Lewis Acid Strength Research Articles

Rational design of Ga-substituted NaY zeolites with controllable acidity for remarkable carbonylation of methyl nitrite to dimethyl carbonate

Lack of high performance and stability catalysts remains a technical bottleneck for the industrialization of methyl nitrite (MN) carbonylation to dimethyl carbonate (DMC). Lewis acidity is crucial for catalytic performance in MN carbonylation reaction, but the regulation of Lewis acid sites and the effects of Lewis acid strength on catalytic performance have not been clarified. Thus, rational design of NaY zeolites with controllable acidity is an effective strategy to promote their catalytic performance. Herein, we substituted Ga for partial framework Al to produce a series of NaGaY-x (x = 0 ∼ 2) zeolites with different Ga content via direct hydrothermal method and applied in this reaction. The results showed that the Lewis acid strength increased upon an appropriate amount of Ga incorporation, thereby the electron density of Pd2+ could be regulated. The optimal electron density of Pd2+ promotes the carbon monoxide (CO) adsorption and activation, which contribute to the formation of the critical intermediate species *COOCH3. Eventually, the Ga-modified PdCu/NaY catalyst exhibited remarkable catalytic activity for MN carbonylation to DMC with the weight-time yields (WTYDMC) of 1251 g·kgcat−1·h−1, and the CO conversion to DMC (CCO) of 69.2 % as well as the DMC selectivity based on CO (SDMC/CO) up to 100 %. Our work paves the way to further understand the synergistic effect between Pd2+ and Lewis acid, assists in the development of high-performance MN carbonylation catalysts.

Probing the Impact of Solvent on the Strength of Lewis Acids via Fluorescent Lewis Adducts.

Various methods have been developed to measure the strength of a Lewis acid. A major challenge for these measurements lies in the complexity that arises from variable solvent interactions and perturbations of Lewis acids as their reaction environment changes. Herein, we investigate the impact of solvent effects on Lewis acids for the first time as measured by the fluorescent Lewis adduct (FLA) method. The binding of a Lewis acid in various solvents reveals a measurable dichotomy between both polarity and donor ability of the solvent. While not strictly separable, we observe that the influence of solvent polarity on Lewis acid unit (LAU) values is distinctly opposite to the influence of donor ability. This dichotomy was confirmed by titration data, illustrating that solvation effects can be appropriately and precisely gauged by the FLA method.

Band gap structure - Lewis acid/base feature correlation for potassium/sodium polyniobates and polytitanates

Polyniobates and Polytitanates are extensively studied because of their characteristic structure, electronic and acid/alkali feature as perovskite oxide based materials. Based on the experimental data from references and theoretical results calculated by density functional theory (DFT), respectively, the qualitative correlation between band gap structure and Lewis acid/base feature is investigated in the present paper. When it comes to band gap structure, the bond angle θ and bond length are considered as the quite influential factors. As the M-O-M bond angle deviates from 180° and the bond length elongates, the band gap energy increases. The conduction band potential and valence band potential are associated with Sanderson charge fraction δi. It can be drawn from this study that the conduction band potential is positively correlated with Lewis acid (Nbδ+ or Tiδ+) strength and valence band potential is negatively correlated with Lewis base (Oδ−) strength for potassium polyniobates and polytitanates with different phase structure. The conduction band potential and valence band potential are positively correlated with Lewis acid (Nbδ+ or Tiδ+) strength and Lewis base (Oδ−) strength for sodium polyniobates and polytitanates with same phase structure. Sanderson electronegativity and the charge for the O 1s confirm the correlation between band gap structure and Lewis acid and base. In addition, the phase structures of polyniobates and polytitanates affect the Lewis acid feature of the conduction band and Lewis base feature of valence band respectively, resulting in changes in their band gap structure.

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.

Effects of Lewis acid strength of monovalent coinage metals and zeolite frameworks on catalytic CO2 cycloaddition with ethylene oxide: A DFT study

The synergistic effect of acid site and zeolite framework type makes Au(i)-ZSM-5 an excellent catalyst for CO2 cycloaddition with ethylene oxide.

Insights into Lewis/Brønsted acidity of metal chlorides and solvent effect of alcohols for synthesis of γ-valerolactone by combining molecular dynamics simulations and experiments

The synthesis of biofuel γ-valerolactone (GVL) from furfural is a multistep process that entails the regulation of Lewis/Brønsted (L/B) acidity of catalyst and the hydrogen donation ability of alcohols. In this study, experiments and molecular dynamics (MD) simulations were combined to get insights into the role of catalysts and solvents during the conversion of furfural to GVL. The capacities of various metal chlorides (divalent to hexavalent metal elements) and alcohols (primary and secondary alcohols) were comparatively investigated. The results showed that chlorides of Group IVB metals (ZrCl4 and HfCl4) exhibited excellent catalytic performance and achieved GVL yields of 61.88 % and 68.13 %, respectively. By preliminary statistical assessment, the intervals in which high isopropyl levulinate (IPL) yields and high GVL yields were found corresponded to different L/B acidity (3.5 < N < 4.8, −2 < Z < 1 and N < 5.7, Z > 1.5) (Z: Lewis acid strength. N: number of molecules in the shell). Secondary alcohols displayed the lower reduction potential but the higher interaction energy with reactants than that of primary alcohols. The distribution of Hf4+ around the reactants was tendentious, whereas the distribution of C3H7OH2+ was more homogeneous. The investigation of reaction mechanism proposed new understandings of the reacted sites of reactants and the hydrogen transfer of solvent to reactants. The hypothesis that IPL was capable of hydrogen/deuterium (H/D) exchange was developed.

Mixed Metal Phosphonates: Structure and Proton Conduction Manipulation through Various Alkaline Earth Metal Ions

Three new layered mixed metal phosphonates [CoMg(notpH2)(H2O)2]ClO4·nH2O (CoMg·nH2O), [Co2Sr2(notpH2)2(H2O)5](ClO4)2·nH2O (CoSr·nH2O), and [CoBa(notpH2)(H2O)1.5]ClO4 (CoBa) were synthesized by reacting a tripodal metalloligand CoIII(notpH3) [notpH6 = C9H18N3(PO3H2)3] with alkaline earth metal ions. Along with an increase in the radius of the alkaline earth metal ions, the 6-coordinate {MgO6}, 7-coordinate {SrO7}, and 9-coordinate {BaO9} geometries are the distorted octahedron, capped triangular prism, and tricapped triangular prism, respectively. Consequently, the metalloligand Co(notpH2)− adopts variable coordination modes to bind the alkaline earth metal nodes, forming diverse layer topologies in the three mixed metal phosphonates. The AC impedance measurements revealed that the proton conductivities at 25 °C and 95% relative humidity (RH) follow the sequence: CoMg·nH2O &gt; CoSr·nH2O &gt; CoBa. As expected, CoMg·nH2O exhibits a 28-fold enhanced value for proton conductivity (4.36 × 10−4 S cm−1) compared with the previously reported isostructural compound, CoCa·nH2O, at 25 °C and 95% RH due to the greater Lewis acid strength of Mg(II) lowering the pKa of the coordinated water.

Aqueous Solution-Grown Crystalline Phosphorus Doped Indium Oxide for Thin-Film Transistors Applications

Solution-grown indium oxide (In2O3) based thin-film transistors (TFTs) hold good prospects for emerging advanced electronics due to their excellent mobility, prominent transparency, and possibility of low-cost and scalable manufacturing; however, pristine In2O3 TFTs suffer from poor switching characteristics due to intrinsic oxygen-vacancy-related defects and require external doping. According to Shanmugam's theory, among potential dopants, phosphorus (P) has a large dopant-oxygen bonding strength (EM-O) and high Lewis acid strength (L) that would suppress oxygen-vacancy related defects and mitigate dopant-induced carrier scattering; however, P-doped In2O3 (IPO) TFTs have not yet been demonstrated. Here, we report aqueous solution-grown crystalline IPO TFTs for the first time. It is suggested that the incorporation of P could effectively inhibit oxygen-vacancy-related defects while maintaining high mobility. This work experimentally demonstrates that dopant with high EM-O and L is promising for emerging oxide TFTs.

One-Pot Transformation of Citronellal to Menthol Over H-Beta Zeolite Supported Ni Catalyst: Effect of Catalyst Support Acidity and Ni Loading

Citronellal was converted to menthol in a one-pot approach using H-Beta zeolite-based Ni catalyst in a batch reactor at 80 °C, under 20 bar of total pressure. The effects of H-Beta acidity (H-Beta-25 with the molar ratio SiO2/Al2O3 = 25 and H-Beta-300 with SiO2/Al2O3 = 300) and Ni loading (5, 10 and 15 wt %) on the catalytic performance were investigated. Ni was impregnated on H-Beta support using the evaporation-impregnation method. The physico-chemical properties of the catalysts were characterized by XRD, SEM, TEM, ICP-OES, N2 physisorption, TPR, and pyridine adsorption–desorption FTIR techniques. Activity and selectivity of catalysts were strongly affected by the Brønsted and Lewis acid sites concentration and strength, Ni loading, its particle size and dispersion. A synergetic effect of appropriate acidity and suitable Ni loading in 15 wt.% Ni/H-Beta-25 catalyst led to the best performance giving 36% yield of menthols and 77% stereoselectivity to ( ±)-menthol isomer at 93% citronellal conversion. Moreover, the catalyst was successfully regenerated and reused giving similar activity, selectivity and stereoselectivity to the desired ( ±)-menthol isomer as the fresh one.Graphical

Fluorescence-based measurement of the Lewis acidities of lanthanide triflates in solution

Despite the prominence of rare-earth complexes to act as Lewis acid catalysts in organic synthesis, the comprehensive measure of the Lewis acid strength of such compounds has yet to be performed due to incompatibilities with existing methods. We report our results in measurement of a sequence of lanthanide triflates via our recently established fluorescent Lewis adduct (FLA) method. The persistence in solution of these Lewis acids as solvated coordination complexes is accurately measurable by the FLA method. However, several of the rare-earth species exhibit fluorescence quenching, which may potentially inhibit the measurement. Nevertheless, meaningful FLA measurements were still possible, and the results correspond to both periodic trends and were even consistent with previous correlated reported data.

Catalytic glycolysis of polyethylene terephthalate (PET) by solvent-free mechanochemically synthesized MFe2O4 (M = Co, Ni, Cu and Zn) spinel

In the present study, the solvent-free mechanochemically synthesized MFe2O4 (M = Co, Ni, Cu, and Zn) spinel was used as a catalyst for PET glycolysis. All catalysts were active for PET glycolysis to produce bis(2-hydroxyethyl) terephthalate (BHET) with the order of PET conversion of ZnFe2O4 > CuFe2O4 > CoFe2O4 > NiFe2O4. The catalytic activity was correlated with the Lewis acid strength of the M2+ in MFe2O4, i.e., a catalyst with strong Lewis acid exhibited high catalytic activity. Although CoFe2O4 was the second-best catalyst among the four in terms of yield of BHET, it possessed the highest saturation magnetization, which is a great advantage for magnetic separation of the catalyst. The reaction over CoFe2O4 exhibited apparent activation energy (Ea) of 188 kJ mol−1, and it was reusable for at least five times. Moreover, a simulation using Aspen Plus® software showed that scale-up is highly feasible.

Highly Active and Selective Sites for Propane Dehydrogenation in Zeolite Ga-BEA.

A highly selective Ga-modified zeolite BEA for propane dehydrogenation has been synthesized by grafting Ga on Zn-BEA followed by removal of Zn in the presence of H2. A propene selectivity of 82% at 19% propane conversion illustrates the high selectivity at 813 K. The kinetic model of the catalyzed dehydrogenation including the elementary steps of propane adsorption, first and second C-H bond cleavage, and propene and H2 desorption demonstrates that the propane dehydrogenation rate is determined by the first C-H bond cleavage at low pC3H8, while at high pC3H8, the rate is limited by the desorption of H2. The active sites have been identified as dehydrated and tetrahedrally coordinated Ga3+ in the *BEA lattice. The low selectivity toward aromatics is concluded to be associated with the high Lewis acid strength of lattice Ga3+ and the low Brønsted acid strength of the hydrated Ga sites.

Insights into glyphosate adsorption on Lewis acidic zeolites from theoretical modelling

The removal of glyphosate via the adsorption process is one of the essential routes for decreasing its harmful impact. Here, we investigate the glyphosate adsorption on the Lewis acidic zeolites through density functional theory (DFT) calculations. We found that Hf-BEA provides the highest adsorption energy (−44.1 kcal/mol) as compared to ones of other zeolites. This result corresponds to its highest ordered Lewis acidity calculated by ammonia adsorption. There is a strong correlation between glyphosate and ammonia adsorption energies, as shown in a linear scaling relationship. This result implies the significant role of the Lewis acidity strength of zeolite in glyphosate adsorption process. The adsorption activity which related to zeolite pore structures (BEA, FAU, ZSM-5 and MOR) are found in the order of Hf-BEA > Hf-ZSM-5 ∼ Hf-MOR > Hf-FAU. The effect of water on the glyphosate adsorption is also investigated. Water is adsorbed on Sn-BEA zeolite much weaker than glyphosate and thus the active site of zeolite cannot be blocked by it. The trend of adsorption energies remains the same as in vacuum. Finally, the hydrolysis of metal-substituted active sites to form open sites strengthens the adsorption of glyphosate.

Aluminum‐Containing Metal‐Organic Frameworks as Selective and Reusable Catalysts for Glucose Isomerization to Fructose

AbstractFructose is a versatile precursor for food and chemicals. Currently, catalytic production of fructose is achieved by enzymatic isomerization of glucose from renewable lignocellulose. Although the catalyst, glucose isomerase, is selective, it is not stable. Here, aluminum‐containing metal‐organic frameworks (Al‐MOFs) are shown to be active, selective, stable, and reusable for glucose isomerization in ethanol. Al‐MOFs achieved 64% fructose selectivity with 82% glucose conversion at 120 °C, superior performance compared with most other solid catalysts. The amino groups in Al‐MOFs enhance Lewis acid strength, which is responsible for the high fructose selectivity at high glucose conversion. Moreover, the Al‐MOF catalyst is stable and reusable at least four times without losing either activity or fructose selectivity. These findings illustrate compelling opportunities for Al‐MOFs in fructose production and other organic reactions, such as fructose conversion to 5‐hydroxymethylfurfural and levulinic acid.

Mesoporous Zirconium Oxide Prepared by Anchoring W, Mo, Nb, Ta Using Peroxo Precursors: Influence of the Oxoanions on the Pores Size and the Hydrothermal Catalysts Stability for Cellulose Conversion

Efficient and sustainable cellulose conversion into chemicals requires the design of heterogeneous catalysts with controlled acid–base properties, mesoporosity and excellent stability in hydrothermal conditions. To this end, modified zirconia with anchored W, Mo, Ta and Nb oxoanions were prepared by ionic exchange using peroxo precursors of the selected transition metals. Modified zirconia samples were characterized by X-ray diffraction, N2 isotherms, FTIR of pyridine adsorption and microcalorimetry of ammonia adsorption. If the nature of the oxoanion tunes slightly the catalysts Lewis acid strength or density, it influences strongly their mesoporosity with mesopores in the range of 10 nm for ZrTa and ZrNb. Upon Pt loading, all the Pt-modified zirconia are effective to catalyze C–C and C-O cleavage during cellulose hydrogenolysis leading to PG as main short polyols with the co-formation of 2,5-hexanedione over ZrNb and ZrTa. Most important, ZrNb and ZrTa exhibit excellent water tolerance without any leaching nor structure modification. Combined to their mesoporosity, these features make ZrNb and ZrTa, true solid Lewis acids with high potential in the frame of bulky biomass conversion in hot waterierGraphical Abstract

Zinc-based cyclens containing pyridine and cross-bridges: X-ray and DFT structures, Lewis acidity, gas-phase acidity, and pKa values

It is well known that catalytic centers containing the zinc(II) ion can act as both Lewis and Bronsted-Lowry acids. In addition to coordination number, the Zn coordination geometry can also strongly impact the acidity of the active site, no matter what measure of acidity is considered. Herein, we report the first pentacoordinate, zinc-ammonia complex containing a pyridine based tetraazamacrocycle, [(pyclen)Zn(NH3)](PF6)2, that has applications in Lewis acid catalysis. From this structure, we obtain binding energies and acidities for a series of related pyclen and cross-bridged cyclen type tetraazamacrocycles comprising the pentacoordinate N4Zn(II)–OH2 entity, collectively referred to as [(R-cyclen)Zn–OH2]2+. Results from gas- and aqueous-phase density functional theory (M05-2X) and ab initio (MP2) calculations reported herein demonstrate that molecular geometry has a substantial influence on both the Zn–OHx binding strengths and the deprotonation energy of coordinated H2O, but not necessarily in a predicable way. While generally more constrained N–Zn–N coordination leads to greater Zn–OH2 binding energies (Lewis acidities), the corresponding Lewis acidities of the complexes don’t always correlate with the Zn–OH2 bond lengths nor the (Bronsted) acidity of the coordinated H2O. Additionally, the order of the Lewis acid strength of the [(R-cyclen)Zn(II)] complexes changes as the basic –OH2 ligand is replaced with its –OH counterpart.

Copper phosphotungstate-catalyzed microwave-assisted synthesis of 5-hydroxymethylfurfural in a biphasic system

In this paper, we have described a novel route to produce 5-hydroxymethylfurfural (HMF), a valuable platform molecule obtained from biomass. Metal-exchanged Keggin heteropolyacid salts were used as catalysts, in microwave-assisted reactions carried out in a water-ethyl acetate biphasic system. To avoid the use of homogenous Brønsted acid catalysts, which are corrosive and difficult to be reused, we have exchanged the protons of the Keggin heteropolyacids with transition metal cations. These salts were evaluated in the fructose dehydration, being the Cu3/2PW12O40 the most active and selective catalyst, achieving 81% of HMF yield, after 15 min reaction at 413 K under microwave irradiation. The effects of metal cation, anion or heteropolyanion present in the catalyst were evaluated. The greatest efficiency of the Cu3/2PW12O40 was attributed to its high Lewis acidic strength which allows its coordinates with the water molecules, consequently generating H3O+ ions in the reaction medium. In addition, after assessing reactions of fructose dehydration in the presence of other Copper salts [i.e., CuCl2 or Cu(NO3)2], we conclude the anion plays too a key role. The higher softness of phosphotungstic anion should stabilize protonate intermediates better than chloride or nitrate anions, favouring this way the reaction. Finally, although the catalyst has been soluble, it was easily reused by removing the aqueous phase and adding a new load of the substrate dissolved in ethyl acetate. The runs were successfully repeated without the loss of activity of the catalyst. Graphical abstract

Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity.

Two series of zinc salts, [EtZn][A] and Zn[A]2, with weakly coordinating anions [A]- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me3NB12Cl11]- [1]-, [Pr3NB12H5Cl6]- [2]-, [Bu3NB12H4Cl7]- [3]-, and [Hex3NB12H5Cl6]- [4]- were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11]- and [HexCB11Cl11]-. The basicity of anion [4]- was determined as being close to that of the triflimide anion [N(SO2CF3)2]-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]2 are lower than those of the Lewis acids B(C6F5)3 and Zn[HexCB11Cl11]2. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

Adsorptive desulfurization using period 4 transition metals oxide: A study of Lewis acid strength derived from the adsorbent ionic-covalent parameter

Pearson’s Hard and Soft Acids and Bases (HSAB) principle has been widely utilized in describing the adsorption trend of sulfur compounds when using transition metal/metal oxides as adsorbents. This study is the first to report on the use of the ionic covalent parameter (ICP) for explaining the adsorption trend of sulfur compounds. Period 4 transition metal oxides (Zn-oxide, Cr-oxide, Mn-oxide, Co-oxide, Fe-oxide, Cu-oxide, Ni-oxide), which act as intermediate Lewis acids, were incorporated into activated carbon (AC), and used in the adsorptive desulfurization (ADS) of model and commercial diesel samples. The study findings show that desulfurization activity in model diesel increased in proportion to the concentration of adsorbent Lewis acid sites. With commercial diesel, the loading of metal oxides significantly enhanced the sulfur adsorption capacity of AC and increased its selectivity for steric sulfur compounds. The results showed a poor correlation between sulfur removal and the Pearson hardness ηth of the intermediate metal oxide cations. However, a strong inverse linear relationship was observed: sulfur removal decreased as the ICPth of the adsorbent metal oxide increased. The study findings suggest a novel concept, i.e. that the ICP is a more accurate measure of the Lewis acidic strength of transition metal oxides with varying crystal structures and metal geometries, and their subsequent desulfurization performance. Kinetic modelling showed that chemisorption was the controlling mechanism. The fixed-bed ADS of commercial diesel when using unmodified AC is best described by the Yoon-Nelson and Thomas models. Adsorption when using the Ni-oxide/AC adsorbent was correlated to the filtration advection–dispersion equation.

Water-Processed Ultrathin Crystalline Indium-Boron-Oxide Channel for High-Performance Thin-Film Transistor Applications.

Thin-film transistors (TFTs) made of solution-processable transparent metal oxide semiconductors show great potential for use in emerging large-scale optoelectronics. However, current solution-processed metal oxide TFTs still suffer from relatively poor device performance, hindering their further advancement. In this work, we create a novel ultrathin crystalline indium–boron–oxide (In-B-O) channel layer for high-performance TFTs. We show that high-quality ultrathin (~10 nm) crystalline In-B-O with an atomically smooth nature (RMS: ~0.15 nm) could be grown from an aqueous solution via facile one-step spin-coating. The impacts of B doping on the physical, chemical and electrical properties of the In2O3 film are systematically investigated. The results show that B has large metal–oxide bond dissociation energy and high Lewis acid strength, which can suppress oxygen vacancy-/hydroxyl-related defects and alleviate dopant-induced carrier scattering, resulting in electrical performance improvement. The optimized In-B-O (10% B) TFTs based on SiO2/Si substrate demonstrate a mobility of ~8 cm2/(V s), an on/off current ratio of ~106 and a subthreshold swing of 0.86 V/dec. Furthermore, by introducing the water-processed high-K ZrO2 dielectric, the fully aqueous solution-grown In-B-O/ZrO2 TFTs exhibit excellent device performance, with a mobility of ~11 cm2/(V s), an on/off current of ~105, a subthreshold swing of 0.19 V/dec, a low operating voltage of 5 V and superior bias stress stability. Our research opens up new avenues for low-cost, large-area green oxide electronic devices with superior performance.