By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C₂H₄)₂(CH₃COCHCOCH₃)] and a crystalline support, dealuminated Y zeolite, giving {Rh(C₂H₄)₂} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by ¹³C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.

Essentially molecular surface catalysis was demonstrated by D. A. Dixon, B. C. Gates, J. F. Haw et al. and is reported in their Full Paper on page 7294 ff. [Rh(C₂H₄)₂(acac)] reacted with a dealuminated faujasite zeolite to give anchored rhodium complexes that retained their ethylene ligands, with each Rh atom bonded to two zeolite oxygen atoms, as shown by EXAFS, IR, and ¹³C NMR spectroscopy. Temperature-dependent NMR spectra demonstrated the structural uniformity of the complexes; acetylene reacted with the complexes, replacing ethylene and initiating catalytic acetylene cyclotrimerization.

Quid pro quo: Der durch feste Säuren katalysierte Wasserstoff-Deuterium-Austausch von Propen wurde bei 573 und 673 K mit einer Reaktionszeit von 3 s untersucht (siehe Schema). Bei niedrigen Partialdrücken und Temperaturen entstand [D₅]Propen als Hauptprodukt, vollständige Deuterierung ([D₆]Propen) trat nur bei höherer Temperatur und mit dem sauersten Katalysator auf.

Poolblick bevorzugt: In Einklang mit neuen theoretischen Arbeiten wurden experimentelle Belege erhalten, die die umstrittene Behauptung stützen, dass im katalytischen Methanol-Olefin-Prozess keine direkte mechanistische Kopplung zwischen Methanol und Ethylen auftritt. Die Ergebnisse schließen unter anderem Wege über Carbene und Oxonium-Ylid-Mechanismen aus (siehe Bild) und sprechen für den Kohlenwasserstoffpoolmechanismus.

Den Beweis liefert die Analyse! EXAFS-Studien belegen das Vorliegen einkerniger Metalleinheiten in einem Rh⁺-Katalysator mit zwei austauschbaren Ethylenliganden auf einem Zeolithträger (siehe Bild; Rh braun, C schwarz, Al grau, O rot, Si blau), und sein dynamisches Verhalten wurde in temperaturabhängigen NMR-spektroskopischen Studien untersucht.

Proton transfer in zeolites: A theoretical and experimental NMR study of cluster formation in HZSM-5 shows that cooperative hydrogen bonding drives the proton-transfer reaction in the case of water (see picture). In comparison, hydrogen sulfide is unable to affect proton transfer at any loading in the zeolite. Thus, the ability of water to transfer the proton from HZSM-5 results from its clustering capability, not its intrinsic proton affinity.

Active phosphate flotsam: Tetramethylphosphonium cations can be prepared in the cages of HSAPO-34 catalysts from PH₃ and methanol by using a ship-in-a-bottle approach (see picture). Calcination of this material leaves phosphate debris in some of the cages, which produces an active methanol-to-olefin catalyst with improved ethylene selectivity.