Catalytic performance of modified kaolinite in pyrolysis of benzyl phenyl ether: A model compound of low rank coal

Abstract

A serials of modified kaolinites were prepared by calcination and further acid treatment and characterized by in-situ XRD, N2 adsorption, NH3-TPD, Py-IR and 27Al MAS-NMR. And their catalytic performance in pyrolysis of methanol/benzyl phenyl ether (MeOH-BPE), a model compound of low-rank coal, were investigated at 400 °C in a fixed-bed reactor to explore the correlation between the structure of modified samples and their catalytic performances. The results show that calcination temperature above 500 °C causes the collapse of kaolinite structure. Further acid leaching facilitates the formation of micropores and mesopores. The calcination of kaolinite leads to the transformation of six-coordinate Al atoms (AlVI) into four and five coordinate species (AlIV and AlV), while the subsequent acid treatment increases the contents of AlIV and AlVI and removes AlV. Total acid sites exhibit a first increase and then decrease tendency with the raising calcination temperature. In the presence of the modified kaolinites, BPE conversion significantly enhances and reaches the highest value of 91.41% over K-A-700 prepared by calcination at 700 °C of kaolinite and further acid leaching. Besides, the maximum content of phenol and toluene is also achieved due to the highest acid sites and AlIV content of K-A-700, which favors the generation of ·H, thus resulting in an obvious inhibition of bibenzyl formation but a significant increase of 2-benzylphenol. In-situ pyrolysis by time-of-flight mass spectrometry suggests that the cleavage of Cal-O bond of BPE to form phenol radicals and benzyl radicals is the primary way, while insufficient ·H results in the formation of dominant product of 2-benzylphenol.