Abstract
The Meerwein–Ponndorf–Verley (MPV) reaction is an environmentally-friendly process consisting of the reduction of a carbonyl compound through hydrogen transfer from a secondary alcohol. This work deals with MPV reduction of furfural to furfuryl alcohol on different ZrOx, MgOx, TiOx, and Mg–Ti, as well as Zr–Ti mixed systems. The solids were synthesized through the sol–gel process and subsequently calcined at 200 °C. Characterization was performed using a wide range of techniques: ICP-MS, N2 adsorption-desorption isotherms, EDX, TGA-DTA, XRD, XPS, TEM, TPD of pre-adsorbed pyridine (acidity) and CO2 (basicity), DRIFT of adsorbed pyridine, and methylbutynol (MBOH) test reaction. ZrOx showed the highest conversion and selectivity values, which was attributed to the existence of acid–base pair sites (as evidenced by the MBOH test reaction), whereas the introduction of titanium resulted in the drop of both conversion and selectivity probably due to the increase in Brönsted-type acidity. As for MgOx, it had a predominantly basic character that led to the production of the condensation product of one molecule of furfural and one molecule of acetone, and thus resulted in a lower selectivity to furfuryl alcohol. The TiOx solid was found to be mainly acidic and exhibited both Lewis and Brönsted acid sites. The presence of the latter could account for the lower selectivity to furfuryl alcohol. All in all, these results seemed to suggest that the MPV reaction is favored on Lewis acid sites and especially on acid–base pair sites. The process was accelerated under microwave irradiation.
Highlights
The transformation of natural residues from agriculture into platform molecules is one of the promising research lines in obtaining high added value chemical products [1,2]
Areas corresponded to solids consisting of Zr and/or Ti, whereas lower values were found for the systems containing magnesium (42–81 m2 /g)
Several solids consisting of pure magnesium, zirconium, titanium, and mixed magnesium–titanium as well as zirconium–titanium gels were obtained through the sol–gel process and calcined at 200 ◦ C
Summary
The transformation of natural residues from agriculture into platform molecules is one of the promising research lines in obtaining high added value chemical products [1,2]. One of those platform molecules is furfural [3], which can be obtained from lignocellulose [4] It contains an aromatic ring and an aldehyde group which makes it a versatile molecule to obtain a wide range of chemical compounds [5], and one of the most important ones is furfuryl alcohol. Catalysts 2018, 8, 539 out through the hydrogen transfer from a donor, which are typically secondary alcohols such as propan-2-ol, using the so-called Meerwein–Ponndorf–Verley (MPV) process [13] This reaction involves the formation of a six-membered ring transition state in which both the reducing alcohol and the carbonyl compound are coordinated to the metal center (Lewis site) [14]. The possibility of carrying out the reaction with microwave-assisted heating was evaluated
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