Adaptation of 27Al CP/MAS NMR to the Investigation of the Adsorption of Molybdate Ionsat the γ-Al2O3/Water Interface

Dimitri Mertens de Wilmar and Olivier ClauseKinetics and Catalysis DiVision, Institut Franc¸ais du Petrole, 1-4, aVenue de Bois-Pre´au,92852 Rueil-Malmaison Cedex, France

Jean-Baptiste d’Espinose de la Caillerie*Laboratoire de Physique Quantique, URA CNRS 1428, Ecole Supe´rieure de Physique et de Chimie Industriellesde la Ville de Paris, 10 rue Vauquelin, 75231 Paris Cedex 05, France

ReceiVed: April 15, 1998; In Final Form: June 22, 1998

Recent studies have demonstrated the importance of the formation of a hydrated aluminum polymolybdatewhen a catalyst is prepared by impregnatingγ-alumina with an aqueous solution of ammonium heptamolybdate[Carrier, X.; Lambert, J. F.; Che, M.J. Am. Chem. Soc.1997, 119, 9, 10137-10146]. We show here howa simple1H 27Al CP MAS NMR experiment performed in the intermediate regime can easily reveal theformation of a hydrated aluminum polymolybdate on the surface of alumina. Taking, simultaneously, advantageof the differences in dipolar polarization transfer and those in quadrupolar effects on spin-locking efficiency,it is possible, after setting the experimental parameters on reference compounds, to partially suppress thesignal resulting from the alumina support at 8.5 ppm while enhancing the 14 ppm resonance of the aluminumpolymolybdate. In this way, the occurrence of a hydrated polymolybdate is evidenced, not only on samplesimpregnated with (NH4)6Mo7O24 but also with Na2MoO4. Furthermore, a dialysis experiment proved that adissolution/reprecipitation mechanism is at least in part responsible for the formation of the Al-O-Mo bond.

Introduction

Solid state nuclear magnetic resonance (NMR) is a provenmethod to study the local environments of active speciesdispersed on the surface of heterogeneous catalysts.1 One ofits main advantage compared to most surface studies methodsis that it does not require any preparation of the surface and istherefore totally noninvasive. On the downside it is a bulksensitive method, and if the nuclei under study are present inthe volume of the catalyst, the signal of the active species mightbe obscured. This has considerably limited the usefulness ofsolid state NMR for studying heterogeneous catalysts when theactive or promoting species is not directly observable.

A recent emphasis has appeared in the literature on theimportance of dissolution/reprecipitation of mixed hydroxospecies during the impregnation of alumina-supported catalysts.Such phenomena have been evidenced in systems of practicalimportance such as Ni(II), Co(II),2 and Mo(VI)3-5 onγ-alumina.Impregnations by paramagnetic Ni(II) or Co(II) ions areevidently not amenable to NMR studies, while for95Mo NMRis difficult in the solid state due to its low gyromagnetic ratioand mediocre natural abundance (15.72%). This has so farlimited the study of poorly crystallized Mo based catalysts toenriched samples.6 By contrast,27Al has an excellent absolutesensitivity, 103 superior to95Mo, and a 100% natural abundance.It is therefore tempting to study the Mo-alumina interactionfrom the aluminum standpoint, but this necessitates being ableto discriminate the weak signal resulting from the few aluminuminteracting with molybdenum.

Cross polarization (CP) from protons is a tool of choice forthe observation of phenomena linked to the surface hydrationof non-hydrous oxides. In this way, NMR is made surfaceselective and remains quantitative for spin 1/2 nuclei when spindynamics is taken into account. The selective detection ofsurface hydroxylated species by proton CP to spinI ) 1/2 nucleihas been very widely used, for example, when the support issilica.7-9 However, for alumina supported catalysts, a straight-forward analysis based on dipolar couplings is hampered bythe quadrupolar nature of27Al ( I ) 5/2). We propose to takeadvantage of the dependence of the efficiency of the polarizationtransfer on quadrupolar interactions to simultaneously enhancethe hydroxylated species and filter out the alumina signal. Weshow that we are able in this way to selectively reveal aluminumengaged in a hydroxylated mixed aluminum-molybdenumspecies during aqueous Mo(VI) impregnation of alumina. Thisfinding is of some interest considering the importance of Mo/Al2O3 materials as oxidation or olefin metathesis catalysts aswell as precursors to hydrorefining catalysts.10 More precisely,the presence of aluminum ions in the deposited phase has animpact on the accessibility to reactants and resistance to thermalsintering of the supported phase, as was observed previouslyfor the MgO/Al2O3 systems.11

Experimental Section

Materials. Bayerite was courtesy of Dr. D. Coster. Anammonium hexamolybdenoaluminate reference compound wasprepared by coaddition of a 0.1 M (NH4)6Mo7O24 and of a 0.1M Al(NO3)3 solution. The pH was fixed at 3.8 by addition ofdiluted nitric acid. The resulting suspension was stirred for 3h at 298 K and then filtered and dried at room temperature.

* Corresponding author: LPQ-ESPCI, 10, rue Vauquelin, 75321 ParisCedex 05, France. Telephone: (33) 01 40 79 46 20. Fax: (33) 01 40 79 4744. E-mail: Jean-Baptiste.Despinose@espci.fr.

7023J. Phys. Chem. B1998,102,7023-7027

S1089-5647(98)01863-X CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 08/18/1998

Chemical and thermogravimetric analyses and XRD are com-patible with (NH4)3[Al(OH)6Mo6O18].12

Impregnation of powderedγ-alumina (Procatalyse, specificsurface area 200 m2 g-1, mean pore diameter 6 nm; Na2Ocontent lower than 40 ppm; ground and pretreated in oven for3 h at 823 K) was conducted by stirring in an excess of solutionat room temperature: Na2MoO4 (0.1 M), 0.5 h of contact, pH) 3.9, 40 mL/g liquid/solid (sample AlMo-1, 7.0 Mo wt %);(NH4)6Mo7O24 (0.05 M), 0.5 h of contact, pH) 3.8, 200 mL/gliquid/solid (sample AlMo-7, 25.0 Mo wt %). The suspensionswere filtered and dried at room temperature. Alumina blankswere prepared following the same procedures but without themolybdenum precursors. Mo contents were determined byX-ray fluorescence.

An impregnation was also performed using a dialysismembrane (1000 molecular weight cutoff). Two dialysis tubescontaining 40 g of alumina spheres in distilled water wereimmersed in 2 L of a0.05 M (NH4)6Mo7O24 aqueous solution.The pH was fixed at 3.8. The spheres were then filtered, dried,and ground (sample AlMo-9, 12.3 Mo wt %). The precipitateoutside the tubes was also filtered and dried (sample AlMo-9bis, 52.6 Mo wt % and 0.21 Al wt %).

MAS NMR. Magic angle spinning nuclear magnetic reso-nance (MAS NMR) experiments were performed on a BrukerASX500 spectrometer at 11.7 T in 4 mm zirconia rotors.27Alone-pulse experiments were performed at 12 kHz with aselective pulse (π/6) duration of 0.5µs, recycle time 1 s, and160 acquisitions. The cross-polarization (CP) pulse sequencewas the usual one with a recycle delay of 5 s, a contact time(tCP) of 500 µs, a1H radio frequency magnetic field strength(ΩS/2π) of 56 kHz, and a spinning frequency (νr) of 8 kHz.13

2400 Acquisitions were performed, except for AlMo-9bis, where102 000 scans were acquired. It was checked that the27Almagnetization followed the phase of the1H, thereby confirmingthat it resulted solely from cross polarization. The27Al radiofrequency (rf) field strength (ΩI/2π) was calibrated within 2kHz by measuring the duration of a pulse in a 0.1 M aqueoussolution of Al(NO3)3. The same solution was the chemical shiftreference. Independently of CP, a two-pulse spin lockingexperiment was performed on27Al: a selectiveπ/(2I+1) pulsefollowed by a spin locking pulse of variable duration.

Theoretical Background

For quadrupolar nuclei such as27Al, quadrupolar effects onspin locking efficiency and polarization transfer does not allowa simple discussion based on the evolution of the spectra underdifferent Hartmann-Hahn matching and contact times. Re-cently, following the work of Vega,14,15several papers attempt-ing a theoretical description of the dynamics of cross polarizationunder MAS involving quadrupolar nuclei have been publishedand important facts have emerged.

Level Matching. For CPMAS of half-integer quadrupolarspins coupled to spin 1/2 nuclei, the Hartmann-Hahn conditionbecomes

whereΩS is the strength of the radio frequency for the spin 1/2nuclei (here1H) and ωnut is the nutation angular frequencyassociated with the central transition (|1/2⟩|-1/2⟩) of theobserved quadrupolar nuclei (here27Al). For static experiments,if the quadrupolar frequencyωQ (ωQ ) 3e2qQ/2I(2I-1)p) issmall, the quadrupolar interaction does not affect the nutationfrequency and the matching condition is simply

with ΩI as the radio frequency strength for the quadrupolar spin.If the quadrupolar frequency is large compared to the radio

frequency strength of the quadrupolar spin, a selective polariza-tion transfer to the central transition is possible but the nutationangular frequency becomes

The matching condition is thus

Spin Locking. Besides energy level matching, cross polar-ization requires the locking of the (|1/2⟩|-1/2⟩) coherence. Vegahas shown that the spin locking of quadrupolar half-integer spinsunder magic angle spinning (MAS) is efficient in the fastpassage and adiabatic regime, that is forR , 1 andR . 1,respectively, where

with ωr being the rotation angular frequency of the rotor. Offundamental interest to our studies, Sun et al. have describedthe efficiency of spin locking in the fast passage regime (R ,1).16 They showed that avoiding resonance conditions (ΩI *ωr), spin-locking efficiency is achieved forR < 0.02 if thesecond-order interaction can be neglected

(with ωO being the Larmor angular frequency). If second-ordereffects need to be taken into account, efficient spin lockingrequires even lower values ofR.

MAS and Nutation. Finally, Ding and McDowell underlinedthe influence of sample spinning on the nutation frequencies.17,18

This is further complicated by the effect of cross relaxation onnutation. Indeed the cross relaxation is governed by theheterodipolar interaction, which, in turn, is affected by therotation of the sample.

It follows that the efficiency of the polarization transferdepends in a complicated manner on the strength of thealuminum radio frequency field (ΩI) and on the spinning speed(ωr). This is exemplified on bayerite (Al(OH)3) (Figure 1)where two maxima appear corresponding to a selective (lowΩI) and nonselective excitation (highΩI) as well as thepossibility of excitation in the intermediate case. This atconstantωQ (there is only one Al site in bayerite) andΩS. Thedependence on the spinning speed of the cross polarized nutationfrequencies due to the cross-relaxation effect on the nutationfrequencies (neglecting the orientation dependence of the dipolarcoupling) is also illustrated by varying the spinning speed.

Therefore, depending on the experimental conditions, crosspolarization efficiency can be expected to be vastly differentfor two sites of different dipolar and quadrupolar characteristics.It follows that in favorable cases one should be able todifferentiate unresolved sites of close chemical shifts by varyingthe intensity of the Al rf field, playing thereby simultaneouslyon the Hartmann-Hahn (mis)matching and the efficiency ofthe Al spin locking. Due to homo- and heterodipolar couplingeffects on cross relaxation and, more importantly, to the spinlocking efficiency dependence on MAS, it is essential that thespinning speed remains constant for a meaningful interpretation

ΩS ) ωnut

ΩS ) ΩI if ΩI/ωQ . 1

ωnut ) (I + 1/2)ΩI

ΩS ) (I + 1/2)ΩI if ΩI/ωQ , 1

R )ΩI

2

ωQωr

ωQ2 /ωO , ΩI

7024 J. Phys. Chem. B, Vol. 102, No. 36, 1998 Mertens de Wilmar et al.

of the results. For the same reason, the contact time also muststay constant throughout the experiment.

Results

Model Compounds. Alumina exhibits an octahedral reso-nance at 8.5 ppm (Figure 2) and ammonium hexamolybde-noaluminate a single sharp line at 14 ppm (Figure 3). Apreliminary experiment was performed on a mixture of hydratedalumina (9 ppm) and [Mo,Al] coprecipitate (14 ppm) atνr ) 8 kHz and varying the strength of the27Al rf field (Figure4). A similar experiment was also performed at 14 kHz, butthe quality of the data was extremely poor, probably becauseat such speeds the cross polarization becomes very sensitive tosmall variations in Hartmann-Hahn mismatches due to insta-bilities of our amplifiers output.19 It appeared that the crosspolarization of the two species differed vastly suggestingapplicability of the method. The frequency of the maxima wasonly slightly different for the two lines indicating that the maineffect was a difference in spin locking efficiency rather than inthe nutation frequency. This was confirmed by a direct spin-lock experiment (Figure 5) under the conditions of maximumpolarization transfer for the non selective case as determinedin Figure 4. It appeared that after 500µs a third of themagnetization of the ammonium hexamolybdenoaluminateremained locked while the magnetization of the alumina hadalmost entirely dephased.

As a result, 1H f 27Al CPMAS performed in theintermediate regime acts as a filter favoring the resonance ofthe ammonium hexamolybdenoaluminate or that of the aluminadepending on the applied27Al rf strength (ΩI). A completetheoretical treatment for CPMAS of quadrupolar nuclei not being

Figure 1. Variable ΩI proton CPMAS27Al NMR experiment onbayerite.ΩS: 56 kHz. tcp: 500µs. O: staticb -: νr ) 5 kHz. sbs:νr ) 12 kHz.

Figure 2. One-pulse27Al MAS NMR spectra of the alumina blank,of Mo impregnated alumina, and of the solid collected outside thedialysis membrane.

Figure 3. One-pulse and proton CP (νr: 8 kHz. ΩS/2π: 56 kHz. tcp:500µs.ΩI/2π: 55 kHz.)27Al MAS NMR of the coprecipitated referenceammonium hexamolybdenoaluminate.

Figure 4. Variable ΩI proton CPMAS27Al NMR experiment on amixture of hydrated alumina (O: 8 ppm) and [Mo,Al] coprecipitate(b: 14 ppm).νr: 8 kHz. ΩS/2π: 56 kHz. tcp: 500 µs.

Figure 5. Variable spin locking duration27Al MAS NMR experimenton a mixture of hydrated alumina (O: 8 ppm) and [Mo,Al] coprecipitate(b: 14 ppm).νr: 8 kHz. ΩI/2π: 55 kHz.

Adsorption of Molybdate Ions J. Phys. Chem. B, Vol. 102, No. 36, 19987025

available at the moment for the intermediate regime (R ≈ 1),the optimumΩI and contact timetcp values have to be setempirically. To favor the aluminum polymolybdate resonance,they were set atΩI/2π ) 55 kHz andtcp ) 500µs on the basisof the results shown in Figures 4 and 5. Conversely, aΩI/2πvalue of 70 kHz would suppress this resonance relative to theone of the hydrated alumina. The relative intensities of thesetwo species therefore depend very sharply on relatively smallvariations of the27Al radio frequency strength.

The CP MAS experiments in this studies were thus performedunder such conditions as to filter the aluminum polymolybdate,hoping to detect it even as a minor occurrence.

Impregnated Aluminas. The one-pulse spectra of theimpregnated samples are almost identical to the one of the blankexperiment and typical of aluminas with an AlIV and an AlVI

resonance at 70 and 8.5 ppm, respectively. A small contributionat 14 ppm is, however, discernible for sample AlMo-9 (Figure2). It is therefore reasonable to assume that the octahedralresonance is actually the superposition of two unresolvedcontributions: the broad and intense AlVI alumina resonanceand a weaker one resulting from a mixed [Mo,Al] hydroxideformed during impregnation. Nevertheless, even with the bestefforts and the sharpest eyes, it was impossible on the basis ofone-pulse27Al MAS NMR alone to positively evidence theformation of a hydrated aluminum molybdate before thecalcination on all samples.

On the contrary, the1H f 27Al CP/MAS spectra obtained,as explained above, under conditions enhancing the 14 ppm lineof the aluminum polymolybdate indicated in a totally unam-biguous manner that this species formed in all cases (Figure6).

The recent work of Carrier et al has definitely shown thatindeed, an Anderson-type heteropolymolybdate is formed insolution when alumina is impregnated by an aqueous solutionof ammonium heptamolybdate, and that furthermore, this mixed[Al, Mo] species is also present on the surface of the uncalcinedcatalyst precursor.4-5 Actually, we obtained in that study similarspectra to the ones of Figure 6. Additionally, the data presentedhere tend to support the idea that the same type of aluminummolybdate also forms when the precursor is sodium molybdate,although it is not possible on the basis of the NMR data alone

to fully characterize the formed species. Moreover, the samplecollected outside the tube in the dialysis experiment (AlMo-9bis) gave a spectra similar to the impregnated samples (Figure6), thereby confirming the fact that a dissolution/reprecipitationmechanism is involved.

Discussion

Although the formation of Al2(MoO4) on calcined alumina-supported Mo catalysts has been reported and studied by27AlMAS NMR for some time, the detection of the formation of aspecies involving the formation of a Al-O-Mo bond duringthe impregnation stage is fairly recent. Incidentally, Edwardsand Decanio have already observed on rehydrated calcinedcatalysts a proton CP27Al resonance at 13 ppm that theyattributed to a compound formed by rehydration of aluminummolybdate on calcined Mo/Al2O3 catalysts.20,21 It must beemphasized that the usual straightforward comparative inter-pretation of one pulse and CP signals to distinguish bulk fromsurface species is obviously flawed when applied to quadrupolarnuclei.22,23 However, far from being a disadvantage, thecomplications arising from the quadrupolar interaction allows,at the price of careful and somewhat lengthy experimentalsettings, even more selective experiments.

A rough back-of-the-envelope calculation easily demonstratesthat more is involved here than simple surface signal enhance-ment. The structure of theγ-Al 2O3 surface is not preciselyknown, but assuming a fully rehydrated surface, one would geta value of the order of 10 surface aluminum per nm2. With aBET surface area of 200 m2/g and within the conservativeassumption that only surface atoms are cross polarized, thesignal arises from about 3 mmol surface Al/g. On the otherhand, all aluminum in [Al(OH)6Mo6O18]3- are hydroxylated andconcerned by the polarization transfer. From a 10% Mo weightloading, if all Mo belonged to the aluminum heteropolymolyb-date, one would get 0.2 mmol Al/g in the molybdate. The signalarising from the heteropolymolybdate would therefore remain1 order of magnitude weaker than the one from the aluminasurface.

Assuming that in our materialsνQ probably ranges from 0.1to 1 MHz, based on typical values measured by SATRAS24 andoff-resonance nutation,25 one can try to describe the relevantcharacteristic of our experiments. First, they were performedunder nonselective conditions atΩS ) ΩI. Second, theycorrespond to the transition between an adiabatic and intermedi-ate regime of passage asR will range between 4 and 0.4depending onωQ. Third, resonance conditions were avoidedasΩI * ωr. Fourth, second order quadrupolar effects on spinlocking need not be involved asωQ

2/ωO (ranging from about100 Hz to 10 kHz in frequency units) was sufficiently smallerthanΩI. Consequently, on the basis of, for the most part, thesecond point, it was the differences in quadrupolar couplingsaffecting the spin locking, compounded with differences inheterodipolar couplings with protons (affecting simultaneouslypolarization transfer and nutation frequencies), which alloweddiscrimination of the two sites. Again, because of the intricatedependency of the signal on quadrupolar and dipolar couplings,we will not attempt a quantitative description of the CPMASresponse but use the experiment as a qualitative filter. Byperforming the CP experiment in the intermediate regime, weare able not only to suppress the bulk alumina signal but alsoto reduce the relative contribution from the alumina surface.

An important drawback is that the experimental conditionsmust be set empirically on model compounds. This impliesthat an a priori assumption must be made on the nature of the

Figure 6. Proton CP MAS NMR27Al spectra of the alumina blank,of Mo impregnated alumina, and of the solid collected outside thedialysis membrane.νr: 8 kHz. ΩS/2π: 56 kHz. tcp: 500 µs. ΩI/2π:55 kHz.

7026 J. Phys. Chem. B, Vol. 102, No. 36, 1998 Mertens de Wilmar et al.

species to be detected. However, the fact that we could evidencethe previously undetected aluminum polymolybdate establishesthe interest of such an27Al CP experiment which we intend toperform on other metal-alumina catalysts systems.

References and Notes

(1) Bell, A. T., Pines, A., Eds.NMR Techniques in Catalysis; MarcelDekker: New York, 1994.

(2) Espinose de la Caillerie, J. B. d′; Kermarec, M.; Clause, O.J. Am.Chem. Soc.1995, 117, 11471-11481.

(3) Mertens de Wilmar, D. Thesis, Universite´ de Paris VI, 1998.(4) Carrier, X. Thesis, Universite´ de Paris VI, 1998.(5) Carrier, X.; Lambert, J. F.; Che, M.J. Am. Chem. Soc.1997, 119,

10137-10146.(6) Edwards, J. C.; Adams, R. D.; Ellis, P. D.J. Am. Chem. Soc.1990,

112, 8349-8364. Edwards, J. C.; Ellis, P. D.Langmuir 1991, 7, 2117-2134.

(7) Maciel, G. E.; Ellis, P. D. InNMR Techniques in Catalysis; Bell,A. T., Pines, A., Eds.; Marcel Dekker: New York, 1994; Chapter 5, pp231-309.

(8) Hommel, H.; Legrand, A. P.; Dore´mieux, C.; Espinose de laCaillerie, J. B. d′ In The Surface Properties of Silica; Legrand, A. P., Ed.;Wiley: Chichester, 1998; Chapter 3B, pp 235-284.

(9) Espinose de la Caillerie, J. B. d′; Kermarec, M.; Clause, O.J. Phys.Chem.1995, 99, 17273-17281.

(10) Gates, B. C.Catalytic Chemistry; Wiley: New York, 1992.(11) Rebours, B.; Espinose de la Caillerie, J.-B. d′; Clause, O.J. Am.

Chem. Soc.1994, 116, 1707-1717.(12) Ohman, L.Inorg. Chem.1989, 28, 3629-3632.(13) Pines, A.; Gibby, M. G.; Waugh, J. S.J. Chem. Phys.1973, 59,

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Adsorption of Molybdate Ions J. Phys. Chem. B, Vol. 102, No. 36, 19987027