396
Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 396-403
Synthesis of Dimethyl Carbonate from Methanol, Carbon Monoxide, and Oxygen Catalyzed by Copper Compounds Ugo Romano, Renato Tesel, Marcello Massi Mauri,' and Pierluigl Rebora ASSORENI, Association for Scientific Research of ENI Group Companies, S. Donato Milanese, Italy
The synthesis of dimethyl carbonate from methanol, carbon monoxide, and oxygen catalyzed by cuprous chloride in a slurry reaction system is reported. The synthesis may be carried out in two steps, oxidation and reduction: the influence of the operating parameters on both these steps was studied separately. The results of the one-step
redox system and a tentative reaction scheme are also reported.
Introduction A current trend in the chemical industry is to reduce the risks connected with the use of highly toxic substances. Besides introducing new criteria in the design and operation of plants, various companies are now studying alternative solutions aimed at the substitution of dangerous products with other safer ones. Our laboratories have been involved for several years in a research project aimed at identifying processes which allow the substitution of phosgene and dimethyl sulfate, used as intermediates in several industrial applications. Although it is a highly toxic compound and often leads in its industrial applications to problems in the removal and disposal of polluting byproducts (hydrochloric acid and chlorides), phosgene's characteristic reactivity in carbonylations leads to its wide application in the production of isocyanates, polycarbonates, and various pharmaceutical intermediates. As a methylating agent, dimethyl sulfate is used with phenols and aromatic amines with the same difficulties described above: namely the very dangerous effects of the compound itself plus pollution by the byproduct alkaline sulfates. The methyl derivatives obtained are mainly used in the fields of fine and specialty chemicals (pharmaceuticals, perfumes, stabilizers, etc.). Dimethyl carbonate is an intermediate whose reactivity makes it useful as a substitute for phosgene and dimethyl sulfate in many of the above applications. Under appropriate operating conditions, it is active toward various substrates in carbonylation (Illuminati et al., 1979) and methylation (Merger et al., 1979) reactions. In fact, the potential areas of utilization are numerous, and we have tested their feasibility to a first approximation at least. They cover a large part of the present industrial uses of phosgene and dimethyl sulfate (Table I). The byproducts formed in these reactions are methanol or carbon dioxide; this fact, plus the reduced hazard of using dimethyl carbonate compared with phosgene and dimethyl sulfate, make the use of these technologies very attractive. This interesting reactivity led us to study a synthesis of dimethyl carbonate of possible industrial interest. This paper treats some important aspects of the synthesis of dimethyl carbonate from methanol, carbon monoxide, and oxygen. This synthesis is now under development in our laboratories, since the low costs and wide availability of the raw materials make this production on a large scale reasonably predictable. Background and Selection of Catalytic System Our process is based on the oxidation of carbon monoxide with oxygen in methanol, to form dimethyl carbonate and water as follows 0 196-4321/80/1219-0396$01 .OO/O
(CH30)2CO + H2O (1) The reaction is run in the presence of copper salts. Reaction 1actually occurs in two steps, oxidation and reduction. In the particular case in which cuprous chloride is the catalyst, the first step is the formation of cupric methoxychloride, as follows 2CuCl+ 2CH30H + ' / 2 0 2 2Cu(OCHJCl +H,O (2) The second step is the reduction of cupric methoxy chloride with carbon monoxide to form dimethyl carbonate and regenerate cuprous chloride 2Cu(OCH3)Cl+ CO (CH30)2CO + 2CuC1 (3) The formation of alkyl carbonates by reduction of transition metal compounds (in particular palladium, mercury, and copper) with carbon monoxide in alcohol solution is a well known reaction described by various authors (Fenton, 1963; Graziani et d., 1971; Perrotti and Cipriani, 1971). In the cases of palladium and mercury, however, the reaction does not seem to be selective and involves reduction to the metal, which cannot be reoxidized directly. The reactivity of copper is of more interest since through a redox process with oxygen and carbon monoxide it gives a system which functions catalytically. However, reduction with carbon monoxide in alcohols of bivalent copper salts (cupric chloride, for example) does not usually show high selectivity. Rather, large quantities of byproducts are formed. In the case of methanol, we have identified these as methyl ether and methyl chloride; for the heavier alcohols, they are the corresponding olefins and alkyl halides (Nefedov et al., 1973). On the other hand, our reaction system consists of the reduction of a cupric compound obtained by oxidizing a cuprous compound in methanol (Romano et al., 1978). In this way the secondary reactions described above are completely avoided, so that the formation of dimethyl carbonate from methanol is almost completely selective. A comparison of reduction tests performed on cupric chloride and cupric methoxychloride under the same reaction conditions (Table 11) shows that the reactivity of the two systems is basically different. When bivalent copper compounds are reduced at different chlorine/copper ratios (Figure l),the maximum yield of dimethyl carbonate is obtained for a ratio of 1. Copper reduction is not complete for lower values, but dimethyl carbonate formation from methanol is completely selective. For higher values, on the other hand, both the yield and selectivity on methanol are reduced, due to the concurrent formation of dimethyl ether and methyl chloride. In this case, a progressive pH decrease was demonstrated in the solution during reduction due to free hydrogen chloride formation. This implies that methanol 2CH30H
+ CO + ' / 2 0 2
-+
+
-
0 1980 American
Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 397
Table I. The Possible Uses of Dimethyl Carbonate as Intermediate reactants
products
end uses
phenols
phenolic ethers diphenyl carbonate
ammines-aliphatic ammines-aromatic alcohols
carbamates alkyl anilines diethyl carbonate other alkyl carbonates aliphatics polyols polycarbonates
glycols
flavoring agents, food antioxidants, pharmaceuticals intermediates intermediate for: aromatic polycarbonate (high performance thermoplastic resins) m o n o isocyanates (pharmaceuticals and pesticides intermediates) diisocyanates (monomers for polyurethanes elastomers) carbamates (pesticides) pesticides dyes intermediates, solvent stabilizers solvent for lacquers, varnishes and insecticides solvents intermediates for polyurethanes lubricant additives
Table 11. Effect of the Cupric Species o n the Reduction Selectivitya ~
molar yield o n methanol, % copper compound CUCl, CuOCH,CI
dimethyl ether
methyl chloride
dime thy1 carbonate
molar yield o n Cuz+,% dimethyl carbonate
12
10
1.5 9.6
15 96
Conditions: Pco = 60 a t m ; T = 100 "C; time = 8 h ; Cu2+= 0 . 0 7 5 mol; CH,OH = 0.75 mol.
The development of this catalytic system was based on previous work in our laboratories involving an in-depth study of the catalytic characteristics of systems consisting of copper complexes with basic organic ligands (Perrotti and Cipriani, 1971). The removal of these ligands allowed us to develop a new system, with better prospects for industrial realization (Romano et al., 1979),on which we have concentrated further development efforts. With the aim of better understanding the chemistry of the reaction system, our preliminary research involved studying the oxidation and reduction steps separately. Figure 1. Effect of CI/Cu ratio on the reduction selectivity. Conditions: Cu2+initial concentration 1.68 mol/L, 90 "C, 50 atm. Cl/Cu ratio wm varied mixing appropriate amounts of CuClz and Cu(OH)~-CUCO~: 0,2DMC/CuZ+; 0 , 2DMC/(2DMC + 2DME + MC). DMC = dimethyl carbonate; DME = dimethyl ether; MC = methyl chloride.
undergoes substitution and etherification equilibrium reactions.
Experimental Section The apparatus used in the tests of both reduction and redox is illustrated in Figure 2. The reactor was a 6-L enamelled autoclave (4 = 17.5 cm) equipped with a variable speed (up to 1300 rpm) impeller type mechanical stirrer. Except for tests designed to evaluate the effect of the stirring speed, all tests were conducted with a stirring speed of 650 rpm.
Figure 2. Experimental apparatus: V1, V2, V3 = flow regulation valves; Al, A2, A3 = CO,, CO, O2 analyzers; S = volumetric gauge.
398
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
g
o
o
0
g
-
;
,
.
p
*
;
g
Gws=
X
Figure 3. ESR of Cu(OCHJC1 dispersion in methanol obtained (a) by metathetical reaction, (b) by CuCl oxidation.
The methanol, carbon monoxide, and cuprous chloride were commercial products used with no further purification. The kinetics of the reduction reaction were determined with dispersions of cupric methoxychloride prepared by oxidizing cuprous chloride in methanol at 70 "C under 8 atm of oxygen. After the oxidation, the reactor was cleared of residual oxygen with nitrogen and brought to the test temperature. Then without stirring and with valve V3 closed, it was pressurized with CO up to the desired value. The test was initiated when stirring was begun, and the absorbed CO was replaced using flow regulation valve V1, whose opening was controlled by pressure regulator PRC. Carbon monoxide flows were measured and recorded automatically by the FRC 1 instrument. The volume of CO absorbed was calculated by subtracting that due to physical absorption, as determined from measurements in pure methanol. The redox tests were conducted with a cuprous chloride dispersion in methanol bringing it to temperature under CO pressure. The gas was continually added at a preestablished rate through valve V1, controlled by the FRC 1 regulator. The gas was continually discharged at constant pressure through valve V3, controlled by the PRC regulator. The redox reaction was initiated by the continuous addition of oxygen through valve V2, controlled by flow regulator FRC2. The entering CO and O2 flow rates were continuously recorded. The discharged gas was passed through CO, COz, and O2 analyzers, and the flow was determined using volumetric gauge S. In both reduction and redox tests, the composition of the liquid was determined at various times using GLC. Toward this end, a 5-m steel separation column (4i = 4 mm) was used filled with 15% Carbowax 1500 on Teflon; temperature was 120 "C and helium was the carrier gas (3600 cm3/h) . Oxidation Step Extensive literature has been devoted to the oxidation of cuprous chloride; in methanol it occurs very quickly to give cupric methoxychloride (Finkbeiner et al., 1966). The product thus prepared differs slightly from that synthesized by metathetical reaction between lithium methoxide and cupric chloride (Brubaker and Wicholas, 1965). The latter shows an ESR spectrum (Figure 3a) consisting of a much enlarged signal characteristic of a product with relaxation times shortened by a ligand field of elevated symmetry, in agreement with the polymeric structure
Figure 4. Effect of pressure on the carbon monoxide consumptions in the reduction of Cu(OCH3)Cl (1.61 mol/L) a t 75 "C. Conditions: X, 20 atm; 0,15 atm; 0 , 10 atm. Dotted line = stoichiometric CO.
proposed for this compound. On the other hand, the product prepared by oxidation shows, in the methanol suspension in, which it was prepared, a spectrum (Figure 3b) with a resolved signal typical of the cupric ion in solution visible over the enlarged signal peculiar to the polymeric species. Such behavior has been ascribed to the fact that the water formed in the oxidation reaction interacts with the methoxychloride to form more soluble cupric species. This assumption is confirmed by the fact that addition of increasing water quantities to the dispersion affords a progressive increase in the intensity of the resolved part of the spectrum. The most important products obtained from methoxychloride dispersions in aqueous methanol as characterized by X-ray diffraction analysis are s o l y
CCUOCH~CI),
+
(
CuCI2* 2H20 2CuC12* CdOH)2
yH2O
CUCI2*3CdOH)2 2CuCI2* ~ C U ( O H )H~2* 0
The dispersed portion of the catalyst was isolated by simple filtration of the slurry, while the dissolved portion was obtained by total evaporation of the solution. The water therefore leads to substitution of the methoxy with hydroxy in the methoxychloride, and the shift of the halogen ligand causes the formation of species of higher halogenation degree in solution and lower halogenation degree in the solid phase. Reduction Step As determined from the absorption of carbon monoxide, the kinetics of the reduction of cupric methoxychloride in methanol are characterized by the presence of an induction period (Figure 4). At constant temperature, this period increases in length as the pressure is lowered. Under certain conditions, the total quantity of carbon monoxide adsorbed may exceed the stoichiometric amount indicated by the dashed line. This same behavior is seen as a function of temperature (Figure 5): at constant pressure, a temperature increase leads to a sensible reduction of the induction step and of the quantity of absorbed carbon monoxide. The rate of formation of dimethyl carbonate is also characterized by an induction phase, after which the carbonate concentration increases linearly with time until the cupric methoxychloride is almost completely reduced (Figure 6). At constant temperature, pressure has within
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19,
No. 3,
1980
399
OMC (mow\.lo2I
80. 7060
"1
i'"
50
0
60
180
120
240
/
t (rninl
d
Figure 5. Effect of temperature on the carbon monoxide consumption in the reduction of Cu(OCH3)Cl (1.61 mol/L) a t 20 atm. Conditions: X, 75 "C; 0 , 90 "C; 0,110 "C. Dotted line = stoichiometric CO.
0 0
30
60
90
120
t.(rnin)
Figure 8. Effect of copper concentration on DMC formation in the reduction of Cu(OCH3)Cl a t 110 "C and 20 atm. Cu(OCH,)Cl concentration: 0,2.23 mol/L; 0,1.68 mol/L; A, 1.12 mol/L. r O M C lM o l / h . l l
oJ.&y
I
,
0
30
-
60
,
90
120
150
180
210
t (min)
Figure 6. Effect of pressure on the DMC formation in the reduction of Cu(OCH3)Cl (1.68 mol/L) a t 90 "C. Conditions: 0 , 7.6 atm; A, 17.6 atm; 0,22.6 atm; 0, 29.6 atm. BO
I
DMC (mol/t.ld)
60.
50
1
Figure 7. Effect of temperature on the DMC formation in the reduction of Cu(OCH,)Cl (1.68 mol/L) a t 15.5 atm. Conditions: A, 90 "C; 0, 110 "C; 0,135 "C.
certain limits a positive effect on the reaction, both decreasing induction time and increasing the rate of the linear section. However, above a certain value the effect of pressure drops off, a t least for the lowest reaction temperatures. Temperature variations at constant carbon monoxide pressure (Figure 7) have similar effects both on the in-
Figure 9. Effect of pressure on the DMC formation rate a t Cu(0CH3)C1concentration 1.68 mol/L. Conditions: X, 85 "C; 0,90 "C; 0 , 95 " c ; 0, 110 " c ; A, 135 "c.
duction and the subsequent steps. The fact that reaction rate after the induction step is independent of the bivalent copper concentration (Figure 8) is proven also by tests carried out at various initial copper quantities. At constant temperature, the rate of formation of dimethyl carbonate in the post-induction period is within certain limits proportional to the carbon monoxide pressure (Figure 9). Under the conditions where this relationship holds, the rate of formation of dimethyl carbonate may be expressed as follows =
(4) At the lowest temperatures Figure 9 shows again that above a certain value the DMC formation rate is independent from the CO pressure. An Arrhenius plot of the k values calculated in the region where (4)is valid (Figure 10) gives an apparent activation rDMC
k'PC0
Ind. Eng. Chem. Prod. Res.
400
Dev., Vol.
19, No. 3, 1980
InK I M o l I h l Atm)
C mol/!
2 52
2 42
2 62
272
107T
K-'282
Figure 10. Arrhenius plot. Cu(OCH3)C1concentration 1.68 mol/L.
401
20.
/
// P
0
0
30
60
90
120
150
t (mln)
Figure 11. Effect of the added water on the DMC formation at 110 "C, 15.5 atm, and Cu(OCH3)CI concentration 1.68 mol/L. Water concentration: 0 , 0.84 mol/L; X, 3.14 mol/L; A, 5.63 mol/L.
energy of 8.0 kcal/mol. This rather low value does not seem to be due to the existence of limiting diffusion phenomena since it was shown that under the experimental conditions used the stirring speed had no effect on the reaction rate. Under these reaction conditions, none of the principal operating variables (pressure, temperature, quantity of copper compound) had any effect on the selectivity of dimethyl carbonate formation from methanol; it remained almost total. The selectivity based on carbon monoxide was also high (approximately go%), although it decreased a t higher temperatures (approximately 80% a t 135 "C). The carbon monoxide which does not form dimethyl carbonate is oxidized to COz. In the redox catalytic system preferred for a continuous process, water is formed from the oxidation of cuprous chloride. The effect of its accumulation on reduction behavior was therefore checked. The curves obtained with the addition of water (Figure 11) show the presence of an induction step here as well. The reduction rate decreases at constant temperature as the quantity of water added is increased. The carbon monoxide selectivity to dimethyl carbonate also decreases as water content in the system is increased, while the quantity of carbon dioxide produced increases. The CO selectivity decreases to a minimum of approximately 60% when the added water reaches a concentration of 10% (by weight) of the methanol. Furthermore, at high
Figure 12. Effect of oxygen flow rate on the DMC formation in the redox system at 94 "C, 20 atm. CO flow rate 2.1 mol/L h and CuCl concentration 1.68 mol/L. 0,0.93 mol of o z / L h; A, 0.69 mol of Oz/L h; +, 0.62 mol of Oz/L h; 0,0.25 mol of Oz/L h.
water concentrations, the order of the reaction with respect to bivalent copper in the post-induction period is approximately 1 instead of 0. Redox System On the basis of the data from the study of the two separate reactions, development of the dimethyl carbonate synthesis process was continued by studying the single-step oxidation-reduction. One significant aspect of this system was that, starting from cuprous chloride, there was no longer any induction period. Also, the concentration of dimethyl carbonate increased over time as a function of the oxygen feed rate (Figure 12). Under the reaction conditions used, the rate of formation of dimethyl carbonate was independent of the initial concentration of cuprous chloride, of the temperature, and of the pressure. It increased linearly as a function of the oxygen feed rate (Figure 13). Within the flow rate limits used (0.2-1.4 mol/L h), oxygen consumption was almost complete. The purged gas essentially contained excess carbon monoxide, C 0 2 formed in the reaction and 0.2-0.4% v/v of oxygen. This should be attributed to the high rate of the oxidation reaction and to the absence of any gas-liquid diffusive phenomena to limit the O2transfer process. It is reasonable to assume that this would not be true under different conditions and that the rate of formation of dimethyl carbonate would no longer depend solely on the oxygen flow rate. For example, this would happen if, again in the absence of phenomena limiting oxygen transfer, ita flow rate exceeded the intrinsic rate of reduction under the same conditions. Another condition under which the behavior would differ is when the rate of oxidation is noticeably decreased. We have reached this last condition by lowering the reaction temperature. In this way, at 70 "C oxygen is recorded in the exit gas only a few minutes after it began to flow. The fact that the system was investigated under kinetically degenerate conditions may mean that the overall results are of little relevance. In reality, however, this behavior is quite important both because of the wide range of operating conditions in which it occurs and because of its striking effect on aspects directly related to development of an industrial process. Since the oxygen is totally consumed in the reaction, the exit gas from the reaction vessel is never an explosive mixture even if the ratio of the two feed gases is well within
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
o'81
401
o.8i
0.4
0 0
90
110
130
1.C
C
d
1.2- 0
-x
- 0 %
,8
0
0
0
0.8-
0.4-
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.
.
.
.
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,
the explosive range. This fact is of particular significance with regard to the reliability and safety of the redox reactor in addition to making the operation extremely flexible. With regard to the selectivity of the catalytic system, it was confirmed that under redox conditions methanol is not oxidized except to an absolutely negligible degree. Therefore the quantity of water formed corresponds to the quantity of dimethyl carbonate produced, according to the indicated stoichiometry. COz, however, is produced at the expense of carbon monoxide. This is also supported by the fact that during the trials run with alternating cycles of oxidation and reduction, it was determined that the COz is formed only during the reduction phase. Figure 14 shows the selectivity of CO to dimethyl carbonate as a function of time and confirms that it decreases, with the water concentration increase, as the reaction proceeds. The appearance of a reaction competitive with the formation of carbonate as water accumulates is consistent with observations (Byerley and Peters, 1968) of the oxidation of carbon monoxide to dioxide by cupric ions in aqueous solution. In addition to the above-mentioned effects on selectivity, as the reaction proceeds the accumulation of water further modifies the system. Due to interaction with the water, the oxidized copper is no longer present as the methoxychloride, but rather in a series of compounds of various degrees of halogenation in equilibrium between the solution and solid phase. Upon separating the dissolved portion of the catalyst from that forming the solid phase, the first when subjected to reduction showed the typical behavior of species with Cl/Cu >1and the second reacted as a Cl/Cu
