High adsorption of ammonia in a titanium-based metal–organic framework

Abstract

We report the high adsorption of NH₃ in a titanium-based metal–organic framework, MFM-300(Ti), comprising extended [TiO₆]_∞ chains linked by biphenyl-3,3′,5,5′-tetracarboxylate ligands. At 273 K and 1 bar, MFM-300(Ti) shows an exceptional NH₃ uptake of 23.4 mmol g⁻¹ with a record-high packing density of 0.84 g cm⁻³. Dynamic breakthrough experiments confirm the excellent uptake and separation of NH₃ at low concentration (1000 ppm). The combination of in situ neutron powder diffraction and spectroscopic studies reveal strong, yet reversible binding interactions of NH₃ to the framework oxygen sites.

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Ammonia (NH₃) is an important feedstock that is produced at a scale of ca. 150 million tonnes per year.¹ In addition, the high volumetric (∼0.105 kg L⁻¹) and gravimetric (17.7 wt%) hydrogen densities of NH₃ make it a promising renewable fuel and a potential hydrogen carrier for on-board storage. However, the highly toxic and corrosive nature of NH₃ poses challenges in its safe storage and transportation. At present, NH₃ is transported as a compressed liquid either at 10 bar at 25 °C or at ambient pressures at low temperature (liquefication point of −33 °C).² Thus, the development of robust sorbents for reversible NH₃ adsorption under mild conditions is of great importance.

Porous materials, such as zeolites,³ active carbons,⁴ mesoporous silica,⁵ and organic polymers⁶ have been tested for NH₃ adsorption. However, these materials demonstrate limited capacities; for example, 9.3 mmol g⁻¹ in 13X zeolite,³ 8.8 mmol g⁻¹ in MCM-41,⁵ and 11.4 mmol g⁻¹ in Amberlyst 15.⁷ Metal–organic frameworks (MOFs) have demonstrated potential for NH₃ adsorption⁸ owing to their high accessible surface area, and their tailored porosity and pore environment. The MOFs displaying top performance for NH₃ adsorption are primarily divalent/trivalent-based MOFs or MOF composites, such as LiCl@MIL-53-(OH)₂⁹ (33.9 mmol g⁻¹ at 298 K and 1 bar) and Ni_acryl_TMA¹⁰ (23.5 mmol g⁻¹ at 298 K and 1 bar). However, the stability of MOFs toward NH₃ adsorption needs to be improved, and typically tetravalent metal-based MOFs (e.g., Zr⁴⁺, V⁴⁺, Hf⁴⁺) exhibit enhanced thermal and chemical stability. However, the reported Zr-based MOFs show limited NH₃ adsorption capacities (8.40 mmol g⁻¹ in UiO-67¹¹). On the other hand, the study of V-based MFM-300(V) for NH₃ adsorption has revealed that the incorporation of V⁴⁺ can effectively enhance the NH₃ adsorption,¹² which is attributed to the charge transfer within the framework and the formation of N₂H₄.

Herein, we report a novel Ti⁴⁺-based MOF, MFM-300(Ti) [Ti₂(O)₂(C₁₆H₆O₈)], which displays a reversible NH₃ uptake of 23.4 mmol g⁻¹ at 273 K and 1 bar, among the best NH₃ sorbent materials showing reversible adsorption to date. Significantly, the packing density of NH₃ in MFM-300(Ti) (0.84 g cm⁻³, calculated using the pore volume derived from crystallographic data) is comparable to that of solid NH₃ at –80 °C (0.82 g cm⁻³), due to the strong guest–guest interaction within the pores. In addition, breakthrough experiments confirm that MFM-300(Ti) can effectively capture and separate NH₃ at low concentrations (1000 ppm). The binding sites of NH₃ in MFM-300(Ti) have been determined by in situ neutron powder diffraction (NPD), and its binding dynamics investigated by a combination of in situ inelastic neutron scattering (INS) and synchrotron IR microspectroscopy.

Solvated MFM-300(Ti), [Ti₂(O)₂(C₁₆H₆O₈)(H₂O)₂(C₃H₆O)_0.7], was prepared via a solvothermal reaction of {Ti₈AF} clusters,¹³ H₄L (biphenyl-3,3′,5,5′-tetracarboxylic acid, H₄bptc), acetic acid, acetic anhydride and methanol at 180 °C for 12 h. Structure solution by NPD (Fig. 1a and b) confirms that MFM-300(Ti) crystallises in the tetragonal space group I4₁22 and is isostructural to other MFM-300 materials.¹⁴ The presence of cis-μ₂-O moieties rather than cis-μ₂-OH groups as observed in M³⁺-based analogues is confirmed by NPD and FTIR spectra (Fig. 1a and b, Fig. S7, ESI†). MFM-300(Ti) exhibits an open framework structure comprising of chains of [TiO₄(O)₂] moieties bridged by tetracarboxylate ligands L⁴⁻ to afford 1D channels along the c axis with a diameter of 7.4 Å. Desolvated MFM-300(Ti) displays a Brunauer–Emmett–Teller (BET) surface area of 890 m² g⁻¹ and pore volume of 0.44 cm³ g⁻¹, as determined from the CO₂ isotherm at 195 K (Fig. S1, ESI†), consistent with the porosity derived from the crystal structure (pore volume of 0.45 cm³ g⁻¹). Crystallites of MFM-300(Ti) show rod-like morphology (Fig. S2, ESI†). The excellent thermal stability up to 450 °C and the chemical stability of MFM-300(Ti) has been confirmed by thermogravimetric analysis (TGA; Fig. S3, ESI†), variable temperature powder X-ray diffraction (VT-PXRD; Fig. S4, ESI†), PXRD analysis of samples soaked in various solutions (Fig. 1c and Fig. S5, ESI†), and the CO₂ adsorption measurements for samples after various treatments to confirm the porosity (Fig. S6, ESI†).

Fig. 1 Views of crystal structures of bare and ND₃-loaded MFM-300(Ti) as determined by NPD at 10 K. The occupancy of each site has been converted to ND₃/Ti for clarity. (a) and (b) Views of MFM-300(Ti) and MFM-300(M^III) along the c-axis, respectively. (c) and (f) Views of packing of ND₃ in MFM-300(Ti) along the c-axis in MFM-300(Ti)·0.84 ND₃ and MFM-300(Ti)·2.54 ND₃, respectively. (d) and (g) Detailed views of host–guest interactions in MFM-300(Ti)·0.84 ND₃ and MFM-300(Ti)·2.54 ND₃, respectively. (e) and (h) Guest–guest interaction along c-axis in MFM-300(Ti)·0.84 ND₃ and MFM-300(Ti)·2.54 ND₃, respectively. Colour code for atoms: Ti, turquoise; O, red; C, grey; H, tan; N, blue (dark blue, blue, and light blue for site I, II, and III, respectively); D, purple.

The adsorption isotherms for NH₃ in MFM-300(Ti) were measured between 273 and 308 K at 1 bar (Fig. 2a). MFM-300(Ti) exhibits an exceptional and fully reversible NH₃ uptake of 23.4 mmol g⁻¹ at 273 K and 1.0 bar, comparable to state-of-the-art sorbents (Table S1, ESI† and Fig. 2e). Hysteresis in the NH₃ isotherms was observed at all temperatures, indicating the presence of strong host–guest interactions. The pressure-swing experiment was conducted at 298 K from 0 to 0.1 bar. Little change in sorption capacity or structure was observed for MFM-300(Ti) after 25 cycles of adsorption and desorption of NH₃ (Fig. 2b and c), confirming the excellent stability of MFM-300(Ti) towards NH₃. The residual NH₃ observed on desorption during cyclic pressure-swing experiments suggests the presence of strong binding sites in the pore, consistent with the observed hysteresis. The dynamic NH₃ uptake of MFM-300(Ti) at low concentration (1000 ppm) was recorded as 0.6 mmol g⁻¹ at 298 K, demonstrating an excellent capture capability (Fig. 2d). The heat of adsorption (Q_st) of NH₃ in MFM-300(Ti) was calculated to be 39–52 kJ mol⁻¹ (Fig. 2f and Table S2, ESI†), comparable with other MOFs incorporating strong binding sites, such as UiO-66Cu(II)¹⁵ (25–55 kJ mol⁻¹).

Fig. 2 (a) Adsorption isotherms of NH₃ in MFM-300(Ti) at 273–308 K up to 1.0 bar (adsorption, solid symbols; desorption, open symbols). (b) Cyclic adsorption–desorption of NH₃ in MFM-300(Ti) at 298 K between 0 and 0.1 bar; dark-coloured bars show the residual NH₃ within the MOF upon pressure-swing desorption at 298 K. (c) PXRD patterns of MFM-300(Ti), (black: as-synthesised, red: after soaking in concentrated HNO₃ for one day, blue: after soaking in aqua regia for one day, dark yellow: after 25 adsorption–desorption cycles). (d) Breakthrough experiment for NH₃ (1000 ppm diluted in He) through a fixed-bed packed with MFM-300(Ti). (e) Plot of NH₃ isothermal adsorption capacities and NH₃ packing density in stable MOF materials (f) isosteric enthalpy and entropy of adsorption for MFM-300(Ti) at different NH₃ loadings.

In situ NPD data for ND₃-loaded MFM-300(Ti) were collected at 10 K (ND₃/Ti = 0.42, 1.27), and Rietveld refinements afforded distinct binding sites for ND₃. At low-loading (ND₃/Ti = 0.42; MFM-300(Ti)·0.84 ND₃) (Fig. 1c and d), two binding sites (I and II) are observed. Site I is anchored by hydrogen bonding between the ND₃ molecules and the carboxylate oxygen atoms [ND₃⋯O_ligand = 2.10(2)–2.97(3) Å]. Site II is adjacent to site I, stabilised by hydrogen bonding interactions [ND₃⋯O_ligand = 2.99(8) Å], electrostatic interactions [ND₃⋯aromatic rings = 2.52(3)–2.73(15) Å], and guest–guest interactions [ND₃⋯ND₃ = 1.87(2)–2.52(4) Å]. Interestingly, when the loading was increased to 1.27 ND₃/Ti (MFM-300(Ti)·2.54 ND₃) (Fig. 2e and f), an additional binding site was observed even closer to the [TiO₆]_∞ chain and stabilised by multiple supramolecular interactions [C–H_aromatic⋯N = 1.50(3)–2.24(3) Å, ND₃⋯O_ligand = 1.92(5)–2.38(6) Å, and ND₃⋯O_bridge = 2.33(6)–2.78(6) Å]. In addition, the adsorbed ND₃ molecules propagated to form a 1D network within the channel of MFM-300(Ti) (Fig. 1e and h). Moreover, the hydrogen bonding distance [N–D⋯N = 2.37(6)–2.52(3) Å] between adsorbed ND₃ molecules in MFM-300(Ti)·0.84 ND₃ decreases to [N–D⋯N = 1.87(3)–2.00(3) Å] in MFM-300(Ti)·2.54 ND₃. This tighter host–guest and guest–guest interaction with increasing NH₃ loading is consistent with the trend of Q_st (Fig. 2f). Notably, the shortest distance between adsorbed ND₃ molecules in MFM-300(Ti) is 1.87 Å, which is notably shorter than those previously observed [2.33 Å in MFM-300(Fe); 3.05 Å in MFM-300(V^IV)]¹⁵ and reflects highly efficient packing of NH₃ molecules in MFM-300(Ti), consistent with the high packing density derived from the adsorption isotherms. Compared with MFM-300(M^III),¹⁵ the replacement of μ₂-OH by μ₂-O in MFM-300(Ti) reduces the steric hindrance between the bridging hydroxyl groups and guest molecules, providing additional binding sites and boosting the storage of NH₃. In MFM-300(V^IV), the presence of the vanadium centre promotes charge transfer between NH₃ molecules, resulting in the observation of both N₂D₄ and ND₄⁺ molecules in the pore,¹⁵ which limits the reusability of the MOF. The utilisation of Ti⁴⁺ ions not only improves the structural stability of the material, but also hinders charge transfer to enhance the adsorption reversibility of NH₃.

The dynamics of host–guest binding has been analysed using in situ synchrotron IR microspectroscopy, INS, and density functional theory (DFT) calculations. The absence of the characteristic IR band of hydroxyl group at ∼3600 cm⁻¹ in activated MFM-300(Ti) (Fig. S7, ESI†) confirmed the presence of cis-μ₂-O as a result of the tetravalent Ti⁴⁺ sites, consistent with the NPD analysis. On loading NH₃, a new band at 3381 cm⁻¹ assigned to a ν(N–H) stretching vibration is observed, confirming the adsorption in the framework (Fig. 3). The band at 1614 cm⁻¹ can be assigned to v_as(COO⁻), which broadens on loading of NH₃. The bands at 1470 cm⁻¹, 1414 cm⁻¹,1360 cm⁻¹, and 1320 cm⁻¹ can be assigned to different vibrations v_s(COO⁻) of the carboxylate group in the framework, and all show redshifts on increasing loading of NH₃. This suggests the presence of strong interactions between NH₃ and the carboxylate groups of the bridging ligand. Simultaneously, as the concentration of NH₃ increases from 2% to 5%, two bands at 1548 and 1500 cm⁻¹ corresponding to aromatic C–C stretching vibrations merge into one band (1525 cm⁻¹), which is not observed in other MFM-300 materials. This indicates a notable change in the conjugated structure of the aromatic rings, which is further evidenced by the elongation of the C–C bonds in the biphenyl linker from 1.458(2) Å in bare MFM-300(Ti) to ∼1.497(4) Å in MFM-300(Ti)·0.84 ND₃ and further to ∼1.500(7) Å in MFM-300(Ti)·2.54 ND₃, as determined by the NPD analysis (Table S9, ESI†).

Fig. 3 In situ synchrotron FT-IR spectra of MFM-300(Ti) as a function of NH₃ adsorption and of regenerated MFM-300(Ti) under a flow of dry N₂ at 100 mL min⁻¹ at 298 K for 2 h.

A combination of INS and DFT calculations has been employed to elucidate the dynamic behaviour of adsorbed NH₃ within MFM-300(Ti). The congruence between experimental and simulated INS spectra for MFM-300(Ti) (Fig. S12, ESI†) and NH₃-loaded MFM-300(Ti) (Fig. S13, ESI†) is remarkable, allowing the assignments of INS peaks. Distinct peaks were observed in the INS difference spectra, which were obtained by subtracting the features of the bare MFM-300(Ti) and the sample cell from the spectra of NH₃-loaded MFM-300(Ti) (Fig. 4b). Translational and rotational modes of adsorbed NH₃ molecules around its C₃ axis are observed between 4.4–14.4 meV and 16.6–23.5 meV, respectively. The bands at 26.5–41.7 meV correspond to the rocking motions of NH₃ around the N centre (Fig. 4c and d). In comparison to the spectrum of NH₃ in the solid state, where each NH₃ molecule forms a 3D hydrogen bonding network with six adjacent NH₃ molecules, the bands in all regions for adsorbed NH₃ exhibit shifts to lower energy and display broader features (for solid NH₃, 6.3–23.1 meV translational modes; 27.6–35.5 meV rotational modes; 36.7–56.9 meV rocking modes) (Fig. 4a). This phenomenon suggests a more dynamic environment for the adsorbed NH₃ molecules within MFM-300(Ti). Specifically, the rotational modes show more significant red shifts, attributed to the higher rotational flexibility of NH₃ in its adsorbed environment in MFM-300(Ti).

Fig. 4 Views of in situ INS spectra and the corresponding vibrational modes for MFM-300(Ti), before and after adsorption of NH₃. Difference spectra were obtained by subtraction of the INS spectra of the bare MOF from that for the NH₃-loaded MOF and are marked as Expt-Diff (experimental difference spectra). (a) Comparison of vibrational modes between solid NH₃ at 10 K (4.4–14.4 meV translational modes; 16.6–23.5 meV rotational modes; 26.5–41.7 meV rocking modes). (b) Experimental difference INS spectra for MFM-300(Ti) upon adsorption of NH₃ in the higher energy range. The scale factor for difference spectrum is 3. (c) and (d) Selected vibrational modes of confined NH₃ molecule and MFM-300(Ti).

The vibrational features of the framework have been observed in the high energy region of the difference-INS spectra. Features at (I) 84.9, (II) 117.6, (III) 126.5, (IV) 145.1, and (V) 155.9 meV can be assigned to C–H out-of-C₆-plane wagging (in-phase along the ring, meaning H atoms move in the same direction), C–H out-of-C₆-plane twisting (anti-phase along the ring, meaning neighbouring H atoms move in opposite directions), C–H in-C₆-plane scissoring mode, in phase, C–H in-C₆-phane scissoring mode, anti-phase, and C–H in-C₆-plane rocking mode, respectively (Fig. 4b and d). Compared with other MFM-300(M) materials, MFM-300(Ti) shows more versatile C–H bending modes, and the changes in the C–H bending peaks upon adsorption of NH₃ indicate the strong host–guest interactions between the benzyl ring of the framework and NH₃ molecules. These results are in excellent agreement with the NPD and FTIR study.

In summary, we report the high capacity and reversible adsorption of NH₃ within a novel titanium-based MOF, MFM-300(Ti). This framework features a unique structure composed of extended [TiO₆]_∞ chains linked by tetra-topic ligands [(C₁₆H₆O₈)⁴⁻]. Notably, at 273 K and 1 bar, MFM-300(Ti) shows an exceptional NH₃ uptake of 23.4 mmol g⁻¹ and a record-high packing density of 0.84 g cm⁻³. The volumetric and gravimetric adsorption capacity is 0.36 g cm⁻³ and 0.28 g g⁻¹, respectively. Dynamic breakthrough experiments confirm excellent adsorption of NH₃ at low concentration (1000 ppm). The combination of in situ NPD, INS, FTIR and DFT studies reveal the molecular details on the host–guest binding interactions. This study has established MFM-300(Ti) as a highly efficient sorbent for NH₃, demonstrating its potential for practical applications.

We thank EPSRC (EP/I011870, EP/V056409), University of Manchester, National Science Foundation of China and BNLMS for funding. This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme (grant agreement no. 742401, NANOCHEM and PoC665632). We are grateful to the STFC/ISIS Facility and Diamond Light Source for access to beamlines WISH/TOSCA and B22(SM30398), respectively. The computing resources were made available through the VirtuES and the ICE-MAN projects, funded by Laboratory Directed Research and Development program and Compute and Data Environment for Science (CADES) at ORNL.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2296493, 2296494 and 2296495. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc01449a

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