PROCESS FOR THE PREPARATION OF OXODISUCCINATES USEFUL AS DETERGENT BUILDERS

Abstract: An improved method is disclosed for preparing sodium and calcium 2,2′-oxodisuccinates from maleic anhydride, or maleic acid, and malic acid, in which ether-bond formation is conducted with both calcium and sodium ions copresent in an ultimately homogeneous, concentrated, basic aqueous environment at temperatures between about 65°C and about 85°C. Under these conditions, 2,2′-oxodisuccinate salt production is an unexpectedly rapid, effective, convenient and potentially economical process.

Description: This application is a continuation-in-part of copending U.S. Patent Application Serial No. 834,185, filed February 24, 1986. BACKGROUND OF THE INVENTION The present invention relates to an improved process for making certain ether carboxylate salts, particularly preferred being 2,2′-oxodisuccinate salts. Sodium forms thereof are particularly useful, safe, biodegradable ether carboxylate sequestrants or builders in detergent compositions for household, institutional and industrial use. A number of types of ether polycarboxylates are known in the art and, along with methods for their preparation, have been disclosed in the patent litera­ ture. For example, Stubbs et al, U.S. Patent 4,017,541, issued April 12, 1977, disclose carboxyalkyl ethers said to include alkali metal salts of propylene glycol mono­ succinyl ether and propylene glycol disuccinyl ether. Pearson et al, U.S. Patent 3,776,850, issued December 4, 1973, disclose polycarboxylates said to have a formula which corresponds to HO-[C(R)(COOH)CR(COOH)O] n -H wherein R can be hydrogen and n ranges from 2 to 4. Berg, U.S. Patent 3,128,287, issued April 7, 1964, and Lamberti et al, U.S. Patent 3,635,830, issued January 18, 1972 both disclose 2,2′-oxodisuccinic acid and salts thereof, which are prepared by heating maleic anhydride or maleic anhydride or maleic acid in the presence of a molar excess of calcium hydroxide, followed by acid treatment of the resulting reaction product. The '830 patent discloses the use of this oxodisuccinic acid material as a detergent builder. Bush, U.S. Patent 4,566,984, issued January 28, 1986, discloses other polycarboxylate builders. All of the above patents are incorporated by reference herein. The disclosed methods for preparing the ether polycarboxylates of the foregoing patents in general involve 1) combining acidic organic reactants and an alkaline earth metal base; 2) carrying out an ether-bond forming reaction wherein the organic reactants couple in the presence of excess alkaline earth metal by a Michael-type addition, and 3) isolation or purification to remove the alkaline earth metal or organic by-products. In the above, the ether-bond forming step 2) is particularly characterized in invariably using a single metal, which is an alkaline earth, present in stoichiometric or superstoichiometric amounts; further, in that a two-phase solid-liquid reaction is involved. Even relatively recent disclosures of ether-bond forming reactions do not focus on this step, which may, according to the art, involve no more than crudely combining organic reactants with base in the absence of solvent. Known ether-forming reactions of particular ether car­ boxylates have slow reaction kinetics with respect to the desired products. Side reactions, e.g., formation of fumarate may occur in parallel to a greater extent than would be the case were the ether-formation reaction to occur rapidly. Ether carboxylate isolation steps sub­ sequent to known ether-forming reaction steps frequently involve difficult, cumbersome and uneconomical purifica­ tion; the finely-divided solid alkaline earth salts and the organic by-products must be removed from the product. These processing disadvantages have proved surprisingly intractable over the years and render particular ether carboxylate materials such as 2,2′-oxodisuccinate salts unattractive for use as builders in detergent products to be commercially marketed in large volume, despite their biodegradability and efficacy. The search for improved ether carboxylate purifica­ tion processes is illustrated by the disclosures of Blumbergs et al, U.S. Patent 3,821,296, issued June 28, 1974; Lindsay et al, U.S. Patent 3,862,219, issued January 21, 1975; and Blumbergs et al, U.S. Patent 3,948,985, issued April 6, 1976. The '296 patent dis­ closes a crystallization enhancement process for convert­ ing to the trisodium salt a crude calcium salt of CMOS (in the form of a mixture obtained by reacting maleic and glycolic acids in the presence of Ca(OH)₂). In the disclosed process, soda ash and the abovementioned mixture are added in about stoichiometric amounts to a preformed "heel". Said heel comprises product of the prior combination of soda ash with some of the above­ mentioned mixture. The '219 patent discloses another purification process; zinc or alkaline earth metal carboxyalkoxysuccinates, preformed from maleic acid and a hydroxyorganic acid, are converted into sodium salt form by treatment with substoichiometric amounts of carbonate. The product derived by mixing these components comprises a solution of sodium carboxyalkoxysuccinate salts, and solid alkaline earth metal salts. The solution is separated and evaporated to obtain sodium salts in solid form. The '985 patent discloses preparation of the acid form of CMOS by acidifying the calcium salt of CMOS (prepared from maleic anhydride and glycolic acid with excess Ca(OH)₂) with sulfuric acid and filtration of CaSO₄.2H₂O. The latter readily precipitates at elevated temperatures between about 40 and 80°C. Fumarate by-product is removed from the carboxymethyl oxysuccinic acid by conventional methods of extraction with organic solvents. Irrespective of these developments, all of which focus on purification of preformed ether car­ boxylates, there has been no recognition that simple modification of the ether formation step might lead more directly to a truly viable commercial process for ether carboxylate synthesis. Accordingly, it is an object of the present invention to provide an improved 2,2′-­ oxodisuccinate synthesis method wherein the ether-bond forming process is carried out efficiently, conveniently and rapidly. Contrasting with the art, homogeneous and, in certain cases, clear liquid reaction mixtures can be formed. These are readily pumpable or otherwise workable at high concentrations of reactants, and provide 2,2′-­ oxodisuccinate calcium and single cation-salts readily isolable or convertible to sodium salt form by conven­ tional process steps. SUMMARY OF THE INVENTION The present invention relates to an improved process for forming sodium and calcium ether carboxylates. A reactant component comprising organic ether-bond forming acids and sodium or calcium salts or mixtures thereof are reacted in a homogeneous alkaline environment wherein both sodium and calcium are also present in particular amounts. The calcium content of the reaction mixture is sub-stoichiometric with respect to the moles of organic reactant component which are used. The process of the invention is particularly suita­ ble for forming sodium and calcium 2,2′-oxodisuccinates which are readily convertible into a form suitable for use as detergency builders by means of conventional, art disclosed processes. Said 2,2′-oxodisuccinate salts are prepared according to the process of the invention by a process which involves combining particular inorganic and organic reactant components at temperatures below about 120°C to form an alkaline aqueous ether-bond forming reaction mixture, which has, as initially formed, a specific range of composition. This mixture is main­ tained at a temperature of at least about 60°C sufficient to permit ether-bond formation to occur. In the initially formed reaction mixture is present an ether-­ bond forming combination of maleate and malate. The maleate is derived from maleate reactants which are acids or salts of maleate (empirical formula C₄H₂O₄) with acidic hydrogen, sodium, calcium or combinations thereof. The malate is derived from malate reactants which are acids or salts of malate (empirical formula C₄H₄O₅) with acidic hydrogen, sodium, calcium or combinations thereof. Likewise present is at least one inorganic base or combination of said base with an inorganic salt, the amount and compositions of which are such that the initially formed reaction mixture has a maleate/malate molar ratio ranging from about 1:1 to about 2:1, a calcium to maleate plus malate molar ratio ranging from about 0.1:1 to about 0.75:1 and a sodium to maleate plus malate molar ratio ranging from about 0.5:1 to about 2.2:1. The amount of inorganic base added in forming the reaction mixture is sufficient both to completely neutralize said mixture and to yield a molar excess of hydroxide ranging from about 0.01 to about 0.4 moles of hydroxide per mole of combined maleate and malate. Maleate plus malate are generally present in the reaction mixtures, as initially formed, in concentrations (calcu­ lated as C₄H₂O₄ and C₄H₄O₅, respectively) ranging from about 31% to about 41% by weight of the total mixture. Reaction mixtures of the invention are particularly useful in being homogeneous low viscosity fluids forming rapidly, conveniently and economically the sodium and calcium 2,2′-oxodisuccinate salts in good yield. Particularly preferred processes of the invention use maleic acid or its anhydride and malic acid as the organic reactant component and sodium and calcium hydrox­ ides together as the inorganic reactant component. Preferred reactant combination and ether-bond forming reaction temperatures range from about 65 to about 85°C, and at the most highly preferred of such temperatures, which is about 75°C, excellent yields of sodium and calcium 2,2′-oxodisuccinate are obtained in periods of from about 1 to about 6 hours. Continuous processes could be readily provided on this basis for high conver­ sions of the maleate and malate reactants into 2,2′-­ oxodisuccinate salts. Also included herein are highly preferred processes for forming the 2,2′-oxodisuccinate salts by combining in particular sequences through intermediate reactant combinations, the above-named preferred reactants; the interrelated process conditions to carry out these combinations and the subsequent ether-bond forming reactions most advantageously, are given in full. DETAILED DESCRIPTION OF THE INVENTION Reactants Organic Reactant Component The processes herein comprise combining an organic reactant component (which may itself include metal cations) and an inorganic reactant component, then conducting an ether-bond-forming reaction. Careful specification of the forms, composition and proportions of the reactants is required. For example, the organic reactant component may change chemical form by hydration, acid-base reaction or cation exchange, which are rapid reactions capable of occurring prior to any of the ether-bond forming reactions of primary interest herein. All the organic reactants herein comprise maleate reactants and malate reactants. These are organic carboxylate compositions wherein essentially no ether (C-O-C) bonds are present, but which are capable of releasing maleate or malate species which form ether bonds under the process conditions described herein. Irrespective of the form of such reactants, physical or chemical, prior to use in the process of the invention, they will be capable of yielding maleate (empirical formula C₄H₂O₄) or malate (empirical formula C₄H₄O₅) in particular molar amounts at some "initial" time after they are combined in an ether-bond forming reaction mixture of the invention. The "initial" time is defined to be that time after combining organic and inorganic reactants at which no ether-bond forming or organic chemical reaction (other than maleic anhydride conversion to acid form) has yet occurred, so that the molar amounts of maleate or malate present are given directly by, and may be calculated from the weights and compositions of the maleate and malate reactants used. The initial molar amounts of maleate and malate can then be used as an unambiguous basis for specifying the relative molar proportions of other reactant species, e.g., sodium, calcium, alkali, which require also to be present in the ether-bond forming reaction mixtures herein. It will also be understood that irrespective of the true degree of dissociation in the concentrated aqueous media of the processes herein, which may suppress such ion dissociation, empirical formula units of maleate and malate herein have structures consistent with (I) and (II), respectively, and would be expected to take the fully dissociated anionic forms depicted, in dilute aqueous solution. It should be recognized that malate can take differ­ ent stereochemical forms, such as DL-malate, herein. The chemically suitable forms of maleate and malate reactants herein need only comprise acids (the anhydride of maleic acid being included), simple water-soluble salts of single cations, especially sodium or calcium, or salts believed to have more complex structures, releas­ ably containing maleate, malate, acidic hydrogen, sodium, calcium or combinations thereof in particular molar amounts based upon given weights of the reactants taken, combinations and hydrated forms of said reactants being included. Illustrative of suitable maleate reactants are maleic anhydride, maleic acid, sodium maleate and calcium maleate. Calcium maleate may be prepared by reacting maleic acid or maleic anhydride with calcium carbonate. Illustrative of suitable malate reactants are malic acid, sodium malate and calcium malate prepared by art-­ disclosed processes. Illustrative of the complex salt compositions included in defining the organic component are a maleate composition of empirical formula [NaH(C₄H₂O₄)] and a malate composition of empirical formula [Ca(Na(C₄H₄O₅))₂] which are acidic- or partial-salt, and dianionic composi­ tions, respectively. Polyanionic compositions are likewise included. Maleate and malate reactant compositions herein are not particularly limited with respect to the inclusion of materials chemically inert under the reaction conditions herein, or limited with respect to physical form, pro­ vided that based upon given weights, said reactant compositions yield particular molar amounts of maleate or malate. Solids, liquids or solutions are acceptable. For practical purposes, however, it is generally desir­ able to use maleate and malate reactants of commercial quality. Such reactants will often contain impurities, for example, in the form of fumarates. Maleate reactants may contain malate, usually in minor amounts. (It will be appreciated that maleate can isomerize to fumarate, at least to some extent, or undergo conversion to malate as is known in the art, thus affecting to some extent the precise compositions of the organic reactant component.) Based on the above definition, it will be understood what is intended in stating that the organic reactant component used in the processes of the present invention consists essentially of a) at least one maleate reactant selected from i) acidic or fully neutralized salts, formed from maleate having the empirical formula C₄H₂O₄, and acidic or neutralizing moieties selected from H, sodium and calcium, and combinations thereof, ii) maleic anhydride, iii) maleic acid, and iv) mixtures of said maleate reactants; and b) at least one malate reactant, in the form of a single isomer or of isomeric mixtures, selected from i) acidic or fully neutralized salts, formed from malate having the empirical formula C₄H₄O₅, and acidic or neutralizing moieties selected from H, sodium and calcium, and combinations thereof, ii) malic acid, and iii) mixtures of said malate reactants. Notwithstanding the above definition of maleate reactants used herein, the ether-bond forming process of the present invention is believed applicable to water-­ soluble reactants derived from a wide variety of acids and salts (of types defined above) yielding in aqueous solution the conjugated structure (III) wherein X and Y are any groups compatible with ether-bond forming reac­ tions, as defined herein, which are conducted in basic aqueous media. Likewise, notwithstanding the above definition of malate reactants used herein, the ether-bond forming process of the present invention is believed applicable to water-soluble reactants derived from a wide variety of moieties having structure IV or V wherein Z, R′, R² and R³ are any group compatible with ether-bond forming reactions conducted in aqueous media, provided that gem-dihydroxy and hemiacetal derivatives are specifically excluded. Inorganic Reactant Component The ether-bond formation reactions herein require, in addition to the above-defined organic reactant compon­ ent, a particular inorganic reactant component, the composition of which should be suitable to form, when combined with said organic reactant component, an alka­ line reaction mixture comprising both sodium and calcium in particular amounts as further defined hereinafter. The inorganic reactant component will in general consist essentially of at least one inorganic base capable of yielding hydroxide anions, at least in dilute aqueous solution. The inorganic base will commonly be selected from the oxides and hydroxides of sodium, calcium or mixtures thereof, though subject to the sodium, calcium and alkalinity constraints upon the ether-bond forming reactions herein, other bases could conceivably be used. The inorganic reactant component can alternatively comprise mixtures of said inorganic base with water-­ soluble inorganic salts capable of yielding sodium, calcium or mixtures thereof together with anions inert for the purposes of the ether bond-forming reactions herein. These salts function exclusively to provide an alternative means of conveying particular amounts of sodium or calcium into the ether-bond forming reaction mixture. As such, their anion composition is not of particular importance, provided that the anions do not interact detrimentally with the ether-bond forming reaction mixture. Suitable salts, for example, include sodium sulfate, sodium bisulfate, sodium chloride and calcium chloride. In contrast, an unsuitable salt is calcium carbonate. Being insoluble, this salt cannot be used in the present process as the inorganic reactant component without modifying said process. Further examples of unsuitable salts are sodium peroxides and hypochlorites in which the anions are capable of chemic­ ally reacting with the maleate and malate in the ether-­ bond forming reaction mixtures. Although it has not been demonstrated that other sodium or calcium salts of inert anions are equally effective or cost-effective, it is conceivable that alternative suitable inorganic salts could be used in the processes herein. The inorganic component herein is not necessarily limited as to physical form. Hydrates, slurries, and solutions or dry materials can be used subject to the composition requirements of ether-bond forming reactions further defined hereinafter. It will generally be convenient to use commercial grades of inorganic reactants. Recognizing that the organic reactant component defined above may provide a source of acidity, of sodium, calcium or all three in the processes herein, and also that said inorganic component may comprise inorganic salts, it is convenient herein to calculate the required total molar composition of sodium, calcium and alkali in the ether-bond forming reaction mixtures based upon the moles of maleate plus malate to be used. By means of the strict sodium, calcium and alkali (i.e., base) molar ratios to maleate and malate required in the ether-bond forming reactions given hereinafter, the practitioner of this invention will be able simply to calculate the molar amounts of the inorganic reactant component needed for reacting any selected amount of maleate and malate according to the process of the invention. It will be appreciated that the processes herein are desirably conducted at high concentrations in aqueous media as further defined hereinafter, both due to their efficacy and high throughput, permitting more economical plant. Preferred reactant compounds, at least in part based upon this observation, are together maleic acid or maleic anhydride, malic acid (particularly preferred is DL-malic acid) and sodium and calcium hydroxides. Maleic acid or maleic anhydride, and malic acid together deliver high maleate and malate molar amounts per unit weight. The combination of sodium and calcium hydroxides is capable of delivering alkalinity in the form of hydroxide and the particular sodium and calcium composition ranges shown unexpectedly herein to be essential to improved ether-bond formation processes. The Ether-Bond Forming Reaction It has been discovered that the use of combinations of sodium and calcium, rather than purely calcium, in aqueous alkaline ether-bond formation reactions, leads unexpectedly to rapid, high yield ether carboxylate production in ultimately homogeneous high-concentration reaction mixtures wherein less than one mole of calcium is present for every mole of maleate and malate used. It is believed that the improved ether-bond forma­ tion may be broadly applicable as indicated above in defining maleate and malate reactants and moieties. For example, processes of the present invention could be used to prepare the novel ether carboxylates comprising mixtures of tartrate monosuccinates and tartrate disuc­ cinates by this process disclosed in U.S. Patent Applica­ tion Serial No. 823,909, R. D. Bush et al, filed January 30, 1986, which is incorporated herein by reference. The following method is particularly applicable to the production of sodium and calcium 2,2′-­ oxodisuccinates. These ether carboxylate salt composi­ tions consist essentially of particular amounts of 2,2′-oxodisuccinate, having empirical formula (C₈H₆O₉), together with particular amounts of sodium and calcium. The structure of the 2,2′-oxodisuccinate tetra-anion is illustrated below. The overall sodium, calcium and minor by-product composition of the 2,2′-oxodisuccinate compositions formed will to some extent be dependent upon which particular embodiment of the process described below is used. The process for forming sodium and calcium 2,2′-­ oxodisuccinates comprises (I) combining, at temperatures below about 120°C, in aqueous media to form a reaction mixture, an organic reactant component, consisting essentially of a) at least one maleate reactant and b) at least one malate reactant, together with an inorganic reactant component, consisting essentially of a) at least one inorganic base or b) of mixtures of said base with particular inorganic salts, all reactants as hereinbefore defined and in specific proportions given below; and (II) maintaining said reaction mixture at a temperature of at least about 60°C, sufficient to permit ether-bond forma­ tion between said maleate and malate present in the reaction mixture. In the reaction mixture of the inven­ tion, maleate:malate molar ratios should range from about 1:1 to about 2:1, more preferably from about 1.1:1 to about 1.6:1, at the "initial" time of combination. Maleate and malate amounts will, of course, decrease, and proportions may vary during the course of the 2,2′-­ oxodisuccinate ether-bond forming reaction since some degree of maleate → fumarate and maleate → malate conver­ sion will concurrently occur. It is furthermore essential that in forming said reaction mixture, the "initial" molar ratio of calcium to maleate plus malate should be from about 0.1:1 to about 0.75:1, more preferably from about 0.31:1 to about 0.57:1. Likewise it is essential that in forming said reaction mixture, sodium should be present in an "initial" molar ratio of sodium to maleate plus malate of from about 0.5:1 to about 2.2:1. When maleate and malate reactants are in acid form and no inorganic salts are used, the sodium to maleate plus malate molar ratio should preferably be within a range of from about 0.9:1 to about 1.48:1. Finally, sufficient inorganic base should be added to the reaction mixture to render said mixture alkaline. Preferably, inorganic base is added in an amount to fully neutralize said reaction mixture and in an amount further sufficient to yield an excess of from about 0.01 to about 0.4, more preferably from about 0.04:1 to about 0.1:1 moles of hydroxide per mole of combined maleate plus malate. Specifying the alkalinity in this manner has proved consistently more reliable than pH measurement; nonetheless by sampling the reaction mixture, cooling the sample to 25°C and measuring pH, pH values obtained should lie from about 9 to about 13; pH values in the upper half of this range are generally preferred. It is also preferable to conduct the ether-bond forming reaction within a high, narrowly defined range of concentrations, by combining the inorganic and organic reactant components in aqueous media to provide total initial concentrations of maleate plus malate, of from about 31% to about 41%, more preferably from about 36% to 40%. Desirably, all organic and inorganic reactants will be combined using physical agitation (with the clear purpose in mind of achieving good mixing as well as of conducting the combination and subsequent reaction safely in view of the exothermic nature of certain of the hydration or neutralization reactions which occur). Temperatures of combination of the reactants should be constrained within a range of from about 60°C to about 120°C, though lower temperatures may be permissible in limited circumstances; e.g., when the organic reactants are in salt form. It is not generally advisable to permit combination temperatures below 60°C owing to the tendency of certain salts, e.g., [HNa(C₄H₂O₄)], to precipitate. Temperatures above 120°C may increase formation of by-products and at temperatures above the boil it is preferable to provide some means of condensing water which evaporates, so that it returns to the reac­ tion mixture. The reaction temperatures defined above are generally maintained for a period of from about 0.5 to about 12 hours. Highly preferred reactant combination and ether-bond forming reaction temperatures range from about 65° to about 85°C. Reaction temperature and time herein are apparently correlated such that preferred combinations involve reaction times of from about 1 to about 6 hours at the most highly preferred reaction temperature, which is about 75°C. Preferred reaction temperatures and times herein are based on optimization of yield and product purity. By sampling the reaction mixtures at intervals during ether-bond formation, and applying techniques such as HPLC chromatography, the yield of 2,2′-oxodisuccinates and the levels of maleate and malate reactants and of fumarate by-products in any individual reaction can be monitored. It is believed that yields of 2,2′-­ oxodisuccinates obtained according to the ether-bond-­ forming process herein may reach a maximum over a period of time and then decrease. Ether-bond forming reactions herein should be arrested by cooling, typically to below 50°C, and prefer­ ably to ambient temperature, at such time as when yields of at least 50% of the organic species present (typically, maleate plus malate plus fumarate plus 2,2′-­ oxodisuccinate) are in 2,2′-oxodisuccinate converted form. The reactions herein proceed at low viscosity, and the ether-formation reaction mixtures herein are readily stirred, pumped, etc. and form ultimately a single phase, which is a clear liquid. Even at the outset of ether-­ bond forming reactions herein, the reaction mixtures (upon formation with careful mixing) are homogeneous, easily pumpable fluids. This contrasts with the art (see, for example, U.S. Patent 3,635,830) wherein large amounts of insoluble salts of the reactants are formed. It is believed that in situ generated complex sodium/­ calcium salts, of the maleate and malate reactants, as well as of the 2,2′-oxodisuccinate products, have much higher solubilities than the single-metal calcium salts, thereby allowing for the convenient high-concentration processes herein. The kinetics of 2,2′-oxodisuccinate formation are also significantly enhanced by the co-presence of all the reactants, including calcium, ultimately in a single phase, thereby providing a workable and efficient ether carboxylate production process. The ether-bond formation reactions herein can generally be carried out in a stainless steel apparatus equipped with a source of agitation, e.g., a mechanical stirring device, and with cooling means, e.g., a cooling surface such as a jacket. Warm, rather than cold, water is used as a coolant and the preferred reaction apparatus will be designed to maximize shear over the surfaces of the cooling surface to ensure that encrustation by reactants or products does not occur, or is minimized. In defining the ether-bond forming process of the invention, it is intended to include both batch and continuous processes which result in ether-bond forming reaction mixtures having composition ranges given herein, optionally through the use of reactant or product recycle steps. Preferred Ether-Bond Forming Reactions Two processes of the present invention are illustra­ tive of the preferred reactants, conditions and composi­ tion ranges in the practice of this invention. These processes are also illustrative of certain intermediate compositions which can be prepared by combining reactants herein. The intermediate composition ranges involved in these processes are selected to result consistently in ether-bond forming reaction mixtures having the particu­ lar composition requirements of the invention. In one such process for forming 2,2′-oxodisuccinate salts, the following sequence is used: i) Dry solid calcium hydroxide is added to an aqueous sodium hydroxide solution, to form an alkaline slurry comprising from about 16 to about 20% Ca(OH)₂ and correspondingly from about 30 to about 34% NaOH; so that the Ca:Na mole ratio therein is from about 0.2:1 to about 0.4:1. ii) Maleic anhydride or maleic acid are combined together, and DL-malic acid, using a maleate/malate mole ratio of from about 1.4:1 to about 1.6:1, is added at a temperature from about 65 to about 85°C, to form an acidic aqueous solution wherein the concentration of maleate plus malate is from about 65 to about 75%. iii) The alkaline slurry of step i) and the acidic solution of step ii) are combined with agitation, at a rate selected to minimize precipitation of partially neutralized salts of maleic acid and to form a homo­ geneous mixture, at a temperature of from about 65 to about 85°C, such that the mole ratio of calcium to maleate plus malate combined is from about 0.35:1 to about 0.45:1 and the mole ratio of sodium to maleate plus malate combined is from about 1.25:1 to about 1.35:1. iv) The reaction mixture which is formed is mixed and maintained at a temperature from about 65°C to about 85°C, until at least 50% of the organic species present are in the form of 2,2′-oxodisuccinates. HPLC and atomic absorption analysis can be used to check organic composi­ tion and sodium/calcium content, respectively. v) The reaction mixture is cooled to arrest chemical reaction. Typically, cooling to less than about 50°C, more preferably to ambient temperature, is carried out. vi) The calcium content of the reaction mixture is reduced using conventional methods of precipitation or sequestration so that calcium comprises from about 0.2 to about 1% of 2,2′-oxodisuccinate salts present. Ethane hydroxy diphosphonate, an acidic resin, sodium carbonate, sodium bicarbonate, or combinations thereof are used for this purpose. An alternative process for forming 2,2′-­ oxodisuccinate salts, uses the following sequence: i) An alkaline slurry is prepared by combining calcium hydroxide and aqueous sodium hydroxide with Ca:Na mole ratios ranging from about 0.35:1 to about 0.45:1. The slurry will comprise from about 16 to about 26% Ca(OH)₂ and from about 24 to about 34% NaOH. At higher Ca(OH)₂ contents, the sodium hydroxide contents will be correspondingly lower, within these ranges of composition. ii) With stirring to maintain a temperature of about 75°C are added with stirring first the slurry of step i) to a concentrated aqueous solution comprising from about 52% to about 62% malic acid; second, solid or liquid maleic anhydride is added to the mixture until maleate/malate molar ratios of addition range from about 1.1:1 to about 1.3:1, calcium to maleate plus malate molar ratios of addition range from about 0.4:1 to about 0.5:1 and sodium to maleate plus malate molar ratios of addition range from about 1.26:1 to about 1:46 to 1. iii) The ether-bond-forming reaction mixture pre­ pared in step ii) is maintained at temperatures of from about 65 to about 85°C for periods of from about 1 to about 6 hours, with stirring. iv) The reaction mixture is then cooled to arrest chemical reaction. v) The calcium content of the reaction mixture is reduced to a range of from about 0.2% to about 1% of the 2,2′-oxodisuccinate salts present by treatment with ethane hydroxy diphosphonate, an acidic resin, sodium carbonate, sodium bicarbonate, or combinations thereof. In either of the above two processes, homogeneous clear liquid reaction mixtures can be obtained. After the ether-bond forming reaction has been com­ pleted to the desired extent, the calcium content of the aqueous reaction mixture may be reduced by conventional means. Removal of calcium can be carried out in a number of ways known in the art. In general, simply adding a calcium precipitating material will suffice. Such calcium precipitating materials include, for example, alkali metal carbonate, ethane hydroxydiphosphonate, pyrophosphate, bicarbonate and/or alkali metal silicate. The resulting calcium precipitate can thereafter be removed from the aqueous reaction product mixture by filtration. In an alternative mode, removing calcium from the aqueous 2,2′-oxodisuccinate reaction product mixture involves treatment of said mixture with an appro­ priate insoluble ion exchange resin or zeolite. No matter what technique is employed, calcium content of 2,2′-oxodisuccinate salts prepared by methods herein should desirably be reduced to the extent that calcium comprises from about 0.2% to about 1.0% of the 2,2′-­ oxodisuccinate in order to form compositions suitable as detergent builders. 2,2′-oxodisuccinate salts formed herein can also be treated, after calcium removal, in a further step, using organic or aqueous solvent extraction to remove excess reactants, such as maleates, or organic reaction by-products, such as fumarates. This can, for example, be accomplished by conventional salt separation pro­ cedures using a solvent such as a mixture of methanol and water (4:1v/v) in which these excess reactants and reaction by-products are relatively soluble and in which the desired ether polycarboxylates are relatively insoluble. At any stage after 2,2′-oxodisuccinate formation, the reaction product can be concentrated by removal of water to the desired extent. Water removal can, for example, involve substantially complete drying of the reaction product mixture, e.g., by spray drying, so that the 2,2′-oxodisuccinates are recovered in solid, e.g., granular, form. Alternatively, the sodium 2,2′-­ oxodisuccinates in the form of an aqueous liquid may be utilized directly in the preparation of detergent compo­ sitions or laundry additive products of the types more fully described hereinafter. After reduction of the calcium content in the reaction product mixture, it is possible, if desired, to acidify the product mixture using conventional acidifica­ tion or ion exchange techniques to convert the 2,2′-­ oxodisuccinate salts therein to their free acid form. Normally, however, the 2,2′-oxodisuccinates materials of this invention can, after calcium depletion or complete replacement by sodium, be used as builders in their water-soluble salt form, and such acidification is therefore not usually necessary or desirable. When converted into suitable form, 2,2′-­ oxodisuccinates can be used as sequestering builders in a wide variety of detergent compositions or laundry addi­ tive compositions. Such builder materials are effective in promoting certain types of fabric cleaning. Such materials retain their efficacy as detergent builders even in relatively low pH cleaning solutions. Detergent compositions incorporating 2,2′-­ oxodisuccinates prepared using the ether-bond forming process improvement of this invention, in detergent-­ suitable form, contain as essential components from about 0.5% to about 98% of a surfactant and from about 2% to about 99.5% of the 2,2′-oxodisuccinate compounds as a detergency builder, generally in sodium-salt form. It is, of course, possible to combine 2,2′-oxodisuccinate detergency builders with known detergency builders in forming such compositions. More complete disclosures of materials suitable for incorporation in detergent and laundry additive composi­ tions of the present invention are found in U.S. Patents 4,056,481, Tate (November 1, 1977); 4,049,586, Collier (September 20, 1977); 4,040,988, Vincent et al (August 9, 1977); 4,035,257, Cherney (July 12, 1977); 4,033,718, Holcolm et al (July 5, 1977); 4,019,999, Ohren et al (April 26, 1977); 4,019,998, Vincent et al (April 26, 1977); and 3,985,669, Krummel et al (October 12, 1976); all of said patents being incorporated herein by reference. In addition to their utility as builders in deter­ gent and laundry additive compositions, the 2,2′-­ oxodisuccinates of the invention can, after reducing their calcium content, also be utilized in other contexts wherein water hardness sequestration is required. Such contents are provided in water softening compositions, devices and methods and boiler descaling compositions and methods. The following embodiments illustrate, but are not limiting of, the processes of the present invention. All percentages and parts herein are by weight unless indi­ cated otherwise. All ratios herein are mole ratios unless indicated otherwise. EXAMPLE I 2,2′-OXODISUCCINATE SYNTHESIS (Procedure 1) DL-malic acid, 134 grams, 1.0 mole, was dissolved in 100 grams of water in a stainless steel reaction vessel equipped with a stirrer and cooling coils. Then, as rapidly as possible subject to temperature constraints, a slurry of sodium hydroxide, 200 grams, 50% solution, 2.5 moles, and calcium hydroxide, 74 grams, one mole, and 75 grams of water was added while the mixture was stirred and cooled by means of warm water passed through cooling coils in the reaction vessel to maintain a temperature of about 75°C. The reaction was clear and yellowish after about half of the slurry was added, but turned milky after complete addition. Then with care to maintain the reaction temperature at about 75°C, powdered maleic anhydride, 117.6 grams, 1.2 moles was added to the reaction solution. The resulting reaction solution was easily stirred and maintained at 75°C for 12 hours at which time the solution was clear and homogeneous. During the reaction small, 0.1 gram or less samples were removed and analyzed by HPLC. The results are tabulated below. A yield of 68.6% was obtained of 2,2′-­ oxodisuccinates after 6.0 hours reaction time. The calcium can be removed from the resulting reaction mixture by treatment with any of the following reagents: ethane hydroxy diphosphonate, an acidic resin or sodium carbonate/sodium bicarbonate. EXAMPLE II 2,2′-OXODISUCCINATE SYNTHESIS (Procedure 2) Maleic anhydride 147 g, 1.5 moles, was dissolved in 150 g of water. DL-malic acid, 134 g, 1 mole, was added and this mixture stirred at 70°C until all solids dis­ solved. With stirring was added a well mixed slurry (consisting of calcium hydroxide 74 g, 1 mole, sodium hydroxide 256 g, 50% solution, 3.2 moles and additional water, 75 g) while the temperature was maintained between 70-85°C. The resulting reaction mixture was stirred and maintained at 75°C for six hours. During the reaction small, 0.1 gram or less, samples were removed and analyzed by HPLC. The results are tabulated below. A yield of 67% was obtained of ODS after six hours.

Claims: 1. A process for forming sodium and calcium 2,2′-oxodisuccinates characterised in that said process comprises I. combining, at temperatures below l20°C, in aqueous media to form a reaction mixture an organic reactant component, consisting essentially of a) at least one maleate reactant selected from i) acidic or fully neutralised salts, formed from maleate having the empirical formula C₄H₂O₄, acidic or neutralising moieties selected from H, sodium and calcium, and combinations thereof, ii) maleic anhydride, iii) maleic acid, and iv) mixtures of said maleate reactants; and b) at least one malate reactant, in the form of a single isomer or of isomeric mixtures, selected from i) acidic or fully neutralised salts, formed from malate having the empirical formula C₄H₄O₅, and acidic or neutralising moieties selected from H, sodium and calcium, and combinations thereof, ii) malic acid, and iii) mixtures of said malate reactants; together with an inorganic reactant component, consisting essentially of a) at least one inorganic base which yields hydroxide anions when placed in water, said base being selected from the oxides and hydroxides of sodium, calcium or mixtures thereof; or b) mixtures of said inorganic base with water-soluble, inorganic salts of sodium, calcium or mixtures thereof; said organic reactant component having a maleate:malate molar ratio ranging from l:l to 2:l; and said inorganic reactant component being used in an amount sufficient to form an alkaline reaction mixture, having initially a molar ratio of calcium to maleate plus malate of from 0.l:l to 0.75:l and a molar ratio of sodium to maleate plus malate of from 0.5:l to 2.2:l; and II. maintaining said reaction mixture at a temperature of at least 60°C, sufficient to permit ether-bond formation between said maleate and malate present in the reaction mixture. 2. A process according to Claim l, wherein in forming said reaction mixture, the inorganic reactant component is an inorganic base which is added in an amount which fully neutralises said reaction mixture, and in an amount which is further sufficient to yield an excess of from 0.01 to 0.4 moles of hydroxide per mole of combined maleate and malate. 3. A process according to either of Claim l or Claim 2, wherein said organic and inorganic reactant components are combined in aqueous media to provide a total initial concentration of maleate plus malate, of from 3l% to 4l%. 4. A process according to any of Claims l to 3, wherein the organic and inorganic reactant components are combined using physical agitation, at a temperature of from 60°C to l20°C, and wherein a reaction temperature of from 60°C to l20°C is maintained for a period of from 0.5 hours to l2 hours. 5. A process according to any of Claims l to 4, wherein all organic and inorganic reactant components are combined with stirring, at a temperature of from 65°C to 85°C, and wherein a reaction temperature of from 65°C to 85°C is maintained for a period of from 0.5 hours to l2 hours and wherein chemical reaction is arrested by cooling when at least 50% of the organic species present are in the form of 2,2′-oxodisuccinates. 6. A process according to any of Claims l to 5, whereby subsequent to ether-bond formation, 2,2′-oxodisuccinate salts in a form suitable for use as a detergent builder are prepared using conventional calcium removal processes requiring calcium removal agents selected from sodium carbonate, sodium bicarbonate, ethane hydroxy diphosphonate, acidic ion-exchange resins and conventional calcium sequestrants. 7. A process according to any of Claims l to 6 wherein said maleate reactants are selected from maleic anhydride, maleic acid and mixtures thereof, said malate reactants are selected from single isomers of malic acid and isomeric mixtures of malic acid; and wherein said inorganic reactant component consists essentially of a mixture of sodium hydroxide and calcium hydroxide. 8. A process according to Claim 7 wherein the reaction mixture is formed using a total initial concentration of maleate plus malate from 3l% to 4l% of said reaction mixture; and is formed using a total amount of sodium hydroxide which comprises from 5% to 29% of said reaction mixture, calculating said sodium hydroxide as NaOH; and is formed using a total amount of calcium hydroxide which comprises from 2% to l9% of said reaction mixture, calculating said calcium hydroxide as Ca(OH)₂. 9. A process for forming 2,2′-oxodisuccinate salts, said process comprising i) adding dry solid calcium hydroxide to an aqueous sodium hydroxide solution, to form an alkaline slurry comprising from l6% to 20% Ca(OH)₂ and from 30 to 34% NaOH; so that the Ca:Na mole ratio therein is from 0.2:l to 0.4:l; ii) combining together in water maleic anhydride or maleic acid, and DL-malic acid, using a maleate:malate mole ratio of from l.4:l to l.6:l, at a temperature of from 65°C to 85°C, to form an acidic aqueous solution wherein the concentration of maleate plus malate is from 65% to 75%; iii) combining the alkaline slurry of step i) and the acidic solution of step ii) with agitation, at a rate selected to avoid precipitation of partially neutralised salts of maleic acid and to form a homogeneous mixture, at a temperature of from 65°C to 85°C, such that the mole ratio of calcium to maleate plus malate is from 0.35:l to 0.45:l and the mole ratio of sodium to maleate plus malate is from l.25:l to l.35:l; iv) mixing and maintaining the reaction mixture which is formed, at a temperature of from 65°C to 85°C, until at least 50% of the organic species present are in the form of 2,2′-oxodisuccinates; v) cooling said reaction mixture to arrest chemical reaction; and vi) reducing the calcium content of said reaction mixture, so that calcium comprises from 0.2% to l% of 2,2′-oxodisuccinate salts present, using ethane hydroxy diphosphonate, an acidic resin, sodium carbonate, sodium bicarbonate, or combinations thereof. l0. A process for forming 2,2′-oxodisuccinate salts, said process comprising i) preparing an alkaline slurry by combining calcium hydroxide and aqueous sodium hydroxide, wherein Ca:Na mole ratios range from 0.35:l to 0.45:l, said slurry comprising from l6% to 26% Ca(OH)₂ and from 24% to 34% NaOH; ii) adding successively with stirring to maintain a temperature of about 75°C, first the slurry of step i) to a concentrated aqueous solution comprising from 52% to 62% malic acid; and second, solid or liquid maleic anhydride to the mixture until maleate:malate molar ratios of addition range from l.l:l to about l.3:l, calcium to maleate plus malate molar ratios of addition range from 0.4:l to 0.5:l and sodium to maleate plus malate molar ratios of addition range from l.26:l to l.46:l; iii) maintaining the ether-bond forming reaction mixture which is the product of step ii) at temperatures of from 65°C to 85°C for periods of from l to 6 hours, with stirring; iv) cooling said reaction mixture to arrest chemical reaction; and v) reducing to a range of from 0.2% to l% of the 2,2′-oxodisuccinate salts present, the calcium content of said reaction mixture, by treatment with ethane hydroxy diphosphonate, an acidic resin, sodium carbonate, sodium bicarbonate, or combinations thereof. 11. A process according to either of Claims 9 or l0 wherein said reaction mixture forms a clear homogeneous liquid.

Inventors: BUSH, RODNEY DEAN, CONNOR, DANIEL STEDMAN, HEINZMAN, STEPHEN WAYNE, KRETSCHMAR, HERBERT CHARLES, MACKEY, LARRY NEIL
Applicants: THE PROCTER & GAMBLE COMPANY
Application year: 1987
Main classification: C07C59
IPC codes: C11D3, C07C51