Acid/Base-Treated Activated Carbon Catalysts for the Low-Temperature Endothermic Cracking of N-Dodecane with Applications in Hypersonic Vehicle Heat Management Systems

Abstract

Hypersonic aircrafts suffer from heat management problems caused by the air friction produced at high speeds. The supercritical catalytic cracking of fuel is endothermic and can be exploited to remove heat from the aircraft surfaces using specially designed heat management systems. Here, we report that an acid/base-treated activated carbon (AC) catalyst shows superior performance to the conventional ZSM-5 catalyst at 4 MPa and 450 °C. Further, under these conditions, coke formation is thermodynamically avoided. Of the prepared catalysts, the AC catalyst treated with NaOH and subsequently with HNO3 (denoted AC-3Na-N) was the most active catalyst, showing the highest selectivity toward light olefins and best heat sink capacity. The acid/base-treated ACs and ZSM-5 catalysts were characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption, and Fourier-transform infrared spectroscopy measurements. Characterization reveals the importance of acid strength and density in promoting the cracking reaction pathway to light olefins observed over the acid/base-treated AC catalysts, which show comparable activity at 450 °C to that of the ZSM-5 catalyst operated above 550 °C. The low-temperature activity suppressed coke and aromatic compound (coke precursors) formation. The stability of the acid/base-treated activated carbon catalysts was confirmed over a time-on-stream of 30 min.

Steam catalytic cracking and lump kinetics of naphtha to light olefins over nanocrystalline ZSM-5 zeolite

“…Although the acidity of nano-ZSM-5 and the associated properties promoted the dodecane conversion levels both in the presence and absence of steam, the concurrent flow of steam in the reaction affected the pore shape selectivity of the catalyst and altered the mechanism more toward the β-scission reaction 59 and achieved higher selectivity towards olefins. 58 However, in the absence of steam, the carbenium ions in ZSM-5(P) promoted higher C–C cracking through the β-scission mechanism and made more dry gas and ethylene products, especially at higher temperatures. On the other hand, dodecane cracking via ZSM-5(ST) in the absence of steam achieved low olefin selectivity and an increase in LPG products (propane and n -butane, and iso-butane), indicating that the zeolite pores inclined toward the hydrogenation reaction route of olefin to LPG.…”

“…Through b-scission via the short-diffusion-length pores of nanoscale zeolite, the carbenium ions formed the essential primary cracking pathway toward promoting higher olen production. Typically, in the presence of steam, the monomolecular reaction is favoured, hindering the hydrogenation and alkylation of olens 58 and lowering the formation of LPG and C5+ byproducts due to the low activation energies. 11 In summary, steam-treated nanoscale ZSM-5 with the extraframework alumina species maintained high activity despite the decline in surface acidity.…”

LDPE cracking over mono- and divalent metal-doped beta zeolites

“…33,34 In the catalytic cracking of polymers, both Lewis and Brønsted acid sites are important because they both attribute to the generation of carbenium ions. 35 We have found that the Lewis acidity of zeolites is important for the catalytic degradation of plastics. 36,37 So far, many efforts have been made to enhance the Lewis acidity of zeolites, and it has been reported that this can be achieved by doping metals ( e.g.…”

Synthesis and characterization of acid-treated melem and graphitic carbon nitride catalysts for biodiesel production

“…For example, Hummer's method involves the use of sulfuric acid, potassium permanganate as an oxidant, water, and heating process [13]. Similarly, our previous studies have shown that large amounts of oxygen functional groups (OFGs) can be introduced onto activated charcoal via sequential NaOH and acid treatments, albeit through laborious and complex steps [14].…”