- Email: [email protected]
Accessibility of the acid sites in dealuminated small-port mordenites studied by FTIR of co-adsorbed alkylpyridines and CO
Microporous and Mesoporous Materials 71 (2004) 157–166 www.elsevier.com/locate/micromeso
Accessibility of the acid sites in dealuminated small-port mordenites studied by FTIR of co-adsorbed alkylpyridines and CO N.S. Nesterenko a, F. Thibault-Starzyk b, V. Montouillout b, V.V. Yuschenko a, C. Fernandez b, J.-P. Gilson b, F. Fajula c, I.I. Ivanova a,* a Department of Chemistry, Moscow State University, Lenin Hills, 119892 Moscow, Russia Laboratoire Catalyse et Spectrochimie, UMR 6506 CNRS-ENSICaen, 6 Bvd Marechal Juin, F-14050 Caen Cedex, France Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b
c
Received 16 January 2004; received in revised form 16 January 2004; accepted 26 March 2004 Available online 7 May 2004
Abstract A novel methodology for characterization and quantification the acidic sites of different accessibility has been developed for dealuminated zeolite catalysts. The method is based on probing the accessibility of acid sites by IR spectroscopy of adsorbed alkylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of subsequently adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-by-step characterization of the nature and strength of sites with different accessibility. The approach proposed was tested on the series of small-port mordenites dealuminated up to Si/Al ratios of 11, 12, 14 and 18 and characterized by various techniques including XRD, TPD-NH3 , nitrogen adsorption–desorption, 1 H MAS NMR and IR spectroscopy. It has been demonstrated that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) the acid sites in the side pockets become accessible for such probe molecules as pyridine due to the partial destruction of side pockets upon dealumination (ii) subsequently, the formation of secondary mesopore system due to further dealumination makes the zeolite crystals completely accessible to relatively bulky molecules such as lutidine, collidine, 2,6-di-tert-butylpyridine. 2004 Published by Elsevier Inc. Keywords: IR spectroscopy of probe molecules; Dealuminated mordenites; Acidity of zeolites; Alkylpyridines
1. Introduction The rational design of efficient heterogeneous catalysts requires a detailed knowledge of the active site(s). In the case of zeolites, much work has been devoted to a detailed description of the active site in its nanoscale (micro-porosity) environment. It has also been repeatedly demonstrated [1–4] that mass transfer limitations play an important role in many of their industrial applications. Creation of controlled mesoporosity has been shown to minimize or even cancel these limitations [5,6]. There is a need and a strong incentive to further improve the active site description of zeolitic catalysts
*
Corresponding author. Fax: +7-95-9328846. E-mail addresses: [email protected] (F. Fajula), iiivanova@ phys.chem.msu.ru (I.I. Ivanova). 1387-1811/$ - see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.micromeso.2004.03.028
and in particular, the accessibility to the active site deserves more attention. FTIR spectroscopy of adsorbed molecules is a wellknown powerful tool to investigate the acid sites of zeolite catalysts, in particular, their nature, number and strength [7–10]. Studies on the location and accessibility of the zeolitic acid sites are available but less widespread. For instance, the distribution of active sites between the external and internal zeolite surface was studied by adsorbing bulky probe molecules, unable to enter zeolitic pores. Corma et al. [11] used 2,6-di-tert-butylpyridine for the investigation of the external surface acidity of MFI and MOR zeolites. This probe molecule, however, was able to penetrate easily into the pore system of 3-dimensional BEA zeolite. The intensity of the N–Hþ stretching band of adsorbed 2,6-di-tert-butylpyridine was shown to correlate with the catalytic activity of
158
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
bulky reactants in cracking. Creyghton et al. used 3,5,di-tert-butylphenyl ether to study the outer surface acidity of MOR and BEA zeolites [12]. Busca et al. selected a range of aliphatic and aromatic compounds as well as acetonitrile and pivalonitrile to discriminate between the external and internal acid sites of MFI zeolite [13,14]. Morterra and co-workers chose for the same purpose acetonitrile and adamantine-carbonitrile [15]. Quinoline was selected in [16] as a suitable probe molecule. The location and accessibility of the active sites inside zeolite pores was investigated by the adsorption of probe molecules of different size. Busca et al. proposed to use branched nitriles such as propionitrile, isobutironitrile and pivalonitrile for the study of location and accessibility of Brønsted and Lewis acid sites in H-MOR [17]. Another approach proposed by Lavalley and coworkers makes use of co-adsorption of Py and CO [18] to distinguish between the acid sites in the main channels and side pockets of MOR. These approaches, using small size probe molecules, were proposed for nondealuminated MOR and can hardly be applied to dealuminated materials, since dealumination creates secondary mesoporosity and changes significantly the accessibility of the sites [18,19]. As far as our knowledge concerned, at present, there is no suitable method to characterize the accessibility of acid sites in dealuminated zeolite materials. The development of such a method is very desired since most commercial catalytic applications of zeolites require variable degrees of dealumination of these materials [6,20,21]. The present work is aimed at the development of such a methodology. The method proposed is based on probing the accessibility of acid sites by alkylpyridines with various sizes and monitoring the nature and strength of the sites not-accessible to alkylpyridines by subsequent CO adsorption. The small-port mordenite with different degree of dealumination was used as model catalyst [22]. The later is an ideal model to study such effects due to the following reasons: It involves acid sites located in two different environments, namely, in often reside small cavities (5.7 · 2.6 and 3.7 · 4.8 A) ferred to as side-pockets and in larger linear channels
In addition, structural defects characteris(6.7 · 7.0 A). tic for small-port MOR impose additional limitations for sites accessibility. Finally, earlier work by Raatz et al. [23,24] has shown that it is possible to ‘‘open-up’’ the porosity of the small-port MOR after removal of ca 20% of framework aluminum by combining calcination and acid leaching treatments.
2. Experimental 2.1. Catalysts The starting mordenites, GP180 and ZM510, were supplied by Societe Chimique de la Grande Paroisse (GP). GP180 is a parent synthetic zeolite in H-form with Si/Al ¼ 5.5, while ZM510 is a dealuminated mordenite with Si/Al ¼ 11 obtained from GP180 by deep-bed calcination of the ammonium form of SP mordenite followed by acid leaching [24,25]. GP180 is assigned as SP and ZM510 as DeAlSP. DeAlSP was further dealuminated by treatment with methanesulfonic acid. In the first preparation, it was treated with 2 M acid at 383 K for 4 h in a stirred vessel to give DeAlSP1 sample. In the second treatment, 6 M acid was used (DeAlSP2). In the third preparation, DeAlSP sample was first calcined at 1023 K and than treated with 2 M acid. After treatments, the samples were filtered, washed, dried at 353 K and calcined in a shallow bed at 723 K for 8 h. The conditions of sample preparation are summarized in the Table 1. 2.2. Characterization The elemental analyses of dealuminated mordenites was performed by atomic absorption. The XRD patterns were obtained with a CGR Theta-60 diffractometer, using CuKa radiation. Sorption–desorption isotherms of nitrogen were recorded at 77 K using an automated porosimeter (Micromeritics Asap 2000). Micro-pore volumes were determined using t-plot method. The total sorbed volumes, including adsorption in the micro-pores and mesopores and on the external surface, were calculated from the amount of nitrogen
Table 1 Characteristics of dealuminated mordenites Samples
Preparation
Si/Al
SP DeAlSP DeAlSP1 DeAlSP2 DeAlSP3
As obtained from GP As obtained from GP Treatment of DeAlSP with 2 M MSa Treatment of DeAlSP with 6 M MSa Calcination of DeAlSP followed by treatment with 2 M MSa
5.5 11 12 14 18
Pore volume (cm3 /g) Vmico
a
MS––methanesulfonic acid.
0.160 0.178 0.179 0.180 0.139
Vmeso 0.014 0.025 0.049 0.061 0.101
Vmicro =Vtotal (%) Vtotal 0.174 0.203 0.228 0.241 0.240
92 88 79 75 58
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
adsorbed at relative pressure p=p0 of 0.96, before the onset of interparticle condensation. The acidic properties were studied by temperatureprogrammed desorption of ammonia (NH3 -TPD), and IR spectroscopy of adsorbed probe molecules. The nature of the acidic sites was also characterized by 1 H MAS NMR and 1 H–{27 Al} TRAPDOR [26,27] NMR experiments. NH3 -TPD was performed in a homemade set-up equipped with TC detector. Prior to NH3 adsorption, the samples were calcined in situ in a flow of dry air at 823 K for 1 h and, subsequently, in a flow of dry nitrogen for 1 h and cooled down to ambient temperature. The NH3 adsorption was carried out for 30 min, at room temperature in a flow of NH3 diluted with N2 (1/1). Subsequently, the physisorbed NH3 was removed in a flow of dry He at 373 K for 1 h. Typical TPD experiments were carried out in the temperature range of 295 to 1073 K in a flow of dry He (30 ml/min). The rate of heating was 8 K/min. Solid-state NMR 1 H and 1 H–{27 Al} TRAPDOR were performed on a Bruker Avance 400 spectrometer (m1 H ¼ 400.33 MHz, m27 Al ¼ 104.23 MHz). Prior to the measurements, the samples were dehydrated using the following procedure: approximately 30 mg of the sample was placed in a glass spoon in a homemade equipment attached to a vacuum line. The sample was evacuated, heated with the rate of 2 K/min up to 723 K and maintained at this temperature under 5 · 105 Torr for 5 h. The dehydrated zeolite was than dropped into the 4 mm-ZrO2 NMR rotor, which was closed with classic Kel-F cap under vacuum using the sample equipment. No signal attributed to residual water has been observed on 1 H MAS NMR spectrum of the sample prepared using the above procedure. The 1 H NMR spectra were recorded using a rotor-synchronized Hahn Echo pulse sequence (p=2–s–p–s-acquire) to avoid the signal from the probe head. The spinning rate was fixed at 10 kHz. The p=2 pulse length was 5.2 ls and 64 scans were accumulated with a 10 s recycle delay. Chemical shifts were referenced relative to tetramethylsilane. In the 1 H– {27 Al} TRAPDOR experiment, the same pulse sequence and conditions were applied to the 1 H channel, while a continuous RF irradiation was applied to the aluminum (27 Al) channel during the first s period. The on-resonance 27 Al irradiation field was set to about 125 kHz for six rotor periods (s ¼ 600 ls). All the spectra have been decomposed to retrieve isotropic chemical shifts and intensity using the Dmfit program [28]. IR spectra were recorded with a Nicolet Magna 550FT-IR spectrometer at 2 cm1 optical resolution, with one level of zero-filling for the Fourier transform. Prior to the measurements, the catalysts were pressed in selfsupporting discs (diameter: 1.6 cm, 7 mg cm2 ) and activated in the IR cell (attached to a vacuum line) at 723 K for 4 h up to 106 Torr. The IR cell described earlier [29] for low-temperature studies was equipped
159
with ZnSe inner windows combined with KBr external windows, which enabled to register spectra in the spectral region down to 600 cm1 . The pressure of the adsorbed gases was measured by two Barocel gauges, the most precise one was attached directly to the samplecontaining compartment of the cell. Another one enabled us to measure a dose of gas in the known volume before admitting it into the cell. The sample temperature during the treatment or recording of spectra was monitored by a chromel–alumel thermocouple inserted into the heater or into the coolant compartment of the cell. For spectra recorded at low-temperature, about 0.5 Torr of He was added into the cell to achieve better thermal contact of the pellet with the inner part of the cell. Adsorption of substituted pyridines: pyridine (Py), lutidine (Lu), collidine (Coll), di-tert-butylpyridine (DTBuPy) was performed at 423 K. The excess of probe molecules was further evacuated at 423 K. The adsorption–evacuation was repeated several times until no changes in the spectra were observed. After adsorption of the substituted pyridines, adsorption of CO was performed at 100 K.
3. Results and discussions 3.1. Characterization of the model catalysts The main characteristics of the parent (SP) and dealuminated (DeAlSP, DeAlSP1, DeAlSP2, DeAlSP3) zeolites are presented in Table 1. The stepwise dealumination results in a gradual increase of the Si/Al ratio from 5.5 to 18. The MOR structure remains intact in all the samples as evidenced by XRD data. On the contrary, the texture of the samples changes significantly upon dealumination. While the parent zeolite, SP, shows a reversible type-I isotherm, typical of a micro-porous solid, the isotherms of the four other samples exhibit a hysteresis loop spreading over the whole range of relative pressures above p=p0 ¼ 0:42, indicating the presence of mesopores with a broad distribution of sizes [30]. The pore volumes of the various samples are reported in Table 1 and point to the following changes occurring upon dealumination: • The first dealumination step (transformation of SP into DeAlSP) leads to an increase of both, microand mesopore volumes. • Further dealumination with increasingly concentrated methanesulfonic acid shows a gradual increase of the mesopore volume, while micro-pore volume remains barely constant. • Combination of heating at 1023 K with an acid attack leads to an increase of the mesopore volume together with a significant decrease of the micro-pore volume.
160
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
As summarized in the last column of Table 1, the increase of the degree of dealumination leads to materials with gradually increasing contribution of mesoporosity. The number of acidic sites in the samples, measured by TPD of NH3 is given in Table 2. As expected, it decreases with the increase of Si/Al ratio upon dealumination. 3.2. Nature and strength of acid sites The nature and strength of OH-groups over dealuminated samples were studied by IR and 1 H MAS NMR spectroscopies. Results obtained by both techniques are reported in Fig. 1 and Table 2. The bridging hydroxyl groups are characterized by an IR band at 3614 cm1 in the m(OH) region and a NMR line at 4.1 ppm. Two types of silanol groups are identified: (i) external: IR band at 3745 cm1 and 1 H NMR line at 1.9 ppm and (ii) internal: IR band at 3734 cm1 [31] and NMR line at 2.1 ppm. Both techniques show that the number of bridging hydroxyl groups decreases significantly with the degree of dealumination. On the contrary, the number of silanol groups increases upon dealumination. While in the starting MOR, external silanol groups give the highest contribution to the spectra, in dealuminated samples the contribution of internal silanol groups increases. It should be mentioned that position and the assignments of the IR bands and NMR lines of bridging hydroxyls and silanol groups are in full agreement with literature data [31–34].
The attribution of the IR band at 3660 cm1 and the corresponding 1 H NMR line at 2.8 ppm deserves some extra comments, since there is no agreement on their nature in the literature. Indeed, several assignments have been proposed for the IR bands of dealuminated zeolites in the region of 3650–3700 cm1 . • Hydroxyl groups of extraframework aluminous species [15,16,31,35]. • Al–OH groups partially bounded to the zeolite lattice [32,33,36]. • Framework Si–OH groups in defect sites of zeolites [37,38]. Tsyganenko et al. [39] observed a band at 3660 cm1 upon growing successive layers of alumina on silica. This band was shown to interact with CO by giving a CO–OH3660 complex confirmed by the appearance of a m(OH) band at 3460 cm1 and a m(CO) vibration at 2174–2165 cm1 . These features were not seen neither in pure silica, nor in alumina, and the band at 3660 cm1 was attributed by the authors to bridging hydroxyls similar to those observed in amorphous silica–alumina. Finally, in [18], it was shown that if activation was not carried out carefully, this band corresponds to water adsorbed on acidic OH groups. In the present study, the line at 3660 cm1 could hardly be attributed to adsorbed water, since the other characteristic bands of water were not observed in the spectral region of 1600–1650 cm1 . The CO adsorption on the OH groups corresponding to the band at 3660
Table 2 Amount, nature and strength of acid sites in dealuminated mordenites Samples
Amount of acid sites (TPDNH3 ) (lm/g)
Type
mOH (cm )
dH , (ppm)
SP
1188
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 – (3780) 3614
DeAlSP
1040
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
DeAlSP1
1105
DeAlSP2
DeAlSP3
Nature of sites
B=L (Py)
Acidic strength DmOH (CO) (cm1 )
1.9 – (2.6) 4.0
3.7
311
3745 3734 3660 3609
1.9 – 2.8 4.1
2.0
326
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 3734 3660 3607
1.9 2.1 2.8 4.1
1.9
330
908
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 – – 3607
1.9 2.0 2.8 4.2
0.9
332
492
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 3739 – 3605
1.9 2.0 – 4.2
0.7
332
1
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
161
Fig. 1. IR (a) and 1 H MAS NMR (b) spectra obtained over MOR samples. Thick lines correspond to 1 H MAS NMR spectra, while thin lines show 1 H–{27 Al} TRAPDOR NMR spectra. (The spectra are normalized to the masse of the samples.)
cm1 results in the appearance of the bands at 3460 cm1 and around 2169–2174 cm1 indicating that, in our case, the 3660 cm1 band could be attributed to OH groups bounded to extraframework silica–alumina species [36]. In order to confirm between these assignments, we acquired a 1 H–{27 Al} TRAPDOR spectra of all the samples. In this experiment, a continuous RF irradiation is applied at the 27 Al frequency during observation of the 1 H signal intensity. The signal of proton, strongly coupled with aluminum atoms is thus significantly diminished or suppressed, while the signal of non-coupled proton remains unaffected. Fig. 1b (gray traces) shows the 1 H–{27 Al} TRAPDOR NMR spectra for all samples. As expected, the unaffected resonances are the silanol species at 1.9 and 2.1 ppm. On the contrary, we observe an important decrease of both 2.6 and 4.0 ppm resonances, indicating unambiguously that these hydroxyl groups are connected to aluminum. The broad 1 H NMR line at 2.6 ppm (Fig. 1b––SP), shows that OH groups corresponding to aluminumcontaining extraframework species are already present in small amounts in the starting MOR. This is likely due to occlusion of an amorphous phase created during the synthesis or to structural damage that occurred during the calcination of the ammonium precursor. Further dealumination, significantly increases the relative number of hydroxyl groups associated with extraframework species (Fig. 1b––DeAlSP). It should be mentioned that the shift to 2.8 ppm of the NMR line corresponding to the extraframework hydroxyls may indicate the formation of another type of extraframework species upon
dealumination. Treatment of DeAlSP with methanesulfonic acid of different concentrations (DeAlSP1 and DeAlSP2) induces a decrease of the contribution of the extraframework hydroxyls in both sets of spectra. Finally, the combined treatment including calcination at high temperature and acid leaching, leads to a material virtually free of extraframework species. Moreover, we only observe external silanol groups indicating that the structure is completely open. (Fig. 1––DeAlSP3). The IR spectra recorded after adsorption of Py in spectral region of the 8a,b and 19a,b ring vibrations of pyridine show bands due to protonated (1545, 1633– 1637 cm1 ) and coordinatively adsorbed pyridine (1454, 1620–1622 cm1 ) on all the samples [40,41]. The ratio of the concentrations of Brønsted to Lewis acid sites (B=L), based on published extinction coefficients of the bands at 1545 and 1454 cm1 [18], are given in Table 2. The results suggested that the first dealumination (DeAlSP1) leads to a significant increase of the intensities of the bands corresponding to both Brønsted and Lewis sites, indicating a higher accessibility of Brønsted sites to Py and the formation of additional Lewis sites in such dealuminated materials. These results are in agreement with the model presented by Raatz et al. [23,24]. Further treatments lead to a decrease of the number of both Lewis and Brønsted sites. The B=L ratio also decreases gradually. This ratio is lower than 1 for DeAlSP3, which is free of extraframework species, indicating that a significant contribution to Lewis acidity arises from framework electron deficient sites, as it was shown previously [42].
162
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
The strength of the acid sites was studied by IR spectroscopy of adsorbed CO (Table 2). Carbon monoxide is a well-established probe molecule for the characterization of the acidity of solids. Its adsorption at low-temperature (100 K) on oxide material leads to the formation of H-bonds on Brønsted acid sites. Upon forming this H-bond, the m(OH) vibration band is perturbed and shifted to lower wavenumbers. The corresponding Dm(OH) shift indicates the strength of the H-bond and thus the strength of the acid site. The m(CO) vibration band of adsorbed CO is also shifted (but to higher wave numbers) from its pseudo-liquid frequency at 2138 cm1 , and its shift corresponds to the strength of the perturbation. Adsorption on Lewis sites or on cations leads to specific m(CO) vibration frequencies, which can be used for identification of the adsorption site. Using CO as a probe molecule brings additional advantage, since its small size allows easily access most of the sites in the framework [43]. The results summarized in Table 2 show that the acid strength of the sites gradually increases upon dealumination. To summarize, the overall surface and textural properties of the series of dealuminated MOR prepared for accessibility studies are characterized by: • a gradual increase of Si/Al ratio and pore volumes, • a decrease of the total amount of acid sites (including bridging and extraframework hydroxyls), • an increase of the number of silanol groups, • an increase of the contribution of Lewis acidity. 3.3. Accessibility of acid sites in dealuminated MOR The methodology proposed for the study of the accessibility is based on the interaction of probe mole lutidine cules of different size, namely, pyridine (5.7 A), collidine (7.4 A), di-tert-butylpyridine (7.9 A) (6.7 A), with OH groups of MOR. The changes upon adsorption of these molecules are monitored by IR spectroscopy and the sites not accessible to the above mentioned
probe molecules are further studied by IR spectroscopy of adsorbed CO. The CO adsorption on neat samples is also investigated for comparison. By increasing the size of the probe-molecules, a gradual liberation of the acid sites available for CO adsorption occurs and affords a stepwise characterization of the nature and strength of the sites of decreasing accessibility. This gives a detailed map of the nature and strength of acid sites of different accessibility. The results obtained over a series of model dealuminated MOR are presented in Figs. 2–6. The IR spectra of OH groups recorded before and after adsorption of alkylpyridines are displayed on the left part of Figs. 2–6. In the right part, the difference spectra observed after subsequent CO adsorption are presented both for the m(OH) and m(CO) spectral regions. Pyridine adsorption over the parent (SP) shows only a partial disappearance of the band at 3614 cm1 , corresponding to bridging hydroxyls (Fig. 2a). The amount of sites not-accessible to pyridine, estimated from the change of the band intensity upon Py adsorption, corresponds to about 32%. As it was suggested previously [44], these sites are most probably located in the side pockets of the MOR structure. External silanol groups are only slightly disturbed by Py adsorption, probably due to their low acidity. Upon subsequent CO adsorption on the sample pre-loaded with Py the bridging hydroxyls band (3610 cm1 ) disappears (Fig. 3b). Simultaneously, the band at 3316 cm1 , corresponding to the CO–OH3614 complex appears in the m(OH) region. This later band is shifted to higher wave numbers with respect to the parent MOR without Py pre-adsorption. This suggests that nonaccessible sites are weaker than those accessible to Py in agreement with previous work [45]. In the m(CO) region, three bands are observed: • 2137 cm1 , attributed to pseudo-liquid CO [18,43,45], • 2165 and 2174 cm1 , assigned to CO interaction with bridging OH groups [18,43,45].
Fig. 2. IR spectra obtained on the SP mordenite before and after adsorption of Py (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
163
Fig. 3. IR spectra obtained on the DeAlSP mordenite before and after adsorption of Py (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Fig. 4. IR spectra obtained on the DeAlSP1 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Fig. 5. IR spectra obtained on the DeAlSP2 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Adsorption of Py causes a significant decrease of the 2174 cm1 band, while the band at 2165 cm1 remains practically intact, suggesting that it corresponds to the less accessible sites in the side pockets. The results point that two sites can be distinguished:
strong bridging hydroxyls accessible to Py and located in the main channels and weaker bridging hydroxyls not accessible to Py and located in side pockets. These results agree with previous studies [36,45].
164
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
Fig. 6. IR spectra obtained on the DeAlSP3 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Upon dealumination of the parent MOR by successive deep-bed calcination and acid leaching [24,25] (DeAlSP) the amount of bridging hydroxyls decreases and OH groups belonging to extraframework silica– alumina species appear (Fig. 3a). It is interesting to note that the band characteristic for CO interaction with bridging hydroxyls in side pockets (2165 cm1 ) is no longer resolved after dealumination (Fig. 3b). The infrared specific feature of the side pocket tends to disappear leading to only one spectral feature for both OH groups. The OH groups of the side pockets are thus loosing their specificity. Meanwhile, the accessibility of bridging hydroxyls to Py increases significantly: more than 80% of sites interact with Py in this sample. These observations suggest that the restrictions limiting access to the side pockets are suppressed upon dealumination. Extraframework OH groups are not accessible to Py, however, they are accessible to CO and form a (CO– OH) complex characterized by the bands at 2169 cm1 in the m(CO) region and at 3640 cm1 in the m(OH) region (Fig. 3b), indicating a low acidity of the associated hydroxyl. The amount of silanol groups also increases upon dealumination. This is confirmed by the increase of the intensity of the band at 3738 cm1 and the appearance of a new band at 2157 cm1 corresponding to CO interacting with silanol groups. Washing the DeAlSP sample with methanesulfonic acid leads to a full accessibility of the bridging hydroxyls to Py (Fig. 4). The accessibility of extraframework OH groups also increases, since no band corresponding to OH–CO complexes is observed after subsequent CO adsorption. The only band observed in the m(CO) region, besides that due to pseudo-liquid CO, corresponds to interactions of CO with silanol groups. Adsorption of a bulkier probe molecule (Lu) shows only a partial disappearance of bridging and extraframework hydroxyls, as confirmed by the spectra in Fig. 4. The results obtained on the DeAlSP1 are consistent with an increase of the volume of the secondary mesoporous
network confirmed by N2 adsorption––desorption isotherms. Further dealumination with 6 M methanesulfonic acid (DeAlSP2) results in a decrease of the amount of extraframework hydroxyls and in a greater accessibility to bulky probe molecules. Thus, bridging hydroxyls become fully accessible to Py and Lu, while extraframework species are fully accessible to Py. Both of these groups are, however, only partially accessible to the more bulky probe Coll as observed in Fig. 5. Washing with more concentrated acid allows for removal of extraframework species and therefore yields a more open structure. Finally, a combination of heating at 1023 K with an acid attack (DeAlSP3) leads to a drastic decrease of the intensity of the bands characteristic of bridging and extraframework hydroxyls and to an increase of the intensity of the signal due to silanol groups (Fig. 6). This brings a further opening of the porous structure: bridging hydroxyls become accessible even to the bulky 2,6-DTBuPy and extraframework sites to Collidine. Thus, a combination of heating at 1023 K with acid attack leads to the most open structure, in which all structural and extraframework acid sites become accessible to molecules with a kinetic diameter of 6.4 and 6.7 respectively. A, Fig. 7 summarizes the above results and gives the distribution of Brønsted acid sites with different accessibility over various samples studied. In the starting material, 30% of the sites are accessible only for CO and therefore are located in side pockets, while 70% are located in main channels and accessible to Py. Upon the first dealumination, the contribution of the sites located in side pockets decreases probably due to the partial destruction of side pockets and the contribution of sites accessible to Py increases up to 80%. Further dealumination makes a part of the sites available to larger probe molecules, suggesting that they are looking into larger pores, formed upon dealumination. The
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
progress to investigate other zeolites, such as the more commercially relevant ‘‘Large-Port’’ MOR and to establish the relationship with catalysts performance in different reactions.
100 80
85
68
80 60
58 60
32 30
33
40
20 10
20 11 0
165
3
4
6
SP DeAlSP DeAlSP1 DeAlSP2 DeAlSP3
N
CO
Acknowledgements The financial support by the NATO StP program is gratefully acknowledged. N.S. Nesterenko, V.V. Yuschenko and I.I. Ivanova are grateful to RFBR (project 02-03-32516), Council for the Grants of Russian President (project NSh-1275.2003.3) and Science support foundation for the financial support. N.S. Nesterenko thanks Haldor Topsoe A/S for a Ph.D. fellowship.
Fig. 7. Distribution of Brønsted acid sites of different accessibility over mordenite samples studied.
References higher the degree of dealumination, the larger are the pores accommodating Brønsted acid sites. The strength of the sites located in larger pores is higher with respect to those located in smaller pores, as evidenced by higher shifts of the bands corresponding to CO–OH complexes observed in m(OH) region of IR spectra.
4. Conclusions A novel methodology to characterize and quantify the acidic sites of varying accessibility has been developed. It was illustrated for the special case of the smallport MOR structure but is of course applicable to other zeolitic catalysts. The method is based on the probing of acid sites accessibility by IR spectroscopy of adsorbed alkylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-bystep characterization of the nature and strength of sites with different accessibility. It has been demonstrated and quantified that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) a full accessibility of the structural micro-pores and (ii) subsequently, an opening up of the secondary mesopore system making the zeolite crystals completely accessible to relatively bulky molecules. This type of quantitative characterization is important since most of the industrial applications of zeolites are generally running under mass transfer limitations. The design of efficient industrial catalysts requires an a priori knowledge of the active site and its location at the micro- and mesoscale. Further work is presently in
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15]
[16] [17] [18] [19] [20]
[21] [22] [23] [24]
P.B. Weisz, V.J. Frilette, J. Phys. Chem. 64 (1960) 382. P.B. Weisz, ChemTech 3 (1973) 498. P.B. Weisz, V.J. Frilette, J. Catal. 1 (1962) 307. N.Y. Chen, C.M. Smith, Molecular Transport and Reaction in Zeolite, VCH, New York, 1994. T.C. Tsai, Sh.B. Liu, I. Wang, Appl. Catal. A: Gen. 181 (2) (1999) 355. C. Song, Acad. Sci. Paris, Ser. IIc, Chem. 3 (2000) 477. J. Lercher, G. Gabriele, Catal. Today 27 (1996) 353. G. Busca, Catal. Today 27 (1996) 323. J. Ryczkowski, Catal. Today 68 (2001) 263. A. Zecchina, G. Ricchiardi, S. Bordiga, F. Bonino, C. Prestipino, C. Lamberti, Stud. Surf. Sci. Catal. 142 (2002) 3. A. Corma, V. Fornes, L. Forni, F. Marquez, J. MartinezTriguero, D. Moscotti, J. Catal. 179 (2) (1998) 451. E.J. Creyghton, J.A. Elings, R.S. Downing, R.A. Sheldon, H. van Bekkum, Micropor. Mater. 5 (1996) 299. T. Armaroli, M. Bevilacqua, M. Trombetta, F. Milella, A. Gutierrez, J. Ramirez, B. Notari, J. Ronald, G. Busca, Appl. Catal. A: Gen. 216 (1–2) (2001) 59. M. Trombetta, T. Armarolli, G. Busca, Appl. Catal. A: Gen. 192 (1) (2000) 125. C. Arean Otero, E. Escalona Platero, M. Penarroya Mentruit, M. Rodriguez Delgado, F.X. Llabres i Xamena, A. GarciaRaso, C. Morterra, Micropor. Mesopor. Mater. 34 (1) (2000) 55. A. Corma, V. Fornes, F. Rey, Zeolites 13 (1993) 56. M. Bevilacqua, G. Busca, Catal. Commun. 3 (2002) 497. M. Maache, A. Janin, J.C. Lavalley, E. Benazzi, Zeolites 15 (6) (1995) 507. R. Dutartre, L.Ch. de Menorval, F. Di Renzo, D. McQueen, F. Fajula, Ph. Schulz, Micropor. Mater. 6 (1996) 311. Y. Sugi, S. Tawada, T. Sugimura, Y. Kubota, T. Hanaoka, T. Matsuzaki, K. Nakajima, K. Kunimori, Appl. Catal. A: Gen. 189 (2) (1999) 251. C. Perego, S. Amarilli, R. Millini, R.G. Bellussi, G. Girotti, G. Terzoni, Micropor. Mater. 6 (1996) 395. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974. F. Raatz, C. Marcilly, E. Freund, Zeolites 5 (1985) 5329. F. Raatz, C. Marcilly, E. Freund, J. Chem. Soc. Faraday Trans. 79 (1) (1983) 2299.
166
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
[25] E. Freund, Ch. Marcilly, F. Raatz, J. Chem. Soc., Chem. Commun. (1982) 309. [26] C.P. Grey, A.J. Vega, JACS 117 (1995) 8232. [27] F. Deng, Y. Yue, Y. Chaohui, Solid State Nucl. Magn. Reson. 10 (1998) 151. [28] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan, G. Hoatson, Magn. Reson. Chem. 40 (2002) 70. [29] F. Mauge, J. Lamotte, N.S. Nesterenko, O. Manoilova, A.A. Tsyganenko, Catal. Today 70 (1–3) (2001) 271. [30] K.S.W. Sing, S.J. Greg, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [31] F. Thibault-Starzyk, A. Vimont, J.P. Gilson, Catal. Today 70 (1– 3) (2001) 227. [32] R. Anquetil, J. Saussey, J.C. Lavalley, Phys. Chem. Chem. Phys. 1 (4) (1999) 555. [33] A. Vimont, F. Thibault-Starzyk, J.C. Lavalley, J. Phys. Chem. B 104 (2000) 286. [34] M. Muller, G. Harvey, R. Prins, Micropor. Mesopor. Mater. 34 (3) (2000) 281.
[35] G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723. [36] M. Maache, A. Janin, J.C. Lavalley, J.F. Joly, E. Benazzi, Zeolites 13 (1993) 419. [37] P.A. Jacobs, J.B. Uytterhoeven, J. Chem. Soc. Faraday Trans. I 69 (1973) 373. [38] E. Loeffler, U. Lohse, C. Penker, G. Oehlmann, L.M. Kustov, V.L. Zholobenko, V.B. Kazansky, Zeolites 10 (1990) 266. [39] A.A. Tsyganenko, O.V. Manoilova, K.M. Bulanin, P.Yu. Storozhev, S. Haukka, A. Palukka, M. Lindblad, Stud. Surf. Sci. Catal. 130 (2000) 3149. [40] A.G. Secerdia, C. Morterra, M.J. Low, J. Colloid Interf. Sci. 99 (1984) 208. [41] E.P. Parry, J. Catal. 2 (1963) 371. [42] D. McQueen, B.H. Chiche, F. Fajula, A. Auroux, C. Guimon, F. Fitoussi, P. Schulz, J. Catal. 161 (2) (1996) 587. [43] R. Shigeishi, B.H. Chiche, F. Fajula, Micropor. Mesopor. Mater. 43 (2) (2001) 211. [44] J. Datka, B. Gil, A. Kubacka, Zeolites 18 (4) (1997) 245. [45] O. Marie, F. Thibault-Starzyk, J.C. Lavalley, Phys. Chem. Chem. Phys. 2 (22) (2000) 5341.
Accessibility of the acid sites in dealuminated small-port mordenites studied by FTIR of co-adsorbed alkylpyridines and CO N.S. Nesterenko a, F. Thibault-Starzyk b, V. Montouillout b, V.V. Yuschenko a, C. Fernandez b, J.-P. Gilson b, F. Fajula c, I.I. Ivanova a,* a Department of Chemistry, Moscow State University, Lenin Hills, 119892 Moscow, Russia Laboratoire Catalyse et Spectrochimie, UMR 6506 CNRS-ENSICaen, 6 Bvd Marechal Juin, F-14050 Caen Cedex, France Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b
c
Received 16 January 2004; received in revised form 16 January 2004; accepted 26 March 2004 Available online 7 May 2004
Abstract A novel methodology for characterization and quantification the acidic sites of different accessibility has been developed for dealuminated zeolite catalysts. The method is based on probing the accessibility of acid sites by IR spectroscopy of adsorbed alkylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of subsequently adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-by-step characterization of the nature and strength of sites with different accessibility. The approach proposed was tested on the series of small-port mordenites dealuminated up to Si/Al ratios of 11, 12, 14 and 18 and characterized by various techniques including XRD, TPD-NH3 , nitrogen adsorption–desorption, 1 H MAS NMR and IR spectroscopy. It has been demonstrated that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) the acid sites in the side pockets become accessible for such probe molecules as pyridine due to the partial destruction of side pockets upon dealumination (ii) subsequently, the formation of secondary mesopore system due to further dealumination makes the zeolite crystals completely accessible to relatively bulky molecules such as lutidine, collidine, 2,6-di-tert-butylpyridine. 2004 Published by Elsevier Inc. Keywords: IR spectroscopy of probe molecules; Dealuminated mordenites; Acidity of zeolites; Alkylpyridines
1. Introduction The rational design of efficient heterogeneous catalysts requires a detailed knowledge of the active site(s). In the case of zeolites, much work has been devoted to a detailed description of the active site in its nanoscale (micro-porosity) environment. It has also been repeatedly demonstrated [1–4] that mass transfer limitations play an important role in many of their industrial applications. Creation of controlled mesoporosity has been shown to minimize or even cancel these limitations [5,6]. There is a need and a strong incentive to further improve the active site description of zeolitic catalysts
*
Corresponding author. Fax: +7-95-9328846. E-mail addresses: [email protected] (F. Fajula), iiivanova@ phys.chem.msu.ru (I.I. Ivanova). 1387-1811/$ - see front matter 2004 Published by Elsevier Inc. doi:10.1016/j.micromeso.2004.03.028
and in particular, the accessibility to the active site deserves more attention. FTIR spectroscopy of adsorbed molecules is a wellknown powerful tool to investigate the acid sites of zeolite catalysts, in particular, their nature, number and strength [7–10]. Studies on the location and accessibility of the zeolitic acid sites are available but less widespread. For instance, the distribution of active sites between the external and internal zeolite surface was studied by adsorbing bulky probe molecules, unable to enter zeolitic pores. Corma et al. [11] used 2,6-di-tert-butylpyridine for the investigation of the external surface acidity of MFI and MOR zeolites. This probe molecule, however, was able to penetrate easily into the pore system of 3-dimensional BEA zeolite. The intensity of the N–Hþ stretching band of adsorbed 2,6-di-tert-butylpyridine was shown to correlate with the catalytic activity of
158
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
bulky reactants in cracking. Creyghton et al. used 3,5,di-tert-butylphenyl ether to study the outer surface acidity of MOR and BEA zeolites [12]. Busca et al. selected a range of aliphatic and aromatic compounds as well as acetonitrile and pivalonitrile to discriminate between the external and internal acid sites of MFI zeolite [13,14]. Morterra and co-workers chose for the same purpose acetonitrile and adamantine-carbonitrile [15]. Quinoline was selected in [16] as a suitable probe molecule. The location and accessibility of the active sites inside zeolite pores was investigated by the adsorption of probe molecules of different size. Busca et al. proposed to use branched nitriles such as propionitrile, isobutironitrile and pivalonitrile for the study of location and accessibility of Brønsted and Lewis acid sites in H-MOR [17]. Another approach proposed by Lavalley and coworkers makes use of co-adsorption of Py and CO [18] to distinguish between the acid sites in the main channels and side pockets of MOR. These approaches, using small size probe molecules, were proposed for nondealuminated MOR and can hardly be applied to dealuminated materials, since dealumination creates secondary mesoporosity and changes significantly the accessibility of the sites [18,19]. As far as our knowledge concerned, at present, there is no suitable method to characterize the accessibility of acid sites in dealuminated zeolite materials. The development of such a method is very desired since most commercial catalytic applications of zeolites require variable degrees of dealumination of these materials [6,20,21]. The present work is aimed at the development of such a methodology. The method proposed is based on probing the accessibility of acid sites by alkylpyridines with various sizes and monitoring the nature and strength of the sites not-accessible to alkylpyridines by subsequent CO adsorption. The small-port mordenite with different degree of dealumination was used as model catalyst [22]. The later is an ideal model to study such effects due to the following reasons: It involves acid sites located in two different environments, namely, in often reside small cavities (5.7 · 2.6 and 3.7 · 4.8 A) ferred to as side-pockets and in larger linear channels
In addition, structural defects characteris(6.7 · 7.0 A). tic for small-port MOR impose additional limitations for sites accessibility. Finally, earlier work by Raatz et al. [23,24] has shown that it is possible to ‘‘open-up’’ the porosity of the small-port MOR after removal of ca 20% of framework aluminum by combining calcination and acid leaching treatments.
2. Experimental 2.1. Catalysts The starting mordenites, GP180 and ZM510, were supplied by Societe Chimique de la Grande Paroisse (GP). GP180 is a parent synthetic zeolite in H-form with Si/Al ¼ 5.5, while ZM510 is a dealuminated mordenite with Si/Al ¼ 11 obtained from GP180 by deep-bed calcination of the ammonium form of SP mordenite followed by acid leaching [24,25]. GP180 is assigned as SP and ZM510 as DeAlSP. DeAlSP was further dealuminated by treatment with methanesulfonic acid. In the first preparation, it was treated with 2 M acid at 383 K for 4 h in a stirred vessel to give DeAlSP1 sample. In the second treatment, 6 M acid was used (DeAlSP2). In the third preparation, DeAlSP sample was first calcined at 1023 K and than treated with 2 M acid. After treatments, the samples were filtered, washed, dried at 353 K and calcined in a shallow bed at 723 K for 8 h. The conditions of sample preparation are summarized in the Table 1. 2.2. Characterization The elemental analyses of dealuminated mordenites was performed by atomic absorption. The XRD patterns were obtained with a CGR Theta-60 diffractometer, using CuKa radiation. Sorption–desorption isotherms of nitrogen were recorded at 77 K using an automated porosimeter (Micromeritics Asap 2000). Micro-pore volumes were determined using t-plot method. The total sorbed volumes, including adsorption in the micro-pores and mesopores and on the external surface, were calculated from the amount of nitrogen
Table 1 Characteristics of dealuminated mordenites Samples
Preparation
Si/Al
SP DeAlSP DeAlSP1 DeAlSP2 DeAlSP3
As obtained from GP As obtained from GP Treatment of DeAlSP with 2 M MSa Treatment of DeAlSP with 6 M MSa Calcination of DeAlSP followed by treatment with 2 M MSa
5.5 11 12 14 18
Pore volume (cm3 /g) Vmico
a
MS––methanesulfonic acid.
0.160 0.178 0.179 0.180 0.139
Vmeso 0.014 0.025 0.049 0.061 0.101
Vmicro =Vtotal (%) Vtotal 0.174 0.203 0.228 0.241 0.240
92 88 79 75 58
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
adsorbed at relative pressure p=p0 of 0.96, before the onset of interparticle condensation. The acidic properties were studied by temperatureprogrammed desorption of ammonia (NH3 -TPD), and IR spectroscopy of adsorbed probe molecules. The nature of the acidic sites was also characterized by 1 H MAS NMR and 1 H–{27 Al} TRAPDOR [26,27] NMR experiments. NH3 -TPD was performed in a homemade set-up equipped with TC detector. Prior to NH3 adsorption, the samples were calcined in situ in a flow of dry air at 823 K for 1 h and, subsequently, in a flow of dry nitrogen for 1 h and cooled down to ambient temperature. The NH3 adsorption was carried out for 30 min, at room temperature in a flow of NH3 diluted with N2 (1/1). Subsequently, the physisorbed NH3 was removed in a flow of dry He at 373 K for 1 h. Typical TPD experiments were carried out in the temperature range of 295 to 1073 K in a flow of dry He (30 ml/min). The rate of heating was 8 K/min. Solid-state NMR 1 H and 1 H–{27 Al} TRAPDOR were performed on a Bruker Avance 400 spectrometer (m1 H ¼ 400.33 MHz, m27 Al ¼ 104.23 MHz). Prior to the measurements, the samples were dehydrated using the following procedure: approximately 30 mg of the sample was placed in a glass spoon in a homemade equipment attached to a vacuum line. The sample was evacuated, heated with the rate of 2 K/min up to 723 K and maintained at this temperature under 5 · 105 Torr for 5 h. The dehydrated zeolite was than dropped into the 4 mm-ZrO2 NMR rotor, which was closed with classic Kel-F cap under vacuum using the sample equipment. No signal attributed to residual water has been observed on 1 H MAS NMR spectrum of the sample prepared using the above procedure. The 1 H NMR spectra were recorded using a rotor-synchronized Hahn Echo pulse sequence (p=2–s–p–s-acquire) to avoid the signal from the probe head. The spinning rate was fixed at 10 kHz. The p=2 pulse length was 5.2 ls and 64 scans were accumulated with a 10 s recycle delay. Chemical shifts were referenced relative to tetramethylsilane. In the 1 H– {27 Al} TRAPDOR experiment, the same pulse sequence and conditions were applied to the 1 H channel, while a continuous RF irradiation was applied to the aluminum (27 Al) channel during the first s period. The on-resonance 27 Al irradiation field was set to about 125 kHz for six rotor periods (s ¼ 600 ls). All the spectra have been decomposed to retrieve isotropic chemical shifts and intensity using the Dmfit program [28]. IR spectra were recorded with a Nicolet Magna 550FT-IR spectrometer at 2 cm1 optical resolution, with one level of zero-filling for the Fourier transform. Prior to the measurements, the catalysts were pressed in selfsupporting discs (diameter: 1.6 cm, 7 mg cm2 ) and activated in the IR cell (attached to a vacuum line) at 723 K for 4 h up to 106 Torr. The IR cell described earlier [29] for low-temperature studies was equipped
159
with ZnSe inner windows combined with KBr external windows, which enabled to register spectra in the spectral region down to 600 cm1 . The pressure of the adsorbed gases was measured by two Barocel gauges, the most precise one was attached directly to the samplecontaining compartment of the cell. Another one enabled us to measure a dose of gas in the known volume before admitting it into the cell. The sample temperature during the treatment or recording of spectra was monitored by a chromel–alumel thermocouple inserted into the heater or into the coolant compartment of the cell. For spectra recorded at low-temperature, about 0.5 Torr of He was added into the cell to achieve better thermal contact of the pellet with the inner part of the cell. Adsorption of substituted pyridines: pyridine (Py), lutidine (Lu), collidine (Coll), di-tert-butylpyridine (DTBuPy) was performed at 423 K. The excess of probe molecules was further evacuated at 423 K. The adsorption–evacuation was repeated several times until no changes in the spectra were observed. After adsorption of the substituted pyridines, adsorption of CO was performed at 100 K.
3. Results and discussions 3.1. Characterization of the model catalysts The main characteristics of the parent (SP) and dealuminated (DeAlSP, DeAlSP1, DeAlSP2, DeAlSP3) zeolites are presented in Table 1. The stepwise dealumination results in a gradual increase of the Si/Al ratio from 5.5 to 18. The MOR structure remains intact in all the samples as evidenced by XRD data. On the contrary, the texture of the samples changes significantly upon dealumination. While the parent zeolite, SP, shows a reversible type-I isotherm, typical of a micro-porous solid, the isotherms of the four other samples exhibit a hysteresis loop spreading over the whole range of relative pressures above p=p0 ¼ 0:42, indicating the presence of mesopores with a broad distribution of sizes [30]. The pore volumes of the various samples are reported in Table 1 and point to the following changes occurring upon dealumination: • The first dealumination step (transformation of SP into DeAlSP) leads to an increase of both, microand mesopore volumes. • Further dealumination with increasingly concentrated methanesulfonic acid shows a gradual increase of the mesopore volume, while micro-pore volume remains barely constant. • Combination of heating at 1023 K with an acid attack leads to an increase of the mesopore volume together with a significant decrease of the micro-pore volume.
160
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
As summarized in the last column of Table 1, the increase of the degree of dealumination leads to materials with gradually increasing contribution of mesoporosity. The number of acidic sites in the samples, measured by TPD of NH3 is given in Table 2. As expected, it decreases with the increase of Si/Al ratio upon dealumination. 3.2. Nature and strength of acid sites The nature and strength of OH-groups over dealuminated samples were studied by IR and 1 H MAS NMR spectroscopies. Results obtained by both techniques are reported in Fig. 1 and Table 2. The bridging hydroxyl groups are characterized by an IR band at 3614 cm1 in the m(OH) region and a NMR line at 4.1 ppm. Two types of silanol groups are identified: (i) external: IR band at 3745 cm1 and 1 H NMR line at 1.9 ppm and (ii) internal: IR band at 3734 cm1 [31] and NMR line at 2.1 ppm. Both techniques show that the number of bridging hydroxyl groups decreases significantly with the degree of dealumination. On the contrary, the number of silanol groups increases upon dealumination. While in the starting MOR, external silanol groups give the highest contribution to the spectra, in dealuminated samples the contribution of internal silanol groups increases. It should be mentioned that position and the assignments of the IR bands and NMR lines of bridging hydroxyls and silanol groups are in full agreement with literature data [31–34].
The attribution of the IR band at 3660 cm1 and the corresponding 1 H NMR line at 2.8 ppm deserves some extra comments, since there is no agreement on their nature in the literature. Indeed, several assignments have been proposed for the IR bands of dealuminated zeolites in the region of 3650–3700 cm1 . • Hydroxyl groups of extraframework aluminous species [15,16,31,35]. • Al–OH groups partially bounded to the zeolite lattice [32,33,36]. • Framework Si–OH groups in defect sites of zeolites [37,38]. Tsyganenko et al. [39] observed a band at 3660 cm1 upon growing successive layers of alumina on silica. This band was shown to interact with CO by giving a CO–OH3660 complex confirmed by the appearance of a m(OH) band at 3460 cm1 and a m(CO) vibration at 2174–2165 cm1 . These features were not seen neither in pure silica, nor in alumina, and the band at 3660 cm1 was attributed by the authors to bridging hydroxyls similar to those observed in amorphous silica–alumina. Finally, in [18], it was shown that if activation was not carried out carefully, this band corresponds to water adsorbed on acidic OH groups. In the present study, the line at 3660 cm1 could hardly be attributed to adsorbed water, since the other characteristic bands of water were not observed in the spectral region of 1600–1650 cm1 . The CO adsorption on the OH groups corresponding to the band at 3660
Table 2 Amount, nature and strength of acid sites in dealuminated mordenites Samples
Amount of acid sites (TPDNH3 ) (lm/g)
Type
mOH (cm )
dH , (ppm)
SP
1188
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 – (3780) 3614
DeAlSP
1040
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
DeAlSP1
1105
DeAlSP2
DeAlSP3
Nature of sites
B=L (Py)
Acidic strength DmOH (CO) (cm1 )
1.9 – (2.6) 4.0
3.7
311
3745 3734 3660 3609
1.9 – 2.8 4.1
2.0
326
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 3734 3660 3607
1.9 2.1 2.8 4.1
1.9
330
908
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 – – 3607
1.9 2.0 2.8 4.2
0.9
332
492
SiOHext SiOHint Si(OH)Alextr Si(OH)Alfr
3745 3739 – 3605
1.9 2.0 – 4.2
0.7
332
1
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
161
Fig. 1. IR (a) and 1 H MAS NMR (b) spectra obtained over MOR samples. Thick lines correspond to 1 H MAS NMR spectra, while thin lines show 1 H–{27 Al} TRAPDOR NMR spectra. (The spectra are normalized to the masse of the samples.)
cm1 results in the appearance of the bands at 3460 cm1 and around 2169–2174 cm1 indicating that, in our case, the 3660 cm1 band could be attributed to OH groups bounded to extraframework silica–alumina species [36]. In order to confirm between these assignments, we acquired a 1 H–{27 Al} TRAPDOR spectra of all the samples. In this experiment, a continuous RF irradiation is applied at the 27 Al frequency during observation of the 1 H signal intensity. The signal of proton, strongly coupled with aluminum atoms is thus significantly diminished or suppressed, while the signal of non-coupled proton remains unaffected. Fig. 1b (gray traces) shows the 1 H–{27 Al} TRAPDOR NMR spectra for all samples. As expected, the unaffected resonances are the silanol species at 1.9 and 2.1 ppm. On the contrary, we observe an important decrease of both 2.6 and 4.0 ppm resonances, indicating unambiguously that these hydroxyl groups are connected to aluminum. The broad 1 H NMR line at 2.6 ppm (Fig. 1b––SP), shows that OH groups corresponding to aluminumcontaining extraframework species are already present in small amounts in the starting MOR. This is likely due to occlusion of an amorphous phase created during the synthesis or to structural damage that occurred during the calcination of the ammonium precursor. Further dealumination, significantly increases the relative number of hydroxyl groups associated with extraframework species (Fig. 1b––DeAlSP). It should be mentioned that the shift to 2.8 ppm of the NMR line corresponding to the extraframework hydroxyls may indicate the formation of another type of extraframework species upon
dealumination. Treatment of DeAlSP with methanesulfonic acid of different concentrations (DeAlSP1 and DeAlSP2) induces a decrease of the contribution of the extraframework hydroxyls in both sets of spectra. Finally, the combined treatment including calcination at high temperature and acid leaching, leads to a material virtually free of extraframework species. Moreover, we only observe external silanol groups indicating that the structure is completely open. (Fig. 1––DeAlSP3). The IR spectra recorded after adsorption of Py in spectral region of the 8a,b and 19a,b ring vibrations of pyridine show bands due to protonated (1545, 1633– 1637 cm1 ) and coordinatively adsorbed pyridine (1454, 1620–1622 cm1 ) on all the samples [40,41]. The ratio of the concentrations of Brønsted to Lewis acid sites (B=L), based on published extinction coefficients of the bands at 1545 and 1454 cm1 [18], are given in Table 2. The results suggested that the first dealumination (DeAlSP1) leads to a significant increase of the intensities of the bands corresponding to both Brønsted and Lewis sites, indicating a higher accessibility of Brønsted sites to Py and the formation of additional Lewis sites in such dealuminated materials. These results are in agreement with the model presented by Raatz et al. [23,24]. Further treatments lead to a decrease of the number of both Lewis and Brønsted sites. The B=L ratio also decreases gradually. This ratio is lower than 1 for DeAlSP3, which is free of extraframework species, indicating that a significant contribution to Lewis acidity arises from framework electron deficient sites, as it was shown previously [42].
162
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
The strength of the acid sites was studied by IR spectroscopy of adsorbed CO (Table 2). Carbon monoxide is a well-established probe molecule for the characterization of the acidity of solids. Its adsorption at low-temperature (100 K) on oxide material leads to the formation of H-bonds on Brønsted acid sites. Upon forming this H-bond, the m(OH) vibration band is perturbed and shifted to lower wavenumbers. The corresponding Dm(OH) shift indicates the strength of the H-bond and thus the strength of the acid site. The m(CO) vibration band of adsorbed CO is also shifted (but to higher wave numbers) from its pseudo-liquid frequency at 2138 cm1 , and its shift corresponds to the strength of the perturbation. Adsorption on Lewis sites or on cations leads to specific m(CO) vibration frequencies, which can be used for identification of the adsorption site. Using CO as a probe molecule brings additional advantage, since its small size allows easily access most of the sites in the framework [43]. The results summarized in Table 2 show that the acid strength of the sites gradually increases upon dealumination. To summarize, the overall surface and textural properties of the series of dealuminated MOR prepared for accessibility studies are characterized by: • a gradual increase of Si/Al ratio and pore volumes, • a decrease of the total amount of acid sites (including bridging and extraframework hydroxyls), • an increase of the number of silanol groups, • an increase of the contribution of Lewis acidity. 3.3. Accessibility of acid sites in dealuminated MOR The methodology proposed for the study of the accessibility is based on the interaction of probe mole lutidine cules of different size, namely, pyridine (5.7 A), collidine (7.4 A), di-tert-butylpyridine (7.9 A) (6.7 A), with OH groups of MOR. The changes upon adsorption of these molecules are monitored by IR spectroscopy and the sites not accessible to the above mentioned
probe molecules are further studied by IR spectroscopy of adsorbed CO. The CO adsorption on neat samples is also investigated for comparison. By increasing the size of the probe-molecules, a gradual liberation of the acid sites available for CO adsorption occurs and affords a stepwise characterization of the nature and strength of the sites of decreasing accessibility. This gives a detailed map of the nature and strength of acid sites of different accessibility. The results obtained over a series of model dealuminated MOR are presented in Figs. 2–6. The IR spectra of OH groups recorded before and after adsorption of alkylpyridines are displayed on the left part of Figs. 2–6. In the right part, the difference spectra observed after subsequent CO adsorption are presented both for the m(OH) and m(CO) spectral regions. Pyridine adsorption over the parent (SP) shows only a partial disappearance of the band at 3614 cm1 , corresponding to bridging hydroxyls (Fig. 2a). The amount of sites not-accessible to pyridine, estimated from the change of the band intensity upon Py adsorption, corresponds to about 32%. As it was suggested previously [44], these sites are most probably located in the side pockets of the MOR structure. External silanol groups are only slightly disturbed by Py adsorption, probably due to their low acidity. Upon subsequent CO adsorption on the sample pre-loaded with Py the bridging hydroxyls band (3610 cm1 ) disappears (Fig. 3b). Simultaneously, the band at 3316 cm1 , corresponding to the CO–OH3614 complex appears in the m(OH) region. This later band is shifted to higher wave numbers with respect to the parent MOR without Py pre-adsorption. This suggests that nonaccessible sites are weaker than those accessible to Py in agreement with previous work [45]. In the m(CO) region, three bands are observed: • 2137 cm1 , attributed to pseudo-liquid CO [18,43,45], • 2165 and 2174 cm1 , assigned to CO interaction with bridging OH groups [18,43,45].
Fig. 2. IR spectra obtained on the SP mordenite before and after adsorption of Py (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
163
Fig. 3. IR spectra obtained on the DeAlSP mordenite before and after adsorption of Py (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Fig. 4. IR spectra obtained on the DeAlSP1 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Fig. 5. IR spectra obtained on the DeAlSP2 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Adsorption of Py causes a significant decrease of the 2174 cm1 band, while the band at 2165 cm1 remains practically intact, suggesting that it corresponds to the less accessible sites in the side pockets. The results point that two sites can be distinguished:
strong bridging hydroxyls accessible to Py and located in the main channels and weaker bridging hydroxyls not accessible to Py and located in side pockets. These results agree with previous studies [36,45].
164
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
Fig. 6. IR spectra obtained on the DeAlSP3 mordenite before and after adsorption of various alkylpyridines (a) and the difference IR spectra recorded after subsequent CO adsorption (b).
Upon dealumination of the parent MOR by successive deep-bed calcination and acid leaching [24,25] (DeAlSP) the amount of bridging hydroxyls decreases and OH groups belonging to extraframework silica– alumina species appear (Fig. 3a). It is interesting to note that the band characteristic for CO interaction with bridging hydroxyls in side pockets (2165 cm1 ) is no longer resolved after dealumination (Fig. 3b). The infrared specific feature of the side pocket tends to disappear leading to only one spectral feature for both OH groups. The OH groups of the side pockets are thus loosing their specificity. Meanwhile, the accessibility of bridging hydroxyls to Py increases significantly: more than 80% of sites interact with Py in this sample. These observations suggest that the restrictions limiting access to the side pockets are suppressed upon dealumination. Extraframework OH groups are not accessible to Py, however, they are accessible to CO and form a (CO– OH) complex characterized by the bands at 2169 cm1 in the m(CO) region and at 3640 cm1 in the m(OH) region (Fig. 3b), indicating a low acidity of the associated hydroxyl. The amount of silanol groups also increases upon dealumination. This is confirmed by the increase of the intensity of the band at 3738 cm1 and the appearance of a new band at 2157 cm1 corresponding to CO interacting with silanol groups. Washing the DeAlSP sample with methanesulfonic acid leads to a full accessibility of the bridging hydroxyls to Py (Fig. 4). The accessibility of extraframework OH groups also increases, since no band corresponding to OH–CO complexes is observed after subsequent CO adsorption. The only band observed in the m(CO) region, besides that due to pseudo-liquid CO, corresponds to interactions of CO with silanol groups. Adsorption of a bulkier probe molecule (Lu) shows only a partial disappearance of bridging and extraframework hydroxyls, as confirmed by the spectra in Fig. 4. The results obtained on the DeAlSP1 are consistent with an increase of the volume of the secondary mesoporous
network confirmed by N2 adsorption––desorption isotherms. Further dealumination with 6 M methanesulfonic acid (DeAlSP2) results in a decrease of the amount of extraframework hydroxyls and in a greater accessibility to bulky probe molecules. Thus, bridging hydroxyls become fully accessible to Py and Lu, while extraframework species are fully accessible to Py. Both of these groups are, however, only partially accessible to the more bulky probe Coll as observed in Fig. 5. Washing with more concentrated acid allows for removal of extraframework species and therefore yields a more open structure. Finally, a combination of heating at 1023 K with an acid attack (DeAlSP3) leads to a drastic decrease of the intensity of the bands characteristic of bridging and extraframework hydroxyls and to an increase of the intensity of the signal due to silanol groups (Fig. 6). This brings a further opening of the porous structure: bridging hydroxyls become accessible even to the bulky 2,6-DTBuPy and extraframework sites to Collidine. Thus, a combination of heating at 1023 K with acid attack leads to the most open structure, in which all structural and extraframework acid sites become accessible to molecules with a kinetic diameter of 6.4 and 6.7 respectively. A, Fig. 7 summarizes the above results and gives the distribution of Brønsted acid sites with different accessibility over various samples studied. In the starting material, 30% of the sites are accessible only for CO and therefore are located in side pockets, while 70% are located in main channels and accessible to Py. Upon the first dealumination, the contribution of the sites located in side pockets decreases probably due to the partial destruction of side pockets and the contribution of sites accessible to Py increases up to 80%. Further dealumination makes a part of the sites available to larger probe molecules, suggesting that they are looking into larger pores, formed upon dealumination. The
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
progress to investigate other zeolites, such as the more commercially relevant ‘‘Large-Port’’ MOR and to establish the relationship with catalysts performance in different reactions.
100 80
85
68
80 60
58 60
32 30
33
40
20 10
20 11 0
165
3
4
6
SP DeAlSP DeAlSP1 DeAlSP2 DeAlSP3
N
CO
Acknowledgements The financial support by the NATO StP program is gratefully acknowledged. N.S. Nesterenko, V.V. Yuschenko and I.I. Ivanova are grateful to RFBR (project 02-03-32516), Council for the Grants of Russian President (project NSh-1275.2003.3) and Science support foundation for the financial support. N.S. Nesterenko thanks Haldor Topsoe A/S for a Ph.D. fellowship.
Fig. 7. Distribution of Brønsted acid sites of different accessibility over mordenite samples studied.
References higher the degree of dealumination, the larger are the pores accommodating Brønsted acid sites. The strength of the sites located in larger pores is higher with respect to those located in smaller pores, as evidenced by higher shifts of the bands corresponding to CO–OH complexes observed in m(OH) region of IR spectra.
4. Conclusions A novel methodology to characterize and quantify the acidic sites of varying accessibility has been developed. It was illustrated for the special case of the smallport MOR structure but is of course applicable to other zeolitic catalysts. The method is based on the probing of acid sites accessibility by IR spectroscopy of adsorbed alkylpyridines, followed by the characterization of the non-accessible sites by IR spectroscopy of adsorbed CO. The gradual increase of the size of alkylpyridine probe molecules in a series of experiments allows a step-bystep characterization of the nature and strength of sites with different accessibility. It has been demonstrated and quantified that upon dealumination of the small-port MOR, two phenomena related to accessibility occur: (i) a full accessibility of the structural micro-pores and (ii) subsequently, an opening up of the secondary mesopore system making the zeolite crystals completely accessible to relatively bulky molecules. This type of quantitative characterization is important since most of the industrial applications of zeolites are generally running under mass transfer limitations. The design of efficient industrial catalysts requires an a priori knowledge of the active site and its location at the micro- and mesoscale. Further work is presently in
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15]
[16] [17] [18] [19] [20]
[21] [22] [23] [24]
P.B. Weisz, V.J. Frilette, J. Phys. Chem. 64 (1960) 382. P.B. Weisz, ChemTech 3 (1973) 498. P.B. Weisz, V.J. Frilette, J. Catal. 1 (1962) 307. N.Y. Chen, C.M. Smith, Molecular Transport and Reaction in Zeolite, VCH, New York, 1994. T.C. Tsai, Sh.B. Liu, I. Wang, Appl. Catal. A: Gen. 181 (2) (1999) 355. C. Song, Acad. Sci. Paris, Ser. IIc, Chem. 3 (2000) 477. J. Lercher, G. Gabriele, Catal. Today 27 (1996) 353. G. Busca, Catal. Today 27 (1996) 323. J. Ryczkowski, Catal. Today 68 (2001) 263. A. Zecchina, G. Ricchiardi, S. Bordiga, F. Bonino, C. Prestipino, C. Lamberti, Stud. Surf. Sci. Catal. 142 (2002) 3. A. Corma, V. Fornes, L. Forni, F. Marquez, J. MartinezTriguero, D. Moscotti, J. Catal. 179 (2) (1998) 451. E.J. Creyghton, J.A. Elings, R.S. Downing, R.A. Sheldon, H. van Bekkum, Micropor. Mater. 5 (1996) 299. T. Armaroli, M. Bevilacqua, M. Trombetta, F. Milella, A. Gutierrez, J. Ramirez, B. Notari, J. Ronald, G. Busca, Appl. Catal. A: Gen. 216 (1–2) (2001) 59. M. Trombetta, T. Armarolli, G. Busca, Appl. Catal. A: Gen. 192 (1) (2000) 125. C. Arean Otero, E. Escalona Platero, M. Penarroya Mentruit, M. Rodriguez Delgado, F.X. Llabres i Xamena, A. GarciaRaso, C. Morterra, Micropor. Mesopor. Mater. 34 (1) (2000) 55. A. Corma, V. Fornes, F. Rey, Zeolites 13 (1993) 56. M. Bevilacqua, G. Busca, Catal. Commun. 3 (2002) 497. M. Maache, A. Janin, J.C. Lavalley, E. Benazzi, Zeolites 15 (6) (1995) 507. R. Dutartre, L.Ch. de Menorval, F. Di Renzo, D. McQueen, F. Fajula, Ph. Schulz, Micropor. Mater. 6 (1996) 311. Y. Sugi, S. Tawada, T. Sugimura, Y. Kubota, T. Hanaoka, T. Matsuzaki, K. Nakajima, K. Kunimori, Appl. Catal. A: Gen. 189 (2) (1999) 251. C. Perego, S. Amarilli, R. Millini, R.G. Bellussi, G. Girotti, G. Terzoni, Micropor. Mater. 6 (1996) 395. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974. F. Raatz, C. Marcilly, E. Freund, Zeolites 5 (1985) 5329. F. Raatz, C. Marcilly, E. Freund, J. Chem. Soc. Faraday Trans. 79 (1) (1983) 2299.
166
N.S. Nesterenko et al. / Microporous and Mesoporous Materials 71 (2004) 157–166
[25] E. Freund, Ch. Marcilly, F. Raatz, J. Chem. Soc., Chem. Commun. (1982) 309. [26] C.P. Grey, A.J. Vega, JACS 117 (1995) 8232. [27] F. Deng, Y. Yue, Y. Chaohui, Solid State Nucl. Magn. Reson. 10 (1998) 151. [28] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan, G. Hoatson, Magn. Reson. Chem. 40 (2002) 70. [29] F. Mauge, J. Lamotte, N.S. Nesterenko, O. Manoilova, A.A. Tsyganenko, Catal. Today 70 (1–3) (2001) 271. [30] K.S.W. Sing, S.J. Greg, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [31] F. Thibault-Starzyk, A. Vimont, J.P. Gilson, Catal. Today 70 (1– 3) (2001) 227. [32] R. Anquetil, J. Saussey, J.C. Lavalley, Phys. Chem. Chem. Phys. 1 (4) (1999) 555. [33] A. Vimont, F. Thibault-Starzyk, J.C. Lavalley, J. Phys. Chem. B 104 (2000) 286. [34] M. Muller, G. Harvey, R. Prins, Micropor. Mesopor. Mater. 34 (3) (2000) 281.
[35] G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723. [36] M. Maache, A. Janin, J.C. Lavalley, J.F. Joly, E. Benazzi, Zeolites 13 (1993) 419. [37] P.A. Jacobs, J.B. Uytterhoeven, J. Chem. Soc. Faraday Trans. I 69 (1973) 373. [38] E. Loeffler, U. Lohse, C. Penker, G. Oehlmann, L.M. Kustov, V.L. Zholobenko, V.B. Kazansky, Zeolites 10 (1990) 266. [39] A.A. Tsyganenko, O.V. Manoilova, K.M. Bulanin, P.Yu. Storozhev, S. Haukka, A. Palukka, M. Lindblad, Stud. Surf. Sci. Catal. 130 (2000) 3149. [40] A.G. Secerdia, C. Morterra, M.J. Low, J. Colloid Interf. Sci. 99 (1984) 208. [41] E.P. Parry, J. Catal. 2 (1963) 371. [42] D. McQueen, B.H. Chiche, F. Fajula, A. Auroux, C. Guimon, F. Fitoussi, P. Schulz, J. Catal. 161 (2) (1996) 587. [43] R. Shigeishi, B.H. Chiche, F. Fajula, Micropor. Mesopor. Mater. 43 (2) (2001) 211. [44] J. Datka, B. Gil, A. Kubacka, Zeolites 18 (4) (1997) 245. [45] O. Marie, F. Thibault-Starzyk, J.C. Lavalley, Phys. Chem. Chem. Phys. 2 (22) (2000) 5341.