Chapter 14
Activation of Silver Powder for Ethylene Epoxidation at Vacuum and Atmospheric Pressures
Downloaded by IOWA STATE UNIV on March 29, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch014
N. C. Rigas, G. D. Svoboda, and J . T. Gleaves Department of Chemical Engineering, Washington University, St. Louis, MO 63130
Transient response techniques are used to investigate the activation of silver powder for ethylene epoxidation at vacuum and atmospheric pressures. Results indicate that the activation process is qualitatively the same in both pressure regimes. Numerical simulation of the process indicates that activation involves the concurrent incorporation of oxygen into surface and subsurface sites. The reaction selectivity parallels the incorporation of oxygen into the subsurface. The heterogeneous selective oxidation of ethylene to ethylene oxide over silver based catalysts has become a significant world wide industry since its development in the 1950's (1-3). Much industrial research has focused on modifying the catalyst composition to maximize catalyst selectivity and activity, and in modern processes ethylene is converted to ethylene oxide with a selectivity of approximately 80%. In spite of these advances, a variety of questions remains regarding the nature of the active silver surface that leads to epoxidation. One important aspect that is not well understood and has received relatively little attention in the literature is the mechanism of the activation process that leads to a catalytic surface. Observing changes in the chemical characteristics of a surface during activation can provide insights into the nature of the active/selective phase that is formed. A clearer understanding of the activation process is also important for producing more selective catalysts. From a fundamental viewpoint, silver's unique character makes it an extremely important catalytic system, and developing a greater understanding of its operation would add to our overall knowledge of oxidation processes. In silver catalyzed epoxidation, the selective oxygen species preferentially adds across the C=C double bond. However, oxygen covered silver surfaces can also give products similar to those found in metal oxide catalyzed reactions. For example, Madix and coworkers (4-6) recently found that butene can be oxidized to butadiene, dihydrofuran, furan, and maleic anhydride by oxygen atoms chemisorbed on silver single crystals. The initial activation of butene involves the abstraction of an acidic allylic hydrogen similar to the activation of propene by bismuth molybdate or the activation of butene by vanadyl pyrophosphate (7,8). In these reactions, oxygen reacts as a Bronsted base and a nucleophile. Atomically adsorbed oxygen on Ag (110) surfaces can also add across the C=C double bond of olefins such as norbornene, 0097-6156/93/0523-0183$06.25/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by IOWA STATE UNIV on March 29, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch014
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CATALYTIC SELECTIVE OXIDATION
styrene, or 2,3-dimethylbutene (6,9). In these cases, the C-H bonds are much more weakly acidic. Thus, chemisorbed oxygen atoms can display decidedly different chemical characteristics depending on the nature of the reacting hydrocarbon. The reactive nature of an oxygen adspecies may also be influenced by the electronic characteristics and structure of the silver surface. Experimental evidence and theoretical calculations indicate that more than one form of atomically adsorbed oxygen exists on a silver surface (10,11). It has been suggested that subsurface oxygen plays a key role in forming the selective surface for epoxidation (10,12). For example, Van Santen and coworkers (13) have proposed that activation of a silver surface initially involves the filling of subsurface sites and that ethylene oxide production does not occur until after the subsurface is fully populated. Recently, Bukhtiyarov and coworkers (14) investigated ethylene epoxidation on polycrystalline silver foils and determined that two forms of atomically adsorbed oxygen are present on surfaces active for ethylene epoxidation. In addition, they observed a strong promoting effect of carbon during the formation of subsurface oxygen (15) and suggested that carbon stabilizes surface defects (14) that are necessary for ethylene oxide production. These findings are consistent with studies by Grant and Lambert (16) that indicate that treatment of a Ag (111) surface with only oxygen is not sufficient for ethylene epoxidation, but that exposure to a reaction medium is necessary. Previous reaction studies have tended to focus on the reactivity of equilibrated silver catalysts of fixed composition. In this paper, ethylene oxidation is studied under nonsteady-state conditions in an effort to unravel aspects of silver activation and the role of subsurface oxygen in forming a selective catalyst. Results from a series of steady-flow and transient studies over polycrystalline silver metal powder are presented. These studies were carried out using a modified TAP reactor system (17) that is capable of performing steady-flow and transient experiments at vacuum conditions and at atmospheric pressures. The results provide some important insights into the mechanism of activation and the nature of the active catalytic surface. In addition, the results demonstrate a new method of monitoring the chemical changes in a catalyst surface during nonsteady-state processes. Modified TAP Reactor System The reactor system used in this study is a modified version of the TAP reactor system (17-19) developed by Gleaves and Ebner. A schematic of the system is shown in Figure 1. The principal modification is a movable high-pressure sealing assembly that permits operation from 10" to 2500 torr. When the sealing assembly is engaged, reactor effluent is split between a vacuum bleed and an external vent that is connected to a back-pressure regulator. The back-pressure regulator is used to control the reactor pressure in the range of 100 to 2500 torr. Effluent from the vacuum bleed is detected by a quadrupole mass spectrometer (QMS), and flow from the back pressure regulator is monitored by a gas chromatograph. When the high-pressure assembly is disengaged, the reactor is continuously evacuated, and all of the reactor effluent is sent to the QMS. Switching between the high-pressure (100-2500 torr) and vacuum (
