Abstract
The effects of the metal incorporation into hydroxyapatites on the deactivation behavior of the solids were examined in the esterification of glycerol (EG) reaction. The introduction of Cu, Co, or Ni ions by ion exchange in calcium-deficient hydroxyapatites resulted in active catalysts for the EG reaction. The metal contents were varied from 2.0 to 17.0%, providing better performances at rather high metal contents. Part of metal species existed in the hydroxyapatite lattice structure and also as isolated Cu2+, Ni2+, and Co2+ entities on the surface, as shown by XPS and EPR. The effects of the reaction temperature, reaction time, and glycerol to acetic acid molar ratios were deeply investigated. The spent solids used in this study were characterized by XRD, FTIR, SEM-EDS, chemical analyses, EPR, and XPS. The Cu2+–OH acid pairs could promote a superior catalytic performance of Cu-containing hydroxyapatites due to the resistance of these solids against leaching of the active species, which is even better than those of Co and Ni-containing counterparts with high metal contents. Cu into hydroxyapatite had a good reusability and long-term utilization for five consecutive cycles of 24 h under a glycerol to acetic acid molar ratio of 0.25 at 80 °C, and longer reaction times provide triacetin formation. This was due to the fact that Cu was stabilized by interacting with Ca, PO4, and OH sites into the hydroxyapatite lattice, being highly active for the EG reaction. The results also revealed that isolated Cu2+ sites played an important role in enhancing the glycerol conversion, intrinsically due to the Cu-containing hydroxyapatites ability to avoid strong adsorption of glycerol oligomers on the catalytic sites.
Highlights
Glycerol is regarded as a renewable and industrially important source of raw material for the production of value-added chemicals, such as glycerol carbonates, esters, and ethylene glycol, among others [1,2,3]
We aim to examine the catalytic properties of the HAP-based catalysts for the esterification of glycerol with acetic acid and to find the relationships between the physicochemical properties and the catalytic behaviors of the solids
The effect of the reaction time on the catalytic performance of solids is investigated in the esterification of glycerol (EG) reaction in the presence of acetic acid
Summary
Glycerol is regarded as a renewable and industrially important source of raw material for the production of value-added chemicals, such as glycerol carbonates, esters (acetins), and ethylene glycol, among others [1,2,3]. Crude glycerol is a chemical raw material obtained from the synthesis of biodiesel [4,5,6,7]. Due to the emerging large-scale worldwide production of biofuels, crude glycerol is forecasted to flood the market and poses especially serious risks. Due to the emerging large-scale worldwide production of biofuels, crude glycerol is forecasted to flood the market and poses especially serious risks to humans because of the improper disposal of large quantities of the crudeoftri-alcohol [1,4]. The increasingly severe of [1,4] This to humans because the improper disposal of large quantities of the crudeproblem tri-alcohol by-product pollution hassevere received great of attention for developing newhas technologies with. The increasingly problem this by-product pollution received great lower costs higher new efficiency conversion crude glycerol.
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