Enhanced CO₂ Conversion into Ethanol by Permanently Polarized Hydroxyapatite through C−C Coupling

Abstract

Hydroxyapatite (HAp) is a naturally occurring mineral form of calcium apatite of biomedical importance due to its similarity with human hard tissues in morphology and composition. Upon polarization at high temperature, applying 3 kV/cm at 1000 °C, the resulting polarized HAp (p-HAp) exhibits enhanced catalytic behavior due charge accumulation at the interface. More specifically, p-HAp was found to catalyse the conversion of mixtures of CO 2 (g) and CH 4 (g) into low carbon organic molecules and into amino acids when N 2 (g) was added to the mixture. In this work, we report how p-HAp facilitates the conversion of CO 2 (g) mainly in ethanol by means of forming CÀ C coupled bonds on its activated surface. After evaluation of a wide range of experimental conditions, we evidence the production of formic acid, methanol and formaldehyde (C1 products); ethanol and acetic acid (C2 products); and acetone (C3 product) from CO 2 (g) using moderate reaction conditions. Moreover, optimization of the reaction parameters led to a significant increase towards ethanol.

Permanently polarized hydroxyapatite, an outstanding catalytic material for carbon and nitrogen fixation

“…CO 2 -enriched air) at atmospheric pressure and performing the reaction at 95 °C. 19 These impressive results show the huge potential of pp-HAp for reducing the environmental impact of the greenhouse gases, working reaction pressures as low as 1 bar. Within this context, it should be stressed that although pp-HAp acts reducing CO 2 , no electric voltage was applied to conduct the reaction, which was attributed to the accumulation of charge at the boundaries and the surface of micrometric grains, minimizing the total energy balance.…”

“…without CH 4 ) on the activated surface of pp-HAp led to the production of formic acid, methanol and formaldehyde (C1 products); ethanol and acetic acid (C2 products); and acetone (C3 product) from CO 2 using mild reaction conditions and without UV illumination. 19 Optimization of the pressure and the temperature, which were varied between 1 and 6 bar and between 95° and 140°, respectively, allowed to optimize the selectivity towards ethanol. Thus, the yield of ethanol relative to all other reaction products increased with the pressure and temperature, varying from 9–13% at 1 bar and 95 °C to more than 60% at 6 bar and 140 °C.…”

“…Thus, the yield of ethanol relative to all other reaction products increased with the pressure and temperature, varying from 9–13% at 1 bar and 95 °C to more than 60% at 6 bar and 140 °C. 19 Another interesting strategy to increase the selectivity was to modify the structure of the catalyst by generating Bru in the HAp crystal lattice. 30 This was achieved by modifying the distance between the electrode plates used in the TSP process, which allowed to create regions with different proton conductivity and adsorption capacity in the catalyst structure.…”

“…non polarized HAp) remains to be conceived mainly as a catalytic support. However, pp-HAp not found to act as a support but to catalyze carbon and nitrogen fixation reactions due to its electrical properties, [17][18][19] which were attributed to both the generation of vacancies and the polarization of OH À groups. 12 Thus, the superficial charge, which exhibited a linear dependence with number of vacancies, was demonstrated to be directly related with pp-HAp catalytic properties.…”

“…Within the context of the latter, the utilization of pp-HAp as catalyst for nitrogen and carbon fixation reactions from CO 2 and N 2 gases, respectively, has provided outstanding results. [17][18][19] Moreover, the mild reaction conditions required to obtain amino acids, ethanol and ammonia from such processes, and the fact that the yield the reaction products has been significantly increased by enhancing the porosity of the catalyst, allow to postulate ultra-porous pp-HAp as a promising alternative to conventional catalysts Even so, the potential of pp-HAp is not only restricted to catalysis as enhanced HAp electrical properties might be desired for other fields, such as the biomedicine. 11 For example, pp-HAp was found to considerably favour the adsorption of inorganic bioadsorbates in comparison to conventional HAp, 20 which may have implications in the fabrication of scaffolds with improved phosphate and phosphonate adsorption capacity for bone regeneration.…”