1030
Ind. Eng. Chem. Res. 1993,32, 1030-1034
Conversion of Methane to Propanal at Ambient Pressure Malcolm L. H.Green,' S h i k Chi Tsang, Patrick
D.F. Vernon, and Andrew P. E. York
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K.
A continuous process is described using three catalysts in sequence for the synthesis of the C3 oxygenate propanal, from methane and air, a t 1atm in 13.2% yield. The three catalytic reactions are the methane oxidative coupling to ethene, methane partial oxygenation to synthesis gas, and the hydroformylation of ethene. The combined process implies that a higher yield of propanal can be obtained than for the formation of ethene from the oxidative coupling reaction alone. We have examined the effect of reaction conditions on the yield of propanal. Introduction Abundant reserves of methane are available; however, most of these reservoirs are situated in remote areas and the cost of compression, storage, and transportation make methane an unattractive proposition as a fuel source (Parkyns, 1988). Currently the most economically viable, operating process for the utilization of methane is that of steam reforming to give synthesis gas (Rostrup-Nielsen, 1984). This synthesis gas can then be used in methanol synthesis or Fischer-Tropsch synthesis to hydrocarbons. These routes are, however, highly capital intensive and require pressures of at least 20 atm. An alternative to using synthesis gas in the production of chemicals is to directly utilize methane, and a large number of research groups worldwide have been concentrating on this problem. Direct conversion of methane to methanol is the major industrial objective (Foster, 1985; Hunter et al., 1984). However,the methanol yield and selectivityreported from the previous studies is very low (Foster, 1985). Recently, a report has claimed that a high yield and selectivity of methanol can be produced from a homogeneous gas-phase reaction of methane and oxygen at elevated pressure (Hunter et al., 1984), although this has proved to be difficultto reproduce (Burchet al., 1989a;Rytz and Baiker, 1991) and is still unable to compete with the present methanol synthesis via synthesis gas. Direct formation of formaldehyde from the reaction of methane and oxygen has also been investigated, and a large number of catalysts have been screened (Baldwin et al., 1991). However, the maximum reported yield is only 3.5 % ,which again is too low for industrial requirements. Much of the research has been concerned with the methane oxidativecoupling reaction to ethane and ethene (Kellerand Bhasin, 1982;Lee and Oyama, 1988;Lunsford, 1990),and high yields of Cp products have been obtained, with reports of improved catalysta appearing regularly in the literature. Although this reaction appears to be more promising, a practical limit, for ethene, of about 2+25 % yield seemsto have been reached without excessivedilution or adding chloride promoter (Labinger, 1988;Burch et al., 1989b). This is due to the products, ethene and ethane, being easier to oxidize than methane and are, therefore, destroyed at the rate competing with their formation. In addition, ethene is a gas at standard temperature and pressure, and therefore, the cost of separation and transportation should be considered if a process is to be adopted. In our laboratory, we have shown that partial oxygenation of methane with air to give synthesis gas in greater
* To whom all correspondence should be addressed.
than 90% yield at atmospheric pressure can be achieved (Ashcroft et al., 1990). Also, hydroformylation, the reaction of ethene with synthesis gas, can occur at 1atm, over a RhH(CO)(PPh3)3catalyst to give propanal. In this paper we report the combined process of the oxidative coupling and partial oxygenation of methane leading to the hydroformylationof ethene to give propanal in 13% yield at atmospheric pressure. A preliminary account of this work has been published (Green et al., 1992). Experimental Procedure The oxidative coupling catalysts, K/BaC03, were prepared by an incipient wetness impregnation method and then calcined in air for 8 h at 860 OC, and the Rh/Alp03 catalyst was prepared via a vacuum incipient wetness technique, from RhC13 and then reduced under flowing Hp at 860 "C for 24 h. The hydroformylation catalyst was prepared as described (Ahmad et al., 1974). The activity of the catalyst is defined as percent conversion of hydrocarbon into all products per mole of carbon basis. The selectivity to product is calculated as follows:
Si= [(moles of the carbon in the hydrocarbon converted to product i)/(moles of carbon in the hydrocarbon converted to all products)] X 100% The activity and selectivity of a catalyst for the partial oxygenation of methane were determined using a quartz tube of 4-mm i.d. and 40-cm length, containing 50 mg of a RWAl203 catalyst. The catalyst for the oxidative coupling of methane to ethene was 0.5 g of 2 mol % K/BaC03 catalyst also in a quartz tube. The arrangement for the combined process is shown in Figure 1. The partial oxygenation and the oxidative coupling reaction were carried out in the same Heraeus tube furnace at 860 "C. A methane to air ratio of 2 5 was used, and the flow rates of methane and air were controlled by Brooke 8744A flow controllers. The input gas stream was split to give 30 mL/min through the oxidative coupling and 6 mL/min to the partial oxygenation catalyst in the combined process. After these catalysts the product gas streams were recombined and passed through a Dreschel bottle containing diethylenetriamine. NH(CH2CH2NHp12, which effectivelyremovedthe Cop and HpO. The exit gases were passed through a cold trap at -78 "C to remove any remaining H20 or amine, before passing into the hydroformylationreactor. This compriseda 50-cmvertical glass column with a sintered frit at the bottom which dispersed the entering gases into fine bubbles. This column contained 2 g of the catalyst RhH(CO)(PPh3)3in 20 mL of di-n-butyl phthalate, and was heated to 90 "C.
0S8S5885/93/2632-~03Q~Q4.O0/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1031 Table I1 (a) Partial Oxygenation: Catalyst Amount = 50 mg; Total Flow Rate = 20 mL/min; C&/air = 2/5; Temperature = 777 OC product selectivity/% CZH~/CZH~ catalyst CH~conv/% CO Hz coz 3.96 4.46 1 % W/WRhIAlzOs 94 97 99 3 3.34 CRG'F' Ni/Al203" 94 97 99 3
Table I. Oxidative Coupling: Temperature = 900 OC; Total Flow Rate = 30 mL/min; CHd/air = 2/5 mtof CH4 catalyst catal/g conv/ % BaC03 1.0 39.31 1.0 46.26 2mol 5% K/BaC03 1.0 33.87 5mol % K/BaC03
CZyield/ % 8.97 18.21 14.71
The exit tube from the reactor was heated to 180 OC to prevent condensation of the propanal. The product gas analysis was carried out on a HewlettPackard HP5890II gas chromatograph fitted with a methanator before the flame ionization detector and separation was achieved on a 3-m Porapak Q column. The response factors for the different gases were determined and taken into account. A Hewlett-Packard HP5971A mass spectrometer detector was also used to determine the identity of the products. Confirmation of the process was achieved by trapping the liquid product over an 18-hperiod followed by analysis using IH NMR spectroscopy. Methane was supplied by Union Carbide plc, ethene was supplied by BOC plc, and all the other gases used were supplied by Air Products plc. They were all greater than 99% purity. Results Before the three processes were combined, we studied each stage of the reaction separately in order to optimize the performance. The stability of the catalysts has also been considered. The three reactions, and then the combined process, are discussed in turn. 1. Oxidative Coupling. The results for the catalysts tested for the oxidative coupling reaction are shown in Table I. Potassium-promoted BaC03 catalysts were chosen and were used at 900 "C, with a total flow rate of 30 mL/min and a CH4:air ratio of 2:5. From Table I, it can be seen that pure BaC03 gives a yield of C2 products of 8.97% under the reaction conditions. Addition of potassium to the barium carbonate gave a significant increase in the C2 yield. The effect of adding alkali metal to increase the yield and selectivity to C2 products is wellknown for the oxidative couplingreaction (Leeand Oyama, 1988;Lunsford, 1990). The highest C2 yield was obtained using the 2 mol % potassium-promoted BaC03 catalyst. This gave a CH4 conversion of 46.26% and a C2 yield of 18.21% ,with a C&/C& ratio of 4.46 (also the highest yield of ethene). These results agree with previous reports (Aika et al., 1986) in which it was found that 2 mol % potassium promotion is the optimium loading. Table I showsthat a 5 % loading of potassium gave a lower methane conversion and slightly lower ethene/ethane ratio. Presumably this high loading with potassium causes most of the active surface to be covered. Potassium-promoted BaC03 catalysts were chosen because they give a relativelyhigh ethene yield and are stable over a long period of time (Aika et al., 1986). Many oxidative coupling catalysts which give a higher ethene yield than our chosen catalyst are short lived, due to, for example, heavy loading with a lithium salt which leaches out of the catalyst at the high reaction temperature. Chloride-promoted catalysts have also been reported as very good couplingcatalysts (Burchet al., 198913);however, they deactivate due to hydrolysis of chloride to HC1. It must, therefore,be noted that in this work the best coupling catalyst, in terms of C2 yield, has not been used in preference for a more industrially practical catalyst. No
(b) COz Reforming: Catalyst Amount = 50 mg; Total Flow Rate = 20 mL/min;CHJCOz = 1; Temperature = 777 OC reactant conv/ 7% % yield catalyst CH4 coz H2 CO 1 % WW / Rh/Az03 86 88 85 87 CRG'F' Ni/Alz03" 88 81 88 85 CRG'F' Ni/AlzO3 reforming catalyst, from British Gas.
noticeable deactivation was detected for the K/BaCOS catalyst after it had been operating for 48 h. 2. Partial Oxygenation. The results from the partial oxygenation experiments are shown in Table IIa. The catalyst was 1%w/w Rh/Al203 at 777 OC. A total flow rate of 20 mL/min was used, at a CH4:air ratio of 25. The CH4 conversionobtained under these conditionswas 94 % , with selectivities to CO and H2 of 97% and 99%, respectively. A similar result was obtained when we used British Gas CRG'F' Ni catalyst. We have previously reported a number of supported metal catalysts which can catalyzethis reaction, for example,Ru, Ir, Pt, and Pd, giving similar results to those with supported Rh (Ashcroft et al., 1990). The prime differencebetween supported Rh and CRG'F' Ni catalyst is that this supported nickel catalyst deactivates very quickly due to the formation of a large amount of carbon (>40% w/w within 5 h) (Ashcroft et al., personal communication). As a result, we have chosen the supported Rh catalyst with which no deactivation was observed within 48 h. Table IIb shows that supported Rh and Ni catalysts can also catalyze the formation of synthesis gas from methane and carbon dioxide under similar reaction conditions. More than 80% yield of synthesis gas can be formed from a methane:carbon dioxide ratio of 1,and this dry carbon dioxide reforming has been previously studied (Richardson and Paripatyadar, 1990). 3. Hydroformylation. Table IIIa showsthe data using cylinder sources of reactants for the hydroformylation reaction in an attempt to optimize this reaction and, also, to examinethe reactivityof the rhodium hydroformylation catalyst toward other gases. The hydroformylation of ethene was found to occur with very high selectivity to propanal (about 99%),with only a small amount of the hydrogenation product ethane being detected. The data in Table IIIa show that when the ratio of reactants was constant, varying the flow rate of added diluent, e.g., nitrogen, caused the conversion to drop very rapidly, but that the selectivities to propanal and ethane remained unchanged. It is also shown that addednitrogen, methane, carbon dioxide, and ethane are all inert, whereas, added oxygen deactivates the catalyst readily at the operating temperature. Therefore, if a combined process is going to work, complete oxygen conversion must be achieved in the earlier oxidative coupling or partial oxygenation processes. Table IIIb shows the effect of reactant composition on the hydroformylation reaction. By increasing either the amount of CO or Hz or both CO/H2 relative to ethene, ethene conversion would increase, but in most cases, more than 98% selectivities to propanal were obtained. It is interesting to find that when the CzH4:CO:Hz ratio was
1032 Ind. Eng. Chem. Res., Vol. 32,No. 6,1993 Table I11 (a) Ethene Hydroformylation: T = 115 OC; 1 atm; Catalvst = RhH(CO)(PPhdn: . .. -. -. Solvent = Di-n-butyl Phthalate reactant product concn of ratio total selectivity/ % c a w CzHd:CO:H2: flow rate C2H4 (g/mL) (dil) (mL/min) conv/ % CzHsCHO C2H6 0 0 1:22:0 15 65.6 99.1 0.9 15 0.008 1:220 34.1 98.9 1.1 30 0.008 1:22:5(N2) 21.4 99.0 1.0 45 0.008 1:2:2:10(N2) 15.8 99.0 1.0 60 0.008 1:2:2:15(N2) 10.5 98.9 1.1 90 0.008 1:2:2:25(N2) 45 21.1 99.1 0.9 0.008 1:2210(CH4) 3.8 96.0 4.0 45 0.008 1:22:10(02) ~
(b) Effect of Reactant Composition: T = 115 "C; 1 atm; Catalyst = RhH(CO)(PPh&; Solvent = Di-n-butyl Phthalate; Catalyst Concentration = 0.005 g/mL; Total Flow Rate = 15 mL/min reactant ratio selectivity/ % C2Hd:COHz C2H4 conv/ % C2HsCHO C2H6 1:l:l 33.76 98.0 2.0 1:1:2 48.99 87.5 12.5 1:2:1 40.98 99.1 0.9 1:2:2 56.58 99.0 1.0 1:4:1 56.00 99.0 1.0 1:4:4 71.46 98.6 1.3 (c) Catalyst Concentration Effect: T = 115 OC; 1 atm; Catalyst = RhH(CO)(PPh3)3;Solvent = Di-n-butyl Phthalae Total Flow Rate = 15 mL/min; CzH4:CO:Hz = 1:22 selectivity/ % catal concn/(dmL) CZH4 conv/% CzHaCHO CzHs 0 0 0.005 56.58 99.0 1.0 0.008 65.60 99.1 0.9 0.014 88.20 98.8 1.2 0.046 99.20 98.2 1.8
(d) Effect of Temperatureon the HydroformylationReaction: Catalyst = RhH(CO)(PPh&; Solvent = Di-n-butyl Phthalate; Total Flow Rate = 30 mL/min; CZ&/CO/HZ= 1:2:2; Catalyst Concentration = 0.025 g/mL selectivity/ % T/"C 25 90 115"
ethene conv/ % 3.40 71.34 75.70
CzHsCHO 100 99.5 98.4
CZHS 0.5 1.6
Trace of benzene detected.
1:1:2, selectivity to propanal dropped to 87.5% with an increase in ethane selectivity to 12.5%, and presumably a higher amount of hydrogen relative to ethene and carbon monoxide will enhance formation of the hydrogenation product. The data in Table IIIc show the effect of catalyst concentration on hydroformylation. The ethene conversion increasessignificantlywhen the catalyst concentration is increased from 0.005 to 0.014 g/mL, while at higher catalyst concentrations there is only a slight increase in ethane selectivity. The temperature dependence of the hydroformylation reaction is reported in Table IIId. The homogeneous rhodium catalyst was so active that it gave 3.4% ethene conversionwith 100% selectivity to propanal even at room temperature. Increasing the temperature gave higher ethene conversion but slightly less selectivityto propanal. At 115 "C traces of benzene were detected, due to the degradation of the rhodium catalyst (Pruett, 1986). Therefore, we have chosen 90 OC to be the reaction temperature in the hydroformylation stage in order to
trap
To
1
GC and trap CzHaCHO
Figure 1. Schematic diagram of apparatus used in the combined process.
prolong the lifetime of the catalyst and decrease the selectivity to ethane. The above process is able to operate at the pressure of 1atm since ethene is very reactive and, for example, has a hydroformylation rate almost 3 times that for propene (Pruett, 1979). 4. The Combined Process. The apparatus used for the combinedprocess is shown in Figure 1. Table IV shows the reactant conversionsand product selectivitiesfor each stage in the process, as well as for the overall reaction. Over the oxidative couplingcatalyst the oxygen conversion was essentially complete (>99.9% and the conversion of methane was 40% ,at a selectivityto ethene of 36%, giving a total ethene yield of 14.4% . The ethene yield was lower in the combined process than in the separate optimization studies due to a small back pressure (about 5 psi) built up between the oxidative coupling reactor and the hydroformylationcolumn. It is known that lower CZyields occur in the oxidative coupling reaction at elevated pressures (Ekstrom et al., 1990). The partial oxygenation catalyst, again,gave essentially complete oxygen conversion with a methane conversion of 97% and selectivitiesto CO and Hz of 99%. Finally, in the hydroformylation stage 76% of the ethene was converted, along with 23% of the CO and 12% of the H2 to give propanal and a small amount of ethane. The yield of propanal from ethene in this stage was 73%. Overall, 49.2% of the CH4 was converted, with a selectivity to propanal of 26.7%, corresponding to an overall yield of 13.2% per single pass. The product yield of propanalwas confirmed by trapping the reaction product over an 18-h period followed by analysis using 'H NMR spectroscopy. This showed the product was essentiallypure propanal together with traces of benzene arising from slight catalyst degradation and confirmed the yield of propanal, based on methane, was 13.2% . Discussion The choice of the partial oxygenation route to synthesis gas rather than the industrial high-pressure steam re-
forming process was made since the former reaction is most thermodynamically advantageous at 1 atm and operates under conditions similar to those of the oxidative coupling reaction. The rhodium catalyst was preferred for the partial oxygenation since it appears not to catalyze the carbon formation reactions, either via methane decomposition (1) or the Boudouard reaction (2). The carbon-formingreactions are thermodynamicallyfavorable under our reaction conditions, and indeed when a sup-
1034 Ind. Eng. Chem. Res., Vol. 32,No. 6,1993
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