Transport Limitations during Phase Transfer Catalyzed Ethyl-Benzene Oxidation: Facts and Fictions of “Halide Catalysis”

The mechanistic interpretation of the catalytic effect of phase transfer catalysts in the selective oxidation of ethyl benzene is hampered by mass transfer effects. We demonstrate that proper experiments lead to a more correct interpretation of the role of the quaternary ammonium salt (QAS) and its counterion. Specifically, experiments unequivocally show that the main action of the counterion is to enhance physical mass transfer processes, while its catalytic effect is limited to a shift in selectivity, not activity. The QAS as a whole accelerates the induction process in the ethyl benzene (EB) oxidation by degenerate branching of its hydroperoxide (EBHP). Proper mechanistic understanding of these phenomena in QAS catalysts is especially crucial under industrially relevant condition

Direct Conversion of Syngas to Olefins over a Hybrid CrZn Mixed Oxide/SAPO-34 Catalyst: Incorporation of Dopants for Increased Olefin Yield Stability

A bifunctional catalyst was developed utilizing a physical mixture of a CrZn-based mixed metal oxide and zeotype SAPO-34 for the direct conversion of syngas to short-chain olefins. A series of promoted CrZn-M (M = Fe, Ga, Al) mixed oxide catalysts were synthesized by coprecipitation and calcined at different temperatures. CrZn-Fe-SAPO-34 catalysts calcined at 400 °C selectively converted syngas to C2–C4 olefins, while maintaining high CO conversion and olefin stability over time. The high olefin yield is ascribed to the stabilization effect of iron on inversed spinel phase ZnCr2O4 and to reduction of the detrimental ZnO phase formed during syngas conditions. At a higher calcination temperature of 600 °C, the stabilization effect is less pronounced. Ga and Al-doped CrZn oxides enabled high and stable olefin selectivity of the hybrid catalysts CrZn-Ga-SAPO-34 and CrZn-Al-SAPO-34, regardless the applied calcination temperature. Spectroscopy analysis demonstrated that these promoters are able to scavenge free ZnO formed on the catalyst, thus stabilizing the inversed spinel. This work demonstrates that a rational design of mixed metal oxide components of the hybrid catalyst process is required to maximize olefin yield and catalyst stability. The selection of dopants capable of stabilizing an inversed spinel phase and scavenging detrimental ZnO is a critical step in successful catalyst design

Mechanistic Insight into the Synthesis of Higher Alcohols from Syngas: The Role of K Promotion on MoS₂ Catalysts

Operando infrared spectroscopy in combination with a kinetic study is used to elucidate the role of potassium on the conversion of carbon monoxide over K-promoted MoS₂ catalysts. More specifically, the initial break-in transient has been studied in detail. Stabilization of reaction intermediates, and effect of promoter on the intrinsic properties of MoS₂ are discussed. Adsorbed alkoxy species were found to play an important intermediate role in the syngas to alcohol route, and it was found that potassium stabilizes these species. Moreover, the electronic properties of MoS₂ change upon promotion, thereby allowing for a relatively easier activation of the CO molecule and a reduced hydrogenation activity toward alkanes

Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer–Tropsch Catalysts

In this combined <i>in situ</i> XAFS, DRIFTS, and Mössbauer study, we elucidate the changes in structural, electronic, and local environments of Fe during pyrolysis of the metal organic framework Fe-BTC toward highly active and stable Fischer–Tropsch synthesis (FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized by decarboxylation of its trimesic acid linker, generating a carbon matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C (Fe@C-400) favors the formation of highly dispersed epsilon carbides (ε′-Fe_2.2C, <i>d</i>_p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (χ-Fe₅C₂, <i>d</i>_p = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (θ-Fe₃C, <i>d</i>_p = 28.4 nm). Thus, the loading, average particle size, and degree of carburization of Fe@C catalysts can be tuned by varying the pyrolysis temperature. Performance testing in high-temperature FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for pyrolysis temperatures up to 600 °C. However, methane formation is minimized when higher pyrolysis temperatures are applied. The material pyrolyzed at 900 °C showed longer induction periods and did not reach steady state conversion under the conditions studied. None of the catalysts showed deactivation during 80 h time on stream, while maintaining high Fe time yield (FTY) in the range of 0.19–0.38 mmol_CO g_Fe^–1 s^–1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts