Effect of Pressure on the Reaction Rate Constant at the Decomposition of Aliphatic Nitrocompounds in Supercritical Water

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1 Effect of Pressure on the Reacton Rate Constant at the Decomposton of Alphatc Ntrocompounds n Supercrtcal Water V.I. Aneev, A. Yermaova Boresov Insttute of Catalyss, SB RAS, Novosbrs, Russa e-mal: an@catalyss.ns.su, Fax: It was shown expermentally that the rate of alphatc ntrocompounds (ntromethane, ntroethane and 1-ntropropane) decomposton n supercrtcal water (SCW) depends on pressure/densty at constant temperature. The rate constant of decomposton n SCW decreased wth ncreasng length of hydrocarbon radcal R. The SCW densty dependence of the apparent rate constant of decomposton of each of three compounds under consderaton was found to correlate wth the densty dependence of the H 3 concentraton n dssocated SCW. The transton state theory has been appled to smulate the effect of pressure on the rate constants. The calculated values of actvaton volumes n the decomposton reactons of ntromethane, ntroethane and 1-ntropropane n SCW were smlar that suggested the same nature of the actvaton complex n all three cases. INTRDUCTIN Alphatc ntrocompounds are among the so-called energetc materals and attract partcular attenton because of strct requrements for utlzaton of explosves, rocet fuel components and ther producton wastes, muncpal wastes and other substances contanng C-N, -- N, N-N, CH-N and some other groups. Decomposton and oxdaton n supercrtcal water (SCW) s the most preferable (promsng) method for the utlzaton these compounds [1,]. Specfc feature of chemcal reactons occurrng n supercrtcal fluds s a strong dependence of a converson rate on pressure/densty [3,4]. Ths effect may be of great mportance for developng the technology of energetc materals processng n supercrtcal solvents. 1. EXPERIMENTAL METHD AND DATA PRCESSING TECHNIQUE Experments on the converson of the chosen compounds n SCW were performed n a tubular flow reactor. n ths bass, the smplest reacton equaton sutable for expermental data processng can be wrtten as SCW, T, P Products (1) It was found that the overall rate of reacton (1) for all compounds under study obeys the frstorder equaton wth respect to : [ ] W, () where s the apparent rate constant, 1/s; [] s the concentraton of a ntrocompound n the soluton, mol/l. It was noted also that at constant temperature of 664 K, the values ncreased exponentally wth ncreasng pressure. Data processng n the framewor of thermodynamc model:

2 ln P lnφ P lnφ P V RT TS V allowed us to calculate the values of actvaton volumes. Integraton of equaton (3) reduces to: V P exp (4) RT Unnown parameters and V n eq. (4) were calculated by the least-squares method. Calculaton showed that the studed ntrocompounds had smlar values of actvaton volume V wthn the confdence nterval: 73±48 cm 3 /mol.. PRESSURE EFFECT IN THE FRAME F ACID-CATALYZED MECHANISM The present paragraph consders a model explanng the effect of pressure on the reacton rate constant on the assumpton that reacton (1) s catalyzed by H 3 ons formed n SCW due to dssocaton of water. It s nown that the onc product of water and hence the H 3 concentraton are hghly senstve to SCW densty alteraton. (3) -4. a) b) -4. с) ln ln -4.8 ln P/RT P/RT P/RT Fgure 1. Pressure dependence of apparent rate constants for the decomposton of alphatc ntrocompounds n SCW: a) NM, b) NE, c) NP Tang nto account strong dssocaton of water n the crtcal regon of ts parameters and the suggested mechansm of acd catalyss, the man reactons nvolved n the mechansm of alphatc ntrocompounds hydrolyss n SCW can be presented as follows: Dssocaton of water: H H 3 H - Isomerzaton of ntrocompound R CH N (where R H, CH 3 or C H 5 ) to ntronc acd: (4) R'CH N R'CH N (5) H

3 3 Protonaton of ntronc acd: R'CH N H H 3 H R'CH N H H (6) Assumng stage (5) to be the lmtng stage of the process and other stages beng n thermodynamc equlbrum, the rate equaton can be presented as [ ] [ H ] [ ] W K 1 3, (6) where K1 s the equlbrum constant of stage (4), and s the rate constant of stage (5). Therefore, accordng to ths mechansm, the apparent rate constant can be presented as a drect rato of the H3 on concentraton: [ H ] K ρ 3 w 1 w, g on/g The dependences of apparent rate constant on the concentraton of [H3 ] ons are presented n Fgs. for NM, NE and NP. The dependences prove the straght-lne relatonshp between and [H3 ]; however, the lnes do not pass through the orgn. Ths fact allows suggeston that together wth the reactons n supercrtcal solvent, the reactons of thermal pyrolyss of occur on metal walls of the reactor, ther rate beng ndependent on the [H3 ] concentraton , 1/sec, 1/sec H 1 7 gon / g. 1 Fgure. Rate constant of ntromethane, ntroethane and 1-ntropropane decomposton vs. the H 3. 1 concentraton n dssocated supercrtcal water c, 1/se.8.6 Expermental data were processed by the least-squares method as

4 4 [ H ] 1 3 (7) Results of statstcal processng are presented n Table 1. Table 1. Constants of equaton (7) and confdence ntervals Reagent ±, 1/s 1 ± 1, g/g on/s σ, % CH ± ± N 3 H5N 3H7N C ± ± C ± ± * σ s the root-mean square relatve devaton of expermental and calculated data ne may see from Table 1 that the model fts expermental data well and constants of equaton (7) were determned relably. Therefore, the hypothess of jont occurrence of thermal and acd catalytc decomposton of ntrocompounds n SCW does not contradct the expermental data. The frst tem n equaton (7) s the rate constant of the frst order reacton of thermal decomposton: [ ], gmol / l / s W (17) 1 The addend contanng the constant s responsble for contrbuton of the rate descrbed by the acd-catalyzed mechansm presented above: [ H ] [ ], gmol / l / s W1 1 3 (18) ne may see from Table 1 that the rate constant of thermal decomposton on the wall remans approxmately constant wth ncreasng length of hydrocarbon radcal n the molecule. At the same tme, the rate constant of decomposton n SCW decreases consderably wth ncreasng length of hydrocarbon radcal. Evdently, ths s caused by decreasng value of equlbrum constant n the R CH N seres (R H, CH 3, C H 5 ) due to strengthenng of the electron-donor propertes of R substtuents. 3. PRESSURE EFFECT IN THE FRAME F TRANSITIN STATE THERY It s mportant to understand the pressure effect on the reacton rate wth the use of thermodynamc models. In the framewor of transton state theory, the man stage of decomposton s the formaton of TS, whch exsts n equlbrum wth the ntal : σ σ [ R ] SCW,equlbrum R N N () Then TS rreversbly decomposes to the reacton products: σ σ Slowly [ R N ] Products The rate of decomposton by scheme ()-(3) n a plug-flow reactor s descrbed by equaton: dy Φ y y, K (4) dτ Φ TS (3)

5 5 where y s the reagent mole fracton; denotes the rate constant of lmtng stage (3), y s the mole fracton of TS; K s the equlbrum constant; Φ Φ ( y,t, P) s fugacty coeffcent. The partal dervatve of fugacty coeffcent wth respect to pressure at constant temperature and mxture composton s equal to partal molar volume ( V ) of a component: ln Φ P T V RT Fndng the logarthm of second equaton of (4) wth ts followng dfferentaton and nsertng eq. (1) gves (3). V VTS V,.e., the dfference between the partal molar volumes of TS and reagent s the actvaton volume of the reacton. In the crtcal regon the V value may be ether negatve or postve. As a rule, so extreme values of V mean that even nsgnfcant pressure varaton n the crtcal regon causes profound alteraton of the flud densty. Accordng to eq. (3), at pressure-ndependent V <, the apparent reacton rate constant ncreases exponentally wth ncreasng pressure. Exactly ths effect s erved at the decomposton of NM, NE and NP n SCW. Note that V NM, V NE, and V NP are negatve values and ndependent of pressure n the nterval of.7-3. MPa at a temperature of 664 K. The expermental data showed that the actvaton volumes n the studed reactons were ndependent of pressure. However, partal molar volumes of reagents and TS must be pressure-dependent. So, n order to verfy the valdty of usng the transton state theory for modelng of the studed reactons, t s necessary to demonstrate that, although partal molar volumes of reagents vary wth pressure, ther dfference remans constant. In order to calculate TS PMV, t s necessary to select the TS model and determne ts physcal parameters. It s reasonable to suggest to a frst approxmaton that crtcal parameters of TS consdered as the polarzed molecule wth loose bonds reman equal to crtcal parameters of the ntal molecule [5], whereas the coeffcents of bnary nteracton and change [6]. j c j (1) 1 Partal molar volume, cm3/mol P, atm

6 6 Fgure 3. The pressure dependence of PMV: curve 1 NM-SCW at j.3, c j.3; curve TS, experment ; curve 3 TS, calculaton at j.5346, c j.54; curve 4 TS, calculaton at varable j и c j. Т 665 K. Fgure 3 llustrates the results of the TS PMV calculatons. Curve 1 represents the pressure dependence of NM PMV n water ( V ) calculated at.3, c. 3. Curve 1 1 represents expermental pressure dependence of TS PMV calculated by equaton exp 3 V V V assumng V 7 cm / mol. Curve 3 s the calculated pressure TS dependence of TS PMV obtaned usng constant values j.53 and c j.5 resultng from the soluton of eq. (13). Curve 4 represents the pressure dependence of TS PMV calculated wth varable j and c j determned by eq. (14) wth coeffcents (15). Fgure 3 shows that the pressure dependence of TS PMV calculated usng varable j and c j agrees well wth the expermental dependence. CNCLUSIN It was shown expermentally that the rate of NM, NE and NP decomposton n SCW strongly depended on pressure at constant temperature. The effect of pressure on the rate constants at the decomposton of NM, NE and NP n SCW was explaned on the bass of two dfferent models. ne of the models based on transton state theory, another on acd-catalyzed mechansm nvolvng H 3 ons formed n SCW due to dssocaton of water. Applcaton of transton state theory allowed provng the expermental fact on the ndependence of the reacton actvaton volume of pressure. The natures of ntal ntrocomponds and TS dffer only n the character of solvent-reagent nteractons. The studed ntrocompounds showed smlar values of TS actvaton volumes that allow suggeston on the same TS nature n all three cases. Modelng of ths phenomenon on the bass on the acd-catalyzed mechansm demonstrated that the effect of SCW pressure on the apparent rate constant of decomposton of alphatc ntrocompounds under consderaton s adequately explaned by the dependence of the lmtng stage rate on the H 3 concentraton n dssocated SCW. Processng of expermental data showed that together wth the reactons n SCW, decomposton of these compounds proceeds on metal walls of the tubular flow reactor. REFERENCES 1. Savage P.E., Gopalan S., Mzan T.I. AIChE Journal, Vol. 41, 1995, p Cocero M.J., Alonso E., Toro R., Vallelado D., Fdz-Polanco F. Ind. Eng. Chem. Res., Vol. 39,, p Naud D.L., Brower K.R. J. rg. Chem., Vol. 57, 199, p Aneev V.I., Yermaova А. Russan J. Physcal Chemstry, Vol. 77, 3, p Glasstone S., Ladler K., Eyrng H., The theory of rate processes. Frc Chemcal Laboratory, Prnceton Unversty, New Yor and London, Aneev. V.I., Yermaova A. Theoretcal Fundamentals of Chemcal Engneerng (n Russan), Vol. 3, 1998, p