Preparation of acid–base bi-functional hydrotalcite based catalyst for converting Vietnamese coconut oil to green hydrocarbons

Acid–base bi-functional hydrotalcite like compounds based on partial incorporation of Al³⁺ into brucite structure of Mg(OH)₂ with various molar ratios were prepared through co-precipitation method. The co-precipitation of the precursors produced precipitations followed by drying at 120 °C for 12 h and calcination in air flow at 500 °C for 6 h to obtain the catalysts (Mg–Al HLCs). Many techniques including XRD, TG–DTA, EDX, NH₃-TPD, CO₂-TPD, GC–MS and XANES were used to characterize and optimize Mg/Al molar ratio based on the thermal stability of the Mg–Al HLCs and their activities in decarboxylation process of coconut oil. The results showed that the best molar ratio of Mg/Al was 3/1 providing a stable hydrotalcite like structure, and the catalyst possessed both acid and base sites on its surface enhancing its activity and selectivity in the decarboxylation process. The catalysts revealed high performance in the decarboxylation process of coconut oil established at 400 °C for 4 h for green hydrocarbons belonging to kerosene fractions.     

Hydrotalcite-derived Co-containing mixed metal oxide catalysts for methanol incineration

The Co–Mg–Al mixed metal oxides were prepared by calcination of co-precipitated hydrotalcite-like precursors at various temperatures (600–800 °C), characterised with respect to chemical (AAS) and phase (XRD) composition, textural parameters (BET), form and aggregation of cobalt species (UV–vis-DRS) and their redox properties (H₂-TPR, cyclic voltammetry). Moreover, the process of thermal decomposition of hydrotalcite-like materials to mixed metal oxide systems was studied by thermogravimetric method combined with the analysis of gaseous decomposition products by mass spectrometry. Calcined hydrotalcite-like materials were tested as catalysts for methanol incineration. Catalytic performance of the oxides depended on cobalt content, Mg/Al ratio and calcination temperature. The catalysts with lower cobalt content, higher Mg/Al ratio and calcined at lower temperatures (600 or 700 °C) were less effective in the process of methanol incineration. In a series of the studied catalysts, the best results, with respect to high catalytic activity and selectivity to CO₂, were obtained for the mixed oxide with Co:Mg:Al molar ratio of 10:57:33 calcined at 800 °C. High activity of this catalyst was likely connected with the presence of a Co–Mg–Al spinel-type phases, containing easy reducible Co³⁺ cations, formed during high-temperature treatment of the hydrotalcite-like precursor.     

CO<Subscript>2</Subscript> capture by Mg–Al and Zn–Al hydrotalcite-like compounds

Hydrotalcite-like compounds (HTC) are distinguished by their properties for CO₂ capture, like high surface area, basic sites, thermal stability and good adsorption/desorption efficiency. Mg–Al e Zn–Al HTCs with Al³⁺ molar ratios x = 0.20, 0.28 and 0.33 were synthesized by coprecipitation, and subsequently calcined at 400 °C. For both HTCs, X-ray diffraction patterns have attested the formation of mixed oxides through calcination. The amount of basic sites, measured by temperature-programmed desorption of CO₂, decreases as x increases. The CO₂ adsorption was performed in a thermogravimetric balance using an adsorption temperature of 50 °C. Mg–Al and Zn–Al samples with x = 0.33 molar composition presented the highest CO₂ adsorption, 0.91 and 0.21 mmol g^?1, respectively. The Langmuir isotherm fitted well to the experimental data. It was also found that increasing the number of adsorption/desorption cycles the CO₂ adsorption decreases, which is associated with the irreversible chemisorption.     

Calcined Mg-Al Layered Double Hydroxide as a Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO2

The calcined Mg‐Al layered double hydroxides (LDHs) with a Mg/Al molar ratio of 3:1 were synthesized and characterized thoroughly by X‐ray diffraction (XRD), temperature‐programmed desorption (TPD) of CO₂, and thermogravimetric analysis (TGA). Thus the calcined Mg‐Al LDHs were used as catalyst for the catalytic synthesis of disubstituted ureas from amines and CO₂. The effects of reaction time, reaction temperature, pressure, solvent and calcined temperature on activity have been investigated. The results indicated that aliphatic amines, cyclohexylamine and benzylamine can be converted to the corresponding ureas selectively over the calcined Mg‐Al LDHs catalysts with N‐methyl‐2‐pyrrolidone (NMP) as solvent without using any dehydrating regent. The catalyst can be recycled several times with only slight loss of activity.     

High temperature adsorption of carbon dioxide on Cu–Al hydrotalcite-derived mixed oxides: kinetics and equilibria by thermogravimetry

The effect of Cu/Al molar ratio on the high-temperature adsorption characteristics of CO₂ on the mixed oxides of Cu–Al hydrotalcite skeletal structure has been studied by thermogravimetry. The Cu/Al molar ratio of the hydrotalcites synthesized was varied between 1.0 and 3.0, and the adsorption temperature ranged from ambient to 600 °C. The hydrotalcite with Cu/Al molar ratio of 2.0 was found to be the most suitable adsorbent for high-temperature CO₂ adsorption, in both the capacity and the rate of adsorption. The activation energy values suggested that the physical adsorption dominates at low temperatures (<400 °C) and the chemisorption dominates at high temperatures (>400 °C).     

Preparation of Mg–Al hydrotalcite from the effluent of Friedel–Crafts reaction and its application as a catalyst in Knoevenagel reaction

Abstract

Hydrotalcite containing magnesium and aluminium (Mg–Al HT) with a molar ratio of Mg(II)/Al(III) = 2.5 has been prepared by a co-precipitation method using the effluent of a Friedel–Crafts acylation reaction. The HT was calcined at 500°C and reconstructed with deionized water. The synthesized HT was characterized by XRD and FT-IR spectroscopy and was successfully used as a catalyst in the Knoevenagel reaction of aldehydes and active methylene compounds. The catalyst was found to be reusable.     

Thermal transformations of Cu–Mg (Zn)–Al(Fe) hydrotalcite-like materials into metal oxide systems and their catalytic activity in selective oxidation of ammonia to dinitrogen

Layered double hydroxides (LDHs) containing Mg²⁺, Cu²⁺ or Zn²⁺ cations in the Me^II positions and Al³⁺ and Fe³⁺ in the Me^III positions were synthesized by co-precipitation method. Detailed studies of thermal transformation of obtained LDHs into metal oxide systems were performed using high temperature X-ray diffraction in oxidising and reducing atmosphere, thermogravimetry coupled with mass spectrometry and temperature-programmed reduction. The LDH samples calcined at 600 and 900 °C were tested in the role of catalysts for selective oxidation of ammonia into nitrogen and water vapour. It was shown that all copper congaing samples presented high catalytic activity and additionally, for the Cu–Mg–Al and Cu–Mg–Fe hydrotalcite samples calcined at 600 °C relatively high stability and selectivity to dinitrogen was obtained. An increase in calcination temperature to 900 °C resulted in a decrease of their catalytic activity, possibly due to formation of well-crystallised metal oxide phases which are less catalytically active in the process of selective oxidation of ammonia.     

No-Glycerol rapid heterogeneous biodiesel production on K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst by coupling process

Biodiesel containing almost no glycerol has been produced by coupling reaction carried out over K₂CO₃ supported by calcium oxide as solid base catalysts. The solid base catalysts synthesized by wet impregnation exhibit an exceedingly high activity in biodiesel production. It was found that the reaction time required for the highest yield of biodiesel, 99.2%, can be shortened to 30 min over K₂CO₃/Al₂O₃ under the optimum reaction conditions: 8: 1: 1 molar ratio of methanol/DMC/oil, 30 wt % K₂CO₃/Al₂O₃ catalyst, and 65°C reaction temperature. Solid basic catalysts examined in the study were characterized by BET surface area, XRD, CO₂-TPD, and SEM techniques. The strong interaction between K₂CO₃ and the support yields a new basic active site, which can be probably responsible for the high activity of K₂CO₃/Al₂O₃.     

Modified hydrotalcites application as precursors for (Na,K)Mg/Al spinel-type compounds formation

In this work, a synthesis route of (Na,K)Mg/Al spinel-type compounds, which combines hydrothermal synthesis at low temperatures (<200 °C) and solid-state sintering (>800 °C) methods, is presented. It was examined that NaOH and KOH additives induce the reaction between initial Mg and Al components and the formation of hydrotalcite during hydrothermal treatment. It should be noted that after 1 h of calcination of synthetic precursors at 850 °C spinel-type compounds are formed only in the samples with alkali addition. Meanwhile in the pure system only traces of the mentioned compounds are observed at 900 °C. Moreover, the increase in solid-state sintering temperature and duration lead to the higher-crystallinity (Na,K)MgAl₂O₄ spinel-type compounds. It should be noted that textural properties of formed (Na,K)Mg/Al spinel-type compounds depend on the chemical composition of precursors. The synthetic and calcined products are characterised by XRD, STA, FT-IR analyses and BET method.     

Synthesis and thermal stability of hydrotalcites based upon gallium

Hydrotalcites based upon gallium as a replacement for aluminium in hydrotalcite over a Mg/Al ratio of 2:1 to 4:1 were synthesised. The d(003) spacing varied from 7.83 Å for the 2:1 hydrotalcite to 8.15 Å for the 3:1 gallium containing hydrotalcite. A comparison is made with the Mg/Al hydrotalcite in which the d(003) spacing for the Mg/Al hydrotalcite varied from 7.62 Å for the 2:1 Mg hydrotalcite to 7.98 Å for the 4:1 hydrotalcite. The thermal stability of the gallium containing hydrotalcite was determined using thermogravimetric analysis. Four mass loss steps at 77, 263–280, 485 and 828 °C with mass losses of 10.23, 21.55, 5.20 and 7.58% are attributed to dehydration, dehydroxylation and decarbonation. The thermal stability of the gallium containing hydrotalcite is slightly less than the aluminium hydrotalcite.     

Synthesis of biodiesel over ZnO/Ca(OH)2/KF catalyst prepared by the grinding method

A novel ZnO/Ca(OH)₂/KF solid base catalyst was prepared by the grinding method and applied to biodiesel synthesis by the transesterification of soybean oil. The effect of various parameters such as KF molar amount, calcination temperature, the amount of catalyst, molar ratio of methanol to oil, reaction temperature, and time on the activity of the catalyst were investigated. The catalysts were characterized by several techniques of thermogravimetry/derivative thermogravimetry, X–ray diffraction, Hammett indicator method, and scanning electron microscopy. The analysis results indicated that the KF interacted with Ca(OH)₂ and formed KCaF₃ phase before calcination of the catalyst. The formed KCaF₃ crystal phase was the main catalytic active component for the catalyst activity. In addition, the basicity of ZnO/Ca(OH)₂/KF was greatly influenced by the different calcination temperates, and the catalyst activity was correlated closely with the basicity. A desired biodiesel yield of 97.6 % was obtained at catalyst amount of 3 %, methanol/oil of 12:1, and reaction time of 1.5 h at 65 °C.     

Suzuki cross‐coupling reaction of aryl and heterocyclic bromides and aromatic polybromides on a Pd(II)‐hydrotalcite catalyst

The Suzuki cross‐coupling reaction of various bromine‐containing substrates and phenylboronic acid in toluene at 90 °C on a Pd(AcO)₂Py₂ catalyst supported on an Mg? Al hydrotalcite, using K₂CO₃ as the base, was studied. The conversion and selectivity results obtained for many of the substrates were excellent and similar to those provided by more active or even homogeneous catalysts. The reactions of aryl polybromides and phenylboronic acid gave the corresponding polyaromatic compounds in variable yields depending on the particular substrate. Arylation occurred in a consecutive manner by substitution of the different Br atoms. ICP‐MS measurements of the palladium content of the catalyst performed prior to and after the reaction revealed that part of the metal is incorporated into the bulk solution; therefore, the catalytic process is not purely heterogeneous. Copyright © 2008 John Wiley & Sons, Ltd.     

Aqueous-phase catalytic hydrogenation of furfural to cyclopentanol over Cu-Mg-Al hydrotalcites derived catalysts: Model reaction for upgrading of bio-oil

A series of Cu-Mg-Al hydrotalcites derived oxides with a (Cu+Mg)/Al mole ratio of 3 and varied Cu/Mg mole ratio (from 0.07 to 0.30) were prepared by co-precipitation and calcination methods, then they were introduced to the hydrogenation of furfural in aqueous-phase. Effects of Cu/Mg mole ratio, reaction temperature, initial hydrogen pressure, reaction time and catalyst amount on the conversion rate of furfural as well as the selectivity toward desired product cyclopentanol were systematically investigated. The conversion of furfural over calcined hydrotalcite catalyst with a Cu/Mg mole ratio of 0.2 was up to 98.5% when the reaction was carried out under 140 °C and the initial hydrogen pressure of 4 MPa for 10 h, while the selectivity toward cyclopentanol was up to 94.8%. The catalysts were characterized by XRD and SEM. XRD diffraction of all the samples showed characteristic pattern of hydrotalcite with varied peak intensity as a result of different Cu content. The catalytic activity was improved gradually with the increase of Cu component in the hydrotalcite.     

Alkali Hydrothermal Synthesis of Na0.1Ca0.9TiO3 Nanorods as Heterogeneous Catalyst for Transesterification of Camelina Sativa Oil to Biodiesel

Orthorhombic perovskite Na_0.1Ca_0.9TiO₃ nanorods were synthesized at low calcination temperature via alkali hydrothermal synthesis. The synthesized nanorods exhibits a square based prism morphology, with a width and length of 200–500 nm and 2–3 μm respectively. The structural, textural and basic characteristics of the catalyst were examined by SEM, TEM, XRD and BET. The growth direction of the nanorods was confirmed to be along the long symmetry [110] zone axis and the exterior surfaces are found to be polar (110) and (002) with either Ti or Ca exposed in those facets. The catalytic activity of the nanorods was investigated for the transesterification of the low-input Camelina Sativa oil and methanol to give the fatty acid methyl ester (FAME). Effects of important reaction parameters such as methanol to oil molar ratio, catalyst dosage, reaction temperature and reaction time on oil conversion were examined. Optimized biodiesel yield of 93 % was achieved with catalyst dosage of 6 % w/w, methanol to oil molar ratio of 36:1 at reaction temperature of 60 °C for 8 h.     

Synthesis and thermal stability of hydrotalcites containing manganese

The hydrotalcite based upon manganese known as charmarite Mn₄Al₂(OH)₁₂CO₃·3H₂O has been synthesised with different Mn/Al ratios from 4:1 to 2:1. Impurities of manganese oxide, rhodochrosite and bayerite at low concentrations were also produced during the synthesis. The thermal stability of charmarite was investigated using thermogravimetry. The manganese hydrotalcite decomposed in stages with mass loss steps at 211, 305 and 793 °C. The product of the thermal decomposition was amorphous material mixed with manganese oxide. A comparison is made with the thermal decomposition of the Mg/Al hydrotalcite. It is concluded that the synthetic charmarite is slightly less stable than hydrotalcite.     

Reactivity of Mg–Al hydrotalcites in solid and delaminated forms in ammonium carbonate solutions

Treatment of Mg–Al hydrotalcites (LDHs, layered double hydroxides) in aqueous (NH₄)₂CO₃ at 298 K leads to composites of dawsonite, hydrotalcite, and magnesium ammonium carbonate. The mechanism and kinetics of this transformation, ultimately determining the relative amounts of these components in the composite, depend on the treatment time (from 1 h to 9 days), the Mg/Al ratio in the hydrotalcite (2-4), and on the starting layered double hydroxide (solid or delaminated form). The materials at various stages of the treatment were characterized by inductive coupled plasma-optical emission spectroscopy, X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetry, and nitrogen adsorption at 77 K. The progressive transformation of hydrotalcite towards crystalline dawsonite and magnesium ammonium carbonate phases follows a dissolution–precipitation mechanism. A gradual decrease of the Mg/Al ratio in the resulting solids was observed in time due to magnesium leaching in the reacting medium. Dawsonite–hydrotalcite composite formation is favored at high aluminum contents in the starting hydrotalcite, while the formation of magnesium ammonium carbonate is favored at high Mg/Al ratios. The synthetic strategy comprising hydrotalcite delamination in formamide prior to aqueous (NH₄)₂CO₃ treatment is more reactive towards composite formation than starting from the bulk solid hydrotalcite.     

Enzymatic Biodiesel Synthesis in Semi-Pilot Continuous Process in Near-Critical Carbon Dioxide

A semi-pilot continuous process (SPCP) for enzymatic biodiesel synthesis utilizing near-critical carbon dioxide (NcCO₂) as the reaction medium was developed with the aim of reducing the reaction time and alleviating the catalyst inhibition by methanol. Biodiesel synthesis was evaluated in both lab-scale and semi-pilot scale reactors (batch and continuous reactors). In a SPCP, the highest conversion (～99.9 %) in four and a half hours was observed when three-step substrate (methanol) addition (molar ratio [oil/methanol]?=?1:1.3) was used and the reaction mixture containing enzyme (Lipozyme TL IM, 20 wt.% of oil) was continuously mixed (agitation speed?=?300 rpm) at 30 °C and 100 bar in a CO₂ environment. The biodiesel produced from canola oil conformed to the fuel standard (EU) even without additional downstream processing, other than glycerol separation and drying.     

ZnO/Mg–Al layered double hydroxides as strongly adsorptive photocatalysts

Nanocomposites of magnesium aluminium layered double hydroxides with carbonate anions (Mg–Al–CO₃-LDHs) and ZnO nanorods were prepared by a homogeneous precipitation process. The ZnO nanorods give the calcined Mg–Al–CO₃-LDHs, strong adsorbents of anionic dyes, photocatalytic activity. The nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the nanocomposites was investigated by degradation of acid red G in aqueous solution, and the nanocomposite with the ZnO-to-Mg–Al–CO₃-LDHs mass ratio of 1:1 had the highest photocatalytic activity in this photocatalytic reaction.     

Synthesis of glycerol carbonate from glycerol and dimethyl carbonate catalyzed by K2CO3/MgO

A series of M/MgO (M?=?CaO, KNO₃, KOH, K₂CO₃) catalysts were prepared by a dry impregnation method and used for synthesis of glycerol carbonate from glycerol and dimethyl carbonate. It was found that K₂CO₃/MgO was the most efficient catalyst, with a glycerol carbonate yield of approximately 99% under the conditions: DMC/glycerol molar ratio 2.5:1, catalyst/raw material weight ratio 1%, reaction time 2?h, and reaction temperature 80?°C. FTIR, BET, TEM, and XRD were used for characterization of the catalyst and showed that the active sites seemed to be K₂O formed on the K₂CO₃/MgO catalyst. Finally, a recycling experiment showed that the catalyst was relatively stable and could be reused up to four times, at least, by regeneration.