3D Assemblies of Amine-Functionalized Graphene Silica

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Surfaces, Interfaces, and Applications

2D/3D Assemblies of Amine Functionalised Graphene Silica (Templated) Aerogel for Enhanced CO Sorption 2

Wenjing Wang, Julius Motuzas, Xiu Song Zhao, and João Carlos Diniz da Costa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07192 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

2D/3D Assemblies of Amine Functionalised Graphene Silica (Templated) Aerogel for Enhanced CO2 Sorption

Wenjing Wang,1 Julius Motuzas,1 Xiu Song Zhao,1 João C. Diniz da Costa1, 2*

1The

University of Queensland, FIM2Lab - Functional Interfacial Materials and Membranes

Laboratory, School of Chemical Engineering, the University of Queensland, Brisbane QLD 4072, Australia. 2LAQV-REQUIMTE,

(Bio)Chemical Process Engineering, Department of Chemistry, Faculty of

Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal.

Abstract This work investigates the one-pot facile synthesis of novel 2D/3D assemblies containing graphene silica (templated) aerogel sorbents for CO2 capture, a greenhouse gas of major global concern. In this synthesis, 3D silica (templated) aerogels were formed along the planes of 2D graphene sheets, resulting in 2D/3D assemblies of flake-like shapes. The templates were burnt off from the 2D/3D assembly, leaving a mesoporous cavity which increased with the carbon chain length used in the synthesis method. As such, morphological features related to surface area and total pore volume increased significantly by over 80 % as compared to blank (no template) samples, and reached maximum values of 734 m2 g-1 and 0.42 cm3 g-1, respectively. The increase in total pore volume allowed for higher content of impregnated amine into the 2D/3D assembly followed by a freezedrying method. The CO2 sorption capacity of the amine functionalised 2D/3D assemblies reached high values at 4.9 mmol g-1 (mass over weight ratio), equivalent to 11.67 mmol cm-3 (mass over total pore volume ratio). The amine functionalised 2D/3D assemblies were stable over 10 cycles of CO2 sorption and desorption. Further, heat of sorption results were generally low, with the lowest value

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reaching 59 kJ mol-1. These results are desirable for the deployment of 2D/3D assemblies as sorbents to capture CO2.

Keywords: 2D/3D assembly; graphene; silica aerogel; template; CO2 sorption.

1. Introduction Sustainability of our planet is of global concern particularly associated with climate change due to the atmospheric emission of CO2. This concern is coupled with our ever increasing desire to use energy dependent technologies. As a result, 60% CO2 emission worldwide comes from point source, represented by fossil fuel-fired power plants1. In the long term, a transition towards renewable resources is envisaged. However, in the short to medium term, fossil fuels still provide the base power that our society needs, and a solution is a process called carbon capture and storage (CCS). This process captures CO2 at the emission point2 which is transported and stored in safe geological reservoirs. Currently, there are four major technologies that can be used to capture CO2 based on absorption,3-5 adsorption,6 membranes7, 8 and cryogenics9. Absorption processes based on aqueous amine started in the 1970s in the chemical engineering industry, and currently is the first generation of CO2 removal technology.10 Cryogenics is also a mature technology, though energy intensive leading to high operation costs. As novel advanced technologies are warranted in CCS, there have been major research efforts in membrane and adsorbents.

Due to lower regeneration energy and ease of maintenance, the solid adsorption process is a promising alternative technology for CO2 capture from flue gases in power plants.11, extensive investigation include activated carbon,13,

14

zeolites,15,

16

12

Adsorbents under

mesoporous silicas,17,

18

metal

organic frameworks (MOFs).19, 20 In order to improve CO2 capture in adsorbents (i.e. solid sorbents), several groups reported the impregnation of amine groups into mesoporous silicas. For instance, amine functionalised benzene silicas,21 macroporous silica,22 SBA-1523 and MCM-4124 reached CO2 2 ACS Paragon Plus Environment

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sorption capacity up to 3.86 mmol g-1. Silica surfaces have a very weak and low CO2 sorption capacity, known as physisorption, whilst the high CO2 sorption capacity is due to the strong reaction with amine, known as chemisorption. Another mesoporous material of interest is silica aerogel which has high porosity (80-99.8 %).25 Lineen et al.26-28 reported a series of amine functionalised silica aerogels where the CO2 sorption capacity increased from 1.76 to 3.5 and 6.1 mmol g-1. The latter higher results were achieved with a silica aerogel with very high total pore volume of 5 cm3 g-1. The advantage of using mesoporous silica is directly related to its high pore volume which provides porous support for the impregnation and homogeneous distribution of amine.

Recently, Wang and co-workers reported a novel 2D/3D graphene SBA-15 mesoporous silica reaching 5.45 mmol g-1 after amine functionalisation.29 This CO2 sorption capacity was 51% higher as compared to an analogous amine functionalised 3D SBA-15 sorbent. Similar results were achieved by intercalating graphene aerogel within the 3D SBA-15 matrix enhanced the porosity,30 and likewise increasing CO2 sorption. A novelty of the 2D/3D graphene SBA-15 sorbents was that the functionalisation was carried out using a freeze-drying method which conferred superior structural integrity of the silica than conventional oven or evaporate drying methods. For instance, the freezedrying method resulted in surface areas (up to 304 m2 g-1) and total pore volumes (up to 0.045 cm3 g-1) at least by ~50% higher as compared to the conventional drying methods,31 thus avoiding the collapse of mesopores and loss of surface area. The freeze-drying method hampers the reaction of water with silanol groups of silica during the drying of the water amine solution impregnation method.32 However, both 2D/3D sorbents were prepared via a two-step method following by the SBA-15 preparation, then adding graphene and/or graphene aerogel. To date, there is no single-step method where both porous silica and graphene can be prepared in a facile single-pot synthesis.

Therefore, this work investigates the preparation of 2D/3D assemblies containing graphene and silica (G-Si) aerogel in a single-pot synthesis. In order to improve the pore volume of the 2D/3D G-Si 3 ACS Paragon Plus Environment


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aerogel, this work explores the use of surfactant with various carbon chains. The resultant 2D/3D assemblies were fully characterized to determine their surface areas and total pore volumes, in addition to microscopic images. Subsequently, the 2D/3D sorbents were functionalised with amine and tested for CO2 sorption at several temperatures and amine loading to determine the optimum operating conditions for CO2 capture. The amine functionalised sorbents were further tested to determine their heat of sorption and operation stability over 10 cycles. Finally, the 2D/3D structural assembly containing graphene and templated aerogels is discussed in light of the results obtained in this work.

2. Experimental All chemicals were purchased from Sigma-Aldrich and used as received including sulfuric acid (H2SO4, 95%), sodium nitrate (NaNO3, technical grade), potassium permanganate (KMnO4, technical grade), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 37%), tetraethyl orthosilicate (TEOS, 98 wt%), tetraethylenepentamine (TEPA technical grade), HNO3 (70 wt%); several surfactants with various carbon length such as tetramethyl ammonium bromide (C1-MAB/TMAB, C4H12BrN), triethylhexyl ammonium bromide (C6-EAB/THAB, C12H28BrN) and trimethyloctadecyl ammonium bromide (C18-MAB/OTAB, C21H46BrN) were also used.

2.1 Synthesis of amine functionalised 2D/3D aerogels Graphene oxide (GO) was synthesized via the modified Hummers method33 by the oxidation of graphite flake. Briefly, 60 ml H2SO4 was added to the mixture of 2.5 g natural graphite and 1.25 g NaNO3. Then 7.5 g KMnO4 was gradually added and the suspension was kept stirring for 2 h in an ice-water bath and 24 h at room temperature. Subsequently, 75 ml deionized water was added slowly to the mixture and the resultant slurry was left stirring for an hour. 25 ml H2O2 was added to reduce the residual KMnO4. The colour of the mixture changed from brown to bright yellow. Finally, the


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mixture was centrifuged and the recovered solids were washed with HCl solution and water. The resultant solid (GO) was dried in an oven at 60 ℃.

The precursor 2D/3D suspension was prepared via a two-step process. In the first step, a predetermined amount of GO was dispersed in deionized water to prepare a GO suspension. A second solution was prepared without template (blank) or with individual surfactants (C1, C6 or C18) dissolved in water where HNO3 (0.5 M) was added to assist the subsequent hydrolysis of silica. This second solution was kept in an ice bath under constant stirring where TEOS was added dropwise and then kept under stirring for 30 min. In the second step, the aqueous GO suspension was mixed into the second solution also under constant stirring. The mass ratio of TEOS: surfactant: GO-H2O: HNO3 was set at 1.0: 0.05: 14.8: 0.27. After 2 h stirring, the final mixture was transferred to an autoclave at room temperature then heated to 60 ℃ for 48 h. The hydrogel formed in the autoclave was then removed and frozen in liquid nitrogen and dried in a freeze drier for 2 days. The final step in this method involved the removal of the surfactant in a tube furnace under oxidative (air) conditions at 450 ℃ for 2 h. Subsequently, the 2D/3D aerogel was kept in the same tube furnace under inert (high purity N2) condition at 550 ℃ for 1 h. The aerogel was named as Cx-G-Si aerogel, where x represents the carbon length of the surfactant, G is for the reduced graphene and Si is for silica.

The 2D/3D Cx-G-Si aerogel was finally functionalised with an amine sorbent (TEPA) via a wet impregnation method followed by freeze-drying to keep the integrity of the aerogel silica structure. A desired amount of TEPA was firstly dissolved into 10 ml H2O and mixed vigorously for 10 min. Subsequently, a desired amount of Cx-G-Si aerogel was added to the solution and kept stirring for 24 h at ambient temperature to ensure full impregnation of TEPA into the porous structure of the 2D/3D assembly. This solution was then frozen in liquid nitrogen for 30 min, and finally dried in a freeze drier for 48 h. A series of Cx-G-Si aerogel/TEPA sorbents were prepared by varying the amine loading from 30 to 90 wt%. 5 ACS Paragon Plus Environment


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2.2 Materials characterization N2 sorption isotherms were obtained from a Micromeritics Tristar 3020 at 77 K to determine the structural properties of the 2D/3D assemblies. The Brunauer-Emmett-Teller (BET) method was used to calculate the surface area. Decomposition analysis was carried out on a thermogravimetric analyser (TGA/DSC 1, Mettler-Toledo) where samples were exposed to air between 25-450 ℃ and then to N2 between 450-700 ℃ with a heat ramping rate of 10 ℃ min-1. Fourier transform infrared (FTIR) spectroscopy was conducted on an IRAffinity-1 spectrometer (Shimadzu) with an attenuated total reflection (ATR) cell in the wavenumber range of 600-4000 cm-1. Scanning electron microscope (SEM) images were obtained at 5 kV using a JEOL field emission microscope (JEOL JSM-7001F). Atomic force microscopy (AFM) images of GO were obtained with a Veeco MultiMode AFM in tapping mode using OLTESPA-R3 silicon probe (Bruker).

The CO2 sorption performance was determined using the same TGA/DSC 1 analyser (Mettle-Toledo). For a typical sorption sampling, ~10 mg sample were activated at 90 ℃ for 30 min in pure N2 at a flow rate of 60 ml min-1. A gas mixture of 15% CO2 and 85% N2 was then introduced to achieve CO2 sorption under 90 ℃. Subsequently, the feed gas was switched to pure N2 again for 30 min to desorb CO2. The above steps were repeated when testing at different temperatures or for the stability tests carried out for 10 cycles. The CO2 breakthrough time was determined by packing a desired amount of sorbent inside a stainless steel tube with glass wool at the ends. This is known as the sorbent bed. Three sets of flow rates (7, 14 and 28 mL min-1) were tested also using a feed gas mixture of 15% CO2 and 85% N2. The feed gas flow rates were controlled by high precision Cole Palmer rotameters, and feed flow rate was measured using a bubble flow meter. The CO2 sorption was carried out by placing the sorbent bed in temperature controlled furnace at 90 ℃. The CO2 breakthrough time was determined by a high precision Testo 535 gas meter for measuring CO2 low concentrations between


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0 and 1 mol%. Subsequently, the sorbent bed was desorbed using high purity N2 for further cycles of sorption and desorption.

3. Results and discussion Fig. 1 shows the N2 isotherms of the 2D/3D Cx-G-Si aerogels. It is observed that by increasing the surfactant carbon chain from blank (no template) to C1 to C18 resulted in a significant increase in the total pore volume of N2 adsorbed. In addition, the N2 isotherms changed in shape. The blank and C1G-Si samples are characterised by type Ⅰ shape. A small hysteresis loop (type IV) at relative pressures p/po of 0.4 to 0.6 were observed for the samples synthesised with C6 and C18 surfactants, clearly indicating capillary formation.

Fig. 1. N2 sorption isotherms of blank (no template) and 2D/3D Cx-G-Si aerogels without TEPA.

Further analysis of the N2 isotherms was carried out and the results are summarised in Table 1 and Fig. 2. It is observed that the blank (no template) sample had the lowest BET surface area (Fig. 2a) and total pore volume (Fig. 2b). In addition, both parameters increased as the carbon chain of the surfactants increased from C1 to C18. For instance, the surface area increased significantly by 64% 7 ACS Paragon Plus Environment


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from 448 to 734 m2 g-1 for the C1-G-Si and C18-G-Si aerogels, respectively. The total pore volume followed similar trends and reached a maximum value of 0.42 cm3 g-1. The increase in surface area and pore volume for the 2D/3D assemblies prepared with surfactants C1 to C18 is attributed to the alkyl chain length as reported elsewhere.34 Surfactants as templates embedded in silica matrices are pore forming agents, and the pore size increases as a function of the alkyl chain length.35 Hence, as the alkyl chain increased from C1 to C18, the larger was the pore size left when the template was burnt off.

Fig. 2. (a) BET surface areas and (b) total pore volumes of 2D/3D Cx-G-Si aerogels where carbon length zero is for the blank sample with no template.

Table 1 Structural properties of blank sample and Cx-G-Si aerogels Samples

BET surface

Total pore

area ( m2g-1 )

volume ( cm3g-1 )

Blank sample

401.7

0.22

C1-G-Si aerogel

448.2

0.24

C6-G-Si aerogel

637.2

0.36

C18-G-Si aerogel

733.9

0.42


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Fig. 3 shows a representative SEM image of a 2D/3D assembly of Cx-G-Si aerogel. It is observed pores were homogeneously distributed along the silica matrix (Fig. 3a). The porosity is derived from the molecular imprinting left by the templates where they were burnt off from the 2D/3D assembly. Fig. 3b displays the formation of several flake-like shapes of different dimensions. It is interesting to observe that these flake-like shapes have a high aspect ratio of side dimensions and very thin thickeness. These results strongly suggest the 3D silica (templated) aerogels are forming along the planes of the 2D graphene sheets. Fig. 3c shows a representative AFM image of the synthesised GO where the line profile displays a height distribution across lateral dimensions with a thickness of ~1 nm, equivalent to the thickness of about 3 graphene sheets together.36 The flake-like assembly conferred by GO in this work is contrary to conventional silica aerogel which tends to form particles. In other words, the 2D graphene sheets were enveloped or sandwiched by the silica aerogel during the one-pot synthesis.

Fig. 3. Representative SEM image of (a) 2D/3D C1-G-Si aerogel, (b) 2D/3D C18-G-Si aerogel and (c) AFM images GO including the height profile plot. 9 ACS Paragon Plus Environment


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Fig. 4a shows the FTIR spectra of blank (no template) and Cx-G-Si aerogel samples. It is observed the bands are assigned to typical silica skeleton, including the stretching mode of Si-O-Si at 807 cm1

and the assymetric streching modes of Si-O-Si at 1055 cm-1 and 1200 cm-1.37-39 The spectra of

individual Cx-G-Si aerogel prepared with the surfactants from C1 to C18 ressembled each other, clearly indicating that the surfactant chain length did not affect the final chemical make up of each 2D/3D assembly. In addition, these infrared bands are common in silica aerogels as reported elsewhere.27, 40 The Cx-G-Si aerogels loaded with 70 wt% TEPA were also studied by exposing them to infrared irradiation. Fig. 4b shows that all 2D/3D assemblies loaded with TEPA exhibited bands corresponding to C-H stretching (2937 cm-1, 2820 cm-1), N-H bending (1654 cm-1, 1604 cm-1, 1458 cm-1, 775 cm-1) and N-H stretching (3361 cm-1, 3268cm-1, 937 cm-1). These results demonstrate the successful incorporation of amine groups on the surface or within the structure of the blank (no template) and Cx-G-Si aerogels as reported elsewhere.41

Fig. 4. FTIR spectra of blank (no template) and 2D/3D Cx-G-Si aerogels (a) without amine functionalisation and (b) with 70 wt% TEPA.


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A C18-G-Si aerogel loaded with TEPA 70 wt% was initially tested for CO2 sorption. Fig. 5 shows that the CO2 sorption curve is characterised by a fast sorption rate within the first 30 min, reaching 4.32 mmol g-1. From there on, the CO2 sorption rate is lower and at 12 h the total CO2 sorption of 4.9 mmol g-1 is achieved. These results reflect the conventional sorption process of amine functionalised mesoporous materials. The fast sorption rate is associated with CO2 contact and reaction with the amine groups as per equations 1 and 2, where R, R1 and R2 represent amine groups.42 The slow sorption rate after 90 min is characterised by sorption towards equilibrium. This is mainly due to the slow diffusion of CO2 through the amine functionalised silica mesoporous structure, particularly to reach the unreacted amine groups, in addition to microporous diffusion. For instance, Fig. 1 shows microporosity associated with the nitrogen isotherms with relative pressures p/po below 0.2. 2RNH2 +CO2↔RNH3+ +RNHCOO ―

(1)

2R1R2NH + CO2↔R1R2NH2+ + R1R2NCOO ―

(2)

Fig. 5. CO2 sorption of 2D/3D C18-G-Si aerogel loaded with TEPA 70 wt% at 90 °C.

Due to the fast CO2 sorption rate within the first 30 min (Fig. 5), where 88% of the total CO2 sorption is reached, the sorption capacity of Cx-G-Si aerogel samples loaded with TEPA 70 wt% were also 11 ACS Paragon Plus Environment


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investigated. Fig. 6a shows that the CO2 sorption capacity of the blank (no template) was the lowest, whilst a trend was observed as values increased as a function of the carbon chain of the surfactant used. For instance, the CO2 sorption capacity of the TEPA functionalised samples for C1-G-Si of 3.42 mmol g-1 increased 26 % to 4.32 mmol g-1 for the C18-G-Si sample. This trend correlates well with the increase of surface area and total pore volume (see Fig. 2). In addition, TGA analyses were carried out to determine mass loss associated with TEPA loading. Fig. 6b shows an average mass loss of 70 wt% for all samples. Fig A1 (Appendix) displays almost zero mass loss curve for a sample without TEPA. Therefore, the results in Fig. 6b clearly confirm that the mass loss is consistent with the TEPA 70 wt% loading.

Fig. 6. (a) CO2 sorption capacity and (b) mass loss of blank (no template) and 2D/3D Cx-G-Si aerogels loaded with TEPA 70 wt%.

Further analysis was carried out for all samples, except the blank sample (no template), as this sample is a 3D matrix material and gave the lowest results as compared to the 2D/3D assemblies. The CO2 sorption capacity as a function of TEPA loading on Cx-G-Si aerogels is displayed in Fig. 7. As the TEPA loading increased, so the CO2 sorption capacity reached a maximum value at TEPA 70 wt% 12 ACS Paragon Plus Environment

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for all 2D/3D aerogel samples. This maximum TEPA loading is consistent with those reported for amine functionalised SBA-15 mesoporous silica.43 The increase of CO2 sorption capacity for TEPA loading up to 70 wt% is associated with the increase in amine groups available for reaction as per equations 1 and 2. For TEPA loading higher than 70 wt%, a sharp reduction in CO2 sorption capacity is observed for all 2D/3D assemblies. At higher TEPA content (80 to 90 wt%), there is excess TEPA which cannot be accommodated in the mesoporous structure of the Cx-G-Si aerogel samples. As such, the excess amine covers the surface of the 2D/3D assemblies, thus causing amine aggregation and mass transfer limitations.44 Under these conditions, the amine groups of TEPA in the mesoporous structure of the 2D/3D assemblies are generally hindered and cannot be reached by CO2.

Fig. 7. CO2 sorption capacity of 2D/3D Cx-G-Si aerogel at various TEPA loading (30 to 90 wt%) measured at 30 min and 90 °C.

Fig. 8 shows that the CO2 sorption capacity increased as a function of the temperature from 25 ℃ and reached a maximum value at 90 ℃. This is a conventional chemisorption behaviour of amine compounds17 according to equations 1 and 2. For temperatures in excess of 90 to 120 ℃, the CO2 13 ACS Paragon Plus Environment


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sorption capacity decreases. This is associated with the exothermic behaviour of the interaction between amine and CO2, an exothermic process, so the equilibrium shifts to the desorption direction at higher temperatures.45 This effect is compounded with amine degradation46 at temperatures higher than 100 °C, as the loss of amine groups reduces the number of sites available for CO2 reaction.

Fig. 8. CO2 sorption capacity as a function of sorption temperature of Cx-G-Si aerogel loaded with TEPA 70 wt%.

In terms of industrial operation, amine efficiency is a good performance measure to determine the industrial deployment of sorbents for CO2 capture. The amine efficiency represents the normalised capacity of CO2 sorption per N content in the amine, and calculated as mmolCO2 mmolN-1.47 The amine efficiency was determined based on the amine content from the decomposition of TEPA based on TGA analysis (see Fig. 6b). Fig. 9a shows that the amine efficiency increased from 0.18 to 0.23 as a function of the carbon length of the surfactant used to prepare the 2D/3D assemblies. The highest amine efficiency for the C18-G-Si aerogel loaded with TEPA 70 wt% is associated with a higher 14 ACS Paragon Plus Environment

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number of amine groups available to react with CO2 as per equations 1 and 2. As the content of TEPA was the same for all samples (see Fig. 6b), these results strongly suggest that as the carbon length of the surfactant increased, a better distribution of amine took place in the mesoporous structure of the 2D/3D assemblies. As a result, the CO2 diffusion was impeded less resulting in superior access and reaction with amine groups.

Fig. 9. (a) Amine efficiency of Cx-G-Si aerogel, (b) cycle performance and (c) CO2 breakthrough time of C18-G-Si aerogel. All samples were loaded with TEPA 70 wt%.



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Other measures of industrial operation are the regeneration performance of amine functionalised mesoporous silica and the CO2 breakthrough time. Fig. 9b displays the performance of the 2D/3D C18-G-Si aerogel loaded with TEPA 70 wt% over 10 cycles. The amine functionalised 2D/3D aerogel was quite stable with an average CO2 sorption of 4.27 mmol g-1. The CO2 breakthrough time refers to the time required to switch a sorbent bed from sorption to desorption for the separation of CO2 from other gases. This is one of the most important parameters in engineering to determine the size of the columns containing the sorbent beds in separation processes. Fig. 9c displays the breakthrough time for three different sets of flow rates (7, 14 and 28 mL min-1). Each time that the flow rate doubled in value, the CO2 breakthrough time consistently reduced by almost half each time from 76.7 to 37.5 and 19 min, respectively. As the sorbent bed was the same with the same amount of the functionalised 2D/3D assemblies, the amount of CO2 sorbed was similar at each flow rate and averaged 4.32 mmol g-1. This result is in line with the cycle performance values in Fig. 4b, thus an attractive sorbent for CO2 capture applications.

The heat of sorption in Fig. 10 was determined from the CO2 sorption isotherm measurements using the Clausius-Clapeyron equation. The heat of sorption was determined from the CO2 sorption isotherm measurements (Fig. 6a) using the Clausius-Clapeyron equation (Eq. 3): (𝑙𝑛𝑝)𝑄 = ―

𝑄𝑠𝑡

1

𝑅

𝑇

( )( ) +𝐶

(3)

where 𝑝 indicates the pressure (Pa), 𝑄 is the adsorbed amount at pressure Pa, 𝑇 represents the temperature (K), and 𝑅 is the universal gas constant (J mol-1 K-1). The heat of sorption 𝑄𝑠𝑡 was obtained from the slope of lnp versus 1/T. The heat of sorption of all 2D/3D assemblies are above 59 kJ mol-1, a characteristic of chemisorption. This is in line with the use of amine, where strong sorption occurs as CO2 reacts with amine groups as per equations 1 and 2. Interestingly, the heat of sorption decreased as the carbon length of the surfactant used to prepare the Cx-G-Si aerogel increased from C1 to C18. This result correlates well with the improved amine efficiency (Fig. 9a) in addition to morphological features such as increased total porosity (Fig. 2b). Heat of sorption reported in the 16 ACS Paragon Plus Environment

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literature varies between 48-90 kJ mol-1 depending on the silica material/structure and amine functionalisation.48-50 The results of 59 kJ mol-1 in this work reaching at full CO2 sorption capacity are on the lower side, which is attractive as less energy is consumed. This is attributed to the physisorption of CO2 on silica (13-25 kJ mol-1),51-53 particularly that silica aerogel has microporosity as observed in Fig. 1, in addition to graphene (25-30 kJ mol-1).54

Fig. 10. Heat of sorption of Cx-G-Si aerogel/TEPA (50 wt%).

It is interesting to note that the CO2 sorption of 4.9 mmol g-1 (Fig.5) is very high indeed. For instance, this sorption value is much higher than those previously reported, particularly for amine functionalised silica aerogels ranging from 1.76 to 3.5 mmol g-1, 26, 27 though lower than the best result of 6.1 mmol g-1. 28 However, the latter had a very high total pore volume of 5 cm3 g-1, and equivalent to 1.22 mmol cm-3 in terms of using the total pore volume instead of mass as the measuring stick. The results of CO2 sorption (as mass sorption over total pore volume) in this work is 11.67 mmol cm-3, almost one order of magnitude higher. This high CO2 sorption value is mainly attributed to the special morphological features of the 2D/3D assembly containing graphene silica aerogel. In this one pot synthesis, the 2D graphene is enveloped by the 3D templated silica aerogel. By burning off the 17 ACS Paragon Plus Environment


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templates in air up to 450 °C, this process leads to the formation of molecular cavities vacated by the combustion of templates. By the same token, graphene has a stronger structure and cannot be combusted below 550 °C.55

Fig. 11 shows an idealised schematic of the 2D/3D assembly in this work, where graphene is intercalated within the silica aerogel structure. These 2D/3D assemblies are characterised by flakelike shapes with a high aspect ratio of wide sides and thin thickness. The mesopores were derived from the molecular cavities left from the burning off the templates. This type of assembly and shape facilitates the amine functionalisation on both surfaces and into the mesopores. Owing to the large aspect ratio and thin thickness, there is a significant large flake-shape surface of the 2D/3D G-Si aerogel available for amine functionalisation.

Fig. 11. Idealised schematic of the 2D/3D assembly containing graphene and mesoporous silica aerogel.

4. Conclusion 2D/3D assemblies containing graphene and silica aerogel were synthesised in a one-pot facile sol-gel method. A template was used to form mesoporous cavities in the 2D/3D structure upon burning them 18 ACS Paragon Plus Environment

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off. The 2D/3D assemblies were characterised by the formation of flake-like shapes with a high side aspect ratio and thin thickness. These morphological features suggest that the 3D silica (templated) aerogel was formed along the planes of the 2D graphene sheets. Both surface areas and pore volumes increased as a function of the carbon length of the template used in the synthesis, reaching maximum values for the 2D/3D assemblies prepared with C18 template of 734 m2 g-1 and 0.42 cm3 g-1, respectively. Similar correlation was determined for the CO2 sorption capacity for amine functionalised 2D/3D assemblies, which also increased as a function of the carbon length of the template. This was mainly attributed to the total pore volume which allowed a better distribution of amine in the 2D/3D structures. The CO2 sorption capacity of the amine functionalised 2D/3D sorbents reached high values at 4.9 mmol g-1 (equivalent to 11.67 mmol cm-3), whilst being stable over 10 cycles of CO2 sorption/desorption. In addition, the amine functionalised 2D/3D graphene silica aerogel delivered low heat of sorption as 59 kJ mol-1. This sorption capacity is still characterised by CO2 chemisorption on amine groups, though the lower heat of sorption was attributed to physisorption on graphene and silica aerogel. In terms of CO2 sorption capacity, stability and low heat of sorption, the overall performance of the amine functionalised 2D/3D graphene silica aerogel is favourable for engineering application in CO2 capture processes to avert climate change concerns. Appendix

Fig. A1. TGA mass loss curve of a 2D/3D assembly of graphene silica (templated) aerogel without TEPA. 19 ACS Paragon Plus Environment


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AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +61 7 3365 6960. Fax: +61 7 3365 4199. Notes The authors declare no competing financial interest.

Acknowledgement W. Wang gratefully acknowledges The University of Queensland scholarship. J.C. Diniz da Costa and X.S. Zhao thank the support from the Australian Research Council (ARC) via the Future Fellowship Program (FT130100405) and Laureate Fellowship Program (FL170100101), respectively.

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