Iron copper zeolite (Fe‐Cu‐ZSM‐5) with aqueous hydrogen peroxide is active for the selective oxidation of methane to methanol. Iron is involved in the activation of the carbon–hydrogen bond, while copper allows methanol to form as the major product. The catalyst is stable, re‐usable and activates methane giving >90 % methanol selectivity and 10 % conversion in a closed catalytic cycle (see scheme).
Although their activity is often inferior to that of other systems, the use of vanadium-based catalysts in homogeneous Ziegler-Natta polymerizations allows the preparation of high-molecular-weight polymers with narrow molecular-weight distributions, ethene/alpha-olefin copolymers with high alpha-olefin incorporation, and syndiotactic polypropene. The main reason for the low activity of these catalysts is their deactivation during catalysis by reduction of active vanadium species to low-valent, less active or inactive species. We here present an up-to-date review of this area with particular emphasis on the attempts to improve catalyst performance and stability by the use of additives or ancillary ligands.
The development of a catalytic, one-step route for the oxidation of methane to methanol remains one of the greatest challenges within catalysis. Of particular importance is the need to develop an efficient route that proceeds under mild reaction conditions so as to avoid deeper oxidation and the economic limitations of the currently practiced syngas route. Recently, it was demonstrated that a copper-and ironcontaining zeolite is an efficient catalyst for such a one-step process. The catalyst in question (Cu−Fe−ZSM-5) is capable of selectively transforming methane to methanol in an aqueous medium with hydrogen peroxide as the terminal oxidant. Nevertheless, despite its high activity and unparalleled methanol selectivity, the origin of its activity and the precise nature of its active species are not yet fully understood. Through a combination of catalytic and spectroscopic studies, we hereby demonstrate that extraframework Fe species are the active component of the catalyst for methane oxidation, although the speciation of these sites from synthesis to catalysis significantly alters the observed activity and selectivity. The analogies and differences between this system and other iron-containing zeolite-catalyzed processes, such as N 2 O-mediated benzene hydroxylation, are also considered.
The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h(-1)), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron- and copper-containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14,000 h(-1) by using H(2)O(2) as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by-production of hydroxyl radicals. The catalytic system in question results in a low-energy methane activation route, and allows selective C(1)-oxidation to proceed under intrinsically mild reaction conditions.
Ein Eisen‐Kupfer‐Zeolith (Fe‐Cu‐ZSM‐5) katalysiert die selektive Oxidation von Methan zu Methanol mit wässrigem Wasserstoffperoxid. Das Eisen aktiviert die Kohlenstoff‐Wasserstoff‐Bindung, während das Kupfer dafür sorgt, dass Methanol als Hauptprodukt gebildet wird. Der Katalysator ist stabil und wiederverwendbar und aktiviert Methan mit >90 % Selektivität und 10 % Umsatz in einem geschlossenen Katalysezyklus (siehe Schema).
Fe-
and Cu-containing zeolites have recently been shown to be efficient
catalysts for the one-step selective transformation of methane into
methanol in an aqueous medium at only 50 °C, using H2O2 as green oxidant. Previously, we have observed that
Fe species alone are capable of catalyzing this highly selective transformation.
However, further catalytic testing and spectroscopic investigations
demonstrate that although these extra-framework Fe species are the
active component of the catalyst, significant promotion is observed
upon the incorporation of other trivalent cations, e.g., Al3+ or Ga3+, into the MFI-framework. While these additional
framework species do not constitute active catalytic centers, promotion
is observed upon their incorporation as they (1) facilitate the extraction
of Fe from the zeolite framework and hence increase the formation
of the active Fe species and (2) provide an associated negatively
charged framework, which is capable of stabilizing and maintaining
the dispersion of the cationic extra-framework Fe species responsible
for catalytic activity. By understanding these phenomena and subsequently
controlling the overall composition of the catalyst (Fe and Al), we have subsequently been able to prepare a catalyst of equal
intrinsic activity (i.e., TOF) but five-times higher productivity
(i.e., space-time-yield) compared with the best catalysts reported
for this reaction to date.
A series of bulky phosphines containing substituted biphenyl, 2-methylnaphthyl, or 2,7-di-tert-butyl-9,9-dimethylxanthene moiety were prepared. They were used in the preparation of new monophosphine-palladium(0)-dvds complexes, which were employed as catalysts for the selective telomerization of 1,3-butadiene with methanol to obtain 1-methoxyocta-2,7-diene (1-MOD), the key intermediate in the Dow 1-octene process. Several ligands showed improved selectivity and yield compared to that of the benchmark ligand PPh(3). Especially 2,7-di-tert-butyl-9,9-dimethylxanthen-4-yl-diphenylphosphine (4, "mono-xantphos") stands out as an excellent ligand in terms of yield, selectivity, and stability.
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