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Cooperative action of heteropolyacids and carbon supported Ru catalysts for the conversion of cellulose
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Cooperative action of heteropolyacids and carbon supported Ru catalysts for the conversion of cellulose María Almohallaa, Inmaculada Rodríguez-Ramosb, Lucília S. Ribeiroc, José J.M. Órfãoc, ⁎ Manuel Fernando R. Pereirac, Antonio Guerrero-Ruiza, a
Dpto. de Química Inorgánica y Técnica, UNED, Madrid, Spain Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain c Laboratory of Separation and Reaction Engineering—Laboratory of Catalysis and Materials (LSRE-LCM), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Portugal b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cellulose Heteropolyacid Alkanediols Ruthenium
Acid hydrolysis and hydrogenation/hydrogenolysis reactions can be combined for catalytic conversion of cellulose into renewable biorefinery feedstocks by using heterogeneous bifunctional catalysts. In the present study a cooperative effect of heteropolyacids (HPA) and Ru nanoparticles supported on two carbon materials is demonstrated. The process can be suitable for the one-pot tandem reaction, yielding the conversion of cellulose into alkanediols (mainly propylene glycol and ethylene glycol). From a mechanistic point of view the differences in the distribution of polyol products, obtained from the cellulose reaction over monometallic Ru catalysts or over bifunctional Ru-HPAs materials, seem to be strongly determined by the competitive reactions of the sucrose (glucose + fructose) intermediate. HPA not only promote, as solid acids, the efficient hydrolysis of cellulose to glucose, but also catalyze the selective cleavage of the CeC bonds in glucose and fructose, leading to the formation of ethylene glycol and propylene glycol. These reactions are in competition with the sugar hydrogenation to the corresponding C6 polyols (e.g. sorbitol), which takes place on the single Ru surface sites. The strong dependence of the product distribution on both catalytic functions is clarified by the kinetic analysis of the three competitive reactions of glucose, including its hydrogenation, isomerization and CeC bond cleavage. Finally, considering the applicability of this reaction, it should be raised that the ball-milling pretreatment of cellulose is compulsory. In fact, during this ball-milling the crystallinity and particle size of cellulose are reduced, which results in a much higher conversion of cellulose. Herein, mixed ball-milling of cellulose and solid catalysts together was presented, which remarkably accelerates the cellulose conversion into valuable products.
1. Introduction The increasing world energy needs, namely depleting fossil resources and growing environmental concerns, have triggered great interest in searching for renewable sources of energy and chemicals. Cellulose, the most abundant source of biomass, consists of carbon, oxygen and hydrogen, and has received considerable attention as a vitally renewable alternative to fossil fuels [1–3]. Cellulose comprises a carbohydrate monomer (glucose) linked by β-1,4-glycosidic bonds. Chemocatalytic cleavage of its CeO and CeC bonds to polyols by hydrolytic hydrogenation or hydrogenolysis in a mild aqueous medium has attracted intensive scrutiny due to the wide variety of possible applications of polyols as value-added chemicals and fuels [4–6]. In particular, sorbitol is regarded as one of the 12 important platform
⁎
compounds in biomass conversion programs [7]. Additionally, 1,2alkanediols such as ethylene glycol (EG) and 1,2-propylene glycol (PG) are used as energy platforms for the production of hydrogen and liquid alkane fuels via the aqueous reforming process or as monomers for polymerization synthesis [1,8,9]. EG is especially consumed in quantities above 20 Mt/yr in the petroleum industry [10]. In view of the importance of EG, its production from sustainable cellulose is critical. The conversion of cellulose to polyols includes two consecutive reaction steps: acidic hydrolysis of cellulose to sugars and the subsequent hydrogenation or hydrogenolysis of the intermediate sugars to polyols over a supported transition metal catalyst under high H2 pressure [11]. However, although some studies have succeeded in converting cellulose into high-valued chemicals, there are still many challenges [12,13], mostly due to the cellulose’s robust crystalline
Corresponding author. E-mail address: [email protected] (A. Guerrero-Ruiz).
http://dx.doi.org/10.1016/j.cattod.2017.05.023 Received 16 November 2016; Received in revised form 17 April 2017; Accepted 5 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Almohalla, M., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.023
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in this type of reaction [28]. Therefore, in this work cellulose and catalyst were also ball-milled together as a pre-treatment to improve the contact between cellulose and the catalyst, as the mixed state can continue in the subsequent hydrolysis reaction thanks to the insoluble properties of the catalyst and the substrate.
structure and insolubility in conventional solvents. The crystal structure and hydrogen bonding in cellulose limit the access to β-1,4-glycosidic bonds by reactants and catalysts [14]. So, in order to facilitate the contact with the catalysts, the crystalline structure of cellulose has to be considered since non-crystalline forms of cellulose will be more reactive. Milling methods such as ball-milling are the typical mechanical techniques for disrupting the crystal structure of cellulose, since cellulose’s hydrogen bonds are cleaved [5]. Acid treatments are the chemical treatments often used for this purpose, but are undesirable due to reactor corrosion and environmental toxicity. Hydrolysis of cellulose catalyzed by sulfuric acid has been implemented on relatively large scales [1]. Although sulfuric acid is an inexpensive and a highly active catalyst for this reaction, it requires an energy-inefficient separation process for catalyst recovery before reuse and, in addition, treatment of the waste sulfuric acid is often not economical. In contrast, ball-milling does not suffer from this problem, making it a great alternative for breaking the robust crystalline structure of cellulose and decreasing its crystallinity in a sustainable way. In a previous report [15], in order to reduce its crystallinity before the reaction and to study its effect on the performance of the process, microcrystalline cellulose was ball-milled in a ceramic pot with two ZrO2 balls (12 mm of diameter) and different cellulose samples were prepared by varying the ball-milling frequencies (from 5 to 20 Hz) for 4 h. It was demonstrated by X-ray diffraction (XRD) the transformation of microcrystalline cellulose into amorphous cellulose, with a decrease of cellulose’s crystallinity from 92 to 23% just by ball-milling at a frequency of 20 Hz for 4 h [15]. From the point of view of green chemistry, successful manipulation of the tandem catalytic process using heterogeneous catalysts would be economical and environmentally viable if the use of chemicals, waste production, catalyst separation and recovery, and processing time are optimized. For example, recent studies showed the one-pot hydrolytic hydrogenation of cellulose to sugar alcohols over bi-functional heterogeneous catalysts which are easily separable from the reaction mixture [16,17]. The reaction involves the acid catalytic hydrolysis over acid sites, combined with in situ hydrolytic hydrogenation/hydrogenolysis over metallic sites in the same reactor. It has a synergic effect due to the fast removal of unstable intermediates of cello-oligosaccharides and sugars, which reduces the side reactions [18]. Heteropolyacids have been demonstrated to be very effective acid catalysts because their strong acid sites fostered the fast and selective hydrolysis of cellulose to glucose and also showed to be active for the CeC bond cleavage [19,20]. Although homogeneous HPAs are more expensive than traditional mineral acids, the required amounts of phosphotungstic and silicotungstic acids for efficient cellulose hydrolysis are usually low [21]. Ru has been considered a good catalyst for hydrogenation reactions with high activity and is relatively cheap compared to noble metals (such as Pt or Au) [22,23]. Carbon materials are known as heat- and water-tolerant supports [5], and for this reason, carbon supported metal catalysts have been extensively studied. Activated carbon (AC) is most often used to stabilize Ru particles and is cheap [22], whereas carbon nanotubes [23,24] and nanofibers [25] are promising supports, but are more expensive. Several authors have recently reported the bifunctional catalytic conversion of cellulose diluted in water (0.8–2 wt%) using catalyst loadings of 0.2–0.4 wt% [26,27], achieving hexitol yields in the range of 30–73%, typically within 24 h. However, the need remains for a catalytic system capable of rapid and selectively transforming more concentrated cellulose feeds into hexitols and alkanediols in high yields. We report here the catalytic conversion of ball-milled cellulose into sorbitol, ethylene glycol and propylene glycol by combining a heteropolyacid (HPA) and Ru supported on active carbon or high surface graphite. According to Kobayashi et al., who considered that the hydrolysis of cellulose is promoted by carbon catalysts and also occurs at the solid–solid interface, the number of collisions is a major obstacle
2. Materials and methods The ultrapure water with a conductivity of 18.2 μS cm−1 was obtained in a Milli-Q Millipore System. Microcrystalline cellulose and sorbitol (98%) were purchased from Alfa Aesar. The metal precursor RuCl3·3H2O (99.9%, Ru 38%) was also supplied by Alfa Aesar. Activated carbon (AC) was produced from olive stones (Oleicola el Tejar, Córdoba, Spain) with 1.25–0.8 mm particle sizes and 1190 m2 g−1 specific surface area. The high surface area graphite (HSAG), with a specific surface area of 400 m2 g−1, was provided by TIMCAL. The commercial heteropolyacid compounds, namely tungstophosphoric acid (TPA, H3O40PW12·nH2O), silicotungstic acid (STA, H4O40SiW12·nH2O) and phosphomolybdic acid (PMA, H3Mo12O40P·nH2O) were supplied by Sigma Aldrich. The heteropolyacid materials (HPAs) were incorporated with 20 wt % loading, both over activated carbon and high surface area graphite, using incipient impregnation. This loading of HPA was selected based on previous studies [29,30] over supported HPAs that demonstrate a maximum catalytic activity when HPA contents are in the range of 15–20 wt%, though these values were not optimized for the cellulose transformation reaction. The second component, Ru, was incorporated to achieve a metal loading of 0.4 wt%, which is based on previous studies for the same reaction [31]. The Ru-based catalysts were prepared by incipient wetness impregnation over the supports (AC, HSAG, AC-HPA and HSAG-HPA). These support materials were firstly introduced into an ultrasonic bath for 30 min. Subsequently the precursor solution (RuCl3·3H2O) was added dropwise, by a peristaltic pump (50 mL h−1), until all the support was wet. Still in the ultrasonic bath, the maturation and drying was allowed for 90 min. Then the catalyst was dried overnight in an oven at 110 °C and then stored for later use. After heat treatment under nitrogen flow (50 cm3 min−1) to 250 °C for 3 h, the catalyst was reduced under hydrogen flow (50 cm3 min−1) for 3 h at the same final temperature. The synthesized catalysts are listed in Table 1. Some textural characterizations of these HPAs supported materials have been recently reported [32]. Considering the high surface area values of the carbon materials used supports, the incorporation of 20 wt.% of HPAs, mainly modified the surface area values just by blocking the pore entries. The ball-milled cellulose was prepared using a laboratory ball mill (Retsch Mixer Mill MM200) by introducing 1.5 g of commercial Table 1 Nomenclature and composition of the synthesized catalysts.
2
Catalyst
Support
HPA (20%)
Metal
AC AC_PMA AC_STA AC_TPA HSAG HSAG_PMA HSAG_STA HSAG_TPA RuAC RuAC_PMA RuAC_STA RuAC_TPA RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA
AC AC AC AC HSAG HSAG HSAG HSAG AC AC AC AC HSAG HSAG HSAG HSAG
– PMA STA TPA – PMA STA TPA – PMA STA TPA – PMA STA TPA
– – – – – – – – 0.4% 0.4% 0.4% 0.4% 0.4% 0.4% 0.4% 0.4%
Ru Ru Ru Ru Ru Ru Ru Ru
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and ball-milled cellulose, an increase in the conversion unrespectively of the catalyst, RuAC-TPA and RuHSAG-TPA, is noticed with the ballmilled cellulose. This behaviour must be due to the physical and structural changes in the cellulose induced by ball-milling that makes glucose chains more accessible to the catalyst active sites. Therefore, ball-milling of microcrystalline cellulose disrupts the robust crystalline structure with a linkage of β-1,4-glycosidic and hydrogen bonds, making easier to directly hydrolyze cellulose and transform it into chemicals and fuels. Compared with the hydrolytic hydrogenation of cellulose to hexitols, hydrolytic hydrogenolysis of cellulose to EG also involves as a first step the depolymerization of the β-1,4glycosidic bonds of cellulosic biomass. Subsequently, as shown in Scheme 1, there are two main pathways for the conversion of cellulose to EG. One includes the retroaldol condensation of glucose to glycolaldehyde followed by the hydrogenation to EG (pathway (i)) [33]. The other pathway is the hydrogenolysis of sorbitol (produced from glucose hydrogenation) to EG and 1,2-PG (pathway (ii)) [34,35]. Also 1,2-PG can be produced following pathway (iii) by retro-aldol condensation of fructose to glyceraldehyde followed by the hydrodeoxygenation to acetol and isomerization to 1,2-PG. The successful selective transformation of cellulose to small molecular alcohols (EG, 1,2-PG) requires the hydrolysis to sugar intermediates (glucose, fructose) and their subsequent hydrogenation/hydrogenolysis. For better understanding these coupled reaction effects, both hydrolysis of cellulose and hydrogenation/hydrogenolysis of glucose steps were studied separately. In order to distinguish the different reaction stages, a catalytic material with metallic and acid properties (bifunctional catalyst) was selected. So, three different reaction experiments were performed with the RuHSAG_TPA catalyst, at the same reaction temperature (205 °C), and using water as solvent: 1) with cellulose as reactant under pure N2 atmosphere to favor the hydrolysis step, 2) with glucose as reactant under pure H2 for the hydrogenation step and 3) with cellulose as reactant under H2 + N2 atmosphere for the combination of the two steps. This latter condition is named as one-pot experiment. Table 2 shows the conversion and distribution of products, after two reaction times (30 and 300 min), obtained under the three above described reaction conditions using RuHSAG-TPA as catalyst. Although the conversions and selectivities to ethylene glycol are higher in experiment 2 (glucose hydrogenation) than in the others, the results obtained for the one-pot reaction are particularly relevant. So, when glucose is the reactant, the previous required transformation of cellulose into glucose has not to be considered, while in the one-pot experiment both steps are simultaneously carried out. Thus, the results presented in Table 2 indicate that this type of bifunctional catalyst, where a metallic function (Ru) is combined with a material with acidic surface sites, is very convenient for the direct transformation of cellulose yielding ethylene glycol. In order to compare all the catalytic materials, the results for AC and HSAG supported catalysts are summarized in Tables 3 and 4, respectively. The presented catalytic values correspond to the one-pot experiments (H2 + N2 atmosphere), being the reported parameters the conversion of cellulose (X) after 300 min of reaction and the yield (Y) toward three target products: sorbitol (YSOR), ethylene glycol (YEG) and propylene glycol (YPG). The different catalysts are classified into two categories: Category I includes bare supports and supported HPA catalysts. Category II includes supported Ru catalysts and binary RuHPA based catalysts. The common feature in the performance of AC supported catalysts (category I, Table 3) is their increased yield toward EG formation compared to bare support, usually approaching 5–6%. However, for HPA catalysts supported on HSAG (Table 4) no improvement in the EG production was observed with respect to the bare HSAG support. So the HSAG support showed some catalytic activity for hydrogenolysis of cellulose, giving 6% of EG yield. In both series of catalysts the highest
microcrystalline cellulose into the ceramic pot with two ZrO2 balls (12 mm of diameter), operating at a frequency of 20 Hz for 4 h. In addition, cellulose and catalyst were ball-milled together (sample denoted as (catalyst)mix) as a pre-treatment to improve the contact between the cellulose and the catalyst. The catalytic tests for hydrolytic hydrogenolysis of cellulose were performed in a stainless steel reactor (Parr Instruments, USA Mod. 5120) equipped with a gas supply system, a manometer, a temperature sensor and a filtered sample outlet, which prevents the catalyst particles to pass through it. Typically, 300 mL of water, 750 mg of cellulose and 300 mg of catalyst were placed in the 1000 mL reactor, and the resulting mixture was stirred at 150 rpm. Then the reactor was flushed three times with N2 to remove air. After heating under this atmosphere to the desired temperature (205 °C), the reaction was initiated by switching from inert gas to H2 (50 bar). Samples (1 mL) were periodically withdrawn for analysis. After reaction (5 h), the catalyst and nonhydrolyzed cellulose were filtered out. The conversion of cellulose was calculated as the ratio of the hydrolyzed cellulose mass to its initial amount; the values were also verified on the basis of total organic carbon (TOC) data. The liquid-phase products were analyzed by high performance liquid chromatography (HPLC). The chromatograph (Elite LaChrom HITACHI) was equipped with a refractive index (RI) detector and the products were separated in an ion exclusion column (Alltech OA 1000). The retention times and calibration curves were determined by comparison with standard samples. The eluent was a H2SO4 solution (0.005 M). An injection volume of 30 μL, a flow rate of 0.5 mL min−1 and a measuring time of 20 min were selected. After the experiments, the samples were filtered to prevent solid particles from entering the columns when the sample is injected into the HPLC. The selectivity (Si) of each product i at time t was calculated as:
Si =
Ci viCoX
where Ci is the concentration of the product i (mol L−1), C0 is the initial concentration of cellulose (mol L−1), X is the conversion of cellulose and vi corresponds to the moles of i produced per moles of cellulose consumed, according to the stoichiometry. For determining the conversion of non-soluble cellulose into water soluble products, TOC data was obtained with a Shimadzu TOC 5000-A and the conversion determined using the equation: X (%) = (moles of total organic carbon in the resultant liquid)/(moles of carbon in cellulose charged into the reactor) x 100. Besides sorbitol, ethylene glycol, propylene glycol and glucose, other reaction products (mannitol, erythritol, xylitol, glycerol and formic acid) were also detected but with yields lower than 5% after 5 h of reaction, as well as unknown products, which would justify the mass balance obtained. Concerning the formation of humins and gaseous products, which were not directly analyzed, it was observed the presence of humins, after the reaction tests, on the reactor wall). In some cases the conversion of cellulose was also calculated as the ratio of the hydrolysed cellulose mass to its initial amount. For instance, the conversion values of microcrystalline cellulose using Ru/AC-TPA, calculated by weight difference or obtained by TOC are within the experimental errors ( ± 3%). However, when cellulose was ball-milled prior to reaction, the conversion measured by weight difference was 100% after 5 h of reaction. These differences can be associated with the formation of humins or gaseous products. Blank experiments with cellulose were firstly performed to determine the background reactivity in the absence of catalyst. Conversion of cellulose lower than 20% was observed after 5 h of reaction. 3. Results and discussion The effect of ball milling of cellulose on its conversion was analyzed using two catalysts: RuAC_TPA and RuHSAG_TPA. As shown in Fig. 1, when comparing the results obtained using microcrystalline cellulose 3
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Fig. 1. Comparative catalytic conversions of ball-milled and microcrystalline cellulose for a) RuAC_TPA and b) RuHSAG_ TPA catalysts.
Scheme 1. Catalytic conversion of cellulose into polyols.
sorbitol, EG and PG increased. RuAC showed higher activity for hydrolytic hydrogenation than hydrogenolysis to EG and PG. This catalyst showed the best sorbitol selectivity. The combination of Ru with HPAs over the carbon support, either AC or HSAG, promotes the hydrogenolysis products. Without ruthenium, cellulose was still converted, but polyols selectivities are low. It has been reported that transition metals are themselves not effective for the CeC cleavage. Ji
yield values toward EG were for TPA-based catalysts, bearing in mind that the sequence of acid strength is TPA > STA > PMA [36], this suggests that stronger acid sites promote cleavage of the CeC bond of glucose, favoring EG production. For this reason, TPA catalysts show a higher yield of EG. The addition of small amounts of Ru (0.4%) modifies significantly the AC and HSAG catalytic properties. In both cases the yields of
Table 2 Conversion and distribution of products for the different conversion steps using RuHSAG_TPA. Experiment/Reaction
t (min)
X (%)
Sglucose (%)
Ssorbitol (%)
SEG (%)
SPG (%)
1/Hydrolysis 2/Hydrogenation/hydrogenolysis 3/One-pot 1/Hydrolysis 2/Hydrogenation/hydrogenolysis 3/One-pot
30
40 87 44 86 100 99
35 – 30 0 – 0
15 9 17 7 7 9
0 18 14 6 26 16
0 1 2 0 4 7
300
4
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et al. [37] found very low yields toward hydrogenolysis products (< 10%) from cellulose over Pt/SiO2, Pt/AC and Ni/AC catalysts. Therefore, the observed promotional effect of ruthenium for polyols production can be due to its excellent hydrogenation activity, which can compensate well the insufficient ability of HPA/AC and HPA/ HSAG. Moreover, the increase of EG and PG yields depends on the heteropolyacid type. Thus, it appears that HPAs species played major role in controlling the selective degradation of sugars into C2 and other unsaturated molecules, whereas Ru carried out the hydrogenation of the unsaturated compounds to EG and polyols [38,39]. A good balance between CeC cracking and hydrogenation would determine the final product distribution. On the other hand, RuAC_HPA catalysts showed higher PG yield than RuHSAG_HPA ones. This higher PG yield for catalysts supported on AC might be a consequence of their improved surface basicity and/ or acidity properties due to the presence of functional groups on the AC surface [40,41]. These surface properties allow AC support to accelerate the isomerization of glucose into fructose, which is likely the intermediate for PG. However, in most cases, the EG yield is higher than that of PG due to the higher yield toward glucose compared with fructose during the primary hydrolysis of cellulose. As can be seen in Tables 3 and 4, the yield values for target products are rather low, but these correspond to the amounts detected by HPLC analysis, so an important quantity of the reacted cellulose is trans-
Table 3 Catalytic results over AC supported samples after 300 min of reaction. Category
Catalyst
X (%)
YSOR (%)
YEG (%)
YPG (%)
I
AC AC_PMA AC_STA AC_TPA RuAC RuAC_PMA RuAC_STA RuAC_TPA
90 87 79 92 86 88 97 98
6 6 6 6 17 8 12 16
0 4 4 6 5 8 13 25
0 0 0 0 2 13 16 10
II
Table 4 Catalytic results over HSAG supported samples after 300 min of reaction. Category
Catalyst
X (%)
YSOR (%)
YEG (%)
YPG (%)
I
HSAG HSAG_PMA HSAG_STA HSAG_TPA RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA
100 99 91 90 98 100 96 99
7 6 6 5 12 8 12 12
6 5 5 6 11 8 12 16
0 0 0 0 6 8 8 7
II
Fig. 2. Comparative catalytic conversions of cellulose for single ball-milled cellulose plus catalysts a), c) and e), and for mixed ball-milling of cellulose and solid catalysts b), d) and f).
5
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were obtained by Kobayashi et al. for conversion of mix-milled activated carbon and cellulose [28]. These authors concluded that the enhancement of the reaction performance by mix-milling is not just due to the mechano-catalytic hydrolysis during the milling treatment, but the intimate contact of reactant and catalysts, which increases the initial reaction rate under the appropriated conditions (temperature and pressure). Also relevant is that three successive tests were performed using the Ru/HSAG-TPA catalyst, in order to evaluate its stability in the reaction. The catalyst showed excellent stability at least up to three successive runs, without significant changes in the conversion or yield of EG, and no leaching to solution was observed. As shown in Table 5, in the case of activated carbon supported catalysts, the conversions of the mixed ball-milled cellulose were higher than those obtained with the single ball-milling. This result would be expected since during ball-milling the grains (1.25–0.8 mm) of the AC are powdered intensifying the contact of the catalyst with the cellulose. The sorbitol yield reached 46% after mixed ball-milling with RuAC. Nevertheless, the mix-milling pretreatment for RuAC-HPA catalyst did not improve the yields toward sorbitol, EG and PG. This may be because hydrogenation of glucose and hydrogenolysis of sugar alcohols (mannitol, sorbitol) take place at the surface of Ru/C simultaneously, hydrogenation of glucose will be the preferable reaction if the glucose is developed quickly [19]. More water-soluble sugars occupy the active sites of Ru/C as the depolymerization rate of cellulose is accelerated in mixed ball-milled cellulose, which could inhibit hydrogenolysis of hexitols over Ru/C. These factors play an essential role in the selectivity of sugar alcohols [45]. On the contrary, for HSAG supported catalysts, the conversion values after both pretreatments are similar, although the yield value toward EG product is maximum for the mixed ball-milled cellulose with (RuHSAG_TPA)mix.
formed into non analyzed (by HPLC) carbon organic products. Provably these compounds are oligosaccharides as indicated in [19]. This behavior is particularly evident for the Ru free catalysts. Binary catalysts based on tungsten showed the most distinguished performance in terms of high EG selectivity. This result agrees with that recently published by Zhang et al. who attributed the good performance of tungsten-containing HPAs to the bifunctional role of W in accelerating cellulose hydrolysis and CeC bond cleavage [42]. Following the set of reactions gathered at Scheme 1 and according to the literature [27,43], in the process of cellulose conversion to EG, the cellulose first undergoes hydrolysis to form oligosaccharides and glucose. The derived oligosaccharides and sugars are further catalytically degraded to glycolaldehyde by retro-aldol condensation in the presence of tungsten species. Although we cannot precisely describe how the tungsten species interact with oligosaccharides and sugar molecules, it can be concluded that the catalytic breakage of the CeC bond selectively takes place at the position between the α‐β carbons. In the degradation of glucose-based oligosaccharides, glycolaldehyde is produced because the sugars have a terminal aldehyde group. Concerning the role of the hydrogenation catalysts, ruthenium is responsible for transforming the unsaturated C2 and C3 intermediates to EG. The metal sites also promote the hydrolysis of cellulose via heterolytic dissociation of H2 [44]. In contrast, when glucose isomerizes into fructose, C3 molecules will be formed, leading to the 1,2-PG and glycerol formation after a series of subsequent reactions, which was demonstrated in an early publication [43]. On the other hand, according to Kobayashi et al. [28], if cellulose and catalytic materials are simultaneous mix-milled in order to improve the contact between the cellulose and the catalyst, the conversions can be enhanced. Several solid catalysts were tested in hydrolytic hydrogenolysis of single and mix-milled cellulose under the same reaction conditions, and the corresponding results are shown in Fig. 2 and Table 5. As shown in Fig. 2, AC supported mixed catalysts without Ru (Fig. 2b) resulted in 90% conversion in less than 5 min, whereas the separately ball-milled catalysts and cellulose provided less than 40% conversion upon the same time in reaction (Fig. 2a). For catalysts with Ru, either supported on AC (Fig. 2c and d) or on HSAG (Fig. 2e and f), and with the presence or not of HPAs, the conversions after 5 min in reaction are remarkably higher for the ball-milled mixtures of cellulose and catalysts, in comparison with the mixed samples without combined milling. Thus, a great increase in the initial reaction rate by the mixmilling pre-treatment is evidenced. These results indicate that the mix-milling pretreatment accelerates the cellulose transformation, being the cellulose conversion much faster in comparison with single ball-milled cellulose mixtures. Similar results
4. Conclusions Hydrolytic hydrogenolysis of cellulose to polyols in one-pot can be regarded as a new approach for the sustainable production of bulk chemicals from biomass. To accomplish this cascade reaction with selectivity control to a desired product, multi-functional catalysts can be an interesting choice. In the present study two carbon materials are used as supports, three commercial HPA and metallic Ru nanoparticles are combined. Over these composite materials, the hydrolysis of cellulose, with CeC bond cleavage, and several hydrogenation steps can proceed cooperatively. While HPAs are efficient catalysts in the cellulose transformation into sugar intermediates, and subsequently for the selective cleavage of the CeC bonds in these sugars, Ru/C can catalyze the hydrogenation reactions. Using this approach the controllable synthesis of ethylene glycol, propylene glycol or sorbitol from cellulose can be achieved. Tungsten-based HPA catalysts proved to be highly active and selective to ethylene glycol. Activated carbon support seems to catalyze the isomerization of glucose into fructose, which facilitates the way of formation of propylene glycol. The addition of small amounts of Ru increases the yields of sorbitol, ethylene glycol and propylene glycol. The promotional effect of Ru can be interpreted by its excellent activity for the hydrogenation of unsaturated compounds to EG and polyols. As a technical issue, ball-milling during the cellulose pretreatment is a critical parameter in order to increase initial catalytic conversions. Furthermore, the mixed ball-milling of cellulose and solid catalysts accelerates cellulose transformation, the catalytic conversion being much faster. These results would be helpful for rational design of multi-functional catalysts and tuning the reaction parameters for controlling the conversion of cellulose to specific products.
Table 5 Catalytic results for single ball-milling of cellulose and mixed ball-milling of cellulose and solid acid catalysts after 300 min of reaction. Catalyst
X (%)
YSOR (%)
YEG (%)
Y
RuAC RuAC_PMA RuAC_STA RuAC_TPA (RuAC)mix (RuAC_PMA)mix (RuAC_STA)mix (RuAC_TPA)mix RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA (RuHSAG)mix (RuHSAG_PMA)mix (RuHSAG_STA)mix (RuHSAG_TPA)mix
86 88 97 98 100 100 100 100 98 100 96 99 93 96 100 92
17 8 12 16 46 6 6 13 12 8 12 12 8 6 6 8
5 8 13 25 5 10 13 20 11 8 12 16 13 10 19 26
2 13 16 10 3 14 15 8 6 8 8 7 4 7 6 6
PG
(%)
Acknowledgments The financial support from the Spanish Government (projects CTQ2014-52956-C3-2-R and CTQ2014-52956-C3-3-R) is recognized. 6
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MAH thanks the MINECO for a FPI predoctoral grant. This work was also financially supported by: project POCI-01-0145-FEDER-006984Associate Laboratory LSRE-LCM funded by FEDER through COMPETE2020 − Programa Operacional Competitividade e Internacionalização (POCI) − and by national funds through FCT − Fundação para a Ciência e a Tecnologia. L.S. Ribeiro acknowledges her Ph.D. scholarship (SFRH/BD/86580/2012) from FCT. References [1] [2] [3] [4] [5] [6] [7]
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Cooperative action of heteropolyacids and carbon supported Ru catalysts for the conversion of cellulose María Almohallaa, Inmaculada Rodríguez-Ramosb, Lucília S. Ribeiroc, José J.M. Órfãoc, ⁎ Manuel Fernando R. Pereirac, Antonio Guerrero-Ruiza, a
Dpto. de Química Inorgánica y Técnica, UNED, Madrid, Spain Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain c Laboratory of Separation and Reaction Engineering—Laboratory of Catalysis and Materials (LSRE-LCM), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Portugal b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cellulose Heteropolyacid Alkanediols Ruthenium
Acid hydrolysis and hydrogenation/hydrogenolysis reactions can be combined for catalytic conversion of cellulose into renewable biorefinery feedstocks by using heterogeneous bifunctional catalysts. In the present study a cooperative effect of heteropolyacids (HPA) and Ru nanoparticles supported on two carbon materials is demonstrated. The process can be suitable for the one-pot tandem reaction, yielding the conversion of cellulose into alkanediols (mainly propylene glycol and ethylene glycol). From a mechanistic point of view the differences in the distribution of polyol products, obtained from the cellulose reaction over monometallic Ru catalysts or over bifunctional Ru-HPAs materials, seem to be strongly determined by the competitive reactions of the sucrose (glucose + fructose) intermediate. HPA not only promote, as solid acids, the efficient hydrolysis of cellulose to glucose, but also catalyze the selective cleavage of the CeC bonds in glucose and fructose, leading to the formation of ethylene glycol and propylene glycol. These reactions are in competition with the sugar hydrogenation to the corresponding C6 polyols (e.g. sorbitol), which takes place on the single Ru surface sites. The strong dependence of the product distribution on both catalytic functions is clarified by the kinetic analysis of the three competitive reactions of glucose, including its hydrogenation, isomerization and CeC bond cleavage. Finally, considering the applicability of this reaction, it should be raised that the ball-milling pretreatment of cellulose is compulsory. In fact, during this ball-milling the crystallinity and particle size of cellulose are reduced, which results in a much higher conversion of cellulose. Herein, mixed ball-milling of cellulose and solid catalysts together was presented, which remarkably accelerates the cellulose conversion into valuable products.
1. Introduction The increasing world energy needs, namely depleting fossil resources and growing environmental concerns, have triggered great interest in searching for renewable sources of energy and chemicals. Cellulose, the most abundant source of biomass, consists of carbon, oxygen and hydrogen, and has received considerable attention as a vitally renewable alternative to fossil fuels [1–3]. Cellulose comprises a carbohydrate monomer (glucose) linked by β-1,4-glycosidic bonds. Chemocatalytic cleavage of its CeO and CeC bonds to polyols by hydrolytic hydrogenation or hydrogenolysis in a mild aqueous medium has attracted intensive scrutiny due to the wide variety of possible applications of polyols as value-added chemicals and fuels [4–6]. In particular, sorbitol is regarded as one of the 12 important platform
⁎
compounds in biomass conversion programs [7]. Additionally, 1,2alkanediols such as ethylene glycol (EG) and 1,2-propylene glycol (PG) are used as energy platforms for the production of hydrogen and liquid alkane fuels via the aqueous reforming process or as monomers for polymerization synthesis [1,8,9]. EG is especially consumed in quantities above 20 Mt/yr in the petroleum industry [10]. In view of the importance of EG, its production from sustainable cellulose is critical. The conversion of cellulose to polyols includes two consecutive reaction steps: acidic hydrolysis of cellulose to sugars and the subsequent hydrogenation or hydrogenolysis of the intermediate sugars to polyols over a supported transition metal catalyst under high H2 pressure [11]. However, although some studies have succeeded in converting cellulose into high-valued chemicals, there are still many challenges [12,13], mostly due to the cellulose’s robust crystalline
Corresponding author. E-mail address: [email protected] (A. Guerrero-Ruiz).
http://dx.doi.org/10.1016/j.cattod.2017.05.023 Received 16 November 2016; Received in revised form 17 April 2017; Accepted 5 May 2017 0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Almohalla, M., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.023
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in this type of reaction [28]. Therefore, in this work cellulose and catalyst were also ball-milled together as a pre-treatment to improve the contact between cellulose and the catalyst, as the mixed state can continue in the subsequent hydrolysis reaction thanks to the insoluble properties of the catalyst and the substrate.
structure and insolubility in conventional solvents. The crystal structure and hydrogen bonding in cellulose limit the access to β-1,4-glycosidic bonds by reactants and catalysts [14]. So, in order to facilitate the contact with the catalysts, the crystalline structure of cellulose has to be considered since non-crystalline forms of cellulose will be more reactive. Milling methods such as ball-milling are the typical mechanical techniques for disrupting the crystal structure of cellulose, since cellulose’s hydrogen bonds are cleaved [5]. Acid treatments are the chemical treatments often used for this purpose, but are undesirable due to reactor corrosion and environmental toxicity. Hydrolysis of cellulose catalyzed by sulfuric acid has been implemented on relatively large scales [1]. Although sulfuric acid is an inexpensive and a highly active catalyst for this reaction, it requires an energy-inefficient separation process for catalyst recovery before reuse and, in addition, treatment of the waste sulfuric acid is often not economical. In contrast, ball-milling does not suffer from this problem, making it a great alternative for breaking the robust crystalline structure of cellulose and decreasing its crystallinity in a sustainable way. In a previous report [15], in order to reduce its crystallinity before the reaction and to study its effect on the performance of the process, microcrystalline cellulose was ball-milled in a ceramic pot with two ZrO2 balls (12 mm of diameter) and different cellulose samples were prepared by varying the ball-milling frequencies (from 5 to 20 Hz) for 4 h. It was demonstrated by X-ray diffraction (XRD) the transformation of microcrystalline cellulose into amorphous cellulose, with a decrease of cellulose’s crystallinity from 92 to 23% just by ball-milling at a frequency of 20 Hz for 4 h [15]. From the point of view of green chemistry, successful manipulation of the tandem catalytic process using heterogeneous catalysts would be economical and environmentally viable if the use of chemicals, waste production, catalyst separation and recovery, and processing time are optimized. For example, recent studies showed the one-pot hydrolytic hydrogenation of cellulose to sugar alcohols over bi-functional heterogeneous catalysts which are easily separable from the reaction mixture [16,17]. The reaction involves the acid catalytic hydrolysis over acid sites, combined with in situ hydrolytic hydrogenation/hydrogenolysis over metallic sites in the same reactor. It has a synergic effect due to the fast removal of unstable intermediates of cello-oligosaccharides and sugars, which reduces the side reactions [18]. Heteropolyacids have been demonstrated to be very effective acid catalysts because their strong acid sites fostered the fast and selective hydrolysis of cellulose to glucose and also showed to be active for the CeC bond cleavage [19,20]. Although homogeneous HPAs are more expensive than traditional mineral acids, the required amounts of phosphotungstic and silicotungstic acids for efficient cellulose hydrolysis are usually low [21]. Ru has been considered a good catalyst for hydrogenation reactions with high activity and is relatively cheap compared to noble metals (such as Pt or Au) [22,23]. Carbon materials are known as heat- and water-tolerant supports [5], and for this reason, carbon supported metal catalysts have been extensively studied. Activated carbon (AC) is most often used to stabilize Ru particles and is cheap [22], whereas carbon nanotubes [23,24] and nanofibers [25] are promising supports, but are more expensive. Several authors have recently reported the bifunctional catalytic conversion of cellulose diluted in water (0.8–2 wt%) using catalyst loadings of 0.2–0.4 wt% [26,27], achieving hexitol yields in the range of 30–73%, typically within 24 h. However, the need remains for a catalytic system capable of rapid and selectively transforming more concentrated cellulose feeds into hexitols and alkanediols in high yields. We report here the catalytic conversion of ball-milled cellulose into sorbitol, ethylene glycol and propylene glycol by combining a heteropolyacid (HPA) and Ru supported on active carbon or high surface graphite. According to Kobayashi et al., who considered that the hydrolysis of cellulose is promoted by carbon catalysts and also occurs at the solid–solid interface, the number of collisions is a major obstacle
2. Materials and methods The ultrapure water with a conductivity of 18.2 μS cm−1 was obtained in a Milli-Q Millipore System. Microcrystalline cellulose and sorbitol (98%) were purchased from Alfa Aesar. The metal precursor RuCl3·3H2O (99.9%, Ru 38%) was also supplied by Alfa Aesar. Activated carbon (AC) was produced from olive stones (Oleicola el Tejar, Córdoba, Spain) with 1.25–0.8 mm particle sizes and 1190 m2 g−1 specific surface area. The high surface area graphite (HSAG), with a specific surface area of 400 m2 g−1, was provided by TIMCAL. The commercial heteropolyacid compounds, namely tungstophosphoric acid (TPA, H3O40PW12·nH2O), silicotungstic acid (STA, H4O40SiW12·nH2O) and phosphomolybdic acid (PMA, H3Mo12O40P·nH2O) were supplied by Sigma Aldrich. The heteropolyacid materials (HPAs) were incorporated with 20 wt % loading, both over activated carbon and high surface area graphite, using incipient impregnation. This loading of HPA was selected based on previous studies [29,30] over supported HPAs that demonstrate a maximum catalytic activity when HPA contents are in the range of 15–20 wt%, though these values were not optimized for the cellulose transformation reaction. The second component, Ru, was incorporated to achieve a metal loading of 0.4 wt%, which is based on previous studies for the same reaction [31]. The Ru-based catalysts were prepared by incipient wetness impregnation over the supports (AC, HSAG, AC-HPA and HSAG-HPA). These support materials were firstly introduced into an ultrasonic bath for 30 min. Subsequently the precursor solution (RuCl3·3H2O) was added dropwise, by a peristaltic pump (50 mL h−1), until all the support was wet. Still in the ultrasonic bath, the maturation and drying was allowed for 90 min. Then the catalyst was dried overnight in an oven at 110 °C and then stored for later use. After heat treatment under nitrogen flow (50 cm3 min−1) to 250 °C for 3 h, the catalyst was reduced under hydrogen flow (50 cm3 min−1) for 3 h at the same final temperature. The synthesized catalysts are listed in Table 1. Some textural characterizations of these HPAs supported materials have been recently reported [32]. Considering the high surface area values of the carbon materials used supports, the incorporation of 20 wt.% of HPAs, mainly modified the surface area values just by blocking the pore entries. The ball-milled cellulose was prepared using a laboratory ball mill (Retsch Mixer Mill MM200) by introducing 1.5 g of commercial Table 1 Nomenclature and composition of the synthesized catalysts.
2
Catalyst
Support
HPA (20%)
Metal
AC AC_PMA AC_STA AC_TPA HSAG HSAG_PMA HSAG_STA HSAG_TPA RuAC RuAC_PMA RuAC_STA RuAC_TPA RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA
AC AC AC AC HSAG HSAG HSAG HSAG AC AC AC AC HSAG HSAG HSAG HSAG
– PMA STA TPA – PMA STA TPA – PMA STA TPA – PMA STA TPA
– – – – – – – – 0.4% 0.4% 0.4% 0.4% 0.4% 0.4% 0.4% 0.4%
Ru Ru Ru Ru Ru Ru Ru Ru
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and ball-milled cellulose, an increase in the conversion unrespectively of the catalyst, RuAC-TPA and RuHSAG-TPA, is noticed with the ballmilled cellulose. This behaviour must be due to the physical and structural changes in the cellulose induced by ball-milling that makes glucose chains more accessible to the catalyst active sites. Therefore, ball-milling of microcrystalline cellulose disrupts the robust crystalline structure with a linkage of β-1,4-glycosidic and hydrogen bonds, making easier to directly hydrolyze cellulose and transform it into chemicals and fuels. Compared with the hydrolytic hydrogenation of cellulose to hexitols, hydrolytic hydrogenolysis of cellulose to EG also involves as a first step the depolymerization of the β-1,4glycosidic bonds of cellulosic biomass. Subsequently, as shown in Scheme 1, there are two main pathways for the conversion of cellulose to EG. One includes the retroaldol condensation of glucose to glycolaldehyde followed by the hydrogenation to EG (pathway (i)) [33]. The other pathway is the hydrogenolysis of sorbitol (produced from glucose hydrogenation) to EG and 1,2-PG (pathway (ii)) [34,35]. Also 1,2-PG can be produced following pathway (iii) by retro-aldol condensation of fructose to glyceraldehyde followed by the hydrodeoxygenation to acetol and isomerization to 1,2-PG. The successful selective transformation of cellulose to small molecular alcohols (EG, 1,2-PG) requires the hydrolysis to sugar intermediates (glucose, fructose) and their subsequent hydrogenation/hydrogenolysis. For better understanding these coupled reaction effects, both hydrolysis of cellulose and hydrogenation/hydrogenolysis of glucose steps were studied separately. In order to distinguish the different reaction stages, a catalytic material with metallic and acid properties (bifunctional catalyst) was selected. So, three different reaction experiments were performed with the RuHSAG_TPA catalyst, at the same reaction temperature (205 °C), and using water as solvent: 1) with cellulose as reactant under pure N2 atmosphere to favor the hydrolysis step, 2) with glucose as reactant under pure H2 for the hydrogenation step and 3) with cellulose as reactant under H2 + N2 atmosphere for the combination of the two steps. This latter condition is named as one-pot experiment. Table 2 shows the conversion and distribution of products, after two reaction times (30 and 300 min), obtained under the three above described reaction conditions using RuHSAG-TPA as catalyst. Although the conversions and selectivities to ethylene glycol are higher in experiment 2 (glucose hydrogenation) than in the others, the results obtained for the one-pot reaction are particularly relevant. So, when glucose is the reactant, the previous required transformation of cellulose into glucose has not to be considered, while in the one-pot experiment both steps are simultaneously carried out. Thus, the results presented in Table 2 indicate that this type of bifunctional catalyst, where a metallic function (Ru) is combined with a material with acidic surface sites, is very convenient for the direct transformation of cellulose yielding ethylene glycol. In order to compare all the catalytic materials, the results for AC and HSAG supported catalysts are summarized in Tables 3 and 4, respectively. The presented catalytic values correspond to the one-pot experiments (H2 + N2 atmosphere), being the reported parameters the conversion of cellulose (X) after 300 min of reaction and the yield (Y) toward three target products: sorbitol (YSOR), ethylene glycol (YEG) and propylene glycol (YPG). The different catalysts are classified into two categories: Category I includes bare supports and supported HPA catalysts. Category II includes supported Ru catalysts and binary RuHPA based catalysts. The common feature in the performance of AC supported catalysts (category I, Table 3) is their increased yield toward EG formation compared to bare support, usually approaching 5–6%. However, for HPA catalysts supported on HSAG (Table 4) no improvement in the EG production was observed with respect to the bare HSAG support. So the HSAG support showed some catalytic activity for hydrogenolysis of cellulose, giving 6% of EG yield. In both series of catalysts the highest
microcrystalline cellulose into the ceramic pot with two ZrO2 balls (12 mm of diameter), operating at a frequency of 20 Hz for 4 h. In addition, cellulose and catalyst were ball-milled together (sample denoted as (catalyst)mix) as a pre-treatment to improve the contact between the cellulose and the catalyst. The catalytic tests for hydrolytic hydrogenolysis of cellulose were performed in a stainless steel reactor (Parr Instruments, USA Mod. 5120) equipped with a gas supply system, a manometer, a temperature sensor and a filtered sample outlet, which prevents the catalyst particles to pass through it. Typically, 300 mL of water, 750 mg of cellulose and 300 mg of catalyst were placed in the 1000 mL reactor, and the resulting mixture was stirred at 150 rpm. Then the reactor was flushed three times with N2 to remove air. After heating under this atmosphere to the desired temperature (205 °C), the reaction was initiated by switching from inert gas to H2 (50 bar). Samples (1 mL) were periodically withdrawn for analysis. After reaction (5 h), the catalyst and nonhydrolyzed cellulose were filtered out. The conversion of cellulose was calculated as the ratio of the hydrolyzed cellulose mass to its initial amount; the values were also verified on the basis of total organic carbon (TOC) data. The liquid-phase products were analyzed by high performance liquid chromatography (HPLC). The chromatograph (Elite LaChrom HITACHI) was equipped with a refractive index (RI) detector and the products were separated in an ion exclusion column (Alltech OA 1000). The retention times and calibration curves were determined by comparison with standard samples. The eluent was a H2SO4 solution (0.005 M). An injection volume of 30 μL, a flow rate of 0.5 mL min−1 and a measuring time of 20 min were selected. After the experiments, the samples were filtered to prevent solid particles from entering the columns when the sample is injected into the HPLC. The selectivity (Si) of each product i at time t was calculated as:
Si =
Ci viCoX
where Ci is the concentration of the product i (mol L−1), C0 is the initial concentration of cellulose (mol L−1), X is the conversion of cellulose and vi corresponds to the moles of i produced per moles of cellulose consumed, according to the stoichiometry. For determining the conversion of non-soluble cellulose into water soluble products, TOC data was obtained with a Shimadzu TOC 5000-A and the conversion determined using the equation: X (%) = (moles of total organic carbon in the resultant liquid)/(moles of carbon in cellulose charged into the reactor) x 100. Besides sorbitol, ethylene glycol, propylene glycol and glucose, other reaction products (mannitol, erythritol, xylitol, glycerol and formic acid) were also detected but with yields lower than 5% after 5 h of reaction, as well as unknown products, which would justify the mass balance obtained. Concerning the formation of humins and gaseous products, which were not directly analyzed, it was observed the presence of humins, after the reaction tests, on the reactor wall). In some cases the conversion of cellulose was also calculated as the ratio of the hydrolysed cellulose mass to its initial amount. For instance, the conversion values of microcrystalline cellulose using Ru/AC-TPA, calculated by weight difference or obtained by TOC are within the experimental errors ( ± 3%). However, when cellulose was ball-milled prior to reaction, the conversion measured by weight difference was 100% after 5 h of reaction. These differences can be associated with the formation of humins or gaseous products. Blank experiments with cellulose were firstly performed to determine the background reactivity in the absence of catalyst. Conversion of cellulose lower than 20% was observed after 5 h of reaction. 3. Results and discussion The effect of ball milling of cellulose on its conversion was analyzed using two catalysts: RuAC_TPA and RuHSAG_TPA. As shown in Fig. 1, when comparing the results obtained using microcrystalline cellulose 3
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Fig. 1. Comparative catalytic conversions of ball-milled and microcrystalline cellulose for a) RuAC_TPA and b) RuHSAG_ TPA catalysts.
Scheme 1. Catalytic conversion of cellulose into polyols.
sorbitol, EG and PG increased. RuAC showed higher activity for hydrolytic hydrogenation than hydrogenolysis to EG and PG. This catalyst showed the best sorbitol selectivity. The combination of Ru with HPAs over the carbon support, either AC or HSAG, promotes the hydrogenolysis products. Without ruthenium, cellulose was still converted, but polyols selectivities are low. It has been reported that transition metals are themselves not effective for the CeC cleavage. Ji
yield values toward EG were for TPA-based catalysts, bearing in mind that the sequence of acid strength is TPA > STA > PMA [36], this suggests that stronger acid sites promote cleavage of the CeC bond of glucose, favoring EG production. For this reason, TPA catalysts show a higher yield of EG. The addition of small amounts of Ru (0.4%) modifies significantly the AC and HSAG catalytic properties. In both cases the yields of
Table 2 Conversion and distribution of products for the different conversion steps using RuHSAG_TPA. Experiment/Reaction
t (min)
X (%)
Sglucose (%)
Ssorbitol (%)
SEG (%)
SPG (%)
1/Hydrolysis 2/Hydrogenation/hydrogenolysis 3/One-pot 1/Hydrolysis 2/Hydrogenation/hydrogenolysis 3/One-pot
30
40 87 44 86 100 99
35 – 30 0 – 0
15 9 17 7 7 9
0 18 14 6 26 16
0 1 2 0 4 7
300
4
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et al. [37] found very low yields toward hydrogenolysis products (< 10%) from cellulose over Pt/SiO2, Pt/AC and Ni/AC catalysts. Therefore, the observed promotional effect of ruthenium for polyols production can be due to its excellent hydrogenation activity, which can compensate well the insufficient ability of HPA/AC and HPA/ HSAG. Moreover, the increase of EG and PG yields depends on the heteropolyacid type. Thus, it appears that HPAs species played major role in controlling the selective degradation of sugars into C2 and other unsaturated molecules, whereas Ru carried out the hydrogenation of the unsaturated compounds to EG and polyols [38,39]. A good balance between CeC cracking and hydrogenation would determine the final product distribution. On the other hand, RuAC_HPA catalysts showed higher PG yield than RuHSAG_HPA ones. This higher PG yield for catalysts supported on AC might be a consequence of their improved surface basicity and/ or acidity properties due to the presence of functional groups on the AC surface [40,41]. These surface properties allow AC support to accelerate the isomerization of glucose into fructose, which is likely the intermediate for PG. However, in most cases, the EG yield is higher than that of PG due to the higher yield toward glucose compared with fructose during the primary hydrolysis of cellulose. As can be seen in Tables 3 and 4, the yield values for target products are rather low, but these correspond to the amounts detected by HPLC analysis, so an important quantity of the reacted cellulose is trans-
Table 3 Catalytic results over AC supported samples after 300 min of reaction. Category
Catalyst
X (%)
YSOR (%)
YEG (%)
YPG (%)
I
AC AC_PMA AC_STA AC_TPA RuAC RuAC_PMA RuAC_STA RuAC_TPA
90 87 79 92 86 88 97 98
6 6 6 6 17 8 12 16
0 4 4 6 5 8 13 25
0 0 0 0 2 13 16 10
II
Table 4 Catalytic results over HSAG supported samples after 300 min of reaction. Category
Catalyst
X (%)
YSOR (%)
YEG (%)
YPG (%)
I
HSAG HSAG_PMA HSAG_STA HSAG_TPA RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA
100 99 91 90 98 100 96 99
7 6 6 5 12 8 12 12
6 5 5 6 11 8 12 16
0 0 0 0 6 8 8 7
II
Fig. 2. Comparative catalytic conversions of cellulose for single ball-milled cellulose plus catalysts a), c) and e), and for mixed ball-milling of cellulose and solid catalysts b), d) and f).
5
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were obtained by Kobayashi et al. for conversion of mix-milled activated carbon and cellulose [28]. These authors concluded that the enhancement of the reaction performance by mix-milling is not just due to the mechano-catalytic hydrolysis during the milling treatment, but the intimate contact of reactant and catalysts, which increases the initial reaction rate under the appropriated conditions (temperature and pressure). Also relevant is that three successive tests were performed using the Ru/HSAG-TPA catalyst, in order to evaluate its stability in the reaction. The catalyst showed excellent stability at least up to three successive runs, without significant changes in the conversion or yield of EG, and no leaching to solution was observed. As shown in Table 5, in the case of activated carbon supported catalysts, the conversions of the mixed ball-milled cellulose were higher than those obtained with the single ball-milling. This result would be expected since during ball-milling the grains (1.25–0.8 mm) of the AC are powdered intensifying the contact of the catalyst with the cellulose. The sorbitol yield reached 46% after mixed ball-milling with RuAC. Nevertheless, the mix-milling pretreatment for RuAC-HPA catalyst did not improve the yields toward sorbitol, EG and PG. This may be because hydrogenation of glucose and hydrogenolysis of sugar alcohols (mannitol, sorbitol) take place at the surface of Ru/C simultaneously, hydrogenation of glucose will be the preferable reaction if the glucose is developed quickly [19]. More water-soluble sugars occupy the active sites of Ru/C as the depolymerization rate of cellulose is accelerated in mixed ball-milled cellulose, which could inhibit hydrogenolysis of hexitols over Ru/C. These factors play an essential role in the selectivity of sugar alcohols [45]. On the contrary, for HSAG supported catalysts, the conversion values after both pretreatments are similar, although the yield value toward EG product is maximum for the mixed ball-milled cellulose with (RuHSAG_TPA)mix.
formed into non analyzed (by HPLC) carbon organic products. Provably these compounds are oligosaccharides as indicated in [19]. This behavior is particularly evident for the Ru free catalysts. Binary catalysts based on tungsten showed the most distinguished performance in terms of high EG selectivity. This result agrees with that recently published by Zhang et al. who attributed the good performance of tungsten-containing HPAs to the bifunctional role of W in accelerating cellulose hydrolysis and CeC bond cleavage [42]. Following the set of reactions gathered at Scheme 1 and according to the literature [27,43], in the process of cellulose conversion to EG, the cellulose first undergoes hydrolysis to form oligosaccharides and glucose. The derived oligosaccharides and sugars are further catalytically degraded to glycolaldehyde by retro-aldol condensation in the presence of tungsten species. Although we cannot precisely describe how the tungsten species interact with oligosaccharides and sugar molecules, it can be concluded that the catalytic breakage of the CeC bond selectively takes place at the position between the α‐β carbons. In the degradation of glucose-based oligosaccharides, glycolaldehyde is produced because the sugars have a terminal aldehyde group. Concerning the role of the hydrogenation catalysts, ruthenium is responsible for transforming the unsaturated C2 and C3 intermediates to EG. The metal sites also promote the hydrolysis of cellulose via heterolytic dissociation of H2 [44]. In contrast, when glucose isomerizes into fructose, C3 molecules will be formed, leading to the 1,2-PG and glycerol formation after a series of subsequent reactions, which was demonstrated in an early publication [43]. On the other hand, according to Kobayashi et al. [28], if cellulose and catalytic materials are simultaneous mix-milled in order to improve the contact between the cellulose and the catalyst, the conversions can be enhanced. Several solid catalysts were tested in hydrolytic hydrogenolysis of single and mix-milled cellulose under the same reaction conditions, and the corresponding results are shown in Fig. 2 and Table 5. As shown in Fig. 2, AC supported mixed catalysts without Ru (Fig. 2b) resulted in 90% conversion in less than 5 min, whereas the separately ball-milled catalysts and cellulose provided less than 40% conversion upon the same time in reaction (Fig. 2a). For catalysts with Ru, either supported on AC (Fig. 2c and d) or on HSAG (Fig. 2e and f), and with the presence or not of HPAs, the conversions after 5 min in reaction are remarkably higher for the ball-milled mixtures of cellulose and catalysts, in comparison with the mixed samples without combined milling. Thus, a great increase in the initial reaction rate by the mixmilling pre-treatment is evidenced. These results indicate that the mix-milling pretreatment accelerates the cellulose transformation, being the cellulose conversion much faster in comparison with single ball-milled cellulose mixtures. Similar results
4. Conclusions Hydrolytic hydrogenolysis of cellulose to polyols in one-pot can be regarded as a new approach for the sustainable production of bulk chemicals from biomass. To accomplish this cascade reaction with selectivity control to a desired product, multi-functional catalysts can be an interesting choice. In the present study two carbon materials are used as supports, three commercial HPA and metallic Ru nanoparticles are combined. Over these composite materials, the hydrolysis of cellulose, with CeC bond cleavage, and several hydrogenation steps can proceed cooperatively. While HPAs are efficient catalysts in the cellulose transformation into sugar intermediates, and subsequently for the selective cleavage of the CeC bonds in these sugars, Ru/C can catalyze the hydrogenation reactions. Using this approach the controllable synthesis of ethylene glycol, propylene glycol or sorbitol from cellulose can be achieved. Tungsten-based HPA catalysts proved to be highly active and selective to ethylene glycol. Activated carbon support seems to catalyze the isomerization of glucose into fructose, which facilitates the way of formation of propylene glycol. The addition of small amounts of Ru increases the yields of sorbitol, ethylene glycol and propylene glycol. The promotional effect of Ru can be interpreted by its excellent activity for the hydrogenation of unsaturated compounds to EG and polyols. As a technical issue, ball-milling during the cellulose pretreatment is a critical parameter in order to increase initial catalytic conversions. Furthermore, the mixed ball-milling of cellulose and solid catalysts accelerates cellulose transformation, the catalytic conversion being much faster. These results would be helpful for rational design of multi-functional catalysts and tuning the reaction parameters for controlling the conversion of cellulose to specific products.
Table 5 Catalytic results for single ball-milling of cellulose and mixed ball-milling of cellulose and solid acid catalysts after 300 min of reaction. Catalyst
X (%)
YSOR (%)
YEG (%)
Y
RuAC RuAC_PMA RuAC_STA RuAC_TPA (RuAC)mix (RuAC_PMA)mix (RuAC_STA)mix (RuAC_TPA)mix RuHSAG RuHSAG_PMA RuHSAG_STA RuHSAG_TPA (RuHSAG)mix (RuHSAG_PMA)mix (RuHSAG_STA)mix (RuHSAG_TPA)mix
86 88 97 98 100 100 100 100 98 100 96 99 93 96 100 92
17 8 12 16 46 6 6 13 12 8 12 12 8 6 6 8
5 8 13 25 5 10 13 20 11 8 12 16 13 10 19 26
2 13 16 10 3 14 15 8 6 8 8 7 4 7 6 6
PG
(%)
Acknowledgments The financial support from the Spanish Government (projects CTQ2014-52956-C3-2-R and CTQ2014-52956-C3-3-R) is recognized. 6
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MAH thanks the MINECO for a FPI predoctoral grant. This work was also financially supported by: project POCI-01-0145-FEDER-006984Associate Laboratory LSRE-LCM funded by FEDER through COMPETE2020 − Programa Operacional Competitividade e Internacionalização (POCI) − and by national funds through FCT − Fundação para a Ciência e a Tecnologia. L.S. Ribeiro acknowledges her Ph.D. scholarship (SFRH/BD/86580/2012) from FCT. References [1] [2] [3] [4] [5] [6] [7]
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