Hydrothermal Carbonization of Fructose: Growth Mechanism and

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Hydrothermal carbonization of fructose: growth mechanism and kinetic model Dennis Jung, Michael Zimmermann, and Andrea Kruse ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02118 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Sustainable Chemistry & Engineering

Hydrothermal carbonization of fructose: growth mechanism and kinetic model †

‡

Dennis Jung* , Michael Zimmermann and Andrea Kruse

†

†

University of Hohenheim, Institute of Agricultural Engineering, Department of Conversion Technologies of Biobased Resources, Garbenstrasse 9, 70599 Stuttgart, Germany.

‡

Karlsruhe Institute of Technology (KIT), Institute for Catalysis Research and Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

corresponding author: [email protected] KEYWORDS Hydrothermal carbonization, carbon spheres, kinetic model, growth mechanism ABSTRACT

Hydrothermal carbonization experiments were performed with fructose solutions as feedstock. A kinetic model is proposed that models hydrochar formation based on molar concentrations. Based on SEM pictures, the mean particles size of the hydrochar was determined, and it could be demonstrated that the growth of the particles is not directly related to the formation rate of hydrochar or the decreasing rate of the intermediates concentration in the aqueous solution. Four additives (salts) have been tested, and the results point out that the ionic strength is an important factor to control the particle size. Together, these observations lead to the conclusion that

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coalescence is an important driver for the growth of the particles, and that a LaMer-like nucleation followed by diffusion-controlled growth is unlikely to be the formation and growth mechanism of hydrochar.

TEXT Introduction In a view of producing renewable energies from agricultural waste streams, hydrothermal carbonization has attained a lot of attention in the past years, due to its ability to convert wet biomasses into a lignite-like fuel1–3. At the same time, the process has been evaluated in terms of its ability to produce functional carbon materials4–13. The chemical conversion pathway is more complex using biomasses compared to monosaccharides, which is mainly caused by the lignocellulose structure of biomasses, but also with biomass as feedstock hydrochar is formed by the conversion of carbohydrates. This allows a wide range of different applications of agricultural or organic waste for the production of renewable fuels or on the other hand, of sugars for advanced carbon materials. The structure of biomass is of special importance in view of modeling the chemical kinetics. The skeleton of lignin, cellulose, and hemicellulose creates a macromolecular structure, which requires some sort of heterogeneous model approach in order to express the process with mathematic formulas. However, it is known that the carbohydrates, once hydrolyzed, undergo dehydration, forming intermediates such as Hydroxymethylfurfural (HMF) and furfural. Those substances further polymerize and form a carbonaceous material, which is referred to as humins14–16, secondary char3 or hydrochar. Usually, this product looks like a bulk of monodispersed or agglomerated spherical particles.


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There have been several approaches to set-up a formation mechanism of these spherical particles. However, those concepts are mostly postulated and poorly supported by experimental evidence. Modeling suffers from a general challenge, which is related to the dehydration of sugars to form HMF, taking place in a homogeneous system, whereas the growth of the particles takes place in a heterogeneous colloid system. The formation of hydrochar is, therefore, a phase separation process, forming an interface. In order to understand the mechanism, it is inevitable to collect data that are in relation to that interface. An example is given by Zhang et al., who overruled the very widely spread concept of a LaMer17 like nucleation13,18,19, by introducing a formation mechanism via hydrophobic ripening20. The authors monitored the decline of the HMF concentration and concluded that it conflicts with a discrete nucleation mechanism. According to them, the formation mechanism is based on the precipitation of hydrophobic molecular clusters, which further agglomerate to spherical particles. Beside those mechanistic discussions, many authors developed reaction flow charts and kinetic models that cover different possible combinations of sugars, HMF or others total organic carbon (TOC) compounds to form hydrochar. However, the generation of a reliable understanding is only possible when data are collected from both systems, the homogeneous reaction pathways, and heterogeneously formed product.

Approach The formation of particles by hydrothermal carbonization has two consequences: First, the concentration of solved compounds decreases. The second effect is the formation of an additional phase. In this study, these consecutive stages of HTC are separated: First, the homogeneous reaction in solution is modeled, then the particle growing is observed and


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documented. This combination of collected data enables evidence-based conclusions on the formation and growth mechanism. The kinetic modeling presented here is based on own results and significantly on literature. The results in the literature are widespread, complex and – partly – contradictive. Therefore now an insertion follows, giving information about the chemistry of HTC from literature and anticipates own results. This is a consequence of the crosslinking of the two stages of HTC, which are methodically separated here and described in the “results” section.

Chemistry of HTC in Literature and Anticipation of Results The process is classified under the subject of thermochemical conversion. Carbohydrates are mixed in water and heated in a closed reactor under autogenic pressure at 180 – 250°C21. Under these conditions, water has an increased ionic product, so that more H+ ions can catalyze hydrolysis and dehydration reactions22. The carbonization of carbohydrates is a hydrolysis followed by several water elimination steps, firstly leading to HMF or furfural, and secondly, the aldol-condensation reactions to hydrochar15. This means that sequences of homogeneous reactions of the dissolved molecules lead to the formation of hydrochar3,23.

13

C-NMR studies

showed that hydrochar is a cross-linked network of furanic moieties24. It has been demonstrated that hexose-based chars are similar to the ones obtained from HMF, whereas pentose-based chars are similar to those obtained from furfural9. The differences are the carbon content, the morphology of the material and the functional groups. The current knowledge on the reaction mechanism is mainly built up from the HMF formation path, whereas little is known from the furfural path. The HMF formation mechanism is assumed to consists of aldol addition/condensation of HMF and 2,5-dioxo-6-hydroxy-hexanal (DHH) forming a dimer that is


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able to grow through HMF addition15. This leads to an oligomer, which is soluble up to a molecular mass of 270 – 650 g mol-1

25

. The phase separation of these oligomers from water

results in primary particles that aggregate to form spherical particles in micrometer size25. This process is mainly observed in water systems, whereas DMSO (Dimethyl sulfoxide) and methanol have demonstrated to significantly reduce the formation of hydrochar26,27. This effect was related to an increased LUMO (lowest unoccupied molecular orbital) energy of HMF in DMSO, reducing its susceptibility to nucleophilic attack28. More important: in the two-phase system, the HMF is solved in DMSO and the acidic solved catalysts stay in aqueous solution. This way, no catalysis of the, in the case of HMF production, unwanted polymerization occurs. With respect to the carbon content of hexose-based hydrochars, it has been demonstrated that almost every sample, produced between 170 and 250 °C has a carbon content between 64 – 73 % (Table 1). It is obvious that this increase in carbon content is a result of dehydration, but only a few calculations describing this process have been proposed. Poerschmann et al. recently proposed that the hydrochar yield is limited by a stoichiometric barrier, which is a result of the dehydration of five water molecules from one hexose29. Falco et al. defined this barrier by the dehydration of four water molecules from one hexose10. Nevertheless, the dehydration of four or five water molecules results in hydrochars, with a carbon content of 66.7 and 80% respectively. In the present work, we propose a stoichiometric calculation using dehydration and decarboxylation reactions, which are in agreement with the measured values. The calculation of the dehydration is used to formulate a hypothetical molecular weight of the hydrochar and thus enables the implementation of hydrochar within a kinetic model that is expressed in molar quantities.


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Table 1 literature.

Carbon content of hydrochars derived by different monomers found in the

Feedstock Temp

Time

Carbon %

Source

Fructose

130

12

61.2

12

180

4

67.1

30

220

6

72

29

190

4

65.2

31

Glucose

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230

66.2

290

73.9

180

24

64.5

9

170

4.5

64.9

13

210

66.3

195

20

66.3

32

HMF

180

24

65.6

9

Sucrose

200

4

65.7

33


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In addition, a focus on the concentration dependency of hydrochar formation is set. Many articles have demonstrated the positive effect of initial concentration of model substances, respectively solid load for biomasses on the hydrochar yield

9,13,34–37

. Obviously, this points to a higher

reaction order of an important step/equation of the mechanism, however, no article currently exist that systematically explores this effect over a large range of concentration and temperature using a model substance such as fructose. The last pillar of this work is the influence of additives during hydrochar formation. Scientific literature covers a huge amount of articles where additives, mostly electrolytic substances have been used in the preparation of hydrochar. From these studies it is remarkable that electrolytic substances promote monodispersed particles, whereas control systems, based on pure water, mostly result in interconnected particles2,10,41– 48,14,18–20,31,38–40

. However, some deviations from that observation exist4,45,49. Different concepts

are proposed to explain that phenomenon, mostly focusing on an individual explanation for a particular additive. In this work, we choose three electrolytic substances. These results are supposed to serve as a basis to develop a unified understanding on the influence of electrolytic substances, rather than focusing on possible impacts of an individual substance. Purpose of the study In here, we report the results of the hydrothermal carbonization of fructose under various conditions. We set a focus on the simultaneous evaluation of microscopic and macroscopic aspects that describe the formation and growth of hydrochar. By this approach, a better understanding of the formation mechanism will be worked out. This is accompanied by the formulation of a kinetic model of the hydrochar formation. Additionally, a set of experiments is performed to explore the impact of different additives on the size and the shape of the particles. Additionally, we postulate a stoichiometric equation for the dehydration reactions that form


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hydrochar, which are in accordance to the measured carbon content. All together this article aims to provide knowledge to improve current semi-empirical50–52 or kinetic models53–55 in the field of hydrothermal carbonization. Materials and Methods Materials and reaction conditions Fructose (VWR Life Science, LOT:2337C420) solutions were prepared with deionized water in 100 ml volumetric flasks adjusting different molar concentrations. An aliquot of 7 ml was transferred to 10 ml autoclaves. Up to five autoclaves were put in a row into a disused gas chromatograph oven with a functioning heating system. A reference autoclave was added to measure temperature and pressure. The gas chromatograph took around 5 minutes to reach the selected temperature in the oven chamber. The heating phase took 45 – 50 min. t = 0 min was set to the time point when the reference autoclave was 1K below the desired temperature. After a defined reaction time, the autoclaves were quenched in cold water. The solid and liquid phases of the HTC slurry were separated by a vacuum filtration device using a 0.45 µm nylon membrane filter (Whatman). The liquid was removed from the flask and the solid residue was washed with approximately 100 ml of deionized water and dried at 105°C for 8h. Salts KHCO3 (99.5% purity, VWR Chemicals), CaCl2 (99% purity, VWR Chemicals), FePO4 (Alfa Aesar) were added to the volumetric flask with water to reach a concentration of 0.025 mol L-1. KCl was added to reach a concentration of 0.075, having the same ionic strength than the CaCl2 solution. Characterisation


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The carbon content of the solid was determined with an Elemental Analyser (Euro EACHNSO) from Hekatech by dynamic, spontaneous combustion and subsequent chromatographic separation. TOC (total organic carbon) was measured with Dimatec 2100 by calculating the difference between total carbon and inorganic carbon. HMF and Furfural were determined by a Lichrosper 100 RP-18 HPLC column by HPLC. Fructose was measured with MetaCarb 87C Agilent column, also by HPLC. Field emission scanning electron microscopy (FESEM, referred to as SEM in this article) was performed with a DSM 982 Gemini from Carl Zeiss Ltd. Mean particle diameters were calculated by measuring the diameter of 50 – 200 particles for monodispersed samples. For the interconnected particles, the diameters of the spherical subunits were measured according to a procedure exemplified in figure 1. However, this approach does not represent any real particle size but enables comparisons among samples with interconnected particles.

Figure 1 The right pictures shows how the spherical subunits were identified from the hydrochar and measured based on the original SE (secondary electrons) picture left. Kinetic model The calculation of the reaction rate constants was performed according to the kinetic model presented in scheme 1. The hydrochar yield, which was gravimetrically recorded, was transformed into molar concentration (mol L-1) using the molecular weight of 108.1 g mol-1. This is derived by the following simplified stoichiometry for hydrochar formation:


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HMF

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Hydrochar + H2O

We justify this assumption by the fact, that such a hydrochar has a theoretical carbon content of 66.7 %, which is in accordance to the majority of measured samples in the literature (Table 1) and in this work (Table 3). The reaction network is expressed in terms of the following differential equations:

(1)

(2)

(3)

1 2

(4)

With k1, k2, k3, k4 the reaction rate constants, m the reaction order of the hydrochar formation, and R1 + R2 as the undefined degradation products from fructose and HMF. [Fruc], [HMF], [Hydrochar] and [R1 + R2] are the molar concentration (mol L-1) of the substances. Scheme 1 shows a schematic representation of the reaction network.


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Scheme 1 Proposed reaction pathways for hydrochar formation. R1 and R2 are unquantified decomposition products.

The calculation of the reaction rate constants (k1 – k4) has been done with MATLAB R2015b by numerical integration of equations 1 - 4 with ODE45 followed by an optimization with the lsqnonlin tool. The reaction order m was fixed to 1 or 2. The starting values for the numerical integration where the measured value at t = 0 min.

Results and discussion Results The kinetic model has been evaluated with the hydrochar produced at 200°C (Equation 1 to 4). Two different optimization procedures were employed: optimization 1 minimizes the least squares for the fructose, HMF, and hydrochar concentration, whereas optimization 2 minimizes the least square solution of the hydrochar concentration only. In table 2 the reaction rate constants for the hydrochar formation are given. The reaction rate constants for optimization 1 and 2 are equal for 0.5 M and a reaction order of 1. The same comparison with a reaction order of 2 results in a rather high difference (= 0.104) of the rate constants. With respect to optimization 1 it is remarkable that the first order model fits much better to the data than the second order model, which can be noted by the R2 value and visualized by the shape of the curve in figure 2A. Within optimization 2, different initial concentrations have been evaluated with the first and second order model. The reaction rate constant is increasing with increasing concentration in each case, however, the values of the second order model have a reduced relative deviation compared to the first order model. Table 2 Reaction rate constant for the hydrochar formation calculated by two different optimization procedures with reaction order 1 or 2. R² is the coefficient of determination and is


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calculated based on measured and modeled hydrochar concentration. The reaction temperature was 200°C. Values for k1, k3, and k4 can be found in Table S1. Optimization

Reaction order

Initial concentration [mol L-1]

k2 [min-1]

R²

m 1

1

0.5

0.024

0.99

1

2

0.5

0.166

0.94

2

1

0.3

0.014

0.99

2

1

0.5

0.024

0.99

2

1

1.0

0.040

0.99

2

2

0.3

0.053

0.99

2

2

0.5

0.062

0.99

2

2

1.0

0.079

0.99

Figure 2A shows the HMF, hydrochar and fructose concentrations from 0 to 80 min reaction time. The figure illustrates the link between HMF concentration and hydrochar formation. The solid line represents the curve of the first order model, and the dashed line those from the second order model. The figure also shows that most of the hydrochar is formed in the absence or at low fructose concentrations. Figure 2B shows the increase in particle diameter with time. It can be noted that the particle diameter is not increasing with the same rate as the hydrochar formed. Between 0 and 40 min, the diameter barely increases, whereas the hydrochar yield has its highest formation rate. This effect reverses in the latter period of the reaction between 60 – 80 min, where the particle size increases strongly at the same time with very low hydrochar formation. As indicated in figure 2C, also the carbon content in the liquid phase scarcely changes after 60


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min. The carbon content of the hydrochar is rather constant (At t=2 min an additional data point is displayed, which was not used for modeling or particle-size determination. ).

Figure 2 A Concentration of hydrochar, HMF, and Fructose over time. The solid line (―) represents the first order model, dashed line (- - -) represents the second-order model. The reaction temperature was 200°C with 0.5 M initial fructose concentration. Hydrochar mass was transformed in molar quantities assuming a molecular weight of 108.1 g mol-1.B Mean particle size of the spherical subunits. Error bar represents one standard deviation. n= number of measured spheres. n0min=25; n20min=69; n40min=77; n60min=64; n80min=86. The reaction temperature was 200°C with 0.5 M initial fructose concentration. C C – solid = carbon content of the hydrochar. C – solid recovery = percentage of carbon recovered in the hydrochar. C – liquid recovery = percentage of carbon recovered in the process water. C – HMF = percentage of carbon bound in HMF. The reaction temperature was 200°C with 0.5 M initial fructose concentration

SEM pictures of the hydrochar are presented in figure 3. It can be observed that hydrochar particles are monodispersed spheres, at the beginning of the reaction (t = 0 min), and then start coalescence, forming a network of interconnected particles. With progressing time, particles regain spherical shape, having a larger diameter but are still interconnected.


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Figure 3 SEM pictures ((A) BSE (backscattered electrons) picture, (B) – (E) SE (secondary electrons) pictures) of hydrochar at different reaction times at 200°C and 0.5 M initial fructose concentration. (A) t = 0 min magnification = 10000x; (B) t = 20 min, magnification = 5000x; (C) t = 40 min, magnification = 5000x; (D) t = 60 min, magnification = 5000x; (E) t = 80 min, magnification = 5000x. Because of charging effects for sample t = 0 min (A) a BSE picture is depicted. The BSE detector is less sensitive to charging effects than SE detectors.

Table 3 shows the carbon content of different hydrochars. The values range from 64.9 – 69.6% in the temperature range of 190-250°C and reaction time up to 4 hours. For 200 and 220°C the investigated reaction time has a low effect on the carbon content. Table 3 Carbon content of hydrochar produced at different reaction conditions. Initial concentration = 0.5 mol L-1. Hydrochar yield [mol-%] is calculated as: Hydrochar mass [g] / 108.1 [g mol-1] / initial amount of fructose [mol]. Hydrochar yield [w-%] is calculated as: Hydrochar mass [g] / initial amount fructose [g]. Due to the higher carbon content at 250°C the mass yield was not transformed into molar yield. Temperature [°C] 190 190 250 250 200

220

Time [min] 120 240 120 240 0 20 40 60 80 0 20 40 60 80

Yield [w-%] 29.4 32.0 35.8 35.8 5.9 20.5 25.0 30.0 31.6 22.0 31.3 32.9 34.8 35.3

Yield [mol-%] 49.0 55.0 9.8 34.2 41.5 50.0 52.6 36.6 52.1 54.7 58.0 58.9

C [w-%] 64.9 65.9 69.0 69.6 65.4 66.3 66.8 66.3 66.6 66.4 67.3 67.3 67.3 67.3


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100

35.5

59.2

67.3

Figure 4 shows how hydrochar yield increases with increasing concentration and temperature. The figure shows that the initial concentration has a strong impact at low molar concentrations from 0 to approximately 0.5 M, above that value the effect reduces; also the effect of temperature is stronger for lower concentrations. In an additional triplicate experiment (220°C, 120 min) we used an initial concentration of 4.44 moles per Liter and received a hydrochar yield of 86.3 ± 0.34 mol-%. The result shows that the yield is increasing over a large concentration range. SEM pictures made at different concentrations show that particles are interconnected (Figure 5).

Figure 4 Hydrochar yield for different initial fructose concentrations and temperature. Hydrochar mass was transformed in molar quantities assuming a molecular weight of 108.11 g mol-1. Hydrochar yield [mol-%] is calculated as: Hydrochar mass [g] / 108.1 [g mol-1] / initial amount of fructose [mol] . Reaction time = 120 min.


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Figure 5 SEM pictures (SE) of hydrochar at different initial concentrations at 200°C and 2 h. Magnification = 5000x. (A) 0.4 M; (B) 0.6 M; (C) 0.8 M; (D) 1.0 M.

Effect of additives The effect of additives was tested with electrolytic substances compared to a unique fructose solution as control (w/o). The concentration was set constant for CaCl2, FePO4, and KHCO3 at 0.025 M, the concentration of KCl was 0.075 M in order to have the same ionic strength as the CaCl2 solution. SEM pictures are given in figure 6 and particles size and yield in figure 7. The samples prepared with CaCl2 and KCl had a rather similar hydrochar yield, whereas FePO4, KHCO3 and the control had a lower yield. The most distinct effect was observed for the particle diameter, following the increasing trend w/o < KHCO3 < FePO4 < KCl < CaCl2. Hydrochars prepared with KHCO3 and w/o are interconnected similarly to those in figure 3 and 5. In contrast to that hydrochar prepared with CaCl2, KCl and FePO4 consist of monodispersed particles with a spherical shape. The sample after FePO4 addition shows crystals beside the spheres, which is a result of the low solubility of FePO4 in water. However, the additives, and especially CaCl2 and KCl influence the size of the particles stronger than reaction time or fructose concentration.


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Figure 6 SEM pictures ((A), (B), (E) SE pictures, (C), (D) BSE pictures) of Hydrothermal carbonization with Fructose and different additives. CFructose=0.5 M; Temperature=200°C; time=20min; CAdditives=0.025 M except for KCl with 0.075 M, which corresponds to the same ionic strength than CaCl2. (A) without additive, magnification = 3000x; (B) KHCO3, magnification = 2000x; (C) CaCl2, magnification = 3000x; (D) FePO4, magnification = 3000x; (E) KCl, magnification =3000x.

Figure 7 Hydrothermal carbonization with Fructose and different additives. CFructose=0.5 M; Temperature=200°C; time=20min; CAdditives=0.025 M except for KCl with 0.075 M, which corresponds to the same ionic strength than CaCl2 A Mean particle size. Error bars represent one standard deviation. Number of measured spheres nCaCl2=76; nKCl=86; nFePO4=98; nKHCO3=189; nw/o=116. B Yield [w-%] of hydrochar. Error bars represent one standard deviation (n=2). Discussion Kinetic model


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The proposed kinetic model in our work assumes a carbon content of 66.7 % of the hydrochar, with a theoretical molecular weight of 108.1 g mol-1; this is justified by experimental results and the stoichiometric calculation of a 4-fold dehydration of a hexose. No further reactions, such as decarboxylation, have been included, as the kinetic model has only been tested for the samples produced at 200°C, which do not show a strong trend towards decarboxylation (Figure 10A,B). The usage of the elemental composition to support stoichiometric calculations allows the application of the kinetic model in molar concentrations. A similar approach has been successfully used for spent brewer grains56. It is, therefore, a useful approach to build a bridge between the homogeneously dissolved HMF and the solid hydrochar. Within optimization 1, the first order model fits very accurately to the data, whereas the second order model has a clear deviation from the measured values. This situation is unsatisfying because the reaction order is obviously higher than 1 in view of hydrochar yield (Figure 4). This has also been observed for other feedstocks as well: cellulose34, glucose13,57,58, macroalgae35, distillers grain36, and organic wastes37. Optimization 2 shows that a second order approach stabilizes (in terms of a lower relative deviation) the reaction rate constant, obtained by concentration variations, compared to the first order approach. However, we note that the validity of optimization 2 is not given straightforward. Comparing optimization 1 and 2 in the first order approach with 0.5 M initial concentration shows that the reaction rate constant is very similar - pointing to a good accordance - the same comparison for the second order in return is not at all satisfying. However, the results indicate that the reaction order of hydrochar formation is an issue, which has been poorly discussed by the scientific community so far. The poor state of the art becomes indicative when Jatzwauk & Schumple talk about “a somewhat higher order” regarding the hydrochar formation. They calculated a reaction order of 1.53 based on the TOC of


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the process water

54

. Knežević et al. used a reaction order of 2 to model the hydrothermal

carbonization of glucose58. Although this is a certain progress, it should be noted that an appropriate calculation of the reaction order should be based on experimental data obtained from experiments with different initial concentrations. Also, the calculated value of 4.29 by Chuntanapum & Matsumura at 350°C does not seem to be applicable to the reaction conditions in this work59. Models that assume first-order approaches will always generate different rate constants for varied initial concentrations. This aspect becomes significant in the following example. The sum of the parameters k2 and k4 (0.0387 min-1) represents the HMF conversion and can be compared, due to the same reaction conditions, to the value reported by Jing & Lü of 0.0039 min-1, which is almost 10 times lower60. The difference in conversion rate can be attributed to the different initial concentration, which was 0.5 M fructose in our work and 0.06 M glucose in the reference. This result outlines the need for an adjustment of the reaction order within the rate equations. The kinetic model in this work is, therefore, a good basis for further work.

Hydrochar properties As mentioned in the section before, the elemental composition of the hydrochar was used to make stoichiometric calculations. The Van Krevelen diagram illustrates that the hydrochar samples produced between 190 and 220°C are located close to a hypothetical sample, resulting from the one-fold dehydration of HMF, which is called hydrochar 1. The reaction mechanism to form so-called humins is very likely an aldol-condensation of HMF molecules or its derivatives. Patil et Lund postulate that the formation mechanism is a reaction of five HMF molecules and DHH by releasing four water molecules15. This oligomer has a carbon content of 61.5 % (own


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calculation), which is in agreement with the humins produced at 130°C12. However, the reaction between HMF and DHH is a hypothesis, as DHH has not been detected so far. Figure 8 shows in a simplified way the formation of a dimer by aldol-condensation. In this type of reaction, a carbon-carbon double bond is formed between a carbon, which was connected to a carbonylgroup and a carbon atom with an “acidic” hydrogen atom. This can be seen in the tautomeric form of HMF in figure 8. After further reaction of the “electron-rich” carbon with the “electrondeficit” carbon, the polymer in Figure 9 is formed. For a large polymer, in average every HMF molecule released one water molecule. In other words: A condensation reaction of two HMF molecules results in a dimer with a carbon content of 61.5 %. Further HMF addition to form a trimer increases carbon to 63.2 %, another one to 64 % etc. Such a reaction chain theoretically converges to a carbon content of 66.7 % (hydrochar 1). This is the upper maximum of the carbon-content which can be achieved through polycondensation of HMF. A graphical representation of the increase in carbon content by a proceeding polymerization is displayed in the supplementary material. We highlight this fact, because a majority of samples, especially those at a lower temperature (190 – 220°C) have a carbon content close to this value (Table 1 and 3). Data in the Van Krevelen plot indicate that there is no further dehydration from hydrochar 1. For longer reaction times and higher temperature, the carbon content increases to approximately 70-73%. This rise can be achieved by the decarboxylation of 1/3 CO2 per hexose (Hydrochar 2). Poerschmann observed a raise from 66 to 72 % carbon at 220°C in a time range of 16 hours29. At 200°C a glucose-based hydrochar was observed to change its carbon content from 65.8 to 68.5 in a time range of 36 hours61. In our experiments we did


20

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OH

O OH

O

O O

O O

-

OH

OH

O

+

- H2O

+

O

CH

OH

O O O

-

OH

+

C

OH O +

H O O

-

OH

O O

O

Figure 8 Tautomeric forms of HMF and the reaction to a dimer via aldol condensation. O

O

O

OH

O O

O n

Figure 9 Polymer of HMF, a very much simplified structure of hydrochar.

A

B

C

Figure 10 A+B Van Krevelen diagram of data from this work. C Van Krevelen diagram for HTC data taken from literature. Feedstock: Glucose (red)34, Glucose (T=200°C) (blue)60, Glucose (yellow) and Starch13, Cellulose and Xylose61.


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not observe any remarkable change in the carbon content with increasing reaction time, however, the increase in temperature from 190-250°C shows that the carbon content is increasing by a few percents. The Van Krevelen diagram indicates that this further raise is related to decarboxylation reactions, which significantly takes place by raising the temperature above 220°C (Figure 10C: Glucose(red)31, Cellulose and Xylose62. Figure 10B: 250°C) or by prolonging the reaction time29,63 (Figure 10C: Glucose (blue)61). An increase in temperature in the range of 170-220°C is not remarkably changing the elemental composition along the decarboxylation line (Figure 10B: 190°C, 200°C, 220°C. Figure 10C: glucose (yellow) and starch13). The experimental observation of dehydration is straightforward due to the accurate measurement of HMF decrease, however, the occurrence of decarboxylation, which can be observed by CO2 in the gas phase64, is not that obvious. However, some hints exist: The reprocessing of process water and hydrochar, each for themselves, resulted both in CO2 emissions58. In the same article experimental evidence is found that the gas yield is not related to the initial concentration of glucose. The same authors showed that the gas yield increases strongly with increasing temperature64. In summary, it can be assumed that the acids providing the carboxyl-groups are distributed in the hydrochar and the liquid phase, and react to CO2 with a similar, rather slow reaction rate. The presence of levulinic acid, as one possible source for CO2, in the hydrochar is likely, according to Baccile et al.24. In summary, it can be concluded that hydrochar firstly emerges through a polycondensation reaction of HMF forming hydrochar 1 (Figure 11). This 4-fold dehydration chain is rather fast and can be completed within 1-2 hours. These reactions also represent the major gain in carbon concentration, as the carbon content raises from 40% in the fructose to 66.7% in hydrochar 1.


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Any further rise in carbon content can only be achieved through slow decarboxylation. In other words: Dehydration has a much lower activation energy than the decarboxylation. This in accordance to the results obtained from spent brewer grains56. However, it should be noted that the release of 1/3 CO2 is chosen arbitrarily to match the experimental results and does not reflect any defined reaction. Likely the CO2 arises from acids, which are incorporated in the hydrochar. A decarboxylation of the polycondensed hydrochar 1, which is based on HMF, to hydrochar 2 is probably a reaction which does not take place like as described above. It is difficult to imagine a loss of one CO2 per fructose/HMF unit in the hydrochar by decarboxylation of incorporated acids (fig.11). We highlight therefore the need for a deeper understanding of the role of the acids during the formation of hydrochar. In addition, possible structure changes, which may lead to CO2 formation should be investigated, and not only postulated53. Table 4 displays the carbon content and the corresponding maximum gravimetrical yield to hydrochar 1 and 2 and their precursor. Table 4 Theoretical carbon content and maximum yield of hexoses after dehydration and decarboxylation. Hydrochar 1 is calculated by the one-fold dehydration of HMF. Hydrochar 2 is calculated by a 1/3 decarboxylation from hydrochar 1. Substance

Molecular formula

Carbon content [wt.%]

Maximum yield [wt.%]

Hexose

40

HMF

57.1

70

Hydrochar 1

66.7

60

Hydrochar 2

.! .

72.9

55

Falco et al. assigned a maximum gravimetrical yield of 60% according to a 4-fold dehydration10, which is not a complete representation of HTC. By subtracting 1/3 CO2 molecule


23


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from the hypothetical hydrochar 1 a carbon content of 72.9% is achieved, which is related to a maximum yield of 55%. Also, Poerschmann et al. mention such a value based on the 5-fold dehydration of fructose, which is unlikely as this would result in a hydrochar with 80% carbon and a H/C value of 0.3332. These are both values that have never been measured under these operating conditions. In a view of designing kinetic models for HTC, a linkage of the elemental composition to the dehydration and decarboxylation reactions can be used. Hydrochar 1 and 2, therefore, serve as hypothetical concepts to illustrate the course of the reaction.

Fast dehydration, 1-2h

Slow decarboxylation, 1-40h

Figure 11 Schematic presentation of the hydrochar formation based on dehydration and decarboxylation reactions. The number of water and CO2 molecules is given relative to one fructose unit converted. The release of 1/3 CO2 per original fructose unit is chosen arbitrarily to match the experimental results.

Formation mechanism of spherical particles In figure 2A it is demonstrated that the formation of hydrochar is related to the decrease of HMF, this formation mechanism has been assumed in many articles before16,20, but purely quantitatively demonstrated. The data proves that most of the hydrochar is formed in absence of fructose, which is in contrast to those concepts which assume that hydrochar formation is a product of sugars alone or with HMF

14,25,60,65–68

. The carbon balance data also shows that yield

increase flattens as soon as the HMF concentration is low, although approx. 40% of the carbon is


24

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in the liquid phase; pointing to the assumption that other organic substances found in the process water apart from HMF only play a minor role in hydrochar formation. This is in contrast to articles that assume such a mechanism54,59. However, the presence of acids in the hydrochar is likely, according to some authors24. The evolution of the sphere diameter shows that the major fraction of HMF, which forms the hydrochar, little affect the growth of the particles (Fig. 2). This is in contrast to concepts assuming a LaMer-like nucleation13,18,19 followed by diffusionlimited growth by monomeric substances4,42. The results are in good accordance with the alternative model, assuming a formation mechanism through hydrophobic ripening20. According to the SEM pictures, the growth of the particles can be described better by primary spherical particles at early stages of the reaction (Figure 2), followed by growth through coalescence. The data show that particle growth takes place after 60 min, although yield formation is very low (Fig. 2A,B) indicating that coalescence is the main force for growth. However, different observations exist where yield increase proceeds along with particle growth in a sucrose and polyacrylic acid sodium system69. Similar to our work, stable particle size together with yield increase has already been observed45. However, the speed of coalescence is very low, leading to a network of interconnected particles, which can be described in terms of a partial coalescence. The reason for partial coalescence can be either spots of crystallinity within the particles or surfactants that inhibit the coalescence70. Additives play a major role in the size of the resulting particles. The literature covers many articles that produced carbon spheres using additives, mostly electrolytic substances. They all have in common that carbon materials consist of monodispersed spheres10,18,19,39,42,44–48 whereas materials based on pure sugar solutions, mostly show partial coalescence2,14,43,44,18,20,31,38–42. By means of the DVLO (Derjaguin, Landau, Verwey, Overbeek) theory, one can argue that this is a


25


Page 26 of 36

result of the lower Debye-length, which is a function of the permittivity, temperature, and ionic strength71. In consequence, repulsion of the particles is reduced and coalescence proceeds faster. For KHCO3 and CaCl2 the valence of the electrolytes 1 and 2 respectively satisfy the prediction of the DLVO theory. This effect is probably disturbed by the reduced solubility of FePO4, where the size of the particles is lower as with CaCl2. In order to focus on the valency an additional experiment with a KCl solution adjusted to the same ionic strength as the CaCl2 solution was performed. In the both experiments, the hydrochar yield and the particle diameter was rather similar. This result is a valuable hint that the ionic strength of an electrolytic solution is a key determiner of the particle size. Electrolytes are more likely to produce monodispersed particles, which are larger in size; this is obviously the case for CaCl2, KCl, and FePO4 in our work. This has been observed in many other cases such as for NH4+

45

, NaCl39, sodium polyacrylate42,69, aluminum chloride19, sodium

sulfate43, cysteine18, NH4Cl44, and levulinic acid46. Those articles reporting increased particle size and discrete monodispersed particles mostly report individual explanations for their finding. Wohlgemuth et al. explained that a variety of Maillard products from cysteine and glucose form seeds at different time point, which further grow according to the LaMer model18. Gong et al. argue that sodium polyacrylate suppresses the cross-linking between the carbonaceous nucleus, leading to more nuclei growth at the same time42. Zhao et al. measured the zeta potential of sucrose-based carbon spheres together with sodium polyacrylate and conclude that monodispersity is a result of the repulsion, which is caused by the negative surface charge69. Zhang et al. mention that an electrolyte can induce coagulation leading to larger particle size44. It has also been proposed that NaCl39 or NH4+ 45 catalyzes polymerization reactions, therefore, promoting the growth of the particles. However,


26

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these concepts assume a reaction-controlled growth, which is not in accordance to the relationship of HMF and the particle size we observed here (Figure 2A and 2B).

Conclusions This work provides new information on the formation mechanism of hydrochar, its structure and the growth of the hydrochar particles. The conclusion of this work can be expressed by the following points: 1. The comparison of time-depending intermediate concentration with the geometrical properties of the particles leads to new insights. The particles grow significantly after the monomer HMF is completely consumed, which points to a growth mechanism through coalescence. 2. We demonstrate that hydrochar formation can be modeled by ordinary differential equations with molar quantities. The model fits adequately with the experimental data and provides the time range (at T=200°C) where the formation of hydrochar from HMF takes place. The data also show that most of the hydrochar forms in the absence of fructose. 3. We link the elemental composition of the hydrochar to the polycondensation of HMF. This reaction is fast compared to the rather slow decarboxylation, leading to an additional increase of the carbon content. 4. Our experiments support the idea of a growth mechanism of the carbon spheres on the basis of the DVLO-theory. According to that electrolytic substances reduce the Debye-length of the particles. The reduced repulsive forces then boost coalescence. We assume that the ionic strength is a determiner in this process.

Outlook


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The evaluation of the kinetic model outlines that the reaction order is a key issue that needs to be addressed in future works. In addition, it remains difficult to explain why additives result in considerable larger particles, which are monodispersed in the same time. One can argue that monodispersity is a result of repulsive forces between the particles, but this is in conflict with the big particles observed. On the order hand, low repulsive forces should trigger coalescence and therefore increase the size, but particles growing together are scarcely observed on the SEM pictures. It can be speculated that the large particle size is a result of a period of fast coalescence, which is terminated by some sort of stabilization mechanism. This has to be subject of further research. Funding Sources Special thanks are given to the Paul and Yvonne Gillet Foundation, who supported Dennis Jung with a generous grant. Supporting Information Table S1: Fitted values for k1, k3 and k4: Figure S1: Plot Number of HMF molecules vs. carbon content References (1)

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Synopsis: The formation of carbon spheres/ hydrochar from fructose is followed by a growth mechanism through coalescence.

Formation

Growth


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Hydrothermal Carbonization of Fructose: Growth Mechanism and

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