(634a) Amine-Based CO2 Capture Aided By Acid/Base Catalyst: Advancement of Amine Regeneration Using Fe-MCM-41 Catalyst

Amine-based CO₂ capture aided
by acid/base catalyst: Advancement of amine regeneration using Fe-MCM-41 catalyst

Xiaowen Zhang ¹, Hongxia Gao ¹, Zhiwu Liang
^1, *, Paitoon Tontiwachwuthikul ¹, Maohong
Fan ²

¹ Joint
International Center for CO₂ Capture and Storage (iCCS), Provincial
Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at
Reducing CO₂ Emissions, College of Chemistry and Chemical
Engineering, Hunan University, Changsha 410082, PR China

    ² College of
Engineering and Applied Science, University of Wyoming, Laramie, WY 82071, USA

Abstract

Carbon
dioxide (CO₂) has become the focus of much attention because it is
the most important human caused greenhouse gas, contributing more than 60% of
the enhanced greenhouse effect. Amine-based CO₂ capture process is
the most developed and the most commercial mature technology that can be used for
CO₂ capture. 5 M monoethanolamine (MEA) solvent is the benchmark
solvent and has been the most frequently studied. However, commercial
implementation of this technology suffers from significant challenges,
particularly the enormous energy consumption for the rich amine solvent
regeneration, resulting in its immensely high operating costs of CO₂
capture.

Our
previous work demonstrated that the introduction of solid acid catalysts to the
CO₂ desorption process is a potential approach to decrease the heat
duty [1]. Several solid acid catalysts including, H-ZSM5, SAPO-34, and SO₄^2-/ZrO₂/
gamma-Al₂O₃ have been reported and presented superior
catalytic desorption performance for this purpose [2, 3]. To further advance
this promising method, in the present work, a composite catalyst MCM-41 (MFe) was
synthesized using a post synthesis of the combined impregnation ultrasound
method and used for the first time as a catalyst in a rich MEA solution
regeneration process. MCM-41 has the advantages of highly ordered porous
structure, suitable Lewis acid sites (LAS), larger surface area, and thermal
stability, but encounters lower B/L ratio and weak Brϕnsted acid sites
(BAS). The MCM-41 is modified by iron oxide (Fe₂O₃) will
generate more BAS and basic active sites. Potentially, the MFe catalyst may show
a superior catalytic CO₂ desorption performance for the CO₂-loaded
amine solvent regeneration process.

In
this work, MFe was prepared and used for the rich 5 M MEA solvent regeneration
process to increase the CO₂ desorption kinetics. Three MFe catalysts
with different Fe₂O₃ loading contents (5%, 10% and 15%) were
synthesized, and marked as MFe5%, MFe10% and MFe15%. The procedure of catalyst
preparation is based on Lan et al. [4], with a modification. All the catalysts
were measured with XRD, FT-IR, ICP, N₂ absorption, Py-IR, and CO₂/NH₃-TPD
techniques.

The
regeneration experiment was conducted using a batch reactor, which is reported in previous work [3]. However,
the mechanical agitator was replaced by a magnetic stirrer with a stirring speed of 500 rpm in this work.
Typically, 1 L rich MEA solutions with the desired
CO₂ loading (0.5 mol CO₂/mol amine) were added into a 2 L
four-necked flask, which was immersed in the oil bath. 12.5 g catalyst was put into the MEA solution. After the desorption
experiment started (initiating from 0 min after reaching the desorption temperature
of 333 K, and the final temperature was kept at 371 K), the regenerated amine
solution was taken to determine the CO₂ loading at 30, 60, 90, 120,
240, 360 and 480 min.

The
heat duty (HD, kJ/mol) for the CO₂ desorption process was monitored
by an electric energy meter, which was calculated by equation (1).

                    (1)

The
relative heat duty (RH, %) was calculated by equation (2).

                
                             (2)

where HD_baseline is
the HD of MEA without catalyst regeneration in first 90 min, while HD_i
is the HD of MEA with different catalysts regeneration in first 90min.

The
CO₂ desorption rates (DR, mol/min) are determined by using a linear fitting
the curves of CO₂ loadings against its corresponding times (at the
first 90 min), and the slope of the fitted curve is defined as the desorption
rate. 

Desorption factor (DF, mol³/(kJ.min)) was applied
to fully assess the catalytic CO₂ activity of the prepared catalysts.
Where, CC (mol) refers to the amount of desorbed CO₂ at the first 90
min.

   
                                                    (3)

The
XRD patterns and FT-IR spectra of MFe10% and MCM-41 are shown in Figure 1,
which certify that the MFe catalysts were successfully synthesized. The ICP
test results also indicate that the actual Fe loading of the MFe catalyst is
close to the normal value. The N₂ physisorption isotherms for all
catalysts are illustrated in Figure 2 (d). All the isotherms are type IV with a specific
hysteresis loop, representing typical mesoporous material characteristics. The
BET surface area and pore volume of the MFe catalysts are slightly decreased
with metal loading increasing up to 15%. However, the acid sites, BAS, B/L
ratio, and basic sites of the MFe catalysts are higher than that of the parent
MCM-41.

Figure 1.
Characterization results of various catalysts: (a) low angle XRD patterns, (b)
high angle XRD patterns, (c) FT-IR spectra, (d) N₂ physisorption
isotherms.

The
CO₂ desorption curves of the 5 M MEA solution with different
catalyst are displayed in Figure 2. These curves manifest the catalytic
performance of five catalysts in MEA solution for the whole 480 min duration conducted at 371 K. It
is noticed that the CO₂ loading decreased rapidly in the first 90
min after which the extent of CO₂ loading change was smaller. Thus,
all analyses for the catalytic performance of each MEA-catalyst system were
performed based on the first 90 min of the desorption processes. As shown in
Figure 2(a), all the catalysts quickened the CO₂ desorption rate
compared with the blank run. Three MFe catalysts demonstrated better catalytic
activity than the parent catalysts MCM-41.
It is important to highlight that the lean CO₂ loading of all the
MEA-catalyst systems tended to the same value at about 0.35 mol CO₂/mol
amine, implying that the catalyst only accelerated the desorption rate but did
not change the thermodynamic equilibrium. With regard to the RH, MFe10% offered the best catalytic activity, which decreased the
heat duty by 32.5% as compared with the blank run (Figure 2(b)). The desorption
factor of MFe10% catalyst is 337.3% higher than the catalyst-free test (Figure 2(c)).
Moreover, it can be seen
from Figure 2(d) that the MFe10% presented
good catalytic stability. Therefore, the MFe10% catalyst can be
considered as a potential catalyst for reducing the energy consumption for CO₂
desorption reaction and for use in industrial CO₂ capture processes.

Figure 2. Catalytic
CO₂ desorption performance: (a) CO₂ loading
change curves, (b) relative heat duty, (c) desorption factor, (d) relative heat
duty of cyclic tests.

The
structure-activity correlation of the
catalyst is investigated. The relationships between the BAS, basic sites and
the catalytic activity (i.e. DF) were explored, and the result is shown in
Figure 3. Note that the catalytic performance increased with increasing BAS.
This confirms that the BAS plays a significant role in its catalytic
performance. With regard to the basic sites, it is noticed that there is no
linear relationship between the basic sites and the catalytic performance
(Figure 3(b)). However, it is inferred that the enhanced catalytic performance
of composite catalysts is in relation to their improved basic sites compared with
the MCM-41 alone. These results demonstrate that the BAS and basic sites play a
predominant role in the catalytic CO₂ desorption activity. Therefore,
the enhanced catalytic performance of composite catalysts can be mainly
attributed to their improved BAS and basic sites. The surface area also has an important effect on the
catalytic performance. However, due to the large surface area of these
composite catalysts, the effect of this factor should be similar in the
catalytic amine solvent regeneration processes with these MFe catalysts.

Figure 3. The structure-activity correlation of the catalyst: (a) BAS, (b) basic sites.

Acknowledgment

The
financial support from the National Natural Science Foundation of China (NSFC-Nos.
21536003, 21706057, 21606078 and 51521006), China Scholarship Council
(201806130065) and Hunan
Provincial Innovation Foundation for Postgraduate (CX2018B154).

References

1. Liang, Z., et al., AIChE
Journal, 2016. 62(3): p. 753-765.

2. Zhang, X., et al., Applied
Energy, 2018. 218: p. 417-429.

3. Zhang, X., et al., AIChE
Journal, 2018. 64(11): p. 3988-4001.

4. Lan, B., et al., Chemical
Engineering Journal, 2013. 219: p. 346-354.