Inclusion chemistry in periodic mesoporous hosts: Growth of quantum-confined materials and gas separation membranes.

摘要

Ordered mesoporous films and membranes refer to materials in a thin layer geometry possessing pore diameters in the 2-50 nm range. Their extremely high surface areas (up to ∼ 1000 m2/g) and precise tuning of pore sizes are among the many desirable properties that have made such thin films and membranes a focus of great attention for emerging applications in the synthesis of nanomaterials and separation membranes. Ordered mesoporous silica films are highly promising hosts to grow semiconductor nanoparticles with fine-tuned size if the film thickness, pore size and orientation can be controlled. This important potential use of ordered mesoporous silica materials is one major focus area of this dissertation. Ordered mesoporous silica thin films can also be grown as membrane layers on porous supports possessing three-dimensionally interconnected pore structures. The investigation of ordered mesoporous silica membranes modified by amine groups for CO2 separation is another major focus area of this work. Potential application of these membranes is to capture CO2 from flue gas which is a critical step preceding a variety of CO2 sequestration approaches to curb its atmospheric emissions.; We systematically investigated the effect of pore size and pore symmetry on the growth of CdS nanoparticles inside the functionalized ordered mesoporous silica hosts. The 2D hexagonal mesoporous silica, SBA-15 possessing 4-22 nm pores and 3D cubic SBA-16 with 5 nm pores was employed to investigate the effect of pore size and pore symmetry on the microstructure of resulting CdS nanoparticles. As the pore size of SBA-15 increased, an increase in the average size of the CdS nanocrystals was observed. The pore size distributions of the CdS/SBA-15 and CdS/SBA-16 composites suggested that CdS nanoparticles nucleated primarily in the micropores of SBA-15 and SBA-16 hosts. Furthermore, SBA-16 possessing smaller pores as compared to SBA-15 resulted in larger crystal sizes of CdS nanoparticles. These results provided new important insights into the mechanism of initial growth of CdS nanoparticles inside the pores of mesoporous silica hosts possessing different pore symmetries.; Ordered mesoporous MCM-48 membranes possessing a cubic pore structure were prepared by solution growth method on alpha-alumina supports. The surfactant was removed by calcination and Soxhlet extraction. The best quality membranes were obtained when the surfactant was removed by extraction. The membranes prepared on asymmetric alpha-alumina supports displayed higher gas permeance (N2 permeance > 2x 10-7 mol/m2.s.Pa) than those fabricated on symmetric supports (N2 permeance 10-7 mol/m2.s.Pa). MCM-48 membranes and powders were successfully modified with a polymer, polyethyleneimine (PEI) containing amino groups. The PEI-modified MCM-48 membranes were highly selective to N 2 permeation from N2/CO2 mixtures at room temperature in the presence of water vapor. The N2/CO2 selectivities greater than 80 were observed with increasing CO2 feed concentration. The observed N2/CO2 selectivity for PEI/MCM-48 membranes is explained by enhanced affinity of PEI towards CO2 in the presence of water vapor hindering CO2 diffusion. These findings suggested that the PEI/MCM-48 membranes and adsorbents are highly promising for the separation of CO2 and other acidic gases from dilute streams, such as flue gas. These studies of inclusion chemistry in periodic mesoporous hosts provided new fundamental insights critical for the rational design of nanomaterials exhibiting quantum-size effect and gas separation membranes.