In an amine-functionalized system, the first CO₂ molecule attacks the NH₂ group through a side-on mode, and the second CO₂ molecule interacts with the first CO₂ molecule through an end-on mode, resulting in high isosteric adsorption enthalpy (40–90 kJ mol⁻¹)^{13, 21, 22}. In hydroxyl-functionalized system NOTT-300, the first CO₂ molecule attacks the OH group through an end-on mode, and the second CO₂ molecule interacts with the first CO₂ molecule through a side-on mode, resulting in a lower isosteric adsorption enthalpy (27–30 kJ mol⁻¹). Similar weak interactions are also observed in binding of SO₂.

The structure was solved from high-resolution PXRD data by ab initio methods. a, Coordination environment for ligand L⁴⁻ and the Al(III) centre. b, View of the corner-sharing extended octahedral chain of [AlO₄(OH)₂]_∞. The μ₂-(OH) groups are highlighted as a space-filling model, and linked to each other in a cis configuration. c, View of the three-dimensional framework structure with a channel formed along the c-axis. The free water molecules in the channel are omitted for clarity. d, View of the square-shaped channel. The μ₂-(OH) groups protrude into the centre of the channel from four directions (aluminium, green; carbon, grey; oxygen, red; hydrogen, white; [AlO₄(OH)₂], green octahedron).

a, Comparison of the gas adsorption isotherms for NOTT-300 at 273 K and 1.0 bar. b, Variation of isosteric enthalpy (Q_st) and entropy (ΔS) of CO₂ adsorption. NOTT-300 exhibits highly selective uptake for CO₂ and SO₂ compared with CH₄, CO, N₂, H₂, O₂ and Ar. The CO₂ selectivities (calculated from the ratio of initial slopes of the isotherm) are 100, 86, 180, >10⁵, 70 and 137 for CH₄, CO, N₂, H₂, O₂ and Ar, respectively. The SO₂ selectivities (calculated from the ratio of the initial slopes of the isotherms) are 3,620, 3,105, 6,522, >10⁵, 2,518 and 4,974 for CH₄, CO, N₂, H₂, O₂ and Ar, respectively. The thermodynamic parameters Q_st and ΔS were calculated using the van't Hoff isochore on CO₂ adsorption isotherms measured at 273–303 K. The Q_st values lie in the range 27.5–28 kJ mol⁻¹ for CO₂ uptakes of 1–2 mmol g⁻¹ and increase continuously thereafter to ∼30 kJ mol⁻¹ at 4.5 mmol g⁻¹. The error in Q_st is estimated as 0.05–0.5 kJ mol⁻¹ (as shown by the error bars). Overall, ΔS decreases continuously with increasing surface coverage over the whole loading range. The error in ΔS is estimated as 0.2–1.6 J K⁻¹ mol⁻¹ (as shown by the error bars).

a, Comparison of the experimental (top) and DFT-simulated (bottom) INS spectra for bare and CO₂-loaded NOTT-300. b, Difference plot for experimental INS spectra of bare and CO₂-loaded NOTT-300. Two distinct energy transfer peaks are labelled as I and II. c, View of the structure of NOTT-300·3.2CO₂ obtained using PXRD analysis. The adsorbed CO₂ molecules in the pore channel are highlighted by the use of ball-and-stick mode. The carbon atom of the second CO₂ site is highlighted in blue. The dipole interaction between CO₂(I,II) molecules is highlighted in orange (O=C=O···CO₂ = 3.920 Å). d, Detailed view of the roles of the OH and CH groups in binding CO₂ molecules in a pocket-like cavity. The model was obtained from DFT simulation. The modest hydrogen bond between O(δ⁻) of CO₂ and H(δ⁺) from the Al OH moiety is highlighted in cyan (O···H = 2.335 Å). The weak cooperative hydrogen-bond interactions between O(δ⁻) of CO₂ and H(δ⁺) from CH are highlighted in purple (O···H = 3.029, 3.190 Å, each occurring twice). Each O(δ⁻) centre therefore interacts with five different H(δ⁺) centres. (Aluminium, green; carbon, grey; oxygen, red; hydrogen, white.)

a, Comparison of the powder diffraction patterns for original, evacuated, SO₂-loaded and final desolvated samples at 273 K. b, View of the crystal structure of NOTT-300·4.0SO₂ obtained from Rietveld refinement of data on SO₂-loaded material at 1.0 bar. The adsorbed SO₂ molecules in the pore channel are highlighted by the use of ball-and-stick mode. The sulfur atom of the second site of SO₂ is highlighted in blue. c, Detailed view of the role of the OH and CH groups in binding SO₂ molecules into a distorted pocket-like cavity. The moderate hydrogen bond between O(δ⁻) of SO₂(I) and H(δ⁺) of OH is highlighted in cyan [O···H = 2.376(13) Å]. The weak cooperative hydrogen-bond interaction between O(δ⁻) of SO₂ and H(δ⁺) of CH is highlighted in purple [O···H = 2.806(14), 2.841(17), 3.111(16), 3.725(18) Å]. Therefore, each O(δ⁻) centre is interacting with five different H(δ⁺) centres simultaneously. The bond distance between S(δ⁺) of SO₂(I) and O(δ⁻) of SO₂(II) is 3.34(7) Å. (Aluminium, green; carbon, grey; oxygen, red; hydrogen, white; sulfur, yellow.)