Structure and Dynamics of Hydrous Surface Species on Alumina

J. Phys. Chem. C 2009, 113, 2475–2486

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Structure and Dynamics of Hydrous Surface Species on Alumina-Boria Catalysts and Their Precursors from 1H, 2H, 11B, and 27Al MAS NMR Spectroscopy 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: August 26, 2008; ReVised Manuscript ReceiVed: NoVember 14, 2008

The hydrous surface of a pseudoboehmite precursor impregnated with 1.3 wt % boron, its dehydration upon gentle heat treatment, and the corresponding alumina boria catalyst obtained by calcination at 750 °C have been studied by 1H and 2H MAS NMR as well as double-resonance 1H{11B}, 1H{27Al} TRAPDOR, and 11 B{1H}, 27Al{1H} CP/MAS NMR experiments. Resonances from bulk and surface AlOH groups, surface BOH sites, and water molecules adsorbed on the surface-aluminate and -borate species have been identified and characterized. The dynamics of the water molecules and their removal from the surface by heat treatment have been studied by the double-resonance experiments. Optimizations of the 1H -11B/27Al cross-polarization conditions for the individual 11B and 27Al sites demonstrate that the BO4 as well as the BO3 sites on the alumina surface exhibit hydroxyl groups in their near vicinity. The same holds for the AlO4, AlO5, and AlO6 sites on the γ-Al2O3 surface of the alumina-boria catalyst. Introduction Aluminas (Al2O3) are widely used heterogeneous catalysts for a number of industrially important reactions and as support materials for hydrotreating catalysts employed in oil refineries for purification of gasoline, fuels, and fine chemicals. The catalytic properties of aluminas are mainly related to their acidity and surface structure1 which both can be modified by addition of minor quantities of other light elements such as silica, phosphates, boria, flourides, and chlorides. Of these additives, borates have recently attracted considerable interest because the reaction of borate species with the alumina surface may result in strong Brønsted acidic centers,2-7 thereby increasing the catalytic activity for acid-catalyzed conversions. In particular, the performance of boron-modified aluminas, i.e., alumina-boria catalysts, can be optimized toward specific catalytic reactions such as selective oxidation of ethane,8 Beckmann rearrangement,9,10 toluene disproportionation,3 skeletal isomerization of n-butenes,6,9 and dehydration of methanol.11 Thus, the catalytic properties are expected to be strongly related to the modifications of the alumina surface achieved, and potentially controlled, by the addition of borates. Therefore, an improved understanding of the surface structure for the alumina-boria support at an atomic/ nanoscale level seems highly relevant in order to achieve new developments of alumina-boria catalysts. This work reports a detailed characterization of the local hydrous environments for the aluminate and borate species on the surface of an alumina-boria catalyst (ABC) and its precursors (ABCP) using 1H, 2H, 11B, and 27Al MAS NMR spectroscopies in combination with cross-polarization (CP) and transfer of populations in double-resonance (TRAPDOR)12 MAS NMR techniques. The studied ABCP and ABC samples, prepared by the impregnation method using a low boron content (1.3 wt % B), are highly hygroscopic, and for this reason it is of interest to study the environments for the hydroxyl ions and water molecules on the surfaces of these catalysts. In a preceding * To whom correspondence should be addressed. Fax: +45 8619 6199. Tel.: +45 8942 3900. E-mail: [email protected].

paper, we investigated the structure of borate and aluminate species on the surface for a series of ABCs and ABCPs with different boron contents (0.25 wt % e B e 3.4 wt %), including those studied in this work, by single- and double-resonance 11 B-27Al MAS NMR experiments.13 In that study it was found that a small fraction of tetrahedrally coordinated boron is incorporated within the first molecular layers of the alumina support and that the BO3 and BO4 units on the surface form a network of (O-)2B-O-B(-O)3 bonds which are anchored to the alumina surface by B-O-Al bonds. 11 B and 27Al MAS NMR spectroscopies have emerged as key techniques in studies of structure and composition of aluminaboria catalysts.4-6,10,11,13-18 Although 11B and 27Al both are halfinteger spin quadrupolar nuclei, their popularity reflects the fact that the 11B and 27Al isotropic chemical shifts allow a clear distinction of BO3 and BO4 units in 11B MAS NMR and of the AlO4, AlO5, and AlO6 coordination polyhedra in 27Al MAS NMR, if the experiments are performed at moderate/high magnetic fields (B0 J 9.4 T) and for spinning speeds νr J 10 kHz. If these conditions are not fulfilled, an improved resolution of resonances from the borate and aluminate sites may be obtained by the multiple-quantum (MQMAS) NMR experiment,19 as utilized in a couple of studies of ABCs.11,18 11B MAS NMR has mainly been used for determination of the BO4 /BO3 ratio for alumina-boria catalysts prepared using different synthetic schemes and quantities of added borates in the syntheses. For example, such studies have shown that the concentration of tetrahedrally coordinated BO4 sites correlates with the number of Brønsted acid sites and increases with increasing boria content on the alumina support.4,5 Information about the borate surface species may also be obtained from 11B{1H} CP/MAS NMR, since this technique probes borate sites in the vicinity of hydroxyl groups or water molecules, if the experiment is performed under optimized conditions. In an earlier investigation of alumina-boria samples (B2O3 loadings of 5-30 wt %) using this method, it was claimed that only the resonance from BO4 units can be observed by 11 B{1H} CP/MAS NMR, thereby indicating that hydroxyl groups

10.1021/jp807595m CCC: $40.75  2009 American Chemical Society Published on Web 01/16/2009

2476 J. Phys. Chem. C, Vol. 113, No. 6, 2009 are only present in the nearest coordination sphere of these boron sites.15 In the present work we have performed a detailed analysis of the cross-polarization dynamics for the BO3, BO4 and AlO4, AlO5, AlO6 species observed by 11B{1H} and 27 Al{1H} CP/MAS NMR, respectively. For example, it is found that the 1H f 11B/27Al magnetization transfer depends strongly on the matching of the 1H and 11B/27Al rf field strengths in the CP experiment, which may explain the lack of any observable cross-polarization from 1H to the BO3 sites in the earlier 11B{1H} CP/MAS NMR study.15 Further information on 1H-27Al and 1 H-11B connectivities and thereby about the hydrous aluminaboria surface is obtained from 1H{11B} and 1H{27Al} TRAPDOR NMR experiments, which in analogy to CP utilize the heteronuclear 1H-11B/27Al dipolar interactions that exhibit an inverse proportionality with the cube of the internuclear distance (D ∝ r- 3). Thus, these NMR techniques are particularly useful in studies of hydrous surface sites for alumina-boria catalysts as demonstrated in the present work. Experimental Section Materials. Three alumina-boria catalyst precursors (ABCPs) were prepared by impregnation of pseudoboehmite with 0.8-1.8 M aqueous solutions of boric acid (H3BO3). After impregnation the samples were dried for 2 h at 110 °C. Chemical analysis revealed that the ABCP samples have a boron content of 0.25, 1.3, and 3.4 wt % B. The alumina-boria catalyst (ABC) was obtained by calcination of the ABCP with a 1.3 wt % boron content in air for 2 h, employing a calcination temperature of 750 °C. The ABCP and ABC samples have specific surface areas of approximately 450 and 360 m2/g, respectively, as determined by the BET method. Although the ABCP and ABC samples are highly hygroscopic, no initial precautions were taken to store or handle them under nonhumid conditions. However, a series of dried samples were prepared by heating the ABCP and ABC samples in an oven overnight in air and at specific temperatures in the range 100-250 °C. At these temperatures, the samples were transferred into airtight containers which after cooling were taken into a glovebox, where the samples were packed into homemade airtight NMR rotors using O-ringequipped end caps. Partially deuterated samples of the ABCPs were prepared by heating approximately 0.5 g of sample at 200 °C followed by cooling in a sealed container which contained a small volume of 99.9% D2O (1-2 mL). The deuterated water was placed in the bottom of the container with the powdered sample of ABCP on a glass plate just above the liquid. The heating/cooling procedure was repeated three times in order to increase the 2H for 1H exchange process. NMR Spectroscopy. The single-pulse and cross-polarization (CP) 11B{1H} and 27Al{1H} MAS NMR experiments were performed on a Varian Unity INOVA-600 spectrometer (14.09 T) using a home-built CP/MAS probe for 4 mm o.d. rotors. For both nuclei the single-pulse spectra employed a short rf pulse width, τp ) 0.5-1.0 µs, for an rf field strength of γB1/2π ≈ 65 kHz (i.e., corresponding to a flip angle of 12-24°) and high-power 1H decoupling (γB2/2π ≈ 65 kHz). The CP/MAS NMR spectra were obtained with this 1H rf field for the 90° 1H pulse (3.8 µs) and the 1H decoupling during acquisition but with a slightly lower and fixed 1H rf field strength during the CP contact time (γB2/2π ≈ 60 kHz). To investigate the specific Hartmann-Hahn matching condition for the individual borate and aluminate species, arrays of 11B and 27Al rf field strengths in the range γB1/2π ≈ 10-60 kHz were employed for a short CP contact time. In the variable contact-time CP experiments, the contact time was varied from 100 µs to 5.0 ms in steps of

Hansen et al. 200 µs. The 1H{11B} and 1H{27Al} TRAPDOR experiments as well as the 1H decoupled 27Al MQMAS spectrum were obtained on a Varian Unity INOVA-300 spectrometer (7.05 T) using a home-built CP/MAS probe for 5 mm o.d. rotors and with transmission-line tuning (TLT) for the high-frequency channel.20 The MQMAS spectrum employed the three-pulse z-filter sequence21 with 1H high-power decoupling (γB2/2π ≈ 100 kHz) during the t1 and t2 evolution periods and 27Al rf field strengths of γB1/2π ) 65 kHz for the first and second pulses and γB1/2π ≈ 25 kHz for the central-transition selective z-pulse. The TRAPDOR experiments employed the standard rotor-synchronized pulse sequence12 with irradiation of 11B or 27Al during the spin-echo dephasing period only and rf field strengths of γB2/2π ) 80-95 kHz for the 1H 90°/180° excitation/refocusing pulses and the 1H decoupling. The dephasing pulses used 11B and 27Al rf fields of approximately 50 kHz. The single-pulse 2 H MAS NMR spectra were recorded on a Varian Unity INOVA-400 spectrometer (9.39 T) using a home-built CP/MAS probe for 5 mm o.d. rotors and a 1.0 µs excitation pulse for an rf field strength of 40 kHz. Prior to all experiments, the magic angle was adjusted by optimization of the line widths for the spinning sidebands in the 23Na MAS NMR spectrum of NaNO3, while stable spinning frequencies were achieved using the Varian rotor-speed controller. The experimental spectra and the isotropic chemical shifts are referenced to TMS (1H, 2H), neat BF3 · O(CH2CH3)2 (11B), and a 1.0 M aqueous solution of AlCl3 · 6H2O (27Al) using a 0.1 M aqueous solution of H3BO3 (δiso(11B) ) 19.6 ppm) and a deuterated sample of chloroform (δiso(1H) ) δiso(2H) ) 7.27 ppm) as secondary references. The quadrupole coupling parameters are related to the principal elements of the electric field gradient tensor (V) by

CQ )

eQVzz , h

ηQ )

Vyy - Vxx Vzz

(1)

where Q is the nuclear quadrupole moment and the principal tensor elements fulfill the condition |Vzz| g |Vxx| g |Vyy|. Simulations of central transition line shapes and manifold of spinning sidebands for 11B and 2H have been performed using the STARS software package.22 Results and Discussion Several synthetic schemes have been employed for preparation of alumina-boria catalysts, the most common being coprecipitation,4 sol-gel synthesis,16-18 and impregnation methods.5,15,23,24 The first two procedures typically employ mixtures of boric acid and an aluminum source (e.g., aluminum nitrate) followed by curing at an elevated temperature (T g 300 °C). Thereby, these methods result in mixed-oxide compounds where the main part of boron is present in the bulk of the support. On the other hand, the impregnation method initially deposits the borate species on the surface of the alumina support, resulting in a material where diffusion of boron into the bulk structure is most likely controlled by the conditions of the heat treatment. The present ABCP and ABC samples all have a boron content well below the value corresponding to one monolayer borate coverage of the alumina support and mainly include borate species dispersed on the alumina surface as shown in our preceding study.13 In contrast to this recent 11B and 27Al MAS NMR investigation of ABCPs and ABCs with boron contents in the range 0.2-3.4 wt % B and ABC calcination temperatures of 500-950 °C,13 some of the NMR experiments performed in the present work are rather time-consuming. Thus, this study focuses on experiments for the ABCP with a boria

Hydrous Surface Species on Alumina-Boria Catalyst

Figure 1. (a) 27Al MAS NMR spectrum (14.09 T) and (b) 27Al MQMAS NMR spectrum (7.05 T) of ABCP recorded using 1H decoupling and spinning speeds of νR ) 12.0 and 10.0 kHz, respectively. The projections onto the isotropic (F1) and anisotropic (F2) axis of the MQMAS spectrum correspond to summations over the 2D spectrum, while the arrows indicate the directions of pure chemical shift (CS) and quadrupolar-induced shift (QIS) broadening. The asterisk in (a) indicates a spinning sideband.

content of 1.3 wt % (denoted ABCP) and the corresponding ABC calcined at 750 °C (denoted ABC), i.e., values that are close to the average of the boron contents and calcination temperatures employed for the series of ABCP and ABC samples studied in our recent work. Characterization of the Precursor, ABCP. 27Al MAS and MQMAS NMR. The 27Al MAS NMR spectrum of ABCP (Figure 1a, 14.09 T) exhibits a single central-transition resonance with the center of gravity, δcg(27Al) ) 9.5 ( 0.1 ppm, in agreement with the presence of Al in octahedral coordination in pseudoboehmite which constitutes the alumina support. Pseudoboehmite may be considered a poorly crystallized alumina, exhibiting a structure similar to boehmite (γ-AlOOH)25 but including a higher content of water.26,27 The absence of a highly ordered Al(OH)6 network in pseudoboehmite is reflected in the 27Al MQMAS NMR spectrum of ABCP (Figure 1b, 7.05 T) where a significant reduction in line width is not observed in the isotropic F1 dimension, indicating a dispersion in chemical shift and/or quadrupole coupling parameters. This spectrum is recorded at a low magnetic field, to enhance second-order quadrupolar effects, and it is observed that the shape of the contours is almost parallel to the plotted axis for the quadrupolarinduced shift (QIS) while the resonance is quite narrow in the direction for pure chemical shift (CS) variations. Thus, the 27Al MQMAS NMR spectrum gives clear evidence for a dispersion in quadrupole coupling parameters (CQ and ηQ) in accord with the less-ordered structure of pseudoboehmite and the fact that the resonance originates from Al within the bulk as well as surface Al sites of the ABCP material. The dispersion in quadrupole coupling parameters can alternatively be probed by the intensity distribution in the manifold of spinning sidebands, observed for the satellite transitions in the 27Al MAS NMR spectra, as illustrated recently for a similar ABCP including 3.4 wt % boron.13 11 B MAS NMR. The 11B MAS NMR spectrum of ABCP (Figure 2a) includes two well-resolved central-transition reso-

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Figure 2. 11B MAS NMR spectra of ABCP (a) before and (b) after heat treatment at 200 °C, recorded using 1H decoupling and a relaxation delay of 6 s. The corresponding 1H MAS NMR spectra are shown in parts (c) and (d), respectively, and all spectra have been obtained at 14.09 T using a spinning speed of νR ) 12.0 kHz. The spectrum in (d) is shown with a vertical expansion factor of 6.5 as compared to the spectrum in (c). The asterisks indicate ssbs.

nances at ∼15 and ∼0 ppm, originating from BO3 and BO4 species, respectively. Furthermore, the dominating BO3 resonance exhibits some features of a second-order quadrupolar line shape, and line shape simulations of this center band gives the parameters CQ ) 2.51 MHz, ηQ ) 0.34, and δiso ) 18.0 ppm, which are very similar to the values reported for the ABCP including 3.4 wt % boron.13 More interestingly, comparison of the 11B MAS spectra before and after drying at 200 °C (Figure 2a and b) reveals a significant reduction in the fraction of BO4 sites and a smeared out line shape of the BO3 resonance for the dried sample. The latter may reflect an increase in ηQ value combined with an increased dispersion in quadrupole coupling parameters which may be caused by a larger strain for the BO3 units on the surface. This observation and the reduced fraction of BO4 sites may be related to dehydration processes of the borate units and the removal of water molecules absorbed on the surface. These reactions may involve condensation of two adjacent -OB(OH)2 trigonal sites to a -OB(OH)-O-(HO)BOunit, resulting in more rigid BO3 surface sites with increased strain (i.e., larger ηQ values). Furthermore, elimination of a water molecule from a trigonal-tetrahedral -OB(OH)-O-(HO)2BQO- unit to a oxygen-bridged unit of two trigonal sites (i.e., -OB(OQ)-O-(HO)B-) may account for the decrease in BO4 sites upon postcuring of the ABCP samples. 1 H MAS NMR. 1H MAS NMR spectra of ABCP before and after drying at 200 °C are shown in Figure 2c and d, respectively, and clearly reveal that a significant amount of water is removed from the surface by the heat treatment at 200 °C. The spectrum before drying is dominated by a rather narrow resonance at 4.5 ( 0.1 ppm, exhibiting first-order ssbs of very low intensity, which shows that homonuclear 1H-1H dipolar couplings are efficiently averaged for these 1H species. Thus, the narrow, high-intensity resonance at 4.5 ppm is assigned to highly mobile water molecules present on the surface of the ABCP sample. The 1H MAS NMR spectrum of the dried sample (Figure 2d) includes a narrow resonance at 1.0 ( 0.2 ppm flanked by a broad high-frequency shoulder at 4-8 ppm. Both of these resonances exhibit first-order ssbs with high intensities as a result of 1H-1H homonuclear dipolar couplings that are

2478 J. Phys. Chem. C, Vol. 113, No. 6, 2009 not fully averaged by the dynamics of these species. This shows that the mobile water molecules are removed from the surface by the heat treatment at 200 °C, as also reflected by the lower overall intensity in the spectrum of the heated ABCP. The dehydration of pseudoboehmite has earlier been studied by 1H CRAMPS NMR,28 which reported three different hydrogen species with chemical shifts at 8.2 ( 0.3, 2.3 ( 0.2, and 4.0 ( 0.2 ppm and assigned these resonances to clustered Al2OH sites, terminal AlOH sites, and physisorbed water, respectively. These results strongly suggest that the resonance at 1.0 ppm (Figure 2d) originates from the terminal AlOH sites (i.e., the surface AlOH sites), thereby exhibiting a chemical shift close to the values δ(1H) ) 1.7-1.8 ppm reported for surface SiOH sites on silicas.29,30 The broad high-frequency shoulder at 4-8 ppm is ascribed to residual water molecules physisorbed on the alumina-boria surface and to clustered Al2OH sites following the 1H CRAMPS NMR study of pseudoboehmite.28 This assignment is also supported by an observed decrease in intensity for this peak in 1H MAS NMR spectra of ABCP samples cured at increasing temperatures in the range 100-250 °C (not shown), reflecting the loss of physisorbed water at low temperatures (∼110 °C) and the condensation of clustered Al2OH sites at higher temperatures (J200 °C). 2 H MAS NMR. Additional information about the hydrogen environments may be obtained from 2H MAS NMR since this isotope benefits from significantly weaker homonuclear dipolar interactions caused by its much lower gyromagnetic ratio relative to 1H. However, this lower value also reduces the chemical shift dispersion (in hertz), and the fact that 2H is an integer-spin quadrupole nucleus (I ) 1) further complicates the MAS NMR spectra. 2H MAS NMR spectra have been obtained for partially deuterated ABCPs dried at 200 °C and including 0, 0.25, 1.3, and 3.4 wt % boron (Figure 3). Generally, these spectra display two partly overlapping manifolds of spinning sidebands (ssbs) from the single-quantum (m ) (1 T m ) 0) transitions, which are quite similar in appearance as illustrated for the ABCP with 3.4 wt % B (Figure 3e) and the pure precursor (Figure 3f). It is noted that the absence of a broad center band at δ(2H) ≈ 4.5 ppm associated with only a few ssbs, as observed for the deuterated ABCPs before curing, shows that the mobile water molecules have been removed from the samples by drying at 200 °C. The best resolution of the two overlapping ssb manifolds is observed for the pure precursor (Figure 3d and 3f), and leastsquares analysis of the ssb intensities in this spectrum gives the 2H quadrupole coupling and chemical shift data, δiso(2H) ) 7.8 ( 0.3 ppm, CQ ) 212 ( 8 kHz, ηQ ) 0.08 ( 0.05 and δiso(2H) ) 2.1 ( 0.5 ppm, CQ ) 264 ( 14 kHz, ηQ ) 0.06 ( 0.10, and the optimized simulation shown in Figure 3g. Comparison of these data with those recently reported for a partially deuterated sample of boehmite (AlOOD, δiso(2H) ) 7.4 ppm, CQ ) 212.4 kHz, ηQ ) 0.04)31 allows a straightforward assignment of the 7.8 ppm resonance to an aluminol site, i.e., clustered Al2OD sites (Figure 3-1), whereas the value δiso(2H) ) 2.1 ppm for the other site is in good agreement with the 1H chemical shift reported for terminal AlOH sites (Figure 3-2).28 The clustered Al2OD sites exhibit a higher intensity than the terminal AlOD sites which agrees with the intensities in the 1H MAS NMR spectrum (Figure 2d) when the larger line width for the Al2OH site and its ssbs are taken into account. Furthermore, the smaller 2H quadrupole coupling for the Al2OH sites relative to the terminal AlOD sites indicates that the clustered sites are less distorted. Thus, these sites may be more accessible to 1H T 2H exchange, resulting in the apparent higher 2 H intensity for the Al2OD sites as compared to the correspond-

Hansen et al.

Figure 3. Expansions of the spectral region for the isotropic resonances in 2H MAS NMR spectra (9.39 T) for partially deuterated ABCP samples with boron contents of (a) 3.4 wt %, (b) 1.3 wt %, (c) 0.25 wt %, and (d) 0 wt %, all cured at 200 °C. The corresponding 2H MAS NMR spectra of the manifolds of ssbs are illustrated for the ABCPs with (e) 3.4 wt % boron and (f) without boron, while (g) shows the optimized result from least-squares fitting of two partly overlapping ssb manifolds to the experimental ssbs observed for the pure ABCP in part (f). The simulation employs the δiso(1H), CQ, and ηQ values listed in the text and relative intensities of 1.0 for the 7.8 ppm (Al2OD) and 0.29 for the 2.1 ppm (AlOD) resonances. The experimental spectra employed νR ) 7.0 kHz, a repetition delay of 4 s, and between 42 000 and 72 000 scans. The asterisks in (e)-(f) indicate the isotropic resonances. The schematic drawing above the spectra illustrates the octahedrally coordinated Al sites in boehmite, i.e., clustered, bulk Al2OH sites (1) and terminal, surface AlOH sites (2).

ing Al2OH/AlOH intensities in the 1H MAS NMR spectrum. The expansions of the spectral region for the isotropic peaks (Figure 3a-d) in the 2H MAS NMR spectra show that the resolution of the two resonances decreases with increasing boron content. Thereby, the spectral features of the two peaks are almost smeared out for the highest boron loading (3.4 wt % B, Figure 3a). The reduced resolution shows that additional resonances are present in-between the 2.1 and 7.8 ppm peaks; i.e., indications of at least one additional peak at 3.5-4.0 ppm are observed for the ABCP samples with 0.25 and 1.3 wt % B (Figure 3b and c). These resonances are tentatively assigned to hydroxyl groups associated with the BO3 and BO4 species on the surface of the pseudoboehmite support. 1 H{11B} and 1H{27Al} TRAPDOR NMR. Improved information about the type of H-Al and H-B environments can be obtained from 1H{11B} and 1H{27Al} TRAPDOR NMR experiments which utilize the heteronuclear dipolar couplings to detect 1 H atoms that are in the near vicinity of either boron or aluminum. This information is gained from the TRAPDOR difference spectrum (S0 - S), which is the difference between


J. Phys. Chem. C, Vol. 113, No. 6, 2009 2479 suppressed in the spin-echo spectrum (S0), as compared to the H MAS NMR spectrum (Figure 2d), which may reflect efficient T2 relaxation, thereby indicating that these 1H atoms are rather rigid as expected. This agrees well with an earlier 1H{27Al} TRAPDOR NMR study of an amorphous aluminum hydroxide where the AlOH resonance was observed to decrease with increasing evolution period in the spin-echo part of the experiment.32 The 1H{11B} TRAPDOR difference spectrum (Figure 4b) resolves a new resonance at 2.5 ppm, which is ascribed to hydroxyl groups associated with boron (BOH sites), following the assignment of the resonances in-between the peaks from the Al2OD and AlOD sites in the 2H MAS NMR spectra (Figure 3). Furthermore, a broad resonance to higher frequency (δ(1H) ≈ 4-8 ppm) is also observed in the 1H{11B} TRAPDOR difference spectrum, indicating that a residual amount of physisorbed water is in the near vicinity of the borate species. 27 Al{1H} and 11B{1H} CP/MAS NMR. Heteronuclear dipolar couplings are also utilized in the 27Al{1H} and 11B{1H} CP/ MAS NMR experiments where polarization can be transferred from 1H selectively to the central transition of the observed quadrupolar nucleus (S). The observation of a quadrupole nucleus by CP/MAS NMR implies that the Hartmann-Hahn matching condition34 of the 1H and 11B/27Al rf field strengths (νH and νS) additionally depends on the spinning speed and the magnitude of the quadrupole coupling (νQ ) 3CQ/{2S(2S - 1)}) with a maximum transfer of magnetization for35,36 1

1 ν ( nνr n ) 1, 2, ... (νQ . νS) 2 S νH ) νS ( nνr n ) 1, 2, ... (νQ , νS)

(

νH ) S +

Figure 4. 1H{11B} and 1H{27Al} TRAPDOR NMR spectra (7.05 T) of ABCP (a) before and (b) after drying at 200 °C. The upper spectra in (a) and (b) are 1H rotor-synchronized spin-echo experiments without 11 B or 27Al irradiation (i.e., the TRAPDOR control experiments, S0), whereas the middle and lower spectra are TRAPDOR difference spectra (S0 - S) for 27Al and 11B, respectively. All spectra employed an evolution period of 4 rotor cycles for a spinning speed of 10.0 kHz. The 1H{11B} TRAPDOR difference spectra are shown with a vertical expansion of a factor of 5 relative to the control experiments.

a rotor-synchronized 1H spin-echo experiment (S0) and a similar spectrum (S) that employs continuous irradiation of the quadrupole nucleus during the spin-echo dephasing period alone or during both the dephasing and refocusing periods.12 The latter spectrum (S) will exhibit an intensity reduction for the 1H atoms dipolar coupled to the irradiated quadrupole nucleus, and thereby, the difference spectrum will only include resonances from 1H sites that are dipolar coupled to the quadrupole nucleus. The 1H{11B} and 1H{27Al} TRAPDOR difference spectra of ABCP before curing (Figure 4a) show a broad resonance at ∼4.5 ppm, demonstrating that aluminum as well as boron exhibit mobile water molecules in their near vicinities, most likely in the form of water molecules physisorbed to the surface of the precursor. In these experiments a short evolution period of 4 rotor cycles (i.e., 4TR ) 400 µs for νR ) 10.0 kHz) is employed to suppress effects from weakly coupled 1H-27Al and 1H-11B spin pairs. For the ABCP dried at 200 °C, the 1H{27Al} TRAPDOR difference spectrum (Figure 4b) shows high intensity for the 1H resonance at 1.0 ppm, demonstrating a strong 27Al-1H interaction for the terminal AlOH sites. However, the resonance from the clustered Al2OH sites at δ(1H) ) 7-8 ppm is

)

(2) (3)

This is illustrated by the CP matching profiles (Figure 5) obtained from 27Al{1H} and 11B{1H} CP/MAS NMR spectra of ABCP recorded using a fixed 1H rf field for an array of 27Al/ 11 B rf field strengths (see caption of Figure 5 for experimental details). For the 27Al resonance from octahedrally coordinated Al, it is apparent that the maximum transfer of magnetization occurs for νH ) 3νAl + νR for the ABCP both before and after drying at 250 °C. According to the matching condition in eq 2, this reflects a CQ value in the megahertz range for the Al sites in the precursor, in agreement with the 27Al quadrupole coupling parameters, CQ ) 2.64 MHz (νQ ) 0.4 MHz) and ηQ ) 0.68, recently determined with high precision for the 27Al site in crystalline boehmite from a 27Al MAS NMR spectrum of the satellite transitions.13 Furthermore, the slight reduction in intensity, observed at all 27Al rf field strengths for ABCP cured at 250 °C, reflects that 1H magnetization is transferred to 27Al from the terminal AlOH and clustered Al2OH sites since a magnetization transfer from the mobile physisorbed water molecules would result in a significant difference in 27Al CP intensity for the ABCP-1.3 before and after curing at 250 °C. A similar analysis of the 11B{1H} CP/MAS NMR spectra for ABCP before curing (Figure 5b) reveals that the maximum magnetization transfer occurs at νH ) 2νB + νR for the BO3 sites, reflecting that these species exhibit a strong quadrupole coupling (CQ ≈ 2.5 MHz,13 i.e., νQ ≈ 1.25 MHz). A somewhat different matching curve is observed for the BO4 sites which displays an almost invariant intensity in the range, νB ) 37-62 kHz, where the limits correspond to matching conditions of νH ) 2νB - νR (eq 2) and νH ) νB + νR (eq 3). This shift in CP efficiency toward higher 11B rf field strength is ascribed to the significantly smaller quadrupole couplings for the BO4 sites (CQ ≈ 0.5 MHz,13 i.e., νQ ≈ 0.25 MHz) as compared to the BO3 units. Thus, νQ is somewhat closer to νS, corresponding to an intermediate situation between the limits νQ . νS and νQ , νS

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Hansen et al.

Figure 5. Variation in (a) 27Al and (b) 11B center band intensities in 27 Al{1H} and 11B{1H} CP/MAS NMR experiments (14.09 T, νR ) 5.0 kHz) as a function of the 27Al and 11B rf field strengths (νrf ) γB1/2π) for ABCP. The CP/MAS experiments employed a fixed 1H rf field strength of γB2/2π ) 50 and γB2/2π ) 68 kHz and CP contact times of τCP ) 0.4 and 0.6 ms for 27Al (a) and 11B (b), respectively. Part (a) includes data for the ABCP before (20 °C) and after (250 °C) heat treatment, while the curves in part (b) correspond to intensities observed before heat treatment for the BO3 and BO4 sites. The intensities are given in arbitrary units.

in eqs 2 and 3, respectively. The matching curves in Figure 5 demonstrate that the optimum observation of BO3 and BO4 sites by 11B{1H} CP/MAS NMR requires the use of different CP matching conditions. Apparently, this fact was not considered in an earlier 11B{1H} CP/MAS NMR study of similar alumina-boria catalysts,15 where only the BO4 tetrahedra were observed by this technique and it was concluded that only these sites contain hydroxyl groups. However, the CP matching profiles in Figure 5b demonstrate that the BO4 as well as the BO3 species include hydrogen atoms in their nearest vicinity. Further information about the 27Al-1H and 11B-1H interactions can be achieved from variable contact-time 27Al{1H} and 11 B{1H} CP/MAS NMR experiments as illustrated in Figure 6b and 6c, respectively, for the ABCP sample cured at different temperatures. The buildup of signal intensity for a quadrupolar nucleus (S) as a function of the contact time (t ) τCP), assuming a matched Hartmann-Hahn condition, may be described by

I(t) )

I0 S H 1 + TSH ⁄ T1F - TSH ⁄ T1F

( ( ) (( exp -

t H T1F

exp -t

1 1 + TSH T S

1F

)))

(4)

using a classical thermodynamic model that considers the inverse spin temperatures for the 1H and S spins.37,38 Here, I0 is the 1H

Figure 6. (a) 27Al NMR intensities for the AlO6 units in ABCP before (20 °C) and after heat treatment at 200 and 250 °C, observed in 27Al spin-lock experiments with different spin-lock times (τ). The data have been fitted (solid lines) with a biexponential function which comprises Al, fast Al, slow ) and a slow (T 1F ) component (eq 5) resulting in the a fast (T 1F data listed in Table 1. (b) and (c) 27Al{1H} and 11B{1H} CP/MAS curves, respectively, for ABCP before and after heat treatment. The 27 Al{1H} and 11B{1H} CP/MAS experiments (νR ) 5.0 kHz) employed 1 H rf field strengths of 50.0 and 67.5 kHz, respectively, with a setting of the Hartman-Hahn matching conditions for the individual sites according to eqs 2 and 3. The CP curves are obtained from a fit to eq Al, slow B, slow 4 using T 1F and T 1F , determined from separate spin-lock Al B and T 1F . The experiments (e.g., part (a)), as fixed parameters for T 1F optimized 27Al and 11B CP NMR parameters are summarized in Tables 1 and 2, respectively.

intensity immediately after the 90° 1H-pulse in the CP experiH S , T 1F are ment, TSH is the S-H cross-polarization time, and T 1F 1 rotating-frame relaxation times for the H and the S spins, H S and T 1F respectively. The T 1F relaxation times cannot be determined independently from a fit of eq 4 to the variation in



H Al, fast Al, slow TABLE 1: 1H and 27Al Rotating-Frame Relaxation Times (T 1G , T 1G , T 1G ) and 1H-27Al Cross-Polarization Times (TAlH) determined from 27Al{1H} CP/MAS and 27Al Spin-Lock MAS NMR Experiments (14.09 T) for the ABCP and ABC Samples Dried at Different Temperaturesa

site ABCP

AlO6

ABCd

AlO4

AlO5

AlO6

tempb (°C)

Al, fast T 1F (ms)

20 200 250 20 150 200 250 20 150 200 250 20 150 200 250

0.078 ( 0.002 0.074 ( 0.002c 0.079 ( 0.002c c

Al, slow T 1F (ms)

H T 1F (ms)

TAlH (ms)

3.44 ( 0.18 2.91 ( 0.14 2.07 ( 0.09 1.32 ( 0.03 2.18 ( 0.05 2.28 ( 0.11 2.07 ( 0.07 1.07 ( 0.11 2.37 ( 0.20 1.90 ( 0.13 2.09 ( 0.11 1.81 ( 0.06 2.50 ( 0.10 2.68 ( 0.16 2.39 ( 0.11

2.15 ( 0.01 3.33 ( 0.02 3.05 ( 0.05 4.5 ( 0.2 10.4 ( 0.8 9.4 ( 0.7 9.9 ( 0.7 5.6 ( 0.7 10.5 ( 0.4 9.5 ( 0.5 11.0 ( 0.6 5.1 ( 0.1 12.0 ( 0.1 10.0 ( 0.6 11.2 ( 0.6

0.130 ( 0.003 0.148 ( 0.003 0.159 ( 0.006 0.47 ( 0.04 0.55 ( 0.04 0.48 ( 0.04 0.60 ( 0.04 0.17 ( 0.05 0.29 ( 0.01 0.34 ( 0.02 0.31 ( 0.02 0.39 ( 0.02 0.50 ( 0.02 0.47 ( 0.03 0.54 ( 0.02

a Al, fast Al, slow The 27Al rotating-frame relaxation constants, T 1F and T 1F , have been determined from spin-lock experiments (see eq 5). The error limits are standard deviations provided by the least-squares fitting routine. b Temperatures used for the overnight drying (postcuring) of the samples prior to the NMR experiments performed at ambient temperature. c The fractions of the fast (C) and slow (D) components (see eq 5) are 22.8 ( 0.2 and 4.1 ( 0.1 (20 °C), 19.1 ( 0.3 and 3.4 ( 0.1 (200 °C), and 15.6 ( 0.3 and 3.0 ( 0.1 (250 °C), respectively. d A Al Al, slow satisfactory fit of the decay of spin-locked 27Al intensity has been obtained by a single-exponential function (i.e., T 1F ) T 1F ).

H B, fast B, slow TABLE 2: 1 H and 11B Rotating-Frame Relaxation Times (T 1G , T 1G , T 1G ) and 1H-11B Cross-Polarization times (TBH) Determined from 11B{1H} CP/MAS and 11B Spin-Lock MAS NMR Experiments (14.09 T) for the ABCP and ABC Samples Dried at Different Temperaturesa

site

tempb (°C)

C

D

B, fast T 1F (ms)

B, slow T 1F (ms)

H T 1F (ms)

T BH (ms)

ABCP

BO3

ABC

BO4 BO3

25 200 25 25 150 200 250 25 150 200 250

2.8 ( 0.1 3.4 ( 0.2 0.58 ( 0.04 3.8 ( 0.3 6.5 ( 0.4 6.2 ( 0.3 4.8 ( 0.4

0.46 ( 0.06 0.41 ( 0.05 0.24 ( 0.07 0.9 ( 0.1 1.4 ( 0.1 1.6 ( 0.1 1.1 ( 0.1

0.080 ( 0.004 0.077 ( 0.001 0.071 ( 0.005 0.128 ( 0.006 0.106 ( 0.007 0.107 ( 0.006 0.104 ( 0.010

2.43 ( 0.28 1.19 ( 0.05 0.64 ( 0.04 0.74 ( 0.04 0.48 ( 0.03 0.44 ( 0.02 0.38 ( 0.04 8.41 ( 1.05 16 ( 6 7(2 3.4 ( 0.5

3.30 ( 0.10 21 ( 3 5.25 ( 0.37 4.60 ( 0.07 14.3 ( 0.9 14.4 ( 1.0 27 ( 5 11.7 ( 1.1 n.d.d n.d. n.d.

0.137 ( 0.009 0.18 ( 0.01 0.26 ( 0.04 0.103 ( 0.007 0.125 ( 0.016 0.123 ( 0.017 0.146 ( 0.029 0.34 ( 0.03 0.38 ( 0.04 0.34 ( 0.02 0.29 ( 0.03

BO4c

a B, fast B, slow The 11B rotating-frame relaxation constants, T 1F and T 1F and the fractions C and D of these components, respectively, have been determined from 11B spin-lock MAS NMR experiments (see eq 5). The error limits are standard deviations provided by the least-squares fitting routine. b Temperatures used for the overnight drying (postcuring) of the samples prior to the NMR experiments performed at ambient B B, slow temperature. c A satisfactory fit of the decay of spin-locked 11B intensity has been obtained by a single-exponential function (i.e., T 1F ) T 1F ). d The 1H rotating-frame relaxation times could not be determined from 11B{1H} CP/MAS NMR experiments with a maximum CP contact time B of 5.0 ms. It is estimated that T 1F > 30 ms for these BO4 sites.

signal intensity as a function of the contact time. For this reason S values (S ) Al, B) are determined from separate 27Al the T 1F 11 and B spin-lock experiments, prior to the analysis of the CP contact-time curves. However, spin-locking of the central transition for a quadrupole nucleus can be rather complex, S from a fit of the intensities preventing a determination of T 1F to a single-exponential decay as generally performed for spin1/2 nuclei. De Paul et al.39 have proposed that the intensity as a function of the spin-lock time (t) may be approximated by a biexponential function S, fast S, slow I(t) ) C exp(-t ⁄ T1F ) + D exp(-t ⁄ T1F )

(5)

which includes a fast and a slow component for the rotatingframe relaxation. This approach is adapted here in the analysis of the 27Al and 11B spin-lock experiments, which results in the S, fast S, slow and T 1F values (S ) Al, B) listed in Tables 1 and 2, T 1F respectively, for ABCP dried at different temperatures. As an example, Figure 6a illustrates the results from the 27Al spinlock MAS experiments for ABCP dried at different temperafast ) dominates the rotating-frame tures. The fast component (T Al, 1F

relaxation (i.e., C ≈ 5-6D) and exhibits almost no variation Al, slow (∼0.08 ms), whereas the slow component (T 1F ) decreases with increasing drying temperature (Table 1). The same fast slow and T B, values observation holds for the corresponding T B, 1F 1F S, fast (Table 2). From the plots in Figure 6a and the fact that T 1F S, slow , T 1F , it is apparent that even after a short spin-lock time, the spin-locking behavior for 27Al and 11B is dominated by the S, slow values have been used as a slow component. Thus, the T 1F S in the least-squares fits (eq 4) of the fixed parameter for T 1F intensities for the CP contact-time curves (Figure 6b and 6c). The cross-polarization time (TSH) can by related to the second moments of the S and 1H spins by the expression33

1 2 ) TSH 3

2π MSH 5 M √ HH

(6)

where MSH and MHH are the van-Vleck second moments, reflecting the strengths of the S-1H and 1H-1H dipolar interactions, respectively. This expression holds for a rigid lattice; however, in the case of motions in the intermediate


Hansen et al.

Figure 7. Single-pulse MAS NMR spectra for ABC: (a) and (b) 1H, (c) and (d) 11B, and (e) and (f) 27Al. The spectra in the upper row are of as-synthesized ABC, whereas the spectra in the lower row are of ABC dried at 200 °C. All spectra have employed a spinning speed of 12.0 kHz. The number in parentheses in (b) is the vertical expansion factor relative to the spectrum (a). The asterisks indicate ssbs.

regime, where the dipolar correlation time (τd) is of the same order as the correlation time for the molecular motion (τc), a similar expression can be derived, the main difference being that 1/TSH is also proportional to τc in addition to MSH/MHH.33 The TAlH and TBH values for the ABCP sample tend to increase with increasing postcuring temperature. This may reflect that mobile 1H species (i.e., water molecules) are removed from the sample thereby causing a decrease in the τc correlation time. S and TSH, the 1H rotating-frame relaxation In contrast to T 1F H ) is a volume property for the 1H atoms in a time (T 1F multicomponent system since it is averaged by 1H-1H spin H diffusion over a spatial proximity of 1-2 nm.40,41 Thus, the T 1F values are dependent on molecular motions and the efficiency of spin diffusion, the latter mediated by the strength of the H values for ABCP homonuclear dipolar interactions. The T 1F 27 1 11 derived from both the Al{ H} and B{1H} CP/MAS NMR H experiments (Tables 1 and 2, respectively) show that T 1F increases upon going from 20 to 200 °C and subsequently decreases when the curing temperature is increased from 200 to 250 °C. This observation suggests that molecular motions of water molecules absorbed on the surface have a dominating H as long as these species are present. When the effect on T 1F mobile water molecules are removed by heating, spin diffusion becomes the dominating mechanism for the rotating-frame H values. Furthermore, after relaxation, resulting in longer T 1F curing at elevated temperatures the 1H environments become more rigid resulting in more efficient spin diffusion and thereby H upon going from a curing the observed decrease in T 1F temperature of 200 to 250 °C. Characterization of the Catalyst, ABC. The 1H environments for the alumina-boria catalyst (ABC), obtained by calcination of the catalyst precursor with a boron content of 1.3 wt % for 2 h in air at 750 °C, are investigated using the

same approaches as in the previous section for the precursor. After calcination the sample was cooled in air without any precautions to avoid absorption of water. Three additional samples were made by postcuring the “hydrated” calcined sample at 150, 200, and 250 °C followed by packing of the airtight rotors under N2 in a glovebox. 1 H, 11B, and 27Al MAS NMR. Single-pulse 1H, 11B, and 27Al MAS NMR spectra of ABC before heat treatment and the corresponding sample after curing at 200 °C are illustrated in Figure 7. The observation of 27Al center bands from Al in tetrahedral and octahedral coordination at approximately 69 and 10 ppm and with a ∼1:2 intensity ratio, respectively, reflects the transformation of pseudoboehmite to γ-alumina upon calcination. In addition, a low-intensity resonance at 32 ppm from pentacoordinated Al is observed (Figure 7e and 7f) which may originate from AlO5 sites formed on the surface of the γ-Al2O3 particles,42 most likely in the vicinity of the small amount of tetrahedrally coordinated boron incorporated in the outer layers of the alumina support.13 The 27Al MAS NMR spectra before and after heat treatment are nearly identical, reflecting that removal of water from the surface does not affect the bulk structure of the support. However, in the corresponding 11 B MAS NMR spectra (Figure 7c and 7d), a minor intensity reduction is observed for the center band from tetrahedrally coordinated boron (∼0 ppm) after the heat treatment. The dehydration process for the borate species is studied in more detail in Figure 8, which depicts the fraction of BO4 species as a function of postcuring temperature for the ABCP and ABC samples. A reduction in number of BO4 sites with increasing temperature up to 150 °C is observed for both samples, while the fractions of BO4 are almost invariant in the range from 150 to 250 °C. Moreover, at these temperatures the fraction of BO4 sites is significantly higher in the catalyst as compared to the



Figure 8. Fraction of BO4 intensity as a function of the postcuring temperature for the ABCP and ABC samples relative to the sum of BO3 and BO4 center band intensities, observed in 11B MAS NMR spectra recorded at 14.09 T.

precursor. The conversion of BO4 into BO3 units is associated with elimination of B-OH hydroxyl groups for the BO4 species attached to the surface. Thus, the higher fraction of BO4 sites in the catalyst samples may reflect that a number of these sites does not include hydroxyl groups. This is the case for the small fraction of BO4 units trapped in the first molecular layers of the support material, as observed in our recent study,13 which remains fixed during the postcuring treatment and thereby accounts for the higher fraction of BO4 sites in the calcined material. In accord with the 1H MAS NMR experiments on ABCP (Figure 2), the corresponding spectra of the catalyst before and after heat treatment (Figure 7a and 7b) clearly illustrate the highly hygroscopic nature of the alumina-boria surface. The spectrum before heat treatment is dominated by a high-intensity resonance at 4.6 ( 0.1 ppm, exhibiting small ssb intensities and assigned to mobile water molecules absorbed to the alumina-boria surface. After heat treatment the intensity of this resonance is significantly reduced, and the spectrum is dominated by two resonances at 3.9 ( 0.3 and 2.7 ( 0.2 ppm which cover the spectral range from about -2 to 10 ppm. The lower intensity of the ssb intensities in this spectrum (Figure 7b), as compared to the corresponding spectrum of the precursor (Figure 2d), reflects a less rigid structure of homonuclear coupled 1H atoms. This agrees well with the fact that water molecules and hydroxyl groups are only present at the surface of the ABC catalyst in contrast to the precursor where the bulk part includes rigid 1H atoms in clustered Al2OH units. 1 H{11B} and 1H{27Al} TRAPDOR NMR. The 1H{27Al} and 1 H{11B} TRAPDOR difference spectra of ABC before heat treatment (Figure 9a) show that both the Al and B sites interact with a minor fraction of the mobile water molecules on the surface, since these spectra exhibit minor resonances at ∼5.2 and ∼4.4 ppm, respectively. The dominating resonance at 4.6 ppm in the S0 reference spectrum originates from loosely bound water molecules to the surface, whereas the shift of the peaks at 5.2 and 4.4 ppm away from this value is ascribed to somewhat stronger 1H-27Al and 1H-11B interactions, respectively, for the water molecules in the near vicinity of the Al and B sites. The TRAPDOR spectra in Figure 9 have been recorded with a total evolution period of 4 rotor cycles (i.e., 400 µs for νR ) 10.0 kHz) to emphasize the TRAPDOR dephasing effect for the Al and B atoms in the closest proximity to the 1H atoms. In the 1H TRAPDOR reference experiment for the ABC cured at 200 °C (Figure 9b), the intensity of the resonance at 3.9 ppm is slightly

Figure 9. TRAPDOR spectra of (a) as-synthesized ABC and (b) ABC dried at 200 °C. In each part of (a) and (b), the upper spectra are the TRAPDOR control experiment (S0) whereas the middle and lower parts are TRAPDOR difference spectra (S0 - S) for 27Al and 11B, respectively. The evolution period for all spectra were set to 4 rotor periods employing a spinning speed of 10.0 kHz. In the TRAPDOR experiments, the carrier frequency of the heteronucleus (11B/27Al) was set in the middle of the spectrum. The number in parentheses in the lower part of (a) is the vertical expansion factor relative to the control experiment.

lower as compared to the standard 1H MAS NMR spectrum (Figure 7b) where this resonance dominates the spectrum. This intensity reduction in the spin-echo reference experiment is most likely associated with a shorter T2 relaxation time for these species, reflecting that they are situated in a stronger coupled 1 H environment. The 1H{27Al} TRAPDOR difference spectrum (Figure 9b) includes a number of nonresolved 1H resonances in the range from -2 to 10 ppm with the most intense component at ∼3.9 ppm. These resonances are assigned to different surface AlOH groups and absorbed water molecules close in space to Al sites. The 1H chemical shift range for the AlOH sites is in reasonable agreement with earlier observations (-0.3 to 7.8 ppm) for a partly deuterated sample of γ-Al2O3.43 The 1H{11B} TRAPDOR difference of ABC (Figure 9b) is very similar to the corresponding spectrum of the precursor (Figure 4b), indicating that the 1H environment surrounding the borate species on the surface is very similar for the anhydrous ABCP and ABC samples. The rather narrow peak at ∼2.3 ppm is shifted slightly toward lower frequency as compared to the corresponding peak at 2.7 ppm in the S0 reference spectrum and in the standard 1H MAS spectrum (Figure 7b). However, in all cases we assign the 2.3-2.7 ppm resonance to BOH hydroxyl groups, while the broader resonance in the 1H{11B} TRAPDOR difference spectrum is caused by absorbed water molecules present in the near vicinity of the borate units on the surface. Finally, we note that similar 1H{27Al} and 1H{11B}


Hansen et al.

Figure 11. 11B intensities from variable contact-time 27Al{1H} CP/ MAS NMR experiments (νR ) 5.0 kHz) for the (a) BO3 and (b) BO4 sites in ABC before drying and after heat treatment at 150, 200, and 250 °C. The experimental intensities have been plotted and analyzed H and TBH values being in the same manner as in Figure 10, the T 1F summarized in Table 2.

Figure 10. 27Al intensities observed in variable contact-time 27Al{1H} CP/MAS NMR experiments (νR ) 15.0 kHz) for (a) AlO4, (b) AlO5, and (c) AlO6 units in ABC before drying and after heat treatment at 150, 200, and 250 °C. The intensities, shown on an arbitrary scale for the individual sites, have been determined by spectral integration of the center band resonances for each type of Al unit. Least-squares Al optimization to the experimental data, using eq 4 and T 1F values from preceding 27Al{1H} spin-lock experiments, gives the T H1F and TAlH values listed in Table 2 and the solid curves in (a)-(c).

TRAPDOR experiments have been reported for BF3 adsorbed on the surface of γ-Al2O3.44 In that study 1H resonances at 2.3 and 3.7 ppm and a shoulder at 5.0 ppm were observed in the 1 H NMR spectra and assigned to an AlOH site associated with -OBF2 species through H · · · F hydrogen bonding, an Al-OH-B Brønsted site, and water molecules adsorbed on the surface of γ-Al2O3, respectively. 27 Al{1H} and 11B{1H} CP/MAS NMR. Information about the dynamics and proximities of the 1H atoms on the surface of the ABC catalyst as a function of the postcuring temperature may

be derived from 27Al{1H} and 11B{1H} CP/MAS NMR experiments. The single-pulse 11B and 27Al MAS NMR spectra, shown for some of these samples in Figure 7c-7f, demonstrate that nearly fully separated center band resonances from the AlO4, AlO5, and AlO6 units as well as the BO3 and BO4 sites can be achieved at 14.09 T. Thus, the 27Al{1H} and 11B{1H} CP/MAS intensities for these species have been obtained by spectral integration, and they are depicted versus the CP contact times in Figures 10 and 11, respectively, using an arbitrary intensity scale for the individual sites. However, it should be emphasized that the CP/MAS experiments employ different Hartmann-Hahn matching conditions for the individual sites, optimized in a similar manner as for the precursor (Figure 5) and reflecting the difference in quadrupole couplings for the individual sites. By this approach, 27Al{1H} CP/MAS resonances for each of the AlO4, AlO5, and AlO6 coordination environments are observed, showing that each of these sites exhibit 1H atoms in their near vicinity. This observation contrasts earlier studies of γ-Al2O3 with different loadings of fluoride45 and of fluorinated NiW/γ-Al2O3 catalysts46 where the resonance from tetrahedrally coordinated Al were strongly suppressed in the 27Al{1H} CP/ MAS spectra, resulting in the conclusion that the hydroxyl groups in the alumina are mainly connected to six-coordinated aluminum. 27 Al{1H} spin-lock MAS experiments have been performed for the studied samples of ABC, and in all cases a satisfactory fit to the decay of spin-locked magnetization can be achieved



Figure 12. Schematic drawing of the hydrous surface structure for the alumina-boria catalyst, combining the findings from the present study about hydroxyl groups and water molecules on the surface with the connectivities and bondings of the borate species from our recent study.13 Al using a single-exponential function, resulting in the T 1F values Al, slow (i.e., T 1F ) listed in Table 1. These data are subsequently used in least-squares refinement of eq 4 for the 27Al intensities of AlO4, AlO5, and AlO6, observed in variable-contact time 27 Al{1H} CP/MAS experiments (Figure 10), to determine the H parameters (Table 1) for the individual sites. The TAlH and T 1F Al H and T 1F values for the three Al environments generally T 1F increase significantly (by a factor of ∼2) when the samples before and after heat treatment are compared. Moreover, the variation in the rotating-frame relaxation times is almost within the error limits for the samples cured at 150, 200, and 250 °C. The cross-polarization times for the AlO5 sites (0.17-0.34 ms) are somewhat smaller than the values for AlO4 (0.47-0.60 ms) and AlO6 (0.39-0.54 ms), again with the shortest value observed before curing for all sites. The increase in rotating-frame relaxation times upon heat treatment is ascribed to a less dominating influence of mobile 1H atoms, caused by the removal of water molecules from the surface. Thus, the smaller values Al H and T 1F before heating may reflect that the hydroxyl for T 1F groups attached to the Al species are involved in 1H exchange processes with mobile water molecules present on the surface of the ABC. The slightly shorter TAlH for the AlO5 sites may reflect a stronger 1H-27Al dipolar interaction (eq 6) and thereby that the AlO5 sites are closer to the surface of the alumina support. This interpretation agrees with the shorter T1 relaxation times observed for AlO5 as compared to AlO4 and AlO6 in a ultrahigh-field 27Al MAS NMR study of γ-alumina.42 FurtherAl H , T 1F , and TAlH times, determined for more, the very similar T 1F the AlO4 and AlO6 sites, show that both of these types are connected to surface hydroxyl sites, in contrast to the interpretation of 27Al{1H} CP/MAS NMR spectra in earlier studies of fluorinated γ-aluminas.45,46 The results from the corresponding 11B{1H} CP/MAS NMR spectra for the ABC samples are illustrated in Figure 11 and summarized in Table 2. The analysis of the preceding 11B{1H} spin-lock MAS experiments required the use of a biexponential function (eq 5) for the BO3 sites while a single-exponential gave B, fast B, slow and T 1F decrease a satisfactory fit for the BO4 sites. T 1F with increasing postcuring temperature for the BO3 sites, in agreement with the observations for the precursor material (Table 2). Following the discussion of those data (Vide supra, H with increasing postcuring Figure 6), the increase in T 1F temperature may reflect that molecular motions have a dominating effect of the relaxation before heat treatment while the removal of water at elevated temperatures results in spin diffusion becoming the dominating relaxation mechanism. H upon going from However, the absence of a decrease in T 1F heat treatment at 150 to 250 °C may reflect that the BO3 sites are not present as rigid species on the alumina surface at these temperatures. The TBH cross-polarization times vary only slightly with the postcuring temperature for the BO3 and BO4 sites, the values being a factor of 2-3 larger for BO4 as compared to the BO3 sites. This variation is tentatively ascribed to the large difference in 11B quadrupole couplings for the two types of

boron environments. Finally, the 11B{1H} CP/MAS experiments recorded here for contact times up to 5.0 ms have not allowed a reliable determination of the T H1F values for the BO4 sites, since a very efficient spin-lock is observed for these sites (Figure 11b). Conclusion The NMR studies in this work have provided new information about the hydroxyl groups and water molecules bound and adsorbed to the surface of an alumina-boria catalyst (ABC) and its pseudoboehmite precursor (ABCP), both exhibiting a low boron content well below the quantity required for full monolayer coverage of the support with borate units. The 1H and 2H MAS NMR spectra of the ABCP have allowed identification of resonances from bulk Al2OH and surface AlOH groups (δ(H) ) 7.8 ppm and δ(H) ) 2.1 ppm) and from BOH sites (δ(H) ) 3.5-4.0 ppm) in-between those from the Al2OH and AlOH sites. Moreover, mobile water molecules adsorbed to the surface are clearly observed by the narrow resonance at δ(H) ) 4.5 ppm. This resonance disappears upon heating, and a new peak with strong spinning sidebands is observed in the 4-6 ppm region, originating from more strongly bound water molecules, i.e., water molecules physisorbed to the surface. The dehydration of the ABCP sample also results in a significant reduction in the fraction of BO4 sites, indicating that the dehydration processes also involve condensation of trigonaltetrahedral -OB(OH)-O-(HO)2BQO- into oxygen-bridged units of trigonal sites (-OB(OQ)-O-(HO)BO-) by elimination of water molecules. Condensation of the bulk Al2OH sites has also been observed for the ABCP heated to temperatures above 200 °C. The AlOH and BOH hydroxyl groups have been unambiguously observed by 1H{11B} and 1H{27Al} TRAPDOR experiments, which also showed that both the surface Al sites and the borate species have mobile water molecules in their near vicinities. The 27Al{1H} CP/MAS NMR experiments have shown that magnetization is transferred to 27Al mainly from 1H of the surface AlOH and bulk Al2OH sites for the precursor and that each of the AlO4, AlO5, and AlO6 sites in the calcined material are observed by CP, demonstrating that each type of Al environment is close to the surface of the support. The 11 B{1H} CP/MAS NMR spectra have demonstrated that both the BO3 and BO4 sites can be observed by 1H-11B magnetization transfer for the ABC and ABCP samples, implying that each of these units have hydroxyl groups or physisorbed water in their nearest vicinity. It should be emphasized that these observations require the Hartmann-Hahn matching conditions of the 1H and 11B/27Al rf field strengths to be optimized for the individual BO3, BO4 and AlO4, AlO5, AlO6 sites, considering the fact that these sites exhibit different quadrupole couplings. Finally, the analyses of the variable contact-time CP/MAS NMR experiments for the ABC and ABCP samples before and after gentle heat treatment have provided valuable information about the dynamics of the individual 1H species and the removal of water molecules upon heating.

2486 J. Phys. Chem. C, Vol. 113, No. 6, 2009 The structural information from the present investigations can be combined with the findings from our recent 11B and 27Al MAS NMR study of the same type of samples,13 which revealed the network of BO4 and BO3 species and their bonding to the alumina support as well as the incorporation of a minor fraction of BO4 in the first molecular layers of the support. This combination results in the sketch of the hydrous surface structure for the alumina-boria catalyst shown in Figure 12. Hydroxyl groups are present at the terminal oxygen atoms of the BO4 and BO3 units and at the parts of the alumina surface which are not covered by borate units. Moreover, water molecules are hydrogen bonded to the borate units and to the alumina surface. The main difference in surface structure for the ABC and ABCP is the absence of surface AlO4 and AlO5 sites for ABCP and the presence of Al2OH sites in the bulk of the ABCP support material. 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 acknowledged. We thank the Danish Technical Science Research Council (J. No. 26-03-0049) for financial support. References and Notes (1) Kno¨zinger, H.; Ratnasamy, P. Catal. ReV.sSci. Eng. 1978, 17, 31. (2) Izumi, Y.; Shiba, T. Bull. Chem. Soc. Jpn. 1964, 1797. (3) Sakurai, H.; Sato, S.; Urabe, K.; Izumi, Y. Chem. Lett. 1985, 1783. (4) Peil, K. P.; Galya, L. G.; Marcelin, G. J. Catal. 1989, 115, 441. (5) Sato, S.; Kuroki, M.; Sodesawa, T.; Nozaki, F.; Maciel, G. E. J. Mol. Catal. A 1995, 104, 171. (6) Flego, C., Jr. Appl. Catal., A 1999, 185, 137. (7) Petre, A. L.; Perdigo´n-Melo´n, J. A.; Gervasini, A.; Auroux, A. Top. Catal. 2002, 19, 271. (8) Colorio, G.; Ve´drine, J. C.; Auroux, A.; Bonnetot, B. Appl. Catal., A 1996, 137, 55. (9) Sato, S.; Hasebe, S.; Sakurai, H.; Urabe, K.; Izumi, Y. Appl Catal. 1987, 29, 107. (10) Forni, L.; Fornasari, G.; Tosi, C.; Trifiro`, F.; Vaccari, A.; Dumeignil, F.; Grimblot, J. Appl. Catal., A 2003, 248, 47. (11) De Farias, A. M. D.; Esteves, A. M. L.; Ziarelli, F.; Caldarelli, S.; Fraga, M. A.; Appel, L. G. Appl. Surf. Sci. 2004, 227, 132. (12) Grey, C. P.; Vega, A. J. J. Am. Chem. Soc. 1995, 117, 8232. (13) Hansen, M. R.; Jakobsen, H. J.; Skibsted, J. J. Phys. Chem. C 2008, 112, 7210. (14) Ferdous, D.; Dalai, A. K.; Adjaye, J. Appl. Catal., A 2004, 260, 137. (15) Bautista, F. M.; Campelo, J. M.; Garcia, A.; Luna, D.; Marinas, J. M.; Moreno, M. C.; Romero, A. A.; Navio, J. V.; Macias, M. J. Catal. 1998, 173, 333.

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JP807595M

Structure and Dynamics of Hydrous Surface Species on Alumina

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