Magic angle spinning NMR investigations on amorphousaluminophosphate oxynitrides

Teresa Blasco, Avelino Corma,* Lorenzo Ferna� ndez, Vicente Forne� s and Rut Guil-Lo� pez

Instituto de (Universidad de V alenciaÈCSIC), Avda. de losT ecnolog•�a Qu•�mica Polite� cnicaNaranjos s/n, 46022-V alencia, Spain. E-mail : acorma=itq.upv.es

Received 26th May 1999, Accepted 28th July 1999

Multinuclear magic angle spinning NMR spectroscopy has been applied to characterise the nitrogen speciespresent in aluminophosphate oxynitrides, i.e. AlPONs, prepared by ammonolysis of high surface areaaluminophosphates. At room temperature, the enters the coordination sphere of some tetrahedral Al inNH3the amorphous to increase its coordination to 5 and 6. The IR spectra show bands characteristic ofAlPO4

and while the 31P spectrum gives a new low Ðeld component that reÑects the inÑuence ofNH3 NH4`,adsorbed and/or When the aluminophosphate precursor is treated with ammonia at temperaturesNH3 NH4`.within the interval 1073È1123 K, a deeper modiÐcation of the occurs giving rise to new nitrogenAlPO4species, which remain after outgassing above room temperature, and are tentatively assigned to ÈPÈNH2terminal groups. When the ammonolysis is carried out at 1273 K an aluminophosphate trydimite phaseappears, and nitrogen must be engaged in the formation of moieties of the type with x [ 1.PO4~x

Nx,

Introduction

Substitution of liquid base catalysts by solids would modifythe routes actually used to obtain Ðne chemicals, with a posi-tive impact on reducing corrosion and environmental prob-lems. Although little attention has been paid to solid basecatalysts when compared with acids, the interest on thesubject has considerably increased in the last decade.1h3Recently, a new family of basic solids has been reported,amorphous aluminophosphate oxynitrides (AlPONs), activein the Knoevenagel condensation of benzaldehyde withmalonitrile, which are prepared by nitridation of high surfacearea aluminophosphate at elevated temperature.4h8 The sameapproach, i.e. thermal ammonolysis, has been applied toincorporate nitrogen into microporous aluminophosphateAlPON-5,9 expanding the potential use of molecular sieves asbase catalysts.

Basicity in aluminophosphate oxynitrides is linked to thepresence of nitrogen atoms substituting for oxygen, and thenit would appear that depending on the type of oxygen atom,i.e., PÈOH, AlÈOH or PÈOÈAl, substituted by nitrogen, theexpected basic sites would be terminal orPÈNH2 , AlÈNH2bridging PÈNHÈAl, each of which exhibits di†erent basestrength.10 It is therefore of interest to determine the type andrelative amount of basic sites to control their concentration,acting on the preparation conditions. However, although avariety of spectroscopic techniques have been used to charac-terise AlPONs, the exact nature of the nitrogen speciespresent is not well established and it is still under discussion.

In N-containing AlPON-5 microporous material, the use ofIR and magic angle spinning (MAS) NMR has allowed thedetection of adsorbed and molecules, and theNH3 NH4`suggestion that the insertion of N into the framework occursthrough the formation of terminal units.9 This conclu-PÈNH2sion was reached assuming, by analogy with SiAlON ceramicmaterials, that the replacement of one O atom by one N atomin the tetrahedra would shift the Al resonance 10 ppmAlO4downÐeld.11 Since ammonolysis of the AlPON-5 above 1073K produced a shift of the 27Al signal from by no moreAlPO4

than 5 ppm, it was concluded that the formation of NH bridg-ing (AlÈNHÈP) and terminal groups in AlPONs isAlÈNH2highly unlikely.9

In spite of the large number of publications devoted toamorphous aluminophosphate oxynitrides,4h8,12h16 only afew of them discuss the nature of the nitrogen sitesformed.8,12h15 In a previous publication including IRcharacterisation8 we suggested that amorphous AlPON con-tains and PÈNHÈAl groups, besides adsorbedPÈNH2 NH3and However, from IR spectroscopy it is not easy toNH4`.di†erentiate between terminal and bridging ÈNHÈÈNH2species.8,9 The use of di†use reÑectance infrared Fourier trans-form (DRIFT) spectroscopy in the characterisation of amorp-hous AlPONs allowed the detection of a peak characteristic of

terminal groups,12,13,15 whereas the presence of aPÈNH2band at 1320 cm~1 was tentatively attributed to P2NÈPgroups.12 The last nitrogen species as well as sites of the NP3type have been suggested to be present in AlPONs on thebasis of X-ray photoelectron spectroscopy (XPS),7 by analogywith the results reported for nitrogen-containing phosphateglasses.17 However, in other publications the XPS peaksN1sare assigned to units,12,13 and the presence of structuralÈNH

xÈNHÈ groups isoelectronic to (ÈOÈ) have also been proposedfrom DRIFT spectra.15

The preferred formation of PÈN bonds has been suggestedto be due to the fact that they are stronger than PÈO, whereasAlÈN linkages are weaker than AlÈO.9,12 Ab initio molecularorbital calculations have been used to obtain thermodynami-cal information on the possibility of creating ÈNHÈ bridgingor terminal groups in aluminophosphates.10 ThePÈNH2results indicate that the formation of terminal is theÈNH2most favourable process, being exothermic, whereas the sub-stitution of a bridging O by an NH group is endothermic.10Therefore, it appears that at low temperatures one shouldpreferentially form terminal groups and the NH bridgingNH2will be formed when increasing temperature.

In this paper, we study the nature of the nitrogen speciesformed in amorphous aluminophosphate oxynitrides bymultinuclear MAS NMR spectroscopy. We show that this

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technique in combination with infrared spectroscopy (IR) isvery helpful to identify the nitrogen species formed when the

precursor is treated with at di†erent tem-AlPO4 NH3peratures.

ExperimentalMaterials

The AlPON samples were obtained by nitridation of highsurface area aluminophosphate precursor prepared fol-AlPO4lowing the solÈgel method developed by Kearby.18 The pro-cedure was as follows : At a temperature of 273 K, a 10% w/wsolution of was slowly added to another one ofNH4OH

and in adequate amounts so that theAlCl3 É 6H2O H3PO4Al/P ratio was 1. Once the was added, the pH of theNH4OHÐnal solution was measured to be approximately 5. Afterstanding overnight at room temperature, the gel obtained wascarefully washed with isopropanol, dried and calcined for 3 hat 923 K.

Nitridation of the aluminophosphate was performedAlPO4by a Ñowing stream of pure ammonia. Time, Ñow rate andtemperature of nitridation were varied. At the end of the acti-vation process, the samples were treated with a pure and drystream of nitrogen. The nitridation conditions and the maincharacteristic of the resultant aluminophosphate oxynitrides(AlPONs) are reported in Table 1.

Elemental analyses were carried out on as-synthesisedAlPONs using a CHNS analyser (Fisons EA 1108). Thesurface areas were obtained on an ASAP 2000 (Micromeritics)using the BET methodology. The AlPON materials werepretreated at 673 K under vacuum prior to the nitrogenadsorption.

FT-IR measurements

Infrared measurements were performed on a Nicolet 710 FTspectrophotometer using self-supporting wafers of 10 mgcm~2 and a Pyrex vacuum IR cell. Prior to the collection ofspectra, the samples were treated under vacuum at 373 and473 K in the cell for 2 h, after which time the spectra wereobtained at room temperature.

Solid-state NMR measurements

Solid-state 31P, 27Al and 1H NMR spectra, both conventionalBloch decays (BD) and cross-polarization (CP) from protons,were recorded under magic angle spinning (MAS) at ambienttemperature on a Varian VXR-400S WB spectrometer, at161.9 MHz, 104.2 MHz and 399.9 MHz, respectively. ACP/MAS Varian probe with zircona rotors (7 mm indiameter) was used for 27Al, 31P and 1H MAS NMR, and asupersonic MAS Doty probe was used to record the 31P and1H NMR spectra at spinning rates above 10 kHz. All spectra,except some of 31P and 1H, were acquired with the samplesspinning at 7 kHz. The MAS rotors were driven by air andthe magic angle was set precisely by observing the 79Br reso-

nance of KBr. Pulses of 5 ls corresponding to a Ñip angle of3p/8 rad and recycle delays of 40 s were used for 31P. The 1Hspectra were recorded with pulses of 7.5 ls to Ñip the magne-tisation by an angle of p/2 rad and delays of 5 s between con-secutive pulses. Only the central ([1/2 ] 1/2) transition isobserved in the 27Al MAS NMR spectra, which were acquiredby Ñipping the magnetisation by an angle of p/20 rad usingpulses of 0.5 ls, and a recycle delay of 0.5 s. The HartmannÈHahn conditions for 1H ] 27Al CP and 1H ] 31P CP wereestablished on samples of kaolinite and respec-NH4H2PO4 ,tively, while the contact times used were optimised on theamorphous aluminophosphate and AlPON samples. 1Hp/2rad pulses of 6.5 and 9.0 ls were used in the CP experimentsto 31P and 27Al, respectively. In both cases, recycle times of 3s were used. 31P NMR spectra were simulated with individualGaussian peaks.

The NMR experiments of the degassed samples were per-formed in sealed glass NMR inserts (Wilmad withconstriction). Before sealing, the samples were packed in theNMR tube, heated under dynamic vacuum at 4 K min~1 upto 473 K and kept at this temperature for 3 h until a Ðnalpressure of 10~6 kPa.

We tried to record the natural abundance 15N MAS NMRspectra under cross-polarization condition from H withoutany success.

Results and discussionTo get an overall understanding of the aluminophosphateoxynitride system, we have studied the interaction ofammonia with a high surface area at room tem-AlPO4perature (AlPO-N), a series of AlPON samples with di†erentnitrogen content prepared by ammonolysis at temperatures inthe range 1073È1123 K under di†erent conditions (AlPON-1to -4), and another sample prepared by treatment of theamorphous with at 1273 K (AlPON-5). The lastAlPO4 NH3treatment leads to the formation of crystalline trydimite, asshown by XRD.

Although MAS NMR is the main characterisation tech-nique applied in this study, we have also used FT-IR to followthe evolution of the nitrogen species on increasing the degass-ing temperature.

FT-IR spectra

Fig. 1 shows the IR spectra of the aluminum phosphate pre-cursor treated with ammonia at room temperature, sampleAlPO-N, which has been used here as a reference. Whensample AlPO-N is heated at 373 K under vacuum to removethe hydration water, the IR spectrum shows a band at 3678cm~1 related to PÈOH, a broad NH stretching band at ca.3300 cm~1 probably due to and/or and twoNH3 NH4`,bending bands at 1620 and 1450 cm~1 associated with NH3and respectively. When the degassing temperature isNH4`,increased to 473 K, the bands of and decrease,NH3 NH4`

Table 1 Main characteristics of the oxynitride and oxide samples

Nitridation Nitridation NH3 Ñow rateCatalysts time/h temperature/K /ml min~1 %N (w/w) SBET/m2 g~1

AlPO4 È È È È 210ALPO-N 8 298 150 4.2 210ALPON-1 10 1073 150 8.8 195ALPON-2 16 1073 150 9.9 207ALPON-3 16 1073 200 10.5 205ALPON-4 16 1123 150 15.0 141ALPON-5 16 1273 150 7.5 *

* Not measured.

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Fig. 1 IR spectra of samples AlPO-N, AlPON-2 and AlPON-5treated under dynamic vacuum at the indicated temperatures.

indicating the desorption of at this temperature, whileNH3that of PÈOH slightly increases (Fig. 1).The IR spectra of AlPONs obtained by treating the alumin-

ophosphate precursor with at 1073 K are also illustratedNH3in Fig. 1 for sample AlPON-2. The following bands can beseen in the spectra : D3300 cm~1, NH or any com-NH

xpound; 1620 and 1445 cm~1, adsorbed and aNH3 NH4` ;broad band centred at 3600 cm~1 assigned to interactingPÈOH groups ; and a band at 1550 cm~1, which can be attrib-uted either to terminal or to bridging AlÈNHÈP.5ÈPÈNH2When the treatment temperature is increased up to 673 K, thebands associated with and decrease, whileNH3 NH4`bending (or AlÈNHÈP) units remain almostPÈNH2unchanged. At the same time, the two narrow stretchingbands at 3370 and 3455 cm~1 assigned to orPÈNH2AlÈNHÈP become clearly visible, and the broad band at 3600cm~1 of interacting PÈOH disappears, while a narrow band at3670 cm~1 of free PÈOH groups becomes evident. Similarresults, with only minor di†erences in the relative intensity ofthe absorption bands, have been obtained for all AlPONsamples studied here.

Fig. 1 also contains the IR spectra of sample AlPON-5,obtained by ammonolysis of AlPON at 1273 K, evacuated at373 K and 473 K. Only very weak bands due to NH4`,

and are detected even when the spectrum isÈPÈNH2 NH3recorded under ambient conditions, which, considering thesample nitrogen content, indicates that other nitrogen speciesmust be formed when the nitridation is carried out at highertemperatures.

Solid-state NMR spectra

Calcined aluminum phosphate precursor. The 31P and 27AlMAS NMR spectra of the starting aluminum phosphateheated at 1073 K in the absence of are shown in Fig. 2.NH3The 31P MAS NMR spectrum consists of a broad and sym-metrical line at [26 ppm typical of tetrahedrally coordinatedphosphorus in amorphous aluminum orthophosphates.19,20Although no additional peaks are detected in the 1H ] 31PCP MAS spectrum, we cannot rule out the presence of PÈOHgroups as they can be masked in the broad resonance. The 1HMAS NMR spectrum (not shown) gives a broad band centredat 5.3 ppm due to water. The 27Al BD MAS NMR spectrumis constituted by a single peak at 38 ppm, which correspondsto aluminum atoms in a tetrahedral environ-Al(OP)4ment.19,20 The 1H ] 27Al CP spectrum shows two additionalresonances at [15 ppm and 9 ppm. As CP selectivelyenhances the nucleus close to protons these two signals most

Fig. 2 31P BD and 27Al BD and CP (contact time of 0.8 ms) MASNMR spectra of the amorphous precursor.AlPO4

probably correspond to Al species coordinated to extra watermolecules or AlOH groups. The resonance at [15 ppm ischaracteristic of octahedral Al. The upÐeld shift of this com-ponent with respect to other aluminum oxides was ascribed tothe presence of P in the second coordination shell of Al.21 Theassignment of the 9 ppm peak is not clear. In crystalline alu-minum phosphates the presence of a resonance at this posi-tion has been attributed either to unreacted pseudobohemiteor to penta-coordinated Al resulting from the interaction ofone molecule with tetrahedral framework Al.22 In anyH2Ocase, the two high Ðeld Al species must be in a very low con-centration, as they are negligible in the BD spectrum.

Amorphous precursor treated with at room tem-AlPO4

NH3perature (sample AlPO-N). Deep modiÐcations in the 31P,

27Al and 1H MAS NMR spectra are observed when theamorphous aluminum phosphate precursor is treated with

at room temperature. Besides the signal coming from theNH3 phase, the 27Al MAS NMR spectrum shows two newAlPO4resonances at 11 and [13 ppm, which are relatively enhancedunder cross-polarization, indicating that they must corre-spond to Al species with protons in its vicinity (Fig. 3). Similarresults have been obtained when the AlPON-5 molecularsieve is treated with in mild conditions, and the twoNH3emerging signals were assigned to framework Al coordinatedto one and two molecules to give penta- and hexa-NH3coordinated Al, respectively.9 Accordingly, the signals at 11and [13 ppm must be due to andAlO4(NH3) AlO4(NH3)2 ,respectively. When the sample is treated under vacuum at 473K, only the tetrahedrally coordinated 27Al is visible in thespectrum, in agreement with the desorption of ammonia mol-ecules from the sample surface observed by IR. This experi-ment supports the assignment of the low frequency signals toAl species in a higher coordination and not to the segregationof an alumina phase.

The 31P MAS NMR spectrum of sample AlPO-N is consti-tuted by the aluminum phosphate peak at [25 ppm and anintense shoulder at ca. [10 ppm, as can be seen in Fig. 3.After degassing the sample at 473 K, the shoulder disappears,and the spectrum of the aluminum phosphate precursor isrecovered (Fig. 3). The mild conditions used to prepare sampleAlPO-N do not allow the replacement of oxygen atoms of theamorphous by N, i.e., AlPON material is not obtained,AlPO4since, as indicated by the IR spectra shown in Fig. 1, PÈNH2and/or bridging PÈNHÈAl moieties are not formed, and only

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Fig. 3 27Al BD and CP (contact time of 0.8 ms), 31P and 1H MASNMR spectra of sample AlPO-N. To record the 31P spectrum afterdegassing, the sample was treated under dynamic vacuum at 473 Kfor 3 h and sealed into a glass insert.

adsorbed and groups are detected. Then, theNH3 NH4`appearance of the resonance at ca. [10 ppm is induced by thepresence of adsorbed and/or that desorb byNH3 NH4`,degassing at 473 K. As mentioned above, when crystallineAlPON-5 is mildly treated with ammonia, the 31P resonanceshows an asymmetry to low Ðelds which was ascribed to theinÑuence of penta- and hexa-coordinated aluminum formedby adsorption of molecules.9 Therefore, we tentativelyNH3assign the signal at ca. [10 ppm to tetrahedra withPO4 NH3molecules coordinated to Al in its proximity. Another possibleexplanation for the appearance of this resonance is the inter-action of ammonia molecules with PÈOH groups, as pre-viously suggested for oxynitrides.7,12,23 The PÈOHÉ É ÉH3Ninteraction via hydrogen bonding or the formation of speciesof the type would be possible because of thePO~É É ÉH4N`certain acid character of PÈOH groups, as also suggested bythe IR results shown in Fig. 1.

The relative intensity of the signal at [10 ppm in theexperimental spectrum (see Table 2) indicates that almost halfof the total phosphorus present is a†ected by the adsorbedammonia and/or ammonium. We believe that this occurs byentering the coordination sphere of aluminum and alsothrough the interaction of ammonia with PÈOH groups.Indeed, the 1H MAS NMR spectrum of sample AlPO-N isformed by a broad resonance of water at 5 ppm with a shoul-der at ca. 6.5 ppm previously attributed to ionsNH4`resulting from the interaction of with available protons.NH3

Ammonolysis of the amorphous precursor at 1073–AlPO41123 K: AlPON. High temperature ammonolysis of amor-

phous induces deep changes in the 31P and 27Al MASAlPO4NMR spectra. Fig. 4 shows the 27Al MAS NMR spectra ofthe oxynitrides studied here. All spectra are mainly formed bytwo broad peaks at around 38 ppm and [10 ppm corre-sponding to tetrahedral and octahedral Al, respectively. Thislatter signal increases with cross-polarization from protonsand must result from the coordination of two additional mol-ecules, probably to form octahedral sites. Although it isNH3 ,not evident from the spectra, we cannot rule out the presenceof some penta-coordinated Al. When the spectra of Fig. 4 arequantitatively compared with similar Al-containing amor-phous aluminum phosphate, some 27Al spectral intensityappears to be missing, indicating the presence of invisible Alin AlPON. Indeed, the tetrahedral signal of sample AlPON-4with the highest N content seems to be formed by the contri-

Table 2 Results of the simulation of the 31P MAS NMR spectra ofoxide and oxynitride samples from individual lines. Chemical shifts(in ppm) are referenced to H3PO4

31P MAS NMR of31P MAS NMR degassed samples

Sample d (ppm) Intensity (%) d (ppm) Intensity (%)

AlPO4 [27 100 È ÈAlPO-N [10.6 47 [24.7 100

[23.6 53ALPON-1 2.6 34 [2.0 23

[8.5 31 [12.0 11[22.3 35 [24.8 66

AlPON-2 3.2 32 [0.8 21[7.9 29 [10.0 6

[21.7 39 [25.2 73AlPON-3 2.6 42 [1.7 22

[7.8 29 [11.3 11[23.2 29 [24.9 67

AlPON-4 2.7 69 2.3 58[7.9 20 9.0 13

[24.3 12 [24 29AlPON-5 25 5

15 123 23

[29 60

bution of two components, one of them quite broad and prob-ably due to Al in a distorted tetrahedral symmetry with a highquadrupole coupling constant. However, inspection of Fig. 4shows that there seems not to be a direct correlation betweenthe N content and the relative intensities of the aluminumsignals. When samples are degassed at 473 K, the octahedralresonance disappears, and only the tetrahedral peak is presentin the spectra indicating the loss of (or water) moleculesNH3from the coordination sphere of Al. When the sample is con-tacted with air, hexa- and penta-coordinated Al signals reap-pear, suggesting its coordination to water molecules. Theexperimental results reported for SiAlON materials11 suggestthat the formation of ÈNHÈ bridging would shift the 27Alresonance of the aluminophosphate to high frequencies. Fromthe results obtained here and given the quadrupolar nature ofthe 27Al nucleus, we can reach no conclusion about the pres-ence or absence of ÈNHÈ species in our samples.

The 31P BD spectra of the aluminum phosphate oxynitridesamples are shown in Fig. 5A. All spectra are characterised bya very broad signal with the maximum at ca. [8 ppm andtwo shoulders at around [25 and 3 ppm. Under cross-polarization conditions at short contact times (see Fig. 5B) themaximum of the spectrum appears at [18 ppm which must

Fig. 4 27Al MAS NMR spectra of samples indicated in the Ðgureand the 27Al CP (contact time of 0.8 ms) MAS NMR spectrum ofsample AlPON-2.

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Fig. 5 (A) 31P MAS NMR spectra of the samples indicated in theÐgure, recorded at spinning rates above 15 kHz. (B) 31P BD and CPMAS NMR spectra of sample AlPON-4 recorded at contact times of1 and 5 ms.

probably be due to PÈOH groups, while at longer contacttime, the component at ca. 3 ppm becomes broader and moreprominent (Fig. 5B). The resonance appearing at the highestÐeld ([25 ppm) corresponds to tetrahedra fromP(OAl)4amorphous aluminophosphate which remains una†ected bythe ammonia treatment. Maximum intensity in the spectra ofFig. 5A appears at chemical shift close to that of the signalemerging upon adsorption of ammonia at room temperature,and then this component at ca. [8 ppm (or [10 ppm) can beassigned to phosphorus tetrahedra interacting with adsorbed

molecules.NH3The IR spectra of AlPON samples indicate the formation ofterminal and/or NH bridging of the AlÈNHÈP typePÈNH2after the high temperature treatment of the starting alu-NH3minum phosphate material, and thus the high frequency phos-phorus resonance (shoulder at ca. 3 ppm) must correspond tothese oxynitride species. This assumption is further supportedby the 31P MAS NMR spectra of samples degassed at 473 K,as the signal from the amorphous compound isP(OAl)4highly recovered mainly at the expense of the resonance at [8ppm, as shown in Fig. 6. From the results presented here andthe thermodynamical data from molecular orbital calcu-lation,10 and in agreement with the previous assignment of theresonance at [ 8 ppm observed after ammonolysis of theAlPO-5 molecular sieves,9 we believe that the low Ðeld signalobserved in our AlPON samples must be mainly due to

species. After degassing at increasing temperature, aPÈNH2

Fig. 6 31P MAS NMR spectra of the samples indicated in the Ðgurerecorded after the treatment at 473 K under dynamic vacuum for 3 h.

certain intensity at [ 8 ppm remains, probably because ofsome residual or molecules left after the thermalNH3 NH4`treatment, although we cannot discard that it could corre-spond to species in somewhat di†erent environment.PÈNH2entities also decrease with thermal treatment,PÈNH2although to a much lesser extent, in agreement with its higherstability observed by IR. From Fig. 6, it can be suggested thatthe contribution of terminal species to the total spec-PÈNH2tral intensity increases when increasing the nitrogen content.

In order to estimate the relative contribution of the di†erentP sites, we have simulated the experimental spectra using indi-vidual Gaussian lines, as illustrated in Fig. 7 for some relevantsamples, leading to the results collected in Table 2. We mustpoint out that the di†erent components are not well resolvedin the experimental spectra and the shape and width of thesignals are unknown, which surely will induce large errors inthe simulations. However we think that the results obtainedcan be useful for comparative purposes in order to have anestimation of the di†erent P signals and their evolution withthe thermal treatment. From the inspection of Figs. 6 and 7,and Table 2, it comes out that a large fraction of the phos-phorus from the parent amorphous aluminum phosphate isa†ected to some extent by the high temperature ammonolysis,especially for the sample with the highest nitrogen content, forwhich the contribution of the signal attributed to terminal

groups becomes more prominent.PÈNH2Typical 1H spectra for AlPON samples are shown in Fig. 8.Besides the resonance of water at 5 ppm there are two broadsignals at 3 and 7 ppm previously assigned to adsorbed NH3and respectively.9 No additional signals which couldNH4`,be attributed to protons from terminal or ÈNHÈ groupsNH2are detected probably because they must be included in otherproton resonances. We tried to determine the chemical shiftposition of the signal from the spectrum of the samplePÈNH2degassed at 473 K, but no reliable results were obtainedbecause of the low proton concentration in the samples andthe background signal coming from the probe. However, apeak at ca. 1.3 ppm was evident, probably coming fromPÈOH groups visible after desorbing NH3 .

Ammonolysis of the amorphous precursor at 1273 K:AlPO4sample AlPON-5. The 27Al spectrum exhibits a peak at 37

ppm of Fig. 9 shows the 31P MAS NMR spectra ofAl(OP)4 .

Fig. 7 Experimental and simulated 31P MAS NMR spectra(simulated from individual Gaussian peaks) of samples AlPON-1 andAlPON-4 under ambient conditions and after the treatment underdynamic vacuum at 473 K for 3 h.

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Fig. 8 1H MAS NMR spectra of the samples indicated in the Ðgurerecorded at spinning rates above 15 kHz.

sample AlPON-5 that consists of a broad resonance centredat ca. 13 ppm and an intense peak at [29 ppm characteristicof crystalline aluminum phosphate, in agreement with theobservation of trydimite by XRD. The best Ðt of the experi-mental data using individual Gaussian peaks, included in Fig.9 and Table 2, contains peaks at 3, 15 and 25 ppm. Undercross-polarization from protons, the low Ðeld region is highlyenhanced, especially at short contact time, with the maximumintensity at ca. 25 ppm, suggesting the proximity of these sitesto protons. Therefore, the spectrum indicates the segregationof trydimite and the presence of phosphorus sites withAlPO4protons in its proximity giving rise to positive chemical shifts.The assignment of this resonance region is not clear to us yet.

It is reasonable to think that ammonolysis at elevated tem-peratures would lead to the formation of nitride-like speciescontaining P2NÈP and/or units, in agreement with theNP3absence of ÈNHÈ vibration bands in the IR spectrum (see Fig.1). A second possible explanation for the low Ðeld signalscould be the formation of phosphorus sites with increasingnumber of ÈN atoms on its Ðrst coordination sphere, in agree-ment with the previous observation.23 Therefore, it could bethought that under these conditions, an aluminum phosphateoxynitride phase with entities is formed, whosePO4~x

Nx

Fig. 9 31P BD and CP (at contact times of 1 and 5 ms) MAS NMRspectra of sample AlPON-5.

nature is not well established yet, but one nitrogen atomshould be bonded to several phosphorus atoms in order to Ðtthe chemical analysis.

ConclusionsThe MAS NMR spectra of AlPON samples prepared byammonolysis of high surface area in the temperatureAlPO4range 1073È1123 K and submitted to di†erent treatments, andthe thermodynamical information obtained by molecularorbital calculation,10 suggest the presence of terminal

entities in AlPONs. Moreover, the interaction ofÈPÈNH2high surface area aluminophosphate with ammonia at roomtemperature takes place through the incorporation of NH3molecules into the coordination sphere of Al which becomespenta- and hexa-coordinated, and also interacting with avail-able PÈOH groups which have a certain acid character.

The results obtained in this paper lead us to propose a reac-tion model for the interaction of and amorphousNH3 AlPO4materials :

O3AlÈOÈPO3] 2NH3 ] O3Al(NH3)2ÈOÈPO3 (1)

O3AlÈOÈPO3] NH3 ] O3AlÈOH] H2NÈPO3 (2)

Reaction (1) occurs at room temperature, whereas reaction (2)takes place during ammonolysis of the high surface area alu-minophosphate in the temperature range 1073È1123 K gener-ating terminal groups. We have not found anyÈPÈNH2evidence for the presence of AlÈOH groups in AlPONs andtherefore we believe that these groups will probably reactfurther with ammonia, increasing its coordination.

On the other hand, it has been previously proposed13 thatthe formation of PÈOH groups occurs by eliminating the

terminal groups upon heating :PÈNH2ÈPNH2 ] H2O ½ NH3(g)] ÈPOH (3)

Ammonolysis of amorphous aluminophosphate at 1273 Kleads to crystalline trydimite and new 31P bands at positivechemical shifts, which are consistent with the presence ofphosphorus with more than one nitrogen in its coordinationsphere, although the nature of these new sites is still unknownto us.

Acknowledgementssupport by the CICYT (project MAT 97-1016-C02-Financial

01) is gratefully acknowledged. Dr. L. Ferna� ndez thanks theSpanish Ministerio de Educacio� n y Ciencia for a contract andR. Guil-Lo� pez thanks Bancaja for a grant.

References1 A. Corma and F. Rey, in Handbook of Heterogeneous Catalysis,

ed. G. Ertl, H. Knozinger and J. Weitkamp, to be published.2 D. Barthomeuf, Catal. Rev., 1996, 38, 521.3 H. Hattori, Chem. Rev., 1995, 95, 537.4 R. Marchand, R. Conanec, Y. Laurent, P. Bastians, P. Grange,

L. M. Gand•� a, M. Montes, J. Ferna� ndez and J. A. Odriozola,Fr. Pat., 94 01081, 1994.

5 P. Grange, P. Bastians, R. Conanec, R. Marchand and Y.Laurent, Appl. Catal. A, 1994, 114, L191.

6 A. Massinon, J. A. Odriozola, P. Bastians, R. Conanec, R. Mar-chand, Y. Laurent and P. Grange, Appl. Catal. A, 1996, 137, 9.

7 A. Massinon, E. Gue� guen, R. Conanec, R. Marchand, Y. Laurentand P. Grange, Stud. Surf. Sci. Catal., 1996, 101, 77.

8 M. J. Climent, A. Corma, V. Forne� s, A. Frau, R. Guil-Lo� pez, S.Iborra and J. Primo, J. Catal., 1996, 163, 392.

9 A. Stein, B. Wehrle and M. Jansen, Zeolites, 1993, 13, 291.10 A. Corma, P. Viruela and L. Ferna� ndez, J. Mol. Catal. A: Chem.,

1998, 133, 241.11 J. Klinowski, J. M. Thomas, D. P. Thompson, P. Korgul, K. H.

Jack, C. A. Fyfe and G. C. Gobby, Polyhedron, 1984, 3, 1267.

4498 Phys. Chem. Chem. Phys., 1999, 1, 4493È4499

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

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on 2

9/10

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4 09

:14:

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View Article Online

12 J. J. Ben•� tez, J. A. Odriozola, R. Marchand, Y. Laurent and P.Grange, J. Chem. Soc., Faraday T rans., 1995, 91, 4477.

13 J. J. Ben•� tez, A. D•� az, Y. Laurent, P. Grange and J. A. Odriozola,Z. Phys. Chem., 1997, 202, 21.

14 J. J. Ben•� tez, A. D•� az, Y. Laurent and J. A. Odriozola, Catal.L ett., 1998, 54, 159.

15 J. J. Ben•� tez, A. D•� az, Y. Laurent and J. A. Odriozola, J. Mater.Chem., 1998, 8, 687.

16 A. D•� az, J. J. Ben•� tez, Y. Laurent, P. Grange and J. A. Odriozola,J. Non-Cryst. Solids, 1998, 238, 163.

17 R. Marchand, D. Agliz, L. Boukbir and A. Quemerais, J. Non-Cryst. Solids, 1988, 103, 35.

18 K. Kearby, in 2nd Int. Congr. Catal., Technips, Paris, 1961, p.258.

19 G. Engelhardt and D. Michel, in High Resolution Solid StateNMR of Silicates and Zeolites, John Wiley and Sons, Chichester,1987.

20 J. Sanz, J. M. Campelo and J. M. Marinas, J. Catal., 1991, 130,642.

21 D. Muller, I. Grunze, E. Hallas and G. Z. Ladwig, Anorg. Allg.Chem., 1983, 500, 80.

22 See for instance : R. Jelinek, B. F. Chmelka, Y. Wu, M. E. Davies,J. G. Ulan, R. Gronsky and A. Pines, Catal. L ett., 1992, 15, 65.

23 B. C. Bunker, D. R. Tallant, C. A. Balfe, R. J. Kirkpatrick, G. L.Turner and M. R. Reidmeyer, J. Am. Ceram. Soc., 1987, 70, 675.

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