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

Abstract

We have examined catalytic oxidation of alkanes on supported Ag catalysts at high temperatures (>800°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 CH 4, C 2H 6, and i-C 4H 10 are examined on 0.25–0.5 wt.% Ag α-Al 2O 3 in fluidized beds while CH 4 and C 2H 6 oxidations and ammoxidations are examined on 4–35 wt.% Ag monoliths supported on α-Al 2O 3. 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 CH 4 ignited on Ag at any feed composition without the presence of NH 3 which is used for light-off. C 2H 6 and i-C 4H 10 are much more reactive than CH 4, 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°C) over at least 16 h of operation. On the Ag monoliths under ammoxidation conditions (alkane + NH 3 + O 2) used for HCN synthesis on Pt monoliths and gauzes, <0.005% HCN is produced from both CH 4 and C 2H 6. For C 2H 6 oxidation, external heat input is necessary to keep the monoliths ignited with C 2H 4 being the dominant product (64% selectivity). Although the performance of Ag is very close to that of Pt, it is difficult to keep C 2H 6 ignited which extinguishes quickly below 800°C. At very high temperatures (>1100°C), metal losses are observed for Ag monoliths. We also examined Au and PtAu monoliths for C 2H 6 oxidation. Although the monoliths can be ignited using NH 3 and O 2, they extinguish as soon as C 2H 6 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. β-H or β-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.