Separation of Dinitrogen Pentoxide from Its Solutions in Nitric Acid

Chapter 9

Separation of Dinitrogen Pentoxide from Its Solutions in Nitric Acid 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on May 20, 2013 | http://pubs.acs.org Publication Date: April 24, 1996 | doi: 10.1021/bk-1996-0623.ch009

Robert D. Chapman and Glen D. Smith TPL, Inc., 3768 Hawkins St., NE, Albuquerque, N M 87109

A convenient process for the separation of nitrogen pentoxide from its solutions in nitric acid (as commercially prepared by electrolysis of N O - H N O ) involves the chemical absorption of the nitric acid by sodium fluoride (producing sodium hydrogen fluoride) in inert organic solvents, such as acetonitrile, suitable for use with many nitrolyzable substrates. Contact time between the N O - H N O solution and the sodium fluoride solid is critical due to a competing sorption of the N O in the later stages of the process, speculated to be due to hydrogen bonding between N O and NaHF . During this stage, continued contact of the solution with the partially spent acid absorbent (sodium fluoride) removes even small amounts of residual nitric acid but also lowers the recoverable yield of N O . 2

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The objective of this work was the development of a process for an improved prepara tion or separation of acid-free nitrogen pentoxide. Improved methodology for N 0 is potentially valuable for numerous applications in synthetic nitro chemistry, including a variety of useful transformations developed recently by researchers in the British Defence Research Agency (RARDE, Fort Halstead) (7). These transformations, many leading to energetic materials of defense interest, include C- and N-nitrations or nitrolyses, oxacyclic ring cleavages leading to vicinal dinitrate esters, and selective Onitrations without affecting sensitive ring structures. Since the latter transformations require nitrations conducted under relatively mild (i.e., non-acidic) conditions, improvements in methodology for acid-free N 0 should be particularly beneficial. 2

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Current address: Research and Technology Division, Weapons Division, Naval Air Warfare Center, Code 474220D, China Lake, CA 93555-6001

0097-6156/96/0623-0078$15.00/0 © 1996 American Chemical Society In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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9. CHAPMAN & SMITH

Separation ofN O from its HN0 Solutions 2

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There are two general, industrially feasible preparative routes to N 0 (7): (1) ozonolysis of N 0 (2-5); and (2) electrolytic oxidation of N 0 solutions in nitric acid (6-11). Although the latter electrochemical reaction was reported as early as 1948 (72), its potential for industrial-scale production of energetic materials was not recognized and exploited until Lawrence Livermore National Laboratory researchers developed an "industrial" process based on it in the early 1980s (6-8). The two halfreactions involved in this electrolytic process are oxidation of N 0 to N 0 (anodic reaction) and reduction of H N 0 to N 0 (cathodic reaction): N 0 + 2NO3- 5 from which acid is to be removed. These systems varied several conditions: N 0 / H N 0 samples (two homemade batches plus a Thiokol sample); ^O^/HNO^ concentrations in chloroform (5-25 w/v%); ratios of sodium fluoride to acid (ranging from 1-13 weight-equivalents); temperatures (rang ing from -20 to 0 °C, though most were run at 0 °C); and contact times (with or with out manual agitation) ranging from -15 min to >2 h. Transfer of liquids was accomp lished under a flow of dry nitrogen or argon or in a glove bag; and manual agitation by swirling the reaction flask was usually performed throughout the reaction (i.e., the contact time). It may be expected that surface area of the acid absorbent (i.e., sodium fluoride) and agitation of the heterogeneous reaction would influence its efficiency.


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In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

82

NITRATION


Results and Discussion A typical result from the chloroform experiments was that significant residual acid remained after short contact times (-15 min), especially when low sodium fluoride/ acid ratios were employed. When longer contact times were allowed (>2 h) at high sodium fluoride/acid ratios, absorption of essentially all of the nitric acid seemed to be achieved (according to initial analyses by *H NMR)—but at the expense of apparently sorbing (absorbing or adsorbing) essentially all of the N2O5 as well. A serendipitous variation in conditions for this acid extraction approach involved a change in solvent to acetonitrile, which was also expected to be convenient for practical nitrations by N2O5. The first experiment in this solvent gave the first indi cation that nitric acid and nitrogen pentoxide were removed at different rates in sys tems employing this acid absorption principle. In particular, -85% of the original nitric acid content was removed in this experiment (according to *Η NMR) while the majority (>98%) of the expected N2O5 content was still present according to quanti tative nitration of benzene following the method described by Bloom et al. (14). This result dictated a more careful analysis of the course of the reaction as acid is absorbed. Thus, a repetition of the conditions of that extraction experiment next entailed careful monitoring of the progress of the sorptions via *Η and N N M R spectrometry. In both N M R analyses, the acetonitrile solvent signals were used as internal standards for quantification of the relative amount of nitric acid (by H NMR) or total nitro content (by N NMR). 1 4

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The results were initially analyzed, in the case of the N data, by plotting the simple ratio of integrals of the nitro peak (δ -47 to -64) and the acetonitrile peak (δ -139)—corresponding to (2[N 0 ]+[HN0 ])/[CH CN]—versus time. The bestcharacterized run was continued up to a reaction time (i.e., contact time between N 0 / H N 0 and sodium fluoride) of 269 min at time intervals of -30 min. The experiment utilized 9.79 g of N 0 5 / H N 0 solution (measured as 17.63% N 0 ) plus 41.40 g of sodium fluoride ( N a F / H N 0 mole ratio = 7.7) in 40 mL acetonitrile, maintained at 0 °C during monitoring of the reaction. After this duration of 4.5 h, real time inspection of the N M R data indicated that residual H N 0 and N2O5 contents were both quite small. Relevant NMR data from this experiment are given in Table I. 2

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The nitro content kinetics from N N M R data are shown in Figure 1. The trend exhibited by the data is also interesting: after an initial exponential decay attributable to absorption of nitric acid by sodium fluoride, the onset of a faster removal of nitro species (presumably N2O5) occurs at about 150-180 min. The trend of the later removal also looks like a decaying exponential, so the observed trend may be a convo lution of two phenomena behaving in this manner. A further consideration of natural phenomena may suggest, however, that events such as the sorption of chemical species tend not to start instantaneously after absolute inactivity for long periods (in the absence of obvious extrinsic factors, such as the addition of reactants). On this

In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

9. CHAPMAN & SMITH

Separation of Ν 0 from 2

its HNO3 Solutions

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Table I. NMR Data From Sodium Fluoride Acid Absorption (Acetonitrile) Time from

«N0


NaF addn δ ι

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integration integration

Η

(min)

( H N 0 ) (lH)

0 29 59 89 117 151 179 210 240 269

13.81 11.40 10.88 10.26 9.73 9.43 9.11 8.49 8.05 7.86

a

HMD

CW CN

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0.666 0.185 0.121 0.0720 0.0252 0.0164 1.00 0.473 0.175 1.027

Ν

+

(N 0 )

!

( H)

3

δΐ4 2

13.898 7.089 7.427 7.494 7.651 7.680 616.55 313.096 304.017 3570.985

5

-47.16 -50.23 -53.90 -57.62 -«0.51 -62.79 -63.39 -63.57 -64.02 -64.23

CH CN 3

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integration

N0 2

5 3

integration ( N ) 14

65.00 66.11 72.99 36.29 36.85 37.75 528.50 37.775 37.209 572.621

13.01 5.86 4.20 1.47 1.13 1.04 13.86 0.647 0.339 0.924

Arbitrary units.

basis, it should be presumed that the later phenomenon is not another simple expo nential decay with an absolute onset at a time several hours into the system reaction. Rather, another physically realistic mathematical function that a chemical phenomenon such as N2O5 sorption may be expected to follow is the logistic sigmoid, which is followed by autocatalytic second-order reactions, for example (22). This function has the general form: C

-C

·

_i_ max ^min

Γ —Γ

^-^min+

1 + e

-*(M

5 0

/

Separation of Dinitrogen Pentoxide from Its Solutions in Nitric Acid

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