In situ single crystal synchrotron IR microspectroscopy of gas-solid 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 NO_x-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.

The method also possesses considerable potential for in situ measurements of single crystals of zeolitic catalysts during their activation and operation. Examples of the former include the calcination of silicoaluminophosphate (SAPO) zeotypes prepared using transition metal complexes of polyamine as ‘templating’ agents in novel, efficient preparation routes for highly active catalysts for SCR of NOx, for example Deka et al.2 and Eschenroeder et al. 3. FTIR microspectroscopy of crystals undergoing calcination shows the degradation of the polyamines, with associated generation and removal of nitrile intermediates (Fig. 3). This liberates the transition metal cations and also results in the generation of catalytically-active Brønsted acidic bridging hydroxyls (Si-O(H)- Al), 3650 cm-1. The next step is to investigate zeolites by FTIR microspectroscopy during catalysis (led by Professor R. F. Howe, Aberdeen University). There, the temporal and spatial distribution of adsorbed precursors and intermediates is being measured with increased resolution during methanol-to-hydrocarbon conversions over single zeolite crystals.

References:

1. Deka, U. et al. Confirmation of Isolated Cu2+ Ions in SSZ-13 Zeolite as Active Sites in NH3-Selective Catalytic Reduction. Journal of Physical Chemistry C 116, 4809-4818
   doi: 10.1021/jp212450d (2012).
2. Deka, U. et al. Changing active sites in Cu-CHA catalysts: deNOx selectivity as a function of the preparation method. Microporous and Mesoporous Materials 166, 144-152
   doi: 10.1016/j.micromeso.2012.04.056 (2013).
3. Eschenroeder, E. C. V. et al. Monitoring the Activation of Copper-Containing Zeotype Catalysts Prepared by Direct Synthesis Using In Situ Synchrotron Infrared Microcrystal Spectroscopy and Complementary Techniques. Chemistry of Materials 26, 1434-1441
   doi: 10.1021/cm403534b (2014).

 

Funding acknowledgements:
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).

Corresponding author:
Professor Paul Wright, University of St Andrews, paw2@st-andrews.ac.uk.