Esterification of glycerol over a solid acid biochar catalyst derived from waste biomass

Abstract

Karanja seed shells were subjected to pyrolysis in an inert atmosphere at different temperatures to prepare a biochar. The biochar was characterized by X-ray diffraction, FT-infrared spectroscopy, laser Raman spectroscopy, thermogravimetric analysis, CHNS-elemental analysis, BET surface area analysis and for the temperature programmed desorption of ammonia. These biochar carbon catalysts were used as catalysts without any functionalization/treatment for the esterification of glycerol with acetic acid. Carbonization at 400 °C led to the formation of biochar with a greater number of strong acidic sites. Amorphous carbon obtained by high temperature carbonization was composed of aromatic carbon sheets oriented in a considerably random fashion. The biochar obtained at 400 °C exhibited the highest glycerol esterification activity. The catalytic activity of the biochar was explained based on its properties derived from the different characterization methods. The biochar catalyst can be reused with consistent activity.

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1. Introduction

The conversion of a variety of biomass materials into useful products, such as liquid, gas and char, is an important area of research due to their abundant availability and low cost.¹ Lignocellulosic biomass can be converted into useful chemicals via different thermochemical processes.² Gasification or pyrolysis of biomass has been widely used to extract the energy content of biomass.³ Pyrolysis is the main process to recover energy from biomass by generating char, oil and gas as major products.⁴ One of the products, namely, biochar, produced from the variety of biomass was found to be useful for the production of active carbon.³ Biomass, such as rice husk,⁵ sunflower shells,² pistachio nutshells,⁵ peanut hulls,⁶ and almond shells^7–9 have been effectively utilized for the production of active carbon. These carbons are utilized for different applications such as adsorbents,^10,11 catalysts and catalyst supports.⁶ The nature of the carbon derived from biomass depends on the type of biomass, treatment temperature and exposure time. These carbons generally possess high surface areas with different pore structures. The biochar produced from different biomass are used directly or after suitable modifications as catalysts for different reactions.¹² The biochar obtained from low temperature slow pyrolysis can generate a highly cross-linked, multi-ringed aromatic structure that is anchored to lignin. This biochar can be easily functionalized to make them strong acid catalysts. Canola meal, a by-product of the canola-based biodiesel industry, has also been explored for the production of biochar.¹³ After suitable modifications, the canola-based acid-activated biochar exhibited a good activity.

The seeds of karanja (general name) or Pongamia pinnata (scientific name) are available abundantly in the world, particularly in tropical and temperate regions such as India, Japan, China, Malaysia, Australia and the Pacific Islands. Karanja comes from a tree belonging to the pea family, Fabaceae.^14,15 Karanja seed is non-edible; however, the oil extracted from the kernels has been identified as being suitable oil for the production of biodiesel.^16,17 Karanja seed deshelling produces 60% to 65% shells and 35% to 40% kernels. The shells of the seeds are major waste components and are typically discarded or used as domestic fuel after removing the oil bearing kernel from the seed. The shell is a major by-product of karanja seed processing industry and adding value to this material would certainly help the economics of karanja-based biodiesel industry. In recent times, numerous investigations have been focused on the utilization of biomass and other agricultural wastes to retrieve their energy content.

Glycerol is the main by-product in biodiesel synthesis by the transesterification of oil with methanol or ethanol. Glycerol is a renewable feedstock, which can be converted into valuable chemicals.^18,19 Glycerol is a highly functional molecule and can undergo oxidation, carbonylation,^20–22 hydrogenolysis,^23,24 esterification and etherification²⁵ to yield useful commodity chemicals. The esterification of glycerol with acetic acid is one of the important reactions to synthesize glycerol acetins, which are known for their fuel additive properties. Different acid-based heterogeneous catalysts have been studied for the esterification of glycerol.^26–30 Glycerol acetylation has been carried out using different heterogeneous catalysts, such as Amberlyst and niobic acid,²⁶ or heteropolyacids, such as dodecamolybdophosphoric acid encaged in USY zeolite,²⁷ dodecatungstophosphoric acid immobilized into a silica matrix and sulfated zirconia.²⁸ Among the abovementioned compounds, some catalysts are not applied at an industrial scale because HPAs are soluble in polar media²⁹ and some are very expensive.³⁰

In the present study, karanja seed shells were subjected to pyrolysis in an inert atmosphere at different temperatures to make a catalytically useful biochar. The biochar was evaluated for its physico-chemical properties using different spectroscopic methods. The biochar was used directly as an acid catalyst without any modifications for the esterification of glycerol with acetic acid.

2. Experimental section

2.1. Materials and preparation of karanja biochar

Karanja kernels were collected from local suppliers. Glycerol and acetic acid were obtained from SD Fine Chem., India. Karanja seeds were collected from local gardens, and the shells were separated from the seeds manually. The shells were ground to small pieces and subjected to drying in an oven at 100 °C for 12 h to remove the moisture content. Biochar was prepared by the pyrolysis of dried karanja seed shell powder under a nitrogen atmosphere at 300, 400 and 500 °C for 4 h. These samples are denoted as KJ-300, KJ-400 and KJ-500, where the number represents the carbonization temperature. The main products of the glycerol esterification with acetic acid are mono (MA), di (DA) and tri acetin (TA).

2.2. Characterization of the catalysts

The BET surface areas of the catalysts were calculated from nitrogen adsorption–desorption data acquired on an Autosorb-1 instrument (Quanta chrome, USA) at liquid N₂ temperature.

The powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a RigakuMiniflex (Rigaku Corporation, Japan) X-ray diffractometer using Ni filtered CuK_α radiation (λ = 1.5406 Å) at a scan speed 2° min⁻¹ and a scan range of 2–80° at 30 kV and 15 mA.

The FT-IR spectra of the samples were recorded on an FT-IR DIGILAB (USA) IR spectrometer using a KBr disc method. Confocal micro-Raman spectra were recorded in air at room temperature in the range of 50–4000 cm⁻¹ using a Horiba–Jobin–Yvon LabRam HR spectrometer with a 17 mW internal He–Ne (helium–neon) laser source with an excitation wavelength of 632.8 nm. The catalyst samples in powder form (about 5–10 mg) were usually loosely spread onto a glass slide below the confocal microscope for the Raman measurements.

The thermal stability of the biochar was examined using a TA500 analyzer in the temperature range of 25–800 °C at a heating rate of 10 °C min⁻¹ with a continuous flow of nitrogen gas at 20 ml min⁻¹. The samples were subjected to thermogravimetric analysis (TGA) to determine the decomposition temperatures.

The temperature-programmed desorption of ammonia was carried out on an apparatus built in our laboratory equipped with a gas chromatograph using a thermal conductivity detector (TCD). In this TPD, around 50 mg of sample was placed in a quartz tube. Then, the catalyst sample was treated at 300 °C for 1 hour by passing pure helium (99.9%, 50 ml min⁻¹). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH₃) at 100 °C at a flow rate of 50 ml min⁻¹ for 1 hour and subsequently flushed with helium at the same temperature to remove any physisorbed ammonia. The TPD analysis was carried out from ambient temperature to 800 °C at a heating rate of 10 °C min⁻¹. The amount of ammonia evolved was calculated from the peak area of the calibrated TCD signal. The NMR spectra were recorded on a Varian 500 MHz spectrometer, and the chemical shifts are reported in ppm. C, H, N and S were measured by elemental analysis on a Vario Micro Cube elemental analyzer.

2.3. Reaction procedure

The esterification of glycerol was carried out in the liquid phase at atmospheric pressure. In a typical experiment, 5 g of glycerol and 9.78 g of acetic acid were placed in a round-bottom flask and 0.2 g of catalyst was added. The reaction mixture was placed in an oil bath maintained at the reaction temperature of 120 °C. The course of the reaction and the products were monitored by gas chromatography (GC) equipped with a flame ionization detector (FID) by separating them on an Equity-5 column.

3. Results and discussion

3.1. Catalyst characterization

The surface area values of the biochar samples are shown in Table 1. The overall surface areas of the biochar materials were low. The surface area and pore volume of the samples increased marginally with increase in the carbonization temperature. The increase in pore volume might be due to the formation of loosely bound carbonaceous material, which led to the creation of channels in the graphitic regions.³¹

Table 1 Physico-chemical properties of the karanja catalyst

Catalyst names	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Acidity (mmol g^−1)
			Moderate acidity	Strong acidity	Total acidity
KJ-300	13	0.015	6.863	1.100	7.963
KJ-400	14	0.021	4.263	2.065	6.328
KJ-500	16	0.027	2.062	1.815	3.877

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The percentages of carbon, hydrogen, nitrogen, sulfur and oxygen present in the biochar were measured by CHNS analysis, and the results are shown in Table 2.

Table 2 CHNS-elemental analysis of the karanja catalysts

Catalyst	Elemental composition (%)
	C	H	N	O
KJ-300	56.90	6.57	1.61	34.91
KJ-400	63.37	4.30	0.82	31.51
KJ-500	76.25	3.52	0.46	19.77

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The KJ-300 sample showed a lower percentage of carbon compared to the KJ-400 and KJ-500 catalysts. The low carbon content in the case of KJ-300 is mainly due to the incomplete carbonization of the karanja seed shell. As the carbonization temperature increases, the percentage of carbon increases and at the same time the contents of hydrogen, nitrogen and oxygen decrease.

The acidic sites present on the surface of biochar, together with the total acidity, were measured by the TPD of NH₃, and the results are shown in Table 1 and Fig. 1. The acidity of the catalysts are due to the presence of –OH and –COOH groups on the surface. The biochar samples showed two desorption peaks in their TPD profiles. These were moderate (lower temperature between 300 to 600 °C) and strong acidic sites (higher temperature between 600 to 750 °C) present in the biochar samples. KJ-400 exhibited stronger acidity than KJ-300 and KJ-500. This was observed due to the fact that upon calcination, the aliphatic groups are converted into an orderly form of graphitic layers. The decrease in total acidity for the KJ-500 is mainly related to the loss of acidic groups because it was treated at higher calcination temperatures.

Fig. 1 NH₃-TPD of the biochar catalysts.

The XRD patterns of the biochar obtained from karanja seed shells are shown in Fig. 2. The XRD pattern exhibits a broad diffraction peak at 2θ = 10° to 30° and few other peaks at 35° to 40° attributed to the amorphous carbon composed of aromatic carbon sheets oriented in a considerably random fashion.³²

Fig. 2 X-ray diffraction patterns of the biochar samples.

Fig. 3 shows the FT-IR spectra of the samples for the detection of functional groups on the catalysts. The bands at 756 and 873 cm⁻¹ are related to the out-of-plane bending modes of the C–H bonds of aromatic and heteroaromatic compounds.^33,34 The bands at 1581 and 1585 cm⁻¹ correspond to aromatic ring modes^35,36 and the band at 1140 cm⁻¹ is related to C–O–C asymmetric stretching. The bands at 1043 and 1083 cm⁻¹ correspond to in-plane C–H bending mode and the band at 1698 cm⁻¹ is related to the C=O stretching modes of carboxylic groups.³⁵ The band at 1215 cm⁻¹ is related to Ar–OH stretching or aromatic acidic groups.³⁶ The bands in the range of 3000–3400 cm⁻¹ are related to the O–H stretching of aromatic phenols and the O–H of carboxylic acids.

Fig. 3 FT-IR spectra of the biochar samples.

The Raman spectra of karanja biochar samples are shown in Fig. 4. The Raman spectra show the main bands at 1370 and 1600 cm⁻¹. The band centered at 1370 cm⁻¹ is related to the stretching vibrations of amorphous carbon and the band at 1600 cm⁻¹ corresponds to the stretching vibrations of graphitic C=C bonds.^37,38 The increase in pyrolysis temperature led to an increase in the intensity of the band located at 1600 cm⁻¹. This suggests the formation of a uniform carbonaceous graphite structure.

Fig. 4 Laser Raman spectra of the biochar samples.

The thermal stability of the karanja seed shell and biochar catalysts were studied by thermogravimetric analysis under a nitrogen flow, and the results are shown in Fig. 5. Karanja seed shell has a high moisture content, which is removed at 100 °C. A maximum weight loss was noticed in between 200 and 450 °C. In this temperature range, most of the biomass was carbonized. The overall weight loss in the case of the karanja seed shells was about 66%. The weight loss for the biochar samples varied, depending on their carbonization temperature. The KJ-300 sample showed a maximum loss of up to 38% weight compared to KJ-500, which showed only a loss of 11% in weight. The maximum loss in weight for the biochar obtained at 300 °C might be related to the decomposition of the oxygen containing functional groups. The decrease in weight loss with the increase in carbonization temperature supports the notion of the loss of surface functional groups.

Fig. 5 Thermogravimetric analysis of the biochar samples.

3.2. Acetylation of glycerol activity of the biochar catalysts

The acetylation of glycerol with acetic acid was carried out over biochar catalysts, and the results are shown in Table 3. The acetylation of glycerol over the biochar resulted in the formation of MA, DA and TA. The biochar derived from karanja seed shells showed reasonable activity towards glycerol acetylation within 1 h of the reaction time. An experiment was carried without using any catalysts and about 8% conversion was observed. The acetylation activity of the biochar increased with the increase in the carbonization temperature from 300 to 400 °C. The sample carbonized at 400 °C showed the highest activity compared to the other samples. The variation in activity with carbonization temperature was related to their physico-chemical properties. The high glycerol acetylation activity for the KJ-400 sample was due to the presence of intense strong acidic sites, as observed from the results obtained for the TPD of ammonia. The low activity of the KJ-300 was related to the presence of different functional groups because the carbonization was carried out at a low temperature. On the other hand, the decrease in activity for KJ-500 is because of the loss of acidic groups (–COOH) due to the high carbonization temperature. Based on the activity profiles over these catalysts, a plausible reaction mechanism was proposed, which is shown in Scheme 1. The acidic site of the catalyst activates acetic acid, followed by the participation of the glycerol hydroxyl group, leading to the mono acetylation of glycerol to yield MA. This MA and another hydroxyl group participate again to yield DA. Similarly, the formation of TA takes place, as shown in Scheme 1.

Table 3 Glycerol acetylation activity over different biochar catalysts^a

Karanja catalysts	Conversion (%)	Selectivity (%)
		MA	DA	TA
a Reaction conditions: temperature: 120 °C, glycerol : acetic acid: 1 : 5, catalyst weight: 0.2 g.
KJ-300	84.5	62.4	30.5	1.8
KJ-400	88.5	56.0	40.0	4.0
KJ-500	83.0	59.0	38.3	2.7

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Scheme 1 Reaction mechanism for the acetylation of glycerol over karanja catalysts.

3.3. Effect of reaction time and temperature

The KJ-400 catalyst showed excellent activity for the esterification of glycerol, and this catalyst was selected as a system catalyst to evaluate the reaction parameters for the acetylation of glycerol. The influence of reaction time and temperature on the acetylation of glycerol was studied, and the results are presented in Fig. 6(A) and (B), respectively.

Fig. 6 Effect of (A) reaction time and (B) reaction temperature on glycerol acetylation over the KJ-400 sample.

The conversion of glycerol increased gradually with time, and attained the maximum at a reaction time of 4 h. The increase in reaction time resulted in the variation of selectivity. The selectivity towards DA and TA increased with reaction time. The increase in DA and TA is expected with time because the acetylation of MA takes place as the availability of glycerol decreases.

The conversion of glycerol increased from 25% to 85% with the increase in reaction temperature from 80 to 120 °C, as shown in Fig. 6(B). With a further increase in temperature to 140 °C, there was no appreciable enhancement in glycerol conversion. The selectivity also varied with reaction temperature. The selectivity towards DA and TA increased continuously with increase in the reaction temperature. The selectivity to MA is very high at a lower reaction temperature and further decreases with reaction temperature. The variation in selectivity is mainly related to the increased activity of the catalysts with reaction temperature.

3.4. Effect of the glycerol to acetic acid mole ratio and catalyst weight

The conversion of glycerol not only depends on the nature of the catalyst but also on the mole ratios of glycerol to acetic acid, as shown in Fig. 7(A).

Fig. 7 Effect of (A) glycerol to acetic acid mole ratio, (B) catalyst weight.

The mole ratios of glycerol to acetic acid were varied from 1 : 3 to 1 : 7. About 75% glycerol conversion was obtained at a glycerol to acetic acid mole ratio of 1 : 3, and reached up to 89% for 1 : 5. For further increases in the mole ratio, there was no considerable variation in glycerol conversion. Most of the reputed solid acid catalysts showed high activity at a glycerol to acetic acid molar ratio of 1 : 9.³⁹ The interesting observation is that the selectivity of DA and TA increased up to 60% with the increase in the glycerol to acetic acid mole ratio. The increase in the selectivity towards DA and TA was mainly due to the higher availability of acetic acid.

The influence of catalyst weight on the esterification of glycerol was studied, and the results are shown in Fig. 7(B). A continuous increase in glycerol conversion with the increase in catalyst weight was observed. The catalysts with the minimum amount of 0.05 g showed about a 79% glycerol conversion. A maximum conversion of 88% was reached at a catalysts weight of 0.2 g. There was not much variation in conversion with a further increase in catalyst amount. The selectivity toward DA and TA increased with the increase in catalyst weight. These results are as expected; the availability of a greater number of active sites leads to a high conversion of glycerol and the simultaneous acetylation of MA and DA as the reaction progresses.

3.5. Comparison with reported catalysts

The active KJ-400 catalyst was compared with the commonly used reported catalysts for the acetylation of glycerol, and the data is tabulated in Table 4. The compiled data suggests that the reported catalysts, such as Amberlyst-36 (ref. 40), heteropoly tungstate supported on active carbon (PW2_AC)⁴¹ and silica matrix (PW-in-S2),⁴² dodecamolybdophosphoric acid encaged in Na USY zeolite (PMo3_NaUSY)²⁷ and mesoporous silicate–niobium silicate type of SAB-15 (MP(5)/NbSBA-15-32),⁴³ require long reaction times or a high glycerol to acetic acid molar ratio to give reasonable activity. Sulfated zirconia catalyst (SZ-1)²⁸ took a long reaction time (24 h) and mainly gave MA as the major product. Moreover, these catalysts require a series of steps in their preparation. Compared to these catalysts, KJ-400 showed a better activity within a reasonable reaction time and reactant molar ratios.

Table 4 Comparison of KJ-400 catalyst with other reported catalysts

Catalyst	Conditions			Conversion (%)	Selectivity (%)			Ref.
	Time (h)	Temp (°C)	Mole ratio (Gly : acid)		MA	DA	TA
Amberlyst-36	12	105	1 : 8	95.6	70.3	4.5	—	40
PW2_AC	3	120	1 : 16	86.0	25	63	11	41
PW-in-S2	7	120	1 : 16	87.0	36	59	4	42
MP(5)/NbSBA-15-32	4	150	1 : 9	94.0	11	51	38	43
PMo3_NaUSY	3	120	1 : 18	68.0	37	59	2	27
SZ-1	24	55	8.2 : 1	54.0	98.9	1.2	—	28
KJ-400	4	120	1 : 5	88.5	56	40	4	Present work

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3.6. Catalyst reusability

The catalyst reusability was tested, and the results are shown in Fig. 8. The used catalyst was recovered after reaction by centrifuging and washed with methanol, followed by drying at 100 °C in a hot air oven and reused without any treatment. This procedure was continued up to five cycles. The activity of the catalyst was almost consistent. A marginal variation in the selectivity towards MA was probably due to the blocking of some active acidic sites on the surface.⁴⁴ It is noteworthy to mention that the biochar catalyst was reusable without any treatment with consistent activity.

Fig. 8 Reusability of the KJ-400 catalyst in the acetylation of glycerol.

4. Conclusions

Catalytically active biochar was prepared by a simple pyrolysis of karanja seed shells at different temperatures without any treatment before or after carbonization. The acidity of the biochar depended on the carbonization temperature. The biochar obtained at 400 °C possessed moderate to strong acidic sites. These catalysts showed an excellent activity for the esterification of glycerol under mild reaction conditions, and the catalyst carbonized at 400 °C showed the highest activity among all the catalysts. The conversion and selectivity for glycerol acetylation depended on the reaction parameters. The catalyst is stable, easy to recover and reusable with consistent activity.

Acknowledgements

The authors JMR, AR and MS thank Council of Scientific and Industrial Research (CSIR), India for financial support in the form of Junior Research Fellowship.

Notes and references

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