Introduction

The use of cheap, fast and different environmentally friendly strategies for the preparation of carbon materials from renewable resources has been on the increase in recent times in the areas of environmental science and technology. This is because of the vital roles these materials play in different applications, such as adsorbents, catalyst supports, energy storage materials, electrode materials, and stationary phases in liquid chromatography [1, 2]. As a result, a number of approaches such as pyrolysis, arc discharge, chemical vapor decomposition, and hydrothermal treatment have been used in the preparation of these carbon materials [3]. Hydrothermal carbonization is a process of decomposing an organic material in hot water under high pressure to produce solid carbon material (hydrochar) and water soluble organics. It is a green and efficient approach for treating organic materials because of its comparatively low emission, and generation of non-toxic waste [4]. Due to its simple operation, mild reaction conditions, and ability to exploit renewable biomass with minimal pre-treatment, it is of a particular environmental advantage when compared to other techniques of carbonization [5–7]. Among potential precursors used in the preparation of carbonaceous materials, glucose is very promising and its hydrothermal carbonization process has been studied several times using the conventional method of oven heating [6, 8–10]. However, the conventional hydrothermal process requires special systems that support pressure and temperature, usually an autoclave with pressure safety device is used. Also, the reaction times are usually in hours, which makes the process expensive and time consuming.

In many applications, the use of microwave heating as an attractive alternative to conventional method of heating has shown to be more energy efficient, because it provides selective, fast and homogenous heating, which reduces processing time and costs significantly [11, 12]. It has also been established that irradiation with microwave produced effective internal heating by direct coupling of microwave energy with solvents, reagents and catalysts, which increased the reactions greatly [13]. The use of microwave heating in hydrothermal carbonization process, and in the preparation of hydrochar from biomasses, glucose and other materials, such as human waste, cellulose and starch, has been reported [4, 14–19]. Some of these processes usually proceed via the degradation of starch or cellulose to form glucose and then further carbonization of the formed glucose. However, the energy properties of hydrochars obtained using glucose as starting material have not been previously reported to the best of our knowledge. Therefore, in this study the energy properties of hydrochars from microwave-assisted hydrothermal carbonization process using glucose as starting material are reported.

Materials and methods

Anhydrous d-glucose was purchased from Fisher Scientific, UK and was used throughout without further purification.

Microwave-assisted hydrothermal carbonization of glucose

5 g each of glucose was dissolved in 5 mL of de-ionized water in 100 ml microwave reaction vessels made of Teflon to form supersaturated solutions. The vessels were sealed and placed in a 2.45 GHz magnetron frequency microwave oven (MARS, CEM, Milton Keynes, UK equipped with XP1500 digestion vessels, and 1600 W at maximum power), and were hydrothermally carbonized at 200 °C in the microwave oven which was set to ramp to a given temperature in 5 min, and then held at the temperature for 5–60 min. The reaction system was allowed to cool to room temperature, and the carbonized materials were filtered off using Whatman filter paper number 3, ashless 11 cm. The solid chars (hydrochars) obtained were washed gently with de-ionized water to remove any left over of the liquid phase of the reaction, and were dried in a conventional oven at 80 °C for 16 h.

Mass yield

In each case, the dry mass of the carbonized material was measured and the mass yield (dry mass percentage of starting material) was calculated as follows:

$${\text{Mass}}\;{\text{yield}}\;(\% ) = \frac{{{\text{Mass}}\;{\text{of}}\;{\text{carbonized}}\;{\text{material}}\;({\text{g}})}}{{{\text{Mass}}\;{\text{of}}\;{\text{starting}}\;{\text{material}}\;({\text{g}})}} \times 100.$$
(1)

Energy properties of the hydrochars

In place of a calorimeter to experimentally determine the energy content, the higher heating value (HHV) was calculated using Eq. 2 (Dulong equation) as previously reported [20]. This formula is one of the first correlations to estimate the HHV of coals, which is still in used by many researchers today and has also been applied to oils [21].

$${\text{HHV}} = 0.3383{\text{C}} + 1.422({\text{H}} - 0/8).$$
(2)

The energy densification ratios of the hydrochars were calculated using the following equation:

$${\text{Energy}}\;{\text{densification}}\;{\text{ratio}}\; = \;\frac{{{\text{HHV}}\;{\text{of}}\;{\text{dried}}\;{\text{hydrochar}}}}{{{\text{HHV}}\;{\text{of}}\;{\text{dried}}\;{\text{starting}}\;{\text{material}}}}.$$
(3)

All experiments were carried out in triplicate, and the results obtained are presented in Table 1.

Table 1 Elemental composition, mass yield, O/C and H/C atomic ratios, and energy properties of glucose and hydrochars, with data from previous studies using the conventional approach for comparison
Full size table

Characterization techniques

The prepared hydrochars were characterized using the following instruments: Fisons instruments EA 1108 CHN analyser (Fison Instrument, Crawley, UK) was used for CHN analysis; samples were ground into fine powder, weighed into tin capsules and placed on the autosampler for analysis. Surface area and pore size distribution were measured using a Micromeritics Tristar BET-N2 surface area analyser (Micromeritics, Hexton, UK). Before the analysis, samples were degassed under nitrogen atmosphere at 120 °C for 3 h. FT-IR spectra were recorded on Thermo Scientific Nicolet 380 FT-IR (Themo Scientific, Hemel Hempstead, UK), equipped with attenuated total reflectance (ATR). The samples were in direct contact with ATR diamond crystal, and each sample was investigated in wavenumber range of 4000–525 cm−1 using 16 scans at a spectral resolution wavenumber of 4 cm−1. The morphologies and particle sizes were visualized with a ZEISS EVO 60 SEM (Carl Zeiss, Cambridge, UK); samples were coated with gold and platinum alloy and impregnated on a sticky disc before analysis. 13C solid state magic angle spinning NMR experiment was carried out on a Bruker Avance II 500 MHz (11.74T) spectrometer (Bruker, Coventry, UK); samples were packed without further treatment into a 4 mm zirconia rotor sample holder spinning at MAS rate ν MAS = 8 kHz. Carbon sensitivity was enhanced by Proton-to-carbon CP MAS: recycle delay for all CP experiments was 3 s and TPPM decoupling was used during signal acquisition. Cross polarization transfer was carried out under adiabatic tangential ramps to enhance the signal with respect to other known methods. CP time t CP = 500 ms. The number of transients for all carbon samples was 200.

Results and discussion

Effect of time on the microwave-assisted hydrothermal carbonization of glucose

The effect of time on the process is presented in Table 1. As the processing time was increased from 15 to 60 min, different mass yields were obtained for the hydrochars. The mass yield increased between 15 and 45 min, and decreased afterwards. Hence, processing time above 60 min was not considered. Maximum mass yield for the hydrothermal carbonization of glucose has been reported to be obtained at 200 °C [6], while further increase in temperature will lead to a gradual decrease in mass yield; reason being that increase in temperature favors gasification reactions, which results in part of the hydrothermal carbon being lost in form of volatile compounds [22, 23]. Hence, temperatures above 200 °C were not considered in this study. Falco et al. [6], using conventional hydrothermal carbonization process, reported a maximum mass yield of about 40 % for glucose hydrochar at 200 °C for 24 h, which is consistent with the maximum mass yield obtained in this study under microwave heating, despite the shorter time (45 min) in the microwave oven.

Elemental composition of the prepared hydrochars

The elemental compositions (C, O, and H) of the starting material (glucose) and different hydrochar samples are listed in Table 1. It was observed that carbon contents increased from 39.84 % in the starting material to about 53–62 % in the hydrochar samples. The oxygen and hydrogen contents of the hydrochars reduced at the same time. These variations, which increased with reaction time, are consistent with hydrothermal carbonization processes. The gradual increase in carbon content, and the decrease in hydrogen and oxygen contents of the hydrochars, with increase in processing time is due to loss of hydrogen and oxygen in deoxygenating, dehydration and decarboxylation reactions that occurred during the microwave-assisted hydrothermal carbonization process [6, 22, 26, 27]. Despite the shorter time used in this study under microwave heating, the result is similar to previous studies [6, 9].

The changes in elemental compositions were further analyzed using the van Krevelen diagram (Fig. 1), by plotting the atomic H/C against the atomic O/C. The diagram provided further evidence about the transformation (dehydration, decarboxylation, and demethanation reactions) that took place in the chemical structure of glucose during the microwave-assisted hydrothermal carbonization process. The transformation from the starting material (glucose) to the hydrochar samples follows a diagonal line due to decrease in the O/C and H/C ratios, suggesting dehydration reactions as prevalent reaction during the process, which is consistent with previous reports [9, 28].

Fig. 1
figure 1

van Krevelen diagram of raw glucose and hydrochars prepared at 200 °C and different processing time

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Energy properties of the prepared hydrochars

An important parameter to test the quality of hydrochars is their higher heating value (HHV). It provides information about the quantity of energy present in the hydrochar. The calculated HHV increased with increase in residence time from 16.90 MJ/kg in 15 min to 21.30 MJ/kg in 45 min, and remained stable afterwards (Table 1). This increase in higher heating value of the hydrochars with increase in processing time is consistent with previous report [29]. The highest HHV in this study showed an increase of 55.5 % when compared to that of the starting material, against 39 % previously reported for loblolly pine [29], 45.20 % for bamboo [30], and 21 % for dry leaves [31].

As stated earlier, dehydration, decarboxylation, and condensation reactions are associated with hydrothermal carbonization process. This leads to the carbonization of the starting material and consequently results in energy densification, which is used to measure the effectiveness of the hydrothermal carbonization processes [32, 33]. In this study, energy densification ratios of the hydrochars (Table 1) increased with increase in reaction time and ranged from 1.23 to 1.55, which is consistent with previous reports [29, 31].

The energy yield is defined as the mass yield of the hydrochar multiply by its energy densification ratio. The result of the energy yield for the hydrochars is presented in Table 1. The lowest energy yield of 25.40 % was obtained when the reaction time was 15 min, and increased steadily until the maximum hydrochar energy yield of 62.06 % was obtained after 45 min. Thus, the hydrochar prepared at 200 °C for 45 min has the highest mass yield, HHV, energy densification, and energy yield. Therefore, further characterization will be focused on this hydrochar.

FT-IR analysis

The FT-IR spectra (Fig. 2) gave further insight into changes in the chemical composition of glucose during the process. Comparing the FT-IR results of glucose (Fig. 2a) and that of the hydrochar prepared at 200 °C for 45 min (Fig. 2b) indicates that dehydration and aromatization of glucose occurred during the process. The allocation of the functional groups was based on previous reports [9, 34]. The broad peak between 3600 and 3000 cm−1 (section 1) corresponds to stretching vibration of aliphatic O–H (hydroxyl and carboxyl), while the peaks between 1500 and 1000 cm−1 (section 5) correspond to C–O stretching vibration from esters, ether, phenols, and aliphatic alcohols. The weak intensity of these peaks (sections 1 and 5) is an indication that dehydration and decarboxylation reactions occurred during the process. The peaks between 1800 and 1650 cm−1 (section 3) present only in the hydrochar result from C=O vibration of esters, quinone, pyrone, carboxylic acids or aldehydes, while the appearance of peaks between 1650 and 1500 cm−1 (section 4) due to C=C vibrations, and well-defined peak below 1000 cm−1 (section 6) from deformation of C–H out of plane bending vibrations in aromatic compounds, showed the aromatic nature of the hydrochar. The peaks between 3000 and 2800 cm−1 (section 2) due to stretching vibration of aliphatic C–H bonds, indicate the presence of aliphatic structures.

Fig. 2
figure 2

FT-IR spectra of a glucose, b hydrochar prepared at 200 °C 45 min

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NMR analysis

13C solid state NMR analysis is usually used as a complementary technique to FT-IR analysis in describing the level of conversion during hydrothermal carbonization process [24]. The 13C solid state NMR spectrum of the hydrochar prepared at 200 °C for 45 min shown in Fig. S1 (Supplementary information) provided information about its chemical composition, and also confirmed the results from the FT-IR. The peaks between 14 and 60 ppm are due to the presence of aliphatic carbons [8, 35], while those between 100 and 160 ppm usually referred to as the aromatic region are all due to C=C bond, but between 140 and 160 ppm are specifically due to the oxygen bound O–C=C (O-aryl) [35]. The peaks between 170 and 200 ppm are due to the presence of carboxylic acid, aldehydes or ketones moieties [36]. The spectrum also shows carbohydrate resonances (specifically CH2OH groups around 62 ppm, CHOH groups around 72 ppm, and anomeric O–C–O carbons around 90 ppm) in the O-alkyl region between 60 and 100 ppm [37, 38]. These functional groups have already been indicated by the FT-IR result. Thus, both results are in good agreement with each other, and clearly show the formation of the hydrochar structure under the microwave-assisted process.

SEM analysis

The scanning electron microscope (SEM) analysis (Fig. 3) provided information about the structural morphologies of the starting material (glucose) and the hydrochar prepared at 200 °C for 45 min. The morphological features of glucose (Fig. 3a) as visualized are completely different from those of the hydrochar (Fig. 3b). The sphere-like microparticles of different sizes (1–10 μm) seen on the SEM image of the hydrochar are in contrast to the block of materials observed in glucose. The mechanism in which these microparticles are generated is based on hydrolysis, dehydration and polymerization of glucose as stated in the mechanism, while the spherical nuclei are formed to minimize the energy of the interface [6, 14].

Fig. 3
figure 3

SEM images of a glucose, b hydrochar prepared at 200 °C 45 min

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Surface area analysis

The hydrochar prepared at 200 °C for 45 min showed a Type II isotherm based on the IUPAC system of classification from the nitrogen sorption measurement in Fig. S2 (Supplementary information). This is a typical isotherm obtained with non-porous materials. It is, therefore, not surprising that the hydrochar has a very small BET surface area of 3.3 ± 0.7 m2g−1, due to poor porosity. The hydrothermal carbonization process involves carbonization and solubilization of organics leading to the formation of tarry substances, which usually contaminate the hydrochars, plugging their pores, and making the apparent BET surface area to be small [39]. Similar results have been reported using the conventional hydrothermal method [9, 40]. Thus, surface area and porosity improvement are usually carried out for the hydrochars to fit into specific applications, such as hydrogen or electrical energy storage [33].

Mechanism for the hydrochar formation

Similar mechanism to that in conventional hydrothermal carbonization process is expected to occur under the microwave-assisted process [41]. However, under the electromagnetic irradiation during the microwave-assisted process, temperature and pressure will play a very vital role by enhancing the different reactions (dehydration, polymerization and aromatization), thereby accelerating the process [12]. Therefore, in this study, glucose fragments by hydrolysis and dehydrates under microwave heating to form different soluble furfural-like compounds. The dehydration reaction is indicated in the hydrochar by the reduction in the OH peak between 3600 and 3000 cm−1. The furfural-like compounds undergo further decomposition to form acids, aldehydes and phenols [18, 42], indicated in the hydrochar by the appearance of the peaks between 1800 and 1650 cm−1 (C=O vibration). The dehydration of water from the equatorial hydroxyl groups could be the mechanism responsible for the appearance of C=O groups [43]. The glucose and its decomposition products then undergo polymerization or condensation reactions, which could be caused by intermolecular dehydration or through aldol condensation leading to the formation of soluble polymers [9]. The aromatization of the polymers also occurs simultaneously with the formation of C=C linkages (indicated by the appearance of peaks between 1650–1500 cm−1, and 100–160 ppm in the FTIR and NMR spectra of the hydrochar, respectively), which could result from keto-enol tautomerism of the dehydrated species, intermolecular dehydration or via the condensation of the aromatized molecules formed during the decomposition or dehydration of the glucose [9, 43]. A burst nucleation process occurs once the amount of aromatic clusters in aqueous phase reaches the critical super saturation point. The formed nuclei then grow through diffusion to the surface of the chemical species that are present in solution (as seen on the SEM image of the hydrochar) to minimize the energy of the interfaces [14]. The reactive oxygen functionalities (as indicated by the FTIR and NMR spectra), such as hydroxyl, carbonyl, and carboxylic, present on the outer surface of the microspheres and in the reactive species helps to quickly link the species to the microspheres [43]. Due to this linkage, stable oxygen groups, such as ether, pyrone or quinone, usually found at the center of the resulting microsphere are formed, and as a result, when the growth process ends, the outer surface of the hydrochar particles will have a high concentration of reactive oxygen groups, while the core will have less reactive oxygen groups [9, 43]. Thus, two types of products are formed in the reaction medium at the end of the reaction, namely the insoluble solid residue (hydrochar), and the aqueous soluble organic phase (consisting of furfural-like compounds, aldehydes and acids). Thus, as the reaction time is increased in this study, it is expected that the aromatization and polymerization processes will also be favoured, which will increase the yield of the hydrochar.

Conclusion

Microwave-assisted hydrothermal carbonization was successfully used to prepare hydrochar from glucose. The chemical and structural characterization of the prepared hydrochar showed that it is similar to those previously prepared using the conventional oven method of hydrothermal carbonization. This implies that the microwave-assisted hydrothermal approach is a fast and simple approach for preparing the hydrochar as it reduced the processing time from hours to few minutes. The energy properties of the prepared hydrochars in relationship to some published results showed a higher increase in HHV of the hydrochars when compared to that of the starting material.