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Quantitative IR characterization of the acidity of various oxide catalysts
Microporous and Mesoporous Materials 82 (2005) 99–104 www.elsevier.com/locate/micromeso
Quantitative IR characterization of the acidity of various oxide catalysts Thomas Onfroy, Guillaume Clet, Marwan Houalla
*
Laboratoire Catalyse et Spectrochimie (UMR CNRS 6506), ENSICAEN-Universite´ de Caen Basse-Normandie, 6 Bd. du Mare´chal Juin, F-14050 Caen cedex, France Received 23 November 2004; received in revised form 21 February 2005; accepted 28 February 2005 Available online 22 April 2005
Abstract The integrated molar absorption coefficients of the infrared bands characteristic of adsorbed lutidine (2,6-dimethylpyridine) were determined for the purpose of quantifying the acid sites of solid catalysts. The integrated molar absorption coefficients were measured for lutidine adsorbed through H-bonding, coordination to Lewis sites and protonation on Brønsted acid sites. The solids were chosen to present all possible bondings with lutidine and to cover a wide range of common types of catalysts or supports: silica, phosphated silica, HY zeolite, alumina, zirconia, WOx supported on zirconia and NbOx supported on zirconia. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Acidity; Oxide catalyst; Infrared spectroscopy; Adsorption of 2,6-dimethylpyridine; Molar absorption coefficient determination
1. Introduction A large number of industrially important reactions require acid catalysts [1–4]. Thus, the characterization of acid sites (type, number and strength) has been widely studied for various catalysts [5–8]. Infrared spectroscopy has often been used to estimate the number of Brønsted and Lewis acid sites in catalytic systems, primarily by monitoring the adsorption of probe molecules such as pyridine and ammonia. Several studies have shown that 2,6-dimethylpyridine (lutidine), because of its basic character and the steric effect of the methyl groups, is an appropriate probe molecule for Brønsted acid sites of solid catalysts [8–16]. This probe molecule was used essentially for qualitative analysis of Brønsted acid sites. Quantitative measurements were hindered because of the lack of availability in the literature of integrated molar absorption coefficients values for the H-bonded, protonated and coordinated lutidine. *
Corresponding author. Fax: +33 231 452 822. E-mail address: [email protected] (M. Houalla).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.02.020
The purpose of the present study is to determine the value of these integrated molar absorption coefficients for a variety of solids commonly used as supports or catalysts (silica, phosphated silica and HY zeolite, alumina and zirconia or zirconia-supported metal oxides). The solids were selected to examine various types of interaction with the probe molecule: H-bonding (silica [12,13,16,17]); H-bonding + coordination (alumina, zirconia [13,16,18–20]); protonation (phosphated silica, dealuminated HY zeolite [17,21]); protonation and coordination (WOx supported on zirconia and NbOx supported on zirconia [6,7,10,22]).
2. Experimental The supports and catalysts used in the present study were SiO2 (Degussa Aerosil; 200 m2/g), c-alumina (Ketjen; 185 m2/g), zirconia (precipitated from zirconium isopropoxide [10]; 50 m2/g), HY zeolite (IFP; isomorphously substituted by treatment with (NH4)2SiF6, Si/ Al ratio: 5.4 [21,23]), PO4/SiO2 containing 1.6 wt% of
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PO4 (260 m2/g, obtained by impregnation of SiO2), WOx/ZrO2 containing 11.6 wt% of W (130 m2/g, obtained by equilibrium adsorption with a solution of ammonium metatungstate) and NbOx/ZrO2 containing 2.7 wt% of Nb (36 m2/g, obtained by impregnation of ZrO2 with Niobium oxalate). Infrared spectra were recorded with a Nicolet Magna 550 FTIR spectrometer (resolution: 4 cm1, 128 scans, apodization: Happ-Genzel). Samples were pressed into discs (ca. 20 mg; 2 cm2), activated successively in vacuum and in O2 at 723 K, and cooled down to 423 K. For all solids, the experiments were performed by adding known amounts of lutidine. For this purpose, increasing pressures were introduced in a small volume (1.59 cm3) and expanded in the cell at 423 K. The probe molecule was allowed to diffuse in the pores for 1 min at this temperature prior to data collection. Following each addition, the spectrum was acquired for the catalysts at room temperature. Lutidine was added until it was detected in the gas phase. The integrated molar absorption coefficient of the HY zeolite was also determined with another setup which consists of an IR cell coupled with a thermogravimetric system (for more details on the setup see Ref. [23]) in which lutidine was adsorbed at 423 K up to saturation and evacuated at various temperatures. With pyridine or lutidine as probe molecules, the bands due to the stretching of the aromatic ring (m8a, m8b, m19a and m19b) are indicative of the sites on which the probe molecule is adsorbed. However, for lutidine, the present study will focus on the m8a and m8b bands because of their higher sensitivity in terms of position or intensity [16]. Difference spectra were obtained by subtracting the spectra of activated catalysts from the spectra obtained after lutidine adsorption (or desorption). They were used to measure the absorbance integrated intensity (noted A) for the considered band. The integrated molar absorption coefficients were determined using an equation similar to the Beer–Lambert law for solutions Z ~m2 n en A¼ ð1Þ Að~mÞ d~m ¼ e ‘ ¼ S‘ S ~m1 )
A n ¼ e S
ðwith e in cm lmol1 Þ
ð2Þ
where Að~mÞ; ~m, e, S, n and ‘ denote respectively the absorbance, the wavenumber, the integrated molar absorption coefficient, the surface of the disc, the amount of lutidine adsorbed and the thickness of the disc assumed as a first approximation to be equivalent to the optical path. The e were calculated directly for each solid (Eq. (2)) using the slope of the linear part of the curves A = f(n). The added lutidine was assumed to be fully adsorbed on the solid until bands corresponding to gas phase species
were detected. This assumption is based on earlier studies dealing with the determination of the integrated molar absorption coefficients of pyridine [24,25]. Using a similar IR cell, Khabtou et al. [25] showed that adsorption on the cell walls was negligible. Their measurements were confirmed by TGA. When necessary, the contribution of H-bond or Lewis sites was subtracted using the eH-bond or the eLewis determined with other solids in order to calculate the amount of lutidine adsorbed through one specific type of bonding. In some instances curve-fitting was also used. Best fits were generally obtained with Gaussian peaks. The use of a Lorentzian profile did not significantly affect the quantitative results. For systems containing several types of bondings between lutidine and the surface (i.e. lutidine adsorbed through H-bonding, on Lewis or Brønsted acid sites), the following equation can be used: S AHbond ALewis ABrønsted þ þ ¼1 ð3Þ n eHbond eLewis eBrønsted Uncertainties in the values of integrated molar absorption coefficients, due specifically to the repeatability of the experiments were estimated to be ca. 0.2 cm lmol1. Errors due specifically to the curve-fitting and measurements were estimated to be ca. ±0.3 cm lmol1. Peak widths and positions were allowed to vary within 5% of the adopted parameters. Areas measurements were repeated for slightly different windows and the mean values were selected. An alternative method for measuring the integrated molar absorption coefficients is to apply Eq. (3) to all the solids examined in the present study. The resulting set of equations is solved by a least-squares procedure. In this case, it is assumed that the eH-bond, eLewis and eBrønsted values did not change with the catalyst.
3. Results and discussions 3.1. Siliceous materials 3.1.1. Silica Fig. 1(a) shows the infrared spectra between 1630 and 1550 cm1 for increasing doses of lutidine (from 0.065 to 1.305 lmol) adsorbed on silica. The spectra exhibit two bands at 1605 cm1 (m8a) and 1585 cm1 (m8b) which were attributed to lutidine adsorbed on the surface by H-bonding [12,13,16]. Analysis of the spectra also suggests the presence of a small shoulder at 1595 cm1 which can be ascribed to physisorbed lutidine. However curve-fitting results indicated that the intensity of the peak at 1595 cm1 remained low with respect to the 1605 and 1585 cm1 bands. Thus, for the sake of simplicity, the spectra were curve-fitted with 2 peaks only. The evolution of the intensity of the integrated bands
T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104
101
1.4
(a)
(b)
0.02
Integrated Intensity (cm-1)
Absorbance (a.u.)
1.2
SiO2
1 0.8 0.6 0.4 0.2
1630
1610
1590
1570
0
1550
0.0
0.5 1.0 µmol of lutidine introduced
Wavenumber (cm-1)
1.5
Fig. 1. Silica: (a) Infrared spectra for increasing amounts of lutidine (from 0.065 to 1.305 lmol); (b) evolution of the bands area at 1605 cm1 (j) and 1585 cm1 (h) with the amount of added lutidine.
silica (upper part) or 1652 and 1627 cm1 for HY zeolite (lower part), characteristic of protonated species [14]. Fig. 2(b) shows the variation of the total integrated area of these two bands as a function of the amount of added lutidine. A linear variation of the intensity is observed with the amount of added lutidine for phosphated silica (up to approximately 2.0 lmol, where physisorbed species were detected) and for the zeolite sample. The integrated molar absorption coefficients of the above mentioned bands, estimated from the slopes of the linear part of the curves, were e(1655+1630) = 7.9 ± 0.5 cm lmol1 for phosphated silica and e(1652+1627) = 6.5 ± 0.5 cm lmol1 for HY zeolite. For the latter sample, the integrated molar absorption coefficient of the
as a function of the amount of added lutidine is depicted in Fig. 1(b). A linear increase (with intercept at the origin) of the bands area with the amount of added lutidine is observed. The slope of these two straight lines was used to determine the integrated molar absorption coefficient of the corresponding band. Similar values were obtained eð1605Þ ¼ eð1585Þ ¼ 1.9 0.5 cm lmol1 . 3.1.2. Phosphated silica and HY zeolite Fig. 2(a) shows the infrared spectra for increasing doses of lutidine adsorbed on phosphated silica and HY zeolite. The spectra exhibit, respectively, the m8a and m8b bands at 1655 and 1630 cm1 for phosphated
60
(a)
(b)
PO4/SiO2
0.2
50 Integrated Intensity (cm-1)
Absorbance (a.u.)
0.05
HY
40
30
20
10
0 1700
1660 1620 Wavenumber (cm-1)
1580
0
5 10 15 µmol of protonated lutidine
20
Fig. 2. Phosphated silica and HY zeolite: (a) Infrared spectra for increasing amounts of lutidine; Upper part: phosphated silica from 0.068 to 2.845 lmol lutidine; lower part: HY zeolite from 0.087 to 8.874 lmol lutidine; (b) evolution of the integrated bands of protonated species as a function of the amount of protonated lutidine on phosphated silica (m), HY zeolite () and HY after thermodesorption (h).
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T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104
two bands characteristic of protonated species was also measured using a thermogravimetric system coupled with an IR cell. With this setup, the amount of adsorbed lutidine (directly measured) and the corresponding infrared spectrum of the sample were obtained simultaneously [23]. The results were also reported in Fig. 2(b) (open squares). Clearly the experimental points fit nicely on the same line defined by the volumetric measurements indicating the same integrated molar absorption coefficient value. This validates the quantitative approach used in the present study.
3.3. Zirconia and zirconia-supported metal oxides 3.3.1. Zirconia The IR spectra for lutidine adsorbed on zirconia are reported in the lower part of Fig. 3(a). As in the case of alumina, two bands at 1609 and 1580 cm1 due to lutidine coordinatively bound to Lewis acid sites [13,20] were observed. Note the asymmetric nature of the band at 1609 cm1. This can be attributed to the presence of a band at ca. 1600 cm1 characteristic of lutidine adsorbed by H-bonding [13]. A second band corresponding to the m8b vibration expected at ca. 1580 cm1 is probably masked by the band attributed to Lewis acid sites at the same position. The spectra were curve-fitted with three bands located at 1609, 1599 and 1580 cm1 as depicted in the lower inset of Fig. 3(a). Using the approach described above for alumina, the area of the band at 1609 cm1 was plotted as a function of the amount of coordinated lutidine (Fig. 3(b)). The results indicated an integrated molar absorption coefficient value of e(1609) = 3.4 ± 0.6 cm lmol1.
3.2. Alumina The upper part of Fig. 3(a) shows the infrared spectra for increasing doses of lutidine (from 0.087 to 2.088 lmol). For the initial doses of lutidine adsorbed on alumina, the spectra exhibit only two bands at ca. 1617 and 1580 cm1 which were attributed to lutidine adsorbed on Lewis acid sites [13,16,19]. With further introduction of lutidine, two shoulders at ca. 1605 and 1570 cm1 attributed to lutidine adsorbed by H-bonding [13,16,19] were also observed. The spectra were curve-fitted with four bands at 1617, 1605, 1582 and 1572 cm1 as shown in the inset of Fig. 3(a). For each dose of lutidine, the area of the band at 1605 cm1 was measured and the corresponding amount of lutidine involved in H-bonding was determined using the e(1605) value measured in the case of silica. This amount was then subtracted from the amount of lutidine introduced to obtain that of coordinatively adsorbed lutidine. The evolution of the area of the band at 1617 cm1 as a function of the amount of coordinated lutidine is shown in Fig. 3(b). The slope of the linear part of the curve yields an integrated molar absorption coefficient of: e(1617) = 5.3 ± 0.6 cm lmol1.
3.3.2. Tungsten oxide and niobium oxide supported on zirconia Fig. 4(a) shows the infrared spectra between 1670 and 1600 cm1 for increasing doses of lutidine on WOx/ZrO2 (upper part) and NbOx/ZrO2 (lower part), respectively from 0.087 to 3.306 lmol and from 0.087 to 1.044 lmol. In addition to the band at 1610 cm1 observed in the case of pure zirconia and attributed to coordinated lutidine, the spectra of lutidine adsorbed on WOx/ZrO2 and NbOx/ZrO2 show two bands at 1643–1644 and 1628 cm1 characteristic of protonated lutidine [10,22]. The spectra were curve-fitted using three bands at 1643–1644, 1628 and 1610 cm1 as depicted in the inset
3.5
(a)
(b)
0.04
Absorbance (a.u.)
Al2O3
0.02
0.02
ZrO2
Integrated Intensity (cm-1)
3.0 0.04
2.5 2.0 1.5 1.0 0.5
1640
1620
1600
1580
Wavenumber
(cm-1)
1560
1540
0.0 0.0
0.5
1.0
1.5
µmol ol of coordinated lutidine
Fig. 3. Alumina and zirconia: (a) Infrared spectra for increasing amounts of lutidine introduced: alumina from 0.087 to 2.088 lmol lutidine; zirconia from 0.087 to 1.218 lmol—Insets: examples of curve-fitting of the spectra; (b) Evolution of the band area at 1617 cm1 for alumina (h) or 1609 cm1 for zirconia (j) as a function of the amount of coordinated lutidine.
T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104 3.0
0.04
(a)
103 10
(b)
Absorbance (a.u.)
WOx/ZrO2
0.02
NbOx/ZrO2
Integrated Intensity (cm-1)
2.5 0.08
8
2.0 6 1.5 4 1.0 2
0.5
0.0 1670 1660 1650 1640 1630 1620 1610 1600
0.0
0.5
Wavenumber (cm-1)
1.0
1.5
2.0
2.5
0 3.5
3.0
ol of protonated lutidine µmol
Fig. 4. Tungsten oxide supported on zirconia and niobia supported on zirconia: (a) Infrared spectra for increasing amounts of lutidine: upper part: WOx/ZrO2 from 0.087 to 3.306 lmol lutidine; lower part: NbOx/ZrO2 from 0.087 to 1.044 lmol—Inset: example of curve-fitting of the spectra for WOx/ZrO2; (b) evolution of the integrated bands of protonated species as a function of the amount of protonated lutidine for WOx/ZrO2 (j; right axis) and NbOx/ZrO2 (h; left axis).
of Fig. 4(a). As described before, from curve-fitting results and the integrated molar absorption coefficient of coordinated lutidine on zirconia previously determined, the amount of protonated lutidine was calculated for each dose of lutidine. The results reported in Fig. 4(b) indicate a linear increase of the integrated area of the bands at 1643 and 1628 cm1 as a function of the amount of protonated species. The integrated molar absorption coefficient values for WOx/ZrO2 and NbOx/ ZrO2 systems were respectively e(1643+1628) = 5.9 ± 0.6 cm lmol1 and e(1644+1628) = 7.3 ± 0.6 cm lmol1. 3.4. Evaluation of the integrated molar absorption coefficient values The integrated molar absorption coefficients determined in the present study for various solid are reported
in Table 1. For the integrated molar absorption coefficients of Lewis acid sites or Brønsted acid sites, the results were approximately within ±18% of the average value. They seem to be little affected by the type of solids (structure or strength of acid site). Similar findings were reported for pyridine adsorption on Brønsted or Lewis acid sites for various zeolites or silica–alumina [24,25]. Emeis [24] did not find any evidence for a dependence of the integrated molar absorption coefficient on the structure of the catalyst or the strength of the acid site. Khabtou et al. [25], using zeolites Y with different Si/Al ratios, also showed that the integrated molar absorption coefficient was independent of the strength of the acid site. As described in the experimental section, an alternative method for calculating e for a given type of site is to assume that its value is independent of the nature
Table 1 Integrated molar absorption coefficients for lutidine adsorbed on various catalysts Integrated molar absorption coefficients e (cm lmol1)
Sites distribution (%) H-bond SiO2 PO4/SiO2 HY zeolite a HY zeolite b Al2O3 ZrO2 WOx/ZrO2 NbOx/ZrO2 Average
Coordination
Protonation
100
0
0
0 0 0 15 15 0 0
0 0 0 85 85 6 8
100 100 100 0 0 94 92
Least-square Nd: not determined. a Determined from adsorption measurements. b Determined from thermogravimetric measurements.
H-bond
Coordination
Protonation
e(1585) = 1.9 e(1605) = 1.9 – – – Nd Nd – – e(1585) = 1.9 e(1605) = 1.9 e(1605) = 1.9
–
–
– – – e(1617) = 5.3 e(1609) = 3.4 Nd Nd e(Lewis) = 4.35
e(1655+1630) = 7.9 e(1652+1627) = 6.5 e(1652+1627) = 6.5 – – e(1643+1628) = 5.9 e(1644+1628) = 7.3 e(Brønsted) = 6.8
e(Lewis) = 4.4
e(Brønsted) = 6.8
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of the solid. Thus, e values for H-bonding (eH-bond), Lewis acid sites (eLewis) and Brønsted acid sites (eBrønsted) can be determined from Eq. (3) by the least-squares method. The values obtained (eH-bond = 1.9 cm lmol1; eLewis = 4.4 cm lmol1; eBrønsted = 6.8 cm lmol1) were close to the average values determined by the previous method. The 95% confidence limits for the least-square determination were estimated at ±18%. Similar deviation was reported for the integrated molar absorption coefficients of pyridine adsorption [24]. It is worth noting that the integrated molar absorption coefficient for lutidine bound to Brønsted acid sites (bands m8a and m8b: e(1642+1625) = 6.8 cm lmol1) is significantly larger than that of the band used to titrate Brønsted acid sites with pyridine (band m19b at 1540 cm1: e(1540) = 1.8– 2.2 cm lmol1 [24,25]). This indicates an additional advantage for the use of lutidine as compared to pyridine for the titration of Brønsted acid sites.
[2] [3] [4] [5] [6] [7] [8]
[9] [10]
[11] [12]
4. Conclusions
[13]
The integrated molar absorption coefficients for bands of adsorbed lutidine were determined, for various modes of bondings, from monitoring the infrared spectra of lutidine adsorbed on different supports and supported catalysts, as a function of the amount of the probe molecule added. The estimated e values for bands characteristic of lutidine adsorbed through Hbonding, coordination and protonation were respectively eH-bond = 1.9 cm lmol1, eLewis = 4.4 cm lmol1 and eBrønsted = 6.8 cm lmol1. The results indicated little variations as a function of the type of solids. The approach adopted for the determination of e was validated by coupled microbalance—IR measurements.
[14] [15] [16] [17] [18] [19]
[20] [21] [22]
References [1] M. Guisnet, in: B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit, J.C. Ve´drine (Eds.), Catalysis by Acids and Bases, Studies
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Quantitative IR characterization of the acidity of various oxide catalysts Thomas Onfroy, Guillaume Clet, Marwan Houalla
*
Laboratoire Catalyse et Spectrochimie (UMR CNRS 6506), ENSICAEN-Universite´ de Caen Basse-Normandie, 6 Bd. du Mare´chal Juin, F-14050 Caen cedex, France Received 23 November 2004; received in revised form 21 February 2005; accepted 28 February 2005 Available online 22 April 2005
Abstract The integrated molar absorption coefficients of the infrared bands characteristic of adsorbed lutidine (2,6-dimethylpyridine) were determined for the purpose of quantifying the acid sites of solid catalysts. The integrated molar absorption coefficients were measured for lutidine adsorbed through H-bonding, coordination to Lewis sites and protonation on Brønsted acid sites. The solids were chosen to present all possible bondings with lutidine and to cover a wide range of common types of catalysts or supports: silica, phosphated silica, HY zeolite, alumina, zirconia, WOx supported on zirconia and NbOx supported on zirconia. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Acidity; Oxide catalyst; Infrared spectroscopy; Adsorption of 2,6-dimethylpyridine; Molar absorption coefficient determination
1. Introduction A large number of industrially important reactions require acid catalysts [1–4]. Thus, the characterization of acid sites (type, number and strength) has been widely studied for various catalysts [5–8]. Infrared spectroscopy has often been used to estimate the number of Brønsted and Lewis acid sites in catalytic systems, primarily by monitoring the adsorption of probe molecules such as pyridine and ammonia. Several studies have shown that 2,6-dimethylpyridine (lutidine), because of its basic character and the steric effect of the methyl groups, is an appropriate probe molecule for Brønsted acid sites of solid catalysts [8–16]. This probe molecule was used essentially for qualitative analysis of Brønsted acid sites. Quantitative measurements were hindered because of the lack of availability in the literature of integrated molar absorption coefficients values for the H-bonded, protonated and coordinated lutidine. *
Corresponding author. Fax: +33 231 452 822. E-mail address: [email protected] (M. Houalla).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.02.020
The purpose of the present study is to determine the value of these integrated molar absorption coefficients for a variety of solids commonly used as supports or catalysts (silica, phosphated silica and HY zeolite, alumina and zirconia or zirconia-supported metal oxides). The solids were selected to examine various types of interaction with the probe molecule: H-bonding (silica [12,13,16,17]); H-bonding + coordination (alumina, zirconia [13,16,18–20]); protonation (phosphated silica, dealuminated HY zeolite [17,21]); protonation and coordination (WOx supported on zirconia and NbOx supported on zirconia [6,7,10,22]).
2. Experimental The supports and catalysts used in the present study were SiO2 (Degussa Aerosil; 200 m2/g), c-alumina (Ketjen; 185 m2/g), zirconia (precipitated from zirconium isopropoxide [10]; 50 m2/g), HY zeolite (IFP; isomorphously substituted by treatment with (NH4)2SiF6, Si/ Al ratio: 5.4 [21,23]), PO4/SiO2 containing 1.6 wt% of
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T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104
PO4 (260 m2/g, obtained by impregnation of SiO2), WOx/ZrO2 containing 11.6 wt% of W (130 m2/g, obtained by equilibrium adsorption with a solution of ammonium metatungstate) and NbOx/ZrO2 containing 2.7 wt% of Nb (36 m2/g, obtained by impregnation of ZrO2 with Niobium oxalate). Infrared spectra were recorded with a Nicolet Magna 550 FTIR spectrometer (resolution: 4 cm1, 128 scans, apodization: Happ-Genzel). Samples were pressed into discs (ca. 20 mg; 2 cm2), activated successively in vacuum and in O2 at 723 K, and cooled down to 423 K. For all solids, the experiments were performed by adding known amounts of lutidine. For this purpose, increasing pressures were introduced in a small volume (1.59 cm3) and expanded in the cell at 423 K. The probe molecule was allowed to diffuse in the pores for 1 min at this temperature prior to data collection. Following each addition, the spectrum was acquired for the catalysts at room temperature. Lutidine was added until it was detected in the gas phase. The integrated molar absorption coefficient of the HY zeolite was also determined with another setup which consists of an IR cell coupled with a thermogravimetric system (for more details on the setup see Ref. [23]) in which lutidine was adsorbed at 423 K up to saturation and evacuated at various temperatures. With pyridine or lutidine as probe molecules, the bands due to the stretching of the aromatic ring (m8a, m8b, m19a and m19b) are indicative of the sites on which the probe molecule is adsorbed. However, for lutidine, the present study will focus on the m8a and m8b bands because of their higher sensitivity in terms of position or intensity [16]. Difference spectra were obtained by subtracting the spectra of activated catalysts from the spectra obtained after lutidine adsorption (or desorption). They were used to measure the absorbance integrated intensity (noted A) for the considered band. The integrated molar absorption coefficients were determined using an equation similar to the Beer–Lambert law for solutions Z ~m2 n en A¼ ð1Þ Að~mÞ d~m ¼ e ‘ ¼ S‘ S ~m1 )
A n ¼ e S
ðwith e in cm lmol1 Þ
ð2Þ
where Að~mÞ; ~m, e, S, n and ‘ denote respectively the absorbance, the wavenumber, the integrated molar absorption coefficient, the surface of the disc, the amount of lutidine adsorbed and the thickness of the disc assumed as a first approximation to be equivalent to the optical path. The e were calculated directly for each solid (Eq. (2)) using the slope of the linear part of the curves A = f(n). The added lutidine was assumed to be fully adsorbed on the solid until bands corresponding to gas phase species
were detected. This assumption is based on earlier studies dealing with the determination of the integrated molar absorption coefficients of pyridine [24,25]. Using a similar IR cell, Khabtou et al. [25] showed that adsorption on the cell walls was negligible. Their measurements were confirmed by TGA. When necessary, the contribution of H-bond or Lewis sites was subtracted using the eH-bond or the eLewis determined with other solids in order to calculate the amount of lutidine adsorbed through one specific type of bonding. In some instances curve-fitting was also used. Best fits were generally obtained with Gaussian peaks. The use of a Lorentzian profile did not significantly affect the quantitative results. For systems containing several types of bondings between lutidine and the surface (i.e. lutidine adsorbed through H-bonding, on Lewis or Brønsted acid sites), the following equation can be used: S AHbond ALewis ABrønsted þ þ ¼1 ð3Þ n eHbond eLewis eBrønsted Uncertainties in the values of integrated molar absorption coefficients, due specifically to the repeatability of the experiments were estimated to be ca. 0.2 cm lmol1. Errors due specifically to the curve-fitting and measurements were estimated to be ca. ±0.3 cm lmol1. Peak widths and positions were allowed to vary within 5% of the adopted parameters. Areas measurements were repeated for slightly different windows and the mean values were selected. An alternative method for measuring the integrated molar absorption coefficients is to apply Eq. (3) to all the solids examined in the present study. The resulting set of equations is solved by a least-squares procedure. In this case, it is assumed that the eH-bond, eLewis and eBrønsted values did not change with the catalyst.
3. Results and discussions 3.1. Siliceous materials 3.1.1. Silica Fig. 1(a) shows the infrared spectra between 1630 and 1550 cm1 for increasing doses of lutidine (from 0.065 to 1.305 lmol) adsorbed on silica. The spectra exhibit two bands at 1605 cm1 (m8a) and 1585 cm1 (m8b) which were attributed to lutidine adsorbed on the surface by H-bonding [12,13,16]. Analysis of the spectra also suggests the presence of a small shoulder at 1595 cm1 which can be ascribed to physisorbed lutidine. However curve-fitting results indicated that the intensity of the peak at 1595 cm1 remained low with respect to the 1605 and 1585 cm1 bands. Thus, for the sake of simplicity, the spectra were curve-fitted with 2 peaks only. The evolution of the intensity of the integrated bands
T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104
101
1.4
(a)
(b)
0.02
Integrated Intensity (cm-1)
Absorbance (a.u.)
1.2
SiO2
1 0.8 0.6 0.4 0.2
1630
1610
1590
1570
0
1550
0.0
0.5 1.0 µmol of lutidine introduced
Wavenumber (cm-1)
1.5
Fig. 1. Silica: (a) Infrared spectra for increasing amounts of lutidine (from 0.065 to 1.305 lmol); (b) evolution of the bands area at 1605 cm1 (j) and 1585 cm1 (h) with the amount of added lutidine.
silica (upper part) or 1652 and 1627 cm1 for HY zeolite (lower part), characteristic of protonated species [14]. Fig. 2(b) shows the variation of the total integrated area of these two bands as a function of the amount of added lutidine. A linear variation of the intensity is observed with the amount of added lutidine for phosphated silica (up to approximately 2.0 lmol, where physisorbed species were detected) and for the zeolite sample. The integrated molar absorption coefficients of the above mentioned bands, estimated from the slopes of the linear part of the curves, were e(1655+1630) = 7.9 ± 0.5 cm lmol1 for phosphated silica and e(1652+1627) = 6.5 ± 0.5 cm lmol1 for HY zeolite. For the latter sample, the integrated molar absorption coefficient of the
as a function of the amount of added lutidine is depicted in Fig. 1(b). A linear increase (with intercept at the origin) of the bands area with the amount of added lutidine is observed. The slope of these two straight lines was used to determine the integrated molar absorption coefficient of the corresponding band. Similar values were obtained eð1605Þ ¼ eð1585Þ ¼ 1.9 0.5 cm lmol1 . 3.1.2. Phosphated silica and HY zeolite Fig. 2(a) shows the infrared spectra for increasing doses of lutidine adsorbed on phosphated silica and HY zeolite. The spectra exhibit, respectively, the m8a and m8b bands at 1655 and 1630 cm1 for phosphated
60
(a)
(b)
PO4/SiO2
0.2
50 Integrated Intensity (cm-1)
Absorbance (a.u.)
0.05
HY
40
30
20
10
0 1700
1660 1620 Wavenumber (cm-1)
1580
0
5 10 15 µmol of protonated lutidine
20
Fig. 2. Phosphated silica and HY zeolite: (a) Infrared spectra for increasing amounts of lutidine; Upper part: phosphated silica from 0.068 to 2.845 lmol lutidine; lower part: HY zeolite from 0.087 to 8.874 lmol lutidine; (b) evolution of the integrated bands of protonated species as a function of the amount of protonated lutidine on phosphated silica (m), HY zeolite () and HY after thermodesorption (h).
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T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104
two bands characteristic of protonated species was also measured using a thermogravimetric system coupled with an IR cell. With this setup, the amount of adsorbed lutidine (directly measured) and the corresponding infrared spectrum of the sample were obtained simultaneously [23]. The results were also reported in Fig. 2(b) (open squares). Clearly the experimental points fit nicely on the same line defined by the volumetric measurements indicating the same integrated molar absorption coefficient value. This validates the quantitative approach used in the present study.
3.3. Zirconia and zirconia-supported metal oxides 3.3.1. Zirconia The IR spectra for lutidine adsorbed on zirconia are reported in the lower part of Fig. 3(a). As in the case of alumina, two bands at 1609 and 1580 cm1 due to lutidine coordinatively bound to Lewis acid sites [13,20] were observed. Note the asymmetric nature of the band at 1609 cm1. This can be attributed to the presence of a band at ca. 1600 cm1 characteristic of lutidine adsorbed by H-bonding [13]. A second band corresponding to the m8b vibration expected at ca. 1580 cm1 is probably masked by the band attributed to Lewis acid sites at the same position. The spectra were curve-fitted with three bands located at 1609, 1599 and 1580 cm1 as depicted in the lower inset of Fig. 3(a). Using the approach described above for alumina, the area of the band at 1609 cm1 was plotted as a function of the amount of coordinated lutidine (Fig. 3(b)). The results indicated an integrated molar absorption coefficient value of e(1609) = 3.4 ± 0.6 cm lmol1.
3.2. Alumina The upper part of Fig. 3(a) shows the infrared spectra for increasing doses of lutidine (from 0.087 to 2.088 lmol). For the initial doses of lutidine adsorbed on alumina, the spectra exhibit only two bands at ca. 1617 and 1580 cm1 which were attributed to lutidine adsorbed on Lewis acid sites [13,16,19]. With further introduction of lutidine, two shoulders at ca. 1605 and 1570 cm1 attributed to lutidine adsorbed by H-bonding [13,16,19] were also observed. The spectra were curve-fitted with four bands at 1617, 1605, 1582 and 1572 cm1 as shown in the inset of Fig. 3(a). For each dose of lutidine, the area of the band at 1605 cm1 was measured and the corresponding amount of lutidine involved in H-bonding was determined using the e(1605) value measured in the case of silica. This amount was then subtracted from the amount of lutidine introduced to obtain that of coordinatively adsorbed lutidine. The evolution of the area of the band at 1617 cm1 as a function of the amount of coordinated lutidine is shown in Fig. 3(b). The slope of the linear part of the curve yields an integrated molar absorption coefficient of: e(1617) = 5.3 ± 0.6 cm lmol1.
3.3.2. Tungsten oxide and niobium oxide supported on zirconia Fig. 4(a) shows the infrared spectra between 1670 and 1600 cm1 for increasing doses of lutidine on WOx/ZrO2 (upper part) and NbOx/ZrO2 (lower part), respectively from 0.087 to 3.306 lmol and from 0.087 to 1.044 lmol. In addition to the band at 1610 cm1 observed in the case of pure zirconia and attributed to coordinated lutidine, the spectra of lutidine adsorbed on WOx/ZrO2 and NbOx/ZrO2 show two bands at 1643–1644 and 1628 cm1 characteristic of protonated lutidine [10,22]. The spectra were curve-fitted using three bands at 1643–1644, 1628 and 1610 cm1 as depicted in the inset
3.5
(a)
(b)
0.04
Absorbance (a.u.)
Al2O3
0.02
0.02
ZrO2
Integrated Intensity (cm-1)
3.0 0.04
2.5 2.0 1.5 1.0 0.5
1640
1620
1600
1580
Wavenumber
(cm-1)
1560
1540
0.0 0.0
0.5
1.0
1.5
µmol ol of coordinated lutidine
Fig. 3. Alumina and zirconia: (a) Infrared spectra for increasing amounts of lutidine introduced: alumina from 0.087 to 2.088 lmol lutidine; zirconia from 0.087 to 1.218 lmol—Insets: examples of curve-fitting of the spectra; (b) Evolution of the band area at 1617 cm1 for alumina (h) or 1609 cm1 for zirconia (j) as a function of the amount of coordinated lutidine.
T. Onfroy et al. / Microporous and Mesoporous Materials 82 (2005) 99–104 3.0
0.04
(a)
103 10
(b)
Absorbance (a.u.)
WOx/ZrO2
0.02
NbOx/ZrO2
Integrated Intensity (cm-1)
2.5 0.08
8
2.0 6 1.5 4 1.0 2
0.5
0.0 1670 1660 1650 1640 1630 1620 1610 1600
0.0
0.5
Wavenumber (cm-1)
1.0
1.5
2.0
2.5
0 3.5
3.0
ol of protonated lutidine µmol
Fig. 4. Tungsten oxide supported on zirconia and niobia supported on zirconia: (a) Infrared spectra for increasing amounts of lutidine: upper part: WOx/ZrO2 from 0.087 to 3.306 lmol lutidine; lower part: NbOx/ZrO2 from 0.087 to 1.044 lmol—Inset: example of curve-fitting of the spectra for WOx/ZrO2; (b) evolution of the integrated bands of protonated species as a function of the amount of protonated lutidine for WOx/ZrO2 (j; right axis) and NbOx/ZrO2 (h; left axis).
of Fig. 4(a). As described before, from curve-fitting results and the integrated molar absorption coefficient of coordinated lutidine on zirconia previously determined, the amount of protonated lutidine was calculated for each dose of lutidine. The results reported in Fig. 4(b) indicate a linear increase of the integrated area of the bands at 1643 and 1628 cm1 as a function of the amount of protonated species. The integrated molar absorption coefficient values for WOx/ZrO2 and NbOx/ ZrO2 systems were respectively e(1643+1628) = 5.9 ± 0.6 cm lmol1 and e(1644+1628) = 7.3 ± 0.6 cm lmol1. 3.4. Evaluation of the integrated molar absorption coefficient values The integrated molar absorption coefficients determined in the present study for various solid are reported
in Table 1. For the integrated molar absorption coefficients of Lewis acid sites or Brønsted acid sites, the results were approximately within ±18% of the average value. They seem to be little affected by the type of solids (structure or strength of acid site). Similar findings were reported for pyridine adsorption on Brønsted or Lewis acid sites for various zeolites or silica–alumina [24,25]. Emeis [24] did not find any evidence for a dependence of the integrated molar absorption coefficient on the structure of the catalyst or the strength of the acid site. Khabtou et al. [25], using zeolites Y with different Si/Al ratios, also showed that the integrated molar absorption coefficient was independent of the strength of the acid site. As described in the experimental section, an alternative method for calculating e for a given type of site is to assume that its value is independent of the nature
Table 1 Integrated molar absorption coefficients for lutidine adsorbed on various catalysts Integrated molar absorption coefficients e (cm lmol1)
Sites distribution (%) H-bond SiO2 PO4/SiO2 HY zeolite a HY zeolite b Al2O3 ZrO2 WOx/ZrO2 NbOx/ZrO2 Average
Coordination
Protonation
100
0
0
0 0 0 15 15 0 0
0 0 0 85 85 6 8
100 100 100 0 0 94 92
Least-square Nd: not determined. a Determined from adsorption measurements. b Determined from thermogravimetric measurements.
H-bond
Coordination
Protonation
e(1585) = 1.9 e(1605) = 1.9 – – – Nd Nd – – e(1585) = 1.9 e(1605) = 1.9 e(1605) = 1.9
–
–
– – – e(1617) = 5.3 e(1609) = 3.4 Nd Nd e(Lewis) = 4.35
e(1655+1630) = 7.9 e(1652+1627) = 6.5 e(1652+1627) = 6.5 – – e(1643+1628) = 5.9 e(1644+1628) = 7.3 e(Brønsted) = 6.8
e(Lewis) = 4.4
e(Brønsted) = 6.8
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of the solid. Thus, e values for H-bonding (eH-bond), Lewis acid sites (eLewis) and Brønsted acid sites (eBrønsted) can be determined from Eq. (3) by the least-squares method. The values obtained (eH-bond = 1.9 cm lmol1; eLewis = 4.4 cm lmol1; eBrønsted = 6.8 cm lmol1) were close to the average values determined by the previous method. The 95% confidence limits for the least-square determination were estimated at ±18%. Similar deviation was reported for the integrated molar absorption coefficients of pyridine adsorption [24]. It is worth noting that the integrated molar absorption coefficient for lutidine bound to Brønsted acid sites (bands m8a and m8b: e(1642+1625) = 6.8 cm lmol1) is significantly larger than that of the band used to titrate Brønsted acid sites with pyridine (band m19b at 1540 cm1: e(1540) = 1.8– 2.2 cm lmol1 [24,25]). This indicates an additional advantage for the use of lutidine as compared to pyridine for the titration of Brønsted acid sites.
[2] [3] [4] [5] [6] [7] [8]
[9] [10]
[11] [12]
4. Conclusions
[13]
The integrated molar absorption coefficients for bands of adsorbed lutidine were determined, for various modes of bondings, from monitoring the infrared spectra of lutidine adsorbed on different supports and supported catalysts, as a function of the amount of the probe molecule added. The estimated e values for bands characteristic of lutidine adsorbed through Hbonding, coordination and protonation were respectively eH-bond = 1.9 cm lmol1, eLewis = 4.4 cm lmol1 and eBrønsted = 6.8 cm lmol1. The results indicated little variations as a function of the type of solids. The approach adopted for the determination of e was validated by coupled microbalance—IR measurements.
[14] [15] [16] [17] [18] [19]
[20] [21] [22]
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