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mesoporous silica monoliths
Accepted Manuscript Modeling the performances of a CO2 adsorbent based on polyethyleniminefunctionalized macro-/mesoporous silica monoliths Nicola Gargiulo, Antonio Verlotta, Antonio Peluso, Paolo Aprea, Domenico Caputo PII:
S1387-1811(15)00294-2
DOI:
10.1016/j.micromeso.2015.05.025
Reference:
MICMAT 7134
To appear in:
Microporous and Mesoporous Materials
Received Date: 14 April 2015 Revised Date:
11 May 2015
Accepted Date: 12 May 2015
Please cite this article as: N. Gargiulo, A. Verlotta, A. Peluso, P. Aprea, D. Caputo, Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized macro-/mesoporous silica monoliths, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.05.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized
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macro-/mesoporous silica monoliths
Nicola Gargiulo, Antonio Verlotta, Antonio Peluso, Paolo Aprea, Domenico Caputo
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Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy
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Corresponding author: Nicola Gargiulo (e-mail: [email protected]; address: Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy; telephone:
Abstract
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+390817682392)
An adsorbent based on polyethylene oxide-templated silica monoliths with hierarchical
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uniform porosity was functionalized with polyethylenimine and was used as a substrate for CO2 adsorption. Such material, denoted PEI-MonoSil, was characterized by means
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of thermogravimetric analysis and N2 adsorption/desorption at 77 K, in order to prove that polymer chains efficiently filled the pores of functionalized samples. CO2 adsorption isotherms on PEI-MonoSil were evaluated at T = 298, 313, 333, and 348 K for pressures up to 100 kPa by means of a volumetric technique. CO2 adsorption data revealed a meaningful dependence of the CO2 adsorption capacity on temperature, with the highest capacity registered at 348 K. CO2 adsorption on PEI-MonoSil was convincingly modeled by means of the Toth isotherm. The comparison between the
ACCEPTED MANUSCRIPT results obtained in the present work and those relative to CO2 adsorption on other nanoporous substrates allowed to highlight a notable affinity of PEI-MonoSil towards CO2 and a high level of heterogeneity in the adsorbent / adsorbate interaction.
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Eventually, the modeling effort also allowed to evaluate the isosteric heat of CO2 adsorption as a function of the fractional coverage of PEI-MonoSil, showing values that are comparable with experimentally estimated values of the enthalpy of reaction
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between CO2 and monoethanolamine solutions.
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Keywords: Macro-/mesoporous silica monolith; Polyethylenimine; Carbon dioxide; Adsorption; Modeling
1. Introduction
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Carbon dioxide (CO2) likely is the most relevant air pollutant and the dominant contributor to the greenhouse effect. Although the attempt to reduce CO2 emissions has been the aim of both many scientific studies and political agreements, their level
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continues to increase [1]. CO2 emissions are mainly due to combustion processes: every time fossil fuels (such as gas, coal or oil) are burnt, carbon dioxide is released into the
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atmosphere. However, other industrial processes, such as hydrogen production, give also relevant contributions [2]. One possible approach to prevent CO2 emissions from such processes is to remove this species from the gaseous streams in which it is contained. CO2 removal from gaseous streams has been usually carried out by physical or chemical absorption, using aqueous solutions, such as amine solutions or potassium carbonate solutions. However, during the last years, many alternative technologies have been under investigation, trying to obtain better performances and smaller costs in the
ACCEPTED MANUSCRIPT CO2 removal process. One of the most important alternative technologies is represented by adsorption [3], commonly performed as pressure swing adsorption (PSA) or, less frequently, as vacuum swing adsorption (VSA). Adsorption-based technologies rely on
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the physical binding of gas molecules to adsorbent materials. The respective forces acting between the gas molecules and the adsorbent material depend on the gas
component and its partial pressure, on the type of adsorbent material and on the
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operating temperature. The materials more often considered for PSA/VSA processes
have been microporous adsorbents, which are characterized by pore sizes below 2 nm.
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Among these, both conventional adsorbing materials (such as silica−gel, activated alumina, and activated carbon) and alumino−silicates of the class of zeolites have been considered [3, 4].
Among zeolites, 13X zeolite, which is characterized by a relatively high surface
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basicity, is considered to be a very suitable adsorbent for CO2 capture by PSA processes [5]. However, in the last two decades, many research efforts have been performed to identify novel, more selective adsorbents, since their development has the potential to
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improve the performance of the adsorption process. As examples, metal organic frameworks (MOFs) revealed themselves to be good candidates for improving
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performances of adsorption-based CO2 removal processes [6-7]. In recent years, the use of immobilized amines [8-11] and, in particular, of polyethylenimine (PEI) stabilized on high-surface-area solids to adsorb carbon dioxide was proved to give very good results even in situations involving life support, such as the “Regenerable CO2 Removal System” (RCRS) of the Space Shuttle [12]. Starting from this concept, Xu et al. developed the so-called “molecular baskets”, which consist of mesoporous silicas functionalized with PEI chains [13]. This work led to a
ACCEPTED MANUSCRIPT very intense investigation activity on the CO2 adsorption properties of such materials in the past decade [14-25]. Among the different substrates studied for preparing molecular baskets, SBA-15
obviously taken into account [14-18, 20, 24, 25].
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mesoporous silica, that is one of the most extensively studied mesostructures so far, was
However, mesoporous materials are usually synthesized as powders, thus being subject
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to a decrease in their textural properties during aggregation/shaping processes. This could be a serious problem when the CO2 removal process has to be scaled up from
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research activity to a practical use. Indeed, for real world applications, it would be better to use substrates directly obtained as aggregate materials. From this point of view, PEIfunctionalized hierarchical macro-/mesoporous silica monoliths, produced by different synthesis routes, can be considered very promising materials for the adsorption of
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carbon dioxide and other acidic gases [26-28]. Nevertheless, literature does not provide an exhaustive thermodynamic description of CO2 adsorption on this kind of substrates. Indeed, when considering the design of an adsorption plant, finding a suitable model for
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the adsorption isotherms concerning the selected adsorbent/adsorbate couple de facto is the very kickoff to outline the packed column.
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For this reason, the aim of this study is the modeling of CO2 adsorption isotherms on a PEI-functionalized adsorbent based on polyethylene oxide-templated silica monoliths with hierarchical uniform porosity first synthesized by Sachse et al. [29] and known as MonoSil silica monoliths. The isotherms were obtained at four different temperatures between 298 and 348 K at pressures up to 100 kPa, and the experimental data were then modeled using the Toth equation to find the values of the isosteric heat of adsorption (i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal
ACCEPTED MANUSCRIPT change in the amount adsorbed) and other relevant parameters such as CO2 maximum adsorption capacity, adsorbent affinity toward CO2 and heterogeneity level of the adsorption process.
2.1. Preparation of the adsorbent (PEI-MonoSil)
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2. Experimental
Macro-/mesoporous silica monoliths with hierarchical uniform porosity (MonoSil) were
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synthesized following a literature recipe by Sachse et al. [29]: 46.3 g of H2O and 4.6 g of HNO3 (65 wt%) were mixed for 15 min at 273 K and 4.79 g of polyethylene oxide
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(PEO, 20 kDa, Aldrich) was added and stirred for 60 min. Then, 37.7 g of tetraethyl orthosilicate (TEOS, Aldrich) were added and the mixture stirred for 30 min. The resulting solution was poured into plastic tubes of 10 cm length and 8 mm internal diameter and was kept at 313 K for 3 days. The monoliths were then washed in water
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and treated in an ammoniac solution (0.1 M) at 313 K for 24 h. They were finally dried at 313 K for 24 h and calcined at 823 K for 8 h. Samples of PEI-functionalized MonoSil-based adsorbent (PEI-MonoSil) were obtained
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using a wet impregnation method similar to what reported by Serna-Guerrero et al. [30]: an amount of 0.2 g of PEI (Aldrich, Mw=800) was dissolved in methanol (Aldrich)
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under stirring for about 15 min and then 0.3 g of Monosil pieces with the size of 0.5-1.0 mm were added to the mixture under continuous stirring. The mixture was stirred overnight at ambient conditions until complete evaporation of the solvent. 2.2. Characterization of MonoSil and PEI-MonoSil MonoSil samples were characterized before modification by field-emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus instrument). Thermogravimetric (TG) analysis before and after modification was carried out with a Netzsch STA 409 Luxx
ACCEPTED MANUSCRIPT device using samples with masses of about 20 mg, which were heated in a nitrogen flow from ambient temperature up to 873 K at a rate of 10 K·min–1 and using α-alumina as reference. Microporosimetric characterization was carried out before and after
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modification by N2 adsorption/desorption cycles at 77 K using a Micromeritics ASAP 2020 volumetric instrument. The specific surface area was evaluated by means of the
BET method, the total mesopore volume was estimated from the N2 adsorbed amount at
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p/p0 = 0.99, and the pore size distribution was obtained by applying the BJH method to the adsorption branch of the isotherms. Prior to characterization, non-functionalized
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MonoSil was degassed at 473 K for 10 h, while PEI-MonoSil, in light of the low thermal stability that usually characterizes molecular baskets [14, 22, 25], was degassed at 348 K for 15 h.
2.3. CO2 adsorption on PEI-MonoSil
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CO2 adsorption isotherms on PEI-MonoSil at four different temperatures, namely, T = 298, 313, 333, and 348 K, and with p ≤ 100 kPa, were obtained using the aforementioned ASAP 2020 instrument. Since ASAP-series devices are meant to work
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at the boiling temperatures of noble/inert gases, the Dewar flask in which the sample tube is usually immersed was substituted by the ad hoc container reported in Figure 1,
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whose outer shell was filled with a flowing thermostatted fluid. Prior to each adsorption experiment, PEI-MonoSil was activated under high vacuum at 348 K for 15 h. Furthermore, in order to verify the reversibility of the adsorption process, the sample tube was weighed after every re-degassing step that followed the single adsorption run. 3. Results and discussion 3.1. Characterization of MonoSil and PEI-MonoSil
ACCEPTED MANUSCRIPT Figure 2 shows FE-SEM micrographs of MonoSil samples before functionalization, revealing a morphology quite similar to that reported by Sachse et al. [29]. Such observation also counts after the functionalization process (Supporting Information
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Figure S1). The macroporous skeleton clearly visible in Figure 2 hosts a mesopore network whose development is induced by the exposure to the basic environment of the 0.1 M ammoniac solution [29]. The presence of such mesopore network is revealed by
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the nitrogen adsorption/desorption isotherm at 77 K reported in Figure 3 together with the corresponding BJH pore size distribution. The shape of the isotherm can be
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distinctly classified as IUPAC type IV, that is characteristic of mesoporous materials [31]; furthermore, a H1-type hysteresis loop can be easily identified. The BET specific surface area came out to be 352 m2·g–1, while the total mesopore volume is 0.79 cm3·g-1, and the main peak of the BJH pore size distribution, as shown in Figure 3b, is
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centered around 15.8 nm.
Figure 4 reports the results of the TG analyses performed on non-functionalized MonoSil, PEI-MonoSil and bulk PEI. The weight loss of about 5% that occurs between
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ambient temperature and 373 K in non-functionalized MonoSil can be easily associated to the removal of adsorbed water vapor. In PEI-MonoSil, such weight loss partially
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overlaps with the first decomposition step of PEI, which ends at about 430 K [22]. Since non-functionalized MonoSil does not undergo any other significant weight loss over 373 K, the second weight loss of PEI-MonoSil, approximately occurring in the 450-650 K temperature range, can be ascribed to the second decomposition step of PEI [22, 25]. Owing that the weight loss at 873 K of non-functionalized MonoSil may be considered constant and not affected by the PEI loading into the pores, it is possible to evaluate the PEI content of PEI-MonoSil, PEI(%), by using the following formula:
ACCEPTED MANUSCRIPT % = % − %
(1)
where % and % are the percentage residual amount of neat and PEI-loaded silica at 873 K, respectively. Results show that the PEI content of PEI-
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MonoSil is about 40% wt/wt, thus corresponding to the PEI/silica weight ratio used during the impregnation process, that then proves to be very effective in loading PEI chains within the pores of MonoSil.
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In order to qualitatively highlight the insertion of PEI chains inside the pore network of MonoSil, a N2 adsorption/desorption cycle at 77 K, whose results are reported in Figure
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5a, was also performed on PEI-MonoSil: indeed, the registered isotherm is still classifiable as Type IV. After PEI loading, the BET specific surface area decreased to 42 m2·g–1, while the total mesopore volume decreased to 0.18 cm3·g–1, and the main peak of the BJH pore size distribution, as shown in Figure 5b, is still centered around
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15.8 nm. Such results are congruent with those reported by Chen et al. for similar materials used as H2S adsorbents [27].
3.2. CO2 adsorption on PEI-MonoSil
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Figure 6 shows CO2 adsorption isotherms on PEI-MonoSil at 298, 313, 333, and 348 K for CO2 pressures ranging from 0 to 100 kPa, together with fits to the Toth equation (see
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below). Figure 7 shows all the isotherms reported in Figure 6 on a single coordinate system, in which the abscissas are plotted on a logarithmic scale. It is considerable to denote that the re–weighing procedure depicted in Section 2.3 did not exhibit any significant weight change, hinting a basically reversible phenomenon. Moreover, comparing CO2 adsorption performances of PEI-MonoSil with those of MonoSil clearly revealed the effectiveness of the functionalization process (Supporting Information Figure S2).
ACCEPTED MANUSCRIPT Examination of Figure 6 shows that the CO2 adsorption capacity gradually increases as temperature increases, rather than decreasing, as usually observed in adsorption processes, including those concerning CO2 capture by activated carbons and zeolites.
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Remarkably, meso-structured monoliths functionalized with nonpolymeric amines are reported to behave similarly to PEI-MonoSil [32], for which the highest CO2 adsorption capacity, i.e., about 2.2 mol·kg–1, was observed at the highest temperature explored, i.e.,
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348 K. Surprisingly, this result is better than others recently reported in the literature for several powdered molecular baskets that host similar or even higher PEI quantities [25,
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33], and also for more traditional adsorbents, such as alkali metal exchanged FER zeolites with different Si/Al ratios, whose CO2 adsorption mechanisms are proved to be fairly different from those of PEI-functionalized nanoporous silicas [34]. Moreover, an accurate observation of Figure 7 reveals that adsorbed amounts in the very low pressure
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range are more pronounced at lower temperatures. Indeed, being adsorption an exothermic process, it is obviously better promoted by low temperatures. However, such trend seems totally subverted for pressures higher than 1 kPa. The explanation of
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this apparently anomalous behavior resides in the so-called “molecular basket” theory, for which the gradual increase of CO2 adsorbed amounts with temperature at pressures
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in the 1-100 kPa range can be explained by considering that, when temperature rises, PEI chains become more and more flexible, heading to an increase in the number of CO2-affine sites [35]. When CO2 molecules reach such sites, the process develops through the formation of ammonium carbamates as reported in the following reaction equations: CO2 + 2RNH2 → RNHCOO- + RNH3+
(2)
CO2 + 2R2NH → R2NCOO- + R2NH2+
(3)
ACCEPTED MANUSCRIPT CO2 + R2NH + R′NH2 → R2NCOO- + R′NH3+
(4)
In order to have a more precise understanding of the phenomena examined, a modeling effort was set out using the semiempirical three parameter Toth isotherm [36]. The Toth
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isotherm is a semiempirical model having significant thermodynamic consistency: indeed, it reduces to Henry’s law in the very low pressure region and has finite
saturation limit in the high pressure region. Moreover, its simple expression does not
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require the definition of the saturation pressure for the adsorbate, thus being suitable to process both subcritical and supercritical isotherms, even when a strong deviation from
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purely physical adsorption exists. According to this equation, the pressure dependence of the adsorbed amount takes the following form: =
! "#⁄!
(5)
where qmax, b, and t are model parameters, and in particular qmax represents the
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maximum adsorption capacity, b is the affinity constant, and t is a parameter that is usually less than unity and is said to characterize the system heterogeneity. Indeed, for t = 1, the Toth isotherm reduces to the Langmuir isotherm, which applies to
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homogeneous adsorbent–adsorbate systems. As reported by Do [37], Toth parameters are usually dependent on temperature. In
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particular, the dependence of the affinity constant on temperature is described by the following equation:
+
% = %& '() *, . - − 101 -
(6)
in which b0 is the value of b at a reference temperature T0, R is the gas constant, and Q is a measure of the heat of adsorption. The parameter t can take the following empirical functional form of temperature dependence [37]:
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2 = 2& + 4 .1 − - 0
(7)
where t0 is the value of t at T0 and α is a dimensionless parameter, having positive value. As regards the temperature dependence of qmax, whose nature is empirical as well [37], a
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functional form similar to that reported in Eq. (7) was chosen:
= ,& *1 + 6 .1 − - 01
(8)
In Eq. (8), qmax,0 is the value of qmax at T0 and χ is a dimensionless parameter, having
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positive value.
The experimental data concerning CO2 adsorption on PEI-MonoSil were submitted to
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nonlinear regression using the Origin Fitting Function Organizer, in order to calculate simultaneously the optimal values of the parameters of Eqs. (6-8), i.e., b0, Q, t0, α, qmax,0 and χ, for the isotherms reported in Figure 6. The computed values of the parameters, obtained using T0 = 348 K, are shown in Table 1, and the comparison between model
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and experimental results can be observed in Figures 6 and 7, in which symbols pertain to experimental data and continuous curves pertain to the best-fit Toth theoretical isotherms. Inspection of Figures 6 and 7 distinctly points out a good fit between model
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curves and experimental data, even in the very low pressure region. This is also proved
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by the value of the coefficient of determination R2 reported in Table 1.
ACCEPTED MANUSCRIPT Table 1. Toth parameters for CO2 adsorption on PEI-MonoSil Best fitting value
Standard error
b0 (kPa-1)
4,24
0.89
Q (kJ·mol-1)
144.94
7.27
t0
0.70
0.06
α
1.77
0.37
qmax,0 (mol)
2.20
χ
2.37
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Parameter
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0.04
0.10
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Coefficient of determination R2 = 0.986
It may be useful to compare the ambient temperature values of Toth parameters b and t, which are easily obtainable from Eqs. (6, 7) and Table 1, with those reported for CO2
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adsorption on other kinds of nanoporous substrates, as shown in Table 2.
ACCEPTED MANUSCRIPT Table 2. Values of Toth parameters b and t calculated at 298 K for CO2 adsorption on different kinds of nanoporous substrates b (kPa-1)
t
Reference
NaX zeolite
0.19
0.56
[38]
Microwave activated carbon
0.60·10-5
0.56
[39]
Mesoporous TiO2 / graphene oxide nanocomposite
1.41·10−2
0.66
[40]
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Adsorbent
1.90·104
0.41
This study
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PEI-MonoSil
It can be clearly noted how PEI-MonoSil is characterized by the highest value of the affinity constant among those listed in Table 2. Indeed, PEI-MonoSil can almost entirely exploit its adsorption capacity at very low CO2 pressures, while a noticeable
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fraction of the adsorption capacity of the other adsorbents reported in Table 2 can only be used at CO2 pressures higher than 100 kPa [38-40]. This is the reason why, even
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though molecular baskets have been proposed for CO2 capture from gaseous streams that are significantly rich in this species (e.g., flue gas) [41], they seem more well-suited
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for scenarios in which a thorough purification from CO2 of dilute gas mixtures is required [21].
As regards the heterogeneity parameter t, PEI-MonoSil exhibits the lowest value among those listed in Table 2, thus revealing that CO2 adsorption on this substrate is connoted by the highest heterogeneity level with respect to the other adsorbents reported in the same table. Indeed, CO2 adsorption on PEI-MonoSil develops through specific, chemisorption-like interactions between adsorptive molecules and active adsorption
ACCEPTED MANUSCRIPT sites represented by the amino groups of PEI chains, whereas, for example, CO2 adsorption on nanoporous carbon-based materials (especially when not doped with heteroatoms) is non-specific (physisorption-like), which makes these adsorbents
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relatively poor in CO2 affinity and selectivity [42]. Using the Toth isotherm, it was possible to elaborate an expression for the isosteric heat of adsorption, i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the
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infinitesimal change in the amount adsorbed, as a function of the fractional coverage of the adsorbent θ = q/qmax. According to Do [37], the isosteric heat of adsorption (∆H) can be computed from the van’t Hoff equation: = −.
: ;<
0
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∆8
, 9
:
(9)
=
After rewriting Eq. (5) in terms of p versus q, substituting Eqs. (6-8) into it and then taking the derivative of its natural logarithm with respect to T, the following expression
−∆> = ? −
, @
A
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for the isosteric heat of adsorption is obtained: ! B . CDE 0 B
! B . CDE 0 B
=CDE
F4GH .
=
0+
I@
L
=CDE @
K − GH M.
J I. - 0 J
@
=
0 − 1NO
(10)
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Eq. (10) can be written in terms of θ = q/qmax, thus leading to: , -
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−∆> = ? −
@
QR!
PQR! F
I@
J I. - 0 J
L
− 4GHSK − @ GHS @ − 1T
(11)
Figure 8 shows the plot of Eq. (11) for T = T0, thus using the best fitting values of the parameters Q, t0, α and χ reported in Table 1. Examination of Figure 8 points out that the isosteric heat of adsorption decreases as the fractional coverage of the adsorbent increases, becoming 0 for θ ≈ 0.9 and then resulting negative for higher values of θ, up to a negative vertical asymptote for θ = 1. This behavior perfectly agrees with what reported by Do for a simpler expression of the isosteric heat obtained from the Toth
ACCEPTED MANUSCRIPT model by keeping qmax constant with temperature [37]. What highlighted for very high values of the fractional coverage is an obvious limit in the physical consistency of this kind of equations, which can be considered reliable for values of θ far enough from
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unity [37]. The fact that ∆H is strongly lower than zero indicates the essentially exothermic nature of the adsorption process. This points out that the rather uncommon increase of the
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adsorption capacity with temperature does not rely on an (otherwise inexplicable)
endothermic adsorption process, but, by all means, on the increase of the number of
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active sites with temperature, thus confirming what previously envisaged by Xu et al. [13]
In the end, inspecting Figure 8 it can be remarked that, for low values of θ, the isosteric heat of adsorption is over 100 kJ·mol-1. Such a high amount of released energy is
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comparable with experimentally estimated values of the enthalpy of reaction between CO2 and monoethanolamine solutions [43], and is notably higher than what reported in literature for both zeolites [44] and other amine-modified silicas [45]. It is known that,
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on plant scale, fixed-bed adsorption is an essentially adiabatic operation, so the isosteric heat of adsorption is responsible for the temperature rise during the process. While this
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occurrence is detrimental for most adsorbents, for which an increase in temperature leads to a decrease in adsorption capacity, a careful management of the heat released by molecular baskets like PEI-MonoSil during CO2 adsorption can help keeping the higher-than-ambient temperature needed by such substrates to perform the best. 4. Conclusions With the aim to thoroughly describe the thermodynamics of CO2 adsorbing molecular baskets suitable for column operations, a PEI-functionalized adsorbent based on
ACCEPTED MANUSCRIPT polyethylene oxide-templated silica monoliths with hierarchical uniform porosity, denominated PEI-MonoSil, was successfully prepared and characterized. The main achievements of this study can be summarized as follows: though being pellet-shaped, PEI-MonoSil showed CO2 adsorption performances that are comparable to those of powdered similar substrates; •
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•
the CO2 adsorption capacity of PEI-MonoSil proved to be significantly dependent
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on temperature, with the highest capacity encountered at the highest temperature considered, i.e., T = 348 K;
CO2 adsorption isotherms on PEI-MonoSil at four different temperatures were
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•
successfully modeled by means of the Toth equation; •
the modeling effort allowed to highlight how, with respect to other adsorbents, PEIMonoSil shows a particularly high affinity towards CO2;
CO2 adsorption on PEI-MonoSil turned out to be connoted by a high heterogeneity
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•
level (chemisorption-like process); •
modeling results permitted to evaluate the isosteric heat of adsorption as a function
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of the fractional coverage of the PEI-functionalized adsorbent. The values that were found for the isosteric heat of adsorption are comparable with experimentally
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estimated values of the enthalpy of reaction between CO2 and monoethanolamine
solutions, and are notably higher than what reported in literature for CO2 adsorption on both zeolites and other amine-modified silicas.
Acknowledgements
The authors acknowledge the help provided by Dr. Ilaria Capasso (Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II) in performing TG analyses.
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ACCEPTED MANUSCRIPT [30] R. Serna-Guerrero, A. Sayari, Chem. Eng. J. 161 (2010) 182-190. [31] S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Kluwer Academic
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[35] W.-J. Son, J.-S. Choi, W.-S. Ahn, Microporous Mesoporous Mater. 113 (2008) 3140.
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ACCEPTED MANUSCRIPT [43] I. Kim, H.F. Svendsen, Ind. Eng. Chem. Res. 46 (2007) 5803-5809. [44] L. Grajciar, J. Čejka, A. Zukal, C. Otero Areán, G. Turnes Palomino, P. Nachtigall, ChemSusChem 5 (2012) 2011-2022.
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ACCEPTED MANUSCRIPT Figure captions Figure 1. Experimental setup for CO2 adsorption runs. Figure 2. Field emission scanning electron microscopy (FE-SEM) images of MonoSil
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obtained at different magnitudes: 655 X (a) and 1500 X (b). Figure 3. (a) N2 adsorption (circles)/desorption (squares) isotherm at 77 K and (b) BJH differential pore size distribution of MonoSil.
MonoSil (dashed curve) and PEI (dotted curve).
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Figure 4. Thermogravimetric (TG) analyses of MonoSil (continuous curve), PEI-
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Figure 5. (a) N2 adsorption (circles)/desorption (squares) isotherm at 77 K and (b) BJH differential pore size distribution of PEI-MonoSil.
Figure 6. CO2 adsorption isotherms on PEI-MonoSil at 298 K (a), 313 K (b), 333 K (c), and 348 K (d). Symbols: experimental data. Continuous lines: best fitting Toth
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theoretical isotherms.
Figure 7. CO2 adsorption isotherms on PEI-MonoSil at T = 298 K (circles), 313 K (squares), 333 K (diamonds), and 348 K (triangles). Continuous lines: best fitting Toth
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theoretical isotherms.
Figure 8. Isosteric heat of CO2 adsorption on PEI-MonoSil at 348 K as a function of the
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fractional coverage of the adsorbent.
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ACCEPTED MANUSCRIPT Highlights > We synthesized a CO2 adsorbent based on a macro-/mesoporous silica monolith. > CO2 adsorption isotherms at different temperatures were collected. > CO2 adsorption
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adsorbent showed very high affinity towards CO2.
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isotherms were well modeled by means of the Toth equation. > The synthesized
ACCEPTED MANUSCRIPT Supporting Information Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized macro/mesoporous silica monoliths Nicola Gargiulo, Antonio Verlotta, Antonio Peluso, Paolo Aprea, Domenico Caputo
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Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy
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Figure S1 shows a FE-SEM micrograph of the adsorbent based on polyethylenimine-functionalized macro/mesoporous silica monoliths (PEI-MonoSil): the substantial similarity of this image with what reported for non-functionalized MonoSil suggests that PEI chains filled the mesopore network hosted in the walls of the depicted macroporous skeleton.
Figure S1 FE-SEM micrograph of PEI-MonoSil
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Figure S2 shows CO2 adsorption isotherms on MonoSil and PEI-MonoSil at 298 K. Indeed, the functionalized sample performs significantly better than its purely siliceous counterpart in terms of CO2 adsorption capacity and affinity.
Figure S2 CO2 adsorption isotherms at 298 K on PEI-MonoSil (squares) and MonoSil (circles). Continuous lines: interpolating B-splines
S1387-1811(15)00294-2
DOI:
10.1016/j.micromeso.2015.05.025
Reference:
MICMAT 7134
To appear in:
Microporous and Mesoporous Materials
Received Date: 14 April 2015 Revised Date:
11 May 2015
Accepted Date: 12 May 2015
Please cite this article as: N. Gargiulo, A. Verlotta, A. Peluso, P. Aprea, D. Caputo, Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized macro-/mesoporous silica monoliths, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.05.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized
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macro-/mesoporous silica monoliths
Nicola Gargiulo, Antonio Verlotta, Antonio Peluso, Paolo Aprea, Domenico Caputo
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Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy
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Corresponding author: Nicola Gargiulo (e-mail: [email protected]; address: Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy; telephone:
Abstract
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+390817682392)
An adsorbent based on polyethylene oxide-templated silica monoliths with hierarchical
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uniform porosity was functionalized with polyethylenimine and was used as a substrate for CO2 adsorption. Such material, denoted PEI-MonoSil, was characterized by means
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of thermogravimetric analysis and N2 adsorption/desorption at 77 K, in order to prove that polymer chains efficiently filled the pores of functionalized samples. CO2 adsorption isotherms on PEI-MonoSil were evaluated at T = 298, 313, 333, and 348 K for pressures up to 100 kPa by means of a volumetric technique. CO2 adsorption data revealed a meaningful dependence of the CO2 adsorption capacity on temperature, with the highest capacity registered at 348 K. CO2 adsorption on PEI-MonoSil was convincingly modeled by means of the Toth isotherm. The comparison between the
ACCEPTED MANUSCRIPT results obtained in the present work and those relative to CO2 adsorption on other nanoporous substrates allowed to highlight a notable affinity of PEI-MonoSil towards CO2 and a high level of heterogeneity in the adsorbent / adsorbate interaction.
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Eventually, the modeling effort also allowed to evaluate the isosteric heat of CO2 adsorption as a function of the fractional coverage of PEI-MonoSil, showing values that are comparable with experimentally estimated values of the enthalpy of reaction
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between CO2 and monoethanolamine solutions.
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Keywords: Macro-/mesoporous silica monolith; Polyethylenimine; Carbon dioxide; Adsorption; Modeling
1. Introduction
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Carbon dioxide (CO2) likely is the most relevant air pollutant and the dominant contributor to the greenhouse effect. Although the attempt to reduce CO2 emissions has been the aim of both many scientific studies and political agreements, their level
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continues to increase [1]. CO2 emissions are mainly due to combustion processes: every time fossil fuels (such as gas, coal or oil) are burnt, carbon dioxide is released into the
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atmosphere. However, other industrial processes, such as hydrogen production, give also relevant contributions [2]. One possible approach to prevent CO2 emissions from such processes is to remove this species from the gaseous streams in which it is contained. CO2 removal from gaseous streams has been usually carried out by physical or chemical absorption, using aqueous solutions, such as amine solutions or potassium carbonate solutions. However, during the last years, many alternative technologies have been under investigation, trying to obtain better performances and smaller costs in the
ACCEPTED MANUSCRIPT CO2 removal process. One of the most important alternative technologies is represented by adsorption [3], commonly performed as pressure swing adsorption (PSA) or, less frequently, as vacuum swing adsorption (VSA). Adsorption-based technologies rely on
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the physical binding of gas molecules to adsorbent materials. The respective forces acting between the gas molecules and the adsorbent material depend on the gas
component and its partial pressure, on the type of adsorbent material and on the
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operating temperature. The materials more often considered for PSA/VSA processes
have been microporous adsorbents, which are characterized by pore sizes below 2 nm.
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Among these, both conventional adsorbing materials (such as silica−gel, activated alumina, and activated carbon) and alumino−silicates of the class of zeolites have been considered [3, 4].
Among zeolites, 13X zeolite, which is characterized by a relatively high surface
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basicity, is considered to be a very suitable adsorbent for CO2 capture by PSA processes [5]. However, in the last two decades, many research efforts have been performed to identify novel, more selective adsorbents, since their development has the potential to
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improve the performance of the adsorption process. As examples, metal organic frameworks (MOFs) revealed themselves to be good candidates for improving
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performances of adsorption-based CO2 removal processes [6-7]. In recent years, the use of immobilized amines [8-11] and, in particular, of polyethylenimine (PEI) stabilized on high-surface-area solids to adsorb carbon dioxide was proved to give very good results even in situations involving life support, such as the “Regenerable CO2 Removal System” (RCRS) of the Space Shuttle [12]. Starting from this concept, Xu et al. developed the so-called “molecular baskets”, which consist of mesoporous silicas functionalized with PEI chains [13]. This work led to a
ACCEPTED MANUSCRIPT very intense investigation activity on the CO2 adsorption properties of such materials in the past decade [14-25]. Among the different substrates studied for preparing molecular baskets, SBA-15
obviously taken into account [14-18, 20, 24, 25].
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mesoporous silica, that is one of the most extensively studied mesostructures so far, was
However, mesoporous materials are usually synthesized as powders, thus being subject
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to a decrease in their textural properties during aggregation/shaping processes. This could be a serious problem when the CO2 removal process has to be scaled up from
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research activity to a practical use. Indeed, for real world applications, it would be better to use substrates directly obtained as aggregate materials. From this point of view, PEIfunctionalized hierarchical macro-/mesoporous silica monoliths, produced by different synthesis routes, can be considered very promising materials for the adsorption of
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carbon dioxide and other acidic gases [26-28]. Nevertheless, literature does not provide an exhaustive thermodynamic description of CO2 adsorption on this kind of substrates. Indeed, when considering the design of an adsorption plant, finding a suitable model for
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the adsorption isotherms concerning the selected adsorbent/adsorbate couple de facto is the very kickoff to outline the packed column.
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For this reason, the aim of this study is the modeling of CO2 adsorption isotherms on a PEI-functionalized adsorbent based on polyethylene oxide-templated silica monoliths with hierarchical uniform porosity first synthesized by Sachse et al. [29] and known as MonoSil silica monoliths. The isotherms were obtained at four different temperatures between 298 and 348 K at pressures up to 100 kPa, and the experimental data were then modeled using the Toth equation to find the values of the isosteric heat of adsorption (i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal
ACCEPTED MANUSCRIPT change in the amount adsorbed) and other relevant parameters such as CO2 maximum adsorption capacity, adsorbent affinity toward CO2 and heterogeneity level of the adsorption process.
2.1. Preparation of the adsorbent (PEI-MonoSil)
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2. Experimental
Macro-/mesoporous silica monoliths with hierarchical uniform porosity (MonoSil) were
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synthesized following a literature recipe by Sachse et al. [29]: 46.3 g of H2O and 4.6 g of HNO3 (65 wt%) were mixed for 15 min at 273 K and 4.79 g of polyethylene oxide
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(PEO, 20 kDa, Aldrich) was added and stirred for 60 min. Then, 37.7 g of tetraethyl orthosilicate (TEOS, Aldrich) were added and the mixture stirred for 30 min. The resulting solution was poured into plastic tubes of 10 cm length and 8 mm internal diameter and was kept at 313 K for 3 days. The monoliths were then washed in water
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and treated in an ammoniac solution (0.1 M) at 313 K for 24 h. They were finally dried at 313 K for 24 h and calcined at 823 K for 8 h. Samples of PEI-functionalized MonoSil-based adsorbent (PEI-MonoSil) were obtained
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using a wet impregnation method similar to what reported by Serna-Guerrero et al. [30]: an amount of 0.2 g of PEI (Aldrich, Mw=800) was dissolved in methanol (Aldrich)
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under stirring for about 15 min and then 0.3 g of Monosil pieces with the size of 0.5-1.0 mm were added to the mixture under continuous stirring. The mixture was stirred overnight at ambient conditions until complete evaporation of the solvent. 2.2. Characterization of MonoSil and PEI-MonoSil MonoSil samples were characterized before modification by field-emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus instrument). Thermogravimetric (TG) analysis before and after modification was carried out with a Netzsch STA 409 Luxx
ACCEPTED MANUSCRIPT device using samples with masses of about 20 mg, which were heated in a nitrogen flow from ambient temperature up to 873 K at a rate of 10 K·min–1 and using α-alumina as reference. Microporosimetric characterization was carried out before and after
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modification by N2 adsorption/desorption cycles at 77 K using a Micromeritics ASAP 2020 volumetric instrument. The specific surface area was evaluated by means of the
BET method, the total mesopore volume was estimated from the N2 adsorbed amount at
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p/p0 = 0.99, and the pore size distribution was obtained by applying the BJH method to the adsorption branch of the isotherms. Prior to characterization, non-functionalized
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MonoSil was degassed at 473 K for 10 h, while PEI-MonoSil, in light of the low thermal stability that usually characterizes molecular baskets [14, 22, 25], was degassed at 348 K for 15 h.
2.3. CO2 adsorption on PEI-MonoSil
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CO2 adsorption isotherms on PEI-MonoSil at four different temperatures, namely, T = 298, 313, 333, and 348 K, and with p ≤ 100 kPa, were obtained using the aforementioned ASAP 2020 instrument. Since ASAP-series devices are meant to work
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at the boiling temperatures of noble/inert gases, the Dewar flask in which the sample tube is usually immersed was substituted by the ad hoc container reported in Figure 1,
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whose outer shell was filled with a flowing thermostatted fluid. Prior to each adsorption experiment, PEI-MonoSil was activated under high vacuum at 348 K for 15 h. Furthermore, in order to verify the reversibility of the adsorption process, the sample tube was weighed after every re-degassing step that followed the single adsorption run. 3. Results and discussion 3.1. Characterization of MonoSil and PEI-MonoSil
ACCEPTED MANUSCRIPT Figure 2 shows FE-SEM micrographs of MonoSil samples before functionalization, revealing a morphology quite similar to that reported by Sachse et al. [29]. Such observation also counts after the functionalization process (Supporting Information
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Figure S1). The macroporous skeleton clearly visible in Figure 2 hosts a mesopore network whose development is induced by the exposure to the basic environment of the 0.1 M ammoniac solution [29]. The presence of such mesopore network is revealed by
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the nitrogen adsorption/desorption isotherm at 77 K reported in Figure 3 together with the corresponding BJH pore size distribution. The shape of the isotherm can be
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distinctly classified as IUPAC type IV, that is characteristic of mesoporous materials [31]; furthermore, a H1-type hysteresis loop can be easily identified. The BET specific surface area came out to be 352 m2·g–1, while the total mesopore volume is 0.79 cm3·g-1, and the main peak of the BJH pore size distribution, as shown in Figure 3b, is
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centered around 15.8 nm.
Figure 4 reports the results of the TG analyses performed on non-functionalized MonoSil, PEI-MonoSil and bulk PEI. The weight loss of about 5% that occurs between
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ambient temperature and 373 K in non-functionalized MonoSil can be easily associated to the removal of adsorbed water vapor. In PEI-MonoSil, such weight loss partially
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overlaps with the first decomposition step of PEI, which ends at about 430 K [22]. Since non-functionalized MonoSil does not undergo any other significant weight loss over 373 K, the second weight loss of PEI-MonoSil, approximately occurring in the 450-650 K temperature range, can be ascribed to the second decomposition step of PEI [22, 25]. Owing that the weight loss at 873 K of non-functionalized MonoSil may be considered constant and not affected by the PEI loading into the pores, it is possible to evaluate the PEI content of PEI-MonoSil, PEI(%), by using the following formula:
ACCEPTED MANUSCRIPT % = % − %
(1)
where % and % are the percentage residual amount of neat and PEI-loaded silica at 873 K, respectively. Results show that the PEI content of PEI-
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MonoSil is about 40% wt/wt, thus corresponding to the PEI/silica weight ratio used during the impregnation process, that then proves to be very effective in loading PEI chains within the pores of MonoSil.
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In order to qualitatively highlight the insertion of PEI chains inside the pore network of MonoSil, a N2 adsorption/desorption cycle at 77 K, whose results are reported in Figure
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5a, was also performed on PEI-MonoSil: indeed, the registered isotherm is still classifiable as Type IV. After PEI loading, the BET specific surface area decreased to 42 m2·g–1, while the total mesopore volume decreased to 0.18 cm3·g–1, and the main peak of the BJH pore size distribution, as shown in Figure 5b, is still centered around
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15.8 nm. Such results are congruent with those reported by Chen et al. for similar materials used as H2S adsorbents [27].
3.2. CO2 adsorption on PEI-MonoSil
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Figure 6 shows CO2 adsorption isotherms on PEI-MonoSil at 298, 313, 333, and 348 K for CO2 pressures ranging from 0 to 100 kPa, together with fits to the Toth equation (see
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below). Figure 7 shows all the isotherms reported in Figure 6 on a single coordinate system, in which the abscissas are plotted on a logarithmic scale. It is considerable to denote that the re–weighing procedure depicted in Section 2.3 did not exhibit any significant weight change, hinting a basically reversible phenomenon. Moreover, comparing CO2 adsorption performances of PEI-MonoSil with those of MonoSil clearly revealed the effectiveness of the functionalization process (Supporting Information Figure S2).
ACCEPTED MANUSCRIPT Examination of Figure 6 shows that the CO2 adsorption capacity gradually increases as temperature increases, rather than decreasing, as usually observed in adsorption processes, including those concerning CO2 capture by activated carbons and zeolites.
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Remarkably, meso-structured monoliths functionalized with nonpolymeric amines are reported to behave similarly to PEI-MonoSil [32], for which the highest CO2 adsorption capacity, i.e., about 2.2 mol·kg–1, was observed at the highest temperature explored, i.e.,
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348 K. Surprisingly, this result is better than others recently reported in the literature for several powdered molecular baskets that host similar or even higher PEI quantities [25,
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33], and also for more traditional adsorbents, such as alkali metal exchanged FER zeolites with different Si/Al ratios, whose CO2 adsorption mechanisms are proved to be fairly different from those of PEI-functionalized nanoporous silicas [34]. Moreover, an accurate observation of Figure 7 reveals that adsorbed amounts in the very low pressure
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range are more pronounced at lower temperatures. Indeed, being adsorption an exothermic process, it is obviously better promoted by low temperatures. However, such trend seems totally subverted for pressures higher than 1 kPa. The explanation of
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this apparently anomalous behavior resides in the so-called “molecular basket” theory, for which the gradual increase of CO2 adsorbed amounts with temperature at pressures
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in the 1-100 kPa range can be explained by considering that, when temperature rises, PEI chains become more and more flexible, heading to an increase in the number of CO2-affine sites [35]. When CO2 molecules reach such sites, the process develops through the formation of ammonium carbamates as reported in the following reaction equations: CO2 + 2RNH2 → RNHCOO- + RNH3+
(2)
CO2 + 2R2NH → R2NCOO- + R2NH2+
(3)
ACCEPTED MANUSCRIPT CO2 + R2NH + R′NH2 → R2NCOO- + R′NH3+
(4)
In order to have a more precise understanding of the phenomena examined, a modeling effort was set out using the semiempirical three parameter Toth isotherm [36]. The Toth
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isotherm is a semiempirical model having significant thermodynamic consistency: indeed, it reduces to Henry’s law in the very low pressure region and has finite
saturation limit in the high pressure region. Moreover, its simple expression does not
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require the definition of the saturation pressure for the adsorbate, thus being suitable to process both subcritical and supercritical isotherms, even when a strong deviation from
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purely physical adsorption exists. According to this equation, the pressure dependence of the adsorbed amount takes the following form: =
! "#⁄!
(5)
where qmax, b, and t are model parameters, and in particular qmax represents the
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maximum adsorption capacity, b is the affinity constant, and t is a parameter that is usually less than unity and is said to characterize the system heterogeneity. Indeed, for t = 1, the Toth isotherm reduces to the Langmuir isotherm, which applies to
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homogeneous adsorbent–adsorbate systems. As reported by Do [37], Toth parameters are usually dependent on temperature. In
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particular, the dependence of the affinity constant on temperature is described by the following equation:
+
% = %& '() *, . - − 101 -
(6)
in which b0 is the value of b at a reference temperature T0, R is the gas constant, and Q is a measure of the heat of adsorption. The parameter t can take the following empirical functional form of temperature dependence [37]:
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2 = 2& + 4 .1 − - 0
(7)
where t0 is the value of t at T0 and α is a dimensionless parameter, having positive value. As regards the temperature dependence of qmax, whose nature is empirical as well [37], a
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functional form similar to that reported in Eq. (7) was chosen:
= ,& *1 + 6 .1 − - 01
(8)
In Eq. (8), qmax,0 is the value of qmax at T0 and χ is a dimensionless parameter, having
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positive value.
The experimental data concerning CO2 adsorption on PEI-MonoSil were submitted to
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nonlinear regression using the Origin Fitting Function Organizer, in order to calculate simultaneously the optimal values of the parameters of Eqs. (6-8), i.e., b0, Q, t0, α, qmax,0 and χ, for the isotherms reported in Figure 6. The computed values of the parameters, obtained using T0 = 348 K, are shown in Table 1, and the comparison between model
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and experimental results can be observed in Figures 6 and 7, in which symbols pertain to experimental data and continuous curves pertain to the best-fit Toth theoretical isotherms. Inspection of Figures 6 and 7 distinctly points out a good fit between model
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curves and experimental data, even in the very low pressure region. This is also proved
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by the value of the coefficient of determination R2 reported in Table 1.
ACCEPTED MANUSCRIPT Table 1. Toth parameters for CO2 adsorption on PEI-MonoSil Best fitting value
Standard error
b0 (kPa-1)
4,24
0.89
Q (kJ·mol-1)
144.94
7.27
t0
0.70
0.06
α
1.77
0.37
qmax,0 (mol)
2.20
χ
2.37
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Parameter
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0.04
0.10
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Coefficient of determination R2 = 0.986
It may be useful to compare the ambient temperature values of Toth parameters b and t, which are easily obtainable from Eqs. (6, 7) and Table 1, with those reported for CO2
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adsorption on other kinds of nanoporous substrates, as shown in Table 2.
ACCEPTED MANUSCRIPT Table 2. Values of Toth parameters b and t calculated at 298 K for CO2 adsorption on different kinds of nanoporous substrates b (kPa-1)
t
Reference
NaX zeolite
0.19
0.56
[38]
Microwave activated carbon
0.60·10-5
0.56
[39]
Mesoporous TiO2 / graphene oxide nanocomposite
1.41·10−2
0.66
[40]
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Adsorbent
1.90·104
0.41
This study
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PEI-MonoSil
It can be clearly noted how PEI-MonoSil is characterized by the highest value of the affinity constant among those listed in Table 2. Indeed, PEI-MonoSil can almost entirely exploit its adsorption capacity at very low CO2 pressures, while a noticeable
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fraction of the adsorption capacity of the other adsorbents reported in Table 2 can only be used at CO2 pressures higher than 100 kPa [38-40]. This is the reason why, even
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though molecular baskets have been proposed for CO2 capture from gaseous streams that are significantly rich in this species (e.g., flue gas) [41], they seem more well-suited
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for scenarios in which a thorough purification from CO2 of dilute gas mixtures is required [21].
As regards the heterogeneity parameter t, PEI-MonoSil exhibits the lowest value among those listed in Table 2, thus revealing that CO2 adsorption on this substrate is connoted by the highest heterogeneity level with respect to the other adsorbents reported in the same table. Indeed, CO2 adsorption on PEI-MonoSil develops through specific, chemisorption-like interactions between adsorptive molecules and active adsorption
ACCEPTED MANUSCRIPT sites represented by the amino groups of PEI chains, whereas, for example, CO2 adsorption on nanoporous carbon-based materials (especially when not doped with heteroatoms) is non-specific (physisorption-like), which makes these adsorbents
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relatively poor in CO2 affinity and selectivity [42]. Using the Toth isotherm, it was possible to elaborate an expression for the isosteric heat of adsorption, i.e., the ratio of the infinitesimal change in the adsorbate enthalpy to the
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infinitesimal change in the amount adsorbed, as a function of the fractional coverage of the adsorbent θ = q/qmax. According to Do [37], the isosteric heat of adsorption (∆H) can be computed from the van’t Hoff equation: = −.
: ;<
0
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∆8
, 9
:
(9)
=
After rewriting Eq. (5) in terms of p versus q, substituting Eqs. (6-8) into it and then taking the derivative of its natural logarithm with respect to T, the following expression
−∆> = ? −
, @
A
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for the isosteric heat of adsorption is obtained: ! B . CDE 0 B
! B . CDE 0 B
=CDE
F4GH .
=
0+
I@
L
=CDE @
K − GH M.
J I. - 0 J
@
=
0 − 1NO
(10)
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Eq. (10) can be written in terms of θ = q/qmax, thus leading to: , -
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−∆> = ? −
@
QR!
PQR! F
I@
J I. - 0 J
L
− 4GHSK − @ GHS @ − 1T
(11)
Figure 8 shows the plot of Eq. (11) for T = T0, thus using the best fitting values of the parameters Q, t0, α and χ reported in Table 1. Examination of Figure 8 points out that the isosteric heat of adsorption decreases as the fractional coverage of the adsorbent increases, becoming 0 for θ ≈ 0.9 and then resulting negative for higher values of θ, up to a negative vertical asymptote for θ = 1. This behavior perfectly agrees with what reported by Do for a simpler expression of the isosteric heat obtained from the Toth
ACCEPTED MANUSCRIPT model by keeping qmax constant with temperature [37]. What highlighted for very high values of the fractional coverage is an obvious limit in the physical consistency of this kind of equations, which can be considered reliable for values of θ far enough from
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unity [37]. The fact that ∆H is strongly lower than zero indicates the essentially exothermic nature of the adsorption process. This points out that the rather uncommon increase of the
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adsorption capacity with temperature does not rely on an (otherwise inexplicable)
endothermic adsorption process, but, by all means, on the increase of the number of
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active sites with temperature, thus confirming what previously envisaged by Xu et al. [13]
In the end, inspecting Figure 8 it can be remarked that, for low values of θ, the isosteric heat of adsorption is over 100 kJ·mol-1. Such a high amount of released energy is
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comparable with experimentally estimated values of the enthalpy of reaction between CO2 and monoethanolamine solutions [43], and is notably higher than what reported in literature for both zeolites [44] and other amine-modified silicas [45]. It is known that,
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on plant scale, fixed-bed adsorption is an essentially adiabatic operation, so the isosteric heat of adsorption is responsible for the temperature rise during the process. While this
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occurrence is detrimental for most adsorbents, for which an increase in temperature leads to a decrease in adsorption capacity, a careful management of the heat released by molecular baskets like PEI-MonoSil during CO2 adsorption can help keeping the higher-than-ambient temperature needed by such substrates to perform the best. 4. Conclusions With the aim to thoroughly describe the thermodynamics of CO2 adsorbing molecular baskets suitable for column operations, a PEI-functionalized adsorbent based on
ACCEPTED MANUSCRIPT polyethylene oxide-templated silica monoliths with hierarchical uniform porosity, denominated PEI-MonoSil, was successfully prepared and characterized. The main achievements of this study can be summarized as follows: though being pellet-shaped, PEI-MonoSil showed CO2 adsorption performances that are comparable to those of powdered similar substrates; •
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•
the CO2 adsorption capacity of PEI-MonoSil proved to be significantly dependent
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on temperature, with the highest capacity encountered at the highest temperature considered, i.e., T = 348 K;
CO2 adsorption isotherms on PEI-MonoSil at four different temperatures were
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•
successfully modeled by means of the Toth equation; •
the modeling effort allowed to highlight how, with respect to other adsorbents, PEIMonoSil shows a particularly high affinity towards CO2;
CO2 adsorption on PEI-MonoSil turned out to be connoted by a high heterogeneity
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•
level (chemisorption-like process); •
modeling results permitted to evaluate the isosteric heat of adsorption as a function
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of the fractional coverage of the PEI-functionalized adsorbent. The values that were found for the isosteric heat of adsorption are comparable with experimentally
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estimated values of the enthalpy of reaction between CO2 and monoethanolamine
solutions, and are notably higher than what reported in literature for CO2 adsorption on both zeolites and other amine-modified silicas.
Acknowledgements
The authors acknowledge the help provided by Dr. Ilaria Capasso (Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II) in performing TG analyses.
ACCEPTED MANUSCRIPT References [1] D.G. Victor, D. Zhou, E.H.M. Ahmed, P.K. Dadhich, J.G.J. Olivier, H-H. Rogner, K. Sheikho, M. Yamaguchi, in: O. Edenhofer, R. Pichs-Madruga, Y. Sokona, E.
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ACCEPTED MANUSCRIPT Figure captions Figure 1. Experimental setup for CO2 adsorption runs. Figure 2. Field emission scanning electron microscopy (FE-SEM) images of MonoSil
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obtained at different magnitudes: 655 X (a) and 1500 X (b). Figure 3. (a) N2 adsorption (circles)/desorption (squares) isotherm at 77 K and (b) BJH differential pore size distribution of MonoSil.
MonoSil (dashed curve) and PEI (dotted curve).
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Figure 4. Thermogravimetric (TG) analyses of MonoSil (continuous curve), PEI-
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Figure 5. (a) N2 adsorption (circles)/desorption (squares) isotherm at 77 K and (b) BJH differential pore size distribution of PEI-MonoSil.
Figure 6. CO2 adsorption isotherms on PEI-MonoSil at 298 K (a), 313 K (b), 333 K (c), and 348 K (d). Symbols: experimental data. Continuous lines: best fitting Toth
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theoretical isotherms.
Figure 7. CO2 adsorption isotherms on PEI-MonoSil at T = 298 K (circles), 313 K (squares), 333 K (diamonds), and 348 K (triangles). Continuous lines: best fitting Toth
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theoretical isotherms.
Figure 8. Isosteric heat of CO2 adsorption on PEI-MonoSil at 348 K as a function of the
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fractional coverage of the adsorbent.
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ACCEPTED MANUSCRIPT Highlights > We synthesized a CO2 adsorbent based on a macro-/mesoporous silica monolith. > CO2 adsorption isotherms at different temperatures were collected. > CO2 adsorption
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adsorbent showed very high affinity towards CO2.
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isotherms were well modeled by means of the Toth equation. > The synthesized
ACCEPTED MANUSCRIPT Supporting Information Modeling the performances of a CO2 adsorbent based on polyethylenimine-functionalized macro/mesoporous silica monoliths Nicola Gargiulo, Antonio Verlotta, Antonio Peluso, Paolo Aprea, Domenico Caputo
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Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università Federico II, P.le Tecchio 80, 80125 Naples, Italy
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Figure S1 shows a FE-SEM micrograph of the adsorbent based on polyethylenimine-functionalized macro/mesoporous silica monoliths (PEI-MonoSil): the substantial similarity of this image with what reported for non-functionalized MonoSil suggests that PEI chains filled the mesopore network hosted in the walls of the depicted macroporous skeleton.
Figure S1 FE-SEM micrograph of PEI-MonoSil
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Figure S2 shows CO2 adsorption isotherms on MonoSil and PEI-MonoSil at 298 K. Indeed, the functionalized sample performs significantly better than its purely siliceous counterpart in terms of CO2 adsorption capacity and affinity.
Figure S2 CO2 adsorption isotherms at 298 K on PEI-MonoSil (squares) and MonoSil (circles). Continuous lines: interpolating B-splines