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The Stereochemistry of Ethylene-L ,2-Dz Epoxidation Over Silver Catalysts

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The Stereochemistry of Ethylene-L ,2-Dz Epoxidation Over Silver Catalysts JOURNAL OF CATALYSIS 65, 297-310 (1980) The Stereochemistry of Ethylene-l ,2-dz Epoxidation over Silver Catalysts MAKOTO EGASHIRA,"~' ROBERT L. KUCZKOWSKI," ANDNOEL W.CANT~ * Deportment c?fChemistry, University qf Michigtrn. Ann Arbor, Michignn 48109, rrnd t School qf Chemistry. Mncqucrrie University, North Ryde, Net%’ South Wtrles 2 113, Austrdict Received November I, 1979; revised March 19, 1980 The stereochemistry of the catalytic epoxidation of cis-ethylene-1 ,211, to ethylene oxide over four unsupported and two supported silver catalysts as well as over A&O and AgO was studied. Variations of the stereochemistry with reaction composition including addition of C,H,CI, to moderate the catalyst and the use of N,O as the oxidant were also explored. Equilibration in the product ranged between 57 and 99%. A correlation in the equilibration with extent of oxidation of the surface was proposed. Several models were discussed which could rationalize the observed partial equilibration and the sensitivity of the randomization kinetics to certain reaction variables such as oxygen concentration or addition of C,H,CI, or N,O but not others such as temperature and catalyst selectivity. INTRODUCTION silver catalysts using similar, controlled ex- perimental techniques during the reaction There have been three previous reports runs and product analyses. These catalysts directed at the stereochemistry of the oxi- varied in their activity and selectivity char- dation of ethylene- 1,2-dz (ET) to ethylene acteristics so that any correlation with ran- oxide (ETO) over unsupported silver cata- domization might be observed. Expeti- lysts. The earliest study indicated 60-64% ments were conducted over about a 140” equilibration in the epoxide (I). In two later temperature range and for various contact reports, Cant and Hall (2) obtained 91.6- times and flow rates. Pretreatments of the 93.2% equilibration, while Larrabee and catalysts and the reaction mixture composi- Kuczkowski (3) reported 84-87% equili- tions were also varied. The latter variation bration for a low-activity high-selectivity included the addition of 1,Zdichloroethane catalyst and 90-9 1% equilibration for a to the reactant gas, since it is often used to high-activity low-selectivity catalyst. These improve the selectivity toward ET0 (4). last authors also reported an absence of Several experiments were conducted in temperature dependence on the equilibra- which nitrous oxide was employed as the tion results and some preliminary experi- source of oxygen. Nitrous oxide has been ments suggesting a weak dependence on shown to produce predominantly mo- catalyst mass and flow rate. noatomic oxygen species on adsorption (5), It appeared valuable to direct additional and it was of interest to determine what stereochemical experiments at understand- correlation this might have with the ran- ing the differences between these studies in domization process. The stereochemistry order to deduce further mechanistic details. of epoxidation was also studied during the Therefore, experiments were performed reduction of silver oxides (Ag,O and AgO) with two supported and four unsupported with ethylene in the absence of any gaseous oxygen. ’ Permanent address: Department of Materials Sci- EXPERIMENTAL ence and Engineering, Faculty of Engineering, Naga- saki University, Nagasaki 852, Japan. Reagents. Four unsupported silver cata- 297 0021-9517/80/100297-14$02.00/O Copyright @ 1980 by Academic Press. Inc. All right? of reproduction in any form reserved. 298 EGASHIRA, KUCZKOWSKI, AND CANT lysts were employed. Ag(1) and Ag(2) were conditions on the randomization, the flow the silver powders previously described in rate of the gas mixture was varied over the Ref. (.3), while the Ag-sponge was similar in range of 15 to 90 cm3 min-‘. For most type to that reported in Ref. (2). Their studies the typical flow rate was 60 cm3 selectivities/activities as well as surface min-‘, where it was confirmed that the area were redetermined as necessary in the reaction was not diffusion controlled. The new system employed here and some differ- typical composition of the reactant gas was ences were noted from the previous re- 3% C2H4, 10.5% 02, and 86.5% N,. A ports. The Ag(3) catalyst was obtained by series of experiments were carried out in reducing Ag,O with C2H, at 180°Cfor more which the concentration of ET was varied than 3 hr. Two supported silver catalysts from 3 to 90% at a constant concentration were 1.5% Ag/a-AlpOa which was prepared of oxygen (10%). In another series, the by Engelhard Industries and 5% Ag/SiC& concentration of oxygen was varied from of the type used by Wu and Harriott (6). 0.2 to 10.5% with 3% C,H,. In both series, Ag,O was prepared by precipitation from a N, gas made up the balance of the mixture. silver nitrate solution with sodium hydrox- In some experiments, 1,2-C,H,CI, was also ide, followed by washing 20 times and included in the feed gas. This was achieved drying at 85-88°C for 2 days (7). AgO was by flowing the oxygen over a small amount obtained by the reaction of silver nitrate of the material condensed in a trap at -63 with potassium peroxydisulfate, followed or -78°C. by thorough washing and drying in air for 3 The oxidation of ethylene by N20 was days (8). conducted over the Ag-sponge catalyst at Matheson CP-grade C,H, and NzO, ex- 280°C by using the reactant composition of tradry Nz, and prepurified O2 were em- 2% CzH4, 2-13% N20, and 85-96% NB. The ployed. cis- and truns-ETd, were obtained reductions of Ag,O and AgO with ethylene from Merck Sharp and Dohme, and their were carried out at 150 and 140°C respec- isomeric purity was the same as specified tively, with the reducing gas comprised of before (3). 2% C2H, and 98% N2. Apparatus and procedure. A single-pass The amount of C2H2D2 reacted in a ste- flow reactor which could be evacuated was reochemistry experiment was usually 0.5- employed. The catalyst (0.15-10.0 g) was 1.0 mmol. In these runs, the N2 stream was held in a 6-, 10.5-, or 12.5-mm-i.d. Pyrex diverted over small amounts of C2H,D2 tube reactor. condensed in a trap at - 145 to - 125°Cafter Pretreatment of the Ag catalysts nor- the C2H, flow had been stopped. mally consisted of reduction in flowing H2 Reaction products were sampled by a at 3 10°C for several hours except for Ag(3) syringe inserted into the effluent for gc which was reduced with C,H,. Reaction analysis or collected in a sample loop at runs were then conducted at a desired - 160°C for spectroscopic analysis. The lat- temperature (140-300°C) for l-3 hr until the ter samples were trap to trap distilled to catalyst stabilized. It was noticed that when concentrate either the ET or the ETO. A a catalyst was exposed to room air its Varian 920 gas chromatograph was em- activity markedly decreased unless it was ployed to analyze reactant and product reduced again. This led to several experi- gases. Three columns were used: 10% Car- ments where room-temperature air was bowax 1000 on Chromosorb W-HP (2.5 m, passed over the catalyst for l-3 days before 45°C) for the analysis of ETO, Porapak Q conducting an oxidation run. These runs of (2.5 m, 25 or 60°C) for COz, NzO, and ET, unreduced catalyst are labeled “air cata- and Molecular Sieve 5A ( 1.5 m, 60°C) for lyst.” 0, and Nz. The conversion/selectivity data To study the effect of diffusion-controlled for the oxidation of ET-d, were also ob- ETHYLENE- 1,2-& EPOXIDATION 299 tained. It was reliable only for the ratio RESULTS of CO, and ET0 formation, that is, the selectivity of reaction, because the reac- Reactor Variables tion time for ET-d, was usually less than The previous report (3) suggested a slight 10 min. increase in the percentage of retention of A Hewlett-Packard 8460A microwave the original stereochemical configuration spectrometer was employed to determine upon decreasing the catalyst mass or in- cisltrans ratios in the product epoxide creasing the flow rate through the reactor. &H,D,O. This analytical technique has The flow rate employed was 14 cm3 min-’ been discussed extensively and the pre- where breakdown of plug flow or diffusion vious report can be consulted for further effects might become noticeable. In order details (3). In most cases three to five pairs to test whether such conditions might have of rotational transitions were compared to an effect on the randomization kinetics, obtain estimates of precision and accuracy. experiments were carried out over catalyst The deviation of each percentage cis-ETO- Ag(2) while the flow rate was varied in the d, value from the average was less than range 15-90 cm3 min-’ at a constant contact -cO.5%. In this paper, data from only one time, W/F = 5.0 g ’ set . cm-” (W = cata- transition ( 111 c OuO)which was closest to lyst weight, F = flow rate). The results at the average are presented. The absolute 190-250°C show effects at flow rates less accuracy in the intensity ratios is believed than 40 cm” min’ : at 210°C for example, to be better than & 1.O% including consider- total conversion of ET at 15 cm” min-’ was ation of a possible systematic error due 23.2% while 33.4% at 60 cm3 min-‘. Also, to the lack of vibrational partition func- an increase in selectivity is observed in tions for cis- and trans-C,H2D,0 (3).
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