Hydrotalcite-like Materials Research Articles

Influence of textural and structural properties of Mg[sbnd]Al and Mg[sbnd]Zn[sbnd]Al containing hydrotalcite derived oxides on Cr(VI) adsorption capacity

A series of hydrotalcite-like materials containing Mg2+/Zn2+ and Al3+ cations in the brucite-like sheets and carbonate or benzoate anions in the interlayer gallery was prepared by the coprecipitation method. The difference in the chemical composition of the synthesized layered double hydroxides resulted in obtaining mixed metal oxides with various phase composition (caused by different cations) as well as various crystallinity and textural parameters (caused by different anions). The MgZnAl oxide systems obtained from the benzoate-intercalated precursors had significantly higher crystallinity and narrower pore size distribution compared to the carbonate-modified samples. The structural and textural properties of the calcined materials influenced their sorption capacities in the chromate removal from an aqueous solution at the optimal pH of 6.0. The most efficient adsorbent was obtained from the calcined magnesium–zinc–aluminum hydrotalcite-like precursor containing interlayer CO32− anions. This material exhibited high efficiency in the chromate elimination even in the presence of competitive anions (Cl− or SO42−).

Effect of the Thermal Treatment on the Capacity of Removal of Cr (VI) of Sol-Gel Hydrotalcite-like Materials

ABSTRACTIn this work the effect on the adsorption of Cr (VI) by thermally treated Hydrotalcite-like Material (HTM) synthesized by the sol-gel method with a Mg/Al = 2 was studied. The characterization of the HTM before and after Cr (VI) removal, as well as the kinetic studies were carried out. When the HTM are thermally treated at 350ºC the hydrotalcite crystalline structure remains and the textural properties improve. The Cr (VI) adsorption capacity of HTM was 125 mg of Cr (VI) / g of HTM. This capacity augments at 132 mg of Cr (VI) / g of HTM after heating HTM at 300°C. The crystallinity as well the porosity of the HTM diminished after adsorption of Cr (VI), due to the obstruction of both the pores and the interlayer space. The adsorption of Cr (VI) occurred before 4 minutes contact time. It can be concluded that HTM exhibits a high Cr (VI) adsorption capacity in a short time.

Sorption enhanced catalytic steam gasification process: a direct route from lignocellulosic biomass to high purity hydrogen

We report for the first time on the one-stage production of high purity hydrogen from raw solid lignocellulosic biomass by sorption enhanced catalytic steam gasification (SECSG) in a combined downdraft flow fluidized bed and fixed bed reactor. A Pd/Co–Ni catalyst derived from a hydrotalcite-like material (HT) and dolomite as a CO2 acceptor are fed together with a biomass (chestnut wood sawdust) as the reactor feed. Almost pure hydrogen (>99.9 vol%) and high H2 yields (up to 90%) can be achieved by this process, which combines conventional gasification/steam reforming, water-gas shift (WGS) reaction and H2 separation by CO2 capture in one step. The in situ removal of CO2 by the carbonation reaction of dolomite shifts the equilibrium of steam reforming and WGS reactions towards H2 production. The hydrotalcite derived Pd/Co–Ni catalyst has shown high activity in the cleavage of C–C and C–H bonds during the conversion of tars and hydrocarbons resulting from fast pyrolysis and catalytic steam gasification (CSG), as a result of which further char formation is reduced. The catalyst, which does not require pre-reduction, has shown a good activity and stability after several reaction–regeneration cycles.

Nanoconfinement of Ni clusters towards a high sintering resistance of steam methane reforming catalysts

This study reports an improvement of the stability of steam reforming catalysts at relatively low temperatures, such as for pre-reforming, and reforming of biomass derived compounds, by enhanced stabilization of Ni nanoparticles through spatial confinement in a mixed oxides matrix. We revealed a simple approach of three dimensional engineering of Ni particles by means of self-assembly of Ni atoms inside the nanoribbon of hydrotalcite-derived mixed oxides. Taking advantage of Transmission Electron Microscopy (TEM), together with electron tomography, the three dimensional (3D) structure of the catalyst was investigated at a nanometer scale, including the Ni particle size, shape, location and spatial distribution, as well as pore size and morphology of the mixed oxides. Porous nano-ribbons were formed by high temperature treatment, adopting the layer structure of the hydrotalcite-like materials. Ni particles formed by selective reduction of mixed oxides embedded in the nano-ribbons with connected pore channels, allowing good access for the reactants. These spatially confined and well distributed Ni particles increased catalyst stability significantly compared to the Ni particles supported on the support surfaces in a commercial catalyst during the steam methane reforming.

Comparação estrutural entre amostras de materiais tipo hidrotalcita obtidas a partir de diferentes rotas de síntese

Three samples of hydrotalcite-like materials (HTC) were synthesized and their structural characteristics were compared with two HTCs obtained commercially. Thermal analyses, FT-IR, PXRD and textural analyses were used to investigate the structural differences between commercial and synthetic samples. Particularly, the memory effect was observed at temperature higher than 600 oC. The Rietveld refinements were obtained with expressive accuracy and the statistical parameters of goodness of fit are quite satisfactory. In conclusion, the procedures adopted in synthesis of HTC produced crystalline materials with high surface area materials.

Synthesis of hydrotalcite-supported shape-controlled Pd nanoparticles by a precipitation–reduction method

A Pd catalyst supported on a layered double hydroxide (LDH or hydrotalcite-like material) has been synthesized in a one step reaction using a precipitation–reduction method in which hydrolysis of hexamethylenetetramine was used to both precipitate the LDH by virtue of the resulting increase in pH and provide formaldehyde which reduces the Pd 2+ precursor to Pd 0. The resulting Pd nanoparticles were mostly tetrahedral in shape, and were highly dispersed on the surface of the LDH. After introducing a capping agent during the synthesis, the morphology of the Pd particles changed to truncated octahedral, although a similar high degree of dispersion was obtained. Compared with a conventional impregnated catalyst composed of pseudo-spherical Pd particles, catalysts prepared by the new method showed both higher activity and selectivity in the hydrogenation of acetylene. The enhanced activity is due to the specific morphology of the Pd particles, which results in their higher dispersion, while the higher selectivity is attributed to the Pd–C phase formed in the catalyst during the reaction. Furthermore, compared with the truncated octahedral Pd particles enclosed by (1 1 1) and (1 0 0) facets, the tetrahedral particles with only (1 1 1) facets exposed showed higher ethylene selectivity, suggesting that the Pd (1 1 1) facet is the preferred facet in the selective hydrogenation of acetylene to ethylene.

Effect of Co-content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation

The present investigation was undertaken in an endeavor to study the effect of the cobalt content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation by varying the Co 2+/Al 3+ atomic ratio, thermal treatment of the samples and the reaction temperature. The samples (Co 2+/Al 3+ = 0.5, 1.5, 3.0) have been synthesized by the co-precipitation method. The unsupported Co 3O 4 has been prepared according to the same procedure as the reference compound in order to reveal the role of Al 3+ ions presence. The physicochemical characterization of the uncalcined, hydrothermally treated, calcined and tested samples has been accomplished appropriately by ICP-AES, N 2 adsorption, Powder X-ray diffraction technique and Diffuse Reflectance Spectroscopy and H 2-TPR measurements. The samples were examined by a number of heating–cooling cycles during the activity tests as a procedure to screen the most active catalyst precursor. It was established that the hydrotalcite-like structure of all uncalcined samples had been completely destroyed during the CO oxidation reaction. A concomitant phase transformation into poorly crystallized spinel-type Co 2+(Co 3+,Al 3+) 2O 4 mixed oxide occurred. This spinel-like mixed oxide phase is better organized in all samples after their calcination at 500 °C. The TPR examinations reveal concomitant presence of high-temperature reduced non-stoichiometric CoAl 2O 4. It was found out that the Co–Al mixed oxide, derived from the sample with the highest cobalt loading (Co 2+/Al 3+ = 3.0) preserves a complete and prolonged CO oxidation ability even after cooling down to ambient temperature. On the contrary, the samples with ratios Co 2+/Al 3+ = 0.5 and 1.5 as well as the Co 3O 4 oxide deactivate more rapidly during the cycles. A hypothetic scheme is proposed for activation/deactivation of the catalysts. It is related to the oxygen ion-radicals O 2 x– stabilization by Al 3+ cation association with the Co 2+/Co 3+ redox couple via anionic vacancy.

Study of structural transformations and phases formation upon calcination of Zn–Ni–Al hydrotalcite nanosheets

In this paper, we describe a general process for the synthesis of highly crystalline Zn–Ni–Al hydrotalcite-like materials. The structure and thermal decomposition of the prepared samples are studied by XRD, FT–IR, TG–DSC, SEM, TEM and N2 adsorption/desorption. The morphology of large-sized, porous and hexagonal platelike Zn–Ni–Al hydrotalcite is affected by calcination temperature. BET specific surface area and pore volume are observed to increase with increase of the calcination temperature up to 700°C followed by a further decrease with increasing temperature.

An influence of thermal treatment conditions of hydrotalcite-like materials on their catalytic activity in the process of N2O decomposition

Hydrotalcite-like materials containing apart from magnesium and aluminum also copper, cobalt, nickel, and iron were prepared by a co-precipitation method. Thermal transformations of hydrotalcite-like materials were studied by thermal analysis methods as well as XRD, UV–vis–DRS, and XPS measurements of the samples calcined at various temperatures (600, 700, and 800 °C). Calcined hydrotalcites, especially those containing cobalt and copper, were found to be active and selective catalysts of N2O decomposition. It was shown that an increase in the calcination temperature significantly activated the Co-containing catalysts. Promotion of the samples with potassium resulted in activation of the hydrotalcite-based catalysts.

Modified electrode with hydrotalcite-like materials and their response during electrochemical oxidation of blue 69

In order to evaluate the electrocatalytic properties, Mg/Al and Mg/Ca/Al hydrotalcite-like materials (LDHs) and their corresponding mixed oxide were immersed in a carbon paste electrode matrix to obtain the so-called modified carbon paste electrode (MCPE). Previous to preparation of MCPE, LDH materials were characterized by different techniques, as XRD, FTIR and BET analysis. The electrochemical response of the electrodes was characterized in neutral conditions using reactive blue 69 dye as a probe molecule. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), multi-sweep cyclic voltammetry (MSCV) and open circuit potential transient techniques were employed. Different current magnitudes in the oxygen evolution reaction (OER) were found as a function of thermal treatment and supporting electrolyte (pH). On the other hand, in a solution containing 250 ppm of the probe molecule at neutral pH, peaks attributed to blue 69 oxidation were found at ca. 0.65 and 0.95 V/SCE. The presence of calcium, as additional divalent cation, and the rehydration of the mixed oxide, due to memory effect, have a positive performance, increasing the magnitude of the electrocatalytic process.

Synthesis and characterization of a new catalyst Pt/Mg(Ga)(Al)O for alkane dehydrogenation

A novel approach is described for preparing Ga-promoted Pt particles for the dehydrogenation of light alkanes to alkenes. The modifying element, Ga, was introduced by transference from the support, a calcined Mg(Ga)(Al)O hydrotalcite-like material. Pt nanoparticles were dispersed onto the calcined Mg(Ga)(Al)O starting from an organometallic precursor, followed by reduction. The formation of PtGa alloy particles is dependent on reduction temperature. Reduction at 723 K produces mainly metallic Pt particles. The average diameter of the Pt nanoparticles increased from 1.4 nm to 2.2 nm with increasing Ga content, and decreasing Al content of the support, demonstrating the importance of support Al atoms in stabilizing the dispersion of Pt. After reduction at 773–873 K, PtGa alloys were observed. It is proposed that at high reduction temperatures, H atoms formed on the surface of the metal particles spill over onto the support where they reduce Ga 3+ cation to atomic Ga, which then interacts with the supported Pt to form PtGa alloys. The activity, selectivity and stability of Pt/Mg(Ga)(Al)O catalysts for ethane and propane dehydrogenation are described in the second part of this study (G. Siddiqi, P. Sun, V. Galvita, A.T. Bell, Journal of catalysis (2010), doi:10.1016/j.jcat.2010.06.016 [40]).

Synthesis of Propylene Glycol Monomethyl Ether Over Mg/Al Hydrotalcite Catalyst

Mg–Al hydrotalcites with different Mg/Al molar ratios were prepared and characterized by XRD, FT-IR, SEM and BET analyses. The calcined hydrotalcite with Mg/Al molar ratio of 4.0 (LDO Mg/Al 4.0) exhibited the highest catalytic activity in the synthesis of propylene glycol methyl ether (PM). The catalytic activity relating to the amount of the basic sites and crystallinity depended on the Mg/Al molar ratio. The optimal equilibrium of acid–base property and high crystallinity made the LDO Mg/Al 4.0 an excellent catalyst in the reaction. Etherification of propylene oxide (PO) with methanol over the LDO Mg/Al 4.0 was researched. The optimized reaction conditions were as follows: 140 °C, catalyst amount 0.9 wt%, methanol/PO molar ratio 4.0 and 6 h. The PO conversion and PM selectivity were 93.2 and 97.4%, respectively. Above all, almost all the PM was 1-methoxy-2-propanol, for no 2-methoxy-1-propanol was detected by GC analysis in the reaction products, and the catalyst could be reused for five times. The particles of the LDH Mg/Al 4.0 showed well-developed hexagonal plates with narrow size distribution (2–4 μm) and were in line with the typical morphology for hydrotalcite-like materials. The optimal equilibrium of acid-base property and high crystallinity made the LDO Mg/Al 4.0 an excellent catalyst in the synthesis of propylene glycol methyl ether (PM) from methanol and propylene oxide (PO).

Selective reduction of nitrogen oxides by hydrocarbons on hydrotalcite Co and Ni catalysts

Hydrotalcitelike Co-Al and Ni-Al catalysts of different compositions (with the atomic ratio M 2+/Al3+ = 0.5–3.0) were studied in the reaction of selective reduction of NO by propane, propylene, and n-decane in the presence of O2. The higher activity of the catalysts with M 2+/Al3+ = 0.5 is connected with high dispersity of Ni or Co cations stabilized by a significant amount of Al3+ ions. Propylene was shown to be the most efficient reducing agent for nitrogen oxide. The highest degree of conversion to the extent of 90–99% was attained at 400 and 420–440°C for Ni-Al and Co-Al samples, respectively. When propane and decane were used as reducing agents, the conversion of both catalysts was characterized by the volcano-shaped dependence on temperature due to the fact that the catalyst took part in the concurrent reaction of hydrocarbon (reducing agent) oxidation. Hydrotalcitelike materials are promising representatives of inexpensive bi- and multicomponent systems. The design strategy for new active catalysts for processes of purification of gas exhausts from NO x , that are stable in the presence of water and sulfur oxides, may be based on usage of hydrotalcites with modified ions introduced into them.

Towards efficient hydrogen production from glycerol by sorption enhanced steam reforming

An increase in glycerol production is a consequence of the rapid growth in bio-diesel production and makes it attractive to utilize glycerol as feedstock, e.g., for production of hydrogen. Here we report integrating the glycerol steam reforming and CO2 capture reaction in one single reactor as a potential approach to enhance the efficiency of hydrogen production. A thermodynamic analysis comparing glycerol steam reforming with or without CO2 capture was made, highlighting that integration of CO2 removal allows high efficiency in the hydrogen production and energy utilization. The experimental assessment on the integrated reaction was carried out with Co–Ni reforming catalysts derived from hydrotalcite-like material (HTls) and dolomite as CO2 acceptor. The applied operating window was at a steam-to-glycerol ratio in a range of 3– 9 and a temperature of 500–650 °C. The obtained hydrogen content or purity was close to the theoretical boundary at temperatures above 575 °C or steam-to-glycerol ratio no less than 4. Particularly, both the hydrogen yield and purity were at approx. 99% with a steam-to-glycerol ratio of 9. At a lower steam-to-glycerol ratio of 3 and temperature lower than 550 °C, hydrogen production was compromised due to the low efficiency of steam reforming and a high tendency of pyrolysis. The elimination of non-catalytic reactions, sufficient methane steam reforming and water gas shift reaction have been identified as critical factors to maximize hydrogen yield in sorption enhanced reforming of glycerol.

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.

Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents

Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M 3+ ∼3 (where M = Al 3+, Fe 3+ or combined) have been synthesized following simple co-precipitation method and were subsequently calcined in air at 450 °C. Both as-synthesized and calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N 2 porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates, being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the sorption efficiency. The maximum uptake of phosphates for both the Mg–Al LDH and Mg–Fe LDH samples containing mainly carbonates as charge-balancing anions was relatively low (ca. ⩽25 mg P/g sorbent) while it was higher for the LDH samples containing mainly chlorides (∼80 mg P/g). On the other hand, the maximum sorption capacity for the calcined Mg–Al LDHs and the calcined Fe(III)-substituted sorbents were very high, ca. ∼250 and ∼350 mg P/g, respectively. The sorption data of both the as-synthesized and calcined LDHs was best fitted by the Freundlich model. Both the Mg–Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal efficiency.

Heterogeneous Fenton Catalyst Supports Screening for Mono Azo Dye Degradation in Contaminated Wastewaters

Degradation of acid orange II (AOII), a dye present in textile industry wastewater, by the heterogeneous Fenton process (HFP) was performed. In order to select an adequate support, heterogeneous Fenton catalysts were prepared using several materials such as carbon nanotubes, carbon nanofibers, activated carbon, hydrotalcite-like materials, mesoporous silica (MCM41), silica, silica xerogel, sepiolite, and zeolite USY, that were tested in the degradation of AOII as the model compound. Color, total organic carbon, chemical oxygen demand removal, and toxicity were studied in the process. The best catalyst was iron supported on sepiolite. The best performing catalytic material was used to investigate the influence of the presence of carbonate ions, pH, and reuse of catalyst. Also, a real textile wastewater was successfully treated with this catalyst.

Approaching Sustainable H2Production: Sorption Enhanced Steam Reforming of Ethanol

Sorption enhanced steam reforming of ethanol (SESRE), featured by yielding high purity of H(2) from one single reaction unit, is a new reaction process with a great potential for realizing sustainable H(2) production. The potential of such process with a CaO-based acceptor has been assessed by thermodynamic analysis and experimental demonstration. As predicted, ethanol can be reformed at relatively low temperatures (500-600 degrees C), still yielding high-quality H(2). Another major advantage of coupling CO(2) capture to the reforming process is predicted to be low risk in carbon formation. The SESRE reaction was carried out over a mixture of hydrotalcite-like material derived Co-Ni catalysts (Co-Ni/HTls) and calcined dolomite with a steam to carbon (S/C) ratio of 3 and temperatures ranging from 500 to 650 degrees C. The chosen reaction system was able to yield H(2) with purity fairly close to the theoretical prediction. Particularly, the best result was obtained over 40Ni and 20Co-20Ni/HTls at 550 degrees C, where the product gas had composition of more than 99 mol % H(2), ca. 0.4 mol % CH(4), 0.1 mol % CO, and 0.2 mol % CO(2). Special emphasis was put on the effect of steam on the stability of the CO(2) acceptor during the SESRE reaction. Hydration of CaO in the acceptor did not cause appreciable induction period, even at the low operating temperatures. However, different from a test under dry atmosphere (CO(2)/argon), the acceptor showed rapid deactivation in a multicycle operation of SESRE. A similar deactivation tend was given by a comparative test in a steam/CO(2)/Ar atmosphere.

Phosphate uptake behavior of ZnAlZr ternary layered double hydroxides through surface precipitation

Tetravalent Zr 4+ ion incorporated ZnAl hydrotalcite-like material with varying Zn/(Al + Zr) atomic ratio (2–4) and Al/Zr atomic ratio 0.7:0.3 with CO 3 2 - as interlayer anion were successfully synthesized and characterized by various physicochemical methods. The phosphate uptake studies were carried out over these materials by batch method with the objective of enhancing the uptake through creation of higher positive charge on brucite-like layers by the incorporation of Zr 4+. The studies revealed that ternary ZnAlZr hydrotalcites show good uptake capacity for phosphate, among which ZnAlZr4-HT depicts a maximum uptake of around 91 mgP/g. We observed that phosphate uptake results in the formation of layered hopeite mineral (Zn 3(PO 4) 3·4H 2O) on the surface of material by the surface precipitation of dissolved zinc ion through the outer-sphere complex formation of phosphate. The phosphate uptake increases with a decrease in Zn/Al and Zn/(Al + Zr) atomic ratios. Effects of initial phosphate concentration, contact time, adsorbent amount and temperature variation on phosphate uptake were studied. Co-presence of competitive anion like nitrate enhances the uptake of phosphate to 148 mgP/g. Recycle studies of the material shows a decrease in the uptake of phosphate with the number of cycles due to an increase in the crystallinity. Very high uptake of phosphate by these materials even in the presence of other anions/cations suggests their potential role in waste water remediation.

Hydrotalcite-like Materials as Catalyst for the Conjugate Addition of Amines to Electron Deficient Alkenes

The novel efficient procedure has been developed for the conjugate addition of amines to electron deficient alkenes. A series of hydrotalcite-like materials were synthesized as catalyst for the conjugate addition of amines and alkenes. After optimizing the reaction conditions, ZnAl-LDHs (3:1) was chosen as the best catalyst for the reaction. The results showed that the catalyst worked very well for the conjugate addition of amines to electron deficient alkenes with the excellent yields in several minutes. Operational simplicity, no solvent, low cost of the catalyst, high yields, reusability, excellent chemoselectivity, wide applicability are the key features of this method.