2.1. Preparation of NaMt-H30 and characterization

Na⁺ exchanged montmorillonite (NaMt) containing 1 wt% of dendrimer H30 (NaMt-H30) has been prepared and characterized in previous works [9, 11, 12, 13]. As a reminder, NaMt originated from the purification of commercial crude bentonite supplied by Aldrich with a 1.58 silica/alumina weight ratio, as previously described. A bulky polyester-alcohol with the chemical formula 2,2-bis(methylol)propionic acid (bis-MPA) with an idealized structure illustrated in Scheme 1 was employed as organic moiety [14, 15]. This compound is commercially designated as dendrimer H30 (supplied by Boltorn Technology Co. Ltd.) and having an average 3607.6 g⋅mol⁻¹ molecular weight.

Clay intercalation with H30 was achieved by mere electrostatic self-assembly in aqueous media between a NaMt suspension and the organic polyol species. For this purpose, attempts were performed by impregnating NaMt samples in powder form (500 mg) with a diluted aqueous solution containing 1 wt% polyol H30 (5 mg). The resulting mixture was then stirred overnight at room temperature. The organoclay sample (organo-Mt) was dried overnight at room temperature until total evaporation of the solvent, and then at 120 °C for 6 h before being smoothly crushed for preventing structure alteration.

NaMt and NaMt-H30 were both characterized by X-rays diffraction (Siemens D5000 instrument, Co-K_α at 1.7890 Å), Fourier Transform infrared ATR-mode spectroscopy (using a KBr IR cell and a Model IR 550 Magna Nicolet instrument) and thermal gravimetric analyses (TG/TDA6200 Seiko Instrument Inc., under a 100 mL⋅min⁻¹ nitrogen stream and a 5 °C⋅min⁻¹ heating rate) [9, 11, 12, 13]. The specific surface area and pore size distribution were assessed by BET method calculations. For this purpose, measurements through N₂ adsorption isotherms were achieved at 77 K by means of a Quantachrome Instruments equipped with an Autosorb automated gas sorption system control. For the sake of conciseness, the related spectra and diagrams are not shown in this work, but their results were only discussed herein in correlation with the adsorptive properties.

2.2. Thermal programmed desorption

The retention strength and CO₂/water retention capacities (CRC and WRC, respectively) were assessed through thermal programmed desorption (TPD). The retention strength was regarded as being proportional to the desorption temperature (desorption peak), while the CRC was defined as the area described by the TPD pattern and expressed in terms of micromole CO₂ desorbed per gram of dry clay material between 20 °C and 200 °C. This upper temperature limit was determined by TG-DTG measurements, which revealed a thermal stability threshold of up to 240 °C for NaMt-H30. Preliminary investigations showed that optimum amount (50 mg) and particle size (0.01–0.1 mm) of the adsorbent sample allowed preventing diffusion phenomenon during TPD runs under nitrogen stream not exceeding 15 mL⋅min⁻¹. Prior to TPD measurements, each sample was dried in a 10 mm tubular glass reactor under a 15 mL⋅min⁻¹ dry nitrogen stream at 160 °C for 1 h, and then cooled to 20 °C. At this temperature, CO₂ was injected without nitrogen (static mode) for accurate CRC assessment. After saturation, the non-adsorbed CO₂ excess was evacuated by the nitrogen stream at the throughput selected for TPD until no CO₂ is detected by Li-840A CO₂/H₂O Gas Analyzer (Li-COR Bioscience Inc., USA) at the TPD device outlet after 40–60 min. TPD measurements were performed between 20 °C and 200 °C at a heating rate of 5 °C⋅min⁻¹ under 1–5 mL⋅min⁻¹ nitrogen stream. Here, care should be taken in CRC measurements, because higher nitrogen flow rates during saturation were found to remove the adsorbed CO₂ by mere convection even without heating.

2.3. Adsorption capacity modeling

Thus, 27 CO₂ adsorption attempts were made according to a 3³ experiment factorial design, by varying the nitrogen flow rate from 1 to 15 °C⋅min⁻¹ (X₁), contact time from 1 to 12 hours (X₂) and quantity of carbon dioxide from 1 to 6 mmol (X₃) in suitable parameters ranges (Table 1).

The model calculations were achieved using non-dimensional or reduced values of these parameters and each parameter was varied on three levels, i.e., (i) −1 for the minimum value, (ii) 0 for the medium value, and (iii) +1 for the maximum value of the respective parameter. The CRC and WRC were considered as optimality criteria for each of the 27 different attempts.

3.1. Structure of NaMt-H30

The structure of NaMt-H30 had been richly investigated in our previous papers [9, 11, 12, 13, 16]. The powder XRD data indicated the transition from a random to a face-to-face parallel layout of the clay sheets and an optimum ratio of 1 wt% H30 for an enlargement of the interlayer spacing without altering the ordered layout of the clay lamellae. The influence of the OH groups from the clay on the incorporated dendrimer was also detailed in our previous work [10, 11, 12, 13, 16, 17, 18]. The uniform H30 spread in the clay surface produced only a slight improvement of the specific surface area from 45–50 m²⋅g⁻¹ to 54 m²⋅g⁻¹, presumably due to slight structure compaction induced by interactions between next-neighboring organoclay lamellae [11, 19].

3.2. Moisture contribution to CO₂ capture

The surface basicity and hydrophilic character expressed in terms of CRC and WRC were the main features investigated herein. Given the higher thermal stability of unmodified NaMt up to 550 °C, repetitive CO₂-TPD attempts (CO₂-TPD1 to CO₂-TPD3) without alternate rehydration after each TPD cycle were run between 20 °C and 450 °C. The results obtained revealed significant changes in the TPD profiles for both CO₂ and water (Figure 1). The significant CRC decay observed between 220 °C and 450 °C accounts for an advanced removal of medium and strong basicities. Indeed, three successive thermal desorption-cooling-dry CO₂ re-injection cycles induced a marked CRC decay for CO₂-TPD2 after TPD1 even more pronounced for CO₂-TPD3 after TPD2. Simultaneously, a marked depletion of the hydrophilic character for H₂O-TPD2 even more accentuated for H₂O-TPD3 took place, as reflected by visible CRC decreases in the WRC (Figure 1b).

It is worth mentioning that repetitive TPD runs after alternate rehydration by injecting moisture saturated CO₂ resulted in TPD patterns (TPD-R) perfectly similar to that of TPD1 for both CO₂ and water. This result is of great importance, because it confirms the thermal stability of the starting clay mineral at least up to 450 °C. It is clear that below this thermal stability threshold, NaMt rehydration revives the CRC and that water also contributes to CO₂ capture.

3.3. Adsorptive properties of NaMt-H30

NaMt-H30 exhibited appreciable thermal stability, barring dehydration between 80 °C and 140 °C, no degradation of the organic moiety was detected up to 240 °C. Such a thermal stability allowed establishing the temperature range for TPD measurements between 20 °C and 200 °C with high accuracy. TPD measurements were achieved for the 27 attempts involved in the factorial 3³ experiment design, by varying three key parameters, namely the carrier gas flow-rate (X₁), contact time (X₂) and impregnating CO₂ quantity (X₃). The CRC, expressed in terms of desorbed CO₂ amount in the investigated temperature range, showed values in the range 34.4–609.1 μmol⋅g⁻¹ for nitrogen throughputs of 1–15 mL⋅min⁻¹ (Table 2).

Preliminary observations showed that lowest flow rates, medium saturation time and medium to high amount of injected CO₂ gave the highest CRC values (441.3–609.2 μmol⋅g⁻¹). Conversely, high carrier gas throughput resulted in lowest CRC levels (34.4–53.1 μmol⋅g⁻¹), thereby confirming CO₂ capture via weak interaction with the OH groups of the organoclay and easy release upon forced convection without heating. The high discrepancy between the highest and lowest CRC levels accounts for 64–94.4% of the nominal CRC value obtained in these experiments. This result is of great importance, because it clearly demonstrates the occurrence of truly reversible capture of CO₂, and that forced convection affects CRC assessments.

3.4. Model calculations and accuracy

CO₂ retention capacity (CRC) values and water retention capacity (WRC) values were modeled on the basis of the 27 experimental attempts, using a Taylor’s second-order polynomial model [20, 21, 22, 23]. Before model refinement, the general Taylor equation for both the CRC and WRC (denoted as Y ) is given by (1):

The calculations of the model coefficients are not described in the present work, but details about the method used are described elsewhere [21]. The calculated values of the model coefficients (Table 3) are assumed to describe the individual effects of the investigated parameters and their possible interactions. The model adequacy strongly depends on the accuracy of the experiment measurements, where the main errors arise from volume and weight measurements. For this purpose, three additional attempts at the central point (0,0,0) are required for estimating the average error in calculating the value of each coefficient on the basis of the random variance (Table 4).

Model refining using the t-student test gave lower average error of 0.133 than the very coefficient values. Therefore, all coefficients were maintained in the CRC model (2). Application of the same Student’s t test to the WRC model gave higher |𝛥a_i| (0.332) than coefficients a₃, a₃₃, a₁₃, a₂₃ and a₁₂₃. Removing these coefficients were removed from the WRC model (3) suggests negligible effect and interactions of their common parameter, namely the injected CO₂ amount (X₃) on the moisture content.

A method of adequacy test was achieved through the Fisher’s test, expressed in terms of the quotient of the random variance, previously calculated and the residual variance estimated in Table 4. In this case also, both calculated models (with 3 degrees of freedom) are considered to fit the CRC and WRC in the same range, because F ≫ F_0.95,3,4 with a 95% confidence.

3.5. Effect simulation and optimization

The individual effects and interactions of the parameters were discussed on the basis of the sign and the absolute value of each coefficient. These coefficient features will define the direction and intensity of the corresponding effect (favorable or detrimental) on CRC and WRC assessment. As expected, the carrier gas throughput displayed the strongest detrimental effect on the CRC and to a lesser extent on the WRC, confirming once again that both CO₂ and water are weakly retained and that the occurrence of forced convection affects the assessment both retention capacities.

As a general tendency, increasing nitrogen throughput reduces the CRC. However, increasing contact time during adsorption induced a much weaker influence on the CRC but a favorable individual effect on the WRC. This must be due to stronger diffusion hindrance of non-linear and more polar water molecules within the organoclay entanglement as compared to CO₂.

The values of the quadratic coefficients, a_ii, are essential criteria for estimating the concavity (convexity) of the response surface. The opposite signs of the quadratic coefficients and concavity decrease in the following sequence: |a₁₁|≫|a₂₂| > |a₃₃|, allowed predicting a pronounced minimum of the CRC response-surface with respect to the nitrogen throughput and flattened maximum with respect to the contact time (Figure 2a). A similar but more flattened saddle-shaped response-surface was observed when replacing the contact time (X₁) by the injected CO₂ (X₃) (Figure 2b). This confirms the strong influences of the investigated parameters.

Informative plots were obtained for CRC = f(X₂, X₃) with X₁ = 0 or 1 (Figure 3). The obtained response-surface revealed CRC maxima at optimum values of the contact time (X₂) and impregnating CO₂ amounts (X₃). The occurrence of optimum contact time must be due to slow gas diffusion with the organoclay dendritic entanglement. The fact that the optima are included within the investigated parameters ranges provides clear confirmation of the model validity. Analytical resolution of the equation system resulting from the first derivation of the CRC models for X₁ = 0 gave optimum values of X₂ = 0.438 and X₃ outside the investigated parameter range (1.872).

Similar calculations for X₁ = 1 gave optimum X₂ = 0.048 and X₃ = 0.71. As expected, calculations using these values in the CRC model gave values below 20 μmol⋅g⁻¹ for highest and medium carrier gas throughputs (+1 and 0). This can be explained by the occurrence of forced convection that can release both CO₂ and water molecules, in agreement with our previous statements. This suggests that other optimum parameters should occur at the lowest carrier gas throughput. The shape of CRC plot for the slowest throughput of the carrier gas (X₁ = −1) allowed predicting a clear optimum (Figure 4). Calculations using the corresponding CRC model resulted in values included within the investigated parameters ranges (X₂ = 0.40 and X₃ = 0.92). Conversion of these non-dimensional values gave optimum contact time (X₂) of 8.7 h and amount of impregnating CO₂ (X₃) of 24.1 mmoL⋅g⁻¹. Qualitative CRC assessment using these optimum parameters in the model resulted in the highest CRC of 35 μmol⋅g⁻¹ among those calculated for various parameter combinations. This CRC is much lower than those obtained by additional triplicate experiments (65.0, 59.4 and 68.2 μmol⋅g⁻¹) under the above determined optimum parameters.

This result confirms once again that the CRC cannot be accurately assessed using such a mathematical model, which is rather intended for parameter optimization. An attempt justified by economic reasons, achieved for minimum contact time (X₂ = −1) failed because of even lower estimated CRC values, below 10 μmol⋅g⁻¹. Thus, it is clear that unlike slow gas stream for avoiding forced convection, minimum contact time cannot be an essential requirement for achieving effective CO₂ capture on NaMt-H30. Here, the unavoidable diffusion hindrance imposes sufficiently long contact times of such adsorbents, but is not necessarily a major drawback being compensated by the non-stoichiometric capture of CO₂. Indeed, a 50 mg NaMt-H30 sample with a 1 wt% polyol H30 (M = 3607.6 g⋅mol⁻¹ and 32 OH/mole) accounts for 88.7 μmoles OH per gram adsorbent.

This is by far lower than the highest CRC value determined in the 3³ factorial design (609.1 μmol⋅g⁻¹) (Table 2) and those obtained by additional triplicate experiments (650.0, 594.0 and 682.0 μmol⋅g⁻¹). It results that each OH group in NaMt-H30 can retain more than 13 CO₂ molecules. This is a significant result that allows envisaging even higher effectiveness by increasing the number of available OH groups. This can be achieved through higher polyol dendrimer dispersion of the clay surface for minimizing the intermolecular H-bridges. The surprising occurrence of an optimum contact time for the CRC can only be explained a similar phenomenon for the WRC for both medium and slow carrier gas throughput (Figure 5).

Insufficient contact time is assumed to affect the moisture content due to slow water molecule diffusion in agreement with previous statements. Conversely, long contact time should probably favor the diffusion of linear CO₂ at the expense of bulkier water molecules. The resulting decrease in moisture content is expected to reduce water–CO₂ interaction and subsequently the CRC. This result is or great importance, because it clearly confirms the narrow interaction between the surface basicity and hydrophilic character in such OH-enriched organoclays.