Synthesis of silica-based porous material through coal combustion

Coal cinder is an abundant byproduct of the extensive consumption of coal in industrial production and daily life. Making full use of the cinder is conducive to a low-carbon economy. In this study, inspired by the burning of coal, a new method for constructing a silica-based composite porous material (SiO₂-CPM) was developed by combusting a siloxane-modified anthracite coal gel (CSiO₂ gel). During this process, the combustion product was directly converted into a porous material, and the calorific value of the coal remained nearly unchanged (~98% of the original calorific value was retained), demonstrating the viability of this method for energy-efficient applications. The SiO₂-CPM exhibited an ultra-low thermal conductivity (0.036 W/(m·K) at room temperature), outperforming conventional insulation materials (e.g., cotton ~0.05 W/(m·K)). Additionally, it showed enhanced mechanical strength (fracture stress of 41.8 kPa) compared to the powder state of the coal cinder. Experimental results indicate that the amount of siloxane, structure-directing agent, and an acidic environment were critical for mechanical enhancement. The SiO₂-CPM showed good dimensional stability against thermal expansion and exhibited excellent thermal insulation and fire resistance even at 900°C. Meanwhile, the SiO₂-CPM with complex geometry could be easily fabricated using this method owing to the excellent shaping ability of the CSiO₂ gel. Compared to conventional methods such as sol–gel synthesis or freeze-drying, this approach for fabricating SiO₂-CPM is simpler and cost-effective and allows the direct utilization of coal cinder post-combustion.