Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 679-685
679
Oxidative Carbonylation of Methanol as a Synthesis Route to Oxamide: Statistically Designed Experiments Robert J. Radel;
Jack M. Sulllvan, and John D. Hatfleld
Division of Chemical Development, National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, Alabama 35660
About one-half the nitrogen in the 10 million tons of nitrogen fertilizer used annually in the United States is lost from the soil. The development of a fertilizer compound which can be used to decrease nitrogen loss might serve to reduce the energy requirements for producing nitrogen fertilizers. The slightly water-soluble compound oxamide has been shown to be a very desirable slow-release nitrogen fertilizer; if it could be produced economically, the nitrogen fertilizer requirements might be significantly reduced. A process developed in this laboratory for the production of oxamide uses the oxidative carbonylation of methanol to produce dimethyl oxalate, which can be readily converted into oxamide. From a statistically designed set of experiments, the parameters of the hyperspheres involving the independent variables molar ratio methanol:qulnone, molar ratio quin0ne:palladium catalyst, pressure, and temperature, with respect to the oxamide yield, the hydroquinone recovery, and the grams of byproduct oil were determined.
Introduction
Several methods are being investigated for the production of nitrogen fertilizers from the products of coal gasification and ammonia synthesis. One of these methods involves the synthesis of oxalate esters via the oxidative carbonylation of methanol utilizing p-benzoquinone as an organic oxidant (Fenton and Steinwand, 1974; Zehner, 1977), as shown by eq 1. The oxalate ester can be sepan
OH
0
OH
I111 CH,OCCOCH,
t ZNH3
-
00
/Ill
HzNCCNHz
t
2CH30H
(2)
E x p e r i m e n t a l Design. Five variables were chosen as the basis for the statistical design (Table I). The first was the discrete variable describing the presence or absence of added acetonitrile as a cosolvent. This cosolvent was selected because of its beneficial effects in several later process steps. The remaining continuous variables were the ratio of the moles of methanol to the moles of quinone oxidant (MEQRATIO),the molar ratio quin0ne:palladium chloride catalyst (QPDRATIO), the starting pressure (at ambient temperature), and the maximum target temperature. In a number of experiments the target temperature was not attained prior to the completion of the reaction (see Experimental Section). The design chosen, adapted from Box and Wilson (1951), was a half-factorial central composite type, including the extremes and center points involving the continuous variables in the absence of acetonitrile. The ordering of the 30 experiments was determined by using a random number generator. The model for this design is the complete quadratic surface between each measured property (response),y , and the independent variables or coded factors, x i , as given by eq 3. 5
y = bo
5
+ iC= (l b i X J + iC(biiXi2) + i#j E((bijXiXj) =2
~3 ~4 ~5
= [log (QPDRATIO)2]- 5
(5)
= (PRESSURE - 1800)/300
(6)
= (MAXTEMP - 140)/20
(7)
where MEQRATIO = mol of methanol/mol of quinone oxidant, QPDRATIO = mol of quinone/mol of PdC12 catalyst, PRESSURE = carbon monoxide starting pressure (at ambient temperature), psig, and MAXTEMP = maximum target temperature, "C.
rated and ammoniated to yield oxamide, or the oxamide can be prepared in situ and recovered by filtration (eq 2). 00
The coded values are related to the actual values (Table I) of the variables by the expressions ~2 = [log (MEQRATIO/12.35)]/log (2) (4)
(3)
E x p e r i m e n t a l Section
The experiments were run in the following manner. One mole of quinone (108.10 g) was charged, along with the appropriate amount of PdClz catalyst, to a 2-L Magnedrive autoclave. The autoclave was sealed and the appropriate amount of methanol and acetonitrile was added through a sample introduction and removal tube. The autoclave was purged with carbon monoxide and was pressurized to the desired pressure with carbon monoxide while stirring. The starting conditions for each experiment are given in Table 11. The reactor then was rapidly heated until the desired temperature was reached or until the reaction was complete. In those cases in which the target temperature was reached, the temperature was maintained within f2 "C until the plot of temperature vs. pressure indicated that the reaction was complete. In several cases the reaction was complete before the maximum target temperature was attained. This detracts from the orthogonality of the design and will be discussed in a later section. The observed reaction parameters and results for the individual experiments are listed in Table 111. After the reaction was complete, the reactor was cooled rapidly by introducing chilled water into the internal cooling coils. The reactor gases were sampled by gas chromatography and the remaining gases were vented. The reactor was purged with dry nitrogen and the contents filtered under nitrogen atmosphere to remove the catalyst. The filtrate was directly transferred to a reaction flask equipped with an overhead stirrer, a dry nitrogen purge, and an ammonia introduction inlet. The methanol solution was cooled to between 0 and 5 "C, and ammonia was bubbled rapidly
This article not subject to US. Copyright. Published 1982 by the American Chemical Society
680
Ind. Ena. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table I. Coded Levels and Actual Values of Five-Factor Box Design ~~~
variable name added acetonitrile solvent ratio of mol of CH,OH t o mol of quinone ratio of mol of quinone to mol of catalysts starting pressure, Psig maximum temperature, "C
1
0
~
24.7
49.4
-2
-1
30% of CH,OH 6.17
nont'
x
x:
~~~
coded design levels
variable designation 2
12.3 5
3.08
IC3
3160
1000
316
100
31.6
x4
2400
2100
1800
1500
1200
x,
180
160
140
120
100
Table 11. Starting Conditions for Individual Experiments expt. no. 1 2 3' 4 5 6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 a
gof PdCl, 1.75 0.175 0.554 0.175 1.75 1.75 1.75 0.175 0.554 1.75 0.554 1.75 5.54 0.554 0.554 0.554 1.75 0.175 0.554 0.556 0.175 0.175 1.75 1.75 0.554 0.554 0.554 0.0564 0.175 0.556
g of quinone 108.04 108.06 54.03 108.13 108.02 108.02 108.13 108.03 108.01 108.06 108.21 108.15 108.05 108.10 108.08 108.06 108.03 108.12 108.01 108.02 108.00 108.00 108.00 108.00 108.00 108.00 108.12 108.00 108.00 108.03
m L of methanol 250 250 1000 250 250 250 1000 250 500 1000 500 250 50 0 500 500 500 250 1000 125 500 1000 1000 1000 1000 500 500 500 500 1000 500
mL of acetonitrile
max target temp, "C ( O F )
0 75 0 0 0 75 0 0 0 330 _.0
I
is
0 0 0 - 0-
In
0 0 0 0 330 0 330 0 0 0 0 330 0
120(248) 120(248) 1 4 0 1284) 160(320) 160(320) 120(248) 160(320) 120(248) 140 (284) 120(248) 140(284) lZO(248) 140 ( 2 8 4 ) 140(284) 140 (248) 140 ( 2 4 8 ) 160 ( 3 2 0 ) 160(320) 140 ( 2 8 4 ) 140 ( 2 8 4 ) 1 2 0 (2481 160(320) lZO(248) 160(320) 140(284) lOO(2121 140(2841 140(284) 1 2 0 (248i 180(3561
starting press., psig 1500 1500 1800 1500 2100 2100 1500 2100 1800 1500 1800 1500 1800 1800 1800 1800 1500 2100 lb0O 1800 1.500 1500 2100 2100 2500 1800 1200 1800 2 1 00
1800
coded design levels x,
x,
+
-
x,
x,
-
-
0 t
0
+
-
2 I
+ I
+
-
t
-
+
0
0
t
-
+
-
J.
0
-
-
I
0
0
L
0
0
t
1-
+ ~
t t
t
+
I
-
+
-
0 0
-
0 0
+
t
0
0 0 0
0
0 0
t
+
0 0
0 0
c
t
0 t
+
0
0
+
. .
I
t
4
+
0
+
t
-2 0
f
-
-
0
+
0
+
+ +
+
-2 0 0
+
x,
4
-
0 0 0 0
0
t 2
t
0 2
L
+
-2 0
0 2 0 0
0
0
0
2
0
0
-
+
This experiment was run using 0 . 5 mol of quinone due t o reactor size limitations.
through the solution while both vigorously stirring and continuously purging the system with nitrogen. The oxamide obtained was filtered directly from this reaction flask and washed with 200 mL of methanol. The oxamide product then was stirred vigorously with two 80C-mL portions of distilled water and one 600- to 800-mL portion of acetone. It then was filtered and air dried. The combined methanol portions from the reaction mixture and methanol washings were evaporated under vacuum and the residue obtained was vigorously stirred with two 800-mL portions of chloroform. The resulting solid was filtered, air dried, and collected as recovered hydroquinone. The chloroform solutions also were evaporated under vacuum, leaving a dark, oily material which also was recovered. The catalyst recovered in the first filtration step was oxidized by fuming to dryness with three portions of concentrated nitric acid. The resulting solid then was fumed to dryness with three portions of concentrated hydrochloric acid, oven dried, and analyzed by X-ray fluorescence (XRF) analysis. Traces of catalyst recovered from the walls and internal sections of the reactor also were analyzed by XRF analysis.
Results Recovered Products. Elemental analyses of the recovered oxamide were obtained, and from these data the empirical formula and percent oxamide were calculated. The purity of the oxamide was determined by comparing the calculated and experimentally observed empirical formula of the recovered product. In all cases the purity of the oxamide was greater than 97.7%. Elemental analyses also were performed for the recovered hydroquinone. As above, the empirical formula and percent recovered hydroquinone were calculated from the analytical data. The hydroquinone products also were examined by lH nuclear magnetic resonance to confirm the purity determined from the difference between the calculated and observed empirical formulas. In at least two cases this provided additional information. In experiment 1,the second crop of recovered material consisted mostly of methyl oxamate. Also in experiment 28, 50% of the recovered material was determined to be unreacted quinone. The quantity of recovered catalyst varied between experiments. A small portion of degraded catalyst remained in the reactor between experiments, which was identified
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 681
Table 111. Observed Reaction Parameters for Individual Experiments
expt no.
max temp,
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
123 (253) 122 (252) 140 (284) 1 6 1 (322) 143 (290) 124 (256) 149 (300) 126 (259) 140 (284) 122 (252) 142 (288) 122 (253) 140 (284) 1 4 1 (285) 142 (287) 142 (288) 154 (310) 160 (320) 145 (293) 142 (287) 122 (252) 157 (315) 123 (254) 140 (284) 142 (287) 102 (216) 142 (287) 142 (287) 124 (255) 157 (314)
"C (OF)
total press. max press., drop at psig T, psig 1925 1995 2305 2090 2766 2671 1790 2888 2230 1790 2269 1925 2418 2195 2295 2278 1928 2747 2590 2278 1900 1940 2515 2572 3209 2300 1520 26 56 2800 23 20
total press. drop at 27 "C (80 O F )
399 393 275.5 313.5 475 475 551 503 522 817 503 3 94 180 53 2 589 570 427 24 7 218 494 413 3 70 679 864 560 503 408 0 1026 513
275 370 228 256 318 380 475 34 2 399 389 309 180 380 399 399 370 3 23 180 399 36 5 370 475 693 465 4 18 351 57 7 50 3 80
app reacn time, h
TPDB," C
oxamide yield, %
hydroquinone yield, %
0.44 1.40 0.12 0.54 0.32 0.44 0.27 0.94 0.38 0.43 0.36 0.66 0.31 0.29 0.43 0.46 0.23 0.57 0.53 0.38 0.92 0.73 0.22 0.22 0.28 0.72 0.44
69 (156) 108 (226) 99 (210) 95 (203) 8 1 (177) 9 1 (196) 86 (186) 80 (176) 86 (187) 93 (200) 87 (189) 98 (208) 50 (122) 82 (180) 68 (155) 67 (152) 89 (193) 107 (224) 114 (238) 84 (184) 93 (200) 104 (220) 8 1 (178) 82 (180) 74 (165) 85 (186) 85 (185)
3.46 69.51 82.96 31.15 65.83 77.38 72.08 66.52 75.46 67.52 72.15 27.83 17.19 70.66 80.14 69.38 59.33 40.32 30.42 79.86 62.25 42.20 77.97 77.17 63.53 85.89 65.16
71.68 89.55 90.42 70.05 81.50 81.75 87.20 84.73 89.85 87.79 85.60 7 2.4 8 33.57 87.43 88.00 89.68 8 8.24 82.77 64.92 83.77 88.38 83.98 84.78 91.57 90.44 92.61 90.99
1.02 0.28
98 (209) 88 (190)
75.63 79.94
92.47 90.68
T P D B = temperature at which the pressure drop began.
as an oxide of palladium (by XRF analysis) and therefore would not be expected to influence any subsequent reactions. The dark, oily material which was recovered from the evaporated chloroform solutions was submitted for analysis as well as being examined by high-pressure liquid chromatography (HPLC) and 'H NMR spectroscopy. Traces of water found in the liquid products varied between 0.1 and 3.4%, with most levels being below 1%. Thus, very little water was contaminating the reaction system. The nitrogen analysis also indicated generally less than 1% nitrogen in the recovered oils, which indicated that only traces of methyl oxamate or other nitrogen-containing materials were present. The carbon analysis indicated that the carbon content of the oil was similar to that of hydroquinone. The 'H NMR spectra indicated that the major component of the oils is p-methoxyphenol. The second major component of the oils was found to be trapped hydroquinone. The gases sampled after the reaction were analyzed by gas chromatographic (GC) analysis. The only component other than carbon monoxide found in these samples was carbon dioxide. However, the quantity of C02 formed during the reaction could not be determined because the starting carbon monoxide was contaminated with carbon dioxide. Discussion The responses evaluated from the above design consisted of the yield of oxamide, the yield of recovered hydroquinone, the yield of byproduct oil, the pressure drop observed for the reaction, the reaction time, and the turnover rate of the palladium catalyst based on the formation of oxamide.
The coefficients of eq 3 were determined by leastsquares analysis method for each of the above responses. This analysis forces the inclusion of all the independent variables, their interactions, etc., regardless of their significance test levels. The values of 'these coefficients are listed in Table IV along with the standard deviation of the replicates (a),the standard deviation (S) of the responses calculated by the model compared with the observed data, and the correlation coefficient (R2). Oxamide Production. The yield of oxamide for the individual reactions was determined using eq 8. yield oxamide = (mol of oxamide recovered X 100)/(mol of quinone charged) (mol of quinone recovered) (8) The optimum conditions for the formation of oxamide were calculated using the coefficients of Table IV in eq 3. The maximum yield of oxamide was calculated to be 82.52% with no acetonitrile solvent (the square terms, bz2 through b55,apply only to this condition) at a molar ratio methano1:quinone of 21.10:1, a quinone:catalyst ratio of 199:1, a temperature of 142.4 "C,and a pressure of 1898 psig of carbon monoxide. The orthogonality of the design in the coded form permits the separation of the main effects of each variable and the two-factor interactions for each variable. The presence or absence of acetonitrile has little overall effect on the yield of oxamide. Adding acetonitrile caused an increase in yield at low temperature but a decrease in yield at high temperature. When varying only the molar ratio methanol:quinone, low yields of oxamide (30%) are obtained at very low ratios (3.08:1), while higher yields are obtained at ratios of about 25:l (80%). Yields of oxamide are low for both large and small values of molar ratios
682
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table IV. Coefficients of t h e Second-Degree Model for the Responses Measured from the Oxidative Carbonylation of Methanol
oxamide
log (100 .- %5 recovd hydroquinone)
b,i b,,
77.960 -2.866 8.416 -6.196 3.969 -4.712 -5.479 -15.994 -2.527 4.804 2.621 2.259 1.796 7.654 -3.656 -4.368 0.108 -5.569 -16.437 -9.578
1.0251 0.0552 -0.1266 0.01 82 -0.0099 0.0075 0.0353 0.1678 -0.0232 -0.0625 0.004 9 --0.02 7 7 -0.0012 -0.01 29 0.0130 -0.0179 -0.0278 0.0175 0.1782 0.0257
ob
4.644
0.0800
2.18
9.077
0.059
0.0269
8.218
0.0977
3.38
40.887
0.141
0.0529
0.959
0.943
0.9895
0.974
0.954
0.840
% yield
coeff bo b: b2 b3 b4
b, b:, b33 b44
b;, bl2 b13 b,4
bl5 b23 b24
b2d b34
SC R2
d
g of recovd oil -0.8728 13.9806 -2.5284 -8.2361 -4.5564 --2.543 5 4.3391 16.8503 0.2585 2.1609 -8.1839 0.7476 4.5111 7.4 75 8 8.1421 2.983 5 2.3834 -6.8342 -2.9711 - 4.3 4 0 7
press. drop, Psig 502.94 105.10 80.50 -16.05 47.23 -18.01 -17.03 -63.38 6.16 9.00 -19.16 -17.77 -25.13 10.36 -48.22 -16.83 -63.13 43.42 -7.94 27.93
reacn time, h
Pd turnover rate a
0.439 -0.0351 -0.0575 0.3330 -0.0462 -0.2094 -0.0051 0.1837 -0.0115 -0.0310 -0.0039 -0.0068 0.0167 0.0324 -0.0019 0.0015 0.0490 -0.0023 - 0.0504 0.0205
0.1442 0.0310 0.0486 0.0179 -0.0062 -0.0 0 24 - 0.00 26 -0.0420 -0.0025 0.0079 -0.0103 0.0107 0.0236 0.0387 0.0171 0.0 179 0.0236 -0.0 186 -0.0528 -- 0.04 0 8
-
The turnover rates for palladium are based on moles of oxamide formed per mole of Pd per second ( m Ox/m Pd/s). o = standard deviation of the replicates. S = standard deviation of the models calculated as observed values. RZ= correlation coefficients. a
quinone:palladium(II) chloride catalyst, but reach a maximum of about 75% a t a ratio of 316:l. Maximum yields of oxamide are obtained a t carbon monoxide pressures of about 2100 psig, with the yields falling off as the pressure is either increased or decreased from this value. On the other hand, the temperature causes a minimum of oxamide to be obtained a t 140 "C, with the yield of oxamide increasing as the temperature is raised or lowered from this value. This temperature effect causes the multidimensional surfaces involving oxamide yield, temperature, and one or more independent variables to exhibit a "saddle", thus creating a point which corresponds to both a minimum and a maximum in oxamide yield. A word must be said here about the "orthogonality" of the temperature factor. As was mentioned in the experimental section, the temperature of the reactor did not always reach that required by the design before the reaction was determined to be complete. This was caused by the relatively long heating time (30 min) required to bring the autoclave up to the reaction temperature as compared to the relatively rapid rates of reaction for several of the experiments. In general, however, the heating rate of the reactor was maintained a t a constant level; thus the temperature profiles of the various experiments (up to the point at which the maximum temperature was reached or to the point a t which the reaction was determined to be complete) are nearly identical. This failure to reach some of the high levels of temperature in several of the experiments could be the cause of the weak minimum in the temperature variable. Experiments currently underway are dealing with this problem by a redesign of equipment and experimental procedure. All of our current results with respect to temperature should be considered with this in mind. Although eq 3 describes a five-dimensional surface, a more useful approach which allows better visualization of the reaction system is to study the shapes of the surfaces describing the significant two-factor interactions while
holding the remaining variables at their zero levels (or at plus one for the absence of acetonitrile). The introduction of acetonitrile into the reaction system causes only small increases or decreases in the yield of oxamide. Its effects on all of the dependent variables will be discussed later. The coefficient of the two-factor interaction term, b12, between the presence or absence of acetonitrile and the molar ratio methano1:quinone is small (Table 111). The coefficients of the interaction terms between the presence or absence of acetonitrile and the ratio quinone:catalyst (bI3)or pressure (b14) are even smaller. A shift in the maximum yield of oxamide to lower quinone:catalyst ratios and lower methano1:quinone ratios in the presence of acetonitrile may be caused by the increased solubility of the quinone, catalyst, or catalyst complex in the methanol-acetonitrile solvent system. Also, higher yields of oxamide are obtained at low temperatures in the presence of acetonitrile. This trend is reversed when no cosolvent is present. The three-dimensional surface describing the relationship between the molar ratio methano1:quinone and the carbon monoxide starting pressure, in the absence of acetonitrile, is shown in Figure 1. For this interaction the yield of oxamide appears to fall off more rapidly when changing the ratio methano1:quinone than when changing the pressure. The maximum yield of oxamide (80.5%)at 140 "C and a QPDRATIO of 316 is predicted to be at a carbon monoxide pressure of 1855 psig and a molar ratio methano1:quinone of 19.95:l. Note the skewed surface caused by this interaction term. The three-dimensional surface describing the relationship between the molar ratio quinone:palladium(II) chloride catalyst and the carbon monoxide starting pressure is shown in Figure 2 at 140 "C and a MEQRATIO of 12.35 in the absence of acetonitrile. The maximum yield of oxamide (80.1%) is obtained at a ratio quin0ne:catalyst of 197:l and a carbon monoxide starting pressure of 2170
Ind. Eng. Chsm. Rod. Res. Dev.. Vol. 21. No. 4, 1982 683
Figure 1. Three-dimensionalsurface deserihing the relationship between the molar ratio of methanol to quinone and pressure with respect to oxamide yield.
Figure 2. Three-dimensionalsurface describing the relationship between the molar ratio of quinone IO catalyst and presawe with respect u)oxamide yield
psig. From the figure we can see that the yield of oxamide falls off more rapidly with changes in ratio quinone:catalyst than with changes in pressure and the surface is skewed toward lower ratios quin0ne:catalyst and higher pressure. The surface describing the relationship between the molar ratio quinone:palladium(II)chloride catalyst and the temperature exhibits the typical saddle shape. The three-dimensional surface describing this interaction is shown in Figure 3 at 1800 psig CO and a MEQRATIO of 12.35 in the absence of acetonitrile. The yield of oxamide in the seat of the saddle is 75.4%. The value of the molar ratioquinone:cataJyst at this point is 2401, while the value for the temperature is 141.6 ' C . The yield of oxamide is increased when the temperature is varied from this minimax point, while the yield of oxamide is decreased as the ratio quinone:catalyst is varied from this point.
Figure 3. Three-dimensional surface describing the relationship between the molar ratio of quinone to catalyst and temperaturewith respect to oxamide yield.
Figure 4. Three-dimensional surface describing the relationship between the temperature and pressure with respect to oxamide yield.
Note also that the surface is skewed such that maximum yields are obtained at high temperatures-low quinone: catalyst ratios and low temperatures-high quinonexatalyst ratios. This effect results from the large negative interaction term, bS5,in Table IV. The three-dimensional surface describing the relationship between the temperature and carbon monoxide starting pressure (Figure 4) exhibits a similar shape. In this case the apexes are located at the corners of the surface, i.e., at the extremes of the design. Maximum yields are obtained at low temperature-high pressure and high temperaturelow pressure conditions. Minimum yields me obtained at low temperature-low pressure and high temperaturehigh pressure conditions. The yield of oxamide in the seat of the saddle is 77.2% when
684
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
MEQRATIO and QPDRATIO are fixed a t their center values and in the absence of acetonitrile. This corresponds to a temperature of 148.82 "C and a carbon monoxide starting pressure of 1785 psig. Other three-dimensional surfaces may readily be constructed for other fixed conditions of the remaining continuous variables and in the absence of acetonitrile. Of particular importance is setting the temperature above or below the stationary point to obtain increased maxima over those shown in Figures 1 and 2. Hydroquinone Recovery. The results for the recovery of hydroquinone were analyzed by regressing the function shown in eq 10. percent hydroquinone recovered = (mol of hydroquinone recovered X 100)/(mol of quinone charged) - (mol of quinone recovered) (9) f ( x ) = log (100 - percent hydroquinone recovered) (10)
This was done to give greater weight to the higher recoveries of hydroquinone in the regression. The fit obtained by using this function was better than that obtained when the percent recovered hydroquinone was used in the regression analysis. The optimum conditions were calculated for the above function using eq 3 and the coefficients listed in Table 111. The recovery of hydroquinone was calculated to be 80.9% at a molar ratio methano1:quinone of 33.3:1, a ratio quinone:palladium(II) chloride catalyst of 354:1, a carbon monoxide starting pressure of 1531psig, and a temperature of 132.8 OC. As in the previous case, these values correspond to a saddle point on the five-dimensional surface, caused in this case by both temperature and pressure giving weak minima rather than maxima. Two- and three-dimensional plots for the dependent variables describing the hydroquinone recovery, the byproduct oil formation, and the combined effects can be found in the supplementary material. The variation in yield of recovered hydroquinone for the presence or absence of acetonitrile is only 0.4%. That is, in the presence of acetonitrile solvent the hydroquinone recovery is 91.0%, while with no acetonitrile solvent the yield is 90.6 7%. The recovery of hydroquinone gradually increases as the molar ratio methano1:quinone is increased to around 30:l. Changing the molar ratio quinone:palladium(II) chloride catalyst has a more pronounced effect on the recovery of hydroquinone. The recovery reaches a maximum at a ratio quinone:palladium(II) chloride of about 316 mol to 1. The hydroquinone recovery then falls off rapidly as the ratio quinone:catalyst is altered to higher or lower values. Altering either the temperature or the pressure produces small effects that result in a minimum for the yield of recovered hydroquinone a t about 1800 psig and 140 O C , respectively. In general, the introduction of acetonitrile into the reaction system produces only small increases or decreases in the recovery of hydroquinone. Also, for all continuous variables except the molar ratio methano1:quinone (which is reversed), higher recoveries of hydroquinone are obtained at, low levels of the variables in the absence of acetonitrile, while higher recoveries of hydroquinone are obtained with acetonitrile present a t higher levels of the variables. The reversal of this trend in the case of the molar ratio methano1:quinone may be due to a lack of homogeneity of the reaction system at very low liquid phase levels. The lack of liquid phase could facilitate thermal reactions of the quinone on the walls of the reactor. Addition of a liquid solvent such as acetonitrile would provide the needed liquid phase to maintain the homogeneity of the reaction system.
The coefficients of the term describing the interactions between the MEQRATIO and the QPDRATIO (b23),the MEQRATIO and the CO starting pressure ( b 2 4 ) , the MEQRATIO and the temperature (&), the QPDRATIO and the CO starting pressure ( b 3 J ,and the CO starting pressure and the temperature (b45) are small. Thus, simultaneous changes in the levels of any two of these variables cause a small change in hydroquinone recovery. The coefficient of the term describing the interaction between the molar ratio quinone:catalyst and the temperature is large. Thus, the recovery of hydroquinone is affected significantly as either of these two factors is varied. Again, the response surface describing this relationship is saddle-shaped. The recovery of hydroquinone in the seat of the saddle was 87.9%. The levels for the molar ratio quin0ne:catalys.t and the temperature at this point were 298.8 mol to 1 and 139.8 "C, respectively. Also the response surface is skewed due to the large interaction term, giving high recoveries at low quinone:catalygt ratio-high temperature conditions and high quinone:catalyst ratiolow temperature conditions. Grams of Oil Produced. As was mentioned above, a dark, oily substance was isolated from the spent solvents after hydroquinone recovery. I t was shown that this oil consisted mainly of p-methoxyphenol, and its presence at the end of the reaction needs to be minimized to maintain adequate hydroquinone recoveries. Thus, the formation of this oil was treated as a dependent variable, and the results of the experimental design were analyzed by the general linear models procedure. The result of this analysis was the equation identified by the coefficients listed in Table IV. From this equation the minimum amount of oil (2.3 g, Table IV) would be formed at a MEQRATIO of 20.34:1, a QPDRATIO of 283:1, a carbon monoxide starting pressure of 1618 psig, and a temperature of 130.4 "C. As can be seen from the signs of the square terms, b , (all positive), the surface corresponding to this equation results in a true minimum of the four continuous variables in the absence of acetonitrile. All four variables reach a minimum value within the limits of the design. The MEQRATIO, QPDRATIO, and temperature exhibit the greatest effects on the formation of the oil, while pressure changes exhibit only minor effects. An inspection of the coefficients also indicates that the coefficient describing the effect of the presence or absence of acetonitrile is quite large, with considerably less formation of oil occurring when acetonitrile was present as a cosolvent. The presence of acetonitrile reduced the amount of oil most effectively a t low values of ratio methanol:quinone, at high values pf ratio quinone:palladium(I1)chloride, at high CO starting pressure, and at high reaction temperature. An inspection of the interaction terms for this response indicates significant interactions between the MEQRATIO and the QPDRATIO ( b 2 3 ) , the QPDRATIO and the CO starting pressure (b%),and the CO starting pressure and the temperature (b4&. This can be easily seen from the skewness of the three-dimensional response surface plots of these variables. Several of the response surfaces show large areas of zero oil formation. Although the actual equations developed in the model predict less than zero yields of oil, these values were arbitrarily set to zero. The remaining variables exhibit much smaller interactive effects and thus the three-dimensional plots are more symmetrical about the x and y axes. Calculation of Optimum Conditions for the Dependent Responses Describing the Pressure Drop, the Reaction Time, and the Catalyst Turnover Rate with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 685
Table V. Calculated Saddle Point or Minimum for Dependent Variables molar
dependent
ratio of methanol
variable
to quinone 21.10 33.28 20.34
% oxamide % hydroquinone
g of oil a
SP = saddle point; MIN
=
molar ratio of quinone to cat. 199 353.9 283.78
press. psig
temp, "C
type pointa
1898 1531 1618
142 134 130.4
SP SP MIN
stationary point
corresp yields at these levels, %
response
oxamide
82.5% 80.9% 2.317
74.95 73.97
hydroquinone
89.78 89.21
minimum.
Respect to Oxamide Formation. The equations describing the remaining responses were developed by applying the general linear models regression technique to the measured responses. This analysis led to the coefficients presented in Table IV. An attempt then was made to calculate the optimum conditions for these variables using the same methods as with the other responses. In all three cases the calculations resulted in predicting optimum conditions which were far outside the range of the experimental design. However, the equations developed by substituting the values listed in Table IV for the coefficients in eq 3 do predict these properties within the precision given for the limits of the design. Effect of Acetonitrile as Cosolvent. The introduction of acetonitrile into the reaction system was examined as a discrete variable, i.e., with acetonitrile present or absent. As a result, the interaction terms hold for this variable and are significant only between values of plus one and minus one. We have already seen the effect that this discrete variable has on the responses. In addition, several interesting observations can be made by comparing the changing effects of the addition of acetonitrile on the oxamide yield, the hydroquinone recovery, and the grams of byproduct oil produced. As an example, the interaction of the presence or absence of acetonitrile and the temperature was examined, with respect to the above responses. At low temperatures (120 "C), the addition of acetonitrile to the reaction medium causes an increase in the oxamide yield, a decrease in the formation of byproduct oil, and an increase in the hydroquinone recovery. At high temperatures, the introduction of acetonitrile causes a decrease in oxamide yield, a continued decrease in oil formation, and an increase in hydroquinonerecovery. Also, the response (in the presence of acetonitrile), while increasing the temperature from 120 to 160 "C,indicates that the oxamide yield is lowered, the hydroquinone recovery is increased, and the oil formation is decreased. The decrease in oxamide yield, coupled with the increase in hydroquinone recovery upon the addition of acetonitrile to the reaction medium at higher temperatures, seems to indicate the reaction of the quinone oxidant by some mechanism which does not yield oxalate ester as its product. This mechanism does allow for the efficient reduction of quinone to hydroquinone which suggests that an alternate oxidation is taking place. At low temperatures, the increase in oxamide yield could be the result of increased solubility of the catalyst system, quinone oxidant, or hydroquinone product in the methanol-acetonitrile solvent mixture. These interactions will be studied further when acetonitrile is included as a continuous variable.
Summary In the absence of acetonitrile, the conditions of the four continuous variables for the yield of oxamide, the recovery of hydroquinone, the formation of oil, and the combined responses discussed previously are summarized in Table V for the stationary points in the various five-dimensional surfaces. The stationary point yields and other corresponding yields are indicated. Experimental results, with respect to the presence or absence of acetonitrile, indicate that it may play an important role in minimizing oil formation and maximizing hydroquinone recovery. As a result, we are planning some additional experiments which would involve the presence of acetonitrile as a continuous rather than a discrete variable. Some studies also are indicated for the conversion of oxalate esters to oxamide in the presence of varying concentrations of hydroquinone, methanol, and acetonitrile. Questions concerning the optimum conditions for oxamide formation in the presence of hydroquinone and the fate of the hydroquinone during the ammoniation reaction need to be resolved. Tests also are underway to determine if the dark oils obtained at the end of the reaction may be used as urease or nitrification inhibitors in the soil systems. The major components of these oils possess structures similar to known nitrification and urease inhibitors. The results of these tests will serve to indicate the necessity of reducing the formation of these oils. Equipment redesign also is underway to permit a more accurate means of keeping both the pressure and temperature constant during the course of the reaction. These two important variables may have been confounded by noncontrollable experimental difficulties relating to our equipment. Acknowledgment All lH NMR spectra were run on a Varian EM-360 with the assistance of Dr. T. P. Murray of the University of North Alabama, Florence, AL. Literature Cited Box, 0. E. P.; Wilson, K. B. J . R . Stat. SOC.( 8 ) 1951, 13(1), 1-45. Fenton, D. M.; Stelnwand, P. J. J . Org. Chem. 1974, 39, 701-704. Zehner, L. R. U S . Patent 4005 130, Jan 25, 1977.
Received for review July 15, 1981 Revised manuscript received June 29, 1982 Accepted July 12, 1982
Supplementary Material Available: Tables providing the analytical data, elemental analyses, and X-ray fluorescence results, as well as additional two- and three-dimensional plots of the effects of the coded variables are included (36 pages). Ordering information is given on any current masthead page.
