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Interplay between carbon monoxide, hydrides, and carbides in selective alkyne hydrogenation on palladium

Interplay between carbon monoxide, hydrides, and carbides in selective alkyne hydrogenation on palladium

Authors :
Javier Pérez-Ramírez
Blaise Bridier
Mónica García-Mota
Núria López
Source :
Journal of Catalysis. 273:92-102
Publication Year :
2010
Publisher :
Elsevier BV, 2010.

Abstract

Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e. , the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1 wt.% Pd/Al 2 O 3 and Density Functional Theory on Pd(1 1 1) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Bronsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3).

Details

ISSN :
00219517
Volume :
273
Database :
OpenAIRE
Journal :
Journal of Catalysis
Accession number :
edsair.doi...........b99c5fbf3ed58a39fdb75e976ce1d0f6
Full Text :
https://doi.org/10.1016/j.jcat.2010.04.018