Real-time online study of the effects of zinc acetate on the distribution of biomass pyrolysis products and the pyrolysis process using photoionisation mass spectrometry coupled with GC-MS
2026-05-11 159
Areas of application: Research into the mechanisms of catalytic pyrolysis of biomass; optimisation of biomass pyrolysis processes;
Introduction
Biomass can be converted into bio-oil containing a variety of high-value-added chemicals through rapid pyrolysis. To achieve the enrichment of high-value-added products, catalysts can be added during the biomass pyrolysis process to regulate product selectivity. Zinc acetate, a common zinc salt, can be used to directionally control biomass pyrolysis by promoting dehydration and cyclisation reactions. In this study, the catalytic effects of zinc acetate on the distribution of biomass pyrolysis products and the reaction process were systematically investigated using a pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) system and an online pyrolysis photoionisation time-of-flight mass spectrometry (Py-PI-TOFMS) system.
Experimental Materials and Methods
Material Preparation:
Rice husks were used as the raw material and subjected to acid washing to remove alkali and alkaline earth metals, thereby minimising their interference with the catalytic effect. Samples with different loading levels were prepared by impregnation pretreatment in zinc acetate solutions of 0.1, 0.3, 0.5, 0.7 and 0.9 mol/L.
Analytical equipment:
Py-GC/MS: Pyrolysis temperatures were set at 400, 450, 500, 550 and 600 °C, with a pyrolysis time of 20 s, to analyse the composition of the products and obtain semi-quantitative data.Py-PI-TOFMS: Pyrolysis temperature was set at 400 °C, with a mass spectrum acquired every 8 seconds to monitor the product formation process in real time.
Results and Conclusions
1. Effects on pyrolysis products
Zinc acetate significantly increased the relative content of acid, furan and anhydrous sugar products, whilst reducing the content of phenolic and aldehyde-ketone products. As the loading increased: the relative content of acids continued to rise (primarily due to an increase in acetic acid); the relative contents of furans, anhydrous sugars and ketones first increased and then stabilised; whilst the relative contents of aldehydes and phenols first decreased and then stabilised.
Figure 1 Product distribution of sample pyrolysis at 550 °C
The selective regulation of high-value-added products is shown in Figure 2: the yields of furfural (FF) and 1-hydroxy-3,6-dioxabicyclo[3.2.1]oct-2-one (LAC) increased significantly; the yield of FF peaks at 500 °C, whilst that of LAC reaches its maximum at 450 °C. The yield of 4-vinylphenol (4-VP) decreases significantly. The mechanism of regulation is attributed to the Lewis acidity of zinc acetate, which promotes dehydration cyclisation reactions and inhibits lignin pyrolysis.
Figure 2: Effect of zinc acetate on the yield of high-value-added chemicals. (a) AA; (b) FF; (c) LAC; (d) 4-VP
2. Effect on the pyrolysis process
Following acid washing, the TIC curve for RH-SX exhibited a “main peak + shoulder peak” pattern, indicating differences in the time at which different products reached their maximum formation rates. For RH-ZN-X (loaded with zinc acetate): as the loading increased, the shoulder peak gradually shifted earlier and eventually disappeared, demonstrating that zinc acetate altered the formation timing of certain products (primarily furans and cyclopentanones).
Biomass pyrolysis products can be broadly classified into four groups: Group A (acids + aldehydes + straight-chain ketones, derived from the ring-opening of sugar units) , Group B (furans and cyclopentanones, derived from the cyclisation of sugar units), Group C (dehydrated sugars, derived from the direct dehydration of sugar units), and Group D (phenols, derived from the cracking of lignin). As shown in Figure 3, the pyrolysis process can be divided into three stages:
① Initial stage: Predominantly dehydrated sugars, acids, aldehydes, straight-chain ketones and phenolic products;
② Intermediate stage: The formation rates of furan and cyclopentanone products reach their peak;
③ Late stage: All reactions gradually subside.
Figure 3: Effect of zinc acetate on the formation processes of various pyrolysis products. (a) RH; (b) RH-SX; (c) RH-ZN-0.1; (d) RH-ZN-0.3; (e) RH-ZN-0.5; (f) RH-ZN-0.7; (g) RH-ZN-0.9
Furfural (m/z 96, FF) and 5-hydroxymethylfurfural (m/z 126, 5-HMF) are the main characteristic ions among the Group B products. As shown in Figure 4, the peak time for FF (~100 s) occurs earlier than that for 5-HMF (~200 s), and the formation rate of FF decreases when 5-HMF peaks (Figures 4a and 4b), demonstrating that the two are in a ‘competitive relationship’ (sharing the same precursor, rather than 5-HMF cleaving to form FF).
Effect of zinc acetate loading: There was no significant effect on the FF peak time, but the 5-HMF peak time was brought forward (e.g. RH-ZN-0.9 was 25 s earlier than RH-SX); the higher the loading, the greater the advance.
Figure 4: Effect of zinc acetate on the formation processes of four pyrolysis products. (a) m/z 96; (b) m/z 126
This study utilised zinc acetate to catalyse the pyrolysis of biomass, thereby achieving the enrichment of acidic, furanic and dehydrated sugar products, providing a theoretical basis for the high-value utilisation of biomass; online mass spectrometry technology offers a new approach to elucidating the kinetic patterns of the pyrolysis process.
References
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