Influence of the Metal Incorporation into Hydroxyapatites on the Deactivation Behavior of the Solids in the Esterification of Glycerol

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.

Catalytic Performances of the Nano FeCo Solids for Biofuel Additives Production: Incorporation of Promoter Effects on the Stability of the Catalysts

“…FTIR spectra depict broad bands at around 4100− 3200 cm −1 that are usually ascribed to the symmetric and asymmetric O−H stretching vibrations of OH groups from physisorbed water. 4,31 Such bands are all close to the ones expected from stretching vibrations of lattice water, as well documented. 4 At 1600, 890, 800, and 600 cm −1 , the absorption band can be identified as the bending of water molecules probably coming from the reaction.…”

“…Thus, the trivalent Co is an inactive entity in EPR measurements with respect to the Co 2+ ions, which is paramagnetic with characteristic EPR signals. 31,41 Hence, the assignment of the EPR signals for all samples demonstrates reasonably that the g value of 4.3 for Co 2+ in an octahedral environment is close to that of the isotropic value g= 4.33. 41 This is indicative that Co 2+ ions are in the Co 3 O 4 form, as suggested by the Raman and XRD results.…”

“…The corresponding C–H bending vibrations are detectable near 1196–1061 cm –1 . The weak C–H bond vibrations appear due to the aldehydes such as benzaldehyde and other by-products of the reaction. ,, These bands along with weak and medium absorption bands located at around 1729, 1686, and 1609 cm –1 are ascribable to the stretching vibration of CO groups. , In agreement, very weak bands near 1278 cm –1 are assigned to the C–O–H stretching vibrations, whereas the appearance of bands in the range of 1051–1100 cm –1 illustrates the presence of C–O groups. ,, Additionally, symmetric stretching of O–C–O groups reflects the infrared absorption of acetal groups at 1450 cm –1 , as found elsewhere. , Moreover, the Me–O–Me (MeMn, Co, Mo, or Cu) bond vibrations are detected below 900 cm –1 . It is interesting to note that the bands observable in the 400–460 cm –1 range split into various weak bands being attributed to the metal–oxygen stretching from Fe–O vibrations in α-Fe 2 O 3 or γ-Fe 2 O 3 and Co–O vibrations originated from Co 3 O 4 .…”

“…FTIR measurements of the spent solids are displayed in Figure a. FTIR spectra depict broad bands at around 4100–3200 cm –1 that are usually ascribed to the symmetric and asymmetric O–H stretching vibrations of OH groups from physisorbed water. , …”

“…The weak C−H bond vibrations appear due to the aldehydes such as benzaldehyde and other by-products of the reaction. 23,31,32 These bands along with weak and medium absorption bands located at around 1729, 1686, and 1609 cm −1 are ascribable to the stretching vibration of C�O groups. 19,20 In agreement, very weak bands near 1278 cm −1 are assigned to the C−O−H stretching vibrations, whereas the appearance of bands in the range of 1051−1100 cm −1 illustrates the presence of C−O groups.…”