Checked Preparation Of Smart Mesoporous Silica
Checked preparation of smart mesoporous silica
in acid medium
G. Tel\’biz, O. Shvets, S. Boron, O. Kochnev, V. Il\’in
Institute of Physical Chemistry, Ukrainian National Academy of Sciences,
Nauky avenue 31, 03028, Kyiv, Ukraine
The past several years have seen explosive growth in research on ordered mesoporous molecular sieves. The mesoporous material may be synthesized in any geometric form, but is preferably in the form of a film, fiber, powder or combinations thereof. Guest particles embedded in molecular sieves hosts are attracting increasing attention with respect to novel applications. However, such applications are limited by the relatively small pore openings; therefore, pore enlargement is one of the main aspects in zeolite chemistry.
Here we report about peculiarities of a checked forming in field pH below 0.1 mesoporous silica, template by cylindrical surfactant micelles. It was established, that the mesoporous fibers are characterized by a high degree of hexagonal ordering, and at transferring to gyroids and spheres the degree of perfection of structure has deteriorated.
As initial substances TBOS or TEOS, CTMABr or 25 % water solution CTMACl, were used as well as the hydrochloric acid – for correction pH of the reactionary solutions.
Reactionary mix of structure xHCl: yCTMABr: zTBOS: nH2O, as a rule was prepared as follows: 0,3 g CTMABr was dissolved up to a full transparency in 65 ml a hydrochloric acid of necessary concentration, transferred a solution in a glass vessel was and 0.52 g TBOS was added, then the vessel was closed and let alone before ending of reaction. Strong acid reactionary environments at pH
High acidity favors to formation and growth in a reactionary mix of fibers whereas gyroids, toroids and other complex forms occur at progressing
decrease of acidity in a direction isoelectric points SiO2 (pH~2). The temperature of synthesis essentially influences, first of all, speed of hydrolysis OS and, accordingly, the growth rate of fibers. So, at rise in temperature in an interval 10-34oC the induction period is reduced from 12 to about 5 days, and the process in 30-10 days is finished.
At lower temperature the growth of fibers of greater diameter is observed, longer and ordered, increase of temperature has the opposite an effect. Growth rate of fibers is influenced essentially also with a ratio of the area of the interface of phases and total amount of a reactionary mix. At transition to hybrid forms and phases with gyroids prevalence the degree of perfection of structure is reduced, however on diffractogram (Figure 1) there are 4 (3) precise intensive peaks. Presence of no more than three peaks characterizes spherical particles, and in case of using TEOS as a source of silicon – is frequent 2 (1), that specifies on significant disorder. Slow calcined (10° C/mines) of all received forms, template silica allows to remove template and to precede to ICM structure without appreciable, on XRD, infringements of structure.
Figure 1. XRD pattern of various forms of mesoporous silica
Supervision by means of an optical microscope of various samples silica has allowed revealing one more important feature of their morphology. In polarized light and at use of optical filters on a anisotropy of optical properties of the fibers, caused precisely parallel by ordering cylindrical micelles on all sections and on all their length is shown: for example, fibers are painted in a range from bright – pink up to flavovirent color, on size of a corner between a plane of polarization of light and the main optical axis of a fiber. Similar it is in part typical and of hybrids – products of merging of fibers with other forms, but not for gyroids, toroids, spirals, snails, spheres etc. The logic explanation of absence of the main optical axis in not fiber forms may be that optical axes of the most part cylindrical silica-liquid crystal micelles, turn to an initial nucleus – germ, make a sharp corner with the main optical axis.
The peculiarities of a forming in field pH below 0.1 mesoporous silica, templated by cylindrical surfactant micelles are investigated. It is shown, that depending on acidity of reactionary medium the spontaneous growth of particles of silica on a phases boundary results in a formation of fibers, gyroids, spheres and other shapes, the overwhelming majority of which one is represented by axially symmetrical bodies.
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