Thermally Enhanced Acidity for Regeneration of Carbon Dioxide Sorbent

Abstract

The thermal regeneration of CO₂ sorbent is the most energy-consuming step in the CO₂-capturing process. Although the addition of an acid can induce CO₂ release, it does not regenerate the sorbent because the acid forms a salt with the basic sorbent and diminishes its capability for capturing CO₂. In this work, a novel approach based on thermally enhanced acidity was studied. This approach utilizes an additive that does not affect the sorbent at room temperature, but its acidity significantly increases at elevated temperatures, which assists the thermal release of CO₂. M-cresol was added to an aqueous solution of morpholine. The CO₂ capture and release of the mixture were compared to those of a control solution without m-cresol. The amounts of carbamate, bicarbonate, and unreacted morpholine were quantitatively determined using ¹H NMR and weight analysis. The results showed that m-cresol did not affect the reactivity of morpholine in the formation of carbamate with CO₂ at room temperature. At elevated temperatures, the acidity of m-cresol increased according to Van’t Hoff’s equation, which resulted in a significantly higher rate of CO₂ release than that of the control. Given the low cost of m-cresol and its derivatives, this approach could lead to practical technology in the near future.

1. Introduction

Carbon dioxide (CO₂), a prominent greenhouse gas, is impactful in accelerating global warming. The surplus of atmospheric CO₂ resulting from the combustion of fossil fuels accounts for approximately 80% of all greenhouse gas emissions [1]. The increase in the atmospheric concentration of CO₂ related to the rise in the global temperature has led to urgent concerns about the potentially catastrophic impacts on civilization and ecosystems [2]. Extensive efforts have been directed toward developing efficient capture technologies to reduce CO₂ emissions and enable CO₂ utilization for various industrial applications.

Among the various materials investigated for capturing CO₂, amine-based sorbents have shown promising results in industrial emission controls for combating the rise of atmospheric CO₂ levels [3,4]. Primary and secondary amines such as monoethanolamine (MEA), diethanolamine (DEA), ethylamine (EN), and morpholine (MOR) are widely utilized at the industrial level due to the capability of forming stable carbamate and bicarbonate species with CO₂ as well as the rate of the capturing reactions [3,4,5,6,7,8,9,10,11,12,13]. As the amine absorbs the CO₂ into the solution, the amine acts as a Bronsted base and a nucleophile, while the CO₂ acts as a Bronsted acid and an electrophile. The resultant solution contains a mixture of bicarbonate and carbamate salts. The carbamate salt requires two amines per addition of CO₂. One of them is protonated and functions as the cationic counter ion of the carbamate anion. The bicarbonate uses one amine per addition of CO₂. The capture and release of CO₂ are governed by the stability of the carbamate and bicarbonate.

Regeneration of the sorbents is necessary for an economically practical technology for CO₂ capture. Different methods have been reported to regenerate the amine sorbents by thermal, chemical, and photo processes [14,15,16,17,18,19,20,21]. Thermal decomposition is the most commonly used method industrially. Because the high thermodynamic stability of the carbamates and bicarbonates is necessary for efficient CO₂ capture, thermal decomposition is highly energy-consuming. In aqueous systems, the energy requirements for sorbent regeneration are further increased by the high heat capacity of water. In addition, the high temperatures needed for sorbent regeneration can cause significant oxidation and thermal degradation of amine sorbents.

Bicarbonates and carbamates are not stable under acidic conditions and decompose to release CO₂. Previous works showed that when a carboxylic acid was added, the rate of CO₂ release from the sorbents was significantly enhanced [15]. However, the sorbent is not regenerated because it forms a salt with the acid and loses its capability for CO₂ capture. The acid must be removed by, for example, crystallization methods after CO₂ is released. In fact, it was shown that even the carboxylic acid residue reduced the capability for CO₂ capture of the regenerated sorbent [15]. The photo-induced pH-swing has attracted much attention in recent years as a process to overcome the issues associated with both thermal and chemical release [16,17,18,19,20,21]. The sorbents were mixed with metastable-state photoacids, which reversibly change from a low-acidity state to a high-acidity state upon photo-irradiation [22,23]. The pH of the system can be switched between basic and acidic conditions with light for CO₂ capture and release, respectively. The high efficiency of photo-induced CO₂ release and sorbent regeneration has been demonstrated [17,18,21]. Although this method is promising, the performance and stability of the photoacids still need to be improved and the cost of the photoacids must be reduced before a practical application can be realized.

In this work, we investigated a novel approach toward a practical technology based on acid-induced CO₂ release. As described above, the addition of acid facilitates the CO₂ release but does not regenerate the sorbent due to the formation of salts. An additive with very low acidity at room temperature cannot protonate the sorbent and thus will not reduce its capability for CO₂ capture. If the acidity of the additive is significantly enhanced at elevated temperatures, the release of CO₂ will be driven by both thermal energy and enhanced acidity. Using such an additive, the thermal energy required for sorbent regeneration can be reduced due to the assistance of the acidity at an elevated temperature.

According to Van’t Hoff’s equation (Equation (1)), if the enthalpy of the reaction is positive, the equilibrium constant increases with the temperature. Applying the equation to the proton dissociation of an acid, we can see that the more positive the enthalpy, the larger the increase in the proton dissociation constant K_a, that is, K in the general Equation (1), with the temperature. A large positive ΔH means that proton dissociation is difficult, thus the acidity is very weak. Therefore, a large enhancement of acidity at elevated temperatures is expected from very weak acids.

(∂[lnK]/∂T) = ΔH/RT²

(1)

2. Results and Discussion

To demonstrate the novel approach, we need to use an additive with very low acidity at room temperature and significantly enhanced acidity at elevated temperatures to reduce the temperature required for sorbent regeneration without diminishing the capability for capturing CO₂ at room temperature. Common amine sorbents have pK_as of around 9. Therefore, if the additive has a pK_a > 10, the proton transfer between the additive and the amine is minimal at room temperature. In this work, we used m-cresol as the additive. M-cresol is an inexpensive industrial material. It is a liquid phenol derivative partially miscible with water. The pK_a of m-cresol is 10.1, and the proton dissociation enthalpy is 23.1 kJ/mol [24]. Using this value and Van’t Hoff’s equation, we calculated that the pK_a decreased by more than 0.5 units when the temperature rose from room temperature to 70 °C and decreased by more than 0.8 units at 100 °C.

The amine sorbent used in this work is morpholine. Morpholine is a secondary amine, which has been identified as a promising sorbent due to its high reactivity with CO₂ and relatively low stripper temperature [11,17,25,26]. The pK_a of morpholine is 8.5. It is 1.6 units lower than that of m-cresol, which prevents the proton transfer from m-cresol at room temperature. Although its basicity is lower than some amine sorbents, morpholine captures CO₂ effectively and forms stable carbamate and bicarbonate. The absorption and desorption of CO₂ by the morpholine/m-cresol system is shown in Scheme 1.

We formulated a CO₂ sorbent by combining m-cresol with morpholine in water to improve the CO₂ desorption upon heating. Common aqueous amine sorbents have ~30% of the amines in water. For the m-cresol amine sorbent, not only are high percentages of morpholine and m-cresol required but the amount of water is also important. Water is necessary for the formation of carbonic acid, which further reacts with amines to form bicarbonates and carbamates. However, when the percentage of water is above ~40%, phase separation occurs after heating. The sample studied in this work contained 40% m-cresol, 40% morpholine, and 20% water. The formula was determined after some compatibility tests. No phase separation was observed at room temperature or at the elevated temperature studied in this work.

The formulated solution was compared with a control solution of 40% morpholine and 60% water to evaluate its CO₂ capture and release efficiency. Precisely, the sample solution contains 2 g of morpholine, 2 g of m-cresol, and 1 g of water. The control solution contains 2 g of morpholine and 3 g of water. A stream of CO₂ was gently passed through the solutions in test tubes for 30 min to load the solutions with CO₂. Since this process could oversaturate the solutions with CO₂, the solutions were stirred under air for 30 min to achieve a pseudo-equilibrium state with air. The amount of CO₂ captured was measured by subtracting the weight of the solution before loading from that after loading. It was found that the sample with m-cresol captured 0.46 g of CO₂ and the control captured 0.53 g of CO₂.

To understand the reason for the 13% decrease in the CO₂ capture by the sample compared to that of the control, we carefully analyzed the amount of the carbamate, the bicarbonate, and the unreacted morpholine using ¹H NMR. The NMR spectra of the control and the sample before and after loading are shown in Figure 1. The peaks of the morpholine protons are between 3.6 and 2.2 ppm. After CO₂ loading, there is a pair of peaks (c d) with equal intensity in the middle and another pair (a b) with equal intensity on the outside. The morpholine carbamate salt contains a carbamate anion and a protonated morpholinium. The morpholine bicarbonate contains a protonated morpholinium. Since both contain morpholinium, but only the carbamate contains the carbamate anion, the intensity of the carbamate anion must be lower than that of the morpholinium, which means the middle pair must represent the protons of the carbamate anion. The outer pair represent both the morpholinium and non-protonated (unreacted) morpholine. The quick proton exchange makes them non-differentiable.

Since the middle pair represent the protons of the carbamate, the molar number of the carbamate can be calculated from the integration of the middle pair over that of all the morpholine protons multiplied by the total molar number of morpholine:

Mol of carbamate = Total mol of morpholine × (integration of middle pair/integration of all morpholine protons)

(2)

Since 2 g of morpholine was added to both the sample and the control, the total molar number of morpholine was 0.023 mol, given the Mw was 87.1 g/mol. The integration ratios in Equation (2) for the sample and the control were found to be 0.42 and 0.41, respectively, for which the molar numbers of the carbamates in the sample and the control were calculated to be 9.7 mmol and 9.4 mmol, respectively. The nearly identical amount of carbamate shows that the nucleophilic reactivity of morpholine was not decreased in the m-cresol/water solution.

The molar number of the bicarbonate cannot be calculated directly from the integration of the outer pair because the outer pair represent both the bicarbonate and the unreacted morpholine. However, it can be calculated from the captured CO₂, given the known amount of the carbamate:

$$\begin{array}{ll}\mathit{Mol} \mathit{of} \mathit{bicarbonate}& =\mathit{Mol} \mathit{of} \mathit{C}{\mathit{O}}_2 \mathit{captured} \mathit{in} \mathit{bicarbonate}\\ & =\mathit{Total} \mathit{mol} \mathit{of} \mathit{C}{\mathit{O}}_2-\mathit{mol} \mathit{of} \mathit{C}{\mathit{O}}_2 \mathit{in} \mathit{carbamate}\\ & =\mathit{Total} \mathit{mol} \mathit{of} \mathit{C}{\mathit{O}}_2-\mathit{mol} \mathit{of} \mathit{carbamate}\end{array}$$

(3)

As described above, the amounts of CO₂ captured by the sample and the control were 0.46 g and 0.53 g, which were 10.5 and 12.0 mmol, respectively. From Equation (3), the molar numbers of bicarbonate in the sample and the control are calculated to be 0.76 mmol and 2.6 mmol. The result shows that the decrease in the CO₂ capture is mainly due to less formation of the bicarbonate. CO₂ reacts with water to form carbonic acid, which further reacts with morpholine to form bicarbonate. The sample has less water than the control, resulting in less bicarbonate formation.

The molar number of the unreacted morpholine can be calculated by subtracting the total molar number of the morpholine by the sum of the morpholine in the carbamate and the bicarbonate:

Mol of unreacted morpholine = Total mol of morpholine
  − mol of morpholine in bicarbonate
− mol of morpholine in carbamate
  = Total mol of morpholine              
                − mol of bicarbonate − 2 × mol of carbamate

(4)

The factor of 2 in the last term of Equation (4) is because 2 equivalents of morpholine are needed for each carbamate. Using Equation (4), we can calculate that the unreacted morpholine in the sample was 2.8 mmol, which was 12% of the total morpholine. For the control, the unreacted morpholine was 1.6 mmol, that is, 7% of the total morpholine.

The CO₂ release was studied by heating the sample and the control at elevated temperatures (50–80 °C) for a certain period. The CO₂ released was measured by the weight loss after heating. The results are plotted in Figure 2. The percentage in the plot is the weight of the CO₂ released divided by the weight of the CO₂ captured multiplied by 100%. Each data point is an average of the results of three experiments. It is possible that a small amount of the solvent evaporated during the heating process. This problem was mitigated by using a long test tube, which serves as an air condenser. Most importantly, the boiling point of m-cresol (202.8 °C) is much higher than that of water. Since the m-cresol, water, and morpholine were miscible under the experimental condition, the boiling point of the sample, which contained the high b.p. m-cresol, was higher than that of the control, and consequently, the sample evaporated less than the control. Therefore, if we consider the evaporation, the enhancement of the CO₂ release by m-cresol would be even more pronounced than that reported below.

Figure 2 clearly shows that the sample released CO₂ much faster than the control. For example, heating the sample at 70 °C for 1 h resulted in 53% of the CO₂ being released, while the control released 29% of the CO₂ under the same condition. If we consider the fact that the total amounts of the captured CO₂ were different, we may also compare the absolute amount of CO₂ released. Under this condition, the sample and the control released 0.24 g and 0.15 g of CO₂, respectively, indicating the much quicker CO₂ release of the sample. For the control solution to release 50% of the CO₂ at 70 °C, it required approximately 3 h, while only 1 h was needed for the sample. Heating the sample at 70 °C for 3 h released ~90% of the CO₂. The 3 h heating at 80 °C led to nearly 100% release.

It is worth comparing the thermal regeneration of the m-cresol/morpholine solution with that of the common sorbent solutions, for example, aqueous solutions of MEA. Although the setups were different in different studies, the temperature for the regeneration of MEA was often near 95 °C. A higher temperature was used for morpholine due to the high stability of its carbamate [11,25,26]. Tertiary amines only form bicarbonates with CO₂, which are less stable than carbamate. Therefore, their thermal regeneration temperature is lower than that of primary and secondary amines. Heating at 70 °C has been used in a systematic study of tertiary amine sorbents [13]. However, the rate of CO₂ capture by tertiary amines is lower than that of primary and secondary amines. The results of this study show that the addition of m-cresol can reduce the regeneration temperature of morpholine, which forms stable carbamate, to a temperature similar to that of tertiary amines.

NMR analysis was applied to understand the decomposition of carbamate and bicarbonate upon heating. Figure 3 shows the ¹H NMR spectra of the sample and the control after being heated at 70 °C for 1 h. The intensity of the carbamate peaks is visually more reduced in the sample spectra than in the control. Using the same method as described above, we can calculate the amount of the carbamate and bicarbonate in the sample and the control. In the sample solution, the amount of the carbamate was reduced from 9.7 mmol to 5.0 mmol, and the bicarbonate was reduced from 0.76 mmol to 0.031 mmol. In the control solution, the amount of the carbamate was reduced from 9.4 mmol to 8.3 mmol, and the bicarbonate was reduced from 0.76 mmol to 0.36 mmol. As expected, the bicarbonate decomposed faster than the carbamate in both the sample and the control. In the sample, essentially all the bicarbonate decomposed, and nearly half of the carbamate decomposed. In the control, 53% of the bicarbonate decomposed, and only 12% of the carbamate decomposed. These results confirm that m-cresol significantly enhanced the thermal decomposition.

As shown in the NMR spectra, there was no observable degradation product of morpholine and m-cresol after heating. In fact, since m-cresol, which is a phenol derivative, is a reductant, it could protect the amine sorbent from oxidation. To demonstrate that the sample with m-cresol can capture CO₂ after the thermal regeneration, we conducted an additional absorption/desorption cycle after the first desorption process at 70 °C for 1 h, which released ~0.23 g of CO₂. The average amount of CO₂ absorbed and desorbed in the additional cycle was 0.21 ± 0.04 g for the four samples tested. Given the inevitable evaporation of the sorbent during the process, the consistency of these results is reasonable.

3. Conclusions

In this work, we tested a novel approach to facilitate the thermal regeneration of CO₂ sorbent. According to Van’t Hoff’s equation, a very weak acid can significantly increase its acidity at an elevated temperature. We showed that it is possible to use an additive with a room-temperature acidity weak enough to avoid protonation of the sorbent but a high-temperature acidity strong enough to assist in the decomposition of the sorbent–CO₂ complex. M-cresol was added to an aqueous solution of morpholine, and the CO₂ capture and release of the mixture were studied. M-cresol has very low acidity at room temperature but significantly enhanced acidity at elevated temperature. Because the carbamate and bicarbonate formed after CO₂ capture are unstable under acidic conditions, the acidity of m-cresol at elevated temperatures can lead to the decomposition of the carbamate and bicarbonate, and to CO₂ release. The experimental results showed that the addition of m-cresol did not reduce the reactivity of morpholine for the formation of the carbamate. The 13% reduction in the CO₂ capture compared with the control is due to there being less water content in the sample, which could be mitigated by optimizing the m-cresol/water ratio in future work. The thermal release rate of CO₂ was largely enhanced at all the temperatures tested in the presence of m-cresol. Given that m-cresol is a low-cost industrial material, this approach could lead to practical technology in the near future.

4. Experimental

4.1. General Instrumentation

Unless otherwise noted, the reagents and solvents were commercially available and used as received without any further purification. The NMR spectra were determined on a Bruker av400 NMR spectrometer. The chemical shifts were reported in delta (δ) units, parts per million (ppm) downfield from the TMS.

4.2. Measurement of CO₂ Loading and Release

The control solution was prepared by mixing 2 g of morpholine and 3 g of water in a test tube. The sample solution was prepared by mixing 2 g of morpholine, 2 g of m-cresol, and 1 g of water in a test tube. A small magnetic stirring bar was added to each test tube. The total weight of the test tubes with the sample (or control) and the stirring bar was measured using an analytical balance. A stream of CO₂ was slowly bubbled through the solutions for 30 min. Fast bubbling must be avoided to prevent evaporation. The solutions were then gently stirred under ambient conditions for 30 min to achieve a pseudo-equilibrium with air. The weight of the test tubes was measured. The mass of CO₂ captured was calculated by subtracting the weight of the test tube before CO₂ loading from the weight after loading.

For the thermal release of CO₂, oil baths were set up at specific temperatures (80 °C, 70 °C, 60 °C and 50 °C). To keep the heating conditions the same for comparable results, two test tubes, one containing the sample and the other containing the control, were put together in the same oil bath for each temperature trial. The solutions were gently stirred while being heated. The weight of the solutions in the test tubes was measured hourly for 3 h. Each experiment was repeated three times, and the average values were used to create the plots in Figure 2.

For the NMR study, D₂O was used instead of water. The formulae of the sample and the control were the same as described above. Approximately 0.2 mL of the solution was taken and analyzed by ¹H NMR after each step, that is, before CO₂ loading, after CO₂ loading, after equilibrating with air, and after thermal releasing.