GUANIDINE FROM CYANAMIDE Preparation in Presence of Ammonium Phosphates
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REPARATIOS of guanidine represents one phase of an investigation of the conversion of calcium cyanamide into other compounds of nitrogen possibly more suitable for utilization in fertilizers. Free cyanamide exhibits three typical reactions, (1) polymerization to dicyanodiamide, (2) hydrolysis to urea, and (3) ammonolysis to guanidine-e. g.,
RUSSELL M. JONES AND J. W. H. ALDRED Tennessee Valley Authority, Wilson Dam, Ala.
volume of each bomb was 20 cc. and the charge was held t o half this volume. CYANAMIDE SOLGTION.Concentrated stock cyanamide solu(3) tions were prepared by the procedure developed by Hetherington and Braham ( 5 ) . One solution so prepared and used in most of The availability of the nitrogen in dicyanodiamide and urea the experiments to be described contained 43.7 per cent total nitrogen, 40.4 per cent cyanamide nitrogen, 0.6 per cent urea has been studied many times, but little is known concerning njtrogen, a trace of dicyanodiamide, and no guanylurea. Pure the use of guanidine salts for fertilizer purposes (6). Present dicyanodiamide was prepared by recrystallization of dicyanoindications are that they form stable ~ i x t U r e Swith various diamide formed by boiling a solution of cyanamide at the proper fertilizer ingredients and are not toxic to plants. Tests compH ( 5 ) . AMMONIUM PHOSPHATE. The monoparing the availability of guanidine ammonium phosphate was a technical to growing plants with the common grade prepared in the laboratory. This nitrogen carriers are now in progress material, analyzing 11.3per cent nitroA study was made of the in several state experiment stations. gen and 62.0 per cent P106,contained some free H3P04. The desired ammopreparation of guanidine Certain properties of guanidine, such nium phosphates were prepared in the as high n i t r o g e n c o n t e n t , strong from cyanamide and the autoclave as required by adding aqua basicity, and nonhygroscopicity of or anhydrous ammonia to a charge of ammonium phosphates. c e r t a i n salts, were considered this monoammonium phosphate in a The maximum yield was solution of cyanamide. sufficiently valuable to warrant a ANALYTICAL METHODS.Cyanamide obtained when a mixture study of a process for its preparation was determined by the ammoniacal of cyanamide and diammoin the form of a salt. The phosphate silver precipitation method ( 7 ) . Diwas chosen for detailed study since cyanodiamide was extracted with acenium phosphate in the tone, converted to guanylurea, and this salt has good physical propermolar ratio HzCNz :NH,: precipitated with nickel (4). Total nities and contains two of the elements trogen and ammonia nitrogen were estiH3P04 = 1:2:1 was autonecessary for plant growth, nitrogen mated by A. 0. A. C. methods (1). claved at 140" C. for one and phosphorus. The Vorzarik ammoniacal picric acid method (8) was used to determine hour. Under these condiguanidine. Urea was estimated by the tions a yield of 70 moles of urease method (S). Experimental Procedure 2H2CNz = H4CzN4 HzCNz HzO = CO(NHz), NHs CH~NB HiCNz
++
APPARATUS. In the study of ammonolysis of cyanamide, use was made of a rocking autoclave. It was essentially a tee turned from a nickel steel forging, closed by blank flanges, and mounted on trunnions with the side branch up. Through the top flange on the side branch, connections were made t o the interior of the autoclave for the ammonia line, pressure gage, blowdown valve, and thermometer well. The thermometer well, which extended almost to the opposite wall, was partly filled with lubricating oil to facilitate heat transfer from the charge to the mercury thermometer. An electrical heating element was embedded in insulation over the main branch. The total volume was 4.5 liters and the volume of the usual charge was about 2 liters. For following the course of the reaction, six small tool steel bombs, similar in design to the bomb for the Parr peroxide calorimeter, were used. The
(2)
guanidine per 100 moles of cyanamide was obtained. The remainder of the cyanamide was converted to urea and to ammonia in about equal amounts. The course of the reaction appears to be the rapid formation of d i c y a n o d i amide at an intermediate temperature followed by conversion of dicyanodiamide to guanidine at higher temperatures. At s t i 11 higher temperatures guanidine d e c o m p o s e s to ammonia and carbon dioxide. 272
EXPERIMENTS I N AUTOCLAVE.In order to determine optimum conditions for the preparation of guanidine phosphate from cyanamide, an experimental study of the reaction between cyanamide and ammonium phosphates was made to determine the effect of (1) concentration of cyanamide in solution, (2) mole ratio of ammonia t o cyanamide, (3) mole ratio of phosphoric acid to ammonia, (4) temperature, and (5) time of heating. Concentration of Cyanamide and Ammonia Preliminary results indicated that considerable conversion of the cyanamidetoguanidineoccurredat 140 "C.
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in the presence of diammonium phosphate. Solutions containing 10, 20, 30, and 40 per cent by weight of cyanamide nitrogen were therefore mixed with varied proportions of monoammonium phosphate, and sufficient ammonia was added to give 2.2 moles of ammonia per mole of H3P04. The data obtained by autoclaving these mixtures for one hour a t 140" C. are shown in Figure 1. Over the range of ammonia concentrations investigated, maximum yields of guanidine were obtained from solutions containing 20 per cent cyanamide nitrogen, Ammonia in the proportion of 1.5 or 2.0 moles per mole of cyanamide produced better yields from all four concentrations of cyanamide than were obtained from the 1: 1 mole ratio.
Ratio of Ammonia to HSP04 The effect of varying the ammonia-phosphoric acid ratio on the yield of guanidine was studied next. In this study the autoclave charges were prepared by adding to the cyanamide solution (20 per cent nitrogen by weight) monoammonium phosphate and sufficient ammonia to vary the mole ratio of ammonia to H3P04from 6to 1. I n each case there were present in the charge 1.5 moles of ammonia for each mole of cyanamide. Each mixture was autoclaved for one hour a t 140" C. The results obtained for these conditions are shown in Figure 2 . The maximum yield of g u a n i d i n e w a s obtained from the use of 2.2 moles of a m m o n i a p e r m o l e of H3P04. When less than 2.2 m o l e s of a m m o n i a w e r e p r e s e n t ! t.he p r o d u c t s from the a u t o c l a v e con-
TEMPERATURE
FIGURE 3. EFFECTOF TEMPERATURE Ratio, moles HaPOa:NHx:H2CN2 = 1:Z.Z:l.l.
have been increased by changing conditions of autoclaving so as to ammonolyze the dicyanodiamide.' The amount of urea in the products showed only slight increase as the mole ratio of ammonia to HaPo4 was increased from 1 to 6.
Temperature and Time of Heating
Effect of temperature was determined by heating a mixture of cyanamide solution containing 20 per cent nitrogen with diammonium phosphate in the autoclave for one hour a t loo", 120°, 130°, 140°, I0 150", and 160" C. Two moles of ammonia were used for each mole of nitrogen in the cyanamide 1 solution. The maximum yield was obtained a t 0 W 140" C . (Figure 3). A change in temperature of P a 10' above 140' caused more ammonia to be formed; a a t 10" below 140 ", considerable dicyanodiamide remained in the product. a Charges of the same composition as were used z for determining the effect of temperature were "= heated a t 140" C. for different intervals of time varying from 3 to 180 minutes. The results are shown in Figure 4. The yield of guanidine increased with time up to one hour, after which a gradual M O L 6 NHs PER MOL WaP04 decrease occurred due to decomposition of guanidine. FIGURE2. EFFECTOF CHANGING Dicyanodiamide was present in mixtures heated TO H3P04 MOLARRATIOOF AMMONIA a short time. The decrease in quantity of dicyanodiamide with increase in time corresponded to the increase in guanidine. tained considerable quantiNo attempt was made to identify the salts present in the ties of ammonium carbonproduct. However, the vapor pressure of the products where ate w h i c h i n c r e a s e d as little ammonia was formed indicated the presence of diamthe acidity of the charge monium and diguanidine phosphate. Both of these salts i n c r e a s e d . With more were isolated by fractional crystallization of the product from than 2.2 moles of ammonia waten From those products in which large quantities of p e r m o l e of H3P04 the ammonia were formed, ammonium carbonate and ammonium y i e l d of g u a n i d i n e d e carbamate sublimed and were deposited in the colder portion creased. At the same time of the autoclave on cooling. dicyanodiamide appeared IO 20 30 40 CONCENTRATION OF CYANAMIDE 8 0 L The product obtained by heating the cyanamide solution in the products in amounts PERCENTABE OF NITR00EN with diammonium phosphate under the optimum conditions which corresponded to the FIGURE 1. PRODUCTSOBfor obtaining the maximum yield of guanidine contained also TAIR'ED WHEK SOLUTIONS decrease i n g u a n i d i n e . urea and ammonia, and was entirely free from dicyanodiamide Since, as is shown later, diCONTAINING 10, 20, 30, AND and insoluble forms of nitrogen. The product offers promise 40 PER CENT CYANAMIDE cyanodiamide is an interas a fertilizer material. NITROGERW E R E HEATED m e d i a t e D r o d u c t in the WITH 1.0, 1.5, AND 2.0 MOLES formation of g u a n i d i n e , 1 A study of the preparation of guanidine from cyanamide and various OF AMMONIA PER MOLE OF the yield of the latter could ammonium salts in the presence of free ammonia is in progress. CYANdMIDE
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Y
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I n another series of one-hour runs a t 140' C., the cyanamide in the charge was replaced partially or wholly by dicyanodiamide. The percentage yield of guanidine based on the sum of cyanamide and dicyanodiamide nitrogen decreased as the proportion of dicyanodiamide increased (Figure 6). With dicyanodiamide alone 1.5hours were required at 140' to yield approximately the same amount of guanidine as given by cyanamide in one hour (Figure 7). This indicates that dicyanodiamide formed from cyanamide in the course of the reaction is more reactive than the dicyanodiamide charged in the form of crystals, The yield of guanidine was less in the small bombs than in the autoclave, partly because of the greater rate of decomposition induced by the increase in proportion of bomb surface to volume of charge. Also the rate of heating the chaTge may change the rate of various reactions involved in the
70
60
40
50
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FIGURE 4. EFFECT OF TIME
-_
200 -
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Reaction Mechanism in Guanidine Formation EXPERIMENTS IN SMALL BOMBS. The data obtained in the autoclave experiments are not adapted to interpretation of the mechanism of guanidine formation because of the various reactions which may occur during the 45 to 50 minutes required to raise the temperature of the charge to the optimum temperature of 140'. In order to ascertain the course of the conversion, solutions of cyanamide and diammonium phosphate in the proportions previously found to give the best yields of guanidine were placed in small steel bombs and brought rapidly to a temperature of 140' C. by immersion in a preheated oil bath. After heating for periods of 2, 10, 20, 40, 60, and 120 minutes, the bombs were withdrawn and chilled immediately in cold water. The results of these experiments are shown in Figure 5. The charge heated for 2 minutes probably did not reach a temperature of 140" C. throughout; of the cyanamide charged, 11.0 per cent remained unchanged, 69 per cent formed dicyanodiamide, and 6 per cent was converted to guanidine. As the reaction period increased, the proportion of guanidine in the product increased and that of dicyanodiamide decreased. Probably dicyanodiamide represents the primary product which, in turn, undergoes conversion to guanidine. The decrease in the sum of dicyanodiamide and the guanidine percentages is indicative of further decomposition.
d e c o m p o s i t i o n of t h e cyanamide. The m e c h a nism of the reaction subsequent to the formation of dicyanodiamide is that outlined by Blair and Braham
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1.1
TIME-
nouns
FIGURE 7. SMALL-SCALE EXPERIMENTS SHOWING EFFECT OF TIME OF HEATINGON YIELDOF GUANIDINE
( 2 ) , where One Of ammonia adds to one of d i c y a n o d i a m i d e forming Charge with mole ratio of HaPO,: NHs:HzCN, = 1:2:1 biguanide which, in turn, reacts with a second molecule of ammonia to give two molecules of guanidine salt. Biguanide was found in small amounts in the runs made a t temperatures below 140' C. These reactions can be represented by the following equations :
Acknowledgment The authors are indebted to K. L. Elmore, J. C. Brosheer, P. C. Gwinn, Ray Lawrence, and John W. Lefforge for analytical data presented in this paper.
Literature Cited
eo
40
so
100
80
T I M E - MINUTES
120
FIGURE5. MECHABISM OF GUANIDINE AS S H O W N BY DURATION OF FORMATION HEATING SMALL-SCALE EXPERIMENTS
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Ratio, moles H~POI:NHI:HICN: 1:2:1.
(1) Assoo. Official Agr. Chem., Methods of Analysis, 3rd ed., Chap. 11, pp. 14-32 (1932). (2) Blair, J. S.,and Braham, J. M., S.Am. Chem. SOC.,44,2342-52 (1922). (3) Fox,E.J., and Geldard, W. J., IND.ENQ.CHEM.,15, 743 (1923). (4) Garby, C.D., Ibid., 17, 266 (1925). (5) Hetherington, H.C., and Braham, J. M., Ibid., 15, 1060 (1923). (6) . _Jacob, K.D.. Allison, F. E., and Braham, J. M., J. Agr. Research, 28,37-68 (1924). (7) Pinck, L.A., IND.ENQ.C H ~ M 17, . , 459 (1925). (8) Vorzarik, 2.angew. Chhem., 15, 670 (1902). R E C E I V September ~D 24, 1935.
