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Design of bifunctionalized hexagonal mesoporous silicas for selective oxidation of cyclohexane
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 215603 (5pp)
doi:10.1088/0957-4484/18/21/215603
Design of bifunctionalized hexagonal mesoporous silicas for selective oxidation of cyclohexane Chen Chen, Lipeng Zhou, Qiaohong Zhang, Hong Ma, Hong Miao and Jie Xu1 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China E-mail:
[email protected] Received 28 January 2007, in final form 27 March 2007 Published 27 April 2007 Online at stacks.iop.org/Nano/18/215603 Abstract Bifunctionalized hexagonal mesoporous silicas, CoPh-HMS, were designed and successfully synthesized by a one-step co-condensation route at ambient temperature. This new material possesses the typical wormhole structure of HMS material. Co2+ ions were incorporated into the frameworks in tetrahedral coordination along with phenyl groups immobilized on the surface, confirmed by a careful spectroscopic (Fourier transform infrared, ultraviolet–visible and 29 Si magic-angle spinning nuclear magnetic resonance) study. It was used in the liquid-phase selective oxidation of cyclohexane with molecular oxygen under solvent-free conditions, and showed excellent performance. The results indicate the potential for intentional design of the catalytic environment at a nanometre level with the combination of transition metal incorporation and surface organofunctionalization for mesoporous siliceous materials.
isomerization, esterification etc [10–12], but for selective oxidation reactions, they may accelerate the further reaction of the primary oxygenated products and decrease the selectivity to these target products. For example, the acidic sample acted as an inhibitor rather than as a catalyst for cyclohexane oxidation and caused the formation of excessive by-products [13]. An approach that may be utilized to overcome this problem is to modify the mesoporous silicas with surface organofunctionalization, which allows rational designing and tailoring of the properties of these materials [14, 15]. Immobilized organic groups can significantly modify the physical properties of mesoporous materials (e.g. their hydrophobicity and adsorption characteristics). Especially for the hydrocarbon oxidation reactions such as cyclohexane selective oxidation in which the substrate is nonpolar molecule while the oxygenated products are strong polar molecules, the introduction of hydrophobic groups into the mesopores can create a hydrophobic reaction environment that will continuously exclude the polar primary oxygenated products from the mesopores and adsorb the substrate into the mesopores. Thus, conversion of the substrate can be facilitated
1. Introduction The discovery of different types of mesoporous siliceous materials attracted great attention in view of their various potential applications in the 1990s [1–3]. In catalytic reaction, these pure siliceous materials are inert and should be functionalized to extend their applications. As a main approach, metal cations were incorporated into the framework to create redox-active sites and these materials (e.g. Ti-HMS, Fe-HMS and Co-TUD-1) showed good performance in the catalytic oxidation reactions [4–7]. In other works, it was found that if the divalent or trivalent metal cations were incorporated, the frameworks of the mesoporous materials would present negative charges that should be compensated by cations such as sodium, calcium, potassium, proton and others; a particular result associated with the incorporation of metal cations is the generation of acid sites by isomorphous substitution of silicon in tetrahedral positions by these cations [8, 9]. The presence of acid sites may be necessary and favourable for the reactions such as cracking, alkylation, 1 Author to whom any correspondence should be addressed.
0957-4484/07/215603+05$30.00
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© 2007 IOP Publishing Ltd Printed in the UK
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while further transformations of the primary products can be inhibited and the goal to increase the conversion and selectivity to primary products may be accomplished. The hexagonal mesoporous silicas (HMS) consists of smaller domain size with short channels and larger textural mesoporosity than their MCM-41 analogues; these properties can provide better transport channels for reactants to access the active sites and better diffusion channels for products to move out [16]. Under these conditions, the deep reaction of the primary products may be inhibited; hence it is a good candidate to be functionalized for cyclohexane selective oxidation. Herein, we present the synthesis of bifunctionalized material CoPh-HMS via one-step co-condensation method and apply them in the selective oxidation of cyclohexane with molecular oxygen under comparatively mild conditions.
Intensity/a.u.
100
c b a 2
4
6
8
10
2 theta/degree Figure 1. Small-angle XRD patterns of (a) HMS, (b) Co-HMS, and (c) CoPh-HMS.
2. Experimental details
by the internal standard method using methylbenzene as an internal standard. The concentration of CHHP was determined by iodometric titration, and the by-products acid and ester by acid–base titration.
2.1. Sample synthesis and characterizations CoPh-HMS was synthesized by the S0 I0 assembly pathway at room temperature [1], using hexadecylamine (HDA) as template, tetraethyl orthosilicate (TEOS) and phenyltriethoxysilane (PTES) as silicon sources, Co(OAc)2 ·4H2 O as Co source and mesitylene as swelling agent, respectively. The molar composition of the gel was as follows: 0.02 Co: 0.8 (TEOS): 0.2 (PTES): 0.27 (HDA): 0.27 (mesitylene): 9 (EtOH): 72(H2 O). Organic template was removed from the as-synthesized material by NH4 NO3 /ethanol extraction. A pure siliceous HMS was synthesized without Co(OAc)2 , mesitylene and PTES, and non-organofunctionalized Co-HMS was synthesized without PTES and Mesitylene. The x-ray powder diffraction (XRD) patterns were obtained using Rigaku D/Max 3400 powder diffraction system with Cu Kα radiation (λ = 0.1542 nm). Fourier transform infrared (FT-IR) spectra were collected on a Bruker Tensor 27 FT-IR spectrometer in KBr media. The surface area and pore volume was determined by N2 adsorption–desorption at 77 K on an ASAP 2010 micromeritics instrument. Ultraviolet– visible diffuse reflectance spectra (UV–vis DRS) were collected on Jasco V-550 UV–vis spectrophotometer equipped with a diffuse reflectance attachment. The microstructures of the materials were examined by transmission electron microscopy (TEM) on a JEOL JEM-2000EX electron microscope. The nuclear magnetic resonance spectra of 29 Si with magic-angle spinning (29 Si MAS–NMR) were taken on a Bruker DRX-400 spectrometer.
3. Results and discussion 3.1. Materials characterizations Figure 1 shows the small-angle XRD patterns of HMS, Co-HMS and CoPh-HMS. Each sample exhibits a single diffraction peak corresponding to the (100) plane at 2θ of 1◦ –3◦ , typical characteristic of HMS materials [2]. The d100 diffraction peak of Co-HMS shifts to the lower angle compared with the pure siliceous HMS, which indicates that a lattice expansion occurred and Co(II) ions were incorporated into the framework of HMS [7]. For the bifunctionalized CoPhHMS, on the contrary, a shift to a larger angle is observed indicating a contraction of the lattice with the phenyl groups immobilized. It is known that HMS series materials are synthesized by the S0 I0 assembly pathway and the effect of hydrogen bonding exists on the interface of surfactant–silicate during the synthesis process [2]. With the emergence of phenyl groups on this interface, the density of silanol groups in this interface decreases and fewer surfactant molecules are required for balance. This situation leads to the contraction of the micelle size in bifunctionalized mesoporous material. On the other hand, this result can be taken as a proof for the successful immobilization of phenyl groups. Transmission electron microscopy (TEM) image (figure 2) shows that CoPh-HMS possesses the typical wormhole structure of HMS material assembled from long alkyl chain neutral amines as template [17]. The BET surface area, pore volume, and average pore radius of CoPh-HMS are 899 m2 g−1 , 0.52 cm3 g−1 , and 2.7 nm, respectively. The coordination geometry of cobalt incorporated in the representative material CoPh-HMS is indicated by the UV–vis DRS (figure 3). The spectrum displays three absorption peaks (525, 584 and 650 nm), which can be unambiguously assigned to the 4 A2 (F) → 4 T1 (P) transition of Co2+ ions in tetrahedral environments as identified in previous study [6]. This suggests that the Co2+ ions in the sample occupy framework positions of the channel walls. Moreover, two intense bands in the UV region centred at 212 and 262 nm exhibiting characteristic
2.2. Catalytic oxidation of cyclohexane Catalytic reactions were performed in a 100 ml autoclave reactor with a Teflon insert inside. When the reaction stopped, the catalyst was separated by filtration after the reaction mixture was diluted with 15.00 g of ethanol to dissolve the by-products. The reaction products were identified by Agilent 6890N GC/5973 MS detector and quantitated by Agilent 4890D GC equipped with OV-1701 column (30 m × 0.25 mm × 0.3 μm) and titration. After the decomposition of cyclohexylhydroperoxide (CHHP) to cyclohexanol by adding triphenylphosphine to the reaction mixture, cyclohexanone and cyclohexanol were determined 2
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Transmittance/a.u.
a
Figure 2. TEM image of CoPh-HMS.
c d
4000
3500
3000
1500
1000
500
-1
Wavenumber (cm ) Figure 4. FT-IR spectra of (a) as-synthesized HMS, (b) HMS, (c) Co-HMS and (d) CoPh-HMS.
212
Absorbance/a.u.
b
262
525
584 650 4
Q 3
Q 3
T
2
T
300
400
500
600
700
(b)
800
Wavenumber/ nm (a)
Figure 3. UV–vis DRS of CoPh-HMS. 50
0
-50
-100
-150
-200
-250
-300
Chemical shift/ppm
vibrational structure are observed. These absorption bands, shifting to longer wavelengths by the substitution, are assigned to π → π ∗ transitions of benzene ring [18]. It can be taken as a strong evidence for the presence of phenyl groups in the material. The infrared spectra of the samples are shown in figure 4. The as-synthesized HMS sample exhibits absorption bands around 2924, 2854 and 1467 cm−1 corresponding to asymmetric and symmetric C–H stretching and C–H bending vibrations of the template molecules. Such bands disappear for the extracted sample indicating the removal of organic template. For the spectrum of CoPh-HMS, typical bands associated with =CH stretching aromatic vibrations (weak peaks around 3030–3080 cm−1 ), with Si–Ph vibrations (around 1137, 740 and 698 cm−1 ), and with phenyl ring vibrations (weak peaks at 1600 and 1437 cm−1 ) are detected. These bands can serve us to confirm the presence of phenyl groups in CoPh-HMS [19]. In addition, the absorption band at 960 cm−1 was assigned to Si–O vibration in Si–OH group [20], which can be observed in the spectra of HMS and Co-HMS, while this band almost disappears in the spectrum of CoPh-HMS. This indicates that the amount of hydrophilic Si–OH on the surface remarkably decreased caused by the substitution with hydrophobic Si–Ph. The 29 Si MAS–NMR spectra of Co-HMS and CoPh-HMS are showed in figure 5. For Co-HMS, distinct resonances characteristic of the silica network [ Q n = Si(OSi)n (OH)4−n , n = 2–4] are observed [21], and Q 4 and Q 3 species are the main components. This indicates that the silica network is well condensed. For CoPh-HMS, besides these two signals, resonances characterizing the organosiloxane network [T m = RSi(OSi)m (OH)3−m , m = 1–3] are
Figure 5. 29 Si MAS–NMR spectra of (a) Co-HMS and (b) CoPh-HMS.
observed [21]. The appearance of organosilane signals between −70 and −90 ppm (T 3 and T 2 ) are characteristics of fully cross-linked organosiloxane species, demonstrating the incorporation of phenyl groups within the framework walls of the mesostructures [22]. Moreover, the ratio of Q 3 silicon species to total silicon in the spectrum of CoPh-HMS20 was 20.7% lower than that of Co-HMS. The apparent diminishment in the Q 3 signal and the appearance of the T 3 (Si(OSi)3 Ph) signal signify the replacement of Si–OH on the surface by Si–Ph, and this is consistent with the result of FT-IR spectroscopy. All these results imply that the surface of CoPh-HMS has turned to be hydrophobic, which can also be directly proved by the observation that CoPh-HMS distributes in cyclohexane (upper phase), while Co-HMS in water (lower phase) (figure 6). 3.2. Catalytic performance The selective oxidation of cyclohexane to yield cyclohexanone and cyclohexanol (the so-called K–A oil) is the centerpiece of the commercial production of nylon. The present industrial process for cyclohexane oxidation is widely carried out above 423 K and 1–2 MPa pressure without catalyst or with metal cobalt salt as homogeneous catalyst. The conversion of cyclohexane is always controlled about 4% to 3
Nanotechnology 18 (2007) 215603
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Table 1. Catalytic oxidation of cyclohexane over the catalysts. (Note: Reaction was carried out with 0.12 g of catalyst and 0.12 g of TBHP in 15 g of cyclohexane at 388 K for 6 h under 1.0 MPa O2 ). Products distribution (mol%)a Catalysts
Conversion (mol%)
A
K
CHHP
Acid
Ester
K–A oil (mol%)
HMS Co(OAc)2 Co-HMS CoPh-HMS CoPh-HMSb
1.2 3.3 4.8 6.7 4.4
— 43.2 39.6 39.8 47.6
44.8 35.0 37.3 40.7 36.4
46.8 4.3 0.4 0.3 0.4
5.7 15.0 15.6 15.4 12.8
2.7 2.5 7.1 3.8 2.8
44.8 78.2 76.9 80.5 84.0
a A, cyclohexanol; K, cyclohexanone; CHHP, cyclohexylhydroperoxide; acid, mainly adipic acid; ester, mainly dicyclohexyl adipate; K–A oil, A and K. b Reaction for 4 h.
Co(II)
Ph Ph Ph
Ph Ph
Co(II)
Ph Ph
Ph
Ph Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph Ph
Ph
Ph
Ph
Ph
Ph Ph
Ph Ph Ph Ph
Ph
Figure 6. Distribution of Co-HMS (left) and CoPh-HMS (right) in cyclohexane (upper phase) and water (lower phase).
Co(II)
Ph Ph
Co(II)
Figure 7. The schematic of the designed bifunctionalized material.
keep 80% selectivity to K–A oil (a mixture of cyclohexanol and cyclohexanone) [23]. Raja and Thomas pointed out that the selective oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone (the so-called K–A oil) with O2 or air under mild conditions is of considerable importance [24], but K–A oil are easily oxidized to the acids and further transformed to other by-products. So, although many attempts have been made in the oxidation of cyclohexane, it continues to be a challenge [23]. In our work (table 1), with Co-free HMS as catalyst, low cyclohexane conversion (only 1.2%) was obtained at 388 K under solvent-free conditions. Co-HMS showed a good activity and the conversion increased to 4.9%, which is higher than the homogeneous catalyst Co(OAc)2 (3.3%) that is widely used in the present industrial process, but the selectivity of ester nearly increased by two times. As mentioned above, acid sites can be generated in Co-HMS by isomorphous substitution of silicon in tetrahedral positions by lower valence Co2+ ions. Acidic catalysts were usually used for the esterification reaction [11], so the presence of acid sites might catalyze the esterification reaction of adipic acid with cyclohexanol and hence increased the selectivity to ester. When the bifunctionalized CoPh-HMS was employed, encouraging results were obtained. It gave 6.7% conversion and 80.3% selectivity to K–A oil. Moreover, compared with Co-HMS, the selectivity of ester decreased remarkably. A proposed schematic of the designed catalyst is shown in figure 7. According to our intention, active sites are generated by Co(II) incorporation, which are modified by the neighbouring hydrophobic organic groups to create an unsuitable hydrophobic environment for polar oxygenated products such as cyclohexanol and adipic acid (the dipolar
moments of cyclohexanol and adipic acid are 1.86 D and 4.04 D, respectively), leading to their difficult proximity of the acid sites. The further reactions of these products on the acid sites were inhibited, and therefore the selectivity of ester decreased while the selectivity of K–A oil increased. Furthermore, this hydrophobic environment would more easily adsorb the nonpolar substrate and facilitate the conversion of cyclohexane resulting high cyclohexane conversion. When the reaction period was shortened to 4 h, the activity was almost same with that of Co-HMS, but the selectivity of K–A oil increased to 84.0% while the selectivity of ester decreased to a value nearest to that obtained from the homogeneous catalyst being used. In addition, in the recyclability test, CoPh-HMS was found to be stable and could be repeatedly used for three times without activity decrease.
4. Conclusions A new bifunctionalized material, CoPh-HMS, was synthesized at ambient temperature via a one-step co-condensation method. Co2+ was successfully incorporated into the framework of HMS along with phenyl groups immobilized on the surface. In the selective oxidation of cyclohexane under solvent-free conditions, CoPh-HMS showed high cyclohexane conversion and selectivity to K–A oil. This work demonstrates the potential of utilizing organic–inorganic hybrid mesoporous materials for the rational design of catalysts at the nanometre scale. 4
Nanotechnology 18 (2007) 215603
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Acknowledgment
[11] Peters T A, Benes N E, Holmen A and Keurentjes J T F 2006 Appl. Catal. A 297 182 [12] Chen X, Chen C, Xu N and Mou C Y 2004 Catal. Today 93–95 129 [13] Vanoppen D L, Vos D De and Jacobs P A 1998 J. Catal. 177 22 [14] Mercier L and Pinnavaia T J 2000 Chem. Mater. 12 188 [15] Brutchey R L, Ruddy D A, Andersen L K and Tilley T D 2005 Langmuir 21 9576 [16] Tanev P T and Pinnavaia T J 1996 Chem. Mater. 8 2068 [17] Williams T, Beltramini J and Lu G Q 2006 Micropor. Mesopor. Mater. 88 91 [18] Lambert J B, Shurvell H F, Lightner D A and Cooks R G 1987 Introduction to Organic Spectroscopy (New York: Macmillan) p 281 [19] Carrado K A, Xu L, Csencsits R and Muntean J V 2001 Chem. Mater. 13 3766 [20] Schwarz S, Corbin D R, Vega A J, Lobo R F, Beck J S, Suib S L, Corbin D R, Davis M E, Iton L E and Zones S I (ed) 1996 Materials Research Society Symposium Proceedings vol 431 (Pittsburg, PA: Materials Research Society) p 137 [21] Engelhardt G and Jancke H 1981 Polym. Bull. 5 577 [22] Mercier L and Pinnavaia T J 2000 Chem. Mater. 12 188 [23] Raja R and Thomas J M 2002 J. Mol. Catal. A 181 3 [24] Schuchardt U, Cardoso D, Sercheli R, Pereira R, da Cruz R S, Guerreiro M C, Mandelli D, Spinac´e E V and Pires E L 2001 Appl. Catal. A 211 1
This work was supported by the National Natural Science Foundation of China (20473088 and 20672111).
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