Abstract
This work introduces a cost and time efficient procedure to specifically increase mesopore volume and specific surface area of biogenic silica (specific surface area: 147 m2 g−1 and mesopore volume: 0.23 cm3 g−1) to make it suitable for applications in adsorption or as catalyst support. The target values were a specific surface area of ~500 m2 g−1 and a mesopore volume of ~0.40–0.50 cm3 g−1 as these values are industrially relevant and are reached by potential concurring products such as precipitated silica, silica gel, and fumed silica. The applied process of partial pseudomorphic transformation was carried out as a single reaction step in a microwave reactor instead of commonly used convective heating. In addition, the conventionally used surfactant cetyltrimethylammonium bromide (CTABr) was substituted by the low-cost surfactant (Arquad® 16-29, cetyltrimethylammonium chloride (CTACl) aqueous solution). The influence of microwave heating, type of surfactant as well as the concentration of NaOH and CTACl on the textural and structural properties of the modified biogenic silica was investigated using nitrogen adsorption as well as scanning and transmission electron microscopy. The results show that the textural parameters of the modified biogenic silica can be exactly controlled by the amount of NaOH in the reaction solution. By variation of the NaOH concentration, specific surface areas in the range of 215–1,001 m2 g−1 and mesopore volumes of 0.25–0.56 cm3 g−1 were achieved after reaction at 393 K for 10 min. The presented microwave route using the low-cost surfactant solution decreases the reaction time by 99% and as shown in an example for German prices, lowers the costs for the surfactant by 76–99%.
Highlights
As environmental awareness is moving into the focus, sustainable, regionally available, and CO2 neutral sources for the synthesis of porous materials are highly demanded
The textural parameters during partial transformation can be controlled by the NaOH concentration in the reaction solution, enabling the synthesis of tailor-made materials with specific surface areas ranging from 215 to 1,000 m2 g−1 and mesopore volumes between 0.25 and 0.56 cm3 g−1
An increase in the NaOH concentration for 0.09 M CTACl resulted in a raise of the specific surface area and the mesopore volume
Summary
As environmental awareness is moving into the focus, sustainable, regionally available, and CO2 neutral sources for the synthesis of porous materials are highly demanded. Porous biogenic silica can be obtained by a valorization process from different types of Si-containing biomass and agricultural waste products like rice, spelt and oat husk, horsetail, cereal remnant pellets, wheat, and rice straw (Alyosef et al, 2013, 2015; Schneider et al, 2018). It is an attractive material with respect to ecological, economic, and environmental reasons. The production of biogenic silica from agricultural waste products combines many benefits. The usage of agricultural wastes solves disposal problems, there are no competing interests with the food sector and the combustion step during the production of biogenic silica can be coupled with energy/steam production
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