Elucidating Keggin site-specific activity of NH4PM catalysts associated with dolomite-added H-MMT in gas-phase glycerol valorisation reactions

Transforming glycerol into value-added chemicals like acrylic acid and allyl alcohol enhances biorefinery sustainability. We report a one-pot gas-phase process using a dual-bed catalytic system: acidified montmorillonite (H-MMT, upper bed) and vanadium-doped ammonium phosphomolybdate (V-doped NH₄PM, lower bed). The combination of H-MMT and primary V-doped NH₄PM (NH₄PMV) catalyst favoured the oxydehydration reaction and was susceptible to the selective conversion of glycerol to acrylic acid (Sel.- 63 %). On the other hand, H-MMT and secondary VO_x-doped NH₄PM ((VO)_x-NH₄PM) catalysts preferred the selective production of allyl alcohol via deoxydehydration reaction. Maximum allyl alcohol selectivity of 61 % was achieved by 1:2 wt. ratio of HMMT and (VO)_0.2-NH₄PM dual bed catalytic system. O₂-TPD and XPS analyses revealed that V doping at the primary Keggin site of NH₄PM increased surface oxygen defects and enhanced Mo^6 + /Mo⁵⁺ charge transfer, promoting acrolein oxidation to acrylic acid. In contrast, secondary V-doped catalysts had fewer oxygen vacancies and were less effective for oxidation but showed high efficiency in hydrogen transfer from formic acid, methanol, or water, favoring allyl alcohol formation. Additionally, the following thermo-kinetic factors were evaluated for the best-performing catalysts, providing insights into the factors contributing to their high catalytic activity. Furthermore, incorporating 15 wt% dolomite into H-MMT reduced coke formation from 34.1 % to 2.54 % post-regeneration and extended catalyst life to 8 h of TOS while maintaining > 40 % product selectivity, offering a practical strategy for coking resistance.