aggregate voids and spaces formed by interparticle contacts. In the sequence of HMS, 4.2%Ni(5.0%)-HMS and 5.9%Ni(10%)-HMS, the inflection at the intermediate relative pressures become less sharp and less tall, indicating less of the well-defined framework mesoporosity, that is, mesostructures gets worse with the increase in nickel content. In the isotherms of 7.5%Ni(15%)HMS and 7.8%Ni(20%)-HMS, there is no inflection at the intermediate relative pressures, while the isotherms belong to the type I isothermal at p/p0 < 0.6. This indicates that the mesopore structure is not formed when nickel content exceeds 7.5%; however, the “textural mesoporous” still exists. The above phenomenon can also be proved by the low-angle XRD results (see Figure 3). As shown in Figure 2, between 2∼5 nm, pure HMS shows a well-BJH-desorbed pore size distribution curve with a peak centered at 3.2 nm; with the increase in nickel content, the peaks in the curves of 4.2%Ni(5.0%)-HMS and 5.9%Ni(10%)-HMS shift to lower value, and there are no peaks in the curves of 7.5%Ni(15%)-HMS and 7.8%Ni(20%)-HMS. Obviously, the increase of nickel content leads to the narrowing of the pores between 2 and 5 nm. Moreover, the decrease of the pore volume contributed by the pores with diameter between 2 and 5 nm becomes smaller. Just so, the textural mesoporous volume in 7.5%Ni(15%)-HMS and 7.8%Ni(20%)-HMS has a

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larger proportion in comparison with other samples, which results in larger BJH average pore diameters of 7.5%Ni(15%)HMS and 7.8%Ni(20%)-HMS (see Table 2). Figure 1, Figure 2, and Table 2 also show that the reduced 5.9%Ni(10%)-HMS (labeled as R-5.9%Ni(10%)-HMS) has similar isotherm and pore diameter distribution curve to the unreduced one, indicating that the reduction did not markedly influence the textural properties. Low-angle XRD is an important method for identifying the mesostructures. As shown in Figure 3, there is a signal broad peak at about 2.0 degree attributed to the (100) reflection. This single low-angle peak is considered to the characterization of short-range hexagonal symmetry with uniform pore diameter. However, the peak intensity becomes weaker with the increase in nickel content, indicating that the ordering of mesostructures gets worse. No clear low-angle reflection is observed for 7.5%Ni(15%)-HMS, that is, there is no “framework-confined mesopore” for the samples with nickel content up to 7.5 wt% or higher. Such phenomena often exist when the no-silicon atoms are incorporated into the silica frameworks.35,36 The more no-silicon atoms are incorporated into the silica frameworks, the worse the mesostructures are. The TEM images of the HMS, 5.9%Ni(10%)-HMS and im-4.2%Ni/HMS samples are shown in Figure 4. All images reveal disordered and wormhole messtructures, similar to the report by Pinnavaia and co-workers.31 Although a long-range packing order is absent, a network of channels is regular in diameter. It is accepted that HMS is synthesized by the neutral S0I0 templeting route.1,33 The formation of HMS mesostructures occurs through the micelles derived from the H-bonding interaction between neutral primary amine surfactant molecules (S0) and neutral Si(OC2H5)4-x(OH)x precursors. Further hydrolysis and condensation of the silanol groups on the micelle-solution interface afford the short-range hexagonal packing of the micelles and the formation of framework wall. However, when the pH value decreases, especially lower than 7.0, H+ ions can bond with DDA. This can destroy H-bonding interaction, and the self-assembly is disturbed. Lin and co-workers37 have been reported that HCl can destroy H-bonding between neutral primary amine surfactant and neutral Si(OC2H5)4-x(OH)x precursor. In sum, low pH value and the incorporation of nickel species in SiO2 matrix induce less well-formed mesostructures. Thus, it is valuable to investigate the effect of the increasing pH values on the mesostructures when high nickel content is introduced to HMS. This work is being carried out. 3.3. H2-TPR Results. Figure 5 shows the H2-TPR profiles of n%Ni(m%)-HMS and im-4.1%Ni/HMS. In the trace of im-4.1%Ni/HMS, there is a sharp peak at 371 °C attributed to bulk NiO; at higher temperatures, two shoulder peaks at about 399 and 493 °C can be attributed to NiO interacting with SiO2 and the surface nickel silicate,38 respectively. For the n%Ni(m%)HMS samples, there are similar beginning reduction temperatures, and the small shoulder peak at about 400 °C can be attributed to the NiO interacting with SiO2, while the main peaks between 600 and 700 °C are attributed to the reduction of nickel silicate, which can be justified by the XRD results (see Figure 6). With the increase in nickel content, the main reduction peak shifts to low temperature. This is perhaps related to the decrease of the pH value of the preparation system. The H2-TPR results indicate that the preparation methods (impregnation and direction synthesis) remarkably influence the existence states of the nickel species.

Figure 4. TEM images of samples: (a) HMS; (b) 5.9%Ni(10%)-HMS; (c) im-4.1%Ni/HMS.

Table 3 lists the actual and the theoretical H2 consumption amounts during the H2-TPR. For each sample, the actual H2 consumption amount is more than the theoretical one. This is related to the hydrogen spillover. During the reduction, H2 can adsorb on the formed metallic nickel and subsequently dissociate, and then the H species spill over to the support surface.

3806 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 3. H2 Consumption and Reduction Degree of Samples during H2-TPR

sample 4.2%Ni(5.0%)-HMS 5.9%Ni(10%)-HMS 7.5%Ni(15%)-HMS 7.8%Ni(20%)-HMS im-4.1%Ni/HMS a

Figure 5. H2-TPR profiles of samples: (a) NiO; (b) im-4.1%Ni/HMS; (c) 4.2%Ni(5.0%)-HMS; (d) 5.9%Ni(10%)-HMS; (e) 7.5%Ni(15%)-HMS; (e) 7.8%Ni(20%)-HMS.

Figure 6. Wide-angle XRD patterns of unreduced samples: (a) HMS; (b) 4.2%Ni(5.0%)-HMS; (c) 5.9%Ni(10%)-HMS; (d) 7.5%Ni(15%)-HMS; (e) im-4.1%Ni/HMS.

The hydrogen spillover is also reflected by the H2-TPD results (see Figure 12) Table 3 also shows the nickel reduction degree of the samples. When the reduction temperature is 450 °C, the nickel reduction degrees increases with the increase in nickel content for n%Ni(m%)-HMS; however, they are lower than 30% and much less than that of im-4.1%Ni/HMS. For 5.9%Ni(10%)-HMS, the nickel reduction degree increases from 23 to 87% with increasing reduction temperature from 450 to 650 °C. 3.4. XRD Result. Figure 6 shows the wide-angle XRD patterns of the unreduced samples. For each sample, there is a broad peak (centered at about 22.5°) due to amorphous SiO2,

actual H2 theoretical consumption H2 consumption (mmol/gcat) (mmol/gcat) 0.78 1.34 1.68 1.78 0.94

0.68 0. 96 1.20 1.22 0.66

reduction degree of nickel (mol%) 22.2a 23.0,a 51.6,b 87.2c 26.2a 27.8a 91.0a

Reduced at 450 °C. b Reduced at 550 °C. c Reduced at 650 °C.

Figure 7. Wide-angle XRD patterns of samples reduced at 450 °C: (a) HMS (unreduced); (b) 4.2%Ni(5.0%)-HMS; (c) 5.9%Ni(10%)-HMS; (d) 7.5%Ni(15%)-HMS; (e) 7.8%Ni(20%)-HMS; (f) im-4.1%Ni/HMS.

that is, the framework of the samples is amorphous. In the pattern of im-4.1%Ni/HMS, the peaks at 2θ ) 37.2°, 43.3°, 62.9°, 75.4°, and 79.4° are attributed to NiO. No NiO is detected in the n%Ni(m%)-HMS samples, while nickel silicate (diffraction peaks at 2θ ≈ 34° and 61°) is detected when n% is higher than 5.9%. This indicates that nickel species exist as nickel silicate and homogenously disperse in the samples. This is consistent to the H2-TPR results (see section 3.3) and the sample colors. The color of HMS is white and that of n%Ni(m%)-HMS is gray; however, the color of im-Ni/HMS is black, similar to that of NiO. Figure 7 shows the wide-angle XRD patterns of the samples reduced at 450 °C for 1 h. There are diffraction peaks (2θ ) 44.4°, 51.8°, and 76.4°) due to metal nickel in the pattern of im-4.1%Ni/HMS. On the basis of the (111), (200), and (220) reflections and calculated with the Scherrer equation, the nickel crystallite sizes are 17.7, 21.8, and 26.2 nm, respectively. However, no metallic nickel is detected in the reduced n%Ni(m%)HMS samples. This indicates that the metallic nickel particles highly disperse in the n%Ni(m%)-HMS samples, which is also justified by the TEM results (see Figure 11) Figure 8 shows the wide-angle XRD patterns of 5.9%Ni(10%)HMS reduced at the different temperatures. With increasing reduction temperature, the diffraction peaks due to nickel silicate become weaker, while the diffraction peaks due to metallic nickel occur for the sample reduced at 650 °C. This is due to the increase of the nickel reduction degree. Even if the reduction degree is 87.2% at 650 °C, the intensities of metallic nickel diffraction peaks are very weak, indicating that the metallic nickel particles are very small. This is also shown in TEM image (see Figure 11d).

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3807

Figure 8. Wide-angle XRD patterns of 5.9%Ni(10%)-HMS reduced at (a) 450; (b) 550; (c) 650 °C.

Figure 9. TEM images of im-4.1%Ni/HMS reduced at 450 °C.

3.5. TEM Results. Figure 9 shows the representative TEM images of im-4.1%Ni/HMS reduced at 450 °C. The size distribution of the metallic nickel particles is not uniform,

Figure 10. Ni particle size distribution of im-4.1%Ni/HMS reduced at 450 °C.

ranging from about 10-100 nm. As shown in the Figure 10, metallic nickel particles mainly distribute between 10 and 30 nm. Figure 11 shows the TEM images of 5.9%Ni(10%)-HMS reduced at 450 and 650 °C. Obtained from Figure 11 panels a and b, there is no conglomeration of the metallic nickel particles, even the sample was reduced at 650 °C. Figure 11c,d shows that the metallic nickel particle size centers at about 3.0 nm. Combining XRD and TEM results, it is clear that there is a good dispersion of the metallic nickel particles in the reduced samples prepared by the direct synthesis. 3.6. H2 Chemisorption and H2-TPD Results. H2 chemisorption amounts of 5.9%Ni(10%)-HMS reduced at 450 and 650 °C, and im-4.1%Ni/HMS reduced at 450 °C are 97, 106, and 96 µmolH2/gNi. Compared with im-4.1%Ni/HMS reduced at 450 °C, 5.9%Ni(10%)-HMS samples reduced at 450 and 650 °C have smaller nickel particles, and they should have higher H2 chemisorption amounts based on the unit mass metallic nickel. In fact, there is no remarkable difference in H2 chemisorption amount among them. This phenomenon may be related to the embedment of metallic nickel particles in the silica matrix. The fact that 5.9%Ni(10%)-HMS samples reduced at 450 and 650 °C have similar H2 chemisorption amounts (µmolH2/ gNi) also indicates that the metallic nickel particles with ∼3 nm exist in a similar environment in two samples. To investigate the interaction between the reduced samples and the hydrogen, HMS, the reduced im-4.1%Ni/HMS and the reduced 5.9%Ni(10%)-HMS catalysts were subject to H2-TPD. The results are shown in Figure 12. As illustrated in Figure 12a, HMS did not adsorb hydrogen after treatment with hydrogen flow at 450 °C; however, hydrogen desorption starts at about 500 °C. The desorbed hydrogen is attributed to that retained on HMS support during the thermal treatment with H2. In other words, the support has the role of hydrogen storage. This phenomenon has been found for MCM-41 and AlMCM-41,18 HY,39 and SiO2-Al2O340 supports. For the reduced 5.9%Ni(10%)-HMS, the H2-TPD curves include two desorption peaks, one below 400 °C and the other above 400 °C. Generally, the hydrogen desorbed below 400 °C is ascribed to that on the metal surface, while the hydrogen desorbed above 400 °C is usually ascribed to the spilt-over one.41,42 Of course, it is not excluded that the hydrogen desorbed above 400 °C is related to the hydrogen storage on the support

3808 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009

Figure 11. TEM images of 5.9%Ni(10%)-HMS reduced at 450 °C (a and c) and 650 °C (b and d).

Figure 12. H2-TPD profiles of HMS and catalysts: (a) HMS treated at 450 °C in H2 before TPD; (b) im-4.1%Ni/HMS reduced at 450 °C; (c) 5.9%Ni(10%)-HMS reduced at 450 °C; (d) 5.9%Ni(10%)-HMS reduced at 650 °C.

(silica and unreduced nickel silicate). Hydrogen spillover is now well established for supported transition metal catalysts where H2 dissociates on the metal into atomic hydrogen which then spills over to the oxide support surfece.42 In the H2-TPR results (see Table 3), it is observed that the actual consumed hydrogen

amount during the H2-TPR exceeds the theoretical amount required for the complete reduction of the nickel component. The excess hydrogen can be attributed to spilt-over and/or storage hydrogen. H2-TPD results can further justify the existence of spilt-over and/or storage hydrogen on the catalyst surface. However, the interaction between spilt-over and/or storage hydrogen and the support surface differs for the (5.9%Ni(10%)-HMS) sample that is reduced at different temperatures. The desorption peak is centered at about 517 °C for 5.9%Ni(10%)-HMS reduced at 450 °C; however, it is centered at about 717 °C for 5.9%Ni(10%)-HMS reduced at 650 °C. Moreover, the amount of spilt-over and/or storage hydrogen on 5.9%Ni(10%)-HMS reduced at 650 °C is more than that on 5.9%Ni(10%)-HMS reduced at 450 °C. The H2-TPD trace of im-4.1%Ni/HMS reduced at 450 °C is different from those of the reduced 5.9%Ni(10%)-HMS. For im-4.1%Ni/HMS, apart from the hydrogen desorption below 400 °C, there are an unremarkable desorption peak centered at about 507 °C and another desorption peak starting at about 677 °C, all of which are related to the spilt-over and/or storage hydrogen. 3.7. Catalyst Activity. The catalyst activity for the hydrodechlorination of chlorobenzene (CB) is expressed as two parameters: one is CB conversion (unit: mol %); the other is the reaction rate (r, unit: molCB · gNi-1 · h-1), which is the average value during each hour. The test has shown that pure HMS does not show any activity.

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Figure 13. Activities of different catalysts reduced at 450 °C. Reaction conditions: 0.1 MPa, 300 °C, total space velocity ) 0.27 mol · gcat-1 · h-1, nH2:nCB ) 6.4.

Figure 13 shows the activities of the different samples reduced at 450 °C. As shown in Figure 13a, there is higher initial CB conversion over 4.2%Ni(5.0%)-HMS than im-4.1%Ni/HMS. When the reaction time exceeds 5 h, the CB conversions over 4.2%Ni(5.0%)-HMS and im-4.1%Ni/HMS are similar; 5.9% Ni(10%)-HMS, 7.5%Ni(15%)-HMS, and 7.8%Ni(20%)-HMS have higher CB conversions than 4.2%Ni(5.0%)-HMS and im-4.1%Ni/HMS. However, based on the same amount of the metallic nickel only, all of n%Ni(m%)-HMS samples exhibit much higher reaction rates than im-4.1%Ni/HMS (see Figure 13b). As shown in section 3.6, 5.9%Ni(10%)-HMS and im-4.1%Ni/ HMS reduced at 450 °C have similar H2 chemisorption amounts based on unit mass metallic nickel, whereas 5.9%Ni(10%)-HMS exhibits higher activity, expressed both CB conversion and reaction rate. This may be related to the nature of the spilt-over hydrogen, the strong interaction between nickel and the support, and the acidity of nickel silicate. In other words, apart from metallic nickel active site, there are other factors influencing the catalyst activity. It is well accepted that spilt-over hydrogen species can promote the hydrogenolysis of C-Cl.43-45 Keane and coworkers45 have investigated the nature and activity of spilt-over hydrogen. They suggest that the spilt-over hydrogen appears to be hydrogenolytic in nature and is responsible for promoting hydrodechlorination; meanwhile, they also found that the spiltover hydrogen strongly interacting with the support, corre-

Figure 14. Activities of 5.9%Ni(10%)-HMS reduced at different temperatures. Reaction conditions: 0.1 MPa, 300 °C, total space velocity ) 0.27 mol · gcat-1 · h-1, nH2:nCB ) 6.4.

sponding to the desorption temperature higher than 677 °C, is less active for HDC. In other words, the spilt-over hydrogen moderately interacting with the support is active for HDC. A reason for superior HDC activity of 5.9%Ni(10%)-HMS reduced at 450 °C to im-4.1%Ni/HMS is perhaps related to a greater amount of active spilt-over hydrogen on the surface of 5.9%Ni(10%)-HMS. Catalytic hydrogenolysis reaction is also strongly influenced by the electronic structure of the metal active sites.46,47 Support induced electronic effect is prevalent for metal particles with diameters below 5 nm,47-49 and the supported transition metal particles remain electron deficient within this size range. In 5.9%Ni(10%)-HMS reduced at 450 °C, the metallic nickel particles with about 3 nm distribute homogenously, and they may embed in the silica matrix and have a strong interaction with the support. As a result, they can possess electron deficiency in comparison with those on im-4.1%Ni/HMS, especially the metallic nickel atoms at the interface of the nickel particles and the support. On the basis of the present results, it is suggested that the electron deficiency of nickel favors HDC. Chlorine is known to act as an electron acceptor with respect to transition metals.50,51 The adsorption strength of chlorine is enhanced as the electronic density of the metal atom increases; however, this is not beneficial to the reduction of the metal chloride, which decreases the catalyst activity and leads to the catalyst deactivation.52,53 Indicated in our previous reports,54,55

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the superior activity of SiO2-supported nickel phosphides to SiO2-supported nickel is closely related to the electron deficiency of nickel in nickel phosphides; however, the electron-enriched nickel species due to the existence of elemental boron are unfavorable to the hydrodechlorination. Similarly, Gopinath and co-workers56 attribute the good HDC activity of Pd/ZrO2 catalyst prepared by the deposition-precipitation to the electron-deficient Pd species. It is also reported by Coq et al.51 that the PdSn catalyst shows a lower activity than the Pd catalyst due to its higher electron density of Pd. It has been known that nickel silicate possesses acidity.57-59 There are both Bro¨nsted acid sites and Lewis acid sites on the surface of the nickel silicate, especially the latter when the nickel silicate is pretreated above 400 °C.59 Ayame and co-workers60 have suggested that Lewis acid sites have a promoting role for the hydrodechlorination of chlorobenzene. The chlorobenzene molecule has a dipole moment, that is, a polar Clδ--Cδ+ bond, the chlorobenzene seems to be easily coordinately adsorbed on the Lewis acid site (L) in a form of LrCl-C6H5, in which chlorobenzene is considered to react easily with the hydrogen species. For the 5.9%Ni(10%)-HMS even reduced at 650 °C, there is still the unreduced nickel silicate. The Lewis acid sites on the surface of the nickel silicate may promote the hydrodechlorination. Though 7.5%Ni(15%)-HMS and 7.8%Ni(20%)-HMS have higher nickel contents and higher nickel reduction degrees than 5.9%Ni(10%)-HMS, they have lower initial HDC activities than 5.9%Ni(10%)-HMS (see Figure 13). This can be due to more micropores, and there is intraparticle mass transfer limitation in 7.5%Ni(15%)-HMS and 7.8%Ni(20%)-HMS. Figure 14 shows the activities of 5.9%Ni(10%)-HMS reduced at different temperatures. With increasing reduction temperature, the reduction degree increases, and as a result, there are more metallic nickel active sites. However, there is no marked difference in chlorobenzene conversion among the samples reduced at 450∼650 °C (see Figure 14a). This is perhaps related to the intraparticle mass transfer limitation, that makes the active sites inside the particles unavailable. Just for this reason, the reaction rate decreases as the reduction temperature increases (see Figure 14b). To eliminate the effect of the intraparticle mass transfer limitation, enlarging the pore diameter of the catalyst is being carried out. 4. Conclusions With increasing the nickel content, the BET surface area, the pore volume, and the pore (2∼5 nm) diameter decrease for n%Ni(m%)-HMS, and the mesostructures attributed to HMS were also discounted. There is no mesostructures in the samples with the nickel content higher than 7.0 wt %. This is related to the decrease of pH value in the preparation system and the incorporation of Ni2+ into the matrix of SiO2. Compared with im-4.1%Ni/HMS, Ni2+ ions disperse more homogenously in n%Ni(m%)-HMS samples. After reduction at 450∼650 °C, metallic nickel particles in n%Ni(m%)-HMS center at about 3 nm, whereas those in im-4.1%Ni/HMS reduced at 450 °C are larger and their distribution is less uniform. This is related to the embeding of Ni particles in SiO2 matrix and the sintering of Ni particles is restricted in n%Ni(m%)-HMS samples. Thus, there is stronger interaction between nickel particles and SiO2. Because of the electron-deficient nickel, more spilt-over hydrogen species, and the acidity of the nickel silicate, n%Ni(m%)HMS samples have better HDC activities than im-4.1%Ni/ HMS. When nickel content is higher than 5.9 wt % and the reduction temperature is between 450 and 650 °C, there is no

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ReceiVed for reView November 22, 2008 ReVised manuscript receiVed February 3, 2009 Accepted February 5, 2009 IE801792H