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Controlled oxidation of methane doped catalysts irradiated by microwaves
catalysis today Catalysis Today 21 ( 1994) 349-355
Controlled oxidation of methane doped catalysts irradiated by microwaves G. Roussy **a,J.M. Thiebaut a, M. Souiri a, E. Marchal a, A. Kiennemann b, G. Maire b aLSTM Unrversitt!de NANCY I, BP 239, 54500 Vandoeuvre, France ’ LERCSI-URA 1418, Umversite de Strasbourg, Strasbourg, France
Abstract The oxidative coupling of methane has been studied on a (SmLi02)0,(CaOMgO)0~2 catalyst by using two different modes of heating: a conventional oven and a microwave irradiation set-up. The C2 selectivity obtained with microwave heating was much higher than with conventional heating especially at low conversions. This difference could arise from the reduction of oxidation products (ethane, ethylene) in the gas phase under microwave irradiation.
1. Introduction There is considerable interest in the catalytic conversion of methane, the principal constituent of natural gas, into C2 hydrocarbons under partial oxidation conditions around 800°C [ 11. The oxidative coupling of methane (OCM) to obtain C2 products is currently under investigation in several laboratories as a possible commercial process for the production of higher hydrocarbons. A potential problem for the use of this technique for higher hydrocarbon production is that under high-temperature oxidative conditions, the desired C2 products are considerably more reactive toward oxidation than is methane itself. In the field of partial methane oxidation, the aim of this present work is to test the potential of microwaves compared to conventional thermal heating for the OCM. Such microwave heating has indeed been claimed to be efficient for the production of Cz hydrocarbons from methane by formation of a plasma induced without a solid [ 21, by using metals (Ni, Fe, W) or finely divided refractory oxides [ 31. The oxidative coupling of methane was studied by Bond et al. [ 41 in the * Corresponding author. 0920-5861/94/$07.00 Q 1994 Elsevier Science B.V. All rights reserved SSDIO920-5861(94)00099-9
G. Roussy et al. /Catalysis Today 21 (1994) 349-355
350
presence of microwaves, using sodium aluminate as a catalyst. Similar selectivity to C,s is obtained using conventional heating and microwave heating but a reaction temperature 400°C lower is used when the sample is irradiated by microwaves. During microwave irradiation, the electric field induces movement of ionic species and polar molecules. The local heat which is generated by the microscopic movement is fundamentally different from that in classical heating where the energy is equally furnished to all the atoms comprising the structure [ 561. Under these conditions, the chemical reactions can be controlled directly inside dielectric matter and can modify the catalyst activity for the OCM.
2. Experimental
section
2.1. Description of the thermal heating device The catalytic reactions are performed in a conventional way. The gases are first mixed before entering a fixed bed quartz reactor (6.6 mm ID), equipped with electronic mass flow meters (Brooks 5878). The catalytic bed is placed between two layers of ground quartz particles which fill all the remaining volume. The sample is pretreated overnight with the feed gas used for the catalyst test. The operating conditions were: catalyst weight 1.4 g; feed gas partial pressures: 0.133 atm CH,, 0.066 atm O2 and 0.8 atm He; gas flow: 4.5 1 h-’ gcat-’ (SIP). 2.2. Description and working of the irradiation device The microwave set-up used for irradiating the catalysts during reaction has been described in more detail elsewhere (Fig. 1) [ 71. The catalyst is placed in a rectangular resonant cavity (TEOl3), the frequency (2.45 GHz) of which is controlled by the displacement of a short circuit with a stepping motor run by a HP 9825 computer. The computer also controls the temperature of the catalyst by varying
CI
CI
s
IY W
”
ST
-
R
T
SC
YC
Fig. I. Microwave set-up for the catalyst study. (C) Chromatograph; (CA) catalyst; (CI) circulator; (G) generator; (MC) microcomputer; (P) input power; (PM) power-meter, (R) reactor; (S) switch; (SC) shortcircuit; (ST) stepping motor; (T) twist; (TH) thermocouple; (W) water load.
G. Roussy et al. /Catalysis
To&y 21 (1994) 349-355
351
Fig. 2. The chemical reactor. (C) Catalyst; (D) f&ted silica disk; (T) silica tube; (TH) thermocouple; (W) wave-guide.
the generator power at regular time intervals. The input power is about 50 W but the adsorbed power is between 20 and 28 W. The catalyst is placed in a tube of quartz, allowing the flow of reactant gas through it. The reactor is located perpendicular to the electric field inside the guide, so that a thermocouple can be introduced without interfering with the electric field (Fig. 2). The catalytic conditions are the same as those used in conventional heating (gas flow, catalyst pretreatment, catalyst weight ...).
3. Catalyst preparation ( SmLiO,) ,,*( CaOMgO),., catalysts were prepared as follows: From an ethanol solution of samarium nitrate, magnesium nitrate and calcium nitrate, a precipitate is formed by adding oxalic acid at pH = 2.2. The precipitate is extracted by evap-
352
G. Roussy et al. / Catalyszs Today 21 (1994) 349-355
oration to dryness at 120°C. Lithium was introduced as Li2C03 or LiOH into the ethanoic solution that contained the precipitate. The resulting material was calcined at 850°C for 24 h.
4. Results and discussion A comparison of the reactivities and selectivities of the (SmLiO,),,s(CaOMgO),., catalyst for both heating modes showed (Fig. 3) that the C2 selectivities are much higher under microwave irradiation (MW) than conventional heating (CH) especially at low conversions. Under microwave irradiation, the C2 selectivity is 100% at low conversion but decreases with methane conversion, while with conventional heating, the C, selectivity is O%, at low conversion and increases with methane conversion. FT-IR study of the catalyst after reaction shows that, at low temperatures, the quantity of adsorbed CO* is low. The ratio of the number of C2 molecules to the number of CO, as a function of the ratio of the number of C2H6 molecules to the number of C2H, molecules for this catalyst (Fig. 4) clearly illustrates that there is a fundamental difference in mechanism between conventional heating and microwave irradiation. The kinetics of each of the five paths in Fig. 5 hold the key to interpreting the results. The separate reaction rates of each reaction (k, to k,) have been crudely evaluated by fitting a large amount of data including the oxidation of C2H6 and C2H4, taking into consideration that the k, follow an Arrhenius low and by assuming that the reaction order is the same as published in Olsbye’s thesis [ 81. In Fig. 6 are shown the calculated ratios ki( MW) lk,( CH) (k, is the reaction rate) as a function of the catalyst temperature T. Fig. 7 shows the initial rate ratios kl /k2 as a function of T. The results of Figs. 6 and 7 have been limited to low temperatures because the
0
10
20
CR4 CONVERSION
30
40
(%)
Fig. 3. Catalyst selectivity plot for ( SmLiO,)O *( CaO-MgO),
z catalyst
G. Roussy et al. /Catalysis
Today 2I (1994) 349-355
353
100
10
CH
x
s 1
0”
650°C 1 I
I
0
,
#
vs. C2H,/C2H,
I
,
3
2
1
‘2 Fig 4. &/CO,
,
,
4
5
H6’C2H4
for (SmLi02)0,(Ca0-Mg0),
z catalyst.
Fig. 5. The methane reaction scheme.
kiMW / kiCH
1:: .l
t”:~~__ -
.Ol
-
,001
-
. .
k5MW/k5CH
k4MWlk4CH
450
500
550
TEMPERATURE Fig. 6. Kinetic rates MW-CH.
Comparison
600
650
(“C) of the methane reactions.
statistical data is only self consistent for temperatures lower than 700°C. Above 700°C our fit is not acceptable because the reaction orders are probably not in agreement with our results; indeed, as Olsbye showed, they change around that temperature during conventional heating. Fig. 6 shows that the ratios k, (MW) lk, (CH) and k2( MW) lk2( CH) are clearly greater than 1. The catalyst is thus more active under microwave irradiation. Fig.
354
G. Roussy et al. /Catalysis
Today 21 (1994) 349-355
kllk2
II 400
.
I
450
I
I
500
I
I
I
550
TEMPERATURE
I
600
I
I
650
(“C)
Fig. 7. Initial selectivity kl/kz.
6 also shows that the ratios %(MW) /k,(CH) and k,(MW) Ik,(CH) are low. The reactions labelled 4 and 5 through which the products C,H, and C2H4 are oxidized, are much slower under microwave irradiation than in conventional heating. We can interpret these results by noting that the degradation reactions (4 and 5) occur in the gas phase, and we might expect them to be slow if the gas phase temperature is less than the temperature of the solid catalyst. Nevertheless, different catalytic pathways for both heating modes can be envisaged. On the other hand, the initial selectivity, which is indicated by the ratio kl/k2 (Fig. 7) is nearly constant in microwave experiments while it varies significantly in conventional heating. The k, and k2 values are extrapolated for temperatures inferior to 550°C. We do not have a clear explanation of why the k,lk, ratio is different under microwave irradiation than conventional heating. However, preferential excitation of specific ionic sites (O-, O*- ...) by the microwave field may be a plausible hypothesis. The specific catalyst used was (SmLiO,,), 8(CaO-MgO)0 2 which gives the best catalysis in methane oxidation and has some dielectric loss. In general, crystallized oxides have low dielectric loss because the movement of an atom is difficult, at best, and often not possible at all. SmLiO, has a small loss factor and a complex structure. When the crystal is doped with CaO and MgO to form some of the Sm atomic sites are occupied by Ca atoms (SmLi02)0.8(CaO-MgO)02, and some of the Li sites are occupied by Mg. Therefore, the electric defect in the solid is enhanced. The dielectric loss in this catalyst is consequently significantly increased by doping. The increased loss factor favours the formation of CH3 radicals because the microwave field excites the specific sites, which extract a proton from a methane molecule to create a CH3 radical.
G. Roussy et al. /Catalysis To&y 21(1994) 349-355
355
5. Conclusion We are studying, in more detail the OCM reaction by varying the catalyst and the geometry of the reactor in order to separate the influences of the thermal conditions from those of the electric field which excites catalytic sites.
References [ 11 J. ho, J.X. Wang, C.H. Lin and J.H. Lunsford, J. Phys. Chem., 107 (1985) 5062. [2] J. Huang and S. Suib, J. Phys. Chem., 97 (1993) 9403-9407. [3] M.Y. Tse, M.C. Depew and J.K.S. Wan, Res. Chem. Interm., 13 (1990) 221. [4] G. Bond, R.B. Moyes and D.A. Whan, Catal. Today, 17 (1993) 427. [5] G. Roussy. A. Zoulalian. M. Charleyre and J.M. Thiebaut, J. Phys. Chem., 88 (1984) 5702. [6] G. Roussy, J.M. Thiebaut, M. Anzarmou, C. Richard, R. Martin, J. Microwave Power Electr. Energy, ( 1987) 189. [7] J.M. Thiebaut, H. Ammor and G.J. Roussy, Chim. Phys., 85 ( 1985) 799. [ 81 U. Olsbye, Thesis, University of Oslo, Norway ( 1991).
22
Controlled oxidation of methane doped catalysts irradiated by microwaves G. Roussy **a,J.M. Thiebaut a, M. Souiri a, E. Marchal a, A. Kiennemann b, G. Maire b aLSTM Unrversitt!de NANCY I, BP 239, 54500 Vandoeuvre, France ’ LERCSI-URA 1418, Umversite de Strasbourg, Strasbourg, France
Abstract The oxidative coupling of methane has been studied on a (SmLi02)0,(CaOMgO)0~2 catalyst by using two different modes of heating: a conventional oven and a microwave irradiation set-up. The C2 selectivity obtained with microwave heating was much higher than with conventional heating especially at low conversions. This difference could arise from the reduction of oxidation products (ethane, ethylene) in the gas phase under microwave irradiation.
1. Introduction There is considerable interest in the catalytic conversion of methane, the principal constituent of natural gas, into C2 hydrocarbons under partial oxidation conditions around 800°C [ 11. The oxidative coupling of methane (OCM) to obtain C2 products is currently under investigation in several laboratories as a possible commercial process for the production of higher hydrocarbons. A potential problem for the use of this technique for higher hydrocarbon production is that under high-temperature oxidative conditions, the desired C2 products are considerably more reactive toward oxidation than is methane itself. In the field of partial methane oxidation, the aim of this present work is to test the potential of microwaves compared to conventional thermal heating for the OCM. Such microwave heating has indeed been claimed to be efficient for the production of Cz hydrocarbons from methane by formation of a plasma induced without a solid [ 21, by using metals (Ni, Fe, W) or finely divided refractory oxides [ 31. The oxidative coupling of methane was studied by Bond et al. [ 41 in the * Corresponding author. 0920-5861/94/$07.00 Q 1994 Elsevier Science B.V. All rights reserved SSDIO920-5861(94)00099-9
G. Roussy et al. /Catalysis Today 21 (1994) 349-355
350
presence of microwaves, using sodium aluminate as a catalyst. Similar selectivity to C,s is obtained using conventional heating and microwave heating but a reaction temperature 400°C lower is used when the sample is irradiated by microwaves. During microwave irradiation, the electric field induces movement of ionic species and polar molecules. The local heat which is generated by the microscopic movement is fundamentally different from that in classical heating where the energy is equally furnished to all the atoms comprising the structure [ 561. Under these conditions, the chemical reactions can be controlled directly inside dielectric matter and can modify the catalyst activity for the OCM.
2. Experimental
section
2.1. Description of the thermal heating device The catalytic reactions are performed in a conventional way. The gases are first mixed before entering a fixed bed quartz reactor (6.6 mm ID), equipped with electronic mass flow meters (Brooks 5878). The catalytic bed is placed between two layers of ground quartz particles which fill all the remaining volume. The sample is pretreated overnight with the feed gas used for the catalyst test. The operating conditions were: catalyst weight 1.4 g; feed gas partial pressures: 0.133 atm CH,, 0.066 atm O2 and 0.8 atm He; gas flow: 4.5 1 h-’ gcat-’ (SIP). 2.2. Description and working of the irradiation device The microwave set-up used for irradiating the catalysts during reaction has been described in more detail elsewhere (Fig. 1) [ 71. The catalyst is placed in a rectangular resonant cavity (TEOl3), the frequency (2.45 GHz) of which is controlled by the displacement of a short circuit with a stepping motor run by a HP 9825 computer. The computer also controls the temperature of the catalyst by varying
CI
CI
s
IY W
”
ST
-
R
T
SC
YC
Fig. I. Microwave set-up for the catalyst study. (C) Chromatograph; (CA) catalyst; (CI) circulator; (G) generator; (MC) microcomputer; (P) input power; (PM) power-meter, (R) reactor; (S) switch; (SC) shortcircuit; (ST) stepping motor; (T) twist; (TH) thermocouple; (W) water load.
G. Roussy et al. /Catalysis
To&y 21 (1994) 349-355
351
Fig. 2. The chemical reactor. (C) Catalyst; (D) f&ted silica disk; (T) silica tube; (TH) thermocouple; (W) wave-guide.
the generator power at regular time intervals. The input power is about 50 W but the adsorbed power is between 20 and 28 W. The catalyst is placed in a tube of quartz, allowing the flow of reactant gas through it. The reactor is located perpendicular to the electric field inside the guide, so that a thermocouple can be introduced without interfering with the electric field (Fig. 2). The catalytic conditions are the same as those used in conventional heating (gas flow, catalyst pretreatment, catalyst weight ...).
3. Catalyst preparation ( SmLiO,) ,,*( CaOMgO),., catalysts were prepared as follows: From an ethanol solution of samarium nitrate, magnesium nitrate and calcium nitrate, a precipitate is formed by adding oxalic acid at pH = 2.2. The precipitate is extracted by evap-
352
G. Roussy et al. / Catalyszs Today 21 (1994) 349-355
oration to dryness at 120°C. Lithium was introduced as Li2C03 or LiOH into the ethanoic solution that contained the precipitate. The resulting material was calcined at 850°C for 24 h.
4. Results and discussion A comparison of the reactivities and selectivities of the (SmLiO,),,s(CaOMgO),., catalyst for both heating modes showed (Fig. 3) that the C2 selectivities are much higher under microwave irradiation (MW) than conventional heating (CH) especially at low conversions. Under microwave irradiation, the C2 selectivity is 100% at low conversion but decreases with methane conversion, while with conventional heating, the C, selectivity is O%, at low conversion and increases with methane conversion. FT-IR study of the catalyst after reaction shows that, at low temperatures, the quantity of adsorbed CO* is low. The ratio of the number of C2 molecules to the number of CO, as a function of the ratio of the number of C2H6 molecules to the number of C2H, molecules for this catalyst (Fig. 4) clearly illustrates that there is a fundamental difference in mechanism between conventional heating and microwave irradiation. The kinetics of each of the five paths in Fig. 5 hold the key to interpreting the results. The separate reaction rates of each reaction (k, to k,) have been crudely evaluated by fitting a large amount of data including the oxidation of C2H6 and C2H4, taking into consideration that the k, follow an Arrhenius low and by assuming that the reaction order is the same as published in Olsbye’s thesis [ 81. In Fig. 6 are shown the calculated ratios ki( MW) lk,( CH) (k, is the reaction rate) as a function of the catalyst temperature T. Fig. 7 shows the initial rate ratios kl /k2 as a function of T. The results of Figs. 6 and 7 have been limited to low temperatures because the
0
10
20
CR4 CONVERSION
30
40
(%)
Fig. 3. Catalyst selectivity plot for ( SmLiO,)O *( CaO-MgO),
z catalyst
G. Roussy et al. /Catalysis
Today 2I (1994) 349-355
353
100
10
CH
x
s 1
0”
650°C 1 I
I
0
,
#
vs. C2H,/C2H,
I
,
3
2
1
‘2 Fig 4. &/CO,
,
,
4
5
H6’C2H4
for (SmLi02)0,(Ca0-Mg0),
z catalyst.
Fig. 5. The methane reaction scheme.
kiMW / kiCH
1:: .l
t”:~~__ -
.Ol
-
,001
-
. .
k5MW/k5CH
k4MWlk4CH
450
500
550
TEMPERATURE Fig. 6. Kinetic rates MW-CH.
Comparison
600
650
(“C) of the methane reactions.
statistical data is only self consistent for temperatures lower than 700°C. Above 700°C our fit is not acceptable because the reaction orders are probably not in agreement with our results; indeed, as Olsbye showed, they change around that temperature during conventional heating. Fig. 6 shows that the ratios k, (MW) lk, (CH) and k2( MW) lk2( CH) are clearly greater than 1. The catalyst is thus more active under microwave irradiation. Fig.
354
G. Roussy et al. /Catalysis
Today 21 (1994) 349-355
kllk2
II 400
.
I
450
I
I
500
I
I
I
550
TEMPERATURE
I
600
I
I
650
(“C)
Fig. 7. Initial selectivity kl/kz.
6 also shows that the ratios %(MW) /k,(CH) and k,(MW) Ik,(CH) are low. The reactions labelled 4 and 5 through which the products C,H, and C2H4 are oxidized, are much slower under microwave irradiation than in conventional heating. We can interpret these results by noting that the degradation reactions (4 and 5) occur in the gas phase, and we might expect them to be slow if the gas phase temperature is less than the temperature of the solid catalyst. Nevertheless, different catalytic pathways for both heating modes can be envisaged. On the other hand, the initial selectivity, which is indicated by the ratio kl/k2 (Fig. 7) is nearly constant in microwave experiments while it varies significantly in conventional heating. The k, and k2 values are extrapolated for temperatures inferior to 550°C. We do not have a clear explanation of why the k,lk, ratio is different under microwave irradiation than conventional heating. However, preferential excitation of specific ionic sites (O-, O*- ...) by the microwave field may be a plausible hypothesis. The specific catalyst used was (SmLiO,,), 8(CaO-MgO)0 2 which gives the best catalysis in methane oxidation and has some dielectric loss. In general, crystallized oxides have low dielectric loss because the movement of an atom is difficult, at best, and often not possible at all. SmLiO, has a small loss factor and a complex structure. When the crystal is doped with CaO and MgO to form some of the Sm atomic sites are occupied by Ca atoms (SmLi02)0.8(CaO-MgO)02, and some of the Li sites are occupied by Mg. Therefore, the electric defect in the solid is enhanced. The dielectric loss in this catalyst is consequently significantly increased by doping. The increased loss factor favours the formation of CH3 radicals because the microwave field excites the specific sites, which extract a proton from a methane molecule to create a CH3 radical.
G. Roussy et al. /Catalysis To&y 21(1994) 349-355
355
5. Conclusion We are studying, in more detail the OCM reaction by varying the catalyst and the geometry of the reactor in order to separate the influences of the thermal conditions from those of the electric field which excites catalytic sites.
References [ 11 J. ho, J.X. Wang, C.H. Lin and J.H. Lunsford, J. Phys. Chem., 107 (1985) 5062. [2] J. Huang and S. Suib, J. Phys. Chem., 97 (1993) 9403-9407. [3] M.Y. Tse, M.C. Depew and J.K.S. Wan, Res. Chem. Interm., 13 (1990) 221. [4] G. Bond, R.B. Moyes and D.A. Whan, Catal. Today, 17 (1993) 427. [5] G. Roussy. A. Zoulalian. M. Charleyre and J.M. Thiebaut, J. Phys. Chem., 88 (1984) 5702. [6] G. Roussy, J.M. Thiebaut, M. Anzarmou, C. Richard, R. Martin, J. Microwave Power Electr. Energy, ( 1987) 189. [7] J.M. Thiebaut, H. Ammor and G.J. Roussy, Chim. Phys., 85 ( 1985) 799. [ 81 U. Olsbye, Thesis, University of Oslo, Norway ( 1991).
22