Abstract
Several layered double hydroxides (LDHs) with general chemical composition (Cu,Zn)1−xAlx(OH)2(CO3)x/2·mH2O have been synthesized by the co-precipitation method, maintaining a (M2+/M3+) molar ratio of 3, and varying the Cu2+/Zn2+ molar ratio between 0.2 and 6.0. After calcination and reduction steps, Cu/ZnO/Al2O3 catalysts were synthesized. These catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 thermoprogrammed reduction (H2-TPR), N2 adsorption-desorption at −196 °C, N2O titration, X-ray photoelectron miscroscopy (XPS), NH3-thermoprogramed desorption (NH3-TPD) and CO2- thermoprogrammed desorption (CO2-TPD). The characterization data revealed that these catalysts are mainly meso-and macroporous, where Cu, ZnO and Al2O3 are well dispersed. The catalytic results show that these catalysts are active in the gas-phase hydrogenation of furfural, being highly selective to furfuryl alcohol (FOL) and reaching the highest FOL yield for the catalyst with a Cu2+/Zn2+ molar ratio of 1. In an additional study, the influence of the aging time on the synthesis of the LDHs was also evaluated. The catalytic data revealed that the use of shorter aging time in the formation of the LDH has a beneficial effect on the catalytic behavior, since more disordered structures with a higher amount of available Cu sites is obtained, leading to a higher yield towards FOL (71% after 5 h of time-on-stream at 210 °C).
Highlights
Layered double hydroxides (LDHs), known as anionic-clays or hydrotalcites, are a group of inorganic lamellar compounds of basic nature
This study reveals that the metal particle size increases directly with the Cu loading, from 5 nm for CuZn_0.2 to 19 nm for CuZn_6, being these data slightly lower than those obtained from X-ray diffraction (XRD) by using the Williamson-Hall method [52]
CuZn_X catalysts were tested in the gas-phase FUR hydrogenation, using FUR dissolved in cyclopentyl methyl ether (CPME)
Summary
Layered double hydroxides (LDHs), known as anionic-clays or hydrotalcites, are a group of inorganic lamellar compounds of basic nature. Much attention is being paid to the study of these inorganic materials due to their chemical and structural properties, which make them useful in interesting applications, such as adsorbents, catalysts, anion exchangers or flame retardants, among others [1,2,3]. These materials exhibit high chemical stability, good biocompatibility and pH-dependent solubility [4]. This inorganic structure results from stacked brucite layers, Mg(OH) , where some Mg2+ ions can be replaced by Al3+ ions, generating an excess of positive charge in layers, which must be counterbalanced by the presence of anions, mainly carbonates (CO3 2− ), in the interlayer spacing [1,2,3].
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