Influence of reaction parameters on the depolymerization of H2SO4-impregnated cellulose in planetary ball mills

Powder Technology 288 (2016) 123–131

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Influence of reaction parameters on the depolymerization of H2SO4-impregnated cellulose in planetary ball mills Robert Schmidt a, Sindy Fuhrmann b, Lothar Wondraczek b, Achim Stolle a,⁎ a b

Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Lessingstr. 12, D-07743 Jena, Germany Otto-Schott-Institute of Materials Research, Friedrich-Schiller University Jena, Fraunhoferstr. 6, D-07743 Jena, Germany

a r t i c l e

i n f o

Article history: Received 23 February 2015 Received in revised form 30 October 2015 Accepted 1 November 2015 Available online 2 November 2015 Keywords: Planetary ball mill Cellulose depolymerization Process parameters Technological parameters

a b s t r a c t The depolymerization of acid-impregnated cellulose in planetary ball mills was investigated under the perspective of the influence of reaction parameters. Several process, technological and chemical parameters were examined. It was found that with a higher rotation frequency νrot, smaller milling balls and a milling ball filling degree ΦMB of approximately 0.3, the highest solubility could be reached and the milling time could be reduced. The use of milling vessels with larger diameter was beneficial. Variation of the milling ball material showed huge influence and a linear correlation between solubility and density of the milling ball material was observed. Kinetic investigations indicate that the degradation of the impregnated cellulose follows a first order model. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The interest in the application of renewable feedstocks is steadily increasing and a particular focus was set on the usage of cellulose. This renewable feedstock offers many benefits, as it represents a non-food, abundant and ubiquitously available material. Cellulose is therefore a material of great interest also with regards to synthesis. Here it can serve as starting material including the synthesis of bioethanol or platform chemicals like furfural and others [1]. Unfortunately, the reactivity of cellulose is low and solubilization is challenging due to its crystalline structure [2]. To overcome these drawbacks, various pretreatment methods can be applied. One of these are chemical methods, among which the treatment with sulfuric acid is common [3,4]. Another approach are physical pretreatments which aim to increase the reactivity by reducing the particle size and crystallinity of cellulose. This can be attained by milling, which is often the first step in cellulose pretreatment or for example by exposing the material to mechanical stress by shear deformation under high pressure [1,3,5,6]. Due to the mechanical energy input, the crystalline content of the cellulose becomes amorphous, the particle size is reduced and as a result, the reactivity and accessibility for reagents is increased [7–9]. Furthermore, ball milling reduces the degree of polymerization (DP) to a certain extent [10–12]. However, a lower DP after milling could not be observed in every case [13]. Although milling led to an increased reactivity of cellulose, the solubility in water is only slightly enhanced, even after milling times of

⁎ Corresponding author. E-mail address: [email protected] (A. Stolle).

http://dx.doi.org/10.1016/j.powtec.2015.11.002 0032-5910/© 2015 Elsevier B.V. All rights reserved.

several hundreds of hours. For instance, Grohn observed that after 300 h milling in a mixer ball mill a solubility of approximately 30% could be achieved [14]. Such long milling time is indeed unfavorable for industrial application. Higher solubility of 44% was reached if mechanical stress was introduced by shear deformation under high pressure (6 GPa) [6]. Beside chemical and physical pretreatment methods, combination techniques like steam-explosion or liquid hot water can be used [3]. Recently, the combination of acidic treatment and ball milling was proven to be an effective way to convert water insoluble cellulose into water-soluble oligomers with up to 100% total conversion [15–19]. Thereby, the application of solid acids as well as H2SO4 or HCl impregnated on cellulose, is possible. The obtained water soluble oligomers have shown to be valuable products that can be further applied for e.g. hydrolysis to sugars, hydrogenation to sugar alcohols or synthesis of platform chemicals, as mentioned above [16–18]. Furthermore, this method is not limited to cellulose, as lignocellulosic substrates like beechwood could be completely solubilized [18]. Milling times of approximately 2 h were necessary for the complete conversion of impregnated cellulose to water soluble oligomers [16]. In the presence of solid acids, the milling time is considerably longer. Especially the chemical aspects of this method of depolymerization, like the type and amount of acid, have been investigated. The influence of milling parameters on the reaction, except for the milling time, has not been reported yet [15,16,19]. However, for an efficient depolymerization of cellulose in planetary ball mills (PBMs) and with regard to a further scale-up, optimization of milling parameters is of major relevance. Our work is focused on the question how reaction parameters influence the cellulose depolymerization, particularly the rotation frequency

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νrot, the milling ball diameter dMB, the material of the milling balls, the size of the milling vessel, the milling ball filling degree ΦMB and the cellulose filling degree ΦCellulose. These parameters appeared to be considerable factors for organic synthesis in ball mills and can significantly affect the outcome of a reaction [20–24]. 2. Materials and methods All chemicals were purchased from Sigma Aldrich or Alfa Aesar and were used without further purification. The reactions were accomplished in a Fritsch Pulverisette P6 classic line (PBM P6) and a Fritsch Pulverisette P7 premium line (PBM P7, Fritsch GmbH, Idar-Oberstein, Germany). As not stated otherwise, milling vessels made from stainless or tempered steel were used with a volume of 250 ml for reactions in PBM P6 and with 45 ml for PBM P7. The milling balls applied were made from magnesia-stabilized zirconia. Particle size distributions were determined by static light scattering using an Analysette 22 MicroTec plus (Fritsch GmbH, Idar-Oberstein, Germany) with wet dispersion unit and water as suspending medium. In order to identify optimal parameter settings and to predict the cellulose solubility for different parameter combinations, a response surface design, based on a quadratic model, was carried out with Design Expert 9.0.3. General procedure for the impregnation of α-cellulose with H2SO4 [16]: 10 g α-cellulose (powder) were added to a solution of H2SO4 (5 mmol, 0.49 g) and 150 ml methyl tert-butyl ether (MTBE). After stirring for 30 min, the solvent was removed under reduced pressure at 40 °C. Experimental procedure for reactions in PBMs: The milling vessels were equipped with the milling balls. Afterwards, the impregnated cellulose was added. Milling was accomplished at the respective frequency νrot and milling time t. In order to reduce the thermal stress of the sample, milling was intermitted by milling pauses. Thereby, pauses of 10 min after t = 20 min in PBM P7 and of 7 min after t = 3 min in PBM P6 were chosen. The stated milling times refer solely to the milling time, without pauses. The overall reaction times tReaction can be calculated with Eq. (1) (for PBM P 7) and Eq. (2) (for PBM P6). 1 t Reaction ¼ t þ t−10 min 2

ð1Þ

7 t Reaction ¼ t þ t−7 min: 3

ð2Þ

Determination of solubility [16,19]: A sample of 1 g was stirred for 5 min in 35 ml water. The solid residue was separated after centrifugation (20 min, 4000 min−1). Afterwards the residue was washed two times with water (35 ml), centrifuged, dried (90 °C, 12 h) and weighed. The solubility was calculated as the difference of the amount of added cellulose and of the solid residue and listed in percentage. Determination of intrinsic viscosity of the solid residue: The solid residue was treated according to DIN54270 with copper ethylenediamine (CUEN). The viscosity η was determined with an Ubbelohde viscometer. The degree of polymerization DP was calculated based on the following equation (Eq. (3)) [25]: η ¼ 2:45  DP 0:7 :

3.1. Influence of process parameters Process parameters influence the energy input directly and can be changed or controlled during ball milling. To such significant parameters can be counted variables like the rotation frequency νrot, milling time t and the temperature T. It could be proven that these three factors are the main ones impacting organic reactions in ball mills [22,28]. 3.1.1. Rotation frequency νrot and milling time t The effect of the rotation frequency was investigated in a range from 450–650 min−1. The results are presented in Fig. 1 and confirm that a higher frequency leads to increased solubility of the cellulose. For instance, low solubility of approximately 30% was observed at 450 min−1 after 20 min milling. An increase to 650 min−1 led to considerably higher yields of 80% soluble material. Regarding t, Fig. 1 illustrates that at constant νrot the solubility increases with longer milling time t. Thus, the solubility increases at 600 min−1 from 20% after 10 min to almost quantitative solubility after 50 min. Conversely, the milling time to achieve high solubility could be reduced when νrot was enhanced. The results can be explained by the influence of the supplied energy. The energy that is provided to the milled substrate depends on the kinetic energy of the milling balls. A change of the rotation frequency νrot directly influences the speed of the milling balls and thus their kinetic energy. Furthermore, the number of stress events increases with νrot and milling time t and in sum more energy is delivered [21, 29]. This higher energy input can lead to a higher solubility. The effect of νrot and t was shown for several examples, covering a wide field of organic chemistry. For most of the reactions, conversion or yield is increased by increasing the two quantities νrot and t [23,24, 28,30,31]. Only in some cases a plateau like state was observed, indicating that an increase of ν does not lead to an enhancement of conversion [32,33]. 3.2. Influence of technological parameters Technological parameters describe the engineering part of variables that can influence the outcome of a reaction. These include the type of ball mill, the milling vessel size, the filling degree of the milling balls ΦMB and all variables connected with the milling balls e.g. material or diameter dMB. These parameters can be significant, as for example by variation of the ball diameter or material, the kinetic energy of the single balls is changed and thus the energy entry and the outcome of a reaction [20,22]. 3.2.1. Vessel size A parameter that gained less attention when regarding the influence of reaction parameters is the size of the milling vessels [20,34]. Thus, we considered it as worthwhile to investigate the depolymerization of cellulose in several milling vessels that differ in terms of volume and geometry (Table 1). The reactions were performed at constant ΦMB

ð3Þ

3. Results and discussion The outcome of a chemical reaction in a ball mill mainly depends on the amount of energy that is supplied. Several reaction parameters directly influence this energy input. The parameters can be arranged in three categories: process, technological and chemical parameters [26,27].

Fig. 1. Influence of the rotation frequency νrot and milling time t on the solubilization of acid-impregnated cellulose. Conditions: PBM P6, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5.

R. Schmidt et al. / Powder Technology 288 (2016) 123–131 Table 1 Size and dimensions of the applied milling vessels. Milling vessel volume VMV [ml]

Inner milling vessel diameter dMV [mm]

Inner milling vessel height hMV [mm]

Solubility at t = 20 min [%]

Solubility at t = 40 min [%]

80 250 330 330 500

65 75 75 100 100

24 69 80 45 69

14 41 39 39 61

18 76 70 80 93

Conditions: PBM P6, steel vessel, νrot = 550 min−1, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5.

and ΦCellulose. The results (Fig. 2, Table 1) show that the solubility of the cellulose was higher if larger milling vessels were used. In the smallest milling vessel with a volume VMV of 80 ml, even after 2 h milling only 50% of the cellulose was transformed into soluble material. The difference in the solubility was minor for vessel volumes of 250 and 330 ml. The highest solubility was observed when using a 500 ml vessel. The volume of the milling vessels can be changed either by increasing the inner vessel diameter dMV or inner vessel height hMV. A closer look on the influence of these variables reveals that dMV strongly affects the reaction. For example, the solubility in the 250 ml vessels achieved 60% after 30 min whereas in the 500 ml vessel a solubility of 80% was reached. The influence of hMV is indifferent. Whereas for 250 and 330 ml vessels (dMV = 75 mm) roughly consistent results were obtained, the solubility for 330 and 500 ml vessels (dMV = 100 mm) differs, with higher solubility in the higher vessel. The results can be assigned to three reasons. (I) The energy input is affected if the size of the milling vessel is changed. The kinetic energy is increased for higher diameters as the distance to the center of the main disc is enlarged and thus higher centrifugal forces occur [20,34]. Kakuk et al. reported a 3-times higher impact energy and a 1.3-times higher power if dMB was doubled [35]. Thus, with a higher energy input a higher solubility was achieved. (II) At constant ΦMB, the number of milling balls is higher in larger milling vessels and thus the number of stress events per second, the stress frequency, is increased. A higher conversion to water soluble oligomers is observed as a result [35]. (III) The curvature at the bottom of the vessel is different for the applied vessels. This might play a role, as ball trajectories and energy input can be influenced by the inner vessel form as demonstrated by Takacs and McHenry in case of a mixer ball mill [36].

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are tungsten carbide, steel, stabilized zirconium oxide, sintered corundum, silicon nitride and agate, differing in the mentioned characteristics [37]. High chemical resistance and low wear are preferable to avoid contamination of the substrate. However, wear can be advantageous for particular reactions e.g. when abrasion of copper milling balls replace the catalyst in a reaction, in addition to mixing and energy entry by the balls [38]. The results presented in Fig. 3a show that the solubility of the acidimpregnated cellulose strongly depends on the milling ball material. Highest solubility was achieved with steel balls and balls made of zirconium oxide. Balls made of agate caused only low solubility. In Fig. 3b it is obvious that the solubility linearly depends on the density of the milling balls. The kinetic energy of the milling balls is determined by the mass and velocity of the milling balls and as the mass is proportional to ρ, the kinetic energy is also proportional to ρ. Thus, as lower the energy input is, as lower is the solubility after a certain milling time. The data of the linear regression are displayed in Table 2. Similar results were found for several organic reactions in ball mills [23,24,39,40]. From these, the general conclusion can be drawn that higher yields can be achieved if milling balls with higher density ρ (e.g. steel, zirconium oxide) were used instead of lighter milling balls made of agate. By the use of materials with higher ρ, more energy is provided, leading to increased conversions. Some authors have however reported that there could not be observed any effect on the yield if the milling ball material is changed [23]. The energy consumption is probably limited to a maximum value that is already reached with lighter milling balls in these cases [21]. Therefore, a further energy increase does not necessarily lead to an enhancement of the reaction.

3.2.2. Milling ball material Chemical resistance, abrasion behavior and the energy entry are important aspects that make the choice of the milling ball material an essential point. Typical materials for milling balls (and milling vessels)

Fig. 2. Influence of milling vessel size and geometry on the solubilization of acidimpregnated cellulose. Conditions: PBM P6, steel vessel, νrot = 550 min−1, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5.

Fig. 3. a–b. Influence of the milling ball material on the solubilization of acid-impregnated cellulose. Conditions: PBM P7, 45 ml steel vessels, νrot = 800 min−1, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.3.

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The overall energy input depends, among others, on the number of milling balls and in several reactions an increase of the yield with an increasing number of milling balls was observed [24,28,30,31,40]. For wet grinding in stirred media mills, ΦMB turned out to be of great influence and it was assumed that an optimum filling degree was reached [41]. Many studies were dedicated to the investigation of the influence of the filling degree, respectively nMB, in PBMs at low values for ΦMB, which did not reveal any optimum [28,40]. The milling ball filling degree ΦMB strongly affects the outcome of the depolymerization of cellulose (Fig. 4). With low ΦMB (low number

of milling balls) the energy entry is reduced and a lower solubility was observed. An increase of ΦMB was accompanied by a higher solubility. In PBM P6, solubility passes an optimum at 0.3 that was also found to be the optimal value in particle refinement and in the condensation reaction of vanillin and barbituric acid [20,42,43]. The solubility decreased for ΦMB N 0.3 due to changed ball trajectories and an interrupted and cumbered ball motion [41,44]. Results for the reaction in PBM P7 showed a similar behavior but the optimal ΦMB was shifted to smaller values. The results are in well accordance with investigations covering broader areas of ΦMB at which an optimal value for ΦMB was found, with regard to yield or heat dissipation [20,24,45]. Beside changes of ΦMB, the number of milling balls can be increased by use of smaller milling balls at a constant ΦMB. The milling ball diameter dMB has strong influence on the energy input. The mass of the milling balls is proportional to the cubic dMB and thus with higher mass the kinetic energy of larger milling balls is higher, which can lead to increased yields [24,28,46]. This is valid for a constant number of milling balls nMB. Different observations can be made if nMB is not constant but ΦMB is. Fig. 5 visualizes the influence of dMB at constant ΦMB. The solubility increases if dMB is reduced. This is in contrast to results of Szuppa et al. who observed no difference in the yield for balls of 5, 10 or 15 mm, with identical overall mass. [33] However, it is known from particle refinement investigations that smaller milling balls lead to smaller particle sizes. Stolle et al. observed a higher yield with smaller milling balls in the condensation reaction of vanillin and barbituric acid [20,29,47,48]. As ΦMB is constant, nMB is higher for smaller milling balls as for larger ones. This leads to two contrary effects. On the one hand, the kinetic energy of the single balls is lower for smaller milling balls [20,21]. This causes a lower stress energy of the single

Fig. 4. a–b. Influence of the milling ball filling degree ΦMB on solubilization of acidimpregnated cellulose. Conditions: (a): PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦCellulose = 0.5. (b): PBM P7, νrot = 800 min−1, 45 ml steel vessels, ZrO2-balls, dMB = 10 mm, ΦCellulose = 0.3.

Fig. 5. a-b. Influence of milling ball diameter dMB on the solubilization of acid-impregnated cellulose. Conditions: (a): PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5. (b): PBM P7, νrot = 800 min−1, 45 ml steel vessels, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.3.

Table 2 Regression function of linear regression y=a+bx of solubility and density of the milling balls. Milling time t [min]

Slope b

Intercept a

20 40 60

16.32164 20.07506 20.01668

−28.38529 −23.79274 −17.56505

3.2.3. Milling ball diameter dMB and milling ball filling degree ΦMB The diameter dMB, the filling degree ΦMB and the number of milling balls nMB are variables that are strongly linked with each other and take strong influence on the outcome of a reaction in a ball mill. The milling ball filling degree ΦMB was calculated as ratio of the milling ball to vessel volume (Eq. (4)) [20]. X ΦΜΒ ¼

V MB

V MV

1

¼



3

6

πdMB  nMB V MV

ð4Þ

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stress events. On the other hand, as nMB is inversely proportional to cubic dMB the number of smaller milling balls is considerably higher and thus the stress frequency is increased. In sum, the supplied power is higher for smaller milling balls [20,21]. In PBM P7, the use of 5 mm balls showed to be preferable compared to the 10 mm balls. However, an application of 2 mm balls does not lead to a further reaction improvement. A lower solubility was observed in this case. The provided stress energy, either for breaking of the bonds or for the particle refinement, was probably too low [47]. This leads to the conclusion that a particular minimum amount of stress energy has to be provided in order to accomplish the reaction successfully. If the stress energy is high enough, an increase of the stress frequency is more beneficial than an increase in the stress energy [21]. To identify the optimal combinations of the factors ΦMB (factor A), dMB (factor B) and t (factor C) a response surface design with quadratic model was applied, which allows to predict the cellulose solubility (nMB can be determined with ΦMB and dMB, thus t was chosen as factor C). The model graphs (Fig. 6) show the influence of ΦMB and dMB at different milling times. Especially for short milling times the choice of ΦMB and dMB shows to be of large influence. At optimized conditions, high yields can be achieved whereas distinctly lower yields were observed outside of the optimal area. The longer the milling time is, as larger is the area in which a variation of ΦMB and dMB shows only slight influence on the outcome of the reaction. Thus, for time efficient milling the selection of ΦMB and dMB is important. The main effects A, B and C as well as the interaction effects AB, A2 and B2 have been determined to be significant model terms. To identify the relative impact of the factors the coded factor coefficients can be compared (Table 3). Regarding the main factors, the influence of ΦMB (factor A) and dMB (factor B) is similar and 1.7-times lower as t (factor C). Interaction factors like AB are as high as main factors and are therefore not negligible. For example, the combination of high ΦMB and dMB is clearly disadvantaged compared with the combination of high ΦMB and small milling balls. Predictions about the response for a given factor level can be done with the final equation in terms of actual factors. Solubility ¼ −52:48 þ 556:44  ΦMB þ 15:20  dMB þ 1:49  t−24:86 2  ΦMB  dMB −887:63  Φ2MB −0:84  dMB :

3.2.4. Cellulose filling degree The influence of the cellulose filling degree ΦCellulose (Eq. (5)) was investigated at constant ΦMB in PBMs P6 and P7. ΦCellulose ¼

V Cellulose V MV

ð5Þ

The results show that the amount of applied cellulose influences the maximum reachable solubility (Fig. 7). A higher solubility was achieved with a low ΦCellulose. This means that, with a higher ΦCellulose, which would be preferable for a scale-up, the milling time t has to be longer. The reason is the decreased stress intensity that is defined as the ratio of stress energy to the mass of substrate that is stressed [21,51]. More substrate is located in the collision zone of the milling balls if ΦCellulose is higher. This leads to higher attenuation of the milling ball movement and the energy is dissipated in a larger amount of substrate. As a result the solubility decreases. When ΦCellulose is changed, the ball-to-powder mass ratio (BtP) is changed, too. This parameter is often used in particle refinement and milling of inorganics and was found to be of strong influence [52]. Usually a BtP between 10:1–20:1 is used, but there are also studies available using a BtP up to 220 [52]. In alloying or particle refinement a high BtP was found to be advantageous, as reaction times can be reduced or a smaller particle size can be reached [52–56]. Unfortunately, only the BtP is stated in many publications but no further information about

Fig. 6. a–c. Response surface model graphs of the solubility as isoline plots for 20 min, 40 min and 60 min. Conditions: PBM P7, νrot = 800 min−1, 45 ml steel vessels, ZrO2balls, ΦCellulose = 0.3. Isolines represent areas of same solubility.

nMB, dMB or the vessel volume are given [52]. Furthermore, the BtP ignores the volume of the milling vessel and is therefore only slightly significant (see section “Vessel size”) [20,34]. Fig. 8 illustrates that the indication of solely the BtP is not sufficient. Three reactions with a BtP

Table 3 Factor coefficients of the significant factors. Factor coefficient

A

B

C

AB

A2

B2

Coded⁎ Actual⁎

8.32 556.44

8.4 15.20

13.98 1.49

13.67 24.86

10.74 887.63

20.93 0.84

⁎ Absolute values.

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R. Schmidt et al. / Powder Technology 288 (2016) 123–131 Table 4 Results of experiments with varied cellulosic substrates and for non-impregnated cellulose. Entry

Substrate

Acid

1

α-Cellulose, powder

H2SO4, impregnated

2

Cellulose microcrystalline, powder

3

Cellulose, fibers, (medium)

4

Cellulose powder from spruce

5a)

α-Cellulose, powder

a)

6 7b) 8b)

p-Toluenesulfonic acid, monohydrate Sodium p-toluenesulfonate Aluminum oxide, acidic Montmorillonite K10

Milling time t [min]

Solubility [%]

20 40 60 20 40 60 20 40 60 20 40 60

41 76 87 52 N99 N99 46 N99 N99 31 73 95

40

23

40 40 40

0 9 13

Conditions: PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5. a) PBM P7, νrot = 800 min−1, 45 ml steel vessel, ZrO2-balls, d MB = 10 mm, Φ MB = 0.3, Φ Cellulose = 0.3, 0.5 mmol/g acid. b) PBM P7, ν rot = 800 min− 1 , 45 ml steel vessel, ZrO 2-balls, dMB = 10 mm, m Cellulose = m Solid mAluminum oxide = 2 g, mMontmorillonite K10 = 1.8 g.

acid .

3.3. Influence of chemical parameters Chemical parameters include all aspects regarding the chemical part of the milling process e.g. type of reaction, substrate amount, use of additional grinding auxiliaries or solvents [49,50]. Fig. 7. a–b. Influence of the cellulose filling degree on the solubilization of acid-impregnated cellulose. Conditions: (a): PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3. (b): PBM P7, νrot = 800 min−1, 45 ml steel vessels, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3.

ratio of 20 were performed in 500 ml vessels but with varied ΦMB and ΦCellulose. Although BtP is identical, the outcome of the reaction is apparently different due to different filling degrees. Thus, the use of the volume based parameters ΦMB and ΦCellulose is preferable over BtP as they are more precise for the description of the reaction conditions.

Fig. 8. Influence of ball-to-powder ratio BtP on the solubilization of acid-impregnated cellulose. Conditions: PBM P6, νrot = 550 min−1, 500 ml steel vessel, ZrO2-balls, dMB = 10 mm. Ball-to-powder mass ratio = 20 (mMB/MCellulose [g/g]). A: ФMB = 0.15, ФCellulose = 0.25, mMB/mCellulose = 430/21.25. B: ΦMB = 0.2, ΦCellulose = 0.33, mMB/mCellulose = 570/28. C: ΦMB = 0.3, ΦCellulose = 0.5, mMB/mCellulose = 860/42.5.

Fig. 9. a–b. PXRD of non-impregnated (a) and impregnated (b) cellulose. Conditions: PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 10 mm, ΦMB = 0.3, ΦCellulose = 0.5.

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3.3.1. Variation of substrate and acid Experiments were performed with different types of celluloses to investigate the influence of the cellulosic starting material (Table 4, entries 1–4). A high solubility was observed for all substrates and a complete conversion to water-soluble products was achieved with microcrystalline cellulose and cellulose fibers (entries 2–3). Entries 5– 8 show the results for the depolymerization under usage of solid acids instead of H2SO4 [15,16]. A lower solubility was achieved in all cases, compared with the results of impregnated cellulose. Thereby p-toluenesulfonic acid monohydrate showed with 23% the best performance. Aluminum oxide (acidic) and Montmorillonite K10 led to poor conversion to water soluble oligomers with 9 and 13%, respectively. 3.4. Mechanism and kinetic investigations The insoluble residue, obtained after removal of the soluble oligomers, was characterized by measuring the particle size distribution, powder x-ray diffraction (PXRD) and determination of the intrinsic viscosity η. The starting material shows the characteristic PXRD signals for cellulose I at 2θ = 14.8, 16.8 and 22.6 (Fig. 9a) [12]. Milling of the nonimpregnated cellulose resulted in degradation of the crystalline part of the cellulose. The crystallinity index (CI) of the non-impregnated cellulose dropped from 66 to 11% during 18 min of milling. CI was calculated according to Eq. (6), with I002 as the intensity of the 002 lattice diffraction at 2θ = 22.6 and Iam as the intensity of the diffraction of the amorphous part at 2θ = 18 [9]. A complete amorphization was

Fig. 10. a–b. Particle size distribution of non-impregnated (a) and impregnated cellulose (b). Conditions: PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 20 mm, ФMB = 0.3, φCellulose = 0.5.

Fig. 11. Change of solubility and intrinsic viscosity as function of the particle size. Conditions: PBM P6, νrot = 550 min− 1, 250 ml steel vessel, ZrO2-balls, Φ MB = 0.3, ΦCellulose = 0.5.

observed after 27 min, indicated by the complete disappearance of the characteristic cellulose I signals [57]. CI ¼

ðI002 −I am Þ  100 I002

ð6Þ

In Fig. 9b the data obtained from the PXRD measurements of the impregnated cellulose is displayed. Even after 60 min milling time, no amorphization of the solid residue was observed. Instead, a new signal at 2θ = 20 occurs after milling for 20 min, which can be assigned to a change of the cellulose structure. The diffractograms suggest the formation of the crystalline cellulose polymorph II [58,59]. It is known from literature that transformation of cellulose I into cellulose II can be accelerated by ball milling if amorphous cellulose is further milled with additional water [12]. Thus, the formation of cellulose II can be explained by a water uptake from air, due to an enhanced hygroscopicity of milled cellulose and of the sulphuric acid [14]. The absence of the broad amorphous signal of milled, impregnated cellulose leads to the conclusion that the amorphous part, which is formed during milling, undergoes the reaction preferentially to the other parts. At least, parts of the milled cellulose underwent transformation to cellulose II before they were converted to water-soluble oligomers. The particle size of the non-impregnated cellulose was slightly decreased during milling (Fig. 10a). Obviously, the grinding limit is already reached [60]. The d50 after 60 min was 66.2 μm. Despite amorphization and particle refinement, the solubility of non-impregnated cellulose was 8% after 6 h of milling in PBM P7, which is in accordance with the results of Meine et al. who observed 10% conversion to water-soluble oligomers after 10 h [16]. Thus, a decrease of CI is not the main reason

Fig. 12. Change of intrinsic viscosity as function of the solubility. Conditions: PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, ΦMB = 0.3, ΦCellulose = 0.5.

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for the increased solubility [19]. In contrast to non-impregnated cellulose, the particle size of impregnated cellulose strongly decreased and a d50 of 4.3 μm was measured after milling for 60 min (Fig. 10b). The d50 of the insoluble residue was 15 times lower as for non-impregnated cellulose, which was milled for the same time range. Furthermore, the intrinsic viscosity of the milled samples was determined. The intrinsic viscosity η is correlated to the degree of polymerization DP. It was found that a small value of viscosity corresponds to a low DP. The calculation of DP from the intrinsic viscosity is possible with empiric formulas [25]. The intrinsic viscosity η of the soluble oligomers was 8.5 ml g−1 which corresponds to a DP of approximately 6, which is in consensus with observations that cellulose becomes soluble with DP b 6 [12,15,19]. The impregnation of cellulose with H2SO4

Table 5 Regression coefficients for linear fittings y=a+bx shown in Fig. 13. Model

Slope b

Intercept a

R2

Zero order First order Second order

−0.01317 −0.03747 0.13181

0.84267 0.14963 −0.73012

0.92681 0.99021 0.94531

decreased the intrinsic viscosity from 563 to 420 ml g−1. Nevertheless, the solubility of impregnated, non-milled cellulose was 8% even after 7 days of impregnation. Thus, the solubility is increased by the acidic impregnation, but milling is necessary to reach high solubility [19]. This indicates the positive effect of ball milling that is based on particle refinement and efficient mixing. In Fig. 11 the decrease of the solubility with the particle size and the simultaneous increase of η are shown. The intrinsic viscosity of the solid residue decreased and the overall solubilization were enhanced during milling. In Fig. 12 the decrease of η as function of the solubility is shown. The almost linear decrease of η reveals an end-cleavage of the cellulose chains and not a random breaking of the bonds. The same was observed by Hick and co-workers [15]. For determination of the reaction rate the mass fraction ω of insoluble cellulose to the initial mass of cellulose was calculated. A zero order model appears to be unlikely as the amount of cellulose influenced the outcome of the reaction (see Fig. 7). A linear correlation was found for the first order model while the linear correlation was lower for the second order model (Fig. 13, Table 5). Thus, a first order reaction can be assumed, which was also found for the amorphization of cellulose [61]. 4. Conclusions The influence of several reaction parameters on the mechanocatalytic depolymerization of cellulose was investigated. The solubility strongly depends on the milling time. For economy of milling time, the variation of the rotation frequency νrot, the milling ball diameter dMB, the milling ball filling degree ΦMB, the milling ball material, the vessel size and the cellulose filling degree ΦCellulose is convenient. With regard to dMB it was found that smaller milling balls are advantageous to a certain limit. ΦMB was found to pass through an optimal value at 0.3, confirming literature results for different kind of reactions. Thus, a ΦMB of 0.3 seems to be generally advantageous. Considering the geometry of the milling vessels, a larger milling vessel diameter was found to be beneficial. The depolymerization could be successfully performed in two types of planetary ball mills and with optimized parameters a scaleup of the factor 18 was achieved regarding the processed amount of cellulose. The kinetic investigations indicate that the depolymerization obeys a first order model. Acknowledgment This study was carried out within the project RESPEKT (Resource efficient chemical synthesis — process development of solvent-free reactions in ball mills) funded by the Deutsche Bundesstiftung Umwelt (DBU; AZ 29622-31). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.11.002. References

Fig. 13. a–c. Kinetic plots for the acid depolymerization of celluloe in PBM P6. Conditions: PBM P6, νrot = 550 min−1, 250 ml steel vessel, ZrO2-balls, dMB = 20 mm, ΦMB = 0.3, ΦCellulose = 0.5.

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