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Effect of sulfur on the oxidative coupling of methane over a lanthana catalyst
Applied Catalysis A: General, 82 (1992) 13-30 Science Publishers B.V., Amsterdam
Effect of sulfur on the oxidative coupling of methane over a lanthana catalyst Ian Campbell*, Sureyya Saricilar, Ian C. Hoare and Suresh K. Bhargava’ CSIRO Division of Coal and Energy Technology, Lucas Heights Research Laboratories, Private Mail Bag 7, Menai, N.S. W. 2234 (Awtraliu), fax. (+61-2) 5436774 (Received 28 May 1991, revised manuscript received 22 October 1991)
Abstract Addition of trace amounts of H&l to the reactants for metbane oxidative coupling results in the formation of SO, and poisoning of a lanthanum oxide catalyst. Poisoning has little effect on Cx+ selectivity until conversion falls to a low level. The factors controlling catalyst poisoning were found to be sulfur concentration, gas flow-rate and catalyst temperature. Poisoned catalyst was characterised by PI-IRS, XRD, 1sOzexchange capacity and TGA, and was found to involve formation of inactive LaxO,SO, which has a decomposition temperature of ca. 900°C under reaction conditions. An equilibrium is set up between formation and decomposition of the LazOzSOI, and the extent of poisoning depends on partial pressure of SO,. Keywords: methane oxidative coupling, lanthanum oxide, lanthanum oxysulphate, sulphur poisoning.
INTRODUCTION
Over the past decade the oxidative coupling of methane to higher hydrocarbons has received considerable attention as a potential process for the conversion of natural gas to transportation fuels and chemical feedstocks [l-6]. One aspect of the reaction that has not received much attention is the effect of impurities in the feed gas on the reaction. Other than methane, the major components in natural gas are higher hydrocarbons, nitrogen, carbon dioxide and water vapour [ 71. Higher hydrocarbons are believed to undergo similar reactions to methane except they are more reactive. Nitrogen is an undesirable diluent in the process streams. Some work has been carried out on the effects of carbon dioxide impurities [ 6-121 mainly in association with stability of lithium based catalysts. A recent paper [ 131 reports the use of water vapour as ‘Present address: Department of Applied Chemistry, Royal Melbourne Institute of Technology, P.O. 2476V, Melbourne, VIC. 3001, Australia.
0926-860X/92/$05.66
0 1992 Elsevier Science Publishers B.V. All rights reserved.
14
additive, but there are no other detailed reports on its effects. The only other studies published on the effects of impurities concern addition of traces of chlorinated hydrocarbons [ 14-181. This lack of data on the effects of impurities has been encouraged by the presupposition that a coupling reactor would be preceded by a gas purification train [ 19,201. Sulfur compounds, often found in natural gas [ 71 in the ppb to low percent range, are well known as severe poisons of many catalysts [ 21-251. The removal of sulfur compounds, while common in gas processing plants, would add considerably to the capital cost of a development project. This paper presents the results of an investigation of the effect of hydrogen sulfide (H,S) on the oxidative coupling reaction over a lanthanum oxide catalyst. Some inferences are drawn about the types of surface sites on the catalyst. The factors controlling poisoning are identified and their impact on the oxidative coupling process discussed. EXPERIMENTAL
Catalyst preparation
and churacterization
The lanthanum oxide catalyst used in this study was prepared by precipitation-gel dehydration techniques. The carbonate was precipitated from 0.01 A4 lanthanum nitrate by addition of ammonium bicarbonate while carbon dioxide was bubbled through the mixture. The product was filtered, washed with distilled water and acetone, and dried at 40°C in a vacuum oven. The catalyst was prepared by drying in air at 12O”C, calcination at 400” C for 2 h and a further 2 h at 700°C. The catalyst was not activated before being used, as further calcination did not enhance performance. The freshly prepared catalyst was characterized by nitrogen adsorption for BET surface area, Fourier transform infrared spectroscopy (FT-IRS ), and powder X-ray diffraction (XRD ). Catalyst which had been exposed to air for several minutes during handling (hereafter called aged catalyst) was characterized by FT-IRS, XRD, 180z exchange capacity and thermogravimetric analysis (TGA). A batch of poisoned catalyst was prepared by exposing 0.120 g of aged catalyst to typical reaction conditions (CH,/O, = 9 and 750” C ) with high concentration of H,S (400 ppm) in the feed gas. After 4 h, its activity for oxidative coupling had fallen to virtually zero. Reactant gases were turned off, and the catalyst removed from the reactor as quickly as possible under an inert atmosphere and characterized as above. During XRD analysis and storage, this catalyst was kept under an inert atmosphere as much as possible to reduce carbon dioxide and water absorption. After analysis the catalyst was regenerated by exposure to reaction conditions at 900°C until conversion became constant and no sulfur was detected in the exit stream. Lanthanum sulfate was prepared
15
by the method of Wendlandt [26]. The activity of this material was determined in the usual manner. The anhydrous form (prepared by calcination at 400” C for 2 h in dry nitrogen) was used as a reference material for the FT-IRS and XRD studies. Reactor system The reactor system consisted of a 0.4 cm I.D. quartz fixed bed reactor with an 11 cm heated length and a 0.25 cm O.D. axial thermocouple well. The catalyst was supported on a quartz wool plug on a 0.35 cm O.D. quartz tube. The feed gases were methane (Matheson, CP grade 99% ), oxygen (CIG, Industrial grade 99.5% ) and a third stream which could be switched between helium (CIG, Industrial grade 99.99% pure), a helium/H,S mixture (CIG, 1220 ppm H&Sin helium) or a nitrogen/sulfur dioxide (SO,) mixture (CIG, 293 ppm SOz in nitrogen). All gas flows were controlled by Brooks 5850 TR electronic mass flow controllers and co-fed to the reactor system. After removing all moisture, the product gases were analysed by GC (Shimadzu GC-SA, Carbosphere packed 1.5 m x l/8 in. S.S. column, temperature programmed from 60 to 250” C, TCD ). The gas flow-rates leaving the reactor were measured with a bubble flow meter. The exit gases were tested for sulfur compounds by sampling as close to the quartz reactor tube exit as possible. After passing through a small glass moisture trap, the gases were conveyed to a GC column in l/l6 in. teflon tubing to reduce dead volumes and sulfur gas adsorption on metal. A GC ( Shimadzu GC8APFp, Chromosorb 107 packed 2 m x l/8 in. teflon column, temperature programmed from 80 to llO”C, sulfur specific flame photometric detector) was used to monitor for the sulfur containing gases (H&J, COS and SO,). The detection limits for H,S and SOZ were estimated as 1 and 0.5 ppm respectively. Catalyst testing procedures All tests were undertaken at atmospheric pressure except the preparation and regeneration of the completely poisoned catalyst which was carried out at 500 kPa resulting from the backpressure exerted by the larger catalyst sample. The catalyst was loaded into the reactor and the thermocouple shield inserted through the bed until it reached the quartz wool plug. The inlet and outlet connections were made and the gas flows adjusted to the desired levels. The temperature was raised so that the hottest point in the catalyst bed was at the desired temperature. The variation in temperature across the catalyst bed depended on the flow-rates used but never exceeded 20 oC for the normal catalyst bed depths. The partial pressures of CHI, 02, He and H,S typically used in the feed were 86.7,9.6, 5.0 and 0.006 kPa respectively. A CH,/O, ratio of 9 was used for all experiments. The products were sampled after 10 min on stream, and at half
16
hour intervals thereafter. Pure helium was used as the third feed for the first one-two hours to establish baseline conversion and selectivity results. After this the feed was switched to one containing the poison.
Characterization techniques XRD spectra were obtained using a Bragg-Brentano focussing diffractometer with a graphite monochromater. The cobalt radiation (KcuI,z with 1= 1.79026 A) was produced with 45 kV and 30 mA tube excitation. 26’ was varied from 3” to 100” with 0.04” resolution and a count time of 1 s. Thermogravimetric analyses were performed with a Stanton Redcroft TG-760 utilising about 5-6 mg of sample in flowing nitrogen (35 ml min-’ ) with a heating rate of 10°C min-‘. 1802 exchange rates were determined in a quartz reactor system [27] with helium and oxygen flows of 106 and 8.6 ml min-l respectively using a 4.3 ml pulse of ‘*02 and 0.020 g of catalyst in each case. Diffuse reflectance spectra were obtained with a nitrogen purged Matson Cygnus Fourier transform infrared spectrometer using a Harrick DR-2A diffuse reflectance apparatus. Samples were prepared as a 5% by weight mixture of the catalyst with dried KBr powder obtained from Spectra Tech KBr crystals. Diffuse reflectance spectra of the aged, poisoned and regenerated catalysts were obtained by collecting 1000 samples scans at 4 cm-’ resolution against a background of 1000 scans of KBr powder at the same resolution. The samples were kept under dry nitrogen at all times. RESULTS AND DISCUSSION
Blank reactor studies The effect of H,S on the blank reaction and the chemical fate of H&S,were determined under typical oxidative coupling conditions. With 10 and 60 ppm H2S, and temperatures up to 9OO”C, no carbon oxides or higher hydrocarbons were detected due to the high flow-rates used. However, at temperatures of 400°C and above, the H,S was partly oxidised to SOP and SOS. Above 7OO”C, all the added H$ was oxidised in the gas phase. Other likely sulfur impurities in natural gas would also oxidise to SO, at or below this temperature. These results are expected thermodynamically and confirm that in the presence of methane and oxygen, and at temperatures in the range 700-9OO”C, H2S is rapidly converted to SO, in the gas phase. The amount of SO, formed is very temperature dependent [ 281. At 4OO”C, the ratio of SOJSO, at equilibrium is 122 but reaction rates are slow, while at 750°C the reaction rate is fast but the equivalent ratio is 0.47. At 900°C the equilibrium ratio of SO,/ SO, is 0.12. Thus under test conditions the catalyst would be exposed to ap-
17
prox. 19 ppm SOS and 41 ppm SOZ at 750°C and at 900” C only 7 ppm SOS and 53 ppm SO,. Catalytic poisoning studies: the effect of temperature Complete poisoning of the lanthanum oxide catalyst occurred at 750” C using 60 ppm H,S and space velocity (SV) of 6.1 (Space velocity of gas hourly space velocity has units of mmol s- ’ g- ’ ) , (Fig. la). The poisoning occurred over a period of about 4 h with conversion falling from 10% to 0.5%. The results cannot be accounted for by competition for available oxygen between methane and H2S since the oxygen consumed by H,S oxidation is negligible. The run was repeated (Fig. lb) confirming the original results, but when the activity had nearly reached zero, the helium/H&I feed was switched back to pure helium. Almost no increase in activity was observed over the next 2 h indicating that the catalyst had been irreversibly poisoned under these conditions. The Cp+ selectivity rose slightly with added sulfur, then decreased as conversion dropped to 1% or less indicating that C2+ selectivity is not a simple or inverse function of conversion. No sulfur compounds were detected in the product stream until the conversion was less than 2% after which the SO, concentration in the reactor effluent increased slowly. The results obtained at 900 oC (other conditions as before) are shown in Fig. 2a. When the H,S feed was begun at this temperature, the conversion decreased slightly while the C,, selectivity increased marginally. Several repeats of the runs at 900’ C have confirmed these observations. If the H,S was turned off, the C,, selectivity fell to its previous value and conversion increased slowly, but not to its original level. Once again, SOz was the only sulfur containing product observed in all analyses after H2S was added to the feed. These results indicate that poisoning of the lanthana catalyst by H&S (or its products) is dependent upon the catalyst temperature, and show that H,S is readily converted to SOP under experimental conditions with a catalyst. Confirmation that the catalyst was being poisoned was obtained by substituting SO, for H2S, and repeating the experiments. Identical trends were observed, suggesting that the H,S is oxidised to SO, before it reached the catalyst. In another experiment conducted at 750 oC (other conditions as above ) , when the catalyst was nearly completely poisoned the temperature was increased to 900°C. Even though the H,S feed continued, methane conversion and C,, selectivity both increased dramatically over a 11 h period (Fig. 2b). Their final values were close to those obtained at 900’ C previously (Fig. 2a). As the temperature was increased, the amount of SO, leaving the reactor increased by a factor of 4, far exceeding that in previous runs. These observations indicate that the catalyst is poisoned by the formation of a surface phase [see eqn. (1) ] which is stable at moderate temperature (750 ’ C ) but not at high temperature (900 oC ) . A catalyst poisoned at 750’ C
16 60
0
4000
6000
12000
16000
20000
24000
26000
6-
20
-
10
42-
Time on Stream (s)
Fig. 1. Poisoning of the lanthana catalyst with time. ( l ) percent CH4 conversion, (m) percent Cz+ selectivity. Conditions were: (a) 75O”C, 60 ppm H2S, 0.020 g catalyst, SV=6, CH,/02=9; (b) same as for (a) but the H,S was turned off after 20 700 s.
will not recover significantly if the flow of poison is turned off because the existing surface phase is stable, but if the temperature is increased even in the presence of H2S, then the surface phase will undergo thermal decomposition until it reaches an equilibrium where the rate of formation of poisoned sites equals the rate of their decomposition. It seems likely that the poisoned surface phase is a form of lanthanum sulfate. LSO,,,, + nSOr(p) = (poisoned La species) (8j
(1)
19 SO
20 16 -
II
r=+
--• 70
1:::
;
T k = 12 .o r %
50
-
.z?
I
a-
‘;; @
.z 40 p
10 -
s v z. 0
60
s
i
yson
30
H,S off
+
3
620 410
2(a) o-
I
1
I
I
I
-
I
20 16
70
16 60 14
';; a.o c 12 r ?
50
.z? .g
10
40
s O6 ? 0
T 5
p d
30
+ 3
6 20 4 10
2 0 0
4000
8000
12000
16000
20000
24000
0 26000
Time on Stream (s)
Fig. 2. The effect of temperature on poisoning of Laz03. ( 0 ) percent CH, conversion, ( n ) percent Cz+ selectivity. Conditions were: (a) as for Fig. la except 900°C; (b) as for Fig. la except after 20 000 s the temperature was increased to 900°C.
C,, selectivity was barely effected by poisoning until the conversion dropped to a low level. This leads to two interpretations of the possible surface sites. The less likely is that there are two types of sites, one for producing methyl radicals and one that gives total oxidation products. To maintain the same C,, selectivity, both must poison at exactly the same rate. The other is that there is only one type of site. This may be responsible for both reactions or just the methyl radical production with oxidation to carbon oxides occurring in the gas phase. While there is evidence that catalyst surfaces can participate in total
20
oxidation [ 29 J, this relates to a different catalyst and Tong et al. [ 301 have shown that there is little reaction between methyl radicals and lanthana. The marginal increase observed in C 2+ selectivity with poisoning probably indicates a few total oxidation sites on the catalyst but not enough to be the major cause of carbon oxide formation. It is likely that the majority of sites only produce alkyl radicals which undergo gas phase oxidation. Thus C,, selectivity is only affected when conversion falls significantly since the oxygen partial pressure in the products increases while the alkyl radical concentration falls, leading to more gas phase non-selective oxidation. Effect of space velocity To examine the effect of gas flow-rate upon catalyst poisoning, experiments were performed at both 750 and 900’ C (Figs. 3 and 4) with 60 ppm H,S in the feed gas at space velocities of 12.1, 6.0 and 3.0. It is clear that at 750°C the catalyst poisons completely at high and moderate space velocities. However, at 9OO”C, complete poisoning does not occur. The amount of deactivation is dependent on the space velocity or gas flow-rate. At the higher space velocity, the deactivation is approximately 4% compared to 1.5% at moderate space velocity and essentially zero at the lowest flow-rate even though the concentration of sulfur in the feed was constant at 60 ppm H&S (see Table 1 for a summary of conditions and results). The explanation of this phenomenon lies 20 18 16
12000
16000
Time on Stream (s)
Fig. 3. The effect of space velocity on poisoning at 750°C. Conditions were: 60 ppm H,S, CHJ 02=9and (A) SV=12, (0) SV=6, (m) SV=3.
21
c
0
4000
8000
12000
16000
20000
24000
28000
Time on Stream (s)
Fig. 4. The effect of space velocity on poisoning at 900°C. Conditions were: 60 ppm H& CH.J O,=Sand (A) SV=12, (0) SV=6, (m) SV=3.
in the feed rates. Prior to poisoning the catalyst activity is above the feed rates, i.e. the reactions were oxygen limited. During poisoning at high space velocity the decreased catalyst activity limited the reaction and conversion fell. At low space velocity the reduced catalyst activity during poisoning remained above the feed rate (i.e. still was oxygen limited) so conversion remained constant. At moderate flows activity was nearly enough to convert all the oxygen so conversion fell only slightly. Thus while the temperature and concentration of sulfur in the feed control the equilibrium in eqn. (1)) the effect on methane conversion is determined by the difference between reduced activity and reactant feed rate. The effect of H2S concentration The effect of varying the concentration of H,S in the feed was studied at a constant space velocity (Fig. 5a). At 900” C, addition of 200 ppm H&S level caused a significant initial poisoning, followed by almost constant conversion. A large amount of SO, was detected in the products in every analysis after the H,S feed was begun. A concentration of 6 ppm H,S at 750 oC (Fig. 5b) caused only a very slight deactivation, controlled by the low feed rate. No H,S and only an occasional, extremely small SO, peak was observed in the products. Comparing these results with those in Figs. 2a and 3a respectively, the differences are consistent with reaction (1). At 750°C the equilibrium is far to the right. However at 900” C the equilibrium is more in the centre with both solids existing in significant proportions determined by the sulfur concentration. The
900 20 11.6 9.0
La20,
La,O,
La203
La203
Laz03
La,O,
La20,
La,O,
La203
La,O,
750 750 900 900 750 750 900 750 900 20 20 10 20 40 20 20 20 20 12.1 3.0 6.1 7.3 3.0 6.1 5.8 6.9 12.1 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 60” 60 60 60 60 60 6 200 60 16.1 9.6 14.4 0.7 10.0 0.9 0.3 15.6 10.0 5.6 10.3 b 59.1 37.4 53.2 99.2 63.3 62.6 8.3 4.4 85.6 90.9 59.2 73.3 67.4 50.1 75.6 53.3 55.3 75.4 76.0 74.9 76.7 8593 10080 10168 11715 9342 15632 13921 19026 12630 16515 174313
750 20 6.1 9.0
La203
“SO2 was used instead of H&3 in this run. bOxygen conversion was not able to be determined accurately due to nitrogen in the feed.
Temp. (“C) Weight (g) sv (mm01 s-l g-l) C&P& &S (mm) CH, Conv. ( R ) O2 Conv. ( % ) C&+Sel. (W) TOS with poison (s)
Catalyst
Summary of experimental conditions and results for the poisoning studies and for runs without poison
TABLE 1
La2(SO4)3
900 900 20 20 7.3 11.8 9.0 9.0 60” 13.5 0.4 6 4.9 74.6 41.1 10101 719
La&
23 20
, L
16
-
16
-
14-O
(a)
-
L c 12 .o L1 210 6 7 *a2
2.. -
H,SO" e---
O6-
l
6000
4000
0
16000
12000
20000
24000
26000
Time on Stream (s)
20
s
16
-
16
-
14
-
(b)
yson
4 20
0
I
I
I
I
I
30
60
90
120
150
160
Time on Stream (x1000 s) Fig. 5. The effect of HzS concentration on poisoning. Note the different time scales. Conditions were: (a) 2OOppmH,S,9OO”C,0.020gcatalyst, SV=7, CH,/O,=9; (b) GppmH,S, 75O”C,O.O20 g catalyst, SV=6, CH,/02=9.
results at 900°C for various space velocities and H2S concentrations are replotted in Fig. 6 as fractional activity (conversion at time tI divided by the conversion at time to preceding the start of poisoning) versus the amount of sulfur (mg) added through the feed. This data demonstrates that the amount
24
I.
I
I,
I.
I.
I
2
4
6
6
10
12
Added
I
I,
I
I1
14
16
16
20
Sulfur (mg)
Fig. 6. Plot of fractional activity against milligrams of sulfur fed to the reactor. The sulfur feed rateisalsogiven. (0) 200ppmH,S,SV=7, (A) SV=12, (0) SV=6, (m) SV=3,andO.O4Og catalyst. Unless stated other conditions were 900” C, 60 ppm HxS, 0.020 g catalyst, CH,/O, = 9. TABLE 2 Oxygen-18 exchange over aged and completely poisoned lanthanum catalysts Conditions were a 4.3 ml pulse of ‘so2 in helium with a total flow of 114.2 ml min-’ and 0.020 g of catalyst Temperature
Time-on-stream
(“C)
(s)
Aged catalyst 1802exchange”
Poisoned catalyst “Ox exchange”
750
24.2
2.9
850
26.1
4.6
26.1
10.1 12.9 16.3 17.1 17.3
950
950 950 950 950
0
50 170 230 340
“Units are lo*’ atoms (g of catalyst) - ‘.
of deactivation is not a function of the total amount of sulfur added but of the rate of sulfur addition (i.e. sulfur concentration and reactant feed rate). Reversibility of oxide-sulfate conversion La2 (SO& was found to be inactive for the oxidative coupling reaction at 750 and 900 ’ C using a space velocity of 12. As expected, at 900 ’ C large amounts of SOz were detected in the exit stream over a period of 30 min indicating that the sulfate was decomposing, after which the material became active giving conversion and C,, selectivity similar to that for the normal catalyst under the same conditions. Following a further hour, SO, was admitted and the conversion fell to 2.5% during the next 3 h. These results confirm that the lanthanum oxide/sulfate systems are interconvertible in a methane/oxygen environment depending on the temperature and gas mixture present. 1802exchange experiments provided further evidence of the inactivity of the poisoned catalyst and its ability to recover when exposed to oxidative coupling conditions at high temperatures. The results given in Table 2 indicate that the oxygen exchange capability of the poisoned catalyst was almost an order of magnitude lower at 750’ C compared to the aged catalyst. At 950’ C it was lower by a factor of 2.5 and showed a significant increase in activity with time-onstream. Identification of catalyst phases The initial catalyst material had a surface area of less than 10 m2/g and was identified by powder XRD as hexagonal La203 with a trace of hexagonal La(OH),. Shafer and Roy [31] noted the rapid absorption of water from air by La20, to form La ( OH)B, while Bernal et al. [32] reported hydration and carbonation in the bulk when lanthana is exposed to air. Analysis of the aged catalyst showed that moisture and carbon dioxide absorption had taken place since hexagonal La (OH), with traces of hexagonal, monoclinic and tetragonal La202C0, were present (Fig. 7~). These compounds were corroborated by FTIRS of the aged catalyst (Fig. 8). The strong bands appearing in the 1600-1300 cm-’ region are due to the presence of the CO:- ion with a site symmetry lowered from DBh [ 331, while the strong band at 496 cm-’ is the result of the lanthanum oxygen stretching mode. This band and the carbonate bands mentioned previously can be attributed to the presence of lanthanum oxycarbonate, while the strong band at 3609 cm-l is due to the presence of lanthanum hydroxide. TGA indicated that La(OH), dehydrates to La,O, in two steps (La(OH),+La00H+La203) andis complete by 500°C. Thus under reaction conditions the hydroxide is completely dehydrated to hexagonal La203. Decomposition of the oxycarbonate material depends on the partial pressure of
26
(a)
90
1
(c) 60
-
go: $
4o _ 20 OO
10
20
30
40
50
60
70
60
90
100
i;ioo 10
20
30
40 2 Theta
(degrees)
Fig. 7. XRD data plotted as percent (I/I,,_) against 28 for (a) anhydrous La2(S04)3, (b) the completely poisoned catalyst [La,O,SO,], and (c) the aged catalyst [La(OH )3 withtracesof La,O&O, 1, (d) the regenerated catalyst,and (e) hexagonal La,O,.
carbon dioxide in the reaction products but under normal reaction conditions occurs in the temperature range of 7504350” C. The XRD spectrum (Fig. 7) of the poisoned catalyst is significantly different from that of the anhydrous sulfate as prepared and that of the parent catalyst (both aged and pure oxide) and was identified as orthorhombic La,O$O,,
27
a4-
v- 0.3
3609
t-.
.
,
I..
‘3500
1358
.
,
3000
.
.
.
.
,
a,
2500
‘_
/
*‘,‘a
2000
Wavenumber
r.,
1500
1-7-7.
1000
,-
500
/ cm-’
Fig. 8. FT-IR diffuse reflectance spectrum of aged catalyst.
<
I
A
111,“‘1,“,,
3500
3000
I,,,
2500
I,,,
2000
Wavenumber
I,,,
1500
I,,,
1000
5 00
/ cm-’
Fig. 9. Comparison of the diffuse reflectance spectra of (a) the poisoned catalyst, (b) the regenerated catalyst and (c ) the aged catalyst.
in close agreement with data obtained by Haynes and Brown [ 341 and Laptev et al. [ 351. The regenerated material was hexagonal La,O, with traces of unconverted La,O,SO, and La20,S. The trace amount of oxysulfide was formed because of the long bed length (approx. 4 cm) necessitated by the amount of catalyst required for XRD analysis. The material at the top of the bed was regenerated and became active for oxidative coupling. This meant the bottom
28
* 3500
y-r-xyw
.(....,....
3000
2500
2000
Wavenumber
1500
.‘,““,’
1000
500
/ cm -’
(b)
3500
3000
2500
2000
Wavenumber
1500
1000
500
/ cm -’
Fig. 10. Diffuse reflectance spectra of (a) the poisoned catalyst and (b) the regenerated catalyst.
of the bed was not exposed to oxygen but to hydrogen and carbon monoxide (and other products ) which are capable of reducing La,O,SO, to La,O,S [ 341. Fig. 9 compares the diffuse reflectance spectra of the aged, poisoned, and regenerated catalysts.There are obvious differences in the regions 1600 to 1300 cm-’ and 1200 to 1000 cm-‘. Poisoning of the catalyst resulted in the disappearance of both the hydroxide, and carbonate bands, accompanied by the appearance of intense bands in the 1180-590 cm-’ region of the spectrum (Fig. 10a). The latter are indicative of the formation of La,O,SO, [ 36-381 and are very similar to those observed for a SOi- ion present as a bridging bidentate complex [39]. Bands at 1177,1113 and 1069 cm-l can be attributed to the vg vibration while those appearing at 654,618 and 596 cm-l are assigned to the v4 vibration. The remaining two vibrations v1 and v, are assigned to the band appearing at 991 cm-’ and the shoulder at 461 cm-‘, respectively. F&generation of the catalyst to its pre-poisoned activity resulted in a substantial reduction in the intensity of the SOi- bands relative to the La-O
29
stretch intensity and was accompanied by the reappearance of the hydroxide band at 3609 cm-’ (Fig. lob). The bands in the 1000-850 cm-’ region are attributed to sulphide stretching modes from the LazOzS impurity. CONCLUSIONS
This investigation of the effects of sulfur on the oxidative coupling reaction of methane has identified the fate of sulfur containing gases under typical reaction conditions. All likely sulfur impurities would be oxidised to SO, or SOS under typical reaction conditions. SO, gases can then poison an oxide type catalyst by formation of surface sulfates which are inactive for methane oxidative coupling. C2+ selectivity is not effected by poisoning until conversion falls to a low level. This result is interpreted as indicating the existence of only one type of surface site. The effect of poisoning, and whether it occurs or not, is controlled by (1) the concentration of sulfur in the feed gas, (2) the flowrate of the feed gas, (3) the catalyst operating temperature and (4) the decomposition temperature of the sulfated form of the catalytic material under reaction conditions (CH,/O, mixture and pressure). In an industrial situation, if sulfur poisoning of the catalyst occurred, it could be avoided by adjusting one (or more) of the above four factors, the easiest being operating temperature. ACKNOWLEDGEMENTS
The authors wish to thank M. Hoang for preparation of the initial catalyst material and the surface area determination, J.L. Lapszewicz for i802 exchange measurements, H.J. Hurst for thermogravimetric analyses and A. Ekstrom for helpful discussions. The work was carried out with financial support from the Industry Research and Development Board of the Department of Industry Technology and Commerce.
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11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
30 31 32 33 34 35 36 37 38 39
S.J. Korf, J.A. Roes, N.A. de Bruijn, J.G. van Ommen and J.R.H. Ross, Appl. Cat& 58 (1990) 131. T. Suzuki, K. Wada and Y. Watanabe, Appl. Cat& 59 (1990) 213. R. Regtop, private communication, 1988. P. Pereira, S.H. Lee, G.A. Somojai and H. Heinemann, Catal. I.&t., 6 (1990) 255. R. Burcb, G.D. Squire and S.C. Tsang, Appl. Catal., 46 (1989) 69. S. Ahmed and J.B. Moffat, Catal. Lett., 2 (1989) 309. S. Ahmed and J.B. Moffat, Appl. CataI., 58 (1990) 83. S. Ahmed and J.B. Moffat, Appi. Catal., 63 (1990) 129. S. Ahmedand J.B. Moffat, J. CataL, 125 (1990) 54. J.M. Fox, T. Chen and B.D. Degen, Chem. Eng. Prog., 86 (1990) 42. R.J. Tyler, Chem. Aust., 57 (1990) 140. B. Dehnon and G.F. Froment (Editors), Catalyst Deactivation, Stud. Surf. Sci. Catal., Vol. 6, EIsevier, Amsterdam, 1980. L.L. Hegedus and R.W. McCabe, Catalyst Poisoning (Chemical Industries, Vol. 17), Marcel Dekker, New York, 1984. R. Hughes, Deactivation of Catalysts, Academic Press, London, 1984. J. Oudar and H. Wise, Deactivation and Poisoning of Catalysts (Chemical Industries, Vol. 20), Marcel Dekker, New York, 1985. B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 1987, Stud. Surf. Sci. Catal., Vol. 34, Elsevier, Amsterdam, 1987. W.W. WendIandt, J. Inorg. Nuclear Chem., 7 (1958) 51. A. Ekstrom and J.L. Lapszewicz, J. Phys. Chem., 93 (1989) 5230. G.T. Austin and R.N. Shreve, Shreve’s Chemical Process Industries (5th ed.), McGraw-Hi& New York, 1984, pp. 329-335. N.A. Cant, C.A. Lukey and P.F. Nelson, J. Catal., 124 (1990) 336. Y. Tong, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem., 93 (1989) 2896. M.W. Shafer and R. Roy, J. Am. Ceram. Sot., 42 (1959) 563. S. Berm& F.J. Botana, R. Garcia, F. Ramirez and J.M. Rodriguez-Izquierdo, Appl. Catal., 31 (1987) 267. N.R. Smyrl, E.L. Fuller and G.L. Powell, Appl. Spectrosc., 37 (1983) 38. J.W. Haynes and J.J. Brown, J. Electrochem. Sot.: Solid State Sci., 115 (1968) 1060. V.I. Laptev, Yu.L. Suponitskii and A.F. Vorob’ev, Russ. J. Inorg. Chem., 32 (1987) 305. V.P. Surgutskii, V.I. Gaivoronskii and V.V. Serebrennikov, Russ. J. Inorg. Chem., 13 (1968) 527. T.F. Tenisheva, T.M. Pavlyukevich and A.I. Lazarev, Izv. Akad. Nauk SSSR, Ser. Khim., (1965) 1771. . G. Pannetier and A. Dereigne, Bull. Chim. Sot. Fr., (1963) 1850. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley, New York, 1986, p. 249.
Effect of sulfur on the oxidative coupling of methane over a lanthana catalyst Ian Campbell*, Sureyya Saricilar, Ian C. Hoare and Suresh K. Bhargava’ CSIRO Division of Coal and Energy Technology, Lucas Heights Research Laboratories, Private Mail Bag 7, Menai, N.S. W. 2234 (Awtraliu), fax. (+61-2) 5436774 (Received 28 May 1991, revised manuscript received 22 October 1991)
Abstract Addition of trace amounts of H&l to the reactants for metbane oxidative coupling results in the formation of SO, and poisoning of a lanthanum oxide catalyst. Poisoning has little effect on Cx+ selectivity until conversion falls to a low level. The factors controlling catalyst poisoning were found to be sulfur concentration, gas flow-rate and catalyst temperature. Poisoned catalyst was characterised by PI-IRS, XRD, 1sOzexchange capacity and TGA, and was found to involve formation of inactive LaxO,SO, which has a decomposition temperature of ca. 900°C under reaction conditions. An equilibrium is set up between formation and decomposition of the LazOzSOI, and the extent of poisoning depends on partial pressure of SO,. Keywords: methane oxidative coupling, lanthanum oxide, lanthanum oxysulphate, sulphur poisoning.
INTRODUCTION
Over the past decade the oxidative coupling of methane to higher hydrocarbons has received considerable attention as a potential process for the conversion of natural gas to transportation fuels and chemical feedstocks [l-6]. One aspect of the reaction that has not received much attention is the effect of impurities in the feed gas on the reaction. Other than methane, the major components in natural gas are higher hydrocarbons, nitrogen, carbon dioxide and water vapour [ 71. Higher hydrocarbons are believed to undergo similar reactions to methane except they are more reactive. Nitrogen is an undesirable diluent in the process streams. Some work has been carried out on the effects of carbon dioxide impurities [ 6-121 mainly in association with stability of lithium based catalysts. A recent paper [ 131 reports the use of water vapour as ‘Present address: Department of Applied Chemistry, Royal Melbourne Institute of Technology, P.O. 2476V, Melbourne, VIC. 3001, Australia.
0926-860X/92/$05.66
0 1992 Elsevier Science Publishers B.V. All rights reserved.
14
additive, but there are no other detailed reports on its effects. The only other studies published on the effects of impurities concern addition of traces of chlorinated hydrocarbons [ 14-181. This lack of data on the effects of impurities has been encouraged by the presupposition that a coupling reactor would be preceded by a gas purification train [ 19,201. Sulfur compounds, often found in natural gas [ 71 in the ppb to low percent range, are well known as severe poisons of many catalysts [ 21-251. The removal of sulfur compounds, while common in gas processing plants, would add considerably to the capital cost of a development project. This paper presents the results of an investigation of the effect of hydrogen sulfide (H,S) on the oxidative coupling reaction over a lanthanum oxide catalyst. Some inferences are drawn about the types of surface sites on the catalyst. The factors controlling poisoning are identified and their impact on the oxidative coupling process discussed. EXPERIMENTAL
Catalyst preparation
and churacterization
The lanthanum oxide catalyst used in this study was prepared by precipitation-gel dehydration techniques. The carbonate was precipitated from 0.01 A4 lanthanum nitrate by addition of ammonium bicarbonate while carbon dioxide was bubbled through the mixture. The product was filtered, washed with distilled water and acetone, and dried at 40°C in a vacuum oven. The catalyst was prepared by drying in air at 12O”C, calcination at 400” C for 2 h and a further 2 h at 700°C. The catalyst was not activated before being used, as further calcination did not enhance performance. The freshly prepared catalyst was characterized by nitrogen adsorption for BET surface area, Fourier transform infrared spectroscopy (FT-IRS ), and powder X-ray diffraction (XRD ). Catalyst which had been exposed to air for several minutes during handling (hereafter called aged catalyst) was characterized by FT-IRS, XRD, 180z exchange capacity and thermogravimetric analysis (TGA). A batch of poisoned catalyst was prepared by exposing 0.120 g of aged catalyst to typical reaction conditions (CH,/O, = 9 and 750” C ) with high concentration of H,S (400 ppm) in the feed gas. After 4 h, its activity for oxidative coupling had fallen to virtually zero. Reactant gases were turned off, and the catalyst removed from the reactor as quickly as possible under an inert atmosphere and characterized as above. During XRD analysis and storage, this catalyst was kept under an inert atmosphere as much as possible to reduce carbon dioxide and water absorption. After analysis the catalyst was regenerated by exposure to reaction conditions at 900°C until conversion became constant and no sulfur was detected in the exit stream. Lanthanum sulfate was prepared
15
by the method of Wendlandt [26]. The activity of this material was determined in the usual manner. The anhydrous form (prepared by calcination at 400” C for 2 h in dry nitrogen) was used as a reference material for the FT-IRS and XRD studies. Reactor system The reactor system consisted of a 0.4 cm I.D. quartz fixed bed reactor with an 11 cm heated length and a 0.25 cm O.D. axial thermocouple well. The catalyst was supported on a quartz wool plug on a 0.35 cm O.D. quartz tube. The feed gases were methane (Matheson, CP grade 99% ), oxygen (CIG, Industrial grade 99.5% ) and a third stream which could be switched between helium (CIG, Industrial grade 99.99% pure), a helium/H,S mixture (CIG, 1220 ppm H&Sin helium) or a nitrogen/sulfur dioxide (SO,) mixture (CIG, 293 ppm SOz in nitrogen). All gas flows were controlled by Brooks 5850 TR electronic mass flow controllers and co-fed to the reactor system. After removing all moisture, the product gases were analysed by GC (Shimadzu GC-SA, Carbosphere packed 1.5 m x l/8 in. S.S. column, temperature programmed from 60 to 250” C, TCD ). The gas flow-rates leaving the reactor were measured with a bubble flow meter. The exit gases were tested for sulfur compounds by sampling as close to the quartz reactor tube exit as possible. After passing through a small glass moisture trap, the gases were conveyed to a GC column in l/l6 in. teflon tubing to reduce dead volumes and sulfur gas adsorption on metal. A GC ( Shimadzu GC8APFp, Chromosorb 107 packed 2 m x l/8 in. teflon column, temperature programmed from 80 to llO”C, sulfur specific flame photometric detector) was used to monitor for the sulfur containing gases (H&J, COS and SO,). The detection limits for H,S and SOZ were estimated as 1 and 0.5 ppm respectively. Catalyst testing procedures All tests were undertaken at atmospheric pressure except the preparation and regeneration of the completely poisoned catalyst which was carried out at 500 kPa resulting from the backpressure exerted by the larger catalyst sample. The catalyst was loaded into the reactor and the thermocouple shield inserted through the bed until it reached the quartz wool plug. The inlet and outlet connections were made and the gas flows adjusted to the desired levels. The temperature was raised so that the hottest point in the catalyst bed was at the desired temperature. The variation in temperature across the catalyst bed depended on the flow-rates used but never exceeded 20 oC for the normal catalyst bed depths. The partial pressures of CHI, 02, He and H,S typically used in the feed were 86.7,9.6, 5.0 and 0.006 kPa respectively. A CH,/O, ratio of 9 was used for all experiments. The products were sampled after 10 min on stream, and at half
16
hour intervals thereafter. Pure helium was used as the third feed for the first one-two hours to establish baseline conversion and selectivity results. After this the feed was switched to one containing the poison.
Characterization techniques XRD spectra were obtained using a Bragg-Brentano focussing diffractometer with a graphite monochromater. The cobalt radiation (KcuI,z with 1= 1.79026 A) was produced with 45 kV and 30 mA tube excitation. 26’ was varied from 3” to 100” with 0.04” resolution and a count time of 1 s. Thermogravimetric analyses were performed with a Stanton Redcroft TG-760 utilising about 5-6 mg of sample in flowing nitrogen (35 ml min-’ ) with a heating rate of 10°C min-‘. 1802 exchange rates were determined in a quartz reactor system [27] with helium and oxygen flows of 106 and 8.6 ml min-l respectively using a 4.3 ml pulse of ‘*02 and 0.020 g of catalyst in each case. Diffuse reflectance spectra were obtained with a nitrogen purged Matson Cygnus Fourier transform infrared spectrometer using a Harrick DR-2A diffuse reflectance apparatus. Samples were prepared as a 5% by weight mixture of the catalyst with dried KBr powder obtained from Spectra Tech KBr crystals. Diffuse reflectance spectra of the aged, poisoned and regenerated catalysts were obtained by collecting 1000 samples scans at 4 cm-’ resolution against a background of 1000 scans of KBr powder at the same resolution. The samples were kept under dry nitrogen at all times. RESULTS AND DISCUSSION
Blank reactor studies The effect of H,S on the blank reaction and the chemical fate of H&S,were determined under typical oxidative coupling conditions. With 10 and 60 ppm H2S, and temperatures up to 9OO”C, no carbon oxides or higher hydrocarbons were detected due to the high flow-rates used. However, at temperatures of 400°C and above, the H,S was partly oxidised to SOP and SOS. Above 7OO”C, all the added H$ was oxidised in the gas phase. Other likely sulfur impurities in natural gas would also oxidise to SO, at or below this temperature. These results are expected thermodynamically and confirm that in the presence of methane and oxygen, and at temperatures in the range 700-9OO”C, H2S is rapidly converted to SO, in the gas phase. The amount of SO, formed is very temperature dependent [ 281. At 4OO”C, the ratio of SOJSO, at equilibrium is 122 but reaction rates are slow, while at 750°C the reaction rate is fast but the equivalent ratio is 0.47. At 900°C the equilibrium ratio of SO,/ SO, is 0.12. Thus under test conditions the catalyst would be exposed to ap-
17
prox. 19 ppm SOS and 41 ppm SOZ at 750°C and at 900” C only 7 ppm SOS and 53 ppm SO,. Catalytic poisoning studies: the effect of temperature Complete poisoning of the lanthanum oxide catalyst occurred at 750” C using 60 ppm H,S and space velocity (SV) of 6.1 (Space velocity of gas hourly space velocity has units of mmol s- ’ g- ’ ) , (Fig. la). The poisoning occurred over a period of about 4 h with conversion falling from 10% to 0.5%. The results cannot be accounted for by competition for available oxygen between methane and H2S since the oxygen consumed by H,S oxidation is negligible. The run was repeated (Fig. lb) confirming the original results, but when the activity had nearly reached zero, the helium/H&I feed was switched back to pure helium. Almost no increase in activity was observed over the next 2 h indicating that the catalyst had been irreversibly poisoned under these conditions. The Cp+ selectivity rose slightly with added sulfur, then decreased as conversion dropped to 1% or less indicating that C2+ selectivity is not a simple or inverse function of conversion. No sulfur compounds were detected in the product stream until the conversion was less than 2% after which the SO, concentration in the reactor effluent increased slowly. The results obtained at 900 oC (other conditions as before) are shown in Fig. 2a. When the H,S feed was begun at this temperature, the conversion decreased slightly while the C,, selectivity increased marginally. Several repeats of the runs at 900’ C have confirmed these observations. If the H,S was turned off, the C,, selectivity fell to its previous value and conversion increased slowly, but not to its original level. Once again, SOz was the only sulfur containing product observed in all analyses after H2S was added to the feed. These results indicate that poisoning of the lanthana catalyst by H&S (or its products) is dependent upon the catalyst temperature, and show that H,S is readily converted to SOP under experimental conditions with a catalyst. Confirmation that the catalyst was being poisoned was obtained by substituting SO, for H2S, and repeating the experiments. Identical trends were observed, suggesting that the H,S is oxidised to SO, before it reached the catalyst. In another experiment conducted at 750 oC (other conditions as above ) , when the catalyst was nearly completely poisoned the temperature was increased to 900°C. Even though the H,S feed continued, methane conversion and C,, selectivity both increased dramatically over a 11 h period (Fig. 2b). Their final values were close to those obtained at 900’ C previously (Fig. 2a). As the temperature was increased, the amount of SO, leaving the reactor increased by a factor of 4, far exceeding that in previous runs. These observations indicate that the catalyst is poisoned by the formation of a surface phase [see eqn. (1) ] which is stable at moderate temperature (750 ’ C ) but not at high temperature (900 oC ) . A catalyst poisoned at 750’ C
16 60
0
4000
6000
12000
16000
20000
24000
26000
6-
20
-
10
42-
Time on Stream (s)
Fig. 1. Poisoning of the lanthana catalyst with time. ( l ) percent CH4 conversion, (m) percent Cz+ selectivity. Conditions were: (a) 75O”C, 60 ppm H2S, 0.020 g catalyst, SV=6, CH,/02=9; (b) same as for (a) but the H,S was turned off after 20 700 s.
will not recover significantly if the flow of poison is turned off because the existing surface phase is stable, but if the temperature is increased even in the presence of H2S, then the surface phase will undergo thermal decomposition until it reaches an equilibrium where the rate of formation of poisoned sites equals the rate of their decomposition. It seems likely that the poisoned surface phase is a form of lanthanum sulfate. LSO,,,, + nSOr(p) = (poisoned La species) (8j
(1)
19 SO
20 16 -
II
r=+
--• 70
1:::
;
T k = 12 .o r %
50
-
.z?
I
a-
‘;; @
.z 40 p
10 -
s v z. 0
60
s
i
yson
30
H,S off
+
3
620 410
2(a) o-
I
1
I
I
I
-
I
20 16
70
16 60 14
';; a.o c 12 r ?
50
.z? .g
10
40
s O6 ? 0
T 5
p d
30
+ 3
6 20 4 10
2 0 0
4000
8000
12000
16000
20000
24000
0 26000
Time on Stream (s)
Fig. 2. The effect of temperature on poisoning of Laz03. ( 0 ) percent CH, conversion, ( n ) percent Cz+ selectivity. Conditions were: (a) as for Fig. la except 900°C; (b) as for Fig. la except after 20 000 s the temperature was increased to 900°C.
C,, selectivity was barely effected by poisoning until the conversion dropped to a low level. This leads to two interpretations of the possible surface sites. The less likely is that there are two types of sites, one for producing methyl radicals and one that gives total oxidation products. To maintain the same C,, selectivity, both must poison at exactly the same rate. The other is that there is only one type of site. This may be responsible for both reactions or just the methyl radical production with oxidation to carbon oxides occurring in the gas phase. While there is evidence that catalyst surfaces can participate in total
20
oxidation [ 29 J, this relates to a different catalyst and Tong et al. [ 301 have shown that there is little reaction between methyl radicals and lanthana. The marginal increase observed in C 2+ selectivity with poisoning probably indicates a few total oxidation sites on the catalyst but not enough to be the major cause of carbon oxide formation. It is likely that the majority of sites only produce alkyl radicals which undergo gas phase oxidation. Thus C,, selectivity is only affected when conversion falls significantly since the oxygen partial pressure in the products increases while the alkyl radical concentration falls, leading to more gas phase non-selective oxidation. Effect of space velocity To examine the effect of gas flow-rate upon catalyst poisoning, experiments were performed at both 750 and 900’ C (Figs. 3 and 4) with 60 ppm H,S in the feed gas at space velocities of 12.1, 6.0 and 3.0. It is clear that at 750°C the catalyst poisons completely at high and moderate space velocities. However, at 9OO”C, complete poisoning does not occur. The amount of deactivation is dependent on the space velocity or gas flow-rate. At the higher space velocity, the deactivation is approximately 4% compared to 1.5% at moderate space velocity and essentially zero at the lowest flow-rate even though the concentration of sulfur in the feed was constant at 60 ppm H&S (see Table 1 for a summary of conditions and results). The explanation of this phenomenon lies 20 18 16
12000
16000
Time on Stream (s)
Fig. 3. The effect of space velocity on poisoning at 750°C. Conditions were: 60 ppm H,S, CHJ 02=9and (A) SV=12, (0) SV=6, (m) SV=3.
21
c
0
4000
8000
12000
16000
20000
24000
28000
Time on Stream (s)
Fig. 4. The effect of space velocity on poisoning at 900°C. Conditions were: 60 ppm H& CH.J O,=Sand (A) SV=12, (0) SV=6, (m) SV=3.
in the feed rates. Prior to poisoning the catalyst activity is above the feed rates, i.e. the reactions were oxygen limited. During poisoning at high space velocity the decreased catalyst activity limited the reaction and conversion fell. At low space velocity the reduced catalyst activity during poisoning remained above the feed rate (i.e. still was oxygen limited) so conversion remained constant. At moderate flows activity was nearly enough to convert all the oxygen so conversion fell only slightly. Thus while the temperature and concentration of sulfur in the feed control the equilibrium in eqn. (1)) the effect on methane conversion is determined by the difference between reduced activity and reactant feed rate. The effect of H2S concentration The effect of varying the concentration of H,S in the feed was studied at a constant space velocity (Fig. 5a). At 900” C, addition of 200 ppm H&S level caused a significant initial poisoning, followed by almost constant conversion. A large amount of SO, was detected in the products in every analysis after the H,S feed was begun. A concentration of 6 ppm H,S at 750 oC (Fig. 5b) caused only a very slight deactivation, controlled by the low feed rate. No H,S and only an occasional, extremely small SO, peak was observed in the products. Comparing these results with those in Figs. 2a and 3a respectively, the differences are consistent with reaction (1). At 750°C the equilibrium is far to the right. However at 900” C the equilibrium is more in the centre with both solids existing in significant proportions determined by the sulfur concentration. The
900 20 11.6 9.0
La20,
La,O,
La203
La203
Laz03
La,O,
La20,
La,O,
La203
La,O,
750 750 900 900 750 750 900 750 900 20 20 10 20 40 20 20 20 20 12.1 3.0 6.1 7.3 3.0 6.1 5.8 6.9 12.1 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 60” 60 60 60 60 60 6 200 60 16.1 9.6 14.4 0.7 10.0 0.9 0.3 15.6 10.0 5.6 10.3 b 59.1 37.4 53.2 99.2 63.3 62.6 8.3 4.4 85.6 90.9 59.2 73.3 67.4 50.1 75.6 53.3 55.3 75.4 76.0 74.9 76.7 8593 10080 10168 11715 9342 15632 13921 19026 12630 16515 174313
750 20 6.1 9.0
La203
“SO2 was used instead of H&3 in this run. bOxygen conversion was not able to be determined accurately due to nitrogen in the feed.
Temp. (“C) Weight (g) sv (mm01 s-l g-l) C&P& &S (mm) CH, Conv. ( R ) O2 Conv. ( % ) C&+Sel. (W) TOS with poison (s)
Catalyst
Summary of experimental conditions and results for the poisoning studies and for runs without poison
TABLE 1
La2(SO4)3
900 900 20 20 7.3 11.8 9.0 9.0 60” 13.5 0.4 6 4.9 74.6 41.1 10101 719
La&
23 20
, L
16
-
16
-
14-O
(a)
-
L c 12 .o L1 210 6 7 *a2
2.. -
H,SO" e---
O6-
l
6000
4000
0
16000
12000
20000
24000
26000
Time on Stream (s)
20
s
16
-
16
-
14
-
(b)
yson
4 20
0
I
I
I
I
I
30
60
90
120
150
160
Time on Stream (x1000 s) Fig. 5. The effect of HzS concentration on poisoning. Note the different time scales. Conditions were: (a) 2OOppmH,S,9OO”C,0.020gcatalyst, SV=7, CH,/O,=9; (b) GppmH,S, 75O”C,O.O20 g catalyst, SV=6, CH,/02=9.
results at 900°C for various space velocities and H2S concentrations are replotted in Fig. 6 as fractional activity (conversion at time tI divided by the conversion at time to preceding the start of poisoning) versus the amount of sulfur (mg) added through the feed. This data demonstrates that the amount
24
I.
I
I,
I.
I.
I
2
4
6
6
10
12
Added
I
I,
I
I1
14
16
16
20
Sulfur (mg)
Fig. 6. Plot of fractional activity against milligrams of sulfur fed to the reactor. The sulfur feed rateisalsogiven. (0) 200ppmH,S,SV=7, (A) SV=12, (0) SV=6, (m) SV=3,andO.O4Og catalyst. Unless stated other conditions were 900” C, 60 ppm HxS, 0.020 g catalyst, CH,/O, = 9. TABLE 2 Oxygen-18 exchange over aged and completely poisoned lanthanum catalysts Conditions were a 4.3 ml pulse of ‘so2 in helium with a total flow of 114.2 ml min-’ and 0.020 g of catalyst Temperature
Time-on-stream
(“C)
(s)
Aged catalyst 1802exchange”
Poisoned catalyst “Ox exchange”
750
24.2
2.9
850
26.1
4.6
26.1
10.1 12.9 16.3 17.1 17.3
950
950 950 950 950
0
50 170 230 340
“Units are lo*’ atoms (g of catalyst) - ‘.
of deactivation is not a function of the total amount of sulfur added but of the rate of sulfur addition (i.e. sulfur concentration and reactant feed rate). Reversibility of oxide-sulfate conversion La2 (SO& was found to be inactive for the oxidative coupling reaction at 750 and 900 ’ C using a space velocity of 12. As expected, at 900 ’ C large amounts of SOz were detected in the exit stream over a period of 30 min indicating that the sulfate was decomposing, after which the material became active giving conversion and C,, selectivity similar to that for the normal catalyst under the same conditions. Following a further hour, SO, was admitted and the conversion fell to 2.5% during the next 3 h. These results confirm that the lanthanum oxide/sulfate systems are interconvertible in a methane/oxygen environment depending on the temperature and gas mixture present. 1802exchange experiments provided further evidence of the inactivity of the poisoned catalyst and its ability to recover when exposed to oxidative coupling conditions at high temperatures. The results given in Table 2 indicate that the oxygen exchange capability of the poisoned catalyst was almost an order of magnitude lower at 750’ C compared to the aged catalyst. At 950’ C it was lower by a factor of 2.5 and showed a significant increase in activity with time-onstream. Identification of catalyst phases The initial catalyst material had a surface area of less than 10 m2/g and was identified by powder XRD as hexagonal La203 with a trace of hexagonal La(OH),. Shafer and Roy [31] noted the rapid absorption of water from air by La20, to form La ( OH)B, while Bernal et al. [32] reported hydration and carbonation in the bulk when lanthana is exposed to air. Analysis of the aged catalyst showed that moisture and carbon dioxide absorption had taken place since hexagonal La (OH), with traces of hexagonal, monoclinic and tetragonal La202C0, were present (Fig. 7~). These compounds were corroborated by FTIRS of the aged catalyst (Fig. 8). The strong bands appearing in the 1600-1300 cm-’ region are due to the presence of the CO:- ion with a site symmetry lowered from DBh [ 331, while the strong band at 496 cm-’ is the result of the lanthanum oxygen stretching mode. This band and the carbonate bands mentioned previously can be attributed to the presence of lanthanum oxycarbonate, while the strong band at 3609 cm-l is due to the presence of lanthanum hydroxide. TGA indicated that La(OH), dehydrates to La,O, in two steps (La(OH),+La00H+La203) andis complete by 500°C. Thus under reaction conditions the hydroxide is completely dehydrated to hexagonal La203. Decomposition of the oxycarbonate material depends on the partial pressure of
26
(a)
90
1
(c) 60
-
go: $
4o _ 20 OO
10
20
30
40
50
60
70
60
90
100
i;ioo 10
20
30
40 2 Theta
(degrees)
Fig. 7. XRD data plotted as percent (I/I,,_) against 28 for (a) anhydrous La2(S04)3, (b) the completely poisoned catalyst [La,O,SO,], and (c) the aged catalyst [La(OH )3 withtracesof La,O&O, 1, (d) the regenerated catalyst,and (e) hexagonal La,O,.
carbon dioxide in the reaction products but under normal reaction conditions occurs in the temperature range of 7504350” C. The XRD spectrum (Fig. 7) of the poisoned catalyst is significantly different from that of the anhydrous sulfate as prepared and that of the parent catalyst (both aged and pure oxide) and was identified as orthorhombic La,O$O,,
27
a4-
v- 0.3
3609
t-.
.
,
I..
‘3500
1358
.
,
3000
.
.
.
.
,
a,
2500
‘_
/
*‘,‘a
2000
Wavenumber
r.,
1500
1-7-7.
1000
,-
500
/ cm-’
Fig. 8. FT-IR diffuse reflectance spectrum of aged catalyst.
<
I
A
111,“‘1,“,,
3500
3000
I,,,
2500
I,,,
2000
Wavenumber
I,,,
1500
I,,,
1000
5 00
/ cm-’
Fig. 9. Comparison of the diffuse reflectance spectra of (a) the poisoned catalyst, (b) the regenerated catalyst and (c ) the aged catalyst.
in close agreement with data obtained by Haynes and Brown [ 341 and Laptev et al. [ 351. The regenerated material was hexagonal La,O, with traces of unconverted La,O,SO, and La20,S. The trace amount of oxysulfide was formed because of the long bed length (approx. 4 cm) necessitated by the amount of catalyst required for XRD analysis. The material at the top of the bed was regenerated and became active for oxidative coupling. This meant the bottom
28
* 3500
y-r-xyw
.(....,....
3000
2500
2000
Wavenumber
1500
.‘,““,’
1000
500
/ cm -’
(b)
3500
3000
2500
2000
Wavenumber
1500
1000
500
/ cm -’
Fig. 10. Diffuse reflectance spectra of (a) the poisoned catalyst and (b) the regenerated catalyst.
of the bed was not exposed to oxygen but to hydrogen and carbon monoxide (and other products ) which are capable of reducing La,O,SO, to La,O,S [ 341. Fig. 9 compares the diffuse reflectance spectra of the aged, poisoned, and regenerated catalysts.There are obvious differences in the regions 1600 to 1300 cm-’ and 1200 to 1000 cm-‘. Poisoning of the catalyst resulted in the disappearance of both the hydroxide, and carbonate bands, accompanied by the appearance of intense bands in the 1180-590 cm-’ region of the spectrum (Fig. 10a). The latter are indicative of the formation of La,O,SO, [ 36-381 and are very similar to those observed for a SOi- ion present as a bridging bidentate complex [39]. Bands at 1177,1113 and 1069 cm-l can be attributed to the vg vibration while those appearing at 654,618 and 596 cm-l are assigned to the v4 vibration. The remaining two vibrations v1 and v, are assigned to the band appearing at 991 cm-’ and the shoulder at 461 cm-‘, respectively. F&generation of the catalyst to its pre-poisoned activity resulted in a substantial reduction in the intensity of the SOi- bands relative to the La-O
29
stretch intensity and was accompanied by the reappearance of the hydroxide band at 3609 cm-’ (Fig. lob). The bands in the 1000-850 cm-’ region are attributed to sulphide stretching modes from the LazOzS impurity. CONCLUSIONS
This investigation of the effects of sulfur on the oxidative coupling reaction of methane has identified the fate of sulfur containing gases under typical reaction conditions. All likely sulfur impurities would be oxidised to SO, or SOS under typical reaction conditions. SO, gases can then poison an oxide type catalyst by formation of surface sulfates which are inactive for methane oxidative coupling. C2+ selectivity is not effected by poisoning until conversion falls to a low level. This result is interpreted as indicating the existence of only one type of surface site. The effect of poisoning, and whether it occurs or not, is controlled by (1) the concentration of sulfur in the feed gas, (2) the flowrate of the feed gas, (3) the catalyst operating temperature and (4) the decomposition temperature of the sulfated form of the catalytic material under reaction conditions (CH,/O, mixture and pressure). In an industrial situation, if sulfur poisoning of the catalyst occurred, it could be avoided by adjusting one (or more) of the above four factors, the easiest being operating temperature. ACKNOWLEDGEMENTS
The authors wish to thank M. Hoang for preparation of the initial catalyst material and the surface area determination, J.L. Lapszewicz for i802 exchange measurements, H.J. Hurst for thermogravimetric analyses and A. Ekstrom for helpful discussions. The work was carried out with financial support from the Industry Research and Development Board of the Department of Industry Technology and Commerce.
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