Catalysis Letters 54 (1998) 113–118 113 Methane coupling to acetylene over Pt-coated monoliths at millisecond contact times K.L. Hohn, P.M. Witt, M.B. Davis and L.D. Schmidt Department of Chemical Engineering, University of Minnesota, Minneapolis, MN 55414, USA Received 5 February 1998; accepted 17 June 1998 We have studied the adiabatic and autothermal oxidative coupling of methane over Pt on α-Al 2 O 3 monoliths at space velocities above 10 5 h −1 . The selectivity to C 2 hydrocarbons (primarily acetylene) reaches a maximum of around 20% at low fuel to oxygen ratios, low dilution, and high space velocities. These conditions promote a large temperature gradient in the monolith, with an exit temperature of nearly 1500 ◦ C and an entrance temperature of less than 200 ◦ C. This temperature gradient appears to be the driving force for C 2 hydrocarbon formation under these conditions. Both homogeneous and heterogeneous reactions may be involved in producing coupling products, and a combustion model predicts C 2 selectivities similar to those observed. Keywords: methane coupling at low contact time, Pt monoliths 1. Introduction The conversion of natural gas into value-added chemi- cals has received great attention in the last twenty years. Recent research has focused on converting CH 4 , the major constituent of natural gas, into synthesis gas [1–3]. This is accomplished at long contact times by steam reforming, CH 4 + H 2 O → CO + 3H 2 (1) over Ni and at shorter contact times by partial oxidation, CH 4 + (1/2)O 2 → CO + 2H 2 (2) over Rh [2,4–8]. Synthesis gas can then be converted to higher hydrocarbons through Fischer-Tropsch chemistry [9] or to methanol. It has also been shown that CH 4 can be converted to C 2 ’s directly by oxidative coupling [10,11] or homologa- tion [12,13]. In oxidative coupling, CH 4 and O 2 are fed over a metal or metal oxide catalyst at moderate tempera- tures (500–800 ◦ C) to produce mainly ethylene. Selectivi- ties in excess of 80% have been reported with CH 4 conver- sions around 20%. The first major study of oxidative cou- pling was done by Keller and Bhasin in 1982 [14]. They investigated numerous metals and found that Pt was not active for coupling. Since then, considerable work has fo- cused on oxidative coupling, particularly on metal oxides, and several review articles have been published on this sub- ject [10,11,15–17]. In homologation, pure CH 4 is passed over a noble metal catalyst to produce a carbon layer. Then H 2 is substituted for CH 4 to remove the carbon as higher hydrocarbons. Recently we reported that significant methane oxidative coupling could be achieved over Rh-coated α-Al 2 O 3 ce- ramic monoliths [8]. As the gas hourly space velocity (GHSV) increased, the selectivity to C 2 ’s increased until O 2 breakthrough occurred. Above a GHSV of 5 × 10 5 h −1 all C 2 products disappeared. By preheating the feed gas and decreasing the CH 4 /O 2 ratio C 2 selectivities as high as 10% were achieved on Rh. An important feature of this work is the temperature profile across the monolith cata- lyst. At high GHSV the entrance of the catalyst is cold, approximately 200 ◦ C, while the exit temperature is almost 1500 ◦ C. It was suggested that this temperature profile al- lows for the production of coupling products which were not seen (<0.5%) in previous research carried out at lower space velocities [14]. We previously reported that Pt, while not a highly se- lective catalyst for syngas, is more selective than Rh for the production of olefins from light alkanes [18–20]. By replacing Rh with Pt, we find that the coupling selectivity can be improved to at least 20%. 2. Experimental Catalysts were prepared as detailed previously [18]. Ce- ramic foam α-Al 2 O 3 monoliths (18 mm diameter, 10 mm long, 45 pores per linear inch) were saturated with an aque- ous solution of H 2 PtCl 6 and dried overnight. The catalysts were then calcined in air at 600 ◦ C and reduced at the same temperature in H 2 . Metal loadings were 2–3% Pt by weight. Experiments were carried out in a quartz tube reactor 18 mm diameter and 40 cm long. Uncoated ceramic mono- liths were positioned before and after the catalyst to reduce axial radiation losses. The monoliths were sealed in the reactor with high-temperature cloth to prevent gas bypass. The outside of the reactor was wrapped with insulation to closely approximate adiabatic operation. The temperature of the catalyst was measured with a Pt/13% Rh thermocou- ple placed at the exit and a chromel/alumel thermocouple  J.C. Baltzer AG, Science Publishers