Reactivity and Stability of Natural Clay Minerals with Various Phyllosilicate Structures as Catalysts for Hydrothermal Liquefaction of Wet Biomass Waste

We evaluated natural clay minerals representing all classes of phyllosilicates as in situ catalysts for hydrothermal liquefaction (HTL) of anaerobically digested cattle manure at 350 °C for 1 h, i.e., kaolinite, montmorillonite, talc, vermiculite, phlogopite, meixnerite, attapulgite, and alumina. The relative compositions of strong Bronsted (SBrA), strong Lewis (SLA), and weak Lewis acidic (WLA) sites and the strong (SBS) and weak (WBS) basic sites of clay minerals significantly affected the formation of HTL products (i.e., biocrude oil, hydrochar, and aqueous- and gas-phase coproducts) and the distribution and speciation of elements. The general mechanistic roles of these active sites are as follows: (1) SBrA catalyzed the biocrude-forming reactions and inhibited the hydrochar-repolymerizing reactions; (2) SLA promoted the production of hydrochar precursors; (3) WLA enhanced the hydrodeoxygenation, hydrodenitrogenation, and hydrodesulfurization of biocrude by utilizing the hydrogen generation catalyzed by WBS; and (4) SBS increased the production of organic acids solubilizing nutrients into the aqueous-phase coproduct (HTL-AP). Montmorillonite was the most suitable for the HTL catalyst due to the optimal composition of these active sites, leading to achieving maximal biocrude energy recovery (i.e., 82%) with low heteroatoms content (i.e., 15% O, 0.24% N, and 0.08% S), minimal hydrochar yield (i.e., 10%), and maximal nutrient yield in HTL-AP, i.e., 71% P, 54% Mg, 29% NH₃-N, and 14% Ca. In addition, the crystalline structure of montmorillonite remained intact after the HTL process. This study informs comprehensive catalytic roles of different surface-active sites of clay minerals useful for future development of clay-based catalysts for more sustainable overall HTL systems.