Purification and decolorization of off-color alkyl alkanolamines

Off-color N-(C.sub.4 -C.sub.10)alkyl di(C.sub.2 -C.sub.3) alkanolamines purified and decolorized by vacuum distillation at a pressure within the range of 1 to 50 mm.Hg (1-50 torr) in the presence of water and or water-soluble metal borohydride whereby the distilled alkanolamine has a reduced APHA color rating which is maintained on storage.

BACKGROUND OF THE INVENTION 
This invention concerns the treatment of certain alkyl diethanolamines to 
inhibit color formation therein during manufacture or to remove 
color-forming contaminants therefrom after manufacture. More particularly, 
it concerns the purification and reduction in color of an off-color 
N-(C.sub.4 -C.sub.10) di(C.sub.2 -C.sub.3) alkanolamine by the addition of 
a color inhibiting amount of a water-soluble metal borohydride to said 
alkanolamine, and subjecting the product in the presence of water to 
vacuum distillation at a pressure of less than about 50 mm.Hg (millimeters 
of mercury). 
There exist a significant demand for high purity/low color N-butyl 
diethanolamine in the photographic market. However, the product, as 
manufactured, contains impurities and is off-color. Vacuum distillation at 
about 100 mm.Hg will remove much of the impurities but high color rapidly 
returns on storage indicating the continued presence of color forming 
compounds. 
THE PRIOR ART 
U.S. Pat. No. 3,207,790 discloses the reduction of color of discolored 
alkanolamines by mixing into the off-color amine a quantity of a 
borohydride of an alkali metal. The process is said to be useful for a 
variety of alkanolamines including N,N-dibutyl ethanolamine and N-ethyl 
diethanolamine t reduce and stabilize color in these products. The 
disclosure does not suggest the subsequent removal of the borohydride or 
any other contaminants. 
U.S. Pat. No. 4,379,024 discloses a manufacturing process for preparing 
alkylaminoalkanols wherein an alkali metal borohydride is added, to the 
crude alkylaminoalkanol before stripping excess reactant. Thereafter, the 
crude material with borohydride is subjected to a final distillation step 
at a reduced pressure in excess of 100 mm.Hg. to recover a pure, 
color-stable alkylaminoalkanol with the borohydride and impurities coming 
off as bottoms. The amine products shown to have been treated in the 
working examples of this patent are dimethyl and diethylaminoethanol. 
STATEMENT OF THE INVENTION 
This invention is a process for the preparation of high purity, low APHA 
color product from an N-(C.sub.4 -C.sub.10) alkyl di(C.sub.2 -C.sub.3) 
alkanolamine containing contaminants including color-forming bodies which 
comprises vacuum distilling said alkanolamine at a pressure of less than 
about 50 mm.Hg in the presence of water and a water-soluble metal 
borohydride whereby the distilled alkanolamine has a reduced APHA color 
rating which is maintained on storage.

DETAILED DESCRIPTION OF THE INVENTION 
To provide a low APHA (American Public Health Association) color rating for 
N-butyl diethanolamine and higher homologs, it has not heretofore been 
known to sufficiently reduce the color-forming impurities in the crude 
product to provide a high purity material which is color-stable on 
storage. Previously, it was known to add an aqueous solution of an alkali 
metal borohydride to alkyl alkanolamines and vacuum distill at above 100 
mm. Hg to provide a color-stable product (See U.S. Pat. No. 4,379,024). 
However, this procedure fails to produce color-stable N-butyl 
diethanolamine or higher alkyl homologs. 
It has now been found that N-butyl diethanolamine and higher homologs may 
be color-stabilized and purified by the addition of an additive amount of 
a water-soluble metal borohydride to the dialkanolamine and vacuum 
distilling in the presence of water at a pressure of less than about 50, 
preferably from about 4 to about 10 mm.Hg, the distillate having a high 
purity and low APHA color on long term storage. 
In the manufacture of N-butyl diethanolamine, one mole of N-butylamine is 
reacted with two moles of ethylene oxide in the presence of water (the 
catalyst). A minor excess of amine is maintained to assure complete 
reaction of the ethylene oxide. The crude reaction product may be 
subjected to multiple distillation steps to produce the pure product. The 
first distillation strips the residual water and excess butylamine. The 
second distillation removes N-butylaminoethanol. The next step isolates 
N-butyl diethanolamine product by vacuum distillation. The amount of N- 
butyl diethanolamine produced with the N-butyl aminoethanol coproduct 
depends on the ratio of the amine to ethylene oxide reactants used. 
The ethylene oxide reactant generally contains low part per million 
concentrations (approx, 10 ppm) of acetaldehyde. N-butylamine, depending 
on its source, may be contaminated with butyraldehyde (e.g., 50 ppm). 
Analysis of N-butyl diethanolamine that has been subjected to normal 
distillation temperatures (&gt;175.degree. C.) indicates the presence of 
additional aldehydes. It is believed that these aldehydes, present in the 
reactants and crude product, react with various components of the crude 
product to produce color bodies. The addition of the borohydride reduces 
the concentration of the aldehydes present in the crude product and the 
low pressure vacuum distillation less than about 50 mm.Hg permits the 
removal of the contaminants (including color-forming materials, color 
bodies and borohydride) at sufficiently low process temperature to avoid 
the reformation of addition 1 aldehydes and color bodies thereby providing 
color-stable and high purity N-butyl diethanolamine. 
While the above discussion has been limited to the manufacture of N-butyl 
diethanolamines, it is also representative of those other alkanolamines 
included in the N-(C.sub.4 -C.sub.10) alkyl di(C.sub.2 -C.sub.3) 
alkanolamines which are a part of this invention. The alkyl groups 
included are, for example, butyl, isobutyl, pentyl, hexyl, isohexyl, 
2-ethylhexyl, octyl, isoctyl, decyl and the like. The alkanol groups may 
be ethanol or isopropanol. 
The process of this invention may include either a single low pressure 
vacuum distillation step or a multiple step or zone distillation, 
procedure including the low pressure vacuum distillation preferably used 
as the last distillation step. The additional distillation steps or zones 
may be operated at pressures above 50 mm.Hg so long as the low pressure 
vacuum distillation step is operated at less than about 50 mm.Hg, 
preferably between about 4 and about 10 mm.Hg. Since subjecting the high 
purity, low APHA color N-butyl diethanolamine and higher homologs of this 
invention to temperatures of 175.degree. C. and higher will generate 
color-forming materials, i.e., aldehydes therein, the dialkanolamines 
should not be exposed to such temperatures either during vacuum 
distillation or thereafter. 
Water concentration in the crude alkyl alkanolamine at the initial phase of 
distillation is preferably from 0.5 to 10% based o the weight of the crude 
material but may be higher. Preferably, an initial concentration of from 
about 1 to 3% is used. If multiple step distillation is employed, much of 
this water is removed overhead usually prior to the step or zone of 
reduction of pressure to the critical range. Water may be introduced as a 
part of the crude material or separately, as from the incorporation of the 
aqueous solution of water-soluble metal borohydride. 
The water-soluble metal borohydride is preferably an alkali metal 
borohydride, particularly, sodium borohydride. However, other 
water-soluble metal salts, e.g., potassium, lithium, cesium and rubidium 
borhydrides will work in this process. The borohydride is generally 
introduced int the freshly manufactured crude alkanolamine product either 
at a point just prior to the removal of water and excess amine in the 
multi step distillation process, or to any off-color product prior to 
distillation. The amount of borohydride preferably ranges from about 0.05 
to about 0.5 percent based on the weight of the crude alkanolamine 
product. The borohydride is preferably used in the form of an aqueous 
solution and may contain an alkali metal hydroxide as a stabilizer. The 
hydroxide is generally present in the aqueous solution in the amount of 
from 5 to 15%, preferably about 10% by weight of said solution. 
The following examples are set forth to demonstrate the process of this 
invention. The use of N- butylamine and ethylene oxide is representative 
of other homologs which produce the N-(C.sub.4 -C.sub.10)alkyl di(C.sub.2 
-C.sub.3) alkanolamines of this invention. 
EXAMPLE 1 
A slight excess of one molar proportion of N-butylamine and two molar 
proportions of ethylene oxide and 0.01 molar proportion of water 
(catalyst) were a charged to reactor and maintained at a temperature of 
150.degree. C. at autogenous pressure for about 5 minutes to complete the 
reaction. A small excess of N-butylamine was employed to assure complete 
reaction of the ethylene oxide. Crude aqueous product was withdrawn from 
the reactor and 0.12% of sodium borohydride, based on the weight of the 
crude product, was added. The sodium borohydride was in the form of an 
aqueous solution containing 10 weight % sodium hydroxide as a stabilizer. 
The water concentration of the crude product after addition of the aqueous 
borohydride solution was about 2 weight %. 
The crude N-butyl diethanolamine product containing sodium borohydride was 
first distilled in a 25 mm., 10 plate column packed with nichrome helices 
where water and N-butylamine was removed overhead at a distillation 
pressure of 600 mm. Hg, an overhead temperature of 80.degree. C. and a 
bottoms temperature of 145.degree. C. The distillation bottoms was passed 
to a second column (25 mm., 10 plate tray and cap type) where coproduced 
N-butylaminoethanol was taken off overhead at a distillation pressure of 
150 mm.Hg, an overhead temperature of 138.degree. C. and a bottoms 
temperature of 172.degree. C. The bottoms from the second distillation 
column was transferred to a third column which was a 25 mm. Oldershaw 
apparatus comprising a tray and cap type column. It contained 25 plates 
through the depth of the column. The reflux ratio (a comparison of the 
volume of condensed vapor returned to the top of the column and the volume 
of condensed vapor taken off as finished product) was maintained at about 
5:1 during distillation. Pure N-butyl diethanolamine was removed overhead 
from this column operated at a pressure of 10 mm.Hg, an overhead 
temperature of 148.degree. C. and a bottoms temperature of 188.degree. C. 
The pure N-butyl diethanolamine contained no low boilers, less than 0.1% 
water, a product assay [analysis by capillary gas chromatography (GC)] of 
99.4% and an APHA color of 20 (APHA Color is determined by Platinum-Cobalt 
Color, Test Method D1209). The N-butyl diethanolamine product showed good 
color stability for over six months. 
The specification requirements for N-butyl diethanolamine photographic 
material are (1) an APHA color rating of no greater than 50, (2) a water 
content of less than 0.1% and (3) an assay (GC) of 99%. 
For comparative purposes, an off-color N-butyl diethanolamine having an 
assay of 99.16% with a water content of 0.22% and a non-determinable color 
rating due to the presence of a gray cast was charged to a 2 liter 
distillation unit in an amount of 1000 ml. The distillation unit consisted 
of a 2 liter flask, glascol heating mantle to heat the flask, thermometer 
well in the flask and a vacuum jacketed 25 plate Oldershaw column. 
Attached to the column was a distilling head adapter followed by a 
Friedrichs water condenser with a side arm. The side arm was connected in 
series to a straight bore stopcock and a 5 place vacuum manifold. The 
verticle sidearm of the distilling head adapter was connected to an Allihn 
condenser followed by a straight vacuum adapter and 500 ml. receiver 
(flask). The 5 place vacuum manifold was attached to a closed end 
manometer, dry ice trap, microvalve and both Allihn and Friedrichs 
condensers. 
Distillation was conducted at 5 mm.Hg pressure, a 10:1 reflux ratio and no 
borohydride or other additive. The temperature of the distilland was 
121.degree. C. Of the seven distillation cuts made, only two passed all 
required specification parameters for photographic material, i.e., APHA 
color, assay and water content. These two cuts represent 17% of the total 
charge. The results for the seven cuts for this distillation run are as 
follows: 
______________________________________ 
Cut No. % H.sub.2 O 
APHA Color Assay Receiving ml. 
______________________________________ 
1 0.09 500+ 94.44 62 
2 0.15 --* 93.91 15 
3 0.07 70 99.32 355 
4 0.08 40 99.64 65 
5 0.11 500+ 98.28 53 
6 0.08 40 99.60 105 
7 0.06 100 99.62 220 
875 
Residue 
100 
Total 975 
______________________________________ 
*The dash line indicates no results available. 
Several other distillations were also run for comparative purposes. In one 
run 1000 ml. of off-color N-butyl diethanolamine was charged to a two 
liter flask connected to the equipment described above for the first 
comparative experiment. To this was added 10 ml. of sodium borohydride 
(12% aqueous solution). The distillation was conducted at 100 mm.Hg and 
the reflux ratio was maintained at 5:1. The temperature of the distillate 
was 222.degree. C. The procedure resulted in no material acceptable for 
the photographic application. 
In another comparative run using identical equipment as above, 1000 ml of 
off-color N-butyl diethanolamine was charged to a two liter flask with no 
additive addition. The distillation was conducted at 100 mm.Hg and the 
reflux ratio was maintained at 6:1. The temperature of the distillate was 
224.degree. C. The distillation resulted in no material acceptable for 
photographic application. 
EXAMPLE 2 
Using the distillation unit and off-color N-butyl diethanolamine of the 
experiment conducted for comparative purposes and reported in the 
foregoing example, two distillations were carried out to determine if 
excess water in the distillation procedure would improve the APHA color of 
the distillation cuts. 
The first distillation consisted of 10 ml. of sodium borohydride (12 wt. % 
solution) and 50 ml. of water added to the 2 liter flask of the 
distillation unit containing 1000 ml. of the off-color N-butyl 
diethanolamine. The distillation was conducted at a pressure of 50 mm.Hg 
and the reflux ratio was maintained at approximately 6:1. The temperature 
of the distilland was 184.degree. C. The results of analyses of cuts taken 
during this distillation are reported below. 
The second distillation was conducted using the same procedure as described 
above for the first distillation except that the addition of 50 ml. of 
water was omitted and the reflux ratio was 5:1. The results of analyses of 
cuts taken during this distillation are reported below. 
For comparative purposes, another distillation run was made as follows: 
2200 ml of the off-color N-butyl diethanolamine described above was placed 
in a 3 liter flask which replaced the 2 liter flask of the above described 
distillation unit. In the absence of sodium borohydride or other additive, 
the distillation was carried out at an average pressure of 49 mm.Hg and a 
reflux ratio of 5:1. 
Twelve cuts were taken during the above reported distillation runs and the 
cuts analyzed for APHA color and purity. The results of these analyses are 
reported below. 
______________________________________ 
Cut No. % H.sub.2 O 
APHA Color Assay Receiver, ml. 
______________________________________ 
First Distillation at 50 mm. Hg. (50 ml. H.sub.2 O) 
1 72.39 500+ -- 76 
2 0.28 50 98.84 140 
3 36.30 500+ -- 54 
4 0.11 25 99.12 380 
5 0.20 -- 93.66 35 
6 0.23 15 97.73 165 
850 
Residue 
190 
Total 1040 
Second Distillation at 50 mm. Hg (No H.sub.2 O added) 
1 22.22 -- 53.07 29 
2 0.13 45 98.05 323 
3 0.26 -- 94.95 42 
4 0.06 25 98.98 330 
5 0.11 65 97.72 130 
854 
Residue 
125 
Total 979 
Comparative Distillation at 49 mm. Hg (No Additive) 
1 1.80 500+ 90.23 110 
2 0.48 500+ 98.16 136 
3 0.57 ** 98.66 172 
4 0.14 65 99.77 338 
5 1.66 500 96.79 64 
6 0.12 ** 99.71 368 
7 0.07 ** 99.74 198 
8 2.07 ** 95.21 26 
9 0.22 ** 99.27 100 
10 0.10 ** 99.56 367 
11 1.58 -- 99.75 18 
12 0.38 175 96.44 80 
1977 
Residue 
75 
Total 2052 
______________________________________ 
** Unable to determine due to presence of gray cast background. 
It has been determined experimentally that distillation alone, regardless 
of vacuum, will not recover appreciable quantities of acceptable N-butyl 
diethanolamine from off-color product. A combination of low distillation 
pressure and borohydride is necessary to produce the high purity, low APHA 
color required to meet the specifications of photographic use. The lower 
the pressure, below the prescribed upper limit, the higher the yield of 
acceptable product.