Destructive Adsorption of CCl₄ over Lanthanum-Based Solids:  Linking Activity to Acid−Base Properties

Abstract

The relative activities of a low-surface crystalline and high-surface amorphous LaOCl, further denoted as S1 and S2, have been compared for the destructive adsorption of CCl 4 . It was found that the intrinsic activity of S2 is higher than that of S1. Both samples were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N 2 -physisorption, and Raman and infrared (IR) spectroscopy. IR was used in combination with CO 2 , CO, and methanol as probe molecules. The CO 2 experiments showed that different carbonate species are formed on both materials. For S1, a high surface concentration of bidentate carbonate species and a lower concentration of monodentate carbonate were observed. In the case of S2, bulk carbonates were present together with bridged carbonates. CO adsorption shows that S2 and S1 have very similar Lewis acid sites. However, methanol adsorption experiments showed that S2 had a higher number of stronger Lewis acid sites than S1 and that twofold coordinated methoxy species were more strongly bound than threefold coordinated methoxy species. Because of the analogy between methanol dissociation and the removal of the first chlorine atom in the destructive adsorption of CCl 4 , the sites enabling twofold coordination were likely to be the same Lewis acid sites actively involved in the destructive adsorption of CCl 4 . La 2 O 3 was less active than the two LaOCl materials, and therefore, the intrinsic activity of the catalyst increases as the strength of the Lewis acid sites increases. S2 contains more chlorine at the surface than S1, which is expressed by the higher number of sites enabling twofold coordination. Moreover, this explains the difference in destructive adsorption capacity for CCl 4 that was observed for the samples S1 and S2. Since LaCl 3 , being the most acidic phase, is not active for the destructive adsorption of CCl 4 , basic oxygen atoms, however, remain needed to stabilize the reaction intermediate CCl 3 as La-O-CCl 3 .

Intermediates in the Destruction of Chlorinated C1 Hydrocarbons on La‐Based Materials: Mechanistic Implications

“…This finding is in agreement with previous experiments with CCl 4 on LaOCl, in which higher intrinsic activity was observed for a more chlorinated surface. [30] An increase in chlorination leads to stronger Lewis acid sites, which are important for the initial reaction step. Once a CÀCl bond has been broken, basic O sites are needed to stabilize the remaining fragment and to convert it into O-containing products.…”

“…For the destructive adsorption of CCl 4 , it was found previously that an optimum combination of Lewis acidity/basicity exists for the active sites. [30] The C À Cl bond becomes elongated when the Cl atom is attracted by a La III Lewis acid site, resulting in the breaking of the CÀCl bond and the formation of a LaÀCl bond. The basic O sites are necessary to stabilize the remaining CCl 3 fragment and enable the conversion of CCl 4 into CO 2 .…”

“…[28,29] Further investigations on LaOCl materials showed a link between activity and the acid-base properties of the catalyst. [30] For the destructive adsorption of CCl 4 , a strong lanthanum Lewis acid site results in high activity. Since chlorine is more electronegative than oxygen, a higher chlorine content at the surface results in higher activity.…”

“…The bands at 1523 and 1333 cm À1 have previously been assigned to polycarbonates. [30] CH 3 Cl: For CH 3 Cl on LaOCl at room temperature, the observed bands are all assigned to gas-phase CH 3 Cl (Figure 1c). At 523 K, a strong band arises at 1178 cm À1 assigned to dimethyl ether.…”

Dehydrochlorination of Intermediates in the Production of Vinyl Chloride over Lanthanum Oxide-Based Catalysts

“…Even though transition metal oxides are very active for the dechlorination of CHCs, the chlorination of the oxide may result in the formation of toxic volatile transition metal oxychlorides [14][15][16][17][18][19][20][21]. The basic oxides have proven to be stable active materials for the conversion of CHCs [22][23][24][25][26][27][28][29][30][31][32][33][34]. In previous work, lanthanum oxide-based catalysts were found to have the highest destructive capacity for the catalytic destruction of chlorinated C 1 and have been studied in detail in our group [29-31, 33, 34].…”