Herein, α-MoO3 micro- and nanoparticles
were synthesized
by a modified Pechini method, and the impact of the crystal structure
and crystal growth orientation on the formation of ionic defects and,
consequently, on the catalytic performance of the materials in the
ethylic transesterification reaction for biodiesel production was
investigated. Structural refinements from X-ray diffraction data and
Raman spectra revealed the formation of α-MoO3 in
a Pbnm orthorhombic phase, with nanoplate-like morphology
at 500 °C (thickness between 100 and 260 nm) or ribbon-like morphology
at 700 °C (thickness between 400 and 900 nm). An anisotropic
crystal orientation along the [010] direction was observed with an
increase of the calcination temperature. We emphasize the dependence
of the orientation change with the elimination of ionic-type defects
(oxygen vacancies and reduced Mo5+ centers) by the temperature
using complementary techniques such as X-ray photoelectron and electron
paramagnetic resonance spectroscopies. The catalytic activity of the
samples depends on the orientation process and the presence of defects
that act as acid-active sites on the catalyst surface and therefore
play an important role in biodiesel production. This effect was confirmed
by surface stability and reactivity simulated by density functional
theory calculations, suggesting that the Mo and O surface terminals
greatly impacted the interface catalytic reaction. The highest catalytic
performance toward the biodiesel conversion (89% of conversion at
150 °C for 2 h) was achieved for the polycrystalline catalyst
calcined at 500 °C, which was correlated with random crystal
orientation and the presence of reduced Mo5+ and oxygen
vacancy centers on the different facets exposed on the surface. The
biodiesel production was confirmed by 1H and 13C NMR spectroscopy and gas chromatography analysis.