UVM Theses and Dissertations
Format:
Print
Author:
Nassivera, Terry W.
Dept./Program:
Chemistry
Year:
2006
Degree:
PhD
Abstract:
Synthetic nanoporous silicas and silicates have become increasingly important in commercial applications over the last 60 years. These materials are typically synthesized using sacrificial organic templates, around which an inorganic alkoxide species is polymerized via an acid- or base-catalyzed reaction. The resulting materials are classified by the International Union of Pure and Applied Chemistry (IUPAC) into three main subdivisions: microporous, mesoporous, and macroporous, based on their pore diameters. Of these groups, mesoporous materials, with pore diameters ranging from 2 to 50 nm, have recently been the subject of intense research. In particular, ordered mesoporous materials with pore diameters less than 10 nm and internal surface areas over 1000 m²/g, have shown great promise for use as chromatographic and catalytic supports. This dissertation presents research involving the synthesis and characterization of nanoporous silicas and silicates, along with the application of mesoporous materials to problems in liquid chromatography. After an introductory chapter that reviews background on porous materials and the techniques used to characterize them, the second chapter focuses on the development of novel macroporous and mixed-phase mesoporous/macroporous materials. These materials were synthesized using pseudo spherical angiosperm pollen grains as templating agents, and as a result the pore surfaces of these materials were found to possess unique textural features. The addition of a secondary organic template, one used to create mesopores, not only formed a mesoporous/macroporous material, but also caused silicification of the pollen grains into solid silica replicas of the grains.
The next chapter describes the synthesis of a new group of mesoporous silicas, APMS-2, or second-generation acid-prepared mesoporous spheres. Much like its predecessor, APMS-I, this silica possessed an amorphous pore structures with a large surface area and a spherical particle morphology. However, it also had a much larger pore volume, particle uniformity, and pore connectivity, which are important features to consider when evaluating its applicability as a catalytic or chromatograplic support. These improved characteristics stem from the addition of ethanol and sodium fluoride to the synthesis mixture. Although these two components have been used individually for years to aid in the synthesis of spherical silicas, the combination of the two allowed for precise control of the polymerization rate and physical properties. This allowed for the effective tuning of particle size and porosity by simply adjusting the reagent concentrations and reaction conditions. The ability to optimize a single reaction mixture for several applications provides important flexibility in designing physical properties for a specific application. The later chapters discuss the use of APMS-1 and APMS-2 in a variety of high performance liquid chromatographic (HPLC) methods. The advantages and disadvantages of these mesoporous materials in these applications were examined and compared to two leading commercial silicas. Chapter 4 focused on the use of these silicas in size-exclusion chromatography applications, where their smaller pore diameters allowed for analysis down to the monomer level. Chapter 5 looked at their performance in both normal-phase (adsorption) and reverse-phase (partition) cliromatography applications. Due to their large surface areas relative to the commercial materials, both versions of APMS were found to perform well as a stationary phase. However, APMS-2 was found to be better suited for these three applications due to its superior monodispersity, particle size, and pore volume.
The next chapter describes the synthesis of a new group of mesoporous silicas, APMS-2, or second-generation acid-prepared mesoporous spheres. Much like its predecessor, APMS-I, this silica possessed an amorphous pore structures with a large surface area and a spherical particle morphology. However, it also had a much larger pore volume, particle uniformity, and pore connectivity, which are important features to consider when evaluating its applicability as a catalytic or chromatograplic support. These improved characteristics stem from the addition of ethanol and sodium fluoride to the synthesis mixture. Although these two components have been used individually for years to aid in the synthesis of spherical silicas, the combination of the two allowed for precise control of the polymerization rate and physical properties. This allowed for the effective tuning of particle size and porosity by simply adjusting the reagent concentrations and reaction conditions. The ability to optimize a single reaction mixture for several applications provides important flexibility in designing physical properties for a specific application. The later chapters discuss the use of APMS-1 and APMS-2 in a variety of high performance liquid chromatographic (HPLC) methods. The advantages and disadvantages of these mesoporous materials in these applications were examined and compared to two leading commercial silicas. Chapter 4 focused on the use of these silicas in size-exclusion chromatography applications, where their smaller pore diameters allowed for analysis down to the monomer level. Chapter 5 looked at their performance in both normal-phase (adsorption) and reverse-phase (partition) cliromatography applications. Due to their large surface areas relative to the commercial materials, both versions of APMS were found to perform well as a stationary phase. However, APMS-2 was found to be better suited for these three applications due to its superior monodispersity, particle size, and pore volume.