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GOVERNWENT OF INBIA
ATOMIC ENERGY COMMISSION
METHYLAMINE-HYDROOEN EXCHANdE., PART III. PHYSICOCHEMICAL PROPBRTIBS OF AMIDE-AMINE SOLUTIONS
K. S.-:niv»»« »»d S..M. Dtve Heavy Wttw Division
BHABHA ATOMIC RESEARCH CENTRE
BOMBAV,. INDIA 1983 1.4.1.C.-1180 mortnu. mmomMr cdKxsani
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Different p&jrtiooeheaicsl properties of potassiua nethylaaide/niethylaMine solutions' have been complied and reviewed* Vhesi properties will b« quite useful in design calculations for plants based on amine-hydrogen exchange for the production of heavy miter* METFTYLAMINE-HYDROGEW EXdHANGE. PART III. JHYSICOOHEUIGAL PROPERTIES OP AMIDE-AMINE
. by K. Srinivaaa and S.M. Dave
1, INTRODUCTION Deuterium exchange between liquid monomethylamine and gaseous hydrogen, catalysed by potassium raethylaraide is emerging as an attractive alternative to the well known ammonia-hydrogen exchange process. The recent studies' * ' indicate that the process has come to a stage where its commercial exploitation ia imminent.
The amlne-hydrogen exchange has many favourable features, such aa lower vapour pressures^' , higher exchange rates^'^' and larger separation factors*°*'*°' as compared to the ammonia-hydrogen system and all these advantages lead to considerable lowering of capital and energy costs for heavy v/ater production.
Eventh.ough the above mentioned parameters are quite favourable, the process chemistry for this exchange is quite complex and could give rise to some formidable problems. In order to assess the feasibility of this process, it is, therefore, absolutely essential that physical properties and chemical behaviour of the aaine/ amide solutions should be precisely known or predloted for different axperimental conditions* Unfortunately rery meagre information about amine-amide system is available in the published literature. This report attempts to collect necessary and pertinent infornation about this system from various scattered sources and present it in more systematic and usable foxn. (2)
PREPARATION OV POTASSIUM METHYMHIEE . ' • Potassium methylamide is most conveniently prepared by the reaction of raonomethylamine with potassium metal''* ', The roaotion is catalysed by traces of transition metal compounds like ferrous chloride or ferrio oxide. The amine rauat be purified from ammonia, moisture and other impuritiea before use.
Hie process involves the dissolution of potassium metal in methylamine before the amide formation reaction.' Potassium metal dissolves in methylamine even at -78 0 giving a deep blue solution which becomes pale yellow with the formation of the amide. The yellow colour is more intense at higher temperatures though the amide solution is almoat colourless at -*78 0» '^ie reaction is aaeomi'anied by the evolution of hydrogen.
K + CH5lffl2 —• CHg-NHK + %R2 , (1)
The evolution of hydrogen is an indication that the reaction is in progress.
Hie dissolution of potassium and its subsequent reaction are much slower than in ammonia. The rate of reaction is negligible below -GO 0.
!Ehe preparation of potassium methylamide is conveniently carried out at about -6 0. , Even then.it requires several hours for completion. The reaction should not be carried out at higher temperature as this will favour the decomposition of the amide. After preparation the (3)
amide solution should be stored below -10*0 to minimise thermal decomposition. In view of the extremely reactive nature of the potassium methylamide the amide solution : . should be handled using vacuum,line and dry box techniques in argon atmosphere*
Potassium methylamide can also be obtained by treating methyleonine with potassium hydride' '•
In yet another methodv ', the amide has been prepared by adding methylamine to a solution of potassium amide' in liquid ammonia.
Frescott and Sanford*": have patented a process for the removal of impurities from potassium raethylaiaide. the method involves converting the KHHOH* into KHHg by reaoting with ammonia and regeneration of KNHOH, by reaction between potassium amide and methylamine.
The amides of other alkali metals with other aliphatic amines can be obtained by similar methods.
PROPERTIES 0? AIIIPE/AMINE SOIUIIONS Potassium methyIsmids is a white crystalline solid with a relatively high melting point. It reacts vigorously with oxygen, oarbon dioxide, water and aloohdls . It reacts explosively with air and oan ignite spontaneously* Solutions of the amide in methyliaine are also extremely, sensitive to air and moisture. The amide may be safely destroyed by reacting it with «iyl alcohol •
Potassium methylamide/methylamine system is similar to (4)
potassium amide/ammonia system in many respects* However, some differences are .expected since there is a high degree of association due to the low dieleotrio constant of methylamine and the high negative oharge density on the • nitrogen. . •
3.1 Solubility of Amide in Aminea Potassium methylamide exhibits an inverse solubility in methylamine, i.e. its solubility decreases with increasing temperature, so that the amide is precipitated at higher temperatures* The solubility has been measured by Hayashitorai, Ishige and Otto^ ' between -78 and +4O#O. The solubility is 39 £ 3 g' K/Kg amine at -78°C and decreases to about 26 g K/Kg aoine at +40°C. This is surprising* particularly since the solubility of potassium amide in ammonia increases considerably with increasing temperature. This indicates that the dissolution of potassium methylamide has a negative enthalphy change which has been roughly estimated to be of the order of -0.5 Kcal/mole.
Table 1 gives the effebt of temperature on the solubility of potassium methylamide in methylamine.
The solubility of potassium ethylamide in ethylamine is relatively high and the system ia similar to the. potassium aaide/ammonia system with the solubility increasing markedly with temperature. She solubility at -40*0 is about 90 g K/Kg amine and at .^78*0 it ia at least 25 g K/Kg aaine. (5)
3«2 Viscosity • No data are available on the visoosity of potassium methylaraide solutions in methylamine. However, the viscosity of methylamine is known for different tempe- ratures and is given in Table 2* •
3.3 ftleotrioal Conductivity Symon and Bonnet*1 *' have described' an apparatus for conductivity measurements on potassium methylamide/ methylamine solutions over the temperature range -55 to 425°C. The eleotrode system based on the recommen- dations of Nichol and Puoss* *' consisted of a vertically mounted pair of concentric platinum cylinders. Temperature control and measurement was better than +0.02o0. Equi- valent oonductanoe values (A) were calculated from resistance data applying correction for the specific conductance of the solvent. The data obtained by them are believed to be accurate within
A plot of A &t constant temperature against the square root of the amide concentration is reproduced in Fig. 1. The dependence of A on temperature for three different concentrations is shown in Fig. 2. A distinct maximum was observed in each case at about ~30°C.
3*4 Dissociation of Amides Potassium methylamide dissolved in methylamine, dissociates Into free ions as shown below :
+ KHIICHj =?=* K + ~NHCH3 ....'(2)
The dissociation constant was reported to be 3 x 10"*^ mol I" (6)
at -90°0 and 4 x 10"7 mol IT1 at -62°C by Rochard^9'16*. Subsequently Symona and Bonnet* "•' found a value of 6.5 x 10~7 mol I."1 at -33.4°C.
Since methylamine has a fairly low dieleotrio constant (about 9.4 at 25°G) concentrated solutions of potassium methylamide are expected to consist of a Eixture of free ions, ion pai.rof triple ions and larger ion aggregates.
Symons and Bonnet' ^' have made a detailed study of the effect of temperature on the dissociation constant of potassium methyl«roide from conductance measurements made between -55°C and 25°0. These data are reproduced In Table 3. The dissociation constant Kj decreased from 9.4 x 10~7 mol IT1 at -55°0 to 6.3 x 10"8 mol I,"1 at 25°C.
The heat of dissociation has been calculated to be -26 kJ mol~ above -15°C. It decreases gradually at progressively lower temperatures. This has been attributed to a change in the nature of the ion pair structures.
3.5 Association of Amides Halliday and Bindner have estimated from kinetic studies, the percentages of the monomer present in potassium methylnmide/methylaininc solutions of different concentrations at different temperatures. These values are given in Table 4.
These calculations predict that at 0°C potassium methylaraide should be pre«ent almost entirely as a monomer over a wide concentration range. This is reported to be in agreement with the degree of association calculated from vapour pressure measurements on IMAAlA solutlona' ''. ,
4.;- REAO'l'IOHS Off AMIDES . '/ ' ' ','.'. Potassium taethylamide dissolved in methylanine is a strong i base and is highly reactive* It reacts with air and with water so violently that special techniques and apparatus are required for handling the system. Some of the reactions with the impurities usually encountered in the process are described below.
4.1 Reaction with water Potassium methylamide as well as its solution in raethylaraine reacts vigorously with water to give potassium hydroxide and methylamine. - .
. CRyiHK + H20 —* CHjNEg + KOH .... (3)
Potasoiiun hydroxide being insoluble in raethylamine can be removed by filtration in inert atmosphere.
When v:ater is in excess the amide is completely decomposed. Amide solutions can be conveniently determined from the quantity of potassium hydroxide formed.
4.2 Reaction v/ith oxygen The reaction of potassium methylamide with oxygen yields potassium dimethylfonnamidine (PBIPA). The reaction is oomplex and is believed to involve several steps.
CHjHlIK + 02 -^ KON, KOH, CHjNCH - ITOHj .*.. (4)
•':•" K (8)
4*3 Reaction with Carbon monoxide and Carbon dioxide Potassium methylamide, like Potassium amide, reacts with Carbon dioxide and Carbon monoxide forming Potassium jr-methyl oartumate and Potassium N-tnethyl foiwamide , respe ctively.
0H3NHK + C02 —* CHjNHCOOK .... (5) 0 CH,HHK + CO * oL^K . . /,-;
4*4 Reaction with Ammonia Potassium raethylauide is reversibly dscanposed by dry gaseous or liquid ammonia to yield potassium amide and free methylamine.
n • ' ' • • ' \ • The equilibrium constant has not been measured directly but estimates reported are 9 and 50\ o/. This reaction is known to occur with other alkali metal alkylamides. The reverse reaction oan be utilised to produce the pure potassium methylamide* *'. Since some ammonia is always present in methylamine solutions of potassium methylanide, either from the solvent itself or from the decomposition of the amide, physicochemical measurements in the. amine/amide system may be subject to significant errors. 4»5 Reaction with Hydrogen Solutions of Potassiun nethylsaide in •ethylamine react with hydrogen under pressure to give potassium hydride* ^'< OHjMHK + Hg r=^ (9) This reaotion has serious implications for the exchange process* the lov solubility of potassiwn hydride In methylaaine shifts the equilibrium to *e right removing the »ajor portion of ito o*t*iy#t fro* the ••lution. This reaotion is largely unaffected by tewperaiure changes* A ainilar reaotion has been found to oeour with eodiwt methyleraide in nethylflftlne and with potassitti itiethylanide in dime thy lamina but not with lithium twrtlqrlairide or with cesium raethylamide. Lithium *etnylan44e l*ei»g » poor catalyst ft suitable raixttire of lithiut «b* potcussiik mettiylanides holds out ooiteider»ble pwj*rt*e«# a potential catalyst for ttie 8mine-*tydrogen exoh«»ee Interestingly^ this reaotioH has not been obeerred in po+ossim amide/antonia ajrvtea. A reaotion between potassium aaidre and hydrogen does occur in ammonia at very high hydrogen pressure** but gives the solvaied electron rather than the hydride precipitate^ ''. 4t6 The thermal decomposition of potaasium nethylamide in methylamine has been well studied" '• She najor produotf of the deooMpositi0n «re pota»«lum KIT* dimethyl- fontamidine (PIMPA), ammonia and hydrogen, the rate constant for the reaetidn In ootwentrated solution* at 50*0 Is about 1.2 x 10"6 8~1. Henoe, thia reaction la fast enough to affect catalyst concentration over a length of time* , She thenaal deeeapoeition of potatsium methylamide hag alee been studied as a funotion of it* qonoentration (10) at 60°C by Halliday, Symohs and Bennett*1*', methylenimine and potasaiuu hydride have been poetulated aa .transient intermediaries for the deoanpoaition. —» KH + m2 * m .... (9) lithium and Sodium methylariides have been reported to yield the respective cyanides u&Jh heating in alethylaaine solutions or in tacUtw. However^ cyanide formation h«a not been detected in the ease of potassium methylafcide '• The thermal decomposition of the catalyat creates serious problems for the «xohaag* process. Apart frow the need lor frequent repieniahawrilt of th* expensive caialyai it also produces amonia and VttifA «tiich have to be removed. She ammonia auat tie romoite* as it reacts with the catalyst - to produce potaaaivaa amide which has a low solubility in methylamine (0.06 moles/Jrg). TMM has a higher volubility (3.3 moles/kg) but it too will have to be removed, though less frequently* A high hydrogen pressure decreases the rate of thermal decomposition but it also causes the precipitation of potassium hydride, as shown in reaction (8), This BUBS up about all the information regarding aaine- amide solutions that ia available in the open literature. Katurally, there are many gaps and serious lapses in the iafowaaticai and more accurate data regarding the stability and solubility of catalyst in aralne as a function of teaperature, carry over of amine, foaitting in amide araine •ystos etc. are very much required for the design of aaine- hydrogtn plant. We have carried out separation factor •easurements at different temperatures^ ^ and More studies regarding stability of catalysts at different process conditions are underway in our laboratory* (11) Authors ar« thankful to Shri V*O. Ba*hpan4e and Ihri H*K. Sadhukhan for the it- ihtemt and encouragement ill this work* 1. U.K. Rae (Ed.)» "Separation of Hydrogen Isotopes"4 ACS Symposium serie* (AneriewR OhMical Society, Washington D.O), 6§# 1978. p. t, 40» 53, 71.. 2» it.P. Wynn, Sulaer lefeh. R«v., 60(4). 162 (197S). t* A.R. Bancroft and H«K. Rae, Atomic Energy Oanada Report No. ABC1-3684 (1970). 4« A.R- Kenyon and D P«pp«r, J. Appl. Chem., 399 (1964). 5. K. Bar - Hi and f.3* EL»l«j 3* Ohem. Soc, 3038 (1962). i 6« S.H. Dave, 8.K. Ghosh and H«X. 8adhuldian. Meth.YJ.mine- Hydro,;en Bsohangi* ?art t.. BARO Report No. 1116 7e 3.M. Dave, N.C* So sr, K* Srinlvaoa and H.K. Sadhukhan* Meth,'fl^)iin»"H,ydr«»fa Bxch^Hiy. Part II., BAIW Report No. 1117 (1981). 8. 3.M. Save, N.C. Ooomer, K. Srinivaea, S.K. GSiosh and H.K. Sadhukhan, laotopeiO'i'ixis* 1§» 362 (1982). 9* 2. Roohard, CEA-R-3835 (t969). 10. M. Hayashitomi, T. Ishlge and 7.9. Otto, J. Ohem. Engg. Data, 18(3), 280 (1973). (12) 11* R.O.iMakhija and H.A. Stairsi dan. J. Oh«m», 49(j?}« 807 i 12* I. Lambert and J. Ravoitfe, Geinah Patent (1971)» 0*A* 76 * . •16262.*- ', . :'..:;•••• '•"•• '' • ^ ?v' V"=? -''• • ;: ' . '. . • ' '.. ; • '' 13« J^P.i'redcot-t and E.C* Sanfordi AEOI Patent, O.A* 76 r 26622. 14*^. E.A. Syraona and J.D. Bonnet, Can. iT. Chem. ^S_t 1518 (1978). 15. J.C. Hiehol and.R.M. Puosa* J. Phya, Ohem;*, 58, 696 (1954)* 16. E. Rochard, J. Ohiffl. PhyB., 68, 1183 (1971). 17* J.B» Halliday and P.E. Bindner, Can. J« Chen., 5,4» 3775(1976)* 13. E.'Bar-Eli'and F.S. Klein, J. Ohim. Soc. 3083 (1962) > 19. J.F. Prescott, Ger. Pat. (1976), O.A. §6 : 57557. 20. E.J. KiracMce and W.L. Joliy, Inorg. Chem. ^, 855 (1967). 21. K.N. Boddeker, G. Tiand and U. ScMndewolf, Angew, Chem. (Int.Ed.) 8, 138 (1969). 22. J^.D. Halliday, E.A. Symons and J.D. Bonnet, Can. J. Chera., 56, 1455 (1978). 23* E. Jusa and E. Hillenbrand, Z. Anorg. Allg. Chera., g73, 297 (1953). Table 1. Solubility of Potassium Methylmide in Methylamine Temperature Solubility +0.1 °C +3g S/Kg Araine -78 ' 39- -40 32 -20 • 30 0 26 23 27 40 26 Table g« fjgooiiiy of Methylamine • ••'• 1 •' '- ' • Temperature Tisooaity •0 •.. ..•:'.-••••• ,.-.-.r•=••.. 1\ (Oentipbise) ' 0*205 : 0.212 - • 20.0 •-"';"•;•"'•>*•. ': 15.0 0.219 10.0 0.228 5.0 0,237 0.0 0*250 -5.0 0.264 -10.0 r 0.280 -15.0 0.298 -20.0 0.318 . -25.0 0.340 -30.0 0.366 -55.0 0.393 -40.0 0.425 -45.0 0*460 -50.0 0.500 -55.0 0.545 table 3. • JfatuarAMcntal Dissociation. Constant of ffffiOfy* in OHjWHw aa a function of 3?o«nperature io7 x •0 «ol I. moi IT 25.0 0.63 -20.0 3.9 20.0 0.77 -25.0 4.7 15.0 0.94 -30.0 5.8 10.0 1«2 -35.0 6.8 5.0 1.4 -4"0.0 7.8 0.0 1.8 -45.0 8.7 -5.0 2.2 -50.0 9.2 r10.0 2.6 -55.0 9.4 -15.0 3.3 - - table 4« Estimated Percentage of Monomer present in KHKOHI/GH^NHQ Solutions. Stoichiometric IMA Monomer cuim OHM ru id vu of BIA 0-0 30°C - 40°0 50»0 0.002 M 100 98 97 95 0.020 M 99 87 77 60 0.200 It 93 43 29 24 #040 500 -30-0 0.& 3030--0 Fig.2. Equivalent conductance of PM A solution as a function of temperature.