Catalytic partial oxidation of alkanes on silver in fluidized bed and monolith reactors

We have examined catalytic oxidation of alkanes on supported Ag catalysts at high temperatures (> 800 degrees C) in both fluidized bed and monolithic reactors at contact times of 50-200 and 5-10 ms, respectively. The results obtained on Ag are compared with Pt for both fluidized beds and monoliths. Oxidation of CH4, C2H6, and i-C4H10 are examined on 0.25-0.5 wt.% Ag/alpha-Al2O3 in fluidized beds while CH4 and C2H6 oxidations and ammoxidations are examined on 4-35 wt.% Ag monoliths supported on alpha-Al2O3. We find that alkanes do not dissociate as easily on Ag as on Pt, and in both types of reactors it is not possible to keep CH4 ignited on Ag at any feed composition without the presence of NH3 which is used for light-off. C2H6 and i-C4H10 are much more reactive than CH4, and in fluidized beds, we observe high selectivities to olefins (60-75%) at 80 to 90% conversion with no problems of extinction. Also, there is no carbon buildup or evidence of any Ag loss from the catalyst at the high temperatures (ca. 800 degrees C) over at least 16 h of operation. On the Ag monoliths under ammoxidation conditions (alkane + NH3 + O-2) used for HCN synthesis on Pt monoliths and gauzes, < 0.005% HCN is produced from both CH4 and C2H6. For C2H6 oxidation, external heat input is necessary to keep the monoliths ignited with C2H4 being the dominant product (64% selectivity). Although the performance of Ag is very close to that of Pt, it is difficult to keep C2H6 ignited which extinguishes quickly below 800 degrees C. At very high temperatures (> 1100 degrees C), metal losses are observed for Ag monoliths. We also examined Au and Pt-Au monoliths for C2H6 oxidation. Although the monoliths can be ignited using NH3 and O-2, they extinguish as soon as C2H6 is introduced. Reactions on Ag appear to be initiated by oxidative H-abstraction from the alkane by a strongly bound oxygen species on the surface to form a surface alkyl. beta-H or beta-alkyl elimination reactions of the adsorbed alkyl then dominate just as on Pt leading to olefins rather than cracking to C-s and H-s.