Enhanced catalytic methane coupling using novel ceramic foams with bimodal porosity

Article abstract of DOI:10.1039/c2cy20458d

Monolithic reactor concepts are currently intensively discussed in the literature. For the oxidative coupling of methane relying on a balance of surface and gas phase reactions, such concepts have previously been claimed to have beneficial effects with respect to obtainable C₂ yields. In order to verify the superior performance in the case of a foam catalyst, ceria and samaria foams were fabricated by a direct foaming process. In both cases, mechanically stable, homogeneous open-cell foams were obtained as revealed by 3D magnetic resonance imaging, Hg-porosimetry, and scanning electron microscopy. As a characteristic feature of the introduced foaming methodology the process resulted in bimodal pore size distributions, ensuring low pressure drops on the one hand and sufficiently large surface areas on the other hand. Oxidative coupling of methane was carried out over the samaria foams. It was possible to obtain C₂ yields that were indeed higher than those obtained with the samaria powder, in contrast to honeycomb monoliths previously studied in the literature. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20458d

Electronic Supplementary Material (ESI) for Catalysis Science & Technology
This journal is © The Royal Society of Chemistry 2013
Supplementary Information
Enhanced Catalytic Methane Coupling using Novel Ceramic Foams with
Bimodal Porosity
B. Neumann1), T. Elkins2), W. Dreher3), H. Hagelin-Weaver2), J.C. Nino4), M. Bäumer1)
1)
2)
Institute of Applied and Physical Chemistry, University Bremen, Leobener Str. NW2,
28359 Bremen, Germany
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611,
USA
Department of Chemistry, University Bremen, NW2-C, 28359 Bremen, Germany
Department of Materials Science and Engineering, University of Florida, Gainesville,
Florida32611, USA
3)
4)
Foaming Process
For the preparation of the ceria and samaria foams, we used commercially available isocyanate PAPI
27 (DOW Chemical). All synthesises were performed using 27.5 %vol of ceramic powder, 0.4 mL
polyethylene glycol
- 200, 0.1mL Tween 80 and 0.8 mL of isocyanate. 1 mg of
Diazabicyclo[2.2.2]octane was added to start the foaming process. To ensure fast and proper
homogenisation of the slurry, we used a drill equipped with a PTFE spatula for mixing. To increase
the amount of gas released during the foaming process, 12 µL of water were added before the end of
the mixing. 24 hours after the preparation, the hybrid-foams were cut into the desired shape and
sintered in a furnace. The maximum temperature of 1300 °C for the samaria and 1650 °C for the ceria
was held for only 2 hours to obtain a high surface area monolith. To ensure slow burning of the PU
without destroying the foam, we selected a heating rate of 1°C/min up to 600 °C and increased the
heating rate afterwards to 2 °C/min. The selected cooling rate was 3 °C/min.

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Powder X-ray Diffraction (p-XRD)
The powder diffraction was performed using a X’PertPro MPD diffractometer (PANalytical) with Cu
Kα1,2 radiation. The 2Theta scanning region was 5-120°. The experiment shows that the samaria has
monoclinic structure, while the ceria is cubic.
Sm2O3
Sm2O3 theoretical
CeO2
CeO2 theoretical
10 20 30 40 50 60 70 80 90 100
Diffraction angle 2 CuK1,2 [°]
Figure 1: X-ray diffractograms of a samaria and a ceria foam. The samaria exhibits the monoclinic high temperature
phase, the ceria is cubic.
Mercury Porosimetry
Hg-Porosimetry was performed using a Quantachrome Autoscan-60 Mercury Porosimeter and the
Quantachrome filling apparatus (Boynton Beach, FL).

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Thermogravimetric Analysis (TGA)
The thermogravimetric analysis (TGA) of the oxides was performed at a heating rate of 5 K/min in air
using a STA 449 F3 Jupiter® Netzsch Germany.
Samaria powder
Ceria powder
200
300
400
500
600
700
Temperature [°C]
Figure 3: Thermogravimetric analysis of the samaria (upper 2 curves) and the ceria (lower 2 curves) ceramic powders
used as a starting material for the foam synthesis. The analysis was carried out during heating and cooling, showing that
the weight loss is irreversible. The dashed lines show the first derivative of the mass loss of the two powders. The ceria
does not show a specific weight loss while the samaria shows a weight loss between 200-350 °C due to the higher
amount of adsorbed water on the surface of the powder.
Diffusive Reflection Fourier Transform Spectroscopy (DRIFTS)
The DRIFTS experiments were performed using an Agilent Cary 670 equipped with a MCT Detector
and a Praying Mantis Diffuse Reflection Cell from Harrick. The collected spectra were referenced to
samaria heated to 140 °C (to remove adsorbed water from the cell and the surface of the powder).

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The reflectivity (R) of each spectrum was corrected by referencing the reflectivity by the reflectivity
of KBr at the specific temperature (Equation 1).
Eq. 1
0,3
0,2
0,1
0,0
Samaria 180 °C
Samaria 220 °C
Samaria 260 °C
Samaria 300 °C
Samaria 380 °C
Samaria 420 °C
-0,1
-0,2
-0,3
4000
3800
3600
3400
3200
3000
Wavenumber [1/cm]
Figure 4: The diffuse reflectance infrared spectra of samaria are presented. Spectra were acquired at different
temperatures between 180 °C and 420 °C. It is clearly visibly that the main loss of OH groups takes place between 260 °C
and 300 °C (represented by the increasing reflectivity in the OH vibration region at ~ 3600 cm-1) already indicated by the
TGA data.
The series of spectra reveal that the weight loss observed with TGA at around 280°C is connected
with the loss of OH groups from the surface of the samaria powder used for the foam synthesis.

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Scanning Electron Microscopy (SEM)The microscopic structure and the shape of all foams were
characterized with a benchtop scanning electron microscope (Neoscope JCM 5000 M, JEOL-Nikon)
operated at 15 kV with a working distance of 12 mm. To decrease charging effect during the
acquisition a thin gold layer was sputtered onto the investigated foams.
Magnet Resonance Imaging (MRI)
All magnetic resonance imaging (MRI) experiments were performed using a 7-Tesla BrukerBiospec
70/20 system (Bruker-Biospin, Ettlingen, Germany) equipped with self-shielded gradients (maximum
strength 400 mT/m, slew rate 4000 mT/m/ms). A quadrature volume coil (72 mm inner diameter)
was employed for both radiofrequency (RF) excitation and signal reception. For the experiments, an
optimized sample holder was machined using Teflon for all parts that are inserted into the RF coil,
ensuring that proton signals exclusively originate from the sample. MRI measurements were
performed using the proton signal of water in which the porous structures were inserted. As rather
*
short effective transverse relaxation times T2 were observed in the porous samples, a zero-echo
time (ZTE) pulse sequence was applied for 3D MRI. In the ZTE sequence, the read gradient used for
spatial encoding is switched on prior to RF excitation achieved by a short rectangular RF pulse. Signal
*
detection is started immediately after RF excitation, thus minimizing signal losses by T2 relaxation.
The ZTE pulse sequence supplied by the manufacturer was modified by adding outer volume
suppression pulses to saturate signals originating from parts of the RF coil. The parameters of the ZTE
pulse sequence were as follows: RF excitation by a 2 s rectangular pulse with a flip angle of 4°,
repetition time (TR) 10.75 ms, 224 complex data points per TR, spectral width 150 kHz, radial
sampling using 158026 projections without polar undersampling, image matrix size 224x224x224,
field-of-view 323 mm3, 4 averages yielding in a total measurement time of about 113 minutes. In
principle, all MRI pulse sequences known from standard MRI applications could be applied. However,
since within the porous structures rather short transverse relaxation times are observed for the
protons of water1, MRI measurements with ultrashort echo times (TE) are required to minimize signal
losses. Using other pulse sequences may lead to misinterpretations as the absence of NMR signals

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could not only be due to the absence of water, but also to very short transverse relaxation times
caused by the interaction of the water with the foam. However, using MRI with ultrashort TE allows
the detection of the water signal within the pores of the foam.
Catalytic Activity of Oxidative Methane Coupling
Product analysis was performed by an on-line Agilent 6890N gas chromatograph equipped with both
a thermal conductivity detector (TCD) and a flame ionization detector (FID) in series along with two
packed columns, a polar Porapak Q capillary column and a molecular sieve, also in series with a
column isolation valve in between the columns. The former was used to separate the CO2, C2H6, and
C2H4 and the latter for CH4, O2, N2, CO, H2 separation. Calibration curves for product formation and
methane conversion were taken by feeding a known flowrate of the calibration gas to the GC and
measuring the calibration gas to N2 TCD peak ratio. All calibration curves were linear and regression
analysis resulted in fit values of >98%. Measurements were taken at each reaction parameter after
20 minutes to allow for the reaction to reach steady state and a repeat was taken to ensure
reproducibility. The water produced during the reaction was separated from the gas phase products
prior to entering the GC using a condenser trap emerged in an ice-water bath
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