Calcium Hydroxyapatite: A Highly Stable and Selective Solid Catalyst for Glycerol Polymerization

Abstract

Calcium-based catalysts are of high interest for glycerol polymerization due to their high catalytic activity and large availability. However, their poor stability under reaction conditions is an issue. In the present study, we investigated the stability and catalytic activity of Ca-hydroxyapatites (HAps) as one of the most abundant Ca-source in nature. A stochiometric, Ca-deficient and Ca-rich HAps were synthesized and tested as catalysts in the glycerol polymerization reaction. Deficient and stochiometric HAps exhibited a remarkable 100% selectivity to triglycerol at 15% of glycerol conversion at 245 °C after 8 h of reaction in the presence of 0.5 mol.% of catalyst. Moreover, under the same reaction conditions, Ca-rich HAp showed a high selectivity (88%) to di- and triglycerol at a glycerol conversion of 27%. Most importantly, these catalysts were unexpectedly stable towards leaching under the reaction conditions based on the ICP-OES results. However, based on the catalytic tests and characterization analysis performed by XRD, XPS, IR, TGA-DSC and ICP-OES, we found that HAps can be deactivated by the presence of the reaction products themselves, i.e., water and polymers.

Development of Hydroxyapatite Coatings for Orthopaedic Implants from Colloidal Solutions: Part 2—Detailed Characterisation of the Coatings and Their Growth Mechanism

“…Powder analysis was performed on apatite mineral to understand how phases emerge within solution and upon solution–surface interaction. TGA analysis of this HA powder revealed the presence of adsorbed water and interstitial water, similar to other HA studies [ 47 , 48 , 49 ]. In the TGA data, Figure 1 a showed a 4% weight loss by 600 °C at a steady rate.…”

Effects of Various Ripening Media on the Mesoporous Structure and Morphology of Hydroxyapatite Powders

“…The simple and low-cost synthesis of micro- and mesoporous HA powders is a challenge for today [ 48 ]. The fields for its application are broadening every day, especially in catalysis [ 49 , 50 , 51 , 52 , 53 ]; in the removal of environmental pollution in the soil, wastewater [ 15 , 54 ], and air [ 55 , 56 , 57 ]; and in the biomedical [ 58 , 59 , 60 ] area. Additionally, mesoporous HA is considered a potential material for optical applications [ 61 ], as an MRI contrast agent [ 62 ], in chromatography matrices [ 63 ], and as a carrier for the food industry [ 64 ].…”

High-temperature phase transformations of hydroxylapatite and the formation of silicocarnotite in the hydroxylapatite–quartz–lime system studied in situ and in operando by Raman spectroscopy

“…However, our observations suggest a complete loss of structural OH groups between a relative narrow temperature interval already between 770 and 850 °C (Table 3). This difference in the dehydration temperature likely reflect difference in the structural state or the degree of disorder of the used HAp samples, including non-stoichiometry, which is very difficult to detect by XRD analysis alone [92][93][94]. Loss of hydroxyl groups bound to the surface of the HAp nano-crystals, however, occurs in distinct overlapping stages below about 600 °C.…”