SUSTAINABLE SELECTIVE PHOTOCATALYTIC EPOXIDATION OF PROPYLENE IN AN ENERGY-EFFICIENT PHOTOREACTOR SYSTEM

PROPYLENE OXIDE (PO) IS A KEY GASEOUS INTERMEDIATE FOR PRODUCING FINE CHEMICALS AND POLYMERS; HOWEVER, ITS INDUSTRIAL SYNTHESIS VIA CHLOROHYDRIN AND HYDROPEROXIDE ROUTES REMAINS ENVIRONMENTALLY AND ECONOMICALLY UNSUSTAINABLE DUE TO HIGH ENERGY DEMAND AND SUBSTANTIAL WASTE GENERATION. THIS PH.D. RESEARCH AIMS TO DEVELOP A SUSTAINABLE PHOTOCATALYTIC PROCESS FOR THE SELECTIVE EPOXIDATION OF PROPYLENE USING MOLECULAR OXYGEN AS THE OXIDANT AND UV-A IRRADIATION AS THE ACTIVATION SOURCE. THE STUDY INTEGRATES MECHANISTIC INVESTIGATIONS, PHOTOCATALYST DESIGN, AND REACTOR ENGINEERING TO OPTIMIZE SELECTIVITY AND ENERGY EFFICIENCY. A CONTINUOUS-FLOW FLUIDIZED-BED PHOTOREACTOR, PREVIOUSLY CONSTRUCTED AT LAB SCALE, WAS UPGRADED WITH AN INNOVATIVE IRRADIATION SYSTEM EMPLOYING UV-A LED MATRICES FEATURING MICROSECOND-SCALE DIMMING CONTROL. THIS ADVANCED SETUP MINIMIZES POWER CONSUMPTION TYPICAL OF CONVENTIONAL PHOTOCATALYTIC REACTORS, ENSURING UNIFORM IRRADIATION, EFFICIENT GAS–SOLID CONTACT, AND ENHANCED HEAT AND MASS TRANSFER, THEREBY IMPROVING BOTH QUANTUM AND ENERGETIC EFFICIENCIES. THE CORE OBJECTIVE OF THE PH.D. WORK WAS TO DEVELOP AN ADVANCED PHOTOCATALYST ENABLING A GREEN AND SUSTAINABLE ROUTE FOR PO PRODUCTION. BOTH COMMERCIAL AND SYNTHESIZED MATERIALS WERE EXPLORED THROUGH FACILE PREPARATION METHODS. INITIALLY, ZNO AND TIO₂ (PC105) WERE TESTED AS REFERENCE PHOTOCATALYSTS. THEN, NOBLE-METAL-MODIFIED TIO₂ CATALYSTS (AG, AU, PT, PD) WERE PREPARED VIA PHOTODEPOSITION TO ACHIEVE UNIFORM NANOSCALE METAL DISPERSION. AMONG THEM, AG/TIO₂ EXHIBITED SUPERIOR PERFORMANCE, ATTAINING A PO YIELD OF 6.4% UNDER UV-A LIGHT. TO FURTHER ENHANCE SUSTAINABILITY AND REDUCE COST, ATTENTION SHIFTED TO NON-NOBLE-METAL-BASED SYSTEMS SUCH AS CUO/TIO₂ AND CUO/NIO/TIO₂ HETEROJUNCTIONS, OFFERING IMPROVED CHARGE SEPARATION AND SURFACE OXYGEN ACTIVATION. THE CUO(1.1%)/NIO(1.1%)/TIO₂ CATALYST DELIVERED OUTSTANDING RESULTS, ACHIEVING 63% PROPYLENE CONVERSION AND 95% SELECTIVITY TO PO, EVIDENCING THE SYNERGISTIC EFFECT OF CU AND NI IN PROMOTING SELECTIVE EPOXIDATION. BASED ON THESE RESULTS, OPERATING CONDITIONS WERE OPTIMIZED FOR THE CUO–NIO–TIO₂ SYSTEM. SYSTEMATIC TESTS ON THE FLUIDIZATION REGIME REVEALED THAT BUBBLING FLUIDIZATION ENHANCES LIGHT TRANSFER TO THE BED CORE. CONCERNING ENERGY CONSUMPTION, THE ELECTRIC ENERGY REQUIRED TO CONVERT ONE MOLE OF PROPYLENE (Eₘ), CALCULATED VIA THE BOLTON ET AL. MODEL, WAS 0.019 KWH MOL⁻¹, ONE OF THE LOWEST REPORTED FOR PHOTOCATALYTIC EPOXIDATION. THIS EFFICIENCY, ATTRIBUTED TO LED DIMMING AND FLUIDIZED-BED CONFIGURATION, CONFIRMS A SUBSTANTIAL ENERGY REDUCTION COMPARED TO CONVENTIONAL FIXED-BED REACTORS. THERMODYNAMIC EQUILIBRIUM ANALYSES DEMONSTRATED THAT PARTIAL OXIDATION IS FEASIBLE ONLY UNDER LIGHT-ASSISTED, NON-THERMAL CONDITIONS. A LANGMUIR–HINSHELWOOD KINETIC MODEL WAS DEVELOPED FOR THE BEST CATALYST AND VALIDATED AGAINST EXPERIMENTS (R² ≥ 0.96). THE DERIVED KINETIC AND ARRHENIUS PARAMETERS REVEALED ACTIVATION ENERGIES OF 36.9 KJ MOL⁻¹ (EPOXIDATION), 47.8 KJ MOL⁻¹ (CO FORMATION), AND 41.8 KJ MOL⁻¹ (TOTAL OXIDATION), INDICATING THAT EPOXIDATION IS BOTH THERMALLY AND PHOTOCHEMICALLY ACCESSIBLE UNDER MILD CONDITIONS, WHILE TOTAL OXIDATION DOMINATES ONLY AT HIGHER TEMPERATURES. THE INTEGRATED EXPERIMENTAL AND MODELING FINDINGS PROVIDE QUANTITATIVE INSIGHT INTO PHOTON–TEMPERATURE–SURFACE CHEMISTRY INTERPLAY, ESTABLISHING A FRAMEWORK FOR SCALING UP ENERGY-EFFICIENT, ENVIRONMENTALLY BENIGN PHOTOCATALYTIC PROCESSES FOR SELECTIVE PROPYLENE OXIDATION. FUTURE EFFORTS WILL ADDRESS CATALYST STABILITY, HIGHER REACTANT PRESSURES, AND VISIBLE-LIGHT-RESPONSIVE PHOTOCATALYSTS TO FURTHER ENHANCE SUSTAINABILITY AND INDUSTRIAL FEASIBILITY.