US2465984A - Production of cycloalkyl nitrites - Google Patents

Description

Match 29, 1949. UM r AL 2,465,984

PRODUCTION OF CYCLOALKYL NITRITES Filed May 21, 1946 ORNEY Patented Mer. 29, 1949 2,465,984 PRODUCTION OF CYCLOALKYL NI1BI1ES Thomas F. Donnant,

Coe, Long Inglewood, pany of Cali Beach, an

poration of California Application May 21, 1946. Serin! No. 671,394

14 Claims. c1. 260-466) This invention relates to a method for the production of esters or nitrous acid and reaction products thereof and relates particulafly to the esterification of saturated hydrocarbons, especially of cycloalkyl hydrocarbons.

Esters of nitrous acid have been prepared in the past by the usual esterification reaction between an alcohol and an acid to form the ester and water. In the case of nitrous acid esters however, the esteriflcation process is complicated by the instability of nitrous acid as well as the reactivity of the esters formed. Possibly this expiains the fact that there is comparatively iittie information available on this esterification. The few processes described in the prior art generally involvethe reaction of an alcohol with nascent nitrous acid formed in situ by acidification of a nitrite sait. Thus the method V described on page 108 of Organic Syntheses,

0011., vol. 2, published by John Wiiey and Sons Inc. of New York city, for the preparation of n-butyl nitrite involves the gradual addition of sulfuric acid and n-butyl alcohoi to a sodium nitrite solution in the cold. As far as applicants are aware th preparation of esters oi nitrous acid by direct esterification of hydrocarbons has L never been carried out.

Applicants have discovered -that esters of nitrous acid, or organic nitrites, as they may also be termed, may be prepared by reacting alcohois, especially cycloalkyl alcohols, with nitrogen tetroxide in the liquid phase, at atmospheric temperatures or beiow, in the absence of cataiysts.

Applicants have also discovered another entirely unexpected reaction, i. e., that the same esters of nitrous acid may aise be prepared by reacting the same reagent, nitrogen tetroxide, with a difierent material, nameiy the hydrocarbon rather than the alcohol. It is most unusuai that the same reagent wiil react with two different materials to form the same product.

It has been discovered that if saturated hydrocarbons are maintained at an elevated temperature in the liquid phase, and contacted for a reIatiVeIy short contact time with a minor proportion of nitrogen tetroxide, good yields of the organic nitrites may be obtained. In this proces it is prterable that the nitrogen tetroxide be dissolved in the hydrocarbon, and that the addition of water or nitric acid be avoided, since the presence of any substantial amount of water or nitric acid in the reaction zone tends to reduce the production of the nitrite and results in the formation of nitro compounds and x1- Los Angeiea. Clarence 8. d Edward C. Attan, Jr., Calit., assignors to Union 011 Comfomia, Los Angeles, Caiif.. a cordation products. Continuons concurrent flow actant. and substantiafly nitrogen te troxide is utilized for the esterificaof the hydrocarbon and the nitrogen tetroxide is also preferred.

The main reaction involved in the process of this invention is believed to be as follows:

(1) Hi H H:C C H1C C\ N104 l e l i E HNOI H: H: H:

C C H] H! Ni en C clohexane Cyclohexyl- Nitrous tetode y nitrite acid Since nitrous acid is very unstabie, the principal products other than the nitrite are nitric-oxid and water rather than nitrous acid; thusf e overall reaction is believed to be:

( 3 l 4 eHu CaEnONO 2N0 21150 Nitr en Cycio Oyciohexyi Nitric Water tetrox de hexane nitrite oxide The nitric oxide of Equation 2 mai be readily oxidized to nitrogen tetroxide by t e following reaction:

(3) 2N0 o. N,O.

Nitric Oxygen Nitrogen oxide (ai tetroxide Thus the by-product nitric oxide n1ay be very readily reconverted to the nitrogen tetroxide reall of the nitrogen cf tion.

It has aiso been discovered that the organic nitrites may be readily converted to additional valuable products. For example, the cycioalkl nitrites have been found to be very readily oxidized to dibasic acids of unusuaily high purity,

This provides an improved process for the production of such dibasic acids, namely, converting the cyclalkanes to the cycioaikyi nitrites, and oxidizing the latter to the dibasic acids." In the case of cyclohexane for example, the cyclohexyl nitrite prepared by equations 1 or 2 above may be oxidized to adipic acid. The overall exidation reaction apparentiy is as foliows, where nitric acid is used as the oxidizing agent:

3 uHI1ONO 7EN03 Cyclohexyl Nitric nitrite acid 3COOH(CH:) COOH 511,0 i0NO Adipic acid Water Nitric oxide The nitric oxide by-product may be reconverted .tored heat, and Water and nitric to the nitrogen tetroxide reactant employed in Equation 1 or 2 by oxidation according to Equation 3 above. It may also be converted to the nitric acid reactant employed in Equation 4 by the following oxidation:

(B) 4NO 302 21120 e 4HNO:

Nitric .Oxygen Water Nitric oxide acid Thus the overall reaction for production of adipic acid by Reactions 2, 3, 4 and 5 is simply:

() 2CqII]: 501 2H20 Cyclo- Oxygen Water hexane It has also been discovered that nitrogen tetroxide is much more soluble in hydrocarbons than are the other nitrogen oxides such as nitrous oxide or nitric oxide or other gases such as nitragen itself. This makes possible a further improvement in the above process for preparing organic nitrites, namely preparing the desired solutlon of nitrogen tetroxide in the feed hydrocarbon by selectively extracting the nitrogen tetroxide from a gaseous mixture containing the tetroxide in admixture with less soluble gases such as other nitrogen oxides, nitrogen, ammonia, air and water vapor. Such gaseous mixtures are ecc- 2CnHxa04 Adipic acid nomically prepared by the oxidation of ammonia. This oxidation is preferably carried out in two steps, the main reaction in the first step being as follows:

Ammoma Nitric Water oxide The second step is the oxidation of the nitric oxide, after separating it from the water, to obtain the tetroxide, according to Equation 3. The resulting gaseous mixture, containing nitrogen tetroxide in admixture with impurities such as other nitrogen oxides, nitrogen, air and the like, is then contacted with the hydrocarbon feed, which selectively dissolves the nitrogen tetroxide to form the reaction mixture.

Thus the primary process of this invention is the esterification of hydrocarbons with nitrogen tetroxide to form the organic nitrites, and this process may be elaborated by the use of the feed preparation process involving the selective extraction of the nitrogen tetroxide by the hydrocarbon feed, and by the subsequent conversion of the cycloalkyl nitrites to the dibasic acids by oxidation.

The invention will become more clear by reference to the attached drawing, in which cyclohexane is indicated as the hydrocarbon feed, although other hydrocarbons may also be employed, as described below. Referring to the drawing, air or oxygen, and ammonia are introduced through limes I and 2 and valves 3 and 4 lnto ammonia oxidation zone 5 in which they are contacted with a catalyst such as platinum gauze heated thus converted to a mixture of oxide as indicated in Equation 6: aThe resulting product is passed through line 8 and cooling means 1 int dehydration zone 8 in which the water is removed by condensation and/or other dehydration and withdrawn through line 9, while the remaining gas, largely nitrogen and nitric oxide, is removed through line in and passed to oxidation zon II. In this zone it is contacted with air introduced through line [2, whereby the nitric oxide is converted to nitrogen tetroxide in accordance with Equation 3. The resulting gas, principally nitrogen and nitrogen tetroxlde is removed through line l3, and the Oxygen nitrogen tetroxide ls selectively dissolved in cyclehexane in absorption system Il. This system may be in the form of a countercurrent absorber in which liquid hydrocarbon feed is introduced in the upper part of the absorber, and the nitragen tetroxide containing gas 15 introduced into the lower part of the absorber. The non-absorbed gas 15 withdrawn through line I! and discarded, or if sufiiciently rich in nitric oxide, may be returned to oxidation zone II. The solution of nitrogen tetroxide in cyclohexane is withdrawn through line [8 and sent to reactiou Zone I, in which the cyclohexane is converted to cyclohexyl nitrite in accordance with Equation 1 or 2. The resulting product is withdrawn through line 20 and separated in separation zone 2l to obtain a gas fraction largely nitric oxide, which may be withdrawu through line 22 and returned to oxidation zone 1 I, a small amount of an aqueous phase which may be withdrawn through line 23, and an 011 phase which may be withdrawn through line 24 and sent to recovery system 25. In the recovery system 25, which may be merely a fractional distillation zone or any other type of separation system, the unreacted cyclohexane may be separated, withdrawn through'line 28 and returned to absorption system l4; the cyclohexyl nitrite may be recovered and withdrawn through line 21, and by-products including nitrocyclchexanemay be withdrawn through line 28. The cyclohexyl nitrite may then be converted to adipic acid by introducing it into oxidation zone 29 where it is contacted with nitric acid introduced through line 30 and converted to adipic acid in accordance with Equation 4. The resulting product may be transferred to recovery zone Il through line 32. A gas fraction, which is pri marily nitric oxide may be withdrawn through line 33 and retumed 1:o oxidation zone H, and the remainlng material may be treated for recovery of the adipic acid which is withdrawn through line 34, and by-products such as small amounts of other dibasic acids, withdrawn through line 35.

It must be emphasized that the process as shown in the drawing is merely illustrative of the invention, and the invention is not llmited thereto, since other cycloalkanes, oxidizers,etc., may be used, as described below. A more complete description of the conditions et operation of each stage of the processwill flrst be presented bowever.

The oxidation of the ammonia in zone 5 ls known, being described for example, in an article by Guy B. Taylor, entitled Oxidation o: ammonia, found in the Journal 01 Industrlaland Engineering Chemistry, vol. 19 (1927), page 1250. It is there shown that various catalytic agents may be employed and that the reaction is substantially complete, V Temperatures above 700 0. are used. An excess of oxygen is desirable, the relative amounts of air and ammonia being generally about air to 10% ammonia. Thls is about 50% more oxygen than required by Equa tion 7.

In the process described in the Taylor article and the other prior art, the mixture of nitllc oxide and water resulting from the above ammonia oxidation ls converted to nitrlc acid rather than to nitrogen tetroxide, i. e., the reaction oi Equation 5 rather than that of Equation 3 is carried out. Both reactions occur very readfly at atmospheric temperatures or slightly elevated temperatures, but for the production of nitrlc acid, water is essential, whereas for production of nitrogen tetroxide it -is most desirable that the presence of any appreciable quantity of water be avoided. Accordingly, the water is removed. The dehydraticn in zone 8 may be merely a cooiing or combination of cooling and compressing suflicient to reduce the water content of the gas to a concentration lower than that of the nitric oxide and preferably less than of the concentration of the nitric oxide. Driers such as calcium chloride, silica gel or the like may also be employed if desired.

The oxidation of the nitric oxide in zone Il may be carried out at atmospheric pressure.but

is preferably carried out under moderate pressures such as about 50 to 500 pounds per sq. in. Temperatures between about 0 C. and about 100 C. may be used, preferably about to 50 C. An excess of air up to 2 or 3 times theoretical should be used. Contact times need not be over about ten minutes and may be as short as one-half second or less.

As an example of the conversion of ammonia t0 nitrogen tetroxide by the 2-stage process described above air and ammonia in the proportions of about 90% air to 10% ammonia are preheated to a temperature of about 700 C. and the mixture is passed at substantially atmospheric pressure through a. platinum gauze heated to red heat. The resulting mixture contains about 9% of nitric oxide together with about 7% of excess oxygen, 13% Water, and less than 1% unconverted ammonia, the remainder being oxygen. This mixture is cooled and compressed to a temperature in the neighborhood et 100 F. and a pressure of about 100 pounds per sq. inch gage, under which conditions the bulk of the water settles out as an aqueous nitric-acid contaiuing condensate, which is withdrawn. The resulting partially dehydrated gas is substantially completely dehydrated by passing it through a silice. gel drier bed. The nitric oxide in the dehydrated mixture is then converted to nitrogen tetroxide by adding air to the mixture in such a. proportion that there is about a 50% excess of oxygen over that required by Equation 3. At 100 F. and 100 pounds per sq. in. gage the conversion of 1e nitric oxide to nitrogen tetroxide is comple. in about 10 to 15 seconds, giving as a product a gas containng about 6% of nitrogen tetroxide (inciuding dioxide), about 4% of oxygen, and less than 1% of nitric oxide, ammonia, and water, the remainder being nitrogen.

The separation of the nitrogen tetroxide from the above gascons mixture is preferably carried out by selective absorption in the hydrocarbon feed in zone 14 as indicated in the drawing, since this method of separaticn automatically accomplishes the also desired effect of dissoiving the nitrogen tetroxide in the hydrocarbon feed stock.

The absorption in zone i4 is preferably carried out at temperatures below about C. down to about 10 0., or below, and superatmospheric pressures such as about pounds to 500 pounds per sq. in. gage. A sufiicient proportion of hydrocarbon to nitrogen tetroxidecontaining gas is employed to remove the bulk of \the nitrogen tetroxide from the mixture. A snall amount of the other gases may also be tolerated. The equipment employed may be a conventional absorber such as employed for natural gasoiine production or the like, i. e., a packed or bubble cap type tower. The gas stream is injected in the lower part of the tower and travels upward countercurrently to a down flowing stream of the hydrocarbon, which is introduced near the top of the tower. The unabsorbed gas is removed from the top of the tower. and may be discarded or further treated for recovery of desired constituents. If there is sufl1- cient nitric oxide in this unabsorbed gas it may be returned to dehydration zone 8 or oxidation zone i l; or if there is sufficient ammonia in this stream, it may be recycled to oxidation zone 5. Conventional methods of separation and concentration of ammonia or nitric oxide may also be employed prior to the recycling as indicated. The solution of nitrogen tetroxide in the hydrocarbon feed is withdrawn from the bottom of the tower in the above operation.

As an example of the above absorption process for separaticn of the nitrogen tetroxide, the mixture previously described, containing about 6% of nitrogen tetroxide, is cooled to a temperature of about 10 C. without substantially reducing the pressure, and is passed countercurrently to a stream cf cyclchexane in a packed tower. The unabscrbed gas withdrawn from the top of the tower contains about 1% of nitrogen tetroxide or less, and the bulk of the nitric oxide, ammonia. oxygen, and nitrogen, while the cyclohexane solution withdrawn from the bottom of the tower contains about 10% by weight of nitrogen tetroxide. Similar absorption processes are carried out with the other hydrocarbon feed stocks of this invention under the same conditions or other conditions within the above ranges, to obtain suitable separaticns of nitrogen tetroxide and to prepare suitahle solutions for oarrying out the esterification process of this invention whereby the hydrocarbon feed stocks are reacted to form the nitrites.

The conditions which we have found favorable for the formation of the nitrites from the corresponding cycloalkanes in reaction zone I9 40 are superatmospheric temperatures below the critical temperature of the feed stock, i. e., temperatures between about 25 C. and 400 C., proferably between about 100 C. and 250 C. superatmospheric pressures up to about 2000. pounds per sq. in. or higher may be used, preferably pressures between about pounds and about 300 pounds per sq. in. gage. The pressure should be suficiently high to insure a substantially liquid phase reaction. By this term it is meant that the hydrocarbon is substantially completely in the liquid phase, and some nitrogen tetroxide is dissolved therein. All of the nitrogen tetroxide need not be dissolved in the hydrocarbon, a1- though it is desirable that great intimacy et contact between the hydrocarbon and any gasoous nitrogen tetroxide be obtained. It has been found desirable to dissolve the nitrogen tetroxide in an excess of the hydrocarbon at a relatively low temperature, such as about 40 C. or below and introduce this solution at this low temporature continuously into the reactor. By this mode of operation continuons concurrent fiow is obtained. and the nitrogen tetroxide is liberated as the solution is warmed to reaction temporature, butis liberated in extremely small bubbles,

which provide the desired intimacy of contact. This type et operation is believed to be of value in any situation in which great intimacy of contact is desired between a liquid and a gas, namel dissolving the gas in the liquid at a lower temperature, and allowing the gas to be liberated in the reactor as the solution is warmed to the reaction temperature.

It has also been found desirable to employ a 5 very short contact time, 1. e., preferably less than action.

about 15 minutes, and no longer than 15 necessary to form the desired nitrite, so as to minimize decomposition and isomerization of the nitrite-t the nitro-hydrocarbon. In the continuous process any accumulation of an aqueous phase in the reactor should also be avoided. As observed from Equation 1, nitrous acid may be formed in the reaction, and nitric acid is formed by its decomposition. Water and nitric oxide are also 1'ormed, as indicated in Equation 2, the water in reiatively small amounts. If allowed to accumulate in the reactor; this water or aqueous phase may accelerate side reactions such as hydrolysis,

nitration, and oxidation resulting in the production of nitro compounds and dibasic acids. Therefore the conditions in the reactor should be such as to prevent accumulation of aqueous phase there, e. 3., the flow through the reactor should .-be sufficiently rapid to prevent retention of the aqueous phase. Obviously, the desired control of reaction time and velocity of flow may be obtained by appropriate reactor design as to length and diameter. Downfiow may also advantageously be employed. The proportion of nitrogen tetroxide to cycloalkane should be relatively low, such as between about 1% and 25%, and preferably in the region of about 5% t0 15% by weight. This permits substantially complete utilization 01 the nitrogen tetroxide in each pass through the reactor. Conditions of temperature and contact time should also be adjusted to reduce the nitrogen tetroxide only to nitric oxide, rather than to nitrogen or nitrous oxide, since the latter two are rather diificult to reoxidize to nitrogen tetroxide, whereas the reoxidation of the nitric oxide to nitrogen tetroxide occurs .substantially in the liquid phase.

spontaneously at atmospheric temperature in contact with air.

The entire reaction product withdrawn from zone I! may be transferred directly to oxidation zone 29 as described below, or may be :flrst separated in zone 2l, merely by stratification, with or without cooling, to obtain a gas phase and a liquid phase. The gaseous fraction obtained from separator 2|, which is largely nitric .oxide, may be merely recycled to zone II, where it is contactd with air and reused in the re- It may also be purified of the bulk of the contaminating water vapor by recycling it to cooler 1 or dehydration zone 8. It may also be recycled to absorption zone I4 if it contains an appreciable quantity of nitrogen tetroxide, thereby purifying it of water, nitrogen, nitric and nitrous oxides and the like.

The liquid reaction product may be transierred directly to oxidation zone 29 as described below, or may be first separated into an oil phase and an aqueous phase. The aquedus phase may be discarded or treated by known methods to recover nitrogen oxides or nitric acid thereirom. The 011 phase may be transfard directly to oxidation zone 29, or may be first treated to obtain an unreacted hydrocarbon fraction which is recycled entirely or in part to the absorption or reaetion step. The remaining material, after part or all of the unreacted hydrocarbon has been removed, may be transferred directly to oXidatien zone 29, or may flrst be treated to separate the desired nitrite and other products of side of separation may also be used. Prier te disfiliation it is desirable to remove residual traces of nitrogen oxides and water from the 011 phase. This may be done by passing air over the vigorously stirred 011 phase to remove the gases, and by dehydrating with anhydrous sodium sulfate. Other conventional methods of purification may also be employed.

As a specific example of the esterification process of this invention a solution containing 9.1% by weight of nitrogen tetroxide in cyclohexane is prepared as above by two-stage oxidatien cf ammonia to nitrogen tetroxide, foliowed by absorption of the latter in cyclohexane. This solution 15 1'ed at an average rate of 100 ml. per minute through a spray nozzle at the bottom of a 6 ft. vertical reactor of H; inch I. D. stainless tubing. packed w th 4 mm. glass Raschig rings. The reactor is equipped with heating elements, so that the Ieed, though introduced at a temperature of about 20 C. is heated to a temperature of about 100 C. within approximately the lower one foot et reactor, and is further heated in the remainder of the reactor to a maximum temperature of about 152 C., the average temperature through the upper of the reactor, which is referred to herein as the reaction tem- .perature, being about 135 C. The pressure on the reactor is approximately 100 pounds per sq. in.- gage, so that the cyclohexane is maintained Under these conditions the average contact time at the reaction temperature et 135 C. is about 3 minutes, and the 'nitrogen tetroxide is substantialiy completely consumed forming a gas which is largely nitric oxide. A very small amount of aqueous phase, largely nitric acid, is formed in the reactor, and is carried through the reactor with the oil phase.

40 The reaction products are withdrawn from the top of the reactor, and cooled to about room temperature. The oil phase is separated from the gas and aqueous phases, and subjected to distillation to rec0ver unreacted cyclohexane and cyclohexyl nitrite thereirom. The remainder is further distilled to recover cyclohexanoi and nitrocyclohexane as successive overhead distillate tractions, leaving a residual bottoms fraction belieVed to contain polymers and poly-substituted cyclohexanes such as nitrocyclohexanols, dinitro. cyclohexanes, nitroso-nitrocyclohexanes, nitritonitrocyclohexanes, and the like.

Under the above conditions, about 8% of the cyclohexane reacts, this being neariy the theoretical amount according to Equation 1, and the remainder of the cyclohexane is suitable for recycling. Of the cycIohexane-ree reaction products, about 27% is cyclohexyl nitrite. The remainder consists of about 6% cyclohexanol, 49%

test is described in the Industrial and Engineering Chemistry, Analytical edition, vol. 13 (1941), page 555.

Under other conditions within the ranges o f temperature, contact time, etc., defined above. cyclohexane is also esterified to obtain substantial yields of the nitrite, as well as other reaction products described above. Similarly. under these conditions other cycloalkane hydrocarbons are esterifled, such as cyclopentane, methylcyclopentane, dimethylcyclopentane, ethylcyclopentane,

methylcyclohcxane, and the like, and the corresponding cycloalkyl nitrites are obtained. Upon oxidation of these nitrites in zone 29 howevcr, as indicated in the drawing, it is clear that adipic acid will not be the only dibasic acid obtained. In the case of cyclopentyl nitrite substantiaily pure giutaric acid is produced, and in the case of the nitrites of the alkylated cycloalkanes, mixtures of adipic, glutaric, succinic and other acids are formed.

The above cycloalkanes need not be pure, since the esterification will also take'place with mixtures of difierent cycloalkanes, and with mixtures of cycloalkanes with other hydrocarbons such as minor proportions et paraffins or aromatic hydrocarbons of similar boiiing point. A mixture containing about 80% of cyclohexane and about 20% of isparafflnszof similar boiling point, for example, may be regdilyobtalnd by' fractionation of petroleum, "andz is an excellent stock.

Although the above description has been confined largeiy to the cycloalkanes, which are the preferred stocks of this invention, it has been found that other saturated hydrocarbons, such as the normal paraflins, pentane, hexane and the like or isoparafiins such as isopentane, isooctane and the like will also form nitrites under the above conditions, and such processes are also within the scope of this invention. As pointed out above pure hydrocarbon feed stocks need not be used, although they are preferred; and pure nitrogen tetroxide also is not necessary although it is preferred. Minor proportions of relatively inert gases such as nitric oxide, nitrogen, or nitrous oxide and ammonia may also be present.

As a speciflc example of the esteriflcaticn of acyclic alkanes, 20 g. of nitrogen tetroxide is dissolved in 180 g. of isooctane and the solution is charged to a bomb, pressured with nitrogen to 100 lb. gage, and heated at about 70 C. for about 26 hours. The nitrogen tetroxide is substantiaily entirely reduced to nitric oxide, and on topping ofi the unreacted isooctane and vacuum distilling the residue, a distillate fraction of about 2 ml;, having a boiling point of about 59.6 C. at 4.4 mm. and a refractive index at 20 C. of about 1.4281, and giving a very positive charactcristic test for nitrite, is obtained.

Normal paraffins may also be esterified as above, and valuable products may be prepared. For example, by esterificaticn of C14 to Cm normal parafins, the corresponding nitrites are obtained, and these are readily hydrolyzed to form the corresponding alcohols, which in turn are sulfated with sulfuric acid to obtain excellent detergents.

Retuming now to the cycloalkyl nitrites prepared.as above, these are very susceptible to oxidation to dibasic acids. For example, liquid cyclohexyl nitrite retained in a glass container on the laboratory shelf exposed to difiused light apparently undergoes auto-oxidaticn and reduction, and over a period of about a week, a substantiai formation of crystals occur,. which crystals. are

round to be adipic acid. In general, the cycloalkyl.

nitrites are readily oxidized to dibasic acids at organic persalts such as sodium perchlorate, po-

tassium permanganate, sodium dichromit and the like may also be used. An excess of oxidizing agent should generally be employed. Reaction temperatures shouid be between about 25 C. and

and 250 C.; pressures may be between about 0 pounds and 1000 pounds per sq. in. gage; and contact times just long enough to obtain the desired conversion may be used. The reaction la preierably carried out in the liquid phase. Meahs should be provided formaintaining good contact between the cycloalkyl nitrite and the 0X1- dizing agent.

When nitric acid is employed as the oxidizfilng agent, temperatures above about 50 C. are preferred since these higher temperatures favor the oxidation reaction over the competing nitration reaction. Excessivcly long reaction times should be avoided, since these tend to degrade the purity of the dibasic acid product. Nitric acid of the strength greater than about by weight 18 prefcrred, aithcugh lower concentrations may be employed at the higher temperatures within the above range and at higher contact times. Atmospheric pressure is suitable although super atmospheric pressures may also be employed. It is desirable to expel the nitrogen oxides from the reaction mixture as rapidly aspossible. and for this purpose an inert gas such as nitrogen may be used as a stripping gas if desired. Because of the high reactivity of the cycioalkyl nitrites, the presence of an inert diluant in the reaction mixture 1s desirable.

As a speciflc example of the above oxidati0n, using nitric acid, to 50 ml. of boiling nitric acid (70%) at atmospheric pressure over a reflux condenser is added duririg 15 minutes, 50 ml. of a 10% solution by volume of cyclohexyl nitrite in cyclohexane. Oxides of nitrogen are rapidly evolved. The mixture is refluxed for five minutes after the addition of all the cyclohexyl nitritecyclohexane solution, and is then cooled, causing the separaticn of crystals of adipic acid from the aqueous phase. The cyclohexane phase la separated from the aqueous phase, and the latter is evaporated to dryness to give a solid white product consisting of adipic acid of over 98% purity. This is a surprising degree of purity. since adipic acid produced by the oxidation of cyclohexancl, cyclohexan0ne, or cyclohexane with nitric acid, by the methods of the prior art.

does not exceed about 60 to 70% purity. The cyclohexane phase in the above speciflc example is found to be entireiy free of cyclohexyl nitrite.

The above example shows that the cyclohexyl nitrite is oxidized selectively to substantially pure adipic acid, without appreciabie reaction of the cyclohexane. ilarly selectively oxidized in the presence cf cycloaikanes, and are also seiectively oxidized in the presence of nitrocycloalkanes. This permits a very efficient combination of the esteriflcaflon reaction with the oxidation reaction, since the entire reaction product from the esteriflcation may be reacted with an oxidizing agent and the nitrite thereby selectively oxidized to the dibasic acid. As an example of this type of oxidation, the above oxidation with boiling 70% nitrlc acid is carried out in the same manner. employing in place of .the above cyclohexane-cyclohexyl n1- Other cycloalkyl nitrites are sim- -clohexane, and other reaction pmducts. It is iound that under these conditions, the cyclohexane and nitrocyclohexane are substantially unaflected, and substantially all of the cyclohexyl nitrite, as well as a portion of the other esteriflcation reaction products, are converted to adipic acid. Similar results are ob'tained by employing the entire esterification reactin product, without separation of gas or aqueous phases; or by employing an oil phase which has been Ireed irom part or all of its unreacted cyclohexane content.

'Ihus a high quality adipic acid may be produced by subjecting the product of the esteriflcation reaction as described above directly to the oxidation reaction as described above; or between the esteriflcati0n and the oxidation there may be a separation of the gas phase or of the liquid phase or ci. 8. portion 01. the unreacted hydrocarbon feed stock, or of any or all of these. Also if desired other products such as nitrocycloalkanes or ccloalkyl alcohols or the like may be separated before the oxidation.

Nitrogen tetroxide is also an excellent oxidizing agent for the conversion 01. the cycloalkyl nitrites to disbasic acids as above. For example, a solution of about of nitrogen tetroxide and 10% cf cyclohexyl nitrite in cyclohexane is decolorized at room temperature in a matter of a few hours, 1. e., all 01 the nitrogen tetroxide is reduced to nitrlc oxide. with the production of adipic acid, which settles as a crystalline solid from the solution. Nitrogen tetroxide may also be employed under other conditions within the above limits, to oxidize cyclohexyl nitrite or other cycloalkyl nitrites to dibasic .acids. The use of nitrogen tetroxide iurthermore, permlts the simultaneous esterification of cyclohexane and oxidation 01. the nitrite ester to adipic acid. The latter reaction proceeds at a much faster rate, so that when a solution 01 about 10% of nitrogen tetroxide in an excess of cyclohexane is allowed to react at room temperature in a clear glass container in the laboratory, crystals of adipic acid appear within about two days, and within about two weeks, the solution is completely decolorized. Substantially no cyclohexyl nitrite or nitrocyclohexane is observed in the product, which is substantially pure adipic acid and unreacted cyclohexane, the nitrogen tetroxide being reduced to nitrlc oxide. This is explainable on the basis that the cyclohexyl nitrlte is the primary reaction product. and this is oxidized to adipic acid at a faster rate than its rate of formation, so that the concentration of the nitrite at any given time is almost negligible. Ii greater yields of the nitrite are desired, without further oxidation to adipic acid, a continuous system should be employed wherein an excess oi liquid cyclohexane containing dissolved nitrogen tetroxide is passed continuoualy and concurrently through a tube at the desired temperature. Temperatures between about 50 C. and 200 C. are preferred. If it is desired to oxidize the nitrite to dibasic acid directly as produced, either a batch system should be employed as described above, or a concurrent system should be used, wherein the cyclohexane is allowed to flow countercurrently to a stream of nitrogen tetroxide.

, 12 In the oxidation of the nitrite with either nitrlc acid or nitrogen tetroxide, nitrlc oxide is produced as a by-product. Since thls is readily A reoxidized to nitrogen tetroxide by air or oxygen, it is advantage0us to introduce oxygen or air into the reaction mixture, either together with the feed in a batch or concurrent system, or at an intermediate stage of the reaction in a countercurrent system, or at a subsequent stage of the reaction in a concurrent system. It has also been discovered that nitrlc oxide has a catalytic eflect on the oxidation, so it is advantageous to introduce nitrlc oxide with the reactants. This is true not only for the oxidation reaction but also for esterlflcatlon reaction. Furthermore, other products of the esteriflcati0n reaction, such as the nitrite, and the other products mentioned above. also exert a catalytic effect on the esterificafion reaction. Ihus the recycling of the unreacted hydrocarbon, and also recycllng of 8. portion of the reaction products, is desirable. Large areas et surface contact also appear to be advantageous in both the esteriflcation and oxidation reaCtionS.

By employing, roxides as oxidizing agents, the cycloalkyl nitrites may not only be converted to dibasic acids, but may also be converted to nitrates. As far as applicants are aware, cycloalkyl nitrates have not previously been prepared. It is also within the scope oi! this invention to prepare cycloalkyl nitrates by the reaction 01 the cycloalkyl nitrites of this invention with Deroxides, whether hydrogen peroxide or organic hydroperoxides such as tertiary butl hydropr oxide, dimethyl cyclopentyl hydromroxide,

methyl cyclohexyl hydroperoxlde and the like, or

other organic peroxides 'such as benzoyl peroxide and the like. In this reaction the hydrogen peroxide is reduced to water; the organic hydroperoxides are reduced to the corresponding alcohols; and the benzoyl peroxide is reduced to benzoic acid anhydride.

In this reaction an excess of the nitrite should be employed. Atmospheric temperature and higher temperatures up to about C. are satisfactory. The reactants are preferably employed in solution in a common solvent so as to obtain a homogeneous reaction. 0rganic solvants are preierred over aqueous solvents. 'Ihus the nitrites and the organic peroxides and hydroperoxides may be dissolved in hydrocarbons such as the hydrocarbon feed stocks oi this invention or other petroleum tractions or the like. Hydrogen peroxide is preferably employed in alcohol or ether solution although it may be used in water solution. In the event that a water solution is employed, the reaction mixture will not be homogeneous. and means should be Provided to assure sufllcient intimacy et contact between the peroxide and the nitrite. If desired, the hydrogen peroxide may also be obtained by the action of water or mineral acids on inorganic hydroperoxides such as barium peroxide, sodium peroxide and the like. Other oxidizing agents et the types described above may also be employed to oxidlze the nitrites to the nitrates.

As an example of the preparation of cycloalkyl nitrates, a solution containing about 25% by weight of dimethylcyclopentyl hydroperoxide in cyclohexane is added nadually to a solution containing about 25% of cyclohexyl nitrite in cyclohexane, the molal proportion of hydroperoxide to nitrite being about 1 to 2. Upon sentie warming, the reaction goes substantiay to completion, and cyclohexane, dimethylcyclopentyl alcohol and 13 l unreacted nitrite are separated from the solution by distillation, leaving a residue which is substantially pure cyclohexyl nitrate. This materla l is an excellent additive for Diesel fuels,to improve their ignition characteristics. For this purpose they are employed in the Diesel fuel in concentrations between about 0.1% and about by weight. If desired, the 'preparation may be carried out by employing the Diesel fuel itself as the diluent. Thus Diesel fuel may be substituted for cyclohexane in the above preparation, and the resulting product diluted with additional Diesel fuel to the desired concentration. In another mode of operation, about 1% of cyclohexyl nitrite may be dissolved in the Diesel fuel, and an equimolal amount of the hydroperoxide dissolved in Diesel fuel may be added thereto. On Slightl Warming, the above reaction will take place. Although the reaction goes substantially to completion, this is not vital, and an excess of unreacted nitrite or hydroperoxide, or both, may be present in the reaction product, since all of these are efiective in improving the ignition characteristics of the Diesel fuel.

The other cycloalky] nitrites and the other peroxides and other oxidizing agents described above may also be reacted as above to obtain the corresponding cycloalkyl nitrates, which may be utilized as above.

Many modifications of the processes of this invention may be employed. For example, the nitrogen tetroxide need not be prepared by the oxidation of ammonia as described above, but may also be prepared by direct oxidation of nitrogen of the atmosphere by the Birkeland-Eyde process, or a modification thereof recently disclosed in the literature, whereby a fuel such as methane is burned in an excess of air at a temperature in the neighborhood of 4,000 F. or above to form nitric oxide, which is oxldized to the tetroxide as described above. In this mode of preparaticn, the concentration of the nitric oxide or nitrogen tetroxide is quite low, and consequently it may be desirable to employ other modes of purification in addition to or in place of those described above, such as for example, low temperature fractional distillation, charcoal adsorption, silica gel adsorption, and the like.

It is also apparent that in the course of the above esterificaticn and oxidation reactions nitric oxide may be liberated, and this is readily oxidized to form the desired nitrogen tetroxide. Furthermore, nitric acid itself may be decomposed to form nitrogen tetroxide, which may be extracted or separated from the product by the methods above. 'Nitric oxide formed by the decomposition of nitric acid may also be oxldized to supply the necessary nitrogen tetroxide.

As mentioned previously, catalysts may be desirable in the esterification reaction as well as the oxidation reaction. It is also desirable to employ a dehydrating agent in the esterificaticn reacticn zone itself, so that the water liberated in the reaction is not allowed to hydrolyze the nitrite esters. Silica gel and diatomacecus earth are suitable dehydrating agents, which also act as catalysts. These are introduced in suspension in the feed, or are placed in the reactor as a bed. Light, especially ultraviolet light, is a catalyst for the above esterificaticn also, and appears to inhibit the production of nitro hydrocarbons as well as catalyze the production of the nitrite and the oxidation products thereof. Also, as previously pointed out, recycling of the esterification reaction products is desirable because of catalytic effects of nitric oxide and other reaction products. Recycling of nitro hydrocarbons has the additional desirable effect of inhibiting the 'nitrite to form cyclohexyl alcohol and ethl thionitrite. The thionitrites are also excellent additives for improving the ignition quality Dieselfuels. This reacticn also takes place at atmospheric or slightly elevated temperatures, and is preferably carried out in 011 solutions.as with the preparation of the nitrates above described.

As indicated above the nitrite esters of-this invention are of great value because of their great reactivity to form other chemicals. However, they are also valuable in themselves as ignition promoters for Diesel fuels or jet propulsion fuels, as effective components ofinsecticides or fungicides, and for other uses.

Although the term nitrogen tetroxide is used exclusively herein it is to be understood that this term includes other forms which may be in equilibrium with the tetroxide such as nitrogen dioxide. As shown by Riebsomer in The reactions of nitrogen tetroxide with organic compounds, in Chemical Reviews, vol. 36, No. 2, April 1945, page 159, the structure of nitrogen tetroxide is not definitely known and undoubtedly changes with changes in temperature or other conditions. We have found however that nitrogen tetroxide is very diflerent in its reactions with hydrocarbons than is nitric acid. In the presence of appreciable amounts of water or nitric acid the organic nitrites are apparently not formed, or possibly are immediately destroyed with the production of oxidation or isomerizaticn products.

Other modifications of this invention which would be apparent to one skilled in the art are to be included within the scope of the invention as defined in the following claims.

We claim:

1. A process for the production of cycloalkyl nitrites which comprises dissolving a minor proportion of nitrogen tetroxide in a cycloalkane, heating the resulting solution in the liquld phase so as to form the corresponding cycloalkyl nitrite and separating from the Iiquid product a fraction rich in said nitrite.

2. A process for the production of a cycloalkyl nitrite which comprises dissolving a minor proportion of nitrogen tetroxide in a cycloalkane,-

maintaining the resulting solution at an elevated temperature so as to form a liquid product containing the corresponding cycloalkyl nitrite and a gascons product containing nitric oxide, separating said nitrite and unreacted cycloalkane from said liquid product, recycling said unreacted cycloalkane, oxldizing said gaseous product containing nitric oxide to obtain additional nitrogen tetroxide, and recycling said nitrogen tetroxide.

3". A process for the production of cycloalkylnitrites which comprises oxidizing ammonia so as to obtain an ammonia oxidation product con-= taining water and nitric oxide, dehydrating the said ammonia oxidation product to remove said water therefrom, oxidizing the dehydrated am:: monia oxidation product to obtain a gaseous mix-=. turc containing nitrogen tetroxide, contacting:

said gascons mixture with a liquid cycloalkane thereby absorbing said nitrogen tetroxide in said cycloalkane to form a solution containing a minor proportion of nitrogen tetroxide, maintaining the resulting solution et au elevated temperature so as to form a liquid product containing the corresponding cycloalkyl nitrite and a gaseous product containing nltric oxide, separating said nitrite and unreacted cycloalkane from said liquid product, recycling said unreacted cycloalkane, and recycling said gaseous product containing nitric oxide to the stage in which the dehyclrated ammonia oxidation product is oxidized to form nitrogen tetroxide.

4. A process for the production of cycloalkyl nitrites which comprises dissolving a-minor proportion of nitrogen tetroxide in a. cycloalkane, introducing the resulting solution having a temperature lower than about 40 C. into the reaction zone, heating the solution in said reaction zone, to a reaction temperature greater than about 100 C., maintaining the solution in the liquid phase for a suflicient contact time to form the cycloalkyl nitrite, removing the reaction mixture from the reaction zone, and recovering said cycloalkyl nitrite theretrom.

5. A process for the production 01 cycloalkyl nitrites which comprises dissolving a minor proportion of nitrogen tetroxide in a cycloalkane, introducing said solution having a temperature below about 40 C. into a reaction zone, maintaining said solution as it passes through said reaction Zone substantially in the liquid phase while heating it to a reaction temperature above about 100 C., maintaining it at said temperature while reacting said nitrogen tetroxide with part of said cycloalkane so as to form a liquid product contalning unreacted cycloalkane and cycloalkyl nitrite, separating said unreacted cycloalkane from said product and recycling it, and recovering said cycloaikyl nitrite from the remainder of said product.

6. A process for the production of cycloalkyl nitrites which comprises dissolving a minor proportion of nitrogen tetroxide in a cycloalkane, introducing said solution having a temperature below about 40 C. into a reaction zone, maintaining said solution as it passes through said reaction zone substantially in the liquid phase while heating it to a reaction temperature above about 100 C. and maintaining it at said temperature while reacting said nitrogen tetroxide with part 01 said cycloalkane so as to form a liquid product containing unreacted cycloalkane and cycloalkyl nitrite and a gaseous product comprising nitric oxide, separating said gaseous product from said liquid product, oxidzing the nitric oxide therein to nitiogen tetroxide and recycling said nitrogen tetroxide, separating said unreacted cycloalkane from said liquid product and recycling it; and recovering said cycloalkyl nitrite from the remainder of said product.

7. A process for the production of cycloalkyl nitrites which comprises ox*dizing ammonia so as to obtain an ammonia oxidation product containing water and nitric oxide, dehydrating the said ammonia oxidation product to remove said water therefrom, oxidizing the dehydratd ammonia oxidation product to obtain a gaseous mixture containing nitrogen tetroxide, contaCting said gaseous mixture with a liquid cycloalkane thereby abmrbing said nitrogen tetroxide in said cycloalkane to form a solution containing a minor proportion of nitrogen tetroxide, introducing said solution having a temperature below about 40 C.

into a reaction zone, maintaining said solution as it passes through said reaction zone substantially in the liquid phase while heating it to a reaction temperature above about C. and maintaining it at said temperature while reacting said nitrogen tetroxide with part of said cycloalkane 50 as to form a liquid product contamine unreaCted cycloalkane and cyCloalkyl nitrite and a gascons product comprising nitric oxide, separating said gaseous product from said liquid product, recycling said gaseous product to the stage in which the dehydrated ammonia oxidation product is oxidized to form nitrogen tetroxide, separating said unreacted cycloalkane from said liquid product and recycling it, and reCovering said cycloalkyl nitritefrcm the remainder of said product.

8. A process according to claim 7 in which an aqueous phase is formed in the reaction zone, and is not permitted to separate from the liquid reaction product until the latter is removed from the reaction zone.

9. A continuous process for the production of cyciohexyl nitrite which comprises oxidizing ammonia with air so as to obtain an ammonia oxidation product containing water and nitric oxide, dehydrating the said ammonia oxidation product to remove said water therefrom, oxidizing the dehydrated ammonia oxidation product to form a gaseous mixture containing nitrogen tetroxide. absorbing said nitrogen oxide in cyclohexane to form a solution containing about 5% to 15% of nitrogen tetroxide, introducing said solution at a temperature below about 40 C. into a reaction zone, maintaining said solution as it passes through said reaction. zone substantially in the liquid phase while heating it to a reaction temperature between about 100 C. and 250 C. and maintaining it at said reaction temperature for a contact time suflicient to react said nitrogen tetroxide with a part of said cyclohexane so as to form a gaseous product comprising nitric oxide and a liquid product contalning cyclohexyl nitrite and unreacted cyclohexane, recycling said gaseous product to the stage in which the dehydrated ammonia oxidation product is oxidizgd to form nitrogen tetroxide, separating said unreacted cyclohexane tram said liquid product, recycling said unreacted cyclohexane to the absorption stop. and recovering said cyclohexyl nitrite from the remainder of said liquid product.

10. A process for the production of nitrous acid esters of cycloalkanes which comprises dissolving a minor proportion of dry nitrogen tetroxide in said liquid cycloalkane and continuously passing the resulting solution through an elongated reaction zone at an elevated teniperature between about 100 C. and 250 C. and a pressure suiflcient to maintain the hydrocarbon substantially in the liquid phase, at a rate suflicient to consume substantiaily all of the nitrogen tetroxide prior to leaving the reactor.

11. A process according to claim 1 in which the cycloalkane is cyclohexane.

12. A process according to claim 1 in cycloalkane is cyclopentane.

13. A process according to claim 4 in which the cycloalkane is cycl0hexane.

14. A process according to claim 4 in which the cycloalkane is cyclopentane.

THOMAS F. DOUMANI. CLARENCE S. 00152.

EDWARD c. AI1AN, JR.

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