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Selective oxidation of methane to C1-products using hydrogen peroxide
Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) @2004 Elsevier B.V. All rights reserved.
637
Selective oxidation of methane to C 1-products using hydrogen peroxide T.M. Nagiev, L.M. Gasanova, E.M. Mamedov, I.T. Nagieva, Z.Y. Ramasanova, and A.A. Abbasov Institute of Chemical Problems, National Academy of Sciences of Azerbaijan, 29 H. Javid av., 370143 Baku, the Republic of Azerbaijan ABSTRACT There were presented the results of experimental investigations on synchronization between natural gas selective oxidation and hydrogen peroxide's decomposition. It was shown that methane synchronous gas-phase oxidation by hydrogen peroxide is flexibly controlled towards both formaldehyde selective formation and hydrogen-containing gas or methanol production. The usage of heterogeneous biomimetic catalyst PPFes+OH/A1SiMg allowed carrying out the process of methane direct selective synchronous oxidation to methanol by hydrogen peroxide under mild conditions with a high yield of methanol (46.5%) at a selectivity of 97%. 1. INTRODUCTION Single-carbon molecules are gaining importance for modem science and industry because of their accessibility and the possibility to synthesize a variety of organic compounds on their basis. There was studied a chemical system comprising synchronized reaction of hydrogen peroxide decomposition and methane oxidation to C~-products. In such system methane oxidation, depending on process parameters, goes in different directions to form respective products. Hydrogen peroxide is applied as an oxidant, because it embraces high inducing activity along with efficiency and specificity of its action. The core of hydrogen peroxide's inducing activity consists in the fact that being transformed to the active form- "OH and H O 2 " radicals (primary reaction), hydrogen peroxide is selectively consumed in another conjugated reaction forming respective products. The way these two conjugated brutto-reactions take is of a synchronized character:
638 H202 + H202 --+ ['OH, HO2"] --+ 2H20 + 02
H202 + CH4
~
CH20 + H20 H2, CO, C02
(primary reaction)
(1)
(secondary reaction)
(2)
CH~OH + H~O since a minimum of molecular oxygen accumulation is in agreement with a maximum of methane conversion [ 1]. 2. E X P E R I M E N T A L
Experimental investigations that revealed chemical conjugation in oxidation reactions by hydrogen peroxide in the gas phase have been carried out in a through-flow integral quartz glass reactor, the construction of which ensured the introduction of H202 into the reaction zone in the undecomposed form. By special experiments it was shown that under H202's adding into the reaction zone through the quartz tube at a velocity of 6-40cm/s, the concentration of H202 stays constant until its reaching the reaction zone. Via injection syringe pressed down by mechanical device (electromotordriven) water solution of hydrogen peroxide (15+25w%) was added directly to pre-reaction zone through quartz tube separately from the substrate being oxidized. Conical quartz tube was 95mm long with a diameter of 5+ 10mm at the input and 4mm at the output. High linear velocity of H202 in the tube (6-40cm/s) excluded any possibility of its decomposition until it is added into the reaction zone. Under these conditions one could keep concentration of H202 as nearly as it was initially adjusted. Any possibility of molecular oxygen's participation as an initial oxidant in the process has been excluded. Through the quartz tube, separately from H202, a natural gas (96% CH4) has been added to the reactor, and then during the reaction it reacted with H202. Hardening zone adjoined the reaction zone (at a distance of 15+20ram). In that place the tube tapered critically. Such a hardening system excluded any possibility of reagents and products being oxidized beyond the reaction zone. Oxidation of methane by hydrogen peroxide was carried out at low contact times in a through-flow system (z=0.8+2.35s), which to a high extent excluded possibility of products further transformation during secondary reactions. As to the participation in the reaction of 02 formed as a result of hydrogen peroxide's decomposition, it should be noted that the reaction medium residence time in the through-flow reactor, in our case, did not allow O2 participating in the process [2].
639
3. D I S C U S S I O N AND RESULTS
There was studied a single-stage process for formaldehyde production via conjugated homogeneous oxidation of methane by hydrogen peroxide [3]. The given process featured the fact that no methanol was formed as a semiproduct. It was shown a high efficiency of hydrogen peroxide as a generator of highly reactive intermediates capable of oxidizing methane selectively to formaldehyde and under optimum conditions (T=520~ x=l.2s, CH4 " H202 - 1 9 1, CH2o2= 25w%) the yield of formaldehyde made up about 40% at a high selectivity of 94% (see Table 1). Formaldehyde was formed as a result of methane direct oxidation by-passing the stage of methanol formation. To support this viewpoint the results of experiments are shown in Table 1.
Table 1 Conjugated oxidation of CH4 and CH3OH by H202 (T=520~
11 "E
VCH4 VH2O2
VCH3OH Yield of products, w %
L/h
mL/h
mL/h
r,.;
CH4:H202 1:0.24 1.2 1:0.5 "-" 1:1 "-" 1:1.2 "-" 1:1.6 "-" CH3OH: H202 1:0.25 1:0.5 1:1 -
1.44 2.32 4.72
CH4:CH3OH:H202 9 0.7:0.4:1 0.8 10 0.8:0.2:1 0.82
5.74 "-"
6 7 8
cCH 4
% r
r...q
1 2 3 4 5
CH2O2 = 25w%)
cCH3OH N, % %
r
~
~
r,.p
CH4-H202-H20 1.44 3.2 5.74 7.2 9.5
-
6 . 8
-
-
14.0 1.0 39.0 2.4 35.5 7.5 30.0 13.0 2.4 CH3OH-H202-H20 4.5 9.2 4.0 "-" 7.9 5.8 CH4-CH3OH-HzOz-H20 0.72 2.5 5.9 0.36 3.7 4.3 -
1 . 4 4
"-"
-
-
-
2 . 0
-
3.2 5.6
6.8 15.3 41.4 45.5 50.0
100 97.5 94.0 78.0 60.0
-
-
1.6 2.5
-
6.5 14.0 16.2
0.5 1.6
4.3 6.2
16.1 13.4
69.0 67.5 48.8
n - molar ratio; c - conversion; N - selectivity; in experiment 8 the yield of C H O O H 1.3w%; in experiments 9 and 10 the yield of C 2 H 6 - 0.9 and 0.7w%; C 3 H 6 - 0.17 and 0.1w%; C 3 H 8 - 0.25 and 0.2w%.
640
In the course of these experimems under the conditions of formaldehyde selective formation methanol was added into the reaction system in various amounts. Given the results of these experiments it follows that the formation of formaldehyde in the presence of methanol is of nonselective character, i.e. it features a big number of by-products - C3H6, C3H8, C2H6, CO and CO2. The formation of formaldehyde in small amounts is caused by the presence of methanol in the initial mixture in the amount up to 40w%, because under these conditions H202 is probably consumed on methanol's deep oxidation. First ever under homogeneous conditions by chemical induction without applying any heterogeneous catalysts it came possible to effectively carry out high-temperature oxidation of methane to molecular oxygen and carbon dioxide by appreciable reduction of reaction temperature (the yield of H2 is up to 74 tool %, CO2- 23%, CO-1.5%) [4]. The experimental data on the effect of contact time on methane oxidation process (Fig. 1) show that an increase in the contact time (z) raises the extent of methane conversion. With an increase in z afteroxidation of CO to CO2 occurs, that leads to a decrease in CO content down to quite small amount even at x=0.6s, whereas the yield of H2 reaches its maximum at 74mo|%.
60
./
2 2O 5 0.O2
0.04
0.06
Fig. 1. Dependence of gas content (mol%) change in reaction gaseous mixture on the contact time (1:) at T=880~ CH2o2= 15w%, CH4: H202=1"3; 1 - H2, 2 - CO2, 3 - CO, 4 - CH4, 5 02.
641 The chemical system being developed features flexibility, which, by intelligent control of reaction rate, enables one, depending on the purpose, to get quantitatively varied composition of products such as H2, CO and CO2. For each of these oxidation reactions there were proposed free radical mechanisms. From the mechanisms proposed it is seen that the relation between indicated synchronous reactions Eq. (1) and Eq. (2) is set through elementary stages by means of intermediary radicals such as "OH and HO2". There was studied the process of direct oxidation of methane by hydrogen peroxide to methanol under pressure, which allowed, compared to other existing processes, to reduce the temperature and pressure, increase conversion and selectivity: at T=400~ p=7atm, the yield of m e t h a n o l - 17.6%, selectivity99.5%. It was studied and developed detail mechanism of methanol production. It was shown that the synchronization between the reactions of hydrogen peroxide decomposition and methane oxidation in the gas phase, inducing gas-phase homogeneous oxidation, takes place in the case of heterogeneous catalyzed oxidation, too. The biomimetic catalyst synthesized by us and mimicking the mechanism of monooxygenase enzyme's (cytochrome P-450) function allowed developing a new highly selective catalytic system. The given system enabled gas-phase oxidation of such inert gas as methane under mild conditions featuring high efficiency" the yield of methanol reached 46.5% at T= 180~ The given works led to the development of extremely active monooxygenase m i m i c - PPFe3+OH/A1SiMg [5]. The studied mimic is capable of catalyzing two interrelated synchronous reactions catalase and monooxygenase. Synchronization mechanism is realized through common reactive intermediate PPFe3+OOH/A1SiMg, which is consumed in both reactions but with different rate. REFERENCES
[1] T.M. Nagiev, Russian J. Phys. Chem. 74 No.11 (2000)1853. [2] T.M. Nagiev, L.M. Gasanova, S.Z. Zulfugarova, Ch.A. Mustafaeva, and A.A. Abbasov, Chem. Eng. Comm., 190 No. 5-8 (2003) 726-748. [3] T.M. Nagiev and L.M. Gasanova, Khim. Fizika, 3 No. 10 (1984) 1455-1461. [4] T.M. Nagiev, L.M. Gasanova, and Z.U. Ramasanova, Zhurnal Fiz. Khim. 68 No. 1 (1994) 23-28 [5] T.M. Nagiev and M.T. Abbasova, Russian J. Phys. Chem. 71 7 (1997) 1088-1092.
637
Selective oxidation of methane to C 1-products using hydrogen peroxide T.M. Nagiev, L.M. Gasanova, E.M. Mamedov, I.T. Nagieva, Z.Y. Ramasanova, and A.A. Abbasov Institute of Chemical Problems, National Academy of Sciences of Azerbaijan, 29 H. Javid av., 370143 Baku, the Republic of Azerbaijan ABSTRACT There were presented the results of experimental investigations on synchronization between natural gas selective oxidation and hydrogen peroxide's decomposition. It was shown that methane synchronous gas-phase oxidation by hydrogen peroxide is flexibly controlled towards both formaldehyde selective formation and hydrogen-containing gas or methanol production. The usage of heterogeneous biomimetic catalyst PPFes+OH/A1SiMg allowed carrying out the process of methane direct selective synchronous oxidation to methanol by hydrogen peroxide under mild conditions with a high yield of methanol (46.5%) at a selectivity of 97%. 1. INTRODUCTION Single-carbon molecules are gaining importance for modem science and industry because of their accessibility and the possibility to synthesize a variety of organic compounds on their basis. There was studied a chemical system comprising synchronized reaction of hydrogen peroxide decomposition and methane oxidation to C~-products. In such system methane oxidation, depending on process parameters, goes in different directions to form respective products. Hydrogen peroxide is applied as an oxidant, because it embraces high inducing activity along with efficiency and specificity of its action. The core of hydrogen peroxide's inducing activity consists in the fact that being transformed to the active form- "OH and H O 2 " radicals (primary reaction), hydrogen peroxide is selectively consumed in another conjugated reaction forming respective products. The way these two conjugated brutto-reactions take is of a synchronized character:
638 H202 + H202 --+ ['OH, HO2"] --+ 2H20 + 02
H202 + CH4
~
CH20 + H20 H2, CO, C02
(primary reaction)
(1)
(secondary reaction)
(2)
CH~OH + H~O since a minimum of molecular oxygen accumulation is in agreement with a maximum of methane conversion [ 1]. 2. E X P E R I M E N T A L
Experimental investigations that revealed chemical conjugation in oxidation reactions by hydrogen peroxide in the gas phase have been carried out in a through-flow integral quartz glass reactor, the construction of which ensured the introduction of H202 into the reaction zone in the undecomposed form. By special experiments it was shown that under H202's adding into the reaction zone through the quartz tube at a velocity of 6-40cm/s, the concentration of H202 stays constant until its reaching the reaction zone. Via injection syringe pressed down by mechanical device (electromotordriven) water solution of hydrogen peroxide (15+25w%) was added directly to pre-reaction zone through quartz tube separately from the substrate being oxidized. Conical quartz tube was 95mm long with a diameter of 5+ 10mm at the input and 4mm at the output. High linear velocity of H202 in the tube (6-40cm/s) excluded any possibility of its decomposition until it is added into the reaction zone. Under these conditions one could keep concentration of H202 as nearly as it was initially adjusted. Any possibility of molecular oxygen's participation as an initial oxidant in the process has been excluded. Through the quartz tube, separately from H202, a natural gas (96% CH4) has been added to the reactor, and then during the reaction it reacted with H202. Hardening zone adjoined the reaction zone (at a distance of 15+20ram). In that place the tube tapered critically. Such a hardening system excluded any possibility of reagents and products being oxidized beyond the reaction zone. Oxidation of methane by hydrogen peroxide was carried out at low contact times in a through-flow system (z=0.8+2.35s), which to a high extent excluded possibility of products further transformation during secondary reactions. As to the participation in the reaction of 02 formed as a result of hydrogen peroxide's decomposition, it should be noted that the reaction medium residence time in the through-flow reactor, in our case, did not allow O2 participating in the process [2].
639
3. D I S C U S S I O N AND RESULTS
There was studied a single-stage process for formaldehyde production via conjugated homogeneous oxidation of methane by hydrogen peroxide [3]. The given process featured the fact that no methanol was formed as a semiproduct. It was shown a high efficiency of hydrogen peroxide as a generator of highly reactive intermediates capable of oxidizing methane selectively to formaldehyde and under optimum conditions (T=520~ x=l.2s, CH4 " H202 - 1 9 1, CH2o2= 25w%) the yield of formaldehyde made up about 40% at a high selectivity of 94% (see Table 1). Formaldehyde was formed as a result of methane direct oxidation by-passing the stage of methanol formation. To support this viewpoint the results of experiments are shown in Table 1.
Table 1 Conjugated oxidation of CH4 and CH3OH by H202 (T=520~
11 "E
VCH4 VH2O2
VCH3OH Yield of products, w %
L/h
mL/h
mL/h
r,.;
CH4:H202 1:0.24 1.2 1:0.5 "-" 1:1 "-" 1:1.2 "-" 1:1.6 "-" CH3OH: H202 1:0.25 1:0.5 1:1 -
1.44 2.32 4.72
CH4:CH3OH:H202 9 0.7:0.4:1 0.8 10 0.8:0.2:1 0.82
5.74 "-"
6 7 8
cCH 4
% r
r...q
1 2 3 4 5
CH2O2 = 25w%)
cCH3OH N, % %
r
~
~
r,.p
CH4-H202-H20 1.44 3.2 5.74 7.2 9.5
-
6 . 8
-
-
14.0 1.0 39.0 2.4 35.5 7.5 30.0 13.0 2.4 CH3OH-H202-H20 4.5 9.2 4.0 "-" 7.9 5.8 CH4-CH3OH-HzOz-H20 0.72 2.5 5.9 0.36 3.7 4.3 -
1 . 4 4
"-"
-
-
-
2 . 0
-
3.2 5.6
6.8 15.3 41.4 45.5 50.0
100 97.5 94.0 78.0 60.0
-
-
1.6 2.5
-
6.5 14.0 16.2
0.5 1.6
4.3 6.2
16.1 13.4
69.0 67.5 48.8
n - molar ratio; c - conversion; N - selectivity; in experiment 8 the yield of C H O O H 1.3w%; in experiments 9 and 10 the yield of C 2 H 6 - 0.9 and 0.7w%; C 3 H 6 - 0.17 and 0.1w%; C 3 H 8 - 0.25 and 0.2w%.
640
In the course of these experimems under the conditions of formaldehyde selective formation methanol was added into the reaction system in various amounts. Given the results of these experiments it follows that the formation of formaldehyde in the presence of methanol is of nonselective character, i.e. it features a big number of by-products - C3H6, C3H8, C2H6, CO and CO2. The formation of formaldehyde in small amounts is caused by the presence of methanol in the initial mixture in the amount up to 40w%, because under these conditions H202 is probably consumed on methanol's deep oxidation. First ever under homogeneous conditions by chemical induction without applying any heterogeneous catalysts it came possible to effectively carry out high-temperature oxidation of methane to molecular oxygen and carbon dioxide by appreciable reduction of reaction temperature (the yield of H2 is up to 74 tool %, CO2- 23%, CO-1.5%) [4]. The experimental data on the effect of contact time on methane oxidation process (Fig. 1) show that an increase in the contact time (z) raises the extent of methane conversion. With an increase in z afteroxidation of CO to CO2 occurs, that leads to a decrease in CO content down to quite small amount even at x=0.6s, whereas the yield of H2 reaches its maximum at 74mo|%.
60
./
2 2O 5 0.O2
0.04
0.06
Fig. 1. Dependence of gas content (mol%) change in reaction gaseous mixture on the contact time (1:) at T=880~ CH2o2= 15w%, CH4: H202=1"3; 1 - H2, 2 - CO2, 3 - CO, 4 - CH4, 5 02.
641 The chemical system being developed features flexibility, which, by intelligent control of reaction rate, enables one, depending on the purpose, to get quantitatively varied composition of products such as H2, CO and CO2. For each of these oxidation reactions there were proposed free radical mechanisms. From the mechanisms proposed it is seen that the relation between indicated synchronous reactions Eq. (1) and Eq. (2) is set through elementary stages by means of intermediary radicals such as "OH and HO2". There was studied the process of direct oxidation of methane by hydrogen peroxide to methanol under pressure, which allowed, compared to other existing processes, to reduce the temperature and pressure, increase conversion and selectivity: at T=400~ p=7atm, the yield of m e t h a n o l - 17.6%, selectivity99.5%. It was studied and developed detail mechanism of methanol production. It was shown that the synchronization between the reactions of hydrogen peroxide decomposition and methane oxidation in the gas phase, inducing gas-phase homogeneous oxidation, takes place in the case of heterogeneous catalyzed oxidation, too. The biomimetic catalyst synthesized by us and mimicking the mechanism of monooxygenase enzyme's (cytochrome P-450) function allowed developing a new highly selective catalytic system. The given system enabled gas-phase oxidation of such inert gas as methane under mild conditions featuring high efficiency" the yield of methanol reached 46.5% at T= 180~ The given works led to the development of extremely active monooxygenase m i m i c - PPFe3+OH/A1SiMg [5]. The studied mimic is capable of catalyzing two interrelated synchronous reactions catalase and monooxygenase. Synchronization mechanism is realized through common reactive intermediate PPFe3+OOH/A1SiMg, which is consumed in both reactions but with different rate. REFERENCES
[1] T.M. Nagiev, Russian J. Phys. Chem. 74 No.11 (2000)1853. [2] T.M. Nagiev, L.M. Gasanova, S.Z. Zulfugarova, Ch.A. Mustafaeva, and A.A. Abbasov, Chem. Eng. Comm., 190 No. 5-8 (2003) 726-748. [3] T.M. Nagiev and L.M. Gasanova, Khim. Fizika, 3 No. 10 (1984) 1455-1461. [4] T.M. Nagiev, L.M. Gasanova, and Z.U. Ramasanova, Zhurnal Fiz. Khim. 68 No. 1 (1994) 23-28 [5] T.M. Nagiev and M.T. Abbasova, Russian J. Phys. Chem. 71 7 (1997) 1088-1092.