Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2000-09-18
2003-04-15
Solola, Taofiq (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06548682
ABSTRACT:
The invention relates to a catalytic gas phase process for the preparation of epoxides from unsaturated hydrocarbons by oxidation with molecular oxygen in the presence of molecular hydrogen and catalysts for this process which are coated with nanoscale gold particles.
In general, direct oxidations of unsaturated hydrocarbons with molecular oxygen in the gas phase do not proceed below 200° C.—even in the presence of catalysts—and it is therefore difficult to prepare oxidation-sensitive oxidation products, such as e.g. epoxides, alcohols or aldehydes, selectively, since the secondary reactions of these products often proceed faster than the oxidation of the olefins employed themselves.
Propene oxide is one of the most important base chemicals of the chemical industry. Its field of use lies in the plastics sector with a proportion of more than 60%, specifically for the preparation of polyether-polyols for the synthesis of polyurethanes. In addition, even greater proportions of the market are covered by propene oxide derivatives in the field of glycols, in particular in lubricants and antifreezes.
About 50% of propene oxide worldwide is currently synthesized via the “chlorohydrin process”. A further 50%, with an increasing trend, is supplied by the “oxirane process”.
In the chlorohydrin process (F. Andreas et al.; Propylenchemie [Propylene chemistry], Berlin 1969), chlorohydrin is first formed by reaction of propene with HOCl (water and chlorine), and propene oxide is then formed from the chlorohydrin by splitting off HCl with a base. The process is cost-intensive, but with appropriate optimization has a high selectivity (>90%) with high conversions. The loss of chlorine in the chlorohydrin process in the form of worthless solutions of calcium chloride and sodium chloride and the associated high salt load in the waste water led early on to the search for chlorine-free oxidation systems.
The oxidation processes use organic compounds instead of the inorganic oxidizing agent HOCl to transfer oxygen to propene. This indirect epoxidation is based on the fact that in the liquid phase organic peroxides, like hydroperoxides, can transfer their peroxide oxygen selectively to olefins to form epoxides. During this process, the hydroperoxides are converted into alcohols and the peroxycarboxylic acids are converted into acids. Hydroperoxides are produced from the corresponding hydrocarbon by autoxidation with air or molecular oxygen. A serious disadvantage of indirect oxidation is the economic dependence of the propene oxide value on the market value of the coupled product and the cost-intensive preparation of the oxidizing agent.
With titanium silicalite (TS 1) as a catalyst (Notari et al., U.S. Pat. No. 4,410,501 and U.S. Pat. No. 4,701,428), it was possible for the first time to epoxidize propene with hydrogen peroxide in the liquid phase under very mild reaction conditions with selectivities of >90% (Clerici et al., EP-A 230 949).
Propene oxidation is also achieved with a low yield in the liquid phase on titanium silicalites containing platinum metal with a gas mixture comprising molecular oxygen and molecular hydrogen (JP-A 92/352771).
U.S. Pat. No. 5,623,090 (Haruta et al.) describes a gas phase direct oxidation of propene to propene oxide with a 100% selectivity for the first time. This is a catalytic gas phase oxidation with molecular oxygen in the presence of the reducing agent hydrogen. Commercially available titanium dioxide coated with nanoscale gold particles is used as the catalyst. Nanoscale gold particles here are understood as meaning particles having a diameter in the nm range. The propene conversion and the propene oxide yield are stated as a maximum of 2.3%. The Au/TiO
2
catalysts described achieve the approx. 2% propene conversion for only a very short time; e.g. the typical half-lives at moderate temperatures (40-50° C.) are still unsatisfactory (Haruta et al., 3rd World Congress on Oxidation Catalysis 1997, p. 965-970, FIG.
6
). This process thus has the disadvantage that the yield of epoxide, which is in any case low, is severely reduced further by rapid deactivation.
For economic use, the development of catalysts with significantly better initial activities with a greatly increased catalyst life therefore continues to be absolutely necessary.
The invention therefore provides a process for the oxidation of unsaturated hydrocarbons in the gas phase in the presence of a hydrogen/oxygen mixture, if appropriate with the addition of an inert gas, on a supported catalyst coated with gold particles, characterized in that a calcined catalyst which has been prepared from optionally doped titanium oxide hydrate and is coated with nanoscale gold particles is employed.
The process according to the invention can be used on all olefins. Since the gas phase oxidation expediently takes place at low temperatures (<120° C.) on the basis of the higher selectivities which can be achieved, it is possible to oxidize all unsaturated hydrocarbons from which are formed those oxidation products of which the partial pressure is sufficiently low for the product to be removed constantly from the catalyst. Unsaturated hydrocarbons having up to twelve carbon atoms, in particular ethene, propene, 1-butene or 2-butene, are preferred.
The preparation of the catalyst has a decisive influence on the catalyst activity. The catalysts are preferably prepared here by the “deposition-precipitation” method. In this, an aqueous solution of an inorganic or organic gold compound is added dropwise to a stirred aqueous suspension of the titanium oxide hydrate used as the catalyst support. A water-containing solvent is preferably used. Other solvents, such as e.g. alcohols, can also be employed. When bases (e.g. sodium carbonate or alkali metal or alkaline earth metal hydroxide solution) are added to this gold(III) salt solution up to a pH of 7-8.5, gold precipitates out on the titanium oxide hydrate surface in the form of Au(III) chlorohydroxo or oxohydroxo complexes or as gold hydroxide. To bring about a uniform deposition of ultrafine gold particles, the change in the pH must be controlled by slow dropwise addition of this alkaline aqueous solution. Since in an excess of alkali metal hydroxide solution the gold compounds deposited dissolve again to form aurates ([Au(OH)
4
]
31
or AuO
2
−
), for this reason a pH of between 7-8.5 must be established.
Precipitated gold(III) hydroxide cannot be isolated as such, but is converted into the metahydroxide AuO(OH) or Au
2
O
3
on drying, which decomposes to elemental gold with the release of oxygen when calcined above 150° C. The nanoscale gold particles generated in this way are immobilized firmly adhering to the support surface, and have particle diameters of <10, preferably <6 nm. The amount of gold applied to the support depends on various variables, thus e.g. on the surface area, on the pore structure and on the chemical nature of the surface of the support. The properties of the support thus play an important role for the catalytic action.
Surprisingly, it has been found that when amorphous hydrated titanium oxide hydrates of high surface area are employed for coating with gold, the catalytic activities in the epoxidation of propene to propene oxide are drastically higher. These titanium oxide hydrates employed have water contents of 5 to 50 wt. % and surface areas of >50 m
2
/g. Initial propene oxide yields of >4% e.g. are obtained with a catalyst which has been prepared on the basis of titanium oxide hydrate and comprises 0.5 wt. % gold.
The water content of the titanium oxide hydrates employed is usually between 5 and 50 wt. %, preferably between 7-20 wt. %. In the preparation of the catalyst, gold is applied to the titanium oxide hydrate in a precipitation step in the form of Au(III) compounds. However, the support loaded in this manner still has no catalytic activity. Only calcining in a stream of air at 350 to 500° C. makes a catalytically active material out of this precursor.
Low sulfate contents in the TiO(
Dilcher Herbert
Dorf Ernst-Ulrich
Lücke Bernhard
Schild Christoph
Schülke Ulrich
Bayer Aktiengesellschaft
Gil Joseph C.
Mrozinski, Jr. John E.
Solola Taofiq
Whalen Lyndanne M.
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