J. CHEM. SOC. FARADAY TRANS., 1995, 91(18), 3281-3284 3281

Reversible Interaction of NH, with the Framework of Template-free Zeolite-type SAPO-34

Remy Vomscheid, Marguerite Briend, Marie-Jeanne Peltre and Denise Barthomeuf Laboratoire de Reactivite de Surface, URA 1106 CNRS, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Pascal P. Man Laboratoire de Chimie des Surfaces, URA 1428 CNRS, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France

The interaction of ammonia with the template-free SAPO-34 framework is studied by XRD, 29Si, 27AI and 31P magic angle spinning (MAS) NMR and IR spectroscopy and temperature-programmed desorption (TPD). A reversible loss of crystallinity upon NH, adsorption-desorption at room temperature is accompanied by the reversible formation of new 27AI and 31 P NMR spectroscopic peaks, and the displacement of the chemical shifts of 29Si, 27AI and 31P nuclei. Rupture of some AI-0-P bonds is proposed. Upon NH, desorption, the broken Al-0-P bonds are reformed. Lewis acidity, detected by IR spectroscopy, is assumed to proceed from the addition of ammonia on Al partly or fully connected to the framework. Such an interaction may affect acidity measurements made using ammonia adsorption.

It has been shown that a molecule such as H,O is able to damage the framework of SAPO-34 (which has a structure similar to that of chabasite) in a reversible way at tem- peratures below 373 K.'s2 At higher temperatures, SAPO-34 is stable up to ca. 1373 K where it is transformed into a dense crystalline phase mainly with a cristobalite-type structure with small amounts of tridymite-like It may be of interest to have information on the framework stability of SAPO-34 with regard to other molecules. Ammonia is of par- ticular importance since it is very often used for the study of acidity of small-pore zeolites such as SAPO-34.'-*

This paper describes the interaction of NH, with SAPO-34 prepared with tetraethylammonium hydroxide (TEAOH) as template. The use of morpholine as templateg gives materials with similar properties.'

Experimental A SAPO-34 sample synthesized with TEAOH'*1'.'2 has the formula Sio~09Alo~4,P,,,202 . Its crystallinity was checked with a Siemens D500 X-ray diffractometer. After the sample had been heated to 873 K to remove the template, the crys- tallinity was referred to as 100%. After NH, adsorption (100 and 750 Torr) the changes in crystallinity were checked by transferring the samples, under a dry atmosphere, to a gas- tight X-ray cell. The percentage of crystallinity was estimated from the (201), (003), (21 l), (104) and (220) peaks.

Magic angle spinning (MAS) "Si, 27Al and 31P NMR spectra were carried out' using a Bruker MSL-400 multinu- clear spectrometer at 79.5, 104.2 and 161.9 MHz, respectively. The chemical shifts were given in ppm from external refer- ences TMS, AI(H,O),,+ in Al(NO,), aqueous solution, and 85% H,P04, respectively. The samples were pretreated in a vacuum line as described above. An excess of ammonia was introduced into the line at a pressure of either 100 or 750 Torr, and equilibrated for 20 h. The samples were then kept sealed until they were transferred to the rotor under a dry atmosphere.

IR studies were performed on self-supporting wafers (30 mg, 18 mm diameter) heated in a vacuum line to 873 K for 11 h under a flow of 0, and then evacuated at the same tem- perature for 6 h. After the cell had been cooled to room tem- perature (rt), ammonia was introduced (100 Torr). After a 16 h adsorption time, ammonia was progressively removed step- wise by a 5 h evacuation at increasing temperatures, from 373

to 673 K. After each step, the spectrum was recorded at rt with a Bruker IFS 66 V spectrometer.

Thermoprogrammed desorption (TPD) was carried out on 50 mg of sample loaded at rt, with ammonia (100 Torr), after a sample pretreatment similar to that used for the IR study. Physically adsorbed ammonia was first removed by a pre- liminary evacuation at 423 K for 6 h. The TPD started at rt, with a heating rate of 300 K h-'. The desorption was observed using a quadrupole mass spectrometer Leybold PGA-100, for m/z values of 2 (H2), 16 (NH, fragment), 28 "2), 32 ( 0 2 ) .

Results and Discussion X-Ray Diffraction Fig. 1 gives the X-ray patterns of the sample in the presence and absence of NH,. The differences observed between the as-synthesized material and that pretreated at 873 K {curves (a) and (b)] are comparable to those usually seen in zeolites, upon removal of the organic template. The adsorption of ammonia [at 100 (c) or 750 (d) Torr] produces a partial loss of crystallinity (around 40%), observable from the decrease in the intensity of lines of spectrum (b), and from the presence of a broad peak around 20 = 25". Desorption of ammonia for 13 h at 673 K (e) regenerates at rt the spectrum of the template-free sample (b). The change in crystallinity is reversible. It has already been reported that water adsorption-desorption below 373 K induces a reversible partial loss of crystallinity,"2 simultaneously with reversible changes in the MAS NMR spectra of 29Si' and 27A1.'4*'5 The results were explained by a reversible breakage of Si-OH-A1 bonds and, to a minor extent, of Al-0-P bonds.'+'2 This prompted an NMR study of SAPO-34 loaded with ammonia.

MAS NMR

29Si The "Si MAS NMR spectra reported in Fig. 2 indicate (as has been already seen') that upon template decomposition, there is a shift of the Si(4Al) peak from -91.7 to -94.4 ppm. This is explained (as in SAPO-3712) through the removal of the template (TEA+ ion) which interacted with the frame- work 0 atom of the Si-0-A1 bond, modifying the bond angles and bond length^.'^.'^ The small peak at - 94.9 ppm

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3282 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

. a

I

21

d

a 21 3 2O/degrees

Fig. 1 Absolute intensity XRD pattern of SAPO-34 in the gas-tight cell: (a), as synthesized; (b), after calcination and evacuation at 873 K; (c), adsorption of NH, (100 Torr); (4, adsorption of NH, (750 Torr); (e), evacuation at 673 K

(a) is assigned to Si(3Al) species existing in Si islands." The adsorption of ammonia at a pressure of 100 or 750 Torr [(c) and (e)] gives rise to the NHf groups seen in IR spectrom- etry (see later). This adsorption moves the peaks back to the

-94.6

-91.3

-95.0

-94.4

r -94.7

f / I

lq- -94.7

1 I I I I I J -80 -100 -120

6 Fig. 2 29Si MAS NMR of SAPO-34: (a), as synthesized; (b), after calcination and evacuation at 873 K; (c), (a) treated as (b) then adsorption of NH, (100 Torr); (d), (c) evacuated at 673 K; (e), (a) treated as (b) then adsorption of NH, (750 Torr); (f), (e) evacuated at 673 K

values of the template-containing sample (a), suggesting an O-/NHZ interaction similar to that of O-/TEA+. The desorption of ammonia at 673 K for 13 h [(d) and (f)] gives the Si(4Al) peak at the same position as found for the template-free materials (curve b), indicating a relaxation of the framework angles. The spectra indicate no significant changes in the type of species present due to NH, adsorption at any pressure. By contrast, water adsorption was shown'*12 to modify the 29Si MAS NMR spectra markedly, creating Q1, Q2 and Q, species (i.e. Si atoms connected to one-three framework T atoms; T = Si or Al) and enhancing Q4 species of the type Si(nA1) with n < 4. The rupture of Si-OH-A1 bonds, proposed to explain the H,O results, does not occur with ammonia. This was proven by cross-polarization experi- ments on the ammonia-loaded sample. The spectra are very similar to that shown in (c) in contrast with that produced from the attack of water on the framework.'.2

2 7 ~ 1 , 3 1 ~ The NMR spectra of 27Al and 31P given in Fig. 3 show some changes upon adsorption of ammonia by the sample (at 100 or 750 Torr).

The decomposition of the template removes the 11.8 ppm 27Al peak (b) assigned in SAPOs to an interaction between the AlO, tetrahedra and the template,I4 and shifts the tetra- hedral A1 peak (Al") from 37.8 to 33.8 ppm. The adsorption of ammonia at a pressure of 100 or 750 Torr moves the Al" peak back to near its initial position and generates peaks near -6.4 to - 10.9 ppm, which can be assigned to hexa- coordinated A1 (Al"). Shoulders at ca. 10 ppm may arise from a secondary coordination of A1 atomsi4 with ammonia giving, for instance, pentacoordinated A1 (Al'). The Alv and AIV' peaks are more intense at the highest NH, pressure (d). The ammonia desorption at 673 K for 13 h gives a spectrum (e) similar to (b).

1 I I I J

100 0 -100 0 -50

Fig. 3 27Al (A) and 31P (B) MAS NMR of SAPO-34: (a), as synthe- sized; (b), after calcination and evacuation at 873 K ; (c), (b) after adsorption of NH, (100 Torr); (4, (b) after adsorption of NH, (750 Torr); (e), (d) evacuated at 673 K

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 3283

The ,'P results vary in a similar way. In the presence of the template (a) or of ammonia [(c) and (41, the chemical shift is ca. -28.3 to -28.6 ppm while for the sample free of adsorbed phase, it is ca. -29.6 to -30 ppm [(b) and (e)]. This shift in 6, is accompanied, in the presence of ammonia by the appearance of a new peak around -17 ppm. This peak appears simultaneously with the AIV and A1IV peaks [(c) and (d)] and with the changes in 27Al and 29Si chemical shifts. The comparison of these results with those from XRD (Fig. 1) strongly suggests that all the observed effects are related. They can be explained, together with the additional coordination of A1 framework atoms by ammonia, by the breakage of Al-0-P bonds by NH,, decreasing the long- range order. Upon ammonia desorption the reversibility observed in NMR and XRD spectra suggests that the broken bonds are reformed. The Si-OH-A1 bonds would not be broken (no new Si environments are indicated from Fig. 2).

Ammoniacal attack of AlPO-5l9 and SAPO-5,' has been reported at temperatures higher than 773 K. For AlPO-5 below 1073 K, only the 27Al MAS NMR spectra were modi- fied while above this temperature, the ,'P MAS NMR spectra show a -6.5 ppm peak assigned to a P-NH, bond.'

In this study, cleavage of Al-0-P bonds by ammonia seems to occur at rt, the extent of the reaction increasing with the ammonia pressure. Fig. 4 suggests a possible reversible interaction which explains the new peaks in the 27Al (AIV, AIV1) and 31P MAS NMR spectra and the partial loss of crys- tallinity. The small -17 ppm 31P peak could arise from P(3A1, 10H) species (i.e. P atoms with 3A1 and 1 0 H as first neighbours). The formation of P(3A1, NH,) species '' is unlikely at rt since a peak at - 7 ppm is not observed.

TPD

Fig. 5 shows the TPD curves. The experiment was carried out in the absence of oxygen (flat curve, a). Most of the ammonia is desorbed as NH, (d) while some is decomposed to N, (b) and H, (c). The maximum of the NH, desorption peak occurs at ca. 620 K, which is comparable to values obtained for dealuminated HY.,' The maxima of the other desorption peaks (N, and H,) appear at the same temperature. Taking into account the different scales used for the NH, spectra and the other curves, this suggests that only a small part of NH, is decomposed to N, and H,, and that this decomposition occurs simultaneously with the removal of NH, . A compari- son with other SAPO-34 samples, synthesized either with TEAOH or morpholine as template, shows that the amount of ammonia desorbed between 423 and 800 K [estimated

P P Fig. 4 Scheme for the reversible opening of Al-0-P bonds in SAPO-34 upon NH, adsorption.

from the area of the Fig. 5(4 peak] is roughly proportional to the framework charge, i.e. to the number of protonic acid sites in the sample.22 For a similar framework charge, the acid strength of SAPO-34 appears higher than that of SAPO-37 (which has a faujasite-like structure) since under identical experimental conditions, the maximum of the NH, desorption spectra are found at 624 K and 548 K, respec- tively.

Infrared Study

Fig. 6 shows the spectra of the sample after pretreatment at 873 K (a), adsorption of NH, and further desorptions from 373 K to 673 K [(b)-(g)]. The peaks at 3625 and 3600 cm-' (due to vibrations of acidic OH group^^*^*'^*^^) disappear almost completely upon NH, adsorption by the sample at rt and further desorption at 373 K (b). They are progressively restored by the stepwise desorption at increasing tem- peratures. The OH bands reach their initial absorbance intensity at 523-573 K [(e) and (f)]. This indicates that removal of acidic hydroxy groups does not occur. Simulta- neously, bands due to adsorbed ammonia disappear com- pletely at the same temperatures. Bands at 1399 and 1454 cm-' are usually assigned to ammonium ions25 and those around 1600-1660 cm-' to NH, adsorbed on Lewis acid

A detailed assignment of bands" compared with results for y-alumina26 and X and Y zeolites25 suggests that adsorbed ammonia gives additional bands at 1750, 2700 and 2930 cm-' for the Brernsted acid sites, and at 3290 and 3414 cm-' for the Lewis centres. Fig. 6 shows that all these bands are removed simultaneously upon desorption. The Lewis sites in Si-A1 zeolites usually retain adsorbed basic molecules at higher temperatures than Brransted sites. This is not the case for SAPO-34, where Lewis and Brernsted acidity are of the same strength. It follows that the Lewis acidity in SAPO-34 is different from that in Si-A1 zeolites, recognized to arise from extraframework Al. No OH groups typical of extra- framework A1 or other extraframework species are seen in the IR spectra of SAPO-34. IR absorption spectra of pyridine

624

a

I 1 i I I 1 500 700

tern peratu re/K Fig. 5 TPD of ammonia for SAPO-34; (a), 0, ; (b), N, ; (c), H, ; (4, NH, . The scale of curve d is ten times lower than for curves a-c.

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

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adsorbed on template-free SAPO-3723 also show bands assigned both to Brsnsted and Lewis acid sites, with no evi- dence of either dealumination or the presence of extra- framework phases. For SAPO materials, the IR bands attributable to Lewis acidity might arise from the additional coordination of A1 atoms with any NH, still adsorbed at temperatures higher than 300 K. The A1 could be tetra- hedrally coordinated to oxygens in the framework, or partly detached from the framework, as shown in Fig. 4. In the case of SAPO-34, the simultaneous decrease in the Brsnsted and Lewis acidity bands shown in Fig. 6 suggests that the Al-0-P bonds would be in some way related to the Brransted sites. NH, might even be able to interact simulta- neously with 'conjugated' Lewis-Brransted sites, as this was proposed to occur for pyridine in Si-A1 zeolite^.^'

In conclusion, the framework of template-free SAPO-34 is very sensitive to the action of ammonia at room temperature. Such an interaction takes place via the formation of AIV and Al"' framework species and the breaking of some Al-0-P bonds. However, no significant modifications of the Si-OH- A1 bonds are observed upon ammonia adsorption. The reversibility of the ammonia interaction shows the high flexibility of the lattice. The question arises as to why water attacks Si-OH-A1 bonds to a greater extent than Al-0-P while ammonia seems to limit its action to Al-0-P bonds. Perhaps the shape, dipole moment and/or the relative permittivity of the two adsorbates are major parameters, together with the bond energy and bond ionicity of the two species, Si-OH-A1 and Al-0-P? Molecular modelling could shed some light on this problem.

Brsnsted and Lewis acid centres are shown to exist after ammonia adsorption by IR spectroscopic experiments. Ammonia is desorbed at the same temperature from both sites, suggesting the presence of 'conjugated' Lewis-Brsnsted sites. Brsnsted sites can unambiguously be assigned to Si-OH-A1 protonic species. Owing to the lack of argu- ments attesting dealumination, A1 framework atoms are sug- gested to constitute the Lewis acid sites by transformation of tetrahedral A1 to penta- or hexa-coordinated Al, still fully or partly bonded to the framework. These results pose questions concerning the interaction of basic molecules with SAPO (and/or AlPO) materials and the best way to measure the Brsnsted acidity of SAPO-34, without simultaneously modifying the solid.

References 1

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Paper 4/07402E; Received 5th December, 1994

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