Zn-Al Mixed Oxides Research Articles

Palladium nanoparticles (Pd NPs) immobilized over polydopamine-functionalized Zn-Al mixed metal oxide as a novel nanocatalyst for synthesis of C-C bond via Suzuki-Miyaura coupling reactions

In recent times modified hydrotalcite like compounds formed with metal oxide layers has gained considerable importance as improved nanoparticle (NP) supports along with their unique electronic properties that inherently stabilizes the immobilized NPs. The present work demonstrates an adapted protocol towards the Pd NPs immobilized polydopamine modified Zn-Al mixed metal oxides (Zn-Al MMO@PDA/Pd NPs). Polydopamine was chosen as a biotemplate and a green reductant for the synthesis Pd NPs in situ without having any harsh reagent. Hitherto, this is the first design and biogenic synthesis of Pd NPs impregnated coordination on Zn-Al mixed metal oxides (Zn-Al MMO) and subsequent catalytic application towards the Suzuki-Miyaura coupling reactions. Subsequently, its physicochemical features were evaluated applying diverse methods like Field Emission Scanning Electron Microscopy (FE-SEM), X-ray elemental mapping (WDX), Energy Dispersive X-ray Spectroscopy (EDX) and X-Ray Diffraction (XRD) study. The Zn-Al MMO@PDA/Pd NPs nanocatalyst was isolated by centrifuger and stable enough to be reused for 8 consecutive times without considerable loss in activity.

Rational strategy for boosting productivity of salicylamide from urea and phenol over metal oxides via modulation their acidic-basic characters

The industrial viability of green synthesis of salicylamide from urea and phenol is obstructed by the low salicylamide productivity using metal oxides as the catalysts. Here, a series of single- and mixed-oxides having different metal compositions (Zn, Mg, Fe, Al) with varied acidic-basic characters were prepared to catalyze all the identified comprising paths of reaction between urea and phenol. Based on in situ FTIR characterization, a bi-functional mechanism that urea and phenol were separately activated on the acidic and basic centers to collaboratively promote the synthesis of salicylamide, was disclosed. Nevertheless, the surface active sites would be blocked due to the strong absorption of urea when catalysts exhibited extra acidity. Additionally, the secondary reactions would aggravate sharply over too basic samples. Thanks to its well-balanced acidity/basicity, Zn-Al mixed oxide provided the best catalytic performance. Herein, we offer a rational approach to develop metal oxides for the effective manufacture of salicylamide.

Amination of methanol for selective production of acetonitrile over Zn-Al mixed oxide catalysts synthesized at different pH

Zn–Al mixed oxides with different acidic and basic natures were synthesized under different pH (8.0–9.5) conditions and applied to the amination of methanol (MeOH). The physical and chemical properties of the samples were characterized using X-ray diffraction (XRD), N2 sorption, and X-ray photoelectron spectroscopy (XPS), as well as temperature-programmed desorption (TPD) with different probe molecules, such as NH3, CO2, iso-propanol (IPA), and MeOH. The conversion of MeOH and selectivity to acetonitrile (ACN) increased as the pH of the synthetic mixture increased to 9.5. The activity of the catalysts was in good accordance with the desorption temperatures of propylene in IPA-TPD and dimethyl ether in MeOH-TPD, while their selectivity to ACN was well correlated with the proportion of the weak base sites on the surface of the catalyst. Particularly, the changes in the acidity and basicity of the catalysts with the synthesis pH were mainly influenced by the degree of disorder of the ZnAl2O4 spinel structure. During the amination of MeOH, the catalytic activity, product selectivity, and deactivation behavior of the catalysts were affected by the reaction parameters. The Zn–Al mixed oxide catalyst was readily regenerated by via simple calcination, which removed the carbonitride and coke that were formed during the reaction.

Reconstruction of the ZnAl Mixed Oxides Into the Layered Double Hydroxide Catalysts Active in the Aldol Condensation of Furfural: The Role of ZnO Particles.

A memory effect is the ability to restore the original, lamellar layered double hydroxide structure. Herein, we have described 1) the changes in the structural and basic properties of ZnAl mixed oxides during their transformation into ZnAl-reconstructed LDHs (RE-LDHs); 2) the extraordinary properties of ZnAl RE-LDHs compared to the original ZnAl LDHs; and 3) the changes of basic properties during the interaction of ZnAl RE-LDHs with atmospheric CO2. Aldol condensation was selected as probe reaction to prove the catalytic potential of ZnAl RE-LDHs. We have described a target method for preparing ZnAl RE-LDHs with a large number of basic sites. ZnAl RE-LDHs possess significantly higher furfural conversion in the aldol condensation of furfural than MOs. The structural, textural, and basic properties of the studied materials were described by temperature-programmed analysis, X-ray diffraction, N2 adsorption, temperature-programmed desorption of CO2, and in-situ diffuse reflectance spectroscopy.

Vapor phase synthesis of alkylpyrazines over ZnAl mixed oxide derived from layered double hydroxides obtained by the mechanochemical method

Vapor phase synthesis of alkylpyrazines over ZnAl mixed oxide derived from layered double hydroxides obtained by the mechanochemical method

CO2 hydrogenation to methanol over partially embedded Cu within Zn-Al oxide and the effect of indium

Developing effective catalysts for CO2 hydrogenation to methanol is an important step to improve the efficiency of a promising process for green synthesis of fuels and chemicals. Optimizing the Cu dispersion is often the main goal in preparing Cu/ZnO-based catalysts due to the strong dependence of the catalytic activity on the Cu surface area. However, the catalytic properties are also related to the nature of the Cu-ZnO interface. Herein, a series of hydrotalcite-derived Cu/ZnO/Al2O3 catalysts were prepared for CO2 hydrogenation to methanol. The preparation method results in partially embedded Cu particles within the Zn-Al oxide matrix. This microstructure exhibits significantly enhanced intrinsic activity and methanol selectivity. Loss of the interfacial area between Cu and Zn-Al mixed oxide phase due to sintering of Zn-Al matrix is identified as the main reason for deactivation of the HT-derived catalysts. The influence of In on Cu/ZnO-based catalysts is also investigated. It is found that In decreases the activity but increases the methanol selectivity and stabilizes the Cu particles and the Zn-Al mixed oxide phase. The lower activity of the In-containing catalysts is linked to the inhibition of Cu active sites by CuxIny species.

Selective synthesis of γ-valerolactone from levulinic and formic acid over ZnAl mixed oxide

After testing different mixed oxides, a very promising catalyst to obtain γ-valerolactone from levulinic acid and formic acid is proposed in this work: a ZnAl hydrotalcite-derived material. Optimum conditions (T = 140 °C, FA/LA = 5/1, aqueous phase stirred batch reactor) excel results previously proposed in the literature, combining high activity (>87%) and selectivity (90% GVL yield) with remarkable stability, the reaction being performed at mild conditions and using a highly available and cheap catalyst.A detailed mechanistic with its kinetic study was accomplished for this optimum catalyst, considering that formic acid decomposition is the limiting step following a second–order kinetic, and the FA dehydration to AL is a pseudo-first–order reversible step. Angelica-lactone is identified as the main by–product and the precursor for the formation of light organic deposits, the main responsible for the slight deactivation observed during the experiments in absence of reactivation procedure.

The role of ZnO in the catalytic behaviour of Zn-Al mixed oxides in aldol condensation of furfural with acetone

The manuscript describes the key aspects of Zn-Al mixed oxides prepared by different methods (the so-called urea method and co-precipitation methods using either NaOH/Na2CO3 alkaline solution or solution of NaOH) affording high conversion and selectivity values in aldol condensation of furfural with acetone. The structure, composition and acid-base properties of Zn-Al mixed oxides were described by using of TGA, XRD, N2-adsorption, TPD-NH3/CO2 and in-situ diffuse reflectance spectroscopy and correlated with the catalytic performance of a series of Zn-Al mixed-oxides catalysts. Along with the impact of specific surface area and basicity on the furfural conversion, the conversion of furfural also increased with the increasing value of the edge of UV-vis spectrum of Zn-Al mixed oxide reflecting ZnO-Al2O3 interaction. We have introduced a simple method based on diffuse reflectance spectroscopy with a potential to predict furfural conversion over Zn-Al mixed oxides in aldol condensation of furfural with acetone.

Robust structured Cu-based film catalysts with greatly enhanced catalytic hydrogenation property

Presently, developing efficient and stable structured heterogenous catalysts with fabulous advantages of excellent ability for heat/mass transfer, facile recovery and separation, and superior stability is of great importance in terms of economic production of chemicals. Here, structured Cu-based film catalysts with novel surface superstructure composed of large quantities of interlaced and nearly vertically grown nanosheets for aqueous-phase hydrogenation of furfural toward the production of furfuryl alcohol (FOL) and cyclopentanone (CPO) were successfully fabricated from structured Cu-Zn-Al layered double hydroxide (CuZnAl-LDH) film precursors. As-fabricated structured Cu-based film catalyst possessing a Cu/(Zn + Cu) molar ratio of 4:5 exhibited a superior catalytic performance in the above reaction to other Zn-free and Zn-rich Cu-based film catalysts, as well as pristine CuZnAl-LDH-derived powdered one, due to the synergistic effect between highly dispersive Cu species and favorable surface acidic sites originating from LDH-derived Zn-Al mixed metal oxide (ZnAl-MMO), as well as the presence of abundant Cu-MMO interface within surface nanosheets, being responsible for the enhanced adsorption and activation of reactant and reaction intermediates. Furthermore, due to strong interactions between Cu species and MMO matrix within nanosheets and between surface nanosheets and anodic aluminum oxide/aluminum substrate, the present structured catalyst showed high structural stability and reusability.

Catalytic performance of zinc-supported copper and nickel catalysts in the glycerol hydrogenolysis

Abstract Gas-phase catalytic conversion of glycerol to value added chemicals was investigated over zinc-supported copper and nickel catalysts. The addition of aluminum in the support was also investigated in glycerol conversion and the results indicate an increase in the acidity and adsorption capacity for both copper and nickel catalysts. HRTEM and XRD analysis revealed NiZn alloy formation in the Ni/ZnO catalyst. The XRD patterns of the prepared ZnAl mixed oxide catalysts show the presence of Gahanite phase (ZnAl2O4). In addition, H2 chemisorption and TPR results suggest a strong metal-support interactions (SMSI) effect between Ni and ZnO particles. Bare supports ZnO and ZnAl (Zn/Al = 0.5) were investigated in the glycerol conversion and they did not present activity. Copper supported on ZnO and ZnAl mixed oxide (Zn/Al = 0.5) was active towards hydroxyacetone formation. Nickel was active in the hydrogenolysis of glycerol both for C–C and C–O bonds cleavage of glycerol producing CH4. Strong metal-support interactions (SMSI) between Ni and ZnO has a remarkable suppression effect on the methanation activity during the glycerol conversion.

Disordered structure of ZnAl2O4 phase and the formation of a Zn NCO complex in ZnAl mixed oxide catalysts for glycerol carbonylation with urea

The effect of a disordered ZnAl2O4 spinel structure on glycerol carbonylation with urea was investigated with pure ZnAl2O4 (c-ZnAl2O4) and ZnAl mixed oxide (c-ZnAlO) prepared by a citrate complex method, and ZnAl physically-mixed oxide (p-ZnAlO). During catalysts preparation, a disordered bulk ZnAl2O4 phase generated disordered sites on the surface: the Al3+ cations substituting for Zn2+ cations at the tetrahedral sites, and the surface oxygen vacancy corresponding to the Zn2+ cations substituting for Al3+ cations at the octahedral sites. The disordered surface sites increased in order of p-ZnAlO < c-ZnAlO < c-ZnAl2O4, which was proportional to the surface acidity. c-ZnAlO exhibited the best reaction performance due to the existence of a solid zinc isocyanate (Zn NCO) complex on the disordered sites. Here, we proposed that the solid Zn NCO complex preferentially generated glycerol carbonate (GC), while the liquid Zn NCO complex produced both GC and zinc glycerolate.

Cu–Zn–Al mixed oxides as catalysts for the hydrogenolysis of glycerol to 1,2-propanediol

This study is aimed to compare Cu–Al–Zn catalysts prepared by different methods and their effectiveness in glycerol hydrogenolysis to 1,2-propanediol. The preparation methods were: co-precipitation and thermal decomposition of hydrotalcite, co-precipitation by KOH and thermal decomposition of the precipitate, sol–gel auto-combustion method, oxalate–gel co–precipitation and thermal decomposition of the precipitate and finally, mechanical mixing of Cu, Zn, Al oxides. It was found that the activity of the catalysts correlates well with the size of Cu crystallites. The smaller Cu crystallites, the higher the activity. However, also the texture of the catalyst, which also depends on the preparation method, has an impact on the catalyst activity. On the other hand, no correlation between activity and acidity, as well as internal surface area was found.

High-sulfur capacity and regenerable Zn-based sorbents derived from layered double hydroxide for hot coal gas desulfurization.

The Zn-Al mixed metal oxide (ZnAl-MMO) with a plate-like structure was derived from Zn-Al layered double hydroxide. The ZnAl-MMO with a Zn/Al molar ratio of 3:1 exhibits superior absorption ability for H2S in a simulated coal gas at 600 ℃ due to the special structure of the ZnAl-MMO. Besides ZnS, elemental sulfur is also produced during the desulfurization process. The deactivation model could well simulate the absorption behavior of H2S. The sulfidation reaction over the sorbent shows large initial reaction rate constants (1110-5390 m3 min-1 kg-1) and low activation energy (29.5 kJ mol-1). The regeneration rate of the used sorbent can reach 99.8% under the optimum conditions. The regenerated sorbents still show high sulfur capacity (ca. 30%), implying its great application potential for industrial-scale desulfurization of the hot coal gas.

Investigating time-dependent Zn species over Zn-based catalysts in glycerol carbonylation with urea and their roles in the reaction mechanism

In this work, two different Zn-based catalysts – ZnO, and ZnAl mixed oxide (ZnAlO) – were employed to investigate Zn-phase-dependent catalysis in glycerol carbonylation with urea as a function of reaction times. ZnAlO catalyst exhibited higher selectivity and yield of glycerol carbonate (GC) over a wide range of glycerol conversion than the ZnO catalyst. The time-dependent Zn species and reaction intermediates were observed in the solid and liquid phases at various reaction times through FTIR and XRD measurements in order to understand Zn-containing intermediates and corresponding reaction routes over each catalyst. The low GC selectivity in the reaction over the ZnO catalyst was closely connected to the formation of zinc glycerolate (ZnGly) in the solid phase. For the ZnO catalyst, ZnGly was formed in the solid phase even at an initial reaction time by the reaction between Zn NCO complex and glycerol, resulting in the loss of GC selectivity. Alternatively, over a ZnAlO catalyst, the formation of the Zn isocyanate (NCO) complex was dominant up to 2 h of reaction time in both the liquid and solid phases. After 2 h of reaction time, ZnGly was observed in the spent ZnAlO catalyst along with decreasing GC selectivity. The relative formation rates of Zn-containing reaction intermediates (ZnGly and Zn NCO complex) over the ZnAlO catalyst were affected by the Zn phases over the solid catalysts and the ratio of urea to glycerol in the liquid phase during the reaction time.

In-situ characterization of the thermal treatment of Zn-Al hydrotalcites with respect to the formation of Zn/Al mixed oxide active in aldol condensation of furfural

Abstract Manuscript firstly describes the transformation of Zn-Al hydrotalcites into the appropriate Zn-Al mixed oxides during their thermal treatment up to 1000 °C. In detail, the structure, composition, morphology and growth of ZnO and ZnAl2O4 phases are analysed by using of TGA, in-situ XRD and in-situ diffuse reflectance UV–vis spectroscopy. Secondly, composition (XRF), structure (XRD), texture (N2 adsorption) and acid-base properties (TPD-NH3, TPD-CO2) of Zn-Al mixed oxides prepared by thermal treatment of Zn-Al hydrotalcites at 400 °C are described. Zn-Al mixed oxides are studied with respect to their utilization in the aldol condensation of furfural with acetone. Manuscript defines the region of the ZnO phase formation and their role on textural, structural, acid-base and catalytic properties of Zn-Al mixed oxides formed by thermal treatment of hydrotalcites. High values of the conversion of furfural well correlated with the low values of ZnO crystallite size and high values of the specific surface area. The contribution of acid-base properties could not be discussed, as studied mixed oxides exhibited approximately the same amount and strength of basic sites.

Dual catalysis over ZnAl mixed oxides in the glycerolysis of urea: Homogeneous and heterogeneous reaction routes

In this work, we prepared ZnO, ZnAl2O4, and ZnAl mixed oxides with different metal molar ratios and applied them for synthesizing glycerol carbonate (GC) from glycerol and urea. The reaction routes related to the Zn species over the ZnAl mixed oxides were investigated. The ZnAl mixed oxides were found to consist of two Zn crystalline phases: ZnO and ZnAl2O4. From the reaction results, the ZnAl mixed oxides showed much higher glycerol conversion and GC yield than the ZnO and ZnAl2O4. During the reaction, the dissolution of the Zn species from the ZnO phase over the ZnAl mixed oxides was observed while the ZnAl2O4 phase remained insoluble. The ZnO phase provided a homogeneous reaction route via the dissolved Zn species, resulting in the formation of a Zn complex containing the isocyanate (NCO) and zinc glycerolate. In contrast, the insoluble ZnAl2O4 phase was responsible for not only a heterogeneous reaction route, but also adsorption of the Zn NCO complex on the catalyst. We proposed that the adsorbed Zn NCO complex could play a role as an active site for an additional heterogeneous reaction route. Therefore, the ZnAl mixed oxides exhibited high GC yields through the dual catalysis routes: the homogeneous reaction route over the ZnO phase and the heterogeneous reaction route over the ZnAl2O4 phase.

Photo-induced properties of photocatalysts: A study on the modified structural, optical and textural properties of TiO2–ZnAl layered double hydroxide based materials

Heterogeneous photocatalytic oxidation technologies, based on the free-radical reaction initiated by a light irradiation of photocatalyst surface, are promising methods for water and air purification, being one of overwhelming concerns in the scientific community regarding the challenges of exponential environmental pollution increase. Mixed oxides derived from ZnAl containing layered double hydroxides (LDH) without or with doping agents (Fe, Sn and Ti) have been recently implemented as efficient and environmental-friendly materials in the photodegradation process of organic compounds in aqueous media. Considering both, the progress in the synthesis of photoactive LDH and evident drawbacks of classical methods, the study is focused on the development of an innovative, simple and cost effective synthesis method of TiO2 containing ZnAl LDH based photocatalysts with improved overall photocatalytic and antibacterial performances in water purification processes. In order to elucidate the relationship between the functional material properties developed in TiO2 impregnation process and photo-induced catalytic and antibacterial activity, comprehensive studies encompassed structural (XRD), textural (low temperature nitrogen adsorption), electronic properties (diffuse reflectance spectroscopy) and morphology (SEM) of developed TiO2-ZnAl LDH based materials, as well as the XPS study of the most active photocatalysts, in correlation to photo-induced catalytic activity (Methylene Blue degradation) and antibacterial activity (Gram-negative E. coli and Gram-positive bacterium S. aureus as indicator strains). The results confirmed that innovative impregnation method using base solution enables favorable interactions among active phases promoting the synergetic effect and complex interactions between the TiO2 (3 mas%) and ZnAl mixed oxides, enhancing photocatalytic activity to the level almost equal to TiO2, simultaneously improving the overall antibacterial performance.

Catalytic behavior of Mg–Al and Zn–Al mixed oxides in the transesterification of rapeseed oil: comparison of batch and fixed bed reactors

The activity and the selectivity of Mg–Al and Zn–Al mixed oxides were studied in the transesterification of rapeseed oil with methanol in both batch and fixed bed reactors. Mg–Al and Zn–Al mixed oxides were prepared by the calcination of Mg–Al and Zn–Al hydrotalcites with the Mg/Al and Zn/Al molar ratios 2. Zn–Al mixed oxide possessed higher ester yield than Mg–Al mixed oxide under the same reaction conditions in a batch reactor. In a fixed bed reactor, Zn–Al mixed oxides possessed high ester yield after 32 h time on stream (ca 88% at 115 °C).

Hydrodeoxygenation of furfuryl alcohol over Cu/MgAl and Cu/ZnAl catalysts derived from hydrotalcite-like precursors: A reaction for bio-oil upgrading

The aqueous phase hydrodeoxygenation (HDO) of furfuryl alcohol over Cu/MgAl and Cu/ZnAl catalysts with different Mg/Al and Zn/Al molar ratios, were investigated. Mg-Al and Zn-Al mixed oxides derived from hydrotalcites precursors were used as supports, which were impregnated with an aqueous solution of copper nitrate by incipient wetness impregnation. The HDO reaction was carried out in a typical batch reactor at 5 MPa of H2 and 200 °C for 4 h. Among the catalysts studied, the Cu/MgAl-0.5 catalyst exhibited the higher furfuryl alcohol conversion (86%) and yield of cyclopentanol (35%), which is the reaction product with the highest hydrogen-carbon (H/C) ratio. With the Cu/MgAl-3 catalyst a high cyclopentanone yield (67%) was achieved. The results obtained, showed that copper supported on mixed oxides catalysts derived from hydrotalcite precursors are a promising alternative to improve the bio-oil quality.

RECONSTRUCTION OF CALCINED Zn -Al LAYERED DOUBLE HYDROXIDES DURING TETRACYCLINE ADSORPSION

Zn-Al mixed oxides containing ZnO different degree crystallinity were obtained by calcinations of Zn-Al layered double hydroxides (LDHs). The reconstruction of calcined Zn-Al LDHs has been performed under stirring in aqueous suspensions. The assynthesized LDHs, its decomposition products, as well as the reconstructed solids upon hydration were characterized by XRD, N2adsorption, differential and thermal gravimetric analysis. It was found that the ability of Zn-Al LDHs to recover a layered structure under the hydration of mixed oxides depends on the degree of ZnO crystallinity. The partial reconstruction of Zn-Al layered structure occurs in tetracycline solutions irrespective to the degree of ZnO crystallinity in calcined LDHs. Calcined Zn-Al LDHs demonstrate the higher adsorption capacity to tetracycline in comparison with as-prepared Zn-Al LDHs. The adsorption of TC on calcined and uncalcined ZnAl LDHs occurs on the centers of one particular type. It is suggested that surface complexation of the A-ring ligand of TC with Al-OH centers takes place.