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In the present study, the possibility of extracting biogenic silica from various European biomass materials was investigated. High-purity biogenic silica (> 90 wt.% SiO 2 ) was obtained from energy crops (miscanthus), agro wastes (wheat straw) and other crop residues (cereal remnant pellets). Three different morphological forms of biogenic silica materials (ash) were obtained by a thermo-chemical treatment of these biomass sources. The wet biomass materials were leached using 5 M sulfuric acid for a defined period of time. After washing and drying the biomass materials, the leached samples were subjected to a heat treatment in a furnace with three sequential temperatures and time stages to determine the minimum combustion temperature of the organic compounds in the biomass materials. The final products were characterized by X-ray diffraction, X-ray fluorescence, carbon content analysis, differential thermal analysis, low temperature nitrogen adsorption, mercury intrusion porosimetry and scanning electron microscopy. The obtained silica materials had a microstructure composed of accessible, interconnected and intra-particle meso-and macropores with sizes ranging from 3 to 1500 nm.

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Pre-shaped mesoporous amorphous rice husk ash (RHA) and MCM-41 derived from RHA as a silica source were transformed into MFI-type zeolites using two different structure-directing agents. Tetrapropylammonium hydroxide (TPAOH) was utilized as an alkali source for silica dissolution and structure control during the direct transformation of RHA into zeolite. A monopropylamine (PA)-containing alkaline solution (NaOH) was used for the pseudomorphic transformation of RHA or MCM-41 into zeolite. The hydrothermal conversion of RHA or MCM-41 into MFI-type zeolites was investigated as a function of reaction time at 175 °C. With PA as template, the crystallization took place inside and on the outer surface of RHA or MCM-41 without losing the original shape of the initial silica sources, while TPAOH led to the formation of conventional MFI-type zeolite crystals due to the complete dissolution of RHA. The final products were characterized by X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and optical emission spectroscopy.