International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.9, pp. 189-192

Section 2.9.3.2. Capillary cells

W. van Beeka* and P. Pattisona,b

aSwiss–Norwegian Beamlines at ESRF, CS 40220, 38043 Grenoble CEDEX 9, France, and bLaboratory for Quantum Magnetism, Institute of Physics, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland
Correspondence e-mail:  wouter@esrf.fr

2.9.3.2. Capillary cells

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Capillary cells, also called microreactors, exist in many variations. Depending on the application (temperature, pressure and chemical environment), capillaries may be made of polyimide, glass, quartz, single-crystal sapphire or steel. They are simple and extremely efficient devices which can accommodate a large number of different applications. The impetus for their development has mainly come from catalysis research, but they have also been successfully employed to perform in situ reactions on intercalation, ion exchange, gas sorption/desorption, cement hardening, hydration–dehydration, light-induced transitions, crystallization processes and polymorphism, to name a few. Capillary cells are almost always used together with temperature- and/or gas-handling (static pressure or flow) devices. In open geometry, or with simple heat shields, one can easily obtain temperatures ranging from 80 to 1000 K with cryogenic and hot-air blowers or resistive heaters close to the sample. Static gas pressures can be as high as 30 MPa and in flow cells 2 MPa is commonly reached, as well as vacuum conditions. Capillary cells were first applied by Clausen (1991[link]) and have been adapted numerous times (Brunelli & Fitch, 2003[link]; Chupas et al., 2008[link]; Jensen et al., 2010[link]; Madsen et al., 2005[link]; Norby et al., 1998[link], 2000[link]; Palancher et al., 2005[link]) together with gas-handling systems (Eu et al., 2009[link]; Hill, 2013[link]; Krogh Andersen et al., 1998[link]; Llewellyn et al., 2009[link]), with large 6 mm-diameter samples (Andrieux et al., 2014[link]), with supercritical solvents up to 40 MPa with a swing-in blower for rapid heating (Becker et al., 2010[link]) or with pulsed supercritical flows (Mi et al., 2014[link]) for following synthesis reactions. A recent review of several capillary cells for high-pressure reactions (Hansen et al., 2015[link]) also contains useful information on how to calculate burst pressures.

One of the most critical issues is how to create a reliable leak-tight connection between the capillary and the metallic or polyether ether ketone (PEEK) gas/liquid supply line(s). For fragile capillaries there are basically two strategies: either to glue the capillary to a metal support with high-temperature epoxy (see Fig. 2.9.1[link]), or to use ferules. When both ends of a capillary have to be tightened with thin-walled capillaries, the use of ferrules needs some skill (see Fig. 2.9.2[link]).

[Figure 2.9.1]

Figure 2.9.1 | top | pdf |

(a) Swagelock VCR gland with an epoxy-glued capillary (red ellipse). (b) VCR capillary cell on a beamline with a cryostream (adapted from Jensen et al., 2010[link]).

[Figure 2.9.2]

Figure 2.9.2 | top | pdf |

(a) An exploded representation of the flow-cell/furnace components, indicating how they fit together. (b) The fully assembled flow cell/furnace. (c) An expanded view of the sample region, indicating the relative position of the sample and thermocouple tip within the furnace hot zone (adapted from Chupas et al., 2008[link]).

If the working temperature permits, it is easy and reliable to use a stainless-steel bracket in which capillaries are glued such that all mechanical forces are transferred to the support instead of being taken up by the thin (glass) capillaries (van Beek et al., 2011[link]), as in Fig. 2.9.3[link]. Gas systems are typically constructed from a combination of pressure reducers, mass-flow meters, valves and a manifold which supplies gases to the cell at controlled pressures and flow rates. It is worth pointing out the less-common backpressure regulator used in Fig. 2.9.3[link]. This unit allows 2 MPa of pressure to be maintained on the sample during flow experiments. Backpressure regulators for much higher pressures are commercially available, but these have so far not been used for in situ work. In situ diffraction has been coupled with stable-isotope analysis to correlate isotope fractionation with crystal structure. For this, a nonmetallic flowthrough capillary cell that avoids any contamination from the components of the cell itself was designed (Wall et al., 2011[link]). Many hundreds of studies have been performed over the last two decades with capillary devices in the above-mentioned fields.

[Figure 2.9.3]

Figure 2.9.3 | top | pdf |

Sketch of a typical experimental setup and a three-dimensional drawing of the in situ flow cell. Note the strain-relief bracket over the capillary. Adapted from Tsakoumis et al. (2012[link]), with permission from Elsevier.

Above 800 K, open capillary systems have severe heat loss and the use of insulation or reflectors around the sample or mirror furnaces is more appropriate. See Lorenz et al. (1993[link]), Margulies et al. (1999[link]), Proffen et al. (1995[link]), Riello et al. (2013[link]) and Yashima & Tanaka (2004[link]) for special designs to minimize heat loss. Finally, it is worth mentioning the recent work of Figueroa et al. (2013[link]), which combines the strong points of the various capillary designs, and also work by Johnsen & Norby (2013[link]), who managed to create and study a working battery in a capillary.

In any powder-diffraction experiment (Warren, 1990[link]), but particularly when using capillary cells, the experimentalist needs to take special care in order to obtain sufficient averaging in terms of grain statistics and to avoid preferred orientation. Typically ex situ capillaries are spun, but when gas lines are attached to the sample, spinning is not possible and only rocking or stationary geometry can be used. In addition, a fine (ground) polycrystalline powder giving perfect homogenous Debye–Scherrer rings, even without spinning, often results in an excessive pressure drop due to its high density and packing. In such cases, the sample may need to be pressed into a pellet and then crushed again to obtain larger agglomerates that allow sufficient gas flow through the sample (Jacques et al., 2009[link]). However, relatively large agglomerates, while reducing the packing density, might still give nonhomogeneous powder rings, which affects the intensities, especially on one-dimensional detector systems (strip detectors or crystal-analyser high-resolution systems). For all of these reasons it is not always straightforward to acquire reliable intensities under in situ conditions. If, however, proper care is taken, then precise structural parameters can indeed be refined from in situ data. For example, Milanesio et al. (2003[link]) obtained a detailed view of the structural rearrangements induced by the template-burning process from 350 to 1000 K on a zeolitic MFI framework. Oxygen flowed through a rocking sample and diffraction data were collected on a translating two-dimensional image plate capable of verifying the reliability of the measured intensities with a time resolution of several minutes (Meneghini et al., 2001[link]). From the temperature-dependent in situ data, the authors were able to extract the overall template occupancy in the framework. This allowed a definition of the key steps in the template-removal process, namely the start of the template decomposition, the start of the template burning and the end of the template burning. These three steps were then used to explain the cell-parameter evolution and atomic displacement parameters of the Si-framework atoms. Kinetic analysis performed on the results from the Rietveld refinements suggested a diffusion-limited reaction of the volatile products of the template leaving the framework (Milanesio et al., 2003[link]).

Conterosito et al. (2013[link]) reduced the time resolution to 100 ms per image using a Pilatus 300 K-W (Kraft et al., 2009[link]) pixel detector installed on an ESRF bending-magnet beamline in combination with a capillary reactor. In these experiments, a mechano-chemical method for fast and clean preparation of exchanged layered double hydroxides (LDHs) was investigated. The inorganic anion in the interlayer region (chloride or nitrate) was exchanged with a series of organic pharmaceutically important molecules. In Fig. 2.9.4[link], one can see the diffraction patterns of the intercalation process of ibuprofen into an LDH, which allowed the complex mechanism to be understood. Firstly, it is striking to note that the signal from the LDH starting material (the 003 reflection, in red) decreases, while the products and intermediates (001 reflections in black) grow already immediately after the first 100 ms image. Secondly, one can see that the low-angle peaks (<1.5°) show a different behaviour in time, suggesting a two-stage process with an intermediate phase and, thirdly, one sees that the intercalation process is over in ∼4 min. The reliability of the in situ procedure was confirmed by comparing the production yield of ex situ and in situ experiments. Owing to the complex two-stage process, kinetic analysis was not possible in the case of ibuprofen. In the same article, however, it was shown that it was possible to perform a full kinetic analysis on single-stage intercalation processes with different molecules reacting at comparable speeds. It is hard to imagine that so much detail on such timescales could be obtained using other tech­niques. For example, related intercalation experiments performed with energy-dispersive diffraction (Williams et al., 2009[link]) were, at that time, still limited to 10 s per pattern. Hence it was not possible to study the kinetics of such fast intercalation processes by other means.

[Figure 2.9.4]

Figure 2.9.4 | top | pdf |

(a) In situ three-dimensional stacked plot of the intercalation of ibuprofen into an LDH. Miller indices are shown in black for the intercalation product peaks and in red for the LDH nitrate starting material peaks. (b) Two-dimensional patterns showing the low-angle peaks during the first instance of the reaction. Adapted with permission from Conterosito et al. (2013[link]). Copyright (2013) American Chemical Society.

Care must be taken to ensure that, when performing experiments at microgram or microlitre levels inside capillaries, the results are still representative of the bulk reaction. Therefore, when studying in situ catalytic reactions, it has become common practice to measure the activity or selectivity of the sample with gas chromotography or mass spectrometry (see Fig. 2.9.3[link]) at the same time. It is also well known from reactor engineering that pressured drops, diffusion effects and flow disturbance are important parameters to take into account (Nauman, 2008[link]). The term operando was introduced by Bañares (2005[link]) during a discussion with colleagues (Weckhuysen, 2002[link]) for these combined experiments coupling structure with the sample activity.

If the miniaturization turns out to be problematic, one could consider measuring bulky (∼1 cm or more) samples and/or using larger reaction vessels in combination with either energy-dispersive diffraction or extremely high X-ray energies (Tschent­scher & Suortti, 1998[link]). This gives the additional advantage that identical sample volumes to those for neutron studies can be used. Hence, one often needs to utilize reaction cells that are specifically designed for the application, as explained in the following section.

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