Hydrotalcite-like precursors

The preparation of mixed oxide catalysts from hy-droxycarbonates is not restricted to Ni/Al203, but also possible for several other systems. One example is the technically very important methanol catalyst based on CuO/ZnO. It can be prepared from a hydrotalcite-like precursor as well [57], but mostly a hydrozincite or au-richalcite precursor is used [58]. On reduction of auri-chalcite precursors, epitaxial relationships between the copper and the ZnO were observed. Such preferential formation of certain orientations is also the decisive factor in the last example discussed here. 

Alejandre, A., Medina, F., Salagre, P., Correig, X. and Sueiras, J. E. (1999). Preparation and study of Cu-Al mixed oxides via hydrotalcite-like precursors. Chem. Mater. 11, 939. 

Cabello, F. M., Tichit, D., Coq, B., Vaccari, A. and Dung, N. T. (1997). Hydrogenation of acetonitrile on nickel-based catalysts prepared from hydrotalcite-like precursors. J. Catal. 167, 142. 

Murcia-Mascaros, S., Navarro, R.M., Gomez-Sainero, L., Costantino, U., Nocchetti, M., and Fierro, J.L.G. Oxidative methanol reforming reactions on CuZnAl catalysts derived from hydrotalcite-like precursors. Journal of Catalysis, 2001, 198 (2), 338. 

Tri-component mixed oxides, Mg/Al/La, Mg/Al/Ce and Mg/Al/Zr, were prepared with the co-precipitation technique, with the aim to improve the basic properties with respect to the reference Mg/AI mixed oxides, obtained from Mg/Al hydrotalcite-like precursors. For samples containing Ce and Zr, the basic properties (as determined by CO2 adsorption and TPD) were considerably improved with respect to Mg/Al/0, whereas for the La-containing system a decrease in basicity was observed. These materials were tested in m-cresol methylation, in both liquid- and gas-phase reactions. A relationship was found between basic features and catalytic performance. 

The aim of the present work was to prepare tri-component materials (Mg/Al/La/0, Mg/Al/Ce/O and Mg/Al/Zr/0) by co-precipitation, a procedure analogous to the preparation of Mg/Al/0 hydrotalcite-like precursors. The catalytic properties of these materials were studied in the reaction of m-cresol methylation. The distribution of products... 

Both the Mg/Al/La/0 and Mg/Al/Ce/0 precursors give XRD patterns corresponding to the presence of a hydrotalcite-like structure and a Me hydroxy carbonate phase (Me = La, Ce). The La containing material is much better crystallized than the Ce containing one. The Mg/Al/Zr/0 precursor is characterised by a pattern similar to that of the Zr/O precursor, indicating the absence of any crystalline precipitate. In this case, therefore, the presence of Zr hinders the development of the crystalline hydrotalcite-like precursor. 

In another study, Grygar et al. [68] investigated the formation of mixed Cu-Mn, Cu-Mn-Al, Cu-Mg-Mn, and Cu-Mg-Mn-Al oxides by the calcination of amorphous basic carbonates, or by the calcination of hydrotalcite-like precursors. A combination of XRD and electrochemical measurements (again reductive dissolutions in aqueous electrolytes) led to the crystalline and amorphous phases being identified, and their formation during the thermal treatment elucidated. 

CuNiAl mixed oxides [43] via hydrotalcite-like precursors have been exploited for the oxidation of phenol aqueous solutions. The results concluded that the phenol conversion decreased continuously with time because of continuous loss of CuO phase by elution and formation of other phases on the surface of the catalyst. 

Carpentier, J., Lamonicr, J.-F., Siffert. S., Laversin, H, and Aboukais. A. (2002). Preparation and characterization of Co-Fe-Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation. Stud. Surf. Sci. Catal., 142B, 1197-1204. 

The hydroxycarbonate precursors showed dif action patterns consistent with the layered double hydroxide structure proposed for hydrotalcite-like compounds, i.e., brucite-like sheets formed by OH groups and cations (Mg and AP ) in octahedral coordination. Water molecules and charge-compensating COs anions are located in the interlayers. The hydrotalcite-like precursor composition is [Mg,, .Al,(0H)2] (C03), . mH20 with r = Al/(A1+Mg) the stoichiometric hydrotalcite compound, Mg6Al2(0H)i6C03.4H20, is obtained for r = 0.25. Upon thermal decomposition at 673 K, the hydroxycarbonate precursors lose... 

Co-precipitation is also applied in the synthesis of hydrotalcite-like precursors (HTlcs) for catalysts e.g. based on Cu-Zn-Al, or Co-Al catalysts. Synthetic HTlcs, with a general formula (OHjg] [A ] nHgO, are hydrated hydroxycarbonates of a random lamellar structure. These materials can be visualized as brucite-type octahedral layers, in which cations partially substitute for cations (x = The cations are located in the center of... 

Nunan, J. G., Himelfarb, P. B., Herman, R. G., Klier, K., Bogdan, C.E., and Simmons, G. W. 1989. Methanol synthesis catalysts based on Cs/Cu/ZnO/M203 (M = Al, Cr, Ga) genesis from coprecipitated hydrotalcite-like precursors, solid-state chemistry, morphology, and stability. Inorg. Chem. 28 3868-74. 

A Bhattacharyya, WD Chang, MS Kleefisch, CA Udovich. Method for Preparing Synthesis Gas Using Nickel Catalysts. US Patent 5,399,537, March 21, 1995. WD Chang, CA Udovich, A Bhattacharyya, MS Kleefisch. Catalyst Prepared from Nickel-Containing Hydrotalcite-Like Precursor Compound. US Patent 5,767,040, June 16, 1998. 

Resini, C., Montanari, T., Barattini, L., et al. (2009). Hydrogen production by ethanol steam reforming over Ni catalysts derived from hydrotalcite-like precursors. Catalyst characterization, catalytic activity and reaction path, Appl.Catal. A Gen., 355, pp. 83-93. 

Zn/Al complex oxides catalyst derived from a hydrotalcite-like precursor with a zinc/aluminum atom ratio of 3.74 1 was used for the transesterification of rapeseed oil with methanol. With oil to methanol ratio of 1 24 at 200° C and 2.5 MPa with 1.4% catalyst, conversion of 84% was observed in 1.5 h [77]. 

Co-precipitation methodology is the most cotmnon synthesis techniqne to obtain LDHs, however nucleation and the kinetics growth can not be controlled easily. An alternative method for the synthesis of nanomaterials is reverse microemnlsion (water-in-oil) in which an aqueous phase is dispersed into an oil phase stabihzed by a surfactant film. Microemulsions can be used as nanoreactors leading to homogeneous nanomaterials with a narrow particle size and better textural properties [3]. In this work, Mn and binary Mn-Cu, Mn-Co hydrotalcite-like precursors synthesized in reverse microemulsion and the effects of preparation methods on the performance of catalysts for deep oxidation of VOCs have been studied. 

In figirre 1 the XRD profiles of the hydrotalcite like precursors substituted by Mn and by binary Mn-Co and Mn-Cu mixes synthesized by the conventional coprecipitation method are shown. The profiles present a group of signals that are characteristic of the hydrotalcite structure (JCPDS No. 89-0460). However, the formation of a contaminant corresponding to rhodocrosite (MnCOs JCPDS No. 44-1472), is identified. This is a crystalline phase which appears with a greater incident in the profiles of mixed binaries, suggesting that the addition of another metal favors the segregation of said phase. 

The microemulsion methodology permitted the obtaining of hydrotalcite like precursor with a much smaller particle size and with different textural properties in comparison with those obtained by the conventional methodology. However, the conditions of synthesis did not permit the obtaining of solids with only one hydrotalcite phase which had repercussions in the catalytic behavior of the resulting oxide. 

---