Ind. Eng. Chem. Res. 1988,27, 551-555
55 1
A Solid-Liquid Phase-Transfer Catalysis in Rotating-Disk Flow J. B. Melville+and J. D. Goddard* Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089-121 1
We present experimental results for the PTC reaction between benzyl chloride and solid potassium acetate (KOAc) in acetonitrile promoted by the quaternary ammonium salt Aliquat 336 to form benzyl acetate. A rotating solid disk of potassium acetate is contacted with a solution of benzyl chloride and catalyst in acetonitrile. Dissolution of KOAc is conducted with and without the addition of benzyl chloride, and the homogeneous reaction of acetylated catalyst with benzyl chloride is studied independently. The results are shown to be consistent with a mechanism consisting of the consecutive steps (i) dissolution of potassium acetate, (ii) rapid homogeneous reaction with Aliquat 336, and (iii) homogeneous reaction between acetylated catalyst and benzyl chloride, which might be generally anticipated for solid-liquid phase-transfer catalysis (SLPTC) in polar aprotic solvents. 1. Introduction
Phase-transfer catalysis (PTC) is a relatively new method for the promotion of two-phase reactions, wherein a so-called phase-transfer catalyst acts to render soluble, in reactive form, one of the reagents in the phase containing the other. PTC usually involves reaction between ionic salts and organic substrates promoted by substances such as onium salts or crown ethers. Table I summarizes the common varieties of PTC. In many cases, PTC provides a convenient and economical alternative to methods requiring the addition of a cosolvent or extreme conditions of heat and pressure in carrying out two-phase reactions between immiscible reagents. Vast literature (Weber and Gokel, 1977; Starks and Liotta, 1978; Dehmlow and Dehmlow, 1983) testifies to the utility of PTC in synthetic organic chemistry. Industrial applications are usually aimed at the synthesis of pharmaceuticals, agrochemical intermediates, and fine chemicals (Reuben and Sjoberg, 1981; Freedman, 1986). The rapid rise of polymer-supported and alumina- or silica-supported catalysts holds promises for increased industrial usage owing to their easy recoverability (Tomoi and Ford, 1981; Ford, 1984). The systematic study of PTC began with the work of Starks, Brandstrom, Makosza, and co-workers in the late 1960s and early 19709, and the following general mechanistic diagram was put forward by Starks (1971) and Starks and Liotta (1978) for liquid-liquid phase-transfer catalysis: Q'Nu-
+ RX
RNu
+ Q+X- (organic phase)
It
It Q+Nu- + M+X-
t M+Nu-
+ Q+X-
(1a)
(aqueous phase)
Much experimental work has been aimed toward the verification of this mechanism in specific chemical systems. It has been found whenever highly lipophilic phase-transfer catalysts are employed that the catalyst remains entirely in the organic phase, with anion exchange then occurring across the liquid-liquid interface as follows (Dehmlow and Dehmlow, 1983): Q+Nu- + R X
t M+NU-
-
RNu
+ Q+X- (organic phase)
1 M+X-
(1 b )
(aqueous phase)
Most early mechanistic studies were hampered by the unknown dependence of interfacial area and mass-transfer coefficient upon the rate of agitation. The study of Evans (1983) employed a diaphragm cell with a Teflon membrane +Presentaddress: Naval Ocean Systems Center, Code 634B, San Diego, CA 92152-5000. 0888-5885/88/2627-0551$01.50/0
Table I. Varieties of PTC organic ionic salt substrate catalyst aqueous liquid organic soluble solid liquid organic soluble aqueous liquid bound to solid polymer, silica, or alumina solid gaseous liquid LLPTC.
* SLPTC.
TPC.
designation liquid-liquid" solid-liquid* triphase catalysisc gas-liquid"
GLPTC.
at the interface to overcome these difficulties and seems
to show that mechanism l a proposed by Starks adequately accounts for a large part of the observed conversion versus time data for the reaction of n-octyl methanesulfonate with potassium iodide catalyzed by tetrabutylammonium iodide. Further mechanistic studies are certainly in order and should be conducted in a system where convective diffusion can be more rigorously accounted for, such as an impinging jet or other flow with uniformly accessible liquid-liquid interface (Goddard et al., 1987). It would be especially interesting to study chemical systems in which shifts from mechanism l a to l b occur as the catalyst lipophilicity is increased. The same state of affairs exists with solid-liquid phase-transfer catalysis (SLPTC), where a precise understanding of the physicochemical mechanism has been hampered by ill-defined hydrodynamics. This has led Melville and Goddard (1985) to propose the rotating disk as a tool for the investigation of SLPTC. The above device, popularized by Levich (1962) in the field of electrochemistry, provides a uniformly accessible surface with controllable mass-transfer coefficient and thereby permits a convenient and accurate accounting for the effects of convective diffusion in solid-fluid reactions. We have developed just such a device for SLPTC (Melville, 1986; Yee et al., 1987), and the present paper reports an experimental study of the reaction between benzyl chloride and potassium acetate promoted by Aliquat 366 (Henkel Corporation) in acetonitrile. By analogy with LLPTC, one can distinguish two mechanisms for SLPTC, illustrated in Figure 1 (Starks and Liotta, 1978; Melville, 1986). In the first of these, illustrated by Figure la, the catalyst Q' can react directly with the solid surface to render the anionic species soluble. In the second, illustrated by Figure lb, the anionic species first dissolves and then complexes with the catalyst. The first mechanism would be expected t o predominate in systems where the inorganic salt is very slightly soluble in the organic solvent and where the catalyst, such as a crown ether, is capable of close approach to the solid surface. The mechanism of Figure l b is to be expected 0 1988 American Chemical Society
552 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
transformer
motor support
motor sample
converter
4 RX
RY
(b) Figure 1. Two distinct mechanisms for SLPTC: (a) heterogeneous and (b) homogeneous solubilization. (0)
when the salt possesses substantial solubility in the solvent and when the catalyst, e.g., a large onium salt, is unable to approach the solid surface closely. In the present work, it was decided to study the reaction of benzyl chloride (RC1) with solid potassium acetate (KOAc) in acetonitrile promoted by Aliquat 336 (QCl) since this system was apparently well characterized and free from complications. Also, similar reactions are covered extensively in the PTC literature, for example, Vander Zwan and Hartner (1978). No previous works on SLPTC have investigated quantitatively the effect of the solubilization of the solid salt on the overall process, although this step is believed by some (Montanari et al., 1982) to be usually rate determining. The rotating disk appears ideally suited for investigating such issues since the hydrodynamics and mass-transfer coefficients are well-known and subject to relatively easy control. 2. Experiments Experimental Method. In order to investigate the overall mechanism of SLPTC in our system, three separate types of experiments were conducted. (1) Solubilization Experiments. Here, a rotating disk of KOAc is allowed to dissolve in a 0.1 M solution of Aliquat 336 (QCl). The KOAc is introduced in the form of a cylindrical pellet inserted in the center of the disk. This pellet is formed from reagent grade KOAc in a hand press. The rotating disk was machined from aluminum and was approximately 7.5 cm in diameter with a central recess in its face approximately 1cm wide by 0.6 cm deep. The disk, mounted on a shaft made from a drill rod, is driven by an electric motor (see Figure 2). The QCl solution is dried by allowing it to stand overnight over molecular sieves. (2) Homogeneous-ReactionExperiments. An acetonitrile solution of the acetate form of Aliquat 336, “aliquat acetate” (QOAc), is reacted in a stirred reactor with benzyl chloride (RCl). The aliquat acetate solution was prepared by slurrying potassium acetate in a 0.1 M acetonitrile solution of Aliquat 336 followed by drying overnight over molecular sieves. (3) Combined (SLPTC) Experiments. The combination of the previous experiments (i.e., SLPTC) is carried out with acetate ion presumably rendered soluble and subsequently reacting with RC1 to form ROAc. All chemicals were reagent grade or equivalent, except the Aliquat 336 which was used as received from the Henkel Corporation. In the KOAc solubilization experiments (l), replicate runs were carried out at three widely differing disk rotation speeds (100,500, and lo00 rpm) in a 500-mL sealed Pyrex reaction flask equipped with a water jacket at a temperature of 25 f 2 “C. Small aliquots of liquid were withdrawn from the reactor of timed intervals and analyzed for QOAc. As the method of analysis, we employed a nonaqueous titration with perchloric acid in dioxane (Fritz, 1953; Siggia and Hanna, 1979) and methyl orange as an
&port
4 ,1
I line power
aluminum disk
i
reoctor -.
disk
reoctor support
Figure 2. Schematic of rotating-disk apparatus.
indicator. The estimated error for this method is 5%. The homogeneous-reaction experiments (2) were carried out in the same 500-mL sealed Pyrex reaction flask agitated by means of a marine-type propeller at about 200 rpm and at a temperature of 25 f 5 “C. The method of analysis was the same nonaqueous titration for QOAc discussed above. The SLPTC experiments (3) were conducted in the same apparatus as experiments 1,but now 20 mL of RC1 was injected into the reaction vessel at the start of the experiment and a different method of analysis was employed. For the latter, the ROAc contained in a small aliquot of solution is converted to the corresponding hydroxamic acid with alkaline hydroxylamine and hence to the purple iron chelate complex (Patai, 1969). The absorbance of the resulting colored solution is measured at a wavelength of 555 nm in a (Bausch & Lomb Spectronic 20) spectrophotometer. The concentration of ROAc is determined by comparing its absorbance to that of known standards. The estimated error for this method is again 5 % . Experimental Results. Figure 3 shows plots of QOAc concentration versus time for the solubilization experiments (1)discussed above. These curves display a decrease in slope with increasing time associated with the decreasing levels of QCl in the system (catalyst saturation). These results will later be interpreted in terms of the solubilization mechanism l b mentioned above. Figure 4 shows a semilog plot of the dimensionless QOAc concentration versus a conveniently scaled time variable for the homogeneous reaction of RC1 with QOAc. For conversions up to about 90%, one observes a straight line characteristic of pseudo-first-order reaction. This is to be expected, since RC1 was present in these experiments in large excess relative to QOAc. The interpretation of these results will be discussed further below. The results for the reaction of RCl with KOAc catalyzed by QCl are displayed in Figure 5. These plots of the ROAc concentration versus time show the same general shape as those obtained for the solubilization experiments. Again, the decrease in slope with time is associated with the decrease in availability of the catalyst caused by increasing conversion to the acetate form. An interpretation in terms of a mathematical model is presented in the following section. 3. Theoretical Model and Conclusions
The Model. Since the molecule employed as catalyst is a relatively bulky quaternary ammonium salt, one sus-
Ind. Eng. Chem. Res., Vol. 27, No. 4,1988 553 -
0
I
I
:
o r u n 1 (V=500ml.) o r u n 2 (V=600ml.) theoretical profile (V=550ml.)
1
0
O
I
o r u n 1 (volume aliquot acetate solution = 500ml.) A r u n 2 (volume aliquot acetate solution 404ml.) 0 r u n 3 (volume aliquat acetate salution=250ml.) theoretical profile (second-order rate constant = averoge of runs 1, 2, 3 )
1
I
-
1
I
I
I
I
400 600 T I M E (min.)
200
0
I
800
"*A
fb
-LO
d0 40 tCRC,(min. Molal)
i0
60
710
Figure 4. Dimensionless aliquat acetate concentration vs scaled time variable during homogeneous reaction with benzyl chloride. Run, symbol, QOAc solution volume (mL): 1, 0,500;2,A, 404;3, 0,250. (-) Theoretical profile with second-order rate constant = arithmetic average of runs 1-3.
0
o r u n 2 (volume OCI A r u n 3 (volume OCI
solution 4 3 4 ml.) solution 430ml.1 theoretical profile (volume OCI solution = average of runs 1,2,3)
-= Q
o
i
0
o
run 1 (V=700ml.) o r u n 2 (V=700ml3 theoretical profile (V=700ml.)
I
I
I
I
1
t
i
B
-
5 -
I
i5
I
I
0
1
I
I
500
io00 T I M E (min.)
1500
Figure 5. Benzyl acetate concentration (M) vs time (min) for SLPTC experiments. Run, symbol, QCl solution volume (mL): 1, 0,462;2, 0 , 434;3,A, 430. (-) Theoretical profile with solution volume = arithmetic average of runs 1-3. Table 11. Intermetation of Solubilization Dataa rotation speed, rpm V, mL (Y P 500 0.00497 0.00374 100 600 0.00434 0.00339 100 600 0.00708 0.00320 500 700 0.001097 0.00309 1000 coeff of av value variation homogeneous solubilization K1' 6.795 8.77 A , cm2 18.68 12.42 heterogeneous solubilization K, 0.0307 9.01 A , cm2 755.82 12.41 cm'js, "Notes and parameters employed: D K O A c = 2.53 X = 9.77 X lo4 cm2/s, C*KOAe = 5 X lo4 M. The coefficient of variation equals standard deviations expressed as percentage of the mean. The geometric area of the potassium acetate insert was approximately 3.88 cm2. The diffusivities of potassium and aliquat acetate were calculated by using the Nernst-Haskell equation cited in Covington and Dickinson (1973). The limiting ionic conductance cited by Covington and Dickinson (1973)was employed. The limiting ionic conductance of the acetate ion was taken to be the average of the values obtained from Walden's rule for the solvents water and formamide, as in Padova (1972). The limiting conductance of the quaternary ammonium ion was obtained from data for other large quaternary ammonium ions in Padova (1972) with account taken of the effect of molecular weight, as in Kortum and Bockris (1951).
DBoAc
554 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
where
with
(This mechanism is to be contrasted to that assumed by Melville and Goddard (1985), which is perhaps appropriate to other solvents, vide infra and Yee et al. (1987).) C*KOA~ denotes the solubility of KOAc and K , the equilibrium constant for the reaction KOAc QCl KC1 + QOAc. If one assumes the reaction of KOAc with QCl to be heterogeneous, (Figure la) and also first order in QCl (Melville and Goddard, 1985), then one obtains an expression of the same form for the QOAc concentration as a function of time, with the parameters a and now given by
+
CY=
-
CQ 1+ 1 / K ,
-
p = -A
DQ V6(1 + l / K J
(4)
+
where K, is the equilibrium constant for QCl KOA&) QOAc + KCl(s). The experimental data for each of the three disk rotation speeds are thus fitted by least squares (Melville, 1986) to a curve of the form (2). The values of a and p thus determined are presented in Table 11. The parameters a and /3 are interpreted in terms of the parameters appearing in both the homogeneous and heterogeneous solubiliition models. The surface area A and reaction constants (Kl’ or K,) are treated as parameters to be determined experimentally. The values of these parameters, determined as averages over all experimental runs, are shown in Table 11. The very large surface area implied for the heterogeneous solubilization mechanism suggests that this mechanism contributes negligibly in the specific system at hand. By contrast, the disk surface area determined for the homogeneous reaction model is quite reasonable. The theoretical curves obtained from the parameters in Table I1 are shown as solid lines in Figure 3. The agreement between the theoretical and experimental concentrations is quite good, with the average relative percentage errors at the different disk speeds being 17.6% at 100 rpm, 5.8% at 500 rpm, and 1.5% at 1000 rpm. The other physical parameters employed in these calculations are listed in Table 11. The mass-transfer film thickness was calculated by using the classical Levich (1962) formula: 6 = ~ . ~ ~ ( D / v ) ’ / ~ ( v / w ) ~ / ~ . In the homogeneous reaction experiments, benzyl chloride is present in large excess relative to that of aliquat acetate. Thus, the observed pseudo-fist-order kinetics is as one expeets for a typical S N 2 substitution reaction such as this. The value for the second-order rate constant is calculated for each run by least squares in Melville (1986). The average of the values thus obtained is 0.142 m mi&, with a standard deviation between runs of less than 170. The theoretical profile obtained using this value of k 2 is shown as the solid line in Figure 4. One observes that the agreement between the experimental and theoretical points is reasonably good, the average deviation being 11.3%. From the results of the experiments discussed so far, we are led to conclude that solid-liquid phase-transfer catalysis occurs by means of a mechanism consisting of dissolution of KOAc and rapid reaction of KOAc with QCl to generate QOAc, with subsequent reaction between QOAc and RC1. With this assumption, one can derive the following expression for the ROAc concentration as a function of time:
b=
1 rl,2= --(-b zk (b2 - 4c)li2) 2 k2 Sk(1 + l/SK1’) k2
1 + l/K1’ C E
+ -K2’+
1 + l/Kl’
Sk(1 f l/SKi’)KZ Kz’(1 + l/K1’)
K2’ K Z C R C ~ C Q C ~ K2 equilibrium constant for the homogeneous reaction of RC1 with KOAc C*QOA~K~’C*KOA~ The constant K2‘ is treated as a parameter to be determined experimentally, and all the other relevant physical parameters (in particular the surface area) are taken to be those employed in the previously discussed solubilization experiments at a disk rotation speed of 500 rpm. The parameter K i is found by least squares in Melville (1986) to be equal to 4.52 which, in order of magnitude, is not inconsistent with the short-time (irreversible) kinetics of the above homogeneous reaction experiments. Unfortunately, the amount of data from these experiments at longer times is insufficient to give any more meaningful comparison. The agreement between the experimental and theoretical concentrations is quite good, the average difference being 5.5%. Conclusions. We have presented experimental results for the solid-liquid phase-transfer catalytic reaction of benzyl chloride with potassium acetate in acetonitrile catalyzed by Aliquat 336. A rotating-disk device was employed, with solid potassium acetate present as a flushmounted cylindrical insert in the face of the disk. SLPTC experiments were carried out, along with supporting experiments on the solubilization of potassium acetate and on the homogeneous reaction of benzyl chloride with aliquat acetate. The experimental data strongly support the conclusion that, in the polar solvent employed here, the process of SLPTC occurs by means of dissolution of potassium acetate followed by rapid reaction with Aliquat 336 to yield aliquat acetate and by subsequent reaction between aliquat acetate and benzyl chloride to yield benzyl acetate. A recent related study of SLPTC by Yee et al. (1987) in a nonpolar organic medium provides evidence for a different mechanism from that postulated above. In the future, it would certainly be of interest to conduct further experimental studies of the mechanism of solid-liquid as well as of liquid-liquid phase-transfer catalysis. As shown here and by Yee et al. (19871, the rotating disk is a convenient device for the study of solid-liquid systems. In future studies of LLPTC, it is hoped to exploit a scheme recently proposed by Goddard et al. (1987) for generating a uniformly accessible liquid-liquid interface. Futher experiment and theory could be of great interest to the chemical engineer, for purposes of scale-up and evaluation of reactor configurations, such as the hollow-fiber device discussed by Stanley and Quinn (1985). Acknowledgment The authors acknowledge support from the University of Southern California, in the form of ARCS and Chevron Graduate Fellowships (to J.B.M.). Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. Part of the work was completed
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
