Effect of Pore Size and Nickel Content of Ni-MCM-41 on Catalytic

Feb 9, 2012 - silica MCM-41 (Ni-M41) for ethene dimerization was investigated as ... The Ni-M41 samples with smaller pore size and higher Si/Ni ratio...
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Effect of Pore Size and Nickel Content of Ni-MCM-41 on Catalytic Activity for Ethene Dimerization and Local Structures of Nickel Ions Masashi Tanaka,† Atsushi Itadani,‡ Yasushige Kuroda,*,‡ and Masakazu Iwamoto*,† †

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-5 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima, Kita-ku, Okayama 700-8530, Japan



S Supporting Information *

ABSTRACT: The catalytic activity of nickel ion-loaded mesoporous silica MCM-41 (Ni-M41) for ethene dimerization was investigated as a function of the pore size and the amount of nickel. In addition, the silica wall and the loading of the nickel species were characterized. The Ni-M41 samples with smaller pore size and higher Si/Ni ratio exhibited greater reaction rate constants. The Fourier transform infrared (FT-IR) spectra indicated the formation of 2:1 nickel phyllosilicate-like species along the pore wall. Furthermore, the IR band at approximately 570 cm−1 and the X-ray absorption fine structure (XAFS) spectra indicated the existence of five-membered rings consisting of Si−O on the M41 pore wall in addition to the typical six-membered ones. On the basis of the UV−vis−NIR diffuse reflectance (UV−vis−NIR DR), FT-IR, and XAFS data, we propose that the three- and four-coordinated Ni2+ ions lie on the five- and six-membered Si−O rings of silica, respectively. Nitrogen monoxide was employed as a probe molecule in the FT-IR and UV−vis−NIR DR experiments and revealed that NO adsorbed as di- and mononitrosyl species on the three- and fourcoordinated Ni2+ ions. The intensity of the dinitrosyl species on the three-coordinated Ni2+ ions correlated with the catalytic activity for ethene dimerization. Therefore, the three-coordinated Ni2+ ions are proposed to act as the active site for the reaction.



INTRODUCTION Mesoporous silica materials such as MCM-411 (M41) and FSM-162 have prompted numerous studies in various areas due to their unique structure and potential applications.3−5 The interesting properties of these materials originate from the large and uniform pore sizes that exist in a regular array, resulting in so-called confinement effects and high surface areas. Many studies have therefore focused on their use as catalysts, supports, and adsorbents. The correlation between the mesopores and catalysis is one of the central topics in this field, and it has been studied.6−10 The amount of OH groups was reported to decrease on the mesoporous silica with larger pores,8 while the catalytic activity of Pt-loaded FSM-16 with larger pores was greater than that with smaller pores.9 In addition, the catalysis of M41 for the acetalization of cyclohexanone was optimal with a pore diameter of 1.9 nm.7c At present, the correlation among the pore diameter, the surface structure, and the catalytic activity has not yet been clarified due to a lack of detailed information about the surface structure of the pore wall. Several approaches have already attempted to solve the local structure of M41 using various spectroscopic methods including X-ray, 11 29Si NMR,12 Raman,12 and theoretical method.13 However, direct evidence connecting the catalytic activity with the surface structure has not been obtained. In general, the pore surface has been © 2012 American Chemical Society

represented as a simple combination of regular six-membered rings.14 Recently, nickel ion-loaded M41 (Ni-M41) prepared by the template ion-exchange (TIE) method has been found to exhibit high catalytic activity for the ethene dimerization reaction and the conversion of ethene to propene including metathesis reaction.15 The former reaction was also reported for Ni-M41 prepared by the usual impregnation method,16 and the latter was further confirmed for Ni-M41 prepared by the equilibrium adsorption method.17 Moreover, Ni-M41 was active for the conversion of ethanol to ethene and propene in one step.15c,18 Nickel complexes are widely used as homogeneous catalysts for industrial dimerization and oligomerization reactions of lower olefins.19 Although a large number of studies have been devoted to the development of heterogeneous processes due to their easy operation,20 the activity of heterogeneous catalysts markedly decreased during the ethene dimerization and could not be employed in a practical industrial process. Minimal deactivation of Ni-M41 during long-term operation at 673 K15,18 might make it possible to employ Ni-M41 as a practical catalyst. The emergence of active and stable Ni-M41 catalysts is a major reason for us to select it as a target material to elucidate Received: October 26, 2011 Revised: February 3, 2012 Published: February 9, 2012 5664

dx.doi.org/10.1021/jp2103066 | J. Phys. Chem. C 2012, 116, 5664−5672

The Journal of Physical Chemistry C

Article

BH capacitance manometer. A constant amount of ethene was introduced into the reaction apparatus containing the Ni-M41 samples. The amount of Ni-M41 mounted was varied to maintain a consistent amount of nickel ions used in the experiment. Namely, the catalytic activity was measured under the normalized conditions for the amounts of ethene and nickel. Prior to the catalytic run, the Ni-M41 catalyst was evacuated at 673 K for 1 h. Characterization. The chemical composition of the template-free samples was determined by an ICP mass spectrometer (ELAN DRC-e, Perkin-Elmer) after the samples were dissolved into an HF solution. X-ray diffraction (XRD) patterns were collected using a Rigaku RINT Ultima/PC diffractometer with a monochromatic Cu Kα radiation. Nitrogen adsorption−desorption isotherms were measured at 77 K with a BEL Japan mini II automatic gas sorption meter after the sample was evacuated at 423 K for 2 h. The specific surface area of the sample was determined by applying the Brunauer−Emmett−Teller (BET) equation to the obtained adsorption isotherm. The pore size distribution was calculated from the adsorption isotherm with the Barrett−Joyner− Halenda (BJH) method.23 The FT-IR spectra were recorded at r.t. using a Perkin-Elmer Spectrum One spectrophotometer with a TGS detector (accumulation 32 scans, resolution 4 cm−1). The powdered sample was pressed into a self-supporting disk with a diameter of 10 mm and placed in a quartz cell with KRS-5 windows. The cell was capable of in situ treatment and gas introduction. Adsorption measurement of ethene or NO was performed by introducing the gas at r.t. to the Ni-M41 sample pre-evacuated at 673 K. For the investigation of the silica framework, the M41 and Ni-M41 samples were separately pressed into a wafer using the KBr pellet technique. The weight ratio of the sample to KBr was 3/100. The XAFS spectra were observed on a beamline 9C equipped with a double crystal monochromator of Si(111) under ring-operating conditions of 2.5 GeV and 300 mA (Photon Factory in the Institute of Materials Structure Science: Interuniversity Research Institute Corporation, High Energy Accelerator Research Organization, KEK, Tsukuba). The photon energy was calibrated by the preedge peak of a copper foil (8.9788 keV). A self-supporting disk was mounted into an in situ cell. The spectral data were analyzed using Maeda’s program.24 In this study, the Fourier transformation of the k3-weighted EXAFS oscillation from k space to r space was performed over the range of 2.0−16.0 Å−1 to obtain the radial distribution function. The back-Fourier transforms were separately calculated in the r ranges of 1.0−2.0 Å for the Ni−O contribution and 2.2−3.2 Å for the Ni−Ni and Ni−Si contributions. The least-squares fitting was applied to obtain the values of the mean free path using the reference materials, Ni(OH)2 and 2:1 nickel phyllosilicate. The 2:1 nickel phyllosilicate sample was synthesized at 423 K according to a previously reported protocol.25,26 The UV−vis−NIR DR spectra were collected at r.t. using a Perkin-Elmer Lambda 19 UV−vis−NIR spectrometer. Barium sulfate was employed as a reference material. The samples were placed in a vacuum reflectance cell, evacuated at 673 K, and then exposed to ethene or NO gas at r.t. Simulation Methodology. The density functional theory (DFT) calculations were performed with the DMol3 in the Materials Studio (version 5.0) program package developed by Accelrys Inc.27 The total energy optimization was performed using the generalized gradient approximations and the BLYP (the exchange correction of Becke and the correlation function

the correlation between the surface structure and the catalytic activity. The surface structure of the nickel species supported by the TIE method was previously reported,15b but the study was limited. More research is required to determine if the structure of silica in M41 is the same as conventional silica and if the nickel phyllosilicate formed on the pore wall is 2:1 or 1:1. Additional studies are needed to determine the reactivity of nickel ions and the location of the active sites in the catalyst. In this work, we examined the catalytic activity of Ni-M41's having different pore sizes and nickel content for the ethene dimerization reaction at ambient temperature to measure the reaction rates. In addition, the local structure and reactivity of the silica and nickel ions were studied by Fourier transform infrared (FT-IR), X-ray absorption fine structure (XAFS), and UV−vis−NIR diffuse reflectance (UV−vis−NIR DR) spectroscopy. Nitrogen monoxide was used as a probe molecule to study the reactivity of nickel ions.



EXPERIMENTAL SECTION Materials. The parent M41 was synthesized according to the previously published protocol.1,21 A fumed silica (Aerosil 200, Degussa AG, Germany) and sodium silicate solution (35 wt % SiO2, Kanto Chemical Co., Inc., Japan) were used as silicon sources, and quaternary ammonium bromide surfactants CnH2n+1(CH3)3NBr (n = 10, 14, 18) (Tokyo Kasei Kogyo Co., Ltd., Japan) were used as templates. The typical preparation procedure is described below. An 18 g sodium silicate sample was mixed with 6 g of Aerosil 200 dispersed into 150 g of distilled water, and the mixture was irradiated with a supersonic wave at 308 K for 2 h. Subsequently, a solution containing 10% tetramethylammonium hydroxide and methanol (Tokyo Kasei Kogyo Co., Ltd.) was added to the silica mixture. A 10 g ndecyltrimethylammonium bromide [C10H21(CH3)3NBr] sample was dissolved in 150 g of distilled water. The solution was stirred at 308 K for 2 h, and the silica solution was added dropwise. Then, the silica−surfactant solution was irradiated with a supersonic wave at 308 K for 2 h, and the pH value of the resulting solution was 11.5. The mixture was transferred to a Teflon bottle and heated in an autoclave at 308 K for 3 days and at 373 K for additional 2 days. The precipitated product was filtered, washed with distilled water, and dried at 343 K overnight to obtain the as-synthesized M41 material. Removal of the template ions was carried out by calcination of the sample at 773 K for 8 h in air. The Ni-M41 with Si/Ni atomic ratios of 20, 40, and 80 was prepared using the TIE method.22 An aqueous solution of nickel nitrate, 50.66 g (0.66 g of salt in 50 g of H2O; Si/Ni = 20), was added to the suspension of the as-synthesized M41 (5 g) and distilled water (50 g) at room temperature (r.t.). The mixture was vigorously stirred at r.t. for 1 h and then kept at 353 K for 20 h. The product was filtered, washed, dried at 343 K overnight, and calcined at 773 K for 8 h in air (the same procedure used for M41). The M41 and Ni-M41 samples are abbreviated as M41-N and NiX-M41-N (N, the number of carbons in the alkyl chain of the surfactant used; X, the Si/Ni ratio determined by an ICP analysis). The ethene (99.5%) gas was purchased from the GL Sciences Co. Japan and the NO gas (99%) from the Sumitomo Seika Chemicals Co., Ltd. Japan. The gases were used in the experiments without any purification. Measurement of Catalytic Activity. The catalytic reaction was performed at 298 K using a conventional volumetric apparatus equipped with an MKS Baratron 310 5665

dx.doi.org/10.1021/jp2103066 | J. Phys. Chem. C 2012, 116, 5664−5672

The Journal of Physical Chemistry C

Article

Table 1. Physicochemical Properties of M41 and Ni-M41 Samples with Different Pore Sizes sample M41-10 Ni73-M41-10 Ni42-M41-10 Ni19-M41-10 M41-14 Ni78-M41-14 Ni42-M41-14 Ni19-M41-14 M41-18 Ni85-M41-18 Ni42-M41-18 Ni23-M41-18

Si/ Nia 73 42 19 78 42 19 85 42 23

Ni contenta (wt %)

SBETb (m2 g−1)

pore diameterc (nm)

pore volumec (cm3 g−1)

d- spacingd (nm)

a0e (nm)

pore wall thicknessf (nm)

0 1.14 2.22 4.22 0 1.03 1.94 4.22 0 0.90 1.98 3.61

988 1077 1042 925 1029 1064 1013 960 887 906 878 838

1.62 1.70 1.70 1.70 2.48 2.54 2.50 2.56 3.40 3.48 3.48 3.56

0.42 0.56 0.51 0.42 0.66 0.59 0.55 0.65 0.69 0.77 0.70 0.60

2.85 2.88 2.88 2.96 3.63 3.72 3.76 3.79 4.80 4.71 4.76 4.80

3.29 3.33 3.33 3.42 4.19 4.30 4.34 4.38 5.54 5.44 5.50 5.54

1.67 1.63 1.63 1.72 1.71 1.76 1.84 1.82 2.14 1.96 2.02 1.98

a

Determined using an ICP mass spectrometer. bSpecific surface area determined from application of the BET equation to the N2 adsorption isotherm at 77 K. cCalculated from the N2 adsorption isotherm with the BJH method. dDetermined by XRD for the hexagonal structure of the mesoporous materials. eThe distance between the pore centers (a0 = 2d100/√3). fEvaluated from the difference beween the pore diameter and a0.

Figure 1. (a) The pressure change as a function of the reaction time at r.t. after the introduction of ethene at 33.3 kPa into the cell. In the absence of sample, the calculated pressure of ethene introduced was 20.0 kPa. The preactivated Ni-M41 samples with various pore sizes and amounts of nickel were used as the catalysts. (b) Dependence of the catalytic activity of Ni-M41 for ethene dimerization on the pore size and the Si/Ni ratio. The reaction rate constants were calculated on the the basis of the results of the part (a) using a second-order rate equation.

of Lee, Yang, and Parr) functional.28 All electrons were included in the calculations. A double numerical basis set with polarization functions (DNP) was used in all of the calculations. The accuracy of this basis set is comparable to the Gaussian 6-31G++ basis set.29

was consistent with the results reported regarding the dependence on the numbers of OH groups8 and the righthand-side dependence of the acetalization of cyclohexanone.7c In these reports, it was claimed that the assemblies of silanol groups decreased on the wider pores. For the Ni-M41 samples, the pore size of M41 is expected to affect the property of nickel species. The interaction of ethene with Ni-M41 was also measured using FT-IR spectroscopy. Figure 2a shows the FT-IR spectra after the introduction of ethene on M41-10 and Ni19-M41-10, which were recorded in the presence of ethene of 13 kPa and depicted after the subtraction of the spectra of the samples evacuated at 673 K. The bands at 3098 and 2978 cm−1 on both samples were attributed to the CH stretching vibrations of ethene adsorbed on silanol groups.30 The 1441 cm−1 band was attributable to the CH2 scissoring band. The spectrum measured at 20 min on M41-10 was the same as that at 2 min, indicating that the adsorption of ethene on M41 was finalized within 2 min. In contrast, the spectrum on Ni19-M41-10 at 20 min was quite different from that at 2 min. The change in the spectra is summarized in Figure 2b as a function of the adsorption time. All of the spectra observed here were very similar to those of 1-butene adsorbed on Cu+-zeolites.31 The bands at 2965 and 2880 cm−1 and at 2928 and 2855 cm−1 were



RESULTS AND DISCUSSION Catalytic Activity for Ethene Dimerization. The properties of the Ni-M41 samples used are shown in Table 1, and the results exhibited reasonable surface areas, pore sizes, and Si/Ni ratios. The changes in ethene pressure as a function of time on the various Ni-M41's at 298 K are summarized in Figure 1a. In all of the experiments, the pressure decreased monotonically with the reaction time. The second-order rate equation was applied to analyze the reaction rates of the ethene dimerization in the initial stage (2−20 min) on the basis of the change in the FT-IR spectra (will be shown in Figure 2). The reaction rate constants obtained are plotted in Figure 1b as a function of the pore diameter and the Si/Ni ratio. The catalytic activity was strongly dependent on the pore diameter and the Si/Ni ratio. Ni73-M41-10 was the most active for the dimerization/ oligomerization of ethene. In particular, Ni-M41, which has smaller pores and a higher Si/Ni ratio, showed greater catalytic activity. The pore size effect that was observed in these studies 5666

dx.doi.org/10.1021/jp2103066 | J. Phys. Chem. C 2012, 116, 5664−5672

The Journal of Physical Chemistry C

Article

Figure 3. FT-IR spectra of Ni-M41's evacuated at 673 K: (1) Ni19M41-10, (2) Ni24-M41-14, and (3) Ni23-M41-18.

has been assigned to the O−H bending vibration of isolated OH groups surrounded by three nickel atoms, and the 670 cm−1 band was attributed to a tetrahedral SiO mode.33 In addition, the O−H stretching band was reported to appear at 3625 and 3645 cm−1 on the crystallized 2:1 and 1:1 nickel phyllosilicates, respectively.33 The observation of bands at 3626, 710, and 667 cm−1 on the present samples indicated the formation of a 2:1 nickel phyllosilicate-like structure. On the Ni-M41's with higher Si/Ni ratios, the bands at 710 and 670 cm−1 were not well recognized but the band at 3626 cm−1 was observed as shown in Figure S1 in the Supporting Information. A low concentration of Ni (Si/Ni ratio >20) or a low temperature of hydrothermal treatment (