Metal-Modified Hierarchical Zeolite Catalysts for Catalytic Pyrolysis of Walnut Shells to Produce Light Aromatics

A series of bifunctional hierarchical HZSM-5 catalysts modified with Zn, Ga, Ni, Cr, or Ag were synthesized via impregnation, and their performance in the catalytic fast pyrolysis of walnut shells was systematically evaluated. The influence of the metal species and concentration of NaOH used for desilication (0.20–0.40 mol·L−1) on the yield of light aromatics was assessed. Ga/HZSM-5 and Zn/HZSM-5 exhibited the most pronounced enhancement at 0.35 mol·L−1, significantly outperforming the unmodified HZSM-5. Building on this finding, Zn-Ga bimetallic hierarchical catalysts were developed, and the effect of the Zn:Ga loading ratio (1%:2%, 1.5%:1.5%, 2%:1%) was investigated. The 1%Zn/2%Ga catalyst delivered the highest performance, achieving a total aromatic yield of 3.876 × 104 a.u.·mg−1, with 82% BTX (benzene, toluene, and xylenes) selectivity. The term “a.u.” stands for “arbitrary units,” typically derived from peak area counts obtained through GC-MS analysis. These values represent the relative signal intensity detected by the instrument, rather than absolute quantities of the substance. To more accurately characterize the aromatic hydrocarbon yield, these data are normalized to the yield of aromatic hydrocarbons per unit mass. These findings demonstrate that the combination of Zn-Ga modification and tailored mesoporosity can markedly enhance the production of high-value benzene, toluene, and xylene (BTX) aromatics from lignocellulosic biomass.