Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (~97%) over a ...copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH₃OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at CuII oxide active centers, followed by CuI reoxidation by water with concurrent formation of hydrogen.
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Zeolites are a class of materials that are of great relevance for industrial catalysis. Several fundamental questions relating to the structure and role of the Lewis acid sites in these materials ...remain unanswered. Proposals for the origin of such species can broadly be classified into three categories, which have distinct structures: extra-framework, framework-associated and framework aluminium. In this Perspective, we review each of these proposals and proceed to analyse their suitability to understand experimental results. Contrary to traditional belief, the number of Lewis acid sites does not always correlate to extra-framework aluminium content. As a result, we highlight that the terms 'extra-framework' and 'framework-associated' aluminium should be used with caution. We propose how the usage of different characterization techniques can enable the closure of knowledge gaps concerning the strength, multiplicity, localization and structure of catalytically active Lewis acid sites in zeolites.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol. ...We consider the varied experimental evidence arising from the application of X-ray diffraction, and vibrational, electronic, and X-ray spectroscopies that exist, along with the results of theory. We aim to establish both what is known regarding these elusive materials and how they function, and also where gaps in our knowledge still exist, and offer suggestions and strategies as to how these might be closed such that the rational design of more effective and efficient materials of this type for the selective conversion of methane might proceed further.
In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol.
Lewis acidic aluminum in zeolites, particularly acidity that is inherent to the framework, is an indeterminate concept. A fraction of framework aluminum changes geometry to octahedral coordination in ...the proton form of zeolite mordenite. Such octahedrally coordinated aluminum is the precursor of a Lewis acid site and its formation is accompanied by a loss in Brønsted acidity. Herein, we show that such Lewis acid sites have a preferred location in the pore structure of mordenite. A greater proportion of these Lewis acid sites resides in the side-pockets than in the main channel. By reverting the octahedrally coordinated aluminum back to a tetrahedral geometry, the corresponding Brønsted acid sites are restored with a concomitant loss in the ability to form Lewis acid sites. Thereby, reversible octahedral-tetrahedral aluminum coordination provides a means to indirectly switch between Lewis and Brønsted acidity. This phenomenon is unique to Lewis acidity that is inherent to the framework, thereby distinguishing it from Lewis acidity originating from extra-framework species. Furthermore, the transformation of framework aluminum into octahedral coordination is decoupled from the generation of distorted tetrahedrally coordinated aluminum, where the latter gives rise to the IR band at 3660 cm
−1
in the OH stretching region.
Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.
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In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure ...of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper-zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.
1,3‐Butadiene was synthesized from ethanol using zirconium‐containing zeolite beta (ZrBEA) catalysts doped with 1 wt % silver. The Zr was planted using post‐synthesis modification by dealumination of ...the parent zeolite followed by treatment with ZrOCl2 in a DMSO solution. FTIR and NMR spectroscopy were used to investigate the planting process by preparing materials with different Si/Al ratios and crystal sizes. The results showed preferential grafting of Zr to the terminal silanols present on the external surface of the zeolite crystals instead of incorporation of Zr into silanol nests. The grafting yielded highly accessible Zr(OSi)3OH open sites with high Lewis acidity, as confirmed by FTIR spectroscopy of adsorbed CO. These sites are shown to be extremely active for the conversion of ethanol to butadiene. Ag/ZrBEA catalysts prepared using the post‐synthesis method showed significant advantages compared with Ag/ZrBEA catalysts synthesized using a conventional hydrothermal procedure. The best catalyst performance in terms of butadiene formation rate (3 μmol g−1 s−1) was observed over Ag/Zr(3.5)BEA(75) (containing 3.5 wt % Zr), which had the smallest crystal size and the highest content of Zr open sites of the prepared catalysts.
The Ag goes to Zeolites! Ag/ZrBEA (zirconium‐containing zeolite beta) catalysts prepared using a post‐synthesis method show significant advantages compared with Ag/ZrBEA catalysts synthesized using a conventional hydrothermal procedure. Parent AlBEA materials with different Si/Al ratios and crystal sizes are used to determine the best catalyst for the conversion of ethanol to butadiene.
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The direct conversion of methane to methanol in a stepwise process over copper-exchanged zeolites is currently one of the most-studied reactions. Despite these studies and the partial identification ...of the active species and reaction mechanism, it has not been possible to design the best material and determine the ideal process conditions for industrial implementation. This study shows how to design a significantly better performing material and the reaction parameters to enhance the methanol yield. This led to the highest methanol productivity to date, using a material that until recently was considered to be inactive, opening an original direction for the design of active materials. It is shown that the zeolite framework affects the structure and redox properties of copper species, formed during ion exchange and treatment procedures. In turn, these CuII-oxo species demonstrate different redox properties, which correlate with the methanol productivity and selectivity: high temperature of copper reduction leads to low activity in the reaction with methane, while low reduction temperature results in overoxidation of methanol to carbon monoxide and formate species. This relationship enables the prediction of the optimal operation temperature of copper-exchanged zeolite, as illustrated for CuFAU, which had been considered to be inactive in methane conversion to methanol. Its multicyclic operation in the isothermal regime at 633 K and ambient pressure gave a stable methanol yield of 90 μmol/g and 92% selectivity, while an increase in the copper loading and methane pressure resulted in the unprecedented yield of 360 μmol/g.
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The synthesis of buta‐1,3‐diene from ethanol has been studied over metal‐containing (M=Ag, Cu, Ni) oxide catalysts (MOx=MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide ...range of ethanol conversions (2–90 %) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonaldehyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol %).
Losing H2: Metal‐promoted oxides supported on silica are efficient catalysts for butadiene synthesis from ethanol. The metal promoters allow ethanol dehydrogenation, the metal oxide components are efficient in acetaldehyde condensation and reduction of crotonaldehyde with ethanol, whereas the silica support allows the dehydration steps. The optimized catalyst provides 74 mol % selectivity to butadiene at 88 % ethanol conversion at 593 K.
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The reaction of methane with copper‐exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be ...possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic CuII species. Several types of surface‐bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono‐ and bidentate formates, CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper‐exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.
It′s all about the ratio: The reaction of methane with copper‐exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The structure of the copper active sites has a significant effect on the relative fraction and the nature of the reaction products.
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Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into ...methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.
Copper‐exchanged samples of the zeolite mordenite with different Si/Al ratios were used to study the direct conversion of methane into methanol in aerobic and anaerobic regimes. The aluminum loading can be used to modulate the nature of the copper sites and the activity of the material.
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