Suppressing C–C Bond Dissociation for Efficient Ethane Dehydrogenation over the Isolated Co(II) Sites in SAPO-34

Abstract

Various Co-based SAPO-34 catalysts were prepared using different methods, including ion exchange (IE), incipientwetness impregnation (IWI), and solid-phase grinding (SPG), to correlate the chemical states of Co species with the C−H and C−C bond scissions in ethane dehydrogenation. The IE-prepared Co/ SAPO-34 led to stable, unreducible, and isolated exchanged Co sites anchored on the zeolite framework with a structure of −Al F − O−Co−O− and showed the highest selectivity to ethylene of close to 98% at 600 °C, which suggests that these Co sites favors suppressing the C−C bond scission in ethane. In comparison, the IWI-and SPG-prepared Co/SAPO-34 catalysts, especially for those with a high Co loading, inevitably give Co oxide clusters that are easily reduced into metallic Co. Together with catalytic results, characterizations, and DFT calculations, it is confirmed that the reduced Co clusters, especially for those outside SAPO-34 channels without the confinement effect, favor both C−H and C−C bond scission, boosting the conversion of ethane into CH 4 or/and coke; however, the ionic-state −Co−O− species can smoothly terminate the ethane dehydrogenation for the ethylene product due to relatively high energy barriers for both C−H and C−C bond scission, avoiding a deep dehydrogenation and C−C cracking. As expected, the unreducible −Co−O− sites are very stable in the title reaction without deanchoring from the zeolite framework in a 100 h cyclic test. This study not only demonstrates the stable −M δ+ −O δ− structure favorable for suppressing C−C bond scission but also highlights a catalyst-constructing strategy for Co-based and similar metal-based catalysts for dehydrogenation of other light alkanes.

Stable co-production of olefins and aromatics from ethane over Co2+-exchanged HZSM-5 zeolite

“…Co 0 clusters boost both C-H and C-C bond scissions, leading to the formation of coke and CH 4 , while the Co species in the form of -Al F -O-CoIJII) -O-Al Fspecies are mainly responsible for C-H bond scission, leading to the formation of ethylene. Previously, we also performed DFT calculations on the dissociation energies of C-H and C-C bonds over Co 0 clusters and isolated CoIJII) sites in SAPO-34 zeolite, 38 giving identical conclusions. By means of the BASs and the appropriate zeolite channel, some ethylene is converted into aromatics.…”

“…2 anchor at the exchange sites in the zeolite, in accord with the characteristic of most metal-exchanged zeolite catalysts. 38,47,54,55 Fig. 3 shows the NH 3 -TPD profiles of all HZSM-5 zeolites and corresponding IE-prepared Co/HZSM-5 catalysts.…”

“…S6 †) that favors both C-C and C-H bond scissions. 38 It should be noted that the Co content in the Co/SiO 2 catalyst was only 1 wt%, less than those in the Co/HZSM-5-IE (12.5) and 2 wt% Co/HZSM-5-IWI catalysts. However, when these Co species were rightly placed into the HZSM-5 zeolite channels with special chemical structures (Fig.…”

“…The formation of Co 0 clusters would not be beneficial to the production of olefins from paraffins, because they could promote both C-C and C-H bond cleavage in the reaction. 38 3.2 Catalytic performance of the co-exchanged HZSM-5 for ethane dehydrogenation and aromatization Fig. 6 shows the catalytic results of ethane conversion over Cobased catalysts, including four IE-prepared Co/HZSM-5 (Si/Al = 12.5∼40) catalysts, one IWI-prepared 2 wt% Co/HZSM-5 (Si/Al = 12.5) catalyst and one IWI-prepared 1 wt% Co/SiO 2 catalyst, at 600 °C, 0.1 MPa and 3000 mL g −1 h −1 (90% of ethane content in the feed).…”

“…However, interestingly, as their atoms had a strong interaction with a support to form a -M-Ostructure without being reduced, the cleavage of the C-C bond could be well suppressed, favoring the formation of olefins with high selectivity. 38 Recently, more efforts including ours have been made to construct Fe-and, especially, Co-based catalysts using various preparation methods, and they all showed good catalytic performance in ethane and propane dehydrogenation. 19,20,[38][39][40][41][42][43][44][45][46] Therefore, it can be seen that if stable Co-O species can be constructed in acidic HZSM-5 zeolite channels, aromatics can be synthesized from light paraffins via the synergetic catalysis of Co and BAS, and Co/HZSM-5 catalysts will be a new member of the catalyst family for alkane aromatization.…”