Acid Base Cooperativity Research Articles

Dual-functionalized aramid fiber as an efficient heterogeneous acid-base bifunctional catalyst for the one-pot tandem reaction

Acid-base bifunctional catalysis represents a significant approach for achieving enzyme-like catalytic activity. In this study, we present a concise and direct approach for constructing acid-base bifunctional aramid fiber catalysts, enabling one-pot tandem deacetalization-Knoevenagel reactions in an aqueous environment. The fiber catalysts were conveniently prepared from commercially available aramid fiber using a straightforward two-step procedure, and comprehensively characterized using various detection techniques. Additionally, the acid-base cooperativity of the AF catalyst was effectively controlled through the manipulation of the acid-base ratios within the catalysts. Notably, the AF-SN1/2 catalyst exhibited superior catalytic activity in the one-pot tandem deprotection-Knoevenagel reaction at room temperature, resulting in the production of high yields. Moreover, the AF-SN1/2 catalyst demonstrated exceptional recyclability and performed admirably in a scaled-up experiment utilizing a straightforward fixed-bed reactor. These findings suggest that the AF-SN1/2 catalyst holds significant promise for future applications in cleaner industrial processes.

La2CuO4 perovskite enhanced ethylene carbonate synthesis via tuning the acid-base cooperation

La2CuO4 perovskite enhanced ethylene carbonate synthesis via tuning the acid-base cooperation

Hill-type pH probes.

Sensitive detection of the minute and yet pathologically significant pH variation is important and in fact challenging for the conventional pH probes following the Henderson-Hasselbalch equation, i.e., HH-type probes. A paradigm shift to Hill-type pH probes is ongoing. Bestowed by their positive cooperative acid-base chemistry, their pH-responsive profile follows the Hill equation, which exhibits a narrower acid/base transition width than HH-type probes and warrants a higher detection sensitivity. A polymer-based Hill-type pH-responsive material was first developed. More recently, there emerged several distinct small-molecular approaches to achieve Hill-type pH-responsive profiles. They complement the polymer-based sensing materials in applications where membrane permeability is a concern. In this trends article, we rationalize the molecular origins of their positive cooperativity in pH sensing and highlight some interesting proof-of-concept applications. We also discussed future directions of this dynamic research area.

Prolinate-based heterogeneous catalyst for Knoevenagel condensation reaction: Insights into mechanism reaction using solid-state electrochemical studies

The carbon-carbon bond formation is essential to producing relevant organic compounds that exhibit pharmacological and biological properties. In this regard, Knoevenagel and multicondensation reactions have been employed to test the acid-base cooperativity of functionalized mesoporous silica nanoparticles with L-prolinate based groups (Prol-MSN) in comparison with choline hydroxide fragments (Chol-MSN). Thus, Prol-MSN material has shown to be a promising heterogeneous organocatalyst for condensation reactions. The use of electrochemical sensor techniques allowed through cyclic and differential pulse voltammetry studies a better understanding of the catalytic mechanism of Knoevenagel reaction for the Prol-MSN material, including the role of silanol surface groups and protic solvents.

Gating of Substrate Access and Long-Range Proton Transfer in Escherichia coli Nitrate Reductase A: The Essential Role of a Remote Glutamate Residue

The Mo/W-bisPGD enzyme superfamily comprises a vast number of mononuclear molybdenum and tungsten enzymes that catalyze a great diversity of vital reactions in prokaryotes. In the past decades, much attention has been devoted to the immediate surroundings of the metal atom highlighting the importance of the inner coordination sphere but has failed to identify molecular determinants of the reactivity. Here, we report the mechanistic importance of a set of conserved residues that line the substrate entry tunnel in Escherichia coli nitrate reductase A (Nar), a paradigmatic enzyme of the Mo/W-bisPGD superfamily. Using mutagenesis, enzyme kinetics, electron paramagnetic resonance spectroscopy, and molecular dynamics, we unveil the pivotal role of Glu-581 motion and a number of polar residues in its close proximity in substrate affinity and proton transfer to the Mo active site. Motion of the side chain of Glu-581 exhibiting a strong acid–base cooperativity with Asp-801 and surrounded by several polar interactions controls the hydration inside the protein core, proton transfer, and substrate selectivity toward the active site. Overall, we identify an additional determinant that fine-tunes the reactivity and selectivity in Nar and propose that a gating mechanism is at play in several other members of the superfamily.

Synergetic Molecular Oxygen Activation and Catalytic Oxidation of Formaldehyde over Defective MIL-88B(Fe) Nanorods at Room Temperature.

Defective MIL-88B(Fe) nanorods are exploited as exemplary iron-bearing metal-organic framework (MOF) catalyst for molecular oxygen (O2) activation at ambient temperature, triggering effective catalytic oxidation of formaldehyde (HCHO), one of the major indoor air pollutants. Defective MIL-88B(Fe) nanorods, growing along the [001] direction, expose abundant coordinatively unsaturated Fe-sites (Fe-CUSs) along extended hexagonal channels with a diameter of ca. 5 Å, larger enough for the diffusion of O2 (3.46 Å) and HCHO (2.7 Å). The Lewis acid-base interaction between Fe-CUSs and accessible HCHO accelerates the FeIII/FeII cycle, catalyzing Fenton-like O2 activation to produce reactive oxidative species (ROSs), including superoxide radicals (•O2-), hydroxyl radicals (•OH), and singlet oxygen (1O2). Consequently, adsorbed HCHO can be oxidized into CO2 with a considerable mineralization efficiency (over 80%) and exceptional recyclability (4 runs, 48 h). Dioxymethylene (CH2OO), formate (HCOO-) species, and formyl radicals (•CHO) are recorded as the main reaction intermediates during HCHO oxidation. HCHO, H2O, and O2 are captured and activated by abundant FeIII/FeII-CUSs as acid/base and redox sites, triggering synergetic ROS generation and HCHO oxidation, involving cooperative acid-base and redox catalysis processes. This study will bring new insights into exploiting novel MOF catalysts for efficient O2 activation and reliable indoor air purification at ambient temperature.

Highly efficient and selective separation of ammonia by deep eutectic solvents through cooperative acid-base and strong hydrogen-bond interaction

Exploring absorbents showing low volatility, high efficiency and good reversibility for NH3 absorption is of great significance. In this work, a novel class of DESs with both weak acid site and strong hydrogen-bond donating site were designed by using imidazolium hydrogen chloride ([ImH]Cl) and glycerol (Gly) as the motifs. The densities, viscosities and thermal stabilities of DESs were measured first. Then, the solubility, reversibility and selectivity of DESs for NH3 absorption were investigated systematically. It is found that the absorption of NH3 in DESs is chemical type, and there is strong interaction existing between DESs and NH3. As a result, the NH3 solubilities of DESs are very attractive especially at low pressures, with the values of 12.934 mol/kg at 298.2 K and 101.3 kPa, and 4.450 mol/kg at 298.2 K and 10.0 kPa. The NH3 absorbed by DESs can be easily desorbed by heating and evacuating. Moreover, the absorption of N2 and H2 in DESs is negligible, and has little effect on the absorption of NH3. Finally, the underlying mechanism of NH3 absorption was examined by theoretical and spectroscopic technique.

The novel acid-base magnetic recyclable catalyst prepared through carbon disulfide trapping process: Applied for green, one-pot, and efficient synthesis of 2,3-dihydroquinazolin-4 (1H) -ones and bis(indolyl)methanes in large-scale

• The synthesis of magnetic acid–base cooperativity catalyst through novel, facile and efficient method. • Synthesis of substituted dihydroquinazolinones and bis(indolyl)methanes with excellent yields in mild and green reaction conditions. • The easy separated catalyst recycled and employed for other eight successive runs. • The important medicinal compounds were successfully performed using introduced catalytic system in large-scale. Herein, a nano acid-base catalyst using magnetic core and carbamodithioic acid functional group have been synthesized and characterized. Its efficiency in the synthesis of dihydroquinazolinones and bis(indolyl)methanes derivatives was investigated. This novel metal-free catalyst exhibited significant catalytic activity in both reactions under green and mild reaction conditions (the yield obtained for the first reaction products: 82–98 % and for the second one: 61–97 %). The catalyst displayed good recyclability with no significant loss of catalytic activity after eight runs (the conversion of the eighth run was found as 83 %, the fresh catalyst conversion was 95 %). The introduced approach is attractive due to its applicability in the large-scale synthesis of important medicinal compounds.

Tuning acid–base cooperativity to create bifunctional fiber catalysts for one-pot tandem reactions in water

A simple and straightforward route to create acid–base bifunctional fiber catalysts for efficient one-pot tandem deprotection-Knoevenagel reactions in water.

Surface wettability modification of amine-functionalized polyacrylonitrile fiber to enhance heterogeneous catalytic performance for aldol reaction in water

Heterogeneous catalytic aldol reaction has widely focused on tethering multiple functional groups upon support that can act together (for example, acid-base cooperation) to improve reaction rate beyond individual moiety alone. Here, a series of amine-functionalized fibers with additional surface wettability modification were prepared to promote aldol reaction in water. The result showed that an enhanced catalytic activity was achieved by introducing hydroxyl group to fiber. Hydrophilic prolinamide-functionalized fiber (PANEF-PLA) exhibited the best catalytic performance, and its activity proved to be higher than those of hydrophobic-modified fiber and even the small molecule prolinamide. This enhancement is attributed to that the favorable hydrophilic microenvironment consisting of hydroxyl group, residual cyano group and prolinamide facilitates the accessibility of reactants to catalytic sites and increases the rate of mass transfer in water for promoting reaction. This work provides an effective protocol to maximize performance of fiber-supported catalyst by appropriate wettability modification.

Acrylic Acid Production by Gas-Phase Dehydration of Lactic Acid over K+-Exchanged ZSM-5: Reaction Variable Effects, Kinetics, and New Evidence for Cooperative Acid–Base Bifunctional Catalysis

This study investigates the effects of reaction variables (including space velocity, temperature, and partial pressures of LA and H2O) and acidic and basic additives on the gas-phase dehydration of lactic acid (LA) for acrylic acid (AA) production, using an aluminum-rich K+-exchanged ZSM-5 zeolite (molar SiO2/Al2O3 = 27, K/Al = 1.0, K/Na = 8) identified among many high-silica zeolites as the most efficient catalyst in our prior work (Yan et al. ACS Catal. 2017, 7, 538−550). Under widely varied reaction conditions (340–380 °C, feed GHSV: 9,500–38,000 h–1, 1.8–11.7 kPa LA, 52.0–73.0 kPa H2O), the catalyst offers not only high selectivity for AA (>75 mol %) but also long-term catalytic stability. Rate equations and their associated kinetic parameters for LA consumption and AA production are obtained by measuring the kinetically relevant catalytic rates under high feed space velocity (GHSV ≥ 57,000 h–1). Acidic (CO2, acetic acid, and trifluoroacetic acid) and basic (NH3) additives are added, respectively, to the reaction feed to investigate their effects on performance of the working catalyst. While addition of either CO2 or acetic acid shows no effect on the catalyst performance, the addition of either NH3 or trifluoroacetic acid causes significant decline in both the catalyst activity and selectivity for AA production, due to poisoning of the surface acidic sites by NH3 or basic sites by trifluoroacetic acid. These data well highlight the excellent catalytic performance of the investigated catalyst and offer a new piece of evidence in support of the cooperative acid–base bifunctional catalysis for the LTA reaction.

Functionalized cellulose nanofibril aerogels as cooperative acid–base organocatalysts for liquid flow reactions

Cellulose nanomaterial aerogels are macroscopic porous solids with relatively high surface areas and are thus an interesting basis for renewable catalyst materials. Cross-linked acid–base bifunctional catalyst aerogels are produced here from TEMPO-oxidized cellulose nanofibrils (TOCNF) and demonstrated in both batch and flow catalysis. Recently established acid–base modification for catalysis is expanded upon for chemical or physical cross-linking with small molecules and polymers. Low density and relatively high surface area (up to 74 m2 g−1) aerogel catalysts are produced with a variety of processing approaches and then freeze-dried from water or tert-butyl alcohol/water mixtures. Finer pore structure and increased surface area are achieved with tert-butyl alcohol as co-solvent. Chemical cross-linking improved aerogel stability to solvents. Homogeneous and aerogel TOCNF catalysts are shown to be effective acid–base cooperative catalysts for aldol condensation reactions in batch reactions. Continuous flow reactions are performed with glass column reactors packed with aerogel catalysts that showed improved rates relative to batch experiments, while also demonstrating physical stability. Catalyst deactivation in flow reactions is observed and observations of deactivation support previously reported mechanisms of site poisoning by competitive chemisorption of reactants in analogous acid–base catalysts. This report is a key demonstration of cellulose nanofibril aerogels for catalysis in continuous liquid flow reactions.

Cooperative acid–base bifunctional ordered porous solids in sequential multi-step reactions: MOF vs. mesoporous silica

Two different catalytic platforms, MOF and mesoporous silica, were compared as porous support for basic amino groups to promote sequential multi-step reactions.

A computational mechanistic insight into H2 activation and CO2 reduction over β-Diketiminato-ligated group 13 metal complexes

The mechanisms of the cascade H2 activation and CO2 reduction over the β-Diketiminato (BDI)-ligated group 13 metal complexes were studied by the density functional theory (DFT) in the framework of ONIOM method. For the initial H2 activation, the intermolecular 1,4-addition pathway is demonstrated to be kinetically more favorable than the intramolecular 1,2 or 1,4-addition pathway. With the intermolecular Lewis acid-base cooperation catalysis, the heterolytic H⋯H bond cleavage adopts an electron transfer model, which involves the concerted donation from the HH σ bonding orbital to the empty 4pz orbital of the GaIII center, and the backdonation from the π-orbital of the exocyclic alkene in the BDI ligand to the HH σ anti-bonding orbital. Furthermore, it is interesting to note that the simultaneous introduction of the electron-withdrawing group (CF3) on the GaIII center and the electron-donating group (OMe or NMe2) on the C6 position of the BDI ligand is effective for enhancing the reactivity of the BDI-GaIII complex in H2 activation. CO2 might be reduced barrierlessly with the more reactive gallium hydride anion that is generated in H2 activation. For both H2 activation and CO2 reduction processes, the BDI-ligated gallium complexes exert the better catalytic effect than the aluminum analogues.

Optimized Cellulose Nanocrystal Organocatalysts Outperform Silica-Supported Analogues: Cooperativity, Selectivity, and Bifunctionality in Acid–Base Aldol Condensation Reactions

Cellulose nanocrystals (CNCs) are demonstrated as effective, ordered supports for cooperative acid–base heterogeneous organocatalysis, offering an alternative to typical silica supports. CNC cataly...

Amine functionalization of cellulose nanocrystals for acid–base organocatalysis: surface chemistry, cross-linking, and solvent effects

Sulfuric acid-derived cellulose nanocrystals (CNCs) are investigated as an alternative support for bifunctional heterogeneous organocatalyst materials. CNC catalysts are developed through direct surface functionalization by TEMPO-mediated oxidation and subsequent EDC amide coupling with alkyl diamines, used to impart both acid and base character to CNC surfaces. Sulfate ester content is controlled by partial acid hydrolysis prior to chemical functionalization, optionally reducing the initial sulfur content by approximately 50%. The resultant acidic and basic catalytic moieties are analyzed qualitatively by FT-IR spectroscopy and quantitatively by conductometric titration and elemental analysis. Relatively high degrees of substitution with amine functionality are achieved with diamine loadings around 0.9 mmol/g. This synthesis partially cross-links the CNCs, the extent of which is analyzed by dynamic light scattering of materials functionalized with 2–6 carbon spacer alkyl diamines. These CNC materials are demonstrated to be effective acid–base catalysts for an aldol condensation test reaction in batch liquid phase kinetic experiments. Relatively slow reaction rates were achieved in water:acetone reaction mixtures, in which decreased sulfate ester content is shown to decrease catalytic activity. Significant rate improvements are achieved by elimination of rate limiting transport and miscibility issues by solvent exchange to organic-only acetonitrile:acetone co-solvent reaction mixtures. Cross-linking and limited acid–base cooperativity are shown to decrease activity in CNCs functionalized with too long and too short diamines, respectively. This study demonstrates CNCs as alternative supports for bifunctional heterogeneous organocatalysis with site-specific activity on the order of that of well-studied aminosilica organocatalysts.

Acid-base sites synergistic catalysis over Mg-Zr-Al mixed metal oxide toward synthesis of diethyl carbonate.

In heterogeneous catalysis processes, development of high-performance acid–base sites synergistic catalysis has drawn increasing attention. In this work, we prepared Mg/Zr/Al mixed metal oxides (denoted as Mg2ZrxAl1−x–MMO) derived from Mg–Zr–Al layered double hydroxides (LDHs) precursors. Their catalytic performance toward the synthesis of diethyl carbonate (DEC) from urea and ethanol was studied in detail, and the highest catalytic activity was obtained over the Mg2Zr0.53Al0.47MMO catalyst (DEC yield: 37.6%). By establishing correlation between the catalytic performance and Lewis acid–base sites measured by NH3-TPD and CO2-TPD, it is found that both weak acid site and medium strength base site contribute to the overall yield of DEC, which demonstrates an acid–base synergistic catalysis in this reaction. In addition, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements reveal that the Lewis base site activates ethanol to give ethoxide species; while Lewis acid site facilitates the activated adsorption of urea and the intermediate ethyl carbamate (EC). Therefore, this work provides an effective method for the preparation of tunable acid–base catalysts based on LDHs precursor approach, which can be potentially used in cooperative acid–base catalysis reaction.

Mechanism of Propylene Oxide Polymerization Promoted by N-Heterocyclic Olefins

We report a mechanistic DFT investigation of the organopolymerization of propylene oxide (PO) promoted by N-heterocyclic olefins (NHOs) in combination with benzylic alcohol (BnOH). Calculations support the experimentally based hypothesis of two competing pathways, namely, the anionic and zwitterionic pathways. The former is based on an acid–base cooperativity between BnOH and the NHO, promoting ring opening of PO by BnO–. The latter occurs through the formation of a zwitterionic adduct by nucleophilic attack of the exocyclic carbon atom of the NHO on the PO, with the concerted ring opening of PO. The two initiating species cannot interconvert, and chain elongation can proceed from both initiation adducts. Potential energy surfaces were computed for a set of NHOs to clarify the effects of the steric and electronic properties of the NHO on the system reactivity. The results achieved represent useful insight toward the synthesis of PPO with better properties with respect to the polymer obtained with the expe...

An Efficient Heterobimetallic Lanthanide Alkoxide Catalyst for Transamidation of Amides under Solvent‐Free Conditions

AbstractA practical heterobimetallic lanthanide‐catalyzed transamidation of primary, secondary and tertiary amides with aliphatic and aromatic amines has been developed. The methodology was also applied to the weakly reactive thioamides to demonstrate its versatility and wide substrate scope. The heterobimetallic lanthanide catalysts showed high catalytic activity and a wide scope of substrates with good to excellent yields under solvent‐free conditions. Efficient activation of the transamidation can be realized by the above complexes acting as cooperative acid–base bifunctional catalysts, which are proposed to be responsible for the higher reactivity in comparison with simple monometallic catalysts.magnified image

A little frustration goes a long way

Chemists have long known that catalytic Lewis acidic and Lewis basic sites on metal and metal oxide surfaces help spark a range of reactions that are important in industrial chemical and fuel production. Yet researchers still have no general theory available to predict all the mechanistic nuances of how these electron-deficient and electron-rich sites lead to that reactivity. As a case in point, a research team led by nanochemistry specialist Geoffrey A. Ozin and computational materials engineer Chandra Veer Singh of the University of Toronto has recently uncovered a previously overlooked phenomenon involving cooperative acid-base reactivity on the surface of the solid photocatalyst indium oxide. In a set of structural and kinetic studies, the researchers show that closely situated Lewis acidic and basic sites on the solid catalyst act like solution-based molecular systems known as frustrated Lewis pairs (FLPs). Ozin, Singh, and their colleagues say this heterogeneous surface analog of