Single [Ga(OH)]2+ species supported on mesoporous hollow-structured H-ZSM-5: A highly efficient light alkanes aromatization catalyst

Abstract

Aromatization of light alkanes is a value-added process in both petrochemical and coal chemical industries. Here, single [Ga(OH)]²⁺ ion-exchanged mesoporous hollow-structured ZSM-5 (Ga-MH-ZSM-5) material was prepared, and it shows unprecedented catalytic performance in light alkane aromatization, considering activity, product selectivity and catalytic stability. The average aromatics yields in ethane aromatization at 600 °C and WHSV of 0.8 h^–1 within 28 h and in propane aromatization at 580 °C and WHSV of 1.1 h^–1 within 20 h reach ~18.4% and ~70.8% with benzene, toluene and xylenes (BTX) accounting for ~96% and ~88% of aromatics, respectively. Ga-MH-ZSM-5-0.41 gave a TON for formation of aromatics (TON_aromatics) from propane as high as 57479, whereas the reported catalysts maximally show a TON_aromatics of 5514. This also holds true for ethane aromatization; the TON_aromatics obtained on Ga-MH-ZSM-5-0.41 was ≥ 3845 in contrast to ≤ 392 on reported non-noble metal catalysts. The catalytic activity of Ga-MH-ZSM-5 highly depends on Ga species structures. [Ga(OH)]²⁺ ions are predominant species at Ga loading ≤ 0.3 wt%, while more [Ga(OH)₂]⁺ and GaO_x oligomers are formed with increasing Ga content. Upon reduction with H₂, [Ga(OH)]²⁺ and [Ga(OH)₂]⁺ are transformed into [GaH]²⁺ and [GaH₂]⁺ species, which show a propane dehydrogenation rate of 300 and 15 times of that of Brønsted acid sites respectively. The light alkanes are mainly dehydrogenated into light olefins on [GaH]²⁺ species, and then, oligomerized and cyclized into (alkyl)cycloalkanes on H⁺ sites, which is followed by possible ring expansion on H⁺ and sequential dehydrogenations into aromatics primarily on [GaH]²⁺.