Solid-state NMR study of ultrastable zeolite Y modified with orthophosphoric acid

121

Magnetic Resonance, 2 (1993) 121-129 Elsevier Science Publishers B.V., Amsterdam

Solid State Nuclear

Solid-state NMR study of ultrastable zeolite Y modified with orthophosphoric acid Waclaw Kolodziejski, Vicente For&s and Avelino Corma * Universidad

Politkcnica

de Valencia

UPV-CSIC,

Znstituto

de Tecnologia

Quimica,

Cam&o

de Vera, s/n.-46071

Valencia,

Spain

(Received 20 March 1993; accepted 31 March 1993)

Abstract

29Si, 27Al and 31P solid-state NMR under magic-angle spinning was used for the study of ultrastable zeolite Y modified with HsPO,. The material is a potential candidate for a new generation of oil-cracking catalysts. Bloch decay and cross-polarization spectra were compared. It was found that amorphous aluminium phosphate is formed during the P-impregnation and the following activation, and that it occupies the zeolite surface and internal voids. For a high P-content under steaming conditions, aluminium phosphate is converted into a crystalline form. No evidence has been found for incorporation of P atoms into the zeolite framework. Keywords:

solid-state NMR; zeolite Y; orthophosphoric

Introduction Zeolite Y is the active component of most commercial cracking catalysts. Its dealuminated ultrastable form (USY) [ 11 combines appropriate pore size as well as desirable selectivity and activity with remarkable thermal stability brought about by the substantially increased Si content. According to X-ray diffraction the USY zeolite retains the topology and crystallinity of the parent faujasite structure, and the hydroxyl nest vacancies in the framework, which are left behind by Al, occur to be healed [2]. Since the SiOitetrahedra are smaller than the AlO:- tetrahedra, the cubic unit-cell parameter decreases in the course of dealumination and therefore can be related to the Si/Al ratio in the framework [31. The parameters controlling the catalytic properties of USY have been thoroughly examined in a

* Corresponding

author.

0926-2040/93/$06.00

acid; impregnation;

ultrastabilization

recent paper from this laboratory [4]. The ultrastabilization can be easily achieved by hydrothermal treatment (steaming) of zeolite NH,-Y, but the process is fairly complicated and, following the paper by Klinowski et al. [5], much solid-state NMR work has been devoted to explaining its nature [6-181. It was found that during ultrastabilization most of the framework Al (FAl) is released from the crystal lattice to form various kinds of extraframework Al (EFAl) having coordination number from 4 to 6 (EFAl’“, EFAIV and EFAl”r). It has been shown that a chemical posttreatment of zeolites with phosphorus compounds modifies the acid-strength distribution of the zeolite Bronsted sites, which in turn affects the carboniogenic catalytic activity [19-231. Solid-state NMR under magic-angle spinning (MAS) is a powerful technique to characterize the interaction between P-containing species and the EFAl and FAl zeolitic components, but until now it has been used only for the study of a medium-pore

0 1993 - Elsevier Science Publishers B.V. All rights reserved

122

u! Kolodziejski

P-impregnated zeolite ZSM-5 with a high Si/Al ratio [20-231. The general conclusions [20-221 are as follows. With increasing concentration of H,PO, an enrichment in P-containing species near the external surface of the crystals is observed. 29Si NMR indicates that the H,PO, treatment followed by calcination dealuminates the framework. The latter result is in agreement with a concomitant intensity loss by the FAl’” signal at 54 ppm. Simultaneously, 6- and 4-coordinated Al signals arise at - 11 and 39 ppm, respectively, that is in the positions characteristic for aluminium phosphate, probably being in an amorphous state. We speculate that the signal at ca. 39 ppm may be overlapped yet with the 4-coordinated Al signal of other EFAl species. The positions of 27A1 signals differ between the samples and the publications by l-2 ppm, although the spectra were recorded at the same magnetic field. The P-impregnated samples show a decrease in Bronsted acidity caused mainly by dealumination (lH and 27A1NMR evidence). After the treatment with H,PO, but before calcination the original intensity of the 4-coordinated Al signal can be restored by elution of H,PO, with hot water. Elution exerts no significant effect on the signals at - 11 and 39 ppm. Besides minor resonances a strong 31P signal with a maximum between -27 and -31 ppm has been detected and assigned on the basis of the chemical shift to aluminium phosphate. Again the signal of aluminium phosphate remains unaffected by the elution experiments and almost disappears if EFAl is removed by acid leaching prior to H,PO, treatment. It turns out that before calcination there is some interaction of H,PO, with FAl, possibly through the bridged hydroxyl groups, and during calcination H,PO, reacts with EFAl to form aluminium phosphate. No evidence has been found for an incorporation of P atoms into the framework and for Si-O-P bond formation. The contradictory conclusions 1231 on the latter problems reached only from 31P NMR are not convincing. First, the spectra from ref. 23 are not much different from those in earlier publications [20-221. Second, the interpretation given in ref. 23 can be easily questioned because AlPO and SAP0 molecular sieves give 31P signals in the same spectral region as non-

et al. /Solid

State Nucl.

Magn.

Reson.

2 (I 993) 121-129

porous AlPO,. Therefore supporting arguments from other than 31P NMR methods are needed. The incorporation of P into zeolites containing more EFAl than ZSM-5 with a high Si/Al ratio is likely to be more effective and also more profitable for catalysis. Thus during ultrastabilization of zeolite Y by steam-calcination appreciable amounts of various kinds of EFAl are generated, which has evident implications for catalytic activity and selectivity [4]. Such EFAl species are potential reactants for H,PO,. Moreover, there is a structural analogue of zeolite Y which contains framework P (SAPO-37) and this makes the study of the USY-H,PO, interaction quite attractive. In our paper this interaction is monitored by 29Si, 27Al and 31P MAS NMR with and without cross-polarization (CP) and clear evidence is given that it is strong and dependent, as concerns its extent and type, on the P-content and the activation conditions.

Experimental

Zeolite Y (LZY-52; Union Carbide) in the NH; form was moderately dealuminated by steam-calcination to give the parent sample USYOOO. This was then P-impregnated by stirring for 2 h at 373 K under reflux in a solution of H,PO, (liquid to solid ratio of 10 ml/g), followed by drying in vacuum at 353 K in a rotary evaporator and then by calcination for 1 h at 773 K. This procedure was used with various concentrations of H,PO, in order to prepare the samples USYO.SPO, USYlPO, USY4PO and USY6PO having 0.5, 1.0, 4.0 and 6.0 wt% of P, respectively, which then were steamed for 5 h at 1023 K to give the corresponding samples USYO.SPS, USYlPS, USY4PS and USY6PS. For comparison the parent sample was steamed under the same conditions (USYOOS). The unit-cell parameters of the samples were determined by X-ray (XRD) diffraction using CuK, radiation and following the ASTM procedure D-3942-S (Table 1). The estimated standard deviation was fO.OO1 nm. The crystallinity was calculated by comparing the peak height of the (5,3,3) reflection and considering LZY-52 to be 100% crystalline. The BET

123

W Kolodziejski et al. /Solid State Nucl. Magn. Reson. 2 (1993) 121-129 TABLE

1

surface area was measured with a Micromeritics model ASAP 2000. Solid-state 31P, 27Al and 29Si NMR spectra, conventional Bloch decays (BD) and with crosspolarization (CP) from protons, were recorded under magic-angle spinning (MAS) at ambient temperature on a Varian Unity VXR-400 spectrometer at 161.9, 104.2 and 79.5 MHz, respectively. A high-speed MAS Doty probe with zirconia rotors (5 mm in diameter) was used for 31P NMR, and a Varian MAS probe with zirconia rotors (7 mm in diameter) was used for 27Al and 29Si NMR. The acquisition parameters are given in Table 2. Precautions were taken in order to avoid intensity distortions of the MAS 27A1signals [24-291. The MAS rotors were driven by air and the magic angle was set precisely by observing the

Characteristics of the samples. The framework Si/Al ratio was calculated from the unit cell parameter (u.c.) according to the equation of Fichtner-Schmittler et al. [3] Sample

USYOOO USYOSPO USYlPO USY4PO USY6PO USYOOS USYO.SPS USYlPS USY4PS USY6PS

U.C.

(nm)

Si/Al ratio

2.448 2.448 2.443 2.440

5.9 5.9 1.7 9.2

2.428 2.426 2.428 2.428

Before

Crystallinity (%o)

9.5 96 96 81 22 91 89 84 80 23

35 62 35 35

BET surface area Cm2/g) 676 526 612 546 335 485 502 500 450 123

2gSi NMR

steaming Si(lAI)

After

steaming

Si(OAI)

-102

-107 -108

USY

000

USY

00s \

II

I

t.'

I \

,- ,j

\ '\

,/'

'\

/u I -6U

-80

-100

-102

-120

-140

I

-60

I -120

I -100

-80

-107

Us‘rlPo

I' ,;'

(4

I -120

-60

ppm

Fig. 1. “Si NMR Bloch decay (full iine) and cross-polarization

from

-80

I \

I \ \ \ \

I -140

'\

\

/I 1'

-100

, -140

\

I

I

',

I' \.:

USY 1 PS

-80

,-\ >.

-108 -101

-60

,'a

-100

\

I -120

-140

TMS

(dashed line) spectra of ultrastable zeolite Y modified with H,PO,.

124

L% Kolodziejski

et al. /Solid

Magn. Reson. 2 (1993) 121-129

State Nucl.

-‘8’ ,‘\,; -;p” *I

USY 4 PS I

l’

I

/I 1’ ,

I \

\

\

\

I’

\

\

\

-108 /P ' f

USY 6PS

= -60

I -80

I

I

-100

-120

I -140

lb)

-60

ppm

from

I/

-80

-100

'1 \

-120

-140

TMS

Fig. 1 (continued).

79Br resonance of KBr [30]. The CP spectra were recorded with single contacts and the contact times for 31P and 29Si were optimized on the TABLE 2 MAS and CP/MAS

NMR acquisition parameters

MAS Resonance

Pulse (CLS)

Flip angle bad)

Recycle delay (s)

MAS rate @Hz)

jlP 27Al 29Si

3.0 0.6 4.0

(3/8hT ?r /20 r/5

15 0.5 40

7.0-7.3 8.5 6.0

Contact time

Recycle delay

MAS rate

(ms)

(4

(kHz)

CP/MAS Resonance

r /2

(ps) 3lP Z7A1 29Si

5.5 9.0 9.0

pulse

1.5 0.8 3.0

3 3 2

6.5 8.5 5.0

original samples. The Hartmann-Hahn condition for 1H-27A CP was established and optimized on a sample of pure and highly crystalline kaolinite [18,31]. Because only the central (- l/2 ++ + l/2) transition is observed, excitation is selective and therefore the Hartmann-Hahn condition is 3~zuB.4 = YHBH where -yA1and yn denote the gyromagnetic ratios of 27A and ‘H, and B is the radio-frequency field strength. 29Si spectra were deconvoluted with standard Varian software and the (Si/Al),,, ratios were calculated from the peak areas using the classic equation given in the literature [6-101. Results and discussion The 29Si BD spectra consist of sets of overlapped Si(nAl) peaks (Fig. 1). Consider the nonsteamed samples. Even by sight inspection of the

W. Kolodziejski et al. /Solid State Nucl. Magn. Reson. 2 (1993) 121-129

peak intensities one can deduce that H,PO, causes some dealumination of the zeolite framework and that the FAl content decreases with increasing acid concentration. Much more pronounced dealumination occurs on steaming and the spectral changes then observed are very similar to those reported in the literature [6-101. We found that spectrum deconvolutions (not shown) are far better if a broad signal at the right slope of the BD spectra is assumed. The signal is well seen for USY6PO. The average chemical shift of this signal is - 111.9 ppm as found from the series of fittings for various samples. The chemical shift value [32,33] and the broadness of the signal (a half width of 500-1000 Hz depending on the sample) suggest that it comes from amorphous silica. From the deconvolutions we found values of 5.4, 7.9, 7.3 and 12.7 for Gi/fW,,,

J

usv OSPO I hk /

ppm

samples USYOOO, USYlPO, USY4PO and USY6P0, respectively, with reasonable agreement with the data given in Table 1. For the same sample series we calculated that 14.1, 6.5, 6.7 and 32.3% of total Si, respectively, contributes to the signal of amorphous silica, thus indicating the extent of the framework destruction caused by dealumination. The steamed samples are highly siliceous, because in our 29Si BD spectra they give only the peak of Si(OAl). No signals of defect sites [34] were detected. The Si(OAl) peak sits at the top of a broad signal of silica, which increases with P-content as is indicated by the gradually raising baseline for samples USYOOS, USYlPS and USY4PS. For sample USY6PS the silica signal dominates the BD spectrum and from the deconvolution we found that it corresponds to 84.1% of the total Si amount.

p; ;;* \,, 11 -1.2 : Y :\

52.3

Fig. 2. 27Al NMR steamed samples,

125

from

usvo.5

PS

‘y

~“7

Alw20)~*

Bloch decay respectively.

spectra of ultrastable zeolite Y modified Asterisks denote spinning sidebands.

with

H,PO,.

from

AI(H>O);’

Full

and dashed

lines

denote

nonsteamed

and

126

W Kolodziejski et al. /Solid State Nucl. Magn. Reson. 2 (1993) 121-129

The 29Si CP spectra look similar to those published by others [5,10]. The spectra contain overlapped signals from zeolite and silica with the enhanced signal at ca. - 101 ppm from Si(OSi),OH sites [32,34-1371. With increasing silica content in the steamed samples the signal at - 101 ppm decreases in tandem with the zeolite peak at - 108 ppm, so the former mainly comes from zeolite. This is confirmed by the CP spectrum of USY6PS containing a broad dominating signal of silica, which is roughly symmetric and only a small shoulder at - 101 ppm can be seen. We note that with increasing P-content in the steamed samples the signal at - 101 ppm decreases slower than the signal at - 108 ppm. It follows that the Si(OSi),OH sites in zeolite cross-polarize better than Si(OSi), sites, probably because the magnetization transfer from nonbonded protons of residual interstitial water to the latter sites is not efficient [341. However, in silica the CP efficiency for both sites is either hardly different or the Si(OSi),OH sites are relatively less abundant than in zeolite. If the second explanation is true, this means that during steaming most of the geminal hydroxyl groups in silica undergo mutual condensation to form Si-0-Si linkages. We note that CP is more efficient for silica than for zeolite, which is evident from the comparison of the BD and CP spectra of sample USY6PS. The latter observation can be explained by the higher content of residual water in amorphous silica than in crystalline zeolite. The 29Si spectra allow us to reject the concept of the Si-O-P bond formation [23], because there is no signal present assignable to Si in such grouping. Furthermore, the P-0-Al-0-Si linkages can be ruled out at least for the steamed samples because the Si(nAl) signals for IZ > 0 are missing. For SAPO-37, which is isostructural with faujasite, a fingerprint signal appears at ca. -90 ppm [38] and this is absent in our spectra of the steamed samples. We conclude that the incorporation of P atoms into the zeolite framework is highly improbable. The 27Al BD spectra of the samples with the low P-content look similar to those of zeolite Y, while those of USY with the high P-content indicate severe structural transformations (Fig. 2).

First of all we note that the spectrum of USY4PS (Fig. 2) is almost identical with the spectrum of sample APAl-A (Fig. 3 in ref. 39) containing amorphous system AlPO,-Al,O, (75 : 25 wt%). Thus the signals from USY4PS at 36.5 ppm (EFAlrV) and at -6.4 ppm (EFAIvr) can be assigned to the similar extraframework material. The spectrum of USY4PO contains signals of moderately dealuminated zeolite (57.5 ppm and - 1.2 ppm; cf. spectrum of USYOOO) together with the signals present in the spectrum of USY4PS. The only difference is that the 4-coordinated Al signal from the amorphous AlPO,Al,O, phase shows up for the nonsteamed sample at 43.7 ppm, while for the steamed material it is located at 36.8 ppm, perhaps because of its different Al,O, content. We also assume this signal to be present in the spectra of the nonsteamed samples with the low P-content, since the 4-coordinated Al signal gradually broadens from USYOOO to USYlPO, before it finally sepa-

2’Al

NW?

Fig. 3. Comparison of 27Al NMR Bloch decay (full line) and cross-polarization (dashed line) spectra of samples USY6PO and USY6PS. Asterisks denote spinning sidebands.

W Kolodziejski et al. /Solid State Nucl. Magn. Reson. 2 (1993) 121-129

rates for USY4PO. For the steamed samples with the low P-content (USYOSPS and USYlPS) the 4-coordinated Al signal from the amorphous AlPO,-Al,O, phase is perhaps strongly overlapped with the FAL’” signal of nonmodified USY, so the joint signal has a maximum at ca. 52 ppm. The signal at 43-45 ppm appears enhanced in the 27Al CP spectra (Fig. 3), so maybe a small quantity of amorphous AlPOd-Al,O, phase such as that in USY4PO is also present in both USY6PO and USY6PS, and if so this phase must contain enough hydrogen to allow CP. Besides, CP enhances the EFAl”’ signal as was already reported [16-W. Hydrothermal treatment of sample USY6PO results in narrowing of the 4-coordinated Al sig-

127

nal at ca. 40 ppm (36.8 ppm for USY6PO and 38.5 ppm for USYGPS) and almost elimination of the 6-coordinated Al signal at - 11.4 ppm (Fig. 2). The signal positions and the spectral effects are the same as observed by Sanz et al. [39] during crystallization of amorphous aluminium orthophosphates. Indeed, we found a strong XRD reflection from USY6PS at 20 = 21.45”, which is absent for USY6PO. The reflection comes from dehydrated AlPO, [40]. Therefore we assign the USY6PO signals at 36.8 and - 11.4 ppm to amorphous AlPO, (EFAl’” and EFAIV1, respectively) and the USY6PS signal at 38.5 ppm to 4-coordinated Al in crystalline AlPO,. The crystallization of AlPO, substantially decreases the BET surface area (Table 1) and causes an increase of silica

USYGPO-

cp 1’ ,/--&+ USYIPO-

/

\-USY \ I

/

6 PS

,I’

\

/‘ ED

Pi

,/‘, ,/

.\

\

/\ ,*(’

‘,

\ 0

40

e#’ USY 05

PO--

,-

=y-

/I y--x

\

\

\

*\ ‘\ -a

‘\yJSYO,5

1’

Ps--

/

BD

/’

-10

‘,I

1

-40

-50

-20

-30

-40

,--(

-50

ppm

Fig. 4. 31P NMR Bloch decay dashed lines denote nonsteamed

,,,,/-qL

USY4 PO -

-\

/’

0

I -30

.

/. ,J, I , , , ( , , I /

11 -20

y

\

\

I’

\

BD

\

‘-

/’

USY 0,5 PO -

II

-10

0

USY 0.5 PS ‘\

/I

I-’

I’

-56

\

,/’

CP

40

40

-20

(BD) and cross-polarization and steamed samples,

from

0

85%

A

/

/’

‘,--USY \ \ \

1’

/ I -0

-20

’

I

4 PS \

k., 1

I

-30

-40

Y modified

with

’

I -50

H3 PO4

(CP) spectra respectively.

of ultrastable

zeolite

H,PO,.

Full

and

128

W Kolodziejski

content by a factor of 2.6 (our 29Si NMR results). The former effect suggests that crystalline AlPO, is formed at the crystal surface of zeolite, the latter that the process occurs also inside the framework resulting in its partial destruction. When discussing the 31P BD spectra (Fig. 4) let us first refer to the samples USY6PO and USY6PS, for which we find similar spectral changes as those reported by Sanz et al. [39]. Amorphous AlPO, gives one broad signal at - 26 ppm (cf. USY6PO). After thermal treatment a sharper signal arises at -30 ppm (cf. USYGPS), which can be assigned to crystalline AlPO, (tridymite structure). Accordingly, we suggest that USY4PO and USY4PS contain phosphorous mainly in the form of amorphous AlPO, (broad BD signals with the maxima at -23.5 and 25 ppm, respectively). For the low P-content there is also a signal at ca. - 15 ppm, which decreases after steaming and this can be assigned to shortchain polyphosphates [41]. Another signal appears at ca. - 37 ppm after steaming and this can be assigned to highly condensed polyphosphates [42]. All the signals are highly overlapped and their positions and relative intensities depend on the sample, rendering the interpretation difficult. CP favours higher-frequency signals, which is proved by the variable contact-time experiment (Fig. 5). For the short contact time (100 ps) species containing protons are preferred and they

State Nucl.

Magn. Reson. 2 (1993)

121-129

give continuous absorption roughly in the - 10 to -22 ppm range. Just in this range a wide POH signal from the amorphous phase was previously found [43]. Therefore it is possible that besides better defined signals as those at - 30 ppm (crystalline AlPO,), - 23 to -26 ppm (amorphous AlPO,), - 15 and -37 ppm (short-chain and highly condensed polyphosphates, respectively), there is a broad-band absorption from a wide distribution of POH sites residing in chemically varying and ill-defined environments. Such mildly acidic Bronsted sites are of special importance in catalysis.

Acknowledgment We express our gratitude to Generalitat Valenciana (a grant for W.K.) and to Comision Interministerial de Ciencia y Tecnologia (MAT91-1152) for financial support. The technical assistance of M.S. Grande and J. Martinez is gratefully appreciated.

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Fig. 5. 31P NMR variable contact-time CP spectra of sample USY4PO. Note the various intensity scaling factors given in the figure.

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