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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Polyethylenimine-Based Solid Sorbents for CO2 Adsorption: Performance and Secondary Porosity Yuan Zhao,† Yidi Zhu,‡ Tianle Zhu,*,† Guiping Lin,§ Mingpan Shao,† Wei Hong,† and Shiyu Hou† †

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School of Space and Environment, Beijing Key Laboratory of Bio-Inspired Energy Materials and Devices, Beihang University, Beijing 100191, China ‡ Institute of electrical engineering, Chinese academy of sciences, Beijing 100190, China § School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China ABSTRACT: The series of polyethylenimine (PEI)-based solid sorbents were prepared by the wet-impregnation of PEI (Mw = 600) on the surface of six porous materials, i.e., mesoporous silicas (MCM-41, SBA-15, and KIT-6), mesoporous carbon (CMK-3), protonated titanate nanotube (PTNT), and fumed silica (FS). The resultant materials, denominated PEI/supports, were characterized by N 2 adsorption/desorption at 77 K, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). CO2 adsorption on the PEI/supports was evaluated using 2 vol % CO2 in flowing air on a self-assembled fixed bed reactor. The observations made here show that relative to the other four sorbent counterparts, PEI/PTNT and PEI/FS exhibit better performance of CO2 adsorption, mainly attributed to their more abundant secondary macropores, together with more homogeneous PEI surface dispersion. For all the PEI/ supports, the adsorption data of CO2 at various pressure equilibriums were modeled by the Langmuir dual-site isotherms. And the values of the fitted model parameters offer some hints that both chemisorption and physisorption are synchronously involved in these sorbent systems, one of which will govern the CO2 adsorption process, dependent on the nature of the supports used.

1. INTRODUCTION

Aeronautics and Space Administration (NASA) to remove CO2 from the crew compartment.13 In recent years, much research done has unanimously confirmed the fact that in addition to the nature of the amine phase, textural properties of supports (e.g., surface area, pore volume, and pore size) also play a crucial role in determining the performance of CO2 adsorption over amine-based solid sorbents. Therefore, the capture and sequestration of CO2 have been extensively investigated over a broad variety of porous solid materials. Many researchers have demonstrated that PEI-based SBA-15 mesoporous silica sorbents exhibited a linearly increased CO2 adsorption capacity with the total pore volume of the bare SBA-15 as support whereas for the same PEI phase, relatively high CO2 adsorption capacities were achieved from mesoporous silicas (KIT-6, silica monolith, and HMS) in view of their 3-D mesopores and/or hierarchical pore structures, as compared to mesoporous analogs (MCM-41, MCM-48, and SBA-15).14−16 The results obtained in the work15 show that PEI-based solid sorbents yielded from MCM-41, MCM-48, SBA-15, SBA-16, and KIT-6 gave an

To better maintain the environmental control and life support in a space craft or a space station in orbit, significant attention has long been paid to the removal of the inorganic metabolite CO2 generated by astronauts onboard. Heretofore, the literature in this regard has been replete with publications largely concerning amine-based solid sorbents for CO2 adsorption.1−10 And the emphasis has been placed on the preferred selection of amine phases and their supports in order to get the desirable capacity of CO2 adsorption over the optimized combination of these two components, sometimes together with other added dopants.11,12 Now, two preparative techniques, i.e., grafting and wet impregnation, are generally adopted for the preparation of such amine-based sorbents, which yield the sorbents with an amine phase covalently bound to or physically coated on the surface of the support, respectively. As far as the amine phase is concerned, PEI has been considered to be one of the most favorable candidates for these amine-based solid sorbents on account of its favorable characteristics such as sufficiently low volatility and high CO2 absorption capacity. For example, Hamilton Sunstrand Space Systems International (HSSSI) developed a regenerable PEIbased sorbent (denoted HSC+) with PEI bonded to an organic polymeric support, which was then used by the National © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

May 14, 2019 July 22, 2019 August 5, 2019 August 5, 2019 DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of the experimental setup.

maintained until methanol evaporated away completely. Finally, the resultant PEI/support samples were dried at 80 °C for 12 h under vacuum. 2.2. Characterization of PEI/Supports. N2 adsorption− desorption measurements were performed at 77 K with a Micromeritics Tristar II 3020 instrument, in which the surface areas were calculated by the Brunauer−Emmett−Teller (BET) method whereas the pore-size distributions were determined by the Barrett−Joyner−Halenda (BJH) model. SEM images were obtained on a Hitachi SU8010 scanning electron microscope. FT-IR data were collected on a Thermo Scientific Nicolet iS10 instrument. XPS data were determined by a PHI Quantro SXM ULVAC-PHI apparatus using an Al Kα (1486.7 eV) X-ray source. MIP data were obtained from a Micromeritics AutoPore IV 9500 V1.09 instrument. 2.3. Evaluation for CO2 Adsorption over PEI/Supports. CO2 adsorption over PEI/supports was investigated on a fixed-bed sorption system, as shown in Figure 1. In a typical sorption run, 0.8 g of sorbent (sieved to −40/+60 mesh) was packed in the U-shaped quartz tube reactor, which was placed in a programmed furnace to achieve the desired temperature. Prior to each measurement, the sorbent was heated at 80 °C for 40 min to eliminate any H2O and CO2 adsorbed from the air and then cooled down to 25 °C, both in 100 mL/min highly pure Ar flow rate (99.999%). After that, the gas stream of 2 vol % CO2 in air with a 30% relative humidity was introduced into the reactor in a 40 mL/min flow rate. The moisture was produced by bubbling air into water, and the relative humidity was measured by using a hygrometer. The flow rates of CO2, water vapor, and air were adjusted by three mass flow controllers (Beijing Sevenstar Electronics Co., Ltd., D07-7 type) to gain the given concentration of CO2 in the gas phase. The inlet and outlet CO2 concentrations of the reactor were monitored by an online gas chromatograph (Techcomp, GC-7890II) equipped with a methane converter and a flame ionization detector. The CO2 adsorption capacity of a PEI/ support sorbent was determined from eq 1 as below, ÄÅ t É Å C − C ÑÑÑ T0 1 1 qs = × ÅÅÅÅ Q× 0 dt ÑÑÑ × × m ÅÅÇ 0 1 − C ÑÑÖ T Vm (1)

equilibrium capacity of CO2 adsorption which was increased with the average pore diameter of supports. Moreover, Goeppert et al.17 found that the particle size of supports can likewise exert an important influence on the performance of CO2 adsorption over such PEI-based solid sorbents, i.e., porous nanosilicas as supports bearing an apparent dimensional advantage over other forms of silicas. In this work, PEI (Mw = 600) as amine phase was impregnated on six types of porous solid materials, namely, MCM-41, SBA-15, KIT-6, CMK-3, PTNT, and FSs (FS1, SBET = 200 m2/g, hydrophilic; FS2, SBET = 400 m2/g, hydrophilic; FS3, SBET = 200 m2/g, hydrophobic), respectively. And the performance of CO2 adsorption was investigated in detail with these PEI-based solid sorbents. With respect to porous solid materials used here, MCM-41, SBA-15, and KIT-6 constitute three traditional mesoporous silicas, which have been widely employed as supports in this regard due to their high surface area, tunable structure, and uniform pores;18−21 PTNT22,23 and FS17,24,25 usually present themselves as agglomerates with plentiful secondary pores and exhibit excellent performance as supports for CO2 adsorption; as for porous carbon materials, several studies have been reported hitherto for CO2 adsorption over carbon-supported PEI sorbents with PEI loaded on mesoporous carbons and carbon nanotubes.26−28 Obviously, the results based on the comparison of the above supports would provide a better understanding of the function manifested by the supports and establish a good basis for the design of optimized support materials and amine-based sorbent systems for CO2 adsorption.

2. EXPERIMENTAL SECTION 2.1. Preparation of PEI/Supports. All chemicals were reagent grade and used without further purification. The porous solid materials as supports, MCM-41, SBA-15, and KIT-6 were obtained from Nanjing Xianfeng Nano Lim. Co., CMK-3 from Nanjing Jicang Nano Lim. Co., as well as three fumed silicas (FS1, SBET = 200 m2/g, hydrophilic; FS2, SBET = 400 m2/g, hydrophilic; FS3, SBET = 200 m2/g, hydrophobic) from Aladdin Reagent Co., Ltd. And PTNT was selfsynthesized according to the previous reports.22,23 To prepare PEI-based solid sorbents, designated xPEI/ support where x represents the weight percentage of PEI in the sorbents, a given amount of PEI (from J&K Scientific, Mw = 600) was first dissolved in 40 mL of methanol at room temperature. Then, 2 g of support (MCM-41, SBA-15, KIT-6, CMK-3, PTNT, or FS1−3) was added into the above PEI methanol solution, respectively, while vigorous stirring was

∫

where qs is the CO2 adsorption capacity of the PEI/support (mmol/g), m the weight of the PEI/support (g), Q the gas flow rate (mL/min), C0 the inlet CO2 concentration (vol %), C the outlet CO2 concentration (vol %), t the CO2 adsorption duration (min), T0 the standard temperature (273 K), T the CO2 adsorption temperature, and Vm standard molar volume (22.4 mL/mmol). B

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research 2.4. CO2 Adsorption Isotherm Measurements. Adsorption isotherm measurements of high purity CO2(99.999%) on the PEI/supports were carried out using a Tristar II 3020 analyzer (Micromeritics, USA) at room temperature and in the pressure range of 0−1 bar. In the current work, the sorbent samples were heated at 100 °C for 4 h under vacuum. The samples were then cooled down to room temperature before exposure to CO2. The experimental data were dealt with using a Langmuir dual-site model, which is based on the postulation that a simultaneous uptake of CO2 proceeds via two independent patterns, i.e., chemical interactions involving amino groups and physical adsorption on the surface of PEI/supports. And the isotherm parameters of the Langmuir dual-site model for CO2 adsorption were fitted out by nonlinear regression implicated in an iterative method with the aid of Matlab (version 8.5.0). By analysis of the obtained results, it is possible to validate the suitability of the proposed model and shed some light on CO2 adsorption on these PEI/support sorbents. The isotherm model of Langmuir dual-site is formulated by a pressure-dependent form of the total adsorption uptake of CO2(qe), as expressed by eq 2: qe = qchem

bphysP bchemP + qphys 1 + bchemP 1 + bphysP

Figure 2. CO2 adsorption capacities of the PEI/supports under the conditions: 2 vol % CO2 in air, 298 K adsorption temperature, 30% relative humidity, 40 mL/min total flow rate, 0.8 g sorbent.

(2)

where P is the equilibrium pressure of CO2 in the gas dosing system, qchem and qphys as characteristic model parameters denote the maximum amounts of CO2 adsorbed by way of chemisorption and physisorption, and b chem and b phys correspond to the affinity constants related to the above two CO2 adsorption mechanisms, respectively. To evaluate the applicability of the aforementioned model, the normalized standard deviation, i.e., Δq (%), has been determined by means of eq 3: Δq(%) =

∑ [(qexp − qmod)/qexp]2 N−1

(3)

Figure 3. Breakthrough curves of CO2 adsorption on the 40% PEI/ supports under the conditions: 2 vol % CO2 in air, 298 K adsorption temperature, 30% relative humidity, 40 mL/min total flow rate, 0.8 g sorbent.

where qexp and qmod stand for the experimental and theoretical calculated uptakes of CO2 adsorbed in order while N refers to the number of data points gathered in each CO2 adsorption isotherm.

observed on all the 50% PEI/supports. For PEI/FSs with the two above PEI loadings, CO2 adsorption follows the capacity order of PEI/FS1 > PEI/FS2 > PEI/FS3, which has no direct connection to the surface area and hydrophilic−hydrophobic behavior of three supports FSs. Therefore, it can be believed that multiple factors have their impact on CO2 adsorption, to which the surface attributes mentioned above may make a minor contribution. In addition, the PEI/supports with 50 wt % PEI loading exhibit CO2 adsorption capacities analogous to those with a 40 wt % PEI loading. The reason for this observation may be a priori that although more active-sites are introduced on the surface of supports at a higher PEI loading, the diffusion resistance can increase concomitantly, thus leading to an encumbrance in CO2 adsorption. If true, more emphasis should be placed on the design of amine-based sorbents with excellent diffusion kinetics in order to gain desirable CO2 adsorption performance. Generally, the breakthrough curves can provide some valuable information about the kinetic performance of CO2

3. RESULTS AND DISCUSSION 3.1. CO 2 Adsorption Performance of the PEI/ Supports. On the consideration that the amine loading range from 40 to 50 wt % is employed for CO2 adsorption in many cases,14,15,29,30 the series of PEI/supports were also prepared with 40 and 50 wt % PEI loadings on MCM-41, SBA15, KIT-6, CMK-3, PTNT, and FS1−3, respectively. The comparison of the performance of CO2 adsorption was conducted over the PEI/supports under the same conditions. As shown in Figure 2, PEI/PTNT and PEI/FSs present much higher CO2 adsorption capacities than those achieved with the other four types of PEI/supports, viz., PEI/MCM-41, PEI/ SBA-15, PEI/KIT-6, and PEI/CMK-3. Concretely speaking, the capacities of CO2 adsorption over 40% PEI/PTNT and three 40% PEI/FSs were found to be 1.66, 2.16, 1.92, and 1.78 mmol/g, respectively, in contrast with those values of 0.71, 1.37, 0.55, and 0.18 mmol/g obtained on 40% PEI/MCM-41, 40% PEI/SBA-15, 40% PEI/KIT-6, and 40% PEI/CMK-3 in order. The similar trend of CO2 adsorption was likewise C

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

3.2. Characterization of Supports and PEI/Supports. As is well-known, the porous structures of the supports can greatly affect CO2 adsorption over the amine-based solid sorbents. So, the porous support materials (MCM-41, SBA-15, KIT-6, CMK-3, PTNT, and FS1) were examined by means of the SEM technique in order to correlate the observed morphologies and CO2 adsorption performance of the PEI/ supports. As shown in Figure 4, both PTNT and FS1 are in an agglomerate state, which yields a considerable amount of interparticular macropores, i.e., secondary macropores. In contrast, MCM-41, SBA-15, KIT-6, and CMK-3 present themselves in a discrete state, in which no such secondary macropores were found. The formation of PTNT agglomerate is reasonably attributed to its unique one-dimensional fibrous morphology. The large ratio of the length to the diameter enables the nanotubes easily to intertwine and further form the large agglomerates. Whereas for FS1 agglomerate, it may be caused by the much smaller size of the nanoparticles. Generally, the small particles have high surface energy, and they are unstable, leading to the gathering of the particles for reaching a more stable state. N2 adsorption−desorption isotherms are shown of the supports and the PEI/supports in Figure 5. As per the IUPAC classification,31 MCM-41, SBA-15, KIT-6, and CMK-3 present a type IV isotherm (Figure 5a, b, c, d), associated with their mesoporous structures. Furthermore, the adsorption hysteresis loops are confined to the P/P0 region from 0.5 to 0.9, consistent with the ordered mesoporosity of these four porous solid materials. For PTNT, the adsorption−desorption isotherm also exhibits itself as a type IV characteristic in the P/P0 region from 0.8 to 0.99 (Figure 5e), mostly resulting from secondary macropores formed between the titanate nanotubes, but not from the eigen pores (pore size: ∼5 nm32) derived from the nanotubes. In the case of FS1, a pseudo type II isotherm with a very slight adsorption hysteresis loop spans a higher P/P0 region from 0.9 to 0.99 (Figure 5f). Such an isotherm has been known to be generally observed over nonporous or macroporous solid materials. With respect to a 40% PEI loading, 40% PEI/FS1 gives a Type IV isotherm distinguished from the pseudo Type II isotherm of FS1 while the other five 40% PEI/supports keep the same isotherm type (Type IV) as that of their supports. Specifically, it is worthwhile to mention that 40% PEI/FS1 shows its adsorption hysteresis loop in the high P/P0 region from 0.8 to 0.9, implying that additional voids as mesopores may evolve from the PEI phase in 40% PEI/FS1. The newly formed mesopores can be obviously observed in the following pore size distributions (Figure 6f). Figure 6 gives the pore size distributions of the supports and the PEI/supports. As shown in Figure 6a, b, c, d, MCM-41, SBA-15, KIT-6, and CMK-3 exhibit a well-defined pore size distribution with a peak value centered at 2.7, 6.5, 6.5, and 3.8 nm, respectively, in accordance with their ordered mesoporous features. With the largest contribution from the secondary pores, PTNT and FS1 display a broad pore size distribution ranging from 0 to 100 nm (Figure 6e, f). From Table 1, it can be seen that after the impregnation of PEI on the supports, both the pore volume and surface area significantly diminished for 40% PEI/MCM-41, 40% PEI/SBA-15, 40% PEI/KIT-6, 40% PEI/CMK-3, and 40% PEI/PTNT as compared to their corresponding supports, definitely by virtue of the filling of PEI into the channels and/or pores, which coincides with the pore size distributions of the five above PEI/supports. And no exact

Figure 4. SEM images of the supports in 10 μm (a1−f1) and 500 nm (a2−f2): (a) MCM-41; (b) SBA-15; (c) KIT-6; (d) CMK-3; (e) PTNT; (f) FS1.

adsorption on the sorbents. Figure 3 shows the breakthrough curves of CO2 adsorption on the 40% PEI/supports. As seen in Figure 3, the CO2 capture took place so rapidly at the initial stage for 40% PEI/MCM-41, 40% PEI/SBA-15, 40% PEI/ PTNT, and 40% PEI/FS 1 that a zero level of CO 2 concentration in the effluent gas at the outlet was measured by the online gas chromatography analysis. In contrast to that, the breakthrough curves of 40% PEI/CMK-3 and 40% PEI/ KIT-6 ramp up abruptly just upon the initiation of CO2 adsorption. As for the final part of the breakthrough curves, a dramatic reduction of CO2 adsorption rate was observed in this stage. As for the fast CO2 adsorption process at the initial stage, the CO2 adsorption mainly arises from the formation of carbamates or bicarbonate species between CO2 and amino groups. After the amine-sites were completed occupied, CO2 continues to be adsorbed with a lower adsorption rate as a result of the internal diffusion resistance in the pore channels, which primarily depends on the pore structure of sorbents. As seen in the area of the red dotted line, after CO2 breakthrough, 40% PEI/FS1 takes a shorter time to reach saturation than the other sorbents do, confirming the minimum diffusion resistance in its pore channels. D

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. N2 adsorption (solid) and desorption (hollow) isotherms of the supports and the PEI/supports.

by MIP is 17.5 cm3/g, exhibiting a different value from the BET result (0.45 cm3/g) whereas it is much closer to the real bulk volume of FS1 (20 cm3/g) mentioned in the literature.17 So far, several studies have unanimously demonstrated that the textural property of the supports exerts a greater influence on the CO2 adsorption over amine-based solid sorbents, among other things.14,15,33 If the voids in supports exist as large mesopores and macropores (i.e., the so-called effective pore volume, Vp, e), the corresponding amine solid sorbents have been usually found to possess excellent CO2 adsorption performance, which is reasonably attributable to the combined effect of facile CO2 diffusion and the subsequent PEI approach. For example, Xu et al.34 used a mesoporous support MCM-41 with a surface area of 1480 m2/g and an average pore size of 2.75 nm, coupled with PEI as amine phase for CO2 adsorption. They found that both the surface area and pore size were dramatically decreased owing to the blockage of PEI packed into the pores, which then leads to a relatively low CO2 adsorption capacity obtained over the MCM-41-supported PEI sorbent. On the other hand, Giannelis and co-workers35 reported that supported PEI sorbents based on a kind of silica

values of pore volume were determined for 40% PEI/MCM-41 due to its zero-type pore size distribution (viz., dV/dw ≈ 0). Despite the remarkable reduction observed in the surface area, 40% PEI/FS1 is unexpectedly subjected to no loss in the pore volume. In agreement with its adsorption hysteresis loop in the high P/P0 region, 40% PEI/FS1 affords a pore size distribution with a cumulative pore volume much larger than that of FS1. Such an anomaly can be well reconciled with a postulation that there may be part of secondary pores between FS1 particles above 500 nm, which cannot be detected by an N 2 physisorption method. When PEI is added to FS1, the size of these larger secondary pores diminishes and falls into the measuring range of the method used (500 nm) can be clearly observed. Also, the total pore volume of FS1 measured E

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Pore-size distributions of the supports and the PEI/supports. (The inset picture in Figure 6f is the PSD curve of FS1 obtained from the MIP method.)

Table 1. Textual Characteristics of the Supports and the PEI/Supports Sample

MCM-41

PTNT

FS1

BET surface area (m2/g) Pore Volume (cm3/g)

1099

799

721

984

260

197

1.09

1.19

0.79

1.32

1.55

0.45

40% PEI/ CMK-3

40% PEI/ PTNT

40% PEI/ FS1

Sample BET surface area(m2/g) Pore Volume (cm3/g)

SBA-15 KIT-6 CMK-3

40%PEI/ MCM-41

40% PEI/ SBA-15

40% PEI/ KIT-6

12

119

20

134

90

49

-

0.26

0.05

0.23

0.69

0.45

foam with ultra large mesopores exhibit fast CO2 adsorption− desorption kinetics and excellent CO2 adsorption capacity. Here, a similar circumstance occurs in our work as follows: (1) as described above, all the PEI/supports have a dramatic reduction in the surface area relative to their supports; (2) despite the existence of a decrease in the pore volume, 40% PEI/PTNT, which still retains a substantial portion of secondary macropores stemming from PTNT, gives a higher

Figure 7. FT-IR spectra of the supports.

F

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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OH groups,38 while for PTNT, protonated titanate nanotube, the one at 906 cm−1 tentatively corresponds to the Ti−O stretching vibration in the Ti−OH groups. In contrast to the above observations, no such OH-related bands were detected for CMK-3, a carbon-based porous material, indicating its absence of the surface hydroxyl groups. As known to all, the polarity compatibility between a liquid and a solid governs to a great extent the ease of dispersion of the former on the surface of the latter. In this context, an excellent dispersion of PEI phase over a support prefers to occur via the interaction between PEI and the surface hydroxyl groups of the support, which results in a more homogeneous dispersion of PEI with more available amino groups, and a higher CO2 adsorption capacity acquired on the five OHcontaining supports in comparison with CMK-3. The two conditions of the dispersion of PEI on the supports with and without OH-containing are separately depicted in Figure 8a and 8b. The N 1s XPS spectra measured from the PEI/supports are shown in Figure 9. One broad asymmetrical peak was resolved into two subpeaks. For the sorbents of PEI/MCM-41, PEI/ SBA-15, PEI/KIT-6, PEI/PTNT, and PEI/FS1, the subpeak with higher binding energy (399.4 eV−399.7 eV) belongs to the primary amino group (−NH2), while the other subpeak with lower binding energy (398.1 eV−398.3 eV) is attributed to the secondary amino group (−NH−). As is well-known, the binding energy of N 1s for an amine molecule generally gains an increment value of 1−1.5 eV as long as its amino group is hydrogen-bonded with other groups, such as amino or hydroxyl.39−41 As a result, for the PEI/CMK-3 sorbent, the peak at 400.6 eV can be assigned to hydrogen-bonded −NH2, whereas the one at 399.8 eV is hydrogen-bonded −NH−. As

Figure 8. Dispersion of PEI on the supports with (a) and without (b) OH-containing.

CO2 absorption capacity than 40% PEI/MCM-41, 40% PEI/ SBA-15, 40% PEI/KIT-6, and 40% PEI/CMK-3, all the four sorbents with heavy loss in the pore volume; (3) due to the super large voids provided by the secondary macropores in FS1, its corresponding sorbent of 40% PEI/FS1 manifests as the best sorbent of the series of 40% PEI/support analogs for CO2 absorption. Therefore, to design a superior support material, more attention should be paid on the secondary pores instead of eigen pores. Figure 7 presents the FT-IR spectra of the six supports. As shown in Figure 7, all the supports except for CMK-3 exhibit several characteristic IR bands in the wavenumber region from 500 to 4000 cm−1. The two IR bands at 3459 and 1645 cm−1 are assigned to, respectively, the stretching and bending vibrations of the O−H bond in the surface hydroxyl groups and some adsorbed H2O molecules.36,37 For the four porous silicas (MCM-41, SBA-15, KIT-6, and FS1), the IR band at 952 cm−1 corresponds to the Si−O stretching vibration in the Si−

Figure 9. N 1s XPS spectra of the PEI/supports: (a) 40% PEI/MCM-41, (b) 40% PEI/SBA-15, (c) 40% PEI/KIT-6, (d) 40% PEI/CMK-3, (e) 40% PEI/PTNT, and (f) 40% PEI/FS1. G

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. CO2 adsorption isotherms for PEI/supports at ambient temperature: the open circles correspond to the experimental data points, and the solid lines are fitted out based on the Langmuir dual-site model.

derived from the FT-IR results, CMK-3 is the only support without −OH groups, resulting in inhomogeneous dispersion or agglomeration of the amine phase. And the serious agglomeration of the amine phase is prone to the formation of intermolecular hydrogen bonds in the amino groups of PEI/ CMK-3 sorbent. 3.3. CO2 Adsorption Isotherms for PEI/Supports. Given that equilibrium plays a critical role in understanding an adsorption process, the experimental adsorption isotherm

data determined at ambient temperature for the PEI-based sorbents are analyzed in this part. On the premise that the equilibrium can be reached in the absence of any diffusion limitation, and physical and chemical interactions coexist in the CO2 adsorption process, the adsorption data were fitted by a Langmuir dual-site model, i.e., eq 2. The fitted model isotherms and parameters are presented in Figure 10 and Table 2, respectively. H

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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wet impregnation method, thus generating the PEI/support sorbents with 40 and 50 wt % PEI loadings, respectively, for CO2 adsorption. The information based on the results in the present work reveals that the CO2 adsorption performance of these PEI/supports depends to a great extent on the nature of supports, with PEI/PTNT and PEI/FS encountered as the two relatively superior sorbents under the selected conditions, which has been reasonably associated with their higher abundance of secondary macroporosity. These secondary pores provide large voids for PEI loading and otherwise enable them homogeneous surface dispersion, which result in the minor diffusion resistance and further increase the accessible amine-sites for CO2 adsorption. The enhanced exposure of amino-groups has been further verified by the CO2 adsorption isotherm simulated by the Langmuir dual-site model. Therefore, in the future study, more attention should be focused on the secondary pores instead of eigen pores to design the superior support material. Additionally, the −OH groups within the support also favor the dispersion of PEI and exposure of amine-sites.

Table 2. Isotherm Parameters of Langmuir Dual-Site Model for CO2 Adsorption on PEI/Supports Sorbent

qchem (mmol/g)

bchem(bar−1)

qphys (mmol/g)

bphys(bar−1)

40%PEI/MCM-41 40%PEI/SBA-15 40%PEI/KIT-6 40%PEI/CMK-3 40%PEI/PTNT 40%PEI/FS1

0.49 1.33 0.32 0.01 1.67 1.89

3012 3024 3012 3011 3025 3021

0.20 0.56 0.11 0.50 0.45 0.26

0.87 0.83 0.95 0.86 0.96 0.98

As shown in Figure 10(d), the total adsorption uptake of CO2 for 40% PEI/CMK-3 kept always increasing with CO2 pressure until the upper limit of 1 bar was adopted. More than that, 40% PEI/CMK-3 was also found to have a much higher value of qphys (0.50 mmol/g) than that of qchem (0.01 mmol/g). In accordance to the early published contributions,42 the conclusion can be arrived at based on the above observations that the process of CO2 capture on 40% PEI/CMK-3 is largely governed by physical adsorption, combined with a minor contribution from chemical adsorption. This may be because the poor dispersion of PEI in CMK-3 channels leads to the yield of some micropores, which is beneficial for the enhancement of the physisorption amount with increasing CO2 pressure, while it is unfavorable for CO2 to contact with amine-sites. On the contrary, the isotherm profiles for the other five 40% PEI/supports (viz., 40% PEI/MCM-41, 40% PEI/SBA-15, 40% PEI/KIT-6, 40% PEI/PTNT, and 40% PEI/ FS1) all exhibit an adsorption saturation plateau subsequent to dramatic CO2 uptake within the extremely narrow region of low pressure. Such a behavior is generally considered to be typical for the Langmuir model used mostly to describe chemisorption.43 As regards the proposed model parameters, Table 2 shows that almost the same values of bchem were observed for the five 40% PEI/supports, together with distinct levels of qchem. To give an appropriate explanation to such variations, it is worth reminiscing about the connotation of bchem and qchem. The parameter bchem essentially embodies the nature of mutual effects from the combination of CO2 and amino groups, independent from the selected adsorption conditions, for which nearly constant values reveal no doubt the similarity in amino functionality or the same interaction mechanism for CO2 adsorption. On the other hand, the parameter qchem as the maximum CO2 uptake in eq 2 changes significantly from one sorbent to another whereas higher values of qchem coincide with more available amino groups for CO2 adsorption. Especially, 40% PEI/PTNT and 40% PEI/FS1 obtain the higher values of qchem, which further verified that the big secondary pores in PTNT and FS1 can provide enough space for PEI loading and guarantee the well dispersion of PEI to expose more aminesites on the surface of sorbents. In addition, except for 40% PEI/CMK-3, the fitted results herein manifest clearly that the proposed model matches sufficiently experimental data of the other five sorbents with low values of Δq (%) less than 3%, hence ascertaining its reliability to describe CO2 adsorption on the PEI/support sorbents in this work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +86 10 61716086. ORCID

Tianle Zhu: 0000-0003-3834-8929 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Key R&D Program of China (No. 2017YFC0211804 and No. 2016YFC0207103).

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REFERENCES

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4. CONCLUSIONS Six mesoporous materials (MCM-41, SBA-15, KIT-6, CMK-3, PTNT, and FS) as supports were PEI-functionalized by the I

DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.9b02659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX