Hydrotalcite-like Research Articles

Enhanced catalytic performance of isobutane direct dehydrogenation over Pt-In catalysts: Effect of different fluorides modified hydrotalcite-like derivatives

A series of MgF2-modified hydrotalcite-like (HT) derivatives supported Pt-In catalysts PtInHTR-X (X = NH4F, NaF and KF, R = reduced catalyst) were prepared by combination of hydrothermal method, alkali-etching and impregnation strategy and applied to direct dehydrogenation of isobutane to isobutene. The catalysts and precursors were characterized by XRD, SEM, TEM, XPS, TG, O2-TPO and low temperature N2 adsorption-desorption isotherms. The formation of MgF2 phase as dopant to modify the catalyst can influence the pore structure and even the surface acid properties of catalysts. Especially, compared with the three catalysts, PtInHTR-KF catalyst had better stability of isobutene selectivity of over 95% after 9 h reaction, and PtInHTR-NH4F and PtInHTR-NaF catalysts exhibited more stable isobutane activity of 60%. One of the reasons for this catalytic performance is that small particles of MgF2 and unsaturated MgF2 can provide weak acidic sites which can inhibit coke deposition. And it was found the improved catalytic performance was also related to the appropriate In3+/In0 ratio, well-dispersed Pt species and the excellent anti-sintering ability of catalysts modified MgF2.

Influence of the 8-quinolinol concentration and solution pH on the interfacial properties of self-healing hydrotalcite coating applied to AZ31 magnesium alloy

The hydrotalcite-like (HT) coatings can provide an enhanced interfacial barrier to protect Mg alloys from corrosion. Therefore, 8-quinolinol (8Q), a green corrosion inhibitor, was intercalated in MgAl-based HT with varying concentration and pH values. The results revealed that the corrosion resistance was significantly improved when the pH of the solution was maintained at 10 with a 0.30 M concentration of 8Q. The hydrophobicity was also introduced to MgAl HT with the intercalation of 8Q, which was retained even after the long immersion. Furthermore, the self-healing performance of 8Q-intercalated HT was confirmed by single-frequency electrochemical impedance spectroscopy (EIS), which is a suitable method for in-situ evaluation of self-healing herein reported for the first time to evaluate the HT coatings. This thorough investigation of the hydrophobic MgAl HT coating is meaningfully supportive for researchers and engineers to have a deep understanding of its possibility for real-world applications in the area of corrosion and prevention.

Co-production of high quality hydrogen and synthesis gas via sorption-enhanced steam reforming of glycerol coupled with methane reforming of carbonates

The sorption enhanced steam reforming (SESR) produces high-purity hydrogen and capture CO2 in the form of carbonates, allowing for controllable CO2 storage or utilization. However, the decarbonation of carbonates at temperatures high up to 800–900 °C makes the SESR process and the subsequent CO2 utilization energy-intensive. Herein, we report the feasibility coupling the SESR of glycerol with CH4 reforming of carbonates, which combines the decarbonation with CO2 utilization to simplify the technology and avoid the energy-consuming temperature swing. A high-efficiency bi-functional Ni-CaO-Ca12Al14O33 catalyst was proposed to this process. The catalyst was derived from a Ni-Ca-Al hydrotalcite-like (HTl) material to achieve homogeneous mixing of Ni, CaO and Ca12Al14O33 which were active metal, sorbent and a spacer to prevent CaO sintering. Thanks to the high activity and stability of Ni-CaO-Ca12Al14O33 catalysts, 98.53% hydrogen-enriched gas could be obtained in SESR of glycerol, and the high CH4 conversion (95.3%) and H2/CO molar ratio (<2) suitable for further Fisher-Tropsch synthesis were achieved during the CH4 reforming reaction. Meanwhile, Ca12Al14O33 as a spacer had provided a stability for ten cycles. Our results highlight the significance of process intensification and high-efficiency bi-functional catalysts for the hydrogen production with low carbon emissions.

Influence of Zr on the performance of Mg-Al catalysts via hydrotalcite-like precursors for the synthesis of glycerol carbonate from urea and glycerol

A series of Zr-containing Mg-Al hydrotalcite-like (HTl) precursors were prepared using co-precipitation method with Zr4+: (Al3+ + Zr4+) from 0 to 0.7. It was demonstrated by X-ray diffraction that the yield of the HTl phase declined on increasing of Zr content. Then, Mg-Al-Zr mixed oxide (CHT-Zr) catalysts were obtained by calcining of the corresponding HTl precursors and they all exhibited mesoporous structure after heat treatment. Based on systemic characterization, it was observed that the amount of Zr additive showed a great effect on the structure and acidic-basic property of CHT-Zr samples. Meanwhile, such CHT-Zr catalysts were effective catalysts for the glycerol carbonate (GLC) synthesis from glycerol and urea. Afterwards, the function of acidic and basic sites in each reaction path identified in this research was elucidated based on the reactant conversion and product selectivity. It proved that the acidic sites were inclined to promote the reaction towards GLC synthesis, while all the paths towards the by-products were significantly catalyzed by the basic sites. However, the catalysts with extra strong acidity did not facilitate the GLC production due to poisoning of surface active sites by urea adsorption. So, a well-balanced acidic-basic property was important to obtain high GLC yield with good selectivity. Among all the catalytic samples, the catalysts with Zr4+: (Al3+ + Zr4+) = 0.3 showed the best catalytic performance with 87.8% yield of GLC. Moreover, different reaction parameters such as reaction temperature, reaction time, catalyst amount and vacuum degree were investigated and optimum conditions were established. Additionally, this catalyst could be readily recycled and reused for at least four times without any activation treatment, while still possessing high activity.

The effect of La on the hydrotalcite derived Ni catalysts for dry reforming of methane

A series of 20Ni-Mg-Al hydrotalcite-like (HT) precursors were prepared to study the influence of lanthanum (La) on the catalytic activity of the catalysts in the dry reforming of methane (DRM). The catalysts were characterized by N2 adsorption-desorption, X-ray diffraction (XRD) and temperature-programmed reduction (TPR). All catalysts presented ordered mesoporous structures with a large specific surface area. XRD confirmed the presence of HT structure for all of the precursors while the La promotion resulted in an additional phase of Lanthanum carbonate hydroxide. TPR study showed larger reduction degree for the catalysts but also reduction peaks that are shifted to higher temperatures. DRM reactions at 600 and 750 °C revealed that the DRM activity was increased by the addition of La, while the stability of the catalysts was reduced at 600 °C.

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol with Dimethyl Carbonate

A series of fluorinated Mg–Al hydrotalcite-like (HTl) compounds were prepared by introduction of different amount of (AlF6)3− into HTl sheets via co-precipitation method. The fluorine-modified Mg–Al mixed oxides (CHT-F) with mesoporous structure were then acquired by thermal decomposition of as-prepared HTl precursors. The influence of fluorine modifier on the physicochemical properties of CHT-F samples was studied in detail and the results demonstrated that their structure and basicity were strongly related to the fluorine content. Simultaneously, the CHT-F samples were test for the glycerol carbonate (GC) production via transesterification between glycerol and dimethyl carbonate without organic solvent. For these catalysts, the role of basic sites was examined based on reactant conversion and product selectivities in overall reaction as well as in side reaction identified in this work. The catalytic results showed that the activity of CHT-F catalysts depended on the total number of surface basic sites. However, further study demonstrated that the undesired product was catalyzed by strong basic sites, resulting in decrease of GC selectivity. Thus, the introduction of appropriate amount of (AlF6)3− into the HTl structure can promote the production of GC, and the best catalytic performance was obtained over the catalyst with F:Al = 1.0. Besides, various parameters including reaction time, reaction temperature and catalyst amount were investigated to optimize the reaction conditions. Furthermore, these CHT-F catalysts possessed high stability on the basis of reusability test and catalyst characterization.

Methanation of CO2 over a (Mg,Al)Ox Supported Nickel Catalyst Derived from a (Ni,Mg,Al)‐Hydrotalcite‐like Precursor

AbstractThe CO2 methanation performance of a Ni catalyst supported on a Lewis basic (Mg,Al)Ox mixed oxide derived from a (Ni,Mg,Al)‐hydrotalcite‐like (HTL) precursor was investigated. The HTL structure of the precursor, synthesized by pH‐controlled coprecipitation, was confirmed by using XRD. Reduction at 900 °C led to metallic Ni particles supported on a spinel‐type (Mg,Al)Ox matrix. Catalytic measurements between 210 and 400 °C revealed a reproducible CO2 conversion into CH4. Kinetic analysis of the data in the lower temperature range resulted in an apparent activation energy of (83±7) kJ mol−1 for this reaction. After a latency period of 10 h, the catalyst showed excellent long‐term stability for up to 50 h on stream under methanation conditions.

A bi-functional Co–CaO–Ca12Al14O33 catalyst for sorption-enhanced steam reforming of glycerol to high-purity hydrogen

Sorption enhanced steam reforming of glycerol (SESRG) enables the production of high-purity hydrogen from the by-product of biodiesel manufacture. To this end, highly stable catalysts and CO2 sorbents are desired to overcome the performance degradation at high temperatures. Herein, a bi-functional catalyst, Co–CaO–Ca12Al14O33, is proposed to couple CO2 sorption and catalytic reforming of glycerol on nanoscale. The catalyst was derived from a Co–Ca–Al hydrotalcite-like (HTl) material to achieve homogeneous mixing of Co, CaO and Ca12Al14O33 as a spacer to prevent CaO sintering. At Ca/Al of 2.8, the highest catalytic activity and sorption capability were reached. Over the optimized catalyst, high-purity H2 of 96.4% was produced at 525°C through SESRG during the pre-breakthrough stage. High stability of the bi-functional catalyst was demonstrated by a cyclic SESRG-calcination operation up to 50 times. The role of Ca12Al14O33 as spacer of CaO was proved by XRD and TEM-EDS analysis. In addition, it was found that Ca3Co2O6, which is formed during the calcination stage and is reduced during the SESRG to release Co catalysts, may act as a dynamic reservoir of Co catalysts to prevent Co from sintering, leading to an excellent SESRG activity and stability.

Pulsed laser deposition of Mg–Al layered double hydroxide with Ag nanoparticles

Powdered layered double hydroxides (LDHs)—also known as hydrotalcite-like (HT)—compounds have been widely studied due to their applications as catalysts, anionic exchangers or host materials for inorganic or organic molecules. Assembling thin films of nano-sized LDHs onto flat solid substrates is an expanding area of research, with promising applications as sensors, corrosion-resistant coatings, components in optical and magnetic devices. The exploitation of LDHs as vehicles to carry dispersed metal nanoparticles onto a substrate is a new approach to obtain composite thin films with prospects for biomedical and optical applications. We report the deposition of thin films of Ag nanoparticles embedded in a Mg–Al layered double hydroxide matrix by pulsed laser deposition (PLD). The Ag-LDH powder was prepared by co-precipitation at supersaturation and pH = 10 using aqueous solutions of Mg and Al nitrates, Na hydroxide and carbonate, and AgNO3, having atomic ratios of Mg/Al = 3 and Ag/Al = 0.55. The target to be used in laser ablation experiments was a dry pressed pellet obtained from the prepared Ag-LDH powder. Three different wavelengths of a Nd:YAG laser (266, 532 and 1064 nm) working at a repetition rate of 10 Hz were used. X-Ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and secondary ions mass spectrometry (SIMS) were used to investigate the structure, surface morphology and composition of the deposited films.

Dependence of chemical composition of calcined hydrotalcite-like compounds for SO x reduction

Binary and ternary hydrotalcite-like (HT) materials containing copper, nickel, zinc, iron, magnesium and/or aluminum were synthesized by coprecipitation and characterized by elemental analysis, powder X-ray diffraction and N 2 adsorption–desorption at −196 °C. Samples were calcined at 700 °C for 4 h, obtaining mixed oxides which were tested for sulfur oxides (SO x ) removal. Their performance was compared with a commercial SO x reducing additive by thermogravimetric SO x uptake. SO x adsorption was carried out by contacting the calcined solid with a mixture of 1% SO 2 in air at 650 °C. The adsorbed species were reduced with H 2 at 550 °C, 650 °C and 700 °C, in order to study their stability and the solid's regenerability. It was found that the isomorphic incorporation of a transition metal in the HT layers increased the mixed oxide's reduction ability. This was reflected on higher reduction rates and sulfate reduction amounts at 550 °C. Furthermore, these materials showed greater adsorption–reduction properties than the commercial additive.

A Simple Environmentally Friendly Method to Prepare Versatile Hydrotalcite-like Compounds

Synthesis of multimetallic hydrotalcite-like (HT) materials is reported, by means of an easy environmentally friendly method. Product washing or purification steps are eliminated. It provides a viable alternative for HT production in large quantities. Therefore, those processes using HTs, and nowadays limited by their unavailability of large amounts, will be feasible.

Anion substitution of nitrate-saturated layered double hydroxide of Mg and Al

In the context of producing industrial-grade hydrotalcite-like (HT) materials containing phosphate and sulfate for use as slow-release fertilizers, the ease with which nitrate-HT can be converted to phosphate-HT and sulfate-HT has been examined. Graded amounts of di-ammonium hydrogen phosphate and ammonium sulfate were added to nitrate-HT, while maintaining pH at 9.5, to produce substitutions ranging from approximately 50% to 100%. Chemical and X-ray diffraction analysis of the products confirmed that the substitution could be easily effected by simple topotactic exchange.

Basicity and Porosity of a Calcined Hydrotalcite‐Type Material from Nitrogen Porosimetry and Adsorption Microcalorimetry Methods.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Basicity and Porosity of a Calcined Hydrotalcite-Type Material from Nitrogen Porosimetry and Adsorption Microcalorimetry Methods

The hydrothermal transformation at 90−95 °C of a slurry of Al2O3/MgO/(CH3COOH)/H2O with molar oxide composition 1:4.4:2.2:1111 produced a hydrotalcite-like (HT) material free from phase impurities and having an approximate unit cell composition of Mg0.66Al0.33(OH)2(CHCOO)0.33. 13C NMR and 27Al NMR spectra have revealed that the air-dried solid contains acetate anions probably as charge-compensating anions in the HT interlamellar space and that all the aluminum is present in octahedral coordination. After calcination in air at 500 °C, the stacks of brucite layers that form the HT crystals exfoliate, forming a stable mesoporous “house-of-cards” structure with pore width in the range 2.5−20.0 nm, characterized by a DFT surface area of 254 m2/g and total pore volume of 0.33 cm3/g at 0.995 p/Po, that is available to sorption and catalysis. A 5-h exposure to 100% steam at 760 °C collapses the HT material forming a residue that contains a well-crystallized MgO and an Al spinel phase identified by XRD. At 30 °C, adsorption microcalorimetry with SO2 revealed that the calcined HT material behaves like a solid base possessing a large (530 μmol SO2/g) population of basic sites with strength near 170 kJ/mol; the strength of the strongest basic site is 190 kJ/mol. The steam-aged HT retains the strength of its strongest basic sites but loses most of its sorption capacity for SO2. Thus, materials of this type should be efficient adsorbents for the removal of sulfur oxides and other acidic air pollutants in industrial applications that do not require the abovementioned hydrothermal stability.

Impregnation-induced memory effect of thermally activated layered double hydroxides

The memory effect of calcined Cu–Co–Zn–Al layered double hydroxides (LDH) upon impregnation with K-containing solutions was studied. A sample of nominal composition CuCoZn 2Al 4O x was prepared by coprecipitation at controlled pH. The resulting hydrotalcite-like (HT) precipitate was calcined at 673, 773, or 873 K. The products were impregnated with different K-containing solutions [K 2CO 3(aq), KOH (aq) and KOH (alc)]. Reconstruction of the HT phase upon impregnation was followed by X-ray diffraction. The HT recrystallization was favored by impregnating calcined LDH samples with K 2CO 3(aq) by wet impregnation. The memory effect was reduced by increasing the calcination temperature of the parent layered double hydroxide. It is proposed that the reconstruction of the HT phase occurs via a retro-topotactic transformation from the Al 3+ and divalent cations located both in the octahedral sites of the oxide matrix. The increase of the calcination temperature causes the solid-state diffusion of divalent cations into tetrahedral positions which results in the progressive formation of stable normal spinels. After calcination at 873 K, all the divalent cations occupy tetrahedral sites and the memory effect disappears.

Electrodes coated by hydrotalcite-like clays. Effect of the metals and the intercalated anions on ion accumulation and retention capability

The influence of the nature of the di- and trivalent metals and of the interlayer anion of different hydrotalcite-like (HT) clays on the anion-exchange ability is investigated. In view of the application of similar clays as electrode coatings in electroanalysis and electrocatalysis, the responses of electrodes modified with the different HT to electroactive species in solution constitute the primary source of information. However, XRD measurements have proved to be invaluable tools when coupled with the electrochemical data. Two organic and one inorganic anion with reversible redox behaviour, i.e. anthraquinone mono- and disulphonates and hexacyanoferrate(II), have been chosen as electroactive species.

Synthesis of Hydrotalcite-Like Precursor Compounds MgCo(11) Co(111).-HTlcs

AbstractPrecursor compounds of MgCo(11)Co(111)-HTlcs for Mg-Co oxide catalysts have been synthesized by coprecipitation in an ammoniacal solution and hydrothermal treatments under various O2/N2 atmospheres. Using elemental analysis (EA) and redox titration, it is found that about 23% of original Co2+ are oxidized to Co+3, resulting in formation of hydrotalcite-like (HTI) structure. The anion (NO3) intercalation increases with the rise in O2 partial pressure. X-ray diffraction pattern shows a conversion from brucite-like to HTl structures with increase of O2 partial pressure. It is found that thermal stability of the HTl-compounds is proportional to the Mg content. Higher specific surface areas are observed for decomposed products with low Co/Mg ratio. A trace amount of CO3O4 is also detected by FTIR for samples synthesized under high O2 partial pressures, which can be attributed to a deeper oxidation of Co2+.

Anion-Exchange Properties of Hydrotalcite-Like Compounds

AbstractIon-exchange isotherms between hydrotalcite-like compounds (HT) of the NO3-, Cl-, and SO4- forms and F−, Cl− Br−, I−, OH−, SO42−, CO32−, and Naphthol Yellow S (NYS2−) ions were determined, and the spacing and the width of the 003 reflection were measured as a function of HT composition. The ion-exchange equilibrium constant for HTs of monovalent anions are in the sequence OH− &gt; F− &gt; Cl− &gt; Br− &gt; NO3− &gt; I−, those for divalent anions are in the sequence CO32− &gt; NYS2− &gt; SO42−. The ion-exchange equilibrium constants tend to increase as the diameters of the anions decrease, and the crystallite size in the 001 direction tends to increase with anions having higher selectivity. The OH-form of HT has the smallest basal spacing and the largest crystallite size in the 001 direction.