Ind. Eng. Chem. Res. 2008, 47, 8005–8010
8005
Preparation, Characterization, and Acidity Evaluation of Perfluorosulfonic Acid-Functionalized Silica Catalysts Gema Blanco-Brieva,† Jose M. Campos-Martin,† M. Pilar de Frutos,‡ and Jose L. G. Fierro*,† Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain, and Centro de Tecnologı´a Repsol YPF, 28931 Mo´stoles, Madrid, Spain
Three different hybrid organic/inorganic solid acid catalysts were prepared by grafting perfluorosulfonic acid onto amorphous silica gel by covalently anchoring β-sultone onto the hydroxyl groups on the silica surface. The anchored sulfonic groups have been characterized by thermal analysis, infrared spectroscopy, and photoelectron spectroscopy (XPS). Particularly, the XPS technique has proven to be an extremely powerful tool to determine the nature and chemical state of sulfur and carbon, as well as the surface density of the acid groups. The hybrid organic/inorganic systems functionalized with sulfonic acid groups were tested in the esterification reaction of acetic acid with methanol in liquid phase. Activity results obtained at 333 K and using an initial molar ratio AcOOH:MeOH ) 1:1 revealed that the acid-functionalized, nonsilylated catalyst reaches acetic acid conversion of ∼50%, which contrasts with the much lower level (∼30%) reached by a commercial Nafion silica composite sample. Treatment of the catalysts in methanol at 333 K under stirring for 48 h emphasized that only the acid-functionalized and silylated sample retains some of the sulfonic groups. Introduction Typical homogeneous mineral acid catalysts, such as H2SO4, HCl, HI, and ClSO3OH, are used in a variety of industrial organic transformations, including aldol condensations, hydrolyses, acylations, nucleophilic additions, and others.1 However, reactor corrosion, waste neutralization, difficult separations, and the inability for reuse have hindered industrial reactions catalyzed by liquid acids. Nevertheless, solid catalysts provide numerous opportunities for recovering and recycling catalysts from reaction environments.1,2 These features can lead to improved processing steps, better process economics, and environmentally friendly large-scale manufacturing. Protonexchanged zeolites appear, in principle, particularly suited to realizing the abovementioned acid-catalyzed conversions; however, the small pore diameter of these acidic zeolites limits their use in processes in which larger molecules cannot penetrate the catalyst’s small micropores. Inorganic supports and particularly amorphous silica, of larger pore size dimension, were covalently grafted onto sulfonic acids to generate strong acid sites on their surface. These methods include, among others, oxidation of immobilized thiols to sulfonic acids,3 hydrolysis of immobilized sulfonic acid chlorides,4 sulfonation of supported phenyl groups,5 and immobilization of perfluorosulfonic acid triethoxysilanes.6 Functionalized amorphous silica surfaces are of great interest owing to their potential applications in environmental and industrial processes. This potential derives from the fact that the covalently anchored functional groups on the silica imbue its surface with specific attributes, such as a stereochemical configuration, binding sites, charge density, and acid-base or redox properties.7–10 Recent developments have been made in the mesoporous ordered silica family incorporating organic groups using either sol-gel methodology4,11–13 or postgrafting techniques.8,13 This chemistry has been successfully exploited * To whom correspondence should be addressed. Fax: +34 915854760. E-mail:
[email protected]. E-mail: http://www.icp. csic.es/eac/index.htm. † Instituto de Cata´lisis y Petroleoquı´mica, CSIC. ‡ Centro de Tecnologı´a Repsol YPF.
to prepare pure Brønsted sulfonic acid-functionalized mesoporous silica, i.e., MCM-41 materials3,14–16 or amorphous silica.6,17–20 A further route to strong heterogeneous Brønsted acids consists of forming Nafion-H/silica composites,6,17 which due to the perfluorinated nature of the organics are stronger acids than hydrocarbon-based materials. However, they suffer from limited availability of the acid groups due to the imperfect dispersion of the resin within the silica pores. This drawback can be avoided by nonoxidative direct synthesis of mesoporous silica-perfluorosulfonic acid materials21 or by grafting perfluorosulfonic acid precursors onto mesoporous silica.22–24 Here, we investigate the viability of incorporating perfluorosulfonic acid (PFSA) onto amorphous silica gel by covalently anchoring β-sultone onto the hydroxyl groups on the silica surface and evaluating the influence of a perfluorated silylant agent. Thermal analysis (TG), infrared spectroscopy, and photoelectron spectroscopy (XPS) techniques have been employed to characterize the nature and surface density of sulfonic acid groups, and the performance of these hybrid functionalized materials was evaluated in the esterification reaction of acetic acid with methanol in liquid phase. Experimental Section Catalyst Preparation. A commercial silica (Grace Davison XPO 2447, surface area 224 m2 g-1, pore volume 1.44 mL g-1) was functionalized with a 1,2,2-trifluoro-2-hydroxy-1-trifluoromethylethanesulfonic acid β-sultone (C3F6O3S) (ABCR) to obtain grafted perfluorosulfonic acid. Accordingly, 2 g of silica was first degassed at 393 K overnight and then cooled at room temperature. A solution of an excess of sultone (1.0 g) in 50 mL of dry toluene was subsequently added to the dried silica. The mixture was refluxed for 4-6 h under an inert atmosphere, and the solid was then filtered and washed thoroughly with toluene to remove any unreacted precursor or byproduct. Finally, the functionalized grafted silica sample was dried at 373 K overnight. A second sample was prepared by treating an aliquot of the first sample (SiSu) with 1H,1H,2H,2H-perfluorooctyldimethylchlorosilane at room temperature in toluene for 4 h. This grafted silica sample was separated by filtration and washed
10.1021/ie800221f CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
8006 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Samples Prepared by Surface Modification of Silica precursor amount per silica gram addition order C3F6O3S (sultone) CF3(CF2)5-(CH2)2Si(CH3)2Cl SiSSu SiSuPF-1 SISuPF-2
0.5 0.5 0.5
1 1
simultaneous successive
thoroughly with toluene to remove any unreacted precursor or byproduct. A third sample was prepared by adding sultone and the silylant agent together by keeping under reflux for 4-6 h under an inert (N2) atmosphere. The solid was then separated by filtration and repeatedly washed with toluene to remove any adsorbed contaminant. All samples were dried at 373 K overnight. The codes used for the synthesized catalysts are shown in Table 1. In addition, a commercial Nafion SAC-13 silica composite sample supplied by Aldrich was used as reference. Catalyst Characterization. The textural properties of the silica-functionalized samples were determined from the nitrogen adsorption-desorption isotherms recorded at 77 K with a Micromeritics TriStar 3000 apparatus. The samples were previously degassed at 423 K for 24 h under a vacuum (10-4 mbar) to ensure a clean dry surface, free of any loosely bound adsorbed species. The specific areas of the samples were determined according to standard BET procedure using nitrogen adsorption data taken in the relative equilibrium pressure interval of 0.03 < P/P0 < 0.3 and with a value of 0.162 nm2 for the cross section of adsorbed nitrogen molecule. Infrared spectra were recorded at room temperature on a Nicolet 510 FT-IR spectrophotometer provided with a KBr beam splitter and a DTGS detector. For each spectrum, 100 scans were accumulated at a spectral resolution of 4 cm-1. Sample wafers were prepared by mixing 20 mg of KBr with 1 mg of the sample. TG of the catalysts was performed on a Mettler Toledo TGA/SDTA 851 apparatus. Typically, 30-40 mg of the sample was heated from 298 to 1373 K at a rate of 10 K/min under nitrogen atmosphere. XPS were acquired with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and Mg KR (hν ) 1253.6 eV) nonmonochromatic X-ray source. The samples were degassed in the pretreatment chamber at room temperature for 1 h prior to being transferred into the instrument’s ultrahigh vacuum analysis chamber. The silicon, oxygen, sulfur, and carbon signals were scanned several times at a pass energy of 20 eV in order to obtain good signal-to-noise ratios. The binding energies (BE) were referenced to the BE of C 1s core-level spectrum at 284.9 eV. The invariance of the peak shapes and widths at the beginning and end of the analyses indicated constant charge along measurements. The peaks were fitted by a nonlinear least-squares fitting routine using a properly weighted sum of Lorentzian and Gaussian component curves after background subtraction. Surface atomic ratios were estimated from the areas of the peaks, normalized to silicon, and corrected using the corresponding sensitivity factors.25 Catalytic Tests. The catalytic performance of perfluorosulfonic acid-functionalized silica samples was evaluated in a liquid-phase glass batch reactor operating at atmospheric pressure. The reaction test selected for the purpose of the present work was the esterification of acetic acid with methanol. The procedure consisted in mixing 1.5 mol of acetic acid with 1.5 mol of methanol (molar ratio 1:1) under vigorous stirring while heating to 333 K. Once this temperature remained stable, 1 g of the catalyst was added to the reaction mixture. Since the objective of this work was to compare catalyst performance of samples differing in the population of sulfonic acid groups, these
Figure 1. Adsorption-desorption nitrogen isotherm at 77 K of the samples and silica precursor. Table 2. Textural Properties of the Solids sample
BET surface area (m2 g-1)
pore volume (mL g-1)
pore diameter (nm)
silica XPO2407 SiSu SISuPF-1 SISuPF-2
224 269 231 247
1.44 1.14 0.75 0.82
21 11 11 13
reaction conditions were maintained constant. Aliquots of the liquid phase were periodically withdrawn from the reactor and acetic acid conversion was evaluated by acid titration with a 2 M NaOH solution. RESULTS AND DISCUSSION The chemical grafting of the silica surface with PFSA was achieved in a first instance through a chemical reaction between the surface hydroxyl groups and the perfluorinated sultone. This procedure is similar to the one described previously in the bibliography for MCM-41 substrates,22,23 but we also studied the influence of the presence of groups perfluorinated in these supports (Table 1). The nitrogen adsorption-desorption isotherms at 77 K of the sulfonic acid-functionalized samples and the naked SiO2 substrate are displayed in Figure 1. All the isotherms are of type IV of IUPAC classification, which are usually attributed to mesoporous solids. These adsorption-desorption isotherms show a narrow hysteresis loop at rather high relative pressures (0.7 < P/P0 < 0.9) (Figure 1). This kind of loop is associated with porous materials that consist of agglomerate or compact packing of approximately regular and uniform spheres, which therefore have relatively narrow pore size distributions. On examining the data reported in Table 2 and Figure 1, a clear effect in both pore volume and pore diameter is observed upon incorporating the perfluorinated compound onto the silica substrate. The pore volume of the resulting surface-functionalized solids is somewhat lower than that of the parent silica substrate. The decrease is even more marked in the samples in which the silylant compound [CF3(CF2)5(CH2)2Si(CH3)2Cl] is incorporated (SiSuPF-1 and SiSuPF-2 samples). This silylant compound may cover the silica particles, block its pores, or both, being reflected in a reduction in pore volume. Nevertheless,
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Figure 2. IR spectra of the samples diluted in KBr. Figure 3. TGA in nitrogen flow of solids.
taking solely the values of the specific surface area (Table 2), no clear correlation can be established, as several counteracting effects, such as pore plugging, coverage of silica particles by the silylant agent, and modification of the density of catalyst particles, compete with each other. Preliminary analyses of the SiSuPF-2 sample (FT-IR, XPS) indicate that the sulfonic groups do not remain anchored onto the surface. Hence, it is considered the reason that sultone leaches during the incorporation of perfluorinated group, and therefore, a more detailed study is not carried out. The functionalized silica samples were analyzed by infrared spectroscopy. At first glance, the spectra of the three samples show the characteristic absorption bands of the silica substrate. However, a detailed study of the region 1300-1500 cm-1 just before the intense band of absorption due to Si-O-Si vibrations shows a weak band located at 1380 cm-1 (Figure 2). This peak, which is also observed in the commercial sample (SAC-13), is attributed to the stretching vibration mode of the SdO bond of sulfonic groups anchored to the silica surface.22–24 The observation of this band in samples SiSu and SiSuPF-1 can be taken as conclusive evidence of the anchorage of perfluorosulfonic groups onto the surface of the silica. The intensity of this band is slightly higher in the SiSuPF-1 sample with respect to the other samples. The thermal stability of the sulfonic acid groups was studied by thermogravimetric analyses (TGA). The thermal decomposition profiles of surface groups incorporated onto the silica surface are displayed in Figure 3. Samples SiSu and SAC-13 record an important weight loss at low temperature (
