Applied Catalysis A: General, 92 (1992) 73-80 Elsevier Science Publishers B.V., Amsterdam

73

APCAT A2385

Rhenium as a promoter for ethylene epoxidation

Yang Jun and Deng Jingfa Department of Chemistry, Fudan University, Shanghai 200433 (People’s Republic of China)

and

Yuan Xiaohong and Zhang Shi Shanghai Research Institute of Petrochemical Technology, Shanghai 201208 (People’s Republic of China)

(Received 5 November 1991, revised manuscript received 2 June 1992)

Abstract

The reaction of ethylene epoxidation was studied over electrolytic silver catalysts with an additive of rhenium in the range 1.9-6.5 ppm, and the effect of the rhenium promoter was also investigated by studying the adsorption of oxygen on the surface of a Ag-Re catalyst containing 4.15 ppm rhenium. It was found that both the conversion of ethylene and the selectivity of ethene oxide could be improved by the use of a rhenium-modified silver catalyst. The temperature deeorption spectra of oxygen showed two adsorbed states, which were different from those on other silver samples. The effect of rhenium was to weaken the silver-oxygen bond, and to reduce the electron density of the adsorbed oxygen, which could be the reason for the enhancement of selectivity of ethene oxide.

Keywords: ethylene epoxidation, oxygen adsorption, rhenium, silver.

INTRODUCTION

The epoxidation reaction of ethylene over silver catalysts is a very important industrial process. In recent years the number of papers devoted to promoters of the reaction has increased. Boron [ 11, thallium [ 2,3], and alkali metals [ 4- 7] have been found to improve selectivity. The most recent work [ 81 has shown that both rhenium and caesium added to Ag/a-Al,O, can improve the selec- tivity greater than the limiting selectivity (86% ) which is predicted by the reaction mechanism of adsorbed molecular oxygen [ 91, but the nature of the rhenium and caesium promoters remains unclear. A series of earlier papers [4- 7] described the results of using caesium as a promoter, but there have been no papers on rhenium, especially not about its role. In the present paper, an electrolytic silver catalyst modified with rhenium was prepared to study the

Correspondence to: Dr. Deng Jingfa, Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China. Tel. (+86-21)5492222-3792, fax. (+86-al)5491875

0926-860X/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

74 J. Yang et aL/Appl. Cata1.A 92 (1992) 73-80

reaction of ethylene epoxidation. The effect of the rhenium promoter was also investigated by studying the adsorption of oxygen on the surface of the catalyst.

EXPERIMENTAL

The silver sample which was used was prepared by electrolytic refining (three times). It had a surface area of 0.022 m2/g measured on a D&orb-2600 phys- isorption analyzer with adsorbed krypton at 77 K and a purity of 99.999%. The Ag-Re catalyst was prepared by impregnating 20-40 mesh grains of electro- lytic silver to a solution of NH,ReO, which was then heated in a muffle furnace under a forced air flow at 160°C for 1 h, and maintained for a further hour. Subsequently, it was further heated up to 25O”C, and maintained at this tem- perature for 3 h before being cooled to room temperature. The catalytic per- formance was evaluated in a microreactor with an on-line gas chromatograph under the following conditions: pressure 1.5 MPa and space time 5000 h-l. The reactant feed consisted of nitrogen (69.01% ), oxygen (7.19% ) and eth- ylene (23.8% ). The catalyst was aged under reaction conditions at 180°C for 30-40 h. No internal or external diffusion effects were observed under these operating conditions.

The temperature desorption spectra (TDS ) experiments and the work func- tion measurements were conducted in a UHV chamber equipped with a quad- rupole mass spectrometer, a cylindrical mirror analyzer (CMA) for Auger elec- tron spectroscopy (AES), a low energy electron gun for measuring work function changes and an argon ion gun. The testing chamber maintained a vacuum pressure of the order of lo-’ Pa during the experiments. The disk of the sample was mounted on a transferable sample holder and was heated by radiation by passing current through two tungsten filaments fixed at the back of the sample. The heating rate was 487 K/min. The sample surface was cleaned by argon ion bombardment and oxidation (PO, of 4 x 10 -’ Pa) -annealing (850 K ) cycles in the chamber until the signals of S, Cl, and C determined by AES were diminished to the noise level (see Fig. 1) .

RESULTS AND DISCUSSION

The catalytic properties of electrolytic silver catalysts with an additive of rhenium in a range of 1.9-6.5 ppm were investigated. The results are shown in Figs. 2 and 3. It was found that all the catalysts showed the same character: the conversion of ethylene increasing with increasing reaction temperature, the selectivity of ethene oxide decreasing with increasing conversion of ethyl- ene. When comparing with the electrolytic silver catalyst, both the activity and selectivity were enhanced on the Ag-Re catalysts. This indicates that rhe- nium added to the silver catalyst can improve activity and selectivity. Fig. 3 shows the variation of activity and selectivity with the quantity of rhenium

J. Yang et al./Appl. Cata1.A 92 (1992) 73-80 75

Re 4

*g I I 1

0 100 200 300 400 500 600

Electron Energy (eV)

Fig. 1. Auger spectrum of the cleaned Ag-Re surface.

76 J. Yang et al.IAppl. Cata1.A 92 (1992) 73-80

I . 0 I . b . ii28 538 548 558

Temperature(K)

89 t

86.

>, 77- .; .- ; 74.

0, 5 * 71 -

681

65: 01234567

Conversion of C&IH~(~/~)

Fig. 2. Activities of the Ag and Ag-Re catalysts for ethylene epoxidation. (0 ) Ag; ( l ) Ag-Rel with 1.91 ppm rhenium; ( + ) Ag-Re2 with 4.15 ppm rhenium; ( A ) Ag-Re3 with 6.38 ppm rhenium.

72L,_, s .

0 1 2 3 4 5 6 7

Re content (ppm)

OOj

Re content (ppm)

Fig. 3. Variation of activity and selectivity with the quantity of rhenium used.

added. The most suitable content was about 4.2 ppm for activity and 2 ppm for selectivity. Therefore, it was necessary to add the correct amount of rhenium in order to achieve the highest activity and selectivity.

IR and thermogravimetry (TG) were used in order to verify the species of rhenium added to the silver catalyst. The TG spectrum of NH,ReO., shows that a weight-loss process took place at 636.8 K, while the spectrum of NH,ReO, treated by the catalyst preparation process mentioned above shows no weight- loss peak. The process of NH4Re04 being dissociated to RezO, was taken at 638.5 K [lo]. Therefore, it can be concluded that NH&so, after being heated

J. Yang et al. jApp1. Cata1.A 92 (1992) 73-80 77

to higher temperatures on a silver sample was dissociated to an oxide com- pound of rhenium. This is confirmed by AES and IR results. In the AES ex- periment, the signal of N was not found on the uncleaned sample surface. In the IR spectra the band at 1402 cm-’ which characterized the stretching vi- bration of the amino-group in NH,ReO, disappeared for the Ag-Re catalyst (Fig. 4b). Weinstock and Schulce [ 111 attributed the band at 920 ? 4 cm-’ to the stretching vibration of Re-0 in ReO, , therefore, the species of rhenium may be ascribed to Re20, due to the band at 910 cm-l as shown in Fig. 4. By the X-ray photoelectron spectroscopy (XPS) results, the binding energy of the Re 4f level is 45.1 eV, this is lower than that of Re’+ (binding energy is 46.0 eV). It means that the valence of rhenium on the silver surface is 7-6, which shows that the rhenium perhaps obtains electrons from the silver lattice.

The spectra of oxygen adsorbed on the surface of the Ag-Re catalyst con- taining 4.15 ppm rhenium are presented in Fig. 5. They show two adsorption states of oxygen coexisting on the surface at room temperature: (a) a low- temperature peak QL at 390 + 10 K and (b) a dispersed peak y beginning to desorb at 620 2 10 K. As shown in Fig. 5, the intensity of peak a! increases with increasing exposure to oxygen, but the temperature of the peak does not change. According to thermal desorption theory, peak a! is ascribed to first-order de- sorption. Numerous experiments [ 12-141 have proven that at room tempera- ture CO can react with atomic oxygen on a silver surface to form COz, but not with molecular oxygen. In this experiment, pure CO is leaked into the preab- sorbed oxygen surface of the Ag-Re catalyst. The exposure to CO is approxi- mately 5 times that of oxygen. After pumping out the residual CO gas, the desorption spectra of oxygen were recorded (shown in Fig. 6), which show that peak (1! also exists after reacting with CO, and that there was no change in the intensity of the peak, whereas the intensity of peak y is lower than before the reaction. Therefore, it may be concluded that peak a! corresponds to molecular

0.6320.

0.0365 2000 1600 1600 1400 1200 1000 600 600 41

Wavenumber (cm-‘)

Fig. 4. IR spectra of the Ag-Re and NH,ReOI samples. (a) NH&O, without heat treatment; (b) Ag-Re with heat treatment.

78 J. Yang et al./Appl. CataLA 92 (1992) 73-80

290 390 490 590 690 790

Temperature (K) .

Fig. 5. TDS of oxygen on the Ag-R.e surface.

491 591 691 791

Temperature (K)

Fig. 6. TDS of oxygen on the Ag-R.e surface with and without carbon monoxide. (a) 30 1 oxygen at room temperature; (b) 30 1 oxygen at room temperature, then 150 1 carbon monoxide at room temperature.

oxygen, and peak y to dissociative atomic oxygen, it can be ascribed to subsur- face oxygen, which was also observed by other investigators [ 15-171.

At a certain level of oxygen exposure, by changing the heating rate Q?) in a range of 50-630 K/min, the plot of In (Ti/j?) versus l/Z’, obtained (where Tp is the peak temperature). From the values of the slope, the activation energy for desorption of peak a! was calculated as being 42.2 kJ/mol.

J. Yang et al./Appl. Cata1.A 92 (1992) 73-80 79

During the adsorption of gas on the metal surface, the change in work func- tion could be related to the transfer of electrons between the adsorbate and the metal. The variation of the work function on Ag-Re and silver as a function of oxygen exposure are shown in Fig. 7. The change in work function increased when oxygen was adsorbed on both the silver and the Ag-Re surface, indicating the transfer of electrons from silver to adsorbed oxygen. This is in accordance with the conclusions of Bao et al. [ 181. The maximum values were 0.6 and 0.18 eV for silver and Ag-Re, respectively. This means that the magnitude of elec- tron transfer from silver to adsorbed oxygen on the Ag-Re surface was less than that on the silver surface, the effective charge of oxygen adsorption on the Ag-Re surface was lower. Grant and Lambert [ 41 proposed that the pivotal factor in ethylene epoxidation is the effective charge state of Ocaj, (where Otaj represents adsorbed atomic oxygen), if it is not very high, an electrophilic at- tack on ethylene is possible, leading eventually to ethene oxide formation. If it is high, hydrogen may be stripped from the hydrogen-carbon interface, leading eventually to ethylene combustion. In our experiments, the adsorbed oxygen compete with rhenium for silver lattice electrons, and lead the reduction of the effective charge state of Oca), Therefore, the enhancement of selectivity in- duced by these species is explained.

Comparing with electrolytic silver [ 141 the adsorption of oxygen on the Ag- Re sample shows three different aspects: (a) there are only two adsorption states of oxygen, which are different from that on other silver samples [ 19,201; (b ) the activation energy for desorption of molecular oxygen is lower than that on electrolytic silver (energy of desorption= 101 kJ/mol); (c) the increase of the change in work function (A#) induced by the adsorption of oxygen on the Ag-Re surface is far lower than that on the silver surface. From the above results, it may be concluded that rhenium added to electrolytic silver substan-

0.6

t

0 1 2 3 4 5 6 7 8 9 10

Time (min)

Fig. 7. Variation of the work function with exposure to oxygen (Pa = 6.7 X IO-’ Pa).

80 J. Yang et al./Appl. Cata1.A 92 (1992) 73-80

tially weakens the silver-oxygen bond, and reduces the electron density of ox- ygen adsorbed on the surface of the Ag-F?R sample.

CONCLUSIONS

It has been shown in this study that rhenium added to a silver catalyst can enhance both the conversion of ethylene and the selectivity of ethene oxide if a suitable quantity of rhenium is used. The enhancement is due to the decrease of the electron density of adsorbed oxygen, and this decrease is induced by the addition of rhenium which competes with adsorbed oxygen for the silver lattice electrons.

ACKNOWLEDGEMENT

The authors are grateful to the National Science Foundation of China for the support to the present work.

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