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.     

Synergic Effect of Active Sites in Zinc‐Modified ZSM‐5 Zeolites as Revealed by High‐Field Solid‐State NMR Spectroscopy

Understanding the nature of active sites in metal‐supported catalysts is of great importance towards establishing their structure–property relationships. The outstanding catalytic performance of metal‐supported catalysts is frequently ascribed to the synergic effect of different active sites, which is however not well spectroscopically characterized. Herein, we report the direct detection of surface Zn species and ¹H–⁶⁷Zn internuclear interaction between Zn²⁺ ions and Brønsted acid sites on Zn‐modified ZSM‐5 zeolites by high‐field solid‐state NMR spectroscopy. The observed promotion of C?H bond activation of methane is rationalized by the enhanced Brønsted acidity generated by synergic effects arising from the spatial proximity/interaction between Zn²⁺ ions and Brønsted acidic protons. The concentration of synergic active sites is determined by ¹H–⁶⁷Zn double‐resonance solid‐state NMR spectroscopy.     

Phosphonate‐Modified UiO‐66 Brønsted Acid Catalyst and Its Use in Dehydra‐Decyclization of 2‐Methyltetrahydrofuran to Pentadienes

Phosphorus‐modified all‐silica zeolites exhibit activity and selectivity in certain Brønsted acid catalyzed reactions for biomass conversion. In an effort to achieve similar performance with catalysts having well‐defined sites, we report the incorporation of Brønsted acidity to metal–organic frameworks with the UiO‐66 topology, achieved by attaching phosphonic acid to the 1,4‐benzenedicarboxylate ligand and using it to form UiO‐66‐PO₃H₂ by post‐synthesis modification. Characterization reveals that UiO‐66‐PO₃H₂ retains stability similar to UiO‐66, and exhibits weak Brønsted acidity, as demonstrated by titrations, alcohol dehydration, and dehydra‐decyclization of 2‐methyltetrahydrofuran (2‐MTHF). For the later reaction, the reported catalyst exhibits site‐time yields and selectivity approaching that of phosphoric acid on all‐silica zeolites. Using solid‐state NMR and deprotonation energy calculations, the chemical environments of P and the corresponding acidities are determined.     

Effect of Brønsted/Lewis Acid Ratio on Conversion of Sugars to 5‐Hydroxymethylfurfural over Mesoporous Nb and Nb‐W Oxides

A series of mesoporous Nb and Nb‐W oxides were employed as highly active solid acid catalysts for the conversion of glucose to 5‐hydroxymethylfurfural (HMF ). The results of solid state ³¹P MAS NMR spectroscopy with adsorbed trimethylphosphine as probe molecule show that the addition of W in niobium oxide increases the number of Brønsted acid sites and decreases the number of Lewis acid sites. The catalytic performance for Nb‐W oxides varied with the ratio of Brønsted to Lewis acid sites and high glucose conversion was observed over Nb₅W₅ and Nb₇W₃ oxides with high ratios of Brønsted to Lewis acid sites. All Nb‐W oxides show a relatively high selectivity of HMF , whereas no HMF forms over sulfuric acid due to its pure Brønsted acidity. The results indicate fast isomerization of glucose to fructose over Lewis acid sites followed by dehydration of fructose to HMF over Brønsted acid sites. Moreover, comparing to the reaction occurred in aqueous media, the 2‐butanol/H₂O system enhances the HMF selectivity and stabilizes the activity of the catalysts which gives the highest HMF selectivity of 52% over Nb₇W₃ oxide. The 2‐butanol/H₂O catalytic system can also be employed in conversion of sucrose, achieving HMF selectivity of 46% over Nb₅W₅ oxide.     

Catalytic Conversion of CO2 to Cyclic Carbonates through Multifunctional Zinc‐Modified ZSM‐5 Zeolite

The production of fine chemicals using CO₂ as C₁ building block through inexpensive heterogeneous catalysts with high efficiency under low pressure is challenging. Herein we propose for the first time the utilization of a multifunctional heterogeneous zinc‐modified HZSM‐5 (ZnHZSM‐5) catalyst for upgrading CO₂ by incorporation into cyclic carbonates from CO₂ and epoxides. Owing to the nice surface properties such as abundant Lewis acid, Brønsted acid and Lewis base sites, large surface area, and plenty of micropores, CO₂ could be concentrated and well activated in ZnHZSM‐5 verified by CO₂‐TPD, TG‐MS, etc. Meanwhile, the epoxides were also activated through metal center and hydrogen bond. Therefore, the reaction can easily assemble at the catalyst interface and show exceptional performance, affording the aimed products with high yield of up to 99% in the presence of commercial tetra‐n‐propylammonium bromide (90% in kilogram scale with 0.004 mol% ZnHZSM‐5 and 0.015 mol% ⁿPr₄NBr).     

Thermodynamics of Carbon Dioxide Adsorption on the Protonic Zeolite H‐ZSM‐5

Adsorption of carbon dioxide on H‐ZSM‐5 zeolite (Si:Al=11.5:1) was studied by means of variable‐temperature FT‐IR spectroscopy, in the temperature range of 310–365 K. The adsorbed CO₂ molecules interact with the zeolite Brønsted‐acid OH groups bringing about a characteristic red‐shift of the O? H stretching band from 3610 cm^?1 to 3480 cm^?1. Simultaneously, the ν₃ mode of adsorbed CO₂ is observed at 2345 cm^?1. From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm^?1, upon changing temperature (and CO₂ equilibrium pressure), the standard adsorption enthalpy of CO₂ on H‐ZSM‐5 is ΔH⁰=?31.2(±1) kJ mol^?1 and the corresponding entropy change is ΔS⁰=?140(±10) J mol^?1 K^?1. These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali‐metal exchanged, zeolites.     

Oxidative Dehydrogenation of Propane over a VO2‐Exchanged MCM‐22 Zeolite: A DFT Study

The adsorption and the mechanism of the oxidative dehydrogenation (ODH) of propane over VO₂‐exchanged MCM‐22 are investigated by DFT calculations using the M06‐L functional, which takes into account dispersion contributions to the energy. The adsorption energies of propane are in good agreement with those from computationally much more demanding MP2 calculations and with experimental results. In contrast, B3LYP binding energies are too small. The reaction begins with the movement of a methylene hydrogen atom to the oxygen atom of the VO₂ group, which leads to an isopropyl radical bound to a HO? V? O intermediate. This step is rate determining with the apparent activation energy of 30.9 kcal mol^?1, a value within the range of experimental results for ODH over other silica supports. In the propene formation step, the hydroxyl group is the more reactive group requiring an apparent activation energy of 27.7 kcal mol^?1 compared to that of the oxy group of 40.8 kcal mol^?1. To take the effect of the extended framework into account, single‐point calculations on 120T structures at the same level of theory are performed. The apparent activation energy is reduced to 28.5 kcal mol^?1 by a stabilizing effect caused by the framework. Reoxidation of the catalyst is found to be important for the product release at the end of the reaction.     

Synthesis and characterization of a novel organic–inorganic hybrid salt and its application as a highly effectual Brønsted–Lewis acidic catalyst for the production of N,N′‐alkylidene bisamides

In this research, a novel organic–inorganic hybrid salt, namely, N¹,N¹,N²,N²‐tetramethyl‐N¹,N²‐bis(sulfo)ethane‐1,2‐diaminium tetrachloroferrate ([TMBSED][FeCl₄]₂) was prepared and characterized by Fourier‐transform infrared spectroscopy (FT‐IR), energy‐dispersive X‐ray spectroscopy (EDX), elemental mapping, field emission scanning electron microscopy (FE‐SEM), X‐ray diffraction (XRD), thermal gravimetric (TG), differential thermal gravimetric (DTG), and vibrating‐sample magnetometry (VSM) analyses. Catalytic activity of the hybrid salt was tested for the synthesis of N,N′‐alkylidene bisamides through the reaction of benzamide (2 eq.) and aromatic aldehydes (1 eq.) under solvent‐free conditions in which the products were obtained in high yields and short reaction times. The catalyst was superior to many of the reported catalysts in terms of two or more of these factors: the reaction medium and temperature, yield, time, and turnover frequency (TOF). [TMBSED][FeCl₄]₂ is a Brønsted–Lewis acidic catalyst; there are two SO₃H groups (as Brønsted acidic sites) and two tetrachloroferrate anions (as Lewis acidic sites) in its structure. Highly effectiveness of the catalyst for the synthesis of N,N′‐alkylidene bisamides can be attributed to synergy of the Brønsted and Lewis acids and also possessing two sites of each acid.     

Cascade Reactions in Tunable Lamellar Micro‐ and Mesopores for C=C Bond Coupling and Hydrocarbon Synthesis

Two‐dimensional MFI zeolite nanosheets contain Brønsted acid sites partially confined at the intercept between micro‐ and mesopores. These acid sites exhibit exceptional reactivities and stabilities for C=C bond coupling and ring‐closure reactions that transform light aldehydes to aromatics. These sites are much more effective than those confined within the micropores of MFI crystallites and those unconfined on H₄SiW₁₂O₄₀ clusters or mesoporous aluminosilicate Al‐MCM‐41. The partially confined site environment solvates and stabilizes the transition states of the kinetically relevant steps during aromatization.     

Chiral Brønsted Acid Directed Iron‐Catalyzed Enantioselective Friedel–Crafts Alkylation of Indoles with β‐Aryl α′‐Hydroxy Enones

A cooperative catalytic system established by the combination of an iron salt and a chiral Brønsted acid has proven to be effective in the asymmetric Friedel–Crafts alkylation of indoles with β‐aryl α′‐hydroxy enones. Good to excellent yields and enatioselectivities were observed for a variety of α′‐hydroxy enones and indoles, particularly for the β‐aryl α′‐hydroxy enones bearing an electron‐withdrawing group at the para position of the phenyl ring (up to 90 % yield and 91 % ee). The proton of the chiral Brønsted acid, the Lewis acid activation site, as well as the inherent basic site for the hydrogen‐bonding interaction of the Brønsted acid are responsible for the high catalytic activities and enantioselectivities of the title reaction. A possible reaction mechanism was proposed. The key catalytic species in the catalytic system, the phosphate salt of Fe^III, which was thought to be responsible for the high activity and good enantioselectivity, was then confirmed by ESIMS studies.     

A Gold‐ and Brønsted Acid Catalytic Interplay Towards the Synthesis of Highly Substituted Tetrahydrocarbazoles

The reaction of indoles and stabilized cyclopropyl alkynes under gold‐ and/or gold & Brønsted acid‐catalysis provided access to highly substituted tetrahydrocarbazoles. A mechanistic study revealed the complex mechanism underlying these processes and the opportunistic cooperation of Lewis and Brønsted acid‐catalysts towards the formation of complex molecular scaffolds.     

Lewis Acid or Brønsted Acid Catalyzed Reactions of Vinylidene Cyclopropanes with Activated Carbon–Nitrogen,Nitrogen–Nitrogen,and Iodine–Nitrogen Double‐Bond‐Containing Compounds

Lewis acid or Brønsted acid catalyzed reactions of vinylidene cyclopropanes (VDCPs), 1 , with activated carbon–nitrogen, nitrogen–nitrogen, and iodine–nitrogen double‐bond‐containing compounds have been thoroughly investigated. We found that pyrrolidine and 1,2,3,4‐tetrahydroquinoline derivatives can be formed in good yields in the reactions of VDCPs 1 with ethyl (arylimino)acetates 2 by a [3+2] cycloaddition or intramolecular Friedel–Crafts reaction pathway. Based on these results, we found that activated carbon–nitrogen and nitrogen–nitrogen double‐bond‐containing compounds, such as N‐toluene‐4‐sulfonyl (N‐Ts) imines 5 and diisopropylazodicarboxylate ( 7 ), can also react with VDCPs 1 to give [3+2] cycloaddition products in moderate to good yields in the presence of a Lewis acid. When N‐tert‐butoxycarbonyl aldimine 9 was used as the substrate, six‐membered cycloaddition products 10 and 11 were formed in moderate yields in the presence of a Brønsted acid, trifluoromethanesulfonic acid (TfOH). The reactions of VDCPs 1 with N‐Ts‐iminophenyliodinane ( 12 ) were also carried out in the presence of (CuOTf)₂ ? C₆H₆ and it was found that nitrogen‐containing indene derivatives 13 were obtained, rather than the aziridination products. Plausible mechanisms for all of these transformations are discussed, based on the obtained results.     

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina‐supported niobium oxide (Nb₂O₅/Al₂O₃) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier‐transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy‐dispersive X‐ray (EDX) analysis indicated that the Nb₂O₅ monolayer domains were highly dispersed over alumina at low Nb₂O₅ loadings, such as 5 wt %, and no Brønsted acid sites were presents. The coverage of Nb₂O₅ monolayer domains over Al₂O₃ increased with increasing Nb₂O₅ loading and almost‐full coverage was obtained at a loading of 16 wt %. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb? O(H)? Nb), which were formed at the boundaries between the domains of the Nb₂O₅ monolayer, acted as thermally stable Brønsted acid sites.     

Synthesis of the first nanomagnetic particles with semicarbazide‐based acidic ionic liquid tag: an efficient catalyst for the synthesis of 3,3′‐(arylmethylene)bis(4‐hydroxycoumarin) and 1‐carbamato‐alkyl‐2‐naphthol derivatives under mild and green conditions

Semicarbazide functionalized with chlorosulfonic acid on the surface of silica‐coated magnetic nanoparticles, {Fe₃O₄@SiO₂@(CH₂)₃Semicarbazide‐SO₃H/HCl}, as a novel magnetic Brønsted acid catalyst according to the aims of green chemistry was synthesized and fully characterized using Fourier transform infrared, UV–visible and energy‐dispersive X‐ray spectroscopies, X‐ray diffraction, scanning electron, transmission electron and atomic force microscopies and thermogravimetric analysis. The capability and excellent activity of this nanoparticle catalyst were exhibited in the synthesis of two series of compounds with important biological activities, namely 3,3′‐(arylmethylene)bis(4‐hydroxycoumarin) and 1‐carbamato‐alkyl‐2‐naphthol derivatives, under mild, green and solvent‐free conditions. To the best of our knowledge, this is the first study of the synthesis and application of {Fe₃O₄@SiO₂@(CH₂)₃Semicarbazide‐SO₃H/HCl} as Brønsted acid solid magnetic nanoparticles. Consequently the present study can open up a novel and promising intuition in the sequence of logical design, synthesis and applications of task‐specific Brønsted acid magnetic nanoparticle catalyst with favourable properties as a full‐fledged efficient material for sustainable approaches. Copyright © 2016 John Wiley & Sons, Ltd.     

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

Commercial bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Brønsted acidic H‐ZSM‐5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable production of ethylene. Now, strong Lewis acidic extra‐framework three‐coordinate Al³⁺ species were introduced into H‐ZSM‐5 zeolites to improve their catalytic activity. Remarkably, Al³⁺ species working with Brønsted acid sites can accelerate ethanol dehydration at a much lower reaction temperature and shorten the unsteady‐state period within 1–2 h, compared to >9 h for those without Al³⁺ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid‐state NMR, shows that strong Lewis acidic EFAl‐Al³⁺ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics‐based cycle to produce ethylene.     

Enantioselective CH Bond Functionalization Triggered by Radical Trifluoromethylation of Unactivated Alkene

An asymmetric unactivated alkene/C? H bond difunctionalization reaction for the concomitant construction of C? CF₃ and C? O bonds was realized by using a Cu/Brønsted acid cooperative catalytic system, thus providing facile access to valuable chiral CF₃‐containing N,O‐aminals with excellent regio‐, chemo‐, and enantioselectivity. Mechanistic studies revealed that this reaction may proceed by an unprecedented 1,5‐hydride shift involving activation of unactivated alkenes and a radical trifluoromethylation to initiate subsequent enantioselective functionalization of C? H bonds. Control experiments also suggested that chiral Brønsted acid plays multiple roles and not only controls the stereoselectivity but also increases the reaction rate through activation of Togni’s reagent.     

Vapor-phase Beckmann rearrangement of oxime molecules over H-Faujasite zeolite.

The Beckmann rearrangement (BR) plays an important role in a variety of industries. The mechanism of this reaction rearrangement of oximes with different molecular sizes, specifically, the oximes of formaldehyde (H₂C?NOH), Z‐acetaldehyde (CH₃HC?NOH), E‐acetaldehyde (CH₃HC?NOH) and acetone (CH₃)₂C?NOH, catalyzed by the Faujasite zeolite is investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6‐31G (d,p) basis set. To enhance the energetic properties, single point calculations are undertaken at MP2/6‐311G(d,p). The rearrangement step, using the bare cluster model, is the rate determining step of the entire reaction of these oxime molecules of which the energy barrier is between 50–70 kcal mol^?1. The more accurate embedded cluster model, in which the effect of the zeolitic framework is included, yields as the rate determining step, the formaldehyde oxime reaction rearrangement with an energy barrier of 50.4 kcal mol^?1. With the inclusion of the methyl substitution at the carbon‐end of formaldehyde oxime, the rate determining step of the reaction becomes the 1,2 H‐shift step for Z‐acetaldehyde oxime (30.5 kcal mol^?1) and acetone oxime (31.2 kcal mol^?1), while, in the E‐acetaldehyde oxime, the rate determining step is either the 1,2 H‐shift (26.2 kcal mol^?1) or the rearrangement step (26.6 kcal mol^?1). These results signify the important role that the effect of the zeolite framework plays in lowering the activation energy by stabilizing all of the ionic species in the process. It should, however, be noted that the sizeable turnover of a reaction catalyzed by the Brønsted acid site might be delayed by the quantitatively high desorption energy of the product and readsorption of the reactant at the active center.     

Thiiranation of 2′‐adamantylidene‐9‐benzonorbornenylidene using 4,4′‐oligothiodimorpholine and brønsted acid

On leaving 4,4′‐dithiodimorpholine 6 powder undisturbed at room temperature over 10 years, it led to the formation of 4,4′‐tetrathiodimorpholine 7 . Reactions of 2′‐adamantylidene‐9‐benzonorbornenyidene 1 with 6, 7 , and 4,4′‐thiodimorpholine 8 and a Brønsted acid in CH₂Cl₂ at room temperature proceeded to afford the corresponding thiiranes, 2 and 3 . The order of reactivity of 4,4′‐oligothiodimorpholines combined with a Brønsted acid is 7 > 6 > 8 . The thiirane 3 was transformed to 1 and 2 under the reaction conditions. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:12–18, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20505     

A New Family of Two‐Dimensional Zeolites Prepared from the Intermediate Layered Precursor IPC‐3P Obtained during the Synthesis of TUN Zeolite

The crystallization of zeolite TUN with 1,4‐bis(N‐methylpyrrolidinium)butane as template proceeds through an intermediate, designated IPC‐3P, following the Ostwald rule of successive transformations. This apparently layered transient product has been thoroughly investigated and found to consist of MWW monolayers stacked without alignment in register, that is, disordered compared with MCM‐22P. The structure was confirmed based on X‐ray diffraction and high‐resolution (HR)TEM analysis. The layered zeolite precursor IPC‐3P can be swollen and pillared affording a combined micro‐ and mesoporous material with enhanced Brunauer–Emmett–Teller (BET) surface area (685 m²g^?1) and greater accessibility of Brønsted acid sites for bulky molecules. This mesoporous material was probed with 2,6‐di‐tert‐butylpyridine (DTBP). IPC‐3P and its modification create a new layered zeolite sub‐family belonging to the MWW family. FTIR data indicate that (Al)MWW materials MCM‐22 and IPC‐3 with Si/Al ratios greater than 20 exhibit a lower relative ratio of Brønsted to Lewis acid sites than MCM‐22 (with Si/Al ratios of around 13), that is, less than 2 versus more than 3, respectively. This is maintained even upon pillaring and warrants further exploration of materials like IPC‐3P with a higher Al content. The unique XRD features of IPC‐3P indicating misaligned stacking of layers and distinct from MCM‐22P, are also seen in other MWW materials such as EMM‐10P, hexamethonium‐templated (HM)‐MCM‐22, ITQ‐30, and UZM‐8 suggesting the need for more detailed study of their identity and properties.     

Transesterification Kinetics Investigation of R‐Substituted Phenyl Benzoates with 4‐Methoxyphenol in the Presence of K2CO3 in DMF

Transesterification of R‐substituted phenyl benzoates 1–5 with 4‐methoxyphenol 6 was kinetically investigated in the presence of K₂CO₃ in dimethylformamide (DMF) at various temperatures. The Hammett plots for the reactions of the 1–5 demonstrate good linear correlations with σ⁰ constants. Low magnitude of ρ_LG values indicate that the leaving group departure occurs after the rate‐determining step. The Brønsted coefficient values for the reactions (?0.2, ?0.16, ?0.13 at 15, 24, 36°C, respectively) demonstrate the weak effect of leaving group substituent on the reactivity of R‐substituted phenyl benzoates 1–5 for the reactions with 4‐methoxyphenol 6 in the presence of K₂CO₃ in DMF. The leaving group substituent effect on free energy (ΔG^≠), enthalpy (ΔH^≠), and entropy (ΔS^≠) of activation was examined. It was shown that the activation parameters obtained depend weakly on the leaving group substituent effect. The reaction is entropy controlled in case the leaving group substituent becomes electron withdrawing.