3 Search Results

Mesoporous silicon nitride (Si3N4) enables access to chemisorbed group IV organometallics catalysts active for propane dehydrogenation (PDH) compared to the organometallic analogues on mesoporous silica under the same reaction conditions. The series of Si3N4-supported materials are active catalysts, (Zr > Hf > Ti k f = 290, 232, and 162 mol mol(Metal)(-1) h(-1) at 450 degrees C with 2% C3H8 in Ar, respectively) with selectivity above 95%, demonstrating additional examples of Ti and Hf systems for PDH. However, the underlying mechanism of the improved performance relative to oxide supported homologues is not well-understood. Characterization of thermally treated samples (DRIFTS, XASmore » and SSNMR) and computational modeling of this catalyst series was utilized to differentiate between potential amido- (C-H activation along the M-N bond) and imido- (C-H activation along the M=N bond) mechanisms. Due to remaining mechanistic ambiguity, a Ga analogue was synthesized and evaluated for PDH activity as an indirect probe to experimentally differentiate pathways. An inversion of the oxide/nitride performance trend is observed for the Ga congener which does not form a Ga=N bond, most consistent with different mechanisms dictating the performance of the group IV/Si3N4 catalysts vs Ga/Si3N4.« less

Amorphous silicon nitride (Si3N4) is an unconventional support for the chemisorption of organometallic complexes and offers potential improvements in active site stability and reactivity through enhanced metal-nitrogen covalency and orbital overlap in bonding interactions with the nitride framework. In this study, we show that silicon nitride-supported iron mesityl complexes display divergent reactivity compared to their silica-supported homologues, resisting metallic particle formation under reducing pretreatment conditions (exposure to excess organolithium reagents) and maintaining active iron/lithium speciation under ammonia synthesis conditions that is absent on the oxide support. When the organometallic iron complex on silicon nitride is exposed to excess n-butyllithium, ironmore » remains isolated, catalyzing the conversion of butyllithium to lithium hydride, resulting in a divalent iron site in a polyhydride environment. In contrast, the silica-supported complex is converted to reduced iron clusters without forming persistent isolated hydrides. These structural differences lead to markedly different catalytic behaviors under ammonia synthesis conditions. The Li/Fe/Si3N4 catalyst is highly active (7.5 mol NH3/mol Fe/h at 300 °C, 10 bar, or 46 mol NH3/mol Fe/h at 400 °C, 10 bar), while both the silica-supported analog and the nonlithiated Si3N4-supported species are inactive. Notably, this activity is enhanced relative to previously reported iron-lithium hydride composite catalysts (0.43–4.1 mol NH3/mol Fe/h at 300 °C, 10 bar) and relative to the industrial benchmark promoted iron catalyst KM1 (3.0 mol NH3/mol Fe/h at 400 °C, 10 bar). The catalyst activation and LiH/LiNHx nitrogen reservoir dynamics for Li/Fe/Si3N4 are studied by X-ray Absorption, Mössbauer, and in situ DRIFT spectroscopies and isotopic exchange kinetics.« less