Ind. Eng. Chem. Res. 1988,27, 555-559
during the tenure of the National Science Foundation Grant CBT-8616201 (to J.D.G.) and of a US. Navy Office of Naval Technology Fellowship (to J.B.M.). Nomenclature A = potassium acetate surface area Ci = bulk concentration of species i C*KOAc = solubility of potassium acetate CQ = CQCl + CQOAC C * Q O A=~ K ~ ‘ C * K O A ~ Di = diffusivity of species i Kif = K I C Q C ~ / C K C ~ K 1 = equdibrium constant for the bulk reaction KOAc + QCl KCl + QOAc K2‘ = K2cRCIjcQCl K 2 = equilibrium constant for the reaction RC1+ QOAc ROAC + QCl K, = surface-reaction equilibrium constant k = (A/V)(DK/~) k2 = second-order rate constant for reaction RCl + QOAc ROAc + QCl V = liquid volume
-
-
Greek Symbols
6 = mass-transfer film thickness u = kinematic viscosity of solvent w = disk rotation speed, rad/s Registry No. CsH5CH2C1, 100-44-7; H3CC02K, 127-08-2.
Literature Cited Covington, A. K.; Dickinson, T. Physical Chemistry of Organic Solvent Systems; Plenum: London and New York, 1973. Dehmlow, E. V.; Dehmlow, S. S.Phase Transfer Catalysis, 2nd ed.; Verlag Chemie: Weinheim, 1983.
555
Evans, K. J. Ph.D. Dissertation, University of Rochester, Rochester, NY, 1983. Ford, W. T. In Crown Ethers and Phase Transfer Catalysis i n Polymer Science; Mathias, L. J., Carraher, C. E., Eds.; Plenum: New York, 1984; p 201. Freedman, H. H. Pure Appl. Chem. 1986,58, 857. Fritz, J. S. Anal. Chem. 1953, 25, 407. Goddard, J. D.; Melville, J. B.; Zhang, K. J. Fluid Mech. 1987,182, 427. Kortum, G.; Bockris, J. OM. Textbook of Electrochemistry; Elsevier: Amsterdam, 1951; Vol. 11. Levich, V. Physicochemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962. Melville, J. B. Ph.D. Dissertation, University of Southern California, Los Angeles, 1986. Melville, J. B.; Goddard, J. D. Chem. Eng. Sci. 1985, 40, 2207. Montanari, F.; Landini, D.; Rolla, F. Top. Curr. Chem. 1982, 101, 147. Padova, J. I. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum: New York, 1972; No. 7. Patai, S., Ed. The Chemistry of Carboxylic Acids and Esters; Interscience: New York, 1969. Reuben, B.; Sjoberg, K. CHEMTECH 1981, May, 315. Siggia, S.; Hanna, J. G. Quantitative Organic Analysis Via Functional Groups, 4th ed.; Wiley: New York, 1979. Stanley, T. J.; Quinn, J. A. ‘Phase Transfer Catalysis in Membrane Reactors”. Paper 98f, Annual AIChE Meeting, Chicago, IL, Nov 10-15, 1985 (submitted for publication in Chem. Eng. Sci.). Starks, C. M. J. Am. Chem. SOC.1971, 93, 195. Starks, C. M.; Liotta, C. Phase Transfer Catalysis Principles and Techniques; Academic: New York, 1978. Tomoi, M.; Ford, W. T. J. Am. Chem. SOC.1981, 103, 3821. Vander Zwan, M. C.; Hartner, F. W. J. Org. Chem. 1978, 43, 13. Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic Synthesis; Springer Verlag: Berlin, 1977. Yee, H. A,; Palmer, H. J.; Chen, S. H. Chem. Eng. Prog. 1987,83, 33.
Received for review June 5, 1987 Accepted October 21, 1987
Kinetic Study of the Substitution Reaction of Benzyl Chloride with Triphenylphosphine To Synthesize Benzyltriphenylphosphonium Chloride. Solvent Effects Maw-Ling Wang,*fAn-Hong Liu,? and Jing-Jer Jwo* Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC, and Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC
Triphenylphosphine (TP) and benzyl chloride (BC) undergo SN2substitution reaction to produce benzyltriphenylphosphonium chloride (BTPPC). The effects of solvent, reactant concentration, agitation rate, and temperature on the conversion rate are investigated in order to find the optimum operating conditions for this reaction. I t is found that no agitation effect is observed when the agitation rate exceeds 700 rpm. The order of relative activities of solvents is methanol > acetic acid > dichloromethane > acetone > ether > benzene > toluene. In methanol, the conversion can be as high as 100% with respect to TP when BC is in great excess. The second-order rate constant at 30 “C for the BC-TP reaction in methanol is 0.135 M-l h-l. The thermodynamic parameters of activation, AH* and A S , for this reaction in methanol are 15.0 kcal/mol and -26.5 cal/mol/K, respectively. The present study has valuable implications in the synthesis of stilbene via the two-phase Wittig reaction. Phase-transfer catalysis (PTC) is one of the most attractive techniques in recent organic syntheses (Starks and Liotta, 1978; Weber and Gokel, 1977; Dehmlow and Dehmlow, 1980). More than 65 different types of organic National Tsing Hua University.
* National Cheng Kung University. 0888-5885/88/2627-0555$01.50/0
compounds have been synthesized by PTC techniques. Polymer chemists have also utilized PTC for various applications in monomer synthesis, polymer modification, and free-radical catalyst activation (Cook and Brooker, 1982; Sherrington, 1984; Mathias, 1981; Rasmussen and Howell, 1984). One advantage of using PTC in synthesis is that high selectivity and high conversion rate can be 0 1988 American Chemical Society
556 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
obtained under appropriate conditions. Thus, it provides a plausible process of simple operation, energy savings, and low cost of product. The Wittig reaction has been extensively applied in olefin synthesis (Wittig, 1974; Vollardt, 1975). In the Wittig reaction, an aldehyde or ketone is treated with a phosphorus ylide to give an olefin. Phosphorus ylides are generally prepared by treating a phosphonium salt with a base, and phosphonium salts are usually prepared by reacting the phosphine with an alkyl halide. The reaction can be illustrated by the following scheme: Ph3P
+X4HRR’
-
[Ph3P+--CRR’ ylide
Ph3P+-CHRR’Xphosphonium salt 2
Ph,P=CRR’]
base
R“CH0
R”CH=CRR’
+ Ph3PO
Mark1 and Merz (1975) showed that alkyltriphenylphosphonium salts react with aqueous NaOH to generate ylides which can then combine with organic-phase aldehydes to produce olefins. This two-phase Wittig reaction has been classified as one of the “Phase Transfer Catalysis of Ylide-Mediated Reactions”, even though the phosphonium salt is consumed in the reaction. Since the alkyltriphenylphosphonium salt plays a very important role in the Wittig reaction, it is worth studying its synthesis from the reaction of alkyl halide and triphenylphosphine. In the present work, we studied the substitution reaction of benzyl chloride with triphenylphosphine to synthesize benzyltriphenylphosphonium chloride. The effects of solvent, temperature, and agitation rate on the conversion rate are investigated. The results have valuable implications in the synthesis of stilbene via the two-phase Wittig reaction. Experimental Section Materials. Benzyl chloride (BC) and benzyltriphenylphosphonium chloride (BTPPC) were G.R. grade products of Merck, West Germany. Other reagents used were all reagent grade chemicals. Procedures. The reactor was a 25-mL four-neck Pyrex flask submerged in a constant-temperature water bath in which the temperature was controlled to within f O . l “C. To start a kinetic run, a known TP organic solution was prepared and introduced into the reactor which was thermostated at the desired temperature for at least 15 min and under constant agitation. A measured quantity of benzyl chloride (BC), also at the desired temperature, was then added to the reactor. An aliquot sample was withdrawn from the reaction solution at a chosen time. The sample was immediately added to a toluene solution (toluene/sample = 10-30 by volume) to retard the reaction and then analyzed quantitatively by the HPLC using the method of external standard. The BTPPC obtained from the reaction was purified and identified by comparing its HPLC and UV absorption data with those of the G.R. grade BTPPC from Merck, West Germany. An LC/ GPC-201 model with WISP 7103 autoinjector and data module from Waters was used for LC analysis. The column used was FBondapak C18 (32.5 mL, 3.9-mm i.d. X 30 cm). The eluent was CH30H/H20 = 5.1. Results and Discussion In organic reactions, two widely accepted mechanisms (SN1and S N 2 ) for nucleophilic substitution reactions were proposed by Hughes et al. (1933). In the s N 1 mechanism, the solvolysis of the substrate molecule by the solvent is
0.010,
0‘0041 8
o rpm A 200 rpm W 5 0 0 rpm
0.
bk” Time ( h r i
Figure 1. Agitation effect: 25 mL of MeOH, 20 mL of BC, 0.01 mol of TP, 30 “C.
an important step and the reaction rate is generally independent of the concentration of nucleophile. In the SN2 mechanism, the substrate and the nucleophile react directly via a transition state to produce products. The rate law of a SN2 reaction generally obeys second-order kinetics, and the reaction rate depends on the concentration of both substrate and nucleophile. Comprehensive reviews of s N 1 and SN2reactions can be found in the literature (Ingold, 1969; Streitwieser, 1962). Some preliminary studies show that the substitution reaction between benzyl chloride and triphenylphosphine follows the SN2mechanism. In order to search for the optimum reaction conditions, several kinds of solvent, the relative amounts of solvent and reactants, the reaction temperature, and the agitation rate are considered. The results are summarized below. (i) Optimum Agitation Rate. The effect of agitation was studied by using the following operating conditions: 25 mL of MeOH, 20 mL of BC, 0.010 mol of TP, 30 “C. The results are given in Figure 1. As shown in Figure 1, no improvement in the reaction rate is observed when the agitation rate exceeds 700 rpm. Therefore, the agitation rate was set at 700 rpm for studying the reaction phenomena from which the reaction is not controlled by mass transfer. (ii) Optimum Solvent. Eight kinds of solvents are classified in two categories depending on the solubility of BTPPC selected for study. Solvents that dissolve BTPPC are acetic acid, dichloromethane, methanol, and water. Solvents that do not dissolve BTPPC are acetone, benzene, ether, and toluene. In general, TP will not dissolve in methanol or water, but BC will dissolve in all the above solvents except water. In the study of solvent effects, the reaction was carried out under the same reacting conditions as in the following: 75 mL of solvent, 25 mL of BC (0.22 mol), 0.010 mol of TP, 700 rpm agitation rate, and 30 “C. Typical results for BTPPC yield vs time are shown in Figure 2. It is obvious that a higher conversion rate is obtained in methanol. The plots of In [TP]/[TP], vs time are shown in Figure 3. A first-order reaction fits the curve. The corresponding pseudo-first-order rate constant and activity free energy are given in a Table I. The order of relative activities of solvents is methanol > acetic acid > dichloromethane > acetone > ether > benzene > toluene. As expected, the BC-TP reaction shows better reactivity in protic or polar solvent since the activated complex is more polar than both reactant molecules. Blank tests show that the solvolysis of BC in these solvents is negligible.
Ind. Eng. Chem. Res., Vol. 27, No. 4,1988 557 .oo
0.010
.80
0.008
2 2
.60
0.006
U
c
2
.40
0.004
w
3
2"
'u
0
0.002
a r o .20
e 0.0
0 .oo
0
40
80
120
160
200
Volume of MeOH (ml)
T i m e (hr)
Figure 2. Effect of solvent on the yield of BTPC: 75 mL of solvent, 25 mL of BC, 0.01 mol of TP, 700 rpm, 30 OC; (D) methanol, (A) acetic acid, (0) water, (0) dichloromethane, (v)acetone, (A)diethyl ether, ( 0 )benzene, I)(' toluene.
Figure 4. Volume effect of methanol: 25 mL of BC, 0.01 mol of TP, 700 rpm, 30 OC.
0.0
-1.0
-0
-2.0
a
\ a
I_
-3.0
Volume of BC Time (hrl
Figure 3. In [TP]/[TP], vs time plot: 75 mL of solvent, 25 mL of BC, 0.01 mol of TP, 700 rpm, 30 OC; (D) methanol, (A)acetic acid, (0) water, (0) dichloromethane, (v)acetone, (A)diethyl ether, ( 0 ) benzene, (I)' toluene. Table I. Effect of Solvent on t h e Pseudo-First-Order Rate Constant and Activity Free Energy of BC-TP Reaction" k , h-' AG*,kcal/mol solvent acetic acid 0.176 23.7 acetone 0.0114 25.4 benzene 0.0014 26.6 dichloromethane 0.0468 24.5 diethyl ether 0.0043 26.0 methanol 0.334 23.3 toluene 0.0008 27.0 water 0.0659 24.3 75 mL of solvent, 25 mL (0.22 mol) of BC, 0.010 mol of TP, 30 "C, 700 rpm.
(iii) Optimum Volume of Solvent. The optimum volume of solvent was only studied for methanol. The following operating conditions were chosen for study: 25 mL of BC, 0.010 mol of TP, 700 rpm, 30 OC. The results are shown in Figure 4 and Table 11. A cume that fits the rate constant k vs the volume ( u ) of methanol has the equation k =
38.01~+ 13.61 u2 + 33.66~+ 417.86
(1)
(mil
Figure 5. Volume effect of BC: 25 mL of MeOH, 0.01 mol of TP, 700 rpm, 30 OC. Table 11. Effect of Methanol Volume on t h e Pseudo-First-Order Rate Constant of BC-TP Reaction" MeOH, mL [TP],, M [BC],, M k, h-' 8.7 0 0.40 0.0448 5 0.33 7.2 0.352 15 0.25 5.4 0.491 20 0.22 4.8 0.500 25 0.20 4.3 0.513 30 0.18 4.0 0.499 3.6 35 0.17 0.492 2.9 50 0.13 0.422 2.2 75 0.10 0.326 100 0.08 1.7 0.278 200 0.04 1.0 0.162 25 mL of BC, 30 OC, 700 rpm.
In general, the reaction rate is controlled by two factors. One is the effect of solvation, and the other is dilution of reactant concentration. For small volumes of solvent (0-15 mL), the solvation effect is dominant. However, the dilution effect becomes more important when a larger volume of solvent (>EmL) is used. At the optimum volume of solvent, both effects are very similar. Thus, from the results in Figure 4,the optimum volume of MeOH is 25 mL under the above-specified conditions. (iv) Optimum Volume of Benzyl Chloride. The following operating conditions are specified for studying the optimum volume of BC: 25 mL of MeOH, 0.010 mol
558 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 Table 111. Effect of BC Volume on the Pseudo-First-Order Rate Constant of BC-TP Reaction" BC, mL [TPIO, M [BC],, M k, h-l 5 0.33 1.45 0.192 0.29 2.48 10 0.260 0.25 3.26 15 0.340 0.22 3.86 20 0.468 0.20 4.35 25 0.513 0.17 5.07 35 0.555 0.13 5.79 50 0.562 0.10 6.52 75 0.512 125 0.067 7.24 0.433 0.044 7.72 0.341 200 25 mL of MeOH, 30 O C , 700 rpm.
TP C O n C e n t r a t l O n
0 0.005mol 0 0.QlOmol
0.324 I
3-
A 0.020mol 5.24
2
$
. 0
3.16
3
P i.38
0.33
of TP, 700 rpm, 30 OC. As shown in Figure 5, the reaction rate increases with the volume of BC (5-50 mL) and then decreases with the volume of BC (50-200 mL). The corresponding pseudo-first-order rate constants are given in Table 111. The rate of dissolution of TP increases with an increase in the volume of BC. In addition, an increase in BC concentration will also make the reaction faster. Therefore, the rate constant increases as the volume of BC increases when the change in TP concentration is not so significant. However, as the volume of BC increases beyond a certain value, the dilution and solvent effects become significant and a decrease in the rate of BTPPC production is observed. It is shown in Table I11 that the optimum volume of BC under these operating conditions is 50 mL. (v) Limitation of Initial TP Concentration. As shown in Figure 6 and Table IV, the rate of BTPPC production is generally proportional to the concentration of TP. However, there is an interference of this trend due to the solubility of TP in methanol. The rate constant is not a constant when [TP], is high. This is probably due to the interference of the reverse reaction. When [TP], is sufficiently low, a limiting value of the rate constant is obtained. (vi) Effect of Temperature. To study the effect of temperature on the rate constant, the following optimum operating variables are used: 250 mL of MeOH, 20 mL of BC, 0.0005-0.02 mL of TP, 700 rpm, 30-70 "C. Typical results are shown in Table IV. The activation energy (E,) can be obtained from the Arrhenius equation (eq 2) by plotting In k vs 1/ T. The results are also given in Table IV.
k = A exp(-EJRT)
-
(2)
A plot of k vs [TP], at different temperatures is given in Figure 7 . The rate constant (k,) at standard state (y = 1)can be obtained by extrapolating the straight line to [TP], = 0 M. By use of eq 3, the thermodynamic param(3)
eters AHt and AS* can be obtained by plotting In ( k o / T ) vs 1/T. The enthalpy of activation (AH') and the entropy
4
8
12
Time l m i n )
Figure 6. Effect of TP concentration on the rate of BTPC production: 25 mL of MeOH, 20 mL of BC, 700 rpm, 70 "C. 0.20,
1
Reaction Temperature Indicated
0.14
0.12
0.06
0'02
t
L
0.0 0.0
0.01
3
I n i t i a l Mole of TP
Figure 7. Rate constants at standard state, y = 1.
of activation ( A S )are 15.0 kcal/mol and -26.5 cal/mol/K, respectively. This result is in excellent agreement with that of the C6H5CH2C1N(CH3)3 reaction (AH*= 14.6 kcal/mol, AS' = -24.0 cal/mol/K) (Bunnett and Reinheimer, 1962). The large negative AS* value is due to the formation and
Table IV. Temperature Dependence of the Pseudo-First-Order Rate Constant of BC-TP Reaction" k, M-' h-l, at TP, mol 30 "C 40 O C 50 "C 60 O C 70 O C 0.0005 0.137 0.326 0.768 1.48 2.88 0.0010 0.135 0.326 0.751 1.47 2.87 0.0050 0.134 0.294 0.736 1.45 2.69 0.010 0.123 0.263 0.691 1.35 2.52 0.15 0.110 0.249 0.635 1.28 2.29 0.020 0.0917 0.233 0.604 1.19 2.12 25 mL of MeOH, 20 mL of BC, 700 rpm.
0.02
Ea/R(k)
7907 7920 7924 8000 8023 8230
I n d . E n g . Chem. Res. 1988,27, 559-565
559
Dehmlow, D. V.; Dehmlow, S. S. Phase Transfer Catalysis, Monographs i n Modern Chemistry; Verlag Chemie: Weiheim, 1980; VOl. 11. Hughes, E. D.; Ingold, C. K.; Patel, C. S. J. Chem. SOC.1933, 526. Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornel1 University Press: Ithaca, NY, 1969. Markl, G.; Merz, A. Synthesis 1975, 295. Mathias, L. J. J. Macromol. Sci.-Chem. 1981, A15, 853. Rasmussen, J. K.; Howell, H. K. Polym. Sci. Technol. 1984,24, 105. Sherrington, D. C. Macromol. Chem. 1984, 3, 303. Starks, C. M.; Liotta, C. Phase Transfer Catalysis, Principles and Techniques, Academic: New York, 1978. Streitwieser, A,, Jr. Solvolytic Displacement Reactions; McGraw Hill: New York, 1962. Vollardt, K. P. C. Synthesis 1975, 765. Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic synthesis; Springer-Verlag: Berlin, Heidelberg, New York, 1977. Wittig, G. Acc. Chem. Res. 1974, 7, 6.
the solvation of the activated complex.
Conclusions The synthesis of BTPPC is the only reaction product. Under appropriate conditions, the conversion can approach 100%. The reaction obeys second-order kinetics. The order of relative activities of solvents is CH30H > CH3CO2H > CHzClz > CH3COCH3 > CZH5OC2H5 > C & j > CH3C6H5. The present study has valuable implications in organic synthesis, especially in the synthesis of stilbene via the two-phase Wittig reaction. Registry NO.BC, 100-44-7; BTPPC, 1100-885; PPh,, 603-350; methanol, 67-56-1; acetic acid, 64-19-7; dichloromethane, 75-09-2; acetone, 67-64-1; diethyl ether, 60-29-7; benzene, 71-43-2; toluene, 108-88-3; water, 7732-18-5.
Literature Cited
Received for review March 20, 1986 Revised manuscript received August 6, 1987 Accepted September 4, 1987
Bunnett, J. F.; Reinheimer, J. D. J.Am. Chem. SOC.1962,85, 3284. Cook, F. L.; Brooker, R. W. Polym. Prepr. 1982,23, 149.
Reduction of Coke Formation during Naphtha Pyrolysis Using Triethyl Phosphite Kalyan K. Ghosh and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur 208016, India
The reduction in the rate of coke formation during naphtha pyrolysis due to the injection of triethyl phosphite has been investigated in a jet-stirred reactor at atmospheric pressure in the temperature range of 1088-1108 K. Coke formation was significantly reduced in the presence of the additive. The data are consistent with the formation of a metal-phosphorus complex which passivates the metal activity for coke formation. Used in the same concentration, phosphorus was more effective than sulfur in reducing the coking rate. The coke formation could be satisfactorily modeled by assuming that the fraction of the metal surface covered with the complex was in equilibrium with the phosphorus-containing free-radical species in the gas phase and that coke simultaneously deposited a t different rates on the bare metal surface and the metal-phosphorus complex. Pyrolysis of naphtha and other hydrocarbons, which is of primary importance in the manufacture of olefins, is always accompanied by the undesirable formation of coke. This coke deposits on the walls of the reactor, reducing the overall heat-transfer coefficient and increasing the pressure drop across the reactor. This results in a gradual increase with run time of both the reactor tube metal temperature and the pressure drop across the reactor, necessitating periodic shutdowns. Rates of coke formation during hydrocarbon pyrolysis have been investigated by several workers. Because it is well established that the reactor walls catalyze coke formation, most of the methods used t~ reduce coking employ some means to passivate the reactor walls. Presulfiding the reactor walls has been reported to reduce coke formation during the pyrolysis of ethane (Albright and McConnel, 1978), propane (Trimm and Turner, 1981), and n-octane (Shah et al., 1976). Bajus and co-workers (1981, 1983) studied the effect of thiophene, dibenzyl sulfide, and dibenzyl disulfide on reaction kinetics and coking of steam cracking of heptane. The metal sulfide, which passivated the reactor surface, inhibited the coke deposition considerably. Similarly, Sahu and Kunzru (1988) found that the rate of coke formation during naphtha pyrolysis was significantly reduced in the presence of thiophene. Brown et al. (1982) deposited a silica coating on preoxidized steel substrates by the decomposition of an alkoxysilane in a carrier gas. With such an inert coating, coke formation during pyrolysis was reduced by a factor of 10 0888-5885/88/ 2627-0559$01.50/0
in short-term tests and by a factor of 3-4 in longer term tests. Tomita (1977) has developed a sintered catalyst consisting of calcium oxide, aluminum oxide, and less than 0.2 wt 70silicon dioxide. Considerable reduction in coke formation during naphtha pyrolysis has been claimed with this catalyst. Attempts have also been made to reduce the rate of coking by catalytically gasifying the coke with steam during pyrolysis. Showa Denko (1983a,b) reported that incorporating alkali metals and their oxides to Ni-Cr alloys or HK-40 was effective in significantly reducing the rate of coke formation during the cracking of steam-hydrocarbon mixtures. Molten alloy steel was modified with lithium and cast, whereas the HK-40 tubes were either microalloyed with barium or flame-sprayed with a powder of HK-40 and lithium oxide. Mandal and Kunzru (1986) investigated the coking rates during n-hexane pyrolysis on potassium carbonate coated metal surfaces and found that the rate of coke formation was significantly reduced in the presence of K2C03due to the catalytic gasification of coke. Similar results were obtained by Bahadur et al. (1987) for the catalytic gasification of coke during naphtha pyrolysis. However, Bahadur (1986) found that the potassium carbonate was gradually removed from the surface during decoking and the rate of coke formation increased with successive decokings. Phosphorus-containing additives have been shown to be effective in inhibiting coking rates. Naberezhnova et al. (1983) found that the organophosphorus compounds 0 1988 American Chemical Society
