(204g) Kinetics and Modeling of CO2 Adsorption On Amine Functionalized Mesoporous SBA-15 Sorbents

Kinetics and Modeling of CO₂ Adsorption
on Amine Functionalized Mesoporous SBA-15 Sorbents

Arunkumar Samanta¹, An Zhao¹,
Partha Sarkar², Rajender Gupta¹

¹Department
of Chemical and Materials Engineering, University of Alberta

9107 - 116 St, ECERF, AB, T6G 2V4,
Canada, Email:asamanta@ualberta.ca, Tel: 780-492-6861, Fax: 780-492-2881

² Environmental
& Carbon Management Division, Alberta Innovates - Technology Futures,
Edmonton, AB, Canada,

1. Introduction

Recently,
adsorption processes using functionalized ordered mesoprous silica sorbents have
shown a great potential for post-combustion CO₂ capture from flue
gas compared to the other separation techniques. Ordered mesoporous silicas
generally provide excellent structural properties as supports, such as large
pore volume, pore size and surface area. In addition to this, numerous active
sites disperse highly on the inner pore surface which benefit and promote the
distribution of amine moieties on the surface. The immobilization of amino
groups anchored to the surface of solid support by means of physical
impregnation or covalent modification via silane chemistry offers stable
interaction with acidic CO₂ molecules, meanwhile largely avoids the
corrosion and toxic problems.

In this work an experimental and theoretical
investigation on the adsorption of CO₂ onto amine- functionalized
mesoporous SBA-15. The adsorption of CO₂ on  the functionalized
sorbents  have been measured by thermogravimetric  method over the CO₂
partial pressure range of 10-100 kPa and temperature range of 303 ? 373 K under
atmospheric pressure.  The functionalization of SBA-15 silicas with
tetraethylenepentamine (TEPA) has been achieved using conventional wet
impregnation technique. The structural properties of the sorbents have been
characterized by nitrogen adsorption/desorption, SAXS, SEM, TEM and FTIR
techniques. Thermal swing adsorption cycles over a range of temperatures and
time in a simulated flue gas were also explored. Different chemisorption
kinetic model has been developed to analyze the experimental data. The model is
validated with the experimental results of isothermal adsorption measurements
of CO₂ on SBA-15/TEPA.

2.
Experimental

2.1
Material Synthesis and Characterization

The mesoporous SBA-15 material
used in this work was synthesized using amphiphilic triblock copolymer Pluronic
P123 (MW_avg = 5800, Aldrich) as the organic structure-directing
agent, tetraethyl orthosilicate (TEOS, Aldrich) as silica source and HCl as pH
controlling agent. The resulting SBA-15 materials, after drying, calcination
and subsequent impregnation, were characterized by N₂ adsorption/desorption,
SAXS, SEM, TEM and FTIR. 

2.2 CO₂ Adsorption and
Regeneration Study

CO₂
adsorption/desorption measurements were performed on a thermogravimetric
analyzer (Q500, TA Instruments). Pure CO₂ (99.99%) or CO₂/
N₂ at 1 atm was used for the adsorption measurements runs and N₂
was used as purging gas for CO₂ desorption. In a typical adsorption
run, about 10 mg of the sorbent were placed in platinum pan. After the sorbent
was heated to 105 °C in a N₂ stream with a flow rate of about 100 cm³.
min^-1 for about 30 min to remove all moisture and CO₂
adsorbed from air, the temperature was decreased to desired adsorption
temperature. The gas was then switched from N₂ to CO₂ and
the temperature was kept constants at desired adsorption temperature for about
1 h. The CO₂ capturing capacity of the solid sorbent was calculated
in mmol.g^-1 dry sorbent from the weight gain of the sample in the
adsorption process.

3. Results and Discussion

3.1
Characterization

The
N₂ adsorption isotherms of calcined SBA-15 shows typical type IV
isotherm indicative of defined mesoporosity in the framework. For calcined
SBA-15, the BET surface area, mesopore area and mesopore volume are about 1099
m².g^-1, 923  m².g^-1 and 1.78 cm³.g^-1,
respectively. The pore size (in the range of 10-30 nm) was estimated using BJH method.
Five distinct reflective peaks are identified from the SAXS diffraction spectra
of SBA-15 and SBA-15/TEPA. The estimated d-spacing (d₁₀₀) is in the
range of 9.7 -10.3 nm.  The morphology of the mesoporous SBA-15 support studied
using SEM and TEM techniques. The SBA-15 is observed to contain rope like
material arranged in bundle of about 4.3 µm diameter and length of 35 µm. Besides,
FTIR spectroscopy of SBA-15/TEPA provided clear evidence for amine
impregnation.

3.2 CO₂ Adsorption Study

CO₂
adsorption capacities of the amine impregnated solid sorbent supports, such as
Norit AC, Sigma AC (C 5510), SBA-15, and MCM-41 has been determined at 303,
323, 348 and 373 K in pure CO₂ and various
partial pressures of CO₂ (in N₂). Typical CO₂
adsorption capacity results are shown and compared in Figure 1. From the
preliminary screening study for TEPA and PEI impregnated sorbents, it has been
found that SBA-15/TEPA outperforms all other sorbents under the same conditions.
In particular, SBA-15/TEPA sorbents shows better CO₂ adsorption
capacity than the sorbent loaded with same amount of PEI. SBA-15/TEPA exhibits
the highest CO₂ adsorption capacity and reached an CO₂
uptake value  as high as 4.51 mmol-CO₂/g-dry sorbent within few minutes
of adsorption test at 348 K and pure CO₂ environment. Amine modified
activated carbon does not show satisfactory performance on adsorption capacity
because of the limitations such as relatively lower pore volume and surface
area.

To study the influence of
adsorption temperature on TEPA impregnated SBA-15 sorbents, adsorption
experiments were carried out at 303, 323, 348 and 373 K. When the temperature
increases from 303 to 348 K, there is a large difference observed in adsorption
capacity that varies from 2.73 to 4.51 mmol.g^-1. One possible reason
is that higher temperature promotes the adsorption reaction and increases the
accessibility of CO₂ to available active sites on the surface of
TEPA. On the other hand, all the sorbents exhibit a lower adsorption capacity
at low CO₂ partial pressure. Preliminary test with ~2.0 (vol) %
moisture in pure CO₂ shows CO₂ adsorption capacity of about
5.05 mmol.g^-1 indicating moisture favors in CO₂
adsorption.

A series of impregnated sorbent using
calcined SBA-15 containing 50- 70 wt% TEPA were also prepared to investigate
the effect of amine loading on adsorption capacity. It is observed that the
adsorption capacity increases from 3.57 mmol.g^-1 to 4.60 mmol.g^-1
 with increasing  the amine loading  from 50 wt% to 70 wt%. However, the
enhancement of adsorption capacity with respect to amine efficiency does not
follow similar trend. For example, SBA-15/TEPA 60 exhibits the highest amine
efficiency 0.290, while SBA-15/TEPA70, which presents the highest adsorption
capacity, has relatively lower amine efficiency value of 0.248.  

About ten cycles of
adsorption-desorption were performed to study the cyclic adsorption-desorption
performance of the SBA-15/TEPA60 sorbent using temperature swing process. From
the cyclic adsorpion-desorpiton performance, it is observed that after ten
cylces of adsorption-desorption, the sorbent shows a gradual decrease in
capacity. This is probably due to the continuous degdation of TEPA during the
repeated adsorption-desorption processes.

3.3 CO₂ Adsorption Kinetics

A fast adsorption kinetics have
been found with more than 90% of the total CO₂ uptake on TEPA-SBA-15
sorbents occurring within the first few minutes (< 5mins ) of adsorption.
All SBA-15/TEPA show a sharp increase of adsorption during the initial
adsorption stage. Three different kinetic models expressed by Eq.(1), Eq. (2)
and  Eq.(3), have been used to analyze the experimental CO₂ uptake data:

where
q_e and q_t are the sorption capacity at equilibrium and at
time t, respectively; k, k_n, n and m are the model constants.   
Figure 2 shows the experimental results of isothermal adsorption measurements
of CO₂ on functionalized SBA-15 sorbents at 348 K and the
corresponding values predicted by various kinetic models. It has been observed
that kinetic model described by Eq. (3) is in good agreement with the
experimental data.

4.
Conclusions

This
work presents a theoretical and experimental investigation on the adsorption of
CO₂ onto amine- functionalized mesoporous SBA-15. The CO₂
sorption/desorption experiments performed by thermogravimetric method revealed
that the well-dispersed TEPA inside SBA-15 exhibited a CO₂ sorption
capacities as high as 4.51  mmol.g^-1 for the sorbent SBA-15/TEPA60 at
348 K. The textural properties of the raw materials and in particular, the
mesopore content seems to be the dominant factor for creating a good dispersion
of the amines into the pore channels of the silica support. The CO₂
capture kinetics is found to be fast and reached 90% of the total capacities
within the first ten minutes. Different chemisorption kinetic model has been
developed to analyze the experimental data. The model is validated with the
experimental results of isothermal adsorption measurements of CO₂ on
functionalized SBA-15 sorbents by thermogravimetric method. Good agreement
between the model results and experimental results indicates that a general
kinetic model described by Eq. (3) can effectively represent the dynamics of CO₂
mass transfer rates on TEPA functionalized SBA-15 sorbents for wide range of
operating conditions. The cyclic CO₂ adsorption-desorption revealed
that the TEPA impregnated SBA-15 sorbents exhibited good cyclic stability. The
results suggest that mesoporous SBA-15 impregnated with TEPA shows a great
potential as adsorbents for post-combustion CO₂ capture.

Acknowledgements:

The
financial support of the Canadian Centre for Clean Coal/Carbon and Mineral
Processing Technology and Carbon Management Canada is acknowledged.