Hydrocarbon cracking can proceed through two different modes: thermal and catalytic cracking. The thermal cracking is proceeded by a radical mechanism, while catalytic cracking is by a carbonium ion mechanism. The goal of this research is to examine the characteristics of pentane cracking over HZSM-5 zeolite. The catalytic cracking is a reaction that mainly takes place at acid sites of solid catalysts and is dependent on the characteristics of acid sites.
HZSM-5 zeolite has the pore size of 5.5 Å and is widely used as a cracking catalyst since it has the proper shape selectivity characteristics and keeps its activity over enough period of time. The synthesis characteristics of HZSM-5 zeolite is discussed in this dissertation. The characteristics of catalytic cracking are affected by the amount of acid sites of the catalysts and diffusion rate of the reactant within catalysts. Thus, the measurement of adsorption of reactants and measuring method of diffusion rate by means of TGA (Thermo-Gravimetry Analysis) are also discussed. The relationships between acidity and activity of catalytic cracking are described to explain the progress of the catalytic cracking at acid sites of catalysts. In addition, the thermal cracking of pentane and the reaction network in catalytic cracking are discussed in this paper.
The results obtained in this work can be summarized as follows:
● The most decisive factors are reaction temperature and PH of the reactant mixture. The optimal temperature is 150℃ and the optimal PH is 9~ 10.5. They give an effect on $TPA^+$ ion, that is, template.
● The physical adsorption of chemicals is related to the pore size of zeolite and the kinetic diameter of adsorbates. o-Xylene with 6.3Å kinetic diameter has lowest adsorption when the pore size of ZSM-5 zeolite is 5. 5Å, while n-pentane with 4.3Å kinetic diameter has the highest adsorption.
● The temperature dependency of diffusion rate can be obtained from the results of TGA. It is in the range of 4~6 Kcal/gmol, which are also in the range of the values obtained by the counter diffusion method. However, the temperature dependency of diffusion rate is lowered when the kinetic diameter of the adsorbate is smaller than the pore size of zeolite.
● The acid sites of ZSM-5 zeolite can be considered as the actual places where the catalytic cracking occurs. The amount of acid sites carrying out the cracking is dependent on a temperature, and the dependency would be higher if the amount of Al is increased. This fact may be explaining that the amount of acid sites would be more dependent on a temperature if the total amount of acid sites is increased.
● In thermal cracking of pentane, the activation energy of n-pentane is 34.4 Kcal/gmol and that of iso-pentane is 30.9 kcal/gmol. The iso-pentane cracking produces a lot of $C_4$ fraction owing to the stability of radicals and β scission.
● The acid sites are considered as places simply carrying out cracking and the conversion at the unit acid site can be regarded as the activity of catlysts. Then, the activity of iso-pentane appeared greater than that of n-pentane. This facts may imply that the activity is related to the kinetic diameter and the stability of produced carbonium ion.
● The uni-molecular cracking is in necessity in order to raise the yield of olefin in n-pentane cracking. But, the bi-molecuar cracking rather than the uni-molecular cracking takes place when the acidity is high and the conversion is lowered when the catalyst has low acidity. Thus, the catalyst posessing proper acidity needs to be selected for the above mentioned purpose. The activity of HZSM-5 zeolite is stable for a considerable period of time and its acidity can be controlled properly in the synthesis process. Therefore, HZSM-5 is estimated as a good catalyst in hydrocarbon cracking.
