The structure–performance relationship is a critical fundamental issue in heterogeneous catalysis, and the size-dependent structure sensitivity of catalytic reactions has long been researched in catalysis. Yet it remains elusive for most of the reactions in a full-size range, from a single atom and subnanometer clusters to nanoparticles. Herein, we report complete size dependence of Pt catalysts used in propane dehydrogenation in terms of activity, selectivity, and stability due to coke formation. The turnover frequency (TOF) of the atomically dispersed Pt/Al2O3 catalyst was approximately 3-fold and 7-fold higher than the subnanometer-sized clusters and the nanoparticles, respectively. A canyon- shaped size dependence of the propene selectivity was observed with a bottom at about 2 nm of Pt particle size. The subnanometer-sized clusters have opposite size dependence of the propene selectivity compared to nanoparticles. Both atomically dispersed Pt and large Pt nanoparticles possess high propene selectivity. The atomically dispersed platinum centers with a positive charge dramatically enhanced the activity, weakened propylene adsorption, and prevented its deep dehydrogenation. Besides, the absence of multiple Pt–Pt sites effectively inhibited undesired side reactions (e.g., C–C cracking), thus improved propylene selectivity and stability. This work demonstrates the promising application of a supported atomically dispersed Pt catalyst for highly selective dehydrogenation of propane.

Chitin-derived hydrogels are commonly used in bone regeneration because of their high cell compatibility; however, their poor mechanical properties and little knowledge of the interaction between the materials and host cells have limited their practical application.Methods: To evaluate osteoinductivity and enhance the mechanical properties of a newly synthesized thermosensitive hydroxypropyl chitin hydrogel (HPCH), a mesenchymal stem cell (MSC)-encapsulated HPCH was infused into a three-dimensional-printed poly (ε-caprolactone) (PCL)/ nano-hydroxyapatite (nHA) scaffold to form a hybrid scaffold. The mechanical properties and cell compatibility of the scaffold were tested. The interaction between macrophages and scaffold for angiogenesis and osteogenesis were explored in vitro and in vivo.Results: The hybrid scaffold showed improved mechanical properties and high cell viability. When MSCs were encapsulated in HPCH, osteo-differentiation was promoted properly via endochondral ossification. The co-culture experiments showed that the hybrid scaffold facilitated growth factor secretion from macrophages, thus promoting vascularization and osteoinduction. The Transwell culture proved that MSCs modulated the inflammatory response of HPCH. Additionally, subcutaneous implantation of MSC-encapsulated HPCH confirmed M2 activation. In situ evaluation of calvarial defects confirmed that the repair was optimal in the MSC-loaded HPCH + PCL/nHA group.Conclusions: PCL/nHA + HPCH hybrid scaffolds effectively promoted vascularization and osteoinduction via osteogenesis promotion and immunomodulation, which suggests promising applications for bone regeneration.

Repair and regeneration of large bone defects is still a challenge, especially for defects which are the irregular and complex. Three-dimension (3D) printing, as an advanced fabrication technology, has been received considerable attentions due to its high precision, customized geometry and personalization. In this study, 3D porous polylactic acid/nano hydroxyapatite (PLA/nHA) composite scaffolds with enhanced osteogenesis and osteoconductivity were successfully fabricated by desktop fused deposition modeling technology. Morphological, composition and structural analysis revealed that nHA was successfully introduced into the PLA system and homogeneously dispersed in the printed PLA/nHA scaffolds. In vitro antibacterial experiment confirmed that the printed porous PLA/nHA scaffolds have good ability for loading and releasing vancomycin and levofloxacin. Meanwhile, MG-63 cells were used to evaluate the cytocompatibility of printed porous PLA/nHA scaffolds by proliferation and cellular morphological analysis. In addition, rabbit model was established to evaluate the osteogenesis and osteoconductivity of printed PLA/nHA scaffolds. All these results suggested that the 3D printed PLA/nHA scaffolds have great potential for repairing and regeneration of large bone defects.