The performance of zeolites in catalysis and adsorption is closely related to their inner architecture beneath the crystal surface, which however remains less studied due to characterization limitations. Here we report the synthesis of two ZSM-5 zeolite samples by changing only the order of mixing of Si and Al sources, resulting not only in morphological differences of the zeolite crystals but most importantly in defined distinction in their inner architecture. The spatial Si and Al distributions and structural properties of the ZSM-5 zeolite crystals were characterized by high-resolution microscopy under chemically unbiased defect-selective NH4F etching. The Al-zoning and structural features in the ZSM-5 zeolite crystals were explained by the biased nucleation in the Si-rich aluminosilicate amorphous precursor followed by multistage crystal growth in a heterogeneous feedstock. This observation was associated with the different solubility and reactivity of the microscopic aluminosilicate domains with various Si/Al ratios in the amorphous precursors. The zeolites with diverse structural properties showed a high cracking activity in n-hexane cracking reaction and different activity, stability, and product selectivity in the ethylene dehydroaromatization (EDA) reaction. The comprehensive understanding of the zeolite synthesis history and their performance in the EDA reaction revealed the chemical mixing-dependent synthesis–structure–performance correlation of the zeolite catalyst.
Natural gas, the cleanest fossil fuel, is an abundant source of methane and expected to play an increasingly important role in powering the world's economic growth over the energy transition of the coming decades. Methane has the potential to be a CO2-free feedstock to cogenerate hydrogen (H2) and added value "building-blocks" chemicals (e.g., olefins and aromatics) for petrochemistry. In this review, the two processes (i) the oxidative coupling of methane (OCM) for production of ethylene and (ii) the nonoxidative methane dehydroaromatization (MDA) producing hydrogen and benzene are discussed. Both routes convert methane directly into valuable products, an advantage over the several-steps syngas route. The performances of various a variety of catalysts reported during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures, M/Zr(SO4)2 with M = Mo, Fe, Pt) under similar reaction conditions are compared. The major drawbacks and the strategies used to mitigate the main challenges related with the performance of the catalysts in both OCM and MDA reactions are critically revealed. For instance, the overoxidation in the OCM is mitigated by optimizing of the operating conditions, using alternative oxidants, and the application of membrane reactor technology are discussed. In the MDA reaction, the major issue is the catalyst deactivation by coke formation and migration and sintering of metallic active phases. Strategies for robust catalysts, methods for mild coke removal, pretreatment under reductive atmosphere are presented. Approaches to improve aromatics yields over coke production by addition of promoters or co-feed reactants to the MDA catalysts are also discussed.
Ethylene dehydroaromatisation (EDA) was investigated at 700 °C under 1 bar of ethylene (5 mol% in N2) over a micro-(M) and a nano-sized (N) H-ZSM-5. On the M zeolite an induction period followed by deactivation was observed, which could be related to the presence of long diffusion path lengths in this sample, leading to mass transfer resistance. During the induction step, the aromatics yield increases, despite a significant loss of the acid site concentration as a result of coking. This induction period corresponds to the formation of an active hydrocarbon pool (HCP) composed of units of 2 to 5 aromatic rings with a molecular weight ranging from 130 to 220 g mol−1 (light coke). A kinetic study revealed that the developing HCP species is two times more active than Brønsted acid sites in the fresh zeolite. Diffusion limitations yet impact the product desorption by promoting coke growth and, therefore the deactivation of the HCP and hence of the catalyst. From MA-LDI/LDI-TOF MS (Matrix Assisted Laser Desorption Ionization—Time of Flight Mass Spectroscopy) characterisation was deduced that even after complete catalyst deactivation, the as-deposited coke continues growing at the external surface of the zeolite by condensation reactions, thus leading to heavy coke composed of more than 100 carbon atoms and a molar mass exceeding 1300 g mol−1. Unlike the micro-sized zeolite, the nano-scaled zeolite features a short diffusion path length and promotes fast formation of the active HCP. As a result, higher activity and selectivity into benzene were observed, whilst catalyst deactivation was significantly mitigated.
Methane dehydroaromatization reaction at 700 °C over Mo/ZSM-5 involves numerous modifications of the molybdenum species from the catalyst preparation and throughout the catalyst lifetime, composed of 4 successive steps: calcination, activation, induction, and deactivation. A thorough kinetic study was undertaken with the aim to understand the transformation phenomena occurring on the catalyst during each stage of the reaction, using methane gas hourly space velocity per gram of catalyst (M-GHSV) from 1 to 29 LCH4 h−1 gcat−1. Here from, unexpected behaviors were observed, supported by molecular modeling results. MoO3 firstly reacts stoichiometrically during the calcination (ΔrH =0.86 eV) with bridged hydroxyl pairs yielding [Mo2O5]2+ species (calcination). Thereafter, [Mo2O5]2+ slowly reduces by methane to form [Mo2C2]2+ (activation). The latter converts methane to ethylene (EA= 1.49 eV), which dimerizes two times faster to butene through hydrocarbon pool catalysis rather than through Brønsted acid sites (induction). The catalyst deactivates through an inhibition effect of aromatics, which adsorb strongly onto [Mo2C2]2+ (ΔHads ~ 0.7 eV) (deactivation). The large amount of autogenous hydrogen produced at lower space velocity allows preventing the active species poisoning, leading to slower deactivation rate.
Among all the proposed catalytic systems (new supports, synthesis post-treatment, change of transition metal, multi-metallic catalysts, etc.) for the methane dehydroaromatization, the initial Mo/ZSM-5 has remained one of the best suitable catalysts, despite its lack of deep understanding. The catalyst evolves throughout four successive stages: calcination, activation, induction, and deactivation. By studying the balance influence between the acid and metal functions throughout its lifetime, the molybdenum and carbon species could be localized, quantified, and identified as well as their roles. An optimal compromise was then established where the catalyst is composed of 4 wt% Mo with the highest possible acidity. Below these targets, the catalysts with minimal Mo content and low Brønsted acidity display poor catalytic performances, whereas zeolite amorphization occurs during the early stages of the reaction independently of the zeolite acidity once the Mo loading exceeds the optimal value.
We report a detailed kinetic study of methane dehydroaromatization on three bifunctional Mo/HZSM-5 catalysts (0.9 wt.%, 2.7 wt.%, and 17.2 wt. % Mo). Two main deactivation modes are present: (i) irreversible damage by zeolite amorphization during the isothermal pre-treatment in an inert atmosphere and (ii) reversible deactivation by coke deposition ("soft", "hard" and carbide species). The former occurs at high Mo loading (> 4.0 w%) and provides low activity catalysts. Catalysts with a lower and well-balanced molybdenum loading (< 4 w%) suffer mainly from the second deactivation mode. The catalyst decay is modelled by semi-empirical laws including time on stream and coke level. The latter initiates textural and structural catalyst modifications leading to an activity loss. Three deactivation descriptors, 1: coke level on the spent samples, 2: loss of micropore volume, and 3: monoclinic/orthorhombic phase transition of the zeolite are identified. Further characterization indicates that both molybdenum and carbon species are responsible for the catalytic deactivation of well-balanced catalysts. At least 50% of the initially dispersed molybdenum migrates to the external surface of the zeolite to form large clusters of molybdenum carbide. The coke extracted from the zeolite micropores are confirmed to be mainly unsubstituted polyaromatics.
Abstract In this article, we describe the synthesis of γ ‐lactones through the reaction of sulfoxonium ylides, aldehydes, and disubstituted ketenes. The one‐pot sequential method provides access to γ ‐lactones from disubstituted ketenes, in moderate to excellent yields, and with good diastereoselectivity favoring the trans ‐diastereomer (dr up to 92 : 8). The reaction mechanism was investigated by performing labeling, crossover, and various control experiments. The results of those experiments support the reaction mechanism involving betaine formation, reaction of the betaine with a ketene to form an enolate intermediate, [3,3]‐sigmatropic rearrangement of an enolate intermediate, and finally, 5‐ exo ‐ tet cyclization to afford the γ ‐lactone product.
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