WO1990015053A1 - Alkylene oxide production using vapor phase oxidation of an alkane or olefin in molten salt followed by a liquid phase co-oxidation of said olefin and an aldehyde in an organic solvent - Google Patents

Abstract

A two phase process for the oxidation of olefins to produce alkylene oxides in increased yields while reducing or substantially eliminating the formation of aldehydes and producing, in place thereof, more desirable carboxylic acids. The first step is a vapor phase oxidation of an alkane or olefin in a molten salt catalyst to produce alkylene oxide and high amounts of aldehyde by-products. The second step is a liquid phase co-oxidation of the olefin and the aldehyde by-products, in an inert organic solvent, to co-oxidize both reactants and form additional amounts of the alkylene oxide and carboxylic acid as a by-product.

Description

ALKYLENE OXIDE PRODUCTION USING VAPOR PHASE OXIDATION OF AN ALKANE OR OLEFIN IN MOLTEN SALT FOLLOWED BY A LIQUID PHASE CO-OXIDATION OF SAID OLEFIN AND AN ALDEHYDE IN AN ORGANIC SOLVENT

This invention relates generally to alkylene oxide production and, more specifically, to a two-step method involving a vapor phase reaction in molten salt followed by a liquid phase reaction in an organic solvent.

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 monoalkyl 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 considerably 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. Patent 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.

⁵ Further developments in the art constituted attempts to maximize the desired olefin oxide production while minimizing by-product formation. For example, U.S. Patent 3,132,156, assigned to Union Carbide Corporation, discloses the vapor phase oxidation of saturated

¹⁰ aliphatic hydrocarbons to olefin oxides. The method described in the '156 patent is said to provide enhanced olefin oxide production as high as 46.2 lbs per 100 lbs of C_ consumed which calculates to be about 33 percent (molar) selectivity. While this level of selectivity

^⁵ constituted an improvement, it remains less than might be desired from a commercial standpoint.

The use of a liquid phase co-oxidation process for the production of alkylene oxides is known in the prior art.

20 By way of illustration, Canadian Patent No. 992,974, issued July 13, 1976, discloses such a process involving the reaction of an admixture of a C3 to C22 olefin, an aldehyde, and an oxygen-containing gas in a benzene medium under specified reaction conditions. However, the

²-^> process disclosed in this patent does not provide as high a selectivity to alkylene oxide from the olefin reactant as might be desired.

A molten salt process for producing alkylene oxides is ³⁰ disclosed in commonly-assigned Pennington U.S. Patent 4,785,123 issued November 15, 1988, and it is an objective of the present invention to modify the process of said Patent in order to provide a new method which substantially increases the yield or level of selectivity of alkylene oxides produced thereby.

Two major problems exist in the known molten salt oxidation processes for converting olefins such as propylene to alkylene oxides such as propylene oxide, namely the lower than desired selectivity to alkylene oxide, which is about 45%, and the larger than desired selectivity to aldehyde formation, such as acetaldehyde, which is from about 18% to 25%.

Known processes which co-oxidize propylene and acetaldehyde require the use of large amounts of acetaldehyde, and produce large quantities of acetic acid by-product whereas the desired end-product is propylene oxide. The overall propylene oxide selectivity of such processes is not increased but acetaldehyde is converted into acetic acid, which is a more marketable material.

The present invention is based upon the discovery that the yield of alkylene oxide produced by known molten salt oxidation processes can be substantially increased by a novel process in which the advantages of the molten salt process are combined with an after-step in which the olefin and the aldehyde by-product are co-oxidized in an organic solvent medium, whereby the overall yield of alkylene oxide is substantially increased to at least about 50% and preferably about 52% to 55%, and acetic acid is produced in marketable quantities. The combined process merges the best features of the molten salt process and the organic solvent process while reducing the disadvantages of each process.

The present-invention relates to a two step process for producing an alkylene oxide comprising:

(a) reacting an alkane or olefin having from 3 to 22 carbon atoms per molecule, or mixture thereof, with an oxygen-containing gas, said alkane or 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 or chloride 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 between about 135 C and about 600 C and a reaction pressure of between about 1 and about 50 atmospheres to produce an alkylene oxide and an aldehyde, and

(b) reacting, in an organic solvent, a mixture of said olefin, said aldehyde and an oxygen-containing gas in a molar ratio of olefin to aldehyde of no greater than 15 to 1 and at a reaction temperature no greater than about 145 C while maintaining a carboxylic acid level in said mixture of below about 13 percent by weight based on the total weight of organic solvent plus carboxylic acid in said mixture, said process providing alkylene oxide in a molar selectivity of at least about 50 percent based upon the amount of said olefin reactant. 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. For example, in step (a) of the process these factors include: 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 (if any).

The oxygen-containing gas useful as a reactant in the present invention can be any such gas, and the same or different such gas can be used in steps (a) and (b). Typically, air is employed as the oxygen-containing gas based upon its ready availability. However, other oxygen-containing gases can be employed in step (a) and/or step (b) such as pure oxygen, and the use of oxygen is expected to be preferred in a commercial setting in one or both of these steps.

The olefin useful in the present invention can be broadly defined as an epoxidizable, olefinically-unsaturated hydrocarbon compound having from 3 to 22 carbon atoms, preferably from 3 to 15 carbon atoms, more preferably from 3 to 12 carbon atoms, most preferably from 3 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:

^•c= ^■O

wherein R_. is hydrogen or an alkyl chain, straight or branched, having 1 to 20 carbon atoms and _ is an alkyl chain, straight or branched, having 1 to 20 carbon atoms; and

wherein R- and R_ are hydrogen atoms or alkyl chains having 1 to 10 carbon atoms and R¹ 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:

wherein R- and R- are olefin radicals having 1 to 4 carbon atoms and R_ and R, represent hydrogen atoms, or one or two alkyl radicals, straight or branched chain, having 1 to 10 carbon atoms. The internal olefins are represented by the following structural formula: ΕL CH=CH ₂

wherein R.. and R« are straight chain or branched chain alkyl radicals having 1 to 10 carbon atoms.

The alkanes, 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 propylene.

Representative other alkylene compounds or olefins are butene-1, butene-2, isobutylene, pentene-1, hexene-1, pentene-2, cyclopentene and cycloctene. Other representative olefins are 2-methylbutene-l, 3-methylbutene-l, heptene-1, octene-1, hexene-2, hexene-3, octene-2, heptene-3, pentadecene-1, octadecene-1, dodecene-2, 2-methylpentene-2, tetramethylethylene, methylethylethylene, cyclobutene, cycloheptene, 2-methylheptene-l,2,4,4-trimethylpentene -1, 2-meth lbutene-2, 4-methylpentene-2, 2-ethyl-3- methylbutene-1, propane, isobutane, pentane, and cyclohexane..

In step (a) of the process of the present invention, the alkane or 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 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 Q in the feed gas line(s).

The molten nitrate or chloride 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 C (275 F) and about 600 C (1,000 F), more preferably between about 150 C and about 450 C, most preferably between about 200 C and about 400 C during the reaction in accordance with the present invention.

The specific temperature selected is based upon the melting point of the particular molten salt chosen. For example, mixtures of molten lithium and potassium nitrate can be suitably employed at a temperature as low as about 280 F, and hence, this temperature may be employed when using lithium nitrate. In the selection of a suitable molten salt bath temperature, it is important to choose a temperature below the thermal decomposition temperature for the particular molten salt chosen. In addition, it is important to maintain a sufficient isotherm across the molten salt bath so as to avoid crust formation of the salt in the bath. Such a crust formation in the 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 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 or chloride salt catalyst used may be any one of the alkali or alkaline earth salts such as lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, or barium or mixtures thereof. In addition, nitrate and chloride salts can be used in mixtures with each other and/or with other salts such as bromides, carbonates, sulfates, and phosphates. Preferably, 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.

In step (a) the ratio of alkane or olefin, or mixture thereof, 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 alkane or olefin fed into the reactor. For example, when reacting propylene with oxygen in a molten potassium nitrate salt column at elevated pressure, a ratio of between about 2 and about 100 parts per volume of propylene per 1 part per volume of oxygen, e.g., about 1 to 35 volume percent oxygen to about 66 to 99 volume percent propylene is found to provide an enhanced selectivity of propylene oxide. A preferred ratio is between 4:1 and 30:1, most preferably between about 8:1 and 20:1. Another consideration in the selection of the amount of 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 on certain olefins such as propylene at an elevated pressure, viz 75 psig, it is preferred to "cut" the amount of propylene to 50 to 75 volume percent and utilize an inert blanket (diluent") gas, such as nitrogen, to provide the remaining volume percent to feed gas. Alternatively, the diluent gas may be comprised of mixtures of oxidation by-product gases generally readily obtainable from the propylene oxide purification operations dcwnstream of the molten salt reactor.

In the selection of the ratio of the volume of oxygen-containing gas relative to the volume of alkane or 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/'propylene reactant mixture at atmospheric pressure, the range of below 7 volume percent of propylene based upon total air plus propylene 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, is employed as a co-catalyst in conjunction with the molten salt catalyst, it is possible to lower the reaction temperature for the particular 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 is 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 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 temperatures.

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. Chloride salt mixtures such as the potassium, cuprous and cupric chlorides also provide excellent results.

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 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 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 alkane, olefin, or mixtures thereof, feed gas(es) can be passed into the molten 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 salt-containing reactor using a sparger. If used, the sparger is preferably positioned in the molten 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. Step (a) of the process can be carried out by feeding a mixture of olefin, inert gas, and oxygen into a reaction vessel containing the molten 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 helps 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.

Step (a) is suitably carried out at atmospheric 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 atomspheres. The most preferred pressure range is between about 1 and about 25 atmospheres.

It is to be understood that by-products in addition to aldehydes are also produced during the reaction. For example, some dehydrogenation of the feed is also effected _r 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.

Step (b) of the present process involves using the substantial amount of aldehyde by-product, isolated by fractional distillation of the gaseous by-product of step (a), in combination with additional olefin and oxygen or an oxygen-containing gas, in order to co-oxidize the aldehyde and the additional amount of olefin, such as propylene, in an organic liquid medium and under pressure with greater than 90% selectivity to alkylene oxide, such as propylene oxide. In this manner the overall yield or selectivity to alkylene oxide is substantially increased, which is the main objective of the present process, while the large amount of aldehyde, such as acetaldehyde, is co-oxidized to form the more marketable acid, such as acetic acid. Thus the selectivity to alkylene oxide is increased to a level of 50% or more, the amount of aldehyde by-product is substantially reduced and a more valuable acid by-product is also produced.

The co-oxidation reaction of Step (b) involves the indirect oxidation of an olefin by way of the oxidation of the aldehyde to a free radical intermediate which epoxidizes the olefin to form the alkylene oxide and the acid. Such co-oxidation processes for producing alkylene oxides from olefins and aldehydes are well known in the art, and reference is made to Aprahamian Canadian patents 992,974 issued July 13, 1976 and 1,012,155 issued June 14, 1977.

Step (b) of the present process is conducted in a suitable inert liquid organic solvent, such as benzene, toluene, xylene, chlorobenzene, acetonitrile, etc., which provides an inert liquid reaction medium for the liquid olefin, aldehyde and oxygen or oxygen-containing gas. The volume of solvent generally is larger than the combined weights of the olefin and aldehyde but may be between 0.5 and 5 parts by weight per combined part by weight of the reactants.

The mole ratio of the olefin to the aldehyde reacted in step (b) is between about 5:1 and 15:1 and most preferably between about 10:1 and 13:1 for ultimate efficiency, the concentrations being maintained relatively constant in the liquid phase during the reaction.

The oxygen or oxygen-containing gas is introduced under partial pressures, such as 50 to 100 psi and in an amount to maintain a molar ratio of oxygen to aldehyde between about 1:1 and 1:6 or more preferably between 1:2 and 1:5 or more.

The co-oxidation reaction of step (b) is carried out in a closed container under a pressure sufficient to maintain a substantial portion of the olefin in the liquid phase, such as between about 300 and 600 psi absolute. The temperature is maintained between about 100 C and 150 C throughout the co-oxidation reaction time which generally is between about 10 minutes and one hour, depending upon the reactants and the reaction conditions.

It is important to the efficiency of the co-oxidation reaction that the percentage of the formed organic carboxylic acid, such as acetic acid, is maintained below about 13% by weight based on the combined weight of the organic solvent and the acid. This result is inherent in the use of starting reaction mixtures in which the weight percent of acetaldehyde is less than 13% by weight of the combined weight of the solvent plus acetaldehyde since only a percentage of the acetaldehyde is converted to the carboxylic acid. However in continuous recirculation processes, in which the propylene and acetaldehyde by-products of. step (b) are recirculated back to the reaction medium of step (b), the organic carboxylic acid and propylene oxide formed in step (b) should be removed from the organic solvent in order to prevent the accumulation of excessive concentrations of either of these desired end products, particularly the organic carboxylic acid, in larger amounts, which tends to retard the desired oxidation of the propylene. This may be accomplished by distilling off the low boiling propylene and propylene oxide and then separating the acetaldehyde, organic solvent and the organic acid by distillation or extraction, after which the organic solvent, propylene and acetaldehyde may be recycled back to the step (b) reaction medium.

The following examples are intended to illustrate, but in no way limit the scope of, the present invention.

Example 1

Propylene, oxygen, and inerts at 300 psig in the proportions by volume of 50/5/45, respectively, were sparged into a 4 liter molten nitrate salt reactor containing 7000 grams of a molten mixture of 60% sodium nitrate and 40% potassium nitrate at 345 C in the step (a). The reactants flowed through the reactor in a continuous manner for four hours. The off gases are absorbed in cold water and certain reaction products are separated and isolated by fractional distillation. There is 14.1 grams of acetaldehyde collected. The oxidation of propylene had occurred to give a 6% per pass conversion of propylene and a 95% conversion of oxygen. Molar selectivities to various produces were as follows:

One Carbon Products Three Carbon Products

carbon monoxide 6.3% propylene oxide 46.2% carbon dioxide 10.4% acrolein 4.0% methane 0.7% acetone 1.9% formaldehyde 0.7% allyl alcohol 0.9% methanol 1.6% Two Carbon Products Four Carbons and Above

ethylene 0.8% butenes 4.2% ethane 0.1% 1,5-hexadiene 1.2% acetaldehyde 20.7% 4-methyl-l,3- 0.3% dioxolane other 0.2%

The 14.Ig (0.32 mol) acetaldhyde from the above step (a) reaction is ^• fed into a one liter stainless steel autoclave reactor containing 250 grams of xylene, 155 g (3.7 mol) liquid propylene, 3.5 liters of oxygen (STP), and inerts at 110 C and 500 psig in step (b). The mixture is agitated and held for 40 minutes at these conditions. Work up on the reaction products gives 7.6 grams of acetic acid and 8.3 grams of propylene oxide. This corresponds to a propylene conversion of about 10 percent in step (b) and an acetaldehyde conversion of about 50% with about 90% selectivity to propylene oxide and an 80% selectivity to acetic acid. The overall propylene conversion from coupling the two reactions of steps (a) and (b) is 6.6% and the overall propylene oxide selectivity is 56%, i.e. 46.2% in step (a) plus 10% in step (b).

Example 2

Co-Oxidation With Recycle of the Acetaldehyde in Step (b)

As in Example 1, 14.1 g (0.32 mol) acetaldehyde produced as a by-product from the molten salt catalyzed oxidation of propylene in step (a) is added to about 14 g (0.32 mol) of additional acetaldehyde, such as from a recycle stream. The total amount of acetaldehyde, about 28 g, is fed into a two liter stainless steel autoclave reactor containing 500 grams of xylene, 320 g (7.6 mol) liquid propylene, 7 liters of oxygen (STP), and inerts in step (b). The temperature is brought up to 110 C at a pressure of 500 psig. The mixture is agitated and held for one hour at these conditions. Work up of the reaction products gives 15.3 grams of acetic acid and 16.7 grams of propylene oxide. This corresponds to complete conversion of 14 grams of acetaldehyde in the co-oxidation reactor. Taking the starting acetaldehyde to 100 percent conversion in the co-oxidation step, in effect, simulates a recycle operation in which the unreacted acetaldehyde and propylene from the co-oxidation reactor are continuously recycled back to the reactor. The overall propylene conversion brought about by coupling the molten salt catalyzed oxidation of propylene in step (a) of Example 1 with the recycle co-oxidation step (b) is 7.5% and the overall selectivity to propylene oxide is 62%.

Example 3

Propylene was oxidized to propylene oxide in a molten salt reactor similar to that used in step (a) of Example 1 except that the molten salt mixture consisted of 30 weight percent potassium chloride, 50 weight percent cuprous chloride, and 20 weight percent cupric chloride at a salt temperature of 375 C and a reactor pressure of 300 psig. The propylene per pass conversion was found to be 4% while the propylene oxide selectivity was found to be 38% and the acetaldehyde selectivity was found to be 25 percent. Acetaldehyde from the molten salt reaction step (a) in the amount of 14 g is used in a second co-oxidation reaction step (b) just as in Example 1 with similar results. Combining the two reaction steps gives an overall propylene conversion of 4.6% and an overall propylene oxide selectivity of 50%.

It is to be understood that the above described embodiments of the invention are illustrative only and that modifications throughout may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein but is to be limited as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A process for producing an alkylene oxide characterized by:

(a) reacting an alkane or olefin having from 3 to 22 carbon atoms per molecule, or mixture thereof, with oxygen or an oxygen-containing gas, said olefin and said oxygen or oxygen-containing gas being gaseous reactants, by contacting said gaseous reactants with a bath, stream, spray or mist of at least one molten nitrate or chloride 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 C and about 600 C and a reaction pressure of between about 1 and about 50 atmospheres to produce an alkylene oxide and an aldehyde, and

(b) reacting, in an organic solvent medium, a mixture of said olefin, said aldehyde produced in step (a) and oxygen or an oxygen-containing gas in a molar ratio of olefin to aldehyde of no greater than 15 to 1 and at a reaction temperature of no grater than 145 C to produce an alkylene oxide and a carboxylic acid of said aldehyde, and maintaining the carboxylic acid level in said mixture below about 13% by weight based on the total weight of organic solvent medium plus carboxylic acid in said mixture;,

said method providing alkylene oxide in a molar selectivity of at least about 50% based upon the amount of said olefin reactant, and said carboxylic acid.

2. A process according to Claim 1 in which said alkylene oxide characterized in that propylene oxide, said olefin comprises propylene, said aldehyde is acetaldehyde and said carboxylic acid is acetic acid.

3. A process according to Claim 1 in which said alkane or olefin and said oxygen-containing gas are present in step (a) in a volume ratio between about 2:1 and 100:1.

4. A process according to Claim 1 in which the reaction of step (a) is conducted at a temperature bbeettwweeeenn aabboouutt 220000 aanndd 440000 CC aamnd under a pressure between about 1 and 25 atmospheres.

5. A process according to Claim 1 which characterized in that bubbling said olefin and oxygen-containing gas reactants through a bath of said molten salt catalyst.

6. A process according to Claim 1 in which the alkylene oxide and aldehyde produced according to step (a) are separated and the aldehyde is used in step (b) in combination with an additional amount of said aldehyde and an additional major amount of said olefin.

7. A process according to Claim 6 in which the aldehyde produced according to step (a) is continuously recycled in step (b) to produce' substantially complete conversion of the recycled aldehyde into the corresponding carboxylic acid.

8. A process according to Claim 1 in which the molar ratio of olefin to aldehyde in step (b) is between about 5:1 and 20:1.

9. A process according to Claim 8 in which said molar ratio is between about 8:1 and 13:1.

10. A process according to Claim 1 in which said organic solvent characterized in that an aromatic hydrocarbon solvent, present in an amount equal to between about 0.5 and 5 parts per weight for each part by weight of combined olefin and aldehyde.

11. A process according to Claim 1 in which the reaction step (b) is conducted at a temperature between about 100 C and 150 C and under a pressure between about 300 and 600 psia.

12. A process according to Claim 1 for producing propylene oxide characterized in that:

(a) reacting propylene with oxygen or an oxygen-containing gas, said propylene and said oxygen or oxygen-containing gas being gaseous reactants, by contacting said gaseous reactants with a bath of at least one molten 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 200 C and about 450 C and a reaction pressure of between about

10 and 30 atmospheres to produce propylene oxide and acetaldehyde, and

(b) reacting, in an organic solvent meditim, a mixture of propylene, said acetaldehyde produced in step (a) and oxygen or an oxygen-containing gas in a molar ratio of propylene to acetaldehyde between about 8:1 and 14:1 and at a reaction temperature of between about 100 C and 150 C to produce a propylene oxide and acetic acid, and maintaining said acetic acid level in said mixture at below about 13% by weight based on the total weight of organic solvent plus acetic acid in said mixture;

said method providing propylene oxide in a molar selectivity of at least about 50% based upon the amount of said propylene, and said acetic acid.