Characterization and catalytic activity of chromium supported on Y-zeolites

Characterization and catalytic activity of chromium supported on Y-zeolites

Journal of Molecular Catalysis, 53 (1989) 155 - 163 CHARACTERIZATION AND CATALYTIC SUPPORTED ON Y-ZEOLITES 155 ACTIVITY OF CHROMIUM JIN-KAI WANG,...

619KB Sizes 0 Downloads 20 Views

Journal of Molecular Catalysis, 53 (1989) 155 - 163

CHARACTERIZATION AND CATALYTIC SUPPORTED ON Y-ZEOLITES

155

ACTIVITY

OF CHROMIUM

JIN-KAI WANG, SEITARO NAMBA and TATSUAKI YASHIMA Department

of Chemistry, Tokyo Institute of Technology,

(Received July 27,1988;

Meguro-ku, Tokyo (Japan)

accepted November 28, 1988)

Summary

Cr/HNa-Y zeolites were prepared by the thermal decomposition of Cr(C0)6 adsorbed on HNa-Y zeolites having various degrees of proton exchange. The average oxidation number (AON) of Cr increased (from 0.02 to 0.26) on increasing the degree of proton exchange (from 0 to 75%) in Y-zeolites dehydrated at 823 K under the same decomposition temperature (473 K) of Cr(C0)6 adsorbed on HNa-Y. Using the Cr/HNa-Y as the catalyst, the reaction of ethylene in the presence of hydrogen was studied. The activity for the hydrogenation of ethylene increased with decreasing AON of Cr, and reached a maximum of 0.02 for the AON of Cr. The Cr(0) species is thus considered to be the active site for the hydrogenation of ethylene. The activity for the polymerization of ethylene was maximized at a AON of 0.26 for Cr. Based on the results of IR studies of NO adsorbed on Cr/HNa-Y, and the fact that the catalysts including only Cr(0) or Cr(I1) are not active for the ethylene polymerization, Cr(1) is suggested to be the active site for the polymerization of ethylene.

Introduction

Zero-valent metal carbonyls have recently been used as precursors to prepare a new class of heterogenous catalysts [l ,2]. In these catalysts the metal atoms can be supported in a lower oxidation state than those in traditional heterogenous catalysts prepared by the conventional impregnation method with aqueous solutions of the metal salts. Banks [3] have earlier reported that catalysts prepared from Cr(C0)6 and alumina or silica-alumina are active for the polymerization of ethylene, but they did not mention the oxidation state of chromium in their catalysts. Recently, Hucul and Brenner [4] reported that a low oxidation state chromium can be prepared using Cr(C0)6 and highly dehydroxylated alumina, and that this chromium species in a low oxidation state is active for the hydrogenation of ethylene. Kazusaka and Howe [5] have reported that, 0304-5102/89/$3.50

0 Elsevier Sequoia/Printed in The Netherlands

156

from the results of ESR and IR measurements, both Cr(I1) and Cr(II1) exist in Cr/A120s prepared from Cr(C0)6 and alumina. When zeolite is used as a support, one can expect to obtain a chromium species which has a higher dispersion and more clearly defined structure than that obtained on amorphous silica or alumina supports, since zeolite has uniform pores in its framework structure. Usually, Cr(I1) or Cr(II1) is supported on zeolites by an ion exchange method with aqueous solutions of each salt. However, the reduction of these Cr ions to an oxidation number lower than two is thermodynamically improbable [6]. In order to obtain the chromium species in a low oxidation state, it is preferable to use chromium complexes such as Cr(CO)+ Using Cr(CO), as a precursor, the chromium is oxidized by the hydroxyl groups of the support during the decomposition of Cr(CO)6 adsorbed on the support [7]. Thus, the oxidation state of chromium can be controlled by varying the concentration of the hydroxyl groups of the support. In our previous paper, the concentration of such hydroxyl groups was changed by varying the dehydration temperature of H(75)Na-Y [B]. The AON of chromium decreased from 1.11 to 0.15 on increasing the dehydration temperature of H(75)Na-Y. In addition, the oxidation of chromium was accelerated at the high decomposition temperatures of Cr(CO)6 adsorbed on HNa-Y zeolites. On the other hand, the concentration of the surface hydroxyl groups of HNa-Y can also be controlled by varying the degree of proton exchange in Y zeolites. In this paper, we have controlled the AON of chromium by varying the degrees of proton exchange of Y zeolite. In addition, we have tried to clarify the effect of the oxidation state of chromium on the activity of this catalyst for both the hydrogenation and the polymerization of ethylene.

Experimental Catalysts Na-Y zeolites (Toyo Soda Ind. Co.) were treated with 0.05 N NH&l aqueous solution at 298 K or 333 K to form NH4Na-Y. After calcination at 743 K in air, H(x)Na-Y was obtained, where x is the percent degree of proton exchange. The percent degree of proton exchange was controlled by varying the amount of NH&I aqueous solution added. A description of techniques used in the preparation of Cr-Y has been provided elsewhere [B]. Namely, two kinds of Cr-Y were prepared by different procedures. One was prepared from the gas-phase adsorption method using Cr(CO), and H(x)Na-Y. The amount of supported chromium in this method is two chromium atoms per supercage. Another kind of Cr-Y is prepared from the ion exchange method using aqueous CrCls and Na-Y. The degree of ion exchange was 45%, which corresponded to one chromium atom per supercage of Na-Y. The degree of proton exchange of HNa-Y and the amount of chromium supported on HNa-Y were determined by flame photometry and atomic absorption spectrophotometry, respectively.

157

Apparatus and procedure The AON of chromium was determined by measuring the amounts of H,, CO, COZ and CH4 evolved during the temperature-programmed decomposition (TPDE) of the Cr(C0)6 adsorbed on HNa-Y from 300 to 473 K. The amounts of these gases were measured with a quadrupole mass spectrometer (ULVAC, MSQ-150A). The reaction of ethylene in the presence of hydrogen was carried out at 333 K with a closed circulation system having 300 ml of dead volume. The initial pressure of ethylene and hydrogen was 100 torr. The reaction products were analyzed by gas chromatography. The activity for ethylene hydrogenation was determined by measuring the amount of ethane produced. The activity for ethylene polymerization was determined from the difference between the amount of consumed ethylene, determined from the pressure decrease in the system, and the amount of ethane produced. The rate of each reaction was determined at the initial 2 min of the reactions [8], and the activity was expressed as formal turnover frequency, N,, which is the number of molecules reacting per min per Cr atom in the catalyst. IR spectra were recorded at room temperature with a JASCO IR-810 spectrophotometer. The Cr/HNa-Y samples were prepared from HNa-Y as a self-supporting wafer placed in a high-vacuum IR cell by the gas-phase adsorption method using Cr(CO),+ The accuracy of band positions was estimated to be +2 cm-i. ESR spectra were recorded at room temperature or 77 K with an X-band spectrometer (JEOL LSF-PE-IX). The g values were calculated by comparison with a manganese standard.

Results and discussion Figure 1 shows the effect of the degrees of proton exchange in Y zeolites on the AON of chromium in Cr/H(x)Na-Y. The AON of chromium was calculated from the relative amounts of H2 and CO evolved, because the amount of CH4 or CO* evolved was extremely small compared with the amount of Hz or CO evolved (CH4/H2 < 10e2, C02/C0 < 10T3) [8]. The AON of chromium increased with increasing degree of proton exchange in Y zeolites, namely with increasing the concentration of hydroxyl groups (i.e. zeolite protons). The AON of chromium could be adjusted in the range from 0.02 to 0.26. Using these catalysts, the reaction of ethylene in the presence of hydrogen was carried out at 333 K. Ethylene and ethane were detected in the residual gas only during the reaction time from 2 min to 60 min. It was suggested that both the hydrogenation and the polymerization of ethylene occurred simultaneously. The activity of each reaction changed largely according to the AON of chromium in Cr/H(x)Na-Y as shown in Fig. 2. The activity for the polymerization of ethylene increased on increasing the AON of chromium. On the other hand, the activity for the hydrogenation of ethylene decreased with increasing AON of chromium. Accordingly, it is suggested that the active site of Cr species responsible for

158

1.5 0.4 3 JL 1.0 ," 'c .I g

P

0.2

z" 0.5

0'

20 40 60 80 Degree of Proton exchonse 1%

I

I

0.1

0.2

( ,3

AON

Fig. 1. Effect of degree of proton exchange in Y zeolites on average oxidation number of Cr in Cr/HNa-Y zeolites. Dehydration temperature of HNa-Y zeolites = 823 K; decomposition temperature of Cr(C0)6 = 473 K. Fig. 2. Effect of average oxidation number of Cr on activity for ethylene polymerization (0) and hydrogenation (0). Dehydration temperature of HNa-Y zeolites = 823 K; decomposition temperature of Cr(CO)s = 473 K.

the respective reactions is different. In the investigation of ethylene polymerization in the absence of Hz, we have concluded that the oxidation state of the chromium in the Cr(CO),/H(75)Na-Y system is lower than 2, based on the results of the ESR studies [3]. The nature of the active site has been extensively characterized using various probe molecules such as NO, CO, etc. In the present study, both reactions were poisoned by NO. Therefore, the correlation between the catalytic activity and the oxidation state of Cr for the respective reaction might be clarified by NO adsorption. The amount of sites catalytically active for ethylene polymerization was examined by NO poisoning (Fig. 3). The catalyst displayed a sharp decrease in activity on adsorption of NO. When the adsorption stoichiometry of NO/Cr was 2, the amount of active site was calculated to be 3.4 X 1O-6 mol, corresponding to -5% of the total chromium content (7.1 X 10m5mol). It is well known that NO is a useful spectroscopic probe for characterizing surface sites on transition metal oxide catalysts [9]. The oxidation state of a metal can be inferred from the shift in NO vibration frequencies. In Fig. 4, the IR spectra of NO adsorbed on various samples of Cr/HNa-Y are shown. These spectra were observed under the equilibrium pressure of 2 ton of NO. The NO bands are shifted to lower wavenumbers on decreasing the degree of proton exchange in Y zeolites. This corresponds to a decrease in the AON of chromium. Accordingly, it is suggested that the oxidation state of the active site for the polymerization of ethylene is higher than that for the hydrogenation of ethylene. In the case of Cr(CO),/Al,Os, Hucul and Brenner [4] have reported that the ethylene hydrogenation

159

I

2000 ho1es.106) NOads

I

1800

I

4

1600

Wavenunber /cm-'

Fig. 3. Activity as a function of adsorbed NO. Dehydration temperature of H(75)Na-Y zeolite = 823 K; decomposition temperature of Cr(CO)e = 473 K; initial pressure of ethylene = 200 torr. Fig. 4. IR spectra of NO adsorbed on various Cr/HNa-Y. Dehydration temperature HNa-Y zeolites = 823 K; decomposition temperature of Cr(CO)e = 473 K.

of

activity of catalysts prepared with fully dehydroxylated alumina supports (dehydroxylated at 1223 K) is higher than that of catalysts prepared with standard alumina supports (dehydroxylated at 773 K). Thus, the authors have suggested that zero-valent chromium provides the active sites for ethylene hydrogenation. In addition, zero-valent MO and W can be prepared from those carbonyl complexes and Na-Y in which no oxidizing sites (i.e. zeolite protons) were found to exist [lo, 111. As shown in Fig. 2, the activity for ethylene hydrogenation increased on decreasing the AON of chromium. The AON of chromium in Cr/HNa-Y, which was highly active for ethylene hydrogenation, was 0.02. Accordingly, it can be concluded that the active site for the hydrogenation of ethylene in the our catalysts is zero-valent chromium (Cr(0)). In Fig. 4, there are two main peaks for every kind of Cr/H(x)Na-Y. A number of models have been proposed to account for these two main peaks. In the case of Cr03/SiOZ [12], these two bands were assigned to the symmetrical and antisymmetrical vibrations of the coupled ligands in [Cr(NO),], on the basis of the comparison of these spectra with the spectra of homogeneous dinitrosyl complexes and the intensity ratios of these peaks being constant. But, in the case of Cr(C0)6/A1,0s [ 51, these two main peaks were assigned to single NO ligands on different isolated Cr(I1) sites because the relative intensities of the peaks were not constant. When a 14NO:15N0 = 1:l mixture was adsorbed on Cr(III)Na-Y prepared by the ion exchange method using CrCls and Na-Y, each of the two main peaks was split into a triplet [ 131. The intensity ratio for the three peaks in each triplet was 1: 2: 1,

160

which was thought to be comprised of 25% [Cr( 14N0)J, 50% [Cr( 14NO) ( 15NO)] and 25% [Cr(i5N0)J. Accordingly, the two main peaks were assigned to the symmetrical and antisymmetrical vibrations of the coupled ligands in [ Cr(NO),] . In order to identify the adsorbed species of nitric oxide on Cr/H(x)Na-Y prepared from Cr(CO), and H(x)Na-Y, 14N0 and isNO were used. The IR spectra of NO chemisorbed on Cr/H(75)Na-Y (AON = 0.26) are shown in Fig. 5. Part A shows the spectra of i4N0 ( . . . . ...) and lSNO (- - -) adsorbed which is the sum individually on Cr/H(75)Na-Y, and the spectrum (-) of i4N0 and 15N0 spectra. Part B shows the spectrum (- - -) which is the ) of a “NO:‘5N0 = sum of 14N0 and “NO spectra, and the spectrum (1:l mixture added to Cr/H(75)Na--Y. When the 14NO:15N0 = 1:l mixture was added to Cr/H(75)Na-Y, two new peaks appeared at 1889 and 1754 cm-‘. Each of the two main peaks was split into a triplet. Accordingly, these two main peaks were assigned to the symmetrical and antisymmetrical vibrations of the coupled ligands in [Cr(NO),]. On this basis, it was found that two NO molecules will adsorb onto not only the Cr(II1) but also the low oxidation state of Cr. However, the intensity ratio for the three peaks in each triplet was not 1:2:1. The different intensity ratio for the three peaks in each triplet can be explained from the following result. When NO was adsorbed on the Cr/HNa-Y prepared from the higher degree of proton exchange in Y zeolite, not only the two main peaks, but also shoulders were observed (Fig. 4). In Fig. 6, the IR spectra of NO were varied by changing the amount of NO added onto Cr/H(75)Na-Y. When the amount of adsorption of NO was low, two peaks were observed at 1755 and 1880 cm-‘. As the amount of adsorption of NO increased, two new peaks appeared at 1775 and 1895 cm-‘. These results suggested that the different oxidation states of Cr were present simultaneously. Thus, the difference in the intensity ratio for three peaks of each triplet is due to the peaks arising from their overlap among the multiple oxidation states of Cr in Cr/H(75)Na-Y. The investigation of the effect of decomposition temperature of Cr(C0)6/H(75)Na-Y on activity for ethylene polymerization showed that the catalytic activity was maximized when the decomposition temperature of Cr(C0)6/H(75)Na-Y was 473 K, under which conditions the AON of Cr was 0.26. On the other hand, the catalytic activity was almost lost when the decomposition temperature of Cr(CO)JH(75)Na-Y was 673 K, while the AON of Cr was 0.52 [8]. Determination of the amount of adsorbed NO indicated that aggregation of chromium occurred at high decomposition temperature. The amount of adsorbed NO on Cr, in Cr/H(75)Na-Y decomposed at 473 K, was 1.2 NO/Cr, while decomposing Cr(CO),/H(75)Na-Y at 673 K resulted in a ratio of 0.6 NO/Cr. The decreasing amount of adsorbed NO indicates that inactivation of the catalyst cannot be entirely due to a change in the aggregation of chromium. On the other hand, the AON of chromium increased from 0.26 to 0.52 on increasing the decomposition temperature from 473 to 673 K. It was suggested that the active Cr species

161

I

h ’

A

:i 1

II

-

"NO:'5NO=1:1

_-mm 14N,,[email protected]

\ ? :; & !\ .

$,\

iI 1 ‘! ‘, \

i\

‘, ‘. \\ \. .._._.-._.

. ... . .. .

(A)

I

___

:a , * I

1

v

14N0

*.

! :..,

i’

‘5NO

14NO+‘5N0

-

I\.

_..... _.--.d __--_s

I 2000

I

I

I

1800 Wovenumber /cm“

‘\

I

I

1600

2000

i I’\. ......I.,“--. ........ ,,,,.: !

‘.., ..__

-.,‘-.-............._...._.. ‘\

.--.,._,_.l ,#--.\

.-

‘.S_

/--.____/

I 1800 Woventier

_,

‘-_________

.-._.-.

-.-.

-___--_-_

I 1600 /cm-l

Fig. 5. IR spectra of NO adsorbed on Cr/H(75)Na-Y. Dehydration temperature of H(75)Na-Y = 823 K; decomposition temperature of Cr(C0)6 = 473 K. For (A) and (B), see text. Fig. 6. IR spectra for successive additions of NO to Cr/H(75)Na-Y. Dehydration temperature of H(75)Na-Y = 823 K; decomposition temperature of Cr(CO)s = 473 K.

was further oxidized at high decomposition temperature, which led to the decrease in activity. It should be noted that at the same decomposition temperature (473 K) a similar amount of adsorbed NO (about 1.2 NO/Cr) was also obtained in the case of Cr/HNa-Y with various degrees of proton exchange (from 0 to 75%) in Y zeolites. This means that the apparent dispersion of Cr in Cr/H(x)Na-Y does not change on varying the degree of proton exchange in Y zeolites. In order to clarify further the oxidation state of the active species, the relation between the IR spectra of adsorbed NO and the catalytic activity for the polymerization of ethylene was examined, and the results are shown in Fig. 7. Spectrum A represents the catalyst with high activity, while spectrum B represents the inactive catalyst. In comparing these two spectra, shoulders at 1755 and 1881 cm-’ were observed in the active catalyst. Development of these shoulders was also observed when the amount of adsorbed of NO was low (Fig. 6). This fact is consistent with the results of the poisoning experiment (Fig. 3). Accordingly, the chromium species corresponding to the 1755 and 1881 cm-’ peaks was concluded to be the active site for ethylene polymerization. In addition, the IR spectrum of NO adsorbed on Cr(II)Na-Y was measured. Cr(II)Na-Y was prepared from the reduction of Cr(III)Na-Y by hydrogen. The reduction of a major fraction of the Cr to the divalent

162 h i! ; ! I i

I 2000

i,

I

I

1800

1600

Wavenumber /cm-'

Fig. 7. IR spectra of NO adsorbed on various Cr-Y zeolites. Samples A and B were prepared from Cr(C0)6/H(75)Na-Y. For sample A, dehydration temperature of

H(75)Na-Y = 823 K, decomposition temperature of Cr(CO)G = 473 K. For sample B, dehydration temperature of H(75)Na-Y = 823 K, decomposition temperature of

SampleC was preparedby ion exchange,then evacuatedat 620 K for 18 h and reducedat 773 K for 1 h.

Cr(CO).s = 673 K.

state was confirmed by the amount of hydrogen consumed (0.9 H/C%) and the disappearance of the ESR spectrum due to Cr(II1). Using Cr(II)Na-Y as the catalyst, no activity was observed in ethylene polymerization as reported by Wichterlova et al. [6]. In Fig. 7 (spectrum C), there are two peaks at 1900 and 1780 cm-‘, as in the IR spectrum of adsorbed NO on Cr(III)Na-Y. Wichterlova et al. [6] reported that the IR spectra of adsorbed NO are identical and cannot be used to distinguish the existence of Cr(II1) or Cr(I1) in the zeolite. However, the peaks at 1900 and 1780 cm-’ are regarded as bands due to NO adsorbed on Cr(II)Na-Y. The peaks corresponding to the active site for ethylene polymerization appeared in the low wavenumber region compared with those for Cr(II)Na-Y. Accordingly, it is suggested that the oxidation state of chromium corresponding to high activity for ethylene polymerization is less than +2. From the results of Fig. 2, the Cr(0) species is not the active site for the polymerization of ethylene. Accordingly, Cr(1) species is suggested to be the active site for the polymerization of ethylene. Hall et al. [14] have characterized the Cr(1) in a NaF crystal host by ESR as an isotropic line at g = 2.001. However, no ESR signal due to Cr(1) was detected at room temperature or 77 K in this study. In the case of Mo(CO)~/AI~O~, Bowman and Burwell have suggested that small clusters of metallic molybdenum anchor to Mo(I1) cations, when the AON of MO is 0.4 [ 151. Accordingly, it is suggested that Cr(0) atoms anchor to Cr(1) or Cr(I1) cations. The lack of detection of Cr(1) species by ESR is thought to be due to the decrease in relaxation times by strong interactions between unpaired electron in the Cr(1) and Cr(0) species. Even though Cr(1) is an unusual valence state, it can possibly be stabilized in the zeolite framework.

163

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

D. A. Hucul and A. Brenner, J. Phys. Chem., 85 (1981) 496. T. J. Thomas, D. A. Hucul and A. Brenner, ACS Symp. Ser., 192 (1982) 267. U.S. Pat. 3 463 827 (1969) to R. L. Banks; Chem. Abstr., 75 (1971) 1513398. D. A. Hucul and A. Brenner, J. Chem. Sot., Chem. Commun., (1982) 830. A. Kazusaka and R. F. Howe, J. Catal., 63 (1980) 447. B. Wichterlova, Z. Tvaruzkova and J. Navakova, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 1573. A. Brenner, D. A. Hucul and S. J. Hardwick, Znorg. Chem., 18 (1979) 1478. J. K. Wang, T. Komasteu, S. Namba, T. Yashima and T. Uematsu, J. Moi. Catal., 37 (1986) 327. M. Kung and H. Kung, Catal. Rev. Sci. Eng., 27 (1985) 425. S. Abdo and R. F. Howe, J. Phys. Chem., 87 (1983) 1713. S. Abdo, J. Gosbee and R. F. Howe, J. Chim. Phys., 78 (1981) 885. A. Zecchina, E. Grarrone, G. Ghiotti, C. Morterra and E. Borello, J. Phys. Chem., 79 (1975) 978. J. R. Pearce, D. E. Sherwood, M. B. Hall and J. H. Lunsford, J. Phys. Chem., 84 (1980) 3215. T. P. P. Hall, W. Hayes, R. W. H. Stevenson and T. Wilkens, J. Chem. Phys., 38 (1963) 1977. R. G. Bowman and R. L. Burwell, J. CataL, 63 (1980) 463.