Chapter 25
Oxidative Coupling of Methane over Praseodymium Oxide in the Presence and Absence of Tetrachloromethane 1
Downloaded by UNIV LAVAL on October 24, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch025
Yasuyuki Matsumura, Shigeru Sugiyama , and John B. Moffat Department of Chemistry and Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada The catalytic oxidative coupling of methane to ethane and ethylene has been investigated on praseodymium compounds. Although praseodymium oxide produces predominantiy carbon oxides from methane, the addition of small quantities of tetrachloromethane (TCM) to the reactant stream significantly improves its catalytic activity. The X-ray diffraction pattern for the catalyst after the reaction with TCM shows the presence of praseodymium oxychloride. Further, the oxychloride, generated from praseodymium chloride by heating under oxygen at 750°C is found to catalyze the reaction effectively. The catalytic activity of praseodymium oxychloride is stabilized with addition of TCM to the feedstream presumably because TCM hinders the formation of praseodymium oxide on the surface during the reaction.
The oxidative coupling of methane to ethane and ethylene is an intriguing process for the utilization of natural gas, which is predominantly methane. Although the search for effective catalysts has been vigorous during the last decade the conversions of methane and selectivities to C hydrocarbons remain less than desirable for an economically practical process (1). The presence of chlorine in the reaction system has been found to enhance the conversion of methane and the selectivity to C compounds. Work in this laboratory has examined the effect of the continuous addition of small quantities of chlorine compounds into the catalyst or the feedstream (2) and work in other laboratories has focused on chlorine-promoted catalysts (3). The addition of a small quantity of tetrachloromethane (TCM) to the reactant stream often results in high conversion of methane and high selectivity to C compounds (4-7). Although participation of TCM in the gas phase reaction cannot be excluded, experimental observations show that TCM interacts with and alters the surface of the 2
2 +
2 +
1
Current address: Department of Chemical Science and Technology, University of Tokushima, Minamijosanjima, Tokushima 770, Japan 0097-6156/93/0523-0326$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
25. MATSUMURA ET AL.
327
Methane over Praseodymium Oxide
catalysts (7). However, the source of the effect of the introduction of chlorine is not yet clear. In this work, we will show that the addition of TCM to the feedstream in the methane conversion process results in the enhancement of the conversion of methane and the selectivity to C hydrocarbons on praseodymium oxide primarily as a result of the formation of praseodymium oxychloride, in contrast with the production of carbon oxides on praseodymium oxide in the absence of TCM (8-10). The surface properties of these catalysts are characterized by application of adsorption experiments and X-ray photoelectron spectroscopy (XPS). 2
Downloaded by UNIV LAVAL on October 24, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch025
Experimental Praseodymium oxide (Pr O ) was obtained from Aldrich and used without further purification. Praseodymium chloride (PrCl ) was prepared from praseodymium chloride hexahydrate (Aldrich 99.9%) by heating at ca. 150°C in air. The catalytic experiments were conducted in a fixed-bed continuous flow reactor operated under atmospheric pressure. The reactor was designed to minimize the free volume in the hottest zone to reduce the contribution of the noncatalytic homogeneous reaction. The sample was placed in a quartz tube (7-mm i.d. and 35 mm in length sealed at each end to a 4-mm i.d. tube) and sandwiched with quartz wool plugs. Pretreatment of the sample was carried out immediately prior to the reaction under a helium stream (0.90 dm h' ) or an oxygen stream (0.75 dm h" ) at the desired temperature. Tetrachloromethane (TCM) was admitted to the main flow of reactants (CH , 0 , and diluent He) by passing a separate stream of helium through a gas dispersion tube in a glass saturator containing the liquid at ice-water temperature. Appropriate adjustments were made to ensure that the residence time was unchanged by the addition of TCM. The total flow rate of the feedstream was 0.90 dm rf\ The reaction gas was analyzed with an on-stream gas chromatograph equipped with a TC detector and integrator. Porapak Τ (5.40 m) and Molecular sieve 5A (1.25 m) were used as separation columns. Blank experiments conducted with methane absent from the feed (0 + He + TCM) indicated that TCM undergoes oxidation producing carbon monoxide and/or carbon dioxide. The data reported were corrected by running duplicate experiments with methane absent under otherwise identical sets of values of the process variables. The surface area of the catalysts was measured by the conventional B.E.T. nitrogen adsorption method. Powder X-ray diffraction (XRD) patterns were recorded with a Siemens Model D500 diffractometer. Patterns were recorded over the range 2Θ = 5-70°. The adsorption of carbon dioxide or oxygen on praseodymium samples was measured by a constant-volume method using a calibrated Pirani vacuum gauge. Praseodymium oxide was heated in oxygen (4 kPa) at 775°C for 1 h, then evacuated at 750°C for 0.5 h just before the measurement. The sample of praseodymium oxychloride was preparedfrompraseodymium chloride by heating under oxygen flow 6
n
3
3
4
1
3
2
3
2
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
1
CATALYTIC SELECTIVE OXIDATION
Downloaded by UNIV LAVAL on October 24, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch025
328
at 750°C for 1 h. This sample and praseodymium chloride were preheated in vacuo at 750°C for 0.5 h and adsorption experiments were carried out at room temperature. The amounts of the adsorbates reversibly adsorbed on the samples were estimated as follows. In the case of carbon dioxide, the adsorbate in the gas phase was removed with a liquid nitrogen trap to 1 Pa and the number of molecules reversibly adsorbed on the sample was determined. For oxygen, two successive adsorption experiments with intervening evacuation at room temperature for 0.5 h provided the number of oxygen molecules reversibly adsorbed. The number of the molecules irreversibly adsorbed on the samples was calculated by subtraction of the amount of the reversible adsorption from that of the total adsorption. Surface analyses by XPS were carried out using a Phi (Perkin Elmer) ESCA 5500 MT spectrometer. The samples were mounted with indium foil (0.1 mm-thick) in air and set into the spectrometer. After measurement xenon-ion etching of the sample was carried out (3 kV, 0.5 min), and the spectra were measured again after etching. Results Methane Conversion. The results for the conversion of methane on praseodymium oxide are shown in Figure 1 and Table I. The major products were carbon monoxide, carbon dioxide, ethylene, and ethane both in the presence and absence of TCM in the feedstream while small amounts of formaldehyde and C compounds were detected. Water and hydrogen were also produced. The catalyst produced low methane conversion (ca. 6%) and selectivity to C compounds (ca. 30%) in the absence of TCM in the feedstream. On addition of TCM the conversion of methane after 0.5 h on-stream was increased by almost two-fold (11.9%) and increased still further to 17.2% after 6 h on-stream. The selectivity to C also increased with time on-stream to 43.3% after 6 h on-stream. It is noteworthy that over the 6 h on-stream with TCM present the C H4/C H ratio increased from 1.0 to 2.1. No methyl chloride was detected in the product stream. After the reaction in the presence of TCM, the colour of the catalyst in the inlet portion of the bed was found to have been converted from the original black to white-green. The quantity of the white-green portion recovered was 0.21 g with main XRD peaks at 25.6, 31.2 and 34.4° in 2Θ which were identical with those of praseodymium oxvchloride (77). The BET surface area of the whitegreen compound was 3.7 m g while that for praseodymium oxide measured after the reaction in the absence of TCM was 7.9 m g" . Praseodymium chloride pretreated in a helium flow at 750°C for 1 h produced a low conversion of methane and selectivity to C compounds after 0.5 h on-stream both in the absence and presence of TCM (Figure 2 and Table I). When TCM was present, the conversion and selectivity increased to 17.1 and 46.4% after 1.8 h onstream, respectively, and then the values remained almost constant. In the absence of TCM, the conversion and selectivity also increased to 16.0 and 54.5%, respectively, after 1.8 h on-stream while in the latter case the values decreased gradually to 11.7 and 37.2% over 6 h on-stream. Although no TCM was added to the feedstream, methyl chloride was formed in the reaction. After 0.5 h on-stream, the selectivity to methyl chloride was 2.6% but decreased to 0.1% over 6 h onstream. The XRD pattern of the catalyst after the reaction with TCM present in the 3
2 +
2 +
2
2
6
2
1
2 +
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
25.
MATSUMURA ET AL.
100
25
M
329
Methane over Praseodymium Oxide
80
20
O
+ TCM .2
60
15
» (D
Ο 10 υ
o
