Use of 129-Xe NMR and HRADS techniques to study coking and decoking of HY zeolites

B. Delmon and G.F.Froment (Eds.) Catalyst Deactivation 1994 Studies in Surface Science and Camlysis, Vol. 88 0 1994 Elsevier Science B.V. All rights reserved.

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Use of 129-Xe NMR and HRADS techniques to study colung and decoking of HY zeolites J.L. Bonardet, M.C. Barrage, J. Fraissard, M.A. Ferrero* and W.C. Conner* Laboratoire de chimie des surfaces, C.N.R.S. URA 1428, Universite P. et M. Curie, 4 place Jussieu, tour 55,75252 Paris Cedex 05, France. *Department of chemical engineering, University of Massachusetts, 159 Goesmann Laboratory, Amherst, MA 01003, USA. ABSTRACT. Location of coke produced during cracking of hexane and ortho-xylene on HY zeolites has been studied by 129-Xe NMR. Coupled with HRADS (high resolution adsorption) technique we obtained information about the distribution of coke during coking and decoking. We also showed that complete reoxidation under relatively mild conditions did not permit to restore the initial catalyst. 1. INTRODUCTION.

The deactivation of catalysts, especially zeolites, during cracking, hydrocracking, methanol conversion, etc, is one of the major technological and economic problems of the chemical industry (1).The interest of these materials lies not only in their high catalytic activity and selectivity but also in the possibility of regenerating them several times so that their "lifetime"is compatible with the cost of their production. Consequently, it is necessary to understand the manner and the rate of catalyst deactivation as well as the nature of the carbonaceous residues formed, commonly called "coke". Many techniques have been developed to study coking, both chemicaI (adsorption of probe molecules having different sizes (2), solvent extraction (3)) and physical (X-ray diffraction (4), electron microscopy (5), IR (6), NMR (7)). Among these techniques 129-Xe NMR applied to microporous systems such as zeolites (8), coupled with HRADS techniques makes it possible to clarify the location of the coke and the blocking of the zeolite micropores. 2.

EXPERIMENTAL.

industrial samples of HY zeolites partially dealuminated (HYD) then acid washed (HYDW) were studied. Coke was produced during cracking of n-hexane (4.7 % (HYDW4.7H) and 8.6% (HYDW8.6H) of coke w / w ) or ortho-xylene (8.7% (HYDW8.7X) of coke w/ w) at 673K. Partial or total oxidation was carried out in a

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stream of pure oxygen (8 1.h-1) at 573 or 623K. The residual coke contents were 4.7, 1.7, 0% and 4.8,1.6,0% for HYDW8.7H and HYDW8.6X samples respectively. These oxidized samples have been denoted HYDW8.7Hoxz and HYDW8.6Xoxz where z represents the percentage of residual coke. Xenon was adsorbed in a volumetric apparatus at ambiant temperature. Adsorption equilibrium was reached after 30 minutes. High resolution adsorption measurements were obtained by adsorption of nitrogen at 77K on a Omicron 360 apparatus; each run took about 12 hours. No HRADS measurement were made on reoxidized samples. The 129-Xe NMR spectra were recorded at ambient temperature on a Bruker CXPlOO spectrometer at 24.9MHz with a classical R /2 pulse sequence.The duration of recycle delay was 0.5s and the number of scans between 1000 and 20000. The NMR spectra for all the samples presented a single relatively narrow signal. Chemical shifts were measured relative to Xe gas extrapolated to zero pressure. 3. RESULTS AND DISCUSSION. 3.1. Principle of 129-Xe NMR. Fraissard et a1 (8) have shown that the chemical shift 6 of adsorbed xenon can be expressed as the sum of several terms corresponding to the various perturbations suffered by the xenon atom in a confined volume :

-as depends on the Xe-wall interactions. -GXe-Xe is due to Xe-Xe collisions in the micropore volume. -@AS represents the contribution of Strong Adsorption Sites (stronger than H+ or Na+) These SAS are often charged (bivalent cations for example). In practice the information obtained by 129-Xe NMR is contained in the shape and the expression of the 6 = f(nXe) curves where nXe is the number of xenon atoms adsorbed by one gramme of anhydrous zeolite. 6s can be obtained by extrapolation of the &plot to zero xenon concentration and related to the mean free path of xenon imposed by the microporous structure (9). If there is fast exchange between xenon adsorbed on the cavity walls and xenon gas in the micropore s the slope dti/ dn of the straight section of the &plot is inversely proportional to the void volume (10). 3.2. Non coked samples. As observed previously (11,12) dealumination of HY zeolites during steaming leads to the formation of extraframework aluminium species (AM)a part of which remains in the microporous structure. These A l m species (AIOx groups) act as adsorption centres stronger than H+ and lead to an increase of the chemical shift of xenon at low concentration. This curvature remains after acid wash, showing that -the A ~ N Fspecies in the micropores are not totally eliminated (Fig. 1-b and 2-b)

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Nevertheless, the slopes of the linear section of the 6-plots are quite similar; this means that A ~ N species F do not very much affect the internal void volume.

I

E20

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" X e g-1

5E20

Figure 1 : 6 = f(nXefp1ots. Continuous line s: non-coked and coked samples (cracking ).(b HYDW; cm HYDW4.7H; of hexane); doted lines: oxidized samples. a@ HY; d(l) HYDW8.6H; e(0) HYDW8.6Hox4.7; HYDW8.6Hox1.7; g o HYDW8.6HoxO

fa)

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5E20

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" X e ' TI

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Figure 2 : 6 = f(nXe) plots. continuous line : non-coked and coked samples (cracking of ortho-xylene); doted lines: oxidized samples. a(& HY; b( ) HYDW; ~ ( 0 ) HYDW8.7X; d ( 0 ) HYDW8.7Xox4.8; e(A) HYDW8.7Xox1.6; f( 0 ) HYDW8.6HoxO

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Now, let us consider the nitrogen adsorption isotherms at 77K (Fig 3). The quantities of N2 adsorbed at saturation show a large decrease of the total microporous volume after steaming, only partially restored by acid wash (table 1). Moreover, the micropore distribution curves (Fig. 4) show that steaming leads to a decrease of the pores whose radii are between 0.6 and 1.4 nm. However, after acid wash, the curve is exactly superimposed on the previous one, proving that acid wash only removes the A~NFspecies located outside the micropores of the framework. This result agrees with the 129-Xe NMR measurements which show identical slopes of the linear sections of the &plots. Table 1 Pore volume at saturation (N2 adsorption measurements)

165 105 135 95

HY HYD HYDW HYDW8.6H

0.252 0.161 0.203 0.142

-f +

0

f

0.0

O-l

H-Y HY Df

THYDW -HYDW 8,6 H

P/PO

I

0.2

Figure 3. Nitrogen adsorption isotherms at 77K.

0.3

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-

12

Y

6

0.4

0.6

0.8

H-Y H-Y D( H-Y D W H Y D W 8,6 H

1.0

1.2

1.4

1.6

1.8

2.0

Pore Radius (nm) Figure 4. Pore distribution curves . 3.3 Coned samp.2~. Table 2 reports the values of and slopes of the 6 = f(nXe) curves for samples HYDW coked by hexane or ortho-xylene then reoxidized. After coking, the &plots are linear on the entire xenon concentration range, the slopes and values increase with coke loading. At the same time, the total micropore volume measured by HRADS decreases by about 30% but the pore distribution curve, if rp > 0.55nm, is the same than for the uncoked sample. The disappearance of the curvature at low xenon concentration proves that interactions of xenon with SAS are masked. Coke is, therefore, deposited first of all on the A~NFspecies. At the same time, the increase of 6s ( from 60 ((HYDW) to 85 ppm (HYDW8.6H and 8.7X)) shows that the mean free path of xenon imposed by the structure has decreased. This decrease is due to the more restricted diffusion of the xenon through the 0.74 nm windows of Y zeolites. From these results it is evident that A ~ N Fspecies are mainly located at the hexagonal windows between the supercages. The increase of the slope (32% and 45% from HYDW to HYDW8.6H and HYDW8.7X) characterizes a loss of micropore volume in agreement with HRADS measurements. This loss of pore volume essentially affects the micropores whose radii are less than 0.55nm and which are not detected by nitrogen adsorption. Nevertheless at high coke content (> 8%), a large part of carbonaceous residues is located on the external surface of the crystallites as shown by the results obtained after partial reoxidation. Analogous results were observed for HYDW coked by orthoxylene.

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Table 2. Parameters of the &plots. Samples HYDW HYDW4.7H HYDW8.6H HYDW8.6Hox4.7 HYDW8.6Hoxlr7 HYDW8.6HoxO HYDW HYDW8.7X HYDW8.7Xox4,8 HYDW8.7Xox1.6 HYDW8.7XoxO 3.4. Reoxidized samples.

m/atom. -1) 78.5 85 83 76 62.5 59.5 85 81 75 60

4.6 5.1 5 4.25 4.6 3.85 5.6 5.6 3.85 8.85

Curvature at low Xe concentration Yes No No No No Yes Yes No No No Yes

The first step of oxidation (12h in a stream of pure oxygen (8 1.h-1) at 573K) leads to samples containing 4.7 %(HYDW8.6Hox4.7) and 4.6 %(HYDW8.7Xox4.8)of residual coke, indicating that in this step, about 50% of the carbonaceous residues are eliminated. Nevertheless, figure 1-e and 2-d show that the b-plots are straight lines, parallel to those of the initial coked samples, and that BS decreases slightly (85 to 83 ppm (HYDW8.6Hox4.7) and 85 to 81 ppm (HYDW8.7Xox4.8)). The fact that the slopes of the &plots for partially decoked samples are the same as those of the initial coked ones proves that under our regeneration conditions the internal micropore void volume is practically unchanged ( this is confirmed by the quantities of xenon adsorbed at saturation (212K) which are virtually identical). Thus a large fraction of the carbonaceous residues (at least 50%) is located on the external surface of the crystallites. The second step of oxidation (same experimental conditions) leads to samples containing a smaller residual coke load (1.6-1.7% w/ w). There is both a decrease in the slopes ,one of which even returns to that corresponding to the non coked catalyst (HYDW8.7Xox1.6), and in 6s values (75-76 ppm) (figures 1-f and 2-e). However the 6 plots remain linear over the whole xenon concentration range. We conclude that after oxidation of external coke, oxygen is able to diffuse more easily within the framework and to eliminate part of the internal carbonaceous residues affecting the supercages. The absence of curvature and the relatively high values of bs (relative to non-coked catalyst) is evidence that the residual coke, the more difficult to remove, is located mainly at the hexagonal windows, on the A ~ N Fspecies of the catalyst.

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After complete oxidation (12h under pure 0 2 at 623K) the shape of the 6-plots is analogous as that of the initial non-coked catalyst. Nevertheless, (figures 1-g and 2-0 we can observe that : - the curvature at low xenon concentration is slightly accentuated. This proves that dealumination of the framework increases. - the slope of the h e a r section is increased by a factor of 2.3 for HYDW8.7XoxO and 1.2 for HYDW8.6HoxO. At the same time, the amount of xenon adsorbed at saturation (212K) decreases in the same proportion. Under our experimental conditions, complete removal of the carbonaceous residues leads to the appearance of structure defects and/ or short-range amorphization of the zeolitic framework, not detectable by X-ray diffraction. The coke produced during ortho-xylene cracking is the most difficult to remove and its elimination leads to pronounced modifications of the structure and of the crystallinity, doubtless because of its greater polyaromatic (or graphitic) character.

CONCLUSION. 129-Xe NMR has proved to be a good tool for studying the deactivation of Y zeolites by coking. This technique allows us to show that even under relatively mild conditions of oxidation, structure defects or short range amorphization, no detectable by X-ray diffraction appears. The complementary HRADS technique give interesting information on the loss of microporosity and the distribution of the pores affected by coking.

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