The role of CO2 in the dehydrogenation of n-octane using Cr-Fe catalysts supported on MgAl2O4

Abstract

• Fe, Cr and Cr/Fe oxide catalysts studied for dehydrogenation of n -octane with CO 2. • The Fe promoter suppressed hexavalent Cr species by forming a Cr 3+ -O-Fe 3+ matrix. • Excellent stability and selectivity to octenes with 5 wt% Cr 2.5 wt% Fe/MgAl 2 O 4. • Role of CO 2 was oxidative over the Cr and non-oxidative over the Cr/Fe catalysts. The effect of CO 2 on the dehydrogenation of n -octane over Cr-Fe oxides supported on MgAl 2 O 4 (MgAl) was investigated. Addition of Fe as a promoter facilitated the formation of Cr-O-Fe polymeric units, stabilizing the CrO x in the +3 state on the catalysts’ surface. Catalytic results revealed that the 2Cr-Fe catalyst was the most active and also stable (ca. 10 % CO 2 conversion, 8 % n -octane conversion, 84 % selectivity to octene isomers) during a 30 h reaction. The stability and high octenes selectivity over this catalyst was reflected in its higher surface basicity. Based on a redox study using CO 2 , it was found that the dominant mechanism for CO 2 activation was oxidative (Mars van Krevelen) over the monometallic Cr catalyst, while a non-oxidative (Reverse Water Gas Shift) mechanism applied over the nCr-Fe bimetallic catalysts. It is proposed that Cr-O-MgAl is the active site in the monometallic Cr catalyst, while the Cr-O-Fe polymeric units are the active sites in the bimetallic catalysts. Coke deposition was shown to be the major cause of deactivation of the catalysts.