Al Hydrotalcite Catalysts Research Articles

Solvent-free oxidative coupling of benzylamine to imine using reusable MnO4−1 incorporated Mg–Al hydrotalcite catalyst

Solvent-free oxidative coupling of benzylamine to imine using reusable MnO4−1 incorporated Mg–Al hydrotalcite catalyst

Production of biodiesel: kinetics and reusability studies of the Mg–Al hydrotalcite catalyst using Jatropha oil

The optimal conditions for biodiesel transesterification are 800 rpm stirring, 12 : 1 methanol to oil ratio, 3% catalyst loading, and 60 °C temperature.

Effect of the Solvent on the Basic Properties of Mg–Al Hydrotalcite Catalysts for Glucose Isomerization

We suggested the existence of a relationship between the base properties of Mg–Al hydrotalcite catalysts and the solvents employed in the industrially important isomerization of glucose produce fructose. We prepared Mg–Al hydrotalcite catalysts with different Mg/Al atomic ratios to tune the basic properties of the catalyst. The prepared catalysts were used in the glucose isomerization conducted in various solvents. Experimental results confirmed that the catalysts exhibited different activities in the different solvents. We also implemented the Hammett indicator method, which allows to analyze the basic properties of the catalysts in various solvents. According to evidence, the basic properties of the catalysts varied substantially in different solvents. Notably, increases in the catalysts’ base properties matched the observed increases in fructose yield of the glucose isomerization. Consequently, we suggested that, in order to prepare efficient Mg–Al hydrotalcite catalysts for glucose isomerization, the interaction between the solvent used to conduct the reaction and the basic properties of the catalyst, which are in turn influenced by the solvent, should be considered.

Ni-based catalysts supported on Mg–Al hydrotalcites with different morphologies for CO2 methanation: exploring the effect of metal–support interaction

Ni-based Mg–Al hydrotalcite catalysts with perfect morphologies were proven to be highly active and stable during CO2 methanation.

Tuning the base properties of Mg–Al hydrotalcite catalysts using their memory effect

Abstract Herein, the relationship between the structure and base properties of Mg–Al hydrotalcite catalysts was comprehensively investigated in relation to heat treatment and rehydration processes, which are well known as memory effects of hydrotalcite. The structure of Mg–Al hydrotalcites changed from layered double hydroxide (LDH) to mixed metal oxide and subsequently to a spinel structure during heat treatment, and it was returned from mixed metal oxide to a LDH structure by rehydration. Based on various characterizations, we successfully proposed a detailed mechanism of memory effect. We also confirmed that the Mg–Al hydrotalcites had weak or strong base sites and that their base properties could be systematically tuned by heat treatment and rehydration. The prepared Mg–Al hydrotalcites were applied to a model reaction, isomerization of glucose to fructose, as base catalysts. Among the catalysts tested, the rehydrated Mg–Al hydrotalcite efficiently produced fructose due to its appropriate base and structure properties. We finally concluded that the base sites of Mg–Al hydrotalcites can be designed as desired by heat treatment and rehydration. Moreover, through systematic design of the base sites of Mg–Al hydrotalcites, these can be promising catalysts for various heterogeneous reactions over base catalysts, giving excellent catalytic performances.

Catalytic Role of Pd(II) Ions in Mg–Al Hydrotalcites for the Oxidation of Styrene

Pd(II) ions were introduced into Mg0.7Al0.3(OH)2(CO3)0.15·mH2O through the co-precipitation of the corresponding nitrate salts at a constant pH value. The presence of Pd in the brucite-like layers affects an increased surface area and decreased crystal domain of the Mg–Al hydrotalcite catalysts. Pd(II) ions may exist in both intra- and extra- brucite-like layers and exhibit different roles in the oxidation of styrene with t-BuOOH, depending on the amount of palladium. The insertion of Pd(II) into the hydrotalcite lattice has significantly improved the catalyst stability. The highest conversion of styrene is about 80% for the production of styrene oxide and benzaldehyde at low temperatures.

Production of green biofuel by using a goat manure supported Ni-Al hydrotalcite catalysed deoxygenation process.

The high oxygen content in natural biomass resources, such as vegetable oil or biomass-pyrolysed bio oil, is the main constraint in their implementation as a full-scale biofuel for the automotive industry. In the present study, renewable fuel with petrodiesel-like properties was produced via catalytic deoxygenation of oleic acid in the absence of hydrogen (H2). The deoxygenation pathway of oleic acid to bio-hydrocarbon involves decarboxylation/decarbonylation of the oxygen content from the fatty acid structure in the form of carbon dioxide (CO2)/carbon monoxide (CO), with the presence of a goat manure supported Ni–Al hydrotalcite (Gm/Ni–Al) catalyst. Goat manure is an abundant bio-waste, containing a high mineral content, urea as well as cellulosic fiber of plants, which is potentially converted into activated carbon. Synthesis of Gm/Ni–Al was carried out by incorporation of pre-activated goat manure (GmA) during co-precipitation of Ni–Al catalyst with 1 : 3, 1 : 1 and 3 : 1 ratios. The physico-chemical properties of the catalysts were characterized by X-ray diffractometry (XRD), Brunauer–Emmet–Teller (BET) surface area, field emission surface electron microscopy (FESEM) and temperature program desorption ammonia (TPD-NH3) analysers. The catalytic deoxygenation reaction was performed in a batch reactor and the product obtained was characterized by using gas chromatography-mass spectroscopy (GCMS) for compound composition identification as well as gas chromatography-flame ionisation detector (GC-FID) for yield and selectivity determination. The optimization and evaluation were executed using response surface methodology (RSM) in conjunction with central composite design (CCD) with 5-level-3-factors. From the RSM reaction model, it was found that the Gm/Ni–Al 1 : 1 catalysed deoxygenation reaction gives the optimum product yield of 97.9% of hydrocarbon in the range of C8–C20, with diesel selectivity (C17: heptadecane and heptadecene compounds) of 63.7% at the optimal reaction conditions of: (1) reaction temperature: 327.14 °C, (2) reaction time: 1 h, and (3) catalyst amount: 5 wt%.

The mechanism of Pd, K co-doping on Mg–Al hydrotalcite for simultaneous removal of diesel soot and NOx in SO2-containing atmosphere

A Pd/K/MgAlO mixed oxide was obtained by coprecipitation. The effect of Pd, K co-doping on the structure and catalytic activity of the Mg–Al hydrotalcite in a high concentration of SO2 atmosphere was characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), Temperature-programmed reduction (H2-TPR), and the transient response method. Studied the reaction mechanism and action mechanism of the doped K and Pd with the polluted gas in the catalytic, and calculated the kinetic parameters of the catalytic reaction. Our results show that potassium was highly dispersed on the surface of Mg–Al hydrotalcite and that incorporating potassium into the catalyst resulted in smaller grains than those of the original catalyst. After potassium doping K, K+ cations substituted protons of the weakly basic OH on MgAlO to form Mg(Al)-O-K., and the formation of this new basic valence bond conducive to the combustion of diesel soot. The ignition temperature of soot decreased from 380 to 217 °C. Moreover, modification of this Mg–Al hydrotalcite catalyst with potassium and palladium significantly reduces the activation energy of the reaction which form CO2 and N2 and benefit the combustion of soot and removal of NOx.

Novel Silica-Encapsulated Cu–Al Hydrotalcite Catalyst: Oxidative Decarboxylation of Vanillyl Mandelic Acid to Vanillin in Water at Atmospheric Pressure

Vanillin is a versatile chemical, having characteristics such as antimicrobial, antioxidant, anticarcinogenic, antimutagenic, hypolipidemic, and antisickling activities and is an intermediate for synthesis of fine chemicals such as vanilla flavoring. Vanilla is the second most expensive spice; it is extracted from vanilla beans and is in short supply. Therefore, to meet demand of vanilla flavor, the synthesis of vanillin via chemical routes is necessary. In this regard, a novel silica-encapsulated copper aluminum hydrotalcite (SECuAlHT) was synthesized. Different techniques such as XRD, FTIR, N2 adsorption–desorption, TGA-DSC, NH3 TPD, CO2 TPD, H2 chemisorption, TEM, and SEM-EDXS were used to characterize SECuAlHT. Its catalytic activity for oxidative decarboxylation of vanillyl mandelic acid (VMA) was investigated. Vanillin was obtained from VMA by oxidative decarboxylation using SECuAlHT in water as a solvent and atmospheric air under base-free conditions. The presence of basic sites in the catalyst was...

Synthesis, Characterization, and Activity Pattern of Ni–Al Hydrotalcite Catalysts in CO2 Methanation

Two nickel–aluminum hydrotalcite samples (HTLCs) were prepared by a coprecipitation method at different pH values and investigated as catalysts for the hydrogenation of carbon dioxide. The newly synthesized samples have been compared with a reference alumina supported nickel-based commercial catalyst, with equal nickel content. The as-prepared and commercial samples were characterized by BET analysis, atomic adsorption spectroscopy (AAS), X-ray diffraction (XRD), and temperature-programmed techniques (H2-TPR and CO-TPD). Catalytic activity of the analyzed samples was investigated toward hydrogenation of CO2 at atmospheric pressure by varying reaction temperature between 250 and 400 °C. The maximum CO2-to-CH4 conversion value achieved by hydrotalcyte was ≈86% at 300 °C. The superior performance of HTLCs has been put in relationship with the major catalysts reducibility nature and with the higher metal surface area and metal dispersion. The stability of the HTLCs was investigated through long-term tests, re...

Magnesium and/or calcium-containing natural minerals as ecologically friendly catalysts for the Baeyer–Villiger oxidation of cyclohexanone with hydrogen peroxide

Magnesium and/or calcium-containing natural minerals of basic character were tested as catalysts for the Baeyer–Villiger oxidation of cyclohexanone to ϵ-caprolactone. Two types of widely available, nontoxic and inexpensive minerals were investigated: carbonates, i.e. magnesite, dolomite and calcite (limestone), and silicates, i.e. talc, sepiolite and hectorite. The minerals were characterized with XRD, SEM, XRF, ICP OES, XPS, N2 adsorption at −196°C, laser diffraction particle sizing, and contact angle measurement. Surface basicity was determined by adsorption of organic acids with different pKa values. Mechanochemical treatment of the samples was performed in a planetary mill. The yields of ϵ-caprolactone over the as received carbonate minerals were comparable with the yield obtained for the reference Mg–Al hydrotalcite catalyst. Of the layered magnesium silicate group, the untreated hectorite gave the best performance, yielding ϵ-caprolactone in the amount similar to that obtained for the reference catalyst. The study of the effect of grinding on the catalytic performance of minerals revealed that the treatment may affect not only the mineral grain size and morphology, but also other factors relevant for the catalytic reaction, such as hydrophilic/hydrophobic properties of the mineral surface and the ease of alkaline earth leaching. The final effect of grinding depended on the interplay between all these factors. Thus, grinding-induced enhancement of catalytic performance was most pronounced for magnesite, less significant for dolomite, and practically negligible for limestone. In the series of magnesium silicates grinding increased the ϵ-caprolactone yield over sepiolite, while little effect was observed for talc and hectorite. The maximum yields of ϵ-caprolactone over ground magnesite and ground sepiolite were by factor 1.4 higher than the yield over the reference hydrotalcite catalyst. Thus, magnesite and sepiolite prove as particularly promising, eco-friendly catalysts for the Baeyer–Villiger oxidation of cyclohexanone to ϵ-caprolactone.

Structure–performance correlations of Mg–Al hydrotalcite catalysts for the isomerization of glucose into fructose

Mg–Al hydrotalcites with different Mg-to-Al molar ratio, texture, and morphology were studied as catalysts for the isomerization of glucose into fructose. The properties of the hydrotalcites were tuned by varying the preparation procedure including pH during co-precipitation, ageing temperature, and solvent. The catalysts were characterized by ICP-OES, N2 sorption, sorption of acrylic acid, XRD, TG-DSC, and SEM. Tuning the conditions of synthesis enables controlling properties such as crystallite size, dispersion of primary particles, and morphology of the agglomerates. These structural parameters of the hydrotalcites critically influence their basicity, which in turn predominantly determines the catalytic performance. The best fructose yields of up to 30% with 89% selectivity were obtained over hydrotalcites synthesized by co-precipitation in (i) an aqueous medium at pH 10 and (ii) an aqueous-ethanol medium at pH 9.5. These synthesis conditions enabled materials with optimal textural properties and the highest amount of accessible basic centres. Although some leaching of magnesium occurred caused by lactic acid formed as by-product of carbohydrate degradation, the hydrotalcites could be recycled without loss of activity and selectivity.

ChemInform Abstract: Synthesis of Substituted Guanidines Using Zn—Al Hydrotalcite Catalyst.

AbstractThe Zn—Al hydrotalcite catalyst can be easily recovered by simple filtration and reused for three cycles without significant loss of its activity.

Production of novel biodiesel from transesterification over KF-modified Ca–Al hydrotalcite catalyst

A series of KF-modified Ca–Al hydrotalcite (xKF/HTL-M) solid base catalysts was prepared in methanol, and used in the production of a novel biodiesel of ethylene glycol monomethyl ether monolaurate.

Synthesis of substituted guanidines using Zn–Al hydrotalcite catalyst

Substituted guanidines were synthesized by the guanylation of amines with carbodiimides using Zn–Al hydrotalcite (Zn-Al HT) catalyst. Zn–Al HT was prepared by co-precipitation method and characterized by X-ray powder diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Raman and Thermogravimetric-Differential Thermal Analysis (TG-DTA). The heterogeneous catalyst afforded moderate to good yields (∼50–60 %) of substituted guanidines in toluene at 110°C in 12 h. The catalyst was recovered quantitatively by simple filtration and reused for three cycles with consistent activity. The XRD and FTIR studies of the used catalyst shows no variation in the structure of the catalyst even after three recycles.

Microwave assisted efficient protocol for the classic Ullmann homocoupling reaction using Cu–Mg–Al hydrotalcite catalysts

Four hydrotalcite catalysts containing different amounts of Cu and Mg cations, with varying atomic ratios of Cu:Mg:Al in the brucite-like layers of 0:2:1, 0.5:1.5:1, 1:1:1 and 2:0:1, were prepared by co-precipitation method. The synthesized catalysts were tested for classic Ullmann homocoupling of iodobenzene to produce biphenyl without using any stoichiometric amounts of base or reducing agent. The Mg–Al hydrotalcite catalyst without Cu ions is inactive, but the doping of Cu ions (Cu/Al=0.5) into the Mg–Al hydrotalcite framework led to 63% conversion of iodobenzene under microwave irradiation. Gradual increase of Cu content and concomitant decrease of Mg content resulted in an increase in the conversion of iodobenzene and selectivity to biphenyl. The physico-chemical properties of hydrotalcite catalysts were studied by chemical analysis, XRD, FTIR, N2-physisorption, SEM, XPS, ESR and TPR techniques to understand the state of Cu ions presented in the hydrotalcite framework. It appears that presence of easily reducible isolated Cu2+ ions is essential to obtain improved performance of the catalyst.

Mg–Al hydrotalcite as an efficient catalyst for microwave assisted regioselective 1,3-dipolar cycloaddition of nitrilimines with the enaminone derivatives: A green protocol

Microwave assisted generation of in situ nitrilimines and their regioselective 1,3-dipolar cycloaddition reaction with enaminones to produce pyrazole derivatives by Mg–Al hydrotalcite (Mg–Al HT) catalyst is introduced. Short reaction times with high yields (90–94%), the absence of an organic solvent and reuse of the catalyst for at least five times without loss of activity could regard as cost effective, environmentally friendly and greener process. The fresh and used catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectroscopy, N2 physisorption and temperature programmed desorption by CO2 (CO2-TPD) to understand the role of hydrotalcite in the reaction mechanism and the robust nature of the catalyst. It appears that better microwave absorptivity of the catalyst with Mg–Al HT structure might be responsible for shorter reaction times with high yields. A plausible mechanism for 1,3-dipolar cycloaddition over the Mg–Al HT is proposed.

Isomerization of glucose into fructose over Mg–Al hydrotalcite catalysts

Abstract Isomerization of glucose into fructose was carried out over as-synthesized Mg–Al hydrotalcite (HT_A), calcined Mg–Al hydrotalcite (HT_C), and rehydrated Mg–Al hydrotalcite (HT_R) catalysts. The experimental results revealed that HT_R catalyst retained abundant weak base sites through exfoliation and vertical breaking of layers in the hydrotalcite structure during the rehydration process, leading to the enhanced catalytic performance of HT_R in this reaction. This means that rehydration process served an efficient method for increasing the catalytic performance of hydrotalcite in the isomerization of glucose into fructose. Reaction temperature and time also played a key role in determining the catalytic performance in this reaction.

Experimental and Modeling Study of Continuous Catalytic Transesterification to Biodiesel in a Bench-Scale Fixed-Bed Reactor

A 500 h endurance test of continuous catalytic transesterification to biodiesel was conducted in a bench-scale fixed-bed reactor. A designed cylinder shape of KF/Ca–Mg–Al hydrotalcite catalyst was stable through the test with high catalytic activity and mechanical strength, converting palm oil to biodiesel with a conversion of more than 95%. Conditions effects on transesterification under relative high pressure (1.0 MPa) were investigated and a one-dimensional heterogeneous model of a fixed-bed reactor was developed to describe the reaction-mass transfer behaviors of continuous catalytic transesterification in a bench-scale fixed-bed reactor. With the given reactor size, optimum conditions were proposed as a LHSV (liquid hourly space velocity) of 0.76–0.25 h–1, molar ratio of methanol-to-oil of 9.16–13.7, and temperature of 338–347 K.

Mg–Al hydrotalcites as efficient catalysts for aza-Michael addition reaction: A green protocol

Mg–Al hydrotalcite was synthesized by a co-precipitation method. We have studied the effect of calcination temperature and hydration of the calcined phases on their catalytic activity for the synthesis of pyrazolo[1,5- a]pyrimidine derivatives (aza-Michael addition product). The structure of the as-synthesized sample and the presence of the anions in the interlayer galleries of hydrotalcites, have been determined by X-ray diffraction and FTIR spectroscopy. On calcining the material at 450 °C, it was amorphous periclase phase. Re-hydration of the calcined phase resulted in the formation of hydrotalcite-like phase. Such treatment to the as-synthesized hydrotalcite significantly changed the pore structure and the BET-surface area as determined from N 2 physisorption at 77 K. The as-synthesized Mg–Al-hydrotalcite catalyst was found to be the most efficient for the aza-Michael reaction relative to the activated solid catalysts tested. The high performance of this catalyst was attributed to the co-operative contribution of its acidic and basic sites. We have shown that this microwave assisted reaction provides an eco-friendly alternative to the conventional syntheses where soluble bases are used. Furthermore, the reaction was performed over a considerably shorter time scale and generated significantly higher yields than traditional methods.