N e w Synthesis of Isoprene Basedon Formaldehyde and Isobutylene David W. Hall’, Frank 1. Dormish, and Edward Hurley, Jr. Denver Research Center, Marathon Oil Co., 7400 South Broadway, Littleton, Cola. 80120
A new synthesis o f isoprene from formaldehyde a n d isobutylene has been developed. The yield of isoprene has been improved compared with other processes which utilize these r a w materials by first converting the formaldehyde t o chloromethyl methyl ether b y reaction with anhydrous hydrogen chloride and methanol. Chloromethyl methyl ether
is a d d e d t o isobutylene t o produce 3-chloro-3-methylbutyl methyl ether. This intermediate is decomposed by a highly efficient pyrolysis reaction t o isoprene, methanol, a n d hydrogen chloride. The latter t w o materials are recycled. Isoprene yield based on formaldehyde
is in excess of 80%
when pure isobutylene i s used. An ultimate yield over 90%
should be possible upon recycle of the various intermediates. When a refinery stream containing dilute isobutylene i s used as the isobutylene source, the yield is somewhat lower.
INCREASING
use of polyisoprene in the tire industry has stimulated research on less costly routes to isoprene monomer. Much of this recent research has been reviewed in the literature (Chemical and Engineering N e w s , 1965; DeMalde et al., 1964). Our interest was in improving the isoprene synthesis which utilizes isobutylene and formaldehyde as the raw materials (Hall and Hurley, 1966, 1967). Institut Franqais du Petrole (IFP) has pioneered development of a commercial isoprene process based on the Prins reaction of formaldehyde with isobutylene (Coussemant and Hellin, 1962a,b), in which formaldehyde is brought in contact with a refinery C4 stream in the presence of aqueous sulfuric acid to obtain the intermediate 4,4-dimethyl-1,3dioxane. 4,4-Dimethyl-1,3-dioxaneis then pyrolyzed over acidic catalysts such as boron phosphate to split our isoprene, water, and a mole of formaldehyde for recycle. A reported disadvantage of the I F P process is the formation of low-value by-products, called “residols,” both in the condensation step and during pyrolysis (Coussemant and Hellin, 1962b; Davidson et al., 1964a,b; Hellin et al., 1964; Parc et al., 1964). I n spite of this drawback, the I F P process produces high purity isoprene suitable for use in stereospecific polymerization reactions. This is probably the reason that additional developmental work has been done in several countries other than France (Arnold, 1966; Gershkovich et al., 1963; Kambara, 1964; Kronig and Swodenk, 1965; Mitsutani, 1966; Noda et al., 1963; Oil and Gas Journal, 1966). The formaldehyde-
’ Present address, Department of Chemistry, Colorado School of Mines, Golden, Cola. 80401 234
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
isobutylene route to isoprene is now being used on a commercial scale in the Soviet Union (Chemical and Engineering News, 1969a,b). Our approach in developing an improved process utilizing formaldehyde and isobutylene had as a first goal conversion of the formaldehyde to a somewhat less reactive and, hopefully, more selective intermediate than is likely to be formed in the Prins reaction. Secondly, we desired to obtain an isobutylene addition product which did not split out formaldehyde in the isoprene-forming step. These modifications would delete those side reactions which lead to residols in the I F P process. The synthesis we developed meets these requirements. I t involves three steps:
HC1+ CHIOH
+ CH2O 2 CHi-O-CH2-C1+ (CME)
CH ,-O-CH?Cl
+ CH,=C-CH
i
H2O
(1)
Metal halid; catalyst
CHi CH i CH ,-c-CH?-CH2-O--CH I
I
c1
(CMBME)
i
(2)
CH, CH:,-C-CH?-CH,-OCH,
+
I
c1 CHs CH,OH
+ HC1+ CHz=C-CH=CH,
(3)
(Isoprene)
Chloromethyl methyl ether (CME) is produced by Reaction 1 in high yields, using known methods. I n Reaction 2 CME is added to isobutylene in the presence of a metal halide catalyst to give 3-chloro-3-methylbutyl methyl ether (CMBME). CMBME is pyrolyzed in the final step to give isoprene, methanol, and hydrogen chloride. The latter two products are recycled to Reaction 1. The ultimate yield of isoprene (maximum possible yield obtainable upon recycle of intermediates) based on formaldehyde should be in excess of 90% when using pure isobutylene as feed and operating under preferred conditions. When operating with mixed butenes from refinery streams, ultimate yields would be in the 70 to 80% range. Results and Discussion
Chloromethyl Methyl Ether. The preparation of chloromethyl methyl ether (CME) from formaldehyde, anhydrous hydrogen chloride, and methanol has been reviewed (Summers, 1955) and is not discussed here. C M E is obtained in better than 95% purity after a simple distillation. In general, the only impurities found are about 1% anhydrous HC1 and 3 to 5% of methylal. For the studies reported C M E was purified to 99+% by a second distillation from titanium tetrachloride, which complexes methylal. Reaction with Pure isobutylene. Reactions of a-haloethers with olefins have been the subject of considerable research since about 1930. Strauss and Thiele in Germany (1936) and Scott in the United States (1935) reported on the reaction of CME with olefins in the presence of catalysts such as HgCL and BiCL. Strauss and Thiele reported yields of 3-chloro-3-methylbutyl methyl ether (CMBME) as high as 65 to 70%. Most of the research on a-haloethers reported during the past 25 years has been done by Russian
Run
(Lapkin et al., 1965; Vartanyan and Tosunyan, 1962) and German (Gross and Jiirgen, 1966) laboratories, but a Japanese group (Tokura et al., 1962) reported on the reaction of C M E with isobutylene in liquid sulfur dioxide. No added catalyst was required in that solvent; however, conversions were low. The literature on this addition reaction was recently reviewed (Gross and Hoft, 1967). We screened a wide variety of Lewis acid catalysts for this step of the isoprene synthesis. Although several catalysts were more effective on the basis of amount required, cost per unit weight, and resulting reaction rate, we are reporting results obtained using titanium tetrachloride. For other than large scale commercial use this catalyst is relatively inexpensive and convenient to use, and it gives the desired products in yields which compare very favorably with those obtained using some of the other catalysts we studied. Certainly, it gives better results in bench-scale reactions than most of the catalysts mentioned by earlier investigators. Run 1 in Table I shows results typical of the reaction of CME with pure isobutylene using T i c & as the catalyst. tert-Butyl chloride is formed to some extent in all these addition reactions. This may be due to the reaction of isobutylene with HC1 split from the product, CMBME. However, it is unclear a t just what stage of the addition reaction the HC1 is split out.
CH, CH3-C-CH?-CH2-O-CH3
+
c1 CMBME CH, HC1+ CH2=C-CH2-CH2-O-CH3 3-Methyl-3-butenyl methyl ether
+
CH, I I
CH,-C
=CH-CH~-O-CH~ 3-Methyl-2-butenyl methyl ether
(4)
Table 1. Reaction of Chloromethyl Methyl Ether (CME) with Isobutylene of Varying Concentration" Material Balance Isobutylene tert-Butyl Hydrocarbon Weight Molar ratio R of hydroReaction CME Selectivity Chloride source of %c , Ticlab to C M E carbon feed Time, Hr Conuersion, 7c to C M B M E Yield, 72 CME isobutylene'
1
0.99
5.10
2 3 4
1.00 6.30 0.96
1.50 1.50 5.10
100.0 13.98 13.96 19Ak
0.7' 1.9' 3.9' 4.0 4.0 0.21' 4.80' 28.05'
71.5 97.6 100.0 58.4 100.0 9.1 61.5 99.4
0.91 1.03 1.01
O.Mh 0.72' 0.37 0.82 0.85
5.6 5.8 5.8 7.6 8.8 0.0 9.5 7.9
93.8 102.6 101.0 92.8 79.4 94.2 89.2 84.6
93.0 91.6 94.3
...
101.4 93.1 96.4 94.2
Experiments conducted in 1-gallon Neucerite-lined Pfaudler autoclaw with s t i m r operating at 450 rpm. Reaction temperature varied between 23" and 29" C. ' Weight per cent of catalyst used based on C M E as limiting reactant. < Maximum relative error in selectivity data for 3-chloro-3-methylbutyl methyl ether ( C M B M E ) determined by standard error treatment to be about 3% . Weight per cent based on C M E converted. e Relative error about 4% . Includes 0.5 hr for addition of CME-catalyst solution to hydrocarbon charge in autoclave. Hydrocarbon feed was a refinery mixed G stream; see experimental section for the analysis. hSelectivities to C M E adducts with 2-butene and I-butene, 0.025 and 0.002 respectidy. ' N o t determined. 'Selectivity to C M E adducts with 2-butene and 1-butene, 0.044 and 0.007, respectidy. 'Diluent was n-butane. Includes 0.133 hr for addition of CME-catalyst solution to hydrocarbon feed in the autoclaue.
'
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 2, 1970 235
The unsaturated ether intermediates shown in Equation 4 are converted to isoprene in Step 3 of the process. By-product tert-butyl chloride may be converted to isobutylene and HCl for recycle, if desired, in that step. Run 1 was permitted to continue well past the time required for 100% conversion of the CME. There is no decrease in selectivity to CMBME or increase in yield of by-product tert-butyl chloride with time. The product is stable in the presence of catalyst and isobutylene, a t least at room temperature. The material balances are considered satisfactory in view of the complexity of the gas-liquid chromatographic (GLC) analyses employed and the problems associated with handling the large excess of isobutylene. Selectivities slightly in excess of 1.0 and CME balances in excess of 100% are due to the error range in the analyses, which was estimated by an error treatment to be 2 to 3% (relative). The large excess of isobutylene (about 5 to 1 on a molar basis) employed in run 1 is not a prerequisite for high yields. We have obtained comparable results using much lower proportions of pure isobutylene. Reactions Using Dilute Sources of Isobutylene. Studies were conducted to determine the effect of replacing pure isobutylene feed with a mixed C4 stream from a refinery catalytic cracking unit. These runs were made using essentially the same conditions as those for run 1, except that the molar ratio of isobutylene to CME was decreased to 1.5 and the CME-catalyst solution was added in 2 rather than 30 minutes. Typical results are listed for runs 2 and 3 in Table I . Reaction rates at room temperatures for comparable catalyst levels were greatly ,decreased in runs employing dilute isobutylene, compared with the rates found in runs where pure isobutylene was used. This is, of course, to be expected for a second-order reaction, which we presume this reaction is. Although second-order kinetics have not been proved for the reaction, they are inferred from the nature of the reaction and from results of pseudo-firstorder treatments not presented here. The following reactions may occur in addition to the reaction of CME with isobutylene when a mixed C4 feed is employed.
+ C1-CHJ-0-CH,
CH,-CH=CH--CH?
+
%Butene
CH?
I
CH3CHCHCHzOCH3
I
c1 3-Chloro-2-methylbutyl methyl ether
CH,-CH,-CH
=CH,
+ Cl-CH2-O-CHT+ 1-Butene
CH~CHZCHCH~CHZOCH~ (6)
c1 3-Chloropentyl m e t h y l ether 236 Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
Pyrolysis of the product from the reaction of CME with 2-butene (Reaction 5) also gives isoprene. More vigorous conditions are required to pyrolyze this compound than to pyrolyze CMBME. The product arising from 1-butene could lead, on pyrolysis, to piperylene, an undesirable contaminant in polymerization grade isoprene. Thus, low selectivity to the product from 1-butene is very important to isoprene purity. This adduct also is more difficult to pyrolyze than CMBME. I n runs 2 and 3 in Table I the selectivity to CMBME is significantly lower than for run 1 using pure isobutylene. The decrease in selectivity to CMBME cannot be accounted for by a simple diversion of the CME to reaction with 1- and 2-butenes. Combined selectivities to all three possible adducts are lower than the selectivity to CMBME alone when pure isobutylene was used. Since it appeared that lower yields of CMBME obtained when a refinery C 4 stream was used as the isobutylene source were due to a dilution phenomenon rather than the reaction of CME with 1- and 2-butenes present, we determined the effect of using isobutylene diluted with an inert diluent. The molar ratio of isobutylene to CME was maintained at a high level approximating that used in run 1 on pure isobutylene. But the amount of isobutylene in the hydrocarbon feed was reduced to about 20% by the addition of n-butane to parallel the conditions for runs 2 and 3 where the refinery C4 stream was used. Results for a typical experiment are listed as run 4 in Table I. These results are comparable to those obtained in runs 2 and 3, and demonstrate that the decrease in yield of CMBME is not due to significant reaction of CME with other olefins in a mixed hydrocarbon feed. Thus, the use of isobutylene diluted to about 20% with n-butane leads to the expected decrease in reaction rate, but also results in lower selectivity to CMBME and a 30% increase in the yield of by-product tert-butyl chloride. The yield of by-product tert-butyl chloride increases to about the same extent, regardless of which source of dilute isobutylene is employed. Further, it is well above the amount that would be predicted to arise by the reaction of isobutylene with HC1 split from CMBME, as estimated from the amounts of the intermediate unsaturated ethers present in the crude reaction mixture. We believe the increase in yield of tert-butyl chloride is associated with an increasing formation of a heavy by-product which is also a result of the dilution effect. The diversion of CME to heavy by-product is shown more clearly by the results for run 5 listed in Table 11. Compared with run 1, where pure isobutylene was employed, we observe in run 5 lower combined selectivities to the CME adducts with all three olefins present at all the conversion levels studied. This suggests that the heavy by-product and the tert-butyl chloride are being formed throughout the course of the reaction. There is, again, no decrease in selectivity or increase in the yield of ted-butyl chloride as the reaction time is extended beyond that required for 100% conversion of CME. So, a transient reaction intermediate, possibly still complexed with the catalyst, would seem a more likely source of the by-products than a subsequent reaction of CMBME itself. I n summary, from the standpoint of reaction rate, selectivity to CMBME, and yield of ted-butyl chloride it makes little difference whether one uses a refinery C, stream containing 1- and 2-butenes as well as inert diluents or
Table II. Effect of Reaction Time on Selectivity Using a Refinery Buty1enes"Feed
Run 5 Temperature. 23-29" C Molar ratio of isobutylene/CME. 1.4211.0 Catalyst. 2.9% TiC1, Reaction
Time, Hr' 0.58 1.83 3.08 4.25 5.67 7.17 22.75
CME Conwrsion, 9;
Isobutylene
47.0 74.8 84.7 90.0 94.3 97.3 100.0
Selectivity to Adduct with 2-Butene I-Butene
0.75 0.80 0.81 0.80 0.82 0.78 0.82
0.046 0.050 0.049 0.044 0.046 0.044 0.046
0.005 0.000 0.004 0.004 0.004 0.003 0.004
Yield tert-Butyl Chloride, R d 12.1 10.7 10.6 10.2 10.7 10.1 10.6
Material Balance, 7; CME Hydrocarbon 90.6 88.7 88.3 86.2 88.0 83.5 86.7
97.8 98.3 98.6 102.1 98.8 97.6 98.4
"Contained 13.9 mole R isobutylene; see experimental section for complete analysis. 'Includes M-hr interval required to add CMEcatalyst solution to hydrocarbon charge in autoclaw. 'Relatiw error in analysis for 2-butene adduct is about 10% and for 1-butene adduct about 20:';. Wt. 7; of CME conwrted.
whether one uses isobutylene with inert diluents. The data in Tables I and I1 confirm that dilution of the isobutylene reactant has a deleterious effect on selectivity and that the loss in selectivity is due to formation of a heavy by-product. Analysis of the heavy by-product recovered from an experiment similar to run 4 listed in Table I indicated that the material was similar in structure to CMBME, but contained a higher ratio of methoxyl methyl groups to side chain methyl groups. This suggested that the heavy by-product is derived from the reaction of CME with one, or possibly both, of the unsaturated ether intermediates, 3-methyl-3-butenyl methyl ether or 3-methyl-2-butenyl methyl ether.
Cl-CH,-O-CH,
i
CH,
I
C H,OC H,C HZ-C-C
HZC H2OC HB
I
(7)
Cl
+
CHa-C =CH-CH,-O-CH,
YH:/L-HI-O-CH, CH
'-1
CC1
H
(8)
CH?-O-CH,
Thus, reaction of CME with 3-methyl-3-butenyl methyl ether (Equation 7 ) would give 1,5-dimethoxy-3-methyl3-chloropentane and the reaction with 3-methyl-2-butenyl methyl ether (Equation 8) would give l-methoxy-2methoxymethyl-3-methyl-3-chlorobutane. A mixture of the two compounds shown above was
prepared by the direct reaction of C M E with a mixture of the two unsaturated ether intermediates. Spectroscopic examination of the crude product mixture showed the essential features found in the infrared and NMK spectra of the authentic sample of the by-product mentioned above. Limited attempts to resolve the mixture into the pure components were not successful because of the thermal instability of these compounds. GLC analysis was also unsuccessful, for the same reason. The spectroscopic comparison indicates that only the product from Equation 7 is being formed in the reaction of CME with the dilute source of isobutylene. A puzzling feature of the side reaction described above is that CME reacts much more slowly with the unsaturated ether intermediates in the presence of TiC1, than with isobutylene under comparable conditions. I t is difficult to see why, then, that any significant reaction of CME with the unsaturated ether intermediates occurs under typical conditions for CMBME synthesis, since isobutylene is present in great excess over the unsaturated ether intermediates. The by-product must arise by some unusual pathway and not simply by the reaction of CME with the unsaturated ether intermediates formed by spontaneous decomposition of CMBME. We found in a separate study, to be reported later, that the product in Equation 7 , 1,5-diniethoxy-3-methyl3-chloropentane, may be converted by the procedures described below to 2-vinyl-1,3-butadiene. Methanol and HCl values are thereby recovered for recycle to the isoprene process. Pyrolysis of CMBME to Isoprene. Two methods were studied for the conversion of CMBME to isoprene. The method studied more thoroughly involved decomposition of the material in selected organic solvents. Pyrolysis over solid catalysts of the crystalline aluminosilicate type was also investigated. The most effective solvents for the pyrolysis reaction were dimethylformamide (DMF) and N-methyl-2pyrrolidone (NMP). N M P was selected for the experiments reported because of its greater resistance to acidcatalyzed hydrolysis compared with open-chain amides. I n addition, N M P can easily be recovered from its hydrolysis product, ymethylaminobutyric acid (McElvain and Vozza, 1949), since the latter splits out water when heated and regenerates N M P . We were unable to detect any Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
237
products from the hydrolysis of N M P used in a typical pyrolysis run which could not be recycled. Results of experiments using N M P as the solvent for converting CMBME to isoprene are listed in Table 111. CMBME used in the decomposition runs listed was of a t least 96% purity. The only impurities were small amounts of the unsaturated ether intermediates, so yield calculations were made presuming the feed to be 100% pure. The combined yield of crude isoprene and recycleable intermediates was 90% or above in all three runs. When the overhead temperature control was set a t 60° C instead of near the boiling point of isoprene a t the prevailing pressure (run 6), the reaction time was greatly diminished, but the yield of isoprene was decreased and the yield of recycleable intermediates was increased. Isoprene yields reported are for isoprene as a mixture with the other low boiling components. The boiling points of the various components are such that after a water wash to remove methanol (methanol must be removed prior to distillation, since it forms an azeotrope with isoprene), only a relatively simple distillation is required to obtain polymerization grade isoprene. Included in the pyrolysis studies listed in Table I11 is the effect of added metal halide catalysts. These metal halides were added with the intention of increasing the reaction rate, since they are known to catalyze the elimination of HC1 from chloroalkanes in dipolar aprotic solvents (Banthorpe, 1963). Thus, the lower reaction time required in run 6 may be due to the nature and amount of the added catalyst as well as the use of a higher overhead temperature control. Preliminary experiments in which no metal halide catalyst was added to the N M P generally required about 3 hours for 100% conversion of CMBME when the overhead temperature was controlled a t about 60°C. The results of these experiments are not reported
here, since the products were analyzed by an uncalibrated GLC analysis. The very high efficiency with which CMBME may be converted to isoprene by decomposition in N M P is perhaps better demonstrated by the experiments discussed below. I n a commercial operation it is likely that the crude CMBME, together with by-product tert-butyl chloride, would be taken from Step 2 and pyrolyzed directly, without prior separation and purification. Experiments carried out in this manner are listed in Table IV. CME was allowed to react with pure isobutylene under the conditions employed for run 1 listed in Table I. The crude reaction mixture in its entirety was mixed with N M P in the distillation apparatus employed for the runs listed in Table 111. Volatiles such as excess isobutylene and a portion of the by-product tert-butyl chloride were removed by partial fractionation a t room temperature and reduced pressure. The temperature was then increased and CMBME was pyrolyzed as described above. The CME balances for the runs listed in Table IV exceed by several per cent the sums of the yields of crude isoprene and the unsaturated ether intermediates. This difference is due to credit taken for unchanged CMBME found a t the end of the runs. The CME balances indicate that an ultimate yield of isoprene of over 90% should be obtainable. This compares very favorably with the results of the pyrolysis experiments listed in Table 111, and further confirms the high efficiency of both Steps 2 and 3 of the process. The data in Tables 111 and IV further demonstrate that at least 90% of the methanol value in the CME charged a t the beginning of Step 2 may be recovered as methanol and methyl chloride after Step 3. Under pyrolysis conditions where a relatively low overhead tem-
Table 111. Decomposition of 3-Chloro-3-methylbutyl Methyl Ether to Isoprene in N-Methyl-2-pyrrolidone Solvent Yield, % d Run
N M p : CMBME Ratio (Wt)"
Temperature, ' c" Reboiler Owrhead
Reaction Time, H f
Crude isoprene
Intermediate ethers
Methanol
Methyl chloride
HCl Balance, %
6 2.3 110-154 60 0.7 80.2 10.8 86.5 6.4 ... 7 1.o 105-165 31 1.5 86.0 4.3 41.9 60.1 97.1 8 1.o 120-165 31 5.5 86.2 6.3 43.9 54.4 96.7 "Following catalysts were added to N M P to promote elimination of HCl: run 6 , ZnCL (15.2 mole 55); run 7 LiCl (10.0 mole 96); run 8, LiCl (10.9 mole %). 'Overhead temperature refers to control temperature maintained until near end of run, when control uas discontinued to permit distillation of most of methanol. Minimum temperature shown for reboiler is lowest temperature the mixture attained as low boilers built up. Maximum temperature shown is that required to distill most of methanol at e& of a run. e Includes 20-minute heat up time. Mole % based onpresumed 100% pure C M B M E charged. C M B M E conversion was essentially 100% in all runs. Material balances were: run 6, 99-% ; run 7, 98.7% ;run 8,9925; , ' N o t determined.
Table IV. Isoprene by Decomposition of Crude Addition Products in N-Methyl-2-pyrrolidone Addition Reaction Conditions"
~
Decompos it ion React ion Conditionsb
Reaction NMPICMBME' Run
Tic/:
time, hr'
wt. ratio
Reboiler
Owrhead
Reaction time, hr
Crude isoprene
Yields, 96' Intermediate ethers Methanol
Methyl chloride
5.1 31.1 60.0 81.5 31 8.5 9 1.03 4.9 1.05 105-165 4.6 31.6 60.0 81.6 31 9.0 103-165 10 0.95 4.9 0.96 Runs conducted according to procedure used for runs listed in Table I. Molar ratios of isobutylene (not diluted) to CME were: run 9, 5.91; run 10, 5.37. 'Decomposition reactions conducted in same manner as reactions listed in Table I I I . Mole Li; based on CME charged to addition step. C M E balances before and after pyrolysis, respectiuely, were: run 9, 82.8 (analysis problems) and 91.1%; run 10, 92.4 and 91.1%. HCl balances were 97.55; and 97.4% for runs 9 and 10, respectiuely. Wt. CZ of C M E charged. 'Includes about 0.5 hr required for adding CME-catalyst solution to isobutylene in autoclaue. ' N M P solution contained 5 mole % LiCl catalyst to promote elimination of HCl. Minimum reboiler temperature occurred as lou boilers were formed. Maximum reboiler temperature occurred at end o f a run when methanol was distilled. Overhead was controlled at 3 1 O C until methanol was to be distilled.
238 Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 2, 1970
perature was maintained, about two thirds of the methanol was converted to methyl chloride. This is due to longer residence time of methanol in the reaction mixture. The amount of methanol converted to methyl chloride may be reduced to 20% or less when a higher overhead temperature is employed in the pyrolysis step (run 6). T o determine better the effect of using a refinery C4 sample as the hydrocarbon feed, the crude CMBME from run 3 in Table I was pyrolyzed in N M P under conditions typical for the reactions described above. The yield of isoprene was 64.7% and the yield of intermediate unsaturated ethers was 3.2%. Conversion of CMBME was > 9 0 5 and conversion of the 2-butene adduct with C M E was about 80%. Within the limits of experimental accuracy, all the 1-butene adduct was recovered unchanged in the N M P residue. Distillation of the crude isoprene gave a small heart cut of over 99.5% purity. Piperylene could not be detected in this very pure sample of isoprene; the limit of detection was estimated to be 50 ppm in this particular sample. The C M E balance after the addition step for this run was 79.4%. The C M E balance after pyrolysis to crude isoprene was 79.0%. This indicates an ultimate yield of isoprene of nearly 80% should be obtainable. The pathway for decomposition of CMBME in N M P to give isoprene is believed to entail loss of HC1, followed by cleavage of the allylic unsaturated ether intermediate, 3-methyl-2-butenyl methyl ether, by the hydrochloride of N M P . This requires that a mobile equilibrium exist between the allylic intermediate and 3-methyl-3-butenyl methyl ether. We have found that ethers not activated by an allylic bond are cleaved only very slowly by the hydrochloride of N M P . The proposed cleavage pathway is in accord with mechanisms for ether cleavage discussed by Burwell (1954). Cleavage of the allylic ether intermediate gives, in addition to methanol, 1-chloro-3-methyl2-butene and, possibly, allylic isomers thereof. Small amounts of l-chloro-3-methyl-2-butene are detected in all the pyrolysis experiments. HCl is then rapidly split from this compound to regenerate the hydrochloride of N M P and give isoprene. Methanol reacts with the hydrochloride of N M P to give methyl chloride and water a t a rate compatible with the proportions of methyl chloride and methanol observed in the present experiments (Hall and Hurley, 1968).
Although CMBME can be converted to isoprene with very high efficiency by heating it in N M P , with little or no loss of NMP, the potential of a few solid catalysts for use in the decomposition step was also briefly explored. The results of studies employing two molecular sieve catalysts are presented in Table V. These experiments were conducted in the vapor phase using a simple borosilicate glass tubular reactor. Runs 11 to 16 were conducted using Linde's SK-400 molecular sieve catalyst. Conversion of CMBME was nearly 100% in all the above runs. The maximum yield of crude isoprene was about 85% a t a temperature of 250°C (run 14). The same sample of SK-400 catalyst was used for a total of about 48 hours with no visible indication of destruction or loss of activity. Runs 17 to 19 were conducted using Linde's AW-300 catalyst. These limited studies indicate that SK-400 is a better catalyst for this pyrolysis reaction than is AW-300. I n all of the above experiments an oil ranging in color from water white to light brown was found as a separate upper phase in the water scrubber. Examination of this oil by infrared and ultraviolet spectroscopy showed that from one half to nearly the entire amount, depending on the particular experiment, was polyisoprene. The molecular weight was apparently rather low, since some of this material could be distilled a t reduced pressure. I n some runs up to one half of the oil was composed of alkylaromatics, p-cymene being the major constituent. Experimental
Analytical. Gas-liquid chromatographic internal standard techniques were used to analyze the reactants and various intermediates in this three-step isoprene synthesis. Chloromethyl methyl ether was analyzed on a 3-meter column of 20% SE-30 on Chromosorb W, using n-hexane as the internal standard. I n this manner such impurities as methylal, trioxane, and bis(chloromethy1) ether were determined quantitatively. Small amounts of anhydrous HC1 and monomeric formaldehyde, which eluted just after the air peak, were determined semiquantitatively. Care was taken throughout sampling and analysis to minimize exposure of C M E to atmospheric moisture and reactive materials such as rubber septums. The column used in analyzing the C M E addition products with isobutylene, 1-butene, and 2-butene was a combination column consisting of 4 meters of 15% SF-96
Table V. Isoprene by Pyrolysis of CMBME over Molecular Sieve Catalysts Yield, Mole si' Run
Space Velocity"
11 12 13 14 15 16' 17 18 19'
0.81 0.77 0.90 1.13 0.87 0.88 0.67 0.65 0.62
Temp., 200 250 250 250 300 300 250 300 350
c6
Isoprene 83.5 82.7 82.0 85.1 76.6 78.3 64.5 71.8 65.8
Intermediate unsaturated ethers
...
< 1.0 ... ... 5.6
... Trace Trace Trace
Methanol
Methyl chloride
21.2 14.1 6.2 15.9 25.2 2.9 56.8 66.9 32.3
72.4 71.6 83.5 73.7 52.8 64.0 34.0 31.0 50.6
HC1
... 17.6
... ... 40.7
...
55.4 59.0 37.1
Heavy Oil Yield, W t O G d 3.6 3.6 4.4 3.2 4.1 4.9 13.5 7.9 6.8
"Space velocity expressed i n terms of weight of C M B M E feed per weight of catalyst per hour. Measured midway down column of catalyst by thermocouple situated outside reactor wall. Freshly distilled C M B M E used i n all r u n s C M B M E conversion is presumed near 100% : judged by spectral analysis of heavy oil. Material balances for r u m 11 to 19, respectively: 96.8; 96.6; 97.9; 96.1; 94.5; 95.2; 98.3; and 97.8%. dMaximum ualues. ' N o t determined. 'SK-400 catalyst used i n r u m 11 to 16 and AW-300 in runs 17to 19.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970 239
followed by 2 meters of XF-1150. Chromosorb W was the support and 3-methylhexane was the internal standard. This column permitted simultaneous quantitative analysis for CME, methylal, tert-butyl chloride, 1-chloro-3-methyl2-butene, 3-methyl-2-butenyl methyl ether, 3-methyl-3butenyl methyl ether, CMBME, 3-chloro-2-methylbutyl methyl ether, and 3-chloropentyl methyl ether. The analysis was satisfactory even in the presence of the large excess of hydrocarbon feed. Residues from CMBME decompositions in N M P were analyzed in two steps. First, N M P was determined using a 2-meter section of 10% Carbowax 20 M on Teflon 6. D M F was the internal standard. The N M P hydrolysis product, y-methylaminobutyric acid, cyclized quantitatively in the hot injection port to regenerate N M P . The HC1 salt of N M P also decomposed quantitatively in the injection port to give free N M P . N M P hydrochloride and y-methylaminobutyric acid hydrochloride were determined in the crude N M P mixture by potentiometric titration (Trusell, 1966). I n the second step of the N M P residue analysis a 4-meter section of 1 5 5 SF-96 on Chromosorb W was employed. 1-Propanol was the internal standard. This analysis provided a quantitative determination for any CMBME, CME adducts with 1- or 2-butenes, and unsaturated ether intermediates which might remain a t the end of a pyrolysis experiment. The GLC scheme developed to analyze crude isoprene produced in the pyrolysis step was calibrated by analyzing a series of synthetic blends containing the various components in the concentration ranges encountered in the pyrolysis experiments. The column used was 4 meters of Ethofat (105) on T-6. No internal standard was used, since the crude isoprene was collected as a distillate and analyzed shortly after collection. In our laboratory conversion, yield, and selectivity are defined as follows: Conversion is the per cent of a given reaction component converted t o other products. This figure includes material unaccounted for. Yield is the per cent of a particular product formed based on the reactants charged. This may be based on total charge of reactants, but usually it will be based on the yield-limiting reactant. Unless otherwise specified, yield will be mole per cent. Selectivity is the fraction of the unrecovered reactant converted to a particular compound. The term "CME balance" is merely a summary, on a molar basis, of the per cent of C M E charged initially which occurs as various CME-derived products such as CMBME and its decomposition products, including isoprene. Materials. Chloromethyl methyl ether was either prepared according to well-known procedures (Summers, 1955) or purchased from Matheson or Eastman. This reagent was readily purified by first distilling it in a simple glass distillation apparatus (1 x 18 inch Vigreux column) which had been assembled while hot and then cooled under a prepurified nitrogen stream. The material was distilled rapidly in a nitrogen atmosphere, with the overhead temperature maintained about 5" C below the normal boiling point by regulating the rate of nitrogen sweep. The forecut contained most of the methylal impurity and excess anhydrous HC1. A heart cut consisting of 60 to 70% of the charge was generally 95+YC pure C M E , with methylal constituting the bulk of the impurity. The distillation residue contained bis(chloromethyl)ether, trioxane, and other forms of formaldehyde which were present 240
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
in the initial charge. I n a continous operation the forecut and distillation residue would probably be recycled to the C M E synthesis reactor. CME of 99+% purity was used in the present investigations. This high purity CME was obtained by mixing about 10% by weight of TiCl, with the heart cut, and then performing a second distillation in the apparatus described above. Methylal was complexed by the TiClr and remained in the distillation bottoms. Overhead fractions were analyzed by GLC for CME purity and by emission spectroscopy to ensure that no TiCL had codistilled. Isobutylene, 1-butene, 2-butene, and n-butane used in these studies were C . P . grades purchased from the Matheson Co. The refinery mixed C, stream employed was obtained from Marathon's Robinson, Ill., refinery. I t analyzed 0.1% Ci and lighter, 15.4% n-butane, 29.65 isobutane, 12.2% 1-butene, 28.75 2-butene, 13.gCc isobutylene, and 0.1% butadiene and C,'s, on a mole per cent basis. Matheson reagent grade N-methyl-2-pyrrolidone was used as received for the liquid phase pyrolysis studies. The solid catalysts used in the vapor phase pyrolysis experiments were purchased from the Linde Division, Union Carbide Co. No pretreatment of these catalysts was performed. SK-400 is a Y-type molecular sieve having a pore diameter of about 10 A. I t contains about 17 nickel ion. This catalyst was used in the form of I%inch cylindrical pellets. AW-300 is a very acid-resistant molecular sieve catalyst having a pore diameter of 3 to 5 A. I t was used in the form of l,Lb-inch extrudate. Addition Step Reactions. A 1-gallon Neucerite-lined Pfaudler autoclave with a stirrer operating a t 450 rpm was used for the experiments listed in Tables I and 11. Hydrocarbon and C M E feeds were charged under pressure from tared stainless steel tanks through polyethylene lines. A nitrogen atmosphere (-70 psig) was maintained in all the runs. Aliquots for analysis were discharged as liquids into dry ice-cooled collecting receivers a t normal pressure. At the end of a run, the entire charge was passed into a 5-liter flask fitted with an upright dry icecooled condenser. In general, 200 to 250 grams of CMEcatalyst solution was added to 1500 to 2000 grams of hydrocarbon feed in the reactor. Addition periods were varied from 2 minutes to hour as desired. Reactor temperature varied between 23" and 29" C during the runs. At the end of run 1 excess isobutylene was allowed to flash away. The residue was decanted into 1 liter of ice water and the mixture was stirred while it warmed to room temperature. The organic phase was separated, washed again with water (200 ml), and then dried over anhydrous sodium sulfate. Crude CMBME was distilled a t 110 mm of Hg and a heart cut was collected a t 81°C. GLC analysis showed this material was 98.1% pure 3-chloro-3-methylbutyl methyl ether (Strauss and Thiele, 1936). Impurities were 3-methyl-3-butenyl methyl ether and 3-methyl-2-butenyl methyl ether. ng was 1.4216 and di: was 0.958. The infrared spectrum of a liquid film showed absorptions characteristic for a gem-dimethyl group a t 1370 and 1385 cm-I, an ether absorption centered a t 1113 cm-', and a moderate intensity absorption a t 582 cm-' believed to be due to the tertiary chlorine atom. The N M R spectrum was determined with a Varian A-60 spectrometer. All resonances are expressed as parts per million downfield from internal standard tetramethyl-
silane (6 = 0). The spectrum of a neat sample showed a singlet for the methyl hydrogens a t 1.57 ppm, a triplet for the hydrogens on the methylene adjacent t o the tertiary carbon atom a t 1.99 ppm, a singlet for the methoxyl hydrogens a t 3.25 ppm, and a triplet for the hydrogens on the methylene group adjacent to the ether linkage at 3.52 ppm. The coupling constant for the two triplets was 6.30 to 6.35 cycles per second. Integration of the spectrum showed the various hydrogens to be present in the proportions expected 3-Chloro-2-methylbutyl methyl ether was prepared by adding C M E (200 grams, 2.5 moles) to 2-butene (167 grams, 2.9 moles) which had 4.0 grams of ZnCl? suspended in it. The C M E was added during a 2-hour period from a pressure-equalizing addition funnel t o the 2-butene maintained a t reflux in a 1-liter round-bottomed, three-necked flask fitted with a magnetic stirrer, thermometer, and upright dry ice-cooled condenser. The exit tube from the condenser led t o a tower of Drierite. After the C M E had been added, the mixture was stirred overnight with no cooling in order t o permit the excess 2-butene to evaporate. The crude product was worked up in a manner similar to that used to isolate the CMBME from run 1 in Table I. Distillation gave a 90-ml fraction collecting a t 79.5" to 80.0"C which was shown by GLC analysis to be just over 9 0 5 pure. A portion of this fraction was redistilled in a microware column. Product collecting a t 39.5"C a t 17 mm of H g was shown by GLC analysis to be 9 8 5 pure 3-chloro-2-methylbutyl methyl ether. The structure was confirmed by infrared and N M R spectroscopic analyses. The index of refraction a t 24OC was 1.4235. 3-Chloropentyl methyl ether was prepared by passing 1-butene gas into C M E (200 grams, 2.5 moles) which contained 6 grams of ZnCL partially dissolved and partially suspended. The C M E was maintained a t 40°C and 1-butene was passed in at a sufficiently low rate that it did not reflux from a dry ice-cooled condenser. After about 15 hours, 58 grams of 1-butene had been absorbed. The mixture was cooled and worked up as described above for the reaction of C M E with 2-butene. The crude product was distilled through a 1 x 18 inch Vigreux section a t 100 mm of Hg. Three fractions collecting between 86.5" and 106°C were each found to be about 70% pure 3-chloropentyl methyl ether by GLC analysis. These fractions were combined and redistilled in a microware column. A fraction collecting a t 90" to 93°C a t 100 mm of Hg had a molecular weight of 131.0 us 136.6 theoretical. The index of refraction a t 24" C was 1.4266. GLC analysis indicated a purity on the order of 9 0 5 . Infrared and N M R spectroscopic analyses were in accord with the structure of this compound. By-product from the Reaction of CME with Dilute Isobutylene. An experiment was conducted in a manner analogous to the run in which C M E was added to 2-butene, but using 20 weight '-c isobutylene in n-butane as the hydrocarbon feed. About 2 moles of C M E containing dissolved TiCl, (3% by weight of the C M E ) was added to an amount of the hydrocarbon feed such that isobutylene was present in a 2 t o 1 molar excess over CME. The reaction was complete within 48 hours a t about -8" C. The crude products were isolated according to the workup procedure described for the C M B M E from run 1 in Table I. The crude product mixture was heated on a steam bath and CMBME removed a t 30 to 40 mm of Hg until
no more material would distill through the short column used. This sample of crude by-product was further purified by fractionation in a microware column a t 1.7 mrn of Hg. A fraction collecting a t 38.5"C (column poorly' equilibrated) to 44.5"C was found to have a molecular weight of 176.0; the calculated molecular weight for 1,5dimethoxy-3-methyl-3-chloropentane is 180.5. Major features of the X M R spectrum of a neat sample using tetramethylsilane internal standard (6 = 0) were a singlet for the methyl group hydrogens a t 1.55 ppm, a singlet a t 3.25 ppm for the methoxy hydrogens, a triplet a t 2.02 ppm for the hydrogens on the methylene groups adjacent t o the tertiary carbon atom, and a triplet at 3.52 ppm for the hydrogens on the methylene groups adjacent to the methoxy groups. The coupling constant for the two triplets was 6.5 cps. The ratio of methoxy hydrogens to methyl hydrogens was difficult to determine accurately because of a small impurity peak shouldering on the methyl hydrogen adsorption, but the ratio was approximately 2 to 1, as required by the structure. The material had an index of refraction a t 20°C of 1.4451. The elemental analysis was in good agreement with theory. Calculated: C , 53.217; H , 9.44%; C1, 19.635. Found (Huffman Laboratories): C, 53.385, 53.605; H, 9.22'~, 9.28'5; C1, 19.885, 19.79%. The low reactivity of the unsaturated ether intermediates was demonstrated by the following experiment, conducted in duplicate in glass pressure tubes. n-Butane, a mixture of the unsaturated ethers (77.37 3-methyl3-butenyl methyl ether and 22.47 of 3-methyl-2-butenyl methyl ether), and C M E were mixed a t dry ice temperature in molar ratios of 12.1:1.12:1.0, respectively. TiC1, was added to the extent of 3.0% by weight of the CME. The mixture was stirred (magnetic stirrer) for 4 days a t room temperature, then chilled and analyzed. The amounts of materials found unchanged were: CME, 86.9%; 3-methyl-3-butenyl methyl ether, 97.6%; 3-methyl-2butenyl methyl ether, 76.55; and n-butane, 99.9%. The duplicate run gave comparable results. Pyrolysis in NMP. The apparatus for the pyrolysis experiments described in Tables I11 and IV consisted of a simple laboratory glassware distillation apparatus. T h e reboiler was a three-necked, round-bottomed flask fitted with a thermometer and magnetic stirrer. A 1 x 18 inch section of Vigreux column led to a variable reflux head cooled with chilled water. T h e gas bypass of this head led to a dry ice trap. The side arm take-off led to a receiver maintained a t room temperature and thence to another dry ice trap. I n the case of runs 9 and 10 (Table IV) residual isobutylene, together with some tert-butyl chloride, was driven past the gas bypass with the head set on total reflux during heatup. I n all runs except number 6, the overhead temperature was set a t 31"C, or just above that of the isoprene-methanol azeotrope. The end of a run was indicated when methanol began to reflux high in the column. At this point the overhead temperature controller was set high enough t o permit methanol to distill, but not the water formed by the reaction of methanol with the hydrochloride of NMP. The reaction was stopped when refluxing water was noted high in the column. Methanol-rich and isoprene-rich phases collected in the receiver were separated and analyzed individually. Materials collected in the dry ice traps were also analyzed. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9 , No. 2, 1970 241
Yield data reported in Tables I11 and IV were based on the total of various crude materials present in these fractions. HC1 remained in the reboiler residue as the salt of NMP. This salt readily liberates HCI when heated to moderately high temperatures. 3-Chloro-2-methylbutyl methyl ether, the product resulting from the addition of C M E to 2-butene, was converted to isoprene by a procedure similar to that described above. 3-Chloro-2-methylbutyl methyl ether (98+% pure, 0.146 mole) was heated for 5 hours a t 150” to 160°C in N M P (100 grams) containing LiCl (0.146 mole). Crude isoprene was collected in a yield of 747. Pyrolysis over Molecular Sieves. The experiments listed in Table V were conducted in an upright tubular reactor constructed from a 16-inch section of 25-mm-0.d. borosilicate glass tube. Heating was accomplished by means of two 6-inch furnaces (Hevi-Duty Electric Co.). The temperature was monitored a t the midpoint of each furnace by thermocouples inserted between the reactor wall and the furnace lining. Feed was vaporized by contact with Carborundum packing placed in the upper section of the column. About 40 grams of feed was passed into the reactor in a reaction time of about 1 hour. A nitrogen flow of 100 cc per minute was introduced a t the top of the reactor above the point where feed was vaporized. A large-bore glass tube led from the reactor, which was also packed with Carborundum a t the bottom section, directly down to a water scrubber. The bulk of any unchanged feed or intermediates was collected over the water, which was maintained a t room temperature, together with any heavy oil by-product. HC1 and methanol were collected in the aqueous phase. The yields of these materials were determined by titration and by GLC analysis, respectively. Isoprene and methyl chloride were entrained in the nitrogen stream and collected in a dry ice trap. Methyl chloride yields in these runs are uniformly low because of loss via entrainment in nitrogen used as a sweep gas. This loss of methyl chloride accounts for the slightly low material balances. I n a few runs, from 1 to 3% of the methanol and methyl chloride values were consumed by the formation of dimethyl ether. CMBME conversion is somewhat ambiguous in many of the runs, since unchanged feed and various intermediates when obtained as a mixture with heavy oil by-product constituted too complex a sample to analyze readily. I n all cases the heavy oil was examined by infrared spectroscopy to ensure that very significant amounts of isoprene precursors were absent. Runs 11 to 16 were conducted using the same 38.6gram sample of Linde’s SK-400. CMBME was passed through a t 250°C until the catalyst had absorbed an equilibrium amount of organic material (10.4 grams). Little difference was noted in the amount of organic material retained by the catalyst over the 200” to 300°C range. Weight hourly space velocity was based on the initial 38.6 grams of catalyst charged. I n runs 17 to 19 the same 39.6-gram sample of Linde’s AW-300 catalyst was used. No equilibration was done before run 17; in that run 3.0 grams of organic material was absorbed a t 250°C.
in this research, to A. L. Schalge and D. D. Conway for spectroscopic analyses, and to F. C. Trusell for potentiometric analyses. C. H. DePuy and M. W. Hanna of the University of Colorado provided valuable consultation throughout this investigation. literature Cited
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Acknowledgment
We are indebted to R. H. Hughes and Ruder Schill for developing the chromatographic analyses employed 242
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
RECEIVED for review August 7, 1969 ACCEPTED January 26, 1970
