Alkylene oxides production from C7 to C22 olefins using molten nitrate salt catalyst

A process for producing an alkylene oxide or mixture of alkylene oxides by a reaction which comprises reacting a C7 to C22 olefin or mixture thereof with an oxygen-containing gas in the presence of at least one molten nitrate salt.

BACKGROUND OF THE INVENTION 
Alkylene oxides (vicinal epoxy alkanes), and particularly propylene oxide, 
are very valuable and widely used chemicals. They have been polymerized 
with a wide variety of monomers to yield polymers which are useful in 
coating compositions and in the manufacture of molded articles. Alkylene 
oxides have also been reacted with alcohols to yield monalkyl ethers which 
have utility as solvents in many commercial processes and which are useful 
as components for synthetic turboprop and turbojet lubricants. 
There are many methods known in the art for the production of alkylene 
oxides and, most notably, propylene oxide. One of the oldest methods is 
the so-called "chlorohydrin process" which involves the reaction of 
chlorine and water to form hypochlorous acid which is then reacted with 
propylene to form propylene chlorohydrin. The propylene chlorohydrin is 
then dehydrohalogenated to yield propylene oxide. Another method to obtain 
propylene oxide is by the liquid phase oxidation of propylene with organic 
peracids. Still another method involves the liquid phase oxidation of 
propylene with t-butyl hydroperoxide and/or ethylbenzene hydroperoxide. 
The aforementioned known methods have serious disadvantages associated 
therewith. For example, the "chlorohydrin process" requires the use of 
chlorine which is relatively expensive and corrosive in nature, requiring 
special handling and expensive equipment. Additionally, the chlorohydrin 
saponification to propylene oxide consumes alkali chemicals such as 
caustic soda or lime, producing a large aqueous waste stream containing 
chloride salts, which require costly treatment prior to discharge from the 
plant. The oxidation of propylene with peracids is a potentially dangerous 
operation and expensive equipment is needed to guard against potentially 
explosive hazards when working with the peracids. Another disadvantage of 
this method is the high cost of peracids. The t-butyl hydroperoxide and 
ethylbenzene hydroperoxide processes have the disadvantages of being 
capital-intensive, multi-step, rather complicated processes. Furthermore, 
these processes require co-feedstocks of isobutane or ethylbenzene, thus 
constraining the practical utility of the processes for propylene oxide 
manufacture. 
Another method which has received considerable attention in the literature 
is the direct oxidation of hydrocarbons with an oxygen-containing gas. 
This method suffers from the disadvantage that it is not specific for the 
production of alkylene oxides but produces a variety of other compounds 
including acids, esters, ethers, and oxides of carbon including carbon 
monoxide and carbon dioxide. The reaction does, however, possess two 
attributes which recommend it highly for commercial utilization, i.e., 
inexpensiveness of starting materials and simplicity of operation. It is 
primarily for these reasons that much attention in recent years has been 
directed to improvements in methods for the production of alkylene oxides 
from the direct oxidation of hydrocarbons even though the producer must 
necessarily contend with the concurrent production of a variety of 
undesired products. 
By way of illustration, the prior art methods which attempted to produce 
propylene oxide by the oxidation of propane such as that disclosed in U.S. 
Pat. No. 2,530,509, assigned to Linde Air Products Company, were only 
partially successful. The majority of the prior art methods used 
conventional vertical columns and differed from each other by variations 
in lengths and diameter of the column, temperature, pressure, etc. 
However, all of these methods suffered one common disadvantage--the 
temperature of the reactants varied throughout the length of the column. 
The temperature variations are easily explained since the oxidation 
reactions are exothermic and the amount of heat evolved differs with each 
reaction which is taking place. Thus, at various increments along the 
tube, conditions existed which favored the direction of the oxidation to 
products other than propylene oxide. These prior art methods necessitated 
the use of elaborate and expensive cooling apparatus. 
Further developments in the art constituted attempts to maximize the 
desired olefin oxide production while minimizing by-product formation. For 
example, U.S. Pat. No. 3,132,156, assigned to Union Carbide Corporation, 
discloses the vapor phase oxidation of saturated aliphatic hydrocarbons to 
olefin oxides. The method described in this '156 patent is said to provide 
enhanced olefin oxide production as high as 46.2 lbs per 100 lbs of 
C.sub.3 consumed which calculates to be about 33 percent (molar) 
selectivity. While this level os selectivity constituted an improvement, 
it remains less than might be desired from a commercial standpoint. 
Likewise, Canadian Pat. No. 968,364, assigned to Union Carbide 
Corporation, discloses the indirect oxidation of olefins via the oxidation 
of methanol to a free radical intermediate which in turn, epoxidizes the 
olefin. However, the indirect oxidation method disclosed in the Canadian 
'364 patent has the disadvantage of requiring the use of a solvent 
together with subsequent solvent separation step(s). Accordingly, new 
methods of producing olefin oxides that combine enhanced selectivity with 
a simple, inexpensive process would be highly desirable. 
SUMMARY OF THE INVENTION 
The present invention relates to a process for producing an alkylene oxide 
or mixture of alkylene oxides by a reaction which comprises reacting an 
olefin having from 7 to 22 carbon atoms per molecule, or mixture thereof, 
with an oxygen-containing gas, said olefin and said oxygen-containing gas 
being gaseous reactants, by contacting said gaseous reactants with a bath, 
stream, spray or mist of at least one molten nitrate salt catalyst, said 
catalyst being present in an amount sufficient to absorb any heat 
generated during said reaction while maintaining an essentially constant 
reaction temperature, said reaction being conducted at a reaction 
temperature of between about 135.degree. C. and about 600.degree. C. and a 
reaction pressure of between about 1 and about 50 atmospheres.

DETAILED DESCRIPTION OF INVENTION 
Several factors will affect the reactant conversion to alkylene oxide and 
the selectivity of alkylene oxide production vis-a-vis by-product 
production in accordance with the process of the present invention. These 
factors include, for example: the contact time of the molten salt with the 
oxygen-containing gas, the temperature of the reactor product gases, the 
molten salt temperature, the molten salt catalyst composition, the feed 
gas temperature, the feed gas composition, the feed gas pressure, and the 
co-catalyst employed. 
The oxygen-containing gas useful as a reactant in the present invention can 
be any such gas. Typically, air is employed as the oxygen-containing gas 
based upon its ready availability. However, other oxygen-containing gases 
such as pure oxygen may be employed if desired, and the use of oxygen is 
expected to be preferred in a commercial setting. 
The olefin useful in the present invention can be broadly defined as an 
epoxidizable, olefinically-unsaturated hydrocarbon compound having from 7 
to 22 carbon atoms, preferably from 7 to 15 carbon atoms, more preferably 
from 7 to 12 carbon atoms, most preferably from 7 to 10 carbon atoms. This 
definition is intended to include terminal olefins selected from the group 
consisting of monofunctional and difunctional olefins having the following 
structural formulas respectively: 
##STR1## 
wherein R.sub.1 is hydrogen or an alkyl chain, straight or branched, 
having 1 to 20 carbon atoms and R.sub.2 is an alkyl chain, straight or 
branched, having 1 to 20 carbon atoms with the proviso that R.sub.1 plus 
R.sub.2 together have at least 5 carbon atoms; and 
##STR2## 
wherein R.sub.1 and R.sub.2 are hydrogen atoms or alkyl chains having 1 to 
10 carbon atoms and R' is from 2 to 10 methylene groups. The definition 
also includes cyclic olefins and internal olefins. The ring portions of 
the cyclic olefins can have up to 10 carbon atoms and one unsaturated bond 
and can be substituted with one or two alkyl radicals having 1 to 10 
carbon atoms. The cyclic olefins are typically represented by the 
following structural formula: 
##STR3## 
wherein R.sub.1 and R.sub.2 are olefin radicals having 1 to 4 carbon atoms 
and R.sub.3 and R.sub.4 represent hydrogen atoms, or one or the two alkyl 
radicals, straight or branched chain, having 1 to 10 carbon atoms. The 
internal olefins are represented by the following structural formula: 
EQU R.sub.1 --CH.dbd.CH--R.sub.2 
wherein R.sub.1 and R.sub.2 are straight chain or branched chain alkyl 
radicals having 1 to 10 carbon atoms with the proviso that R.sub.1 plus 
R.sub.2 together have at least 5 carbon atoms. 
The olefins, and mixtures thereof, useful as reactants in accordance with 
the present invention generally have up to, but do not exceed, 22 carbon 
atoms per molecule, preferably not more than 12 carbon atoms per molecule. 
When a straight-chain molecule is employed, it is more preferred that such 
molecule not have more than ten carbon atoms. When a cyclic compound is 
used, it is more preferred that the cyclic compound not have more than 12 
carbon atoms per molecule. A preferred reactant within this group is 
styrene. 
Representative other olefins are heptene-1, octene-1, hexene-2, hexene-3, 
oxtene-2, heptene-3, pentadecene-1, octadecene-1, dodecene-2, 
cycloheptene, 2-methylheptene-1, and 2,4,4-trimethylpentene-1. 
The olefin gas is preferably preheated to prevent condensation in the line 
delivering this gas to the reactor. Alternatively, both the 
oxygen-containing gas and the olefin gas (collectively referred to herein 
as "the feed gases") can be preheated to prevent condensation in any of 
the feed lines. However, in the absence of preheat, the molten nitrate 
salt will rapidly heat the feed gases up to reaction temperature. If the 
feed gas is preheated, it preferably is maintained at at least about 
100.degree. C. in the feed gas line(s). 
The molten nitrate salt(s) catalyst is generally maintained at a 
temperature sufficient to keep the salt(s) in a molten condition. 
Preferably, the temperature is maintained between about 135.degree. C. 
(275.degree. F.) and about 600.degree. C. (1,000.degree. F.), more 
preferably between about 200.degree. C. and about 600.degree. C., more 
preferably between about 250.degree. C. and about 550.degree. C. during 
the reaction in accordance with the present invention. 
The specific temperature selected is based upon the melting point of the 
particular molten nitrate salt chosen. For example, mixtures of molten 
lithium and potassium nitrate can be suitably employed at a temperature as 
low as about 280.degree. F., and hence, this temperature may be employed 
when using lithium nitrate. In the selection of a suitable molten nitrate 
salt bath temperature, it is important to choose a temperature below the 
thermal decomposition temperature for the particular molten nitrate salt 
chosen. In addition, it is important to maintain a sufficient isotherm 
across the molten nitrate salt bath so as to avoid crust formation of the 
nitrate salt in the bath. Such a crust formation in the nitrate salt bath 
can cause localized overheating of gases trapped by the crust in the bath 
and an associated "runaway" oxidation reaction due to overheating of the 
gases in the bath. In order to maintain a bath isotherm, constant stirring 
of the molten nitrate salt bath is preferred. Alternatively, the molten 
salt can be circulated by conventional means, such as the use of internal 
draft tubes or external pumping loops. 
The nitrate salt catalyst used may be any one of the alkali or alkaline 
earth nitrates such as lithium, sodium, potassium, rubidium, cesium, 
magnesium, calcium, strontium, or barium or mixtures thereof. In addition, 
the nitrate salts can be used in mixtures with other salts such as 
chlorides, bromides, carbonates, sulfates, and phosphates. Generally, the 
content of the other salt(s), when present, should be restricted to less 
than 60 percent by weight based upon the weight of the total melt and in 
most cases their contents should not exceed about 25 percent of the total 
melt. 
The ratio of olefin to oxygen in the oxygen-containing gas in the reactor 
can vary over a wide range. However, in accordance with the present 
invention, it has now been found that enhanced selectivity of alkylene 
oxide product is achieved by maintaining a relatively low amount of oxygen 
relative to the amount of olefin fed into the reactor. For example, when 
reacting styrene with oxygen in a molten potassium nitrate salt column at 
atmospheric pressure, a ratio of between about 1 and about 20 volume 
percent of oxygen, e.g., about 5 volume percent oxygen to about 95 volume 
percent styrene is found to provide an enhanced selectivity of styrene 
oxide. When using air as the oxygen-containing gas, it is preferably 
employed in an amount of between about 5 and about 75 volume percent based 
upon the total amount of air plus styrene employed in the reaction. 
Another consideration in the selection of the amount of styrene or other 
olefin to use as a feed is the high partial pressure of the olefin which 
in high concentrations can cause thermal cracking of the olefin reactant 
itself. Therefore, when conducting the oxidation reaction at an elevated 
pressure, viz 75 psig, it is preferred to "cut" the amount of styrene in 
the illustrative example to 75 volume percent and utilize an inert blanket 
("diluent") gas, such as nitrogen, to provide the remaining 20 volume 
percent of feed gas. Alternatively, the diluent gas may be comprised of 
mixtures of oxidation by-product gases generally readily obtainable from 
the styrene oxide purification operations downstream of the molten salt 
reactor. 
In the selection of the ratio of the volume of oxygen-containing gas 
relative to the volume of olefin employed in the reaction mixture, the 
range of ratios which might pose a flammability hazard should be avoided, 
as is well known. For example, when utilizing an air/styrene reactant 
mixture at atmospheric pressure, the range of below 7 volume percent of 
styrene based upon total air plus styrene should be avoided. 
A co-catalyst can also be utilized in accordance with the present 
invention. For example, when an elemental metal, or the oxide or hydroxide 
thereof, such as palladium, silver or molybdenum oxide, is employed as a 
co-catalyst in conjunction with the molten nitrate salt catalyst, it is 
possible to lower the reaction temperature for the particular nitrate salt 
selected and/or enhance the selectivity or conversion to the desired 
olefin oxide. By way of illustration, a palladium on alumina co-catalyst 
or a silver co-catalyst such as silver nitrate is expected to similarly 
reduce the required reaction temperatures. The use of these metal 
co-catalysts are preferred when the reaction is conducted at atmospheric 
pressure. At superatmospheric pressure, an alkali metal hydroxide 
co-catalyst, such as sodium hydroxide, has been found to be particularly 
advantageous in providing enhanced selectivity to the desired product. In 
addition, in a continuous process employing caustic recycle, the alkali 
metal hydroxide is expected to enhance the desired product distribution by 
removing by-product carbon dioxide by forming alkali metal carbonate. 
If used, the co-catalyst is generally employed in a catalytically effective 
amount, generally in an amount of less than about 5 (preferably between 
about 0.5 and about 5, more preferably in an amount between about 0.5 and 
about 3) weight percent based on the total amount of co-catalyst plus 
molten salt catalyst. 
The molten salt catalyst in which the co-catalyst (if used) is suspended or 
dispersed, helps to maintain the co-catalyst at a constant desired 
temperature or isotherm. The maintenance of the co-catalyst in such an 
isotherm makes it possible to reduce or avoid the problems of co-catalyst 
de-activation that might otherwise be encountered in a non-isothermal 
system due to overheating of the co-catalyst itself or due to thermal 
degradation of product to a tarry by-product which can coat, and thus 
de-activate, the catalyst. 
Typically, the molten salt(s) is employed in an amount on a weight basis of 
between about 5 times and about 100 times (preferably between about 5 
times and about 50 times) the total weight of the reactants employed. 
The molten salt(s), in addition to functioning as a catalyst and as an 
isothermal medium for the co-catalyst, if used, also serve as a 
temperature regulator. More specifically, the molten nitrate salt(s) have 
a high heat absorption capacity, enabling them to absorb large quantities 
of heat during the exothermic oxidation reaction while maintaining an 
essentially constant reaction temperature and thereby preventing a runaway 
reaction. The absorbed heat of reaction from this exothermic oxidation may 
be employed in the process of the present invention to help maintain the 
molten salt in a molten state and/or to heat the gaseous reactants to 
reaction temperature. 
In a preferred embodiment of the present invention, a mixture of potassium 
and sodium molten nitrate salts is employed comprising between about 20 
and about 80 weight percent of sodium nitrate, preferably between about 45 
and about 65 weight percent of sodium nitrate based upon the total amount 
of sodium nitrate and potassium nitrate in the molten salt mixture. 
Another preferred molten mixture is a mixture of sodium, lithium and 
potassium nitrate salts, preferably in a ratio of between about 10 and 
about 30 weight percent of lithium nitrate and between about 15 and about 
75 weight percent of sodium nitrate based on the total amount of the 
mixture. 
One method of contacting the gaseous reactants in the presence of the 
molten nitrate salt is by bubbling the reactants through a bath of the 
molten salt. If the gaseous reactants are bubbled into the bottom of the 
bath or column containing the molten nitrate salt, the contact time of the 
reactants with the molten salt catalyst is equal to the "rise time" of the 
reactants through the bath or column. Thus, the contact time can be 
increased by increasing the length of the molten nitrate salt bath or 
column. An alternate method of contacting the gaseous reactants in the 
presence of the molten salt would be to pass the gaseous reactants through 
a reactor countercurrently to a spray or mist of the molten salt. This 
latter method is preferred since it provides for enhanced surface area 
contact of the reactants with the molten salt. Still another method of 
contacting the gaseous reactants with molten salt would be to inject the 
reactants into a circulating stream of molten salt, wherein the kinetic 
energy of both streams is utilized to provide intimate mixing through the 
application of nozzles, mixers, and other conventional equipment. This 
latter method is expected to be preferred in a commercial setting. These 
methods are only illustrative of types of reaction systems which may be 
employed in the practice of this disclosure. Other conventional methods of 
gas-liquid contact in reaction systems may also be employed. 
The olefin feed gas(es) can be passed into the molten nitrate 
salt-containing reactor using a separate stream (e.g. feed tube) from the 
stream delivering the oxygen-containing gas to the reactor. Alternatively, 
the reactant gases can be fed into the reactor together in a single 
stream. In a preferred embodiment of the present invention, two 
co-axially-mounted feed gas tubes are employed. The co-axial mounting of 
the feed gas tubes has been found to reduce or minimize the back-up of 
molten salt into an unpressurized feed tube if pressure is temporarily 
lost in either (but not both) feed tube. Mixing of the gaseous reactants 
prior to, or at the point of, the gas(es) inlet into the reactor is 
desired in order to facilitate the oxidation reaction. Mixing is suitably 
accomplished using an impingement mixer or sparger tube. 
If a molten salt bath is used, the feed gas is preferably bubbled into the 
molten nitrate salt-containing reactor using a sparger. If used, the 
sparger is preferably positioned in the molten nitrate salt to a sparger 
exit port depth of between about 2 and about 1000 centimeters, preferably 
between about 10 and about 200 centimeters, depending upon the size of the 
reactor utilized and the overall depth of the molten salt in the reactor. 
Alternatively, the gas can be fed directly into the bottom of the reactor 
by a feed tube. The feed gas tubes are preferably co-axially mounted so 
that in the event of a loss of pressure in either gas tube, the gas in the 
other tube will maintain sufficient pressure to keep the molten salt from 
backing up into the unpressurized feed gas tube. 
This process can be run in a batchwise or continuous operation, the latter 
being preferred. The order of introduction of the reactants is determined 
by the operator based on what is most safe and practical under prevalent 
conditions. Generally, the desirability of avoiding flammable gas mixtures 
throughout the reaction and subsequent product separation systems will 
dictate the desired procedures. 
The process can be carried out by feeding a mixture of olefin, inert gas, 
and oxygen into a reaction vessel containing molten nitrate salt. The 
reaction vessel can be glass, glass-lined metal, or made of titanium. For 
example, a glass-lined stainless steel autoclave can be used, although, 
even better from a commercial point of view, is an unlined type 316 
stainless steel autoclave (as defined by the American Iron and Steel 
Institute). A tubular reactor made of similar materials can also be used 
together with multi-point injection to maintain a particular ratio of 
reactants. Other specialized materials may be economically preferred to 
minimize corrosion and contamination of the molten salt and products, or 
to extend the useful life of the reaction system. 
Some form of agitation of the molten salt(s)/feed gas mixture is preferred 
to avoid a static system and insure the homogeneity of the molten salt, 
agitation help prevent crust formation of the salt(s) at the head gas/salt 
interface in the reactor. This can be accomplished by using a mechanically 
stirred autoclave, a multi-point injection system, or a continuous 
process, e.g., with a loop reactor wherein the reactants are force 
circulated through the system. Sparging can also be used. In the subject 
process, it is found that increased rates of reaction are obtained by good 
gas-liquid contact provided by agitation of the molten salt/gas mixture. 
The process of the present invention is suitably carried out at 
atmospheric, subatmospheric or superatmospheric pressure. Typically, the 
process is effected at superatmospheric pressures of up to about 100 
atmospheres, preferably between about 1 atmosphere and about 50 
atmospheres, more preferably between about 1 atmosphere and about 35 
atmospheres. The most preferred pressure range is between about 1 and 
about 25 atmospheres. 
It is to be understood that by-products are also produced during the 
reaction. For example, some oxidative cracking of the styrene feed is also 
effected, particularly at higher temperatures within the hereinabove noted 
temperature range, and therefore, the reaction conditions are generally 
controlled to minimize such production. The separation of the resulting 
by-products in order to recover the desired product may be effected by a 
wide variety of well-known procedures such as: absorption in water 
followed by fractional distillation, absorption, and condensation. 
The following examples are intended to illustrate, but in not way limit the 
scope of, the present invention. 
PROPOSED EXAMPLE 1 
Styrene Oxidation to Styrene Oxide 
A 6 liter cylindrical stainless steel autoclave reactor approximately 68 cm 
deep and 9 cm in diameter is filled with 3600 g of sodium nitrate and 2400 
g of potassium nitrate. The salt mixture is melted and brought up to 
350.degree. C. by way of externally wrapped electrical resistance heating 
coils. Styrene at a feed rate of 0.5 ml/min is injected into a feed tube 
extending into the salt to a depth of about 1/2 inch from the bottom of 
the reactor. Oxygen, at a feed rate of 200 cc/min, and nitrogen at a feed 
rate of 1000 cc/min, are fed through the same tube in order to sweep the 
styrene into the reactor. The end of the feed tube is equipped with a 
sparging element in order to get better dispersion inside the reactor. The 
reactor pressure is held at 150 psig for 30 minutes with continuous gas 
flow into and out of the reactor. After this time period, the reactant 
feeds are stopped, ending the reaction. A dry ice-isopropanol trap in-line 
after the reactor is used to condense reaction products and unreacted 
styrene. The gas exiting the trap is collected in a sample cylinder. Gas 
chromatographic and GC/MS analysis of the condensate and reaction off 
gases shows that the major reaction products are styrene oxide, 
benzaldehyde, phenylacetaldehyde, carbon monoxide, and carbon dioxide. 
PROPOSED EXAMPLE 2 
1-Tert-Butylcyclohexene Oxidation to 1-Tert-Butylcyclohexene Oxide 
A reaction is carried out in the same way as in PROPOSED EXAMPLE 1, except 
that 1-tert-butylcyclohexene is used instead of styrene. After a reaction 
run time of 30 minutes, the reactant flows are stopped. Gas 
chromatographic and GC/MS analysis of the reaction condensate and off 
gases shows that the major products are 1-tert-butylcyclohexene oxide, 
1-tert-butyl-2-cyclohexenone, and 6-keto-7,7-dimethyloctanal. 
PROPOSED EXAMPLE 3 
2-Methyl-1-Undecene Oxidation to 2-Methyl-1,2-Epoxyundecane 
A reaction is carried out in the same way as in EXAMPLE 1, except that 
2-methyl-1-undecene is used instead of styrene. The run is allowed to 
proceed for 30 minutes and then reactant flows are stopped. Gas 
chromatographic and GC/MS analysis of the reaction condensate and off 
gases shows that the major products are 2-methyl-1,2-epoxyundecane, 
methylnonyl ketone, carbon monoxide, and carbon dioxide.