The coupling mechanism between chemical functional groups and physical nanopore structures during biomass thermal conversion remains challenging. The synergistic evolution of functional groups and nanopores during rice straw pyrolysis (60°C–500°C) was investigated by in situ synchrotron diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and very-small-angle neutron scattering (VSANS). DRIFTS analysis revealed that intense cleavage of aliphatic functional groups occurred at 200°C–400°C, resulting in a 66% reduction in the aliphatic structural parameter, while the aromatic condensation degree concurrently rose by 61%. VSANS showed that the nanopore porosity significantly surged from 1.48% to 2.25% within the same reactive window (200°C–400°C). During the subsequent carbonization phase (400°C–500°C), the pore surface fractal dimension decreased from 2.79 to 2.40, with this stage accounting for 85% of the total roughness reduction. Pearson correlation analysis confirmed a negative correlation (-0.97) between the aliphatic parameter and nanopore porosity, whereas the aromatic condensation degree was tightly coupled with the reduction in surface roughness (-0.96). These insights demonstrate that the thermal cracking of aliphatic matrices directly drives nanopore volume expansion, while the ordered reorganization of aromatic clusters smoothens the internal pore morphology. This dual-driven coupling mechanism offers a fundamental framework for upgrading biomass into tailored, high-performance biochar.
 

Biomass pyrolysis; Aliphatic structural parameters; Porosity; Synchrotron DRIFTS; VSANS