Presented in this paper is a deep investigation into the defect chemistry of UiO-66 when synthesized in the presence of monocarboxylic acid modulators under the most commonly employed conditions. We ...unequivocally demonstrate that missing cluster defects are the predominant defect and that their concentration (and thus the porosity and composition of the material) can be tuned to a remarkable extent by altering the concentration and/or acidity of the modulator. Finally, we attempt to rationalize these observations by speculating on the underlying solution chemistry.
Ni-containing porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addition, it remains unclear why the ...aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses density functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calculations employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calculations targeted Ni-containing SSZ-24 zeolite as a representative catalyst and considered Ni+, Ni2+ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee–Arlman mechanisms that have been proposed in the literature, in addition to a new proton-transfer mechanism. Free energy profiles were derived at a typical experimental reaction temperature of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee–Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calculated intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available experimental data. We further propose a mechanism for formation of the active nickel–alkyl species by reaction between ethene and isolated Ni2+ ions. The results hence provide a solid starting point for experimental investigations of the catalytic cycle, to validate our predictions and ultimately determine the atom-scale properties that control catalytic activity.
Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer ...goods. In a post‐oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO2. The methanol‐to‐hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline‐rich (methanol to gasoline; MTG) or olefin‐rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.
The active site of the methanol‐to‐hydrocarbons reaction has been identified as a hybrid site consisting of an organic molecule in close interaction with a Brønsted acidic proton on the zeolite lattice. With increasing internal volume, the organic intermediates change from alkenes via a mixture of alkenes and arenes to mainly arenes. Product selectivity reflects the site composition.
The direct conversion of methane to methanol (MTM) is a reaction that has the potential to disrupt a great part of the synthesis gas-derived chemical industry. However, despite many decades of ...research, active enough catalysts and suitable processes for industrial application are still not available. Recently, several copper-exchanged zeolites have shown considerable activity and selectivity in the direct MTM reaction. Understanding the nature of the active site in these materials is essential for any further development in the field. Herein, we apply multivariate curve resolution analysis of X-ray absorption spectroscopy data to accurately quantify the fraction of active Cu in Cu-MOR (MOR = mordenite), allowing an unambiguous determination of the active site nuclearity as a dicopper site. By rationalizing the compositional parameters and reaction conditions, we achieve the highest methanol yield per Cu yet reported for MTM over Cu-zeolites, of 0.47 mol/mol.
The development in the field MOF materials is moving from the discovery of new structures toward applications of the most promising materials. In most cases, specialized applications require ...incorporation of functional chemical groups. This work is a systematic investigation of the effect that simple substituents attached to the aromatic linker have on the stability and property to the parent MOF. A family of isoreticular MOFs, based on the UiO-66 structure was obtained from the three different linker ligands H2N−H2BDC, O2N−H2BDC, and Br−H2BDC. The physicochemical and chemical investigation of these materials demonstrate that this class of MOFs retains high thermal and chemical stabilities, even with functional groups present at the linker units. The results demonstrate the possibility of incorporating active functional groups into the UiO-66 structure almost without losing its exceptionally high thermal and chemical stability. It has been established that the functional groups, at least in the amino functionalized UiO-66 sample, are chemically available as evidenced by the H/D exchange experiment, making the tagged UiO series MOFs very interesting for further studies within the field of catalysis.
Cu-exchanged zeolites possess active sites that are able to cleave the C–H bond of methane at temperatures ≤200 °C, enabling its selective partial oxidation to methanol. Herein we explore this ...process over Cu-SSZ-13 materials. We combine activity tests and X-ray absorption spectroscopy (XAS) to thoroughly investigate the influence of reaction parameters and material elemental composition on the productivity and Cu speciation during the key process steps. We find that the CuII moieties responsible for the conversion are formed in the presence of O2 and that high temperature together with prolonged activation time increases the population of such active sites. We evidence a linear correlation between the reducibility of the materials and their methanol productivity. By optimizing the process conditions and material composition, we are able to reach a methanol productivity as high as 0.2 mol CH3OH/mol Cu (125 μmol/g), the highest value reported to date for Cu-SSZ-13. Our results clearly demonstrate that high populations of 2Al Z2CuII sites in 6r, favored at low values of both Si:Al and Cu:Al ratios, inhibit the material performance by being inactive for the conversion. ZCuIIOH complexes, although shown to be inactive, are identified as the precursors to the methane-converting active sites. By critical examination of the reported catalytic and spectroscopic evidence, we propose different possible routes for active-site formation.
Sulfated metal–organic framework-808 (S-MOF-808) exhibits strong Brønsted-acidic character which makes it a potential candidate for the heterogeneous acid catalysis. Here, we report the isomerization ...and oligomerization reactions of light olefins (C3–C6) over S-MOF-808 at relatively low temperatures and ambient pressure. Different products (dimers, isomers, and heavier oligomers) were obtained for different olefins, and effective C–C coupling was observed between isobutene and isopentene. Among the substrates investigated, facile oligomerization occurred very specifically for the structures with an α-double bond and two substituents at the second carbon atom of the main carbon chain. The possible oligomerization mechanism of light olefins was discussed based on the reactivity and selectivity trends. Moreover, the deactivation and regeneration of S-MOF-808 were investigated. The catalyst deactivates via two mechanisms which predominance depends on the substrate and reaction conditions. Above 110 °C, a loss of acidic sites was observed due to water desorption, and the deactivated catalyst could be regenerated by a simple treatment with water vapor. For C5 substrates and unsaturated ethers, the oligomers with increased molecular weight caused deactivation via blocking of the active sites, which could not be readily reversed. These findings offer the first systematic report on carbocation-mediated olefin coupling within MOFs in which the Brønsted acidity is associated with the secondary building units of the MOF itself and is not related to any guest substance hosted within its pore system.
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•Five zeolite catalysts are compared for the conversion of methanol to hydrocarbons.•Topology is the decisive factor for both activity and deactivation.•An array of complementary ...methods is required to quantify the deactivation.•Extended carbon species dominate in three dimensional and large pore catalysts.•Molecular coke predominates in diffusion restricted catalyst systems.
Catalyst deactivation during the methanol-to-hydrocarbons (MTH) reaction was investigated using five different commercially prepared microporous catalysts, including Mordenite, ZSM-22, ZSM-5, zeolite Beta and SAPO-34. The reaction was carried out in a fixed bed reactor at a constant feed rate per gram of catalyst. Deactivated and partially deactivated catalysts were obtained at increasing reaction times. The whole of the catalyst beds was characterized using nitrogen adsorption, thermogravimetric analysis, a dissolution-extraction protocol, and UV-Raman spectroscopy, focusing primarily on methods suitable for the quantification of the coke. The results illustrate that topology is the dominant parameter that influences not only catalyst lifetime and product distribution, but also the nature of the species causing the deactivation. For all catalyst topologies, when the entire catalyst bed is examined together, the micropore volume and BET surface area decrease more rapidly than total coke from TGA increases at short reaction times. In the materials with the more restricted access to the internal voids, such as ZSM-22 and SAPO-34, the loss of activity is to a large extent due to species which are soluble in dichloromethane and give rise to distinct features in the Raman spectra. For the Mordenite and Beta catalysts, which have larger pores comprising three dimensional networks, and to some extent for the ZSM-5 catalyst employed, the accumulation of more coke species which are insoluble in dichloromethane, presumably on the external surface of the zeolite crystals, is observed. This is linked to the appearance of more pronounced D and G bands in the Raman spectra, indicative of extended carbon species.
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