Modified wollastonite sequestrating CO₂ and exploratory application of the carbonation products

Abstract

The feasibility of wollastonite carbonation for CO₂ sequestration was systematically investigated, and the recovery of the by-products (SiO₂, NH₄Cl) in the carbonation process, the controllable preparation of carbonation product (CaCO₃) with different polymorph and morphology during the carbonation process, and the application of the carbonation product were evaluated. The ammonia dosage and volume of the Ca²⁺ leaching solution showed a prominent effect on the wollastonite carbonation ratio. The synthesis of the carbonation product (CaCO₃) with different polymorphs and morphologies was successfully realized by controlling the temperature and ammonia dosage during the carbonation process. A reaction mechanism of wollastonite carbonation was also proposed by thermodynamic research of the gas–liquid–solid reaction. The carbonation ratio could reach up to 93%, and 1 ton wollastonite could sequester 350 kg of CO₂ under optimized conditions. The crystalline phase of the as-obtained CaCO₃ was spherical vaterite and cubic calcite, which could meet the relevant standards for the industrial precipitated calcium carbonate CaCO₃ (HG/T 2226-2010). The physical properties of the corresponding PP/CaCO₃ composites were very close. The cost of wollastonite carbonation was estimated to be 120 $ per t without industrial scale experiment. The wollastonite carbonation strategy showed potential application for CO₂ sequestration.

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1. Introduction

Human beings have played a very important role in controlling the global environment over the past century.¹ Modernization and industrialization of countries have been changing our environmental conditions. Undoubtedly, energy has played a very important role in the course of human development.² The consumption of fossil fuels was accompanied by a large amount of carbon dioxide emissions.^3–5 According to the Emission Database for Global Atmospheric Research,⁶ excessive fossil fuel consumption has increased the global CO₂ emission to 36.1 billion tonnes in 2013, which is 48% more than that of two decades ago. Over the past century, atmospheric CO₂ level increased more than 39%, from 280 ppm during pre-industrial times to the record high level of 400 ppm in May 2013 with a corresponding increase in global surface temperature of about 0.8 °C.⁷ The global carbon cycle cannot absorb all the anthropogenically produced carbon dioxide in the coming centuries, so adaptation technologies are urgently required. There are two main strategies for lowering the atmospheric concentration of CO₂: developing a clean energy and sequestering CO₂. Owing to the high cost of other energy sources, societal pressure and the established infrastructure of countries, fossil fuels will remain as the world's primary energy sources for the foreseeable future.^8–10 Methods for sequestering CO₂ currently being considered by industrialized countries include the enhancement of terrestrial carbon sinks as well as geological ocean and mineral sequestration.^11–15

Mineral carbonation, due to its rich in raw material, has recently been considered as a leading route for CO₂ sequestration.^16–19 Yan et al.²⁰ systematically studied the mineral carbonation reaction characteristics of wollastonite, serpentine and olivine under low-medium pressure conditions. At T = 150 °C, P = 40 bar and particle sizes <30 μm, the highest carbonation conversion efficiency of 83.5%, 47.7% and 16.9% was achieved for wollastonite, serpentine and olivine, respectively. By developing an attrition-leaching hybrid process for direct aqueous mineral carbonation, Julcour et al.²¹ reported that the carbonation yield for olivine and serpentinised ores could reach up to 35% in 5 h, 80% in 24 h in water, and 70% in 5 h with inorganic additives. Vance et al.²² discussed the carbonation of portlandite by carbon dioxide in the liquid and supercritical states as a potential route toward CO₂-neutral cementation. It is only slightly sensitive to the effects of temperature, pressure, and the state of CO₂ over the range of 6 MPa ≤ P ≤ 10 MPa and 8 °C ≤ T ≤ 42 °C. More than 80% carbonation of Ca(OH)₂ was achieved in 2 h upon contact with liquid CO₂ at ambient temperatures. During the Mg(OH)₂ slurry carbonation, Fricker et al.²³ investigated the different carbonate phases and their formation kinetics, and they found that the reaction temperature was a dominant parameter driving the formation of specific carbonate phases. Wang et al.²⁴ examined the serpentine carbonation of different flue gases and maintained the pH in the system using ammonium salts. An average carbonation efficiency of about 90% was achieved at 80 °C after 30 min. Overall, previous studies have described the different mineral carbonation processes in detail, but the carbonation ratio so far is still rarely able to surpass 90%. The researchers focused on CO₂ sequestration amount and carbonation ratio; there was little attention on the carbonation products. The cost of mineral carbonation for CO₂ sequestration was still very high.

In this study, wollastonite was used in a carbonation experiment, where hydrochloric acid and ammonia were employed as the reaction media to speed up the reaction. Many researchers have reported wollastonite carbonation,^25–32 where they all focused on how to improve the carbonation ratio. In this study, we investigated the feasibility of the route using an experimental approach and studied the entire procedure of wollastonite carbonation for CO₂ sequestration, including acid-leaching of wollastonite, carbonation of wollastonite, recovering of byproducts and application of carbonation product. To broaden the application field of the carbonation product, polymorph and morphology controllable preparation of carbonation product during the carbonation process was discussed. Moreover, we explained the underlying mechanisms of the carbonation process by thermodynamics analysis, and the economic cost of wollastonite carbonation was also simply estimated.

2. Experimental

2.1. Materials preparation

Pristine wollastonite used in this study was obtained from Jiangxi, China. Detailed analysis of the raw material was in the previous study.²⁵ Polypropylene pellets (PP, ρ = 0.91 g cm⁻³, ∼3 mm in size) was obtained from China Petroleum & Chemical Co., Ltd. The titanate coupling agent, hydrochloric acid, ethylalcohol and ammonia were used without further purification. Carbon dioxide was industry-grade with a purity of 99.9%.

From our previous research into acid-leaching for clay minerals,^33–35 pristine wollastonite was first acid-leached at 80 °C for 120 min with a 6 M hydrochloric acid solution. 20 g of pristine wollastonite was placed into a 500 mL reaction vessel, and 200 mL of hydrochloric acid solution was then added. A magnetic stir bar was used to ensure adequate mixing throughout the reaction. A hot bath was used to maintain the desired reaction temperature. After the desired reaction time, the solid (SiO₂) was then washed and dried for further analysis. The acid-leaching filtrate was neutralized using ammonia for the carbonation experiment. A carbonation experiment was performed by adding ammonia and carbon dioxide into the neutral acid-leaching filtrate. The CO₂-rich gas was injected at a predetermined flow rate at normal pressure. A pH meter was installed to monitor the progress of the carbonation reaction. The pH value was recorded at 1 min interval until the pH was relatively constant. Carbonation experiment was referred to the desulfurization residue carbonation.³⁶ After a desired reaction time, the mixture was cooled and filtered to separate the solids (CaCO₃) from the solution. The solids were then washed with distilled water until they reached a neutral pH, before being dried in an oven at 110 °C for 24 h. The carbonation filtrate was evaporated at 80 °C to obtain NH4Cl for further analysis. The modification process and preparation of the PP/carbonation product composites are shown in ESI† in detail.

2.2. Characterization

The composition of the sample was determined by X-ray fluorescence (XRF) and chemical titration. Powder XRD measurements of the samples were conducted with a DX-2700 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) at a scanning ratio of 0.02 deg. s⁻¹ with a voltage of 40 kV at 40 mA. SEM was performed using a JEOL JSM-6360LV scanning electron microanalyzer with an accelerating voltage of 5 kV. Fourier transform infrared (FTIR) spectroscopy of the samples over the range of 4000–400 cm⁻¹ was performed on a Nicolet Nexus 670 FTIR spectrometer. The samples were ground with KBr crystals, and the mixture was pressed into a pellet for the FTIR measurements. The particle size was measured using a particle size analyzer (LS-POP(6), Zhuhai OMEC Instrument Co., Ltd, China). The physical properties of the additive-PP composites were measured using a mechanical testing machine (WDW-1000, China) and a softening temperature testing machine (XRW-300MA, China) according to the standards of GB/T 1042-1992 and GB/T 9341-2000, respectively. The calculation methods of the carbonation ratio (η) and the molar content (%) of CaCO₃ polymorphs are described in ESI,† respectively.

3. Results and discussion

3.1. Wollastonite carbonation process

The effects of ammonia dosage, CO₂ flow rate, volume of Ca²⁺ leaching solution and temperature on the carbonation of wollastonite are discussed by monitoring the pH of the reaction solution and carbonation ratio. The detailed analysis is shown in Fig. S1–S4 in ESI.† According to the analysis, the ammonia dosage and the volume of Ca²⁺ leaching solution were the most important process variables influencing the wollastonite carbonation process. The effects of either the CO₂ flow rate or temperature was not as notable as the ammonia dosage and volume of Ca²⁺ leaching solution. Under the optimized conditions (8 mL, 100 mL min⁻¹, 150 mL, 30 °C), the carbonation ratio can reach about 93%.

3.2. Application of carbonation product (CaCO₃)

The abovementioned research has clearly demonstrated the feasibility of using wollastonite for CO₂ sequestration with industry-grade using the proposed scheme. By following the optimized process, the carbonation ratio (η) can reach 92.5%, and 1 ton wollastonite was able to sequester 350 kg of CO₂, a value greater than the vast majority of the other wollastonite carbonation obtained previously (Table 1). Fig. 1 shows the XRD and SEM results of the acid-leached wollastonite, carbonation by-product and carbonation product obtained under the optional conditions. There was a prominent amorphous package of SiO₂ in the acid-leached wollastonite (Fig. 1a), and the crystal phase was hydrogen silicate. The morphology of the particles was granular (Fig. 1c), and the average particle size (d₅₀) was 21.8 μm (Fig. 1b). The content of SiO₂ in the samples was 98.3%. The whiteness of the samples was 92%. The only crystal phase was sal ammoniac in the carbonation by-product (Fig. 1a). The purity of the NH₄Cl byproduct evaluated by the XRD pattern was 99%, which could meet the relevant standards for the ammonium chloride NH₄Cl (GB/T2946-2008). The main crystal phase of carbonation product (CaCO₃) was of vaterite containing some calcite. The morphology of the particles was spherical and cubic (Fig. 1d), and the average particle size (d₅₀) was 16.7 μm (Fig. 1b). The content of CaCO₃ in the samples reached 99.1%. The whiteness of the samples was 93%. The performance indices of the carbonation product (CaCO₃) met the relevant standards for the industrial precipitated calcium carbonate CaCO₃ (HG/T 2226-2010); therefore, it could possibly replace calcium carbonate used in plastics field. In the following study, we experimentally analyzed the carbonation product (CaCO₃) used in PP-packing in detail.

Table 1 Comparisons of CO₂ sequestration by wollastonite carbonations

No.	Process conditions	Carbonation ratio (%)	References
1	Carbonation at 80 °C and 3 MPa for 30 min	20	27
2	Particle size < 106 μm, T = 100–225 °C for 15 min	45	50
3	Carbonation at T = 185 °C, P = 150 atm for 1 h and particle sizes < 37 μm	82	51
4	NaHCO3 was used as agent, carbonation at T = 150 °C, P = 40 bar and particle sizes < 30 μm	83.5	20
5	150 μm < particle size < 250 μm after 2 days, T = 90 °C, pCO2 = 250 bar	85	26
6	NH4OH was used as agent, carbonation at room temperature and atmospheric pressure	91.1	25
7	Carbonation at T = 100 °C and P = 40 atm	100	28
8	NH4OH was used as agent, carbonation at room temperature and atmospheric pressure	92.5	This work

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Fig. 1 (a) XRD patterns of the acid-leached product (SiO₂), carbonation product (CaCO₃), and by-product (NH₄Cl) for mineral carbonation of wollastonite, (b) particle size distribution of SiO₂, and SEM images of (c) SiO₂ and (d) CaCO₃.

CaCO₃ was first surface-modified by the titanate coupling agent, and then added to polypropylene (PP) to prepare PP/CaCO₃ composites. The detailed modification analysis is shown in Fig. S5.† The XRD patterns of PP and PP/modified CaCO₃ composites are shown in Fig. 2. The reflections at 2θ 13.68°, 16.50°, 18.16°, and 21.34° were characteristic of α-PP crystalline phase (Fig. 2a), corresponding to crystal planes (110), (040), (130), and (041),³⁷ respectively. PP/modified CaCO₃ composites had similar diffraction patterns to pure PP, indicating that no other PP crystalline phases appeared. The results illustrated that the morphology of modified CaCO₃ for fillers could not induce the crystalline phase of PP. The intensities of the calcium carbonate crystal diffraction reflections clearly increased with increasing modified CaCO₃ powder content (Fig. 2a); however, this did not lead to a crystalline transformation of PP. Fig. 2b–f show the SEM images of the PP/modified CaCO₃ composites, indicating that CaCO₃ particles increased with the addition of modified CaCO₃ powder content.

Fig. 2 (a) XRD patterns, SEM images of PP/modified CaCO₃ composites with different percentages of modified CaCO₃ ((b) 10%, (c) 20%, (d) 30%, (e) 40%), and (f) the fractured surface of the corresponding sample (e).

To illustrate intuitively the feasibility of carbonation product (CaCO₃) for applications in plastics, we compared the physical properties of PP/commercial CaCO₃ composites, PP/CaCO₃ composites, PP/modified commercial CaCO₃ composites and PP/modified CaCO₃ composites. The variation of the physical properties of PP/CaCO₃ composites as a function of CaCO₃ and surface modification is shown in Fig. 3, and the impact strength and tensile strength decreased with increasing modified CaCO₃ content (Fig. 3a and b). However, the addition of modified CaCO₃ to PP could enhance the flexural strength and soften temperature of the composites from 28 MPa and 63 °C for PP to 39.3 MPa and 92.6 °C for PP/modified CaCO₃ composites (Fig. 3c and d). While at the addition of 40% CaCO₃, the flexural strength and softening temperature of PP/CaCO₃ composites could increase up to 34.3 MPa and 119.6 °C. Compared to the PP/CaCO₃ composites with the same conditions, the mechanical properties of PP/modified CaCO₃ composites to some degrees increased, and the soften temperature accordingly decreased. The results further demonstrated that the morphology of CaCO₃ had an obvious effect on the strength of PP-composites. The physical properties of PP-composites filled with commercial CaCO₃ and surface modification are also shown in Fig. 3. Compared to CaCO₃ and surface modification, they had the same influence on the PP materials. The physical properties of PP/CaCO₃ were close to that of PP/commercial CaCO₃ (Fig. 3). Therefore, the wollastonite carbonation product (CaCO₃) can basically replace commercial CaCO₃ used in the plastics field.

Fig. 3 Physical properties of PP/CaCO₃ composites with different CaCO₃ content (10–40%): (a) impact strength, (b) tensile strength, (c) flexural strength and (d) softening temperature.

3.3. Polymorph and morphology control of carbonation product

Precipitated calcium carbonate, an important industrial chemical, has been utilized widely for various types of functional application fields such as paper-making, rubber, food, coating, pharmaceuticals and cosmetic manufacturing.^38,39 It is generally accepted that the crystal type, morphology and their properties are closely related, and different forms of particles sometimes gave different particle properties even for the same substance.^40–42 Therefore, it was very important to design and synthesize CaCO₃ with well-controlled polymorph and morphology. In the wollastonite carbonation process, we simultaneously obtained the carbonation product (CaCO₃) with a single calcite structure or a single vaterite structure by controlling the reaction temperature or ammonia dosage (Fig. 4 and 5). The carbonation product (CaCO₃) with single crystal phase or morphology could potentially be applied to much wider fields.

Fig. 4 Polymorph and morphology-controllable preparation of the carbonation product with different temperatures: (a) influence of temperature on the XRD patterns of the carbonation product, (b) influence of temperature on the composition of carbonation products: calcite and vaterite, influence of temperature on the SEM images of carbonation product: (c) 20 °C, (d) 25 °C, (e) 30 °C, (f) 40 °C, (g) 50 °C and (h) 60 °C.

Fig. 5 Polymorph and morphology controllable preparation of the carbonation product with different ammonia dosages: (a) influence of the ammonia dosage on the XRD patterns of the carbonation product, (b) influence of the ammonia dosage on the composition of carbonation products: calcite and vaterite, influence of the ammonia dosage on the SEM images of the carbonation product: (c) 5 mL, (d) 8 mL, (e) 10 mL and (f) 20 mL.

Pure CaCO₃, as the product of wollastonite carbonation for CO₂ sequestration, was successfully obtained under the action of hydrochloric acid and ammonia through the abovementioned research. Herein, the process was further investigated to elucidate CaCO₃ polymorphism as a function of the reaction temperature (Fig. 4) and ammonia dosage (Fig. 5). Fig. 4 revealed the influence of temperature on the structure and morphology of the products. Pure spherical aragonite was detected at 20 °C (Fig. 4a and c). The product composed of spherical vaterite and calcite with a cube shape was synthesized at 25–50 °C (Fig. 4a and d–g). With an increase in temperature from 25 °C to 50 °C, the molar content of vaterite decreased from 90.8% to 62% and that of calcite increased from 9.2% to 38% (ref. 43) (Fig. 4b). Furthermore, pure calcite could be obtained at 60 °C (Fig. 4a), which showed that the increased temperature led to an increase in calcite and a decrease in vaterite. The FT-IR spectral analysis of the products formed at different temperatures were also performed. Fig. S6a† shows the absorption peaks at 877 cm⁻¹ and 744 cm⁻¹ for the ν₂ and ν₄ vibrations of CO₃²⁻ in vaterite,⁴⁴ respectively, and the peaks at 848 cm⁻¹ and 714 cm⁻¹ that are characteristic absorption peaks of calcite⁴⁵ are consistent with XRD analysis. Fig. 4c–h show the SEM images of the carbonation product at different temperatures.

The influence of the ammonia dosage on the morphology and XRD patterns of the products is also discussed in Fig. 5. Calcite, which was the only crystal phase when using a stoichiometric amount of ammonia, was replaced by vaterite upon the addition of excess ammonia (Fig. 5a). The molar content of calcite with different ammonia dosages was 100%, 26.75%, 0 and 0, respectively, whereas the molar content of vaterite was 0, 73.25%, 100% and 100%, respectively (Fig. 5b). The result was consistent with the previous study.⁴⁶ The results of the morphology and FT-IR spectra analysis of calcite and vaterite produced with different ammonia dosages (Fig. 5c–f and S6b†) were consistent with the corresponding results under different temperatures.

3.4. Mechanism of wollastonite carbonation

The aqueous carbonation of materials with low porosity such as wollastonite is an irreversible and heterogeneous gas–liquid–solid reaction. In the gas–liquid–solid system, the overall reactions for the mineral carbonation of wollastonite through the proposed process are expressed by eqn (1)–(5). To discuss the reaction mechanism of wollastonite carbonation for CO₂ sequestration following the HCl–NH₄OH system, the thermodynamic parameters at the standard state in the carbonation process of wollastonite were calculated.⁴⁷

CaSiO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + SiO₂(s), ΔG (kJ mol⁻¹) = −201, K = 1.71 × 10³⁵ (1)

CaCl₂(aq) + 2NH₄OH(aq) → Ca(OH)₂(s) + 2NH₄Cl(aq), ΔG (kJ mol⁻¹) = −302, K = 8.66 × 10⁵² (2)

Ca(OH)₂(s) + CO₂(g) → CaCO₃(s) + H₂O(l), ΔG (kJ mol⁻¹) = −122, K = 2.43 × 10²¹ (3)

2NH₄OH(aq) + CO₂(aq) → (NH₄)₂CO₃(aq) + H₂O(l), ΔG (kJ mol⁻¹) = −276, K = 2.40 × 10⁴⁸ (4)

CaCl₂(aq) + (NH₄)₂CO₃(aq) → CaCO₃(s) + 2NH₄Cl(aq), ΔG (kJ mol⁻¹) = 107, K = 1.75 × 10⁻¹⁹ (5)

From the thermodynamic calculation results, the Gibbs free energy changes (ΔG) are −201, −302, −122 and −276 kJ mol⁻¹ for eqn (1)–(4). They are all less than zero. Their corresponding equilibrium constants (K) are 1.71 × 10³⁵, 8.66 × 10⁵², 2.43 × 10²¹, and 2.40 × 10⁴⁸. They are all greater than zero. Therefore, eqn (1)–(4) can proceed spontaneously at the standard state and regarded as an irreversible exothermic reaction. However, the Gibbs free energy change (ΔG) is 107 kJ mol⁻¹ for eqn (5), more than zero. Its corresponding equilibrium constant (K) is 1.75 × 10⁻¹⁹, close to zero. Eqn (5) cannot proceed spontaneously at the standard state. Compared to the equilibrium constant for eqn (4) (K = 2.40 × 10⁴⁸), the equilibrium constant for eqn (2) (K = 8.66 × 10⁵²) is greater, which indicates that ammonia was prone to reaction with calcium chloride to from calcium hydroxide. In the desulfurization residue³⁶ (or desulfurization gypsum⁴⁸) carbonation process, the role of ammonia was to form ammonium carbonate (or ammonium bicarbonate). The reaction mechanism was different in the two systems. Through the abovementioned analysis, the probable mechanism of wollastonite carbonation could be summarized in the following three steps: (a) leaching of Ca²⁺ from pristine wollastonite, (b) calcium chloride transformed into calcium hydroxide under the action of ammonia, and (c) CO₂(g) dissolved in water and reacted with calcium hydroxide to form calcium carbonate.

3.5. Cost estimation of the entire process for wollastonite carbonation

The abovementioned research clearly demonstrated the feasibility of this route proposed in the paper using wollastonite for CO₂ sequestration. The entire process consisted of four steps (Fig. 6): (a) Ca²⁺(aq) leached from the natural wollastonite under the action of hydrochloric acid, (b) CO₂(g) dissolved in water to generate CO₃²⁻(aq), (c) carbonation product precipitated from the solution under the influence of ammonia, and (d) carbonation product was applied in plastics as a filler. Approximately 1 ton of wollastonite can absorb 350 kg of CO₂, where the process requires 0.6 ton of hydrochloric acid, 0.9 ton of ammonia and 3.2 ton of water. In addition, 1 ton wollastonite was also able to produce 510 kg SiO₂, 840 kg NH₄Cl, and 797 kg CaCO₃. These products can also produce a certain economic value. From previous research,^28,36,49 we estimated the total cost of the entire wollastonite carbonation process to be around $120, including energy consumption, the value of the by-products and carbonation product (Table 2).

Fig. 6 Proposed schematic of the wollastonite carbonation for CO₂ sequestration.

Table 2 Price of raw materials and treatment energy consumption of the proposed methods^a

Feed material/products	Costs ($ per ton)	Treatment methodology	Treatment energy consumption (kW h per ton)	Total costs ($)
a Note: all the cost of the feed material and products were determined by market price.
Wollastonite	133	Heat treatment	300	120
Water	0.7
HCl	23	Stirring (400 rpm)	50
NH4OH	92
SiO2	75	Filtering	100
NH4Cl	85
CaCO3	92	Drying	50

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4. Conclusions

The mineral carbonation of wollastonite for CO₂ sequestration under the action of hydrochloric acid and ammonia was studied through experiments and thermodynamic calculations. The ammonia dosage and volume of the Ca²⁺ leaching solution were the most important process variables influencing the carbonation process. The influence of either the CO₂ flow rate or temperature was not as notable as the ammonia dosage and volume of the Ca²⁺ leaching solution. The carbonation ratio can reach about 93%, and 1 ton of wollastonite was able to sequester 350 kg of CO₂. The main crystalline phase of the carbonation product was spherical vaterite, which could meet the relevant standards (HG/T 2226-2010), and their physical properties of PP/CaCO₃ composites were very close. The cost of wollastonite carbonation could be reduced by recovering the carbonation byproducts and making full use of carbonation product (CaCO₃) with a single calcite structure or a single vaterite structure prepared by controlling the reaction temperature or ammonia dosage during the carbonation process. This mineral carbonation strategy could have interesting potential in CO₂ sequestration.

Acknowledgements

This study was supported by the National Science Fund for Distinguished Young Scholars (51225403), the National Natural Science Foundation of China (41572036), the State Key Lab of Powder Metallurgy, Central South University (2015-19), the Hunan Provincial Science and Technology Project (2015TP1006) and the Hunan Provincial Co-Innovation Centre for Clean and Efficient Utilization of Strategic Metal Mineral Resources (2014-405).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: A detailed description of the experimental setup, analytical techniques, and additional results. See DOI: 10.1039/c6ra13908f

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