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Layered double hydroxides/multi-walled carbon nanotubes– based composite for high temperature CO adsorption 2
Lakshminarayana Kudinalli Gopalakrishna Bhatta, Seetharamu Subramanyam, Chengala D. Madhusoodana, Umananda Manjunatha Bhatta, Puspendu Guha, Raghavendra Prasad Havenje Dinakar, and Krishna Venkatesh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00141 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 7, 2016
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Layered Double Hydroxides/Multi-Walled Carbon Nanotubes–Based Composite for High
2
Temperature CO2 Adsorption
3
Lakshminarayana Kudinalli Gopalakrishna Bhatta,
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Madhusoodana D Chengala, Umananda Manjunatha Bhatta, Puspendu Guha, Raghavendra
5
Prasad Havenje Dinakar, Krishna Venkatesh
6 7 8 9 10
∗,†
§
†
†
‡
Seetharamu Subramanyam, †
£
†
Centre for Emerging Technologies, Jain University, Ramangaram District-562 112, Karnataka,
India ‡
§
£
Central Power Research Institute, Sir C. V. Raman Road, Bangalore-560 080, India Ceramic Technological Institute, BHEL, Bangalore-560 012, India Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, Odisha, India
11
ABSTRACT: Layered double hydroxides (LDH) derived mixed metal oxides (MMO) are
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considered as promising solid sorbents for CO2 capture in the temperatures range of 350–500 °C.
13
Accordingly, they find potential applications in the sorption enhanced water–gas shift (SEWGS)
14
process and in removal of CO2 from hot flue gas/syngas. Numerous strategies have been
15
explored to improve the CO2 capture property of LDH-derived MMO under the conditions of
16
intended applications. These strategies include novel sorbents by replacement of cations and
17
intercalation of organic anions on Mg–Al LDH, development of LDH based hybrid/composite
18
materials, optimization of synthesis conditions to control particle size, and method development
19
for different types of alkali impregnation. The present work involves synthesis of Mg–Al
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LDH/multi-walled carbon nanotubes (MWNTs) composite and explores its applicability for CO2
21
capture under dry conditions. Additionally, K2CO3 is impregnated onto the composite to study 1 ACS Paragon Plus Environment
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the effect of alkali promotion. The K2CO3-promoted Mg–Al LDH/MWNT composite exhibited a
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fresh adsorption capacity of 1.12 mmol g-1 at 300 °C under a total pressure of 1 bar. The
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enhanced CO2 sorption capacity of composites in comparison with their pristine counterparts can
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be attributed to improved particle dispersion. Further, K2CO3-promoted Mg–Al LDH/MWNT
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composite shows an average working capacity of 0.81 mmol g-1 over 10 cycles in the
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temperature range of 300 – 400 °C. The deactivation model provides excellent predictions of the
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experimental CO2 breakthrough curves obtained with various sorbents. The values of model
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parameters are comparable with those reported in literature.
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1. INTRODUCTION
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Climate change due to anthropogenic emission of greenhouse gases (GHGs) is a major global
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challenge of the present time. The surmised consequences of climate change such as extreme
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weather events, ecosystem degradation, and species extinctions have compelled the humankind
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to explore various GHG mitigation strategies. Among the many GHG mitigation strategies
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explored so far, the carbon dioxide capture and storage (CCS) technology is considered as a mid-
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term commercially viable option due to the fact that the fossil fuels will continue to be a major
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source of energy in foreseeable future. This technology mainly consists of three steps; CO2
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capture, transportation, and utilization/storage. Among these, CO2 capture is very expensive and
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hence considered as the most critical one. Absorption, membranes, cryogenic distillation
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systems, and adsorption are the four prominent techniques that are in either use or under
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development for CO2 separation and capture. Among these techniques, adsorption of CO2 on
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solid sorbents has been widely investigated as a means of an alternative to benchmark absorption
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technology, which is having many challenges.1 The increased research interest in solid sorbents
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can be noticed from the fact that there have been nearly more than 1500 new publications in this
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area from the year 2011 to 2014. The numerous classes of CO2 solid sorbents that have been
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reported in literature include zeolites, carbon based sorbents, metal organic frameworks (MOFs),
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metal oxides, and alkaline ceramics. Further, the derivatives, hybrids, and composites of these
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materials have also been investigated for enhanced CO2 capture.2 However, each of these
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materials presents its own intrinsic limitations, stipulating the strong need for the development of
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robust adsorbents that are capable of working under industrial relevant conditions.3
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The layered double hydroxides (LDHs), also known as hydrotalcites, are anionic basic clays,
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represented by the general formula (M12−+x M3x+ (OH)2 ) •(Anx −n mH2O ) ; where M2+ = Mg2+, Ni2+,
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Zn2+, Cu2+, or Mn2+; M3+ = Al3+, Fe3+, or Cr3+; An- = CO32 -, SO42-, NO3-, Cl- , or OH-, and x = 0.1
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- 0.5.4,5 Among numerous types of LDHs, the Mg–Al CO3 has been extensively investigated for
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CO2 capture. It is widely accepted that LDHs as such do not exhibit any CO2 adsorption capacity
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due to poor basicity and the presence of entities that hinder CO2 adsorption. Upon thermal
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treatment, they are transformed into nearly amorphous metastable mixed metal oxides (MMO)
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with a poorly defined 3D network. These LDH-derived MMO are known to adsorb CO2 at
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temperatures in the range of 350–500 °C and accordingly, identified as intermediate-temperature
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CO2 sorbents. Consequently, several investigations have demonstrated their applicability in the
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sorption enhanced water–gas shift (SEWGS) process for effective CO2 capture. They also find
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potential application in removal of CO2 from hot flue gas/syngas.6 In general, LDH-derived
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MMO present several advantages such as high CO2/N2 selectivity, easy regenerability, good
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hydrothermal stability, compatibility with SER catalysts, cost-effectiveness compared with
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lithium ceramics, and fast kinetics. The demerits of this sorbent system include low adsorption
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capacity (particularly at low CO2 partial pressure), low kinetics in comparison with
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physisorbents and poor mechanical stability during multicycle adsorption-desorption process.7
x+
x−
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Hence, in order to improve the performance of LDH-derived MMO under the conditions of
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intended applications, several strategies have been explored since the last two decades. Such
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efforts of current research can be summarized into following aspects: (1) novel sorbents by
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replacement of cations and intercalation of organic anions on Mg–Al LDH, (2) development of
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LDH based hybrid/composite materials, (3) optimization of synthesis conditions to control
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particle size, (4) method development for different types of alkali impregnation, and (5) further
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investigation on CO2 adsorption mechanism.7,8
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In the current work, adsorption of CO2 on Mg–Al LDH/multi-walled carbon nanotubes
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(MWNTs) composite has been investigated using a laboratory scale fixed bed reactor.
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Additionally, K2CO3 is impregnated onto the composite to study the effect of alkali promotion.
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In this regard, the following samples were synthesized and characterized using different
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techniques: neat Mg−Al LDH (MO), K2CO3-promoted Mg−Al LDH (KMO), Mg–Al
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LDH/MWNT composite (MOC), and K2CO3-promoted Mg–Al LDH/MWNT composite
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(KMOC).
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2. EXPERIMENTAL
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2.1. Materials. The chemicals used in this work were of analytical reagent (AR) grade and
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purchased from SD Fine-Chem Ltd. (India), except for aluminium nitrate nonahydrate, which
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was purchased from Merck (India). Long, CVD-grown, multi-walled carbon nanotubes
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(MWNTs) were procured from an Egyptian petroleum research institute (Egypt). Polycarbonate
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membranes were from Millipore (HTTP Isopore membrane). Double distilled water was used
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throughout the study.
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2.2. Synthesis of MO. The neat Mg–Al LDH with a Mg/Al atomic ratio of 1:1 was synthesized using the co-precipitation at low supersaturation method. In order to obtain about 4.0
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g of sample, 5.75 g of Mg(NO3)2·6H2O and 8.4 g of Al(NO3)3·9H2O were dissolved in 22.5 ml
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of water. Dropwise addition of this solution to an aqueous solution (15 ml) containing 4.24 g of
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Na2CO3 and simultaneous dropwise addition of another aqueous solution (17 ml) containing 6.4
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g of NaOH resulted in white gelatinous precipitate. During addition, the pH of resulting solution
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was continuously monitored using pH meter (Systronics, µ pH 361) and kept in the range of 10
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to 12. After aging the mixture at room temperature for 24 h with continuous stirring, the
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precipitate was separated by high-speed centrifugation and washed with water until the pH value
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reduced to 7. Finally, the sample was dried overnight at 120 °C and then calcined at 400 °C in air
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for 5 h to obtain mixed metal oxides.
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2.3. Oxidation of MWNTs. About 100 mg of MWNTs was dispersed in 20 ml of a 3:1
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mixture of concentrated H2SO4/HNO3 and sonicated for 30 min at room temperature. The
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mixture was stirred for 2 h at 60 °C, diluted with water, and cooled to room temperature.9 The
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mixture was then filtered using 0.4 µm polycarbonate membranes and washed with 200 ml of
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0.01 M NaOH. Finally, it was washed with water until the pH value reduced to neutral.
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2.4. Synthesis of MOC. In order to synthesize LDH/MWNT composite containing 70 wt%
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of LDH, 1.68 g of MWNTs was dispersed in an aqueous solution (32 ml) containing 6.4 g of
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NaOH and 4.24 g of Na2CO3 using ultrasonic liquid processor (Johson Plastosonic Pvt. Ltd). To
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this mixture, was added another aqueous solution (22.5 ml) containing 5.75 g of
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Mg(NO3)2·6H2O and 8.4 g of Al(NO3)3·9H2O. The resulting black suspension was aged at 60 °C
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for 12 h under vigorous stirring. The precipitate was then separated by high-speed centrifugation
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and washed with water until the pH value reduced to 7. The sample was dried at 120 °C and then
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calcined at 400 °C under inert N2 atmosphere.
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2.5. Synthesis of KMO and KMOC. Impregnation of nominal 20 wt% of K2CO3 onto MO
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was achieved by employing an incipient wetness method. An aqueous solution (15 ml)
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containing 1.0 g of K2CO3 was added to 5.0 g of MO to the point of incipient wetness. Decanting
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the excess liquid after 1 h, the resulting paste was dried at 120 °C for 16 h, followed by
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calcination at 400 °C for 4 h. The KMOC was synthesized in an analogous manner, but using
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MOC instead of MO and calcining the sample under inert N2 atmosphere.
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2.6. Characterization. The textural properties of samples were evaluated from N2
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adsorption–desorption isotherms obtained at 77 K with a NOVA 1000 Quanta Chrome high-
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speed gas sorption analyzer. Prior to the measurement, the samples were outgassed at 523 K for
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several hours. The specific surface areas (SSA) were calculated using Brunauer–Emmett–Teller
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(BET) relation applied to the adsorption branch of the isotherms in the range of relative
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pressures: 0.05–0.3. The total pore volume and their size distribution (PSD) were obtained by
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applying the Barrett–Joyner–Halenda (BJH) analysis to the isotherm desorption branch.
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Powdered X-ray diffraction (PXRD) patterns were recorded using Panalytical Xpertpro X-ray
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diffractometer with Cu Kα radiation (λ = 0.154 nm) in 2θ range 5o to 90o at 40 kV and a scanning
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rate of 2 min-1. The surface morphology of samples was analyzed using scanning electron
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microscope (SEM) images obtained on a Carl Zeiss, Neon 40 FESEM instrument. High-
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resolution transmission electron microscopy (HRTEM) images were obtained on a FEI Tecnai
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T20; operating at 200 keV equipped with EDAX EDS. EDX was used to identify the elemental
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composition of samples.
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2.7. CO2 adsorption measurement. A fixed-bed reactor (SS, 320 mm length, 10 mm
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internal diameter) was employed to perform the CO2 adsorption/desorption experiments. While
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adsorption was carried out at 300 °C, desorption proceeded at 400 °C. The reactor was loaded
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with a known quantity of calcined sample (~ 2.0 g) and positioned vertically inside in an electric
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furnace. The reactor is equipped with a K-type thermocouple for monitoring bed temperature and
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two pressure transducers; one at entrance and another at the exit of reactor. Both adsorption and
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desorption were performed at atmospheric pressure. The pressure drop across the bed was found
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to be negligible for all samples during the experiment. Prior to adsorption, the loaded sample was
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pre-treated at 400 °C for 1 h under helium flow (30 ml min-1) and cooled to 300 °C. A gas
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mixture of 10% CO2 balanced by helium was passed through the bed at a flow rate of 20 ml min-
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1
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and a portion of that gas was fed to gas chromatograph (Mayura, 1100) equipped with a thermal
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conductivity detector and a porapak Q column. The breakthrough capacity (qBT) and saturation
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adsorption capacity (qSA) of sample were evaluated using the data of CO2 concentration at exit,
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obtained from integrating the gas chromatograph peak area with necessary corrections for blank
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experiment. After attaining saturation, the sample was flushed with helium at a rate of 30 ml min-
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1
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estimate multicycle stability for selected samples. The experiment was repeated at least twice for
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first adsorption capacity to ascertain reproducibility of results.
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3. RESULTS AND DISCUSSION
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. Subsequently, the column effluent was cooled to near-ambient temperature in a heat exchanger
for 1 h followed by desorption at 400 °C. The adsorption and desorption were continued to
3.1. Synthesis of sorbents. MWNTs were functionalized using acid treatment in order to
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improve the purity and the solubility in water. Subsequent treatment with NaOH converts
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carboxylated carbonaceous fragments resulted from oxidation of MWNTs into their conjugate
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salts. Finally, the water washing removes other impurities leaving carboxylate sites on the
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MWNTs, which will increase the compatibility with LDH deposition.10 All the adsorbents were
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calcined at 400 °C under air/N2 atmosphere prior to adsorption as this temperature had been
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reported to be optimal for calcination of Mg−Al LDH.11–13 Alkali metal carbonates have been
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mainly used to improve the CO2 capture properties of different types of sorbents, including
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LDH-derived MMO, MgO, CaO, and alkaline ceramics. Potassium carbonate scores over other
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alkali metal carbonates as promoter in terms of thermodynamic/kinetic factors.14 It is reported
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that ~ 20 wt% of K2CO3 loading increases the sorption capacity of HTlcs due to an increase in
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the number of active sites on surface, despite reduction in surface area and pore volume.15–17 The
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exact mechanism of CO2 adsorption on LDH-derived MMO and optimal Mg/Al atomic ratio still
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remain unclear. Additionally, the Mg/Al atomic ratio of tested LDHs has been in the range of 1
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to 3.7,17–21 Investigating the MWNTs supported LDH with Mg/Al atomic ratio of 2:1, Garcia-
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Gallastegui et al.10 found that use of a MWNT support improved the absolute capacity and cycle
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stability of the hybrid adsorbent under dry conditions. The ionic interaction between the base-
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washed, negatively-charged, oxidized, MWNTs and positively-charged LDH platelets resulted in
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successful synthesis of hybrid. The optimum loading of MWNTs was in the range of 35 to 50
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wt%. However, the presence of large amount of inert material in sorbent could increase the bed
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volume due to low weight fraction of active material, leading to higher capital and operating
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costs.2 In the light of these aspects, 20 wt% K2CO3 promoted 30 wt% MWNTs supported LDH
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with Mg/Al atomic ratio of 1:1 was selected for the present study.
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3.2. Characterization. Figure 1 shows N2 adsorption–desorption isotherms of samples. In all
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cases, isotherms correspond to type IV of IUPAC classification of physisorption isotherms.
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Isotherm of this type is characteristic of mesoporous material with strong adsorbent-adsorbate
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interaction. The isotherm shows distinct hysteresis at higher values of P/P0, which is associated
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with capillary condensation taking place in mesopores. The hysteresis is closest in shape to the
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H3 type, corresponding to aggregates of plate-like particles giving rise to slit-shaped pores.
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While the absence of knee implies negligible micropore volume, the steep rise in the adsorbed
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volume of nitrogen at a relative pressure > 0.7 implies the broad pore size distribution.22 The
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volume adsorbed was much higher after supporting with MWNTs, indicating a higher total
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surface area. The SSA, BHJ pore diameter, and pore volume of adsorbents as well as MWNTs
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are presented in Table 1. The SSAs and pore volumes of MOC and KMOC were significantly
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lower than those of MWNTs. This can be attributed to effective loading of LDH-derived MMO
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onto the MWNTs surface. As expected, K2CO3 impregnation (KMO) resulted in the reductions
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of SSA and pore volume of MO due to blocking of pores. The pore size distributions (Figure 2)
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show similar distribution shape and modal pore-size values. Additionally, the average pore
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diameter (varied from 7.1 nm to 11.8 nm) of adsorbents indicates that pore diffusion should not
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be the rate-determining step for the subsequent CO2 adsorption.23
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Figure 1. N2 adsorption-desorption isotherms at 77 K 9 ACS Paragon Plus Environment
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Figure 2. Pore size distribution of adsorbents from BJH method
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The PXRD spectra of samples are shown in Figure 3. The reflection peak at 26o and 42o in the
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MWNTs sample are indexed to the (002) and (001) planes of graphitic carbon, respectively
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(JCPDS 23-0064). The broad peak reflections at ∼ 37°, 43°, and 62° for MO and KMO
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correspond to diffractions of periclase MgO from (111), (200), and (220) planes, respectively
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(JCPDS 43-1022), indicating smaller crystallite size of periclase phase. The spectra of MOC and
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KMOC predominantly display the diffraction pattern of MWNTs since it was the base material.7
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However, the more broadened characteristic peaks of LDH-derived MMO appeared in some
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angle ranges, indicating a good dispersion of MMO phase on MWNTs support.24,25
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Figure 3. PXRD diffraction patterns of adsorbents and MWNTs
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The electron microscope images of samples are shown in Figure 4. The surface of MO (Figure
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4a and 4c) appears to be highly porous with loosely connected nanostructures (10s and 100s of
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nm), resulting in inter particle voids. The SEM image of KMO (Figure 4b) shows flatter
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morpholgy, which is complemented by TEM image (Figure 4d). The image shows no
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interparticle voids but for a few light patches in between resulting from K2CO3 impregnation.
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This observation supports the results of N2 adsorption-desorption isotherm, in which reduction in
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SSA and pore volume was observed upon K2CO3 impregnation (Table 1). The TEM images of
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MOC and KMOC (Figure 4e and 4f) show MMO nano structures/clusters embedded in MWNTs
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network. Magnified images of MOC (Figure 5(a) and (b)) and KMOC (Figure 5(c) and (d)) show
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isolated MO particles/fragments attached to MWNTs. The EDX analysis confirmed the presence
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of Mg and Al in all samples. The presence of potassium in K2CO3-promoted samples and the
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presence of carbon in MWNTs-supported samples are also confirmed.
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Figure 4. SEM micrographs of (a) MO, (b) KMO; TEM micrographs of (c) MO, (d) KMO, (e)
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MOC, and (f) KMOC
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Figure 5. Magnified TEM images of MOC (a and b) showing small cluster of MO particles attached to MWNTs and larger fragments of KMO attached to MWNTS (c and d)
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3.3. CO2 sorption results. In order to determine the adsorption capacities of synthesized
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adsorbents, the dynamic column breakthrough (DCB) method was employed. This provides a
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laboratory-based adsorption experiment that can more closely match the conditions in an
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industrial-scale gas separation process than achievable with static adsorption experiments. The
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DCB method is more practical, representing the material characteristic in a packed bed under
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flow system. The representative dynamic adsorption data are particularly valuable to the gas
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separation by means of methods like pressure swing adsorption (PSA), vacuum swing adsorption
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(VSA), or temperature swing adsorption (TSA). Consequently, the data produced with the DCB
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method improve the prospects for reliable scale-up of the adsorption technology with an
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assessment of overall energy requirements of a cyclic operation. It is a necessary step towards
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the process development as it provides information about the macroscopic performance of the
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adsorption column.26–28 The amount of CO2 adsorbed on an adsorbent (q, mmol g-1) at a certain
241
time (t, min) has been estimated using the relation (1) after making necessary corrections for
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blank run
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q=
244
Where W = the weight of adsorbent (g); Q = the feed flow rate (normal ml min-1); Cin = the
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influent (vol%); and Cout = the effluent CO2 concentrations (vol%). Integrating the equation 1
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from t = 0 to breakthrough time when Cout reaches 10% of Cin gives the breakthrough capacity of
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adsorbent (qBT)29,30, while integrating the equation from t = 0 to saturation time (ts) when Cout
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reaches Cin gives the saturation capacity of adsorbent (qSA). The integral term in eq. 1 can be
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easily obtained by a graphical method. Figure 6 shows the experimental CO2 breakthrough
t
Q Cin C ( 1 − out ) dt ∫ W × 22.4 0 Cin
( 1)
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curves obtained with adsorbents at 300 °C, 1 bar. The breakthrough time is longest for KMOC,
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indicating highest sorption capacity since the breakthrough time is proportional to sorption
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capacity provided adsorption temperature and sorbent quantity were normalized. The efficiency
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of adsorption process depends on the overall dynamics of the fixed-bed reactor wherein mass
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transfer resistances play an important role. The width and shape of mass-transfer zone depend on
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the adsorption isotherm, flow rate, mass-transfer rate to the particles, and diffusion in the pores.
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For a narrow mass-transfer zone, the breakthrough curve is very steep and most of the bed
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capacity is used at the break point. In a typical fixed-bed reactor, the efficiency of adsorption
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process is determined by the overall dynamics of the system, in which mass transfer resistances
259
are important. The mass-transfer zone width and shape depends on the adsorption isotherm, flow
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rate, mass-transfer rate to the particles, and diffusion in the pores. For a narrow mass-transfer
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zone, the breakthrough curve is very steep and most of the bed capacity is used at the break
262
point. In the present study, the curves show steep shape at the breakthrough times, indicating
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narrow mass-transfer zone and no diffussional constraints.
264 265
Figure 6. Experimental CO2 breakthrough curves obtained with adsorbents at 300 °C, 1 bar
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The adsorption capacities of adsorbents, presented in Table 2, follow the order KMOC > MOC >
267
KMO > MO. The CO2 adsorption capacity of pristine MWNTs was found to be negligible. If
268
specific adsorption capacity (adsorption capacity per mass of LDH) is considered the increase in
269
capacity would be much more. However, adsorption capacity per mass of total adsorbent is a
270
more meaningful metric for practical applications. The enhanced capacities of LDH/MWNT
271
composites can be attributed to better contact between gas and surface of active material due to
272
improved particle dispersion. The improved dispersion of LDH is evident from characterization
273
results. As expected, the positive effect of K2CO3 impregnation on adsorption capacity is also
274
evident from the data. The increase in the sorption capacity of KMO compared with MO can be
275
ascribed to an increase in the number of active sites on surface, despite reduction in surface area
276
and pore volume (Table 1). However, the TEM images of MOC and KMOC (Figure 4e and 4f)
277
show predominantly MWNTs, which is also evident from sharp rise in C peak from the
278
corresponding EDX spectra. Hence, it can be reasonably deciphered that the optimal loading of
279
MWNTs might not be 30 wt% for LDH with Mg/Al atomic ratio of 1:1, providing scope for
280
future work.
281
3.4. Deactivation model for breakthrough analysis. The adsorption of CO2 on materials
282
investigated in the present study can be considered as a non-catalytic heterogeneous gas–solid
283
reaction.31,32 This type of reaction typically involves number of steps, including diffusion of CO2
284
molecules into surface and pores, chemisorption onto active sites, and consequent formation of
285
carbonate product layer. Subsequently, this product layer offers resistance to further diffusion of
286
CO2 molecules. One can also expect changes in pore structure, active surface area, and active site
287
distribution during the course of reaction. All these changes result in decrease in the activity of
288
the adsorbent with respect to time. Several studies have shown that the deactivation model can be
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successfully applied to non-catalytic heterogeneous gas–solid reaction to obtain kinetic
290
parameters.33–35 This model provides a better description of reaction kinetics in comparison with
291
other proposed models such as shrinking-core model and the homogeneous model.36 In the
292
deactivation model, a single activity term ‘a’ is employed to denote the decrease in reactivity and
293
textural changes of adsorbent with the reaction extent.
294
Among different forms of deactivation model, the one proposed by Yasyerli et al.37 has been
295
found to be more realistic since the model was modified to include the concentration dependence
296
of the deactivation term.38 According to this model, considering the pseudo steady state
297
assumption and neglecting the axial dispersion term, species conservation equation for the
298
packed column and the rate of change of activity of the solid reactant (a) can be expressed using
299
relations 2 and 3, respectively. dC A − ko a C A = 0 dW da − = kd C A a dt −Q
300
(2) (3)
301
Using an iterative procedure, the following approximate expression was obtained for the
302
breakthrough curve.
303
ko W 1 − exp ( − kd t ) ) exp ( − kd t ) ( 1 − exp CA Q = exp C Ao 1 − exp ( − kd t )
304
Where a = the activity of the solid sorbent, CA = effluent CO2 concentrations (vol%), CAo =
305
influent CO2 concentrations (vol%), Q = volumetric flow rate (cm3 min-1), ko = initial sorption
306
rate constant (cm3 min-1 g-1), kd = deactivation rate constant (min-1), W = weight of solid sorbent
307
(g), and t = time (min).
(4)
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The iterative non-linear least squares regression analysis of experimental breakthrough data of all
309
studied sorbents showed an excellent agreement with eq. 4. The model parameters along with
310
correlation coefficients are reported in Table 2. While the deactivation rate constant (kd) was
311
found to be highest for MO, KMOC exhibited highest value of initial sorption rate constant (ko).
312
The values of rate parameters for all adsorbents were found to be consistent with their adsorption
313
capacities. The predictions of breakthrough curves from eq. 4, using these rate parameters are
314
shown in Figure 7, together with the experimental data points. The excellent fit of the
315
experimental data with the deactivation model predictions indicates the decrease of activity of
316
the sorbent with time along with probable changes in textural characteristics of sorbents. Further,
317
it can be seen from Table 3 that the values of the rate constants from this study are almost the
318
same order of magnitude as those reported in the literature.
319 320 321
Figure 7. Experimental breakthrough curves (data points) and deactivation model predictions (lines)
322
3.5. Cyclic stability. A good sorbent should maintain a constant working capacity during
323
multiple operation of adsorption-desorption. High cyclic stability is always desirable as it
324
reduces the operational cost due to low frequency of sorbent replacement in large scale
325
operation. Studies have shown that alkali impregnation improves the cyclic stability of parent 17 ACS Paragon Plus Environment
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LDH as well as supported LDH at ordinary pressure in the temperature range of 200 − 450 °C.7,39
327
Figure 8 shows the normalized CO2 adsorption capacity on KMOC and MO for 10 cycles of
328
adsorption-desorption. The KMOC exhibited better cyclic stability than MO and maintained an
329
average working capacity of 0.81 mmol g-1 over 10 cycles in the temperature range of 300 – 400
330
°C. It has been thought that the dispersion of the LDH on the MWNTs support apparently
331
mitigated the sintering during calcination leading to a more active and stable material.10
332 333
Figure 8. Normalized CO2 adsorption capacities of MO and KMOC
334
4. CONCLUSION
335
The Mg–Al LDH/multi-walled carbon nanotubes (MWNTs) composite was synthesized using
336
the co-precipitation method. Additionally, K2CO3 was impregnated onto the composite to study
337
the effect of alkali promotion on CO2 adsorption. The synthesized materials were characterized
338
using N2 physisorption isotherms, PXRD, and electron microscopy techniques, which revealed
339
the successful synthesis of composite adsorbents. The LDH/MWNT composites exhibited better
340
CO2 sorption capacities than those of their counterparts. The enhanced capacity can be attributed
341
to better contact between gas and surface of active material due to improved particle dispersion.
342
The deactivation model provided a good prediction of the experimental CO2 breakthrough data. 18 ACS Paragon Plus Environment
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The values of the model parameters obtained are almost the same order of magnitude as those
344
reported in the literature. Further, the KMOC exhibited better cyclic stability than MO and
345
maintained an average working capacity of 0.81 mmol g-1 over 10 cycles in the temperature
346
range of 300 – 400 °C, apparently due to mitigation of sintering during calcination. The
347
possibility of further improvement in CO2 adsorption capacity of KMOC demands a deeper
348
study on optimization of MWNT.
349 350 351
AUTHOR INFORMATION Corresponding Author ∗
352 353 354 355 356 357
Phone: +91 80 2757 7200. E-mail:
[email protected],
[email protected] ACKNOWLEDGEMENTS
358
The authors would like to acknowledge Prof P. V. Satyam (IOP, Bhubaneswar) as well as and
359
Prof S. M. Shivaprasad (JNCASR, Bangalore) for helping in electron microscopy measurements,
360
and Mr. Omkar S Power, Jain University for assistance in conducting the experiments. The
361
authors also thank the authorities of Bangalore Institute of Technology and St. Joseph's College,
362
Bangalore for their help in characterization of samples (BET and PXRD).
363
REFERENCES
364
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Table 1. BET surface area and pore characteristics of samples
Wang, X. P.; Yu, J. J.; Cheng, J.; Hao, Z. P.; Xu, Z. P. Environ. Sci. Technol. 2008, 42,
Sample
Surface area (m2 g-1)
Pore volume (cm3 g-1)
Average Pore Diameter (nm)
MO
95.6
0.29
11.8
KMO
84.7
0.15
7.1
MOC
460.3
0.90
7.8
KMOC
431.9
0.93
8.6
MWNTs
587.2
1.71
11.6
429 430
Table 2. CO2 adsorption data and deactivation model parameters Adsorption capacity
Deactivation model parameters
(mmol g-1 of total mass of sorbent) Adsorbent
qBT
qSA
ko (cm3 min-1 g-1) a
kd (min-1) x 102 R2
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MO
0.30
0.45
85.02
54.56
0.9999
KMO
0.43
0.65
93.27
39.01
0.9999
MOC
0.53
0.85
108.00
33.68
0.9999
KMOC
0.84
1.12
126.07
25.01
0.9999
a
with temperature correction for the flow rate
431 432
Table 3. Comparison of deactivation model parameters Adsorbent
Feed gas (vol%)a
Sorption
ko (cm3 min-1 g-1)
kd (min-1)
Ref.
conditions MgAlO
8% CO2
350 °C, 1 bar
148.7
0.51
40
CoAlO
8% CO2
350 °C, 1 bar
162.6
0.68
40
CaAlO
8% CO2
350 °C, 1 bar
163.2
0.59
40
MgO/Al2O3
16% CO2
527 °C, 1 bar
195.3
0.68
41
Wood ash
10% CO2+12% H2O
100 °C
480
0.72
31
TiNT-TEPA
15% CO2
70 °C
672.4
1.67
42
MgO/Al2O3
8% CO2
150 °C, 1 bar
47.92
0.46
32
KMOC
10% CO2
300 °C, 1 bar
126.07
0.25
This study
a
balanced by N2/He.
433
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