Date Published: April 26, 2019
Publisher: Public Library of Science
Author(s): Shangxing Chen, Peng Wang, Guorong Fan, Shengliang Liao, Hongyan Si, Zongde Wang, Richard G. Haverkamp.
Using a mixture of neutral primary amine dehydroabietylamine (DHAA) and long-chain cetyltrimethyl ammonium bromide (CTAB) as the template, ordered lamellar supermicroporous silicas were synthesized with NaOH as the base source and tetraethylorthosilicate (TEOS) as the silica source. The concentrations of DHAA, CTAB, and NaOH in the synthesis system had great effects on the structural properties of the samples. When the molar ratio of components was nTEOS:nCTAB:nDHAA:nNaOH:nH2O = 1:0.114:0.00457:0.5:60, the material showed a lamellar phase with the highest ordering degree. By adding only a trace amount of DHAA into the synthesis system, the structure of the samples could be transformed from cubic phase to lamellar phase, since the added DHAA solubilized in CTAB micelles to change the effective surfactant ion pair packing parameter. The dosage of CTAB should be moderate; too high or too low will decay the ordering degree of the lamellar structure.A much higher concentration of NaOH resulted in an ethanol-rich solvent in which the DHAA did not solubilize in the micelles of CTAB, but adsorbed at the hydrophilic headgroup–solvent interface. Accordingly, a structural transformation from lamellar phase to hexagonal phase occurred.
According to the International Union of Pure and Applied Chemistry (IUPAC) classification, materials with pore size less than 2 nm can be denoted as microporous materials, while materials with pore size in the range of 2–50 nm are mesoporous materials. Usually, the pore size of ordinary microporous zeolites does not break the boundary of 1.2 nm. Meanwhile, it is also very hard to fabricate well-ordered mesoporous materials with pore size less than 2.0 nm through the ordinary templating route. Actually, materials with pore size of 1.2~2.0 nm, which can be denoted as supermicroporous materials, are important due to their application potential in the field of shape-selective catalysis as well as adsorption and separation of compounds with specific molecular sizes[3, 4]. Therefore, there is an urgent demand for the preparation of ordered supermicroporous materials to fill the gap between microporous zeolites and mesoporous molecular sieves. However, because of the lack of suitable templating agents, synthesizingordered supermicroporous materials is still a scientific challenge.
Ordered supermicroporous silicas were synthesized by using a mixture of neutral primary amine dehydroabietylamine and long-chain cetyltrimethylammonium bromide as the templating agents. The dosages of DHAA, CTAB, and NaOH were the key factors for the formation of the lamellar supermicroporous structure. Even if only a small amount of DHAA is added to the sol-gel system of CTAB, the structure of the samples can be changed from cubic phase to lamellar phase. This phenomenon may have been caused by the solubilization of DHAA in CTAB micelles, which change the effective surfactant ion pair packing parameter. The dosage of CTAB should be moderate; too high or too low will decay the ordering degree of the lamellar structure. Only a suitable concentration of NaOH is beneficial to the formation of lamellar phase. A further increase inNaOH concentration will result in an ethanol-rich solvent, in which the DHAA will not solubilize in CTAB micelles, but adsorb at the hydrophilic headgroup–solvent interface. Accordingly, a transformation from lamellar phase to hexagonal phase occurred. When the molar ratio of components was nTEOS:nCTAB:nDHAA:nNaOH:nH2O = 1:0.114:0.00457:0.5:60, the lamellar sample possessed the highest ordering degree and the pore size was about 2 nm.