Amorphous calcium carbonate (ACC), with the highest reported specific surface area of all current forms of calcium carbonate (over 350 m² g^-1), was synthesized using a surfactant-free, one-pot method. Electron microscopy, helium pycnometry, and nitrogen sorption analysis revealed that this highly mesoporous ACC, with a pore volume of ∼0.86 cm³ g^-1 and a pore-size distribution centered at 8-9 nm, is constructed from aggregated ACC nanoparticles with an estimated average diameter of 7.3 nm. The porous ACC remained amorphous and retained its high porosity for over 3 weeks under semi-air-tight storage conditions. Powder X-ray diffraction, large-angle X-ray scattering, infrared spectroscopy, and electron diffraction exposed that the porous ACC did not resemble any of the known CaCO₃ structures. The atomic order of porous ACC diminished at interatomic distances over 8 Å. Porous ACC was evaluated as a potential drug carrier of poorly soluble substances in vitro. Itraconazole and celecoxib remained stable in their amorphous forms within the pores of the material. Drug release rates were significantly enhanced for both drugs (up to 65 times the dissolution rates for the crystalline forms), and supersaturation release of celecoxib was also demonstrated. Citric acid was used to enhance the stability of the ACC nanoparticles within the aggregates, which increased the surface area of the material to over 600 m² g^-1. This porous ACC has potential for use in various applications where surface area is important, including adsorption, catalysis, medication, and bone regeneration.

Amorphous calcium carbonate (ACC) stabilized by various carboxylic additives was synthesized by incorporating the additives into an ACC suspension in methanol. The additives studied included polyacrylic acid, citric acid, adipic acid, 6-aminocaproic acid, 4-aminobutyric acid and hexanoic acid. The stabilized ACC samples (ACC-additives) exhibited similar characteristics to ACC alone. They appeared X-ray amorphous, contained characteristic infrared bands and had the same nanoparticle aggregated microstructure as ACC. The porosity of the ACC-additives was, however, markedly improved, with Brunauer-Emmett-Teller (BET) surface areas of up to ~640 m²/g. The BET surface area of ACC-citric acid was close to double that of a highly porous ACC sample. The structure and amount of the additive had a noticeable effect on the porosity of the ACC-additives. When the additive was adsorbed onto the surface of the ACC nanoparticles, their growth was restricted. The restricted growth reduced the size of the ACC nanoparticles, which increased the BET surface area of ACC. Finally, the long-term stability study revealed that the stability of all the ACC-additives was markedly enhanced when stored in ambient or semi-airtight conditions (in a closed falcon tube). In particular, ACC stabilized with adipic acid (ACC-AA-267) had excellent stability, remaining in an amorphous phase for more than one year under ambient conditions and retaining ~87% porosity for 48 weeks under semi-airtight conditions. The extremely high porosity and excellent long-term stability make these ACC-additives promising candidates for applications where porosity and stability are critical, such as those involving adsorption, bone regeneration or drug delivery.

The morphology controlled synthesis of porous vaterite microparticles from amorphous calcium carbonate (ACC) nanoparticles via mesoscale transformation and self-assembly is presented. The morphology of vaterite microparticles ranging from ellipsoidal to spherical can be controlled by adjusting the amount of adipic acid (AA) additive during synthesis. Electron microscopy and electron diffraction reveal that the vaterite microparticles are formed by the oriented self-assembly of vaterite nanocrystals. The Brunauer-Emmett-Teller (BET) surface area of the vaterite microparticle varies between similar to 30 and similar to 80 m(2)/g. The coverage of AA on the surface of the ACC nanoparticle plays the pivotal role in the morphology controlled synthesis of vaterite microparticles. 6-Aminocaproic acid (6A), benzoic acid (BA), citric acid (CA), and poly(acrylic acid) (PAA) are also tested as additives and their effect on the morphology of vaterite microparticles is presented. Morphology control of functional materials can be beneficial for application where the morphology and porosity are critical, such as drug delivery. This work demonstrates a possible method to finely adjust the morphology of vaterite microparticles with the assistance of additives through mesoscale transformation and self-assembly using amorphous nanoparticles as precursors.

The emission of greenhouse gases such as CO2and SF6 is believed to contribute significantly toward globalwarming. One way to reduce their release is by adsorption atpoint sources using a suitable adsorbent. In this work we presentthe synthesis of two hierarchical porous carbon materials(referred to as PC-CaCit and PC-MgCit) with a high uptake ofSF6 (5.23 mmol/g, 0 °C, 100 kPa) and a reasonable uptake ofCO2 (>3 mmol/g). PC-CaCit and PC-MgCit were obtained bypyrolysis of the most porous calcium citrate and magnesiumcitrate ever reported, which were synthesized by us. TheLangmuir specific surface area of PC-CaCit and PC-MgCit wasover 2000 m2/g (BET surface area also close to 2000 m2/g). Wecharacterized PC-CaCit and PC-MgCit using a range of advanced characterization techniques including N2 adsorption, highresolutionelectron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy. PC-CaCit and PC-MgCit alsoshowed a SF6-over-N2 selectivity of ∼33 at 0 °C (100 kPa), good cyclic performance, and moderately low heat of adsorption.The porous carbons synthesized in this work are good candidate adsorbents for greenhouse gases.