Investigation on the Catalytic Behavior of Alkali Metals and Alkaline Earth Metals on the Biomass Pyrolysis Assisted with Real-Time Monitoring

In this study, the catalytic behavior of alkali metals and alkaline earth metals (AAEMs) on the pyrolysis of lignocellulosic biomass was investigated, assisted with the online monitoring of synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Rice husk was pretreated by acid washing and AAEMs (Na+, K+, Ca2+, Mg2+) infusion before pyrolysis. Py–GC/MS and Py–SVUV-PIMS analysis were conducted at 550 °C. The effects of AAEMs on the distribution and the time-evolution of pyrolysis products were comprehensively discussed. With the catalysis of alkali metals, the reaction process was promoted while the duration of the whole pyrolysis process was shortened. The fragmentation followed by ring-fission of sugar units was enhanced, resulting in an increase of aldehyde, ketones, and acids. The ether bond fracture in lignin and the secondary thermal cracking of big molecule phenolics were enhanced, resulting in an increase of phenolics. Phenol and m-cresol were found partly formed by the secondary decomposition of bigger phenolic molecules or oligomers in the late pyrolysis stage. Compared with alkali metals, alkali earth metals have stronger Lewis acidity, which are more conducive to the dehydration of cellulose to form anhydrosugars as well as the secondary dehydration conversion of anhydrosugars to form other dehydrated products. Under the catalysis of alkali earth metals, most of the furfural is generated by the decomposition of hemicellulose in the early pyrolysis stage and a small part came from the secondary conversion of anhydrosugars in the late pyrolysis stage. The dehydration capacity of alkali earth metals induced the repolymerization between phenolic products, resulting in a decrease of the yield of phenolics products.