Process for preparing solid amine oxides

A solid amine oxide is prepared by reacting a tert-amine with at least a stoichiometric amount of an aqueous hydrogen peroxide having a concentration of at least 50% by weight in the presence as the reaction medium of a normally gaseous material which (a) has a critical temperature <160.degree. C. and a critical pressure <12 MPa, (b) is in a liquefied, densified, or supercritical state in which it has a density of at least 0.1 g/cc, and (c) is present in an amount sufficient to maintain the reaction mixture stirrable throughout the reaction.

FIELD OF INVENTION 
The invention relates to a process for preparing solid amine oxides. 
BACKGROUND 
It is known that amine oxides are useful materials, those that are most 
attractive commerically being the mixed amine oxides, i.e., amine oxides 
having at least one long-chain group and at least one short-chain group 
attached to the amino nitrogen. These oxides are used in many formulations 
in which their surface activity is an attribute, e.g., laundry detergents, 
rinses, and dryer sheets; shampoos and hair conditioners; soaps, and other 
personal hygiene products. 
As taught in U.S. Pat. No. 4,748,275 (Smith et al.-I) and the references 
discussed therein, there are many known methods of preparing amine oxides 
by reacting tert-amines with aqueous hydrogen peroxide. The syntheses most 
commonly employed are the aqueous processes utilizing sufficient water to 
provide the products as aqueous solutions, e.g., the processes of U.S. 
Pat. No. 4,247,480 (Murata et al.) and European Patent Application 0307184 
(Bauer et al.) in which carbon dioxide is used to promote the reaction. 
Less commonly, the amine oxides are prepared in organic solvents, as in 
U.S. Pat. No. 3,776,959 (Stalioraitis et al.). 
The aforementioned solvent processes are quite satisfactory for the 
products which are to be used in applications in which their water or 
organic solvent content can be tolerated. However, the utilization of 
these processes necessitates the performance of after-treatments, such as 
spray-drying or evaporation, when the amine oxides are intended for use in 
applications, such as dry solid laundry detergent formulations, in which 
the presence of the solvent cannot be tolerated. 
Smith et al.-I teach that the use of a temperature high enough to maintain 
the product in a molten state permits some amine oxides to be prepared in 
the solid form that makes them more desirable than the dissolved oxides 
for some purposes. 
Copending application Ser. No. 07/591,426 (Smith et al.-II) discloses a 
process in which stirrability of the reaction mixture is maintained by 
conducting at least the latter part of the tert-amine/hydrogen peroxide 
reaction in an organic solvent in which the amine and amine oxide are 
soluble at the reaction temperatures but in which the amine oxide is 
insoluble at a lower temperature, thus permitting a relatively easy 
recovery of the amine oxide in solid form. In some respects this process 
is more attractive than the process of Smith et al.-I. However, when it is 
used to prepare a substantially pure amine oxide, it requires the use of 
centrifugation, crystallization, and drying steps which add to its cost. 
SUMMARY OF INVENTION 
It has now been found that amine oxides can be more economically prepared 
in solid form by reacting a tert-amine with at least a stoichometric 
amount of an aqueous hydrogen peroxide having a concentration of at least 
50% by weight in the presence as the reaction medium of a normally gaseous 
material which (a) has a critical temperature &lt;160.degree. C. and a 
critical pressure &lt;12 MPa, (b) is in a liquefied, densified, or 
supercritical state in which it has a density of at least 0.1 g/cc, and 
(c) is present in an amount sufficient to maintain the reaction mixture 
stirrable throughout the reaction.

DETAILED DESCRIPTION 
The process of the invention is applicable to the oxidation of any 
tert-amine which can be reacted with hydrogen peroxide to form an amine 
oxide. As is known, these amines include a variety of tert-amines 
corresponding to the formula RR'R"N wherein R, R', and R" are 
independently selected from alkyl, hydroxyalkyl, cycloalkyl, and aralkyl 
groups containing up to 30 carbons and any two of those groups may form a 
non-aromatic heterocyclic group, such as a morpholine or piperidine ring, 
with the nitrogen. However, they are generally tert-amines of that formula 
in which R, R', and R" are independently selected from primary alkyl and 
hydroxyalkyl groups containing 1-30 carbons. 
Because of greater interest in the oxides prepared from them, the 
tert-amines which are apt to be preferred for use in the process are those 
in which R is methyl, ethyl, or hydroxyethyl; R' is a primary alkyl group 
containing 6-24 carbons; and R" is independently selected from methyl, 
ethyl, hydroxyethyl, and primary alkyl groups containing 6-24 carbons. Of 
these preferred tert-amines, those which are particularly preferred are 
those in which the primary alkyl groups have a straight chain in at least 
most of the molecules, generally at least 70%, preferably at least 90% of 
the molecules. 
Exemplary of the tert-amines that may be used are trimethylamine, 
triethylamine, N-isobutyldimethylamine, trihexylamine, 
N,N-dimethyl-2-ethylhexylamine, N-eicosyldimethylamine, 
N-isobutyl-N-triacontylmethylamine, N-benzyldimethylamine, 
N-ethyldibenzylamine, N,N-diisobutyl-4-t-butylbenzylamine, 
tri-2-hydroxyethylamine, and, more preferably, (1) the N-alkyldimethyl-and 
N,N-dialkylmethylamines in which the alkyl groups are hexyl, octyl, decyl, 
dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, and/or 
tetracosyl, (2) the corresponding amines in which the methyl groups are 
replaced with ethyl or hydroxyethyl groups, and (3) mixtures of such 
amines. 
The aqueous hydrogen peroxide employed in the reaction may be any aqueous 
hydrogen peroxide having a concentration of at least 50% by weight. 
However, to minimize the amount of water in the product, it is preferred 
to use a hydrogen peroxide having a concentration of at least about 70%, 
generally about 70-90% by weight. 
As is customary in oxidations of tert-amines, the amount of hydrogen 
peroxide utilized is at least the stoichiometric amount. It is 
undesirable, however, to use too great an excess of the oxidizing agent, 
so the amount employed is generally about 1.1-1.2 times the stoichiometric 
amount. 
The fluid used as the reaction medium for the process may be a liquefied, 
densified, or supercritical form of any normally gaseous material which 
has a critical temperature &lt;160.degree. C. and a critical pressure &lt;12 
MPa; has a density of at least 0.1 g/cc, preferably at least 0.15 g/cc, 
and more preferably at least 0.2 g/cc under the reaction conditions; and 
is inert in the sense that it will neither prevent the reaction from 
occurring nor react with the product. Such normally gaseous materials 
include, e.g., air, oxygen, carbon dioxide, nitrogen, argon, ethylene, 
methane, ethane, propane, butane, isobutane, methyl fluoride, 
trifluoromethane, tetrafluoromethane, chlorotrifluoromethane, and mixtures 
thereof, as well as the other suitable gases mentioned in Lange, Handbook 
of Chemistry, Ninth Edition, McGraw-Hill Book Company, Inc. (New York), 
1956, pp. 1494-1498, the teachings of which are incorporated herein by 
reference. 
For economic reasons, it is frequently preferred that the reaction medium 
be one formed from a normally gaseous material which has a critical 
temperature that is above or not much below room temperature, generally a 
critical temperature of at least 0.degree. C., preferably at least 
20.degree. C., most preferably 20.degree.-50.degree. C., e.g., materials 
such as ethylene, carbon dioxide, chlorotrifluoromethane, methyl fluoride, 
trifluoromethane, ethane, propane, butane, and isobutane. 
Liquefied, densified, or supercritical carbon dioxide is generally the most 
preferred reaction medium because of its being able to serve the 
additional function of promoting the reaction, and a medium formed from 
air can also speed the reaction. However, any of the other media of the 
invention can be employed when increasing the rate of the reaction is not 
a prime consideration. 
Most commonly, the medium which is employed is one that is commercially 
available as a liquefied gas and can simply be introduced into the 
reaction vessel in liquid form and maintained in liquid form by the use of 
pressure. However, if desired, it may be acquired in the gaseous state and 
introduced into the reaction vessel via a compressor to convert it to a 
liquefied, densified, or supercritical state. 
The manner in which the process is conducted can be varied considerably as 
long as at least the latter part of the reaction is conducted in an amount 
of the reaction medium sufficient to maintain the reaction mixture 
stirrable throughout the reaction. The process may be a batch, semi-batch, 
or continuous process; and the reaction medium may be present throughout 
the reaction, or it may be added to the reaction mixture only when the 
reaction has proceeded to the stage where a solvent is needed to maintain 
the reaction mixture stirrable. Also, the ingredients of the reaction 
mixture can be combined in many different ways. For example: 
(1) the hydrogen peroxide can be gradually added to a solution of the 
tert-amine in the reaction medium, 
(2) separate streams of the tert-amine and the hydrogen peroxide can be 
gradually added to the reaction medium, 
(3) separate streams of the hydrogen peroxide and the reaction medium can 
be gradually added to the tert-amine, or 
(4) the hydrogen peroxide can be gradually added to the tert-amine and 
allowed to react therewith until a substantial amount of amine oxide has 
been formed before the reaction medium is added. 
As in conventional processes, it is preferred to combine the reactants at a 
controlled rate because of the exothermic nature of the reaction; the 
reaction may be conducted in the presence of a chelating agent, such as 
diethylenetriaminepentaacetic acid or ethylenediaminetetraacetic acid, if 
desired; and it is generally preferred to maintain contact between the 
reactants until the reaction is substantially complete. 
When the reaction medium is not added until needed, the temperatures at 
which the reaction may be conducted during the earlier portion of the 
reaction may be any temperatures conventionally employed for such 
reactions, usually temperatures in the range of 20.degree.-90.degree. C. 
It is sometimes desirable to continue using such temperatures after the 
reaction medium is added when they are appropriate for maintaining the 
reaction medium in the desired liquefied gas, densified gas, or 
supercritical fluid state. However, even when the temperatures would be 
appropriate for the portion of the reaction conducted in the reaction 
medium, it can sometimes be desirable to use a lower temperature for the 
latter part of the reaction, e.g., when that portion of the reaction is 
conducted in the presence of liquefied, densified, or supercritical carbon 
dioxide. 
When the reaction medium is such a carbon dioxide, the reaction proceeds 
much more rapidly than conventional carbon dioxide-promoted amine oxide 
syntheses because of the larger amounts of carbon dioxide used as a 
solvent, thus permitting lower temperatures to be used without slowing the 
reaction to a commercially-unacceptable rate. Accordingly, it could be 
desirable to lower the temperature from one in an upper portion of the 
20.degree.-90.degree. C. range to a lower temperature in that range or to 
a temperature even below 20.degree. C. for the portion of the reaction 
during which the reaction medium is present. 
The temperature and pressure conditions employed for the reaction while the 
reaction medium is present will vary with the particular reaction medium 
used. Thus, e.g., (1) a temperature and pressure below the critical 
temperature and critical pressure of the normally gaseous material are 
used when it is desired to maintain the reaction medium as a liquefied 
gas, (2) a temperature and pressure above the critical temperature but 
below the critical pressure are utilized when the reaction medium is to be 
maintained as a densified gas, (3) temperatures and pressures above the 
critical temperature and pressure are used when the reaction medium is to 
be a supercritical fluid, (4) and variations among those conditions are 
permitted when it is wished to change the reaction medium from being in 
one of the acceptable fluid states (i.e., liquefied gas, densified gas, 
and supercritical fluid) to another of those states during the course of 
the reaction. The temperatures and pressures which can be used to maintain 
the utilizable normally gaseous materials in these states are, of course, 
already known to those skilled in the art. 
In the preferred embodiment of the invention wherein the reaction medium is 
liquefied carbon dioxide, the reaction may be virtually instantaneous at 
room temperature and the pressure required to keep the carbon dioxide 
liquid. It could therefore be desirable to conduct the entire reaction at 
room temperature when the liquefied carbon dioxide is present throughout 
the reaction, although higher temperatures are preferably used for any 
portion of the reaction conducted before the liquefied carbon dioxide is 
added, and higher temperatures could also be used after the addition of 
the liquefied carbon dioxide when it is desired to convert the liquefied 
carbon dioxide to densified or supercritical carbon dioxide during the 
reaction. 
In general, (1) when the reaction medium is an appropriate form of carbon 
dioxide, it is usually preferred to conduct the reaction at temperatures 
in the range of 20.degree.-80.degree. C.; and it is sometimes even more 
preferred to have the temperatures in the range of 20.degree.-40.degree. 
C., most preferably 20.degree.-30.degree. C., and (2) when an appropriate 
form of another normally gaseous material is used, it is preferred to 
employ one which permits the reaction to be conducted at temperatures not 
higher than 80.degree. C., preferably at temperatures not higher than 
60.degree. C. 
The reaction medium is used in solvent amounts which, if desired, may be 
only the minimum required to keep the reaction mixture stirrable. However, 
unlike the process of Smith et al.-II, in which it is preferable to 
minimize the amount of solvent employed, there is no reason to minimize 
the amount of solvent used in the present process. Since the reaction 
medium can be easily vented from the reaction vessel by reducing the 
pressure at the end of the reaction, and the vented gas can be easily 
recycled, the economic advantages of the process are not lost when a 
considerable excess of the solvent is utilized. Thus, the amount of 
reaction medium employed is generally apt to be such that it constitutes 
about 10-75% by volume of the reaction mixture and the reaction 
medium/reactant weight ratio is at least 1/1. 
As already mentioned, the process is conducted under a pressure sufficient 
to maintain the reaction medium in the desired state, but the pressure is 
not otherwise critical. Ordinarily, however, it is desirable to use an 
amount of pressure consistent with conducting an economical process, 
usually a pressure in the range of about 4.9-8.5 MPa. 
After completion of the reaction, the system is vented to remove the 
reaction medium, and the amine oxide product is recovered from the 
reaction vessel. Any water remaining in the product may be removed by 
conventional means, if desired. However, except when the less concentrated 
hydrogen peroxides have been employed, there is generally no more than a 
negligible amount of water present in the product, and drying therefore is 
usually unnecessary. 
The invention is advantageous in that it provides an economical means of 
preparing amine oxides which can be used in the preparation of powdered 
compositions, such as dry laundry detergent formulations, without first 
being subjected to after-treatments which could increase their cost and/or 
contaminate them with materials used in the after-treatments or with 
decomposition products formed during the after-treatments. It does not 
require the centrifugation and crystallization steps of Smith et al.-II 
and, in its preferred embodiments, requires no drying step either, so it 
permits the preparation of amine oxides at considerably less cost. 
The following example is given to illustrate the invention and is not 
intended as a limitation thereof. 
EXAMPLE 
Part A 
A mixture of 150 g of N-tetradecyldimethylamine and 0.5 g of 
diethylenetriaminepentaacetic acid was heated to 65.degree. C., and 35 g 
of 70% hydrogen peroxide was added over a period of five minutes while 
maintaining the temperature. The reaction mixture was then stirred at 
75.degree. C. until gelation occurred after about 15 minutes, and the hot 
product was transferred to a storage vessel. 
Part B 
Part A was repeated twice to form additional product which was combined 
with the product of Part A. 
Part C 
A 2L 316SS Parr autoclave was charged with 347.5 g of a crude reaction mass 
which was composed of the combined products of Parts A and B and which was 
determined by NMR to be at the stage of 88% conversion. The autoclave was 
flushed three times with nitrogen and carbon dioxide. Then 1080 g of 
liquid carbon dioxide was added, and the reaction mixture was stirred and 
heated to 25.degree. C., a temperature that was maintained for one hour. 
Exotherm was noted for the first 15 minutes, and the pressure went from 
4.9 MPA at the beginning of the heatup to 25.degree. C. to a peak pressure 
of about 8.5 MPa when the temperature reached 25.degree. C. After the 
reaction conditions had been maintained for one hour, the autoclave was 
vented to the atmosphere. The product was a white solid powder which 
showed a 100% conversion to N-tetradecyldimethylamine oxide by NMR.