Effect of Boron Substitution in Chabazite Framework: IR Studies on the

Effect of Boron Substitution in Chabazite Framework: IR Studies on the Acidity ... Oslo, Norway, and ChevronTexaco Energy Research and Technology Comp...
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J. Phys. Chem. C 2007, 111, 2992-2999

Effect of Boron Substitution in Chabazite Framework: IR Studies on the Acidity Properties and Reactivity Towards Methanol L. Regli,† S. Bordiga,*,† C. Lamberti,† K. P. Lillerud,‡,§ S. I. Zones,| and A. Zecchina† Department of Inorganic, Physical and Materials Chemistry, NIS Centre of Excellence, and Centro di Riferimento INSTM, UniVersity of Turin, Via P. Giuria 7, I-10125, Torino, Italy, Department of Chemistry, UniVersity of Oslo, P.O. Box 1033, N-0315, Oslo, Norway, Centre of Materials Science and Nanotechnology, P.O. Box 1162, N-0318, Oslo, Norway, and CheVronTexaco Energy Research and Technology Company, 100 CheVron Way, Richmond, California 94802 ReceiVed: June 28, 2006; In Final Form: NoVember 20, 2006

The importance of methanol to light olefins (MTO) process in today’s economy, using H-SAPO-34 as a catalyst, has stimulated the investigation of new zeolitic material with the same CHA topology but characterized by a different acidic strength. In this work, we report a detailed IR investigation on the symmetry, accessibility, acidic strength, and reactivity of B sites in a CHA framework. As the prepared material exhibits [B(OSi)4] units in a Td-like geometry, upon template burning, the break of a B-O-Si bond results in [B(OSi)3] units in a D3h-like geometry, testified by the appearance of the IR fingerprint at 1390 cm-1. Interaction with a strong base as H2O allows the Td-like geometry to be reversibly restored. Weak bases, such as H2 and CO, are unable to displace B species from the D3h-like geometry but testify the weak Brønsted acidity of internal OH species. Interaction with CH3OH gives rise to a much more complex spectroscopy, reflecting a rich reactivity of methanol with both [B(OSi)3] units and adjacent SiOH species. This reactivity, discovered in the present IR investigation, justifies on a molecular level the unsuitability of the B-CHA system for the MTO process.

1. Introduction The methanol-to-hydrocarbon (MTH) process represents a possible route for the upgrading of natural gas or coal to higher value products, such as gasoline or small alkenes. The zeolite and zeotype characterized by Chabazite topology have attracted a large interest as they show a specific shape selectivity for converting methanol to light olefins (MTO) without the formation of aromatics.1-4 Norsk Hydro and UOP have developed the MTO variant of the reaction where ethane and propene are the main products formed over a catalyst based on H-SAPO34.5,6 The H-SAPO-34 is a silicoaluminophosphate zeotype (CHA framework) with fairly large cages (about 7 × 10 Å) connected by eight-ring windows (3.4 Å diameter).5,7 The narrow pore openings give rise to products selectivity as only small linear alkenes may diffuse through the apertures and go out of the catalyst crystals. The MTO reaction is carried out at about 650-700 K in a fluidized bed reactor with continuous catalyst regeneration, due to a very fast deactivation made by coke formation. The catalytic process involves a complex series of reactions still not completely disclosed; nevertheless, there is a large consensus about the fact that Brønsted acidity is required and that the products are not obtained from a direct combination of single methanol molecules with a consecutive mechanism.8 Most probably, the reaction develops through a more complex scheme * Corresponding author. Fax: +39011-6707855; e-mail: silvia.bordiga@ unito.it. † University of Turin. ‡ University of Oslo. § Centre of Materials Science and Nanotechnology. | ChevronTexaco Energy Research and Technology Company.

known as a Hydrocarbon pool based on the formation of a methyl-substituted benzyl ring.9,10 The idea to modulate the Brønsted acidity by the isomorphus substitution of boron inside the zeolitic framework has been previously exploited with success in the case of vapor phase Beckman rearrangement of a cyclohexanone oxime to -caprolactam performed on B-ZSM-5.11,12 It is consequently expected that a B-substituted-Chabazite should be less acidic than the corresponding H-SAPO-34, H-SSZ-1313 materials and thus possibly less prone to form coke. To try to improve catalyst performances in terms of lifetime (i.e., to minimize the coke formation) and to see the effect of chemical compositions on the selectivity of C2-C4 products, a comparison between H-SAPO-34, H-SSZ-13, and B-SSZ-13 has been performed.14 This study has evidenced that H-SSZ-13 gives a much higher alkane product than H-SAPO-34, while B-SSZ-13 is completely inactive. Authors explain the total inactivity of B-SSZ-13 toward MTH process in term of its lower Brønsted acidity as singled out by NH3 TPD measurements.14 As far as H-SSZ-13 (Si/Al ) ∼12) and H-SAPO-34 with a similar Brønsted site content, they have been recently characterized by IR spectroscopy by following the reactivity toward probe molecules such as H2, CO, H2O, and CH3OH.15-17 The studies have shown that both materials are characterized by two families of OH groups that react similarly toward CO and H2. In the literature, these two families of Brønsted sites are known as high-frequency (HF) and low-frequency (LF) sites.6,13 On the basis of the perturbations of the Brønsted sites and the corresponding polarization of adsorbed CO and H2, it appears that H-SAPO-34 interacts more weakly with probe molecules than H-SSZ-13.16-18 The higher Brønsted acidity of H-SSZ-13 has been confirmed also by considering the reactivity toward H2O and CH3OH. In

10.1021/jp064048w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007

Effect of B Substitution in Chabazite Framework

Figure 1. Powder XRD pattern of the B-SSZ-13 sample collected at the GILDA BM8 beamline19,20 of the ESRF synchrotron with an image plate detector. Although collected with λ ) 0.688835 Å, the pattern has been converted as obtained with Cu KR to allow a direct comparison with database patterns. Inset: magnification of the high 2θ region.

particular, CH3OH interacts strongly with the Brønsted sites in H-SSZ-13, and at high coverages, protonated species are formed extensively. This behavior was not observed for H-SAPO-34 where only strong H-bond interactions are observed. Upon heating, the formation of (CH3)2O has been evidenced in both materials even if more extensively in the case of H-SSZ-13.15 As far as the B-SSZ-13 material is concerned, an extensive spectroscopic characterization has not been reported until now, and no data are available regarding the reactivity toward methanol. This lack will be covered by this paper, where we report a description of the spectroscopic features of boronsubstituted Chabazite and of its reactivity toward H2, CO, H2O, and CH3OH. 2. Experimental Procedures 2.1. Synthesis of B-SSZ-13. A total of 106 mmol of N,N,N,trimethyl-1-adamantammonium hydoxide, in 175 mL of water, was used to dissolve 2.15 g of boric acid (∼35 mmol). After the solution was clear, 26.13 g of Carbosil M-5 (97% SiO2, remainder H2O) was added with stirring. The reactants were mixed in the Teflon liner with a 600 cm3 Parr overhead stirrer reactor. The reactor was closed up, and while being stirred at 75 rpm, it was ramped to 433 K over an 8 h period. The reaction was held at this temperature for 3 more days. An analysis of the settled reaction product shows it to be a borosilicate Chabazite pattern with some symmetry differences from the usual aluminosilicate material. The crystallinity of the material and its structural features were checked on the GILDA BM8 beamline at the ESRF, equipped with a flat Fuji 200 400 mm2 image plate (IP) supported by a magnetic plate, by using a 0.688835 Å wavelength. A more detailed description of the experimental setup can be found elsewhere.19,20 The XRPD pattern converted as obtained with Cu- KR to allow a direct comparison with database patterns is shown in Figure 1. The purity of the compound was testified by the absence of peaks not belonging to the CHA structure. The material obtained shows a Si/B ratio of 11, which implies that there is one heteroatom per unit cell and an average of one B(III) species in any cage. This composition is the most favorable when a large template (N,N,N,-trimethyl-1-adamantammonium hydroxide) is used because any template molecule is neutralized by a B(III) species. The synthesis of this material, as all zeolites with a Chabazite framework and a low Al content are not trivial, as a non-commercial template is needed. A larger set of samples

J. Phys. Chem. C, Vol. 111, No. 7, 2007 2993 with an increasing boron content would be interesting, but at the moment, they are not available. 2.2. Characterization Methods. The B-SSZ-13 zeolite, after a pretreatment of the samples at 773 K in vacuum, has been characterized by FTIR spectroscopy in transmission mode. The vibrational information relevant in our study lies in three different regions: (i) the 1400-700 cm-1 range covering framework modes able to highlight the presence and the nature of the B species (tetrahedral-like or trigonal planar); (ii) the 3800-2400 cm-1 range representative of the Brønsted OH species, either unperturbed or after interaction with probes; and (iii) the region covering the specific modes of the used probe molecule. As region (i) is dominated by the strong silica modes, an optimum sample thickness resulting in high-quality IR spectra on the three regions could not be found. We decided to duplicate the IR experiments. To be able to follow in detail the spectral evolution in the 1400-700 cm-1 range, a very thin layer of B-SSZ-13 was deposited on a silicon wafer. Conversely, when the attention is turned toward the reactivity of the surface species and in particular of the hydroxyls, a compressed self-supporting zeolite wafer was used. Parallel studies on thin films of B-SSZ13 deposited on a silicon wafer and on self-supported pellets have been performed when the reactivity toward CH3OH has been considered. The experiments have been performed at room temperature adopting a 2 cm-1 resolution on a Bruker IFS 66 FTIR spectrometer, equipped with an MCT detector. The interaction with very weak bases like H2 and CO can be observed at low temperatures only. As negligible perturbation of the framework modes are expected with these probes, only samples prepared as pellets have been investigated. The measurements have been performed by using a homemade cryogenic setup that allows (i) in situ high-temperature activation of the sample (773 K) under high vacuum conditions; (ii) the ability to perform FTIR adsorption experiments in transmission mode at fixed temperatures, as low as ca. 15 and 60 K for H2 and CO measurements, respectively (estimated at the sample level), and variable gas pressure; (iii) the ability to record IR spectra in the 300-15 K temperature interval, monitoring simultaneously the gas phase equilibrium pressure of the adsorbed species. A detailed description of the cryogenic setup (consisting of a properly modified closed circuit liquid helium Oxford CCC 1204 cryostat) is given elsewhere.21 The spectra were acquired at a resolution of 1 cm-1 by averaging 128 interferograms on a Bruker Equinox-55 FTIR spectrometer whose sample compartment was modified ad hoc to accommodate the cryogenic IR setup. 3. Results and Discussion This section is divided into three subsections. The first one (section 3.1.) is devoted to the characterization of the vibrational properties of [B(OSi)3] in trigonal planar geometry (calcined sample) and in distorted tetrahedral geometry (upon interaction with a strong ligand such as the template itself or water molecules). Section 3.2. deals with the characterization of the acidic properties of the B(OH)Si Brønsted sites probed by the interaction with weak bases: CO (at 60 K) and with H2 (at 12 K). Finally, the reactivity of the B(OH)Si Brønsted sites has been tested toward interaction with CH3OH in section 3.3. 3.1. Vibrational Properties of B-SSZ-13: Calcination and Interaction with Water. The [B(OSi)3] unit in trigonal planar geometry is characterized by an intense IR mode at 1390 cm-1, due to asymmetric B-O stretching.22-24 The intensity of this band, measured by in situ IR spectroscopy, consequently gives the fraction of boron atoms in trigonal planar symmetry. As

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Figure 2. (a) IR spectra of the skeleton stretching modes of B-SSZ13 zeolite before (gray curve) and after (black curve) in situ calcination. (b and c) Effect of increasing H2O dosages from 8 × 10-3 mbar to 18 mbar (equilibrium pressures) on the activated sample (bold curve); dotted curve corresponds to water vapor pressure (18 mbar). The ν(OH) and δ(OH2) regions are reported in panels a) and b), respectively. All the spectra here reported have been collected at room temperature on a thin layer of B-SSZ-13 on a silicon wafer.

described in the Experimental Procedures, due to the high adsorption coefficient of this mode, the related band can be observed on extremely thin samples only. The IR spectrum, in the skeleton mode region, of the material in the presence of the template molecule (N,N,N,-trimethyl-1adamantammonium hydoxide) is reported in Figure 2a, gray curve. The spectrum is dominated by a huge and broad absorption in the 1225-1000 cm-1 region, due to the asymmetric stretching of the tetrahedral [SiO4] unit.25,26 At a lower frequency, two well-defined bands were observed at 890 and 800 cm-1. The latter, unusually sharp, is due to the total symmetric stretching of [SiO4] units and is the mode dominating the Raman spectrum of the material.25,27 The former is tentatively ascribed to the perturbation of the asymmetric stretching of the tetrahedral [SiO4] unit by adjacent Si vacancies.25 Note that a defective silicalite exhibits a similar band, although less intense and broader, around 880-920 cm-1.28 An alternative explanation could be that we are dealing with a much better defined species such as [BO3OH] or [BO2(OH)2] units perturbing the adjacent [SiO4] units. Note that the presence of a hydroxylated boron species has been evidenced in the NMR study of Hwang et al.29 Conversely, the hypothesis that the 890 cm-1 band could be due to strained [Si-O-Si] units does not hold because the intensity of the band is not increased upon calcination (compare black and gray curves in Figure 2a) and because its perturbation upon interaction with probe molecules (H2O and CH3OH) is totally reversible, vide infra.28 Several minor features, although perfectly defined and visible, are due to the vibrational modes of the template (bands at 1492, 1475, 1452, 1415, 1369, 1350, 1307, 958, and 383 cm-1). The disappearance of all these well-defined minor features occurs upon template removal, performed in situ in the IR cell (black bold spectrum in Figure 2a). A blue shift occurs for the bands previously observed at 890 and 800 cm-1 (gray spectrum), now at 912 and 810 cm-1. The broadening of the huge absorption due to the asymmetric stretching of the [SiO4] tetrahedra is the consequence of an increased heterogeneity of the [SiO4] species upon template removal. Notwithstanding these aspects, the most important modification undergone by the IR spectrum of B-CHA upon template removal is the appearance of a strong IR mode at 1390 cm-1 due to the asymmetric B-O stretching of the [B(OSi)3] unit in trigonal planar geometry.30,31

Scheme 1 represents pictorially the evolution of the B local environment upon template removal. The trigonal planar geometry observed for the [B(OSi)3] units of the calcined sample measured under vacuum conditions can be destroyed by interactions with strong adsorbates like water, either via an indirect or direct way, as depicted in Scheme 2, structures I or II, respectively. This is observed in the sequence of IR spectra reported in the remaining parts of Figure 2, where the dosage of progressive equilibrium pressures of water (from vacuum, bold spectrum, to water vapor pressure (18 mbar), dashed spectrum) is reported in the ν(O-H) and δ(OH2) regions, panels b) and c), respectively. In particular, the trigonal planar [B(OSi)3] fingerprint band at 1390 cm-1 is progressively eroded, while the δ(OH2) mode at 1622 cm-1 grows up in an antiparallel way (Figure 2c). In the ν(O-H) region, the spectrum of the activated B-CHA sample is characterized by silanol bands at 3745 cm-1 (shoulder) and a doublet at 3733 and at 3715 cm-1. These components, by analogy with what is already observed for H-SAPO-34 and for H-SSZ-13, can be assigned to HF and LF Brønsted sites characterized by a very weak acidity, being associated with the B(OH)Si group.18,32 The closeness of both HF and LF components to the band of free silanols reflects the very weak acidity of such sites. All components are progressively eroded, resulting in broad and structured absorption extending down to 3000 cm-1 with components around 3645, 3420, and 3200 cm-1 due to the formation of H-bonded complexes of water adsorbed on such sites and in mutual interaction in the zeolite pores.25,27,28,33-35 3.2. Acidity of B(OH)Si Brønsted Sites as Probed by CO and H2 Adsorption. Simple diatomic molecules such as CO and H2 have been widely used as probes to characterize the acidic properties of zeolites and zeotypes.36,37 The effect of interaction between Si(OH)B sites of B-SSZ-13 and these probes is reported in Figure 3a,b, respectively. Black bold curves report the IR spectrum of the sample in the ν(O-H) region as it appears after activation. The spectrum is characterized by three main bands at 3745, 3733, and at 3715 cm-1 due to isolated silanols and to HF and LF B(OH)Si weak acidic sites, respectively. A total of 70 mbar of CO was sent on the sample cooled at 60 K. The equilibrium pressure was 3 mbar. Then, a series of spectra were collected, decreasing progressively the CO pressure at 60 K. Starting from low CO coverages (solid curves in panel a), we observe the progressive erosion of Brønsted and isolated silanols bands and the parallel appearance of a slightly broader band at 3653 cm-1 (∆νOH ) -80 cm-1). In particular, upon increasing the CO equilibrium pressure (PCO),

Effect of B Substitution in Chabazite Framework

Figure 3. (a) 60 K IR spectrum of B-SSZ-13 activated at 773 K (bold curve) and effect of increasing CO coverages (solid curves); bold gray curve corresponds to the effects of the formation of a liquid-like phase inside the cages (equilibrium pressure ) 3 mbar). The inset reports the background subtracted spectra in the C-O stretching region. (b) As panel a for H2 coverages at 15 K. Spectra are collected on a selfsupported pellet. a.u. ) absorbance units.

we observe the progressive disappearance of the component at 3733 cm-1 first, followed successively by that at 3715 and 3745 cm-1. The new band centered at 3653 cm-1, growing upon increasing PCO, is easily ascribed to the formation of OH‚‚‚CO adducts. It is important to notice that the three distinct species detected in the starting spectrum originate from a single broad component upon interaction with CO. This suggests that the presence of three components is associated to structural differences more than to different acidic strengths. A similar conclusion is achieved by considering the ν(CO) region (inset in Figure 3a). The weak acid sites are able to interact with CO and perturb its stretching mode, with respect to the free molecule (2143 cm-1), generating a single band at 2156 cm-1 (∆νCΟ ) +13 cm-1). This evidence leads us to conclude that the CO molecule is not able to distinguish isolated silanols and Brønsted acid sites in B-SSZ-13. The weak character of the OH‚‚‚CO adducts is testified by the high reversibility of the CO band associated to the complex, which is nearly equivalent to that associated to the liquid-like phase (band centered at 2139 cm-1 with a shoulder at 2135 cm-1).38 At high PCO coverages, CO condenses inside the zeolite cavities and completely fills the pores of the zeolite (see bold gray curve), causing a change in the local dielectric constant. In this condition, the ν(OH) responds, giving rise to a further red shift of the maximum observed at 3617 cm-1. No evidence of absorptions at frequencies higher than 2156 cm-1 excludes that CO is able to interact directly with boron acting as a Lewis site. The weak strength of the B3+ Lewis acid sites has already been observed in B2O3SiO2 where weak bases such as CO are not able to form B3+‚ ‚‚CO adducts.39 This lack of coordination has been explained in terms of the π-character of B-O bonds, which disfavors the conversion of boron from a trigonal planar conformation to a tetrahedral conformation upon adsorption of probe molecules of weak basicity.39 Moving toward panel b of Figure 3, it is possible to observe the effect of H2 interaction with B-SSZ-13. The spectra correspond to a decreasing H2 coverage starting from an equilibrium pressure of 3 mbar. The curves are collected, keeping the system at 15 K. Starting from low coverages, it is rather clear that the HF peak is the first component that is eroded

J. Phys. Chem. C, Vol. 111, No. 7, 2007 2995 after interaction with H2, as previously observed by CO (see Figure 3a). In this case, the HF component is perturbed by H2, and its vibrational frequency is shifted to lower wavenumbers, giving rise to a band at 3720 cm-1 (∆νOH ) -13 cm-1). Unfortunately, this absorption is superimposed on an LF band, which is located at 3715 cm-1, making it difficult to understand what happens to the LF component. However, as the erosion of the silanol band occurs, in analogy with what has been observed with CO, it is possible to affirm that H2 interacts and also perturbs the LF acid sites. Also, in this case, the resulting adducts (OH‚‚‚H2) originate a single component, preventing any differentiation in acidity of the three original species. In the ν(HH) region (inset of Figure 3b), we observe the presence of only one weak band at 4130 cm-1, characterized by a low red shift (∆νHH ) -30 cm-1), indicating the low acidity of the sites.37 Note that previous data obtained in the case of the aluminum homologue have shown a ∆νHH value in the range of 80-100 cm-1.16,17,18b 3.3. Reactivity of B(OH)Si Brønsted Sites toward CH3OH. Materials with a Chabazite framework like H-SAPO-34 are wellknown as catalysts for MTO processes. As the major problem with this process is the too fast deactivation of the catalyst due to coke formation, the study of a material with a lower acidity but the same topology is of great interest. In this context, a spectroscopic study related to the interaction of methanol with B-SSZ-13 zeolite has been performed. The effect of increasing CH3OH coverages is reported in Figure 4 a-c, while panels d-f report the spectra obtained upon decreasing coverages. Panels a), b), d), and e) were obtained on a self-supported pellet, allowing us to see sufficiently strong the signals associated with the hydroxyls and C-H modes. Conversely, panels c) and f), obtained on a very thin film deposited on a silicon wafer, allow us to see some components associated with the framework modes, in particular, being able to follow the evolution of the band at 1390 cm-1 and the shoulder at 890 cm-1 (vide supra section 3.1). Corresponding to a very low methanol equilibrium pressure (0.05-0.1 mbar), similar to what is observed when H2O was dosed (Figure 2), the component at 3745 cm-1 is progressively eroded (negative component in the difference spectra) with the parallel formation of a broad and complex band centered at about 3450 cm-1 (∆νOH ) -295 cm-1). Simultaneously, an anomalous phenomenon has been evidenced: the growth of two sharp bands at 3730 and 3715 cm-1, suggesting the hydrolysis of some T-O-T bridges with the formation of Si-OH and/or B-OH species (curve 1 in Figure 4a). Unfortunately, the spectral region where the corresponding δ(OH) modes should be observed is obscured by framework vibrations. In the lowfrequency range (Figure 4b), no substantial changes are seen. By progressive increase of methanol equilibrium pressure, we observe the erosion of all the OH groups (negative components in the difference spectra are developing in the range of 3745-3700 cm-1) and the progressive growth of a complex broadband extending down to about 2600 cm-1 and showing two components at 3480 and 3300 cm-1, due to the formation of medium-strength H-bonded species. Note that the band formed at 3730 cm-1 and a shoulder at 3715 cm-1 undergo a progressive decrease in intensity (curves 2 and 3 in Figure 4a): this indicates that at higher coverages, CH3OH interacts with all hydroxiles, and a liquid-like phase is formed in the cavities as evidenced also by the growth of the band at 3630 cm-1, associated with the ν(OH) of end of the chain methanol molecules, which becomes clearly visible.15,40 In the ν(CH) stretching region, a progressive growth of a multiplet is evident.

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Figure 4. Panel b) reports the evolution of the OH and CH species upon increasing equilibrium pressures of CH3OH dosed at room temperature on B-SSZ-13 in the form of a self-supported pellet activated at 773 K (curves 1-11). Spectra have been reported in difference. Bold gray and black spectra refer to the maximum (17 mbar) and minimum (0.05 mbar) equilibrium pressure, respectively. Panel a): enlarged view from panel b) in the hydroxyl region. Panel c) same experiment repeated on a B-SSZ-13 sample in the form of a thin film on a silicon wafer (curves 1-8). Curve 1 has been collected before CH3OH dosage. Panels d) and e) as panels a) and b) for desorption experiments (curves 12-22). Bold gray and black curves refer to the maximum and minimum coverage (17 and 0.02 mbar equilibrium pressures), respectively. The bottom dashed curve in panel e) reports the IR spectrum of B(OCH3)3 model compound. Panel f) as panel c) for desorption experiments (curves 9-16). Curves 9 and 16 refer to the maximum and minimum coverage (17 and 0.02 mbar equilibrium pressures), respectively.

Starting from the spectra collected in the presence of a very small amount of methanol, we observe an anomalous number of components (peaks at 2996, 2971, 2955, 2920, 2886, and 2857 cm-1), with respect to what is expected for methanol simply interacting with silanols or with acidic zeolites.15,40 In those cases, a doublet at around 3000 and 2950 cm-1 was ascribed to the asymmetric modes of CH3, while the component at 2857 cm-1 was associated with the symmetric mode of the CH3 groups. Finally, the band at 2928 cm-1 was assigned to an overtone of the bending δ(CH3) mode at about 1460 cm-1. It is also evident that the bands at 2971, 2920, and 2886 cm-1 are due to the ν(CH) of methanol differently engaged. We can hypothesize that they are due to the ν(CH) of methanol directly bonded to boron species. Upon increasing the coverages, even if the single components are less distinguishable, the complexity of the spectra is maintained, and a further band at 2845 cm-1 appears, testifying to the formation of a physisorbed phase. In the framework stretching region (Figure 4c), we observe the progressive consumption of the band at 1390 cm-1 and of the shoulder at 890 cm-1, testifying the rupture of the D3h symmetry of the [B(OSi)3] species and the perturbation of Si-O modes. The parallel growth of a doublet at 1483 and 1453 cm-1, associated with asymmetric CH3 deformation and the presence of a complex absorption (1380-1350 cm-1) with two clear maxima at 1376 and 1357 cm-1, is observed. The latter complex absorption contains both the symmetric CH3 deformation band and the absorption due to H-bonded OH bending mode; furthermore, new species derived by the reactivity of methanol with boron could be associated to bands in this region. For the time being, we can only state that the original mode due to the trigonal [B(OSi)3] species is strongly perturbed and that some

boron sites change from trigonal to tetrahedral-like coordination upon interaction with methanol. All these features suggest that methanol is not only interacting with silanols but also that it is in direct contact with the boron species. Another important characteristic is the absence of the bands so-called A, B, and C, which are the typical spectroscopic features of a strong H-bond interaction. As Paze` et al.41 explains, in the presence of a molecule with a medium-high proton affinity (PA), such as H2O or CH3OH (PAH2O ) 166.5 kcal mol-1 and PACH3OH ) 181.9 kcal mol-1), a zeolite, which is characterized by the presence of strong Brønsted sites, is able to realize a strong H-bond interaction, which produces two main effects: (i) the band associated to the OH stretching mode is strongly red-shifted, generating an intense and broad absorption extending until 1000 cm-1 and (ii) the bands due to the overtones of the δ and γ bending modes of the OH groups are deeply blueshifted, falling in the region where the intense and broad absorption due to the ν mode is present.41 In this case, a peculiar phenomenon, called a Fermi resonance effect, causes the appearance of three maxima identified as the A, B, and C components separated by two Evan’s windows (minima) that normally corresponds to the expected positions of the components due to the 2δ and 2γ bending modes. The lack of these spectroscopic features gives a further confirmation that B-SSZ13 is a material characterized by a very low Brønsted acidity. The effect of progressive pumping at room temperature is shown in Figure 4d-f. Because of the low acidic character of the Brønsted sites present in B-SSZ-13, we would have expected to observe a large CH3OH removal and a substantial restoration of the original spectrum. Conversely, the last spectrum reported as a bold gray curve has been obtained after prolonged pumping

Effect of B Substitution in Chabazite Framework

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SCHEME 4

at room temperature (residual equilibrium pressure of 1.0 × 10-3 mbar) and definitely shows a low reversibility (Figure 4d,e). Main spectroscopic features observed during the outgassing can be summarized as follows: (i) progressive, but not complete, restoration of original silanols (band centered at 3745 cm-1); (ii) disappearance of the component at 3630 cm-1 associated to the nearly free ν(OH) of the end of the chain methanol molecules; (iii) decreasing of the broadband associated to H-bonded OH groups (maximum at about 3450 cm-1); (iv) decrease of the intensity of bands in the ν(CH) bands: the spectrum in this region becomes more simple, and a component at 2878 cm-1, never seen when CH3OH has been adsorbed in proton exchanged zeolites, becomes clearly visible; (v) the nearly total restoration of the band at 1390 cm-1 and of the shoulder at 890 cm-1; and (vi) the persistence of the component at 1483 cm-1 associated to the asymmetric CH3 deformation band, being the second overlapped with the main band at 1390 cm-1. Points (i) and (iii) indicate that the physisorbed phase is not completely eliminated. To clarify the assignment of the band at 2878 cm-1, the result of a blank experiment on the B(OCH3)3 compound is reported at the bottom of Figure 4d, dashed curve. The spectrum reports clearly a complex absorption band in the range of 3010-2920 cm-1 and a distinct peak at 2874 cm-1. In the low-frequency region, B(OCH3)3 is characterized by a complex spectrum characterized by two strong bands at 1484 and 1358 cm-1 and a shoulder around 1380 cm-1. The high-frequency one is clearly due to asymmetric bending modes of the [CH3] units; the assignment of the remaining two to the symmetric bending modes of the [CH3] units and to the [B(O)3] stretching mode in D3h symmetry is not straightforward. Consequently, this portion of the spectrum has not been introduced in Figure 4f.

The similarity with the result obtained upon CH3OH dosage on B-SSZ-13 suggests the formation of partially irreversible B-OCH3 species formed upon the hydrolysis of a Si-O-B bridge. The formation of these species could also be associated with the residual complex absorption visible in the 1380-1360 cm-1 range in Figure 4f. The complete set of experimental evidence previously described requires more than one reaction path. In fact, we have to explain spectroscopic features that are associated with both OH and [B(OSi)3] groups, some of which are irreversible at room temperature. We have summarized in Scheme 3 (structures I-V) the most probable possibilities. Species I justifies the appearance of a B-OCH3 species and the growth of some new silanols. Species II explains the formation of B-OH groups. Species III justifies the irreversible disappearance of some silanols. Species IV justifies the reversible change in the symmetry of the framework boron species and the erasure of some silanols now engaged by methanol. Species V justifies the presence of some methanol molecules slightly perturbed by boron, the change in the symmetry of the framework boron species, and the formation of medium-strength H-bonds. Moreover, the irreversible adsorption of methanol on external silanols (no complete restoration of the band at 3745 cm-1 and no elimination of the broad band associated to H-bonded OH groups (maximum at about 3450 cm-1)) has to be considered. The presence of some adsorbed methanol, having a ν(CH) very similar to those observed in the case of the SiOCH3 groups40, does not allow us to give an estimation of the ratio between the B-OCH3 and the Si-OCH3 species on the basis of the intensity of the two bands at 2878 and 2857 cm-1. At high CH3OH loading, the formation of multi-adducts must be taken into account. This means that the symmetry of species

2998 J. Phys. Chem. C, Vol. 111, No. 7, 2007 I-III can be even further decreased with the formation of a distorted tetrahedral species. Scheme 4 reports one of the possible evolutions of structure I upon increasing CH3OH coverage, as an example. The whole set of data evidence that in B-SSZ-13, the Brønsted acidity is very weak, but some Lewis acidity is present, as testified by the coordination of methanol as reported in structure V of Scheme 3. A second important feature is that upon a prolonged desorption procedure, an abundant irreversible phase has been found. The sample outgassed at room temperature has been heated at 573 K, and no reactivity has been observed. Note that, on zeolites with strong Brønsted acidity, it is well-known that upon mild thermal treatment, methanol reacts with the acidic sites giving rise to (CH3)O2 species and water.8,42 Our observation is in agreement with the results presented by Yuen and co-workers in which it is shown that the interaction between boron-substituted CHA and AFI with methanol, the formation of dimethylether, which is the first step on any MTH process, is not observed.14 4. Conclusion In this work, we exploited the capabilities of IR spectroscopy of adsorbed molecules (H2O, CO, H2, and CH3OH) to investigate the symmetry, accessibility, acidic strength, and reactivity of B sites in the B-SSZ-13 framework. Template removal causes the modification of the local B geometry from a Td- to a D3hlike symmetry, moving from [B(OSi)4] to [B(OSi)3] units. Progressive water dosages are able to restore the Td-like geometry upon insertion of a strong fourth ligand in the [B(OSi)3] units. This insertion is reversible after prolonged evacuation at room temperature. When weak bases such as H2 or CO are used to probe the reactivity of the [B(OSi)3] units and of the adjacent SiOH groups, only SiOH‚‚‚H2 (SiOH‚‚‚ CO) adducts have been observed, testifying the virtual absence of the Lewis acidity of the [B(OSi)3] species hosted in the CHA framework. These results agree with the similar observation found by Travert et al.39 for the B2O3-SiO2 system. Methanol interacts with internal silanols, resulting in both a simple adsorption via H-bonds and in the methoxylation of the OH group. The interaction of CH3OH, a stronger base than water (PAH2O ) 166.5 kcal mol-1 and PACH3OH ) 181.9 kcal mol-1), with [B(OSi)3] units does not result only in the formation of adducts where B lies in a Td-like geometry. IR spectroscopy reveals the transformation of a fraction of [B(OSi)3] units, upon interaction with CH3OH, into new species such as (i) [B(OSi)2OCH3] + SiOH or (ii) [B(OSi)2OH] + SiOCH3. These results are in agreement with the fact that the formation of dimethylether, which is the first step on any MTH process, is not observed in B-SSZ-13.14 Acknowledgment. We thank Mr. Lun Teh Yuen for the synthesis of the boron SSZ-13 material, D. Cocina for the support during the low-temperature IR experiments (Figure 3), and Prof. P. Ugliengo for fruitful discussions. The whole staff of the GILDA BM8 beamline at the ESRF (in particular, C. Meneghini) is acknowledged for support during the synchrotron radiation XRPD study. The financial support of Compagnia di San Paolo is gratefully acknowledged. L.R. acknowledges the Regione Piemonte for her Ph.D. grant. References and Notes (1) Xu, Y.; Grey, C. P.; Thomas, J. M.; Cheetham, A. K. Catal. Lett. 1990, 4, 251-260. (2) Gale, J. D.; Shah, R.; Payne, M. C.; Stich, I.; Terakura, K. Catal. Today 1999, 50, 525-532.

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