(721d) Bimetallic Ni/Ga Modified HZSM-5 Catalyst for Ethane Aromatization

Increased production of shale gas has prompted great interest in the field of catalysis toward conversion of ethane into aromatics (benzene, toluene and xylene (BTX)). Although, aromatization of C₃₊ alkanes has been fully realized in industry, ethane aromatization is still undergoing fundamental studies. Ga, Zn or Pt modified aluminosilicate zeolites have recently received attention for their unique catalytic properties to convert ethane to aromatics. However, the activity, selectivity and thermal stability of the aforementioned catalysts require significant improvement. Here, we report that the bimetallic Ni -Ga modified HZSM-5 to be a promising catalyst for ethane aromatization. Such bimetallic non-noble metal modified HZSM-5 is either more active or more selective than the mono-metallic Ga and Ni modified HZSM-5 catalysts. Additionally, the present Ni/Ga/HZSM-5 catalyst also has demonstrated higher stability than the traditional Pt supported and zinc exchanged HZSM-5 catalysts.

We prepared a series of bimetallic catalysts consisting of Ni/Ga alloy supported on ZSM-5 zeolite (Si/Al: 30) with different Ni to Ga molar ratio through incipient wetness impregnation. The performance of each catalyst was examined for ethane aromatization at atmospheric pressure, constant reaction temperature (823 K), and constant gas hourly space velocity (2000 hr^-1). For the optimal Ni/Ga molar ratio of 1/1 with Ni loading of 2 wt%, BTX selectivity up to ∼75% was obtained at low methane selectivity (∼7%). The space time yield of BTX at steady state is 2.4 mmol/(g.hr). However, the samples with Ni to Ga ratio greater than 1 resulted in higher rate of methane formation and lower BTX selectivity. Decreasing Ni to Ga ratio to less than 1 resulted in higher BTX selectivity and lower activity. For all of the catalysts, the selectivities of benzene and toluene are 30-40 vol% and 25-35 vol%, respectively, while the selectivity towards xylene is only 1.5-3%.

The early-stage catalytic performance of the present catalytic system is also intriguing. It has been found that monometallic Ni/HZSM-5 catalyst exhibits an induction period of 10 min, during which 100% of ethane is converted to methane and coke through hydrogenolysis and cracking. However, the induction period for the bimetallic Ni/Ga catalysts was shortened significantly, and for the catalyst consisting of Ga to Ni ratio greater than 3/5, the induction period lasts only a few seconds. The early-stage catalytic behavior has confirmed that Ni particles are more active in dehydrogenation while the presence of Ga increases the BTX selectivity and decreases the rate of ethane hydrogenolysis. Finally, the basic and acid sites were characterized via NH₃-TPD, Propylamine-TPDec., and Pyridine-IR. TEM and XRD were performed to determine segregation and/or agglomeration of catalysts and to quantitate the crystalline phases, respectively. Most plausible relationships between the catalytic performance and the surface/bulk properties/structure will also be discussed.