Acid Sites Research Articles

Boosting catalytic efficiency of nanostructured CuO-supported doped-CeO2 in oxidative coupling of benzyl amines to N-benzylidenebenzyl amines and benzimidazoles: impact of acidic and defect sites

This study presents the rational synthesis of Cu-supported doped-CeO2 catalysts designed for the oxidation of benzylamine, both in the absence and presence of 1,2-diaminobenzene. The catalysts were prepared using a two-step method and characterized by various techniques, including XRD, Raman spectroscopy, BET surface area analysis, NH3-TPD, pyridine-FTIR, H2-TPR, XPS, SEM, and TEM. Raman and XPS analyses confirmed the presence of oxygen vacancy sites, with CuO/CeO2-ZrO2 displaying the highest concentration of these sites. H2-TPR revealed strong metal-support interactions, while NH3-TPD indicated that CuO/CeO2-ZrO2 possessed the greatest number of acidic sites. The pyridine-FTIR results indicates both the acidic sites present on the catalyst surface. The Cu/CeZr sample exhibits the lowest Iu////ITotal ratio (0.0567) compared to the Cu/Ce (0.0843) and Cu/CeSi (0.0672) samples, indicating a higher number of Ce3+ species or a greater number of oxygen defect sites in the sample. The catalyst demonstrated excellent performance in converting benzylamine to imines and was also highly effective in the synthesis of benzimidazole from benzylamine and 1,2-diaminobenzene, broadening its application potential. The superior catalytic activity is attributed to the abundant oxygen vacancies, redox properties, strong metal-support interactions, and acidic sites. Furthermore, the CuO/CeO2-ZrO2 catalyst maintained its efficiency over five consecutive cycles, exhibiting robustness, high functional group tolerance, and reduced reaction times, making it a promising system for diverse catalytic applications.

Silicate-Confined Hydrogen on Nanoscale Zerovalent Iron for Efficient Defluorination Reactions.

Defluorination reactions are increasingly vital due to the extensive use of organofluorine compounds with robust carbon-fluorine (C-F) bonds; particularly, the efficient defluorination of widespread and persistent per- and polyfluoroalkyl substances under mild conditions is crucial due to their accumulation in the environment and human body. Herein, we demonstrate that surface-modified silicate of pronounced proton affinity can confine active hydrogen (•H) onto nanoscale zerovalent iron (nZVI) by withdrawing electrons from nZVI to react with bound protons, generating confined active hydrogen (•H*) for efficient defluorination under ambient conditions. The exposed silicon cation (Siσ+) of silicate functions as a Lewis acid site to activate the C-F bond by forming Siσ+...F--C and substantially lowers the energy barrier of nucleophilic •H* attack, thereby facilitating selective C-F hydrodefluorination and subsequent fluorine immobilization. In a column flow reactor, silicate-modified nZVI efficiently removes perfluorooctanoic acid (PFOA) of concentrations ranging from 0.24 to 24 μmol/L with 75-92% defluorination efficiencies, 8 times higher than those of nZVI, generating environmentally friendly alkyl carboxylic acids as the primary products. Besides PFOA, this novel nZVI also realizes deep defluorination of other organofluorine compounds, including perfluorooctanesulfonates and fluoroquinolones, demonstrating its superior defluorination potential.

Structure of Hydrothermally Stable Acid Sites and their Catalytic Role in P-Modified ZSM-5 Zeolite Revealed by Solid-State NMR Spectroscopy.

The ZSM-5 zeolite is the key active component in high-severity fluid catalytic cracking (FCC) catalysts and is routinely activated by phosphorus compounds in industrial production. To date, however, the detailed structure and function of the introduced phosphorus still remain ambiguous, which hampers the rational design of highly efficient catalysts. In this work, using advanced solid-state NMR techniques, we have quantitatively identified a total of seven types of P-containing complexes in P-modified ZSM-5 zeolite and clearly revealed their structure, location, and catalytic role. The experimental findings indicate that the introduced phosphorus can stabilize a portion of the aluminum atoms in the lattice by forming an energetically favorable framework-bound Si-OH-Al-O-P structure inside the zeolite channels, preserving more acidic bridging hydroxyl groups under the working conditions of the catalyst. Besides, two other types of hydrothermally stable phosphoric acid species (H3PO4 and H4P2O7) captured by neighboring silanol groups (H3PO4···HO-Si(SiO4)3 and H4P2O7···HO-Si(SiO4)3) are identified. For the FCC process, the framework-bound Si-OH-Al-O-P structure is proven to be the active site in the conversion of the FCC reactant, while the two phosphoric acid species can promote the yield of C2-C4 olefins.

Striking Improvement of N2 Selectivity in NH3 Oxidation Reaction on Fe2O3-Based Catalysts via SiO2 Doping.

The emission of NH3 has been reported to pose a serious threat to both human health and the environment. To efficiently eliminate NH3, catalysts for the selective catalytic oxidation of NH3 (NH3-SCO) have been intensively studied. Fe2O3-based catalysts were found to exhibit superior NH3 oxidation activity; however, the low N2 selectivity made it less attractive in practical applications. In this work, aimed at improving the N2 selectivity on Fe2O3-based catalysts, a simple SiO2 doping strategy was proposed. Although the NH3 oxidation activity showed almost no change on Fe2O3 after SiO2 doping, the N2 selectivity was significantly improved. Systematic characterizations revealed that SiO2 doping could increase the specific surface area of Fe2O3, and a strong interaction of Fe-O-Si was formed in Fe2O3-SiO2 mixed oxide catalysts. Furthermore, abundant Brønsted acid sites were formed on Fe2O3-SiO2 catalysts due to the facile hydrolysis of the Fe-O-Si structure into Si-OH and Fe-OH. Although SiO2 doping was found to weaken the redox ability of Fe2O3, the abundant Brønsted acid sites on Fe2O3-SiO2 catalysts could facilitate NH3 oxidation reaction through an internal SCR (i-SCR) pathway, thus achieving superior N2 selectivity. This work can provide new insights into constructing efficient NH3-SCO catalysts with high N2 selectivity.

Interfacial Oxygen Vacancy-Copper Pair Sites on Inverse CeO2/Cu Catalyst Enable Efficient CO2 Electroreduction to Ethanol in Acid.

Renewable electricity-driven electrochemical reduction of CO2 offers a promising route for production of high-value ethanol. However, the current state of this technology is hindered by low selectivity and productivity, primarily due to limited understanding of the atomic-level active sites involved in ethanol formation. Herein, we identify that the interfacial oxygen vacancy-neighboring Cu (Ov-Cu) pair sites are the active sites for CO2 electroreduction to ethanol. A linear correlation between the density of Ov-Cu pair sites and ethanol productivity is experimentally evidenced. Moreover, a high Faradaic efficiency of 48.5% and a partial current density of 344.0 mA cm-2 for ethanol production are achieved over the inverse CeO2/Cu catalyst with a high density of Ov-Cu pair sites in acid. Mechanistic studies that combine density functional theory calculations and spectroscopic techniques propose an Ov-involved mechanism where interfacial Ov sites directly activate and dissociate CO2 into *CO in a thermodynamically spontaneous manner, thus favoring the subsequent *CHO formation and asymmetric CHO-CO coupling. Besides, the asymmetric Ov-Cu pair sites could preferentially stabilize the *CH2CHOH intermediate, resulting in the favorable formation of ethanol over ethylene. Our findings provide new atomic-level insights into CO2 electroreduction to ethanol, paving the way for the rational design of future catalysts.

Functionalized covalent organic frameworks for catalytic conversion of biomass-derived xylan to furfural.

A direct synthesis strategy was employed to prepare functionalized COFs enriched with acidic sites, using various precursor monomers. The functionalized COFs were applied in the catalytic conversion of biomass-derived xylan to furfural in the liquid phase. The study further assessed the recyclability and reusability of these COFs, explored the relationship between their structural features and catalytic performance, and investigated the reaction mechanism underlying the COF-catalyzed conversion of xylan to furfural. The results revealed the successful solvothermal synthesis of four COFs (COF-LZU1, TP-PA, TB-DAB, and TP-DAB). Among them, TP-DAB exhibited the highest acidity, reaching 2.092mmol/g, due to its rich content of -OH and -COOH. Additionally, TP-DAB demonstrated a well-developed porous structure and excellent thermal stability, essential characteristics for a highly efficient solid acid catalyst. When employed as a catalyst in a mixed solvent system of H2O/THF at 160°C for 180min, TP-DAB achieved complete xylan conversion with furfural yield of 63.49%. Moreover, TP-DAB maintained good stability and reusability, being effective for at least five cycles. This study presents a novel and highly effective green catalyst for the sustainable conversion of biomass-derived sugars into furfural, providing new avenues for the efficient conversion of biomass into fine chemicals.

DFT investigation of the impact of inner-sphere water molecules on RE nitrate binding to internal pore and external surface of MCM-22.

The impact of inner-sphere water molecules on the binding of rare earth (RE) nitrates to MCM-22 aluminosilicates is analyzed. We used cluster models of MCM-22 to investigate the binding phenomena through localized-basis density functional theory (DFT) calculations. We also conducted plane-wave DFT calculations for a few selected binding configurations using the entire periodic MCM-22 unit cell to check for consistency. Two different MCM-22 cluster models are developed to represent an internal pore and an external surface. Starting with pure silica MCM-22, we substituted one Si with Al and added a H atom on the O bridging the Si and Al to create a Brønsted acid site (BAS), Si-{OH}-Al. Specifically, we investigated the binding of two RE nitrate aqua complexes, [X(NO3)3(H2O)n] where n = 4 (3) for X = Nd (Yb) via the reaction X(NO3)3(H2O)n + Si-{OH}-Al → Si-{OX(NO3)2(H2O)n}-Al + HNO3 at BASs, and via an analogous reaction at silanol sites, Si-{OH}. The above analysis just includes the inner coordination sphere H2O. Actually, for the Nd (Yb) complex, after binding at the T1 and T2 sites (T1 site) within the internal pore, one of the H2O molecules leaves this inner sphere. The binding strength at BASs and silanol sites is calculated from the energy change during the above reactions. One finds that Nd complexes prefer binding at the internal pore, while Yb complexes have a comparable binding preference both at the internal pore and external surface. The cluster calculations show good agreement with periodic calculations, implying that the cluster models are suitable for binding studies. Compared to the binding of non-hydrated RE nitrates, the explicit H2O molecules have a minimal impact on overall binding energy trends, but they do increase individual binding energy values. This study also demonstrated the stronger binding affinity of BASs over silanol sites.

Characterization of zeolitic imidazole framework (ZIF-8) catalyst for potential biodiesel production from waste cooking oil (WCO)

Heteropoly acids (HPAs) catalysts prove effective in waste cooking oil biodiesel production, considering their high density of Brønsted acidic sites, exhibit significant resilience to elevated levels of free fatty acid (FFA) and moisture content. However, the separation of HPA catalysts after biodiesel production is challenging due to their homogeneous catalytic nature. This study aims to develop magnetic vanadium-substituted HPAbased ZIF-8 composites to create a catalyst for biodiesel production from WCO that is more efficient and easier to separate. In this work, a range of analytical methods was utilized to characterize the catalyst, such as Fouriertransform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), highresolution transmission electron microscopy (HRTEM), and a vibrating sample magnetometer (VSM). The successful incorporation of HPA acid into the magnetite ZIF-8 nanocomposite was indicated by prominent bands in the FTIR analysis, and this formation was further validated by EDX analysis. The VSM results also revealed that the nanocomposite has good magnetic responsiveness, facilitating catalyst separation and recycling. The magnetic ZIF-8 composites functionalized with H6PV3MoW8O40 demonstrated significant potential for sustainable biodiesel production from WCO.

Assignment of IR spectra of ethanol at Brønsted sites of H-ZSM-5 to monomer adsorption using a Fermi resonance model.

Understanding how alcohol molecules interact with the Brønsted acid sites (BAS) of zeolites is a prerequisite to the design of zeolite catalysts and catalytic processes. Here, we report IR spectra for the adsorption of ethanol on a highly crystalline sample of H-ZSM-5 zeolites exposed to ethanol gas at increasing pressure. We use density functional theory in combination with a FERMI resonance model to assign the measured spectra to a single adsorbed ethanol molecule per BAS. Specifically, we assign the bands at 2450 cm-1 and 1670 cm-1 to a FERMI resonance between the fundamental (Z)O-H stretching band of a single-ethanol-loaded BAS and the first overtone of the (Z)O-H out-of-plane bending. We conclude that adsorbed dimers do not contribute in a noticeable way up to a concentration of almost one ethanol molecule per BAS site. We further show that hybrid functionals (B3LYP) are required to get a close match between the predicted and experimental spectra, whereas commonly used generalized gradient type functionals such as PBE incorrectly describe the potential energy surface. They overestimate the redshift of the OH stretching band on hydrogen bond formation which results in an erroneous assignment of the IR bands.

Construction of mesoporous lanthanum oxide and supported Co-N catalysts for high-efficient synthesis of furfuryl alcohol

The mesoporous lanthanum oxide materials were designed via the solvent evaporation-induced self-assembly method, and Co-N/La2O3 catalysts were prepared to evaluate performance in the reaction of furfural hydrogenation. The characterization analyses proved that Co/La2O3 and Co-2N/La2O3 catalysts had high specific surface area and open mesoporous channels (> 20nm), moreover doped nitrogen not only promoted the distribution of Co species and regulated size of metallic Co nanoparticles (~17.37nm), but also increased electron-rich Co0 (57.8%) and CoNx sites. The highest conversion (>99.9%) and selectivity (98.6%) were achieved on Co-2N/La2O3 catalyst under 140oC, 1.5MPa H2 and 3h, which was attributed to the combination of Co0 and CoNx sites enhancing the dissociation of H2 and adsorption of furfural by DFT calculation, meanwhile the doped nitrogen brought more acidic sites (~ 2.81mmol NH3/g) of Co-2N/La2O3 catalyst. Additionally, the Co-2N/La2O3 catalyst showed the stable catalytic activity for 5 recycles and excellent universality for hydrogenation reactions of biomass platform molecules.

Selective photocatalytic glucaric acid production from TEMPO-mediated glucose oxidation on alkalized carbon nitride

Biomass photorefining is a promising approach for sustainable clean energy and high-value chemical production. However, selectively converting glucose into glucaric acid, the most valuable derivative, still poses a significant challenge due to the difficulty in transforming the terminal hydroxyl group into a carboxy group. Here, we demonstrate that highly selective glucose photorefining into glucaric acid can be achieved by synergistically coupling alkalizing modification of polymeric carbon nitride (CN) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediation, which promotes the oxidation of the primary alcohol group attached to the C6 site of gluconic acid. Density functional theory (DFT) calculations affirm the enhanced performance of modified CN in glucaric acid production. When medicated with TEMPO, the optimized photocatalysis achieves ~100% glucose conversion and >30% glucaric acid yield, setting a record for photocatalytic glucaric acid production. This work showcases the significance of combining photocatalyst modification and redox mediation for inspiring high-efficiency photocatalysis systems for biomass photorefining.

Effect of catalyst synthesis of bimetallic nickel-cobalt supported iron-based catalysts on converting palm kernel oil into bio-jet fuel via deoxygenation reaction.

Bimetallic nickel-cobalt supported magnetite catalyst has been synthesized using varies techniques such as hydrothermal reactor (R), wet-impregnation (W) and co-precipitation (C) approaches in order to study the effect of preparation methods on the catalytic efficiency of NiCo/Fe3O4 catalyst in transforming the PKO into bio-jet fuel via deoxygenation (DO) reaction. Noted, co-precipitation method rendered greater changes on textural morphological changes, with NiCo/Fe3O4 (C) specific surface area were 157 m2/g followed by Fe3O4 (25 m2/g), NiCo/Fe3O4 (W) (7 m2/g) and NiCo/Fe3O4 (R) (2 m2/g ), respectively. Indeed, NiCo/Fe3O4 (C) showed highest weak+strong acidic sites (15,038.1μmol/g). The efficiency of the DO followed NiCo/Fe3O4 (C) > NiCo/Fe3O4 (W) > NiCo/Fe3O4 (R) > Fe3O4, whereby the highest hydrocarbon yield was 90% and 21% is the lowest. Noteworthy to mention, all of the liquid product exhibited selectivity of over 90% towards the kerosene range. The deoxygenation reaction was optimized using the most effective catalyst, NiCo/Fe3O4 (C) catalyst, through the OVAT technique. The results showed that the optimal conditions were achieved at a temperature of 350 °C within 3 h by using 5 wt% of catalyst loading. The reusability and stability of the NiCo/Fe3O4 (C) catalyst were examined, revealing that the catalyst may be reused for up to 8 runs. During these runs, the catalyst achieved a hydrocarbon yield of up to 55% and a BJF selectivity of over 95%. The decrease in the yield and selectivity was attributed to coking, specifically in the form of graphitic carbon.

The lead-free halide perovskite/UiO-66 with dual Lewis acid sites for efficient photocatalytic NO removal

The lead-free halide perovskite/UiO-66 with dual Lewis acid sites for efficient photocatalytic NO removal

Selective production of lactitol from lactose using the Lewis acid site of MOF catalysts without exogenous hydrogen

Lactose is found in the residue obtained from the separation of high value-added proteins from whey and can be used to solve the problem of waste generated by the agri-food...

Increasing Lewis acidic sites and promoting electron transfer of Mn2O3 by C-hybridization to improve the peroxymonosulfate activation for Bisphenol A degradation.

The surface acidity and electron transfer performance of manganese oxide catalysts significantly affected its performance of peroxymonosulfate (PMS) activation. In this work, Mn2O3 catalyst was prepared by the precipitation method. The C-hybridization Mn2O3-D (Mn2O3-D) catalyst prepared with disodium oxalate as a precipitant had more Mn3+ and Lewis acid sites on the surface, promoting the binding of PMS on the catalyst surface, which exhibited the best performance in inducing PMS activation to degrade bisphenol A (BPA). Quenching experiments and in situ electron spin resonance (ESR) results indicated that radicals and singlet oxygen were not the main reactive oxygen species (ROSs) in the advanced oxidation process. The chemical probe experiment of phenylmethylsulfone (PMSO) showed that the ≡Mn-OOSO3- metastable intermediate formed by the binding of PMS with Mn sites on the catalyst surface was important active species for contaminants degradation. Contaminants combined with intermediates on the catalyst surface to form the electron transfer channels, which were directly degraded through oxygen-atom-transfer pathway and single-electron-transfer pathway. And the hybridization of C promoted the electron transfer during this process. This work further elucidated the reaction mechanism of PMS activation by manganese oxides, and proposed new ideas for the design of MnOx catalysts for efficient activation of PMS.

Heterogeneously catalyzed lignin depolymerisation in continuous flow: Assessing feedstock pretreatment and catalyst deactivation

Due to the large-scale production of lignin, continuous flow reactors are preferred for its further valorization through depolymerization. However, there is limited literature on their use in lignin depolymerization, especially concerning feedstock pretreatments and catalyst deactivation. Therefore, in this study, the impact of a batch solvolysis pretreatment on the mild reductive catalytic depolymerization of Soda lignin, with and without a Pd catalyst, is evaluated and catalyst deactivation is systematically assessed. The Pd catalyst substantially enhances Mw reductions, e.g., without feedstock pretreatment, it increases the Mw reduction from approximately 70 % to 90 % at low time-on-streams (T.O.S.). It also yields products with more functionalities from the untreated feedstock, i.e., at low T.O.S., the phenolic and aliphatic OH content increases from 4.21 to 4.89 mmol/g and 2.30 to 3.72 mmol/g, respectively. The differences are less pronounced with the pretreated feedstock due to condensation of the lignin structure. Furthermore, the high space–time that can be achieved at a short residence time negates the need for a pretreatment. Regarding deactivation, no significant Pd leaching or poisoning by S or Na was observed. The Pd nanoparticles grew from about 8 nm to 20 nm (pretreated feed) and 17 nm (untreated feed), which is likely due to the phase change from PdO to metallic Pd rather than deactivation through sintering. The support material changed from γ-Al2O3 to boehmite, leading to the loss of acid sites and morphological changes. While some indications for small amounts of coke deposits were observed, they could not be differentiated from interstitial water within the boehmite structure. Thus, the main cause for deactivation of the Pd catalyst was identified to be the hydrothermal instability of the alumina support.

Cs-Doped BCCF perovskite with enhanced surface proton acid sites for high-performance R-PCECs

Cs-Doped BCCF perovskite with enhanced surface proton acid sites for high-performance R-PCECs

Bifunctional chlorhexidine-based covalent organic polymers for CO2 capture and conversion without a co-catalyst.

Two new cobalt/zinc-coordinated bifunctional covalent organic polymers (COP-Co and COP-Zn) based on chlorhexidine are prepared as heterogeneous catalysts for carbon dioxide (CO2) conversion. Due to the Cl- nucleophile and cobalt/zinc Lewis acid sites, COP-Co and COP-Zn can efficiently convert CO2 and epoxides into cyclic carbonates under mild conditions without a co-catalyst.

TRANSACETALIZATION OF GLYCEROL WITH 2,2-DIMETHOXYPROPANE USING ZEOLITES UNDER MILD CONDITIONS

The acetalization or transacetalization reactions produce acetals or ketals used in cosmetics, food, pharmaceuticals, and fuel additives. Besides synthesizing several products, the acetalization or transacetalization reactions can protect carbonyl groups, 1,2 and 1,3-diols in multi-step synthesis. This work studies the transacetalization of glycerol with 2,2-dimethoxypropane (2,2-DMP) to yield 2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane (solketal) by catalyzing with USY, HBeta, and HZSM-5 zeolites under cleaner and mild reaction conditions (reaction temperature: 25 °C; 2,2-DMP/glycerol molar ratio: 1; reaction time: 1 h; and catalyst concentration: 0.05-5 wt.%). For catalyst concentration equal to or greater than 2,5 wt.%, glycerol conversion stabilized near 96-97% for the three zeolites tested. The product selectivities also assume a constant value, close to 97% for solketal. For the lowest concentration of catalyst, the influence of the zeolite properties on the glycerol conversion, such as acidity and the strong/weak acid site ratio, was observed. Concerning the selectivity of the products, the results suggested that it can be based on zeolite topology, active sites at the particle surface, and the relative amount of micro and mesoporous HBeta, the most selective catalyst for acetals production, was reused at least once without loss of activity and selectivity.

Quantifying Protein-Nucleic Acid Interactions for Engineering Useful CRISPR-Cas9 Genome-Editing Variants.

Numerous high-specificity Cas9 variants have been engineered for precision genome editing. These variants typically harbor multiple mutations designed to alter the Cas9-single guide RNA (sgRNA)-DNA complex interactions for reduced off-target cleavage. By dissecting the contributions of individual mutations, we attempt to derive principles for designing high-specificity Cas9 variants. Here, we computationally modeled the specificity harnessing mutations of the widely used Cas9 isolated from Streptococcus pyogenes (SpCas9) and investigated their individual mutational effects. We quantified the mutational effects in terms of energy and contact changes by comparing the wild-type and mutant structures. We found that these mutations disrupt the protein-protein or protein-DNA contacts within the Cas9-sgRNA-DNA complex. We also identified additional impacted amino acid sites via energy changes that constitute the structural microenvironment encompassing the focal mutation, giving insights into how the mutations contribute to the high-specificity phenotype of SpCas9. Our method outlines a strategy to evaluate mutational effects that can facilitate rational design for Cas9 optimization.