Mg-Al Mixed Oxides Research Articles

Development of Active and Stable Low Nickel Content Catalysts for Dry Reforming of Methane

Methane dry reforming (DRM) was investigated over highly active Ni catalysts with low metal content (2.5 wt %) supported on Mg-Al mixed oxide. The aim was to minimize carbon deposition and metal sites agglomeration on the working catalyst which are known to cause catalyst deactivation. The solids were characterized using N2 adsorption, X-ray diffraction, temperature-programmed reduction, X-ray photoelectron spectroscopy, and UV-Vis diffuse reflectance spectroscopy. The results showed that MgO-Al2O3 solid solution phases are obtained when calcining Mg-Al hydrotalcite precursor in the temperature range of 550–800 °C. Such phases contribute to the high activity of catalysts with low Ni content even at low temperature (500 °C). Modifying the catalyst preparation with citric acid significantly slows the coking rate and reduces the size of large octahedrally coordinated NiO-like domains, which may easily agglomerate on the surface during DRM. The most effective Ni catalyst shows a stable DRM course over 60 h at high weight hourly space velocity with very low coke deposition. This is a promising result for considering such catalyst systems for further development of an industrial DRM technology.

Oxidative steam reforming of ethanol (OSRE) over stable NiCo–MgAl catalysts by microwave or sonication assisted coprecipitation

The present paper reports the physicochemical and catalytic characterization of NiCo–MgAl mixed oxides prepared from hydrotalcite (HT) precursors using ultrasound (US) or microwave radiation (MW)-assisted coprecipitation. The US or MW use reduces the preparation time and enhance both basicity and redox properties of the material compared to the solid obtained by the conventional method (coprecipitation).The oxidative steam reforming of ethanol (OSRE) was performed during 100 h (stability test) with or without solid pre-reduction in order to understand some aspects of the OSRE mechanism when transition metal catalysts are used without activation. The results demonstrate the potential of using 1Ni1Co mixed oxides obtained by MW or US assistance which exhibited better properties and catalytic stability employing shorter synthesis times. Furthermore, experiments conducted without material pre-reduction indicate the sensitivity of OSRE to the basic strength of the surface. A marked increase in basicity decrease H2 yield because of the formation of C3 byproducts.

Reconstructed Mg-Al hydrotalcites prepared by using different rehydration and drying time: Physico-chemical properties and catalytic performance in aldol condensation

Aldol condensation in presence of reconstructed hydrotalcites as basic catalysts is well documented and attracts an increased attention because it allows producing complex molecules starting from relatively simpler ones. Nevertheless, more studies are necessary for the better understanding of preparation factors that affect the physico-chemical properties of these materials and their catalytic performance. This work considers the effect of the duration of the liquid-phase rehydration of Mg-Al mixed oxide as well as the drying duration of prepared samples on their physico-chemical properties and performance in aldol condensation of furfural and acetone. The properties of all materials were investigated by XRD, TGA, TGA-MS, SEM and DRIFT. Experiments on aldol condensation of furfural and acetone were performed by using a stirred batch reactor at T=25°C with Ac:F=5 molar ratio. The characterization of the prepared materials showed that the recovery of HTC structure occurred within a few seconds of the contact between Mg-Al mixed oxide with liquid water. The content of functional species in all reconstituted HTCs was nearly constant and independent on preparation conditions while the amount of physisorbed water in the prepared samples decreased with increasing drying time. The calcination of as-prepared HTC followed by the subsequent rehydration of Mg-Al mixed oxide resulted in the replacement of >90% carbonate groups on hydroxyls which are Brønsted basic sites. Consequently, the activity of the reconstructed HTCs in aldol condensation of furfural and acetone was significantly higher compared to Mg-Al mixed oxide. The catalytic performance of rehydrated materials was almost independent on rehydration time while furfural conversion decreased if carrying out the reaction in an aqueous medium with increased water content. On the other hand, properly dried reconstructed HTC with the absence of physisorbed water also exhibited a very low activity in the reaction. The effect of Cannizzaro reaction as a simultaneous process with aldol condensation on the composition of reaction products was also considered. The obtained results may be useful for optimizing the preparation conditions of reconstructed HTC as a basic catalyst as well as for evaluating their performance during long-term catalytic tests.

Structure-activity correlations of calcined Mg-Al hydrocalcites for aliphatic polycarbonate synthesis via transesterification process

Mg-Al mixed oxides with different Mg/Al molar ratio were prepared by thermal decomposition of hydrotalcitelike precursors at 500 °C for 5.0 h and used as catalysts for the transesterification of diphenyl carbonate with 1,4-butanediol to synthesize high-molecular-weight poly(butylene carbonate) (PBC). The structure-activity correlations of these catalysts in this transesterification process were discussed by means of various characterization techniques. It was found that the chain growth for the formation of PBC can only be obtained through connecting ―OH and ―OC(C)OC6H5 end-group upon removing the generated phenol, and the sample with Mg/Al molar ratio of 4.0 exhibited the best catalytic performance, giving PBC with Mw of 1.64 × 105 g/mol at 210 °C for 3.0 h. This excellent activity depended mainly on the specific surface area and basicity rather than pore structure or crystallite size of MgO.

Preparation of mesoporous material from hydrotalcite/carbon composite precursor for chromium(VI) removal

The composite (LDH/C) of MgAl hydrotalcite and carbon was synthesized in the presence of glucose, which was used as the precursor of the corresponding MgAl mixed oxide (MgAlOC). The as-synthesized hydrotalcites and their corresponding mixed oxides were characterized by XRD, FTIR, SEM, BET/BJH and TG/DTA. The results showed that carbon in the LDH/C could act as a mesoporous template, and the MgAlOC had higher surface areas and more mesopores as well as higher Cr(VI) adsorption performance than the MgAl mixed oxide (MgAlO) prepared in the absence of glucose. The experimental data in the Cr(VI) adsorption by the MgAlOC was analyzed using various adsorption kinetic and thermodynamic models. The pesudo-second order and Elovich models fitted the experimental data. And the intraparticle diffusions model fitted the experimental data well at the second stage of adsorption (>30min), indicating that the intraparticle diffusion in the Cr(VI) adsorption process was rate-limiting step. Combined with thermodynamic analysis, the activation energy (21.38 kJ·mol−1) suggested that the adsorption process was chemisorption nature. The results demonstrated that the intercalation of CrO42− into the interlyer spaces of the MgAlOC occurred by the reconstruction, and followed by the anion-exchange of interlayer –OH and CrO42− anions companying with diffusion in the entire adsorption process.

Methanol and ethanol reactivity in the presence of hydrotalcites with Mg/Al ratios varying from 2 to 7

Mg-Al mixed oxides, prepared from hydrotalcite precursors with Mg/Al ratio varying from 2 to 7, were tested in the Guerbet reaction of methanol and ethanol to heavier products. Acidic-basic properties were followed by the adsorption calorimetry of SO2 and NH3 molecules and correlated with the catalytic activity. The results showed that the selectivity to higher alcohols is increasing with the increase of both kinds of active sites.

The occurrence of Cannizzaro reaction over Mg-Al hydrotalcites

Mg-Al mixed oxides and reconstructed hydrotalcites prepared from mixed oxides by rehydration attract much attention as solid basic catalysts for organic reactions, such as aldol condensation. Therefore, the understanding of parameters that govern their catalytic performance is of great importance. Cannizzaro reaction in the presence of mixed oxide and reconstructed HTC as basic catalysts was used as a model reaction to characterize both catalytic systems and to understand their behavior in aldol condensation. The performance of the samples was investigated in the conversion of aqueous furfural mixture using stirred batch reactor operating at T=50°C. The properties of both initial samples and catalysts after reaction were studied by different physico-chemical methods, such as XRD, DRIFT and TGA. It was shown that both mixed oxide and reconstructed HTC possessed poor activity in the conversion of dried furfural proving the importance of water presence for the reaction to take place. Addition of water to the reaction mixture resulted in growth of the yield of both furfuryl alcohol and furoyl furoate as reaction products. Furoic acid was not identified by GC analysis, but the interaction of the acid with the basic sites of catalysts was confirmed by DRIFT and TGA analyses indicating that at optimum reaction conditions, most of the basic sites were involved in the formation of furoate species. The formation of the surface furoate species influenced the behavior of the basic catalysts in aldol condensation of furfural and acetone. It allows concluding that the occurrence of Cannizzaro reaction should be taken into account when considering the behavior of basic catalysts in organic reactions with furfural as a reactant.

Storage capacity and oxygen mobility in mixed oxides from transition metals promoted by cerium

The oxygen mobility and storage capacity of Ce-Co/Cu-MgAl or Ce–MgAl mixed oxides, obtained by hydrotalcite precursors, were evaluated using Toluene-temperature-programmed-reaction, 18O2 isotopic exchange and O2-H2 titration. The presence of oxygen vacancies-related species was evaluated by means of Electron Paramagnetic Resonance. A correlation was found between the studied properties and the catalytic activity of the oxides in total oxidation processes. It was evidenced that catalytic activity depends on two related processes: the facility with which the solid can be reduced and its ability to regenerate itself in the presence of molecular oxygen in the gas phase. These processes are enhanced by Cu-Co cooperative effect in the mixed oxides. Additionally, the incorporation of Ce in the Co-Cu catalysts improved their oxygen transport properties.

One-pot synthesis of biomorphic Mg-Al mixed metal oxides with enhanced methyl orange adsorption properties

Biomorphic Mg-Al mixed metal oxides with hierarchical structures were synthesized by a one-pot method employing cotton fibers as templates. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). It was found that the as-prepared biomorphic Mg-Al mixed metal oxides retained the tubular morphology of cotton fibers. Adsorption of biomorphic Mg-Al mixed metal oxides for methyl orange was investigated in detail. Different conditions were evaluated, including adsorbent dosage, initial methyl orange concentration, adsorption time and temperature. The adsorption mechanism was also discussed according to the adsorption kinetics and thermodynamics. The results indicated that the adsorption process was well fit to Freundlich isotherm model and pseudo-second-order kinetic model.

Effect of One-Pot Rehydration Process on Surface Basicity and Catalytic Activity of MgyAl1-aREEaOx Catalyst for Aldol Condensation of Citral and Acetone

The liquid phase synthesis of pseudoionones (PS) by the cross-aldol condensation of citral and acetone was investigated over MgAl mixed oxides containing rare earth elements (REE = Y, La, Eu), which were obtained from corresponding REE-modified hydrotalcite materials after calcination. The results showed that the unmodified and La(Eu)-modified MgAl mixed oxide catalysts showed relatively low activity, and Y-modified MgAl mixed oxides presented an unexpected high catalytic activity. PS selectivity of ∼85% and citral conversion of 100% were achieved at 60 °C for 3 h. On the basis of the characterizations of the structural, textural, and basic properties, it was found that Mg3Al1-aYaOx catalysts exhibited relatively well-developed small flake morphology with high surface area and pore volume, resulting in exposure of more basic sites on the catalyst surface. The formation of PS over Mg3Al1-aYaOx may be accompanied by gradual modification of the catalyst surface to form re-Mg3Al1-aYaOx through a rehydration p...

A La-doped Mg-Al mixed metal oxide supported copper catalyst with enhanced catalytic performance in transfer dehydrogenation of 1-decanol.

In the present work, a La-doped Mg-Al mixed metal oxide supported copper catalyst (Cu/La-MgAlO) was synthesized through a layered double hydroxide precursor route. The materials were characterized by powder X-ray diffraction, transmission electron microscopy, CO2-temperature programmed desorption, Fourier transform infrared spectra of CO2 absorption, and X-ray photoelectron spectroscopy. The results revealed that the introduction of a trace amount of La could significantly improve the surface basicity of the Cu/La-MgAlO catalyst, especially strong Lewis basicity. Compared with the undoped supported Cu catalyst, Cu/La-MgAlO exhibited much higher activity and selectivity in the liquid-phase transfer dehydrogenation of 1-decanol with a 1-decanal yield up to 89%. The excellent catalytic efficiency was mainly ascribed to the surface cooperation between the Lewis basic sites and the adjacent Cu(0)/Cu(+) species. That is, basic sites, especially strong-strength basic sites, held the key to the abstraction of protons from the hydroxyl group in 1-decanol, while the adjacent Cu(0) and Cu(+) species were responsible for the hydrogen transfer and the adsorption of styrene in the transfer dehydrogenation and hydrogenation reactions, respectively. This study provides a new method for designing cost-effective supported copper-based catalysts highly efficient for the transfer dehydrogenation of primary aliphatic alcohols by modifying the surface basicity of metal oxide supports.

Insight into reaction pathways in CO hydrogenation reactions over K/MoS2 supported catalysts via alcohol/olefin co-feed experiments

Reaction pathways for higher alcohol synthesis from syngas are studied over K/MoS2 domains supported on mesoporous carbon (C) and mixed MgAl oxide (MMO) via addition of methanol, ethanol, and ethylene co-feeds.

Transition metal-modified mesoporous Mg-Al mixed oxides: Stable base catalysts for the synthesis of diethyl carbonate from ethyl carbamate and ethanol

A series of transition metal-modified hydrotalcite precursors (HT-M, M=Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) were prepared by using the “memory effect” of Mg-Al hydrotalcite. Then, the mixed oxides with different composition were obtained by thermal decomposition of as-prepared hydrotalcite precursors at 773K, and their physicochemical properies were characterized by thermogravimetry and differential thermogravimetry (TG-DTG), X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectroscopy (XPS), scanning electron micrograph (SEM) and CO2 temperature-programmed desorption (CO2-TPD) technique. It was found that the mesoporous structure was formed during calcination. More important, the basicity of the calcined hytrotalcites (HTC-M) samples was depended on the type of transition metal. Simultaneously, they were used as solid base catalysts and evaluated for the synthesis of diethyl carbonate (DEC) from ethyl carbamate and ethanol. Among them, HTC-Mn exhibited maximum activity with the DEC yield of 56.7% under optimum reaction conditions. Then, the correlation between their basic properties and catalytic performance was studied in detail for the HTC-M catalysts. Furthermore, HTC-Mn catalyst could be readily recycled while maintaining high catalytic activity toward this reaction.

Glycerol steam reforming over layered double hydroxide-supported Pt catalysts

Layered double hydroxides containing Mg and Al (Mg/Al ratios of 3 and 5) were used as support for Pt-based catalysts for glycerol steam reforming. Additionally, catalysts supported on the parent MgAl mixed oxides were also evaluated. Fresh catalyst samples were characterized by XRD, BET, TPD-CO2 and XRF whilst the spent catalysts were examined by TEM and TPO/TGA-MS. All catalysts revealed to be active, leading to a hydrogen-rich gas stream but with distinct resistance to deactivation. The catalyst synthesized directly from the layered double hydroxide precursors with Mg/Al ratio of 3 was shown to be more effective since global and gas conversion are similar, varying within 60–25%. Major incidence of weak to moderate basic surface centers rendered catalysts more selective, reaching up to 68% selectivity to hydrogen. However, they were not enough to suppress deactivation. It was found that the formation of more stable carbon deposits play a key role on deactivation and only a minor contribution from the carbonaceous material formed from the intermediate organic liquid compounds was proposed. Highly dispersed metal centers were suggested to be important for in situ catalyst surface cleanness.

Tuning of higher alcohol selectivity and productivity in CO hydrogenation reactions over K/MoS2 domains supported on mesoporous activated carbon and mixed MgAl oxide

Higher alcohol synthesis from syngas is studied over K/MoS2 domains supported on mesoporous carbon (C), mixed MgAl oxide (MMO), or mixtures thereof. While the carbon support offers high ethanol productivity, the MMO support yields enhanced C3+OH selectivity. MoKMMO-C, whereby Mo is initially contained on MMO then ground with carbon, behaves similar to the parent MoKMMO catalyst, as Mo on MMO has limited mobility during reaction. In contrast, on MoKC-MMO, significant Mo migrates from C to MMO during reaction, giving reactivity associated with Mo species on both supports (high C3+OH selectivity and productivity). MoS2 domain structures are correlated with the selectivity of the catalysts (C3+OH selectivity∼double MoS2 layers, total hydrocarbon selectivity ∼ single MoS2 layers). This study advances the understanding of the support’s effect on structure–reactivity relationships for this family of catalysts and introduces a new catalyst composition with desirable reactivity.

Enhanced NOx conversion by coupling NOx storage-reduction with CO adsorption-oxidation over the combined Pd−K/MgAlO and Pd/MgAlO catalysts

NOx conversion was enhanced by coupling NOx storage-reduction (NSR) with CO adsorption-oxidation (CAO), which was observed in NSR lean-rich cycle tests with CO as a reductant on catalysts of Mg-Al mixed oxides supported Pd (Pd/MgAlO) and Pd–K (Pd–K/MgAlO). CO was adsorbed on Pd sites in the rich condition and then oxidized by gaseous NOx via Eley–Rideal mechanism during the transition from the rich condition to the lean condition, which results in an enhanced NOx conversion. This is confirmed by different configuration experiments of Pd–K/MgAlO and Pd/MgAlO, acting as NSR and CAO catalysts, respectively. The intimate mixture of Pd–K/MgAlO and Pd/MgAlO showed much higher activity due to the resultant coupling effect of NSR with CAO in comparison with a zone-arranged catalyst system no matter whether Pd–K/MgAlO or Pd/MgAlO was in the front. The coupling also worked in the presence of H2O and CO2 despite that the efficiency were spoiled. This strategy is ease to be implemented and thereby is potential for the practical after-treatment technology for lean-burn and diesel engines.

Adsorption microcalorimetry characterization of K-doped MgAl mixed oxide catalysts for soybean oil transesterification synthesized by impregnation and ball milling techniques

Layered double hydroxides were prepared by pH-controlled co-precipitation method with a Mg/Al molar ratio between 1.5 and 4.0 and used as precursors for obtaining, through calcination, a series of MgAl mixed oxides, which were used as catalysts for soybean oil transesterification with methanol. The mixed oxide with the highest Mg/Al ratio was doped with potassium for obtaining highly basic catalysts. Three different potassium salts (K2CO3, KNO3, CH3COOK) were used for loading potassium on the support (K loading ca. 3 mass%) by two different techniques, namely conventional incipient wetness impregnation and innovative mechanical milling. All the catalysts were characterized as to their chemical composition, structure and texture by inductively coupled plasma atomic emission spectroscopy, X-ray diffraction and N2 physisorption, respectively. Their basic and acid features were assessed by adsorption microcalorimetry, using CO2 and NH3 as probe molecules, respectively. Catalytic testing was carried out in a slurry batch reactor operated at 343 K and atmospheric pressure. The occurrence of potassium leaching into the liquid phase was checked for the K-doped catalysts. The initial activity of the heterogeneous catalysts was interpreted in terms of surface basicity, by taking into account a possible role of acidity in determining the mechanism.

이소프로판올의 탈수반응에 있어서 Mg-Al 혼합 산화물의 산점 특성

Mg/Al 몰비를 1에서 3 범위에서 변화시켜 Mg-Al 혼합산화물을 공침법으로 제조하였다. 제조된 Mg-Al 혼합산화물은 X-선회절분석, 주사전자현미경분석, 질소흡착에 의한 비표면적 및 기공부피 측정, 이소프로판올의 승온탈착모델 반응으로 제조된 산화물의 특성분석을 수행하였다. 소성된 Mg-Al 혼합산화물은 이소프로판올의 탈수소/탈수반응에 적용하여 산염기 특성을 비교하였다. Mg-Al 혼합산화물에서 Al 함량이 증가함에 따라 비표면적 및 산점세기가 비례적으로 증가하였다. 이러한 산점세기 변화는 이소프로판올의 전환율 및 프로필렌 선택도에 직접적인 영향을 미쳤다. Mg-Al mixed oxides with molar ratio of Mg/Al = 1-3 were prepared by co-precipitation and characterized by using X-ray diffraction, scanning electron microscopy, BET surface area and pore volume measured by <TEX>$N_2$</TEX> sorption analysis, and temperature programmed desorption of iso-propanol. As Al content in Mg-Al mixed oxide increased, the acidity and BET surface area proportionally increased. This increase of acidity directly influenced the catalytic activity of iso-propanol conversion and selectivity to propylene.

Synthesis of Glycerol Carbonate by Transesterification of Glycerol with Dimethyl Carbonate over Mg-Al Mixed Oxides Supported on MCM-41

The Mg-Al mixed oxides were deposited on the MCM-41 via the coprecipitation followed by thermal decomposition and characterized by many techniques. In the transesterification of glycerol (GL) with dimethyl carbonate (DMC), the resulting supported catalysts exhibited much higher catalytic performance than the pure Mg-Al mixed oxides, which was ascribed to the increased basicity. Under the optimal reaction conditions, the obtained data showed that at DMC/glycerol molar ratio of 3:1, catalyst loading of 0.3 g and reaction temperature of 373 K, the glycerol conversion and glycerol carbonate yield from the process was 98.7 % and 92.5%, respectively.

Fabrication of supported PdAu nanoflower catalyst for partial hydrogenation of acetylene

Three-dimensional PdAu nanoflowers have been designed and effectively synthesized through a procedure of dropping addition of the reductant, in which morphology evolution was recorded by capturing HRTEM images at continuous synthetic stages. HRTEM, positron annihilation spectroscopy, and in situ CO-IR results showed that synthesized flowerlike PdAu nanoalloys with a size of 30–55nm were composed of several small PdAu building units and have abundant crystal defects as well as synergetic effects. The obtained colloidal PdAu nanoflowers were then immobilized onto MgAl mixed metal oxides as a novel catalyst for partial hydrogenation of acetylene. Due to its complex morphology, rich defective sites, and synergetic effect caused by the cooperation between building units, supported PdAu nanoflower-like catalyst exhibited not only much higher activity and selectivity but also preferable stability compared with the catalyst whose active component was shape- and size-similar to the building units of PdAu nanoflowers.