Improving Yields and Catalyst Reuse for Palmitic Acid Aromatization in the Presence of Pressurized Water

ZSM-5 was evaluated for chemical production in a reaction mixture consisting of palmitic acid and water at conditions near the critical point of water (400 °C, 23 ± 2 MPa). Two types of ZSM-5, a microscale variety with particle diameters determined by scanning electron microscopy in the range from 1.66 to 2.56 μm (micro-ZSM-5) and a nanoscale variety with 350–730 nm diameters (nano-ZSM-5), and three water loadings (0, 15, and 65 wt %) were evaluated for their effects on conversion and product selectivity. Palmitic acid conversion and yields of one-ring aromatics, including toluene and xylenes, were greatest for the combination of nano-ZSM-5 and 15 wt % water loadings, showing that reducing particle size and optimizing water content help achieve desired reaction outcomes. Subsequently, the use of nano-ZSM-5 combined with 15 wt % water loading was studied in greater detail, finding that the catalyst could be reused up to four times at these conditions without reduction of aromatic yields and while retaining a fraction of the original acid sites. Time-resolved studies and molecular-level analysis using two-dimensional gas chromatography and isotopic resolution mass spectrometry provided information on the reaction pathway, which consists of a combination of homogeneous and heterogeneous steps. The results of this study motivate future work on water-promoted catalytic cracking of oils to produce valuable chemicals.