Study of H2/CO/CO2 adsorption on MFI zeolites for CO2 selective WGS membrane reactor concept

Licentiatavhandling, 2006

The single-component adsorption behavior of CO, CO2 and multi-component adsorption of H2/CO2, CO/CO2, and H2/CO/CO2 on silicalite-1 and cation exchanged MZSM-5 zeolites (M = H, Na, and Ba) were investigated by a step change response method. Temperature programmed desorption (TPD) and in-situ fourier transform infrared (FTIR) spectroscopy were also performed to study the type of adsorbed species and their thermal stabilities. The adsorption experiments were carried out at temperatures from 323 to 473 K and under atmospheric total pressure. The results of single gas adsorption experiments indicated that the CO2 adsorption capacity was significantly higher than that of CO on the zeolite samples studied due to the higher polarity and acidity of CO2. It was found that CO2 was adsorbed on silicalite and HZSM-5 as one type of adsorbed species, and on NaZSM-5 and BaZSM-5 zeolites as at least two types of adsorbed species over the temperature range studied. One was a weakly adsorbed species probably due to interaction with the framework of ZSM-5, and the other was a strongly adsorbed species caused by interactions with the Na and Ba cations of ZSM-5. The CO2 adsorption behavior for silicalite and HZSM-5 was adequately described by a single site Langmuir adsorption model but for NaZSM-5 and BaZSM-5 a dual site Langmuir model was required. However, the CO adsorption on the silicalite-1 and BaZSM-5 was well described by a single-site Langmuir model. FTIR-spectra at various elevated desorption temperatures after adsorption of CO2 at low temperature revealed that CO2 was adsorbed on MZSM-5 and silicalite in several ways and formed carbonate bands, which seemed to be caused by different adsorption sites. The multi-component adsorption results show that there is an adsorption competition between CO and CO2 on the zeolite surface. A slight decrease in CO2 adsorption was observed compared to that for single-component CO2 adsorption. On the other hand, a significant decrease in CO uptake could be seen with increasing CO2 uptake. This behavior is adequately predicted by an extended Langmuir model. In contrast, the extended Langmuir model failed to describe the CO2 adsorption on BaZSM-5 for a H2/CO2 gas mixture where it was found that the presence of H2 in the gas mixture enhanced the adsorption of CO2. The adsorption behavior is needed in order to evaluate whether these materials are suitable for use in a WGS reactor equipped with a CO2 selective membrane.