Phosphorylation of alcohols

Phosphorylation of alcohols by contacting the alcohol with a phosphorous sulphide under the impact of ultrasound. The phosporylated produce can be converted into a metal salt, which is a suitable anti-wear additive for lubricating oils.

FIELD OF THE INVENTION 
The present invention relates to a process for the phosphorylation of 
alcohols with phosphorous sulphide. The product of this phosphorylation is 
a dithiophosphoric acid derivative which is an intermediate in the 
preparation of anti-wear additives for lubricating oils. 
BACKGROUND OF THE INVENTION 
These additives are conventionally prepared in two steps. The first step 
comprises the phosphorylation of the alcohol with phosphorous sulphide and 
the second step comprises the treatment of the phosphorylated product with 
a metal oxide, hydroxide and/or salt. 
The first step of this process is slow. In order to obtain commercially 
feasible reaction rates it may be necessary to heat the reaction mixture 
to such temperatures that decomposition of the phosphorylation product may 
occur. In particular in the case of the phosphorylation of phenol or 
substituted phenols this step is very slow, and on heating, decomposition 
of the phosphorylated product is likely to occur. 
SUMMARY OF THE INVENTION 
It has now been found that satisfactory reaction rates can be obtained at 
low temperatures if ultrasound is applied during the phosphorylation. 
Accordingly, the present invention provides a process for the 
phosphorylation of alcohols with a phosphorous sulphide by contacting the 
alcohol with the phosphorous sulphide under the impact of ultrasound.

The alcohol employed can be any conventional primary, secondary or tertiary 
alcohol which will react with phosphorous sulphide to form the 
phosphorylated product. Suitable alcohols include alkanols, such as the 
pentanols and hexanols, cycloalkanols, such as cyclohexanol, bi- or 
tri-cycloalkanols, aromatic alcohols, such as phenol and naphtol, 
arylalkyl alcohols such as benzyl alcohol, all of which may be substituted 
by one or more alkyl or alkoxy groups; the alcohol can contain from 1 to 
25 carbon atoms and equivalent kinds of alcohols. Preferably the alcohol 
is selected from a C.sub.1-20 alkanol, C.sub.5-8 cycloalkanol optionally 
substituted with one or more C.sub.1-4 alkyl group, phenol and C.sub.1-25 
-alkyl phenol. In case of alkylphenols it was found that the p-alkyl 
phenols were somewhat more reactive than the O- or m-alkyl phenols. The 
phosphorous sulphide employed can be any phosphorous sulfide which will 
react with an alcohol to form a phosphoroylated products, including 
tetraphosphorous heptasulphide or tetraphosphorous trisulphide and the 
like, but is preferably phosphorous pentasulphide. 
The process according to the invention can be carried out in a diluent. 
However, it is preferred to work with a reaction mixture which only 
consists of phosphorous sulphide and the alcohol. The molar ratio between 
the two reactants may vary within wide ranges, such as from 0.1 to 10 
equivalent alcohol per equivalent phosphorous sulphide; it is however 
preferred to employ substantially stoichiometric quantities of the 
reactants. This implies that when phosphorous pentasulphide is used the 
molar ratio of sulphide:alcohol is about 1:4. 
The sound intensity influences the reaction rate but limits to the 
intensity are generally set by economical circumstances. On the one hand 
the sound intensity should not be too high so that carrying out the 
process would become very expensive. On the other hand the sound intensity 
should not be too low so that the reaction is hardly speeded up. To avoid 
either situation the sound intensity is preferably from 30 to 300 
W/cm.sup.2. Good results were obtained by using sound frequencies between 
10 and 100 kHz. 
Ultrasound may be employed during part of the reaction only. However, it 
appeared that the reaction was only accelerated during the period in which 
ultrasound was actually applied. Therefore, it is preferred to apply 
ultrasound during the entire reaction time. 
The reaction temperature can be selected from as low as feasible to the 
temperature where the phosphorylated product gets unstable. Preferably the 
temperature ranges from 0.degree. to 150.degree. C., in particular from 
20.degree. to 110.degree. C. The reaction may be carried out at normal or 
elevated pressure. 
As stated hereinbefore, the phosphorylated product is an intermediate in 
the preparation of additives for lubricating oils. These additives are the 
metal salts of such products. Accordingly, the present invention further 
provides a process for the preparation of metal salts of dithiophosphoric 
acid derivatives which comprises the phosphorylation of an alcohol with 
phosphorous sulphide as described above, followed by conversion of the 
phosphorylated product to the the metal salt by using a metal oxide, 
hydroxide or by using a base and a metal salt. Preferred metals include 
group I and group II metals, such as sodium and zinc. In particular zinc 
salts are preferred. These zinc salts are preferably obtained by 
converting the phosphorylated product with zinc oxide. 
It is further possible to add more than the stoichiometric amount of metal 
oxide or hydroxide to the phosphorylated product to create a basic metal 
salt prepared according to the invention. This basic salt is also formed 
by the reaction of a neutral metal salt of a dithiophosphoric acid 
derivative with a metal oxide or hydroxide. The formation of the (basic) 
metal salt from the phosphorylated product and (an excess of) the metal 
oxide or hydroxide, can also be promoted by the use of ultrasound. This 
fining, combined with the teaching of the present invention enables a 
convenient one-step synthesis of a metal salt of dithiophosphoric acid 
derivative. Accordingly, the present invention further relates to a 
process for the preparation of a metal salt of a dithiophosphoric acid 
derivative comprising mixing an alcohol, a phosphorous sulphide and a 
metal oxide and/or metal hydroxide and subjecting the resulting mixture to 
ultrasound. The metal salt includes a neutral salt and a basic salt. In 
this one-step process the alcohol is first phosphorylated, and 
subsequently the phosphorylated product reacts with the metal oxide or 
hydroxide. 
The metal salts, in particular the zinc salts, of the dithiophosphoric acid 
derivatives prepared according to the invention are extremely useful as 
anti-wear additives in a lubricating oil. Therefore, the present invention 
also provides lubricating oil compositions comprising a major amount of a 
lubricating base oil and a minor amount of the metal salt of the 
invention. 
The lubricating base oil will conveniently comprise more than 50% of the 
composition. It can be selected from mineral lubricating oils of varying 
viscosities, but it also includes a synthetic lubricant, such as 
conventional ester-type lubricants or polyolefin-type fluids, or a 
vegetable oil, or a grease. 
The lubricating oil compositions can further contain a number of other 
additives, such as antioxidants, foam inhibitors, corrosion inhibitors, 
viscosity index improvers, pour paint depressants and the like as can be 
established by a person of skill in the art. 
The invention will be illustrated by means of the following Examples which 
should not be regarded as limiting it in any way. 
EXAMPLE 1 
In a vessel 40.5g (110 mmol) of C.sub.16-18 alkyl phenol was mixed with 
6.4g (29 mmol) of phosphorous pentasulphide and the reaction mixture was 
kept at a temperature of 50.degree. C. The reactants were subjected to 
ultrasound at a frequency of 20 kHz and a sound intensity of 150 
W/cm.sup.2. The reaction was stopped after 55 hours, and 38 g of 
di(C.sub.16-18 aklylphenol)dithiophosphoric acid was obtained after 
purification of the reaction mixture by filtration (yield 72.8%). 
As comparison a similar experiment was carried without the application of 
ultrasound. The yield of the dithiophosphoric acid derivative was 18.8%. 
EXAMPLE 2 
Similar procedures as described in Example 1 were carried out but at 
different temperatures during different reaction times and optionally 
under interrupted ultrasound application. 
In experiment 2a ultrasound was applied in a pulsed fashion: 1/3 s sound, 
2/3 s no sound, yielding an ultrasound employment during 33% of the time. 
In experiment 2b no ultrasound was applied, but the reactor mixture was 
continuously stirred. Experiments 2c and 2e were carried out under the 
continuous impact of ultrasound whilst in experiments 2d and 2f, like in 
2b, only stirring occurred. The reactions between C.sub.16-18 alkylphenol 
and P.sub.2 S.sub.5 (in stoichiometric amounts) gave the results as 
indicated in the Table I below. 
TABLE 1 
______________________________________ 
Yield 
Ultrasound dithio- 
Fre- Inten- Duration Reaction phosphoric 
Exper. 
quency sity % react, 
Temp. Time acid 
No. kHz W/cm.sup.2 
tune C..degree. 
h derivative % 
______________________________________ 
2a 20 150 30 38 70 15 
2b -- -- -- 38 70 2 
2c 20 150 100 65 46 75 
2d -- -- -- 65 47 34 
2e 20 80 100 95 1.5 87 
2f -- -- -- 95 7.5 54 
______________________________________ 
From the above results it is clearly evident that the application of 
ultrasound provides substantially accelerated reaction rates and enhanced 
product yields.