On methanol to hydrocarbons reactions in a hierarchically structured ZSM-5 zeolite catalyst

Abstract

Two type ZSM-5 catalysts (Si/Al ratio of ∼30) with different mesoporosity were synthesized by using a structure directing agent of tetra-n-butylphosphonium hydroxide. In particular, the molar compositions of ethanol and water in the synthetic precursor were changed in order to acquire the two type ZSM-5 catalysts. The resulting ZSM-5 catalysts were formed via the interconnection of very thin pillars or lamellae; (1) ∼6nm thick with marked mesoporosity (H_30; high mesoporous ZSM-5) and (2) ∼13nm thick without any considerable mesoporosity (L_30; low mesoporous ZSM-5). The pyridine-based acid titration reveals that H_30 had internal Brønsted acid sites similar to those in the commercially available ZSM-5 with a Si/Al ratio of 75 (referred to as C_75), though H_30 contained a large amount of external Brønsted acid sites. The methanol to hydrocarbons (MTH) reaction performance of these two ZSM-5 catalysts demonstrates that H_30 preferred to produce propene over ethene compared to C_75, while L_30 showed a very poor MTH performance mainly due to the lower amount of internal Brønsted acid sites. More desirably, a very short diffusional length (∼18,600 times lower than that in C_75) in H_30 considerably disfavored the aromatic dealkylation that is known to produce ethene. With this, H_30 allowed for achieving the ratio of propene to ethene as high as ∼9.1, which is, to the best of our knowledge, a highest value among the MTH results on ZSM-5 catalysts without any co-feed. Furthermore, ceria-doped H_30 not only enhanced the stability for the MTH reaction via a passivation of the external Brønsted acid sites, but also improved a propene to ethene ratio up to ∼15.0.