Applied Catalysis, 44 (1988) 223-237

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

223

Support Participation in Chemistry of Ethylene Oxidation on Silver Catalysts

JAMES K. LEE, XENOPHON E. VERYKIOS’ and RANGASAMY PITCHAI*

Department of Chemical Engineering, Drexel L’niuersity, Philadelphia, PA 19104 (LT.S.A.)

(Received 22 March 1988, revised manuscript received 18 May 1988)

ABSTRACT

Activity and selectivity of supported silver catalysts in ethylene epoxidation and combustion is shown to be a strong function of the carrier employed. All supports used in this study, except for [t-alumina, were found to exhibit significant activity in ethylene oxide isomerization and oxida- tion. Their activity was found to be proportional to their surface acidity. Poisoning of surface acid sites by alkali and alkaline earth metal impregnation resulted in suppression of isomerization activity and reduction of oxidation activity. These phenomena are explained evoking the concept of acid-base bifunctional catalysis.

INTRODUCTION

Industrial production of ethylene oxide is predominantly based upon direct oxidation of ethylene. Commercial catalysts for the selective conversion of eth- ylene to ethylene oxide consists of silver supported on carriers of low surface area, almost exclusively a-alumina. Although t.his catalytic system has been studied extensively over the past fifty years, many fundamental phenomena continue to elude understanding [ 1,2].

An important practical question which has not been systematically investi- gated concerns the role of the support in ethylene oxidation catalysis. The uniqueness of a-alumina as a support material in producing highly selective silver catalysts has been attribut.ed to its low surface area. Nevertheless, there are other support materials of equally low surface area which result in non- selective silver catalysts. There is some evidence in the literature [ 31 indicat- ing that the supports themselves might be act.ive in the conversion of ethylene oxide to acetaldehyde and carbon dioxide. However, no systematic study of ethylene oxide conversion over traditional support materials has ever been

*Present address: Institute of Chemical Engineering and High Temperature Chemical Processes, Department of Chemical Engineering, University of Patras, 26110 Patras, Greece.

0166.9834/88/%03.50 c 1988 Elsevier Science Publishers B.V.

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made with a view t.o understand the role of the support in this important com- mercial process.

Current commercial catalysts for ethylene epoxidation consist of lo-15 wt.- % silver on low surface area a-alumina. High metal content is necessary to provide large active surface area, albeit with a very low degree of dispersion. However, if a higher degree of dispersion of the metal could be achieved, the same active area could be obtained with a significantly smaller amount of sil- ver. As a result, the precious metal would be utilized more efficiently and the process would become more economical. A high degree of metal dispersion can usually be achieved employing high surface area supports.

The purpose of the present investigation was to elucidate the role of the support in ethylene oxidation catalysis over supported silver catalysts and to ident.ify the reasons for the uniqueness of a-alumina in the formulation of selective catalysts for this process. The reactivity of ethylene oxide on surfaces of conventional and non-conventional support materials was investigated for this purpose.

EXPERIMENTAL

All supports used in this study were obtained from commercial suppliers. The origin and the properties of the supports used are given in Table 1. Alkali- impregnated supports were prepared according to the following procedure: a known amount (10 g) of support (average size of particles = 0.4 mm) was added to an aqueous solution (100 ml) of alkali or alkaline earth salt of appro- priate concentration at 90’ C under vigorous stirring. When all the water was evaporated, the support was dried in an oven overnight at 110°C. It was then calcined in air at 400 5 C for 5 h.

Supported silver catalysts were prepared by impregnation of the unmodified support with an aqueous solution of silver nitrate, drying at 110°C overnight and reducing at 200’ C in hydrogen flow for 6 h. The alkali-impregnated silver catalysts were prepared by impregnation of the reduced silver catalysts with an aqueous solution of alkali or alkaline-earth salt. After drying at 110 ‘C over- night, they were oxidized at 200 a C in flowing oxygen and then reduced in hy- drogen flow for 6 h.

Kinetic data were obtained in a fixed-bed, single pass, plug flow reactor, consisting of a U-shaped, 1 cm I.D. stainless steel tube. The reactor was im- mersed in a constant-temperature fluidized sand bath controlled by a PID con- troller, equipped with a digital readout. Temperature of the catalyst bed was measured by inserting a thermocouple into a thermowell which runs through the center of the reactor. Gas flow-rates were controlled and measured with electronic thermal mass flow controllers. The feed stream was thoroughly mixed and preheated to the reaction t.emperature in the preheating section of the reactor. The catalyst bed, about 4 to 7 cm long, was held in place with glass

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TABLE I

Surface area and impurity contents of supports

Support Supplier Surface Purity Comments

area (m”/g) level (% 1

Al,0 i American 195.6 r98 ~0.1% Fe, 0.3% CaO,

(Boehmite) Cyanamid <O.l% Na,O, ~0.5% SiOa

MTG-S-1905

n-All0 I United 0.5 99.5 Powdered the pellets and sieved.

Catalysts SAHT-99

MgO Alfa-11853 10.0 98

Nb,O, Alfa-51103 1.5 99.5

SIC Alfa-881’75 1.1 98.8

SiO, Cabot 195.3 99.8 < 2 ppm Al, < 1 ppm Ba, < 2 ppm Ca, Cab-0-Sil < 2 ppm Fe, < 2 ppm MO, < 5 ppm Na, MS-7 < 0.1 ppm Li, and < 2 ppm Ti.

TiO, Degussa 42.0 >97 < 0.3% A1103, < 0.2% SiO,

P 25 < 0.01% Fe,O,

V,O: Alfa-81110 0.8 99

YLOI Alfa-87829 1.7 99.99

ZrO., Alfa-86122 1.0 98.5

TiO, - 6.4 _ Prepared by the calcination of Degussa

(Rutile) P25 at 900°C.

wool plugs. The average particle size of the catalyst was 0.4 mm. The reactor was always operated under the differential mode. Temperature gradients along

the catalyst bed were determined to be negligible. After the catalyst was loaded, the reactor temperature was raised to 200°C

with nitrogen flowing through it. When the temperature reached 2OO’C, the flow was switched to hydrogen and the catalyst was further reduced in hydro- gen at 200’ C for 1 h. The system was then purged with nitrogen for 1 h. Upon completion of this pretreatment, the reactor pressure was increased to 200 psi (1380 kPa) and the reactor temperature was adjusted to the required value. A temperature range of 210-240’ C was used for all kinetic experiments. The feed compositions used were: 3% (by volume) ethylene, 12% oxygen and 85% ni- trogen for ethylene epoxidation and 0.13% ethylene oxide, 12% oxygen, bal- ance nitrogen, for ethylene oxide oxidation experiments.

On-line analysis of the products was achieved by injecting the exit stream into the GC column with an &port gas sampling valve with two sampling loops. Products were separated by a 6 feet long, 0.125” O.D., stainless steel tube col- umn packed with 80-100 mesh porapack Q. A thermal conductivity detector

was used with helium as the carrier gas. Surface acidities of the supports were

“26

measured by the amine titration technique using Hammett indicators [4]. The experimental procedure used was the same as that employed by Benesi [5].

Supported silver catalysts were characterized in terms of total surface area with the BET method using argon as adsorbate at liquid nitrogen temperature and in terms of exposed metallic surface area employing selective adsorption of oxygen at. 200 ’ C and oxygen pressures up to 25 Torr (3.3 kPa). Blank ad- sorption experiments were conducted with all the supports to determine any ox>-gen uptakes by these materials, under identical conditions.

RESl’LTS AND DISCUSSIOS

Eth>,lene oxidation on silver supported on r;arious carriers

The effects of the support on the performance of silver catalysts under eth- ylene oxidation conditions were investigat.ed employing various conventional and nonconventional supports. Catalysts containing 2 wt.-% silver were for- mulated and their performance under ethylene oxidation conditions was in- \-estigated. Results in terms of total rate of ethylene consumption per unit exposed metallic area of silver and selectivity to ethylene oxide formation are shown in Table 2. In preliminary investigations it was determined that none of the unmetallized supports exhibit.s any catalytic activity towards ethylene conversion, under the conditions employed in the present investigation.

It is apparent from Table 2 that, among the supports employed in this study, only a-Al,O,,, SiOp, ZrO,, SIC and TiO, of the rutile structure result in cata- lysts selective towards ethylene oxide formation. Furthermore, total specific

TABLE 2

Effects of support in ethylene oxidation over silver catalysts at 235’ C

Catalyst BET Surf. area Total rate- 10”

(m’/g) (mol/h m’ silver)

Selectivity

(W)

Ag/TiO, (A) 42.0 25.0 0

AgjTiO? (R) 6.4 39.4 19.0

Ag/Nb,O, 1.5 69.7 0

Ag/V,O, 0.8 4.1 0

Ag/Al_O : (B) 195.6 38.3 0

Ag/ ZrO? 1.0 6.6 42.5

Ag/SiC 1.1 13.5 37.7

Ag/(i-Al,O,, 0.5 183.8 64.5

Ag/SiO, 195.3 310.5 48.1

Ag/MgO 10.0 2.4 0

Ag/k’,O < 1.7 8.5 0

(A): Anatase; (B): Boehmite: CR): Rutile.

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activity per unit. exposed metallic area varies by as much as a factor of 130. This observed variation of specific catalytic activity is not due to intraparticle diffusional resistances of heat and mass which were carefully eliminated ex- perimentally. Furthermore, there seems to be no direct or inverse correlation between overall activity and selectivity, and no correlation between total sur-

face area of the catalyst and selectivity. These factors have often been proposed to explain similar variations of selectivity observed over different catalysts.

A possible explanation of the observed phenomenon could be sought at the different degrees of dispersion of silver on the different supports. It is well- known [6, 71 that ethylene oxidation is somewhat sensitive to the size of the metal (silver) crystallites. Nevertheless, turnover frequencies of both epoxi- dation and combustion reactions have been observed to vary by a factor of 2 to 3 at. the most [ 7, 81. Furthermore, small silver crystallites, as those found in silica-supported catalysts, have been shown to exhibit reduced activity as com- pared to larger silver crystallites, as those observed in a-alumina-supported catalysts [ 81. Thus, the effect of metal crystallite size on turnover frequency can not explain the results reported in Table 2. No reasonable explanation of the variation of catalytic activity with the support employed can be offered, based on the results reported in the present communication. The primary pur-

pose of this study was to elucidate the participation of the support in ethylene oxidation chemistry. This topic is discussed in the following section.

Ethylene oxide isomerizationjoxidation on various supports

In the previous section it was reported that none of the supports employed in this study exhibit any activity towards ethylene conversion. However, the low or zero apparent selectivities observed with many catalysts could be caused by further reactions of ethylene oxide on sites supplied by the support material. To test this hypothesis, most of the supports were tested for ethylene oxide conversion activity under conditions similar to those employed for ethylene

oxidation. The partial pressure of ethylene oxide in the feed stream was kept at very low levels in order to simulate the composition of reaction mixtures during ethylene oxidation experiments. Kinetic results obtained at 240°C are reported in Table 3. It is apparent that all supports, with the exception of a- alumina, exhibit significant isomerization/oxidation activity for ethylene ox- ide. Measurable activity over a-alumina was observed at temperatures exceed- ing 280’ C. The supports which exhibit high isomerization/oxidation activity (TiO, of the anatase structure, boehmite Al,O,, Nb205, V205) have also re- sulted in non-selective catalysts for ethylene oxidation. Similarly, supports which show weak activity for ethylene oxide conversion (a-Al,O,, TiO, of the rutile structure, ZrO,, Sic and SiO,) have resulted in selective catalysts for ethylene oxidation. Thus, the apparent zero or low selectivities reported in

Table 2 are most probably due to isomerization of ethylene oxide (formed on

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0.2

0.1

0 % Na

02 2.2 % Na

0.1 _

,”

i 4.9% No

0.1

0 7 5 3 1 -1 -3 -5 -7 -9

Acid Strength, H,

Fig. 3. Amount and strength of surface acidic sites of alumina as a function of sodium content.

0 % Na

0 5 ‘/. Na

0.9 % No

1 9 % Na

02

01

I

Ruble Ti02

0 7 5 3 1 -? -3 -5 -7 -9

Acid Strength, H,

Fig. 4. Amount and strength of surface acidic sites of titania as a function of sodium content.

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Fig. 5. Effect of sodium on activity of alumina in ethylene oxide reactions.

‘6t A - lsomerlzation I

0 Oxidatmn

0 - Total

o:-_l;dLi 11 12 13

Wt .-% Na

Fig. 6. Effect of sodium on activity of titania in ethylene oxide reactions.

Surface acidity of alumina and titania modified by varying alkali metal con- tent, was measured by the amine titration method [4, 51 and the results are presented in Figs. 1 and 2. As expected, total acidities of alumina and titania decrease with increasing alkali content. In the case of alumina, 85% of surface acidity was eliminated by addition of 4.9 wt.-% sodium, while no measurable acidity remained upon addition of 6.7 wt.-% sodium. In the case of titania, most of the acidic sites were neutralized by addition of 1.9 wt.-% sodium. Acid

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0

TI02 A’203 0 Isomermtion n

0 Oxlc!atlor n l Oxidation on

Ruth Ti02

10’. 0123456789 10 11 12 13 1L

Wt.-% Na

Fig. 7. Effect of sodium on activation energy for ethylene oxide reactions on alumina and titania.

T = 235’C

C’ 3 0 : Oxidat,on

A : lsomerizOtl0n

Wt.-% Addhve

Fig. 8. Effect of sodium, potassium and calcium on activity of silica in ethylene oxide reactions.

strength distribution of pure and alkali-modified alumina and titania is shown in Figs. 3 and 4, respectively. In the case of titania, there is a monotonic de- crease in acid strength with increasing sodium content, whereas in the case of alumina, an initial redistribution in acid strength followed by a continual de- cline is observed.

The effect of sodium addition on catalytic activity of alumina and titania for

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Fig. 9. Effect of sodium, potassium and calcium on activation energy of ethylene oxide oxidation

on silica.

TABLE 3

Kinetic results of ethylene oxide isomerization and oxidation on various supports at 240°C

Support Rate. 10’ (mol/h g)

Isomerization Oxidation Total

TiO, (A) 225.7 117.5 343.2 A1,03 (B) 148.4 187.9 336.3

Nb,O, 35.1 29.7 64.9

V,O, 1.2 48.3 49.5 TiOa (R) 0.0 12.4 12.4 SiO, 7.0 7.8 14.8 Sic 0.3 0.8 1.1 ZrO, 0.3 0.2 0.5 a-A&O, 0.0 0.0 0.0

(A ) : Anatase; (B ) : Boehmite; (R): Rutile.

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ethylene oxide isomerization and oxidation is shown in Figs. 5 and 6, respec- tively. It is obvious that tot.al activity (isomerization and oxidation) decreases with increasing sodium content. Furthermore, apparent isomerization of eth- ylene oxide to acetaldehyde is completely inhibited by sodium addiCon. HOW-

ever, the apparent oxidation rate increases with the disappearance of isomerization. The effect of sodium addition on the activation energy of eth- ylene oxide oxidation is shown in Fig 7. A significant increase of the activation energy of the oxidation reaction is observed upon addition of sodium on alu- mina or titania. The concentration of sodium does not influence activation

energy. It is interesting to observe that although the activation energy on the unmodified materials is somewhat different, it reaches essentially the same value on alumina and titania upon sodium addition.

Kinetic resu1t.s obtained on sodium-, potassium- and calcium-modified silica are shown in Fig. 8. As with alumina and titania, isomerization activity is com- plet.ely eliminated by alkali/alkaline eart.h impregnation. Oxidation activity decreases with increasing sodium and potassium content. On the contrary, calcium impregnation of silica results in a significant increase of the rate of ethylene oxide oxidation. The effect of alkali/alkaline earth addition on acti- vation energy is shown in Fig. 9. It is apparent that activation energy is influ- enced by the type of metal silica is impregnated with, but it is independent of the concentration of the metal.

The activity patterns presented in Figs. 5 and 6 correlate well with the ac- idity patterns presented in Figs. l-4. It is apparent that catalytic activity of

alkali-modified alumina and titania, in ethylene oxide reactions, is propor- tional to the acidity of these materials. Total activity of both materials de- creases in parallel with their acidity as the sodium content increases. Although the effect of alkali-impregnation on the acidity of silica was not investigated, it is reasonable to expect the effects to be similar to those measured on alumina and titania. This observed correlat.ion between catalytic activity and surface acidity is reasonable since isomerization reactions are known to proceed on acidic sites. Furthermore, isomerization of ethylene oxide to acetaldehyde is known to be an intermediat.e step in its conversion to combustion products

[91. Figs. 5, 6 and 8 show that the isomerization reaction of ethylene oxide to

acetaldehyde is completely suppressed even when the alkali-impregnated ma- terials show a large amount of surface acidity. This observation can be inter- preted in either of two ways: It could be argued that not all of the acid sites on the catalyst surface are capable in promoting ethylene oxide isomerization. It is obvious from the acid strength distribution diagrams (Figs. 3 and 4) that strong acidic sites are the ones which are being neutralized first by sodium impregnation. It appears, then, that ethylene oxide isomerization requires acid sites with strong acid strength, probably with H,, < - 3.0. This interpretation is consistent with the fact that no acetaldehyde is formed on rutile titania, which has weak acid sites only, with acid strength + 1.5 <H, < + 6.8. Following

236

TABLE 4

Knietic results of ethylene oxidation over alkali-impregnated silver catalysts

Catalyst Temperature Reaction rate**lO* Reaction rate**lO’

(‘Cl (mol/h g Ag) (mol/h m2 Ag)

Epoxidation Combustion Epoxidation Combustion

2% Ag/AI.LO 1 (Boehmite)

217 0 337.0 0 18.7

225 0 467.0 0 25.9

235 0 691.5 0 38.3

2% Ag/AI,O, 217 0 72.7 0 5.2

(Boehmite) 225 0 97.5 0 7.0

+6.7 wt.-% Na 238 0 149.2 0 10.7

2% Ag/TiOii

(Anatase)

217 0 182.0 0 11.0

225 0 263.5 0 16.0

235 0 413.5 0 25.0

2% Ag/Ti02

( Anatase )

+ 1.9 wt.-% Na

225 0 231.0 0 15.0

236 0 388.0 0 25.2

2% Ag/TiOL

(Anatase)

t12.1 wt.-% Na

236 0 218.2 0 27.1

*Reaction rate is expressed in terms of moles of ethylene reacted to give the product.

line-earth ion on the oxidation activity of silica appear to be opposite to those of alkali ions. The oxidation activity was found to increase with increasing calcium content. A plausible explanation for this observation is the possibility of calcium oxide agglomeration on the surface. Similar agglomeration of bar- ium oxide has been observed when ZnO was impregnated with barium nitrate [ 141. As a result, ethylene oxide can be oxidized by calcium oxide. Moreover, calcium has been shown to be less effective in neutralizing acid sites compared to potassium [ 131. As can be seen in Fig. 9, the amount of alkali ion content does not affect appreciably the activation energy for ethylene oxide oxidation. This is a good indication that the activity in oxidation is dependent on the number of sites created by the addition of alkali ions. The relatively high ac- tivation energy obtained for calcium-impregnated silica is consistent with the earlier explanation of ethylene oxide oxidation on agglomerated particles of calcium oxide.

As discussed previously, ethylene oxide isomerization and oxidation activity of support materials is significant enough to critically affect selectivity of silver supported on such carriers. Moreover, it has been shown that alkali impreg-

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nation is effective in suppressing ethylene oxide isomerization and combus- tion. Therefore, silver catalysts supported on carriers impregnated with alkali would be expected to exhibit enhanced selectivity towards ethylene oxide for- mation. This expectation was not realized experimentally. Alkali-impregnated Ag/Al,O, (Boehmite) and Ag/Ti02 catalysts did not show any selectivity to ethylene oxide formation (Table 4)) but exhibited greatly reduced ethylene combustion activity. Alkali-impregnated Ag/SiO, catalysts exhibited reduced selectivity to ethylene oxide and reduced overall activity. These results are probably due to poisoning of silver surfaces by deposition of alkali metals on them, either during preparation of the catalysts or by surface migration under reaction conditions. In order to determine whether this was a phenomenon occurring during preparation or a reaction-induced migration, attempts were made to prepare the catalyst using the reverse order of impregnation. This procedure always resulted in very poorly dispersed silver catalysts due to the fact that during the impregnation of silver using silver nitrate, alkali already impregnated on the support dissolved in water and precipitated silver oxide from the solution.

In a recent single crystal study, Campbell [ 151 has shown cesium to affect the ethylene epoxidation and combustion activity of Ag (111). In the present study it can be reasonably assumed that alkali impregnation of Ag/SiO, results in a certain coverage of the silver surface as well. The observed decrease of activity and selectivity of these catalysts is then similar to the same effects observed by Campbell [ 151. It can be concluded, therefore, that alkali-impreg- nation of supported silver catalysts is not the proper technique of annihilation of support surface acidity so as to eliminate its detrimental participation in ethylene oxidation catalysis.

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