Catalytic
transformation of light alkanes could have considerable
practical value, yet remains one of the most challenging areas in
catalysis research due to the inertness of the C–H bond. Here,
we proposed an efficient ammodehydrogenation (ADeH) catalytic system
for the direct C–N linkage between light alkanes and ammonia
for CH3CN and H2 (CO
x
free) production. This breakthrough is achieved over bifunctional
metal-modified HZSM-5 catalysts, through the tandem dehydrogenation–amination–dehydrogenation
mechanism. We show that ethane ADeH over the Pt/HZSM-5 catalyst can
be realized under atmospheric pressure at temperatures as low as 350
°C. The specific rate of CH3CN is ∼60 μmol/(g
min), and the selectivity is up to 99% under such mild conditions.
The yield of CH3CN increases with increasing temperature;
however, the selectivity decreases due to the formation of HCN, C2H4, and CH4. Additionally, the Pt/HZSM-5
catalyst is coke-resistant during the ADeH owing to the strong interaction
between NH3 and the acid sites of the catalyst. We anticipate
that the proposed ADeH could be extended for the transformation of
various n/iso-alkanes with tunable selectivity to
alkene and nitriles.
Catalytic conversion of ethane to aromatics (BTX) over metal/HZSM-5 catalysts involves significant catalyst deactivation due to coking. Consequently, true acidity/performance relationships are escaped if the intrinsic catalyst acidity was correlated to the "steadystate" performance. Here, the effect of acidity on the early-stage performance and time-dependent deactivation kinetics has been investigated. The early-stage ethane conversion and BTX selectivity both increased with decreasing Si/Al 2 ratios. Specifically, the space−time yields of BTX increase linearly with increasing Brønsted acidity, indicating Brønsted acids as the main active sites for BTX. Further evidence can be found from the transient experiment (C 2 H 6 /Ar ↔ C 2 H 6 /NH 3 ) and C 2 H 4 -TPSR. A promotion effect of the Zn(II) sites (mainly responsible for ethane dehydrogenation) on BTX formation was also observed. With time-on-stream, the catalytic performance attenuated due to coking, which can be modeled as "r(t) = r 0 /(1 + kt α )" kinetically, and the parameters (for aromatics) k decreased and α increased with decreasing acidity.
The conventional reforming produces H 2 with stoichiometric amounts of CO and CO 2 from hydrocarbons. Here, we show that CO x -free H 2 can be produced from ammoniaassisted reforming (ammoreforming) of natural gas liquids (C n H 2n+2 + nNH 3 = nHCN + (2n + 1) H 2 , n = 2 or 3) at the same conditions as the steam reforming. Such a process coproduces HCN, which can be easily separated from H 2 and used as value-added chemicals or for NH 3 recycling through hydrolysis. The ammoreforming of ethane and propane was realized over the Re-modified HZSM-5 zeolite rather than the traditional Pt-based catalyst for the BMA process (methane ammoreforming). The specific activity of the Re/HZSM-5 catalysts at 650 °C is up to 1 mol H 2 /g Re /min (or 180 min −1 ) during ethane ammoreforming. The catalyst is highly coke resistant and shows only slight deactivation with a time-on-stream up to 20 h. Characterization of the fresh and used catalysts by X-ray absorption and Raman spectroscopies suggested that the isolated ReO x site grafted by AlO 4− tetrahedral in the zeolite framework is responsible for the outstanding catalytic activity and coke resistibility. KEYWORDS: light alkanes, ammonia reforming, anti-coke, isolated ReO x , CO x -free H 2
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