| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 90 === In this chapter, we will report the organic and inorganic modification on the mesoporous silica. The modification, including silylation, esterification and metal oxide grafting are investigated in the thesis. All the organic and inorganic compounds are anchored on the surface via acting with silanols (Si-OH) to form the stronger chemical bonding. The reactions are represented as following:
A) Si-O-H + R1O-SiR23 --> Si-O-SiR23 + H-O-R1
B) Si-O-H + HO-R3 --> Si- OR3 + H-O-H
C) Si-O-H + (R4O)3Al --> Si- O Al (R4O)2 + H-O- R4
R1 R2 R3 R4 can be alkyl or aryl groups during modification
The modification is processed in the mild condition at refluxing for 3~24h. Silanes or metal alkoxides is mixed with the NO3-made mesoprous silica (NMMS) in the solution of EtOH or 1-PrOH. Due to the weaker hydrogen interaction of S+X-...I0 and neutral silica wall of the NMMS, the neutral organic template (S+X-) could be facilely extracted by polar solvent without a necessity of ion exchange for the S+I- ones. The strong chemical covalent-bonding is formed and substitutes the original weak hydrogen-bonding between the interfaces of the silica surface.
The larger amounts of loading are achieved in this procedure compared to the previous reports [Zhao1998a] [Kimura1999a] [Corma 2001a]. Moreover, the friendly alcohols are used as solvents, instead of toxic silanes [Jaroniec 1999a]. The extracted surfactants can be recovered and reproduced as templates of the mesoporous silica, avoiding the huge waste of quaternary ammonium surfactants. The modification results will be elucidated and discussed in the following chapter:
The silylation modification is discussed in the Chapter 3.1.1 to 3.1.4. The detail data of the silylated NMMS are analyzed and listed in the 3.1.1, while 3.1.2 will show some application of the functionalized NMMS to the absorption of organic and inorganic substrates. The silylation is taken in the liquid-solid condition, which can be regarded as the chemical absorption of organic silanes. The isotherm curve can be gotten by adjusting different concentration of silanes at refluxing condition. Hence, extended and modified Langmuir equation can be used to calculate the equilibrium constants in 3.1.3. The residue of silane solution can be further used to fabricate the organic-inorganic hybrid mesoporous silica. In 3.1.4, the co-condensation with TEOS and silanes of the acid-made mesoporous silica can be processed under the modified procedure and the mesoporous silica of the non-hexagonal mesophase is also made.
The thermo and hydrothermal stability of NMMS we used are tested and strengthened in the chapter 3.2.1. MCM-41 and MCM-48, made in alkaline condition, cannot be directly modified in this one-step process, because the bonding between the surfactants and silica are too strong to be replaced by the silanes, except in pure silane solvent [Jaroniec 1999a]. The acidification is the key process to weaken the bonding and makes the MCMs and other alkaline-made mesoporous silica materials accessible in this one-step modification. The bonding transformation and modification of silanes are studied in 3.2.2. Moreover, the extraction of templates in NMMS, MCM-41 and SBA-15 also reported in 3.2.3. The removal of templates via extraction is more economic and preserves more active silanols on the surface. Because the high temperature calcinations (560~580oC) makes the contraction of silica matrix and loss of silanols for the silanol condensation.
Chapter 3.2.4 and 3.2.5 will report some results of esterification from high carbon chain and organic functional groups of the alcohols. Esterification is very important in the application of silica materials to enhance the hydrophobic property. Notwithstanding the hydrolytic stability is always the drawback during application, some interesting compounds and metal ions can be introduced into the nano-channels of mesoporous silica in non-aqueous solution after surface modification via esterification. Furfuryl alcohol, a heterocyclic compounds, is also anchored on the silica surface during esterification in 3.2.6. In high temperature (100~200oC), the heterocyclic ring will polymerize and form the graphite-like carbon inside the nanochannels after 800oC pyrolysis of N2. The property of the silica surface can be tuned to very hydrophobic and graphite-like surface can be very interesting in fuel-cell applications and gas storage studies.
Ti, Zr and Al are three common metals of the heterogeneous catalysts. Chapter 3.3.1 and 3.3.2 will discuss the results of TiO2 and ZrO2 grated on the mesoporous silica. Their thermal and hydrothermal stabilities are strengthened which regarded as the monolayer of metal oxides coating on the silica surface. Other detail physical characterization and chemical analysis are also reported in the parts of thesis. Besides, Al2O3 is also grafted onto the mesoporous silica surface via the similar coating of the one-step process. The additional Al2O3 coating will further protect the mesophase from the thermal and hydrothermal stabilities tests in Chapter 3.3.5. The extra high loading of Al2O3 (Si/Al<4) will be very impressive and active toward the cracking and alkylation reaction.
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