Article pubs.acs.org/JPCC
IR Characterization of Homogeneously Mixed Silica−Alumina Samples and Dealuminated Y Zeolites by Using Pyridine, CO, and Propene Probe Molecules Yuichi Matsunaga, Hiroshi Yamazaki, Toshiyuki Yokoi, Takashi Tatsumi, and Junko N. Kondo* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: Homogeneously mixed silica−alumina samples with various Si/Al ratios (Si/Al = 9/1, 7/3, 5/5, 3/7, and 1/9) were prepared and characterized by infrared spectroscopy. Pyridine, propene, and CO were used as probe molecules to detect Brønsted and Lewis acid sites. The homogeneity of Si and Al in mixed oxides was confirmed by comparison with a physical mixture of pure silica and alumina. The continuous change of spectral appeared by increasing or decreasing the Si/Al ratio in silica−alumina samples. The frequency of the broad band due to surface OH groups hydrogen-bonded to CO was found to be a measure of the Si/Al ratio in silica−alumina samples roughly. This information was used to characterize steam-treated HY and NH4Y zeolites. Dealumination from the framework structure and generation of extra-framework silica−alumina were apparent for both steam-treated zeolites, which were not identified by X-ray diffraction patterns and microscope images. The Si/Al ratios of the formed silica−alumina on steam-treated YH and NH4Y zeolites were roughly estimated to be 5/5 and 7/3, respectively, by the peak position of hydrogen-bonded OH bands when CO was adsorbed. The acid strength of both steam-treated samples was weakened, and it was more apparent for HY than NH4Y zeolite.
1. INTRODUCTION As an essential modification step of as-synthesized zeolites, dealumination, the removal of Al atoms from the framework of zeolites with maintaining the crystal structures, is one of the most commonly used procedures. Dealumination is known to improve the hydrophobicity, thermal and hydrothermal stabilities, and the catalytic performances.1 The characterization of dealuminated zeolites has been carried out by X-ray diffraction (XRD), N2 adsorption−desorption isotherm measurements, thermogravimetric (TGA) analysis, various solidstate NMR spectroscopic measurements, and infrared (IR) spectroscopy as well as catalytic tests.2−6 The research on dealumination of zeolites has been performed on ZSM-5 in detail even at a single-crystal level.7 However, less is discussed about the dealuminated Al species. Extra-framework Al(OH)3 and Al(OH)2+ species in supercage and Al(OH)2+ species in sodalite cage of HY zeolites have been proposed to enhance the Brønsted acidity as Lewis acid sites,2 and the formation of silica−alumina upon realumination of dealuminated zeolite beta is suggested.8 We found that because of the formation of silica− alumina phase on the thermally treated HY zeolite, IR characterization with pyridine became very complex because pyridine coordinated on extra-framework Al sites and hydrogen-bonded to weakly acidic OH group of silica−alumina, giving rise to the same absorption band as that of pyridine coordinated to strong Lewis sites on the framework of HY zeolite.9 Thus, CO probe was proposed to carefully analyze dealuminated zeolite samples by IR method.9 The silica− alumina species formed by dealumination are considered to be © 2013 American Chemical Society
localized as small clusters. So, the Si/Al ratio in silica−alumina clusters may be varied in a large range depending on the numbers of Al and Si removed from the framework. Therefore, to roughly estimate the Si/Al ratio in silica−alumina on the extra-framework, reference data are required. Amorphous silica−alumina samples are widely known to be of importance as both acid catalysts and catalyst supports.10−12 The acidic nature, atomic structures of acid sites, and acid strengths of silica−aluminas have been, therefore, still often investigated.13−16 The complexity of the material arises from the variation of the samples depending on the preparation method16−18 in addition to the difficulty in analysis due to the amorphous structure. The atomic structure and the acid strength of OH groups on a silica−alumina sample (Si/Al = 0.19) have been recently reported by IR observation of lutidine (2,6-dimethylpyridine) adsorption in combination with density functional theory (DFT) calculations, and the origin of the proton transfer from surface OH groups to lutidine molecules is discussed.13 More recently, various aluminosilicates including silica-alumna and zeolite samples with various Si/Al ratio were prepared, and the quantitative analysis on the amount and the strength of surface OH groups was carefully carried out by some catalytic reactions, temperature-programmed desorption (TPD) of isopropylamine, and IR studies using CO and pyridine adsorption.14 While the mixing of Si and Al in silica− Received: April 2, 2013 Revised: June 15, 2013 Published: June 18, 2013 14043
dx.doi.org/10.1021/jp403242n | J. Phys. Chem. C 2013, 117, 14043−14050
The Journal of Physical Chemistry C
Article
alumina samples was reported to be inhomogeneous,16−18 the coprecipitation method from absolute ethanol solution of aluminum nitrate nonahydrate (ANN; Al(NO3)3·9H2O) and tetraethylalthosilicate (TEOS; Si(OC2H5)4) was found to produce porous and homogeneously mixed silica−alumina xerogels at various Si/Al ratios.19 Silica−alumina samples with various Si/Al ratio (Si/Al = 1/9, 3/7, 5/5, 7/3 and 9/1) were prepared according to the ref 19 and characterized by IR spectroscopy using typical probe molecules (pyridine, CO, and propene), together with γ-alumina and silica. In addition, dealuminated HY and NH4Y zeolites by steam treatment were observed by IR to estimate the average Si/Al ratios in extraframework silica−alumina clusters.
2. EXPERIMENTAL SECTION Silica (Aerosil 200, Evonik) and γ-alumina (Evonik, denoted as alumina below for simplicity) were provided by Aerosil Japan. Silica−alumina samples at different Si/Al ratios (Si/Al = 9/1. 7/3, 5/5, 3/7, and 1/9) were prepared according to a reference using aluminum nitrate nonahydrate (ANN; Al(NO3)3·9H2O) and tetraethylalthosilicate (TEOS; Si(OC2H5)4). A certain amount of ANN was first dissolved in absolute ethanol (200 mL), followed by the addition of TEOS. The concentration of total ANN and TEOS was adjusted to 1.2 mol·L−1. Then, silica−alumina was obtained by coprecipitation with ammonium aqueous solution (20%, 100 mL) after vigorous stirring at room temperature for 3 h. The precipitates were dried at 333 K for 2 h and 383 K for 15 h. Samples were finally clacined at 823 K for 4 h. HY zeolite, JRC-Z-HY5.6 (Si/Al = 2.8), was supplied by the Catalysis Society of Japan. NH4Y zeolite was obtained by dispersing and stirring 5 g of HY zeolite in 300 mL of aqueous ammonium nitrate solution (1 M) at 353 K for 1 h. This procedure was repeated three times, where the original zeolite structure was found by XRD to be maintained. The steam treatment of both HY and NH4Y zeolites was performed by exposing samples to the steam with 20% moisture at 773 K for 2 h. Samples were characterized by XRD (Rigaku Rint Ultima III with a Cu Kα X-ray source), field-emission scanning electron microscopy (FE-SEM, Hitachi S-5200, 1 kV), and IR spectroscopy (Jasco FT-IR 4100). Elemental analyses of the samples (Si/Al ratio) were performed on an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu ICPE-9000). About 30−40 mg of each sample was pressed into a self-supporting disk 20 mm diameter and was placed in a quartz cell connected to a conventional closed gascirculation system. The sample disk was pretreated by evacuation at 773 K for 1 h. The pretreated sample was exposed to a small amount of CO at 153 K, and the CO pressure was gradually increased from 2.5 to 1000 Pa. Propene adsorption was conducted at 173 K under 30 Pa of propene. For pyridine adsorption, sufficient amount of pyridine was introduced at 443 K, followed by evacuation at 623 K to remove weakly adsorbed species, then spectra were measured at 443 K. The IR spectrometer was equipped with an MCT detector at a 4 cm−1 resolution, and 64 scans were averaged to obtain each spectrum.
Figure 1. Background IR spectra of silica−alumina with Si/Al ratios of (a) 10:0, (b) 9:1, (c) 7:3, (d) 5:5, (e) 3:7, (f) 1:9, and (g) 0:10. (h) Physically mixed sample of silica and alumina (5:5). Samples a and b are pure silica and γ-alumina, respectively. The indicated numbers are peak-top frequencies of OH stretching absorption bands.
and alumina (Si/Al = 5/5) after pretreatment. Strong absorption bands were observed for all samples between 4000 and 3000 cm−1 due to OH groups. Small bands appearing between 3000 and 2800 cm−1 are attributed to CH stretching bands of residual hydrocarbon species. A sharp band due to surface isolated silanol groups at 3745 cm−1 is apparent for silica (spectrum a) with a tailing to low-frequency side, which is assigned to hydrogen-bonded silanol groups. The spectral feature was almost the same for silica−alumina (Si/Al = 9/1), and it was not drastically changed for silica−alumina samples of Al-substitution up to a half of Si. The presence of a considerable amount of hydrogen-bonded OH groups on the silica−alumina with Si/Al = 7/3 (spectrum c) prepared in this study is consistent with that of the silica−alumina, which was provided by Catalysis Society of Japan with similar Si/Al ratio (Si/Al = 1.9),9 the reason for which is not clear. For Al-rich samples (Si/Al = 3/7 and 1/9, spectra e and f), the presence of some alumina domains is found by comparison of spectra with that of alumina (spectrum g): a peak at the high-frequency side of the main band appeared, and the band at 3674 cm−1 became gradually evident. IR spectrum of physically mixed silica and alumina is interpreted as a sum of spectra of silica (a) and alumina (g), where a sharp peak of silanol groups is superimposed by the absorption bands due to OH groups on alumina. The absence of bands of OH groups on alumina for Si-rich silica−alumina samples (spectra b−d) confirms the homogeneous mixing of Si and Al in silica−alumina, assuming that the silica monolayer formation on small alumina bulk domains20 did not occur in the present study. This is also supported by the difference in spectra d and h, where samples consisted of the same ratio of Si and Al.
3. RESULTS AND DISCUSSION 3.1. Characterization of Silica−Alumina with Various Si/Al Ratios. Figure 1 compares background spectra of silica− alumina samples, silica, alumina, and a physical mixture of silica 14044
dx.doi.org/10.1021/jp403242n | J. Phys. Chem. C 2013, 117, 14043−14050
The Journal of Physical Chemistry C
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Figure 2. Subtracted IR spectra of background spectra from spectra measured after pyridine adsorption at 443 K, followed by evacuation at 623 K for 1 h of silica−alumina with Si:Al ratios of (a) 10:0, (b) 9:1, (c) 7:3, (d) 5:5, (e) 3:7, (f) 1:9, and (g) 0:10. (h) Physically mixed sample of silica and alumina (5:5). Samples a and g are pure silica and γ-alumina, respectively. Spectra were measured at 443 K.
Figure 3. Subtracted IR spectra of background spectra from spectra measured after CO adsorption at 153 K and at (A) 500 Pa to 1 kPa and (B) 2.5 to 10 Pa on silica−alumina with Si:Al ratios of (a) 10:0, (b) 9:1, (c) 7:3, (d) 5:5, (e) 3:7, (f) 1:9, and (g) 0:10. (h) Physically mixed sample of silica and alumina (5:5). Samples a and g are pure silica and γ-alumina, respectively. CO pressure was gradually increased from bottom spectrum to top in each series of spectra.
1545 and 1455 cm−1 attributed to pyridinium ions and pyridine molecules coordinated to Lewis acid sites, respectively, were observed in spectrum b. The formation of pyridinium ions was accompanied by the consumption of OH groups, which appeared as a negative peak corresponding to the sharp band at ∼3745 cm−1. The formation of pyridinium ions was evident in spectra b−d in Figure 2, indicating the Brønsted acidity of surface OH groups for samples with Si/Al = 1/9, 3/7, and 5/5. Because surface OH groups of alumina do not possess Brønsted acidity, pyridine adsorbed on Al atoms, Lewis acid sites, was found (spectrum g). The amount of acidic OH groups
Next, acidic sites of samples were characterized by adsorption of pyridine, CO, and propene. The acidity of surface OH groups is well-clarified by pyridine adsorption: a characteristic band due to pyridinium ions is observed at ∼1545 cm−1, which is formed by proton transfer from OH groups to pyridine molecules.21 Background-subtracted IR spectra measured after pyridine adsorption on various samples are shown in Figure 2. Any types of adsorption were not observed on silica (spectrum a), confirming the nonacidic nature of OH groups and the absence of any Lewis acid sites. The substitution of only 10% of Si to Al generated both Brønsted and Lewis acidities: bands at 14045
dx.doi.org/10.1021/jp403242n | J. Phys. Chem. C 2013, 117, 14043−14050
The Journal of Physical Chemistry C
Article
2230 cm−1 band when CO was adsorbed.17 CO interacting with weakly acidic OH groups on alumina was observed at 2159 cm−1. The consecutive change of silica−alumina samples upon increasing the Al content verifies the homogeneous mixing of silica and alumina in prepared samples. The inconsistency of prepared and physically mixed samples (Si/Al = 5/5) further supports the homogeneity of silica−alumina samples. In addition to pyridine and CO, light olefins can be also used as a probe molecule. Propene adsorption at low temperature (153 K) was used in this study to avoid any reaction and because of the originally IR-active CC stretching band. (The CC stretching band of free ethene is IR-inactive and the absorption coefficient is even small for adsorbed species.) The CC stretching band of propene in gas phase24 and Ar matrix25 (free molecule) appears at 1653 and 1650 cm−1, respectively, and shifts to lower frequency for adsorbed molecules depending on the strength of the interaction of π electrons and adsorption sites.26 In the case of adsorption of propene on Brønsted acid sites of H-ZSM-5 with methyl groups, at extremely low temperatures (
