ABSTRACT
We present novel titanium-porphyrinic gels (TPGs) and titanium-porphyrinic aerogels (TPAs), in which porphyrinic ligand tetrakis(4-carboxyphenyl)porphyrin is coordinated to Ti-oxo clusters. These hierarchically porous TPAs, with micro-, meso-, and macropores and reactant-concentration-dependent Brunauer-Emmett-Teller surface areas of 407-738â m2 g-1 , are prepared by CO2 critical point drying of TPGs. Although the Ti4+ â Ti3+ photoreduction of TPAs is less efficient than that of crystalline microporous Ti-porphyrinic framework DGIST-1, prompt diffusion of O2 and spin-trapping agents into the TPA pores causes the rapid generation of reactive oxygen species (ROS), as observed by EPR spectroscopy. When used as an ROS scavenger, large 1,3-diphenylisobenzofuran is degraded by the best-performing TPA 10 times faster than by DGIST-1, suggesting that the accessibility of molecules (reactants) to pores (reactive centers) strongly influences photocatalytic activity.
ABSTRACT
Herein, we report titanium-organic gels (TOGs) as new Ti-oxo-based materials that exhibit stimuli-responsive sol-gel transformations and hierarchical porosity upon the removal of solvent molecules. Heating a solution of Ti-oxo clusters and pyromellitic acid as a tetratopic ligand produces TOGs that readily become sols by applying physical stimuli such as shaking or vortexing under ambient conditions. Porous titanium-organic aerogels (TOAs) were obtained by the CO2 supercritical point drying (CPD) of the TOGs, and their porous structures were characterized by N2 adsorption and desorption isotherm measurements. These TOAs, based on the Ti-oxo clusters, possess hierarchical micro-, meso-, and macropores. Furthermore, accompanying the prominent photochromic phenomena, reduction of Ti4+ to Ti3+ was observed upon UV irradiation. The TOAs were successfully applied in the adsorption and photocatalytic degradation of several dye molecules. This research introduces a versatile method for preparing stimuli-responsive and porous Ti-oxo-based photocatalysts.
ABSTRACT
Titanium(IV)-based metal-organic frameworks (Ti-MOFs) have received significant attention in recent years due to their numerous photocatalytic applications. We herein prepare the single-crystalline Ti-carboxylate MOF (DGIST-1) composed of an unprecedented Ti-oxo chain cluster and the porphyrinic ligand, TCPP (tetrakis(4-carboxyphenyl)porphyrin). Preformed Ti-oxo clusters were used as Ti4+ sources to avoid the spontaneous hydrolysis and condensation reactions of traditional Ti-alkoxide precursors, thus, enabling the formation of the highly crystalline Ti-MOF. The successfully activated DGIST-1 exhibited a higher surface area (i.e., 1957.3â m2 g-1 ) than previously reported Ti-MOFs due to its high crystallinity. Furthermore, the visible-light-responsive photocatalytic activity of DGIST-1 was confirmed by the simultaneous generation of singlet oxygen (1 O2 ) and superoxide (. O2 - ) species, in addition to the highly efficient and selective oxidation of benzyl alcohol to benzaldehyde.
ABSTRACT
Reactive oxygen species (ROS) are highly reactive oxidants that are typically generated by the irradiation of semiconducting materials with visible or UV light and are widely used for the photocatalytic degradation of toxic substances, photodynamic therapy, and selective organic transformations. In this context, TiO2 is considered to be among the most promising photocatalysts due to its high redox activity, structural stability, and natural abundance. In view of the extensive development of highly active photocatalysts, we herein briefly introduce TiO2 and the mechanisms of TiO2-mediated ROS generation, subsequently focusing on key advances in the design and synthesis of Ti-containing porous materials, such as porous TiO2, Ti-based metal-organic frameworks, and Ti-based metal-organic aerogels. In particular, this review highlights the significance of porosity and the structure-function relationship for the development of Ti-based photocatalysts. The structures, porosities, and ROS generation mechanisms of these materials as well as the related efficiencies of ROS-mediated photocatalytic organic transformations are discussed in detail to provide a useful reference for future researchers and to inspire the exploration of high-performance photocatalysts.