Direct Detection of Supramolecular Reaction Centers in the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 by 13C–27Al Solid‐State NMR Spectroscopy

Hydrocarbon‐pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon‐pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H‐ZSM‐5 zeolite by advanced ¹³C–²⁷Al double‐resonance solid‐state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the ¹³C nuclei (associated with HP species) and the ²⁷Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.     

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.     

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.     

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.     

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by ¹⁷O DNP surface‐enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub‐picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.     

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.     

Halogen‐Bonding‐Induced Conjugate Addition of Thiophenes to Enones and Enals

Herein, we report the conjugate addition of α,β‐unsaturated carbonyl compounds to thiophene derivatives. We used a 2‐iodoimidazolinium triflate salt as a halogen‐bonding donor, which afforded moderate‐to‐excellent yields of the corresponding alkylated thiophenes. Insight into the catalytic process was obtained from ¹H NMR spectroscopy studies and DFT calculations, which indicated a halogen‐bonding‐supported mechanism with limited Brønsted acid catalysis.     

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C₁–n‐C₄ alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C₁–n‐C₄ alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.     

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

Heterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by ¹⁷O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.     

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.     

Brønsted acid‐catalyzed polymerization of ε‐caprolactone in water: A mild and straightforward route to poly(ε‐caprolactone)‐graft‐water‐soluble polysaccharides

The polymerization of ε‐caprolactone (ε‐CL) has been assessed in water using various Brønsted acids as catalysts. The reaction was found to be quantitative at 100 °C, leading to number–average molecular weights up to 5000 g mol^?1. The Brønsted acid‐catalyzed polymerization of ε‐CL in water was further conducted in the presence of water‐soluble polysaccharides thereby affording graft copolymers. The approach enables an easy, mild access to dextran hydroxyesters. For low degree of substitution, the latter self‐assembles in water to form nanoparticles. Poly(ε‐CL)‐graft‐methylcellulose copolymers can also be obtained via a similar approach. It is noteworthy that the methodology reported herein is a one‐step route to poly(ε‐CL)‐graft‐water‐soluble polysaccharides, operating in mild conditions, that is, at low temperatures, using readily available metal‐free catalysts and water as a solvent. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2139–2145     

Evidence that protons can be the active catalysts in Lewis acid mediated hetero-Michael addition reactions

The mechanism of Lewis acid catalysed hetero‐Michael addition reactions of weakly basic nucleophiles to α,β‐unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone–catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of π‐olefin–metal complexes or for carbonyl–metal‐ion interactions was obtained. The reactions did not proceed in the presence of the non‐coordinating base 2,6‐di‐tert‐butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono‐ and dicarbonyl substrates have been rationalised. A ¹H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid‐catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.     

Impacts of Composition and Post‐Treatment on the Brønsted Acidity of Steam‐Treated Faujasite: Insights from FTIR Spectroscopy

A detailed FTIR study of the effects of steaming and acid leaching on protonated Y faujasite (FAU) and EMT zeolites is provided and the results are thoroughly analysed. In particular, emphasis is placed on the Brønsted acidic evolution and acidic strength measurements for a large series of as‐modified zeolites using CO as a sensitive probe to distinguish various protonic sites. While an increase of acidity for framework OH groups is observed during the strengthening of dealumination for both FAU and EMT series, the steaming process also generates a large variety of additional Brønsted acidic groups. Regarding acidic strength, these heterogeneous OH groups are sensitive to post‐treatments and their existence strongly depends on the initial composition of the zeolites. The presence of residual Na⁺ cations in the starting materials induces dramatic Brønsted acidic changes after steaming. As a result, steamed zeolites that initially contain traces of sodium possess unusual acidic Brønsted groups with low acidity. This result contradicts the trend generally observed with framework OH groups, for which steaming results in an increase of Brønsted acidic strength. The study reveals that the situation is indeed more complex, as some compositions and post‐treatments strongly influence the Brønsted acidity of as‐steamed zeolites both in their nature and their corresponding acidic strength. By linking these IR‐compiled features to the as‐exposed modifications, a large acidity scale better suited to characterizing catalysts having Brønsted acidity expanding from lowest to highest strength is proposed.     

Solid state NMR characterization of formation of poly(ϵ‐caprolactone)/maghnite nanocomposites by in situ polymerization

Results of multinuclear MAS NMR spectroscopy are reported for poly (ε‐caprolactone)/maghnite nanocomposite formation, with ε‐caprolactone in situ polymerized in the presence of maghnite, a proton exchanged montmorillonite clay. Exfoliated and intercalated materials with different maghnite loading in the range 3–15 wt % were investigated. ¹H NMR evidences Brønsted acid hydroxyl groups in the silicate layers and shows that their broad signal at 7.6 ppm present in the parent clay disappears in the nanocomposite material. ²⁷Al MAS NMR results show that beside the hexacoordinated aluminum signal, two additional peaks corresponding to two different tetrahedral Al sites are present in the clay framework. The NMR signal intensity of only one of them was found to be affected in the nanocomposites compared with the parent maghnite, suggesting that these specific aluminum sites are the reactive ones at the initial stages of the polymerization. However almost no changes occurred in the ²⁹Si NMR spectra, confirming that the polymer grafting, as indicated earlier by atomic force microscopy, took place on the aluminum tetracoordinated sites rather than on the silicon sites. A mechanism of maghnite surface catalyzed polymerization of ε‐caprolactone was proposed, involving Brønsted and Lewis acid sites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3060–3068, 2007     

MIL‐101‐SO3H: A Highly Efficient Brønsted Acid Catalyst for Heterogeneous Alcoholysis of Epoxides under Ambient Conditions

For the first time, a ～100 % sulfonic acid functionalized metal–organic framework (MOF), MIL‐101‐SO₃H, with giant pores has been prepared by a hydrothermal process followed by a facile postsynthetic HCl treatment strategy. The replete readily accessible Lewis acidic and especially Brønsted acidic sites distributed throughout the framework as well as high stability endow the resultant MOF exceptionally high efficiency and recyclability, which surpass all other MOF‐based catalysts, for the ring opening of epoxides with alcohols (especially, methanol) as nucleophiles under ambient conditions.     

Importance of Size and Distribution of Ni Nanoparticles for the Hydrodeoxygenation of Microalgae Oil

Improved synthetic approaches for preparing small‐sized Ni nanoparticles (d=3 nm) supported on HBEA zeolite have been explored and compared with the traditional impregnation method. The formation of surface nickel silicate/aluminate involved in the two precipitation processes are inferred to lead to the stronger interaction between the metal and the support. The lower Brønsted acid concentrations of these two Ni/HBEA catalysts compared with the parent zeolite caused by the partial exchange of Brønsted acid sites by Ni²⁺ cations do not influence the hydrodeoxygenation rates, but alter the product selectivity. Higher initial rates and higher stability have been achieved with these optimized catalysts for the hydrodeoxygenation of stearic acid and microalgae oil. Small metal particles facilitate high initial catalytic activity in the fresh sample and size uniformity ensures high catalyst stability.     

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.     

Acid/Vanadium‐Containing Saponite for the Conversion of Propene into Coke: Potential Flame‐Retardant Filler for Nanocomposite Materials

Vanadium‐containing saponite samples were synthesized in a one‐pot synthetic procedure with the aim of preparing samples for potential application as fillers for polymeric composites. These vanadium‐modified materials were prepared from an acid support by adopting a synthetic strategy that allowed us to introduce isolated structural V species (H/V‐SAP). The physicochemical properties of these materials were investigated by XRD analysis and by DR‐UV/Vis and FTIR spectroscopy of CO that was adsorbed at 100 K; these data were compared to those of a V‐modified saponite material that did not contain any Brønsted acid sites (Na/V‐SAP). The surface‐acid properties of both samples (together with the fully acidic H‐SAP material and the Na‐SAP solid) were studied in the catalytic isomerization of α‐pinene oxide. The V‐containing solids were tested in the oxidative dehydrogenation reaction of propene to evaluate their potential use as flame‐retardant fillers for polymer composites. The effect of tuning the presence of Lewis/Brønsted acid sites was carefully studied. The V‐containing saponite sample that contained a marked presence of Brønsted acid sites showed the most interesting performance in the oxidative dehydrogenation (ODH) reactions because they produced coke, even at 773 K. The catalytic data presented herein indicate that the H/V‐SAP material is potentially active as a flame‐retardant filler.     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.     

The synthesis of metal–organic framework Al‐MIL‐53‐derived Brønsted acid catalyst and its application in the Mannich reaction

A metal–organic framework Al‐MIL‐53‐NH₂‐derived Brønsted acid catalyst (Al‐MIL‐53‐RSO₃H) has been synthesized employing a post‐synthetic modification strategy under mild conditions. The Al‐MIL‐53‐RSO₃H catalyst was successfully utilized in the nitro‐Mannich reaction taking advantage of its strong Brønsted acidity. Good to excellent yields of Mannich adducts were achieved for a variety of acylimine substrates in the presence of 0.1 mol% Al‐MIL‐53‐RSO₃H. Furthermore, the Al‐MIL‐53‐RSO₃H catalyst can be recycled five times without decreasing the yield and selectivity of Mannich adducts.