Article pubs.acs.org/IECR
Nafion-Modified Large-Pore Silicas for the Catalytic Acylation of Anisole with Acetic Anhydride Rafael van Grieken, Fernando Martínez, Gabriel Morales, and Antonio Martín* Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain ABSTRACT: Large-pore mesoporous SBA-15-type and FDU-12-type silicas have been synthesized and functionalized with perfluorosulfonic acidic Nafion resin using a postsynthetic impregnation method. The remarkable physicochemical properties of these large-pore ordered silicas (high surface area, open structure, large pore size, and thermal stability) make them particularly attractive for the immobilization of strongly acidic perfluorosulfonic sites. The loading of Nafion resin was 15 wt % and the impregnation method provided ca. 100% yield of incorporation. The synthesized Nafion-modified materials were evaluated in the liquid-phase acylation of anisole with acetic anhydride, and compared with other commercially available catalysts (Nafion NR50, Nafion SAC-13, and zeolite ZSM5). The prepared catalysts provided much higher activities than the commercial samples, the performance of the material Nafion(15)/LP-FDU-12 being especially outstanding. This material takes advantage from both a large-pore size and 3D interconnected cubic pore structure, facilitating the diffusion of chemicals.
1. INTRODUCTION Over the last years, ordered mesoporous silica materials have attracted much attention as a result of their unique properties, such as large surface areas, narrow pore size distributions, and tunable pore sizes and structures.1 The pore size of mesoporous silica materials can be expanded up to 30 nm by using different strategies such as adding swellings agents (e.g., 1,3,5trimethylbenzene,2−4 hexane,5−7 or 1,3,5-triisopropylbenzene8), varying the hydrothermal step time and temperature during the synthesis,9 employing alkanes and NH4+ at low temperature synthesis conditions,5,6 or changing the silica source.10 Increasing the mean pore size, while keeping a narrow pore size distribution, is a crucial issue for the functionality of the material, with an aim to reduce physical constraints or mass transfer limitations or to allow the allocation of larger molecules within the pore system. This has expanded the use of silica mesoporous materials to many diverse applications requiring such large pore sizes: catalysis,11−13 drug delivery systems,14 or high-surface-area polymer brushes.15 In other cases, they have been used as silica templates for the synthesis of mesoporous carbons as sorbents.16 A promising application for these large-pore silica supports in the field of catalysis is their functionalization with acid polymers or resins to yield acid catalysts showing high accessibility to the acid sites and remarkable diffusion properties. In this context, Nafion-perfluorosulfonic acid resin, generally considered a superacid material17−19 with acid strength comparable to that of pure sulfuric acid,20 is a good target for being supported on porous silica. The catalytic performance of pure Nafion is limited by the restricted surface area and low availability of acid sites in the polymeric resin. For this reason, several attempts have been reported to incorporate Nafion perfluorosulfonic acid resin over different silica supports in order to enhance its catalytic performance in several acid-catalyzed reactions.21−23 For this task, different techniques have been developed using a postsynthetic grafting process24 or a direct one-step synthesis strategy.25 Recently, we reported a postsynthetic method for © XXXX American Chemical Society
the surface impregnation of Nafion over mesoporous 2Dhexagonal SBA-15 silica material.26 The resultant catalyst showed good catalytic performance in the Friedel−Crafts acylation of anisole with acetic anhydride. However, a low availability of the perfluorosulfonic acid sites was detected for the samples with higher contents of resin. A similar impregnation method was also reported for Nafion over a cubic mesoporous SBA-16 silica material,22 which showed the benefits of an interconnected 3D pore system for the diffusion of reactants, although it displayed the formation of nondesired polymeric aggregates. The use of silica supports with a more open structure and a wider mean pore size presents an interesting challenge to increase the capacity of Nafion loading without noticeable loss of accessibility to the acid centers. In the present study, we propose an impregnation method of Nafion resin over two large-pore silica supports, a 2D-hexagonal SBA-15-like mesophase and a 3D cubic FDU-12 structure, in order to increase the resin loading, the diffusion properties, and the availability of the perfluorosulfonic acids sites. The catalytic performance of the synthesized materials has been assessed in a typical high acid strength-demanding reaction such as the Friedel−Crafts acylation of anisole with acetic anhydride to produce aromatic ketones, which are useful intermediates in the fine chemical industry as pharmaceuticals and fragrances.27
2. MATERIALS AND METHODS 2.1. Synthesis of SBA-15, SBA-16, Large-Pore SBA-15, and FDU-12 Silica Mesoporous Supports. Conventional SBA-15 and SBA-16 silica were prepared using the typical methods described in the literature.2,3 SBA-15-type large-pore Received: April 29, 2013 Revised: July 5, 2013 Accepted: July 8, 2013
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dx.doi.org/10.1021/ie401360b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 1. SAXS patterns of mesoporous silica supports (a), and Nafion-modified materials with a content of resin of 15 wt % (b).
pressure under magnetic stirring for 2 h. The completion of solvent evaporation was performed at room temperature for 24 h. The resultant hybrid materials were denoted as Nafion(15)/ support. 2.3. Characterization Techniques. Small-angle X-ray scattering (SAXS) patterns were recorded on a Hecus MBraun small angle X-ray scattering instrument with a MBraun PSD 50m position gas detector. Structural characterization was completed by transmission electron microscopy (TEM) on a PHILIPS TECNAI-20 electronic microscope operating at 200 kV. Textural properties of the Nafion-mesostructured silica composites were evaluated by means of nitrogen adsorption and desorption isotherms at 77 K using a Micromeritics TRISTAR 3000 equipment. Silica samples were outgassed at 200 °C in N2 flow, while impregnated samples were outgassed at 150 °C in order to preserve the organic load. The specific surface area was estimated according to the BET method, and the total pore volume was calculated at P/P0 = 0.975. The BJH method was employed to estimate the mean pore diameter from the adsorption branch of the isotherm. Nafion resin content was determined via weight loss by thermogravimetric analysis in air (SDT 2960 Simultaneous DSC-TGA, from TA Instruments) in the range of 300−500 °C. Cation-exchange capacities corresponding to the perfluorosulfonic acid-modified mesostructured materials were determined using saturated solutions of NaCl in ethanol as cationic-exchange agent. In a typical experiment, 0.05g of solid was added to 15g of alcoholic solution containing the sodium salt. The resulting suspension was allowed to equilibrate and thereafter it was titrated potentiometrically by dropwise addition of 0.01 M NaOH (aq). Sulfur content was determined by means of elemental analysis (HCNS) in a Vario EL III apparatus. 2.4. Catalytic Reaction Tests. The catalytic runs of acylation were performed in a two-necked 100 mL roundbottom flask equipped with a reflux condenser and a magnetic stirring bar. Equimolar amounts of anisole/acetic anhydride were used (0.05 mols), employing nitrobenzene as solvent (50
size material was synthesized according to an already described method:11 Typically, surfactant Pluronic 123 (2.4 g) and NH4F (0.027 g) were dissolved in 1.3 M HCl aq. solution (84.0 mL) at room temperature under mechanical stirring. This solution was then placed in a water bath set at 17 °C, and after 1 h a mixture of 5.5 mL of tetraethylorthosilicate (TEOS) and 1.2 mL of 1,3,5- triisopropylbenzene (TIPB) was added. The solution was mechanically stirred for 24 h at 17 °C and atmospheric pressure, and then aged under static conditions and an autogenous pressure at 100 °C for 24 h. The resultant solid was isolated by filtering, washing with water, and drying at 60 °C in a vacuum oven. Finally, the sample was calcined under air at 550 °C for 5 h (heating ramp, 2 °C min−1). The assynthesized large-pore SBA-15-type material was designated LP-SBA-15. The synthesis of cubic pore-expanded FDU-12 silica support followed a similar procedure to that described in the literature.4 3 g sample of Pluronic F127 copolymer was dissolved in 185 mL of 2 M aq. HCl. The mixture was placed in a water bath set at 15 °C under mechanical stirring. Then 4.2 mL of TIPB and 15.0 g of KCl were added. After 2 h, TEOS (12.5 g) was added, and the mixture was stirred for 24 h at 15 °C. Next, the mixture was aged under static conditions and autogenous pressure (100 °C, 24 h). The solid product was isolated as mentioned previously for the LP-SBA-15 support. To achieve the selected pore expansion, 0.5 g of the assynthesized sample was then placed in 30 mL of 2 M aq. HCl and heated at 100 °C for 48 h. Finally, the surfactant was removed by calcination in air at 550 °C for 5 h (heating ramp 2 °C min−1). The synthesized large pore FDU-12 material was designated LP-FDU-12. 2.2. Incorporation of Perfluorosulfonic Nafion Resin by Impregnation. The incorporation of the perfluorosulfonic Nafion resin was carried out by an impregnation method comprising the dispersion of a Nafion solution (5 wt % in lower aliphatic alcohols, Aldrich) over the different silica supports.26 Resin loading has been established at 15 wt %. The impregnation was carried out at 60 °C and atmospheric B
dx.doi.org/10.1021/ie401360b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 2. N2 adsorption−desorption isotherms of the synthesized mesoporous materials. (a) SBA-15 samples; (b) LP-SBA-15 samples; (c) SBA-16 samples, and (d) LP-FDU-12 samples.
Table 1. Textural and Organic Properties of the Synthesized Materials and Selected Commercial Catalysts material
structure
SBA-15 Nafion(15)/SBA-15 LP-SBA-15 Nafion(15)/LP-SBA-15 SBA-16 Nafion(15)/SBA-16 LP-FDU-12 Nafion(15)/LP-FDU-12 Nafion-SAC-13 Nafion-NR50 ZSM-5
2D-hexagonal 2D-hexagonal large-pore 2D-hexagonal large-pore 2D-hexagonal 3D-cubic 3D-cubic large-pore 3D-cubic large-pore 3D-cubic nonordered silica organic resin MFI (microporous)
Nafion wt %a S contentb (meq S/g)
titrationc (meq H+/g) SBET (m2/g)
15
0.13
0.13
15
0.14
0.13
15
0.17
0.15
15 13 100
0.14 0.13 0.90
0.14
653 477 447 264 956 608 535 408 200
