Technology
New catalyst aids oxidation of propylene Silver ketenide promotes direct air oxidation of ethylene and propylene with high selectivity and conversion On paper, the oxidation of ethylene and propylene to their corresponding oxides appears to be fairly straightforward. In commercial practice, however, it is fraught with problems requiring careful control of operating conditions. And in the case of propylene oxide, interme diate steps are involved and coproducts that are often unwanted are formed. Sil ver-based oxidation catalysts are a key factor in producing ethylene oxide. But these aren't suitable for converting propylene to propylene oxide in com mercial yields. Now, chemistry professor Derek Bryce-Smith, Dr. Ernest Blues, and their associates at the U.K.'s University of Reading, near London, have developed what they claim to be an unusual type of silver catalyst which promotes direct air oxidation of ethylene and propylene to the oxides. Ethylene oxidation pro ceeds with a selectivity of 80 to 90% or more and at a conversion rate per pass of 50 to 80%, without the need for pro moters. By comparison, conventional, unpromoted silver-based catalysts gen erally have a selectivity factor of 65% or lower, and a conversion factor of about 30%, Dr. Bryce-Smith points out. An extension of the work has led to discovery of a further class of new,
highly active, transition metal catalysts. These promise to be useful in a number of oxidation reactions, including the conversion of hydrocarbons in automo bile exhaust gases to carbon dioxide and water. Dr. Bryce-Smith's discovery stems from his studies of metal ketenides, a hitherto unknown class of compounds. They may be formed in a variety of ways. Silver ketenide, for example, precipitates out in quantitive yields on mixing silver acetate, in pyridine or triethylamine, with acetic anhydride at room temperature. The anhydride interacts with the tertiary amine to form ketene which, in turn, converts the silver acetate to silver ketenide. "The reaction reflects the acidic na ture of ketene's hydrogen atoms, a prop erty that hadn't been realized before now," Dr. Bryce-Smith notes. Silver ketenide crystals differ mark edly from those of conventional silver compounds. They have a graphitic, or lamellar, structure consisting of layers of silver atoms laid down one on top of another. In each layer, the distance between adjacent silver atoms is 2.84 Α., which is remarkably short. Pendent ketene moieties separate the layers by a distance of 6 A. This distance may be more than doubled by insertion of other molecules such as pyridine. Con ventional silver metal crystals, by com parison, have a face-centered cubic structure, with the adjacent atoms more widely spaced at 2.88 A. apart. Also, conventional silver is diamagnetic, whereas silver ketenide can be made in nondiamagnetic form, its magnetic susceptibility being positive in sign and varying strongly with the field
Dr. Bryce-Smith (left) and Dr. Blues discuss crystal makeup of new catalyst
strength. This magnetic susceptibility likely reflects the unusual bonding of the silver atoms in the uniatomic layers. By heating or chemical treatment, it's possible to remove the ketene moi eties from silver ketenide. (The deg radation is accompanied by evolution of carbon suboxide gas—C3O2—in a high state of purity, suggesting the reaction as a practical preparative route to the hard-to-make chemical.) When the ketene "props" are elimi nated, the silver layers collapse. X-ray diffraction analyses of the resulting silver crystals show that they have the conventional face-centered cubic struc ture. "We believe, however, that they contain 'islands' or regions of abnormal structure inherited from the ketenide crystal form," Dr. Bryce-Smith con tends. Supporting this view is the fact that silver resulting from silver ketenide decomposition retains its abnormal magnetic characteristic. Both silver ketenide and the silver that remains after its decomposition promote direct air oxidation of ethylene. In some cases, the silver alone is a more active catalyst than its ketenide pre cursor. More important, though, is the fact that unlike silver ketenide, the allsilver crystals can withstand heating to several hundred degrees centigrade, which, in turn, boosts the oxidation rate. The Reading chemists claim that their studies and those of others show that the new form of silver "handsomely outperforms" conventional silver cata lysts in air oxidation of ethylene to eth ylene oxide. Moreover, the reaction is extremely "clean." Carbon dioxide is the only by-product and it is formed in small quantities. "This should be an enormous advantage in an industrial process," Dr. Bryce-Smith points out, "because the degradation of ethylene to carbon dioxide is highly exothermic, resulting in severe heat-transfer prob lems in present-day operations." Indeed, it is this fact that compels ethylene oxide makers to hold conversion rates down to about 30% per pass, he notes. To date, the Reading chemists' pro pylene oxidation studies using the new silver catalyst aren't as far along as those for ethylene oxidation. But pre liminary trials have resulted in pro pylene oxide yields between 20 and 30%. Although these would have to be in creased to 50% and beyond to make the route commercially attractive, existing ethylene oxidation catalysts aren't ef fective in promoting propylene oxida tion.
EVANS
BENZYL MERCAPTAN (a-Toluenethiol)
CHrSH 2
Inquiries are invited regarding the manufacture of your spe cial requirements for organic sulfur compounds in com mercial quantities. Data Sheets and Samples available on request.
Ε\^Γ\Ξ CHemenci 90 Tokeneke Road
irvo
Darien, Connecticut 06820 Phone: 203-655-8741 Cable: EVANSCHEM
TWX: 710-457-3356
Dr. Bryce-Smith and his team have also made a wide series of ketenides of other metals. There is evidence that some of these may be better for the direct oxidation of propylene. "It is incredible," he remarks, "that such simple molecules have so long escaped observation." The most promising aspect of the propylene oxidation studies so far is that the reaction is direct and coproducts aren't involved. In contrast, current commercial routes to propylene oxide entail intermediate steps. For instance, propylene is first converted to the chlorohydrin, which is then treated with a base to obtain the oxide. Alternatively, a compound such as acetaldehyde is first peroxidized and then reacted with propylene to form propylene oxide to gether with acetic acid coproduct. "The important thing to realize is that these new silver catalysts so far are ' u n t u n e d , ' " Dr. Bryce-Smith says. "Developing a catalyst system usually entails a considerable degree of 'tuning' to optimize its activity." For example, there is still much work ahead to deter mine the best support system to use. As to the catalysts' effective life, he notes that there has been no detectable dete rioration of the catalysts during pro longed periods of bench-scale trials. In a parallel development, Dr. BryceSmith and Dr. Blues are looking into the formation of metal crystals from unstable naphthalene-metal complexes. The crystals precipitate out when a metal halide is added to a solution of an alkali metal naphthalide or benzenide in diglyme. Crystals of gold, silver, copper, plati num, palladium, ruthenium, rhodium, iron, cobalt, and nickel that the Reading workers have obtained in this way have unique properties. Gold and platinum, for example, dissolve readily in hot, 68% nitric acid. All of them, except copper, silver, and gold, are instantly pyrophoric in air. The "active" metals have conven tional crystal structures. But their crystallite size is unusually small. "Active" cobalt has a crystallite size too small, probably less than 10 Α., to provide an x-ray diffraction pattern. The "active" metal crystals are out standing in their ability to catalyze oxidation of organic compounds. "Ac tive" ruthenium, palladium, and plati num, for example, promote oxidation of benzene and toluene to carbon dioxide and water in quantitative yields under mild conditions, and they oxidize pri mary and secondary aliphatic alcohols to the corresponding aldehydes and ketones at 20° C. "Active" palladium oxidizes propylene to propylene oxide, acetone, and other products. And active silver can convert ethylene to very high yields of ethylene oxide. Proper choice of catalyst support systems, adds Dr. Bryce-Smith, can radically change the character of some of the catalysts and result in complete burning of hydro carbons to carbon dioxide and water.
ALUMINUM
Alcoholates · Chelates • Acylates ALCOHOLATES AIP ASB
Aluminum Isopropoxide Aluminum Tri-(secButoxide )
INTERMEDIATES AISE
ASE-M
ASB-.4S
AIE-M
Aluminum Isopropoxide Stéarate Acetoacetic Ester Chelate Aluminum Disecondary Butoxide Acetoacetic Ester Chelate Aluminum (sec-Butoxide) 2.6 (Stéarate) Aluminum Diisopropoxide Acetoacetic Ester Chelate
OR PICK YOUR OWN COMBINATION! Chattem makes organo-aluminum compounds for coatings, lubricants, carbonyl reduction, resin systems, w a t e r p r o o f i n g compounds, printing inks and custom or development items.
WRITE OR PHONE COLLECT
§ chattem chemicals DEPT. CEN, 1715 W. 38th ST., CHATTANOOGA, TENN. 37409 PHONE 615/821-4571
Nov. 20, 1972 C&EN
23
