Propylene Hydrogenation and Propane Dehydrogenation by a Single-Site Zn²⁺ on Silica Catalyst

Abstract

This study reports the highly selective (more than 95%) dehydrogenation of propane to propylene as well as the reverse hydrogenation reaction by silica-supported single-site Zn(II) catalyst. The catalyst is thermally stable at dehydrogenation temperature (550 °C and above), and catalytic byproducts are small. In situ UV-resonance Raman, XANES, and EXAFS spectra reveal that tetrahedrally coordinated Zn(II) ions are chemisorbed into the strained three-membered siloxane rings on the amorphous silica surface. Under reaction conditions, the Zn(II) ion loses one Zn–O bond, resulting in a coordinatively unsaturated, 3-coordinate active center. The infrared spectrum of adsorbed pyridine indicates that these are Lewis acid sites. Theoretical calculations based on hybrid density functional theory suggest that the catalyst activates H–H and C–H bonds by a nonredox (metal) mechanism consisting of heterolytic cleavage of C–H bonds, in contrast with the homolytic mechanisms such as oxidative addition/reductive elimination pathways. The computed minority catalytic pathway consists of undesired C–C bond cleavage at Zn(II) site, follows a slightly different mechanism, and has a significantly higher activation energy barrier. These mechanisms are consistent with the high olefin selectivity observed for single-site Zn(II) on SiO2.

Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation

“…[51][52] Heating the catalyst 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 with strongly adsorbed propylene to 200 °C under hydrogen atmosphere produced the same intermediate as propane dehydrogenation, but only propylene desorption was observed under helium atmosphere. In the hydrogen atmosphere, propylene is first partially hydrogenated to vanadium propyl species, which then transforms to propenyl-vanadium.…”

Silicalite-1 zeolite acidification by zinc modification and its catalytic properties for isobutane conversion

“…19,20 Zn/SiO 2 catalyst exhibited high propylene selectivity in the dehydrogenation of propane. 21 However, the stability is also a big challenge for the industrial applications of Zn/SiO 2 catalyst since the activity can only maintain nearly 50% of the initial value during 12 h test. Therefore, Zn x /S-1 catalysts exhibited both higher activity and excellent stability in comparison with zinc titanate and Zn/SiO 2 catalysts in the dehydrogenation of alkanes.…”

“…It has been accepted that Zn-containing catalysts were active in the dehydrogenation of alkanes. [14][15][16][19][20][21] Zn-modied HZSM-5 zeolites (Zn/ZSM-5) have been reported to be effective catalysts for low-temperature activation of light alkanes, with the modied Zn species playing a key role in light alkane dehydrogenation. [14][15][16] However, their commercial use in such processes is impeded by their poor alkene selectivity with respect to the formation of large amounts of cracked dry gas byproducts (C 1 and C 2 hydrocarbons), resulting from the strong acidic character of HZSM-5 zeolite.…”

Catalytic hydrogenation of liquid alkenes with a silica-grafted hydride pincer iridium(iii) complex: support for a heterogeneous mechanism

“…45 The common features of these three systems are the mechanism based on a constant oxidation state and the occurrence of σ-bond metathesis involving alkyl and metalbound H 2 . The key steps of this mechanism are alkene insertion into the Zn-H bond and proton transfer from silanol to release the alkane.…”