Construction of hierarchical ZSM-5 molecular sieves by quaternary ammonium salt-assisted acid hydrolysis of a silicon source to enhance the catalytic pyrolysis of n-pentane

ZSM-5 molecular sieves are potential catalysts for the catalytic pyrolysis of light naphtha. Nevertheless, traditional ZSM-5 is constrained by its low yield of light olefins and poor stability, limiting its use as a pyrolysis catalyst. In this work, a method involving quaternary ammonium salt (QAS)-assisted acid hydrolysis of silicon sources was developed to construct hierarchical and highly crystalline ZSM-5 molecular sieves for the efficient catalytic pyrolysis of n- pentane. The mesoporous sizes in hierarchical ZSM-5 were controlled by varying the alkyl chain length of the QASs. Diffusional measurements revealed that the diffusion time constant of n- pentane increased with increasing mesoporous size in ZSM-5. The n- pentane pyrolysis tests revealed that a suitable mesoporous size (10.8 nm) facilitated the n- pentane monomolecular cracking pathway while the hydrogen transfer reactions were inhibited. Among the series of ZSM-5 molecular sieves, the D-ZH5 catalyst synthesized by dodecyl trimethyl ammonium bromide (DTAB)-assisted acid hydrolysis of ethyl orthosilicate (TEOS) achieved the highest yields of light olefins (41.69% (mass) for ethylene and propylene) and the lowest deactivation rate (0.13%·h –1 ).