Copper and Nickel Modified MCM-41 An Efficient Catalyst for

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Copper and Nickel Modified MCM-41 An Efficient Catalyst for Hydrodehalogenation of Chlorobenzene at Room Temperature Dharitri Rath and K. M. Parida* Institute of Minerals and Materials Technology, Bhubaneswar, India 751013 ABSTRACT: The paper reports preparation, surface, and textural characterization of Cu and Ni modified MCM-41 and its application for hydrodehalogenation of chlorobenzene. A series of Cu/Ni modified MCM-41 samples were synthesized by the co-condensation method by varying the amount of Ni and further characterized by various physicochemical methods. The incorporation of metals in the framework of mesoporous silica matrix was confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed the formation of uniform spherical particles on the support surface. The catalytic activity of the samples was evaluated toward the hydrodehalogenation reaction of chlorobenzene at room temperature. Samples containing Si/(Cu þ Ni) = 10 showed the highest chlorobenzene (98%) conversion with 100% selectivity for benzene. Among the various solvents studied for the reaction, isopropyl alcohol gave the best results at 2 h of reaction time. The effect of various halogen substituent and different electron donating and withdrawing groups on chlorobenzene is also studied to optimize the reaction conditions.

1. INTRODUCTION The mesoporous MCM-41 molecular sieves, a member of the M41S family, possesses large internal surface area (∼1000 m2/g) and massive accessible pore volume (normally over 0.8 mL/g), and one-dimensional pore geometry has attracted much research attention, owing to their potential application as catalysts, ion exchangers, molecular clusters, catalyst support, and adsorbent.1,2 However, due to the absence of active sites in their matrixes, pure siliceous mesoporous molecular sieves are of limited use as catalysts. Incorporation of metal centers in the silicate framework is, therefore, necessary to make the materials as potential catalysts. The presence of high surface area, porosity, and amorphous structure offer the possibility of transition metal ion incorporation inside the mesoporous framework by substitution of Si atoms in regular tetrahedral positions. A series of single metal ions such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, La, and Ru have been used to modify the MCM-41 silica framework.3-7 The incorporation of two different metals might, therefore, create materials with different or new redox and acid properties.8,9 Supported bimetallic catalysts are very interesting materials in general terms since one metal can fine tune or modify the structural and electronic properties of the other. Therefore, bimetallic catalysts usually improve catalytic activity, selectivity, and stability of the single component metal catalysts.10,11 The disposal of chlorinated organic wastes is a serious environmental problem. Polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) are carcinogenic, mutagenic, cumulative, and stable chemicals. Also, in the recent years, dechlorination in water has attracted much attention due to pollution problems of organic halides in water systems. Various metal particles and bimetallic systems have been employed to investigate the aqueous dechlorination.12 Among the methods proposed for destruction of r 2011 American Chemical Society

chlorinated organic compounds,13 the catalytic hydrodechlorination (HDC) is receiving more and more attention because it is simple, effective, and safe. Noble metal catalysts, like Pd, Rh, and Pt on various supports, are active for the reaction under mild conditions.14-16 Chlorobenzene and its derivatives were hydrodechlorinated on Pd/C catalyst in a flow system; Pd/AlPO4-SiO2 and Pd/C catalysts17,18 were tested in liquid phase. A few of these catalysts are introduced to large-scale applications because of their high cost.19,20 Transition metal catalysts, such as Ni, Ni-Mo on γ-alumina, or silica or carbon composite,21-24 required high temperature (>473 K) or high hydrogen pressure (2 MPa) to reach significant activity in the gas phase dechlorination process. The electronic factors affecting the hydrodechlorination of chlorobenzene derivatives have been the focus in several studies. For example, Menini et al.25 observed a correlation between reaction rates and donor properties of the substituents in the gas phase reaction of substituted chlorobenzenes over a supported Ni/SiO2 catalyst. Konuma et al.26 have observed that the hydrogenolysis activities of chlorobenzenes are accelerated by the presence of both the electron donating and electron withdrawing groups. In the present work, we have synthesized copper-nickel bimetallic supported MCM-41 (Cu/Ni-MCM-41) catalyst by the co-condensation method. The materials have been tested for hydrodehalogenation of various haloarenes. The enhancement of catalytic activity has been explained on the basis of synergistic, electronic, and structural interaction of copper and nickel. Received: June 25, 2010 Accepted: December 29, 2010 Revised: December 29, 2010 Published: January 24, 2011 2839

dx.doi.org/10.1021/ie101314f | Ind. Eng. Chem. Res. 2011, 50, 2839–2849

Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Mesoporous silica containing different amounts of Cu and Ni were synthesized at room temperature. In a typical synthesis procedure, about 1.3 g of cetyl trimethyl ammonium bromide (CTAB) was added to a solution containing 50 mL of H2O and 47.5 mL of ethyl alcohol. To this mixture, about 8.5 mL of an aqueous NH3 solution (28%) containing 0.205 g of Cu(NO3)2 3 3H2O and 0.99 g of Ni(NO3)2 3 6H2O was added to maintain the Si/(Cu þ Ni) ratio of 10. This resulted in the formation of a dark blue color solution to which about 6.4 mL of TEOS was introduced. After stirring the mixture for about 2 h at room temperature, the blue color product was filtered, washed with deionized water, and then dried in an oven at 75 C overnight. The as-synthesized samples were calcined in flowing air at 540 C for 18 h to obtain modified MCM-41. Keeping the amount of Cu fixed, we synthesized different catalysts by varying the amount of Ni (0.45-1.8 g). The catalysts are further termed as (x) Cu/Ni-MCM-41, where x = Si/(Cu þ Ni) = 7.5-15. 2.2. Physico-Chemical Characterization. Powder X-ray diffraction (PXRD) patterns of the samples were taken in the 2θ range of 1-30o at a scanning rate of 2/min in steps of 0.01o (Rigaku Miniflex set at 30 kV and 15 mA) using Cu KR radiation. The high angle X-ray diffraction (XRD) patterns of powdered samples were taken in the 2θ range of 20-80o at a rate of 1.2o/min (Philips analytical 3710) using Cu KR radiation. The FTIR spectra of the samples were recorded using Varian 800-FTIR in KBr matrix in the range of 4000-400 cm-1. The Bruner-Emmett-Teller (BET) surface area, average pore diameter, mesopore distribution, total pore volume, and micropore volume were determined by the multipoint N2 adsorption-desorption method at liquid N2 temperature (-196 C) by an ASAP 2020 (Micromeretics). Prior to analyses, all the samples were degassed at 300 C and 10-6 Torr pressure for 5 h to evacuate the physically adsorbed moisture. The mesopore structure was characterized by the distribution function of mesopore volume calculated by applying the Barrett-Joyner-Halenda (BJH) method. The co-ordination environment of Cu/Ni-MCM-41 samples was examined by UV-vis diffuse reflectance spectroscopy. The spectra were recorded in Cary 100 UV-visible spectrophotometer in the wavelength range of 200-800 nm. The H2 temperature programmed reduction was carried out in a CHEMBET-3000 (Quantachrome) instrument. About 0.1 g of sample was taken inside a quartz “U” tube and degassed at 300 C for 1 h with nitrogen gas flow. The sample was then cooled to 30 C, and at this temperature, the gas flow was changed to 5% H2 in nitrogen. It was then heated at a heating rate of 10 C/min up to 700 C, and the reduction patterns were recorded. The scanning electron microscopic figures of 10Cu/NiMCM-41 sample were recorded using Hitachi S3400N. The transmission electron microscopic figures of 10Cu/Ni-MCM-41 sample were recorded using TECNAI-G2 with a charge coupled device (CCD) camera. The X-ray photoelectron spectra of Cu, Ni, and O were recorded using KRATOS with Mg, Al, and Cu KR as X-ray sources. The electronic spin resonance (ESR) spectra are recorded at 77 K and X-band frequency on a Bruker EMX ESR spectrometer. Elemental analysis by atomic absorption spectroscopy (AAS) was performed using Perkin-Elmer Analysis 300 with acetylene flame. 2.3. Catalytic Reaction. Prior to each reaction, the catalyst was pretreated for 4 h in flowing H2 while being heated at a rate of 10 C/min to 400 C. Then, the catalyst is used for the reaction.

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Scheme 1. Schematic Representation of the Hydrodehalogenation of Chloro Benzene

Figure 1. (a) Low angle (0-10) XRD patterns of MCM-41 (a), 10Cu/Ni-MCM-41 (b), and 15Cu/Ni-MCM-41 (c). (b) High angle (20-80) XRD patterns of 10Cu/Ni-MCM-41 (a), 10Ni-MCM-41 (b), and 15Cu/Ni-MCM-41 (c).

The hydrodehalogenation of chloro benzene is tested in a three necked round-bottom flask with a reflux-condenser at atmospheric pressure.27 In a typical experiment, 3 mmol of chlorobenzene is dissolved in 10 mL of methanol containing 4.5 mmol of triethylamine to trap the evolving HCl. The reaction was carried out by vigorous stirring of the reaction mixture with 50 mg of the catalyst under H2 gas flow (10 mL min-1) at room temperature for a period of 2 h. The heat and mass transport limitations are negligible in the present reacting conditions. 2840

dx.doi.org/10.1021/ie101314f |Ind. Eng. Chem. Res. 2011, 50, 2839–2849


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Figure 2. N2 adsorption-desorption isotherms of MCM-41 (a) and 10Cu/Ni-MCM-41 (b).

The reaction products were analyzed by off line gas chromatography (GC) equipped with a flame ionization detector (FID; Shimadzu-2010) and a ZB-MAX (30 m 0.25 mm) column. The schematic representation of the reaction is shown in Scheme 1. The mass balance is >98% for carbon and >97% for chlorine. In order to study the hydrogen donating effect of the solvent in the hydrodehalogenation reaction, we have performed the above reaction in deuterated methanol keeping all other parameters constant. After analyzing the reaction products by offline GC, we found no deuterated benzene from which we confirmed that the solvent does not act as a source of hydrogen in the hydrodehalogenation reaction.

3. RESULT AND DISCUSSION 3.1. Surface Characterization. 3.1.1. XRD. The low angle XRD patterns of all the samples are shown in Figure 1. It is observed that all the materials exhibit a strong peak in the 2θ range of 1.5-2.2 due to the d100 plane. Also, small peaks due to higher order (110), (200), and (210) plane reflections indicated the formation of a well-ordered mesoporous material. The mesoporosity remained intact after modification of the silica network with metals. There is a little bit of reduction and broadening of the (100) peak after metal modification, indicating a slight reduction in hexagonal symmetry. The d spacing and, hence, the unit cell parameter (ao) are found to be large for metal modified samples compared to the neat MCM-41. The d spacing value for MCM-41 is 37.41 Å and that of Ni and Cu modified samples is found to be 42.43 Å. There should be enlargement in the unit cell parameter as the

Figure 3. BJH pore size distribution curves of MCM-41 (a) and 10Cu/ Ni-MCM-41 (b).

incorporation of bimetal cations with ionic radius larger than Si4þ results in a larger M-O bond distance. The ionic radii of Ni2þ (69 pm) and Cu2þ (73 pm) are larger than that of Si4þ (40 pm), so the increase in d spacing indicates the incorporation of metal in the framework of MCM-41. The wide angle diffraction peaks of Ni/Cu modified MCM-41 samples are shown in Figure 1b. The 10Cu/Ni-MCM-41 sample showed no wide angle peaks due to the absence of any cubic phase of the metal, but 15Cu/Ni-MCM-41 exhibits peaks at 2θ = 37.3, 43.3, 62.9, and 75.4 which correspond to (111), (200), (220), and (222) planes of cubic NiO (JCPDS 78-0643), respectively. Due to low Cu loading, no diffraction peaks are available for CuO species. 3.1.2. N2 Adsorption-Desorption Isotherm. The nitrogen adsorption-desorption isotherms are shown in Figure 2. It is observed that there are three different well-defined stages in the isotherm. The initial increase in nitrogen uptake at low P/P0 may be due to monolayer adsorption on the pore walls. A sharp steep increase at intermediate P/P0 may indicate the capillary condensation in the mesopores and a plateau portion at higher P/P0 associated with multilayer adsorption on the external surface of 2841



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Table 1. Surface Properties of MCM-41 and Metal Modified MCM-41 sample code

a

Si/(Cu þ Ni) molar ratio

BET surface area

unit cell parametera

pore diameter

pore volume

wall thickness

(m2/g)

(Å)

(Å)

(cm3/g)

(Å) 17.6

MCM-41

¥

1380

43.1

25.5

1.28

Cu-MCM-41

10 (Cu = 10, Ni = 0)

860

48.9

24.9

1.04

24.0

Ni-MCM-41

10 (Cu = 0, Ni = 10)

835

49.0

24.2

0.98

24.8

7.5Cu/Ni-MCM-41

7.5 (Cu = 5, Ni = 2.5)

814

49.3

23.9

0.91

25.4

10Cu/Ni-MCM-41

10 (Cu = 5, Ni = 5)

770

49.7

22.8

0.88

26.9

15Cu/Ni-MCM-41

15 (Cu = 5, Ni = 10)

733

49.9

22.1

0.85

27.8

Unit cell parameter = 2 d100/(3)1/2 (from xrd).

Figure 5. TPR spectra of 10Ni-MCM-41 (a), 10Cu/Ni-MCM-4 (b), and 15Cu/Ni-MCM-41 (c).

Figure 4. (a, b) FTIR Spectra of MCM-41 (a), 10Ni-MCM-41 (b), and 10Cu/Ni-MCM-4 (c).

the materials. The sharpness and height of the capillary condensation step are the indications of pore size uniformity. Deviations from a sharp and well-defined pore filling step are the indication of an increase in pore size heterogeneity.28 Parent MCM-41 sample exhibits N2 uptake at a relative pressure of 0.32 which corresponds to the precondensation loop. The isotherm shows a H4 type hysteresis loop (according to IUPAC nomenclature) with well-developed step in the relative pressure range of ≈0.9. The incorporation of Cu and Ni in the MCM-41 framework is found to lower the P/P0 for capillary condensation step, indicating the shift in pore size to a lower value due to metal incorporation. The pore diameter and pore volume of parent MCM41 is found to be 2.5 nm and 1.28 cm3/g, respectively. These values are found to decrease with increasing metal incorporation over all the catalysts (Figure 3). All the samples have high BET surface area, which is characteristic of mesoporous materials. The parent MCM-41 has a surface area of 1380 m2/g, but there is a decrease in the value with

metal modification. 10Cu/MCM-41, 10Ni/MCM-41 and 10Cu/ Ni-MCM-41 samples show the surface area of 860, 835, and 790 m2/g, respectively. Table 1 shows the detailed physicochemical characteristics of all the samples. 3.1.3. Fourier Transform Infrared Spectroscopy (FTIR). Infrared spectroscopy had been used extensively for the characterization of bimetal modified MCM-41 (Figure 4). Strong bands associated with OH stretching vibrations of water and surface hydroxyl groups occur between 3200 and 3700 cm-1. A strong sharp absorption band in the region of 3400-3500 cm-1 characterizes the hydroxyl groups. Water of hydration usually exhibits one strong sharp band near 3600 cm-1 and one or more strong sharp bands near 3400 cm-1. Water of hydration can be easily distinguished from hydroxyl groups by the presence of the H-O-H bending motion, which produces a medium band in the region of 1623-1650 cm-1.29 The substitution of silicon by copper and nickel causes shifts of the lattice vibration bands to lower wave numbers. The wavenumber of the antisymmetric Si-O-Si vibration band of 10NiMCM-41 and 10Cu/Ni-MCM-41 samples decreases to 1086 and 1080 cm-1, receptively (Figure 4a,c). These shifts should be due to the increase in the mean Si-O distance caused by the substitution of the small silicon (radius 40 pm) by the larger size of Ni2þ (69 pm) and Cu2þ (73 pm). The observed shifts, which depend on the change in the ionic radii as well as on the degree of substitution, are comparatively small. In addition, the rocking motion of bridging oxygens perpendicular to the Si-O-Si plane can be correlated with the 445 cm-1 band which is common to all the spectra. The vibration peaks at 960-970 cm-1 are assigned to metal incorporation into the framework of the mesoporous silica 2842



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Table 2. Reducible Sites from TPR Study catalyst

a

reduction temp range

reducible sites

Cu content in the sample

Ni content in the sample

(C)

(μ mol/g)

(mmol 10-1/g)a

(mmol 10-1/g)a

Ni-MCM-41

330-420

1243

0

28

10Cu/Ni-MCM-41

250-420

8165

8.3

29

15Cu/Ni-MCM-41

200-420

7832

8.3

45

Analyzed by AAS.

Figure 7. SEM figures of 10Cu/Ni-MCM-41 (a) and 15Cu/Ni-MCM41 (b).

Figure 6. DRS figures of 7.5Cu/Ni-MCM-41 (a), 10Cu/Ni-MCM-4 (b), and 15Cu/Ni-MCM-41 (c).

materials. This band is mainly assigned to the Si-OH vibrations, but when metals are incorporated, the intensity of the band increases and is shifted to lower wavenumber. This is generally considered to be a proof of the incorporation of heteroatom into the framework. 3.1.4. Temperature-Programmed Reduction (TPR). The reducibility of copper in the MCM-41 materials has been investigated by the TPR measurements. The TPR profiles of Ni-MCM41 and Cu/Ni-MCM-41 are shown in Figure 5. Ni-MCM-41 showed only one peak where Cu/Ni-MCM-41 samples showed two peaks. The reduction temperature of bare NiO is 370 C in Ni-MCM-41.30 The improvement of the reducibility of Ni (lowering of reduction temperature) when Cu is present in the sample is due to the synergistic, electronic, and structural interaction of copper and nickel.31 This finding satisfies the property of the group 11 elements of the periodic table to decrease the reduction temperature of other metals. The Cu-MCM-41 sample exhibits two reduction zones,32 i.e., one in the range of 250-300 C and another within 350400 C. The first one is for highly dispersed CuO clusters, and the second is for bulk CuO. In case of 10Cu/Ni-MCM-41, the maximum rate of H2 consumption is around 270 C. H2 consumption in this region corresponds to the reduction of Cu2þ or finely dispersed CuO to copper metal in the channels of MCM-41 materials. Hence, the presence of Ni in these materials impedes the reduction of Cu2þ. A similar inhibition of Cu2þ reducibility has also been found in CuZnY zeolites, CuMCM-41, CuMCM-48, and CuO-ZnO catalysts.33 This may be due to the fact that the

Figure 8. TEM figures of 10Cu/Ni-MCM-41 (a-d).

presence of Ni in the CuMCM-41 materials enhances the dispersion of Cu2þ ions by preventing the formation of CuO clusters. The highly dispersed Cu2þ ions in the MCM-41 framework would strongly interact with the support, thereby diminishing its reducibility. The increase in Ni content in the case of 15Cu/Ni-MCM-41 sample compared to 10Cu/Ni-MCM-41 broadens the reduction peaks. This may be due to the increase in particle size of the sample. From the TPR study, we confirmed that there exists a synergetic interaction between Cu and Ni in the MCM-41 framework so that they affect the reduction of each other. The amount of H2 consumed with respect to Cu2þ content calculated from the TPR peak (Table 2) areas infers a complete 2843



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Figure 10. ESR spectra of 10Cu-MCM-41 (a) and 10Cu/Ni-MCM-41 (b).

Figure 9. XPS of Ni 2p (a), Cu 2p (b), and O 1s (c).

reduction of Cu2þ ions. This also provides evidence that the majority of the Cu2þ ions incorporated in the MCM-41 framework are accessible to H2 molecules. 3.1.5. UV-Vis Diffused Reflectance Spectra (DRS). UV-vis diffuse reflectance spectra of the copper- and nickel-modified samples are shown in Figure 6. The DR spectra of the samples showed three main bands; the first one is around 250 nm, the second one is at 430 nm, and the last one is around 780 nm. The band around 780 nm, corresponding to the 2Eg (D) f 2T2g spin and allowing Lapporte-forbidden transition of Cu2þ in the octahedral coordination, is very broad in these samples. Hence, it suggests a further distortion in the symmetry of the copper environment toward square pyramidal coordination. The strong band at 240 nm in all the samples can be attributed to the ligand to metal (Cu and Ni) charge transfer (LMCT) transition. The

peak intensity increases with an increase in Ni content. It should be noted that a band around 450 nm in the samples has been assigned to the formation of Cu1þ three-dimensional cluster in the CuO matrix.34-36 3.1.6. Scanning Electron Microscopy (SEM). To study the surface topography and to assess the surface dispersion of the active components over the MCM-41 substrate, SEM investigation was performed on Cu and Ni modified MCM-41 and the images are shown in Figure 7. It has been found that the catalysts are well-ordered spherical particles. The metal particles are uniformly distributed over the support surface. The particle size is calculated from SEM figures and found as ∼0.5 μm for 15Cu/ Ni-MCM-41 and ∼0.2 μm for 10Cu/Ni-MCM-41. Of all the samples, 10Cu/Ni-MCM-41 possesses more porous texture. 3.1.7. Transmission Electron Microscopy (TEM). The TEM images of 10Cu/Ni-MCM-41 sample are shown in Figure 8. TEM images of the bimetallic mesoporous molecular sieves are characteristics for the mesoporous materials with hexagonal channel array, showing high quality in organization of channels of these catalysts. From the figure, it is confirmed that the particles are spherical in nature. The metal particles are well dispersed throughout the silica framework, which is clearly seen in the figure. The particle size can be confirmed from the TEM images. The particle size is calculated to be in between 0.5 and 0.2 μm. Hence, the SEM and TEM results are complementing each other. 3.1.8. XPS. Figure 9a-c shows the XPS spectrum of Ni 2p, Cu 2p, and O 1s, respectively. For clarity, the deconvoluted spectra of Ni 2p and Cu 2p are shown separately. In the XPS spectra of Ni 2p3/2, three main peaks appeared in the binding energy (BE) range of 850-865 eV. The BE of 853 eV can be attributed to Ni2þ species and that of 860 eV is assigned for shake up satellite. The third peak is around 857 eV, which may correspond to Ni3þ cations or surface Ni hydroxide species.37 The result can be confirmed from the XPS data of O 1s. In Figure 9c, there is only one peak at 529 eV. According to the literature,38 the peak at 529.7 eV corresponds to O2- bonded to metal in the catalyst framework. Also, the single peak confirms that there are no free surface hydroxyl groups on the catalyst. As Ni3þ is an unstable species, the third peak in Figure 9a is assigned 2844



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Table 3. Effect of Various Catalysts on Hydrodehalogenation of Chloro Benzenea sample code

a

Si/(Cu þ Ni) molar ratio

conversion (%)

yield (%) first cycle

2nd cycle

third cycle

rate 10-6 (mol L-1 s-1)

MCM-41

¥

0

0

0

0

Cu-MCM-41

10 (Cu = 10, Ni = 0)

65

82

80

77

1.4

Ni-MCM-41

10 (Cu = 0, Ni = 10)

80

91

89

86

2.5

7.5Cu/Ni-MCM-41

7.5 (Cu = 5, Ni = 2.5)

90

97

94

92

3.3

10Cu/Ni-MCM-41

10 (Cu = 5, Ni = 5)

98

100

96

95

4.16

15Cu/Ni-MCM-41

15 (Cu = 5, Ni = 10)

99

100

97

95

4.26

Temperature = room temperature; chloro benzene = 3 mmol; catalyst amount = 0.05 g; methanol = 10 mL; triethylamine = 4.5 mmol; time = 2 h.

Scheme 2. Schematic Representation of Probable Mechanism of Hydrodehalogenation of Chloro Benzene

Figure 11. Influence of time on concentration of chlorobenzene. Catalyst = 0.05 g, solvent = methanol, room temperature.

)

)

to nickel hydroxide. The peaks at higher BE correspond to Ni 2p1/2. Figure 9b shows four peaks in the range of 930-960 eV. According to the literature, the first peak at 932 eV corresponds to bivalent copper species in both Oh and Td sites. The third peak around 940 eV is the satellite peak for copper. The order of BE in the Cu 2p3/2 region is as follows: Cuþ (Oh) < Cuþ (Td) < Cu2þ (Oh) < Cu2þ (Td). From the O 1s XPS data, it is confirmed that there is O-M bonding in the material. Thus, the peak around 937 eV may be due to copper hydroxide species on the surface of the catalyst. According to the literature, the XPS peak for Ni 2p3/2 should be at 856 eV.39 However, in case of Cu/Ni-MCM-41 bimetallic sample, the peak shifts to a lower value (853 eV), so the presence of Cu affects the electronic environment of Ni. As the BE of Ni2p3/2 decreased in Cu/Ni-MCM-41 compared to that of NiMCM-41, there is a flow of electron from the support to Ni. This process favors the reduction of chlorobenzene to benzene on the catalyst surface. 3.1.9. ESR. ESR spectroscopy has been employed to further substantiate the UV-vis DRS results on the chemical environment of Cu2þ ions in the MCM-41 materials. ESR is indeed a very sensitive tool for obtaining information on the co-ordination states of Cu2þ ions in these materials. Figure 10 shows the X-band ESR spectra of Cu-MCM-41 and Cu/Ni-MCM-41 samples recorded at liquid nitrogen temperature. The spectrum of 10Cu-MCM-41 can be described with the spin Hamiltonian parameters g = 2.382 and g^ = 2.068. Hence, we propose an octahedral or elongated octahedral coordination of Cu2þ ions with oxygen of silica in the MCM-41 framework. This is consistent with the results of UV-vis DRS. A small decrease in g for 10Cu/Ni-MCM-41 compared to that of 10Cu-MCM-41 indicates that the Cu2þ ions in the former sample containing Ni are

Table 4. Effect of Solvents on Hydrodehalogenation of Chloro Benzenea solvent methanol ethanol

conversion (%) 98 99

iso-propanol

100

acetonitrile n-hexane

45 13

benzene

5

a

Temperature = room temperature; chloro benzene = 3 mmol; catalyst amount = 0.05 g; triethylamine = 4.5 mmol.

Table 5. Effect of Different Halogen Substituents on Hydrodehalogenation of Chloro Benzenea substrate

a

conversion (%)

iodobenzene

32

bromobenzene

99

chlorobenzene

98

flourobenzene

35

Temperature = room temperature; halo benzene = 3 mmol; catalyst amount = 0.05 g; methanol = 10 mL; triethylamine = 4.5 mmol.

relatively more distorted toward square pyramidal coordination. A similar type of results is reported by Karakassides et al.40 3.2. Catalytic Hydrodehalogenation Reaction. Parent MCM-41 and samples with different Si/(Cu þ Ni) ratio were tested for the hydrodechlorination of chlorobenzene at room temperature, and the results are shown in Table 3. Parent MCM-41, 2845



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Table 6. Effect of Substituents on Hydrodehalogenation Reaction

without any metal loading, gave no conversion toward the hydrodehalogenation reaction. Ni-MCM-41 showed 80% chlorobenzene conversion at room temperature using methanol as solvent. In the case of bimetal (Cu and Ni) modified MCM-41, conversion increased from 90% to 98% with an increase in Ni content. The sample with Si/(Cu þ Ni) = 15 showed the highest conversion (99%) and selectivity (100%) toward the formation of benzene. These results can be confirmed from the TPR study which states that the presence of Ni in the Cu-MCM-41 materials enhances the dispersion of Cu2þ ions by preventing the formation of CuO clusters. The highly dispersed Cu2þ ions in the MCM-41 framework strongly adsorb the reactants and, hence, act as active surface for the reaction. Gioia et al.41 studied the hydrodechlorination reaction mechanism of chlorobenzenes on Ni-Mo/Al2O3 catalysts. According to him, the hydrodechlorination rates were expressed assuming the surface reaction controlled by the Langmuir-Hinshelwood mechanism and the homolytic dissociation of hydrogen molecule. The rate data of the hydrodehalogenation reaction for all

the catalysts is presented in Table 3 and, the concentration vs time plot for 10Cu/Ni-MCM-41 is given in Figure 11. The initial concentration of chlorobenzene gradually decreased with an increase in reaction time, hence following the first order reaction kinetics. According to the spillover mechanism of H2,42 the metal surface provides the sites for the adsorption of active species, i.e., hydrogen and chlorobenzene. The majority of investigators postulate that at the initial stage of the spillover hydrogen is adsorbed dissociatively on the metal phase. The unpaired electrons on the surface of the metal pair with the electrons of hydrogen to bind the hydrogen to the surface. The C-Cl bond in chlorobenzene possesses some double bond character as the chlorine atom loses p electrons due to resonance.43 The surface bearing adsorbed hydrogen causes adsorption of chlorobenzene as well, and a subsequent stepwise transfer of hydrogen atoms occurs. Then, benzene and HCl are formed as the final products. Triethyl amine is present in the reaction mixture to neutralize the HCl formed. The detailed mechanism of the hydrodechlorination 2846


Industrial & Engineering Chemistry Research reaction is given Scheme 2. The influences of various reaction parameters such as different reaction time, temperature, solvent, and catalyst loading have been investigated to optimize the reaction conditions. 3.2.1. Effect of Solvent. The hydrodechlorination reaction was carried out using various solvents such as methanol, benzene, nhexane, and acetonitrile, and the results are summarized in Table 4. In the reaction conditions mentioned above, methanol gave the maximum conversion (98%). The dielectric constant of the solvent plays an important role in stabilizing the electrophilic intermediate. At a higher solvent dielectric constant, the stronger ionic forces stabilize the arenium intermediate.44,45 The nonalcoholic solvents showed less conversion and selectivity due to a lower value of dielectric constant, especially benzene (conversion = 5%). Kaneda et al.46 reported the same order of solvent effect toward the dehalogenation of various aryl halides over hydroxyapatite-supported Pd nanocluster catalysts. The solvent effect can be explained according to the extent of solubility of the reactants. Chlorobenzene and hydrogen are better soluble in polar solvents than the nonpolar ones. Solubility of the reactants increases with an increase in temperature, pressure, and carbon chain length of the alcohol molecules. Iso-propanol and ethanol gave better conversion than methanol as solvent in this reaction. The overall order of reactivity of all the solvents used in hydrodechlorination of chlorobenzene using 10Cu/Ni-MCM41 is as follows: iso-propanol > ethanol > methanol > acetonitrile > n-hexane > benzene. 3.2.2. Effect of Reaction Time. The effect of reaction time on the percentage of conversion of chlorobenzene is shown in Figure 11. The conversion increases from 52% to 98% as the time increased from 0.5 to 2 h, at room temperature using 10Cu/ Ni-MCM-41 as catalyst. However, with further increase in reaction time, the yield of benzene marginally increases. As the time increases from 0.5 to 2 h, there is no appreciable change in selectivity of benzene. 3.2.3. Effect of Substituents 3.2.3. a. Effect of Different Halogen Substituents. The effect of different halogen substituents in hydrogenolysis of halo benzenes is shown in Table 5. The bond energies of different carbon-halogen bonds are 548.1, 340.2, 281.4, and 222.6 kJ/mol for C-F, C-Cl, C-Br, and C-I bonds, respectively. Therefore, the expected order of reactivity of different halobenzenes is iodobenzene > bromobenzene > chlorobenzene > flourobenzene, but the rates of reduction in hydrogenolysis did not follow the above order. The availability of hydrogen is the factor that determines the order of reactivity of different halobenzenes. Wiener et al.47 studied the same reaction with formate ion on Pd/C. The low conversion of iodobenzene is attributed to nearly full coverage of the Pd/C surface with iodobenzene, and no sites are available for adsorption of formate ion. The presence of iodobenzene hinders the hydrogen evolution via formate ion decomposition. In the case of iodo benzene, there is nearly full coverage of catalyst surface and less chance of adsorption of hydrogen.48 The percentage of conversion of iodo benzene is only 32%, but that for chloro, bromo, and fluoro benzene are 98%, 99%, and 35%, respectively. In the case of fluoro benzene, the conversion is less due to the high bond energy in the C-F bond. Thus, the order of reactivity of different halo benzenes is bromobenzene > chlorobenzene > fluorobenzene > iodobenzene. 3.2.3. b. Eect of Dierent Substituents in Chloro Benzene. The effect of different substituents in hydrogenolysis of chloro

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benzene is tested, and the results are shown in Table 6. The results showed that the introduction of an electron-withdrawing group (-NO2, -COCH3, etc.) at the para position of chloro benzene decreased the percentage of conversion and yield, where as an electron donating group (-CH3, -OH, -OCH3, etc.) increased it. This is due to the fact that the presence of an electron donating group stabilizes the electrophilic intermediate through the inductive and mesomeric effect of the substituents where the electron withdrawing group destabilizes the same. The electron releasing effect of the substituents increased the rate of hydrogenolysis of the C-Cl bond in chloro benzene. In the case of p-nitro chloro benzene, there is a chance of formation of aniline due to simultaneous hydrogenation of the nitro group with a hydrogenolysis reaction. A similar type of substituent effect was observed by Selvam et al.49 over Pd-MCM-41 (Si/Pd = 100). 3.3. Reusability of the Catalyst. The reusability of the catalyst for hydrodehalogenation of chlorobenzene was studied using 10Cu/Ni-MCM-41. After 2 h of reaction, the catalyst was separated by filtration, washed several times with distilled water, dried at 110 C, and finally calcined at 500 C. Then, the material was used in the reaction with a fresh reaction mixture. In the regenerated sample after three cycles, the yield decreased by 5% (the quantitative measurement is shown in Table 3).

4. CONCLUSIONS XRD and nitrogen adsorption-desorption studies revealed that the modified samples retained the mesoporosity. The pore diameter and pore volume decreased with an increase in metal loading. The information obtained from the TPR study reveals that both Cu and Ni affect the reduction of each other due to the synergistic, electronic, and structural interaction between the two metals. SEM and TEM results showed that the particles are spherical and are arranged in a uniform manner throughout the support. Cu and Ni modified samples can be used as reusable heterogeneous catalyst for the hydrodehalogenation reaction at room temperature. The sample containing 10Cu/Ni-MCM-41 showed a high conversion of chloro benzene (98%) with 100% selectivity for benzene in methanol medium. ’ AUTHOR INFORMATION Corresponding Author

*Address: FRSC, CChem, C & MC Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar, 751013 Orissa, India. E-mail: [email protected]. Phone: þ916742581637(425). Fax: 91674-2581637.

’ ACKNOWLEDGMENT The authors are thankful to Prof. B. K. Mishra, Director, IMMT, Bhubaneswar for his keen interest, encouragement, and kind permission to publish this work. Mrs. D. Rath is gratefully acknowledged, CSIR, for the award of SRF. The financial support from DST is highly appreciated. ’ REFERENCES (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Order mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710–712. (2) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 2373–2419. 2847


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Copper and Nickel Modified MCM-41 An Efficient Catalyst for

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