Clinoptilolite, a natural zeolite, is mined in Wolsungkun, Kyungsang-bukdo. Because of high $SiO_2/Al_2O_3$ ratio, clinoptilolite zeolite shows great stability to heat-and acid-treatments. The application of natural zeolite as adsorbents and catalysts, however, is limited by its small pore size. By treating with acids, clinoptilolite can be modified to adsorb considerable amount of large molecules such as benzene.
In this study clinoptilolite samples were treated with hydrochloric acid, sulfuric acid and phosphoric acid of different strength. The adsorption characteristics and crystal structure of the original and acid-treated elinoptilolite have been studied. Also their catalytic activities in cumene cracking and toluene disproportionation reactions have been investigated.
The adsorbed amount of nitrogen, methanol and benzene on acid-treated clinoptilolites were measured in volumetric adsorption unit. By treating with hydrochloric acid, the adsorbed amount increased to 5-fold for nitrogen, to 3-fold for benzene, but for methanol no significant change was observed. As acid strength increased further, there were declines both in adsorption capacity and crystallinity. The results showed that the increase of adsorbed amount was caused by the rearrangement of the pore entrance and cation exchange.
A method for determination of clinoptilolite content in natural mineral based on benzene adsorption on acid-treated sample could be proposed. By this method, the original sample used in this study was found to contain approximately 45% of clinoptilolite.
The acidities of hydrochloric acid-treated clinoptilolites were determined by Benesi method. The samples treated with 0.25N, 0.5N and 1N acids where the crystal structure were maintained, showed strong acid sites, those were absent from the original sample and that treated with high concentration acids.
Using pulse technique in micro-reactor system, the catalytic activities of hydrochloric acid-treated clinoptilolites in cumene cracking and toluene disproportionation reactions were measured. For cumene cracking reaction, the maximum conversion was observed for the 0.5N hydrochloric acid-treated sample. The maximum benzene adsorption also was observed for same sample. Adsorbed amount of benzene for sample treated with acid concentrations higher than 3N were about the same as original sample, but the conversion rate of the former was higher than that of the latter. These facts suggested that the conversion rate was determined mainly by diffusion of cumene and the product through the pore structure.
In the toluene disproportionation reaction, the same trend was observed. But the rate of deactivation was severe for sample with strong acid sites. Since catalyst with strong activity was deactivated easily in the condition without supply of hydrogen, the maximum conversion was shifted to the sample treated with higher concentration of acid, 1N.
