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In situ thermogravimetry in an infrared spectrometer: an answer to quantitative spectroscopy of adsorbed species on heterogeneous catalysts
Microporous and Mesoporous Materials 67 (2004) 107–112 www.elsevier.com/locate/micromeso
In situ thermogravimetry in an infrared spectrometer: an answer to quantitative spectroscopy of adsorbed species on heterogeneous catalysts Frederic Thibault-Starzyk *, Barbara Gil 1, Sebastien Aiello, Therese Chevreau, Jean-Pierre Gilson Laboratoire Catalyse and Spectrochimie, CNRS-ENSICAEN-Universite de Caen, 6 Boulevard Marechal Juin, Caen Cedex 14050, France Received 1 September 2003; received in revised form 29 October 2003; accepted 31 October 2003
Abstract A technique for the quantitative determination of adsorbed species on zeolites and other solids was designed by coupling infrared spectroscopy and in situ thermogravimetry. The weight of H-Y zeolites in an infrared spectrometer was monitored upon pyridine adsorption by a McBain type microbalance. Improved extinction coefficients for the pyridinium band and for m(OH) bands were determined on these reference materials. The influence of the temperature used for desorption of physisorbed pyridine was studied. 2003 Elsevier Inc. All rights reserved. Keywords: IR spectroscopy; Pyridine; Acidity; Quantitative; Y zeolite
1. Introduction Infrared spectroscopy is an important tool for the study of the surface chemistry of solids, for example in heterogeneous catalysis. It can be used to characterize surface adsorption sites and catalytically active sites [1]. In the case of solid acids, probe molecules are often employed, and their interaction with the adsorption site is studied (e.g. H-bond). Quantitative data are obtained by comparing the signal intensity with one of a reference sample, in infrared spectroscopy or with any other spectroscopic technique. The reference sample is used to obtain an absorption coefficient for a given band, it is used later on to determine the amount of species of interest in the actual sample from the intensity of the corresponding band [2]. The determination of absorption coefficients is therefore a key step for quantitative and in situ/operando studies of working catalysts [3,4].
*
Corresponding author. Tel.: +33-2-31452810; fax: +33-231452822. E-mail address: [email protected] (F. Thibault-Starzyk). 1 On leave from the Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.10.016
Unfortunately, absorption coefficients are hardly available for solid samples under the conditions of a catalytic study, particularly in infrared spectroscopy. The use of absorption coefficients in the Beer–Lambert law is not rigorous in the case of powder samples. As reminded recently [5], the absorption coefficients for adsorbed species depend on the optical properties of the solid, on the experimental conditions, and on the interaction between the solid and the adsorbed molecule. Such interactions are critical for zeolite materials, in which confinement effects strongly influence adsorbed molecules [6]. Estimates are often used from reference studies [2], performed with great care ex situ on similar materials or even under the assumption that the values of absorption coefficients are intrinsic to the probe molecule and independent on the type of solid [7]. We describe here a method for the exact determination of the mass of the sample and thus of the amount of adsorbed species on a solid catalyst with simultaneous recording of the infrared spectrum, in conditions very near to that of the real reaction conditions, by in situ thermogravimetry in the infrared spectrometer. The spectrum is measured in the transmission mode, since diffuse reflection spectroscopy (DRIFTS) is not adequate for quantitative measurements. Indeed, DRIFTS
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112
10
32
8
31
6
30
4
29
mass/mg
-1
does not follow a linear quantitative law, and is very much influenced by complex parameters such as the size of the zeolite crystals and the temperature [8]. Transmission infrared is therefore always preferred for quantitative and operando studies. Using this exact determination of the amount of adsorbed species in the infrared spectrometer, we give here a much improved absorption coefficient for pyridine (py) on Y and dealuminated Y zeolites.
PyH + area/cm
108
2 28
2. Experimental part Two zeolite samples were used. The first one (socalled mother zeolite) is a non-dealuminated NH4 -Y, it is denoted as HYo (Zeocat E2268, Si/Al ¼ 2.72). The second sample, denoted as HYi, was produced by isomorphous substitution (dealumination by ammonium hexafluorosilicate) up to Si/Al ¼ 5.53. Its crystallinity is 95%. Both solids have been described previously [9]. The new measurement technique allowed the simultaneous determination of the mass of the sample and the recording of the corresponding infrared transmission spectrum. Our design is based on a MacBain’s balance, fitted to an infrared cell used for in situ studies of adsorption on heterogeneous catalysts (Fig. 1). The cell can be heated up to 873 K, and evacuated to 106 mbar. A small calibrated volume on the gas line was used to introduce known doses of probe molecules. The sample was a self supporting wafer (4.91 cm2 , 38.2 and 57.8 mg dry weight for HYo and HYi, respectively, prepared by pressing at 5 · 107 Pa or 0.5 ton during 2 min). It was attached inside the cell to a spring, which extended 400
0 27 0
500
1000
1500
2000
time/s Fig. 2. Typical coupled IR/TG experiment. Dependence of sample mass and 1545 cm1 band area (Hþ -py) on time upon adsorption of incremental Py doses. r: PyHþ band area.
mm/g. The vertical position of the sample in the infrared cell was kept constant by a motor compensating the elongation of the spring to maintain the sample at the same position in the infrared beam of the spectrometer (Nicolet Avatar 360 at 4 cm1 optical resolution and one level zero-filling). The motor was controlled by an optical switch. A linear output from the motor control unit was plotted and used for mass measurement after calibration. The mass resolution was better than 10 lg. Prior to the adsorption experiment, the samples were activated by heating (1 K/min) up to 720 K under vacuum (103 Pa) and kept at this temperature for 12 h. During the experiment, incremental small doses of the probe molecule (here pyridine) in the gas phase were introduced into the cell, and the mass gain was monitored together with the pressure changes, as shown in Fig. 2. The pressure first increased in the cell when the doses were introduced, and then decreased rapidly when the equilibrium was reached between the adsorbed and gas phases. Infrared spectra were collected after stabilisation of the mass for each pyridine dose, and the intensity of the 1545 cm1 band for Hþ -py species was measured. The low residual pyridine pressure in the cell did not alter the mass measurements, and stayed below 10 Pa. 3. Results and discussion 3.1. Influence of adsorption temperature on quantitative results
Fig. 1. Setup for coupled IR/TG measurement.
Routine studies of pyridine adsorption are usually performed by saturating the sample with pyridine, heating at a certain temperature to ease its diffusion in the sample, and evacuating the sample at the same temperature to desorb non-protonated pyridine (often denoted as physisorbed pyridine). This technique can be
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112
used for the routine comparison of samples in a series. However, the best temperature for removal of nonprotonated species is difficult to establish using the classical setups, i.e. without in situ weighing of the sample. Most studies use a desorption step at 425 K, which has already been questioned [3]. Fig. 3A shows the changes with temperature of the intensities of the protonated (1545 cm1 ) and physisorbed (1490 cm1 ) pyridine bands during a desorption experiment on HYo. Fig. 3B highlights the corresponding temperature and mass loss of the sample. During the whole experiment, the intensity of the 1545 cm1 band remains constant, indicating that no pyridinium is desorbed between 420 and 520 K. At the same time, the intensity of the 1490 cm1 band (which has contributions from most of adsorbed pyridine species) decreases slightly. On the other hand, the mass of the sample, and thus the amount of adsorbed pyridine, gently decreases up to 430 K, and then strongly decreases between 430 and 450 K. The mass remains constant between 470 and 500 K. We thus show here that an important desorption of weakly adsorbed pyridine takes place up to 470 K, which does not lead to a significant decrease of the intensities of bands associated to adsorbed pyridine, and is therefore usually not noticed when looking only at the spectrum. This weakly adsorbed pyridine can however cause an overlap with bands used for pyridine quantitative measurements. Since the intensity of the 1545 cm1 band for protonated pyridine is not perturbed by heating at 470 K, this desorption temperature is correct for the quantitative measurement of acidity by pyridine adsorption. The usual desorption step at 425 K is not sufficient for obtaining reproducible results, quite likely one of the reasons for discrepancies between molar absorption coefficients obtained by different authors [10–13]. By desorption at 420 K, we observed more than 15% variations in molar absorption coefficient for pyridinium in a series of preliminary experiments.
3.2. Pyridine adsorption experiments Figs. 4A (HYo) and 5A (HYi) show the IR spectra of the activated samples before pyridine adsorption. m(OH) vibration bands for bridging zeolitic hydroxyls are around 3600 cm1 : • 3550 cm1 : low frequency OH (OHLF ) located in the hexagonal prisms; • 3630 cm1 : high frequency OH (OHHF ) located in the supercages of the Y zeolite structure. The smaller band at 3745 cm1 corresponds to the vibration band of SiOH groups, the so-called terminal silanols. The two samples used in this study were not steamed and were heated with great care for the thermal pre-treatment: they contain no Lewis sites, as seen by the absence of any band at 1450 cm1 upon pyridine adsorption. No structure defects can be detected either. Spectra obtained after adsorption (b–e) of pyridine on the mother HYo zeolite are shown in Fig. 4. On this sample, it leads to the formation of pyridinium ions PyHþ (1545 cm1 band) and to a concomitant disappearance of the m(OHHF ) band. The band corresponding to the OHLF remains almost unperturbed. It only slightly shifts towards lower wavenumbers, and its integrated intensity increases slightly. This is probably due to a very weak long distance interaction between pyridine and OHLF . It is known that on a non-dealuminated Y zeolite, interaction between pyridine and OH groups in small cavities is hardly possible. The very weak interaction observed here on OHLF does not alter our quantitative measurements for OHHF on HYo and will be neglected. The spectra obtained after pyridine addition on the isomorphously dealuminated HYi sample are shown in Fig. 5. The results are markedly different, since both types of OH groups, at high and low frequency, are simultaneously perturbed by the probe molecule. Pyridinium ions are formed, together with a relatively
10
550
9
7 6
1545 cm-1
500 0.1 450
temperature/K
0.2
1490 cm-1
8
mass lost/mg
IR band area
109
5 0.0
4 420
(A)
440
460 480 500 temperature/K
520
0
(B)
1000
2000 3000 time/s
4000
400
Fig. 3. Determination of the optimum adsorption/desorption temperature. Desorption of pyridine under vacuum (103 mbar) at increasing temperature after saturation of the sample (HYo). A––infrared intensities for bands at 1490 cm1 (physisorbed pyridine) and 1545 cm1 (protonated pyridine); B––temperature and sample mass loss recorded during the experiment.
110
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112 3
4
a
(A) Absorbance
Absorbance
3
b c
2
(A)
b
a
d e
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c
2
d e e
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1 a
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3000
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0
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2000
3500
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3000
2500
2
3
(B)
1.0
1.5
c
1
0.5
e’
d
d’
e
c’
3800
3400 -1
ν/cm
e’
1
c
d’
0.5
d
c’ b’
0.0
0
0.0 3600
1.0
b
e
b’
0
(C)
a
Absorbance
Absorbance
b
1650
3800
1500
3600
3400
1650
ν/cm
1500 ν/cm-1
ν/cm-1
-1
Fig. 4. IR spectra of sample HYo at 470 K: (a) activated catalyst, (b–e) with pyridine adsorbed. A––spectra in the region 3800–1400 cm1 , B–– m(OH) region, C––pyridine cycle vibration region; spectra b0 –e0 are obtained by subtraction of spectrum a.
1500
(B)
(C)
a
2
2000
ν/cm-1
-1
ν/cm
Fig. 5. IR spectra of sample HYi at 470 K: (a) activated catalyst, (b–e) with pyridine adsorbed. A––spectra in the region 3800–1400 cm1 , B–– m(OH) region, C––pyridine cycle vibration region; spectra b0 –e0 are obtained by subtraction of spectrum a. 20
250
3.3. Determination of the molar absorption coefficients
200 OH band area
weak H-bond with OHLF (as shown by the broadening of the m(OHLF ) band). This kind of interaction was also observed elsewhere [2,14].
15
y = -2.9867x + 219.23 R² = 0.9761
150 10 y = 1.3641x R² = 0.996
100
5
50
Infrared intensities on absorbance spectra recorded in the transmission mode depend on the amount of vibrating (absorbing) species in the optical path of the infrared beam. On a non-diffusive and homogeneous sample, infrared absorbance can be used for quantitative analyses. Although solid powders do not satisfy all theoretical criteria for its application, satisfactory data can often be obtained by the Beer–Lambert law: A ¼ eðn=SÞ
ð1Þ
• A integral absorbance (in cm1 ), • n=S surface concentration of the observed species on the wafer (moles per cm2 wafer), • e is the apparent integral absorption coefficient (in lmol1 cm). For a discussion of the computing of e for powder solids, see [5]. The purpose of this work is to test the validity of this law, and to determine the best absorption coefficients.
PyH+ band area
0
y = -6.7632x + 69.147 R²= 0.9949
0
0 0
2
4
6
8
10
adsorbed Py [micromoles/cm²]
Fig. 6. Dependence of OH group and PyH+ band area on the concentration of adsorbed pyridine for HYo; j––pyridinium ion; r–– OHHF band; N––OHHF + OHLF bands.
Fig. 6 shows the results obtained by adsorbing increasing quantities of pyridine on HYo at 470 K. In our experiments, the pyridinium surface concentration increases linearly with the amount of pyridine introduced, and we already established that it is, in such conditions, the only pyridine species on the surface. The extinction coefficient for the 1545 cm1 band on that sample is thus 1.36 ± 0.03 cm lmol1 . As already mentioned, only the OHHF groups are affected by pyridine on that sample, which allows an unambiguous and easy determination of the absorption
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112 30
200 R² = 0.9976
OH band area
160
20 120 80
y = -5.3853x + 153.88
10
R2= 0.9814
PyH + band area
y = 1.349x
40 0
0 0
5
10
15
20
25
adsorbed Py [micromoles/cm²]
Table 1 Amounts of OH groups measured by pyridine adsorption compared to chemical analysis of the Al content HYo
HYi
OHHF Molar absorption coefficients (cm lmol1 ) Measured amounts (mol/u.c.) from m(OH) bands Chemical analysis of Al content (mol/u.c.) a b
Fig. 7. Dependence of OH group and PyH+ band area on the concentration of adsorbed pyridine for HYi; j––pyridinium ion; r–– OHHF band; ––OHLF band; N––OHHF + OHLF bands.
111
OHLF
OHHF
OHLF
6.76 15.1
5.39 42.8a 51.6
11.6b
19.9 30.5
Obtained using e(OHLF ) from HYi. Obtained using e(OHHF ) on HYo.
coefficient. The intensity of the m(OHHF ) comes to 0 when the amount of pyridine introduced is 10.22 lmol/ cm2 . Assuming that one pyridinium ion is formed for each acidic OHHF , this corresponds to 1.31 lmol/mg and 15.1 OHHF per unit cell of dry acidic zeolite. The extinction coefficient of the m(OHHF ) band is given by the slope of the line, which is 6.76 cm lmol1 . On the dealuminated HYi sample (Fig. 7), the situation is not so simple, since both types of OH groups are perturbed simultaneously. We are able to measure the extinction coefficient for pyridinium groups, and obtain a value of 1.35 cm lmol1 , very similar to HYo. Although both OH groups are affected upon introduction of the first pyridine dose, the OHHF group is more quickly perturbed than the OHLF : after 13 lmol/cm2 pyridine has been added, the OHHF has disappeared while the OHLF continues to decrease. We use the slope of the OHLF band decrease between 15 and 20 lmol pyridine/cm2 wafer to measure the absorption coefficient for m(OHLF ), the value is 5.39 cm lmol1 . It should be noticed that this perturbation of the OHLF located in the small cavities of the FAU structure is not due to the formation of pyridinium ions, since the intensity of the 1545 cm1 band does not increase correspondingly. The dealumination by isomorphous substitution preserves the structure of the framework. On such a sample, pyridine can’t easily reach the OH groups in the hexagonal prisms or sodalite units, and therefore cannot be protonated by them (although there might be a trace amount of pyridine protonated on these sites). Only the e(OHHF ) for HYo can be rigorously measured, but assuming identical absorption coefficients of m(OH) in HYo and in HYi, we have been able to estimate the amount of OH groups in both samples (Table 1). The amount of OHLF groups is determined on the mother sample HYo using the e(OHLF ) measured on HYi, where the perturbation of OHLF is easily observed. This amount is thus only an approximation, but the overall amount of OH groups are in good agreement with chemical analysis, and give a reasonable distribution between the two possible locations.
4. Conclusion A quantitative spectroscopic determination of the amount of adsorbed species, for mixtures or even for a single pure compound is always difficult, particularly on the surface of zeolites. We illustrate here a new technique for the exact determination of the mass, amount and infrared absorption coefficient for adsorbed species on solid catalysts, namely in situ thermogravimetry in an infrared spectrometer. We first apply this technique to pyridine adsorbed on H-Y zeolites with Si/Al ratio of 2.7 and 5.5. The absorption coefficient for the pyridinium band at 1545 cm1 is 1.36 lmol1 cm at 470 K. An improved removal of physisorbed pyridine is obtained at 470 K. This temperature parameter might be the key to the discrepancy often observed between experiments in various laboratories. Absorption coefficients are also determined for m(OH) vibration bands, and the distribution between the two possible locations for OH groups in the structure is determined. The study of the absorption coefficients for these bands or for other probe molecules in more complex samples (with extraframework phase or with various pore size and particle) is in progress. Such an approach is required to better quantify the complex spectra recorded under in situ or even operando [3,4] IR spectroscopy.
Acknowledgement We thank J.-C. Lavalley for having initiated this study.
References [1] M.L. McKelvy, T.R. Britt, B.L. Davis, J.K. Gillie, F.B. Graves, L.A. Lentz, Anal. Chem. 70 (1998) 119. [2] S. Khabtou, T. Chevreau, J.C. Lavalley, Microporous Mater. 3 (1994) 133. [3] M.O. Guerrero-Perez, M.A. Banares, Chem. Commun. (2002) 1292.
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[4] J. Saussey, F. Thibault-Starzyk, in: B. Weckhuysen (Ed.), In Situ Spectroscopy of Catalysts, American Scientific Publishers, Stevenson Ranch, CA, 2003. [5] C. Morterra, G. Magnacca, V. Bolis, Catal. Today 70 (2001) 43. [6] K.S. Smirnov, F. Thibault-Starzyk, J. Phys. Chem. B 103 (1999) 8595. [7] E. Selli, L. Formi, Microporous Mesoporous Mater. 31 (1999) 129. [8] R. Craciun, D.J. Miller, N. Dulamita, J.E. Jackson, Prog. Catal. 5 (2) (1996) 55.
[9] S. Jolly, J. Saussey, J.C. Lavalley, J. Mol. Catal. 86 (1994) 401. [10] J. Take, T. Yamaguchi, K. Mijamoto, H. Ohyjama, M. Misono, Stud. Surf. Sci. Catal. 28 (1986) 495. [11] J. Datka, B. Gil, A. Kubacka, Zeolites 17 (1996) 428. [12] N. Echoufi, P. Gelin, Catal. Lett. 40 (1996) 249. [13] M. Guisnet, P. Ayrault, C. Coutanceau, M.F. Alvarez, J. Datka, J. Chem. Soc. Faraday Trans. 93 (1997) 1661. [14] M.A. Makarova, K. Karim, J. Dwyer, Microporous Mater. 4 (1995) 243.
In situ thermogravimetry in an infrared spectrometer: an answer to quantitative spectroscopy of adsorbed species on heterogeneous catalysts Frederic Thibault-Starzyk *, Barbara Gil 1, Sebastien Aiello, Therese Chevreau, Jean-Pierre Gilson Laboratoire Catalyse and Spectrochimie, CNRS-ENSICAEN-Universite de Caen, 6 Boulevard Marechal Juin, Caen Cedex 14050, France Received 1 September 2003; received in revised form 29 October 2003; accepted 31 October 2003
Abstract A technique for the quantitative determination of adsorbed species on zeolites and other solids was designed by coupling infrared spectroscopy and in situ thermogravimetry. The weight of H-Y zeolites in an infrared spectrometer was monitored upon pyridine adsorption by a McBain type microbalance. Improved extinction coefficients for the pyridinium band and for m(OH) bands were determined on these reference materials. The influence of the temperature used for desorption of physisorbed pyridine was studied. 2003 Elsevier Inc. All rights reserved. Keywords: IR spectroscopy; Pyridine; Acidity; Quantitative; Y zeolite
1. Introduction Infrared spectroscopy is an important tool for the study of the surface chemistry of solids, for example in heterogeneous catalysis. It can be used to characterize surface adsorption sites and catalytically active sites [1]. In the case of solid acids, probe molecules are often employed, and their interaction with the adsorption site is studied (e.g. H-bond). Quantitative data are obtained by comparing the signal intensity with one of a reference sample, in infrared spectroscopy or with any other spectroscopic technique. The reference sample is used to obtain an absorption coefficient for a given band, it is used later on to determine the amount of species of interest in the actual sample from the intensity of the corresponding band [2]. The determination of absorption coefficients is therefore a key step for quantitative and in situ/operando studies of working catalysts [3,4].
*
Corresponding author. Tel.: +33-2-31452810; fax: +33-231452822. E-mail address: [email protected] (F. Thibault-Starzyk). 1 On leave from the Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland. 1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.10.016
Unfortunately, absorption coefficients are hardly available for solid samples under the conditions of a catalytic study, particularly in infrared spectroscopy. The use of absorption coefficients in the Beer–Lambert law is not rigorous in the case of powder samples. As reminded recently [5], the absorption coefficients for adsorbed species depend on the optical properties of the solid, on the experimental conditions, and on the interaction between the solid and the adsorbed molecule. Such interactions are critical for zeolite materials, in which confinement effects strongly influence adsorbed molecules [6]. Estimates are often used from reference studies [2], performed with great care ex situ on similar materials or even under the assumption that the values of absorption coefficients are intrinsic to the probe molecule and independent on the type of solid [7]. We describe here a method for the exact determination of the mass of the sample and thus of the amount of adsorbed species on a solid catalyst with simultaneous recording of the infrared spectrum, in conditions very near to that of the real reaction conditions, by in situ thermogravimetry in the infrared spectrometer. The spectrum is measured in the transmission mode, since diffuse reflection spectroscopy (DRIFTS) is not adequate for quantitative measurements. Indeed, DRIFTS
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112
10
32
8
31
6
30
4
29
mass/mg
-1
does not follow a linear quantitative law, and is very much influenced by complex parameters such as the size of the zeolite crystals and the temperature [8]. Transmission infrared is therefore always preferred for quantitative and operando studies. Using this exact determination of the amount of adsorbed species in the infrared spectrometer, we give here a much improved absorption coefficient for pyridine (py) on Y and dealuminated Y zeolites.
PyH + area/cm
108
2 28
2. Experimental part Two zeolite samples were used. The first one (socalled mother zeolite) is a non-dealuminated NH4 -Y, it is denoted as HYo (Zeocat E2268, Si/Al ¼ 2.72). The second sample, denoted as HYi, was produced by isomorphous substitution (dealumination by ammonium hexafluorosilicate) up to Si/Al ¼ 5.53. Its crystallinity is 95%. Both solids have been described previously [9]. The new measurement technique allowed the simultaneous determination of the mass of the sample and the recording of the corresponding infrared transmission spectrum. Our design is based on a MacBain’s balance, fitted to an infrared cell used for in situ studies of adsorption on heterogeneous catalysts (Fig. 1). The cell can be heated up to 873 K, and evacuated to 106 mbar. A small calibrated volume on the gas line was used to introduce known doses of probe molecules. The sample was a self supporting wafer (4.91 cm2 , 38.2 and 57.8 mg dry weight for HYo and HYi, respectively, prepared by pressing at 5 · 107 Pa or 0.5 ton during 2 min). It was attached inside the cell to a spring, which extended 400
0 27 0
500
1000
1500
2000
time/s Fig. 2. Typical coupled IR/TG experiment. Dependence of sample mass and 1545 cm1 band area (Hþ -py) on time upon adsorption of incremental Py doses. r: PyHþ band area.
mm/g. The vertical position of the sample in the infrared cell was kept constant by a motor compensating the elongation of the spring to maintain the sample at the same position in the infrared beam of the spectrometer (Nicolet Avatar 360 at 4 cm1 optical resolution and one level zero-filling). The motor was controlled by an optical switch. A linear output from the motor control unit was plotted and used for mass measurement after calibration. The mass resolution was better than 10 lg. Prior to the adsorption experiment, the samples were activated by heating (1 K/min) up to 720 K under vacuum (103 Pa) and kept at this temperature for 12 h. During the experiment, incremental small doses of the probe molecule (here pyridine) in the gas phase were introduced into the cell, and the mass gain was monitored together with the pressure changes, as shown in Fig. 2. The pressure first increased in the cell when the doses were introduced, and then decreased rapidly when the equilibrium was reached between the adsorbed and gas phases. Infrared spectra were collected after stabilisation of the mass for each pyridine dose, and the intensity of the 1545 cm1 band for Hþ -py species was measured. The low residual pyridine pressure in the cell did not alter the mass measurements, and stayed below 10 Pa. 3. Results and discussion 3.1. Influence of adsorption temperature on quantitative results
Fig. 1. Setup for coupled IR/TG measurement.
Routine studies of pyridine adsorption are usually performed by saturating the sample with pyridine, heating at a certain temperature to ease its diffusion in the sample, and evacuating the sample at the same temperature to desorb non-protonated pyridine (often denoted as physisorbed pyridine). This technique can be
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112
used for the routine comparison of samples in a series. However, the best temperature for removal of nonprotonated species is difficult to establish using the classical setups, i.e. without in situ weighing of the sample. Most studies use a desorption step at 425 K, which has already been questioned [3]. Fig. 3A shows the changes with temperature of the intensities of the protonated (1545 cm1 ) and physisorbed (1490 cm1 ) pyridine bands during a desorption experiment on HYo. Fig. 3B highlights the corresponding temperature and mass loss of the sample. During the whole experiment, the intensity of the 1545 cm1 band remains constant, indicating that no pyridinium is desorbed between 420 and 520 K. At the same time, the intensity of the 1490 cm1 band (which has contributions from most of adsorbed pyridine species) decreases slightly. On the other hand, the mass of the sample, and thus the amount of adsorbed pyridine, gently decreases up to 430 K, and then strongly decreases between 430 and 450 K. The mass remains constant between 470 and 500 K. We thus show here that an important desorption of weakly adsorbed pyridine takes place up to 470 K, which does not lead to a significant decrease of the intensities of bands associated to adsorbed pyridine, and is therefore usually not noticed when looking only at the spectrum. This weakly adsorbed pyridine can however cause an overlap with bands used for pyridine quantitative measurements. Since the intensity of the 1545 cm1 band for protonated pyridine is not perturbed by heating at 470 K, this desorption temperature is correct for the quantitative measurement of acidity by pyridine adsorption. The usual desorption step at 425 K is not sufficient for obtaining reproducible results, quite likely one of the reasons for discrepancies between molar absorption coefficients obtained by different authors [10–13]. By desorption at 420 K, we observed more than 15% variations in molar absorption coefficient for pyridinium in a series of preliminary experiments.
3.2. Pyridine adsorption experiments Figs. 4A (HYo) and 5A (HYi) show the IR spectra of the activated samples before pyridine adsorption. m(OH) vibration bands for bridging zeolitic hydroxyls are around 3600 cm1 : • 3550 cm1 : low frequency OH (OHLF ) located in the hexagonal prisms; • 3630 cm1 : high frequency OH (OHHF ) located in the supercages of the Y zeolite structure. The smaller band at 3745 cm1 corresponds to the vibration band of SiOH groups, the so-called terminal silanols. The two samples used in this study were not steamed and were heated with great care for the thermal pre-treatment: they contain no Lewis sites, as seen by the absence of any band at 1450 cm1 upon pyridine adsorption. No structure defects can be detected either. Spectra obtained after adsorption (b–e) of pyridine on the mother HYo zeolite are shown in Fig. 4. On this sample, it leads to the formation of pyridinium ions PyHþ (1545 cm1 band) and to a concomitant disappearance of the m(OHHF ) band. The band corresponding to the OHLF remains almost unperturbed. It only slightly shifts towards lower wavenumbers, and its integrated intensity increases slightly. This is probably due to a very weak long distance interaction between pyridine and OHLF . It is known that on a non-dealuminated Y zeolite, interaction between pyridine and OH groups in small cavities is hardly possible. The very weak interaction observed here on OHLF does not alter our quantitative measurements for OHHF on HYo and will be neglected. The spectra obtained after pyridine addition on the isomorphously dealuminated HYi sample are shown in Fig. 5. The results are markedly different, since both types of OH groups, at high and low frequency, are simultaneously perturbed by the probe molecule. Pyridinium ions are formed, together with a relatively
10
550
9
7 6
1545 cm-1
500 0.1 450
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8
mass lost/mg
IR band area
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460 480 500 temperature/K
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4000
400
Fig. 3. Determination of the optimum adsorption/desorption temperature. Desorption of pyridine under vacuum (103 mbar) at increasing temperature after saturation of the sample (HYo). A––infrared intensities for bands at 1490 cm1 (physisorbed pyridine) and 1545 cm1 (protonated pyridine); B––temperature and sample mass loss recorded during the experiment.
110
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112 3
4
a
(A) Absorbance
Absorbance
3
b c
2
(A)
b
a
d e
e
c
2
d e e
1
1 a
3500
3000
a
0
2500
2000
3500
1500
3000
2500
2
3
(B)
1.0
1.5
c
1
0.5
e’
d
d’
e
c’
3800
3400 -1
ν/cm
e’
1
c
d’
0.5
d
c’ b’
0.0
0
0.0 3600
1.0
b
e
b’
0
(C)
a
Absorbance
Absorbance
b
1650
3800
1500
3600
3400
1650
ν/cm
1500 ν/cm-1
ν/cm-1
-1
Fig. 4. IR spectra of sample HYo at 470 K: (a) activated catalyst, (b–e) with pyridine adsorbed. A––spectra in the region 3800–1400 cm1 , B–– m(OH) region, C––pyridine cycle vibration region; spectra b0 –e0 are obtained by subtraction of spectrum a.
1500
(B)
(C)
a
2
2000
ν/cm-1
-1
ν/cm
Fig. 5. IR spectra of sample HYi at 470 K: (a) activated catalyst, (b–e) with pyridine adsorbed. A––spectra in the region 3800–1400 cm1 , B–– m(OH) region, C––pyridine cycle vibration region; spectra b0 –e0 are obtained by subtraction of spectrum a. 20
250
3.3. Determination of the molar absorption coefficients
200 OH band area
weak H-bond with OHLF (as shown by the broadening of the m(OHLF ) band). This kind of interaction was also observed elsewhere [2,14].
15
y = -2.9867x + 219.23 R² = 0.9761
150 10 y = 1.3641x R² = 0.996
100
5
50
Infrared intensities on absorbance spectra recorded in the transmission mode depend on the amount of vibrating (absorbing) species in the optical path of the infrared beam. On a non-diffusive and homogeneous sample, infrared absorbance can be used for quantitative analyses. Although solid powders do not satisfy all theoretical criteria for its application, satisfactory data can often be obtained by the Beer–Lambert law: A ¼ eðn=SÞ
ð1Þ
• A integral absorbance (in cm1 ), • n=S surface concentration of the observed species on the wafer (moles per cm2 wafer), • e is the apparent integral absorption coefficient (in lmol1 cm). For a discussion of the computing of e for powder solids, see [5]. The purpose of this work is to test the validity of this law, and to determine the best absorption coefficients.
PyH+ band area
0
y = -6.7632x + 69.147 R²= 0.9949
0
0 0
2
4
6
8
10
adsorbed Py [micromoles/cm²]
Fig. 6. Dependence of OH group and PyH+ band area on the concentration of adsorbed pyridine for HYo; j––pyridinium ion; r–– OHHF band; N––OHHF + OHLF bands.
Fig. 6 shows the results obtained by adsorbing increasing quantities of pyridine on HYo at 470 K. In our experiments, the pyridinium surface concentration increases linearly with the amount of pyridine introduced, and we already established that it is, in such conditions, the only pyridine species on the surface. The extinction coefficient for the 1545 cm1 band on that sample is thus 1.36 ± 0.03 cm lmol1 . As already mentioned, only the OHHF groups are affected by pyridine on that sample, which allows an unambiguous and easy determination of the absorption
F. Thibault-Starzyk et al. / Microporous and Mesoporous Materials 67 (2004) 107–112 30
200 R² = 0.9976
OH band area
160
20 120 80
y = -5.3853x + 153.88
10
R2= 0.9814
PyH + band area
y = 1.349x
40 0
0 0
5
10
15
20
25
adsorbed Py [micromoles/cm²]
Table 1 Amounts of OH groups measured by pyridine adsorption compared to chemical analysis of the Al content HYo
HYi
OHHF Molar absorption coefficients (cm lmol1 ) Measured amounts (mol/u.c.) from m(OH) bands Chemical analysis of Al content (mol/u.c.) a b
Fig. 7. Dependence of OH group and PyH+ band area on the concentration of adsorbed pyridine for HYi; j––pyridinium ion; r–– OHHF band; ––OHLF band; N––OHHF + OHLF bands.
111
OHLF
OHHF
OHLF
6.76 15.1
5.39 42.8a 51.6
11.6b
19.9 30.5
Obtained using e(OHLF ) from HYi. Obtained using e(OHHF ) on HYo.
coefficient. The intensity of the m(OHHF ) comes to 0 when the amount of pyridine introduced is 10.22 lmol/ cm2 . Assuming that one pyridinium ion is formed for each acidic OHHF , this corresponds to 1.31 lmol/mg and 15.1 OHHF per unit cell of dry acidic zeolite. The extinction coefficient of the m(OHHF ) band is given by the slope of the line, which is 6.76 cm lmol1 . On the dealuminated HYi sample (Fig. 7), the situation is not so simple, since both types of OH groups are perturbed simultaneously. We are able to measure the extinction coefficient for pyridinium groups, and obtain a value of 1.35 cm lmol1 , very similar to HYo. Although both OH groups are affected upon introduction of the first pyridine dose, the OHHF group is more quickly perturbed than the OHLF : after 13 lmol/cm2 pyridine has been added, the OHHF has disappeared while the OHLF continues to decrease. We use the slope of the OHLF band decrease between 15 and 20 lmol pyridine/cm2 wafer to measure the absorption coefficient for m(OHLF ), the value is 5.39 cm lmol1 . It should be noticed that this perturbation of the OHLF located in the small cavities of the FAU structure is not due to the formation of pyridinium ions, since the intensity of the 1545 cm1 band does not increase correspondingly. The dealumination by isomorphous substitution preserves the structure of the framework. On such a sample, pyridine can’t easily reach the OH groups in the hexagonal prisms or sodalite units, and therefore cannot be protonated by them (although there might be a trace amount of pyridine protonated on these sites). Only the e(OHHF ) for HYo can be rigorously measured, but assuming identical absorption coefficients of m(OH) in HYo and in HYi, we have been able to estimate the amount of OH groups in both samples (Table 1). The amount of OHLF groups is determined on the mother sample HYo using the e(OHLF ) measured on HYi, where the perturbation of OHLF is easily observed. This amount is thus only an approximation, but the overall amount of OH groups are in good agreement with chemical analysis, and give a reasonable distribution between the two possible locations.
4. Conclusion A quantitative spectroscopic determination of the amount of adsorbed species, for mixtures or even for a single pure compound is always difficult, particularly on the surface of zeolites. We illustrate here a new technique for the exact determination of the mass, amount and infrared absorption coefficient for adsorbed species on solid catalysts, namely in situ thermogravimetry in an infrared spectrometer. We first apply this technique to pyridine adsorbed on H-Y zeolites with Si/Al ratio of 2.7 and 5.5. The absorption coefficient for the pyridinium band at 1545 cm1 is 1.36 lmol1 cm at 470 K. An improved removal of physisorbed pyridine is obtained at 470 K. This temperature parameter might be the key to the discrepancy often observed between experiments in various laboratories. Absorption coefficients are also determined for m(OH) vibration bands, and the distribution between the two possible locations for OH groups in the structure is determined. The study of the absorption coefficients for these bands or for other probe molecules in more complex samples (with extraframework phase or with various pore size and particle) is in progress. Such an approach is required to better quantify the complex spectra recorded under in situ or even operando [3,4] IR spectroscopy.
Acknowledgement We thank J.-C. Lavalley for having initiated this study.
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