Hydrotalcite-derived Materials Research Articles

Catalytic Performance of CuZnAl Hydrotalcite-Derived Materials in the Continuous-Flow Chemoselective Hydrogenation of 2-Methyl-2-pentanal toward Fine Chemicals and Pharmaceutical Intermediates.

Hydrotalcite-derived materials are eco-friendly, cheap, and efficient catalysts of different reactions. However, their application in liquid-phase hydrogenation could be more extensive. Hence, this work concerns the application of three hydrotalcite-derived materials with different CuZnAl molar ratios in the liquid-phase continuous-flow hydrogenation of 2-methyl-2-pentenal (MPEA) at a wide range of temperature (298-378 K) and pressure (1 × 106-6 × 106 Pa). The catalytic investigations were supported by catalysts characterization by ICP-OES, TPR, in situ XRD, XPS, NH3-TPD, CO2-TPD, and TEM measurements on different stages of their biography. It was shown that the catalytic activity of these samples is related to the Cu0/Cu+ ratio. Depending on the reaction conditions, selectivity control is possible. All catalysts were 100% selective to 2-methylpentanal (MPAA)-sedative drug precursor, with low conversion, at temperatures ≤ 338 K at every pressure. However, the selectivity of the second desired product, fragrance intermediate, 2-methyl-2-penten-1-ol (MPEO), increased significantly at higher temperatures and pressures. It reached the unique value of 54% with 60% substrate conversion at 378 K and 6 × 106 Pa for the catalyst with the highest Cu loading. It was revealed that the production of significant amounts of MPEO is related to the reaction conditions, the Cu+ predominance on the surface, the hydrogen spillover effect, and the acid-base properties of these systems.

Photo- and Thermocatalytic CO2 Methanation: A Comparison of Ni/Al2O3 and Ni-Ce Hydrotalcite-Derived Materials under UV and Visible Light.

Catalysts derived from Ni/Al/Mg/Ce hydrotalcite were prepared via a co-precipitation method, varying the Ce/Al atomic ratio. All of the catalytic systems thus prepared were tested for CO2 methanation under dark and photocatalytic conditions (visible and ultraviolet) under continuous flow with the light intensity set to 2.4 W cm-2. The substitution of Al by Ce formed a solid solution, generating oxygen vacancies and Ce3+/Ce4+ ions that helped shift the dissociation of CO2 towards the production of CH4, thus enhancing the activity of methanation, especially at lower temperatures (<523 K) and with visible light at temperatures where other catalysts were inactive. Additionally, for comparison purposes, Ni/Al2O3-based catalysts prepared via wetness impregnation were synthesized with different Ni loadings. Analytical techniques were used for the characterization of the systems. The best results in terms of activity were as follows: Hydrotalcite with Ce promoter > Hydrotalcite without Ce promoter > 25Ni/Al2O3 > 13Ni/Al2O3. Hydrotalcite, with a Ce/Al atomic ratio of 0.22 and a Ni content of 23 wt%, produced 7.74 mmol CH4 min-1·gcat at 473 K under visible light. Moreover, this catalyst exhibited stable photocatalytic activity during a 24 h reaction time with a CO2 conversion rate of 65% and CH4 selectivity of >98% at 523 K. This photocatalytic Sabatier enhancement achieved activity at lower temperatures than those reported in previous publications.

Research Progress in the Field of Adsorption and Catalytic Degradation of Sewage by Hydrotalcite-Derived Materials.

With the rapid development of industry and agriculture and the greatly improved living conditions, the resultant gradually deteriorated environments threaten the human beings. Refractory or even toxic pollutants, which are from different industries such as printing and dyeing, pesticides, chemicals, petrochemicals, plastics and rubber, seriously threat the ecosystems and human health. Having the advantages of flexible composition, unique structure, high stability, memory effect, easy preparation and low cost, hydrotalcite compounds have a great potential in sewage degradation and environmental protection. This study focuses on the adsorption and catalytic properties (such as photocatalysis, electrocatalysis and photoelectrocatalysis) of hydrotalcite-derived materials for treating organic, inorganic and heavy metal ion sewage. The types of adsorption and catalysis, and the effects of various influencing factors on the degradation efficiency were discussed as well.

Hydrogen production through steam reforming of toluene over Ce, Zr or Fe promoted Ni-Mg-Al hydrotalcite-derived catalysts at low temperature

Ni-Mg-Al hydrotalcite-derived materials promoted with Ce, Zr or Fe were prepared by the co-precipitation method, which exhibited excellent performance in steam reforming of toluene (SRT) at low temperature. Especially, Fe-doped catalyst had higher catalytic activity and stability at 400 °C, which maintained almost 100% toluene conversion during the SRT reaction (3 h). Moreover, it also had higher hydrogen production and better selectivity for hydrogen production. The physico-chemical characterizations and model simulation indicate its high dispersion of active sites and accessibility of reactants to active sites: uniform dispersion of active sites (SEM, TEM and EDX), large specific surface area (151 m2/g), pore volume (0.84 cm3/g) and average pore diameter (22.1 nm), small crystallite size (34.9 nm), desirable Ni species reducibility (TPR), high basicity (TPD), low metal carbide formation (XPS), and good adsorption affinity with toluene (−18.391 kJ mol−1, DFT). Moreover, less carbon was deposited on the Fe-containing catalyst through measuring the amorphous carbon and CNTs by the TG-MS method, which was due to the tip-growth mechanism of CNTs. Considering lower toluene conversion temperature, higher hydrogen yield, and better resistance to carbon deposition, this study may be meaningful in development of SRT at low temperature.

Hydrotalcite-Derived Mixed Oxides for the Synthesis of a Key Vitamin A Intermediate Reducing Waste.

The synthesis of hydroxenin monoacetate, a key intermediate in the manufacture of vitamin A, relies on the undesirable use of stoichiometric amounts of organic bases such as pyridine. Although the final product (vitamin A acetate) can be produced from hydroxenin diacetate, using the monoacetylated intermediate improves the overall process yield. Aiming to identify more efficient, environmentally benign alternatives, this work first studies the homogeneous acetylation reaction using pyridine. The addition of the base is found to enhance the rate of hydroxenin monoacetate formation, confirming its catalytic role, but also yields non-negligible amounts of hydroxenin diacetate. On the basis of these insights, Mg- and Al-containing hydrotalcites are explored because of their broad scope as base catalysts and the ability to finely tune their properties. The reaction kinetics are greatly enhanced via controlled thermal activation, forming high surface area mixed metal oxides displaying Lewis basic sites. In contrast, a Brønsted basic material synthesized by the reconstruction of a mixed oxide performs similarly to the as-synthesized hydrotalcite. Variation of the Mg/Al ratio from 1 to 3 has no significant impact, but activity losses are observed at higher values because of a reduced number of basic sites. After optimizing the reaction conditions, hydroxenin monoacetate yields >60% are obtained in five consecutive cycles without the need for any intermediate treatment. The findings confirm the potential of hydrotalcite-derived materials as highly selective catalysts for the production of vitamins with reduced levels of organic waste.

Mechanism of activity enhancement of the Ni based hydrotalcite-derived materials in carbonyl sulfide removal

In this paper, a series of Ni based hydrotalcite-like compounds (HTLCs) were prepared by co-precipitation method. Carbonyl sulfide (COS) removal was investigated over the hydrotalcite-derived oxides (HTO). The properties of desulfurizer and its changes in the desulfurization process were systematically characterized by XRD, SEM, FTIR, CO2-TPD, XPS, XAFS and In-situ DRIFTS. The results showed that the activity series for removal of COS decrease in the following sequence: Ni3Al-HTO > Ni3Fe-HTO > Ni3Cr-HTO. In-situ DRIFTS and quantum chemistry study implied that carbonate and sulfate was formed on the desulfurizer surface. Lewis basic sites and hydroxyl was determined to play a pivotal role for the COS removal. The enhanced activity after trivalent cations substitution is attributed to their high ability to provide electrons as evidenced by results of CO2-TPD, XPS and XAFS. This work is useful to explore feasible routes to improve the performance of desulfurization material for the feed gas purification.

Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition

Ni/Al and Ni/Mg/Al hydrotalcite-derived materials containing various Ni loadings were synthesized and subsequently promoted with Ce-species via adsorption from [Ce(EDTA)]− complexes. The obtained materials were characterized by elemental analysis (ICP-MS), XRD, H2-TPR, CO2-TPD, TG and low temperature N2 sorption experiments. The amount of the introduced Ce was dependent on the nickel and magnesia content in catalysts precursors, and it influenced materials properties (i.e. basicity, reducibility of Ni species) in various ways and the catalytic performance in the dry reforming of methane (DRM). The promoted catalysts showed improved performance with CH4 and CO2 conversions at 550 °C in the range of 35–55 and 35–45%, respectively. The extent of the improvement was dependent on the nickel content and the presence of magnesia. In general, Ce promotion increased materials stability by changing of the type of carbonaceous deposits. Ce modification hindered the transformation of amorphous carbonaceous deposits to graphitic carbon. The former may be easier oxidized and contribute to syngas production. Two selected catalysts were additionally tested in DRM at elevated temperatures (650 and 750 °C) and over various feed gas compositions.

A Short Review on the Catalytic Activity of Hydrotalcite-Derived Materials for Dry Reforming of Methane

Nickel-containing hydrotalcite-derived materials have been recently proposed as promising materials for methane dry reforming (DRM). Based on a literature review and on the experience of the authors, this review focuses on presenting past and recent achievements on increasing activity and stability of hydrotalcite-based materials for DRM. The use of different NiMgAl and NiAl hydrotalcite (HT) precursors, various methods for nickel introduction into HT structure, calcination conditions and promoters are discussed. HT-derived materials containing nickel generally exhibit high activity in DRM; however, the problem of preventing catalyst deactivation by coking, especially below 700 °C, is still an open question. The proposed solutions in the literature include: catalyst regeneration either in oxygen atmosphere or via hydrogasification; or application of various promoters, such as Zr, Ce or La, which was proven to enhance catalytic stability.

Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming

Zr and Ce promoted hydrotalcite-derived materials were tested as a catalysts for dry reforming of methane. Zirconium species were introduced into hydrotalcite brucite-like layers, while cerium species were deposited on the external surfaces of crystallites. Three catalysts with Ce/Zr molar ratio of 1.2, 0.6 and 0.3 were synthesized, tested in dry reforming at 550 °C and subsequently characterized with the aid of XRD, XRF, FTIR, H2-TPR, CO2-TPD and TG techniques. Promotion with both Ce, Zr resulted in decrease catalytic activity, however, stability of materials was significantly increased, which could be attributed to increased concentration of strong basic sites. The effect of Ce/Zr molar ratio was mainly dependent on the position of Zr species in brucite-like layers or in mixed ceria–zirconia on catalyst surface and influenced materials basicity, reducibility and stability in DRM.

Catalytic partial oxidation of methane over nickel and ruthenium based catalysts under low O 2 /CH 4 ratios and with addition of steam

Abstract Catalytic partial oxidation (CPO) of methane to synthesis gas at low O 2 /CH 4 ratios and in the presence of steam was investigated over nickel and ruthenium catalysts supported on hydrotalcite-derived materials. The influence of catalyst properties and composition on activity, temperature profile and deactivation by carbon formation was examined. All catalyst presented high methane conversions, close to the values predicted by thermodynamic equilibrium and such conversions increased in proportion to the metal surface of the catalyst tested. The temperature profiles at O 2 /CH 4 = 0.2 and H 2 O/CH 4 = 0.3 and a constant exit temperature of 700 °C varied depending on the catalyst type; it was possible to examine catalyst deactivation from the change in the shape of the profile of each catalyst. Since the O 2 /CH 4 and H 2 O/CH 4 ratios were low, the risk or potential for carbon formation was thermodynamically favorable along the entire catalytic bed; however, this potential was qualitatively higher when the temperature profile of the catalyst presented a pronounced maximum peak at the inlet of the reactor. During catalytic reaction tests and methane decomposition experiments, the ruthenium catalyst did not formed appreciable amounts of carbon while a bimetallic catalyst (Ni and Ru) form only small amounts (in comparison with the nickel catalysts). For the ruthenium catalyst, a higher O 2 /CH 4 ratio favored conversions closer to the equilibrium value. The observations presented in this work indicate that during the CPO of methane, at low O 2 /CH 4 ratios and in the presence of steam, the catalyst properties and composition will have a substantial influence on the extent of the combustion and reforming reactions along the catalytic bed. This will in turn define the temperature profile, and therefore the risk or potential for carbon formation; this risk might effectively be overcome by the use of ruthenium-containing catalysts.

Pt-MgZnCuAl hydrotalcite-derived catalysts in the reduction of nitrates using continuous and batch reactors

The role of acid–base properties and reconstruction degree of the hydrotalcite-derived materials (MgZnCuAl) in the catalytic hydrogenation of nitrates reaction is studied. In addition, the catalysts have been tested in a batch and continuous reactor to evaluate the influence of the reactor configuration on the catalytic performance. The characterizations of the samples have been performed by XRD, TPR, BET, TEM and MBOH test reaction. A clearly influence of the acid–base properties of the catalysts, modulated by the Zn incorporation, is observed with an increase in the nitrogen selectivity. Besides the common operational parameter differences between work in continuous and discontinuous systems, it shows great differences in the reconstruction degree of the samples. Higher reconstruction leads to higher nitrate reduction and a decrease of nitrite production.

Performance of New Pt and Pt–Cu on Hydrotalcite-Derived Materials for NOxStorage/Reduction

A series of Pt and Pt,Cu supported catalysts was prepared by impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds calcined at different temperatures. These NOx storage-reduction (NOxSR) catalysts show improved performances in NOx storage respect to Pt–Ba/Al2O3 NOxSR catalysts at reaction temperatures lower than 473 K. These catalysts also show improved resistance to deactivation by SO2. The co-presence of Pt and Cu (at different ratio Pt/Cu and using different impregnation methods) slightly worsens low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in presence of SO2. The overall Pt–Cu/HT performances are thus higher than those of the Pt–Ba/Al2O3 Toyota-type NOxSR catalysts.