Nikolay Kosinov

Selective coke combustion by oxygen pulsing during Mo/ZSM‐5‐catalyzed methane dehydroaromatization

Abstract Non‐oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H2/CO ratio close to two is the main side‐product of coke combustion. Using 13C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one‐fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.

High flux high-silica SSZ-13 membrane for CO2 separation

High-silica (gel Si/Al = 100) SSZ-13 membranes were prepared by hydrothermal secondary growth on the surface of α-alumina hollow fiber supports. The membranes were evaluated for their performance in the separation of CO2 from equimolar mixtures with CH4 or N2. The maximum CO2–CH4 and CO2–N2 separation selectivities were found to be 42 and 12 respectively, with a high CO2 permeance of 3.0 × 10−7 mol m2 s−1 Pa−1 at 293 K and total feed pressure of 0.6 MPa. At the low aluminum content, the prepared membranes contain a very low number of defects, as follows from their H2/SF6 ideal selectivity of over 500 in the 293–473 K temperature range. Due to their hydrophobicity, water in the feed mixture has only a small influence on the permeance at temperatures above 353 K. Water improves the CO2–N2 and CO2–CH4 selectivity, which is attributed to preferential blocking of the hydrophilic, non-zeolitic defect pores. The hydrothermal stability of the high-silica SSZ-13 membrane was evaluated by a long (220 h) CO2–N2 separation test with a humidified (9.5 kPa H2O) feed mixture at 393 K and 0.6 MPa feed pressure. The permeance and selectivity were stable during this endurance test, underpinning the promise of high-silica SSZ-13 membranes for application in the separation of hot and humid gas mixtures.

Competitive Adsorption of Substrate and Solvent in Sn-Beta Zeolite During Sugar Isomerization

Abstract The isomerization of 1,3‐dihydroxyactone and d‐glucose over Sn‐Beta zeolite was investigated by in situ 13C NMR spectroscopy. The conversion rate at room temperature is higher when the zeolite is dehydrated before exposure to the aqueous sugar solution. Mass transfer limitations in the zeolite micropores were excluded by comparing Sn‐Beta samples with different crystal sizes. Periodic density functional theory (DFT) calculations show that sugar and water molecules compete for adsorption on the active framework Sn centers. Careful solvent selection may thus increase the rate of sugar isomerization. Consistent with this prediction, batch catalytic experiments show that the use of a co‐solvent, such as tetrahydrofuran, that strongly interacts with the Sn centers suppresses glucose isomerization. On the other hand, the use of ethanol as cosolvent results in significantly higher isomerization activity in comparison with pure water because of decreased competition with glucose adsorption on zeolitic Sn sites.

Relevance of the Mo-precursor state in H-ZSM-5 for methane dehydroaromatization

Although the local geometry of Mo in Mo/HZSM-5 has been characterized before, we present a systematic way to manipulate the configuration of Mo and link it to its catalytic properties. The location and geometry of cationic Mo-complexes, the precursor of the active metal site for methane dehydroaromatization, are altered by directing the way they anchor to the framework of the zeolite. The feature used to direct the anchoring of Mo is the location of Al in the zeolite framework. According to DFT calculations, the local geometry of Mo should change, while UV-vis and pyridine FTIR spectroscopy indicated differences in the dispersion of Mo. Both aspects, however, did not influence the catalytic behavior of Mo/HZSM-5, indicating that as long as enough isolated Mo species are present inside the pores of the zeolite, the catalytic behavior is unaffected. This paves the way to better understand how the Mo oxo precursor transforms into the active phase under the reaction conditions.

Fluoride-assisted synthesis of bimodal microporous SSZ-13 zeolite

The presence of small amount of fluoride in alkaline hydrothermal synthesis of SSZ-13 zeolite yields bimodal microporous particles with substantially improved performance in the methanol-to-olefins (MTO) reaction. Hydrocarbon uptake measurements and fluorescence microspectroscopy of spent catalysts demonstrate enhanced diffusion through micropores at the grain boundaries of nanocrystals running through the zeolite particles. Fluoride-assisted SSZ-13 synthesis is a cheap and scalable approach to optimize the performance of MTO zeolite catalysts.

Probing the influence of SSZ-13 Zeolite pore hierarchy in MTO catalysis by NASCA microscopy and positron emission profiling

An understanding of the role of the hierarchical pore architecture of SSZ‐13 zeolites on the catalytic performance in the methanol‐to‐olefins (MTO) reaction is crucial to guide the design of better catalysts. We investigated the influence of the space velocity on the performance of a microporous SSZ‐13 zeolite and several hierarchically structured SSZ‐13 zeolites. Single catalytic turnovers, as recorded by nanometer accuracy by using stochastic chemical reactions (NASCA) fluorescence microscopy verified that the hierarchical zeolites contain pores larger than the 0.38 nm apertures native to SSZ‐13 zeolite. The amount of fluorescent events correlated well with the additional pore volume available because of the hierarchical structuring of the zeolite. Positron emission tomography (PET) using 11C‐labeled methanol was used to map the 2 D spatial distribution of the deposits formed during the MTO reaction in the catalyst bed. We used PET imaging to demonstrate that hierarchical structuring not only improves the utilization of the available microporous cages of SSZ‐13 but also that the aromatic hydrocarbon pool species are involved in more turnovers before they condense into larger multiring structures that deactivate the catalyst.

Engineering of Transition Metal Catalysts Confined in Zeolites

Transition metal–zeolite composites are versatile catalytic materials for a wide range of industrial and lab-scale processes. Significant advances in fabrication and characterization of well-defined metal centers confined in zeolite matrixes have greatly expanded the library of available materials and, accordingly, their catalytic utility. In this review, we summarize recent developments in the field from the perspective of materials chemistry, focusing on synthesis, postsynthesis modification, (operando) spectroscopy characterization, and computational modeling of transition metal–zeolite catalysts.

Structure and Evolution of Confined Carbon Species during Methane Dehydroaromatization over Mo/ZSM-5

Surface carbon (coke, carbonaceous deposits) is an integral aspect of methane dehydroaromatization catalyzed by Mo/zeolites. We investigated the evolution of surface carbon species from the beginning of the induction period until the complete catalyst deactivation by the pulse reaction technique, TGA, 13C NMR, TEM, and XPS. Isotope labeling was performed to confirm the catalytic role of confined carbon species during MDA. It was found that “hard” and “soft” coke distinction is mainly related to the location of coke species inside the pores and on the external surface, respectively. In addition, MoO3 species act as an active oxidation catalyst, reducing the combustion temperature of a certain fraction of coke. Furthermore, after dissolving the zeolite framework by HF, we found that coke formed during the MDA reaction inside the zeolite pores is essentially a zeolite-templated carbon material. The possibility of preparing zeolite-templated carbons from the most available hydrocarbon feedstock is important for the development of these interesting materials.

High-silica zeolite membranes for gas and liquid separation

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