Thermodynamics of Guest-Host Interactions in Nanoporous Silica Matrixes and Carbonhydrate Metal-Organic Frameworks

This thesis consists of two major parts detailing the thermochemical study of guest-host interactions in two groups of nanoporous materials: ordered mesoporous silicas (MCM-41 and SBA-15) and metal-organic frameworks (CD-MOF-2).;The first part includes two sub-projects: the host-guest interactions of a rigid organic molecule (TMAAI) in porous silica frameworks and small molecule - silica interactions in porous silica structures. The enthalpies of interaction between various guest molecules (liquid and solid phases) and the host silica frameworks were investigated employing hydrofluoric acid (HF) calorimetry, immersion calorimetry, and thermogravimetric analysis and differential scanning calorimetry (TGA-DSC). Solid-state nuclear magnetic resonance (NMR), powder X-ray diffraction (XRD), infrared (IR) and X-ray photoelectron spectroscopy (XPS), and chemical analysis characterized the material properties and support thermochemical measurements.;Molecular level interactions at organic - inorganic interfaces play important roles in many areas, such as industrial catalysis, biomedical engineering and geochemical processes. This project investigates organic - inorganic interactions in porous framework materials using the a modeled nanoconfinement system including a rigid organic guest TMAAI (N,N,N-trimethyl-1-adamantammonium iodide) and inorganic porous silica frameworks with various pore sizes (SSZ-24, MCM-41 and SBA-15). The enthalpies of interaction of confinement were measured by HF solution calorimetry (25 % HF aqueous solution at 50 °C), and ranged from -56 to -176 kJ per mole of TMAAI. The phase evolution was investigated as a function of pore size (0.8 nm to 20.0 nm) by XRD, IR, TGA-DSC and solid-state NMR. The results from these measurements suggest the existence of three types of inclusion depending on the framework pore dimension: single molecule confinement, multiple molecule confinement/adsorption and nanocrystal confinement. When the pore size is a tight geometric fit to a TMAAI molecule, the guest is tightly confined by the surrounding silica pore wall and single molecule confinement occurs. As the pore size of the framework increases, TMAAI molecules are confined or adsorbed as monolayers or multilayers. Once the pore size becomes large enough to host at least a unit cell, the TMAAI molecules form nanocrystals as guests in the pores. These changes in structure probably minimize the free energy of the system for each pore size, as indicated by trends in the enthalpy of interaction.;A series of porous silica frameworks with different pore sizes (SSZ-59, MCM-41 and SBA-15) were investigated by immersion calorimetry in various solutions at 25 °C. In water, ethanol, triethylamine, NaCl brine (1 M) and Na 2CO3/NaHCO3 buffer (1M), the enthalpies of immersion for the calcined porous silica samples range from -0.7 to -9.6, -3.4 to -10.5, -5.7 to -15.1, -0.5 to -8.0 and -1.5 to -11.4 kJ per mole of SiO2, respectively. For each solution, the enthalpies of immersion vary from -0.3 to -2.3 kJ per mole of H2O, -49.1 to -62.5 kJ per mole of ethanol and -65.5 to -95.6 kJ per mole of triethylamine. These thermochemical results suggest that guest-host interactions in silica based porous media are governed both by pore size and concentration of surface hydroxyl groups. The organic molecules (ethanol and triethylamine) bond more strongly than aqueous solutions to the generally hydrophobic surfaces of porous silica materials. This suggests the energetics of such functionalized organic-rich silica surfaces could play a significant role in geologic CO2 sequestration, enhanced oil recovery, and geochemistry in low temperature diagenetic and sedimentary environments.;In the second part of this thesis, the enthalpy of adsorption of CO 2 on an environment friendly metal-organic framework, CD-MOF-2, has been determined for the first time by direct CO2 adsorption calorimetry at 25 °C. The differential enthalpy of CO2 adsorption at near zero coverage is -113.5 kJ/mol CO2. This strongly exothermic adsorption is assigned to the most reactive primary hydroxyl groups. The calorimetric data show two plateaus of differential enthalpy versus coverage; one near -65.4 kJ/mol CO2 is assigned to adsorption on less reactive hydroxyl groups and one near -40.1 kJ/mol CO2 is related to physisorbed CO2. The adsorption appears irreversible on the minority most reactive hydroxyl groups while the adsorption on less reactive hydroxyls and physisorption are reversible at 25 °C. (Abstract shortened by UMI.).