(698g) Catalytic CO2 Desorption in CO2-Loaded Aqueous MEA Solution over so42-/ZrO2/?-Al2O3 Catalysts

Catalytic CO₂ desorption in CO₂-loaded
aqueous MEA solution over SO₄^2-/ZrO₂/gamma-Al₂O₃ catalysts

Xiaowen
Zhang ^a, Helei Liu^b, Jieling Hong ^a, Paitoon Tontiwachwuthikul ^a,b, and Zhiwu
Liang ^a*

^a 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

^b Clean Energy Technologies Research Institute (CETRI),
Faculty of Engineering and Applied Science, University
of Regina, Regina, Saskatchewan, S4S 0A2, Canada

Abstract

Greenhouse gases, especially carbon dioxide (CO₂),
have attracted significant focus as one of the predominant contributors to
climate change. Chemical absorption of CO₂ using amine solvents is the
most commercial mature technology that can be adopted for CO₂
capture. 5 M monoethanolamine (MEA) solvent is the baseline solvent and has
been the most frequently investigated. However, this solvent suffers from high
energy requirement for the rich solution regeneration process, resulting in its
immensely high operating costs of CO₂ capture.

A potential approach to greatly reduce energy
requirement was proposed by Idem et al. ¹. They demonstrated that
the addition of solid acid catalyst into the rich amine solution for CO₂
desorption is an effective method to decrease the heat duty. Several zeolite
molecular sieve catalysts such as, H-ZSM5 and gamma-Al₂O₃
have been studied and showed higher catalytic activity for this purpose. To
further develop this promising method, in the present work, a composite
catalyst SO₄^2-/ZrO₂ (SZ) supported on gamma-Al₂O₃
(SZA) was prepared by the precipitation-impregnation method and used for the
first time as a catalyst in a rich anime solvent regeneration process. SZ is a representative
solid superacid catalyst, which possesses the superior Brϕnsted acid sites
(BAS), but encounters small surface area and rapid deactivation. gamma-Al₂O₃
is an acid catalyst and the most extensively
employed support materials, and has large physical properties, strong Lewis
acid sites (LAS), and high thermal stability. Potentially, the composite
catalyst SZA may show a superior catalytic CO₂ desorption
performance for a CO₂-loaded amine solvent regeneration process.

In this work, SZA was prepared and applied for the
rich 5 M MEA solvent regeneration process to increase the CO₂
desorption performance. Three SZA catalysts with different ZrO₂ and gamma-Al₂O₃
mass ratios were synthesized. The procedure of catalyst preparation is based on
Hua et al. ², with a modification. The
SZA with ZrO₂/gamma-Al₂O₃ mass ratio of 0.5, 1, and 2 was marked as SZA1/2,
SZA1/1, and SZA2/1, respectively. All the catalysts were measured with
XRD, FT-IR, N₂ absorption, Py-IR, and NH₃-TPD techniques.

The regeneration experiment was conducted using a
batch reactor, which is displayed in
Figure 1. Typically, 1 L rich MEA solutions with the CO₂ loading of
0.5 mol CO₂/mol amine were introduced into a 2 L four-necked flask,
which was immersed in the oil bath. 12.5 g
catalyst was put into the MEA solution. The time was recorded until the amine
solvent temperature reached at 338 K. The final temperature of this
system was kept at 371K for 540 minutes. The CO₂ loadings of
solution at time of at 15, 30, 45, 60, 90, 120, 240, 360 and 540 minutes were
measured by Chittick apparatus.

Figure 1. Schematic of the batch reactor.

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 120 minutes,  while
HD_i is the HD of MEA with different catalysts regeneration in first 120minutes.

The CO₂ desorption rate (DR, mol/min) was
calculated by equation (3).

                                   
(3)

The XRD patterns and FT-IR spectra of these catalysts
are shown in Figure 2, which certify that the SZA catalysts were successfully
synthesized. The N₂ physisorption isotherms for all catalysts are
illustrated in Figure 2 Panel C. All the isotherms are type IV with a specific hysteresis loop, representing typical mesoporous
material characteristics. The structural
and acid properties of the five catalysts are summarized in Table 1. It can be
seen that the introduction of gamma-Al₂O₃ as a carrier
contributes to the increase of textural properties for SZ. For the acid sites,
the addition of gamma-Al₂O₃ enhanced both of weak acid
sites and strong acid sites of SZA catalysts, and the total acidity of SZA
catalysts increased with the increasing gamma-Al₂O₃
content. SZA1/1 possesses the greatest strong acid sites and total acid sites
as 1.105 and 2.317 mmol/g. Also, the BAS was greatly increased after loading gamma-Al₂O₃
support.

Figure 2. Characterization results
of various catalysts.

Table 1 Physicochemical properties of various catalysts.

a. Measured by NH₃-TPD.  

b. Measured by
Py-IR.

The CO₂ desorption curves of the 5 M MEA
solution with different catalyst are displayed in Figure 3. These curves
manifest the catalytic performance of five catalysts in MEA solution for the
whole 540 min duration
conducted at 371K. It is noticed that the CO₂ loading decreased
rapidly in the first 120 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 120 min of the desorption
processes. As shown in Figure 3 Panel
A£¬all the catalysts quickened the CO₂
desorption rate as compared with the blank run. Among these catalysts, three
SZA catalysts demonstrated better catalytic activity than the parent catalysts SZ or gamma-Al₂O₃. It is
important to highlight that the lean CO₂ loading of all the
different MEA-catalyst systems tended to the same value at 0.354 mol CO₂/mol
amine, implying that the catalyst only accelerated the desorption rate but did
not change the thermodynamic equilibrium. As
shown in Figure 3 Panel B, with regard to
the RH, SZA1/1 offered the best catalytic activity, which decreased the heat
duty by 36.9% as compared with the blank run. Moreover, the reusability of the
SZA was studied, and the result indicates that SZA presented good
catalytic stability. Therefore, the SZA catalysts 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 3. Catalytic CO₂ desorption performance.

The structure-activity correlation of the catalyst is investigated.
The relationship between the BAS, strong acid sites and the catalytic activity
were explored, and the result is shown in Figure 4. It can be seen that the
catalytic performance increased with increasing BAS. The same trend is observed
for strong acid sites as well. This confirms that the BAS and strong acid sites
of a catalyst play a significant role in its catalytic performance. Moreover,
because the mesopore surface area (MSA) of a catalyst affects the fraction of
the active sites available for reaction, the combinations effect of the product
of MSA and BAS (BAS*MSA) is
studied. As shown in Figure 4 Panel C, an increase of BAS*MSA results in an increase in CO₂
desorption rate as well as a decrease in the heat duty. This reveals that the
more BAS distributed on the catalyst surface, which are potentially provided by
the larger mesopore surface area, the better catalytic performance for the MEA
solvent regeneration process.

Figure 4. The structure-activity correlation of the catalyst

Acknowledgment

The financial support from the National Natural
Science Foundation of China (NSFC-Nos. 21536003, 21776065, 21476064, and
51521006), China Scholarship Council (201606135004), and China Outstanding
Engineer Training Plan for Students of Chemical Engineering & Technology in
Hunan University (MOE-No.2011-40) are gratefully
acknowledged. The authors also acknowledge support from the NPRP Grant #
7-1154-2-433 from the Qatar National Research Fund (a member of Qatar
Foundation). Furthermore, the authors gratefully acknowledge the support of K.C
WONG education foundation.

References

1. Shi H et al., International Journal
of Greenhouse Gas Control. 2014;26:39-50.

2. Hua W et al., Journal of Catalysis. 2001;197(2):406-413.