Lewis Acid Ti Research Articles

Comparison of Brønsted Acidic Silanol Nests and Lewis Acidic Metal Sites in Ti-Beta Zeolites for Conversion of Butenes

The Lewis acidic framework Ti sites in Ti-Beta and Si-Beta catalysts were compared by FT-IR and NMR characterization methods before they were applied to the conversion of four butenes. The results showed that Si-Beta has fewer Lewis acid sites and abundant weak Brønsted acidic silanol nests, which play an important role in conversions between n-butene, cis-2-butene, and trans-2-butene. The conversions for these butenes over Si-Beta were always higher than those over a series of Ti-Beta catalysts with gradient-varied Lewis acidic framework Ti sites and silanols. This is because isobutene can only oligomerize, which requires stronger acidity, so its conversion over Si-Beta was lower than those over Ti-Beta zeolites. For a series of Ti-Beta catalysts with different abundances of Lewis acidic Ti sites, the more Lewis acid sites it had, the higher the conversions for the four butenes.

Boosting electrochemical ammonia synthesis via dynamic nitrogen carriers coupled with electron-rich Lewis acidic sites on TiO2−x nanofiber

In light of the high energy consumption and substantial carbon emissions associated with traditional NH3 production based on the Haber–Bosch process, the aqueous electrochemical nitrogen reduction reaction (NRR) offers a clean and sustainable alternative production route. Nevertheless, activating the NN bonds at room temperature is challenging due to the high bond energy, severely hindering the development and commercialization of the electrochemical NRR. Herein, we report a synergistic strategy for achieving efficient N2 activation at ambient conditions that combines electrolyte engineering with catalytic site-modulated TiO2−x nanofiber electrocatalysts. The synthesized TiO2−x nanofiber electrocatalysts contained abundant intrinsic oxygen vacancies and were further modified with hydroxyl groups to create electron-rich Lewis acidic Ti sites. Additionally, BF3 was engineered into the electrolyte microenvironment, and it could form adducts with N2, serving as a dynamic carrier for N2 transport. The electron-rich Lewis acidic sites and the dynamic carriers exerted a ‘pull–pull’ effect on N2, thereby weakening the NN bonds. Through electrochemical performance evaluation, the designed electrocatalytic scheme achieved an NH3 yield of ∼57.15 μg h−1 mg−1 and a Faradaic efficiency of ∼15.14 %. We anticipate that this methodology will provide new insights into the development of electrochemical ammonia synthesis, particularly in relation to multifaceted design.

Design of frustrated Lewis pair in defective TiO2 for photocatalytic non-oxidative methane coupling

The efficient C–H polarization is the prerequisite for the low-temperature photocatalytic CH 4 conversion, which is restricted by the poor C–H stretching ability of short-distance adjacent lattice atoms. Here, a frustrated Lewis pair (FLP) composed of doped metal in TiO 2 as Lewis acid (LA) and neighboring Ti–OH as Lewis base (LB) with a long distance (0.31–0.37 nm) was designed through DFT calculation and fabricated by hydrogenation treatment of metal-doped TiO 2 –SiO 2 with macroporous-mesoporous structure. Benefitting from the long LA-LB distance and matched acid-base intensity, hydrogenated Ga-doped composite achieves superior C–H stretching with a high photocatalytic CH 4 conversion rate (139 μmol g −1 h −1 ). The photo-irradiation causes electron excitation from Ga to Ti–OH according to the time-dependent DFT calculation and in situ EPR analysis, which promotes the formation and coupling of ·CH 3 . This work provides a key underpinning for regulating the characteristics of FLP for C–H activation and C–C coupling via light irradiation. • Frustrated Lewis pair (FLP) in defective TiO 2 was designed by DFT calculation • 139 μmol g −1 h −1 CH 4 conversion rate in NOCM is achieved under irradiation • FLP with long distance and suitable Lewis intensity is conducive to C–H stretching • Photo-irradiation promotes the formation and coupling of CH 3 The serious greenhouse effect caused by the massive use of fossil energy is an inevitable environmental problem in today’s world. In particular, the greenhouse effect of methane is 25 times that of CO 2 . However, the traditional methane conversion process consumes a great deal of energy. This study emphasizes the efficient conversion of methane driven by green and renewable solar energy under mild conditions. Its fundamental principle is to generate a charge by using light in semiconductors. The efficient conversion of methane is attributed to the unique active sites designed on TiO 2 , such as magnets, which attract C and H, respectively, effectively break the C–H bond, and generate high-value products under the action of a photogenerated charge. This research does not require the high temperature and high pressure of traditional industry and is of great significance to realize energy development and environmental protection that is beneficial to all. Long LA–LB distance and strong acid and base intensities promote the stretching of the C–H bond. The effect of light irradiation on improving the intensities of LA Ga and LB Ti–OH was confirmed based on the electron transition from LA to LB, which is the key to forming methyl and hydrogen radicals for the further coupling production of C 2 H 6 .

Ketonization of Propionic Acid on Lewis Acidic Zr-Beta Zeolite with Improved Stability and Selectivity

Ketonization reactions provide a feasible approach to remove oxygen and increase carbon chain length for conversion of biomass derived carboxylic acids. However, the reaction suffers from fast catalyst deactivation and low ketone selectivity. In this work, Lewis acidic heteroatom Ti-, Zr-, Ce-, and Sn-Beta zeolites were prepared using a two-step post-synthesis method and applied in vapor phase ketonization of propionic acid at 350 °C. Among these zeolites, Zr-Beta shows both the highest activity and selectivity. Characterizations indicate that Zr prefers the vacant tetrahedral site when the Zr content is 96% at a space time of 2 h. The amount of coke deposition on Zr-Beta is about 1/3 of that on H-Beta, and the structure of Zr-Beta is preserved after 60 h of reaction. The results of this work indicate that Zr-Beta zeolite is a promising catalyst for ketonization with good stability and high selectivity toward ketone.

Tighter Confinement Increases Selectivity of d-Glucose Isomerization Toward l-Sorbose in Titanium Zeolites.

Aqueous-phase isomerization of d-glucose to d-fructose and l-sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks. Glucose isomerization rates (per Ti, 373 K) are undetectable when Ti sites are confined within mesoporous voids (Ti-MCM-41, TiO2 -SiO2 ) and increase to detectable values when Ti sites are confined within the smaller 12-membered ring (12-MR) micropores of Ti-Beta. Isomerization rates decrease to lower values (by ≈20×) with further decreases in micropore size as Ti sites are confined within 10-MR pores (Ti-MFI, Ti-CON), likely because of intrapore reactant diffusion restrictions, and reach undetectable values within the 8-MR pores of Ti-CHA as size exclusion prevents glucose from accessing active sites. Remarkably, the selectivity toward l-sorbose over d-fructose increases systematically as spatial constraints around Ti sites become tighter, and is >10 on Ti-MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.

Tighter Confinement Increases Selectivity of d‐Glucose Isomerization Toward l‐Sorbose in Titanium Zeolites

AbstractAqueous‐phase isomerization of d‐glucose to d‐fructose and l‐sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks. Glucose isomerization rates (per Ti, 373 K) are undetectable when Ti sites are confined within mesoporous voids (Ti‐MCM‐41, TiO2‐SiO2) and increase to detectable values when Ti sites are confined within the smaller 12‐membered ring (12‐MR) micropores of Ti‐Beta. Isomerization rates decrease to lower values (by ≈20×) with further decreases in micropore size as Ti sites are confined within 10‐MR pores (Ti‐MFI, Ti‐CON), likely because of intrapore reactant diffusion restrictions, and reach undetectable values within the 8‐MR pores of Ti‐CHA as size exclusion prevents glucose from accessing active sites. Remarkably, the selectivity toward l‐sorbose over d‐fructose increases systematically as spatial constraints around Ti sites become tighter, and is >10 on Ti‐MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.

2,3-Diaminopropanols Obtained from d-Serine as Intermediates in the Synthesis of Protected 2,3-l-Diaminopropanoic Acid (l-Dap) Methyl Esters.

A synthetic strategy for the preparation of two orthogonally protected methyl esters of the non-proteinogenic amino acid 2,3-l-diaminopropanoic acid (l-Dap) was developed. In these structures, the base-labile protecting group 9-fluorenylmethyloxycarbonyl (Fmoc) was paired to the p-toluensulfonyl (tosyl, Ts) or acid-labile tert-butyloxycarbonyl (Boc) moieties. The synthetic approach to protected l-Dap methyl esters uses appropriately masked 2,3-diaminopropanols, which are obtained via reductive amination of an aldehyde prepared from the commercial amino acid Nα-Fmoc-O-tert-butyl-d-serine, used as the starting material. Reductive amination is carried out with primary amines and sulfonamides, and the process is assisted by the Lewis acid Ti(OiPr)4. The required carboxyl group is installed by oxidizing the alcoholic function of 2,3-diaminopropanols bearing the tosyl or benzyl protecting group on the 3-NH2 site. The procedure can easily be applied using the crude product obtained after each step, minimizing the need for chromatographic purifications. Chirality of the carbon atom of the starting d-serine template is preserved throughout all synthetic steps.

Regulatory norms driving the demand for bio-based lubricants & increasing adoption in the automotive industry: Global bio-lubricants market to 2024

The application of three-dimensional, mesoporous titanosilicates, Ti-SBA-12 and Ti-SBA-16, as reusable solid acid catalysts for producing biodiesel and biolubricants is reported for the first time. Biodiesel was prepared by esterification of oleic acid with monohydric methanol and biolubricants by esterification with polyhydric alcohols (glycerol, trimethylolpropane, neopentyl glycol and pentaerythritol). High catalytic activity of these catalysts is attributed to the presence of dispersed Lewis acidic Ti sites and mesoporosity in the structure. Biodiesel yields as high as 90 mol% were obtained at 443 K. Surface hydrophobicity of the catalyst has also a predominant role, especially, in reactions involving polyhydric alcohols. In those reactions, Ti-SBA-16 showed higher catalytic activity than Ti-SBA-12.

Cooperatively Catalyzed Henry Reaction through Directed Metal‐Chitosan Interactions

AbstractThe main focus of the present work is on identifying the critical parameters involved in inducing acid‐base cooperative catalytic interaction in hybrid metal‐polymer materials. To makes these hybrid materials Ti or Sn acid sites are grafted to the hydroxy groups of chitosan (CS). The cooperative catalytic interactions between the Ti/Sn and the primary amine sites on CS are tested for the nitro‐aldol condensation of nitromethane with 4‐nitro‐benzaldehyde. The observed results show that the synthesis conditions have a pronounce effect on the 3D arrangement of the hybrid materials, which in turn affect the reaction activity and selectivity. The catalysts are characterized using N2‐pysisorption, XRD, FTIR, XPS and HRTEM in conjunctions with reaction kinetic analysis. Using the combined results, a reaction mechanism is proposed, which emphasizes the effect of proximity and orientation of the Lewis acid (Ti or Sn) and base (NH2) active sites on cooperative catalysis in the Henry reaction.

Theoretical investigation of dissociative and non-dissociative acetic-acid on TiO2-B surfaces

Interaction between acetic acid and TiO2-B surfaces are investigated by first-principles calculations based on density functional theory (DFT). We found non-dissociative adsorption configurations are energetic favorable. We also found that the favorable adsorption configuration is not only determined by the matched interface structures, but also by the electrostatic potential distribution on the Lewis acid (Ti*) and basic (O*) sites. Although the TiO2-B (100) surface exhibits the highest surface energy among the all low-index clean surface, the TiO2-B (010) surface displaces a stronger oscillation of electrostatic potential between the Lewis acid and basic sites which provides an extra driving force to promote acetic acid dissociation and leads to larger adsorption energy. Such strong oscillatory electrostatic potential provides an extra driving force to promote acetic acid dissociation, which leads to larger adsorption energy. Our results provide a detailed insight into the adsorption structure and mechanism and further shed light on the photochemistry on surface at the molecular level.

Dominant Role of Entropy in Stabilizing Sugar Isomerization Transition States within Hydrophobic Zeolite Pores

Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than within hydrophilic micropores; however, relative contributions of competitive water adsorption at active sites and preferential stabilization of isomerization transition states have remained unclear. Here, we employ a suite of experimental and theoretical techniques to elucidate the effects of coadsorbed water on glucose isomerization reaction coordinate free energy landscapes. Transmission IR spectra provide evidence that water forms extended hydrogen-bonding networks within hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase glucose isomerization turnover rates measured on Ti-Beta zeolites transition from first-order to zero-order dependence on glucose thermodynamic activity, as Lewis acidic Ti sites transition from water-covered to glucose-covered, consistent with intermediates identified from modulation excitation spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically higher (by 3-12×, 368-383 K) when Ti sites are confined within hydrophobic micropores. Apparent activation enthalpies and entropies reveal that glucose and water competitive adsorption at Ti sites depend weakly on confining environment polarity, while Gibbs free energies of hydride-shift isomerization transition states are lower when confined within hydrophobic micropores. DFT calculations suggest that interactions between intraporous water and isomerization transition states increase effective transition state sizes through second-shell solvation spheres, reducing primary solvation sphere flexibility. These findings clarify the effects of hydrophobic pockets on the stability of coadsorbed water and isomerization transition states and suggest design strategies that modify micropore polarity to influence turnover rates in liquid water.

A Density Functional Theory Study of the Mechanism of Direct Glucose Dehydration to 5‐Hydroxymethylfurfural on Anatase Titania

AbstractA new mechanism for glucose dehydration to HMF without the intermediate isomerization to fructose is discussed for surface models of anatase TiO2 using periodic density functional theory calculations. Activation of the glucose at glucose's C3‐OH position by titania starts the reaction resulting in adsorbed 3‐deoxyglucosone. The mechanistic study reveals two possible pathways for the acyclic form of glucose. Comparison of different surface models shows that the presence of tetrahedral Ti4+ species on a defective TiO2(101) anatase surface is essential for explaining the activity. Synergy between a strong Lewis acidic Ti site and a vicinal basic oxygen site of a TiOH group is essential to establish the direct conversion of glucose to HMF. IR spectroscopy of adsorbed chloroform in the presence of water confirms the presence of water tolerant Lewis acid sites in close proximity to basic sites. In conventional glucose dehydration, Lewis acid sites and proton donors act as catalytic sites for efficient sequential isomerization‐dehydration to HMF, while direct dehydration to HMF requires the cooperation of Lewis acid‐base pairs.

Three-dimensional, mesoporous titanosilicates as catalysts for producing biodiesel and biolubricants

The application of three-dimensional, mesoporous titanosilicates, Ti-SBA-12 and Ti-SBA-16, as reusable solid acid catalysts for producing biodiesel and biolubricants is reported for the first time. Biodiesel was prepared by esterification of oleic acid with monohydric methanol and biolubricants by esterification with polyhydric alcohols (glycerol, trimethylolpropane, neopentyl glycol and pentaerythritol). High catalytic activity of these catalysts is attributed to the presence of dispersed Lewis acidic Ti sites and mesoporosity in the structure. Biodiesel yields as high as 90mol% were obtained at 443K. Surface hydrophobicity of the catalyst has also a predominant role, especially, in reactions involving polyhydric alcohols. In those reactions, Ti-SBA-16 showed higher catalytic activity than Ti-SBA-12.

NHC-Catalyzed Addition of Homoenolate Equivalents to Unsaturated Keto Esters

A N-heterocyclic carbene catalyzed addition of homoenolate equivalents derived from enals 1 to unsaturated β,γ-unsaturated α-keto ­esters 2 has been reported. Lewis acidic Ti(Oi-Pr)4 was compatible with NHC catalysis and crucial for the formation of the desired products. N-heterocyclic carbene precatalyst 3 delivered substituted cyclopentanes 4 possessing four contiguous stereocenters in moderate to good yields with high diastereoselectivity and excellent enantio­selectivity.

1,4-Diazepanes derived from ( S)-serine – Homopiperazines with improved σ 1 (sigma) receptor affinity and selectivity

Starting from the proteinogenic amino acid ( S)-serine chiral non-racemic 1,4-diazepanes 4 with a hydroxymethyl residue in position 2 are synthesized and pharmacologically evaluated. The key step in the synthesis is the formation of the bicyclic system 8 by consecutive nucleophilic substitution of the chloropropionamide 7 with primary amines and intramolecular aminolysis. Both reaction steps require catalysis with the Lewis acid Ti(O- iPr) 4. Homologation of the piperazine to the 1,4-diazepane ring results in a remarkable improvement of σ 1 receptor affinity and σ 1/σ 2 selectivity. The 1,4-dibenzyl derivative 4a interacts with a K i value of 7.4 nM with σ 1 receptors and shows a 53-fold selectivity for σ 1 receptors over σ 2 receptors.

A theoretical analysis of the Ti(IV)-catalyzed glycoaminocyanation reaction

The chelation of the Lewis acid Ti(O iPr) 4 provides the necessary conformational control to induce π-facial selectivity on the cyanation process. The steric hindrance imposed by the α-substituent restrains the addition for l-amino acid derivatives.

Spectroscopic and catalytic properties of SBA-15 molecular sieves functionalized with acidic and basic moieties

SBA-15, grafted with Lewis acidic Ti 4+ ions and basic amines (imidazole, guanine or adenine) exhibited high catalytic activity in the cycloaddition of CO 2 to epoxides yielding selectively cyclic carbonate precursors of polycarbonates. In the second class of materials, the surface of SBA-15 was grafted with thiol or sulfonic acid functionalities. Mn(Salen)Cl was, then, covalently bound to these acidic groups. The resulting catalysts show enhanced chemo-, regio- and stereo-selective catalytic activity in the epoxidation of terpenes such as R-(+)-limonene, α-pinene and Δ-carene with molecular oxygen. The acid–base properties of these modified SBA-15 materials were investigated by temperature-programmed desorption and diffuse-reflectance infrared Fourier transform spectroscopy. Ammonia and pyridine were used as probe molecules for acidic sites and CO 2 for basic sites. Acidic Ti sites activated epoxides and the grafted basic amines activated CO 2. The simultaneous presence of both acidic and basic sites on amine/adenine-functionalized Ti-SBA-15 enabled synergistic enhancement in catalytic activity in the cycloaddition reaction of CO 2 to epoxides. When Mn(Salen)Cl was immobilized on thiol or sulfonic acid-functionalized SBA-15, the oxidation state of Mn decreased from +3 to +2. The organo-acidic functional groups (thiol or sulfonic acid) axially coordinated to Mn ions modified the electronic structure of Mn ions enhancing the activity and selectivity of Mn(Salen) complexes in aerial oxidation reactions.

Dual Lewis acid - Lewis base activation in enantioselective additions to aldehydes

Abstract Reaction of benzaldehyde with ethyl cyanoformate in the presence of Lewis acidic Ti(IV) complexes of bispyridylamide or salen ligands and Lewis basic amines affords the O-alkoxycarbonylated cyanohydrin. In the presence of the salen-based catalytic system, acetyl cyanide can also be added to benzaldehyde, providing a highly enantioselective direct route to the O-acetylated cyanohydrin.

CO 2 activation and synthesis of cyclic carbonates and alkyl/aryl carbamates over adenine-modified Ti-SBA-15 solid catalysts

Cyclic carbonates were synthesized by the cycloaddition of CO 2 to epoxides (epichlorohydrin, propene oxide, and styrene oxide), and carbamates were synthesized by reaction of alkyl or aryl amines, CO 2, and n-butyl bromide. Solvents and cocatalysts/promoters like N,N-dimethylaminopyridine (DMAP) and quaternary ammonium salts, which are usually essential with conventional catalyst systems, could be avoided with the use of adenine-modified Ti-SBA-15 catalysts. The catalysts were reused in several recycle experiments. The structural and textural properties of the catalysts were determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), N 2 adsorption, and Fourier transform infrared (FTIR) and diffuse reflectance ultraviolet–visible (UV–vis DRS) techniques. Acid–base properties of the solid catalysts were investigated by temperature-programmed desorption (TPD) of NH 3 and CO 2 and DRIFTIR spectroscopy of adsorbed pyridine and CO 2 techniques. The studies reveal that the Ti 4+ ions increase catalytic activity by enhancing the adsorption of the epoxide and alkyl or aryl amine substrates. CO 2 molecules are activated at the basic nitrogen groups of adenine. Increasing the surface concentrations of either CO 2 (by anchoring basic molecules like adenine or increasing the partial pressure of CO 2) or epoxides and alkyl or aryl amines (by increasing the concentration of Lewis acidic Ti 4+ ions) enhances the catalytic activity. CO 2 molecules activated at the covalently anchored adenine sites react with epoxide/amines adsorbed on the silica surface to form carbonates/carbamates.

The Synthesis and Reactivity of Group 4 Zwitterionic Complexes of the Type Mt+CH2AlCl3−: One‐Component Stereoselective Polymerization and Oligomerization Catalysts for Olefins and Acetylenes

AbstractA reinvestigation of the interaction of TiCl4 with 2 equiv. of Me3Al in toluene between −78 °C and 25 °C over 24 h has now established that the ultimate black product obtained is an associated zwitterion of the type [Ti+−CH2−AlCl3−]n, supported by multinuclear NMR spectroscopy and mass spectrometric and gasometric analyses of the gases evolved (CH4, H2) upon its protolysis. Chemical reactions of the zwitterion have corroborated specific aspects of its proposed structure: 1) its methylene character, by its transformation of benzophenone into 1,1‐diphenylethylene; 2) its divalent titanium content, by the substantial reductive dimerization of benzophenone to tetraphenylethylene, and 3) its Lewis acidic Ti center, by its catalytic isomerization of trans‐stilbene oxide to 1,1‐diphenylacetaldehyde. Similar individual reactions of ZrCl4 or HfCl4 with Me3Al have led to the analogous zwitterions [Zr+−CH2−AlCl3−]n and [Hf+−CH2−AlCl3−]n, respectively. These zwitterions of Ti, Zr and Hf have been proven to be capable of the cyclotrimerization and/or polymerization of acetylenes with varying facility, as evidenced by their catalytic action on 1‐hexyne, phenylacetylene, di‐n‐butylacetylene, and diphenylacetylene. Furthermore, all three zwitterions were able to polymerize ethylene, without any added cocatalyst, with an activity following the order Zr > Ti > Hf. The Ti and Zr zwitterions effected the stereoselective polymerization of propylene to yield 50% of isotactic polymer, and all three catalysts induced the polymerization of 1‐hexene to yield 85% (Zr, Hf) or 100% (Ti) of isotactic polymer. These oligomerizations and stereoselective polymerizations of acetylenes and olefins can be rationalized through a model for the active site resembling a three‐membered metallacyclopropa(e)nium ion intermediate formed from the attack of the Group 4 metal zwitterion on the unsaturated hydrocarbon. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)