A team of researchers recently published a paper in the journal ACS Applied Nano Materials that demonstrated the feasibility of using nanoporous polarized hydroxyapatite (HAp) scaffolds as a green catalyst in chemical reactions.
Study: Tailorable Nanoporous Hydroxyapatite Scaffolds for Electrothermal Catalysis. Image Credit: Artur Wnorowski/Shtuterstock.com
Importance of Nanocatalysts
The confinement of chemical reactions to the nano- and micro-scales offers several advantages, such as the control of the mass and heat transfer and increased surface-to-volume ratio, leading to enhanced efficiency and final selectivity. Thus, the development of nanocatalysts and nanoreactors in the form of porous or hollow structures has gained significant attention.
Significance of HAp Catalysts and their Limitations
Recently, HAp catalysts utilized for carbon and nitrogen fixation conversion in value-added molecules under mild reaction conditions demonstrated exceptional selectivity behavior, which indicated the viability of HAp as a cheap and green alternative to conventional catalysts.
Catalytic activation of HAp was realized using a thermally stimulated polarization (TSP) treatment on sintered HAp pellets. Although the TSP treatment conferred electrochemical and electrical properties to the sintered HAp, the poorly porous structure of the HAp catalysts limited their catalytic efficiency and the overall yield of reactions.
Polarized Nanoporous HAp Catalysts as a Potential Solution
The HAp scaffolds recently developed for air filtration and tissue engineering applications represent a promising approach for three-dimensional (3D) catalyst designing with a larger specific surface area.
3D printing technology has gained prominence due to its ability to model the architecture and control the dimensions, pore shape, pore size, and porosity of samples, allowing the differentiation between micropores and macropores.
3D printing through direct-ink writing/robotic material extrusion was explored extensively for biological applications to create tailored printable HAp inks by adding proper biocompatible polymers based on the requirements of every application.
However, selecting a suitable polymer can be extremely challenging as both synthetic polymers and natural biopolymers have certain drawbacks for the synthesis of printable HAp inks. Thus, a suitable additive should be identified to create customized 3D nanoporous HAp and polarized catalysts.
The polymeric additive must have adequate rheological properties to precisely control the porosity and final architecture of the samples without impacting their crystal structure. Moreover, the additive must be eliminated through the sintering process after forming HAp scaffolds without affecting their mechanical and dimensional stability.
Pluronic F-127 hydrogel was utilized in a recent study to create 3D-printed highly porous yttrium-stabilized zirconia scaffolds meeting all conditions discussed above. Thus, researchers selected pluronic hydrogels in this study to control the porosity and final architecture of HAp scaffolds for the preparation of highly nanoporous polarized HAp catalysts.
Fabrication and Evaluation of 3D Polarized Nanoporous HAp Catalysts
In this study, researchers fabricated 3D nanoporous polarized HAp catalysts for the first time using Pluronic F-127 hydrogel and evaluated their catalytic efficiency.
Fabrication of 3D Nanoporous HAp Scaffolds
HAp powder obtained using the hydrothermal method was freeze-dried for 72 hours to remove the water content. Pluronic F-127 was mixed with water to prepare the hydrogel.
Subsequently, HAp inks with different concentrations of Pluronic hydrogel were obtained in the form of a white paste by slowly adding the hydrogel to the HAp powder and then vigorously stirring the mixture using a Fisherbrand digital vortex mixer at a temperature of four degrees Celsius.
The white paste was then left aging for 24 hours at four degrees Celsius to ensure homogenous distribution of Pluronic F-127 hydrogel. Eventually, HAp inks were modeled using the 3D printing method at low temperatures to fabricate the 3D nanoporous HAp scaffolds with customized architecture.
The scaffolds were then sintered at 1000 degrees Celsius for two hours to remove the hydrogel content. The final obtained sample was designated as x-HAp, where x indicates the pluronic hydrogel mass percentage in the scaffold.
Evaluation of the Fabricated Scaffolds
Raman microscopy, wide-angle X-ray diffraction (WAXD), energy dispersive X-ray (EDX) analysis, and scanning electron microscopy (SEM) were used for structural characterization of the 3D nanoporous HAp scaffolds.
A contact angle measuring equipment was employed to obtain the water absorption capabilities. The mechanical properties of fabricated HAp scaffolds, such as elastic modulus and hardness, were measured at a nanometric length scale using the nanoindentation technique.
Catalytic Activation of the HAp Scaffolds and their Performance Evaluation
The nanoporous HAp scaffolds were catalytically activated using the TSP treatment. Researchers also synthesized conventional HAp catalysts and compared the catalytic performance of both nanoporous and conventional HAp catalysts for different nitrogen and carbon fixation reactions.
These reactions include ammonia synthesis from nitrogen gas, ethanol production from gas mixtures of methane and carbon dioxide, and amino acid synthesis from gas mixtures of methane, carbon dioxide, and nitrogen.
The catalytically activated nanoporous and conventional HAp catalysts were used as inert reactors in the nitrogen and carbon fixation reactions. Auxiliary coatings were applied on the catalysts during amino acid production to obtain amino acids from nitrogen and carbon fixation. Proton nuclear magnetic resonance (1H NMR) spectroscopy was used to quantify the yields of these reactions.
Significance of the Study
3D nanoporous HAp scaffolds with customized architecture were fabricated successfully by mixing Pluronic F-127 hydrogel and HAp powder. A 60-weight percentage of Pluronic F-127 hydrogel was considered suitable for preparing HAp ink owing to its good control over the porosity and pore size without affecting the mechanical stability of the fabricated scaffolds.
The HAp ink with optimal paste properties obtained from the mixture of a 60-weight percentage of Pluronic F-127 hydrogel and HAp powder (60-HAp) fulfilled the 3D printing requirements. The adequate mechanical stability of the fabricated scaffolds was ensured by sintering them at high temperatures.
Sintered 60-HAp scaffolds demonstrated high crystallinity and purity, indicating effective dehydration of the scaffolds after sintering. Thus, the addition of Pluronic F127 hydrogel to prepare printable inks and the nanopores generated due to them did not affect the scaffold structure used for the preparation of polarized HAp-based catalysts.
The aminotris(methylenephosphonic acid) (ATMP)/ zirconyl chloride (ZC)/ATMP coating on the polarized nanoporous 60-HAp catalyst during amino acid production reduced its catalytic efficiency as the nanopores generated by the incorporation of Pluronic hydrogel were completely blocked by the coating layers, leading to a substantial reduction in reaction yield compared to conventional nonporous HAp catalyst.
However, the non-coated 60-HAp catalysts with high nanoporosity demonstrated an exceptional increase in reaction yields due to higher exposed surface and enhanced water absorption capability.
The reaction yields obtained using 60-HAp catalyst during the nitrogen fixation reaction to produce ammonia and carbon fixation reaction to produce ethanol were 2000 percent and 3000 percent higher than conventional nonporous HAp catalyst. The microcavities present within the nanoporous 60-HAp catalysts promoted the heterogeneous catalytic processes used for ammonia and ethanol production.
Taken together, the findings of this study demonstrated that non-coated nanoporous scaffold-based 60-HAp catalysts possess excellent catalytic properties and significant scalability potential. Thus, these catalysts can be used as an eco-friendly, cheaper, and solid alternative to conventional catalysts in chemical reactions, specifically nitrogen and carbon fixation reactions.
Turon, P., Sans, J., Arnau, M. et al. Tailorable Nanoporous Hydroxyapatite Scaffolds for Electrothermal Catalysis. ACS Applied Nano Materials 2022. https://pubs.acs.org/doi/10.1021/acsanm.2c01915
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