(664e) The Synthesis of Highly Efficient Ca-Based CO2 Sorbents Containing a Hierarchical Pore Structure

In 2010 the global CO₂ emissions
increased by more than 5 % compared to 2009, reaching 33 billion tonnes.¹One possible, mid-term solution to reduce the release of CO₂ into
the atmosphere is the implementation of CO₂ capture and storage (CCS)
technologies.² However, the currently available CO₂
capture technology, i.e. amine scrubbing, is associated with a large penalty on
plant efficiency. A
promising, new CO₂ capture technology is the use of alkaline earth
metal based CO₂ sorbents. Due to its (i) high theoretical
CO₂ capture capacity of 0.78 g CO₂/ g sorbent, (ii) low
cost and (iii) relatively vast abundance in naturally occurring minerals, e.g. limestone or
dolomite, CaO
is the most promising candidate in this class of
materials. The selective capture and release of CO₂ is achieved by
the following, theoretically reversible, reaction:

CaO(s) + CO₂(g)  « CaCO₃ (s), DH⁰_298K = ± 178 kJ/ mol               (1)

However, CaO produced via the calcination of naturally occurring materials has a
serious drawback, viz. the
rapid decrease in the CO₂ capture capacity with repeated cycles of
carbonation and calcination.³ The carbonation reaction can be
divided into two regimes.⁴ It has been argued that the fast,
kinetically controlled reaction stage, is associated with the filling of small pores.
The importance of the available pore volume can be clearly seen by comparing
the molar volume of the product, CaCO₃ (M_V = 36.9 cm³/mol), with that of CaO (M_V
= 16.7 cm³/mol). Once the pores have been
filled, the second, diffusion controlled reaction stage takes over, in which
CaCO₃ is deposited on the partially carbonated CaO
grains. Owing to the typically short contacting times in circulating fluidized
beds, the reactor of choice
for the commercial implementation of the calcium looping processes, only the CO₂ uptake achieved during the
fast reaction stage will be of practical relevance. Since the Tammann temperature (temperature at which thermal sintering
starts) of CaCO₃ (533 °C) is relatively low compared to the
operating temperatures of 650-950 °C, the dramatic reduction in CO₂
uptake has been attributed to the reduction of surface area and the blockage of
pores with diameter d_pore<
100 nm. One potential
strategy to increase the CO₂ capture stability of Ca-based sorbents is to stabilize CaO
with high Tammann temperature matrix. However, work
in this field has largely focused on the use of ?simple? synthesis techniques,
which do not allow easily to adjust key structural
properties, such as the pore volume. In this work we report the preparation of carbon gel templated,
Ca-based CO₂ sorbents. Here, calcium
nitrate and aluminium nitrate were used as the calcium
and aluminium precursors, respectively, whereas
resorcinol and formaldehyde were the precursors of the carbon gel. In addition,
the diameter of the carbon
sphere template was varied by changing the surfactant (CTAB) to resorcinol
ratio. Upon removal of
the carbon gel via calcination in air, hollow micro-spheres were obtained. The synthetic
material was investigated using (i) scanning electron
microscopy (SEM), (ii) surface area (BET) and pore volume (BJH) measurements,
(iii) X-ray diffraction (XRD) measurements and (iv) mercury
porosimetry measurements. The material synthesized possessed an excellent CO₂
capture capacity when compared to limestone or alternative CO₂ sorbents,
such as hydrotalcites or metal organic frameworks. Furthermore,
the current synthesis techniques allow the amount of Al₂O₃
required to stabilize the cyclic CO₂ capture capacity of the
material to be reduced to only 5 wt.%.

References
1. Olivier, J. G. J.; Janssens-Maenhout, G.; Peters, J. A. H. W.; Wilson, J. Long-term trend in global CO₂
emissions. 2011 report; PBL
Netherlands Environmental Assessment Agency: The Hague, 2011

2. Herzog, H. What future for carbon capture and
sequestration?
Environ. Sci. Technol. 2001, 35, 148-153.
3. Grasa,
G. S.; Abanades, J. C. CO₂ Capture
Capacity of CaO in long series of
carbonation/calcination cycles. Ind. Eng.
Chem. Res. 2006, 45,
8846-8851.
4. Dennis, J. S.; Pacciani, R. The
rate and extent of uptake of CO₂ by a synthetic, CaO-containing
sorbent.
Chem. Eng. Sci. 2009, 64, 2147-2157.