Structural analysis and characterization of synthesized ordered mesoporous silicate (mcm-41) using small angle x-rays scattering and complementary techniques

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Akinlalu, Ademola
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Nuclear engineering and science
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2. A comparison of the SAXS spectra for the different molar concentration sample reveals that the 0:5M samples shows a deteriorating structural characteristics as compared to the 0:25 and 0:75M samples respectively and a clear decrease in the (100) reflection planes. Also noticed is the slight rightward shift in the overall spectrum prole. This observation suggest that further analysis is needed so as to better understand the result.
Mesoporous silicate have widespread potential applications, such as drug delivery, supports for catalysis, selective adsorption and host to guest molecules. Most important in the area of scientific research and industrial applications is their demand due to its extremely high surface areas (> 800m2g-1) and larger pores with well-defined structures.
Mesoporous silicate (MCM-41) samples were prepared by hydrothermal method under various chemo-physical conditions and various experimental methods such as small angle X-rays scattering (SAXS), Nitrogen adsorption-desorption analysis at 77 K, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were employed to investigate the changes in the structural morphology and subtle lattice parameter changes. With regards to the subtle changes in the structural characteristics of the synthesized mesoporous silicate, we seek to understand the electron density function changes as the synthesis parameter are varied from low molar concentration of ATAB/Si to higher concentration, the system becoming more acidity due to increase in the hydrolysis time of pH regulator as a result of increased production of ethanol and acetic acid and the changes due to extended reaction time.
This Ph.D. research tries to understand the influence of various parameters like surfactant-Si molar ratio, reaction time, and the hydrolysis of the pH regulator on the orderliness/disorderliness of the lattice order, lattice spacing and electron density function. The stages during synthesis are carefully selected to better understand where the greater influence on the overall structural morphology exist so as to be able to ne tune this parameter for any desired specification and application.
The SAXS measurement were conducted on a HECUS S3-Micro X-ray system at Rensselaer Polytechnic Institute, Troy, NY. while the data evaluation and visualization were carried in 3DView 4.2 and EasySWAXS software. The electron density functions were generated with proprietary software called edens.
In this dissertation, the following observations have been revealed resulting from SAXS measurement.
1. As one increases the hydrolysis duration of ethyl acetate, a gradual collapse of the lattice spacing of the mesoporous silicate MCM-41 is observed. We found from SAXS that there is a slight right shift of the spectra toward the higher q values indicating that we are gradually losing orderliness in the lattice spacing and hexagonal structure of the mesoporous silica. Also, the intensity of the peak of second and third peaks are diminutive when compared to sample with shorter hydrolysis time.
3. We establish that during MCM-41 synthesis, longer reaction time is needed to produce quality sample with well defined structurally characteristic for its intended application because according to spectrum for the sample with a longer reaction time (aging), a shift towards the lower q-values indicates that a sample with a larger lattice parameter and wall thickness but the intensities of its peak are diminishing when compared to the other of relatively shorter reaction time.
Other complementary techniques were used to corroborate the result obtained from SAXS. Nitrogen adsorption-desorption analysis at 77K was used to generate the isotherms while B.E.T method was used in conjunction with the isotherms to obtained the very important surface area information. SEM provide a visual structural morphology of the samples and FTIR gave the fingerprint detail of the bonds and vibration types between particle present.
August 2016
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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