Greenaway A, Gonzalez-Santiago B, Donaldson PM, Frogley MD, Cinque G, Sotelo J, Moggach S, Shiko E, Brandani S, Howe RF, Wright PA. In situ Synchrotron IR Microspectroscopy of CO2 Adsorption on Single Crystals of the Functionalized MOF Sc2(BDC-NH2)3. Angewandte Chemie International Edition 126, 13701-13705, doi:10.1002/ange.201408369 (2014).
Turrina A, Eschenroeder ECV, Bode BE, Collier JE, Apperley DC, Cox PA, Casci JL, Wright PA. Understanding the structure directing action of copper-polyamine complexes in the direct synthesis of Cu-SAPO-34 and Cu-SAPO-18 catalysts for the selective catalytic reduction of NO with NH3. Microporous and Mesoporous Materials 215, 154-167, doi:10.1016/j.micromeso.2015.05.038 (2015).
Synchrotron infrared microspectroscopy; FTIR; CO2; MOFs; Zeolite catalysis; SCR.
Highly porous materials such as metal-organic frameworks (MOFs) and zeolites are very effective at adsorbing gases and catalysing reactions, due to their ability to trap molecules in their pores. Infrared (IR) microspectroscopy experiments on the Multimode InfraRed Imaging and Microspectroscopy (MIRIAM) beamline, B22, have now revealed the processes involved in gas adsorption and catalysis in more detail than ever before. In particular, B22 was used to determine (i) how carbon dioxide (CO2) is taken up by a MOF made of scandium cations joined together by organic linkers functionalised with amino groups, and (ii) the mechanism of activation of zeolitic silicoaluminophosphate (SAPO) catalysts for the reduction of nitrogen oxides (NOx) found in car exhaust fumes. The IR spectral data generated by B22 revealed rapid diffusion of CO2 into the pores of the MOF and confirmed that the amino groups enhanced the uptake. B22 was also able to collect IR spectral data from polarised IR radiation, which could be used to determine the orientation of the adsorbed CO2 molecules. This revealed that the CO2 molecules align themselves along the pore channels. With a SAPO catalyst, the spectral data produced by B22 showed how it becomes catalytically active through the release upon heating of metal cations from metalcomplex templates within the pores and the simultaneous generation of Brønsted acid sites. Ongoing work also provides evidence that IR spectroscopy with B22 facilitates the study of the adsorption of mixtures of gases - difficult to do using conventional methods - and operando monitoring of catalytic reactions.
Microporous solids find increasing application as adsorbents and catalysts in gas separations and chemical conversions. The uptake of the greenhouse gas CO2 from fuel gases and power station emissions and the selective catalytic reduction (SCR) of NOx-containing exhaust gases from diesel engines are topical examples. To understand and optimise porous solids for these processes, spectroscopic techniques are needed to observe them operando, i.e. under working conditions1. IR microspectroscopy is a powerful and well-established method for this approach, offering quantitative, time-resolved molecular information on adsorbates, and for catalytic conversions, on the molecular species present as intermediates, but there are technical challenges of high absorbance and insufficient time resolution in the laboratory IR spectroscopy of powders.
Figure 1: a) Multiple experiment collection sites (10 x 10 μm2) on the same crystal of Sc2(BDC-NH2 )3 ; b) Spectra from a single site as temperature, T, is varied at 100 mbar CO2 showing asymmetric CO2 stretch intensity decreasing with increasing T; c) Isobar at 100 mbar of CO2 calculated from quantitative analysis of CO2 and NH2 stretches at varying T.
Recent developments at the beamline, where the synchrotron radiation (SR) IR radiation has a 100-fold higher photon flux density than conventional (e.g. globar) laboratory sources, address these issues. The uptake and conversion of molecules in small volumes (<10-15 m3) of single microcrystals (themselves a few tens of microns in dimensions) can be followed by the measurement of IR spectra with good signal-to-noise ratios on a sub-minute time-scale by adapting the Fourier transform IR (FTIR) microscope to accommodate a Linkam environmental cell. Such a cell has been adapted to enable temperature control up to circa 450 °C and a customised gas flow control unit for a wide variety of gases.
This technique was well suited for the study of the adsorption of CO2 on the small pore scandium metal organic framework, Sc2(BDC-NH2)3 , where the linker is 2-amino-1,4-benzenedicarboxylate (Fig. 1). The amino-groups in this highly stable, hydrophobic MOF enhance the strength of CO2 adsorption at partial pressures present in flue gases. The MOF single crystals are 80 x 20 x 20 μm3 in size, with a readily recognised morphology and crystal faces indexed by single crystal X-ray diffraction. Using SR IR microspectroscopy, the CO2 uptake is clearly observed by its asymmetric stretch at 2335 cm-1. The symmetrical and asymmetric N-H stretches from the MOF together provide an important internal reference that (together with a single isotherm collected on a bulk sample) enable the quantification of CO2 uptake by selected single crystals from CO2/ He mixtures (up to 200 mbar of CO2). The time resolution of spectral collection confirms rapid diffusion of CO2 through the pores. This permits equilibrium adsorption isobars and isotherms to be measured that indicate uptakedependent adsorption enthalpies for CO2 (31±3 kJ mol-1) ca. 10 kJ mol-1 higher than those of the same material without amino-groups. Careful examination of the positions of the peak maxima of the -NH2 stretches show a shift to lower wavenumber by around 5 cm-1, confirming the weak physical interaction.
Figure 2: a) Polarised IR spectra from a crystal of Sc2(BDC-NH2 )3 with the (0 0 1) face perpendicular to the beam with adsorbed CO2 ; unpolarised (black), polarisation perpendicular (red) and parallel (blue) to the long morphological axis of the crystal; b) Model of adsorbed CO2 molecules within the pore structure, closely aligned to the channel axis. Disordered NH2 groups are represented by blue spheres.
The high brightness of the SR also permits the collection of polarised IR spectra on microcrystals, where the IR signal is strongly reduced. The intensities of the NH2 stretches show strongly anisotropic behaviour under polarised IR (Fig. 2). The orientations of the crystals with respect to the IR linear polarisation can be established from the relationship between the polariser and the welldefined crystal shapes, so the anisotropy can readily be explained. For example, the NH2 symmetric stretches have their major component parallel to the channels of the MOF (that run along its a axis), which corresponds to the crystals’ long dimension. When CO2 is adsorbed, similar effects are seen for the relative intensity of the asymmetric CO2 stretch (enhancement when the IR is polarised along the a axis direction). This reveals that CO2 molecules align with their long axes close to parallel to the channel axis. Furthermore, since crystals were found to settle in one of three ways, with their (001), (011), or (010) faces parallel to the supporting window of the Linkam cell, it was possible to triangulate the orientation of the CO2 molecules, even at uptakes so low that its determination by crystallography would be impossible. The method also allows the possibility of the measurement of uptakes and molecular orientation when gas mixtures are present, which is very time-consuming by conventional adsorption measurements and usually requires large sample volumes.
Figure 3: In situ calcination of a single SAPO-18 crystal prepared using a transition metal complex of 1,2-bis(3-aminopropylamino)ethane as the template. Bands in green represent stretches characteristic of the polyamine, while red regions cover nitrile groups (2250 cm-1) and hydroxyl stretches (3650 cm-1) that arise during template removal.
We thank the EPSRC (EP/J02077X/1) and Johnson Matthey plc for funding and Diamond Light Source for beamtime on B22 (SM8875-1 and SM10014-1).
Professor Paul Wright, University of St Andrews, firstname.lastname@example.org.
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