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Methanol‐to‐olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C?C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid‐state NMR spectroscopy with UV/Vis diffuse reflectance spectroscopy and mass spectrometry on an active H‐SAPO‐34 catalyst, we provide spectroscopic evidence for the formation of surface acetate and methyl acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C?C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.  相似文献   

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The role of N‐heterocyclic carbenes in the chemistry of ionic liquids based on imidazolium salts has long been discussed. Here, we present experimental evidence that 1‐ethyl‐3‐methylimidazolium‐2‐ylidene (EMIm) can coexist with its protonated imidazolium cation (EMImH+) at low temperatures. If the vapor of the ionic liquid [EMImH+][AcO?] is trapped in solid argon or nitrogen at 9 K, only acetic acid (AcOH) and the carbene, but no ionic species, are found by IR spectroscopy. This indicates that during the evaporation of [EMImH+][AcO?] proton transfer occurs to form the neutral species. If the vapor of [EMImH+][AcO?] is trapped at 9 K as film in the absence of a host matrix, a solid consisting of EMImH+, EMIm, AcO?, and AcOH is formed. During warming to room temperature the proton transfer in the solid to form back the IL [EMImH+][AcO?] can be monitored by IR spectroscopy. This clearly demonstrates that evaporation and condensation of the IL [EMImH+][AcO?] results in a double proton transfer, and the carbene EMIm is only metastable even at low temperatures.  相似文献   

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Over zeolite H‐ZSM‐5, the aromatics‐based hydrocarbon‐pool mechanism of methanol‐to‐olefins (MTO) reaction was studied by GC‐MS, solid‐state NMR spectroscopy, and theoretical calculations. Isotopic‐labeling experimental results demonstrated that polymethylbenzenes (MBs) are intimately correlated with the formation of olefin products in the initial stage. More importantly, three types of cyclopentenyl cations (1,3‐dimethylcyclopentenyl, 1,2,3‐trimethylcyclopentenyl, and 1,3,4‐trimethylcyclopentenyl cations) and a pentamethylbenzenium ion were for the first time identified by solid‐state NMR spectroscopy and DFT calculations under both co‐feeding ([13C6]benzene and methanol) conditions and typical MTO working (feeding [13C]methanol alone) conditions. The comparable reactivity of the MBs (from xylene to tetramethylbenzene) and the carbocations (trimethylcyclopentenyl and pentamethylbenzium ions) in the MTO reaction was revealed by 13C‐labeling experiments, evidencing that they work together through a paring mechanism to produce propene. The paring route in a full aromatics‐based catalytic cycle was also supported by theoretical DFT calculations.  相似文献   

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We report the first generation and characterization of elusive Breslow intermediates derived from aromatic N‐heterocyclic carbenes (NHCs), namely benzimidazolin‐2‐ylidenes (NMR, X‐ray analysis) and thiazolin‐2‐ylidenes (NMR). In the former case, the diamino enols were generated by reaction of the free N,N‐bis(2,6‐diisopropylphenyl)‐ and N,N‐bis(mesityl)‐substituted benzimidazolin‐2‐ylidenes with aldehydes while the dimer of 3,4,5‐trimethylthiazolin‐2‐ylidene served as the starting material in the latter case. The unambiguous NMR identification of the first thiazolin‐2‐ylidene‐based Breslow intermediate rests on double 13C labeling of both the NHC and the aldehyde component. The acyl anion reactivity was confirmed by benzoin formation with excess aldehyde.  相似文献   

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Microporous vanadosilicates with octahedral VO6 and tetrahedral SiO4 units, better known as AM‐6, have been hydrothermally synthesized with different morphologies by controlling the Na/K molar ratio of the initial gel mixtures. The morphology of the AM‐6 materials changed from bulky cube to nanofiber aggregates as the Na/K molar ratio decreased from 1.9 to 0.2. Raman spectroscopy revealed that the VO3? intermediate species plays an important role in the formation of the nanofiber morphology. The orientation of ‐V‐O‐V‐ chains in nanofiber aggregates was examined by confocal polarized micro‐Raman spectroscopy. It was found that these aggregates are assemblies of short ‐V‐O‐V‐ chains perpendicular to the axis of nanofibers. The obtained AM‐6 nanofibers greatly increase the exposed proportion of V? O terminals, and thus improve the catalytic performance.  相似文献   

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