Ex situ catalytic pyrolysis of biomass through a packed-bed catalysis close coupled with microwave pyrolysis was investigated to convert Douglas fir sawdust pellets to aromatic hydrocarbons by Zn/ZSM-5 catalyst. A comparison test from five different Zn loadings (0, 0.5, 1, 2, 5 wt.%) was first conducted, and it was found that the highest amount of aromatic hydrocarbons was produced from 0.5% Zn loaded on ZSM-5. Then a central composite experimental design (CCD) was used to optimize the upgraded bio-oil and syngas yields with 0.5% Zn loaded in ZSM-5. In comparison to the non-catalytic experiment, all the catalysts decreased the bio-oil yield and increased the syngas production. The product yields from Zn/ZSM-5 were sensitive with reaction conditions as the bio-oil yields varied between 22.3% and 44.8% compared with 32.2% and 37.8% over ZSM-5 catalyst, and syngas yields from 33.3% to 55.5% vs. 38.8% to 43.7% on ZSM-5 catalyst. GC/MS analysis showed that aromatic hydrocarbons become the most abundant compounds in the bio-oil. The high amount of aromatic hydrocarbons in the upgraded bio-oils from GC/MS analysis was confirmed by the FTIR analysis. The aromatic hydrocarbon was increased when the packed-bed temperature and inverse weight hourly space velocity (WHSV) (1) were increased. The comparison of coking on ZSM-5 and Zn/ZSM-5 catalysts at different reaction conditions showed that the coking increased with increasing (WHSV) (1) and decreasing packed-bed temperatures. Zn/ZSM-5 had lower coking than ZSM-5 on all the reaction conditions except packed-bed temperature at 269 degrees C and (WHSV) (1) at 0.048. (C) 2014 Elsevier Ltd. All rights reserved.