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 classesofphyllosilicates as in situ catalysts for hydrothermal liquefaction(HTL) of anaerobically digested cattle manure at 350 & DEG;C for 1h, i.e., kaolinite, montmorillonite, talc, vermiculite, phlogopite,meixnerite, attapulgite, and alumina. The relative compositions ofstrong Bronsted (SBrA), strong Lewis (SLA), and weak Lewis acidic(WLA) sites and the strong (SBS) and weak (WBS) basic sites of clayminerals significantly affected the formation of HTL products (i.e.,biocrude oil, hydrochar, and aqueous- and gas-phase coproducts) andthe distribution and speciation of elements. The general mechanisticroles of these active sites are as follows: (1) SBrA catalyzed thebiocrude-forming reactions and inhibited the hydrochar-repolymerizingreactions; (2) SLA promoted the production of hydrochar precursors;(3) WLA enhanced the hydrodeoxygenation, hydrodenitrogenation, andhydrodesulfurization of biocrude by utilizing the hydrogen generationcatalyzed by WBS; and (4) SBS increased the production of organicacids solubilizing nutrients into the aqueous-phase coproduct (HTL-AP).Montmorillonite was the most suitable for the HTL catalyst due tothe optimal composition of these active sites, leading to achievingmaximal biocrude energy recovery (i.e., 82%) with low heteroatomscontent (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 crystallinestructure of montmorillonite remained intact after the HTL process.This study informs comprehensive catalytic roles of different surface-activesites of clay minerals useful for future development of clay-basedcatalysts for more sustainable overall HTL systems.