Direct conversion of methane to methanol and formaldehyde over a double-layered catalyst bed in the presence of steam

Chunlei Shi, Qun Sun, Hangchun Hu, Richard G. Herman, Kamil Klier, Israel E. Wachs
1996 Chemical Communications  
Using a double-bed reactor configuration with an up-stream 1 mass% S042-/l mass% SrO/La203 catalyst to generate methyl radicals and a down-stream 1 mass% V205/Si02 catalyst to stabilize the methyl radicals for reaction to form hydrolysable methoxide species, the methanol and formaldehyde space-time yields are increased up to five-fold for partial oxidation of methane in the presence of air and steam at moderate temperatures of 550-600 "C as compared with the single-bed 1 mass% V205/Si02
more » ... , while little of these oxygenates are formed over a mechanically mixed catalyst bed. The direct conversion of natural gasl.2 to easily transportable chemicals and fuels has attracted the attention of the scientific and engineering communities for more than a decade. Recently, there has been a renewed interest in the direct conversion of methane to oxygenates. Various oxide catalysts have been studied for the partial oxidation of methane to oxygenate^,^,^^ among which silica-supported Moo3 and V205 catalysts have been reported to be the most active and selective for oxygenate production, with V2OS outperforming Moo3 according to most reports .5.6 Dowden et al.7 proposed that Moo3 was capable of an oxygen insertion into a C-H bond of CH4, while V205 was capable of both hydrogen abstraction from and oxygen insertion into the CH4 molecule, which explains the better performance of the latter. Recent re~earch~38.9 also suggested that higher yields of oxygenates could be obtained when the silica surface was partially covered by surface molybdenum oxide and vanadium oxide species. 10,11 To date, most efforts on direct conversion of methane to methanol and formaldehyde have focused on developing a single catalyst that is capable of both activating methane and producing oxygenates. A different approach has been adopted in the present research by using a double-layered catalyst bed in an attempt to circumvent the difficulty of abstracting a hydrogen atom from CH4 and reacting the methyl radical with activated 0 by using the same catalyst. Pitchai and Klier-1 proposed that the role of steam in partial oxidation of methane to oxygenates was to hydrolyse the surface methoxide species formed during the reaction. Using IR spectroscopy, Lunsford and coworkersI2 reported evidence for the formation of methoxide ions during partial oxidation of methane with N20 as the oxidant over an Mo03/Si02 catalyst. The present paper demonstrates that the abstraction of hydrogen from methane and the subsequent formation and hydrolysis of methoxide species could be accomplished more efficiently by utilizing two separate catalysts closely packed one after the other. The first presently used catalyst bed, the function of which is that of a methane C-H bond activating catalyst, is a 1 mass% S042-/1 mass% SrO/La203 catalyst which is a very good methyl radical generating catalyst at relatively low temperatures.13 The second catalyst bed, on which methoxide species are proposed to form and subsequently hydrolyse, is a 1 mass% V20s/Si02 catalyst. The 1 mass% SrO/La203 catalyst was obtained from AMOCO Corporation after being prepared as described elsewhere.14.15 The catalyst was impregnated with aqueous (NH4)2S04, stirred until dry, and then dried at 120 "C overnight. The resulting catalyst was calcined in air at 600 "C for 6 h. The silica-supported V205 catalyst was prepared by using VO-(0P1-i)~ and amorphous silica. The amorphous silica (Cabosil EH-5, surface area = 380 m2 g-1) was first mixed with deionized water and then the mixture was stirred and dried at 140 "C to increase the density of the silica.16 A methanol solution of VO(OPri)3 was prepared and mixed with the dried silica in a glove box under N2 in order to avoid reaction with atmospheric moisture. The resulting mixture was thoroughly mixed by vigorous stirring on a magnetic hot plate until a thick paste was formed, which was dried overnight under N2 and then calcined in air at 600 "C for 6 h. Raman spectroscopy and solidstate 5 *V NMR characterization demonstrated that only surface vanadium species were present and that crystalline V205 particles were absent.16 The initial catalytic experiments were carried out over both the single-layered catalyst bed (100 mg of 1 mass% V20s/Si02 catalyst) and the double-layered catalyst bed (consisting of 100 mg of 1 mass% S04*-/l mass% SrOLa203 followed by 100 mg of 1 mass% V20s/Si02) at 525-600°C. The space-time yields were calculated based on the total mass of the catalyst bed consisting of both catalyst layers. Addition of water vapour to the reactant mixture was achieved by injecting distilled water into the heated volume preceding the catalyst bed with an ISCO liquid metering pump (Model 3 14) . The water was vapourized and the resulting steam was then mixed with CH4-air (1.5 : 1). The analyses of the reactant and product gases were carried out using a Hewlett-Packard 5890 gas chromatograph with a molecular sieve 13X column and a Chrompack Model 7550 PoraPLOT Q fused silica capillary column in parallel and TCD detectors. Condensable products, methanol and formaldehyde, were trapped by using an ice bath at 0°C and were separately analysed by a Hewlett-Packard 5970 GC MS instrument. The GC MS formaldehyde analysis was calibrated by a series of standard solutions that were quantified by iodiometric titration. Comparison of the direct conversion of methane from CH4air-steam (1.5 : 1 .O : 0.2) at ambient pressure to methanol and formaldehyde over the single and double-layered catalyst beds is shown in Fig. 1 . It is evident from Fig. 1 that at temperatures below 600 OC, the amount of methanol and formaldehyde produced by the single-bed 1 % V205/Si02 catalyst is relatively low. However, when the double-bed configuration was used, a significant enhancement of the oxygenate space-time yields was observed. At 575 and 6OO0C, the increase in oxygenate space-time yield was nearly five-fold. Over the single V2OS/ Si02 catalyst, the C atom selectivities toward oxygenates at 525, 550, 575 and 600 "C were 93.5, 87.3, 41.8 and 33.470, respectively, but the CH4 conversion levels were low. The remaining products over this catalyst were carbon oxides. Over the double-layered S042-/SrO/La20311V20S/Si02 catalyst bed, the selectivities of the oxygenates over this temperature range
doi:10.1039/cc9960000663 fatcat:fdj7knjrujdgzfch3fsyvyrmjm