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Preparation and Characterization of Mg‐Zr Mixed Oxide Aerogels and Their Application as Aldol Condensation Catalysts
Author(s) -
Sádaba Irantzu,
Ojeda Manuel,
Mariscal Rafael,
Richards Ryan,
López Granados Manuel
Publication year - 2012
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201200440
Subject(s) - x ray photoelectron spectroscopy , phase (matter) , crystallography , calcination , solid solution , dissolution , desorption , oxide , chemistry , materials science , analytical chemistry (journal) , inorganic chemistry , catalysis , adsorption , chemical engineering , organic chemistry , engineering
A series of Mg‐Zr mixed oxides with different nominal Mg/ (Mg+Zr) atomic ratios, namely 0, 0.1, 0.2, 0.4, 0.85, and 1, is prepared by alcogel methodology and fundamental insights into the phases obtained and resulting active sites are studied. Characterization is performed by X‐ray diffraction, transmission electron microscopy, X‐ray photoelectron spectroscopy, N 2 adsorption–desorption isotherms, and thermal and chemical analysis. Cubic Mg x Zr 1− x O 2− x solid solution, which results from the dissolution of Mg 2+ cations within the cubic ZrO 2 structure, is the main phase detected for the solids with theoretical Mg/ (Mg+Zr) atomic ratio ≤0.4. In contrast, the cubic periclase ( c ‐MgO) phase derived from hydroxynitrates or hydroxy precursors predominates in the solid with Mg/(Mg+Zr)=0.85. c ‐MgO is also incipiently detected in samples with Mg/(Mg+Zr)=0.2 and 0.4, but in these solids the c ‐MgO phase mostly arises from the segregation of Mg atoms out of the alcogel‐derived c ‐Mg x Zr 1− x O 2− x phase during the calcination process, and therefore the species c ‐MgO and c ‐Mg x Zr 1− x O 2− x are in close contact. Regarding the intrinsic activity in furfural–acetone aldol condensation in the aqueous phase, these MgOZr sites located at the interface between c ‐Mg x Zr 1− x O 2− x and segregated c ‐MgO display a much larger intrinsic activity than the other noninterface sites that are present in these catalysts: MgOMg sites on c ‐MgO and MgOZr sites on c ‐Mg x Zr 1− x O 2− x . The very active MgOZr sites rapidly deactivate in the furfural–acetone condensation due to the leaching of active phases, deposition of heavy hydrocarbonaceous compounds, and hydration of the c ‐MgO phase. Nonetheless, these Mg‐Zr materials with very high specific surface areas would be suitable solid catalysts for other relevant reactions catalyzed by strong basic sites in nonaqueous environments.