Selective epoxidation of conjugated diolefins

Details Selective epoxidation of conjugated diolefins Selective epoxidation of conjugated diolefins

2001-05-24

2002-05-14

Trinh, Ba K. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S534000, C549S537000

Reexamination Certificate

active

06388106

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to a gas phase process for the selective epoxidation of conjugated diolefins, or polyolefins, that contain allylic carbon-hydrogen bonds. More specifically, this invention pertains to the preparation of mono-epoxides by contacting in the gas phase a conjugated diolefin, or polyolefin, that contains allylic carbon-hydrogen bonds with molecular oxygen in the presence of a modified silver catalyst.
BACKGROUND OF THE INVENTION
Processes for the selective epoxidation of olefins which contain no allylic hydrogen atoms (non-allylic olefins) or olefins which contain hindered allylic hydrogen atoms are described by Monnier and Muehlbauer in U.S. Pat. Nos. 4,897,498, 4,950,773, 5,081,096, 5,138,077 and 5,145,968. Stavinoha and Tolleson disclose in U.S. Pat. No. 5,117,012 the selective epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene (EpB) by contacting a mixture comprising 1,3-butadiene, oxygen and methane with a supported silver catalyst at elevated temperatures. Similarly, Stavinoha et al. U.S. Pat. No. 5,362,890 disclose a continuous process for the preparation of the monoepoxide of an olefin reactant selected from norbornene, norbornadiene and olefins having the general formula
wherein R
1
is hydrogen or alkyl and R
2
is an aryl radical or the group
provided that the olefin reactants contain more than 2 carbon atoms and do not contain any allylic hydrogen atoms, by the steps of:
(1) continuously feeding a gas comprising about 3 to 30 mole percent of said olefin reactant, about 3 to 30 mole percent oxygen and about 40 to 90 mole percent of a paraffin hydrocarbon containing 2 to 6 carbon atoms wherein the oxygen:paraffin hydrocarbon mole ratio is in the range of about 0.03:1 to 0.75:1 to an epoxidation zone containing a supported, silver epoxidation catalyst and maintained at a temperature of about 175 to 230° C.; and
(2) continuously removing from the epoxidation zone a gas comprising about 0.5 to 3.5 mole percent of said monoepoxide of the olefin reactant, about 2 to 28 mole percent of said olefin reactant, about 2 to 28 mole percent oxygen and about 40 to 90 mole percent of said paraffin hydrocarbon.
U.S. Pat. No. 6,011,163 describes a process for the preparation of the monoepoxide of an olefin reactant selected from norbornene, norbornadiene and olefins having the general formula
wherein R
1
is hydrogen or alkyl and R
2
is an aryl radical or the group
provided that the olefin reactants contain more than 2 carbon atoms and do not contain any allylic hydrogen atoms. The process comprises contacting norbornene, norbornadiene and olefins of formula (I) with molecular oxygen in the presence of a modified silver catalyst at elevated temperature and pressure. U.S. Pat. No. 4,897,498 discloses the epoxidation of 1,3-butadiene to produce 3,4-epoxy-1-butene as the selective reaction product. However, the same modified or promoted catalysts, or even unmodified silver catalysts, are essentially non-selective for the epoxidation of propylene to form propylene oxide. Attempted epoxidation yields only carbon dioxide and water as the reaction products. Propylene is the simplest example of an allylic olefin, which is defined in Morrison and Boyd,
Organic Chemistry,
Allyn and Bacon, Inc., Boston, 1959, pages 154-155, as an olefin containing hydrogen atoms attached to a carbon atom that is adjacent to a C═C double bond. Thus, propylene has one —CH
3
group (and three allylic hydrogen atoms) which is allylic to the C═C double bond of propylene, e.g., C═C—CH
3
. On the other hand, olefins such as ethylene and 1,3-butadiene contain only vinylic hydrogen atoms that Morrison and Boyd define as hydrogen atoms that are attached to doubly-bonded carbon atoms, e.g., C═C—H. Furthermore, Morrison and Boyd state on page 155 that allylic hydrogen atoms are even easier to abstract than tertiary hydrogen atoms. In fact, Morrison and Boyd state that the reactivity sequence of hydrogen atoms can be written as
allylic>tertiary>secondary>primary>CH4>vinylic.
Furthermore, U.S. Pat. No. 5,770,746 discloses that the C—H bond energy of the allylic C—H bonds in propylene are 77 kcal/mole, while the C—H bond energy of the vinylic C—H bond in ethylene is 112 kcal/mole. Thus, it is readily apparent that abstraction of one of the allylic C—H bonds of propylene is preferable to addition of oxygen to the C═C double bond to form propylene oxide. Based on bond energetics alone, it is understandable why epoxidation of propylene, or any other allylic olefin, using molecular oxygen and promoted or unpromoted silver catalysts, results in non-selective oxidation, forming primarily carbon dioxide and water and little, if any, of the corresponding epoxide.
In view of the above stated relationships of olefin structure and reactivity, one would expect that the oxidation of any olefin which contains reactive allylic C—H bonds with molecular oxygen in the presence of a modified silver catalyst would not selectively produce an olefin epoxide. Thus, one skilled in the art would expect that the oxidation of conjugated diolefins such as isoprene (2-methyl-1,3-butadiene) or piperylene, (1,3-pentadiene), with molecular oxygen in the presence of a modified silver catalyst would not selectively produce an olefin epoxide, since each molecule contains a —CH
3
group which is allylic to a C═C double bond.
Epoxidation of conjugated diolefins containing allylic alkyl groups with allylic C—H bonds is described in U.S. Pat. No. 2,879,276 using molecular oxygen in a liquid phase process. Accoding to this U.S. Pat. No. 2,879,276, no catalyst was necessary although in the presence of catalysts considered to be free radical initiators, such as azodiisobutyronitrile or benzoyl peroxide, the rates of olefin epoxide formation were higher. The oxygen concentrations in the process also were critical, and it was necessary to maintain oxygen concentrations between 0.1-40 mm (or torr) oxygen pressure, with oxygen concentrations of 3-6 mm being most preferable.
The epoxidation of propylene using very extensively modified silver catalysts is disclosed by Gaffney and coworkers in U.S. Pat. Nos. 5,703,254, 5,770746, and 5,780,657. The catalyst compositions investigated by Gaffney comprised: 30-60% Ag, 0.5-3% K, 0.5-1% Cl, 0.5-2.5% Mo, 0.5-1% Re, 0.5-1% W, and balance CaCO
3
. These catalyst were prepared by ball milling slurries of the components with powdered CaCO
3
, thoroughly mixing the CaCO
3
with the other components. Thus, these catalyst compositions are not maintained on a support in the conventional sense of the term defined as supported catalysts.
Gaffney and coworkers demonstrated that the complex mixture of catalyst components and gas phase promoters was required for both activity and selectivity to propylene oxide. Thus, a catalyst composed of 50 weight percent Ag and 50 weight percent CaCO
3
gave only 3% selectivity to propylene oxide whereas a catalyst containing Ag, K, and CaCO
3
without any organic chloride or NO (nitric oxide) feed additive gave low conversion (<1%) and low selectivity (<3%). Co-feeding ethyl chloride and NO in the feedstream along with C
3
H
6
and O
2
enhanced both conversion (up to 10%) and selectivity (up to 60%). The levels of ethyl chloride used with the Gaffney process (typically about 200 ppm) are many times higher than the levels typically used in the epoxidation of the olefins. Ordinarily, organic chloride levels higher than 10-50 ppm cause catalyst deactivation.
BRIEF SUMMARY OF THE INVENTION
We have discovered that certain conjugated diolefins that contain allylic carbon-hydrogen bonds may be selectively converted to olefin epoxides by contacting in the gas phase the conjugated diolefin containing allylic carbon-hydrogen bonds with molecular oxygen in the presence of a modified silver catalyst. The present invention therefore provides a process for the preparation of the monoepoxide of an olefin reactant having the formula:
wherein R
1
, R
2
, R
3
, R
4
, R
5
, and R
6
are independently selected from hydrogen,

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