Enhanced Lewis Acidity Research Articles

Efficient CeVO4/SnO2 and CeVO4/sulfated-SnO2 catalysts with high SO2 resistance for selective catalytic reduction of NOx with NH3: Influence of acid sites

Developing eco-friendly catalysts with high NOx removal efficiency and SO2 resistance in a wide temperature range is crucial. A series of efficient CeVO4/sulfated-SnO2 (CeV/Sn-S) and CeVO4/SnO2 (CeV/SnO2) catalysts were developed, and the optimal mass fraction of CeVO4 was 9 wt%. 9 %CeV/SnO2 and 9 %CeV/Sn-S achieved over 90.0 % NOx conversion at 220–385 and 240–430 °C, respectively, and they both showed notable SO2 resistance. CeV, 9 %CeV/SnO2 and 9 %CeV/Sn-S displayed different acid sites and SO2 adsorption capabilities. CeV exhibited low surface acidity, primarily Brønsted acidity. The introduction of SnO2 enhanced Lewis acidity, while sulfated SnO2 addition increased both Lewis and Brønsted acidity, and reduced SO2 adsorption, which benefited to the SO2 resistance. 9 %CeV/Sn-S had more lattice defects, a lager surface area, and higher Ce4+ and V5+ ratios than 9 %CeV/SnO2, indicating a stronger interaction between CeVO4 and sulfated SnO2. Then, 9 %CeV/Sn-S catalyst’s strong NH3 adsorption and moderate redox properties contributed to its higher NOx conversions and N2 selectivity above 330 °C, along with a broader operational window. Both Eley–Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms were observed in these catalysts. However, the presence of sulfates on 9 %CeV/Sn-S limited NOx adsorption, thereby reducing the contribution of L-H mechanism. The 9 %CeV/SnO2 catalyst’s strong Lewis acidity and abundant adsorbed NOx species favored the L-H mechanism, resulting in its higher NOx conversion below 250 °C compared to 9 %CeV/Sn-S. This work contributes to sharpening comprehension for the influence of acid sites and steering the design of novel SCR catalysts for efficient NOx abatement in practical applications.

Synthesis of quadruply boron-doped acenes with stimuli-responsive multicolor emission

Boron-doped acenes have attracted attention due to their unique structures and intriguing luminescent properties. However, the hitherto known boron-doped acenes have only one or two boron atoms, limiting the chemical space of this unique family of compounds and the capability to tune their optical properties. Herein, we report the synthesis of quadruply boron-doped acenes, including pentacene, heptacene, and nonacene. The importance of the boron doping level on the luminescent properties of acenes is demonstrated. The title compounds manifest enhanced Lewis acidity as compared with dihydrodiboraacenes, leading to Lewis-base-responsive emission in the solid state. Moreover, quadruply boron-doped nonacene displays mechanochromic luminescence in addition to Lewis-base-responsive properties, realizing high-contrast solid-state multicolor emission. This work greatly expands the chemistry of boron-doped acenes and offers opportunities for developing boron-based luminescent materials.

Vanadium-Modified TS-1 Catalysts with Enhanced Lewis Acid and Charge Density for Efficient Hydroxylation of Benzene to Phenol

Vanadium-Modified TS-1 Catalysts with Enhanced Lewis Acid and Charge Density for Efficient Hydroxylation of Benzene to Phenol

Surficial engineering of active hydroxyls for ambient formaldehyde oxidation via enhanced Lewis acidity over Zr-doped cryptomelane materials

Lewis acids of solid catalysts have been featured for a pivotal role in promoting various reactions. Regarding the oxidation protocol to remove formaldehyde, the inherent drawback of the best-studied MnO2 materials in acidic sites has eventually caused deficiency of active hydroxyls to sustain low-temperature activity. Herein, the cryptomelane-type MnO2 was targeted and it was tuned via incorporation of Zr metal, exhibiting great advances in not only the complete HCHO-to-CO2 degradation but also cycling performance. Zr species were existent in doping state in the MnO2 lattice, rendering lower crystallinity and breaking the regular growth of MnO2 crystallites, which thereby tripled surface area and created larger volume of smaller mesopores. Meantime, the local electronic properties of Mn atoms were also changed by Zr doping, i.e., more low-valence Mn species were formed due to the electron transfer from Zr to Mn. The results of infrared studies demonstrate the higher possession of Lewis acid sites on ZrMn, and this high degree of electrophilic agents favored the production of hydroxyl species. Furthermore, the reactivity of surface hydroxyls, as investigated by CO temperature programmed reduction and temperature programmed desorption of adsorbed O2, was obviously improved as well after Zr modification. It is speculated jointly with the characterizations of the post-reaction catalysts that the accelerated production of active hydroxyls helped rapidly convert formaldehyde into key intermediate—formate, which was then degraded into CO2, avoiding the side reaction path with undesired intermediate—hydrocarbonate—over the pristine MnO2, where active sites were blocked and formaldehyde oxidation was inhibited. Additionally, Zr decoration could stabilize Lewis acidity to be more resistant to heat degeneration, and this merit brought about advantageous thermal recyclability for cycled application.

Beta Zeolite-Supported Double-Metal Cyanide Catalysts with Enhanced Lewis Acidity for the Synthesis of High-Performance Poly(1,2-butylene oxide) Lubricating Oils

Beta Zeolite-Supported Double-Metal Cyanide Catalysts with Enhanced Lewis Acidity for the Synthesis of High-Performance Poly(1,2-butylene oxide) Lubricating Oils

Biomimetic single Al-OH site with high acetylcholinesterase-like activity and self-defense ability for neuroprotection

Neurotoxicity of organophosphate compounds (OPs) can catastrophically cause nervous system injury by inhibiting acetylcholinesterase (AChE) expression. Although artificial systems have been developed for indirect neuroprotection, they are limited to dissociating P-O bonds for eliminating OPs. However, these systems have failed to overcome the deactivation of AChE. Herein, we report our finding that Al3+ is engineered onto the nodes of metal–organic framework to synthesize MOF-808-Al with enhanced Lewis acidity. The resultant MOF-808-Al efficiently mimics the catalytic behavior of AChE and has a self-defense ability to break the activity inhibition by OPs. Mechanism investigations elucidate that Al3+ Lewis acid sites with a strong polarization effect unite the highly electronegative –OH groups to form the enzyme-like catalytic center, resulting in superior substrate activation and nucleophilic attack ability with a 2.7-fold activity improvement. The multifunctional MOF-808-Al, which has satisfactory biosafety, is efficient in reducing neurotoxic effects and preventing neuronal tissue damage.

Simultaneous Mesoporosity Generation and Lewis Acidity Enhancement in Zr‐Based Metallosilicalite for Promoting the Conversion of Ethanol‐Acetaldehyde to 1,3‐Butadiene

AbstractZr based metallosilicalites, especially Zr‐ß, is promising catalyst for the conversion of ethanol to 1,3‐butadiene, which is considered to be a sustainable alternative to petroleum steam cracking. However it is suffering from deactivation derived from coking and unsatisfied catalytic activity derived from deficient Lewis acidity. For these issues, a dissolution‐recrystallization process through tetraethyl ammonium hydroxide treatment (TEAOH) for enhancing porosity and Lewis acidity of Zr‐ß zeolite was developed in this study. A balance of dissolution and recrystallization existed in this process, which was produced by OH− etching and templating of TEA+ ions, creating additional mesoporosity. Zirconium active sites maintained tetrahedral coordination, while the Lewis acidity was enhanced by creating higher proportion open sites in framework. The recrystallized Zr‐ß exhibited higher catalytic activity and stability in the conversion of ethanol‐acetaldehyde to butadiene due to the compromise of microporosity and mesoporosity, as well as the appropriate enhancement of Lewis acidity.

Electrochemical and Spectroscopic Properties of Europium in AlCl3-NaCl Melts

The physicochemical properties of Eu cations in AlCl3-NaCl melts have been studied for the purpose of spent nuclear fuel recycling via electrochemical and spectroscopic methods. Eu metal dissolves in the AlCl3-NaCl melts to generate divalent europium and Al metal while releasing chlorine gas. The effect of Lewis acidity of melt on the electrochemical behaviors of Eu(II) has been studied. In Lewis base melt of AlCl3-NaCl, Eu(III), and Al(III) can be co-reduced at the potential between Al deposition and Eu(II) oxidation. With the enhanced Lewis acidity of the molten salt, Eu(II) diffusion becomes more facile. The in situ absorption spectra of Eu(II) in AlCl3-NaCl melts reveal that the complexation ligand of Eu(II) changes with the melt Lewis acidity, thereby affecting the electrochemical behaviors of Eu(II) species. In addition, the redox potential of the Eu(II)/Eu(III) conversion reaction is more positive than that of Sm(II)/Sm(III).

Acid-base bifunctional Fe-NC catalyst with Fe-N4 and Fe nanoparticles active sites derived from Fe-doped ZIF-8 boosted microalgal lipid conversion

To enhance catalytic acid-base sites in ZIFs and efficiently promote conversion of microalgal lipids, a bifunctional catalyst with single-atom Fe-N4 sites and Fe nanoparticles (NPs) was synthesized by pyrolyzing Fe-doped ZIF-8. Compared with ZIF precursors, Fe nanoparticles and coordinated Fe ions in Fe-N4 sites enhanced Lewis acidity, while increased uncoordinated N atoms enhanced Lewis basicity. Therefore, the acidity of Fe-NC increased from 0.473 to 1.845 mmol/g, while basicity increased from 0.172 to 0.439 mmol/g. DFT calculations showed that the adsorption energies of acetic acid on the Fe–N4 and Fe NP were −0.536 eV and −0.397 eV, respectively, exceeding that (-0.190 eV) on the Zn–N4 site. The energy barriers for rate-determining steps over the Fe-N4 and Fe NP were + 0.32 eV and + 0.21 eV, respectively, which were lower than that (+1.01 eV) over the Zn-N4 site. The maximum biodiesel yield reached 98.3% over Fe-NC catalyst, higher than that (84.5%) over ZIF-8 catalyst.

Polymeric catalyst with polymerization-enhanced Lewis acidity for CO2-based copolymers

Ring-opening copolymerization of CO2 and epoxides is a promising way to manufacture high value-added materials. Despite a variety of catalyst systems have been reported, the reaction is still limited by low activity and polymer selectivity. Herein, a strategy of polymerization-enhanced Lewis acidity is reported to construct a series of highly efficient polymeric aluminum porphyrin catalysts (PAPCs). The characterization of the coordination equilibrium constant (Keq) showed significantly enhanced Lewis acidity of PAPC (Keq = 18.2 L/mol) compared to the monomeric counterpart (Keq = 6.4 L/mol), accompanied with increased turnover frequency (TOF) from 136 h−1 to 5500 h−1. Through detailed regulation of Lewis acidity, the highly Lewis acidic PAPC-OTs displayed a record high TOF of 30,200 h−1 with polymer selectivity of up to 99%.

Zeolitic imidazolate framework (ZIF-8)-derived acid-base bifunctional single-atom catalysts with Zn-Nx coordination for microalgal lipids conversion

To strengthen active sites in zeolitic imidazolate frameworks (ZIFs) to effectively convert microalgal lipids into biodiesel, acid-base bifunctional single-atom catalysts with Zn-Nx coordination were synthesized through pyrolyzing ZIF-8. The saturated coordinated Zn-N4 sites were destroyed and converted to unsaturated coordinated Zn sites and surrounding N atoms at 700 °C, which enhanced Lewis acidity and basicity, respectively. The Zn-N content in the catalyst decreased from 77.68 % to 43.26 %, while that of pyridinic N increased from 8.14 % to 36.73 %. The acidity of the catalyst increased from 0.498 to 3.979 mmol/g, whereas basicity increased from 0.219 to 0.650 mmol/g. According to density functional theory calculations, unsaturated coordinated Zn-Nx (x = 1, 2, 3) sites exhibited lower free energies and reaction energy barriers in free fatty acid esterification. The energy barriers for rate-determining steps over Zn-N4, Zn-N3, Zn-N2, and Zn-N1 sites were + 1.01 eV, +0.82 eV, +0.63 eV, and + 0.54 eV, respectively. Therefore, the optimum catalyst exhibited a high conversion efficiency of 97.9 %, which was better than that of ZIF-8 (85.4 %).

Synergetic metal-Nx and metal nanoparticles derived from Zn/Co-zeolitic imidazolate frameworks boosted conversion of microalgal lipids

In order to enhance acid-base active sites and their utilization efficiency to efficiently convert microalgal lipids into biodiesel, an acid-base catalyst was synthesized through pyrolyzing ZnCo bimetallic zeolitic imidazolate frameworks (ZIFs). Pyrolysis destroyed saturated coordinated metal-N4 sites in ZIFs and produced more unsaturated metal-Nx and metal nanoparticles, which enhanced Lewis acidity. The increased pyridinic N and unsaturated metal-Nx promoted Lewis basicity. Meanwhile, Zn doping reduced sizes of catalyst crystals and metal nanoparticles, which improved utilization efficiency of active sites. Hence, compared with pyrolyzed ZIF-67, the acidity of optimal catalyst increased from 0.205 to 1.535 mmol/g, whereas basicity increased from 0.069 to 0.405 mmol/g. DFT calculations revealed that catalytic performances of metal nanoparticles and unsaturated metal-Nx were better than that of saturated metal-N4. Therefore, the catalytic efficiency of C-Zn/Co-ZIF = 4 (molar ratio of Zn to Co was 4) catalyst was highest at 96.2%, because of the minimal catalyst crystals and metal nanoparticles.

Evolution of Lewis acidity by mechanochemical and fluorination treatment of silicon carbide as novel catalyst for dehydrofluorination reactions

Transforming potent greenhouse gasses, hydrofluorocarbons (HFCs) to hydrofluoroolefins (HFOs) is an environmentally friendly and low energy input route compared with traditional incineration method. During the conversion process, Lewis acidity of the catalysts play a vital role in the activity. Here, inspired by the fluorinated metal oxide, a novel nonmetallic catalyst SiC was fabricated via sequentially mechanochemical treatment and fluorination. With series characterizations, it demonstrated that on the super surface of SiC, there are thin layers of graphite, COOH species, SiO2 and Si4C4-xO2 substrates. Through the mechanochemical treatment, the layers of graphite, COOH species and partial SiO2 could be grinded off. Hence, the exposed SiO2 residue could be removed via the fluorination by HFC-152a and HF forming volatile SiF4. At the same time, the transition layer, Si4C4-xO2 was fluorinated to Si4C4-xF2 accompanied with enhanced Lewis acidity. As a result, the activity of the treated SiC was significantly superior than the unmilled SiC. These findings provide a feasible way to develop the catalytic performances of nonmetallic catalysts for the dehydrofluorination.

The regulation of electron distribution on Fe Lewis acidic sites within silicon skeleton and its contribution to Ketoprofen ozonation

Lewis acid site was vital center for activating O3 during catalytic ozonation process. Herein, we proposed a strategy to enhance the Lewis acid strength of Fe-MCM-48 (FeM) by fluorination. Fluorinated Fe-MCM-48 (FFeM) exhibited the better catalytic ozonation efficiency towards Ketoprofen (KET), a kind of nonsteroidal anti-inflammatory drug. 87.4 % TOC removal was obtained by F2.5Fe0.8M/O3 process, but only 27.5 % and 52.7 % were found by sole ozonation and Fe0.8M/O3 processes, respectively. Comparative characterizations, experiments and DFT calculations showed that fluorinated FFeM was regular sphere with three dimensional duct channel and large SBET. Doping of F reduced particle size, implanted electron withdrawing Si-F group and formed the electron poor center around Fe atom, which enhanced Lewis acid strength of FFeM. Protonated hydroxyl groups on Lewis acid sites were the main active sites for O3 activation. Greater amount of O3 were absorbed on the surface of FFeM and the greater O3 utilization rate, •OH and •O2– generation rates were obtained by FFeM/O3 process. Common anions such as Cl−, SO42− and NO3– and Humic acids (HA) had little effect on KET removal but inhibited TOC removal in FFeM/O3 process. •OH, •O2– and Fe(IV) = O oxidation were accounted for KET degradation. 16 kinds of intermediates generated from KET degradation and 4 pathways were deduced.

Atom-economic access to cationic magnesium complexes.

Cationic alkaline-earth complexes attract interest for their enhanced Lewis acidity and reactivity compared with their neutral counterparts. Synthetic protocols to these complexes generally utilize expensive specialized reagents in reactions generating multiple by-products. We have studied a simple ligand transfer approach to these complexes using (NacNac)MgR and ER3 (NacNac = β-diketiminate anion; E = group 13 element; R = aryl/amido anion) which demonstrates high atom economy, opening up the ability to target these species in a more sustainable manner. The success of this methodology is dependent on the identity of the group 13 element with the heavier elements facilitating faster ligand exchange. Furthermore, while this reaction is successful with aromatic ligands such as phenyl and pyrrolyl, the secondary amide piperidide (pip) fails to transfer, which we attribute to the stronger 3-centre-4-electron dimerization interaction of Al2(pip)6.

Experimental and Computational Studies of Ruthenium Complexes Bearing Z-Acceptor Aluminum-Based Phosphine Pincer Ligands.

Reaction of [Ru(C6H4PPh2)2(Ph2PC6H4AlMe(THF))H] with CO results in clean conversion to the Ru-Al heterobimetallic complex [Ru(AlMePhos)(CO)3] (1), where AlMePhos is the novel P-Al(Me)-P pincer ligand (o-Ph2PC6H4)2AlMe. Under photolytic conditions, 1 reacts with H2 to give [Ru(AlMePhos)(CO)2(μ-H)H] (2) that is characterized by multinuclear NMR and IR spectroscopies. DFT calculations indicate that 2 features one terminal and one bridging hydride that are respectively anti and syn to the AlMe group. Calculations also define a mechanism for H2 addition to 1 and predict facile hydride exchange in 2 that is also observed experimentally. Reaction of 1 with B(C6F5)3 results in Me abstraction to form the ion pair [Ru(AlPhos)(CO)3][MeB(C6F5)3] (4) featuring a cationic [(o-Ph2PC6H4)2Al]+ ligand, [AlPhos]+. The Ru-Al distance in 4 (2.5334(16) Å) is significantly shorter than that in 1 (2.6578(6) Å), consistent with an enhanced Lewis acidity of the [AlPhos]+ ligand. This is corroborated by a blue shift in both the observed and computed νCO stretching frequencies upon Me abstraction. Electronic structure analyses (QTAIM and EDA-ETS) comparing 1, 4, and the previously reported [Ru(ZnPhos)(CO)3] analogue (ZnPhos = (o-Ph2PC6H4)2Zn) indicate that the Lewis acidity of these pincer ligands increases along the series ZnPhos < AlMePhos < [AlPhos]+.

Synchronously constructing the optimal redox-acidity of sulfate and RuOx Co-modified CeO2 for catalytic combustion of chlorinated VOCs

CeO2-based catalysts are highly active but easily deactivated for the catalytic combustion of chlorinated VOCs (CVOCs), and enhanced redox and acidity are also considered to be effective strategies. However, the screening of practicable modifiers and their preparation method to achieve the promotion and balancing of redox-acidity is still desired. In this work, sulfate and RuOx were simultaneously introduced into CeO2 by a simple co-precipitation method, and the optimal redox-acidity was constructed. The catalytic combustion of 1,2-dichloroethane (DCE) indicated that 2.5Ru-CeO2-S0.05 presented an outstanding oxidation activity and selectivity of chlorinated byproducts and superior cycle and long-term stability (at least 60 h at 280 °C), even in the presence of 5 % H2O. A series of control experiments, such as the introduction methods, the precursors and contents of Ru and sulfate, and their property characterization revealed that the bulk doping of Ru mainly contributed to the promoted activity and durability through more exposed Ce3+/4+ and oxygen vacancies and the rapid removal ability of the inorganic chlorine species via a semi-Deacon reaction pathway, as well as to the formation of chlorinated byproducts. Meanwhile, the synchronous introduction of sulfate as Brønsted acid sites efficiently inhibited these chlorinated byproducts, while its retained redox ability and the enhanced Lewis acid sites owing to the strong electron withdrawing of the sulfate also further promoted DCE oxidation. Moreover, 2.5Ru-CeO2-S0.05 showed a high activity and excellent durability for the catalytic oxidation of ethylene, vinyl chloride and their mixtures with 1,2-DCE. This work provided a demonstration for balancing the oxidation performance and acidity of CeO2-based catalysts, which contributed to the development of applicable catalysts for catalytic oxidation of CVOCs under actual working conditions.

Triarylborane‐Functionalized Au8Ag8(CCR)16 Nanocluster with Enhanced Lewis Acidity

AbstractFunctionalization of ligated metal nanoclusters with versatile ligands is highly desired for wide applications. Lewis acids have shown promise in multiple realms such as self‐assembly and catalysis, which may endow nanoclusters with diverse functions. However, metal nanoclusters with Lewis acid surface have not been reported. Here, the functionalization of a bimetallic nanocluster Au8Ag8(CCR)16 with triarylborane (TAB) is reported, which exhibits enhanced Lewis acidity. By using single‐crystal X‐ray crystallography, the structure of the nanocluster with 16 boron receptor sites is confirmed, which presents ≈15‐fold enhancement of quenching efficiency in sensing fluoride compared to that of free triarylborane ligand. By the combined analysis using ultrafast transient absorption and photoluminescent (PL) decay measurements, the quenching mechanism of Au8Ag8 to fluoride is elucidated. The PL quenching is realized via two pathways with enhanced non‐radiation energy dissipation, i.e., prolonged electron–phonon coupling and reverse intersystem crossing (RISC). This work may open a new route to diverse applications of metal nanoclusters.

The role and performance of isolated zirconia sites on mesoporous silica for aldol condensation of furfural with acetone

Furfural is an important bio-based platform chemical in the production of valuable products such as biofuel intermediates and polymeric materials. In this work, isolated Zr on mesoporous silica catalysts at varied Zr loading were prepared and evaluated for aldol condensation of furfural with acetone to 4-(2-furyl)− 3-buten-2-one(FAc) and 1,5-bis-(2-furanyl)− 1,4-pentadien-3-one(F2Ac). A remarkable effect of the Zr loading on the catalyst performance is revealed. The catalysts are characterized by XRD, BET, NH3-TPD, Py-FTIR, XPS, UV–visible, and TGA. The connectivity of Zr was characterized by XPS and UV–visible spectroscopy. For the 25 wt% Zr loading, Zr is present predominantly as isolated monomeric species on mesoporous SiO2. The types and densities of different acid sites of the catalysts are measured with Py-FTIR analysis. The results show that isolated Zr on mesoporous SiO2 enhanced Lewis acid site densities and exhibited superior catalytic activity for aldol condensation between furfural and acetone to produce FAc and F2Ac.

Special direct route for efficient transfer hydrogenation of nitroarenes at room temperature by monatomic Zr tuned α-Fe2O3

Determining the route of a heterogeneous catalytic reaction is an important step in studying the reaction mechanism as well as improving reaction activity. As for the widely reported transfer hydrogenation of nitroarenes to aniline, many reported works have made simple judgements about the reaction route based on the “direct route” and “condensation route” proposed by Haber in 1898. Herein, detailed mechanistic experiments and DFT calculations helped us identify a special direct reduction route that goes through Ph-NO2 → Ph-NHOH → Ph-NH2 without the widely recognized Ph-NO intermediate for the transfer hydrogenation of nitroarenes to anilines by monatomic Zr tuned α-Fe2O3. In this catalytic system, Zr0.025Fe0.975Oy-350 can achieve complete transfer hydrogenation of nitroarenes at room temperature, where the parent α-Fe2O3 is inactive. Accompanying spectroscopic characterizations and calculations illustrate that the incorporation of Zr into the crystal lattice of α-Fe2O3 can effectively reduce the electron cloud density of surface iron species. The resulting special Fe(3+α)+-O(2+β)--Zr(4-γ)+ structure has greatly enhanced Lewis acidity, that promotes the decomposition of N2H4·H2O to form active [H] for the reduction of nitroarenes. Moreover, the structure is also beneficial for the adsorption of nitroarene with weakening of the NO bond. The present work can be extended to the application of non-noble metal oxides in the transfer hydrogenation of nitrobenzene under mild condition, and provides a significant method for exploring the reaction process of a heterogeneous catalytic reaction.