Direct Mechanism of the First Carbon–Carbon Bond Formation in the Methanol‐to‐Hydrocarbons Process

In the past two decades, the reaction mechanism of C−C bond formation from either methanol or dimethyl ether (DME) in the methanol‐to‐hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface‐activated DME, by in situ solid‐state NMR spectroscopy, a species crucial to the first C−C bond formation in the MTH process. New insights into the first C−C bond formation were provided, thus suggesting DME/methanol activation and direct C−C bond formation by an interesting synergetic mechanism, involving C−H bond breakage and C−C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.     

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol‐to‐Hydrocarbons Process

After a prolonged effort over many years, the route for the formation of a direct carbon?carbon (C?C) bond during the methanol‐to‐hydrocarbon (MTH) process has very recently been unveiled. However, the relevance of the “direct mechanism”‐derived molecules (that is, methyl acetate) during MTH, and subsequent transformation routes to the conventional hydrocarbon pool (HCP) species, are yet to be established. This important piece of the MTH chemistry puzzle is not only essential from a fundamental perspective, but is also important to maximize catalytic performance. The MTH process was probed over a commercially relevant H‐SAPO‐34 catalyst, using a combination of advanced solid‐state NMR spectroscopy and operando UV/Vis diffuse reflectance spectroscopy coupled to an on‐line mass spectrometer. Spectroscopic evidence is provided for the formation of (olefinic and aromatic) HCP species, which are indeed derived exclusively from the direct C?C bond‐containing acetyl group of methyl acetate. New mechanistic insights have been obtained from the MTH process, including the identification of hydrocarbon‐based co‐catalytic organic reaction centers.     

Disentangling Reaction Processes of Zeolites within Single‐Oriented Channels

Establishing structure–reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well‐defined model system was developed to build structure–reactivity relationships for specific zeolite‐channel orientations during various catalytic reaction processes, for example, the methanol‐ and ethanol‐to‐hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single‐oriented zeolite ZSM‐5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on‐line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.     

π‐Interactions between Cyclic Carbocations and Aromatics Cause Zeolite Deactivation in Methanol‐to‐Hydrocarbon Conversion

The understanding of catalyst deactivation represents one of the major challenges for the methanol‐to‐hydrocarbon (MTH) reaction over acidic zeolites. Here we report the critical role of intermolecular π‐interactions in catalyst deactivation in the MTH reaction on zeolites H‐SSZ‐13 and H‐ZSM‐5. π‐interaction‐induced spatial proximities between cyclopentenyl cations and aromatics in the confined channels and/or cages of zeolites are revealed by two‐dimensional solid‐state NMR spectroscopy. The formation of naphtalene as a precursor to coke species is favored due to the reaction of aromatics with the nearby cyclopentenyl cations and correlates with both acid density and zeolite topology.     

Large Zeolite H‐ZSM‐5 Crystals as Models for the Methanol‐to‐Hydrocarbons Process: Bridging the Gap between Single‐Particle Examination and Bulk Catalyst Analysis

The catalytic, deactivation, and regeneration characteristics of large coffin‐shaped H‐ZSM‐5 crystals were investigated during the methanol‐to‐hydrocarbons (MTH) reaction at 350 and 500 °C. Online gas‐phase effluent analysis and examination of retained material thereof were used to explore the bulk properties of large coffin‐shaped zeolite H‐ZSM‐5 crystals in a fixed‐bed reactor to introduce them as model catalysts for the MTH reaction. These findings were related to observations made at the individual particle level by using polarization‐dependent UV‐visible microspectroscopy and mass spectrometric techniques after reaction in an in situ microspectroscopy reaction cell. Excellent agreement between the spectroscopic measurements and the analysis of hydrocarbon deposits by means of retained hydrocarbon analysis and time‐of‐flight secondary‐ion mass spectrometry of spent catalyst materials was observed. The obtained data reveal a shift towards more condensed coke deposits on the outer zeolite surface at higher reaction temperatures. Zeolites in the fixed‐bed reactor setup underwent more coke deposition than those reacted in the in situ microspectroscopy reaction cell. Regeneration studies of the large zeolite crystals were performed by oxidation in O₂/inert gas mixtures at 550 °C. UV‐visible microspectroscopic measurements using the oligomerization of styrene derivatives as probe reaction indicated that the fraction of strong acid sites decreased during regeneration. This change was accompanied by a slight decrease in the initial conversion obtained after regeneration. H‐ZSM‐5 deactivated more rapidly at higher reaction temperature.     

Hydrogen‐Abstraction/Acetylene‐Addition Exposed

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent in the interstellar medium (ISM) and also in carbonaceous meteorites (CM) such as Murchison. However, the basic reaction routes leading to the formation of even the simplest PAH—naphthalene (C₁₀H₈)—via the hydrogen‐abstraction/acetylene‐addition (HACA) mechanism still remain ambiguous. Here, by revealing the uncharted fundamental chemistry of the styrenyl (C₈H₇) and the ortho‐vinylphenyl radicals (C₈H₇)—key transient species of the HACA mechanism—with acetylene (C₂H₂), we provide the first solid experimental evidence on the facile formation of naphthalene in a simulated combustion environment validating the previously postulated HACA mechanism for these two radicals. This study highlights, at the molecular level spanning combustion and astrochemistry, the importance of the HACA mechanism to the formation of the prototype PAH naphthalene.     

Formation and Fate of Formaldehyde in Methanol‐to‐Hydrocarbon Reaction: In Situ Synchrotron Radiation Photoionization Mass Spectrometry Study

HCHO has been confirmed as an active intermediate in the methanol‐to‐hydrocarbon (MTH) reaction, and is critical for interpreting the mechanisms of coke formation. Here, HCHO was detected and quantified during the MTH process over HSAPO‐34 and HZSM‐5 by in situ synchrotron radiation photoionization mass spectrometry. Compared with conventional methods, excellent time‐resolved profiles were obtained to study the formation and fate of HCHO, and other products during the induction, steady‐state reaction, and deactivation periods. Similar formation trends of HCHO and methane, and their close correlation in yields suggest that they are derived from disproportionation of methanol at acidic sites. In the presence of Y₂O₃, the amount of HCHO changes, affecting the hydrogen‐transfer processes of olefins into aromatics and aromatics into cokes. The yield of HCHO affects the aromatic‐based cycle and the formation of ethylene, indicating that ethylene is mainly formed from the aromatic‐based cycle.     

Experimental and modeling study of the oxidation of 1‐butyne and 2‐butyne

Ignition delays were measured behind shock waves in the cases of hydrocarbon–oxygen–argon mixtures containing 1‐butyne or 2‐butyne (1 or 2% of hydrocarbons for equivalence ratios from 0.5 to 2). Reflected shock waves permitted to obtain temperatures from 1100 to 1600 K and pressures from 6.3 to 9.1 atm. A detailed mechanism of the reactions of 1‐butyne and 2‐butyne has been explained in the line of the mechanism developed previously for the reaction of C₃–C₄ unsaturated hydrocarbons (propyne, allene, 1,3‐butadiene) [Int J chem Kin 1999, 31, 361]. It is based on the most recent kinetic data values published in the literature and is consistent with thermochemistry. This mechanism has been validated by comparing the results of our simulations to the experimental results obtained for ignition delays in our shock tube and to measurements of species obtained during thermal decomposition [Int J Chem Kin 1995, 27, 321; J Phys Chem 1993, 97, 10977]. The main reaction pathways have been derived from flow‐rate and sensitivity analyses. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 172–183, 2002; DOI 10.1002/kin.10035     

C−C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM‐5 Zeolite

Despite significant progress achieved in Fischer–Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn²⁺‐ion exchanged ZSM‐5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO₂) at 20 % CO conversion. NMR spectroscopy, X‐ray absorption spectroscopy, and X‐ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM‐5. Quasi‐in‐situ solid‐state NMR, obtained by quenching the reaction in liquid N₂, detects C₂ species such as acetyl (‐COCH₃) bonding with an oxygen, ethyl (‐CH₂CH₃) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Brønsted acid site of the catalyst, respectively. These species could provide insight into C?C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn²⁺‐ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ‐13, MCM‐22, and ZSM‐12) all give ethane predominantly. By contrast, a physical mixture of ZnO‐ZSM‐5 favors formation of hydrocarbons beyond C₃₊. These results provide an important guide for tuning the product selectivity in syngas conversion.     

Mechanistic dichotomy in the gas‐phase addition of NO3 to polycyclic aromatic hydrocarbons: Theoretical study

The gas‐phase addition mechanism of the NO₃ radical (an important tropospheric nocturnal oxidizing species) to some selected polycyclic aromatic hydrocarbons (PAHs), important pollutants of the troposphere, has been computationally analysed. Purpose of this work is to verify whether the reaction can take place through a mechanism different from the simple radical addition to the π aromatic system. This mechanism could consist in an Electron Transfer (ET) from the aromatics to NO₃, thus generating an aromatic radical‐cation and a nitrate anion at long C O distances. The coulomb attraction should finally bind the two species and generate the radical adduct without any electronic energy barrier. The CASPT2 results show that, while benzene and naphthalene react with NO₃ through a plain radical mechanism, anthracene reacts by a mechanism with a partial ET character, and pentacene reacts with a sort of inner‐sphere ET pathway. These results concur to explain the high reactivity of NO₃ with larger PAHs whose ionization energy is below 7 eV and could be important in studies of environmental PAH oxidative degradation.     

Highly Diastereoselective Synthesis of Methylenecyclobutanes by Merging Boron‐Homologation and Boron‐Allylation Strategies

A highly diastereoselective synthesis of methylenecyclobutanes possessing a quaternary stereocenter is reported, in which boron homologation of an easily‐generated cyclobutenylmetal species is performed, followed by an allylation reaction. Combining three steps in a one‐pot process further optimized the method, which afforded the expected adducts in excellent yields and stereoselectivity, starting from commercially available 4‐bromobutyne.     

Metal‐Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons

This study explores the kinetics, mechanism, and active sites of the CO₂ electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon‐based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO₂ to CO/H₂ mixtures outperforming a low‐area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low‐cost, abundant electrocatalysts for synthetic fuel production from CO₂.     

A stepwise retro‐imino‐ene as a key step in the mechanism of allene formation via the Crabbé acetylene homologation

The mechanism of the acetylene homologation procedure accidentally discovered and further developed by Crabbé and coworkers is unknown. Kinetic isotope effect (KIE) experiments, however, suggest that an intramolecular hydrogen shift is the key step of the transformation. In this work, we present a computational study of this mechanism. We found that the reaction proceeds via an unexpected stepwise retro‐imino‐ene rearrangement. This mechanism justifies the role of Cu(I) as a reaction catalyst and is also compatible with the KIE experiments reported. © 2012 Wiley Periodicals, Inc.     

Telescoped,Divergent, Chemoselective C1 and C1‐C1 Homologation of Imine Surrogates: Access to Quaternary Chloro‐ and Halomethyl‐Trifluoromethyl Aziridines

A conceptually novel, high‐yielding, mono‐ or bis‐homologation was realized with lithium halocarbenoids and enables the one‐step, fully chemocontrolled assembly of a new class of quaternary trifluoromethyl aziridines. Trifluoroacetimidoyl chlorides (TFAICs) act as convenient electrophilic platforms, enabling the addition of either one or two homologating elements by simply controlling the stoichiometry of the process. Mechanistic studies highlighted that the homologation event, carried out with two different carbenoids (LiCH₂Cl and LiCH₂F), leads to fluoromethyl analogues in which the first nucleophile is employed for constructing the cycle and the second for decorating the resulting molecular architecture.     

Telescoped,Divergent, Chemoselective C1 and C1‐C1 Homologation of Imine Surrogates: Access to Quaternary Chloro‐ and Halomethyl‐Trifluoromethyl Aziridines

A conceptually novel, high‐yielding, mono‐ or bis‐homologation was realized with lithium halocarbenoids and enables the one‐step, fully chemocontrolled assembly of a new class of quaternary trifluoromethyl aziridines. Trifluoroacetimidoyl chlorides (TFAICs) act as convenient electrophilic platforms, enabling the addition of either one or two homologating elements by simply controlling the stoichiometry of the process. Mechanistic studies highlighted that the homologation event, carried out with two different carbenoids (LiCH₂Cl and LiCH₂F), leads to fluoromethyl analogues in which the first nucleophile is employed for constructing the cycle and the second for decorating the resulting molecular architecture.     

甲醇制烃反应机理研究进展

综述了甲醇制烃(MTH)反应机理的研究概况.分别介绍了MTH反应过程中的二甲醚生成、烃池机理、深度反应、催化剂失活原因和副产物甲烷的生成途径,重点综述了烃池机理的研究进展和存在的争议.指出了明确分子筛结构和酸性对烃池物种类型、低碳烯烃生成路径的影响以及初始C—C键形成机制是未来MTH机理研究的方向,据此指导并开发出抗积炭失活能力强、目标产物选择性高的催化剂仍是改进MTH工艺的关键.     

Ordered Mesoporous Organosilica with Ionic‐Liquid Framework: An Efficient and Reusable Support for the Palladium‐Catalyzed Suzuki–Miyaura Coupling Reaction in Water

The preparation of a novel palladium‐supported periodic mesoporous organosilica based on alkylimidazolium ionic liquid (Pd@PMO‐IL) in which imidazoilium ionic liquid is uniformly distributed in the silica mesoporous framework is described. Both Pd@PMO‐IL and the parent PMO‐IL were characterized by N₂‐adsorption–desorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), TEM, and solid‐state NMR spectroscopy. We have demonstrated that Pd@PMO‐IL is an efficient and reusable catalyst for the Suzuki–Miyaura coupling reaction of various types of iodo‐, bromo‐, and even deactivated aryl chlorides in water. It was also found that although the PMO‐IL nanostructure acts as reservoir for soluble Pd species, it can also operate as a nanoscaffold to recapture the Pd nanoparticles into the mesochannels thus preventing extensive agglomeration of Pd. This observation might be attributed to the isolated ionic liquid units that effectively control the reaction mechanism by preventing Pd agglomeration and releasing and recapturing Pd nanoparticles during the reaction process. The catalyst can be recovered and reused for at least four reaction cycles without significant loss of activity.     

A Free‐Radical Prompted Barrierless Gas‐Phase Synthesis of Pentacene

A representative, low‐temperature gas‐phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C₂₂H₁₄) along with its benzo[a]tetracene isomer (C₂₂H₁₄) is unraveled by probing the elementary reaction of the 2‐tetracenyl radical (C₁₈H₁₁^.) with vinylacetylene (C₄H₄). The pathway to pentacene—a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes—is facilitated by a barrierless, vinylacetylene mediated gas‐phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low‐temperature pathway can launch isomer‐selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free‐radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.     

Experimental and modeling of oxidation of acetylene,propyne, allene and 1,3‐butadiene

The ignition delays of insaturated hydrocarbons‐oxygen‐argon mixtures were measured behind shock waves in the cases of acetylene, propyne, allene and 1,3‐butadiene. Reflected shock waves permitted to obtain temperatures from 1000–1650 K and pressures from 8.5 to 10.0 atm. A particular effort has been made to build a detailed mechanism of the reactions of C₃‐C₄ unsaturated species and benzene. This mechanism is based on the most recent kinetic data values published in the literature and is consistent with the thermochemistry. This mechanism has been validated by comparing the results of our simulations to the experimental results obtained in our shock tube experiments and to profiles of radicalar and molecular species measured in three premixed flames (acetylene [1–2] and 1,3‐butadiene [3]) coming from the literature. The main reaction pathways have been derived in the case of the oxidation of these four insaturated hydrocarbons and for the formation of benzene. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 361–379, 1999     

Disentangling Reaction Processes of Zeolites within Single-Oriented Channels

Establishing structure–reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well-defined model system was developed to build structure–reactivity relationships for specific zeolite-channel orientations during various catalytic reaction processes, for example, the methanol- and ethanol-to-hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single-oriented zeolite ZSM-5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on-line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.