Lewis Acid Centers Research Articles

Design and synthesis of ZnGlu MOF/SBA-16 nanocomposite, and its performance as an environmentally friendly nanocomposite for solvent-free chemical fixation of CO2 to epoxides for high-yield synthesis of cyclic carbonates

In this study, ZnGlu MOF has been immobilized into hexagonal mesoporous silica SBA-16 to fabricate ZnGlu MOF/SBA-16 hybrid material. The prepared hybrid material was introduced as a sustainable green nanocomposite for CO2 epoxidation to cyclic carbonates under solvent-free conditions at low pressure of CO2 (5 bar). The structure of the obtained nanocomposites is authenticated using various analytical techniques like FT-IR, XRD, SAXRD, CO2-TPD, NH3-TPD, BET, TEM, FE-SEM, EDX, EDX-mapping, and ICP-OES. For making a greener process, ZnGlu MOF/SBA-16 nanocomposite showed remarkable catalytic activity for converting CO2 with diverse epoxides toward five-membered cyclic carbonates prepared without using any toxic metals and reagents. The presence of the mesoporous silica SBA-16 has improved reactants diffusion and molecular accessibility for suitable interactions in Lewis acid centers, which led to MOFs outperforming in the efficient chemical fixation of CO2. Also, the essential role of free amino groups in ZnGlu MOF was shown as base sites and H-bonding groups when suggesting a possible reaction mechanism. This efficient, and convenient catalytic system can be separated by filtration, and its original activity was maintained for five successive cycles under the same reaction conditions. The above nanocomposite, with great potential, can be used to catalyze different chemical reactions for industrial purposes.

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand.

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]+ in which the NiIII and CuIII centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]+, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent MIII species, suggesting the locus of oxidation for both [ML]+ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent MIII species. Notably, [ML]+ can also be obtained via O2 oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]+: -1.16 V, NiL/[NiL]+: -1.01 V vs Fc/Fc+ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..

Diiminium Nucleophile Adducts Are Stable and Convenient Strong Lewis Acids.

Strong Lewis acids are essential tools for manifold chemical procedures, but their scalable deployment is limited by their costs and safety concerns. We report a scalable, convenient, and inexpensive synthesis of stable diiminium-based reagents with a Lewis acidic carbon centre. Coordination with pyridine donors stabilises these centres; the 2,2'-bipyridine adduct shows a chelation effect at carbon. Due to high fluoride, hydride, and oxide affinities, the diiminium pyridine adducts are promising soft and hard Lewis acids. They effectively produce acylpyridinium salts from carboxylates that can acylate amines to give amides and imides even from electronically intractable coupling partners.

Robust Pd/Al2O3 bifunctional catalyst for single reactor tandem synthesis of furan and tetrahydrofuran derivatives from furfural

In this study, we report a reusable catalytic system based on a bifunctional Pd/Al2O3 spheres to conduct the single-reactor tandem aldol condensation/crotonization reaction between furfural and methylisobutyl ketone, followed by hydrogenation, both in batch and continuous mode. In batch mode, the process achieved an overall 86 % yield of 1-(tetrahydrofuran-2-yl)-5-methylhexan-3-one at 180 °C after 9 h at (5.2 wt% furfural concentration). The reaction rate for the aldol condensation/crotonization step decreased at higher FF concentration, suggesting the inhibition of Lewis acid centers due to adsorbed FF as inferred from kinetic modeling. In continuous mode, the catalyst was operated in a dual-type fixed bed reactor and reached 55 % steady-state conversion and 60 % and 35 % selectivity to 1-(furan-2-yl)-5-methylhexan-3-one and (E)-1-(furan-2-yl)-5-methylhex-1-en-3-one, respectively, after 12 h operation at 170 °C and a weight-hourly space velocity of 0.50 h−1 at high furfural concentration (18 wt%). NH3/CO2 temperature-programmed desorption and FT-IR spectroscopy implemented with adsorbed pyridine, NH3 and CO2 were used to assess the nature, strength, density and stability of the acid and basic sites.

Insights into adsorption of various gases on extra-framework cations of zeolite: A dispersion corrected DFT study on zeolite cluster models with Li[formula omitted], Na[formula omitted] and K[formula omitted] charge compensating ions

Design of an economical and sustainable gas separation material is relevant in several industrial processes. Zeolites with tunable pore sizes are ideal molecular sieves of many gases. The adsorption centers of these molecular sieves are extra-framework Lewis acid centers. In this study, we attempt to delineate the electronic properties of such centers (Li+, Na+ and K+) and their sorption properties towards N2, O2, CO, CO2 and H2. Negative framework of zeolites are modeled using different cluster models that present distinct electronic environment and role of this environment on the Lewis acidity of the cation. The sorption property towards different gases is evaluated using dispersion corrected DFT studies. The results obtained are benchmarked for one of the studied model using CCSD calculations. The results indicate that while the local environment modulates the adsorption properties, the relative adsorption properties between different ions follow the same order irrespective of the type of negative framework modeled. This reveals that intrinsic atomic properties of the charge compensating cations drive the sorption properties of the zeolites. Adsorption energies compounded with the analysis of IR stretching frequencies of the adsorbed gases reveals that Li centers shows molecular adsorption (charge donation to the cationic centers) towards N2 as compared to O2 and towards CO2/CO as compared to H2, demonstrating the applicability of Li-Zeolites as ideal membranes for oxygen concentrators and syngas separation. These adsorption studies are ratified by the BOMD simulations at 300 K, where H2 and O2 desorbs while N2, CO and CO2 remains adsorbed to the cationic site.

An Important Photocatalysis Oxidation: Selective Oxidation of Aniline to Azobenzene over a {P4Mo6}-based Crystalline Catalyst Under Visible-light Irradiation at Room Temperature.

Selective oxidation of aniline to azobenzene with a higher economic value is significant for modern industrial production. However, the high reaction temperature and undefined catalytic reaction center always limit the promotion of this reaction. Here, we designed and synthesized a new [P4MoV6O31]12--based cobalt-containing complex (Hbiz)5{Co(H2O)3[Co(H2O)2]2}{Co[Mo6O12(OH)3(PO4)(HPO4)3][Mo6O12(OH)3(PO4)2(HPO4)2]}·3H2O (1; biz = benzimidazole) under solvothermal conditions. Using 1 as the photocatalyst, the visible-light-driven conversion of aniline to azobenzene at room temperature was realized for the first time (conversion rate 97%, selectivity 96% of azobenzene, reaction time of 12 h). Additionally, real sunlight irradiation can produce a comparable outcome in 48 h. The mechanism investigation manifested that the Mo and Co centers in 1 acted as Brönsted and Lewis acid centers, synergistically promoting the aniline oxidation reaction. This research advances the application of polyoxometalate-based photocatalysts while also offering a sustainable and effective method for producing azobenzene.

BIOETHANOL CONVERSION INTO HIGH-OCTANE COMPONENTS OF MOTOR FUEL IN THE PRESENCE OF COBALT-CONTAINING HZSM-5 TYPE ZEOLITES

The influence of the concentration of cobalt introduced into HZSM-5 on its acidic and catalytic properties in the transformation of ethanol into high-octane components of motor fuels has been studied in the plug-flow unit within the temperature range of 300-450 °С. The unmodified НZSM-5 zeolite is shown to be highly selective to aromatization products (27.5-47.6 %) and С1-С3 alkanes (25.8-37.1 %). A significant increase in selectivity to isomerisation products (from 17.2-19.8 to 34.0-42.7 %) is established for cobalt-modified (1.0-3.0 wt. %) zeolite catalysts within the temperature range 300-350 °C. The highest increase in the selectivity of modified catalysts to aromatisation products (32.7-40.7 %) is observed within the temperature interval of 400-450 °С. The effect of modification on the acid properties of HZSM-5 zeolite has been studied using the temperature-programmed desorption of ammonia. It is shown that the changes of the catalytic properties of HZSM-5 zeolite are connected with a considerable decrease in the concentration of strong Brønsted acid centres (from 538 to 317 μmol/g), with the formation of new stronger Lewis acid centres (37-80 μmol/g) including cobalt ions and acid-base Coδ+-Oδ- type pairs capable of strong polarization and activation of C-H bonds in the intermediates. Selectivity to the products of isomerisation and aromatization on the modified catalysts is regulated by cobalt concentration and process temperature. The highest selectivity to isomerisation products (42.7 %) is achieved over the 2 % Co/HZSM-5 catalyst at a temperature of 300 °C. The catalysate obtained over the 2 % Co/HZSM-5 catalyst at 300 °С corresponds to Euro-5 standard in the content of isoparaffin (56.4 %) and aromatic (29.8 %) hydrocarbons, benzene (0.6 %) and olefins (1.1 %).

Acid Treatment for Regulating Hf Sites of the Hierarchical Hf-B-BEA Zeolite, Thereby Boosting Its MPV Reaction Performance

Catalyst engineering plays a key role in bridging gaps between the structure of the Lewis acid center and its corresponding catalytic performance. In this regard, work that has been done in tuning active sites of Hf-containing zeolites, which hold promising catalytic activity for a variety of hydrogen-shift reactions, is far from complete. Herein, a Hf-B-BEA catalyst was rapidly prepared (within hours) by a hydrothermal semi-reconstruction strategy, involving partial framework etching of parent B-BEA and Hf grafting during the reassembling process. Then, an acid treatment was performed for expelling the boron atoms from the framework; more importantly, Hf migration into the silanol nests was triggered. The acid treatment worked powerfully as the driving force for transforming the semi-framework Hf into well-defined tetrahedrally coordinated framework Hf, thus endowing Lewis acidity of the obtained Hf-BEA. Consequently, this operation greatly boosted the Meerwein–Ponndorf–Verley reduction performance of Hf-BEA, which was almost inactive over the pristine Hf-B-BEA. The rational regulation of Hf sites provides new concepts in designing Hf-containing zeolites and understanding the structural–activity relationship.

A bifunctional polycentric-affinity MOF/MXene heterojunction-based molecularly imprinted photoelectrochemical organophosphorus-sensing platform

The analytical functionalization of metal organic frameworks (MOFs) has been an extremely attractive topic in photoelectrochemical (PEC) sensing. However, most PEC sensors based on photoactive MOFs are more focused on the acquisition and amplification of semiconductor photoelectric signals, and the study of affinity mechanisms for specific targets is more limited. Here, we find that PCN-224 (Zn) can effectively sense organophosphorus structures through a multicenter affinity mechanism, as evidenced by x-ray photoelectron spectroscopy (XPS) and computational chemistry. Further, we coupled PCN-224(Zn) to Nb4C3 to form a Schottky junction that enhances the photoelectric signal response of the sensor. In particular, the heterojunction exhibited a higher photocurrent density than pristine PCN-224(Zn) (17.3 μA vs. 26.5 μA cm−2). On the basis, we developed a novel MOF-based polycentric-affinity PEC sensing strategy for dimethoate (DIM) detection in concert with molecular imprinting technology. The affinity effects of the Lewis acidic center sites (Zn(II), Zr(IV)) and spatial matching through molecular imprinting synergistically ensure that DIM is precisely captured. Ultimately, a trace DIM-PEC sensor was constructed by a competitive strategy using dopamine as an electron donor. The photocurrent intensity was linearly related to the logarithm of DIM concentration in the range of 0.1 nM to 1000 nM, with a low detection limit of 26.1 pM (3σ/S). This study demonstrates the potential of bifunctional polycentric-affinity MOF in the PEC sensing.

XeF2 Coordination Complexes of the [BrO2]+ Cation, [O2Br(FXeF)n][AsF6] (n = 1, 2) and [O2Br(FXeF)2][SbF6]; Their Syntheses and Structural Characterizations.

The syntheses and structural characterizations of the first XeF2 coordination complexes of the [BrO2]+ cation are described. The reactions of [BrO2][PnF6] (Pn = As, Sb) with XeF2 in anhydrous HF solvent yield the salts [O2Br(FXeF)n][AsF6] (n = 1, 2) and [O2Br(FXeF)2][SbF6], which were characterized by low-temperature (LT) Raman spectroscopy and single-crystal X-ray diffraction (SCXRD). The XeF2 ligands and [PnF6]- coordinate to the Lewis acidic [BrO2]+ cation through primarily electrostatic BrV---FXe σ-hole bonds that result from coordination of the F atoms into regions of high positive electrostatic potential on the Br(V) atom and have bond trajectories that avoid the stereoactive valence electron lone-pair of Br(V). The complexes and their structural characterizations by LT Raman spectroscopy and SCXRD significantly extend the coordination chemistry of Br(V) and provide rare examples of a noble-gas difluoride coordinated to a strong oxidant main-group Lewis acid center.

In Situ Construction Channels of Lithium-Ion Fast Transport and Uniform Deposition to Ensure Safe High-Performance Solid Batteries.

Solid-state lithium-ion batteries (SLIBs) are the promising development direction for future power sources because of their high energy density and reliable safety. To optimize the ionic conductivity at room temperature (RT) and charge/discharge performance to obtain reusable polymer electrolytes (PEs), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-hexafluoro propylene) (P(VDF-HFP)) copolymer combined with polymerized methylmethacrylate (MMA) monomers are used as substrates to prepare PE (LiTFSI/OMMT/PVDF/P(VDF-HFP)/PMMA [LOPPM]). LOPPM has interconnected lithium-ion 3D network channels. The organic-modified montmorillonite (OMMT) is rich in the Lewis acid centers, which promoted lithium salt dissociation. LOPPM PE possessed high ionic conductivity of 1.1 × 10-3 S cm-1 and a lithium-ion transference number of 0.54. The capacity retention of the battery remained 100% after 100 cycles at RT and 0.5 C. The initial capacity of one with the second-recycled LOPPM PE is 123.9 mAh g-1 . This work offered a feasible pathway for developing high-performance and reusable LIBs.

Poly (ionic liquid)s-silica spheres as effective core-shell catalysts for the conversion of glucose and fructose into 5-hydroxymethylfurfural

5-Hydroxymethylfurfural (5-HMF), as a significant, bio-derived platform chemical molecule, has been under extensive research due to its wide application in petrochemical industry. In this work, three kinds of poly (ionic liquid)s-silica spheres (PIL@SiO2) with various lengths of alkyl chains as effective core-shell catalysts were prepared by the polymerization and coating methods, and were used to prepare 5-HMF from glucose and fructose with optimal results. When the alkyl carbon number of PIL monomer was 18, PIL@SiO2-18 displayed good catalytic activity in 5-HMF production, resulting in the highest yields of 77.36% and 72.8% from glucose and fructose in methyl isobutyl ketone (MIBK)/ saturated solution of sodium chloride (Sat.NaCl) biphasic solvent. A possible reaction mechanism was proposed that the core-shell synergistic catalysis effectively promoted the preparation of 5-HMF from monosaccharide, in which Lewis acidic polycationic centers contributed to driving the isomerization process. The promising approach provides an efficient strategy for silica-based core-shell synergistic catalysts in the synthesis of 5-HMF from carbohydrates.

Competitive adsorption of alcohols over heteroatom zeolites hinders the catalytic conversion of sugar

In the catalysis of glucose from bulk methanol to Lewis acid centers via zeolite pores, MES-ATR-IR and PSD detected competitive adsorption driven by hydrogen bonding ascribed to the hydroxyl of the solvent, substrate, and interface. The phase-resolved spectra show a change from methanol- to glucose-silicon hydroxyl bonds, which slows the rate of Lewis acid center carbonyl activation. Moreover, increased silicon hydroxyl binding to glucose results in lower acidity of metal centers in the process. It almost eliminates the variations in reactivity caused by metal identity. In contrast, the absence of hydroxyl at the solvent, substrate, or interface prevents competitive adsorption and brings out the acidity of different metal centers, with Sn > In > Hf > Ti > Zr being the order of binding ability to the carbonyl. This study contributes to the development of zeotype metal center local environments for substrate and solvent matching in various reaction networks.

Reactivity of a Cationic Bismuth Amide towards Unsymmetric Heterocumulenes.

Invited for this month's cover is the Lichtenberg group of the Philipps-University Marburg (Germany). The front cover picture shows "bismuth" dressed in colors reminiscent of those found on the surface of this element. In the graphic, bismuth is craving for soft ice cream. This represents the preference of Lewis acidic bismuth centers for soft donor atoms, as demonstrated in the insertion of heterocumulenes into the Bi-N bond of a cationic bismuth amide. More information can be found in the Research Article by Crispin Lichtenberg and co-workers.

Front Cover: Reactivity of a Cationic Bismuth Amide towards Unsymmetric Heterocumulenes (ChemPlusChem 7/2023)

I scream, you scream, we all scream for ice cream! The front cover picture shows “bismuth” dressed in colors reminiscent of those found on the surface of this element. In the graphic, bismuth is craving for soft ice cream. This represents the preference of Lewis acidic bismuth centers for soft donor atoms, as demonstrated in the insertion of heterocumulenes into the Bi-N bond of a cationic bismuth amide. More information can be found in the Research Article by Crispin Lichtenberg and co-workers.

C–H and H–H Bond Activation by a Cationic Germylyne Complex

Reactivities of a cationic germylyne complex [Cp*(CO)2W≡Ge(IPr)](BAr4F) (1) [IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene] toward several unsaturated organic substrates and dihydrogen were investigated. Complex 1 underwent activation of the formyl C–H bond of benzaldehyde and the ortho C–H bond of pyridine to afford base-stabilized germylene complexes, indicating that coordination of the heteroatom (O/N) of the substrates to the Lewis acidic Ge center facilitates the C–H bond activation on W. The reactions with alkenes resulted in the terminal C–H bond cleavage on Ge by coordination of the C═C double bond to W, giving η3-germaallyl complexes. The reactions with terminal alkynes, on the contrary, proceeded through no C–H bond cleavage to produce metallacyclopropenylgermylenes, which have an unprecedented bonding situation with a lone pair on Ge. Complex 1 also reacted with dihydrogen at room temperature to afford a dihydrido(dihydrogermyl) complex finally through a hydrido(hydrogermylene) complex. Characterization of these products including X-ray crystal diffraction study (XRD) and possible reaction mechanisms based on cooperative action at the W–Ge unsaturated bond platform are described.

Investigation on the catalysis performance of metal-doped zeolites on ring-opening acidolysis reaction

Metal-incorporated zeolites as heterogeneous Lewis acid were applied in the ring-opening acidolysis of ethylene oxide with acrylic acid to prepare 2-hydroxyethyl acrylate (HEA). The catalysis characteristics, including Lewis acid strength and internal mass transfer resistance, were systematically investigated to facilitate the rational design of zeolite catalysts. Through modifying the Lewis acid strength of zeolites via different metals (Sn, Zr and Ti) incorporation and the internal mass transfer resistance of zeolites from the particle size, pore diameter or pore structure, it was found that the increased Lewis acidity can accelerate both the main and side reactions, resulting in the high activity but low selectivity of zeolite catalysts; the enhanced internal diffusion was the key to limiting subsequent cascade alcoholysis side reactions as well as the decline of catalytic performance. Based on these findings, Zr-containing zeolites featured with moderate Lewis acidity and uniform intracrystalline mesoporosity were designed and prepared, presenting excellent catalysis and recycling performance. These results demonstrated the great potential of the developed zeolite catalysts for the green synthesis of HEA as well as the similar ring-opening acidolysis of epoxides.

Construction of mesoporous Ru@ZSM-5 catalyst for dichloromethane degradation: Synergy between acidic sites and redox centres

Ruthenium supported on hierarchical pore HZSM-5 zeolite (named Ru@mZ5) as an efficient catalyst for dichloromethane (DCM) oxidation was synthesized in this work. Ru component was introduced without the expense of acid site thanks to the more accessibility of mesopores. Instead, moderate Lewis acid (Ruδ+) that increased the surface acidity formed on the hierarchical pores. Ru@mZ5 also showed enhanced chemisorption and oxidation of DCM due to the synergy between the acidic properties and the ruthenium active centres. Compared with the bare microporous HZSM-5 (Ru/Z5) and mesoporous KIT-6 (Ru@K6) loaded Ru, the reaction rate of Ru@mZ5 was the highest at 200℃ (0.167 mmol/g h), the TOFRu was 0.0488 s−1, and the adsorption capacity of DCM was the highest at 50℃ (223 μmol/g) (600 ppm DCM, GHSV = 25,000 mL/g h). Moreover, the hierarchical pore structure of Ru@mZ5 were conducive to the desorption of products, which improved the reusability and stability of the catalyst under wet and dry conditions without chlorination byproducts.

Application of functional metal anionic Lewis acid ionic liquids in the alkylation of chlorobenzene/SOCl2.

4,4'-Dichlorodiphenyl sulfoxide is the main raw material for the manufacture of polysulfone, polyether sulfone and other engineering plastics. It is also an intermediate for medicines, dyes and pesticides, which has been widely utilized in engineering plastics, fine chemicals and other fields. The alkylation of chlorobenzene with thionyl chloride can give 4,4'-dichlorodiphenyl sulfoxide as a product using Lewis acidic ionic liquids. In this work, metal-based methylimidazolium ionic liquids were synthesized, which were found to be efficient catalysts for alkylation reactions. The molar ratio of different metal chlorides to 1-butyl-3-methyl imidazole chloride and the influence of different metal chlorine additives on the catalytic Lewis acid center were investigated. The fissionable species of AlCl3 in ionic liquids will enhance the acidity of ionic liquids and, thus, promote the catalytic performance of ionic liquids. Under the optimized reaction conditions, the conversion rate of excess chlorobenzene was 45.3% and the selectivity of 4,4'-dichlorodiphenyl sulfoxide was 31.9%.

Facile fabrication of superior Pd and Ce containing SiO2 aerogel composite adsorbents for deep desulfurization of model fuels and aromatics

A series of PdO/CeO2/SiO2-z aerogel composites with different molar ratio of Si/Ce were facilely fabricated via sol-gel method combined with atmospheric drying technology, where a major of Ce3+ species could be oxidized to Ce4+ species by adding hydrogen peroxide. They were used to remove thiophenics from model fuels and aromatics, compared with PdO/SiO2 and CeO2/SiO2 aerogel composites. Adding Ce and Pd led to an appearance of weak Lewis acid centers on the surface of adsorbents. Compared with PdO/SiO2 and CeO2/SiO2, the adsorption capacity of PdO/CeO2/SiO2-z significantly increased, in which PdO/CeO2/SiO2-100 performed best and the adsorption capacity for thiophene was 15.0 mg-S/gads, which was about the sum of PdO/SiO2-636 and CeO2/SiO2-100. The case was attributed to π-complexation of Pd2+ and S-M (Pd, Ce, acted as Lewis acid centers) bond (σ) with thiophenics, where the adsorption process was dominated by S–Ce bond. Combined with the same chemical state of Pd and Ce in single metal and bimetal adsorbents verified by XPS, it can be deduced that no synergistic interaction between Pd and Ce species was formed in the adsorption process. PdO/CeO2/SiO2-100 exhibited a higher resistance for competitive adsorption of aromatics and a good solvent washing regeneration performance. The adsorption process of PdO/CeO2/SiO2-100 for thiophene was well fitted by Langmuir isothermal and pseudo-secound-order model and were spontaneous and endothermic. The solvents (n-heptane, benzene and p-xylene) have no obvious effect on the adsorption thermodynamics and kinetics of PdO/CeO2/SiO2-100.