PREPARATION OF NITROALKANES WITH ACTINIC LIGHT

Abstract: Nitroalkanes having from 1 to 4 carbon atoms are made by reacting the analogous aliphatic hydrocarbon with nitric acid, or NO₂, in the vapor phase in the presence of actinic light. Lower temperatures and pressures are required than known to the art and the product is obtained in better yields and with fewer by-products.

Description: PREPARATION OF NITROALKANES WITH ACTINIC LIGHT Nitroalkanes are an essential stabilizing ingredient employed in l,l,l-trichloroethane when it is used in vapor degreasing and cold cleaning. All manufacturers throughout the world add nitromethane and/or nitroethane to their commercial l,l,l-trichloroethane-based solvents. Normally nitroalkanes are manufactured by a vapor phase nitration of the alkane with either nitric acid or NO2. There is a mixture of products formed due to carbon-carbon scission. Thus, for example, when propane is nitrated, the products include l-nitropropane, 2-nitropropane, nitroethane and nitromethane. Because of the oxidative conditions other oxygen-containing compounds are also produced, e.g. aldehydes, acids, and carbon oxides. Patents disclosing such a process are U.S. 2,844,634 and 2,905,724. Improvements in these vapor phase nitrations are claimed by employing the nitric acid or nitrogen oxides together with oxygenated sulfur compounds, e.g. SO2, His04, (U.S. 3,272,874) and by conducting the nitration in the presence of ozone (U.S. 3,113,975). Other processes involve nitration of paraffins by nitrogen peroxide (NO2)2 in the presence of oxygen (air) under pressure at 1500-3300C (U.S. 3,780,115); reacting an olefin with nitric acid in the presence of a lower aliphatic monocarboxylic acid anhydride to produce a nitroester and subsequently reducing with an alkali borohydride to form the nitroalkane (U.S. 3,706,808) and reacting organic amines with ozone (U.S. 3,377,387). The present process is an improvement which reduces the amount of by-products and obtains improved yields of the desired products by conducting the nitration of paraffins by employing nitric acid at lower temperatures and pressures in the presence of ultraviolet light. It has now been discovered that the lower alkanes containing from 1 to 4 carbon atoms can be nitrated to yield the analogous nitroalkanes and/or a nitroalkane having fewer carbon atoms than the starting alkane. The process reacts nitric acid with an alkane in the vapor phase in the presence of actinic radiation. Thus, for example, one obtains a mixture of nitromethane and nitroethane by conducting the reaction of nitric acid and ethane in the presence of ultraviolet light, having wavelengths within the range of 3200 to 14000 , at atmospheric pressure and a temperature of 1300C at a contact time of about 8 seconds. The reaction to produce nitroalkanes at lower pressures and temperatures is conducted by exposing the lower alkane and nitric acid reactants in the vapor phase to actinic light. Temperatures are desirably in the range of 1000 to 1500C and the pressure is desirably O to 700 kPa gauge. A molar ratio of lower alkane to nitric acid of from 0.75 to 3.0 moles of alkane per mole of acid and a contact time of 4 to 10 seconds are employed. Preferred temperature, pressure, contact time and molar ratios of reactants are from about 1250 to about 1400C, from 0 to 350 kPa gauge, 6.5 to 9 seconds and from 1.2 to 2.5 moles of alkane per mole of acid. Although not required, it is preferred to conduct the reaction in a glass reactor so the reactant may be easily exposed to actinic light. Alternatively, the reactor may have a light well for the admission of the light. Conventional ultraviolet lamps are a suitable source of light. Example 1 In a representative example of the invention nitric acid was fed by a positive displacement metering pump through a vaporizing preheater, mixed with nitrogen diluent and a gaseous alkane, in this case, ethane. The molar ratio of ethane to HUN03 was 1.2/1. The mixed gas stream was then introduced into a reactor equipped with a light well (essentially a glass window which admits an ultraviolet light source) and steam-jacketed walls. The steam pressure was regulated for temperature control. At 125-1300C, atmospheric pressure, and 7.8 seconds contact time, 18.1 percent of the ethane being fed was converted to 15.3 percent nitromethane and 43.5 percent nitroethane. The effluent from the reactor was scrubbed with water in a packed trickle bed column and the water analyzed by gas chromatography for the nitro -alkanes. Ethane conversion was determined by taking gas samples of the mixed feed and reactor effluent and analyzing by gas chromatograph. Example 2 A source of U.V. light (Conrad-Eanovia lamp, #679A-0360) was employed using Vycore Pyrex and quartz glass reactors, which selectively transmitted certain sections of the spectrum, and employed as in Example 1 an ethane/HNO3 mol ratio of 1.2/1 at 1300C, atmospheric pressure and 7.8 seconds reaction time. Table I shows the conditions and results of these runs using ethane and nitric acid. TABLE I Reactor W Wavelength % Conversion % Selectivity to Material range of C,Hs NM* NE* o o Quartz 2200A-14000A 8.8 12.6 35.9 o o Vycor 2800A-14000A 11.8 12.6 37.9 o o Pyrex 3200A-14000A 18.9 14.3 40.5 *NM = nitromethane, NE = nitroethane Example 3 In another experiment run under the same conditions, methane and 30 percent nitric acid were employed and the molar ratio of the reactants was varied to determine the effect on conversion and selectivity. The ratios used and results are shown in Table II. TABLE II CH4/HN0 % Conversion % Selectivity mol ratio ¯ of CH4 to CH,NO, 0.89/1 19.0 18.9 1.06/1 16.1 23.6 1.48/1 14.6 21.5 2.97/1 3.8 51.8 Example 4 The contact time was varied to determine its effect on conversion and selectivity employing a temperature of 1300C and atmospheric pressure. The HNO3, employed as 68 percent nitric acid, was used at a ratio of 1.2 moles methane per mole of acid. Table III shows the results. TABLE III Contact Time % Conversion % Selectivity (seconds) of CH4 to NM 5.0 4.3 50.7 6.5 13.1 21.2 7.0 15.9 17.6 9.4 17.3 17.0 Example 5 The contact time was varied at a mol ratio of 1.76 moles ethane per mol of HNO3 employing a temperature of 1300C and a pressure of 0 psig. Results are shown in Table IV. TABLE IV Contact Time % Conversion % Selectivity to (seconds) of C,H NM NE 8.4 5.8 18.0 51.6 9.2 7.4 16.5 45.5 10.2 10.0 15.4 41.9 Example 6 Under the same conditions of mol ratio and pressure as in Example 5, ethane and HNO, were reacted at substantially the same contact time but at different temperatures. Results are shown in Table V. TABLE V Temperature Contact Time % Conversion % Selectivity to ( C) (seconds) of C,R NM NM 128 8.4 6.0 17.5 48.5 135 8.6 19.7 8.9 22.1

Claims: CLAIMS 1. A process of making nitroalkanes which comprises reacting HN03 and a lower alkane in the vapor phase at a temperature below 1500C and a pressure of less than 700 kPa in the presence of actinic light. 2. The process of Claim 1 wherein the mol ratio of lower alkane to HNO3 is 0.75 to 3 moles alkane per mol of Hun03. 3. The process of Claim 2 wherein the reaction temperature is maintained at 125 to 1400C. 4. The process of Claim 2 wherein the pressure of the reaction is maintained at from 0 to 350 kPa. 5. The process of Claim 2 wherein the contact time is from 4 to 10 seconds. 6. The process of Claims 1, 2, 3, 4 or 5 wherein the light employed has a wavelength within the range of from 2200 to 14000A. 7. The process of Claim 6 wherein the high intensity light employed has a wavelength within the range of from 3200 to 14000 . 8. A process of making nitroalkanes by reacting HNO3 and a C1 to C4 alkane in the vapor phase, the improvement of which comprises, conducting the reaction (a) at a pressure of less than 700 kPa; (b) at a temperature of less than 1500C; and (c) in the presence of actinic light.

Inventors: HAYES, WILLIAM VERNE, JONES, MARK BERRY
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1983
Main classification: C07C79
IPC codes: C07C67, B01J19, C07C205, C07C201