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Nonoxidative methane conversion into higher hydrocarbons
H.E.Cuny-Hyde and R.F. Howe (Editors), Natural Gas Conversion !I 0 1994 Elsevier Science B.V. All rights reserved.
497
Nonoxidative methane conversion into higher hydrocarbons V.D. Sokolovskii (l),S.S.Shepelev (2)
1) 2)
Chemistry Department, University of Witwatersrand, Johannesburg, Wits 2050 , RSA. Institute of Catalysis RAN, 630090,Novosibirsk, Russia (*)
Nonoxidative methane conversion into higher C', hydrocarbons with and without catalysts and initiators has been investigated. Under relatively mild conditions, when just a low methane conversion occurs heterogeneous catalysts and homogeneous initiators accelerate the methane transformation into higher hydrocarbons but reduce selectivity due to a coke formation. At more severe conditions providing an essential conversion of methane by the gas phase reaction catalysts and initiators has not affected on the methane conversion and only result in the selectivity decrease. The dependence of the C,' hydrocarbons yield on contact time at different temperatures has been studied. It was shown by a thorough selection of temperature - contact time conditions the yield of C,' hydrocarbons can be achieved as higher as 25 - 28% at selectivity 60 - 70 %. 1,
INTRODUCTION
The direct conversion of methane to higher C,' hydrocarbons is a process of an immense practical importance [I]. A number of ways of the such conversion are known: the direct oxidation of methane to methanol [2] with a following transformationto higher hydrocarbons by the MTG process; oxidative oligomerization of methane to aromatic compounds [3] ; methane oxidative coupling to C, hydrocarbons [4], which can be converted to liquid hydrocarbons (ARCO process). The yield in the ARCO process is about 20 % per pass, which was claimed to be near of practical demands [5]. Methane can be transformed into higher C,+ hydrocarbons by a conversion without oxidant too. This process has definite advantages in a comparison to the oxidative methane conversion, namely the less complicated technology and the absence of a dilution of gaseous products with carbon oxides and nitrogen ( in the case air use in an oxidative process). The nonoxidative conversion by-product hydrogen - can be isolated and used separately. However the yield of higher hydrocarbons was achieved in this process with or without catalysts is rather low (about 8-18% per pass [6,7]).
* Present address: Department of Chemistry, University of Texas at Dallas, TX 75083-
0688,USA
498
The present work is devoted to the investigation of regularities of the methane nonoxidative conversion in the presence and in the absence of heterogeneous catalysts and homogeneous initiators. 2.
EXPERIMENTAL
The nonoxidative methane conversion was carried out in flow quartz reactors. Reactors 100, 30, 3 and 1.9 cm3in volume were used. Methane of 99.9 % purity was passed through the reactor with the volume space velocity of 0.1 - 2.0 cm3 / sec. The reaction temperature was varied in the range 800 1230 "C. Alumina, magnesia and silica used as catalysts were pelleted, crushed and the fraction 0.5 - 0.25 mm was taken for experiments. These catalysts have been calcined in air at 1100 " C before experiments, their surface area was in the range of 1 - 5 m2 / g. GC analysis of reaction products was performed with the use of column filled with Porapak-Q and glass capillary column with SE-30 liquid phase. Analysis of carbon deposits on the reactor walls performed by burning down these deposits has been shown shows the deposits to consist of carbon and practically do not contain hydrogen.
-
3.
RESULTS AND DISCUSSION.
An appreciable methane conversion in a quartz reactor without catalysts and initiators begins at temperature above 850 C. Ethane, ethylene, benzene and other higher hydrocarbons,as well as hydrogen have been discovered in reaction products. Carbonaceous deposits were formed on the reaction walls. The product composition of the methane thermal conversion in the empty reactor at 1000 "C and the reaction mixture contact time 60 s. are presented in Table 1. Table 1. Product composition of methane thermal conversion at 1000 " C and contact time 60 s. in 30 cm3 quartz reactor. substance
concentration vol. %
substance
concentration VOI. %
methane ethane ethylene + acetylene benzene toluene
69.50 0.156
aromatic C, aromatic C,+ naphthalene methylnaphthalene dimethylnaphthalene hydrogen
0.001 0.002 0.082 0.001 0.002 28.07
1.775 0.401 0.007
methane conversion hydrocarbons selectivty
19.4 % 36.4 %
3.1. The influence of catalysts and initiators in the reaction of methane nonoxidative conversion. Several catalysts and gaseous initiators have been tested in this process, the results being presented in Table 2.
499
Table 2 Effect of catalysts and initiators on the nonoxidative methane conversion. T= 1000 'C, methane feed 0.5 ml/s. Catalyst, initiator
methane conversion, %
C, + selectivity, %
reactor volume 3 cm3 without catalyst AI,03 (1.06 g) AI,03 (0.10 g) MgO (0.10 g) SiO,
1.o 21.0 6.0 3.0 1.2
80.6 10.1 31.5 46.4 68.8
Oxygen (0.4 %) H,S (2.2 %)
2.0 2.1
63.4 51.0
m c t o r volume 30 cm3 without catalyst AI,03 (1.10 g)
19.4 21.0
36.4 5.4
Oxygen (0.5 %) H,S (1.6 %)
22.5 21.2
31.2 33.4
One can see at the low methane conversion (low contact time in the absence of catalyst and initiators) catalysts and initiators increase the methane conversion, but lead to the decrease of selectivity by the acceleration of the coke formation. In the conditions of a more higher methane conversion without catalyst and initiators, the only result of the catalyst and initiators introduction was the decrease of selectivity. The data obtained support the conclusion made in [6,7]: in the reaction of nonoxidative methane pyrolysis the nature of the catalyst surface does not determine its catalytic properties. According to literature [8], methane pyrolysis can be initiated by an addition of small amounts of higher hydrocarbons. This is probably due to their higher reactivity comparatively to methane in the pyrolysis conditions, and radicals formed from higher hydrocarbons initiate the methane molecules decomposition. We have studied the influence of the propane addition on the methane conversion and C,' yield in the pyrolysis process. Fig. 1 shows the methane conversion and C,' yield as a function of the concentration of the propane added. These results can be probably explained as follows. The reaction temperature 900 C is not sufficient for effective propagation of radical chains, that is why these chains initiation as a result of a decomposition of propane molecules leads to only a poor reaction rate increase. At 950 C conditions for a chain propagation are already exist, but in the absence of initiating additives the reaction rate is low. In these particular conditions the homogeneous initiators cause the most pronounced effect on methane pyrolysis process. This process, however, can be initiated also by its own products, and at higher methane conversion (10% at 950 "C) this way of the initiation prevails. At 1000 "C and above the initiation by reaction products prevails even at zero propane concentration. An additional initiation
500
'L (J9
(A)
1 ooooc
K
0
m
c
+
n
v
Eu
u
0.0
1 *o
0.0
1 -0
Propane Inlet Concentration, vol Z yield (A) and the methane conversion (B) as a function of the Fig.1. The C,' concentration of propane added at the different temperatures. Contact time 60 s. by propane can not increase significantly the rate of the radical chains initiation, and the methane conversion is independent on the concentration of propane added. 3.2. Regularities of the nonoxidative methane conversion without catalysts and initiators. The dependence of the higher hydrocarbons yield and selectivity in the thermal methane conversion on the volume feed at the different temperatures has been studied. The higher was the contact time, the higher was the methane conversion, but the lower the selectivity of C ,' hydrocarbons formation. The C,' hydrocarbons yield dependence on contact time goes through the maximum (Fig.2) The maximal yield of higher hydrocarbons increases with the rise of temperature, the maximum positions being shifted towards lower contact times. The regularities presented are in a compliance with the consecutive scheme of the higher hydrocarbons formation as intermediate products of the methane to carbon conversion: methane -- > higher hydrocarbons -- > carbon The maximum yield was achieved as higher as 12%, but analysis of kinetic data obtained was shown the more high yield of C,' hydrocarbons with a high selectivity might be achieved at the moderate-high temperature range (about 1200 "C)by the thorough selection of temperature - contact time parameters to provide a reasonable conversion without an essential loss of the selectivity. Results obtained are presented in Table 3.
501
0
100
500
300
700
Methane Volume Feed, h-' Fig.2. C,' hydrocarbons yield as a function of methane volume feed at the different temperatures. Table 3. Thermal methane conversion into higher hydrocarbons (*). Quartz reactor volume 1.9 cm3. Selectivity, % ml/s
C,-C,
Arom
Naph
Total C,'
%
%
1190
1.6 1.2 1.o 0.8
38.8 34.2 32.8 31.4
24.9 23.8 23.6 23.7
10.9 13.2 13.8 14.1
74.6 71.2 70.2 69.2
22.9 30.2 33.3 35.5
17.1 21.5 23.4 24.6
1210
1.5 1.3 1.1 1.o 0.9
42.9 36.4 32.8 31.1 30.1
22.8 20.5 20.3 19.8 19.3
10.4 13.3 14.2 14.5 14.9
76.1 70.2 67.3 65.4 64.3
26.8 33.1 37.9 39.4 40.4
20.4 23.2 25.5 25.8 26.8
1230
1.2 1.0
31.4 30.9
17.8 16.8
15.3 13.9
62.9 60.1
44.3 47.6
27.9 28.2
* C,-C, consist of 85-95 % C, hydrocarbons; Aromatic consist of 95 % benzene.
502
It can be seen form Table 3 the yield of C', hydrocarbons of 25 - 28 % at selectivity 60 - 70 % can be achieved by the nonoxidative methane conversion without catalysts and initiators, that even higher then for the methane oxidative coupling reaction. Besides a large amount of hydrogen has been produced (the 30 - 70 % yield of hydrogen, calculated as a moles of hydrogen formed on the mole of methane passed x 100 ) in the process of the nonoxidative conversion.
4. CONCLUSION. Our observations show that the effect of catalysts on the reaction rate at various reaction conditions can be dramatically different. At the low methane conversion the catalyst or the radical producing additive can play a significant role in the initiation of radical chains and speeding up the process of methane pyrolysis. At the high conversion of methane the concentrationof radicalsproduced by the reaction itself is high enough to maintain the initiation process. At these conditions methane pyrolysis behaves as an autocatalytic reaction [9],and the introduction of other catalysts can not change substantially the reaction rate. This work demonstrated that the application of initiators or catalysts can accelerate the nonoxidative methane transformation at rather mild conditions, and does not permit to improve the yield of higher hydrocarbons at more severe reaction conditions, when just the gas phase reaction provides an essential methane conversion. For gas phase reaction without catalysts and initiators the yield of higher C,' hydrocarbons of 25 - 28 % at selectivity 60 - 70 % has been achieved. ACKNOWLEDGEMENT Richard Ward Endowment Fund is greatly appreciated for the financial support. REFERENCES
1. J.R. Anderson, AppLCatal., 47 (1989)177 2. P.S. Yarlagadda, L.A. Morton, N.R. Hunter, H.D. Gesser, Ind. Eng. Chem. Res., 27 (1988)252. 3. J.B. Claridge, M.L.H. Green, S.C. Tsang, A.P.E. York, Appl. Catal. A: General, 89 (1992)103. 4. V.D. Sokolovskii, E.A. Mamedov, Catal. Today, 14 (1992)331. 5. G. Renesme, J. Saint-Just, Y. Muller, Catal. Today, 13 (1992)371. 6. F. Fisher, Brennstoff Chemie, 9 (1928)309. 7. Van der Zwet, P.A.J.M. Hendriks, R.A.van Santen, Catal. Today, 4 (1989)365. 8. Yu. G. Egiasarov, B.H. Cherkes, L.L. Potapova, Ya.M. Paushkin, Dokl. AN USSR, 300 ( 1988)412. 9. J.M. Roscoe, M.J. Thompson, Intern. J. Chem. Kinet., 17 (1985)967.
497
Nonoxidative methane conversion into higher hydrocarbons V.D. Sokolovskii (l),S.S.Shepelev (2)
1) 2)
Chemistry Department, University of Witwatersrand, Johannesburg, Wits 2050 , RSA. Institute of Catalysis RAN, 630090,Novosibirsk, Russia (*)
Nonoxidative methane conversion into higher C', hydrocarbons with and without catalysts and initiators has been investigated. Under relatively mild conditions, when just a low methane conversion occurs heterogeneous catalysts and homogeneous initiators accelerate the methane transformation into higher hydrocarbons but reduce selectivity due to a coke formation. At more severe conditions providing an essential conversion of methane by the gas phase reaction catalysts and initiators has not affected on the methane conversion and only result in the selectivity decrease. The dependence of the C,' hydrocarbons yield on contact time at different temperatures has been studied. It was shown by a thorough selection of temperature - contact time conditions the yield of C,' hydrocarbons can be achieved as higher as 25 - 28% at selectivity 60 - 70 %. 1,
INTRODUCTION
The direct conversion of methane to higher C,' hydrocarbons is a process of an immense practical importance [I]. A number of ways of the such conversion are known: the direct oxidation of methane to methanol [2] with a following transformationto higher hydrocarbons by the MTG process; oxidative oligomerization of methane to aromatic compounds [3] ; methane oxidative coupling to C, hydrocarbons [4], which can be converted to liquid hydrocarbons (ARCO process). The yield in the ARCO process is about 20 % per pass, which was claimed to be near of practical demands [5]. Methane can be transformed into higher C,+ hydrocarbons by a conversion without oxidant too. This process has definite advantages in a comparison to the oxidative methane conversion, namely the less complicated technology and the absence of a dilution of gaseous products with carbon oxides and nitrogen ( in the case air use in an oxidative process). The nonoxidative conversion by-product hydrogen - can be isolated and used separately. However the yield of higher hydrocarbons was achieved in this process with or without catalysts is rather low (about 8-18% per pass [6,7]).
* Present address: Department of Chemistry, University of Texas at Dallas, TX 75083-
0688,USA
498
The present work is devoted to the investigation of regularities of the methane nonoxidative conversion in the presence and in the absence of heterogeneous catalysts and homogeneous initiators. 2.
EXPERIMENTAL
The nonoxidative methane conversion was carried out in flow quartz reactors. Reactors 100, 30, 3 and 1.9 cm3in volume were used. Methane of 99.9 % purity was passed through the reactor with the volume space velocity of 0.1 - 2.0 cm3 / sec. The reaction temperature was varied in the range 800 1230 "C. Alumina, magnesia and silica used as catalysts were pelleted, crushed and the fraction 0.5 - 0.25 mm was taken for experiments. These catalysts have been calcined in air at 1100 " C before experiments, their surface area was in the range of 1 - 5 m2 / g. GC analysis of reaction products was performed with the use of column filled with Porapak-Q and glass capillary column with SE-30 liquid phase. Analysis of carbon deposits on the reactor walls performed by burning down these deposits has been shown shows the deposits to consist of carbon and practically do not contain hydrogen.
-
3.
RESULTS AND DISCUSSION.
An appreciable methane conversion in a quartz reactor without catalysts and initiators begins at temperature above 850 C. Ethane, ethylene, benzene and other higher hydrocarbons,as well as hydrogen have been discovered in reaction products. Carbonaceous deposits were formed on the reaction walls. The product composition of the methane thermal conversion in the empty reactor at 1000 "C and the reaction mixture contact time 60 s. are presented in Table 1. Table 1. Product composition of methane thermal conversion at 1000 " C and contact time 60 s. in 30 cm3 quartz reactor. substance
concentration vol. %
substance
concentration VOI. %
methane ethane ethylene + acetylene benzene toluene
69.50 0.156
aromatic C, aromatic C,+ naphthalene methylnaphthalene dimethylnaphthalene hydrogen
0.001 0.002 0.082 0.001 0.002 28.07
1.775 0.401 0.007
methane conversion hydrocarbons selectivty
19.4 % 36.4 %
3.1. The influence of catalysts and initiators in the reaction of methane nonoxidative conversion. Several catalysts and gaseous initiators have been tested in this process, the results being presented in Table 2.
499
Table 2 Effect of catalysts and initiators on the nonoxidative methane conversion. T= 1000 'C, methane feed 0.5 ml/s. Catalyst, initiator
methane conversion, %
C, + selectivity, %
reactor volume 3 cm3 without catalyst AI,03 (1.06 g) AI,03 (0.10 g) MgO (0.10 g) SiO,
1.o 21.0 6.0 3.0 1.2
80.6 10.1 31.5 46.4 68.8
Oxygen (0.4 %) H,S (2.2 %)
2.0 2.1
63.4 51.0
m c t o r volume 30 cm3 without catalyst AI,03 (1.10 g)
19.4 21.0
36.4 5.4
Oxygen (0.5 %) H,S (1.6 %)
22.5 21.2
31.2 33.4
One can see at the low methane conversion (low contact time in the absence of catalyst and initiators) catalysts and initiators increase the methane conversion, but lead to the decrease of selectivity by the acceleration of the coke formation. In the conditions of a more higher methane conversion without catalyst and initiators, the only result of the catalyst and initiators introduction was the decrease of selectivity. The data obtained support the conclusion made in [6,7]: in the reaction of nonoxidative methane pyrolysis the nature of the catalyst surface does not determine its catalytic properties. According to literature [8], methane pyrolysis can be initiated by an addition of small amounts of higher hydrocarbons. This is probably due to their higher reactivity comparatively to methane in the pyrolysis conditions, and radicals formed from higher hydrocarbons initiate the methane molecules decomposition. We have studied the influence of the propane addition on the methane conversion and C,' yield in the pyrolysis process. Fig. 1 shows the methane conversion and C,' yield as a function of the concentration of the propane added. These results can be probably explained as follows. The reaction temperature 900 C is not sufficient for effective propagation of radical chains, that is why these chains initiation as a result of a decomposition of propane molecules leads to only a poor reaction rate increase. At 950 C conditions for a chain propagation are already exist, but in the absence of initiating additives the reaction rate is low. In these particular conditions the homogeneous initiators cause the most pronounced effect on methane pyrolysis process. This process, however, can be initiated also by its own products, and at higher methane conversion (10% at 950 "C) this way of the initiation prevails. At 1000 "C and above the initiation by reaction products prevails even at zero propane concentration. An additional initiation
500
'L (J9
(A)
1 ooooc
K
0
m
c
+
n
v
Eu
u
0.0
1 *o
0.0
1 -0
Propane Inlet Concentration, vol Z yield (A) and the methane conversion (B) as a function of the Fig.1. The C,' concentration of propane added at the different temperatures. Contact time 60 s. by propane can not increase significantly the rate of the radical chains initiation, and the methane conversion is independent on the concentration of propane added. 3.2. Regularities of the nonoxidative methane conversion without catalysts and initiators. The dependence of the higher hydrocarbons yield and selectivity in the thermal methane conversion on the volume feed at the different temperatures has been studied. The higher was the contact time, the higher was the methane conversion, but the lower the selectivity of C ,' hydrocarbons formation. The C,' hydrocarbons yield dependence on contact time goes through the maximum (Fig.2) The maximal yield of higher hydrocarbons increases with the rise of temperature, the maximum positions being shifted towards lower contact times. The regularities presented are in a compliance with the consecutive scheme of the higher hydrocarbons formation as intermediate products of the methane to carbon conversion: methane -- > higher hydrocarbons -- > carbon The maximum yield was achieved as higher as 12%, but analysis of kinetic data obtained was shown the more high yield of C,' hydrocarbons with a high selectivity might be achieved at the moderate-high temperature range (about 1200 "C)by the thorough selection of temperature - contact time parameters to provide a reasonable conversion without an essential loss of the selectivity. Results obtained are presented in Table 3.
501
0
100
500
300
700
Methane Volume Feed, h-' Fig.2. C,' hydrocarbons yield as a function of methane volume feed at the different temperatures. Table 3. Thermal methane conversion into higher hydrocarbons (*). Quartz reactor volume 1.9 cm3. Selectivity, % ml/s
C,-C,
Arom
Naph
Total C,'
%
%
1190
1.6 1.2 1.o 0.8
38.8 34.2 32.8 31.4
24.9 23.8 23.6 23.7
10.9 13.2 13.8 14.1
74.6 71.2 70.2 69.2
22.9 30.2 33.3 35.5
17.1 21.5 23.4 24.6
1210
1.5 1.3 1.1 1.o 0.9
42.9 36.4 32.8 31.1 30.1
22.8 20.5 20.3 19.8 19.3
10.4 13.3 14.2 14.5 14.9
76.1 70.2 67.3 65.4 64.3
26.8 33.1 37.9 39.4 40.4
20.4 23.2 25.5 25.8 26.8
1230
1.2 1.0
31.4 30.9
17.8 16.8
15.3 13.9
62.9 60.1
44.3 47.6
27.9 28.2
* C,-C, consist of 85-95 % C, hydrocarbons; Aromatic consist of 95 % benzene.
502
It can be seen form Table 3 the yield of C', hydrocarbons of 25 - 28 % at selectivity 60 - 70 % can be achieved by the nonoxidative methane conversion without catalysts and initiators, that even higher then for the methane oxidative coupling reaction. Besides a large amount of hydrogen has been produced (the 30 - 70 % yield of hydrogen, calculated as a moles of hydrogen formed on the mole of methane passed x 100 ) in the process of the nonoxidative conversion.
4. CONCLUSION. Our observations show that the effect of catalysts on the reaction rate at various reaction conditions can be dramatically different. At the low methane conversion the catalyst or the radical producing additive can play a significant role in the initiation of radical chains and speeding up the process of methane pyrolysis. At the high conversion of methane the concentrationof radicalsproduced by the reaction itself is high enough to maintain the initiation process. At these conditions methane pyrolysis behaves as an autocatalytic reaction [9],and the introduction of other catalysts can not change substantially the reaction rate. This work demonstrated that the application of initiators or catalysts can accelerate the nonoxidative methane transformation at rather mild conditions, and does not permit to improve the yield of higher hydrocarbons at more severe reaction conditions, when just the gas phase reaction provides an essential methane conversion. For gas phase reaction without catalysts and initiators the yield of higher C,' hydrocarbons of 25 - 28 % at selectivity 60 - 70 % has been achieved. ACKNOWLEDGEMENT Richard Ward Endowment Fund is greatly appreciated for the financial support. REFERENCES
1. J.R. Anderson, AppLCatal., 47 (1989)177 2. P.S. Yarlagadda, L.A. Morton, N.R. Hunter, H.D. Gesser, Ind. Eng. Chem. Res., 27 (1988)252. 3. J.B. Claridge, M.L.H. Green, S.C. Tsang, A.P.E. York, Appl. Catal. A: General, 89 (1992)103. 4. V.D. Sokolovskii, E.A. Mamedov, Catal. Today, 14 (1992)331. 5. G. Renesme, J. Saint-Just, Y. Muller, Catal. Today, 13 (1992)371. 6. F. Fisher, Brennstoff Chemie, 9 (1928)309. 7. Van der Zwet, P.A.J.M. Hendriks, R.A.van Santen, Catal. Today, 4 (1989)365. 8. Yu. G. Egiasarov, B.H. Cherkes, L.L. Potapova, Ya.M. Paushkin, Dokl. AN USSR, 300 ( 1988)412. 9. J.M. Roscoe, M.J. Thompson, Intern. J. Chem. Kinet., 17 (1985)967.