HZSM-5 catalyst

Applied Catalysis A: General, 103 (1993)

259-268

Elsevier Science Publishers B.V., Amsterdam APCAT

A2594

Study on the active center of Ga,O,/HZSM-5

catalyst

Songling Jia, Suochuan Wu and Zhongyue Meng Department of Chemistry, Nanjing Normal University, (People’s Republic of China)

Nanjing 210024,

(Received 2 February 1993, revised manuscript received 25 May 1993)

Abstract The study of a physically mixed Ga203/HZSM-5 catalyst shows that propene aromatization on the catalyst may be improved by activating it in hydrogen at high temperature. The optimal pretreatment conditions are at 773 K in hydrogen for 2 h and about 1 wt.-% Ga content. Ga”’ in Ga20s/HZSM-5 catalyst cannot be reduced by the high temperature pretreatment in hydrogen. The enhancement of propene aromatization on the catalyst may be due to the migration of gallium species, possibly into the channels of the zeolite, and the improvement of hydrogen transfer properties of the catalyst. The hydrogen adsorption-desorption process is completely reversible.

Key words: active center; hydrogen adsorption-desorption;

physical mixture; propene aromatization

INTRODUCTION

Recently, a Ga-containing ZSM-5 zeolite catalyst, which is a good catalyst for the aromatization of light paraffins, has received considerable attention. The reaction mechanism of paraffin aromatization and the role of the promoter gallium species have been well reviewed by Seddon [ 11. But the state of gallium species on the zeolite is still a matter of controversy. Gnep et al. [ 2,3] and Meriaudeau et al. [4] suggest that the active species could be gallium cations in an exchange position or gallium oxide well dispersed on HZSM-5. However, some patents [ 561 reveal that gallium catalysts can be improved by hydrogen treatment. Price et al. [7-91 also report a similar phenomenon and state that the active species is probably Ga’ as a zeolite cation. The main objective of this paper is to examine the performance of propene aromatization on physically mixed Ga20,/HZSM-5 catalyst so as to optimize conditions for hydrogen pretreatment, and to further study the active center of aromatization. Correspondence to: Dr. S. Jia, Department of Chemistry, Nanjing Normal University, ghai Road, Nanjing 210024, People’s Republic of China, Fax: ( + 86 - 25) 307448.

0926-860X/93/$06.00

0 1993 Elsevier Science Publishers B.V.

All rights reserved.

122 Nin-

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Songling Jia et al./Appl. Catal. A 103 (1993) 259-268

EXPERIMENTAL

Sample preparation GazO, was prepared with pure gallium metal and XRD analysis showed it to be 8Ga203. Gallium-containing catalysts were prepared by mixing Ga,O, with HZSM-5 or NaZSM-5 zeolite and then the mixtures were calcined at 773 K for 6 hr. Ga20JHZSM-5 and Ga,O,/NaZSM-5 samples are referred to as xGaHZ (y ) and xGaNaZ (y ) respectively, in which x stands for weight percent content of gallium and y for the pretreatment conditions of catalysts, such as temperature, time and gas atmosphere. (Unless otherwise stated, y indicates 773 K, 2 h).

Activity measurements Catalytic conversion was carried out in a pulse-microreactor with Nz as carrier gas. Products were analyzed using a Varian 3700 GC with a FID detector, and results were recorded by a HP3390A integrator.

X-ray photoelectron spectroscopy XPS measurements were performed on VGS LARMKII ESCA spectrometer with an aluminum anode (h v = 1486.6 eV) as X-ray source. The base pressure of the spectrometer was less than 3 x lo-’ Pa during the experiments. Charging effects were corrected by adjusting the adventitious carbon peak C1, to the position at 234.6 eV. The accuracy of the binding energy with respect to this standard value was within 0.3 eV.

X-ray difiaction The pretreated samples were measured using a Rigaku D/MAX-RC X-ray diffractometer with a Cu Ko+ radiation source operated at 6 kW. The scan rate was 4 ’ /min. The d value (error + 0.002)) 20 and the peak intensity were given directly by the on-line computer.

Temperature programmed reduction with Hz Samples of 150 mg size were pretreated in a quartz U tube and then reduced in N2-H2 gas (Hz lo%, N2 90%) from room temperature to 1123 K at 13.6 K/ min. The signal was analysed by GC with a TCD detector.

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H2 chemisorption The equipment was similar to that used in the activity measurement. Hz was injected at a given temperature into the pretreated sample until H2 adsorption saturation. Then the temperature was altered, the system was purged with N2 and Hz was again injected. Any Hz exuded was detected with a TCD Varian 3700 GC, and the results were recorded with a HP 3390A integrator. The relative amount of adsorption of Hz is defined as: ; Ati= -~~~~;;~~U~;~

injection

(A is peak area of exuded H,; n is the number of injections before saturation of adsorption). RESULTS AND DISCUSSION

The reactivity of propene The product distributions of propene aromatization for several catalysts are summerized in Table 1. It seems that under N2 pretreatment conditions, even when the content of Ga supported on HZSM-5 zeolite is 10 wt.-%, the conversion and BTX yield improve slightly. It appears that over GaHZ ( N2), just like over HZSM-5, the acid sites of the catalyst account for the reactivity of propene. But it is worth noting that after GaHZ is pretreated at high temperature in Hz flow, both the conversion of propene and the yield of BTX are increased TABLE 1 Product distribution of propene aromatization Reaction temp. 773 K; pretreatment temp. 773 K Sample

HZSM-5 2 GaHZ (N,) 10 GaHZ (N, ) 0.75 GaHZ (Hz) 0.93 GaHZ (H,) 2 GaHZ (H,) 3 GaHZ (H,) 10 GaHZ (H,)

Product distribution ( % )

G-2

C3

15.07 14.78 12.84 12.90 13.25 11.56 15.72 9.61

29.39 24.09 23.84 17.62 17.63 17.67 15.73

29.03

BTX 26.68 27.46 23.64 23.19 16.69 18.58 11.71 17.21

3.69 3.26 6.87 4.06 2.50 1.66 0.90 4.75

0.82 0.58 2.61 1.37 0.61 0.14 0.70 1.26

24.67 24.43 29.89 34.08 49.34 50.32 53.29 51.45

Songling Jia et al./Appl. Catal. A 103 (1993) 259-268

262 100

100 m-.1 ,/-.f-

*-

.-m-g

c

./

80

. _ ._.-*

604 .

/‘%

,/’

./‘-*

55

’

./

40

./

20

20 673

773 tmp.

873

973

(JO

Fig. 1. Dependence of propene reactivity on pretreating temp. in H, over 2 GaHZ (Hz) (pretreatment time 2 h). - . -. -Reaction at 923 K; ~ reaction at 773 K.

0

.O 0

3

6

time(hr.

)

9

Fig. 2. Dependence of propene reactivity on pretreating time in H2 flowing over 2 GaHZ (H,) (pretreating temp. 773 K). -. -. -Reaction at 923 K; reaction at 773 K.

markedly, while the cracking products C,_, are depressed to some extent. In addition, propene conversion and BTX yield increase with Ga content, and the appropriate Ga content of GaHZ catalyst is about 1 wt.-%. Figs. 1 and 2 show the dependence of propylene aromatization on H, pretreatment temperature and time, respectively. It is clear that the optimal pretreatment temperature and time for GaHZ catalyst are about 773 K and 2 h. Studies on active center The results in Table 1 indicate that pretreating GaHZ at high temperature

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Songling Jia et al. jApp1. Catal. A 103 (1993) 259-268

in flowing Hz can improve the performance of propene aromatization. Price et al. [ 81 held that high temperature pretreatment in flowing H, generates a highly active species, which is probably Ga’, as a zeolitic cation. However, the results listed in Table 2 show that there is no change in performance for propene conversion by either pretreating GaHZ (H,) at 773 K in air or leaving it in air for two months. This implies that either Ga’ on GaHZ (H,) is difficult to oxidize, or Ga”’ on GaHZ is not reduced by Hz pretreatment at high temperature to form Ga’. A further study is made in this paper. From the XPS results in Table 3 it can be seen that neither the valence state of Ga nor the surface composition of 6-GazO, change after Ga,O, is pretreated at 773 K in flowing Hz. When &Ga,O, is supported on HZSM-5, both the binding energy and the half-peak width of Ga 2ps12increase slightly because of the interaction between &Ga203 and HZSM-5. Nevertheless, altering the TABLE 2 Effect of pretreatment procedure on propene reactivity Reaction temp. 773 K; pretreatment temp. 773 K Sample

2 GaHZ (H2) 2 GaHZ (Hz, air) 2 GaHZ (He)”

Product distribution ( % )

Cl-2

G

Cd

G

C6

A”

BTX

12.06 11.70 11.38

17.77 17.56 18.19

17.16 16.87 16.25

1.60 1.56 1.55

0.16 0.26 0.16

no no no

51.15 52.77 52.41

“Alicyclics. ‘Storage in air for 2 months. TABLE 3 XPS results Hz pretreatment temperature: 773 K

(Ga 2p3,,)

Sample

BE. (ev)

6-Gaz03

1117.90 1118.10 1118.70 1118.80 1118.80 1118.70 1118.65

6-Ga& (I%) 2 GaONb 2 GaHZ (Nx) 2 GaHZ (H,) 2 GaHZ (H,, air) 2 GaHZ (H,, 923 K)

Ga surf. content” (%)

Half peak width (ev)

35.67 6.26 5.08 3.72 3.11 1.70

2.54 2.52 2.74 2.72 2.74 2.87 2.74

‘Theoretical Ga content of GaaO, is 40%. bGaON is the sample by mixing GaxO, and HZSM-5 directly without further treatment.

Songling Jia et al./Appl. Catal. A 103 (1993) 259-268

264

pretreatment conditions does not lead to a shift of the binding energy or the half peak width of Ga 2p,,, in &Ga,O, supported on HZSM-5. All the Ga 2p,,, peaks are symmetric single peaks. XPS measurements of GaHZ sample pretreated at high temperature in situ in Hz give the same results (Fig. 3). From the above it can be concluded that the valence state of Ga in GaHZ pretreated at high temperature in H2 is the same as that in 6-Gaz03. This is quite different from the result of Price et al. [ 81. Note in Table 3 that the surface Ga content of the catalysts decreases from 2GaON to 2GaHZ (H2, 923 K). In particular, pretreatment in flowing Hz decreases the surface Ga content more extensively, indicating that pretreatment in Hz can make more gallium species migrate into the channels of the zeolite, a result similar to that observed by Minachev et al. [lo]. In contrast to the Ga 2p,,, peak, some change takes place for the 01, peak of the 2 GaHZ catalyst treated with hydrogen. For the untreated 2GaHZ, there exist two types of oxygen: 02- (B.E. =531.0 eV) in 6-Ga20, and 02; (B.E. = 532.1 eV) of zeolite oxygen (Fig. 4). However, for the catalysts treated in HP, only one symmetric 01, peak at 532.1 eV is observed. It appears that by the treatment in H2, the surface phase of Ga20, on HZSM-5 disappears, possibly by forming Ga3+ (-O-Al<), in which the binding energy of 01, is 532.1 eV. 6-Ga203 is metastable and can be easily be transformed to e-Ga,O, by high temperature treatment [ 111. Comparing the XRD spectrum of 10 GaHZ with HZSM-5 (Fig. 5), one finds that some new bands of c-Ga203 as well as 6-Ga203 appear due to the treatment at 773 K during the preparation of 10 GaHZ. When S-Ga,O, supported HZSM-5 is treated at 773 K in HP, the changes observed are that the bands of &Ga,O, at d=2.840 (28=31.840) and d=2.364 (20=38.040) decrease while those of r-Ga,O, at d=2.915 (28=34.640) and d=2.642 (28=33.900) increase; there is no new band. The on-line computer

A a

b

1122.00 1118.00 1114.00 binding energy(w)

Fig. 3. In-situ XPS spectra of 2 GaHZ. (a) After H2 treatment; (b) before Hz treatment.

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Songling Jia et al./Appl. Catal. A 103 (1993) 259-268

536.00

528.00

532.00 binding

energy

(ev)

Fii. 4. In-situ XPS spectra (0,s) of ZGaHZ. (a) After Hz treatment, (b) before H2 treatment.

30.00

35.00

40.00

28 (degree) Fig. 5. XRD spectra of catalysts.

analysis of the intensity of Ga,O, bands shows that the amount of intensity increase is almost equivalent to the decrease. Thus, hydrogen treatment only leads to a partial change in Ga203 crystalline form. Price et al. [9] suggest that the decrease of the XRD band intensity of /IGa,O, by the treatment in H2 is due to the reduction of Ga,O,, but the high

Songling Jia et al./Appl. Catal. A IO3 (1993) 259-268

266

temperature treatment cannot lead to the crystalline transformation of P-Ga,O, [ll]. However, XRD band intensities can be influenced by various factors, such as the size of particles and the surface effect [ 121. The high temperature treatment in H2 can improve the dispersity of the supported Ga203, which results in the change of the band intensity. Therefore, the decrease of band intensity of &Ga,O, is probably not the result of the reduction of Ga”‘. From the TPR profiles (Fig. 6), it can be seen that HZSM-5 can adsorb hydrogen to some extent from about 303 K, but the intensity distribution of its H, adsorption sites is dispersive. On the other hand, a peak near 323 K appears in the TPR profiles of all the supported HZSM-5 catalysts except in that of Ga203 itself. This is in agreement with Petit [ 131, who states that this peak is due to an adsorption of Hz on the zeolite material. Besides, Petit et al. [ 131 believe that another peak at 883 K is likely to be from partial reduction of Ga,O, supported on the zeolite. Similarly, a peak occurs near 963 K in the TPR profiles of GaHZ catalysts, but this peak is probably not due to the reduction of Ga203. First of all, the similarity of TPR profiles between GaHZ ( H2) and GaHZ (Hz, air) indicates that the property of H2 adsorption changes little when a GaHZ catalyst is treated first in Hz and then in air. This agrees with the results of propene conversion. Moreover, the areas of two peaks appearing at 963 K for GaHZ (N,) and 663 K for &Ga,O, are almost equal, but the amount of S-Ga,O, used in the experiment is 50 times the Ga content in the GaHZ (N,) used. Consequently, we deduce that the peak at 963 K is not attributable to the partial reduction of Ga,O, supported on HZSM-5. By contrast to GaHZ (N,), for GaHZ catalysts pretreated in H, at 773 K and 923 K, a peak near 633 K is observed in the TPR curves. These peaks are

0aHz(H2,air) aam *IIz(z,)

HZSU-g

0

473

673

tmP* w Fig. 6. TPR profiles.

873

1073

Songling Jia et al./Appl. Catal. A 103 (1993) 259-268

267

in addition to the peaks near 323 K and 963 K. The peak at 633 K is not due to the reduction of Ga203 because the peak temperature is much lower than the hydrogen pretreatment temperature of the catalysts. The appearance of this peak implies that pretreating the catalyst in Hz is favorable for H2 adsorption. In addition, for the GaHZ (Hz, 923 K) sample, the peak at 963 K almost totally disappears in comparison with that of GaHZ (N2) and the area of the peak at 663 K matches that at 963 K of GaHZ (N,). Both of these results suggest that pretreating in H2 can increase the capacity of H2 adsorption of the catalyst at relatively low temperature and decrease it at higher temperature. In short, the TPR results show that Ga,O, supported on HZSM-5 is not reduced by H2 treatment. This.+agrees with the XPS results. An earlier publication [ 141 also held that Gaz03 is difficult to reduce. From the dependence of the relative amount of Hz adsorption on temperature (Fig. 7), for GaHZ (N,) the amount increases with temperature, whilst for GaHZ (H,), two extremes of H2 adsorption are obtained at temperature ranges of 523 K-673 K and 773 K-973 K. This corresponds with two peaks near 633 K and 963 K in the TPR profiles. This gives further evidence that both TPR peaks result from H, adsorption by the catalyst. From the results of H2 adsorption-desorption cycle experiment in Table 4 it is clear that H, adsorption-desorption over the 2 GaHZ (H,) catalyst is completely reversible; in other words, the hydrogen adsorbed at relatively low temperature can be removed easily by a high temperature purge and vice versa. In addition, this reversibility is further evidence that H, consumption peaks in TPR profiles near 633 K and 963 K are not due to the reduction of Ga”’ in the GaHZ catalyst. It is well known that the aromatization of light alkanes over Ga/HZSM-5 catalyst follows a bifunctional mechanism, in which dehydrogenation is a key

a 4o 1. k

473

673

873

1073

temp.(K) Fig. 7. Relation between relative amount of hydrogen adsorption and temperature.

Songling Jia et al./Appl. Catd. A 103 (1993) 259-268

268 TABLE 4

Relative amount of Hz adsorption on 2 GaHZ (Hz) at different temperature steps

Relative amount of Hz adsorption ( % )

Rise

Drop

Temp. program

>

Rise 673 K

723 K

773 K

723 K

673 K

773 K

122.0

152.4

198.0

162.3

124.2

196.5

step [ 21. Therefore, the improvement of Hz transfer ability and the reversibility of H, adsorption-desorption are in favor of propene aromatization. CONCLUSION

Propene aromatization over HZSM-5 can be greatly enhanced with GapOs supported on it, followed by high temperature pretreatment in Hz. The optimum pretreatment conditions for the physically mixed catalyst Ga,O,/HZSM5 should be in hydrogen at 773 K for 2 h with a suitable Ga content about 1 wt.-%. The pretreatment of catalyst in Hz at high temperature does not lead to the reduction of Ga”’ in Ga,O,/HZSM-5, but results in the migration of gallium species, possibly into the channels of the zeolite. This improves the Hz transfer ability of the catalyst. The adsorption-desorption of H2 over GaHZ is completely reversible and thus promotes propene aromatization. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

14

D. Seddon, Catal. Today, 6 (1990) 351. N.S. Gnep and J.Y. Doyemet, Appl. Catal., 43 (1988) 155. N.S. Gnep, J.Y. Doyemet and M. Guisnet, in H.G. Karge and J. Weitkamp (Editors), Zeolites as Catalysts, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989, p. 153. P. Meriaudeau and C. Naccache, J. Mol. Catal., 50 (1989) L7. E.P. Kieffer, Austrahan Patent 565365 (1985). B.R. Gane and P. Howard, Australian Patent Appl. 25132 (1984). V. Kanazirev, G.L. Price and K.M. Dooley, J. Chem. Sot., Chem. Commun., 9 (1990) 712. G.L. Price and V. Kanazirev, J. Catal., 126 (1990) 267. G.L. Price and V. Kanazirev, J. Mol. Catal., 66 (1991) 115. Kh.M. Minachev, O.V. Bragin and T.V. Vasina, Dokl. Akad. Nauk. SSSR, 304 (1989) 1319. R. Roy, V.G. Hill and E.F. Osbom, J. Am. Chem. Sot., 74 (1952) 719. R. Jenkins, R.W. Gould and D. Gedcke, Quantitative X-ray Spectrometry, Marcel Dekker, New York, 1981. L. Petit, J.P. Bournonville and F. Roatz, in P.A. Jacobs and R.A. Santen (Editors) Zeolites: Facts, Figures, Future (Studies in Surface Science and Catalysis, Vol. 49), Elsevier, Amsterdam, 1989,1163. I.A. Sheka, I.S. Chaus and T.T. Mityureva, The Chemistry of Gallium, Elsevier, Amsterdam, 1966, p. 28.