(158f) Continuous Process for Synthesis of Nanocrystalline Zinc-Substituted Hydroxyapatite Catalyst for Transesterification Reactions

Hydroxyapatite (Hap) is a biocompatible material, widely used as bioceramic material and as adsorbent for separation of biomolecules [1, 2]. Zinc-substituted hydroxyapatite (ZnHAp) is known to improve the bioactivity of HAp [3] and to have antibacterial effects [4]. Additionally, nanometric HAp shows to have better osteogenic and mechanical properties than micrometric Hap [5]. In fact, human bone has a nano-scale structure and a Zn content ranging from 0.0126 wt% to 0.0217 wt% [3]. Moreover, ZnHAp is used as selective adsorbent for proteins separation, where the increase of zinc content leads to a higher β2-microglobulin (β2-MCG) selectivity (in a solution of BSA) [6]; and as a selective catalyst for transesterification reactions [7]. Application of nanosized particles as catalysts and adsorbents represent an area of growing interest, due to its high surface area. Besides, for protein separation, β2-microglobulin/BSA selectivity increases due to steric exclusion of BSA by tuning the size of the meso pores formed in the agglomerates of nano-crystalline ZnHAp particles [8].

There are several processes to synthesize Hap, such as chemical precipitation hydrothermal method, sol?gel, microemulsion, ultrasonic irradiation and microwave irradiation. Among these, chemical precipitation from aqueous solutions at low temperatures (from 40 to 100 °C) is the most versatile and economic one. However, traditional implementation using stirred reactors enables a poorer micromixing quality, and therefore it is not suitable for nanoparticles formation, unless when organic modifiers are used, such as ethylene glycol, citric acid, sodium dodecyl sulphate and sodium dodecylbenzene sulphonate [9]. Another alternative is the use of microreactors, suitable for small scale production, where the production is increasing by numbering-up instead of scaling-up the microreactor unit [10]. Recently, a new technology - the NETmix® reactor - provided excellent performance to synthesize nanometric Hap at high through put [1, 11], since it offers a higher molecular-level mixing quality in comparison with conventional methods.

This work concerns the synthesis of nanocrystalline zinc-substituted hydroxyapatite, by a wet chemical precipitation method by means of a continuous process using the NETmix® static mixer at high throughputs. Calcium citrate, zinc citrate and phosphoric acid where used as precursors and ammonia was used for pH controlling. The influence of zinc content on the crystal and particles size, crystallinity, purity and specific surface area of the ZnHAp was analyzed. The performance of the prepared catalysts was evaluated for the transesterification of triacetin with methanol in terms of reaction rate and selectivity. Moreover, deactivation studies were performed.

References:

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