Supercritical catalytic cracking of n-dodecane over air-oxidized activated charcoal

Highlights

• Activated charcoal was compared with the counterpart ZSM-zeolite as a catalyst for n-dodecane catalytic cracking.

• Activated charcoal catalysts oxidized at 673 K have been found to be most suitable for n-dodecane cracking.

• The acidity of the activated charcoal was specially controlled by the oxidation treatment temperature conditions.

• The correlation between the structure, acid properties and activity of the activated charcoal catalyst was studied.

• The relationship between the acidity of the catalyst and the reaction mechanism has been proposed.

Abstract

The direct conversion of n-dodecane to light alkenes with an efficient catalyst can enhance the combustion efficiency and cooling performance of endothermic heat sinks for the development of propellants suitable for supersonic vehicles. In this work, a pretreated activated charcoal showed excellent performance as a novel cracking catalyst superior to the conventional ZSM-5 zeolite catalyst. Activated charcoal was oxidized at 298, 473, and 673 K, leading to surface modification, and n-dodecane cracking experiments were carried out at 4 MPa and 723 K. The activated charcoal exhibited a higher light alkene selectivity and heat sink capacity compared with the conventional ZSM-5. The charcoal oxidized at 673 K showed the highest light alkene selectivity of 28% among the tested catalysts, exceeding that of the reference ZSM-5 by 18%. The oxidizing pretreatment of the charcoal at high temperatures was found to generate carboxylic functional groups acting as Brӧnsted acid sites based on characterization by X-ray photoelectron spectroscopy, FT-IR, and temperature-programmed desorption with NH₃. The activated charcoal oxidized at 673 K had the largest amount of strong acid sites and Brӧnsted acid sites, which led to the highest conversion of n-dodecane and the selectivity of light alkene.

Introduction

As the speed of propulsion vehicles increases with the development of aircraft propulsion technology, the corresponding engine heat capacity requirements continue to increase in tandem because of the stagnation temperature effect. Propulsion vehicles that are constantly exposed to the atmosphere during flight use fuel as their primary coolant because they require large heat sinks [1], [2], [3]. Light hydrocarbon fuels, such as methane and hydrogen, can only act as heat sinks with respect to sensible and latent heat, whereas high-density hydrocarbon fuels can supply additional cooling potential through endothermic cracking reactions. In addition, the light hydrocarbons produced by the cracking reactions will increase the rate of combustion by reducing the ignition time [4], [5], [6].

Up to now, catalytic cracking based on heterogeneous acid catalysts has been developed to significantly improve the heat sink capacity compared with the simple thermal cracking process [7], [8], [9]. For hydrocarbon fuel catalytic cracking, zeolites and complex oxides have been widely used as catalysts [9], [10], [11], [12], [13], [14]. Zeolite catalysts have been used primarily for hydrocarbon catalytic cracking due to various advantages such as their bare acidity and abundant micropores [15], [16]. Representatively, ZSM-5 zeolites exhibit catalytic activities at lower temperatures (673–873 K) than complex oxide catalysts for hydrocarbon cracking and are easily modified to control the product selectivity by changing their Si/Al molar ratios [17]. However, conventional ZSM-5 zeolites containing strong acid sites and simple microporous structures with long diffusion pathways are easily deactivated by carbon deposition blocking the pores, which occurs often under supercritical reaction conditions [18]. On the other hand, complex oxide catalysts with their excellent thermal stability and carbon deposition resistance have been mostly used at temperatures above 873 K [19]. However, their activities are poorer than that of ZSM-5 zeolites, which critically decreases their heat sink capacities.

The mechanism of hydrocarbon cracking over heterogeneous solid acid catalysts has been reported to proceed through a carbenium ion chain mechanism. According to this mechanism, the acid sites on the catalyst surface directly affect the catalyst performance [20]. Sommer and co-workers studied the cracking of isobutane over H-ZSM5 and Y-zeolite and discovered that Brӧnsted acid sites were directly related to the activation step, during which a carbenium ion forms to yield smaller molecules [19]. Fornseca and co-workers investigated the series/parallel mechanism in n-decane catalytic cracking over H-Y-zeolite and suggested that the strong Brӧnsted acid sites can simultaneously cause the formation of carbenium ions and initiate the reaction, while the weak or Lewis acid sites can contribute to the bimolecular process [21]. Wojciechowshi [20] suggested that the acid density and strength of the solid acid catalyst affects the product selectivity and coke formation during n-paraffin cracking due to faster interionic reactions.

Activated carbon (AC) is used in a variety of industrial applications owing to its low cost, high specific surface area, and good thermal and chemical stabilities. Generally, partial oxidation during the activation process generates two forms of ACs: H-type AC, which can adsorb strong acids, and L-type AC, which can neutralize strong bases [22]. L-type AC is characterized by oxygen-containing functional groups resulting from various surface modifications depending on the oxidant and activation temperature. In particular, low-temperature oxidation generates carboxylic acids that act as strong acid groups, while high-temperature oxidation can produce phenolic hydroxyl groups that act as weak acid groups [23]. Moreover, because this activation process can lead to well-developed mesoporous carbon structures, properly controlled ACs could be particularly effective in catalytic systems controlled by pore structure sensitivity such as the n-paraffin cracking process.

The purpose of this study was to explore the catalytic properties of air-oxidized activated charcoal for n-dodecane cracking. Activated charcoal was thermally treated at 298, 473, and 673 K in an air atmosphere. The activity and heat sink capacity of n-dodecane cracking over the catalysts were investigated and compared with those of a ZSM-5 zeolite catalyst under supercritical reaction conditions. The effect of high-temperature oxidizing pretreatment of the charcoal was analyzed by X-ray photoelectron spectroscopy (XPS), FT-IR, and temperature-programmed desorption with ammonia (NH₃-TPD). The activated charcoal exhibited a higher light alkene selectivity and heat sink capacity compared with the conventional ZSM-5 zeolite catalyst.