US3426054A - Preparation of metal dialkyl dithiophosphates - Google Patents

Description

United States Patent 01 ice 3,426,054 PREPARATION OF METAL DIALKYL DITHIOPHOSPHATES Helmuth G. Schneider, Westfield, and Thomas P. Mc-

Namara, Atlantic Highlands, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed July 16, 1964, Ser. No. 383,219 U.S. Cl. 260--429.9 9 Claims Int. Cl. C07f 9/16, 3/07 ABSTRACT OF THE DISCLOSURE In preparing a polyvalent metal salt of a dialkyl dithiophosphoric acid having alkyl groups of from 1 to 30 carbon atoms, an alkali metal salt is first prepared and the latter is then converted to a polyvalent metal salt by double decomposition. The contribution of the present invention to the art consists in improving the quality of the products by keeping the proportion of dialkyl dithiophosphoric acid to alkali metal base within the narrow limits of one mole of the former to from 1.1 to 1.15 moles of the latter and by also keeping the proportion of polyvalent metal salt to dialkyl dithiophosphoric acid within very narrow limits in the double decomposition step, these limits being from 0.5 to 0.55 mole of divalent metal salt, or from 0.35 to 0.40 mole of trivalent metal salt, per mole of dialkyl dithiophosphoric acid.

The present invention is concerned with an improved process for the preparation of polyvalent metal salts of dialkyl dithiophosphoric acids. The invention is particularly directed to the preparation of the zinc salts of dialkyl dithiophosphoric acids for use as additives in lubricating oil compositions wherein they serve as antioxidants and wear-reducing agents.

Polyvalent metal salts of dialkyl dithiophosphoric acids whose alkyl groups contain in the range of from about 1 to about 30 carbon atoms and more particularly those having from about 3 to about 12 carbon atoms are well known in the art as additives for lubricating oil compositions. Metal salts of this type, and especially the zin salts, are particularly valuable as antiwear and antioxidant additives for lubricating oils that are intended for use in the crankcases of internal combustion engines. The nickel salts have been similarly employed, as have those of cadmium and lead. Other polyvalent metal salts of dialkyl dithiophosphoric acids have been found to have antioxidant, corrosion inhibiting or detergent properties in lubricants, fuels, and related oleaginous compositions.

It is common practice to prepare dialkyl dithiophosphoric acids by reacting phosphorus pentasulfide with an aliphatic alcohol or a mixture of alcohols containing the desired range of alkyl groups in a molar ratio of approximately 4 moles of alcohol for each mole of phosphorus pentasulfide. The acids are then ordinarily neutralized in accordance with prior art processes With an oxide, hydroxide or carbonate of a polyvalent metal or alternatively with a reactive polyvalent metal salt.

In the processes of the prior art, certain disadvantages have been encountered in preparing polyvalent metal dialkyl dithiophosphates, particularly when neutralizing the dithiophosphoric acids with metal oxides. For example,

utilization of the metal oxide to obtain the desired degree of neutralization of acid has been very erratic both as to the proportion of metal oxide required relative to the acid that is being neutralized and as to the reaction time that is needed to bring about complete neutralization. In the case of zinc oxide neutralization of dialkyl dithiophosphoric acids, for example, it has been necessary to 3,426,054 Patented Feb. 4, 1969 employ from 5 to 20% excess of zinc oxide and this in turn increases the time that is required for the final filtration step to remove excess oxide.

In accordance with the present invention it has been found that the conversion of dialkyl dithiophosphoric acids to their polyvalent metal salts can be much more closely controlled and the preparation of those salts can be greatly accelerated if a double-decomposition method is employed rather than the direct neutralization procedure. While it is known in the prior art to prepare polyvalent metal salts of dialkyl dithiophosphoric acids by neutralizing those acids with an alkali metal oxide or hydroxide and then converting the salts to polyvalent metal salts by a double-decomposition reaction, the process of the present invention differs from the prior art methods in that the proportions of dialkyl dithiophosphoric acid to alkali metal oxide or hydroxide and the proportions of polyvalent metal to dialkyl dithiophosphoric acid are kept within very narrow limits.

The principles of this invention are applicable not only to the preparation of zinc salts but also to the preparation of salts of other metals of Group II of the Periodic Table, such as mercury, cadmium, calcium, and barium, as well as amphoteric metals that will form basic salts, including aluminum, tin, copper, lead, cobalt, nickel, iron, bismuth, and beryllium.

To prepare polyvalent metal salts of dialkyl dithiophosphoric acids in accordance with the present invention, those acids are first converted to alkali metal salts by neutralizing the salts with an alkali metal hydroxide in which the mole ratio of dialkyl dithiophosphoric acid to alkali metal hydroxide is kept within the narrow range of 1 mole of acid to from 1.10 to 1.15 moles of alkali metal hydroxide. The resulting alkali metal salts are converted to polyvalent metal salts by a double-decomposition reaction in which the amount of polyvalent metal salt employed in the double-decomposition reaction is in the range of from 0.5 to 0.550 mole of inorganic salt of a. divalent metal, or from 0.35 to 0.40 mole of inorganic salt of a trivalent metal, per mole of dialkyl dithiophosphoric acid.

If the neutralization of dialkyl dithiophosphoric acids and the subsequent conversion of the alkali metal salt to polyvalent metal salts are conducted with stoichiometric proportions of alkali metal hydroxide and of polyvalent metal inorganic salt, the products will not be satisfactory in that they will have low pH values, will tend to evolve hydrogen sulfide on storage, and will also tend to be corrosive toward copper. In the case of zinc salts of mixed C -C dialkyl dithiophosphoric acids, for example, it has been determined that the product made with stiochiometric proportions of reactants has a pH of about 4.0 to 4.5, whereas, prior experience has indicated that it is desirable to have a minimum pH value in the range of about 5.65 to 5.8 in order to obtain a product that is stable with respect to H 8 evolution. To obtain the desired minimum pH value, it is necessary to impart a slight degree of alkalinity to the product. At the same time, if the excess alkalinity is too great, the yield falls oif rapidly. The dialkyl dithiophosphoric acids that are employed in the practice of this invention may be obtained from any suitable source. The usual method for preparing these acids is to react about 4 moles of an aliphatic alcohol or a mixture of alcohols with 1 mole of phosphorus pentasulfide at a reaction temperature in the range of about to 250 F. for in the range of from about 1 to 6 hours. However, the present invention is not limited by the manner in which the dialkyl dithiophosphoric acids are prepared.

The dialkyl dithiophosphoric acids used in this invention include not only those made from a simple aliphatic alcohol such as isopropyl, n-butyl, n-decyl, etc., but also mixed aliphatic alcohols such as C C or C oxo alcohols obtained by a reaction of olefins with carbon monoxide and hydrogen and subsequent hydrogenation of the resultant aldehydes. Mixed alcohols obtained by the hydrogenation of natural fats and oils may also be employed. For example, mixed aliphatic alcohols in the (I -C range consisting chiefly of lauryl alcohol can be obtained by hydrogenating cocoanut oil. These are sold under the trade name Lorol. Mixed C C alcohols consisting principally of C and C aliphatic alcohols can be obtained from tallow by hydrogenation and/or by sodium reduc tion. Primary aliphatic alcohols of 22 carbon atoms or more can be obtained by the hydrolysis of Ziegler-type ethylene polymers and are available commercially from the Continental Oil Company under the name of Alfol alcohols. All of these higher alcohols can be used for dialkyl dithiophosphoric acid manufacture. Dithiophosphoric acid-s obtained from mixtures of alcohols are particularly preferred. Such mixtures include, for example, a combination of isopropyl alcohol and methyl isobutyl carbinol; a combination of primary amyl alcohol and isobutanol; a combination of mixed amyl alcohols and technical lauryl alcohol; a mixture of isopropyl alcohol and C oxo alcohol, and the like. It is also within the contemplation of the invention to employ in the preparation of the metal salts mixed acids obtained by reacting individual alcohols separately with P 8 The products of reaction of phosphorus pentasulfide with aliphatic alcohols do not constitute entirely of dialkyl dithiophosphoric acids but, in addition, contain hydrogen sulfide and also materials which have been referred to variously as inerts or neutrals because they contain unacidified sulfur. It is believed that these materials are primarily sulfides, disulfides or trisulfides of the dialkyl dithiophosphoric acids. To the extent that hydrogen sulfide is present in the dialkyl dithiophosphoric acids, it consumes alkali metal hydroxides in the neutralization step. The alkali metal sulfides or hydrosulfides thus formed react with the polyvalent metal salt in the subsequent double-decomposition step to form the corresponding polyvalent metal sulfide. It is thus evident that the presence of hydrogen sulfide needlessly runs up chemical composition in the process. For this reason it is advantageous to blow the dialkyl dithiophosphoric acids with air or with an inert gas until they are essentially H S free before undertaking the neutralization step.

As has been already stated, the dialkyl dithiophosphoric acids also contain inerts. To determine the amount of alkali metal hydroxide that will be needed for proper neutralization in accordance with the present invention, it is first necessary to determine the percentage of free dialkyl dithiophosphoric acids that are present, which is done by titrating small samples. Then based on the results of this titration, the quantity of alkali metal hydroxide needed to react with the dialkyl dithiophosphoric acid in the desired 1.1 to 1.15/1 molar proportion can be readily determined. The neutralization of the dialkyl dithiophosphoric acids is an ionic reaction and, therefore, occurs quite rapidly; however, hydrolysis of the neutrals or inerts is much slower and requires about 15 minutes. The time needed for complete neutralization for any dialkyl dithiophosphoric acid system can be determined by noting the change in free alkalinity with time, which can be done conveniently by titration of a sample. When no further change in alkalinity occurs, hydrolysis is complete.

The neutralization step can be conducted at any suitable temperature, ranging from ambient temperatures to about 180 F., and it can be carried out in any suitable vessel, including mixing columns and storage tanks. External cooling will be desirable because the heat of reaction -will raise the temperature. It is advantageous to keep the neutralization temperature as low as practical, consistent with the rate of hydrolysis of the inerts because there is a tendency for the filterability of the final metal dithiophosphate to decrease with increase in neutralization temperature. For mixed C -C dialkyl dithiophosphoric acids, the preferred temperature. range is about to F.

When neutralizing the dialkyl dithiophosphoric acids with sodium hydroxide, it is generally advantageous to use as high a strength of NaOH as possible, the only limitation being that the sodium salt that is formed should not set up to a gel. Gel formation depends on the time of aging and the type of alcohols that have been used. For alcohols of 5 carbon atoms, it has been found that the sodium hydroxide concentration can be as high as 19 to 20 weight percent. With higher molecular weight dialkyl dithiophosphoric acids, there is a greater tendency toward gel formation upon neutralization with NaOH. For example, if the acids have been derived from hexylalcohols, it is desirable to lower the NaOH concentration to 18 weight percent. Gel formation per se does not affect the quality of the final additive formed on double decomposition; it does, nevertheless, make the handling of the alkali metal salt of the dialkyl dithiophosphoric acids difiicult. Gel formation also has a tendency to render the polyvalent metal salt of the dialkyl dithiophosphoric acids of such small particle size that the times required for subsequent washing and drying are unduly lengthened.

Although it is not essential to the process, it may be desirable in some cases to subject the neutralized product to a simple filtration to remove ferric hydroxide, for example, and any deleterious material that may have been introduced into the system with the dialkyl dithiophosphoric acids. Any suitable fibrous material may be used in this filtration step as for example a drum filled with wood shavings, a bag filter, or the like.

The second step of converting the alkali metal salt to the polyvalent metal salt composition is preferably conducted sufficiently soon after the neutralization step so that the formation of a gel of the alkaline metal salt cannot occur. For example, in the case of the sodium salt, gel formation may take place at the end of about 18 hours at room temperature. If gel formation is permitted to occur before the double-decomposition reaction, the polyvalent metal salts that are produced on double decomposition are of such small particle size that the times required for subsequent washing and drying of the polyvalent metal salt are unduly lengthened.

The step of converting the alkali metal salt to the polyvalent metal salt by double decomposition is also a fairly rapid reaction and involves at the most no more than about 10 to 20 minutes. This reaction involves very little evolution of heat and can be carried out in the same range of temperatures as used in the neutralization step.

One important factor which affects the filterability of the final polyvalent metal dialkyl thiophosphate is the concentration of electrolytes during the double-decomposition step. This is probably associated with the colloidal nature of the product when it is first formed. When Working with divalent metal chlorides, it has been found that for proper filterability of the final product, the concentration of alkali metal chloride ions should be at least 2.5 moles per liter, and preferably in the range of 2.5 to 4 moles per liter. This desired concentration of ions can be controlled by using the proper strength of alkali metal hydroxide in the neutralization step and/or by using the proper concentration of polyvalent metal salt in the double-decomposition step. For example, in the preparation of zinc salts of mixed C C dialkyl dithiophosphates the minimum NaOH concentration should be 18 to 19 weight percent; and in the double-decomposition step the concentration of zinc chloride should be about 50 weight percent.

Following the double-decomposition reaction, the product is washed and then filtered.

At the end of the double-decomposition step, two phases will be formed, one of them being the polyvalent metal dialkyl thiophosphate and the other being an aqueous phase. The separation of these phases and the waterwashing steps can be done in any convenient manner. If settling is to be conducted in tankage, it is advantageous to dilute the mixture to give a greater spread in gravity between the phases and thereby facilitate settling and washing. Generally dilution of the system with an equal volume of water will be satisfactory for this purpose. It is to be understood, of course, that other methods of Washing and separating of phases can be employed, such as the use of centrifuges or other mechanical equipment. The number of water washes to which the product must be subjected will depend largely upon the etficiency of phase separation; usually two washes are sufiicient. It is advisable to check the efliciency of the washing step by determining the alkali metal content of the additive, which can be done conveniently by flame photometry. It is desirable to reduce the alkali metal content to less than 0.1%. The washing step is preferably conducted at a fairly high temperature, e.g., 150 to 175 F., so as to keep the viscosity of the product at a low level and thus speed up phase separation.

If the system has been washed and centrifuged at an elevated temperature, it may not be necessary to dry the product prior to filtration. If drying is required, one suitable method is to blow it with air or inert gas at 180 to 200 F.

The following examples will serve to illustrate the manner in which this invention is to be performed, as well as the benefits derived by operating within the defined limits of the invention. In these examples sodium Then, to the resulting solution containing the sodium salts of the dialkyl dithiophosphoric acids, there was added 136.5 g. of zinc chloride in the form of a 50% aqueous solution. At the end of an additional 15 mintues of agitation, the product was diluted with an equal volume of water, agitated and allowed to settle, after which the Water layer was decanted. After there such washings the product was filtered through diatomaceous earth and then blown with air at 180 F. until dry, which required about minutes of air blowing.

In the above preparation, the mole ratios of dialkyl dithiophosphoric acid to sodium hydroxide to zinc chloride were 1/1.1/0.5. A conversion of acid to polyvalent metal salt of 96. 8% was obtained, it being determined that 9.1 g. of unreacted dialkyl dithiophosphoric acid remained.

Example 2 To establish the effect of varying the ratios of dialkyl dithiophosphoric acid to sodium hydroxide to zinc chloride in the preparation described in Example 1, a number of preparations were made using the procedure of Example 1 but varying the ratios as set forth in Table 11. Table I also gives information as to the quantity of unreacted dialkyl dithiophosphoric acid in each instance and the corresponding percent conversion figure, as well as the measured pH of the product and the mole ratios of sulfur to phosphorus, phosphorus to zinc, and sulfur to zinc in each of the products.

TABLE I Product characteristics Grams reactants Moles per mole DDPA Percent conversion pH Mole ratios DDPA NaOH ZIlOlz NaOH ZllCl2 S/P P/Zn SlZn 283 224 137 1. 10 0. 3 97 5. 80 1. 84 2. 17 3. 98 283 235 137 1. 15 0. 5 90 5. 90 1. 91 2. 05 3. 92 283 247 137 1. 20 0. 5 84 5. 85 l. 79 2. 01 3. 61 283 258 137 1.25 0.5 81 5.90 1. 79 1. 84 3.30 283 224 143 1. 1O 0. 525 99 6. 15 1. 91 2. 09 4. 01 283 235 143 1. l5 0. 525 95 6. 85 1. 88 1. 99 3. 74 283 247 143 1. 20 0. 525 90 7. 0 2. 10 1. 80 3. 80 283 258 143 1. 0. 525 85 7. 3 2. 08 l. 72 3. 57 283 247 137 1. 20 0. 5 85 5. 85 1. 79 2. 01 3. 61 283 247 143 1. 20 0. 525 90 7. 0 2. 10 1. 80 3. 80 283 247 154 1. 20 0.575 100 Washing diflicult (light mayonnaise) 283 247 160 1. 20 0. 6 Completely emulsified (mayonnaise) hydroxide is employed for preparing the alkali metal salts, since this reactant is generally the most economic and convenient one for this purpose. It will be understood, however, that other alkali metal oxides or hydroxides may be similarly employed, e.g., Na O, KOH, LiOI-l, and the like. Also, the polyvalent metal salt is exemplified by ZnCl It is readily apparent that other salts may be substituted in the reaction such as the chlorides, nitrates or other soluble salts of cadmium, nickel, barium, aluminum, and other metals previously mentioned.

Example 1 Mixed dialkyl dithiophosphoric acids were prepared by reacting a mixture of weight percent of primary amyl alcohols and 65 weight percent of isobutyl alcohol with phosphorus pentasulfide in a mole ratio of alcohols to P S of 4 to 1. The reaction was conducted at about 170 F. for a period of about 4 hours until a specific gravity of about 1.05 was attained, measured at 78 F. After the product was stripped of hydrogen sulfide by the use of a stream of nitrogen, it was cooled to a temperature between 90 and 100 F. and then filtered.

A quantity of the dialkyl dithiophosphoric acids obtained as described above (283 g.) was neutralized by stirring into it 224 g. of sodium hydroxide in the form of a 19.4 weight percent solution. This neutralization was conducted at 125 to 135 F., the temperature being maintained by external cooling. After all of the caustic had been added, the mixture was agitated for -15 minutes.

It will be noted from the data of Table I that although the target pH of at least 5.8 was attained in each instance, the yields were undersirably low when the molar ratios fell outside the range of .1 mole of dialkyl dithiophosphoric acid, 1.10 to 1.15 moles of sodium hydroxide and 0.5 to 0.525 mole of zinc chloride.

If excessive amounts of sodium hydroxide and zinc chloride are employed, an inversion of the colloid takes place so that instead of having a dispersion of additive in the aqueous phase, the additive becomes the continuous phase, with the result that large amounts of salt are entrained and connot be extracted by water Washing. The product resembles mayonnaise and contains occluded water and salts that cannot be removed by washing.

Example 3 To establish the effect of decreased concentrations of the electrolyte on the filterability of the zinc dialkyl dithiophosphates, a number of prepartions were made using the procedure of Example 1 with mole ratios of dialkyl dithiophosphoric acid to sodium hydroxide to zinc chloride of 1/1.15/=0.525. Varying amounts of water were added to the aqueous solution of the sodium dialkyl dithiophosphate prior to the step of double decomposition with zinc chloride. In each case the products were washed and dried, and their filtration rates were then determined under standardized conditions. The concentration of electrolyte in each case and the filtration rates that were noted are shown in Table 11. These data show that the filtration rates drop 011 as the concentration of electrolyte decreases from 3.7 to 1.0 moles of sodium chloride per liter and that there is a marked change when the concentration has dropped below 3 moles per liter.

The filtration tests in which the data for Table II were obtained were conducted as follows:

Filtration was done with a 9 cm. Buchner funnel under 20 mm. vacuum. The funnel was first precoated with a slurry of 25 g. of Hyfio diatomaceous earth filter aid in 250 g. of a mineral lubricating oil, the slurry having been heated to 180 F. After the mineral lubricating oil had run through the filter, the sample under test was added to the filter. To prepare the sample for test, 2 weight percent of Hyfio filter aid was added to it, and it was heated to 180 R, which was the temperature of filtration. Filter rate times were measured after 25 cc. of filtrate from the sample had been collected, thus making adequate allowance for displacement of essentially all of the lubricating oil from the filter cake precoat.

It is to be understood that the invention is not limited to the specific examples herein presented, as they are merely illustrative of the invention and the manner in which it may be practiced. The scope of the invention is to be determined by the appended claims.

What is claimed is:

1. An improved method of preparing a polyvalent metal salt of a dialkyl dithiophosphoric acid having alkyl groups in the range of from 1 to 30 carbon atoms which comprises reacting the said acid with a basic material selected from the group consisting of the oxides and hydroxides of alkali metals in the proportion of 1 mole of dialkyl dithiophosphoric acid and from 1.1 to 1.15 moles of alkali metal basic material and thereafter converting the resulting alkali metal salt to a polyvalent metal salt by a double-decomposition reaction by means of an aqueous solution of an inorganic polyvalent metal salt, the proportion of polyvalent metal inorganic salt to dialkyl dithiophosphoric acid being in the range of 0.5 to 0.550 mole of salt per mole of dialkyl dithiophosphoric acid when said salt is a salt of a divalent metal, and in the range of 0.35 to 0.40 mole of inorganic salt per mole of dialkyl dithiophosphoric acid when said salt is a salt of a trivalent metal.

2. Improved preparation method as defined by claim 1 wherein the concentration of alkali metal salt ions is at least 2.5 moles per liter during the d0uble-decomposition step.

3. An improved method of preparing a zinc salt of a dialkyl dithiophosphoric acid having alkyl groups of from 1 to 30 carbon atoms which comprises the steps of neutralizing said acid with sodium hydroxide in the proportion of 1 mole of dialkyl dithiophosphoric acid and from 1.10 to 1.15 moles of sodium hydroxide and thereafter subjecting the resulting sodium salt to a double-decomposition reaction with an aqueous soluton of an inorganic salt of zinc in the proportion of 1 mole of dialkyl dithiophosphoric acid to from 0.5 to 0.550 mole of inorganic zinc salt.

4. Improved preparation method as defined by claim 3 wherein said double-decomposition reaction is conducted prior to the time that gelation has occurred following said neutralizing step.

5. Improved method as defined by claim 3 wherein said dialkyl dithiophosphoric acid is derived from the reaction of a mixture of primary amyl alcohols and isobutyl alcohol with P 5 6. Improved method defined as by claim 3 wherein said neutralization step is conducted with a sodium hydroxide solution of about 18 to 20 weight percent concentration and said double-decomposition reaction is conducted with a zinc chloride solution of about 50 weight percent concentration.

7. Improved method as defined by claim 1 wherein said dialkyl dithiophosphoric acid has alkyl groups of from 3 to 12 carbon atoms.

8. Improved method as defined by claim 1 wherein said polyvalent metal is a metal of Group II of the Periodic Table.

9. Improved method as defined by claim 1 wherein said polyvalent metal is zinc.

References Cited UNITED STATES PATENTS 2,476,037 7/1949 Giammaria 260429] 2,488,662 11/1949 Farrington et al 260-429 3,234,250 2/ 1966 Schneider et al. 260429 XR 3,290,347 12/1966 Miller 260435 1,939,951 12/1933 Buchanan et al. 260987 X 2,193,965 3/1940 Hochwalt 260987 X 2,391,184 12/1945 Nelson et al 260987 X 2,595,170 4/1952 Rudel et al. 260987 X 2,838,557 6/1958 Verley 260987 3,014,940 12/1961 Lynch et al 260987 X 3,291,817 12/ 1966 Rockett 260429.9

TOBIAS E. LEVOW, Primary Examiner.

H. M. S. SNEED, Assistant Examiner.

US. Cl. X.R.