Nsted Acid Research Articles

Post-synthesis metal (Sn, Zr, Hf) modification of BEA zeolite: Combined Lewis and Br⊘nsted acidity for cascade catalysis

Zeolites modified by Lewis acidic metal centers such as Sn, Zr, and Hf are promising catalysts for numerous reactions relevant to biorefining, such as isomerization of carbohydrates and Meerwein-Ponndorf-Verley (MPV) reduction of furans. Preferred catalysts contain these metal ions in the framework of large-pore BEA zeolite, requiring often complex multistep preparation procedures based on expensive organometallic precursors. Herein, we developed a facile approach for obtaining highly dispersed isolated Sn, Zr, and Hf incorporated in dealuminated BEA zeolite with high metal content (Si/M ratio of 50–75) via a solid-state grinding approach using simple inorganic metal precursors. The efficient incorporation of isolated metal sites in the BEA framework with high content was achieved by methanol treatment before calcination, which removes excess metal. The Lewis acid sites derive from isolated metal ions in open sites for Sn, Zr, and Hf, while Sn-modified BEA also contains closed Sn sites. The open Sn sites display the highest Lewis acidity. The Br⊘nsted acidity stems from silanols perturbed by Lewis acidic metal ions of open metal sites and the OH group connected to the open metal sites. The metal-modified zeolites are active in the cascade reductive etherification of cinnamaldehyde, involving the MPV reduction to cinnamyl alcohol and the subsequent etherification to cinnamyl propyl ether with the isopropanol solvent over Lewis and Br⊘nsted acid sites, respectively. Sn-modified BEA was the most active sample, which stems from its strongest Lewis acidity, which is crucial for the first MPV step. Sn modification of the optimized solid-state ion-exchange method was applied to various BEA zeolites with different morphologies (nanocrystalline, hierarchical, and conventional BEA), showing that pore hierarchization can further benefit cascade reductive etherification reaction.

Electrostatic Fingerprints of Catalytically Active Amino Acids in Enzymes

The computed electrostatic and proton transfer properties are studied for 20 enzymes that represent all six major EC classes and a variety of different folds. The properties of aspartate, glutamate, and lysine residues that have been previously experimentally determined to be catalytically active are reported. All catalytic aspartate and glutamate residues studied here are strongly coupled to at least one other aspartate or glutamate residue and often to multiple other carboxylate residues with intrinsic pKa differences less than 1 pH unit. Sometimes these catalytic acidic residues are also coupled to a histidine residue, such that the intrinsic pKa of the acidic residue is higher than that of the histidine. All catalytic lysine residues studied here are strongly coupled to tyrosine or cysteine residues, wherein the intrinsic pKa of the anion‐forming residue is higher than that of the lysine. Some catalytic lysines are also coupled to other lysines with intrinsic pKa differences within 1 pH unit. Some evidence of the possible types of interactions that facilitate nucleophilicity is discussed. The interactions reported here provide important clues about how side chain functional groups that are weak Br⊘nsted acids or bases for the free amino acid in solution can become strong acids, bases or nucleophiles in the enzymatic environment. Such interactions are important considerations in enzyme design.

Ranking the Bronsted Acid Strength of Protonic Zeolites with VTIR Spectroscopy --- An Overview of Current Research

Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Br nsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Br nsted acid strength. The enthalpy change, Delta H0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Br nsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O-H stretching frequency, Deltaν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously Delta H0 and Deltaν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Deltaν(OH) and Br.nsted acid strength can be misleading when ranking zeolite acidity. Keywords: VTIR spectroscopy, br nsted acidity, H-zeolites

Acid Properties of GO and Reduced GO as Determined by Microcalorimetry, FTIR, and Kinetics of Cellulose Hydrolysis-Hydrogenolysis

Graphene oxide addresses increasing interests as a solid acid catalyst working in water for carbohydrate conversion. If there is a general agreement to correlate its unique catalytic performances to its ability to adsorb sugars, the origin of its acidity remains controversial. In this article, we study the acid strength of graphene oxide (GO) prepared by modified Hummers method and that of reduced GO by calorimetry of NH3 adsorption and by FTIR of pyridine adsorption. Very strong acid sites are detected on GO by calorimetry, while reduced graphene oxide (reGO) is not very acidic. The FTIR of pyridine adsorption shows the prevailing presence of Br∅nsted acid sites and a unique feature, the presence of pyridine coordinated by hydrogen bonds. This exceptionally strong Br∅nsted acidity is tentatively explained by the presence of graphene domains decorated by hydroxyl, carboxylic, or sulfonated groups within the GO sheet, resulting in a high mobility of the negative charges which makes the proton free and explains its strong acidity. Accordingly, only GO is active and selective for native cellulose hydrolysis, leading to 27% yield in glucose. Finally, we show that sugar alcohols cannot be formed directly from cellulose using GO combined with Pt/re-GO under hydrogen, explained by the reduction of oxygenated functions of GO. The instability of the functional groups of GO in a reducing atmosphere is the weak point of this peculiar solid acid.

Observation of Low-γ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy.

We introduce a novel NMR approach that extends the capabilities of indirect dynamic nuclear polarization (DNP) under magic-angle spinning to probe the local environment of half-integer spin quadrupolar nuclei. Compared to cross-polarization, this novel method based on the refocused INEPT scheme with adiabatic dipolar recoupling is easier to optimize and does not distort the quadrupolar line shapes. Furthermore, the use of this technique, instead of the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) scheme or direct DNP, greatly improves the sensitivity of DNP-NMR for the detection of quadrupolar isotopes with small dipolar couplings to protons, including notably those located in the subsurface of inorganic materials or with low gyromagnetic ratio (γ). This technique has been applied to identify the atomic-level structure of Brønsted acid sites of hydrated titania-supported MoO3, MoO3/TiO2, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated 17O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo2 and HOMo3 Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of 95Mo and 47,49Ti low-γ quadrupolar isotopes. This possibility has been demonstrated by detecting the signals of these nuclei near the surface of MoO3/TiO2. This technique has allowed the observation of 49Ti surface sites, which are absent from the bulk region of TiO2. Furthermore, both 95Mo and 47,49Ti DNP spectra have shown an increased structural disorder of TiO2 and MoO3 phases near the surface of the particles and notably the preferential location of the amorphous TiO2 phase at the surface of the particles. The proposed polarization transfer is also employed to acquire the first DNP-enhanced spectrum of 67Zn, another low-γ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained 17O, 27Al, and 67Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as α-Al2O3 and partially inverse ZnAl2O4 spinel.

Fundamental Mechanisms of Lewis Acid Doping of Organic Semiconductors

One key challenge in the field of organic semiconductors (OSCs) and their application in organic electronics is controlling the electronic properties of the OSC by p-/n-doping in a predictable manner. It has been established that upon dopant admixture either integer or fractional charge transfer can occur, where the parameters deciding one over the other are still under debate. In numerous studies integer charge transfer was found for conjugated polymers, while small molecular OSCs show a tendency towards fractional charge transfer forming ground state charge transfer complexes (1). The typical approach to assure integer charge transfer is using molecular dopants with a high electron affinity, exceeding the ionization energy of the OSC (for p-doping), as the dopant then exhibits an empty orbital lying lower in energy than the occupied frontier molecular orbital of the semiconductor. Recently, however, a transition between the two scenarios has been reported for polythiophene and the strong molecular p-dopant F4TCNQ, where aging of the samples appeared to promote the formation of charge-transfer complexes showing fractional charge transfer (2). Another study highlighted the key role of the microstructure for the doping process, where different polymorphs show either fractional or integer charge transfer (3).This lack of predictability of traditional doping employing strong donors/acceptors recently led to intense research efforts to find alternative approaches. There, using Lewis acids (LAs) as molecular p-dopants for OSCs has been found to be particularly promising, as these compounds react selectively with Lewis bases to form robust adducts. This makes LAs, in principle, chemically selective dopants that attack OSCs only at the sites where electron lone pairs are located. Interestingly, despite an electron affinity far below the ionization energy of polythiophene, it has been found that the LA BCF can readily ionize this material, which is incompatible with the conventional picture of integer charge transfer as detailed above (4). Very recently, it has been suggested that LA doping does not follow from the formation of an adduct, but instead, that the LA simply act as a protonating agent after forming a complex with water and, hence, becoming a Br∅nsted acid (5).In this presentation, I will briefly review these ideas and discuss how the concepts of electron affinity, ionization energy, integer, and partial charge transfer as used in the field relate to the doping using LAs. By correlating experimental data from optical and infrared spectroscopy with DFT calculations, I will provide alternative descriptors that allow to clarify the discussion and, more importantly, that have predictive power towards the use of Lewis acids as dopants for OSCs. Finally, I will show that the use of these descriptors, in the case of polythiophene doped with BCF discussed above (4), lift the apparent theoretical inconsistency, and as such, will be of high value for a comprehensive larger-scale exploration of OSCs doping using LAs. 1. Salzmann et al., Acc. Chem. Res. 2016, 49, 3702. Watts et al., Chem. Mater. 2019, 31, 6986−69943. Jacobs et al., Materials Horizons. 2018, 5, 6554. P. Pingel et al., Adv. Electron. Mater. 2016, 2:1600204.5. B. Yurash et al., Nature Materials. 2019, 18, 1327–1334

Lewis–Br⊘nsted induction acidity in SBA-15 modified with Zr and P

Abstract In this work, we present the synthesis of SBA-15 materials modified with 6 and 9 mol.% of Zr and P, respectively. Silanol SBA-15 groups were detected by Fourier transform infrared spectroscopy. X-ray diffractograms revealed that the typical hexagonal arrangement of SBA-15 was preserved after Zr and P introduction, and the structure was confirmed by transmission electron microscopy. After the Zr introduction, the morphology of SBA-15 changed from fibrous particles to a semi-spherical shape according to scanning electron microscopy, while nitrogen physisorption revealed the stability of the textural materials after the P introduction. The infrared spectra of pyridine adsorption indicated that the Zr and P incorporation into SBA-15 generated adequate Lewis and Br⊘nsted acidities to carry out methanol dehydration and direct the selectivity towards dimethyl ether, with medium-strong acid sites being responsible for obtaining up to 99% selectivity towards dimethyl ether.

Practical Asymmetric Synthesis of Sitagliptin Phosphate Monohydrate.

Optically pure sitagliptin phosphate monohydrate is efficiently and practically synthesized through a chiral hemiacetal as the key intermediate in 54% overall yield starting from (E)-4-(2,4,5-trifluorophenyl)but-2-enal and N-boc-protected hydroxylamine. The chiral hemiacetal fragment is constructed by a tandem aza-Michael/hemiacetal reaction catalyzed by an organocatalyst and the influence of acidity of Brønsted acid on tandem aza-Michael/hemiacetal reaction is researched in detail.

Br⊘nsted Acid Catalyzed gem‐Difluoroallylation of Aldehydes and Ketone with β‐Tosyloxy‐γ,γ‐difluroallylboronic Acid Pinacol Ester

AbstractA shelf‐stable gem‐difluorinated reagent β‐tosyloxy‐γ,γ‐difluroallylboronic acid pinacol ester has been prepared, which can be easily used for the preparation of gem‐difluorinated homoallylic alcohols through Br⊘nsted acid (PhCO2H) catalyzed gem‐difluoroallylation of aldehydes and ketone.

ChemInform Abstract: Reactions of 1,2‐Cyclopropyl Carbohydrates

AbstractReview: progress in the synthesis and reactions of 1,2‐cyclopropyl carbohydrates since 2000

ChemInform Abstract: 9‐Membered Carbocycle Formation: Development of Distinct Friedel—Crafts Cyclizations and Application to a Scalable Total Synthesis of (.+‐.)‐Caraphenol A.

While forming any medium-sized ring requires an approach capable of overcoming significant entropic and enthalpic penalties,[1] the challenge in meeting that requirement is often more pronounced in systems composed only of carbon atoms, such as the 9-membered rings of caraphenol A (1)[2] and α-viniferin (2),[3] two oligomeric forms of the natural product resveratrol (3)[4,5] which are bioactive (including acetylcholinesterase inhibition, anti-inflammatory properties, and potential value as probes for elucidating key pathways in Alzheimer’s disease and drug resistance in tumors).[6] Indeed, without the more obvious and established array of productive cyclization strategies afforded by heteroatoms, only a handful of approaches have proven capable of accessing fully carbocyclic variants. For 9-membered rings, that includes Grob fragmentations (such as 4→5),[7] highly context specific C–C bond constructions such as the acetylide addition leading to enediyne 6,[8] and ring-closing olefin metathesis (not shown).[9] Given this landscape, as well as the unique and sp2-rich patterning of 1 and 2 (and 11 other related molecules)[10] that effectively precludes the application of these three established strategies, we sought to identify additional approaches capable of forging strained, 9-membered carbocycles. Herein, we show that a variety of Friedel–Crafts-type reactions promoted by either Au(III) or Br∅nsted acids are competent in this regard with functionally rich substrates. In addition to providing what we believe is the first example of a 9-exo-dig ring closure, late-stage application of one of the developed approaches has enabled the completion of an efficient and highly scalable total synthesis of caraphenol A (1).

Primary‐Tertiary Diamine/Br⊘nsted Acid Catalyzed α‐Allylation of Carbonyl Compounds with Allylic Alcohols

AbstractA simple and direct α‐allylic alkylation of unmodified aldehydes with allylic alcohols catalyzed by primary amine/Br⊘nsted acid has been developed. The N,N‐dimethylethylenediamine/TfOH was identified as the optimal catalyst to promote this transformation via an enamine process in high yields (up to 88%) and with good diastereoselectivities (up to 9:1).

Preparation of a novel solid acid catalyst with Lewis and Br{\o}nsted acid sites and its application in acetalization

A novel melamine--formaldehyde resin (MFR) supported solid acid with Lewis and Bronsted acid sites was synthesized through the immobilization of acidic ionic liquid and cuprous ion on MFR. The scanning electron microscopy (SEM) characterization showed that addition of PEG-2000 in the synthesis of MFR could promote the formation of regular particles with diameters around 3.7 m m. The XRD pattern demonstrated that some cuprous ions were aggregated. The catalytic performance of this acid catalyst was evaluated by acetalization. The results showed that the catalytic activity of MFR with Bronsted acid could be improved by addition of Lewis acid. The solid acid was very efficient for the acetalization of carbonyl compounds and diols with moderate to excellent yields and there was no loss of catalytic activity even after being recycled for 6 runs.

Autothermal reforming of butanol to butenes in a staged millisecond reactor: Effect of catalysts and isomers

Dehydration and isomerization of butanol is studied in an autothermal short contact-time reactor containing a 1wt% Pt stage followed by a zeolite or γ-Al2O3 stage downstream to convert butanol into butenes with up to 95% yield at residence times on the order of 100ms. CH4 is fed as a sacrificial fuel to the Pt stage and butanol is fed between the stages to avoid undesired oxidation and reforming reactions of butanol over Pt. The energy released by CH4 catalytic partial oxidation drives downstream butanol dehydration and isomerization.The effect of catalyst is studied by comparing the performance of HZSM-5, HFER, and γ-Al2O3 catalysts. Higher yields (20%) of butenes were obtained with γ-Al2O3 and HFER than with HZSM-5. The absence of Br∅nsted acid sites in γ-Al2O3 and the small pore structure of HFER lead to reduced yields of large side products such as higher hydrocarbons that promote oligomerization reactions. A 95% butene yield is obtained with HFER at temperatures ranging from 280–350°C and a 95% yield with γ-Al2O3 at temperatures between 320 and 350°C. Only a 75% butene yield was obtained with HZSM-5 at 230°C.The effect of hydrocarbon structure on product formation is studied by comparing conversions of each butanol isomer using a heated tube reactor at temperatures between 200 and 400°C. The reactivity of butanol follows as: t-butanol>2-butanol>iso-butanol>1-butanol. trans-2-Butene and cis-2-butene are primarily formed from linear butanol isomers, while isobutene forms from branched butanol isomers. Conversions and product distributions of butanol isomers formed over HZSM-5 in a staged reactor are comparable (<10% difference across all species) with data using a heated tube reactor at similar temperatures.We successfully demonstrate an alternative pathway to dehydrate butanol into butenes with an autothermal staged reactor compared to conventional methods for applications in small-scale biomass utilization. The largest advantage of this reactor is the integration of highly exothermic autothermal stage and endothermic alcohol dehydration stage which provides an alternative processing technique to maintain the bed temperature.

High Carbon Ash from Stoker Fired Boiler of a Thermal Power Station as Support in the Reduction of Ketones: A Novel Application

Reduction of a few substituted cyclohexanones adsorbed on size classified high carbon ash fractions from a stoker fired boiler of a thermal power station using NaBH4 as the reducing agent with ethanol as the medium resulted in the formation of two epimeric alcohols. However, trans-4-t-butyl and trans-4-methylcyclohexanols are exclusively formed when the corresponding ketones adsorbed on one particular size classified ash fraction are reduced with NaBH4. In all other cases, a fairly high degree of stereoselectivity has been obtained. It has been observed that low carbon ash does not activate the ketones towards similar stereoselectivity, clearly establishing that the high carbon ash has a definite role to play. The stereochemical outcome of the reductions has been rationalized on the basis of a probable transition state involving Lewis/Br⊘nsted acid sites present in the ash. Mesopores and macropores present in the ash are most likely to act as pockets for adsorption of ketones. Product analysis has been carried out using 1HNMR and 13CNMR.

Si-OH-Al原子配列制御による高濃度ブレンステッド酸点の作製

Si-OH-Al原子配列制御による高濃度ブレンステッド酸点の作製

SDS-Catalyzed Esterification Process to Synthesize Ethyl Chloroacetate

Using sodium dodecyl sulfate (SDS) as the catalyst, ethyl chloroacetate has been synthesized by esterification of chloroacetic acid and ethanol. Various factors that affected the esterification have been investigated, such as reaction time, different water-carrying agents, and amount of catalyst. The esterification yield of ethyl chloroacetate reached 97.1% with 1.2/1 molar ratio of ethanol to chloroacetic acid, 1.0% catalyst (molar percentage of chloroacetic acid), 2.5 hr reaction time, 5 mL cyclohexane as water-carrying agent, and reflux temperature. The catalytic activity of SDS is almost equal with Br⊘nsted acids, such as sodium hydrosulfate or p-toluene sulfonic acid. Its catalytic activity is greater than that of most Lewis acids. SDS can easily hydrolyze into sodium hydrosulfate, which plays an essential role during the esterification process. The advantage of this catalytic process is that it is environmentally friendly, with high esterification yield, which may meet the standards of green chemistry.

Effect of Modified Zeolite on One-Step Process of DME Synthesis

HZSM-5 zeolites modified by MgO, CaO, and ZnO were prepared by impregnation and bifunctional catalysts were prepared by modified HZSM-5 zeolites and JC207 catalyst (a catalyst for methanol synthesis, which has been industrialized in China). Evaluation of catalytic activity was conducted in a fixed-bed reactor. Results show, with proper basic oxide-modified zeolite, a Br⊘nsted acid site (strong acid sites) shift to Lewis acid site (weak acid sites). Weak and less strong acid sites on HZSM-5 zeolite are activity centers for the formation of dimethyl ether (DME), whereas strong acid sites for by products such as hydrocarbons. Consequently, the selectivity of CO2 and hydrocarbons decrease, especially with zeolite modified by CaO.

Si-OH-Alブレンステッド酸点の高分散によるプロトン伝導性複合酸化物の作製

Si-OH-Alブレンステッド酸点の高分散によるプロトン伝導性複合酸化物の作製

Effects of Heat Treatment on the Structure and Properties of the Tungstophosphoric Acid-Modified Zirconia Aerogel Solid Acids

Based on detailed experimental results,the effects of heat treatment on the structure,acidity,and catalytic performance of the 12-tungstophosphoric acid(TPA) modified zirconia aerogel(ZA)(TPZA) solid acids were investigated.TPZA samples,with TPA mass fraction of 25% and heat-treated at 120-750 ℃,were characterized by N2 physical adsorption,X-ray diffraction(XRD),Fourier transform infrared(FTIR) spectroscopy,FTIR spectroscopy of adsorbed pyridine(Py-FTIR),and temperature programmed desorption of NH3(NH3-TPD).And they were used as catalysts in the polymerization of tetrahydrofuran(THF).The results show that,although the heat treatment might greatly influence both the surface density and type of tungsten species,the TPZA materials with the TPA loading of 25% could acquire suitable Brnsted and Lewis acidity and accordingly acceptable catalytic activities over the wide temperature ranges of 120-300 ℃ and 550-750 ℃.In the TPZA samples treated at 120-300 ℃,the presence of Brnsted acid sites can be associated with the presence of highly dispersed and somewhat distorted but intact Keggin-anions,whereas the Brnsted acid sites in the samples treated at 550-750 ℃ might originate from the zirconia-anchored,highly polymerized tungstate surface species.Moreover,all the samples show obvious Lewis acidity,which is associated with the presence of coordinatively unsaturated Zr4+ and phosphorous oxide produced by the decomposition of TPA.