J. Phys. Chem. B 2006, 110, 6763-6767
6763
Influence of V Content on the Nature and Strength of Acidic Sites in VSiβ Zeolite Evidenced by IR Spectroscopy K. Go´ ra-Marek,† J. Datka,*,† S. Dzwigaj,*,‡ and M. Che‡,§ Faculty of Chemistry, Jagiellonian UniVersity, 30-060 Cracow, Ingardena 3, Poland Laboratoire de Re´ actiVite´ de Surface, UMR 7609-CNRS, UniVersite´ Pierre et Marie Curie, 4, Place Jussieu, 75252 Paris Cedex 05, France, and Institut UniVersitaire de France ReceiVed: NoVember 1, 2005; In Final Form: January 17, 2006
VSiβ zeolites prepared by a two-step postsynthesis method have been characterized by physical techniques. A significant reduction of intensity of the IR band near 3515 cm-1 after impregnation of dealuminated β zeolite with aqueous NH4VO3 indicates that V ions specifically react with hydrogen-bonded SiO-H groups of vacant T sites. IR bands at 3618 and 3645 cm-1 are assigned to SiO-H groups interacting with V and to VO-H groups, respectively. In VSiβ, diffuse reflectance UV-visible data show that below 1.9 wt % V is present as lattice tetrahedral species and at higher content as extra-lattice octahedral species (mononuclear and polynuclear). VSiβ samples are EPR-silent at 298 or 77 K suggesting that there are no paramagnetic VIV ions. IR studies show that V-OH groups are less acidic than Si-OH-Al groups of parent HAlβ zeolite. IR results of CO adsorption evidence three kinds of Lewis acidic sites, related to lattice mononuclear and extralattice mononuclear and polynuclear V species. Quantitative IR studies of ammonia and pyridine adsorption reveal that only about half of V introduced into zeolite is able to form either Brønsted or Lewis acidic sites.
Introduction Silica-supported vanadia catalysts have been widely used in selective oxidation of hydrocarbons1 with high selectivity to olefins for the oxidative dehydrogenation (ODH) of short chain paraffins.2-5 Recently, V-containing microporous materials have been proposed as alternative catalysts.6,7 Isolated tetrahedral VV species have been shown to be responsible for the high selectivity to alkenes for ODH of paraffins. For V2O5/SiO2 catalysts,8 the selectivity drastically decreases above 1.9 V wt %. Such behavior has been related to the change of structure or nature of supported vanadium species. It was suggested9-13 that isolated tetrahedral V species are predominant at low V loading (below 1.9 wt %), whereas at higher loading, octahedral V species and polyvanadate V species, containing V-O-Vgroups, are also present on the silica surface. To study the effect of V content on the nature of acidic centers, we prepared a series of VSiβ samples with various V contents by a two-step postsynthesis method which consisted first of creating vacant T-sites by dealumination of β zeolite with nitric acid and then impregnating the resulting Siβ zeolite with aqueous NH4VO3 solution, used as VV precursor.9,10 VSiβ samples were characterized at the macroscopic (chemical analysis, BET, XRD) and molecular (IR, DR (diffuse reflectance) UV-visible and EPR) levels. Experimental Section Materials. A tetraethylammonium beta (TEAβ) zeolite (Si/ Al ) 11) from RIPP (China) was dealuminated by treatment * Corresponding authors. (S.D.) E-mail:
[email protected]. Fax: 33 1 44 27 60 33; (J.D.) E-mail:
[email protected]. Fax. 48 12 6340515. † Jagiellonian University. ‡ Universite ´ Pierre et Marie Curie. § Institut Universitaire de France.
with a 13 mol L-1 HNO3 solution for 4 h at 353 K under stirring, as reported earlier.10,11,14 The resulting dealuminated Siβ zeolite (Si/Al > 1300) was recovered by centrifugation, washed with distilled water, and dried overnight at 353 K. This material was impregnated with an aqueous solution of ammonium metavanadate (2.3 g of zeolite in 10 mL of solution with different concentrations of NH4VO3 varied from 0.6 × 10-3 to 2.6 × 10-3 mol L-1).10,11 Because of its concentration and pH (10-3 mol L-1 and 2.4, respectively), the aqueous NH4VO3 solution is known to contain mononuclear VO2+ ions.15 The suspension obtained was left standing for 3 days at room temperature. The samples obtained after centrifugation and drying at 353 K overnight contained 0.45, 1.27, 1.96, and 2.05 V wt % and were labeled V0.45Siβ, V1.27Siβ, V1.96Siβ, and V2.05Siβ, respectively. The V0.45Siβ and V1.27Siβ samples were white suggesting the presence of tetrahedral VV only.14 In contrast, V1.96Siβ and V2.05Siβ samples were bright and pale yellow, respectively, suggesting the presence of octahedral (mononuclear and/or polynuclear) V species.14 Techniques. Nitrogen adsorption-desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2010 apparatus. The samples were initially outgassed at 298 then at 623 K to a pressure < 0.2 Pa. Pore size distributions were obtained by a single point t-plot procedure and microporous volumes were calculated using Barrett, Joyner and Halenda’s equation.16 Powder X-ray diffractograms were recorded on a Siemens D5000 using the Cu KR radiation (λ ) 154.05 pm). Electron paramagnetic resonance (EPR) spectra were recorded at 298 and 77 K with a computerized BRUKER ESP 300 spectrometer at 9.3 GHz (X band) and with a 100 kHz field modulation and a 10 G modulation amplitude. Diffuse reflectance UV-visible (DR UV-visible) spectra were recorded at 298 K with a Cary 5E spectrometer equipped with an integrator and a double monochromator. The parent V-free materials were used as references.
10.1021/jp0582890 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/16/2006
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TABLE 1: Concentration and Strength of Brønsted (B) and Lewis (L) Acidic Sites Determined by Pyridine and Ammonia Adsorption concentration pyridine acidic sites/u.c.b
strength NH3 acidic sites/u.c.b
pyridine A570/A0c
NH3 A490/A0d
sample
V/u.c.a
B
L
B+L
B
L
B+L
B
L
B
L
V0.45Si β V1.27Si β V1.96Si β V2.05Si β HAl β (Si/Al ) 11) Si β
0.38 1.07 1.65 1.73 5.3 Al/u.c. 0
0.05 0.19 0.24 0.24 2.8
0.16 0.20 0.31 0.55 1.2
0.21 0.39 0.55 0.79 4.0
0.05 0.17 0.24 0.27 3.0
0.16 0.29 0.48 0.61 1.1
0.21 0.46 0.72 0.88 4.1
0.86 0.81 0.40 0.35 0.90
0.56 0.60 0.69 0.65 0.90
0.70 0.70 0.40 0.40 0.90
0.70 0.79 0.81 0.75 1.0
0
0
0
0
0
0
a
b
Number of V atoms per unit cell (u.c.) of β zeolite. Number of acidic sites per unit cell of β zeolite. c Fraction of pyridine bonded to Brønsted or Lewis acidic sites remaining after desorption at 570 K. d Fraction of ammonia bonded to Brønsted or Lewis acidic sites remaining after desorption at 490 K.
Transmission IR spectra of self-supported wafers (5-8 mg/ cm2) were recorded at 170 or 440 K with an Equinox 55 Bruker spectrometer equipped with an MCT detector with a resolution of 2 cm-1 and normalized to 10 mg of sample. Before measurements, the wafers were evacuated in a vacuum (10-4 Pa) at 723 K for 1 h. CO was adsorbed at 170 K on Siβ and VSiβ samples pretreated (10 K/min) at 773 K for 1 h in a vacuum (10-4 Pa) and further cooled to 170 K. The IR spectra were then recorded at this temperature after adsorption of severals doses of CO up to saturation of all Lewis sites. The nature, number, and strength of V acidic sites were determined by IR using pyridine, ammonia and carbon monoxide as probe molecules. The concentrations of Brønsted and Lewis acidic sites were determined by quantitative IR studies of pyridine and ammonia adsorption. Excess pyridine or ammonia (sufficient to neutralize all acidic sites) were adsorbed at 440 and 320 K, respectively. Physisorbed molecules were subsequently removed by evacuation at the same temperature. The concentrations of Brønsted and Lewis acidic sites were determined from the intensities of bands at 1542 and 1450 cm-1 (due to PyH+ and NH4+ respectively) and at 1448 and 1620 cm-1 (due to PyL and NH3L respectively) after neutralization of all acidic sites, by using the extinction coefficient of the bands. The extinction coefficients were determined from the amounts of pyridine or ammonia adsorbed in H-Mordenite (containing only Brønsted acidic sites)17 or on Al2O3 (containing only surface Lewis acidic sites).18 The plots of band intensity versus the concentration of molecules adsorbed are linear, and the values of extinction coefficients were obtained from the slopes of the lines: 0.070 cm/µmol (PyH+), 0.11 cm/µmol (NH4+), 0.100 cm/µmol (PyL), and 0.026 cm/µmol (NH3L). These values were used to determine the concentrations of Brønsted and Lewis acidic sites for different V contents (Table 1). Results and Discussion Macroscopic Characterization of Samples before and after Introduction of V. Both porosity and crystallinity are preserved after impregnation with NH4VO3 solution, as indicated by similar XRD patterns of Siβ, V0.45Siβ, V1.27Siβ, V1.96Siβ, and V2.05Siβ and by absence of mesoporosity in the pore size histograms (results not shown). There is no XRD evidence for extra lattice crystalline phase or long-range zeolite amorphization. Molecular Characterization of Samples before and after Introduction of V. OH Groups in Siβ and VSiβ Zeolites. The IR spectrum of Siβ outgassed at 723 K for 1 h (10-4 P) (Figure
Figure 1. IR spectra recorded at 170 K of OH groups in Siβ and VSiβ zeolites with various V contents.
1) shows bands at 3735, 3705, and 3515 cm-1, assigned to isolated internal, terminal internal, and hydrogen-bonded SiO-H groups respectively, located in vacant T sites forming hydroxyl nests.10,19 The introduction of V in Siβ induces significant changes (Figure 1). First, the intensity of the broad band near 3515 cm-1, associated with hydrogen bonded silanols groups is significantly reduced, showing that silanol groups are consumed. Second, two additional IR bands at 3645 and 3618 cm-1 appear in dehydrated VSiβ samples (Figure 1), particularly for large V contents (V1.96Siβ and V2.05Siβ samples). Both bands correspond to acidic OH because they disappear upon adsorption of pyridine (result not shown), in line with earlier results.10 The band at about 3650 cm-1, already observed for V supported on titania20 and on titania/silica21 materials has been assigned to VO-H groups.10 The band at about 3620 cm-1 has been attributed to SiO-H groups experiencing strong Si-Oδ-‚‚‚δ+V interactions leading to Brønsted acidic character. Nature and EnVironment of V. VSiβ samples are EPR-silent either at 298 or at 77 K suggesting that there are no VIV paramagnetic ions, consistent with the absence of d-d transitions in DR UV-visible spectra in 600-800 nm range (Figure 2) as reported earlier.22-24 DR UV-visible spectra of samples with low V content (V0.45Siβ and V1.27Siβ) exhibit two main bands at 265 and 340 nm (Figure 2 and Table 2). These bands are assigned to π(t2)
Nature and Strength of Acidic Sites in VSiβ Zeolite
Figure 2. Diffuse reflectance UV-visible spectra recorded at 298 K of V0.45Siβ, V1.27Siβ, V1.96Siβ, and V2.05Siβ zeolites. These spectra have been measured using Siβ zeolite as reference.
f d(e) and to π(t1) f d(e) oxygen-tetrahedral VV charge transfer (CT) transitions, involving bridging (V-O-Si) and terminal (VdO) oxygen, respectively, in agreement with earlier data.10,11,13,25 Samples with higher V content (V1.96Siβ and V2.05Siβ) also exhibit bands at 265 and 340 nm related to tetrahedral mononuclear V species and a third broad band at around 400-480 nm corresponding to octahedral (mononuclear and polynuclear) V species involved in the oxygen-octahedral VV CT transition, in line with earlier data.12,13 The broad band at around 400-480 nm is probably related to vanadium species in extra-lattice position because it (i) increases in intensity with V content (above 1.9 wt %) and (ii) is removed upon washing V1.96Siβ and V2.05Siβ samples with an aqueous NH4OAc solution. Nature, Number, and Strength of Vanadium Acidic Sites. CO Adsorption. CO adsorption experiments give information on OH group acidity in both AlSiβ and VSiβ, as well as on the nature and acid strength of vanadium Lewis acidic sites. It is wellknown, that CO is a suitable molecule to probe Lewis acidic sites.26,27 Difference spectra between IR spectra recorded after and before CO adsorption at 170 K are given in Figure 3 for V1.96Siβ and the parent HAlβ (Si/Al ) 11). Sample V1.96Siβ shows distinctly lower frequency shifts of IR bands due to O-H groups (from 3647 to 3487 cm-1, ∆ν ) 160 cm-1) than the parent HAlβ (from 3616 to 3303 cm-1, ∆ν ) 313 cm-1) indicating that the acid strength of O-H groups is lower in V1.96Siβ than in HAlβ. Analysis of the difference spectrum suggests that CO is hydrogen-bonded to most V-OH groups.18 IR spectra of CO adsorbed at 170 K on Siβ and VSiβ (Figure 4) exhibit narrow and well resolved bands in the 2135-2250 cm-1 range. The spectrum of CO adsorbed on Siβ (Figure 4) shows only bands related to physisorbed CO (∼ 2135 cm-1) and CO
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6765 hydrogen-bonded to silanol groups (∼ 2154 cm-1), in line with an earlier report.18 The introduction of a low V content (V0.45Siβ and V1.27Siβ) leads to a decrease of intensity of the band at ∼2154 cm-1 and to the appearance of two new bands at ∼2157 and 2173 cm-1 likely due to CO bonded to acidic hydroxyls (VO-H groups evidenced by IR spectra (Figure 1)) and to Lewis acidic sites (mononuclear V species shown by UV-visible spectra (Figure 2)), respectively. For higher V content (V1.96Siβ and V2.05Siβ) two new bands appear at 2192 and 2001-2205 cm-1 which can be related to the presence of two kinds of Lewis sites of different acid strength: the higher the frequency, the stronger the Lewis acidic site. The latter correlation is related to a little antibonding character of the σ molecular orbital of CO. Thus, when this orbital interacts with an empty orbital of the transition metal cation center, the CtO bond is strengthened leading to an increase of stretching frequency.27 The latter depend on the electron-acceptor character of the Lewis acidic site. For zeolites27 and mesoporous aluminosilicates,18 adsorption of CO on extra-lattice Al ions leads to νCO at 2190-2200 cm-1, and on stronger acidic ones, formed by dehydroxylation, to νCO above 2220 cm-1. The presence of only one band related to Lewis acidic sites at 2173 cm-1 for low V content (V0.45Siβ and V1.27Siβ) suggests that lattice tetrahedral mononuclear VV species have the lowest Lewis acid strength. Moreover, the presence of the IR bands at 2192 and 2001-2205 cm-1 for the samples with higher V content (V1.96Siβ and V2.05Siβ) and characterized by the broad UV-visible band at 400-480 nm (Figure 2), suggests that they probably are related to CO adsorbed on extra-lattice mononuclear (band at 415 nm) and polynuclear (containing V-O-V groups) (broad band at 415-480 nm) V species, respectively (Table 2). The higher frequencies of CtO adsorbed on these V species than that adsorbed on tetrahedral mononuclear VV ions suggest that the former are stronger electron-acceptors than the latter and can be associated to Lewis sites of intermediate and high acid strength, respectively. The amounts of both types of Lewis sites increase with V content (Figure 4). Our studies show that lattice and extra-lattice V species can act as Lewis acidic sites and that the acid strength of extralattice V species is higher than that of lattice V species. It is well-known that relatively weak Lewis acidity, intermediate reducibility of transition metal ions and high oxygen mobility are essential for selective ODH of alkanes to occur.28 Our experimental results29 show that isolated tetrahedral VV species present in VSiβ at a low V content are more active in selective ODH of propane to propene than extra-lattice VV species. It seems that the lower Lewis acidity of lattice tetrahedral VV species leads to less favorable adsorption of alkenes and in consequence to higher alkene yields in the products obtained on samples with mononuclear VV species than those obtained on VSiβ with extra-lattice polynuclear VV species. These data suggest that the relative adsorption strength of alkanes and alkenes involved in selective ODH of alkanes can strongly influence the relative reaction rates of these two types of hydrocarbon and that this adsorption effect depends on the Lewis acidity of cations involved in π-bonding of alkenes to VSiβ surface. Pyridine and Ammonia Adsorption. IR spectra of pyridine adsorbed on Siβ and VSiβ are presented in Figure 5. Only very weak bands at 1597 and 1445 cm-1 due to pyridine bonded to Lewis acidic sites are present for Siβ. The introduction of V results in the formation of Brønsted (bands at 1542 and 1637
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TABLE 2: Nature of Acidic Sites (Brønsted (B) or Lewis (L)) Determined by Pyridine, Ammonia, and CO Adsorption, and Type of V Species and O2- f VV Charge Transfer Bands Determined by Diffuse Reflectance UV-Visible Spectroscopy nature of acidic sites determined with: sample and V contenta
pyridine
NH3
CO
V0.45Si β V1.27Si β V1.96Si β
Bb B B
Lc L L
Bd B B
Le L L
Bf B B
L1g L1 L1
L2 h
L3 i
V2.05Si β
B
L
B
L
B
L1
L2
L3
type and nuclearity of V species
O2- f VV charge transfer bands (nm)
Td mono Td mono Td mono and Oh mono Td mono and Oh poly
265j, 340j 265, 340 265, 340 415k 265, 340 415-480l
a The change of behavior of samples due to V content (below or above 1.9 V wt %). b Brønsted (B), IR band of PyH+ at 1542 cm-1. c Lewis (L), IR band of PyL at 1448 cm-1. d Brønsted (B), IR band of NH4+ at 1450 cm-1. e Lewis (L), IR band of NH3L at 1620 cm-1. f Brønsted (B), IR band of CO-H-OV at 2156-2157 cm-1. g Lewis (L1), IR band of OC-VV latticemononuclear at 2173 cm-1. h Lewis (L2), IR band of OC-VV extralatticemononuclear at 2192 cm-1. i Lewis (L3), IR band of OC-VV extra-latticepolynuclear at 2201-2005 cm-1. j Tetrahedral (Td) mononuclear V species. k Octahedral (Oh) mononuclear V species. l Octahedral (Oh) polynuclear V species.
Figure 3. Difference IR spectra recorded at 170 K of OH groups interacting with CO adsorbed on VSiβ and HAlβ. Figure 5. IR spectra recorded at 440 K of pyridine adsorbed on SiB and VSiβ with various V contents.
Figure 4. IR spectra recorded at 170 K of CO adsorbed on Siβ and VSiβ with various V contents.
cm-1) and Lewis (bands at 1445-1448 cm-1 and 1594-1607 cm-1) acidic sites. Moreover, the increase with V content of the frequency of the bands of pyridine bonded to Lewis sites suggests that there is an accompanying increase of the Lewis acid strength of V species. The concentrations of both Brønsted and Lewis acidic sites determined from ammonia adsorption are presented in Table 1. The concentrations of Brønsted sites determined with either ammonia or pyridine are practically the same, whereas pyridine leads to a lower concentration of Lewis sites than ammonia probably because it is more bulky. The concentrations of Brønsted and Lewis sites both increase with V content. The total concentration of Brønsted and Lewis acidic sites (B + L) for each sample is about half that of the vanadium content, indicating that less than half of the vanadium is involved in the formation of either Brønsted and Lewis acidic sites. It suggests that some V ions (probably as non-hydroxylated
(SiO)3VdO sites with structures γ and β22) are incorporated inside the zeolite structure at particular sites (such as S122) where they are poorly accessible to ammonia and pyridine. The acid strength of both Lewis and Brønsted sites was measured by IR experiments of ammonia and pyridine adsorption-desorption. Pyridine and ammonia in excess were adsorbed at 440 and 300 K, respectively, and physisorbed molecules were removed by evacuation at those temperatures. The intensities of bands related to Brønsted and Lewis acidic sites are denoted A0. Adsorbed bases were further desorbed at 490 (ammonia) or 570 K (pyridine) and the intensities of the corresponding bands noted A490 and A570, respectively. The ratios A490/A0 and A570/ A0 thus represent the fraction of ammonia or pyridine bonded to Brønsted and Lewis acidic sites remaining after desorption. These values were taken to measure the acid strength and are given in Table 1. Table 1 shows that the acid strength of Lewis sites increases with V content up to about 1.7 V atoms per unit cell (V/u.c.) of β zeolite. This is well seen for ammonia that can reach more easily the acidic sites than pyridine. The results of ammonia desorption experiments (Table 1) agree also with the results of CO adsorption experiments (Figure 4) which show an increase of the contribution of the bands of CO bonded to stronger sites (2192 and 2205 cm-1), and a decrease of the contribution of weakly acidic sites represented by the band at 2173 cm-1 and also with data presented in Figure 5 (higher frequencies of PyL bands). According to the data of Table 1, the acid strength of Brønsted sites decreases when the amount of V increases, suggesting that the incorporation of the first fractions of V into the lattice
Nature and Strength of Acidic Sites in VSiβ Zeolite produces hydroxyl groups which are more acidic than those created for the latter fractions. Conclusions The aim of this work is to study the effect of V content on the nature and environment of V species, and on the nature, number, and strength of vanadium acidic sites in VSiβ prepared by a two-step postsynthesis method. Upon impregnation in air of Siβ zeolite, prepared by dealumination to generate vacant T sites followed by drying, V is introduced, from an aqueos solution of NH4VO3, as tetrahedral mononuclear VV species at low V content (V0.45Siβ and V1.27Siβ samples) and as extra-lattice octahedral (mononuclear and polynuclear) VV species at higher content (above 1.9 wt. %, V1.96Siβ and V2.05Siβ), as shown by FT-IR, EPR, and DR UV-visible. The IR bands observed at 3618 and 3645 cm-1 correspond to OH groups with acidic character assigned to SiO-H groups interacting with V and to VO-H groups, respectively. Their acid strength is found to be lower than that of Si-OH-Al groups in aluminosilicate zeolites. VSiβ samples do not reveal any EPR signal at 298 or 77 K suggesting the absence of paramagnetic VIV ions. Quantitative IR studies of ammonia and pyridine adsorption reveal that only about half of the V introduced into zeolite is able to form either Brønsted or Lewis acidic sites. According to IR results of CO adsorption, three kinds of Lewis acidic sites of different acid strength can be distinguished in VSiβ zeolites related to framework tetrahedral (mononuclear) and extra-lattice octahedral (mononuclear and polynuclear) V species. Further studies are underway to understand the role of Lewis acidity of VV ions in the selective oxidative dehydrogenation of alkanes to alkenes. Acknowledgment. This study was sponsored by the Ministry of Scientific Research and Information Technology of Poland under Grant No. 4 T09A 184 24. S.D. gratefully acknowledges the CNRS (France) for financial support as Chercheur Associe´. References and Notes (1) Bielanski, A.; Haber, J. Catal. ReV.-Sci. Eng. 1979, 19, 1.
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