Theoretical and Experimental Studies on the Ability of Intracrystalline Pores of β-La2(SO4)3 To Accommodate Various Gas Species with a Special Focus on Ammonia Insertion Behaviors.

Abstract

β-La2(SO4)3 is a microporous inorganic crystal with one-dimensional perforated pores where H2O molecules can be inserted. To evaluate the nature of the pores and extend the application range, we investigate the ability to accommodate various hydrogen compound molecules XHn (CH4, NH3, HF, H2S, HCl, and HI) by insertion. The stable structures of the XHn molecules in the pores of β-La2(SO4)3 and the change in the Gibbs energy for XHn insertion ΔinsertG (T) are estimated by first-principles calculations. The guest XHn molecules are stabilized by forming H-O and X-La bonds with the β-La2(SO4)3 host structure. Based on the values of ΔinsertG (T), NH3, H2O, and HF are energetically stable in the crystal even above 0 °C. Correspondingly, thermogravimetry (TG) of β-La2(SO4)3 in NH3, CH4, and CO2 gases revealed that NH3 can be inserted into β-La2(SO4)3 below 360 °C, but CH4 and CO2 cannot. Unlike the case of H2O insertion, NH3 insertion proceeds via two steps. The first step is a single-solid-phase reaction of β-La2(SO4)3·yNH3, where NH3 molecules are inserted into the host structure with a continuously changing nonstoichiometric y value between 0 and 0.1. The second step is a two-solid-phase reaction between β-La2(SO4)3·0.1NH3 and β'-La2(SO4)3·0.3NH3, which is a phase formed after further NH3 insertion into β-La2(SO4)3·0.1NH3 with a minor change in the host structure. The fact that both NH3 and H2O can be inserted confirms that the pores of β-La2(SO4)3 allow for the insertion of molecules with a strong polarity. This nature is similar to zeolites and metal-organic frameworks (MOFs) with polar surfaces in the pores.