United States Patent Office Patented Aug
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3,524,872 United States Patent Office Patented Aug. 18, 1970 1. 2 3,524,872 prepared ammonium thiocyanate to produce a clean, PRODUCTION OF METHYLENE BIS THOCYA slightly off-white product of methylene bis thiocyanate NATE FROM AMMONUM THIOCYANATE AND DHALO METHANES (MT). Joseph Matt, Chicago, Ill., assignor to Nalco Chemical CONTROL OF THE pH Company, Chicago, Ill., a corporation of Delaware The instant reactant, NHSCN, much more than its No Drawing. Filed May 15, 1967, Ser. No. 638,624 alkali metal cousins, is susceptible to polymerization. It Int, C. C07 c 161/02 is believed that when the pH of the reaction mixture is U.S. C. 260-454 9 (Caims less than 6 and to a certain extent less than 6.2, then ammonia is present in the reaction mixture as NH3 or O NH, and the unstable HSCN polymerizes to a dark ABSTRACT OF THE DISCLOSURE brown or red mass. The product yield is cluttered with The present invention relates to an improvement in the a polyymeric by-product if the initial pH is below 6. general process of producing methylene bis thiocyanate Conversely, where the pH is >6.5 ranging up into a (MT) by the following equation: basic medium, the polymer formation in the product is 5 apparently chargeable to increased sensitiveness of the 2NHCNS -- CH2 X2 - CH(SCN)2 + NHX desired methylene bis thiocyanate and the unstable mono (AT) (MT) halide intermediate in the reaction medium also giving Timm polymer by-product. Thus, beset by different polymer formation problems Applicant has now found that in dealing with the for 20 either way on the pH scale, the present process has Sur merly non-preferred reactant ammonium thiocyanate, cer mounted these difficulties by finding a critical initial pH tain critical pH limitations are necessary in order to to make available as a reactant the cheaper but previously achieve operable or improved results as related to inhibi scorned ammonium thiocyanate (AT). tion of polymer formation occurring in brown or red by It has been found that the reaction mixture containing products. It has now been found that in using a process 25 an organic solvent for the dihalo methane is generally an involving an added organic solvent for the dihalo methane acid pH medium of about 5. In order to practice the im (e.g., toluene), there is a critical initial pH range for con provement of the present invention, this acid pH in the ducting the reaction within the operable limits of a pH reaction mixture must be adjusted to the range pH 6.0- of 6.0-6.5, a preferred range of 6.2-6.5, and an optimum 6.5, preferably to 6.2–6.5 and/or an optimum pH of 6.5 value of about 6.5. 30 by the addition of a suitable base, such as KOH or NaOH Variation of critical process limitation, such as provid or basic salt, such as K2CO3 or Na2CO3. ing a completely aqueous solvent medium for the dihalo Table I below illustrates the reaction using the mixed methane reactant during the reaction, is taught in the re dihalo methane, chlorobromomethane (MCB). The re lated copending application Ser. No. 604,122 filed Dec. sults are shown for three groups of tests showing the 23, 1966, entitled "Method for Preparing Methylene Bis 35 practice of the products and variation of pH. (Thiocyanate)’ of Matt et al. The runs at initial pH of 6.2-6.5 uniformly showed a Relative to the background of the invention, it is to striking lack of polymer formation. The results also be noted that the product of the improved process of the showed that where the pH shifted slightly upwardly from present invention has evoked much interest due to its re 6.5 towards a less acidic medium, there was medium cently discovered propetries as an algaecide and as a 40 high polymer formation at 6.6. bactericide for sulfate reducing bacteria. U.S. Pats. Nos. The results further showed that the practice of the proc 3,252,855 and 3,300,375 to Wehner and Pat. No. 3,306, ess at a neutral pH of 7 or alkaline in the range 7.0-8.0 810 to Buckman et al. are all devoted to methylene bis uniformly led to unworkable high polymer product. thiocyanate and its activity as an industrial microbiocide. The results further showed that below an initial pH of The patent and literature art point out that uniquely about 6.0, there is a medium or high polymer formation methylene bis thiocyanate is active against many different which accentuates the narrow operational pH range for bacteria and algae in a concentration of only a few parts the process. per milion which, considering its relatively low unit cost, TABLE I: REACTIONS OF CHLOROBROMOMETHANE makes it an ideal active compound for agriculture and (MCB) WITH AMMONIUM THIOCYANATE (AT) AT 90° C. other industrial compositions. 50 pH vs. polymer formation in NH4SCN, reaction THE THIOCYANATE REACTANT No. pH NHSCNpercent 1, In general, the reaction may be viewed as between a thiocyanate MCNS or NH4CNS and a dihalo methane Little polymer formation in MT product CH2X2. Alternatively, the latter may be expressed in 6.5 50 6.2 50 nomenclature as a methylene halide. In the aforemen 6, 5 50 tioned copending application Ser. No. 604,122, it was 6.5 50 noted that in the general thiocyanate reaction, the alka Medium polymer formation in MT product line earth metals were inoperable or too expensive, and 808-6------------------------------- 6.6 50 the ammonium thiocyanate reactant was grouped with 60 808-20- 6.5 50 the alkali metals, sodium and potassium, which were pre 746-262- 5.7 50 ferred. The preference of the alkali metals over ammoni 746-293. - 5.7 50 um thiocyanate as the reactant in an industrial process High polymer formation in MT product is strange since ammonium thiocyanate enjoys a favorable 805------------------------------ 7.8 50 cost differential over the related sodium and potassium 65 S08-8------------------------------- 6.6 50 808-9------------------------------- 6.6 66 alkali metal salts. For example, recently a cost compari 808-12 6.5 66 son showed the cost per pound mol of the ammonium S08-30 ... O 50 S08-48 7.4 50 salt was about two-thirds that of sodium thiocyanate. 746-242 5.7 50 However, it is well known that ammonium thiocyanate 746-255 5, 7 P0 presents some problems as a reactant due to polymer for 70 746-260----------------------------- 5.7 50 mation leading to poor yields and/or a "red gunk' prod 1 NH4SCN calculated as percent strongth of aqueous solution. NOTE.-The runs prior to 803-40 and the series 746 sequence used uct. This problem is partially solved by using freshly reagent grade A.T. Other's used technical grado A.T. 3,524,872 3 4 THE DIHALOMETHANE REACANT AND dihalomethane is necessary to obtain an optimum yield REACTION SOLVENT of MT. Too much AT reactant clutters the MT product with an undesired excess of inorganic salts. Therefore, As taught in the copending application Ser. No. an operable molar range for the AT reactant is 2 and 4 604,122 noted above, the dihalomethane reactant for the moles per mole of organic halide reactant. production of methylene bis thiocyanate (MT) is prefer 5 It has further been found that strength of solution of ably dibromomethane and this compound illustrates a pre about 50% for the ammonium thiocyanate reactant is ferred class of reactants where the halo constituents are preferred as when the AT is made up with an equal similar. Also operable is diiodomethane. Operable, but weight of water. not preferred, are mixed dihalomethanes, such as chloro Below about 40% AT aqueous strength of solution, bromomethane CH2ClBr, and bromoiodomethane O CHBr and chloroiodomethane CH2Cl. In any event, it the yields and reaction times are adversely affected. At is desired that at least one, and preferably both, of the 60% and above AT aqueous strength of solution, high dihalo constituents be either bromo or iodo. In other polymer formation was observed even within the pre words, dichloromethane and difluoromethane are not pre ferred initial pH range of 6.2-6.5. ferred. In the past, it has been customary to add an or 5 The best results are obtained by use of reagent grade ganic solvent to the methylene dihalide reactant to solu ammonium thiocyanate reactant (AT) although this is bilize the reactant as well as the MT product. Since the not required. particular AT reactant in this process is conventionally in aqueous solution, these prior art solvents were organic PURIFICATION AND RECOVERY solvents which were hydrophilic in nature or miscible 20 Where the product is fouled by polymer formation, with water. Preferred known water-miscible solvents in inorganic salts, or when necessary, especially where mixed cluded acetone, methanol, dimethyl formamide, etc. A halides are used, the MT product may be recovered in pure more extensive list appears hereinafter under "Purifica form, preferably using a water-miscible purification sol tion and Recovery.” vent. Lower aliphatic alkanols are preferred and a 20% Although water-miscible solvents are generally oper 25 aqueous Solution of isopropanol is the solvent of choice. able in this process as added reaction solvents, as well as The solvent/HO ratio may vary from 1 to 10 to 10 to 1. purification solvents, a group of sequestering solvents with a preferred ratio of about 1 to 5. The permissible more amply described in the cofiled application of Joseph variation of solvents include other substantially water Matt entitled “Sequestering Solvent Process for Produc miscible lower alkanols, such as methanol and n-propanol tion of Methylene Bis Thiocyanate (MT)" are preferred 30 etc., as well as other classes of water-soluble solvents, as solvents during the reaction.