Utilization of Ornamental Rock Waste as a Catalytic Support for α-MoO₃ in Biodiesel Production

Large amounts of waste are generated during the beneficiation process of ornamental rocks (granite/marble), leading to a serious environmental issue, particularly regarding their final disposal. In this context, this study aimed to explore the application of ornamental rock waste as a catalytic support for the α-MoO₃ catalyst to develop efficient and fully sustainable heterogeneous systems for biodiesel production. The samples were characterized using physicochemical techniques such as X-ray diffraction (XRD) with phase quantification by Rietveld refinement, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy with chemical mapping (SEM-EDS), transmission electron microscopy (TEM), N₂ adsorption/desorption for surface area measurement using the BET method, particle size distribution, acidity measurements by temperature-programmed desorption of ammonia (TPD-NH₃), magnetic measurements, and gas chromatography (GC) to evaluate the products of simultaneous transesterification/esterification (TES) reactions and determine the percentage of methyl and ethyl esters achieved. The diffractograms revealed the presence of crystalline phases such as mica-biotite, ferro-tschermakite, albite, quartz, and iron-magnesium silicate, which are components of the catalytic support, as well as the characteristic crystalline phases of the orthorhombic α-MoO₃ system. XRD also confirmed the presence of these phases in the heterogeneous systems. FTIR and Raman spectroscopy analyses exhibited bands and vibrational modes characteristic of the synthesized materials. SEM images showed morphologies composed of agglomerates of irregularly shaped particles of various sizes, which TEM observed. The N₂ adsorption/desorption isotherm analyses indicated the formation of mesoporous structures with surface areas ranging from 0.615 to 3.87 m².g⁻¹. Particle size distribution analysis revealed that the samples had an average particle diameter (D₅₀) between 5.02 and 20.00 µm, with total acidity ranging from 77.0 to 245 µmol/g of NH₃. Magnetic tests revealed materials with ferrimagnetic characteristics. Catalytic tests demonstrated that ornamental rock waste exhibits remarkable efficiency as a support for the orthorhombic α-MoO₃ catalyst, as the heterogeneous systems showed auspicious performance. Specifically, the system containing 40% Mo ions (40%σ-MoO₃:Waste) exhibited the best catalytic performance, with conversions ranging from 78.6–95%. Thus, the results indicated that the studied heterogeneous systems have substantial potential to generate positive impacts both environmentally and within the technological community. The ability to transform ornamental rock waste into effective catalysts highlights the economic feasibility and environmental relevance of this approach. This study presents a promising pathway for reusing industrial waste, contributing to more sustainable and efficient practices in biodiesel production from waste frying oils.