Hydrotalcite-type Compounds Research Articles

Effect of heat treatment on the synthesis of Mg/Al-NO3 hydrotalcite-type compounds for fluoride removal

Hydrotalcite-type compounds (HTs) are crystalline ion-exchange materials formed by positively charged sheets of metal hydroxides stabilized with interlayer anions. The synthesis methods significantly influence their crystallographic and morphological properties. Alternative methods, such as microwave or ultrasound irradiation, can reduce the synthesis time and obtain materials with specific characteristics. HT with the interlayer anion of NO3 − was synthesized via conventional aging (hydrothermal), microwave, ultrasound, and combined (microwave/ultrasound) methods. The obtained compounds were characterized via X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS), N2 adsorption-desorption, Fourier-transform infrared spectroscopy (FTIR), and point of zero charge. A larger crystal size ( t = 5.9 nm) and a higher surface area ( S = 6.70 m2 g−1) were obtained when HT was irradiated by microwaves/ultrasound compared to the synthesized compound via hydrothermal aging ( t = 5.5 nm, S = 1.83 m2 g−1). Although the removal capacity of the combined-mode irradiated compound was slightly lower ( q max = 41.7 ± 4.8 mg g−1) than the conventionally aged sample ( q max = 44.4 ± 6.8 mg g−1), the synthesis time was drastically decreased from 24 h to 5 min, and the combined-mode irradiated compound could remove fluoride as efficiently as the compounds obtained conventionally.

Effect of Fe and La on the Performance of NiMgAl HT-Derived Catalysts in the Methanation of CO2 and Biogas

Bulk Ni-based catalysts derived from hydrotalcite-type (HT) compounds show enhanced stability in the hydrogenation of CO2 into CH4, in comparison to their supported counterparts. The composition of the parent hydrotalcite determines the Ni loading and particle size, as well as the basicity of the catalysts. In this work, a comparison between the effect of Fe and La on the performance of NiMgAl HT compounds (Ni/Mg/Al = 25/50/25 atomic ratio) calcined at 600 °C is investigated. After a reduction pretreatment at 750 °C, both promoters enhance the activity in the low-temperature range; however, the La-containing catalyst reaches a better CH4 productivity rate at high oven temperature (i.e., 200 L gNi–1 h–1 at 325 °C). Although the formation of the NiFe alloy improves the activity, lanthanum keeps smaller Ni0 particles and provides a higher basicity to the mixed oxide than Fe. The activity of the NiMgLaAl catalyst can be further tailored by increasing the Ni loading from 25.6 wt % (Ni25Mg50La5Al20) to 44.6 wt % (Ni50Mg25La5Al20). Lastly, the feasibility of HT-derived catalysts in the direct methanation of clean biogas is proved.

Simultaneous Microwave/Ultrasound Irradiation of Al/Mg Hydrotalcite-Type Compound Synthesis to Remove Fluoride

Simultaneous Microwave/Ultrasound Irradiation of Al/Mg Hydrotalcite-Type Compound Synthesis to Remove Fluoride

Liudongshengite, Zn4Cr2(OH)12(CO3)·3H2O, a new mineral of the hydrotalcite supergroup, from the 79 mine, Gila County, Arizona, USA

ABSTRACTA new mineral species, liudongshengite, ideally Zn4Cr2(OH)12(CO3)·3H2O, has been found in the 79 mine, Gila County, Arizona, USA. It occurs as micaceous aggregates or hexagonal platy crystals (up to 0.10 × 0.10 × 0.01 mm). The mineral is pinkish and transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼1.5, with perfect cleavage on (001). No twinning or parting is observed macroscopically. The measured and calculated densities are 2.95 (3) and 3.00 g/cm3, respectively. Optically, liudongshengite is uniaxial (−), with ω = 1.720 (8), ε = 1.660 (7) (white light). An electron microprobe analysis, combined with the carbon content measured using an elemental combustion system equipped with mass spectrometry, yielded the empirical formula (Zn3.25Mg0.17Cr2.58)Σ6.00(OH)12(CO3)1.29·3H2O, based on (M2+ + M3+) = 6 apfu, where M2+ and M3+ are divalent and trivalent cations, respectively.Liudongshengite belongs to the quintinite group within the hydrotalcite supergroup and is the Cr-analogue of zaccagnaite-3R, Zn4Al2(OH)12(CO3)·3H2O. It is trigonal, with space group Rm and unit-cell parameters a = 3.1111(4), c = 22.682(3) Å, and V = 190.12(4) Å3. The crystal structure of liudongshengite is composed of positively charged brucite-like layers, [M2+1–xM3+x(OH)2]x+, alternating with negatively charged layers of (CO3)2–·3H2O. Compared to other minerals in the quintinite group, liudongshengite is remarkably enriched in M3+, with an M2+:M3+ ratio of 1.33:1. Like zaccagnaite-3R and many other hydrotalcite-type minerals, liudongshengite may also possess polytypes, as a series of synthetic hydrotalcite-type compounds with a general chemical formula [Zn4Cr2(OH)12]X2·4H2O, where X = Cl–, NO3–, or ½ SO42–, but with unit-cell parameters different from those for liudongshengite, have been reported previously.

Memory Effect on a LDH/zeolite A Composite: An XRD In Situ Study.

In this memory effect study, hydrotalcite-type compounds in the lamellar double hydroxide-like (LDH)/zeolite A composite material were analyzed using X-Ray Diffration XRD) in situ experiments. Three samples were analyzed: Al,Mg-LDH, Al,Mg-LDH/ZA composite, and a physical mixture (50/50 wt%) of zeolite A and Al,Mg-LDH. The Al,Mg-LDH sample was treated at 500 °C in an O2 atmosphere and subsequently rehydrated. The Al,Mg-LDH/ZA composites had three treatments: one was performed at 300 °C in a He atmosphere, and two treatments were performed with an O2 atmosphere at 300 and 500 °C. In the physical mixture, two treatments were carried out under O2 flow at 500 °C and under He flow at 300 °C. Both went through the rehydration process. All samples were also analyzed by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The results show that the LDH phase in the Al,Mg-LDH/ZA compounds has memory effects, and thus, the compound can be calcined and rehydrated. For the LDH in the composite, the best heat treatment system is a temperature of 300 °C in an inert atmosphere.

Steam reforming of clean biogas over Rh and Ru open-cell metallic foam structured catalysts

The valorisation of clean biogas (CH4/CO2 = 60/40 v/v) by steam reforming over metal open-cell foam-based structured catalysts was investigated. NiCrAl foams were coated by RuMgAl or RhMgAl hydrotalcite-type compounds through electrodeposition to obtain, after calcination, a thin and stable catalytic film of oxides. The active sites for the reforming reactions are highly dispersed Rh or Ru nanoparticles stabilized by a strong metal support interaction, large Ni particles segregated from the support during reduction and reaction, and Rh/Ni bimetallic particles formed by the interaction of the two formers. Rh-based catalysts show superior activity and stability with the time on stream than Ru catalysts, and a low carbon deposition, which is mainly ascribable to the presence of large Ni particles. In comparison to a pelletized catalyst, the structured catalysts are allowed to operate at high space velocities and low Steam to CH4 ratio, increasing the biogas valorisation and thus the productivity.

Sulfate removal by Mg–Al layered double hydroxide precipitates: Mechanism, settleability, techno-economic analysis and recycling as demulsifier

Abstract This study investigated sulfate (SO42−) removal by Mg–Al hydrotalcite-type (HT) compound (Mg6Al2SO4(OH)16•mH2O), and analyzed applicable condition of the method from techno-economic perspective. Under optimized condition of pH 8.80, molar ratio of Mg2+ and SO42− (Mg/S ratio) of 6.63 and molar ratio of Al3+ and SO42− (Al/S ratio) of 2.53, HT compound achieved a greatly lower concentration of SO42− (3.76 mmol/L) than conventional gypsum precipitation. The composition and morphological analysis of precipitates showed that HT compound was transformed to Mg(OH)2 at pH > 9.0, and Mg(OH)2, Al(OH)3 and MgAl2(OH)8 at Al/S ratio > 2.5. HT compound had better settleability than Mg(OH)2 and ettringite, and was easier to be flocculated by anionic polyacrylamide to obtain a lower SV30 of 16.4%. Turbiscan showed that HT compound had the highest instability and aggregation property among the three systems. Techno-economic analysis showed that HT compound coupled ettringite route was more economic than Mg(OH)2 coupled ettringite route at Mg/S ratio > 6.08, but was not applicable when concentrations of Mg2+ and SO42− were very low. The precipitated HT compound was recovered as demulsifier and showed more efficient COD removal than conventional demulsifier for chemical cleaning wastewater treatment.

Adsorbents derived from hydrotalcites for the removal of diclofenac in wastewater

Emerging compounds have received increasing attention because of their potential environmental hazards. The drug diclofenac has acute toxicity and has been detected in surface water and wastewater. Conventional treatment processes are not able to totally remove this compound. Adsorption has been widely used to remove pollutants and standing out as an unspecific and versatile operation. In this context, the present study investigated the application of mixed oxides of Mg, Ni, or Zn with Al, derived from hydrotalcite-type compounds, as adsorbents in the removal of sodium diclofenac from synthetic aqueous solutions. The solids were synthesized by the continuous coprecipitation method and characterized by the SBET, XRD, TGA, FTIR, and SEM techniques. The influence of adsorbent dosage and contact time parameters were evaluated. Adsorbent regeneration by thermal treatment was also evaluated. The results showed that sodium diclofenac highest removal rates were found applying the best experimental conditions: residence time and adsorbent dosage of 30 min and 2 g L−1 for C-Zn with maximum adsorption removal of 76.5%. The thermal treatment after the adsorption confirmed the regeneration of the solids, allowing reuse in the adsorption for up to 8 cycles. The application of mixed oxides as sorbent solids was technically feasible for the removal of diclofenac sodium in wastewater treatment.

Layered Double Hydroxides for the Catalytic Isomerization of Linoleic Acid to Conjugated Linoleic Acids (CLAs)

Several hydrotalcite-type compounds with different divalent (Mg2+, Ni2+, Cu2+, Zn2+) and trivalent (Al3+, Cr3+) cations and different ratio compositions were tested for the isomerization of linoleic acid in order to study their role on the obtention of specific conjugated linoleic acids (CLAs) with anticarcinogenic and nutritional properties. This is a complex reaction due to the high number of possible isomers of linoleic acid together with the significant competition of the isomerization reaction with other secondary undesired reactions. All catalysts showed very high conversions of linoleic acid, but condensation products were mainly obtained, especially for the hydrotalcite-type compounds with higher Mg/Al ratios due to their higher Brønsted basicity and for the catalysts with higher Ni2+ content or with the presence of Cu2+, Zn2+ in the layers because of the influence of the higher acidity of these cations on the Brønsted basicity of the hydroxides. The best results were achieved for the catalysts with Mg/Al ratio around 2.5–3, resulting in 29–38% of selectivity to the identified CLAs.

Novel Cu-Zn-Al catalysts obtained from hydrotalcite-type precursors for middle-temperature water-gas shift applications

Water gas shift (WGS) reaction is a key step in the steam reforming industrial processes, allowing to increase the H2 production reducing the CO content in the exiting stream. Currently the WGS reaction is performed in two steps, at high and low temperature, but in recent years, the possibility to perform it in only one step using one reactor operating at middle temperature (about 300 °C) has received increasing interest. Cu/Zn/Al hydrotalcite-type (HT) compounds having a Cu-content lower than 30 wt% showed to be useful precursors of highly active and stable catalysts for the WGS reaction in the 250–350 °C temperature range. The best performances were observed for a catalyst containing 20 wt% of copper and a M(II)/M(III) atomic ratio equal to 2.0 that reached the equilibrium values of CO conversion in all the temperature range with a behaviour similar to one of the most widely employed commercial catalyst, associated to a very good stability with time-on-stream. The catalytic behaviour may be rationalized considering both the Cu0 surface area and the Cu/ZnO interactions, which are promoted by the homogeneous distribution of the cations in the brucite-type layers of the HT precursors.

Sulfate removal from wastewater using ettringite precipitation: Magnesium ion inhibition and process optimization

One of the main challenges in industrial wastewater treatment and recovery is the removal of sulfate, which usually coexists with Ca2+ and Mg2+. The effect of Mg2+ on sulfate removal by ettringite precipitation was investigated, and the process was optimized in the absence and presence of Mg2+. In the absence of Mg2+, the optimum conditions with sulfate removal of 99.7% were obtained at calcium-to-sulfate ratio of 3.20, aluminum-to-sulfate ratio of 1.25 and pH of 11.3 using response surface methodology. In the presence of Mg2+, sulfate removal efficiency decreased with increasing Mg2+ concentration, and the inhibitory effect of Mg2+ matched the competitive inhibition Monod model with half maximum inhibition concentration of 57.4 mmol/L. X-ray diffraction and Fourier transform infrared spectroscopy analyses of precipitates revealed that ettringite was converted to hydrotalcite-type (HT) compound in the presence of Mg2+. The morphology of precipitates was transformed from prismatic crystals to stacked layered crystals, which confirmed that Mg2+ competes with Ca2+ for Al3+ to form HT compound. A two-stage process was designed with Mg2+ removal before ettringite precipitation to eliminate the inhibitory effect, and is potential to realize sludge recovery at the same time of effective removal of sulfate and hardness.

Reactions involved in the electrodeposition of hydrotalcite-type compounds on FeCrAlloy foams and plates

The cathodic electrodeposition of Mg/Al hydrotalcite-type (HT) compounds on FeCrAlloy plates and foams by the electro-base generation method, using nitrate salts as precursors, was studied. In order to characterize the main mechanisms governing the precipitation of HT compounds, potential sweeps or pulses were applied at both plates and foams dipped in nitrate and chloride baths, with and without Mg2+ and Al3+ precipitating cations. Moreover, nitrate reduction products were analyzed. The features of coatings and pH near foam supports were correlated to electrochemical results.Nitrate and oxygen reduction, as well as H2 evolution, contributed to a pH increase during the electrodeposition of HT compounds, but nitrate reduction was responsible for the highest pH increase at the electrode-electrolyte interface. FeCrAlloy showed a low activity in nitrate reduction to nitrite and ammonia. Mg2+ and Al3+ precipitating cations fostered electrochemical reduction and modified reaction products. The shape of the support, foam or plate, did not significantly alter the electrical conductivity and reduction potentials, but it did affect current density and H2 bubble formation. A higher current density was recorded at the foam, while H2 bubbles were smaller and dissipated rapidly; subsequently, a thicker coating was electrodeposited.

Stable Rh particles in hydrotalcite-derived catalysts coated on FeCrAlloy foams by electrosynthesis

Rh-based structured catalysts for the Catalytic Partial Oxidation of CH4 to syngas were prepared by electrosynthesis of Rh/Mg/Al hydrotalcite-type compounds on FeCrAlloy foams and calcination. The effects of Rh content, total metal concentration, and partial replacement of Mg2+ by Ni2+ in the electrolytic solution on coating thickness, Rh speciation, metallic particle size, and catalytic activity were investigated by SEM/EDS, μ-XRF/XANES and tests under diluted and concentrated reaction conditions.The amount of Rh species, present as Mg (RhxAl1−x)2O4, depended on the thickness of the electrosynthesised layer as well as the Rh particle size and dispersion. Smaller and more dispersed particles were obtained by decreasing the Rh concentration in the electrolytic solution from Rh/Mg/Al=11/70/19 to 5/70/25 and 2/70/28 atomic ratio% (a.r.%) and in thinner rather than thicker layers. Despite the improvement in metallic particles features, the CH4 conversion was negatively affected by the low amount of active sites in the coating, the high metal support interaction and possibly the oxidation of metallic particles and carbon formation.A larger amount of solid containing well dispersed Rh particles was deposited by increasing the total metal concentration from 0.03M to 0.06M with the Rh/Mg/Al=5/70/25 a.r.%, and the catalytic performances were enhanced. The partial replacement of Mg2+ by Ni2+ gave rise to a very active bimetallic Rh/Ni catalyst, CH4 conversion and selectivity to syngas were above 90%, however, it slightly deactivated with time-on-stream.

Erratum To: Sorption of chromium (VI) by Mg/Fe hydrotalcite type compounds

Erratum To: Sorption of chromium (VI) by Mg/Fe hydrotalcite type compounds

Role of Coating-Metallic Support Interaction in the Properties of Electrosynthesized Rh-Based Structured Catalysts

Rh-structured catalysts for the catalytic partial oxidation of CH4 to syngas were prepared by electrosynthesis of Rh-containing hydrotalcite-type (HT) compounds on FeCrAlloy foams followed by calcination at 900 °C. During the calcination the simultaneous decomposition of the layered HT structure and formation of the protective FeCrAlloy outer shell in alumina occurred. Here, we studied the role of the coating-metallic support interaction in the properties of the catalysts after calcination, H2 reduction, and catalytic tests, by a combination of electron (FEG-SEM/EDS) and synchrotron X-ray (XRF/XRPD and XRF/XANES) microscopic techniques. The characterization of crystalline phases in the metallic support and coating and distribution of Rh active species was carried out on several samples prepared by modifying the Rh content in the electrolytic solution (Rh/Mg/Al = 11.0/70.0/19.0, 5.0/70.0/25.0, 0/70.0/30.0 atomic ratio). A sample was also prepared with no aluminum in the electrolytic solution (Rh/Mg/Al = 13...

Similarities between Zinc Hydroxide Chloride Monohydrate and Its Dehydrated Form: A Theoretical Study of Their Structures and Anionic Exchange Properties

The electronic structures of zinc hydroxide chloride monohydrate (ZHC) and its dehydrated form were calculated by ab initio density functional theory methodology in order to study some of their properties, such as anionic exchange. It was observed that the dehydrated structure showed no major differences in the unit cell parameters compared to hydrated one, except for the basal parameter c, which was smaller in the dehydrated structure. Charge difference plots and projected densities of states were computed to study the interactions between water molecules and the grafted chlorides. Differences between this kind of layered hydroxide salt and hydrotalcite-type compounds could be noticed and understood by the calculations. It could be observed that there were no drastic changes due to the dehydration. The calculated dehydration potentials were used to predict the water loss temperature and the thermodynamic features of the chloride anionic exchange reactions of Zn5(OH)8Cl2·H2O using different halide anions, and it was found that only the exchange with iodine can be performed spontaneously at 298.15 K.

Bacterial Cellulose as a Template for Preparation of Hydrotalcite-Like Compounds

Bacterial cellulose (BC) was used as a template for preparation of Hydrotalcite-type compounds or layered double hydroxides (LDHs) by co-precipitation method, and submitted to three aging times. Isolated composites containing LDH supported on BC (BC-LDH) were calcined to remove the organic matrix. Mixed metal oxides formed after thermal decomposition underwent reconstruction reaction in Na2CO3 solution producing again hydrotalcite-like materials (RBC-LDH). To evaluate the influence of BC on the LDH formation, it was also analyzed LDH fractions not attached to the BC fibers and LDH synthesized in absence of the polymeric membrane. XRD pattern of BC-LDH composites show broadened peaks related to the organic semi-crystalline matrix, indicating a deep level of interaction between organic and inorganic phases. Interaction is also evidenced by changes in the composites thermal decomposition profiles compared to pristine polymer. SEM images revealed formation of submicrometrical round-shaped LDH particles after removal of membrane (RBC-LDH), differently from the plate-habit exhibited by pristine LDH. Aging plays a key role in growth of all LDHs samples, leading to the formation of larger inorganic particles as time is increased. RBC-LDH aging for three days shows significant improvement in the surface area (more than three times) if contrasted to LDH prepared in BC absence.

Improved Hydrotalcite-type Compounds for Post-combustion CO2 Abatement

Among the greenhouse gasses, CO2 emissions are of great concern. Releasing 4Gt fossil carbon results in an annual increase in the atmospheric concentration of CO2 by 1ppm. As the demand for fuel increases progressively with worldwide economic progress, CO2 emissions become a stringent problem; scientists are looking for solutions to deal with this problem. CO2 capture and storage is one of the only viable options for capturing CO2 emissions from stationary point sources. Post-combustion CO2 capture systems could be retrofit to existing and new point CO2 sources, such as power plants. The current state of the art technology utilizes an aqueous amine solution in a temperature swing for CO2 capture. There are several potential advantages of using solid materials, such as a lower specific heat capacity and less evaporation of moisture. However, significant improvements in sorbent performance are still necessary. Specifically, the cost, stability, attrition, and interaction with flue gas constituents are of concern.This work exploits the possibility to change the characteristics of the hydrotalcite-type materials by increasing their CO2 physisorption properties in order to make them more suitable for the post- combustion technologies. By inducing changes during the preparation of hydrotalcite-type compounds and/or addition of dopants such as Zr4+ it has been possible to modify the structure of these materials by lowering the strength of the water bonding to the surface of the material. This leads to the decrease of the temperature where the dehydration takes place. Moreover, this gives higher than normal CO2 capacities at low temperatures which are explained through the existence of weak basic sites. This behaviour is temperature dependent and gives scope to exploit the structural characteristics of the hydrotalcite type compounds for developing new materials which to compete with amine based systems in terms of energy efficiency.The hydrotalcites have been evaluated at the laboratory scale. The importance of activation temperature, moisture, and testing procedure has been highlighted. The deactivation is only related to the dehydration process and the process is reversible as no structural modification at the level of octahedral layers can be observed.

Ni–Fe catalysts derived from hydrotalcite-like precursors for hydrogen production by ethanol steam reforming

Nickel–iron mixed oxides derived from reevesite, a hydrotalcite-type compound, were tested in steam reforming of ethanol for hydrogen production. The influence of iron content (Ni/Fe ranging from 3 to 1) and the calcination temperature of the catalyst precursor (773 and 1073K) on the catalytic performance were investigated. Both parameters were essential to optimize the reforming performance. Increasing the amount of iron in the reevesite precursors affected both the chemical and activity properties of the derived mixed oxide catalysts. Iron displays a positive role in nickel-based catalysts due to the enhancement of catalytic activity and hydrogen selectivity induced by the improved dispersion of nickel and the alleviation in carbon deposition. The calcination temperature led to variations in phase composition consisting of Ni(Fe)Ox solid solution and NiFe2O4, which affected the final size and dispersion of nickel species formed during the reaction. The best catalyst, with a Ni/Fe ratio of 1 and calcined at 773K, rendered high and stable hydrogen and carbon dioxide selectivity of up to ca. 60% and 40%, respectively, low methane content, and consisted of a Ni(Fe)Ox+NiFe2O4 mixture with high surface area and small Ni0 crystallites. A higher percentage of crystalline NiFe2O4 attained at high calcination temperature (1073K) associated with a lower carbon deposition resistance and probably Ni0 sintering brings about lower activity and fast deactivation. The improved performance over catalysts calcined at lower temperature and with lower Ni/Fe ratio is motivated by the effect of iron on the structural and electronic properties of the mixed oxides, thus inducing a slow formation of metallic nickel particles and coke deposits. Features like high surface area, higher iron content, lower reducibility of nickel species and small nickel crystallite size well dispersed on the surface of the catalyst with high iron content lead to a higher activity in ethanol dehydrogenation, acetaldehyde decarbonylation and reforming, and WGS.

Novel Rh-based structured catalysts for the catalytic partial oxidation of methane

Novel Rh-based structured catalysts were prepared by the electrosynthesis of Rh/Mg/Al hydrotalcite-type (HT) precursors on a FeCrAlY foam. The catalysts obtained by the calcination of HT compounds were investigated in the catalytic partial oxidation (CPO) of CH 4. The effects of the electrosynthesis conditions (potential and pH of the plating solution) on the surface morphology and chemical composition of the samples as well as on the catalytic activity were investigated. The control of the pH of the solution favoured the precipitation of Rh as hydroxide rather than of metallic particles, whereas the potential applied determined the pH value reached near to the foam. By increasing the cathodic potential from −1.2 to −1.3 V, but keeping the synthesis time constant (1000 s), the required conditions to obtain HT precursors were achieved faster and, therefore, a larger amount of them precipitated. Due to the different coverage and chemical nature of the electrosynthesized species, catalytic performances depended on the synthesis conditions, the best values being achieved by the catalyst obtained from the HT precursor prepared at −1.3 V for 1000 s.