Afnan
Altwala
ab and
Robert
Mokaya
*a
aSchool of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK. E-mail: r.mokaya@nottingham.ac.uk
bDepartment of Chemistry, College of Science Al-Zulfi, Majmaah University, Al-Majmaah, 11952, Saudi Arabia
First published on 23rd March 2022
Potassium oxalate (PO) was trialled as a non-corrosive and less toxic activating agent for the direct activation of biomass (sawdust, SD). The PO + SD mixtures were activated either in powder form or after compaction into pellets. The resulting activated carbons are highly microporous with surface area in the range of 550 to 2100 m2 g−1 and pore volume between 0.3 and 1.0 cm3 g−1. The porosity of the directly activated and compactivated carbons is similar to that of conventionally activated (via hydrothermal carbonisation) equivalents. In general, pelletized (i.e., compactivated) carbons achieved higher levels of porosity for any identical set (with respect to amount of PO and temperature) of preparation conditions. Unlike hydroxide activation, the amount of PO used, for PO/SD mass ratio between 2 and 4, does not have a significant effect on porosity. On the other hand, the activation temperature plays a critical role in determining the textural properties at any given PO/SD ratio. The porosity of the carbons is dominated by pores of size 6–8 Å, which are suitable for post-combustion (low pressure) CO2 storage. At 25 °C, the carbons capture up to 1.6 and 4.3 mmol g−1 of CO2 at 0.15 bar and 1 bar, respectively. Our findings show that the use of potassium oxalate as a mild activating agent via direct activation succeeds in addressing the need for non-corrosive and less toxic activators and also negates the need for hydrothermal treatment or pyrolysis of biomass prior to activation. The present carbons are attractive as sustainable energy materials especially for post-combustion CO2 capture and storage.
Another important consideration is that, for new activated carbons to be of interest, they should fall into one of the following categories: have new or improved properties, be easy/cheap to prepare, and be sustainable. Lower cost/easier to prepare materials typically result from a reduction in the number of fabrication steps required. Biomass is normally activated after the process of hydrothermal carbonisation (HTC) or pyrolysis, which converts the biomass into carbon-rich carbonaceous matter. Hydrothermal carbonisation involves heating (typically at 180–300 °C) in water under pressure to enable thermochemical decomposition of biomass to carbon-rich carbonaceous matter.19–22 The pyrolysis process, on the other hand, involves generation of carbonaceous matter via enrichment of carbon content during thermal treatment at temperatures of 600–900 °C under oxygen-free conditions.23–29 We have recently explored a direct and cheaper route to KOH activated biomass-derived carbons, which excluded the need for HTC or pyrolysis prior to activation.30 Recently, we have also reported on the process of compactivation, also known as mechanochemical activation, wherein mixtures of the precursor and KOH are compacted into pellets prior to activation.31 Given that the activation process is initially based on solid–solid interaction between the activating agent and the precursor, the compactivation method was developed with the aim of increasing proximity (solid–solid contacts) between the precursor and KOH.29,31 The desired outcome of compactivation is to improve the efficiency of the activating agent with respect to the porosity generated. It is now known that compactivation with KOH can generate carbons with higher (compared to conventional powder activation) surface area and pore volume.29,31 However, so far, compactivation has only been explored for KOH activation, and has not been performed for direct activation of raw biomass. It of interest to explore what impact the milder nature of PO has on the process and whether overall yields and porosity are affected by use of raw biomass rather than enriched carbonaceous matter derived from pyrolysis or HTC.
In this report, we explore the use of PO for the direct activation and compactivation of raw biomass (sawdust). This approach potentially offers several advantages that have, so far, not been probed in any one study, namely (i) use of a milder non-hydroxide activating agent, (ii) direct activation of biomass that negates the need for HTC or pyrolysis, and (iii) porosity modulation and optimisation (with respect to amount of activating agent) via the compactivation route. Overall, therefore, this approach offers a direct process that is simpler, cheaper, and more sustainable. Importantly, it is necessary that these advantages not compromise the porosity of the resulting carbons, i.e., the carbons should have similar properties to analogous carbons prepared via conventional methods. Based on the porosity of the resulting activated and compactivated carbons, we investigated their CO2 uptake and show that they offer very attractive trends especially for low-pressure (post-combustion) uptake.
Sample | Yield [wt%] | C [%] | H[%] | N[%] | O[%] |
---|---|---|---|---|---|
Sawdust | 47.0 | 5.6 | 0.4 | 47.0 | |
DSD2600 | 28 | 71.3 | 1.0 | 0.5 | 27.2 |
DSD2700 | 28 | 65.6 | 1.1 | 0.2 | 33.1 |
DSD2800 | 22 | 65.5 | 0 | 0 | 34.5 |
DSD4600 | 28 | 65.5 | 0.5 | 0.3 | 33.7 |
DSD4700 | 26 | 71.5 | 0.7 | 0.2 | 27.6 |
DSD4800 | 22 | 72.5 | 0.1 | 0 | 27.4 |
DSD2600P | 34 | 75.0 | 1.2 | 0.6 | 23.2 |
DSD2700P | 20 | 73.0 | 0.8 | 0.2 | 26.0 |
DSD2800P | 20 | 80.0 | 0.5 | 0.0 | 19.5 |
DSD4600P | 30 | 72.0 | 0.5 | 0.5 | 27.0 |
DSD4700P | 26 | 76.5 | 1.3 | 0.7 | 21.5 |
DSD4800P | 20 | 86.0 | 0.1 | 0.1 | 13.8 |
For the present direct activation process, it is noteworthy that compaction of the PO/SD mixtures before activation (i.e., activation of pelletized mixtures) generates generally similar yields to activation of powder mixtures. Compaction of the PO/SD mixture is expected to engender closer contact between the PO and SD particles and under any given activation conditions (i.e., amount of PO and temperature) should lead to greater levels of activation and therefore lower activated carbon yields. Such a trend is now well established for KOH activation; in effect, compactions acts to improve the efficient use of KOH.29,31,32 The fact that the trend is not observed here for PO activation suggests that, unlike for hydroxide activation where the amount of KOH is critical, the PO/SD ratio is not a critical factor in determining the level or extent of activation. It is clear from the data in Table 1 that, in general, only the activation temperature determines the carbon yield; change of PO/SD ratio from 2 to 4 at any given temperature does not appear to have any significant effect on the carbon yield. On the other hand, higher activation temperatures lead to a lowering of carbon yield.
As expected, following activation, the carbon content (given as wt%) increases from 47% for the sawdust to a high of 86% for the sample compactivated at 800 °C (DSD4800P). The effect of activation temperature is such that carbons activated at the highest temperature (800 °C) have the most elemental C content (72.5% and 86% for powder and compacted samples, respectively). In general, compactivated carbons have higher C content compared to powder samples under any given activation conditions. The content of N, H and O decreases at higher activation temperature, which is the expected trend at higher levels of activation and is consistent with previous reports.23,24,29–32 Thermogravimetric analysis of the activated carbons, under flowing air conditions, was performed in order to assess the carbon purity (i.e., lack of inorganic matter) and thermal stability. The TGA curves (Fig. S1, ESI†) show a large mass loss between 450 and 650 °C due to carbon combustion, with an initial mass loss below 120 °C due to water removal. The TGA curves show that the residual mass, after heating in air at 800 °C, is typically lower than 5 wt%, which indicates that the activated carbons are generally free of inorganic residues. Samples activated at 600 °C have maximum burn-off at 500 °C compared to up to 650 °C for carbons activated at 800 °C, which exemplifies the effect of activation temperature on thermal stability. Higher activation temperatures results in greater thermal stability in the activated carbons, which is consistent with previous observations.33–39
The powder XRD patterns of the activated carbons (Fig. S2, ESI†) show that the amount of PO does not have any significant effect on the nature of the carbons – the XRD patterns of representative samples suggest a comparable level of graphitic ordering at any given activation temperature. The XRD patterns have very broad peaks at 2θ = 26° and 43°, which correspond to a very weakly graphitic/turbostratic nature in the carbons. The XRD patterns of carbons activated at higher temperature show a very weak peak, which indicates greater disruption of any graphitic ordering. Given that higher activation temperature leads to more thermally stable activated carbons, the ‘disruption’ of graphitisation as evidenced by the XRD patterns may be more to do with decrease in size of ‘graphitic’ domains rather than actual overall lowering of the level of graphitisation. The XRD patterns of some of the activated carbons exhibit sharp peaks, which could indicate the presence of inorganic residues left over by the activating agent. However, the TGA curves suggest that the presence of any inorganic impurities occurs only at a minor level.18–21
Scanning electron microscopy was used to probe the morphology of the carbons. The SEM images of the raw sawdust and activated carbons DSD4800 and DSD4800P are shown in Fig. 1. The morphology of the raw sawdust is comprised of extended fibrous structures typical for woody matter (Fig. S3, ESI†). After activation, the morphology shows some retention of fibrous and honeycomb-like structures. The morphology of the activated carbons includes smooth surfaces characterised by large conchoidal cavities, which is similar to what has previously been reported for carbons generated via direct or flash carbonisation activation routes.22,30,34 The aforementioned retention of some woody morphology is consistent with the milder nature of PO activation compared to KOH where the morphology is completely altered (compared to that of the biomass) especially at high levels of activation.35–37
Fig. 3 shows the nitrogen sorption isotherms and corresponding pore size distribution curves for carbons prepared at PO/SD ratio of 4. The isotherms are typically type I and very similar to those in Fig. 2 both in terms of shape and amount of nitrogen adsorbed. Clearly, increasing the amount of PO does not appear to have any significant effect on the porosity, which is consistent with the results of XRD analysis as described above. The PSD curves in Fig. 3 are comparable to those of in Fig. 2, which means that increase in the PO/SD ratio does not lead to pore size expansion. This means that control of the porosity of the carbons is best achieved by choice of activation temperature rather than the amount of PO or a combination of both. This is a departure from what is observed for direct activation of sawdust with KOH wherein increase in the KOH/SD ratio from 2 to 4 generated carbons with larger pores and also included the presence of small mesopores.30 According to Fig. 3 (B and D), direct activation at PO/SD ratio of 4 results in the formation of larger micropores with hardly any mesopores even for samples activated at 800 °C. The presence of supermicropores for samples activated at 800 °C could be related to CO2 and CO gases released during the activation at high temperature.8 Generally, though, the compactivated samples show higher nitrogen sorption compared to equivalent powder samples, which is in agreement with previous studies using KOH as activating agent.31,32 The increase in porosity induced by compactivation is, however, not as high as that observed for KOH activation. This is consistent with the fact that the amount of PO is not a critical factor in determining the porosity. It is likely, therefore, that PO exists in an excess amount, which means that the close contact with sawdust particles engendered by the compaction does not have any significant effect of the overall efficiency of the activator.
One of the key aims of this study was to explore the impact of a simpler and more direct activation route on the porosity of activated carbons. This is an important consideration given the mild nature of PO as an activating agent. Table 2 summarizes the textural properties of the directly activated and compactivated carbons. The surface area of activated carbons is in the range of 550–1860 m2 g−1 with pore volume of between 0.3 and 0.96 cm3 g−1. The surface area and pore volume are mainly determined by the activation temperature. Both textural parameters increase as activation temperature rises from 600 to 800 °C. The amount of PO (given as PO/SD ratio of 2 or 4) does not appear to have any effect of the surface area and pore volume especially for activation at 600 and 700 °C. However, for activation at 800 °C, a higher amount of PO generates greater surface area and pore volume. Thus the surface area and pore volume of DSD4800 (1859 m2 g−1 and 0.96 cm3 g−1, respectively) is higher than that of DSD2800 (1238 m2 g−1 and 0.59 cm3 g−1, respectively). The activated carbons exhibit high to very high microporosity as indicated by the magnitude and proportion of surface area and pore volume arising from micropores (Table 2). The proportion of surface area arising from micropores is between 80 and 86% while for pore volume it is 59 to 71%. The activation temperature has no effect on the proportion of microporosity. However, a higher amount of PO appears to very slightly reduce the microporosity. It is also clear from the textural data in Table 2 that compactivation generates carbons with higher surface area and pore volume compared to equivalent activated samples and that, interestingly, this increase in overall prosity does not compromise the microporosity. The microporosity of the compactivated carbons is either similar of slightly higher than that of equivalent activated carbons.
Sample | Surface area (m2 g−1) | Micropore surface areaa (m2 g−1) | Pore volume (cm3 g−1) | Micropore volumeb (cm3 g−1) | CO2 uptake. (mmol g−1) | ||
---|---|---|---|---|---|---|---|
0.15 bar | 1 bar | 20 bar | |||||
a The values in the parenthesis refer to % micropore surface area. b The values in the parenthesis refer to % micropore volume. | |||||||
DSD2600 | 682 | 574 (84) | 0.36 | 0.23 (64) | 1.0 | 2.7 | 4.9 |
DSD2700 | 945 | 813 (86) | 0.48 | 0.33 (69) | 1.2 | 4.1 | 8.5 |
DSD2800 | 1238 | 1053 (85) | 0.59 | 0.42 (71) | 0.9 | 3.4 | 12.3 |
DSD4600 | 556 | 465 (84) | 0.30 | 0.18 (60) | 1.0 | 2.5 | 4.8 |
DSD4700 | 1131 | 906 (80) | 0.63 | 0.37 (59) | 0.9 | 3.0 | 6.8 |
DSD4800 | 1859 | 1497 (81) | 0.96 | 0.60 (63) | 0.8 | 3.2 | 11.5 |
DSD2600P | 730 | 638 (87) | 0.36 | 0.26 (72) | 1.2 | 2.9 | 5.1 |
DSD2700P | 822 | 699 (85) | 0.39 | 0.28 (72) | 1.0 | 3.6 | 7.7 |
DSD2800P | 1893 | 1545 (82) | 0.92 | 0.61 (66) | 1.0 | 4.0 | 13.0 |
DSD4600P | 793 | 693 (87) | 0.41 | 0.28 (68) | 1.2 | 2.9 | 5.2 |
DSD4700P | 1242 | 1123 (91) | 0.61 | 0.45 (74) | 1.6 | 3.8 | 7.8 |
DSD4800P | 2121 | 1816 (86) | 1.01 | 0.73 (72) | 1.1 | 4.3 | 12.2 |
It is interesting to note that the porosity of the directly activated carbons is comparable (Table S1, ESI†) to that of equivalent (in terms of amount of PO and activation temperature) conventionally activated samples prepared via HTC prior to activation of the hydrochar. The microporosity of the two sets of samples is also similar especially for carbons prepared at PO/SD or PO/hydrochar ratio of 2. At PO/SD or PO/hydrochar ratio of 4, the directly activated carbons have very slightly lower levels of microporosity. The overall picture that emerges is that direct activation or compactivation of biomass with PO does not compromise the porosity of the resulting carbons. This is similar to what has previously been observed when the harsher activating agent, KOH, is used.30 Thus direct activation or compactivation of biomass with PO offers simplicity but without introducing any disadvantages with respect to achievable porosity.
Fig. 4 CO2 uptake isotherms of carbons directly activated from sawdust (SD) at 600, 700 or 800 °C, and PO/SD ratio of 2 (A) or 4 (B). |
Fig. 5 shows the CO2 uptake isotherms for compactivated samples, and Table 2 summarises the uptake at 0.15 bar, 1 bar and 20 bar. For compactivated carbons, the CO2 uptake at 1 bar is in the range 2.9–4.3 mmol g−1, and the highest uptake is for sample DSD4800P. For compacted samples, the rise is from 2.9 mmol g−1 (600 °C) to ca. 3.8 mmol g−1 (700 °C) and up to 4.3 mmol g−1 for the sample compactivated at 800 °C (DSD4800P). The uptake data, therefoe, suggest that at 25 °C and 1 bar, the total surface area does not determine CO2 uptake capacity but rather samples with the largest proportion of micropore surface area have the better performance. This is consistent with previous studies in which the preponderance of small pores (6–8 Å) has been noted to determine low pressure CO2 uptake. It is noteworthy that the highest CO2 uptake at 1 bar of 4.3 mmol g−1 for the present sawdust-derived direct PO activated carbons is comparable to that previously reported for conventionally (via HTC) PO activated sawdust derived carbons8 and also for directly activated sawdust-derived carbons with KOH as activating agent.30 A benefit of the current PO activation route is that it is milder and more environmentally friendly and appears to offer a simpler optimisation of pore size by choice of activation temperature. Thus despite being prepared via a simpler, milder, potentially cheaper and more direct route, the directly activated and compactivated carbons show attractive performance for gravimetric CO2 uptake, which is comparable or better than for current benchmark porous carbons (Table S2, ESI†).40–52
Fig. 5 CO2 uptake isotherms of carbons directly compactivated from sawdust (SD) at 600, 700 or 800 °C, and PO/SD ratio of 2 (A) or 4 (B). |
The CO2 uptake at 0.15 bar is considered to be a close mimic of capture performance from post-combustion flue gas streams emanating from power stations, which typically contain of ca. 15–20% CO2 with the remainder being mainly N2 (70–75%), and water (ca.%).53,54 For activated (powder) carbons, the uptake at 0.15 bar is in the narrow range of 0.8 and 1.1 mmol g−1 with samples activated at 800 °C having the lowest storage capacity. For compactivated carbons, the uptake is higher at between 1.0 and 1.6 mmol g−1. The trend generally matches that of uptake at 1 bar and is clearly related to the microporosity of the carbons. It is noteworthy that sample DSD4700P achieves a very high uptake at 0.15 bar of 1.6 mmol g−1, which is one of the highest reported for porous carbons (Table S2, ESI†).40–52
The trend is reversed for the CO2 uptake at the higher pressures of 20 bar. The highest uptakes are achieved for samples activated at 800 °C, and the lowest values are achieved for samples activated at 600 °C. This trend mirrors the variation in surface area and confirmed the fact that the CO2 uptake at moderate to high pressure is dependent on the total surface area rather than pore size, whereas at lower pressure the level of microporosity plays an important role in determining the uptake capacity. Additionally, the CO2 uptake at 20 bar is not affected by the amount of PO, which is a consequence of the fact that changing the PO/SD ratio from 2 to 4 does not alter the porosity in any significant way. This confirms that in optimising or tailoring the textural properties of the present carbons for targeted CO2 uptake applications, the activation temperature is more effective than the PO/SD ratio. The possibility of using low amounts of PO whilst still achieving the full range of porosity is also a positive outcome of the present synthesis approach.
Footnote |
† Electronic supplementary information (ESI) available: Three additional figures; TGA curves, XRD patterns and SEM images along with two tables with comparative textural data and CO2 uptake. See DOI: 10.1039/d1ya00085c |
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