# Contrasting Catalytic Pathways in Lignin Pyrolysis: Deoxygenative Cracking over HZSM-5 Versus Repolymerization–Coking over Activated Carbon

## TL;DR

This paper compares how two catalysts, HZSM-5 and activated carbon, affect lignin pyrolysis, showing different pathways for deoxygenation and coke formation.

## Contribution

The study reveals contrasting catalytic pathways of HZSM-5 and activated carbon in lignin pyrolysis, offering insights for catalyst design.

## Key findings

- HZSM-5 reduces activation energy by 83 kJ/mol and suppresses coke formation during lignin pyrolysis.
- Activated carbon increases activation energy and promotes methoxy group conversion rather than removal.
- AC leads to a 2.5% increase in coke yield compared to HZSM-5.

## Abstract

Catalytic pyrolysis is a crucial technology for lignin valorization, where the catalyst support itself can play a pivotal role in influencing the catalytic process. This study systematically investigates and compares the distinct catalytic effects of two commonly used catalyst supports, HZSM-5 zeolite and activated carbon (AC), during lignin pyrolysis. Macrokinetic analysis was conducted using TGA coupled with the Friedman kinetic model to determine the apparent activation energies (Ea) and coke yields. The evolution of functional groups was analyzed using Py-GC/MS coupled with quantitative functional group indexing. Additionally, the evolution of small-molecule gases during catalytic pyrolysis was monitored using TGA-FTIR. The results demonstrate differences in the catalytic pathways promoted by HZSM-5 and AC. HZSM-5 effectively deoxygenated lignin by removing methoxy and hydroxyl groups, resulting in a reduction in Ea by 83 kJ/mol at 80% conversion and suppression of coke formation. In contrast, AC, exploiting its large specific surface area as a reaction platform, promoted the conversion of methoxy groups into methyl and hydroxyl functional groups, rather than directly removing them. Moreover, the use of AC led to a marked increase in Ea, and the coke yield increased by 2.5%. This study provides valuable insights for the rational design of efficient catalyst systems for biomass conversion.