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J. Phys. Chem. C 2008, 112, 7210-7222
Structural Environments for Boron and Aluminum in Alumina-Boria Catalysts and Their Precursors from 11B and 27Al Single- and Double-Resonance MAS NMR Experiments Michael Ryan Hansen, Hans J. Jakobsen, and Jørgen Skibsted* Instrument Centre for Solid-State NMR Spectroscopy and Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark ReceiVed: NoVember 20, 2007; In Final Form: February 19, 2008
Alumina-boria catalysts, obtained by calcination (550-950 °C) of pseudo-boehmite precursors impregnated with 0.25-3.4 wt % boron, have been characterized by 11B and 27Al MAS NMR of the central and satellite transitions, MQMAS, 11B{27Al} REDOR, and TRAPDOR NMR, and by homonuclear dipolar recoupling MAS NMR experiments. The 27Al MAS NMR spectrum of the satellite transitions for boehmite (γ-AlOOH) has allowed the first precise determination of the 27Al quadrupole coupling parameters for this important aluminate. The 27Al MAS NMR spectra of the alumina-boria catalysts show that the fraction of tetrahedral AlO4 sites decreases with increasing boron loading. This finding and the results from 11B MAS NMR suggest that a small fraction of tetrahedrally coordinated boron is incorporated within the first molecular layers of the γ-Al2O3 support. The observation of slightly stronger 11B-27Al dipolar couplings for BO4 relative to the trigonal BO3 sites from 11B{27Al} REDOR NMR supports this finding. Information on the dispersion of BO4 and BO3 environments have been obtained from 11B MAS NMR spectra of the satellite transitions and from 11 B MQMAS NMR experiments. The presence of BO4 and BO3 species on the surface of the γ-Al2O3 support is demonstrated by 11B{27Al} REDOR and TRAPDOR NMR, while 11B homonuclear correlation experiments indicate BO4-BO3 connectivities and thereby a network on the surface for these units. For the aluminaboria catalyst corresponding to the highest boron content and calcination temperature, an additional BO3 resonance is observed in the 11B MQMAS spectrum, which is assigned to a separate B2O3 phase exhibiting a high degree of local order.
Introduction Hydrotreating catalysis includes a number of important hydrogenation reactions for unsaturated hydrocarbons and removal of undesired light heteroatoms and even some metals from crude oil and petroleum fractions in oil refineries.1 The industrially most important hydrotreating processes are the removal of sulfur (hydrodesulfurization, HDS) and nitrogen (hydrodenitrogenation, HDN) from oil products,2 since these processes are essential for the production of purified gasoline, fuels, and fine chemicals in order to meet the increasing environmental demands of low sulfur and nitrogen contents. Commercial HDS and HDN hydrotreating catalysts typically contain molybdenum/tungsten, promoted by cobalt or nickel, and dispersed in their sulfidized forms on an alumina support.1,3 Nitrogen can also be removed by selective catalytic reduction, where nitrogen oxides (NOx) are reduced by ammonia to form dinitrogen and water, using catalysts typically based on vanadia supported on anatase (TiO2).4 The most commonly used support materials for hydrotreating catalysts are γ- and η-aluminas, which are metastable Al2O3 polymorphs with high surface areas prepared by dehydroxylation upon heat treatment of aluminum hydroxide (Al(OH)3, gibbsite or bayerite) or oxyhydroxide (AlOOH, boehmite).5,6 The catalytic properties of hydrotreating catalysts depend strongly on the surface structure and acidity of the support material. As a consequence, improved catalytic materials can be obtained using modifier elements such as silica, phosphates, * To whom correspondence should be addressed. Fax: +45 8619 6199. Tel: +45 8942 3900. E-mail:
[email protected].
boria, fluorides, and chlorides, which affect either the solubility and stability of the transition-metal species in the impregnation process or improve the mechanical and thermal stability of the alumina support. For example, the introduction of small quantities of phosphorus (in the form of phosphates) on the alumina support significantly improves the HDN activity of the resulting catalyst,7 most likely because phosphorus modifies the surface structure and acidity of the support. Although the surface modifications obtained by phosphorus loading are not fully understood, significant information on phosphate species absorbed on alumina supports has been obtained from solid-state 31P MAS NMR,8,9 27Al MAS, and MQMAS NMR studies10-12 as well as from double-resonance NMR experiments, which utilize 27Al-31P dipolar couplings to probe phosphorusaluminum connectivities.13 More recently, applications of borates as a modifier to the alumina support have attracted considerable interest14-19 since borate species on the surface lead to strong Brønsted-acid centers, which contribute to improvements in catalytic activity and performance. In addition, the loading of borate species may also affect the dispersion and reactivity of the active metals on the surface. Thus, an improved understanding of the surface structure for the alumina-boria support on an atomic/nanoscale level seems highly relevant for further developments of borialoaded alumina support materials. In this study, we investigate the structural environments for the borate and aluminate species at the surface of aluminaboria catalysts (ABC) and their precursors (ABCP), prior to transition-metal impregnation, using 11B and 27Al MAS NMR
10.1021/jp7110346 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/09/2008
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spectroscopy of the central and satellite transitions,20 MQMAS NMR,21 homonuclear dipolar recoupling experiments,22 and double-resonance 11B{27Al} rotational-echo double-resonance (REDOR)23 and transfer of populations in double-resonance (TRAPDOR)24 NMR techniques. The precursors (ABCP) are pseudo-boehmite impregnated with different concentrations of boric acid solutions while the catalyst samples (ABC) are obtained by subsequent calcination at temperatures in the range 550-950 °C. For comparison, a commercial sample of boehmite is also investigated by 27Al MAS NMR for which the spectrum of the satellite transitions provides the first precise determination of the 27Al quadrupole coupling parameters for this important phase. The 11B{27Al} REDOR and TRAPDOR experiments are utilized to independently prove 11B-27Al connectivities for both the trigonal and tetrahedrally coordinated borate species in the ABC and ABCP samples. Although REDOR-type techniques principally allow determination of internuclear distances,23 the present experiments are complicated by the fact that 11B and 27Al are both quadrupolar spin-nuclei, which makes a quantitative evaluation of the experiment quite difficult. Thus, the REDOR spectra are analyzed qualitatively, providing connectivity information only, in analogy to a previous 27Al{31P} and 31P{27Al} REDOR study of phosphorus-impregnated γ-aluminas with different phosphorus loadings.13 Experimental Section Materials. The catalyst precursors (ABCP) were prepared by impregnation of pseudo-boehmite with 0.8-1.8 M aqueous solutions of boric acid (H3BO3). After impregnation, the samples were dried for 2 h at 110 °C. The catalysts (ABC) were obtained by calcination of the ABCP materials in air for 2 h, employing calcination temperatures of 550, 650, 750, 850, and 950 °C. The ABCP and ABC samples have specific surface areas of approximately 450 and 360 m2/g, respectively, as determined by the BET method. Chemical analysis revealed that the ABCP samples have boron contents of 0.25, 0.9, 1.3, 2.1, and 3.4 wt % B. Although the ABC samples are highly hygroscopic, no initial precautions were taken to store or handle them under nonhumid conditions. The “polycrystalline” sample of boehmite is a commercial product (Pural-200) obtained from Condea Chemie (Hamburg, Germany). NMR Spectroscopy. 11B and 27Al MAS and MQMAS NMR spectra were acquired on a Varian Unity INOVA-600 spectrometer (14.09 T) using a home-built (11B background-free) CP/MAS probe for 4 mm o.d. rotors. For both nuclei, the singlepulse experiments employed a short rf pulse width, τp ) 0.5 µs, for an rf field strength of γB1/2π ≈ 65 kHz and 1H decoupling (γB2/2π ≈ 60 kHz) to obtain wideband excitation and quantitatively reliable spectra for sites experiencing different quadrupole couplings.25 The triple-quantum MQMAS spectra employed the three-pulse z-filter sequence26 with a delay of one rotor period between the second and third pulses and with the latter being a selective 90° pulse using a moderate rf field strength (γB1/2π ≈ 25 kHz). The first and second pulses used rf fields of γB1/2π ≈ 65 kHz (11B and 27Al) and high-power 1H decoupling (γB /2π ≈ 60 kHz) was employed during both 2 the t1 and t2 periods of the MQMAS experiment. The 11B{27Al} REDOR and TRAPDOR experiments (14.09 T) used a triple-resonance (X-Y-1H) MAS probe from Doty Scientific Inc. for 5 mm o.d. rotors and the standard rotor-synchronized REDOR23 and TRAPDOR24 pulse sequences (Figure 1a and b) with 1H decoupling during both the evolution and detection periods (γB2/2π ≈ 50 kHz). Rf field strengths of γB1/2π ) 28 kHz and γB1/2π ) 23 kHz were used for 11B and 27Al,
Figure 1. Radio frequency pulse sequences used in this study for the (a) 11B{27Al} REDOR, (b) 11B{27Al} TRAPDOR, and (c) the 11B and 27 Al 2D homonuclear dipolar recoupling experiments. The solid black pulses are π pulses. The REDOR experiment (a) employs strings of xy-4 phase-cycled π pulses applied to the S (11B) spins. The mixing period in the homonuclear correlation experiment uses a supercycle of 4 windowed R441 elements as shown in (d). All pulses in (c) are selective to the central transition except for the initial RAPT loop, which employs short, hard pulses to enhance the central transition polarization.
respectively. The two-dimensional homonuclear dipolar recoupling experiments (14.09 T, 4 mm home-built CP/MAS probe) were carried out by the pulse sequence shown in Figure 1c for correlating central transition single-quantum (SQ) coherences reported by Ede´n et al.22 The mixing period for the zeroquantum-zero-quantum (ZQ-ZQ) homonuclear dipolar recoupling used a supercycle ((R441R44-1)41) of the symmetrybased R441 element27,28 shown in Figure 1d. Except for the initial rotor-assisted population-transfer (RAPT)29 loop, all pulses were soft and selective for the central transitions, employing rf field strengths of γB1/2π ) 18 kHz and γB1/2π ) 4.5 kHz for the π/2 and π pulses, respectively, in the 27Al SQ-SQ experiment. For the corresponding 11B SQ-SQ experiment γB1/2π ) 16 kHz (π/2) and γB1/2π ) 2.5 kHz (π) were employed. The RAPT excitation used short (τpulse ) 1.0 µs), hard (γB1/2π ≈ 65 kHz) pulses and nRAPT ) 16-26 repetitions of the loop to enhance the central transition intensity. The quadrupole coupling constants (CQ) and the associated asymmetry parameters (ηQ) are related to the principal elements of the electric-field gradient tensors (V) by CQ ) eQVzz/h and ηQ ) (Vyy - Vxx)/ Vzz, where Q is the nuclear quadrupole moment and the principal elements of the V tensor fulfill the condition |Vzz| g |Vxx| g |Vyy|. Simulations of the 11B and 27Al central-transition line shapes and manifolds of spinning sidebands for the satellite transitions have been performed using
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the STARS software package.20 11B and 27Al isotropic chemical shifts are relative to neat F3B‚O(CH2CH3)2 and a 1.0 M aqueous solution of AlCl3‚6H2O, respectively, employing a secondary reference of a 0.1 M aqueous solution of H3BO3 (δiso(11B) ) 19.6 ppm) for the 11B NMR spectra. Error limits for δiso, CQ, and ηQ are not reported for 11B and 27Al sites experiencing a dispersion in their structural environments. For these sites, the reported parameters should be considered average values for the distribution in these parameters. Thus, error limits are only given for data related to structurally well-defined sites. Results and Discussion 27Al
MAS NMR of the Satellite Transitions for Boehmite and ABCPs. The alumina support of the ABCP samples is a pseudo-boehmite, which can be considered a poorly polycrystalline aluminum hydroxide with a structure similar to boehmite but including a higher content of water.30,31 The structure of boehmite (γ-AlOOH) contains an edge-sharing arrangement of AlO6 octahedra that form double layers, interconnected by O-H‚‚‚O hydrogen bonds with OH groups in the neighboring layers.32 In agreement with this basic structure, the 27Al MAS NMR spectrum of the central and satellite transitions for boehmite (Figure 2a) shows only a single resonance from Al in octahedral coordination. The distinct features in the manifold of spinning sidebands (ssbs) for the satellite transitions demonstrate the presence of a single Al site in an ordered structural arrangement. This is further supported by a least-squares optimization of simulated to integrated ssb intensities, which results in the 27Al parameters, δiso ) 11.1 ( 0.3 ppm, CQ ) 2.64 ( 0.04 MHz, ηQ ) 0.68 ( 0.03, and the simulated spectrum in Figure 2b, which reproduces the main features of the experimental ssb manifold quite well. To our knowledge, this represents the first precise determination of the 27Al quadrupole coupling parameters for a polycrystalline sample of boehmite. From these parameters and the isotropic chemical shift, the centers of gravity calculated for the central transition cg at 78.15 and 156.4 MHz of δ1/2,-1/2 ) 3.2 and 9.1 ppm, respectively, are in good agreement with the earlier reported positions of 3.4 33 and 9 ppm34 at these Larmor frequencies. The 27Al MAS NMR spectrum (11.7 T) of the satellite transitions for a sample of boehmite has been reported earlier.37 However, a distribution of CQ and ηQ values employing parameters in the range CQ ) 1.8-2.8 MHz and ηQ ) 0.5-1.0 along with an isotropic chemical shift of δiso ) 11.1 ( 0.5 ppm was required for a decent analysis. In addition to our results showing the presence of Al in an ordered structural arrangement, the observation of a well-defined ssb manifold also reflects that the AlO6 octahedra from the bulk boehmite crystallites dominate the spectrum, and therefore, the crystallites on an average must be relatively large in our sample. This is in accord with the average crystallite size of 280 Å, estimated using the Scherrer formula from the width of the most intense reflection (d020) observed in a powder-XRD diffractogram for the boehmite sample. 27Al MAS NMR spectra of a few other commercial boehmites with significantly smaller crystallite sizes (XRD: ∼20-70 Å) have also been recorded (not shown), which exhibit manifold of ssbs over the same spectral range as observed in Figure 2a, but with a featureless ssb envelope. The analysis of these ssb manifolds requires consideration of a distribution in quadrupole coupling parameters in the simulations, in analogy to the above-mentioned earlier reported 27Al MAS NMR spectrum of a boehmite.37 This reflects the fact that these spectra are dominated by resonances from slightly different AlO6
Figure 2. 27Al MAS NMR spectra (14.09 T) of the central and satellite transitions for (a) a polycrystalline sample of boehmite, (c) the boehmite precursor before boron loading, and (d) the ABCP material including 3.4% boron. The spectra were acquired using a spinning speed of νR ) 13.0 kHz, a 0.5-µs excitation pulse, 1H decoupling, and a 2-s relaxation delay. (b) Simulated spectrum of the satellite transitions in (a) corresponding to the parameters δiso ) 11.1 ppm, CQ ) 2.64 MHz, and ηQ ) 0.68. The insets show the center band for the central transition, which is cut off at approximately 1/20 and 1/60 of its total height in (a) and (c, d), respectively.
octahedra, most likely from Al sites close to the surface of the boehmite particles, which dominate the sites of very small crystallites. The 27Al MAS NMR spectra of the pure pseudo-boehmite (Figure 2c), used in the preparation of the ABCP samples, and of this material following impregnation with the highest concentration of boric acid (i.e., ABCP 3.4 wt % B, Figure 2d) appear quite similar. These spectra are dominated by resonances from the AlO6 octahedra of the bulk part of the crystallites, and their strong similarity shows that the impregnated borate ions do not affect the bulk structure. Thus, the borate species
Alumina-Boria Catalysts and Their Precursors are dispersed on the surface of the pseudo-boehmite crystals, as expected from the method of impregnation for the sample preparation and in agreement with earlier investigations of similar samples14,16,35,36 that utilize other methods (e.g., temperature-programmed desorption experiments, X-ray photoelectron spectroscopy, FT-IR) to study borate species on alumina surfaces. However, comparison with the spectrum of “polycrystalline” boehmite (Figure 2a) reveals that the spectra in Figure 2c and d exhibit (i) ssb manifolds with different and less features corresponding to a quadrupolar broadened ssb pattern from a single Al site, (ii) an approximately doubling in line widths for the individual ssbs (from ∼7 to ∼14 ppm), and (iii) line shapes for the central transitions, which tail toward lower frequency. The latter observation indicates a distribution of 27Al quadrupole coupling parameters, as reported earlier for 27Al MAS NMR spectra of poorly crystalline materials38 and of alumina hydrates.39 Such a distribution may also account for the lack of spectral features observed for the ssb envelopes and the increased line widths of the ssbs. The distribution in CQ and ηQ (i.e., in the EFG elements) reflects the reduced crystalline nature of pseudo-boehmite combined with its small crystallite size. The latter makes a significant contribution to the ssb intensities from AlO6 species in the vicinity of the surface, which exhibit slightly different local geometries. 27Al MAS and MQMAS NMR Spectra of ABCs. Thermal treatment of boehmite at 750 °C results in the metastable alumina polymorph, γ-Al2O3, which has a defect spinel structure with an fcc packing of oxygen atoms.6,40 The spinel structure includes 8 tetrahedral Al sites (AlIV), 16 octahedrally coordinated Al atoms (AlVI), and 32 oxygen sites per unit cell, implying that 8/3 aluminum vacancies are present to fulfill the Al2O3 stoichiometry. 27Al MAS NMR spectra of the central transition for γ-Al2O3 have been reported in several studies,10,13,33,34,39,41-43 and generally, these spectra are dominated by rather featureless resonances from AlIV and AlVI. However, in some cases, a third low-intensity center band in between these resonances is observed and assigned to a pentacoordinated Al site (AlV). The 27Al MAS NMR spectrum of the central transition for the pure pseudo-boehmite precursor, calcined at 750 °C, is shown in Figure 3a. The absence of a second-order quadrupolar line shape and the low-frequency tail for the center band resonances from the AlIV and AlVI sites indicate some disorder in the γ-Al2O3 structure. It is well-known that the Al atoms near the surface of the γ-Al2O3 crystallites may exhibit very large quadrupole couplings that hamper a reliable 27Al NMR detection of these sites.10,44-46 However, the visibility of these 27Al sites is significantly improved by high-speed spinning at high magnetic fields, as demonstrated for γ- and η-Al2O3 samples by Kraus et al.46 Since the 27Al MAS NMR spectra in Figure 3 are recorded at a high magnetic field using high-speed spinning and a short excitation pulse,25 a reliable estimate of the fraction for the different Al sites can be obtained from these spectra. A quantitative evaluation of the center band intensities for the pure precursor calcined at 750 °C (Figure 3a) gives fractions of 30.7, 3.7, and 65.6% for the AlIV, AlV, and AlVI sites, respectively. These intensities correspond to a AlVI/AlIV ratio of 2.13, which shows that the 8/3 cation vacancies per unit cell are distributed over tetrahedral as well as octahedral interstices (a uniform distribution corresponds to a AlVI/AlIV ratio of 2:1).40 The AlVI/AlIV ratio for the calcined precursor agrees very well with earlier reported ratios for γ-Al2O3 of 2.13 from 27Al MAS NMR10 and of 2.19 from Rietveld refinement of XRD data.47 Thus, these studies strongly suggest that the cation vacancies slightly favor the tetrahedral sites in γ-Al2O3.
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Figure 3. 27Al MAS NMR spectra (νR ) 15.0 kHz, 1H decoupling) of the central transitions for (a) the alumina support calcined at 750 °C and the ABC samples calcined at 750 °C and corresponding to boron contents of (b) 0.9, (c) 2.1, and (d) 3.4 wt % for the precursors. The intensities are normalized relative to the center band for octahedrally coordinated Al. The low-intensity resonance observed at 108 ppm in all spectra is a spinning sideband from the center band at 10 ppm.
Obviously, these AlVI/AlIV intensity ratios, which reflect the order with respect to the aluminum arrangement, also depend on the nature and purity of the precursor as well as on the specific thermal treatment. The ABC samples with different boron contents employ the same precursor and heating scheme, which justify a comparison of the 27Al intensities for these samples. A quantitative evaluation of the 27Al MAS NMR spectra for the ABCs with boron contents of 0.9, 2.1, and 3.4 wt % (Figure 3b-d) reveals that the AlIV intensity decrease with increasing boron loading, i.e., AlIV intensities of 29.9, 28.9, and 24.4%, are found for these samples, respectively. The corresponding intensities for AlVI are almost independent of the boron content (66.2, 66.5, and 68.7% for the ABCs with 0.9, 2.1, and 3.4 wt % boron) while the intensity for the small AlV resonance increases as 3.5, 4.2, and 7.0%, respectively, for these samples. The AlIV, AlV, and AlVI intensities observed for the ABCs containing 0.25 and 1.3 wt % boron (not shown) further support these observations. Thus, the 27Al MAS NMR spectra for the ABC samples show that the borate species interact with the cation sublattice of the γ-Al2O3 support, where the reduction in the fraction of AlIV sites with increasing boron loading indicates that some of these sites are substituted by tetrahedral boron sites. Therefore the BO4 species are not only present at the surface of the support but incorporated in the bulk of the
7214 J. Phys. Chem. C, Vol. 112, No. 18, 2008 alumina, e.g., within the first molecular layers of the calcined alumina-boria catalyst support. The fraction of AlV for the pure ABC calcined at 750 °C (3.7%, Figure 3a) agrees excellently with the mole percent of pentacoordinated Al3+ ions of 3.3%, reported very recently for γ-alumina from a ultrahigh field (21.1 T) 27Al MAS NMR spectrum,48 where the increased chemical shift resolution allows observation of the AlV center band almost without any overlap with the AlVI resonance. 27Al spin-lattice relaxation times (T1) were also reported in that study for the three different coordination environments, and it was proposed that the pentacoordinated Al3+ sites are located on the surface of the γ-alumina particles, since these sites exhibit a significantly shorter (T1) relaxation time as compared to the AlIV and AlVI sites. The increase in AlV intensity with increasing boron loading (Figure 3b-d) may potentially reflect the presence of these sites on the γ-Al2O3 surface if these sites are formed in the vicinity of the tetrahedrally coordinated B3+ ions incorporated in the outer layers of the support. However, care should be exercised in the interpretation of small changes in AlV intensity since it is our experience that the intensity and spectral appearance for the AlV center band are considerably more sensitive to exact magic angle setting than the dominating AlIV and AlVI center bands. Furthermore, the low-frequency part of the AlV center band overlaps with the AlVI resonance at the magnetic field of 14.09 T, and thus, a very high field, e.g., as employed in the most recent study of γ-Al2O3 supports,48 may be required to derive more conclusive results about the AlV intensity. The asymmetric line shape of the 27Al central transitions for AlIV and AlVI, tailing toward lower frequency (Figure 3) indicates a distribution in isotropic chemical shifts or quadrupole coupling parameters.38,39 Supplementary information on this issue can be obtained by 27Al MQMAS NMR, since this method removes the second-order quadrupolar broadening in the isotropic dimension.21 The 27Al MQMAS NMR spectra (Figure 4) of the pure alumina and the ABC with highest boron loading, both calcined at 750 °C, show only a minor reduction in line width for the AlIV and AlVI resonances in the isotropic (F1) dimension of these spectra as a result of the Al disorder in the support material. A comparison of the shapes for the contours of the AlIV and AlVI resonances with the axes for a pure chemical shift (CS) and quadrupolar-induced shift (QIS), Figure 4a, indicates that the AlVI resonance is mainly affected by a dispersion in quadrupole coupling parameters since the tail of the resonance is parallel to QIS. This is further supported by a similar MQMAS spectrum of the ABC 3.4 wt % B sample recorded at 7.1 T (not shown) where the tail for the AlVI resonance along QIS is more clearly observed as a result of the inverse proportionality of the quadrupolar-induced shift with magnetic field strength. On the other hand, the AlIV resonance is to a much smaller extent affected by dispersions in δiso, CQ, or ηQ since the contours largely follow the F2 direction of the MQMAS spectrum. The 27Al MQMAS NMR spectra in Figure 4 appear to be of somewhat higher quality than those reported in an earlier 27Al MQMAS NMR study of P- and Mo-loaded γ-Al2O3 samples,10 in which it was suggested that both the AlIV and AlVI sites are affected by large distributions in δiso and quadrupole coupling parameters from the resonance shapes in the MQMAS NMR spectra. The 27Al MQMAS NMR spectra of the ABC and ABC 3.4 wt % B samples in Figure 4 are quite similar, the main difference being a reduced intensity for the AlIV resonance in the B-loaded sample. Thus, the spectra indicate the absence of any new Al resonances associated with the loading of boron. It is noted that
Hansen et al.
Figure 4. 27Al MQMAS NMR spectra (νR ) 15.0 kHz) of (a) the alumina support calcined at 750 °C and (b) the ABC 3.4 wt % B sample calcined at 750 °C. 1H decoupling is employed during the whole 3-pulse sequence and acquisition of the FIDs. The projections onto the F1 and F2 axes correspond to summations in these dimensions over the 2D spectra. The asterisks indicate a spinning sideband from the AlVI site in the isotropic (F1) dimension.
amorphous phases of AlPO4 and an aluminate molybdate compound have been detected in 27Al MQMAS NMR spectra of γ-Al2O3 samples with high loadings of P (10 wt %) and Mo (12 wt %).10 11B MAS NMR of the Satellite Transitions for ABCP and ABC. The 11B MAS NMR spectrum of the alumina-boria catalyst precursor with the highest boron content (ABCP 3.4 wt % B, Figure 5b) clearly reveals the presence of borate species in trigonal (BO3) and tetrahedral (BO4) coordination, considering either the central transitions or the ssb manifolds originating from the satellite transitions. Simulation of the second-order quadrupolar line shape for the central transition of the BO3 site gives the isotropic chemical shift and quadrupole coupling parameters, δiso ) 18.4 ppm, CQ ) 2.51 MHz, and ηQ ) 0.31. An adequate estimation of these parameters for the BO4 site is most conveniently obtained by analysis of the ssb’s from the satellite transitions in Figure 5b, which have been observed here for the first time for 11B in an alumina-boria catalyst and at a level of only 3.4 wt % B. Least-squares optimization of simulated to experimental ssb intensities for the satellite transitions of the BO4 site gives the parameters, δiso ) 0.7 ppm, CQ ) 0.48 MHz, and ηQ ) 0.55. This method provides a significantly higher precision of these parameters as compared to line shape simulations of the central transition, recently
Alumina-Boria Catalysts and Their Precursors
Figure 5. 11B MAS NMR spectra (14.09 T) of the central and satellite transitions for (b) the ABCP 3.4 wt % B sample and (c) the corresponding ABC calcined at 750 °C. Both spectra are recorded using a spinning speed of νR ) 14.0 kHz, high-power 1H decoupling, a repetition delay of 2.0 s, and approximately 120.000 scans. The central transitions are cut off at ∼1/100 of the total height for center band of the BO3 site. (a) Simulation of the satellite transitions for the spectrum in part (b), employing the “average” parameter set δiso ) 0.7 ppm, CQ ) 0.48 MHz, ηQ ) 0.55 for the BO4 site and δiso ) 18.4 ppm, CQ ) 2.51 MHz, ηQ ) 0.31 for the BO3 site, as determined from optimizations to the ssb intensities and to the center band line shape in (b), respectively. The inset in (a) illustrates the corresponding simulation for the central transitions of the two 11B sites employing the same parameters.
employed in the analysis of 11B MAS NMR spectra (9.4 T) for the BO3 and BO4 sites in alumina-boria catalysts prepared by sol-gel synthesis.17,49,50 The simulated spectrum in Figure 5a for the experimentally observed satellite transitions (Figure 5b), employing the 11B quadrupole coupling parameters determined from the central (BO3) and satellite (BO4) transitions, reveals a number of characteristic features for the envelopes of ssb’s (“horns”, shoulders, and edges), which are somewhat smeared out in the manifold of ssb’s for the experimental spectrum. In analogy to the corresponding 27Al MAS NMR spectrum (Figure 2c), this reflects the fact that the alumina surface does not include BO3 and BO4 units in highly ordered arrangements. Thus, the 11B NMR parameters given above should be considered an average parameter set for the slightly different BO3 and BO4 environments on the alumina surface. The 11B MAS NMR spectrum of the corresponding aluminaboria catalysts (ABC 3.4 wt % B, calcined at 750 °C, Figure 5c) includes a manifold of ssb’s from the BO3 site over the same spectral range as observed for the BO3 site in the precursor. Simulation of the second-order line shape for the central transition in Figure 5c gives the parameters δiso ) 17.7 ppm, CQ ) 2.56 MHz, and ηQ ) 0.02, which indicates a minor change in asymmetry parameter toward axial symmetry of the BO3 species as compared to the ηQ value for the BO3 site of the precursor. Some changes are also observed in the ssb manifold
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Figure 6. 11B MQMAS NMR spectra (νR ) 15.0 kHz) of the ABC 3.4 wt % B samples calcined at (a) 750 and (b) 950 °C. 1H decoupling is employed during the full 3-pulse sequence and acquisition of the FIDs. The projections onto the F2 axes correspond to summations in this dimension over the 2D spectra. The asterisks indicate spinning sidebands in the isotropic (F1) dimension. which at the actual spinning speed result in overlap with the center band resonances in the summations of the F1 dimension (cf., Figure 7).
for the BO4 site, which exhibits a bell-shaped envelope, indicating a larger distribution in BO4 quadrupole coupling parameters. Least-squares analysis of the ssb intensities results in the parameters δiso ) 0.7 ppm, CQ ) 0.49 MHz, and ηQ ) 0.60, and thereby a quadrupole coupling of similar magnitude as observed for the precursor. However, the small increase in the “average value” for ηQ reflects a more pronounced bellshaped form of the ssb manifold compared to Figure 5b and thereby an increased variation in BO4 environments for the ABC material. 11B MQMAS NMR Spectra of ABC. To obtain additional information on the variation in local BO3 and BO4 environments, the ABC 3.4 wt % B samples calcined at 750 and 950 °C have been investigated by 11B MQMAS NMR (Figure 6), despite the low overall boron content in these samples. This experiment has recently been used for similar alumina-boria materials, however, with the principal aim of resolving the BO3 and BO4 resonances at lower magnetic fields.19 To suppress phase distortions and artifacts, the MQMAS NMR spectra employ rotor-synchronized acquisition in t1 and t2 with time increments of 1/(3νR) for νR ) 15.0 kHz. Unfortunately, this spinning frequency results in first-order ssb’s from the BO3 and BO4 sites, which overlap in the isotropic (F1) dimension with the center
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Figure 7. Summations of the isotropic dimensions in the 11B MQMAS NMR spectra shown in Figure 6 for the ABC 3.4 wt % B samples calcined at (a, b) 750 and (c, d) 950 °C. The summations in (a) and (c) include only contributions from the spectral region for BO3 sites in the F2 dimension, i.e., slices from 20 to 5 ppm while (b) and (d) show the corresponding summations for the resonance from BO4 tetrahedra (slices from 4 to -4 ppm in the F2 dimension).
band resonances from BO4 and BO3, respectively. This overlap gives a broadening of the peaks in the summation over the total projections on to the F1 and F2 dimensions, in particular for the low-intensity BO3 center band. Thus, sliced summations over the individual resonances, using spectral ranges in the F2 dimension of -4 to 4 ppm and 5 to 20 ppm for BO4 and BO3, respectively, have been obtained as illustrated in Figure 7. In contrast to the 11B MAS NMR spectrum of ABC calcined at 750 °C (Figure 5c), the MQMAS spectrum displays the BO3 center band with lower height than the BO4 resonance (cf., F2 projection, Figure 6a). This reflects an inefficient excitation of the triple-quantum coherences for this site due to its relatively large quadrupole coupling combined with the use of a moderate 11B rf field (i.e., γB /2π ≈ 65 kHz). The contours of the BO 1 4 center bands are almost identical in the MQMAS spectra of the ABCs at both calcination temperatures, and from the CS and QIS directions (Figure 6a), it is apparent that the BO4 sites exhibit a significant dispersion in chemical shifts. On the other hand, the contours for the BO3 species are almost aligned along the F2 direction, reflecting a somewhat smaller chemical shift distribution for these species (obviously, the low intensity for BO3 must be considered in this evaluation). Overall, the experimental 11B MAS and MQMAS NMR spectra (Figures 5-7) indicate that the calcination mainly affects the local environment for the BO4 tetrahedra, since these sites in ABC 3.4 wt % B exhibit a larger dispersion in 11B quadrupole couplings and chemical shifts as compared to the precursor. This observation indicates that a minor fraction of BO4 species is incorporated in the bulk of the alumina support after calcination, which thus supports the results from 27Al MAS NMR (Figure 4). Fractions of BO3 and BO4 Species for ABCP and ABC. The spectral features of the 11B MAS NMR spectra for the ABCP samples with boron loadings in the range 0.25-3.4 wt % and the corresponding ABCs after calcination are generally quite similar to those observed for the 11B central transitions shown in Figure 5 for a loading of 3.4 wt % B. Thus, the main variation between these spectra is related to the different
Figure 8. Results from quantitative analysis of the intensities for the central transition observed in 11B MAS NMR spectra of the ABCP and ABC materials. (a) Quantities of boron in the form of BO3 and BO4 species in the ABCP samples as a function of the total boron content. Fraction of BO4 species, determined as I(BO4)/{I(BO4) + I(BO3)}, as a function of (b) the total boron content for the ABCPs and (c) the calcination temperature for the ABC samples. In (c), the circles (O), crosses (×), and triangles (4) correspond to ABCs with total boron contents of 0.25, 1.3, and 3.4 wt % boron, respectively.
fractional content of BO3 and BO4 species in the samples. The results from spectral analysis of the intensity variation in the 11B MAS NMR spectra for the ABCP samples with different boron loadings are summarized in Figure 8a and b, where the concentrations of the BO3 and BO4 species are calculated from
Alumina-Boria Catalysts and Their Precursors the BO4/BO3 intensity ratios for the 11B central transitions in combination with the total boria content of the samples. The quantities of the BO3 and BO4 sites both increase almost linearly with increasing boron content (Figure 8a) while the fraction of BO4 sites (i.e., BO4/[BO3 + BO4]) tends to increase slightly with the total boron content (Figure 8b). These results agree well with an earlier 11B MAS NMR study of alumina-boria catalysts by Sato et al.;14 however, they observed a somewhat larger fraction of BO4 sites, which increased almost linearly from 14 to 28% for boron contents in the range 0.5-2.3 wt % B. The fractions of BO4 sites for the ABCPs (Figure 8b) are very similar to the value of 0.13 reported from 11B MAS NMR for low concentrations of boron (0.03-0.35 wt %) on a boehmite surface.51 Moreover, it was found that the fraction of BO4 sites does not vary significantly with either pH (from 3 to 11) or concentration of the borate solution. The results in Figure 8a and b indicate that the borate coverage of the precursor surface does not exceed one monolayer when the specific surface area of the support material (450 m2/g) is taken into account. If it is assumed that borate is present as planar BO3 units on the surface only, it can be estimated that a 0.25-3.4 wt % boron loading corresponds to borate surface areas in the range 10-120 m2/g and thereby coverages well below one monolayer. The small decrease in the fraction of BO4 species at a boron loading of 2.1 wt % (Figure 8b) could possibly reflect the formation of BO3 chains at the surface, in line with the results from a recent study of alumina-boria xerogels.49 This interpretation implies that the BO3 chains are partly formed at the expense of BO4 sites and as consequence of a significant coverage of a borate monolayer on the alumina surface. On the other hand, the fraction of BO4 sites increases slightly again for the highest boron-loading (3.4 wt %, Figure 8b). For the calcined material (ABC) with lowest boron content, the fraction of BO4 sites is almost independent of the calcination temperature (Figure 8c) and equal to the value of the corresponding ABCP sample. This invariance may reflect that the boron content is well below the value for a monolayer boria coverage on the alumina surface. At higher boron contents, the proportion of BO4 sites (Figure 8c) increases significantly with increasing calcination temperature. However, the ABC of highest boron content and exposed to calcination at 950 °C does not follow this trend. The 11B MAS NMR spectrum of this sample (Figure 9b) clearly reveals a shift of the BO3 center band by ∼2.5 ppm to lower frequency as compared to the BO3 center bands for the other ABCs and shows distinct features of a second-order quadrupolar line shape, indicating the presence of an additional species/phase of BO3 units with a high degree of local order. The 11B quadrupole coupling and chemical shift parameters are determined for this site from optimization of two simulated quadrupolar line shapes to the BO3 central transition in Figure 9b, employing fixed parameters for the site that corresponds to BO3 species dispersed on the alumina support and the average data determined for this site in the ABC 3.4 wt % B sample at 750 °C (δiso ) 17.7 ppm, CQ ) 2.56 MHz, and ηQ ) 0.02, Figure 5c). This optimization results in the parameters δiso ) 16.9 ( 0.2 ppm, CQ ) 2.65 ( 0.04 MHz, and ηQ ) 0.14 ( 0.05, and a relative BO3 intensity of 55 ( 5% for the highly ordered BO3 sites. The slightly larger quadrupole coupling and lower δiso value, as compared to the BO3 species on the surface, are in line with the data reported earlier for B2O3 (CQ ) 2.69 MHz, ηQ < 0.05, δiso ) 14.6 ppm)52 and suggests that an ordered B2O3 phase has been formed by sintering a part of the large amount of borate in this sample.
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Figure 9. 11B MAS NMR spectra (νR ) 12.0 kHz, 1H decoupling, and a 10-s repetition delay) illustrating the spectral region for the central transitions for the ABC samples with (a) a 3.4 wt % boron content and calcined at 850 °C, (b) 3.4 wt % boron and calcined at 950 °C, (c) 1.3 wt % boron and calcined at 950 °C, and (d) 0.25 wt % boron and calcined at 950 °C. The dashed lines illustrate the frequencies for the singularities of the second-order quadrupolar line shape observed in (b) for the dominating BO3 site in the ABC 3.4 wt % B calcined at 950 °C.
Additional information about the BO3 species may be obtained from the 11B MQMAS NMR spectra and the corresponding F1 summations (Figures 6 and 7) for ABC 3.4 wt % B calcined at 750 and 950 °C. While a rather broad and featureless BO3 resonance is observed at δ3Q ) 45 ppm in the isotropic dimension for the ABC calcined at 750 °C (Figure 7a), the corresponding summation for the 950 °C sample (Figure 7c) shows an additional peak at 40 ppm, originating from the ordered B2O3 phase. The distinct spectral features for this phase are also apparent from a careful inspection of the contours for BO3 in the 11B MQMAS NMR spectrum (Figure 6b), since features of a second-order quadrupolar line shape are observed in slices along the F2 dimension at a frequency of δ3Q ≈ 41 ppm. Thus, the 11B MQMAS spectrum demonstrates that the ABC 3.4 wt % sample calcined at 950 °C includes two BO3 species, i.e., BO3 sites associated with the alumina surface and a separate, ordered phase with 11B NMR characteristics that resemble those reported for B2O3. 27Al and 11B Homonuclear Dipolar Recoupling Experiments for ABC. Connectivities between different AlO4, AlO5, and AlO6 polyhedra as well as BO3 and BO4 units on the surface of the ABCs may be elucidated from 27Al and 11B homonuclear correlation experiments, employing a recently developed pulse sequence22 for half-integer spin quadrupolar nuclei shown in
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Figure 10. Contour plots of the (a) 27Al and (c) 11B SQ-SQ correlation MAS NMR spectra (14.09 T, 1H decoupling) for ABC 1.3 wt % B obtained with the pulse sequence for homonuclear dipolar recoupling shown in Figure 1c. The 27Al spectrum (a) employed νR ) 14.0 kHz, a 2-s relaxation delay, 160 t1 increments, 240 scans, a mixing time of 4.571 ms, and nRAPT ) 26. The corresponding 11B spectrum (c) was obtained with νR ) 15.0 kHz, a 6-s relaxation delay, 160 t1 increments, 64 scans, a mixing time of 2.133 ms, and nRAPT ) 16. Parts b and d illustrate slices in the F2 dimension corresponding to the dashed lines shown in the contour plots. The slices in (b) for the AlV and AlIV center bands are vertically expanded by factors of 8 and 4, respectively, relative to the intensity in the slice for the AlVI center band.
Figure 1c. Using this sequence for 27Al in a study of ABC 1.3 wt % (calcined at 750 °C) results in the 27Al single-quantumsingle-quantum (SQ-SQ) correlation spectrum shown in Figure 10a. From a comparison with the center bands in 27Al MAS NMR spectra of similar ABCs (Figure 3), it is apparent that cross-peaks between the AlIV and AlVI resonances are clearly observed, in accordance with the fact that these polyhedra share oxygen atoms in γ-Al2O3.6,40 The cross-peaks are best appreciated from the slices of the 2D spectrum (Figure 10b) which also reveal a AlV T AlVI cross-peak (despite low intensity) in the slice taken at 35 ppm in the F1 dimension. On the other hand, a cross-peak between the AlIV and AlV center bands can hardly be identified from this slice, which indicates the absence of (O-)3Al-O-Al(-O)4 bondings. It has earlier been proposed that the AlV sites are located at the surface of the alumina particles43 and the absence of AlV T AlIV connectivities potentially reflects that the surface AlV sites share oxygen atoms only with AlVI units. However, a decisive conclusion regarding the AlV environment cannot be derived from the present spectrum, considering the quite low intensity for the AlV center band and cross-peak(s). Application of the homonuclear 11B dipolar recoupling experiment to ABC 1.3 wt % (calcined at 750 °C) seems more challenging, considering the low boron content for the sample. Nevertheless, the 11B SQ-SQ correlation experiment (Figure 10c) clearly reveals a cross-peak between the BO3 and BO4
center bands in the F1 low-frequency part of the 2D spectrum, i.e., corresponding to the slice at 0 ppm in the F1 dimension. Although the intensity of this cross-peak is low (∼1% of the maximum intensity for the diagonal BO3 resonance), it is comparable to the intensity of the BO4 resonance in the same slice when the difference in line widths is taken into account. Thus, this cross-peak shows the presence of BO3 T BO4 connectivities, which strongly suggests that these units are connected by (O-)2B-O-B(-O)3 bonds. Thereby a network of BO3 and BO4 units exists on the surface of the alumina particles. We note that the second cross-peak, expected from the BO3 T BO4 connectivities (i.e., at 15 ppm in F1 and 0 ppm in F2), can vaguely be observed in a vertical expansion of the 2D spectrum, however, with an intensity below 0.5% of the maximum value. The reason for the low intensity of this crosspeak is at the moment not clear. Potentially it could arise from a minor truncation of the data in the F1 dimension caused by a long T2 relaxation associated with the BO4 units. 11B{27Al} REDOR and TRAPDOR NMR Spectra of ABCP and ABC. Direct evidence for the presence of borate species in the vicinity of aluminum may be achieved from 11B{27Al} REDOR experiments, an approach that has been successfully employed in the evaluation of site connectivities in aluminoborate glasses.53 In the present study, the standard REDOR sequence23 has been used to observe 11B for borate species that are dipolar coupled to 27Al in the ABCP and ABC
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Figure 11. 11B{27Al} REDOR NMR spectra (νR ) 12.0 kHz, 1H decoupling) of the precursors (a) ABCP 0.25 wt % B, (b) ABCP 3.4 wt % B, and the corresponding samples calcined at 750 °C, (c) ABC 0.25 wt % B and (d) ABC 3.4 wt % B. The 11B spin-echo reference spectra (S0) are shown in the upper row, the REDOR spectra (S) including 27Al irradiation in the middle row, while the REDOR difference spectra (∆S ) S0 - S) are shown in the lower row. All spectra employ a total evolution period (NTr) of 12 rotor cycles (1.0 ms) and their intensities are normalized relative to the intensity of the BO3 resonance in the individual 11B spin-echo reference spectra. The carrier frequencies for 11B and 27Al were set in between the resonances for the BO3-BO4 and AlIV-AlVI central transitions, respectively.
Figure 12. REDOR curves for the BO3 (4) and BO4 (0) sites plotted as the normalized REDOR fraction (∆S/S0) versus the evolution time (NTr) obtained from 11B{27Al} REDOR NMR spectra (νR ) 12.0 kHz, 1 H decoupling, cf. Figure 11) of (a) ABCP 3.4 wt % B, ABC 3.4 wt % B calcined at 750 °C and (b) ABCP 1.3 wt % B, ABC 1.3 wt % B calcined at 750 °C. Open symbols correspond to the precursor samples while solid symbols represent the catalysts calcined at 750 °C. The dashed lines are guides to the eye.
(750 °C) samples with lowest (0.25 wt %) and highest (3.4 wt %) boron content (Figure 11). The 11B spin-echo reference spectra (S0), employing a short evolution time (NTr), along with those employing 27Al irradiation (S), result in the REDOR difference spectra (∆S ) S0 - S), which only include resonances from borate sites that are strongly dipolar coupled to AlOx (x ) 4,5,6) species of the support. For the highest boron content, these spectra show that the BO3 and BO4 sites are in the near vicinity of Al sites for both the ABCP precursor (Figure 11b) and the calcined ABC material (Figure 11d). Furthermore, even
Figure 13. 11B{27Al} TRAPDOR NMR spectra (νR ) 10.0 kHz, 1H decoupling) of the ABC 3.4 wt % B sample calcined at 750 °C. (a) 11 B spin-echo reference spectrum, (b) TRAPDOR spectrum (S) including 27Al irradiation during the first half of the evolution period, and (c) TRAPDOR difference spectrum (∆S ) S0 - S), all employing a total evolution period (NTr) of 12 rotor cycles (1.2 ms). The carrier frequencies for 11B and 27Al were set in between the BO3-BO4 and AlIV-AlVI center band resonances, respectively. (d) TRAPDOR curves (∆S/S0) for the BO3 (4) and BO4 (0) sites as a function of the NTr evolution time from 11B{27Al} TRAPDOR NMR spectra, recorded under similar conditions as in (a)-(c), for the ABCP and ABC (750 °C) samples containing 3.4 wt % B. Open symbols correspond to the precursor while solid symbols represent the calcined material. The dashed lines are guides to the eye.
for the lowest boron content (Figure 11a and c), a REDOR effect is clearly observed for the BO3 sites whereas the resonance from
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Figure 14. Schematic drawing for the calcined alumina-boria catalysts (with a boron content: wt % B j 3.4), illustrating the possible boria surface species and their connectivity to the outer layers of the alumina support based on the single- and double-resonance 11B and 11Al MAS NMR experiments performed in this work. The terminal oxygen atoms of the borate units are most likely hydroxyl groups.
BO4 is more vague in the difference spectra, most likely as a result of the low boron concentration and low fraction of BO4 sites in the ABCP and ABC samples containing 0.25 wt % B. 11B{27Al} REDOR NMR spectra have also been obtained for the ABCP and ABC (750 °C) materials containing 1.3 wt % B (not shown), which reveal REDOR difference spectra similar to those in Figure 11 and with resonances from both the BO3 and BO4 sites. Determination of the ∆S/S0 fraction in REDOR spectra, incrementing the evolution period for the dipolar recoupling in terms of rotor periods (NTr), may provide information about the strength of the 11B-27Al dipolar interactions. For the ABCP and ABC (750 °C) samples with boron contents of 3.4 and 1.3 wt %, these ∆S/S0 curves (Figure 12a and b, respectively) increase almost linearly with increasing recoupling time for both the BO3 and BO4 sites and without any characteristic dipolar oscillations as observed for isolated pairs of spin-1/2 nuclei.23,54 This behavior may reflect a nonuniform dispersion of BO3 and BO4 species on the alumina surface, effects from 11B-27Al multiple-spin systems, or the fact that both nuclei are half-integer spin quadrupolar nuclei.55,56 However, for short evolution times, the slope of the ∆S/S0 curves can be related to the dipolar interactions using second-moment analysis; i.e., the steepness of the curve is proportional to the second moment and thereby the average I-S dipolar coupling strength.54,57 Qualitatively, this approach reveals that the slightly steeper slopes, observed for BO4 as compared to the BO3 sites in the calcined ABCs only, reflect the presence of a somewhat stronger 11B-27Al dipolar coupled environment for the BO4 species. Thus, considering the disordered nature of the material, this finding also supports the assumption that part of the BO4 tetrahedra is incorporated within the first molecular layers of the alumina support, in agreement with our interpretation of the relative intensities observed in the 11B and 27Al MAS NMR spectra (vide supra). We note that the intensity of the BO4 resonance is very low for long recoupling times in the 11B{27Al} REDOR spectra of the ABCP samples, implying that reliable ∆S/S0 curves cannot be obtained. This accounts for the scatter in ∆S/S0 values for the BO4 sites of the ABCPs in Figure 12. However, the corresponding ∆S/S0 data for the BO3 sites clearly reveal smaller values of the slope for the ABCPs as compared to the ABCs. A plausible explanation of this observation may be that the BO3 sites are more strongly bonded to the alumina surface for the calcined materials as compared to the hydrous precursors. For the ABCPs, boron may be present in hydrous units containing several BO3 and BO4 units that on calcination dehydrate and form additional B-O-Al bonds to alumina surface. Complementary information on 11B-27Al connectivities in the ABC and ABCP samples may be achieved from 11B{27Al} TRAPDOR NMR experiments,24 where recoupling of heteronuclear I-S dipolar couplings is achieved by interchange of population numbers for the 2I + 1 Zeeman states for the nonobserved nucleus (I) during rf irradiation and MAS (cf., pulse
sequence in Figure 1). Thereby, TRAPDOR should in principle give a more efficient recoupling, since all Zeeman states are involved, as compared to REDOR, where the π pulse train used for recoupling is selective with respect to the central transitions. However, a requirement for the TRAPDOR experiment is that the population transfer can be characterized as “adiabatic”, which can be fulfilled by adjustment of the rf field strength or the spinning speed if the approximate magnitude of the quadrupole couplings is known for the nonobserved spin species.58 11B{27Al} TRAPDOR spectra have been obtained for the ABCP and ABC (750 °C) samples with boron contents of 1.3 and 3.4 wt % as illustrated in Figure 13 for the 3.4 wt % B samples. The TRAPDOR difference spectrum (Figure 13c) clearly reveals resonances from both the BO3 and BO4 sites with intensities that are quite similar to those observed by 11B{27Al} REDOR NMR (Figure 11d) for nearly the same recoupling time. Furthermore, the ∆S/S0 intensity variation as a function of recoupling time is very similar for the two sites in the calcined material, with a slightly stronger dipolar dephasing for BO4 relative to BO3 in agreement with the results from the REDOR experiments. However, the steepness of the ∆S/S0 curve for BO3 in the ABCP sample is also similar to the two curves from the ABC in contrast to the observations by REDOR (Figure 12a), where the slopes of the curves for BO3 in ABCP and ABC are quite different. Considering the hydrous nature of the surface for the ABCP, this difference may potentially reflect that the TRAPDOR experiment, under the actual experimental conditions, is less sensitive to “interfering” 1H dipolar couplings with 11B and 27Al than the REDOR technique. Moreover, in contrast to our expectations, the steepness of the TRAPDOR curves is generally lower than for the REDOR curves for the individual 11B sites in Figures 13d and 12a, respectively. This indicates a less efficient recoupling by TRAPDOR, which may be ascribed to the low rf field strength (γB1/2π ) 23 kHz) employed for the 27Al pulse in the TRAPDOR experiment. The intensities for BO4 in the ABCP sample observed by TRAPDOR show that a reliable ∆S/S0 curve cannot be obtained for long recoupling times by this technique either. Finally, we note that the 11B{27Al} TRAPDOR spectra for the ABCP and ABC (750 °C) samples containing 0.25 and 1.3 wt % B exhibit the same spectral features as presented in Figure 13 for the sample with high boron loading. Thus, the TRAPDOR experiments primarily serve as an independent proof of the presence of 11B-27Al connectivities for both the BO3 and BO4 sites, thereby resulting in the same conclusions as derived from the REDOR experiments. Conclusion From the NMR spectroscopic point of view, the present work demonstrates the successful application of 11B MQMAS and MAS NMR of the satellite transitions in the characterization of the BO3 and BO4 environments for alumina-boria catalysts,
Alumina-Boria Catalysts and Their Precursors despite the low concentration of boron (0.25-3.4 wt % B) in these samples. In particular, the clear reflection of such variations in the 11B MAS NMR spectra of the satellite transitions show that care should be exercised when quadrupole coupling parameters are extracted for boria-alumina catalysts or similar materials from line shape simulations of the central transitions alone. A dispersion in quadrupole coupling parameters, and in some cases also the chemical shifts, for the individual 11B and 27Al sites have been observed in 11B and 27Al MQMAS NMR spectra as well as in the MAS NMR spectra of the satellite transitions. The dispersion in quadrupole coupling parameters has been evaluated qualitatively, although a quantitative measure for these distributions may be established from simulations that consider distributions for the principal elements of the electric field gradient tensor in the simulations. The 11B{27Al} REDOR and TRAPDOR NMR experiments are found to be strong tools for obtaining heteronuclear connectivity information for the surface species of the catalysts, in accordance with an earlier study employing similar 31P-27Al REDOR NMR experiments for phosphorus-impregnated aluminas.13 Finally, the application of a homonuclear correlation experiment for quadrupolar nuclei22 to probe correlations between disordered borate sites in low abundance shows great promises for this experiment in studies of inorganic materials. From the catalyst materials point of view, the present NMR studies have provided new insight into the surface structure of alumina-boria catalysts with relatively low boron contents (0.25-3.4 wt % B). Considering the specific surface area of the alumina support, these boron contents are below the quantity required for full monolayer coverage of the support by borate units. The structural information gained from the NMR experiments is sketched in Figure 14, which summarize the type of borate species and their connectivities that may be found on the surface of the support material for the calcined catalyst samples. In this representation, the terminal oxygen atoms of the borate species are most likely hydroxyl groups, considering the method of sample preparation and the hygroscopic nature of alumina surfaces. The 11B homonuclear correlation experiment as well as the 11B{27Al} REDOR and TRAPDOR experiments have shown that BO3 and BO4 units form a network including (O-)2B-O-B(-O)3 bonds and that both borate species are anchored to the alumina surface by B-O-Al bonds. Moreover, the single- and double-resonance 11B and 27Al MAS NMR experiments have indicated that a small fraction of the BO4 species may be incorporated in the first molecular layers of the alumina support. The same borate species and network illustrated in Figure 14 may be found on the surface of the alumina-boria catalyst precursor, the main difference being that the surface of the precursor includes a significant amount of absorbed water. A detailed MAS NMR study of the hydrous surface structure for similar alumina-boria catalysts, using single- and double-resonance 1H MAS NMR techniques, is in progress in our laboratory. Finally, it has been observed that a high calcination temperature (950 °C) combined with a high boron loading results in an additional type of BO3 species, which according to its 11B NMR characteristics most likely originate from a separate B2O3 phase exhibiting a high degree of local order. Acknowledgment. The use of the facilities at the Instrument Centre for Solid-State NMR Spectroscopy, University of Aarhus, sponsored by the Danish Natural Science Research Council, the Danish Technical Science Research Council, Teknologistyrelsen, Carlsbergfondet, and Direktør Ib Henriksens Fond, is acknowl-
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