Synthesis and characterization of hierarchical titanium-containing silicas using different size templates

Abstract

The development of hierarchical catalytic heterogeneous catalytic systems with a controlled micro/mesopore ratio is an important issue in catalysis. The main purpose of the paper is to the establish influence of the size of the template molecule on the main characteristics of hierarchical titanium-containing silicas (H-TS). The H-TS were successfully synthesized by a bi-template method using tetrapropylammonium hydroxide (TPAG) as permanent template of MFI topology and tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrabutylammonium hydroxide (TBAG), dodecyltrimethylammonium bromide (DTMAB), cetyltrimethylammonium bromide (CTAB), didecyldimethylammonium chloride (DDAC), tetradecylbenzyldimethylammonium bromide (TBDAB) as variable structure-directing agent (SDA) of a different size. The structure and morphology were characterized by X-ray diffraction, a Fourier transform infrared spectroscopy, a Raman spectroscopy, a scanning electron microscopy and low-temperature nitrogen adsorption-desorption. The catalytic properties of the prepared titanium-containing micro-mesoporous silicas for hydroxylation of phenol with $$\hbox {H}_{2}\hbox {O}_{2}$$ have been evaluated, and their activities have been compared with those of TS-1 with only micropores. The findings of the research illustrate that with an increase in the molecule volume of the structure-directing agent, there is a decrease in the proportion of the crystalline phase and in the degree of the inclusion of $$\hbox {Ti}^{4+}$$ atoms in the structure of the zeolite material. Increase in the volume of the structure-directing agent leads to the decrease of the crystallinity of the samples and decrease in the specific surface area of the material and in the specific volume of micropores. The result of this research will be used to create a theoretical model of controlled micro-/mesoporous ratio in silicas and it may open new perspectives for their potential application in selective oxidation reactions involving large molecules. SYNOPSIS Synthesis and characterization of hierarchical titanium-containing silicas produced by a bi-template method are reported. The findings of the research illustrate that the size of the template influences the main textural properties of titanium-containing silicas.