Recycle and Extraction: Cornerstones for an Efficient Conversion of

Apr 14, 2017 - Recycle and Extraction: Cornerstones for an Efficient Conversion of Cellulose into 5-Hydroxymethylfurfural in Ionic Liquids. Cinzia Chi...
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Research Article pubs.acs.org/journal/ascecg

Recycle and Extraction: Cornerstones for an Efficient Conversion of Cellulose into 5‑Hydroxymethylfurfural in Ionic Liquids Cinzia Chiappe,*,† Maria J. Rodriguez Douton,† Andrea Mezzetta,† Christian S. Pomelli,† Giulio Assanelli,‡ and Alberto R. de Angelis*,‡ †

Dipartimento di Farmacia, Università di Pisa, via Bonanno 33, 56126 Pisa, Italy ENI Downstream R&D Development, Operations and Technology, 20097 S. Donato Milanese, Milan, Italy

‡

S Supporting Information *

ABSTRACT: Hydrolysis and subsequent degradation of microcrystalline cellulose in five ionic liquids (ILs) using metal salts and/or Brønsted acids as catalysts allowed for the direct access to 5-hydroxymethylfurfural (HMF), an important renewable biofuel precursor and a useful building block. For each catalytic system, several reaction parameters (temperature, reaction time, catalyst, and cellulose loading) have been selectively changed. Four systems ([BMIM]Cl-CrCl 3 , [BMIM]Cl-FeCl3, [BMIM]Cl-[MIMC4SO3H][HSO4] and the not yet investigated [BMIM]Cl-TiOSO4) were found to be effective for cellulose degradation into HMF. The extraction of HMF from the reaction media represents however the weak point of all these processes being able to affect negatively both HMF recovery and IL recyclability. The critical step which causes the drastic decrease in HMF yield starting from the first recycle has been clearly identified. Furthermore, the possibility to use TiOSO4 as a sustainable and robust catalyst for the conversion of saccharides (or polysaccharides) in HMF has been shown. The present study could open new perspectives for the one-pot synthesis of HMF starting from cellulose and/or other sugars. KEYWORDS: Cellulose, Ionic liquids, HMF, Metal catalyst, Brønsted acids

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Zeolites8,9 or associations of mineral acids, metal chlorides, and boronic acids have been tested as well as systems able to transform cellulose in HMF.10 The use of Brønsted acidic ILs, as single catalyst or in synergy with Lewis acids, represents another area of intensive investigation: for example, 1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogensulfate [MIMC4SO3H][HSO4] in association with chloride metal salts (FeCl2, MnCl2) or Co(SO4) has been employed11−13 by Tao et al. to obtain HMF and furfural from cellulose (yield ca. 45%). Inspired by these dual catalytic systems, other research groups obtained HMF in yields up to 69.7% starting from cellulose, using CuCl2 and 1-(4sulfonic acid)butyl-3-methylimidazolium methylsulfate [MIMC4SO3H][CH3SO3] in 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]),14 or the dual-core sulfonic acid [bisC3SO3HMIM][CH3SO3] and MnCl2 in [BMIM]Cl.15 Li et al. explored a different approach, which avoids the use of ILs as solvent, and isolated HMF in a 45.3% yield from cellulose by employing 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogensulfate [MIMC3SO3H][HSO4] and InCl3 as catalysts in DMSO.16 The immobilization of Cr3+ ions with SO3H functionalized solid polymeric ILs allowed instead for obtaining

INTRODUCTION The inability of common organic solvents to dissolve natural cellulose under mild conditions has significantly delayed the development of large-scale applications of this abundant biopolymer. The turning point was probably reached in the early 2000s when Swatloski et al. reported1 the ability of a chloride based IL to dissolve significant amounts of cellulose at moderate temperatures. Subsequently, many efforts have been devoted to find the best experimental conditions and the most suitable ILs not only for cellulose solubilization but also for its transformation, in the presence of suitable catalysts, into useful biofuels and chemicals, like 5-hydroxymethylfurfural (HMF), furfural, and levulinic and formic acids. The essential step of this challenging process is generally considered to be the selective conversion of the pyranose ring of hexose sugars, derived from cellulose hydrolysis, into furans. In 2007, Zhao et al. showed the possibility to convert efficiently glucose in HMF using metal chlorides in ILs.2 Later, a plethora of papers has been published reporting more or less efficient conditions to obtain HMF from simple carbohydrates, such as glucose or fructose, or from lignocellulosic biomass. Metal chlorides3,4 or less common organometallic catalysts,5 dissolved in 1-butyl-3methylimidazolium chloride ([BMIM]Cl), in ammonium-based ILs,6 or in mixed systems (such as DMSO-[BMIM]Cl, 10 wt %),7 represent the most investigated conditions to obtain HMF from sugars or cellulose in moderate to-good yields (45−60%). © 2017 American Chemical Society

Received: March 22, 2017 Revised: April 12, 2017 Published: April 14, 2017 5529

DOI: 10.1021/acssuschemeng.7b00875 ACS Sustainable Chem. Eng. 2017, 5, 5529−5536

Research Article

ACS Sustainable Chemistry & Engineering

physical chemical properties of the medium (i.e., like viscosity) and to modify the anion’s ability to act as hydrogen bond acceptor. On the other hand, anion and cation structure determine also the IL cost, a parameter that significantly affects the possibility to make of an IL-based biomass (pre)-treatment process a practical reality.29,30 Therefore, cellulose dissolution tests were initially carried out in three less expensive protic Brønsted acidic tetramethylguanidinium based-ILs arising from simple neutralization reactions: tetramethylguanidinium oxalate, [TMG][HC2O4], tetramethylguanidinium acetate [TMG][OAc], and tetramethylguanidinium hydrogensulfate [TMG][HSO4]. Furthermore, [BMIM]Cl and [BMIM][OAc], whose ability to dissolve cellulose is well-known, were included as reference systems (Scheme 1). For these experiments, two

HMF from cellulose in lower yield (31%) also using the DMSO-[BMIM]Cl mixture as reaction medium.17 Paired metal chlorides, such as CuCl2/CrCl2 or CuCl2/ PdCl2, in [EMIM]Cl have been extensively used by Su et al. to transform cellulose in HMF: a 57.5% yield was reached after the fine-tuning of the metal salts ratio.18,19 Comparable results have been obtained also using CrCl2 and RuCl3 in the same reaction medium (HMF, 60% yield from cellulose).20 More recently the effective combination of microwave irradiation (MI), ILs, and metal salts has been proposed21 by the Zhang and Zhao group, which has also confirmed the possibility of using solid acid catalysts, such as H-form zeolites with a low Si/Al molar ratio.22 Nonetheless, more “peculiar systems”, based on metal chloride catalysts in N,N-dimethylacetamide and lithium chloride (DMAc-LiCl), with or without [BMIM]Cl or other ILs, have been investigated.23,24 As claimed by Rinder and Raynes, DMAc-LiCl can actually act as an efficient solvent for the one-pot conversion of cellulose and lignocellulosic biomass into HMF.25 On the other hand, a 54% yield of HMF has been obtained from cellulose using CrCl2 and HCl as catalysts and [EMIM]Cl as additive, a yield that is moreover reasonably close to the 53% obtained when [EMIM] Cl was used as solvent, under similar reaction conditions. Unfortunately, despite the large interest raised by the potential use of ILs in several biorefinery contexts, including the transformation in new biobased materials,26 and the wide variety of catalysts employed under different reaction conditions, many challenges in achieving industrially affordable processes remain.27 Several aspects, important for large scale processes, have been indeed only marginally investigated. The literature dealing with this issue is principally focused on small scale laboratory experiments, finalized to select reaction conditions by which side-product formation is avoided, disregarding product separation and purification: the yield of HMF has been generally determined by HPLC, analyzing the crude IL reaction mixtures. Unfortunately, HMF recovery from ILs is not a trivial issue.28 Moreover, most of the above-cited papers do not address the important topic of the effective recyclability of the employed systems. ILs are today quite expensive solvents, and their recyclability is an inalienable condition for application in biomass processing to reduce costs and increase sustainability.27,29 In this paper, we report a comparative study on the direct transformation of cellulose into HMF using metal salts and/or Brønsted acids in ILs. Furthermore, working with amounts of chemicals up to 100 g, we tried to set our study far away from the standard laboratory experiments (usually carried out at small scale level, i.e. 1 g). Problems related to product isolation and solvent recyclability have been extensively investigated showing clearly the principal factors that negatively affect the different steps of the process. Finally, the potential of TiOSO4 as catalyst has been discussed considering starting material structure and catalytic medium recyclability.

Scheme 1. Tested Ionic Liquids

types of cellulose, namely microcrystalline (MCC) and filter paper cellulose were used. Furthermore, since the three tetramethylguanidinium based-ILs are solid at room temperature, dissolution experiments were carried out at a relatively high temperature (130 °C), sometimes in the presence of a small amount of water (5%) to decrease IL melting point, although this latter solvent is normally used to favor cellulose precipitation from ILs solutions (Table 1). Table 1. Cellulose Dissolution in ILs (wt%)

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RESULTS AND DISCUSSION It is well-known that IL anion structure plays a critical role in cellulose dissolution. Anions able to act as hydrogen bond acceptors (i.e., chloride, acetate, dimethylphosphate) favor indeed the process due to their ability to intercalate themselves among the polysaccharide chains, through the formation of hydrogen bonds with the hydroxy groups of cellulose.27 However, cation structure can also affect the dissolution process; its role is mainly due to the ability to modulate the

cellulose

[TMG] [OAc]

[TMG] [HC2O4]

[TMG] [HSO4]

[BMIM] [OAc]

[BMIM]Cl

MCC filter paper

15 9

17 11

13 11

16 11

20 15

All five investigated ILs were able to dissolve at least 10 wt % of cellulose. Therefore, solutions containing this amount of MCC were employed to carry out hydrolysis tests simply adding to these solutions an excess (30 mol %) of the mineral acid corresponding to the IL anion and maintaining the resulting mixtures at 130−135 °C under stirring for 24 h. Unfortunately, the only system able to degrade cellulose to 5530

DOI: 10.1021/acssuschemeng.7b00875 ACS Sustainable Chem. Eng. 2017, 5, 5529−5536

Research Article

ACS Sustainable Chemistry & Engineering Table 2. Cellulose (3 g) Degradation in IL (30 g) in the Presence of Catalyst(s) IL 1 2 3 4 5 6 7 8 9 10 11 12

[BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Cl [BMIM]Clf [BMIM]Cl [BMIM]Cl

13 14

[BMIM]Cl [BMIM]Cl

15 16 17

[TMG][HSO4]g [TMG][HSO4] [BMIM]Cl/[TMG]Cl 2:1

H2O (g)

1.5

3.7 3.2

1.5 3.5 17.5

catalyst (wt %)a CrCl3, (1) FeCl3, (1) TiOSO4, (1) CuCl2, (1) ZnCl2, (1.2) [MIMC4SO3H][HSO4], (2.2) [MepyrrC4SO3H][HSO4], (2) H3BO3, (3) H3BO3, (2) CrCl3, (0.8) H2SO4, (1.9) FeCl3, (2.2) H2SO4, (1.5) CrCl3, (1.5) [MIMC4SO3H][HSO4], TiOSO4, (1) (1.7) CrCl3, (0.8) [MIMC4SO3H][HSO4], (2) [MIMC4SO3H][HSO4], MnCl2, (1) CuCl2, (0.2) (1.7) FeSO4.7H2O, (0.8) [MIMC4SO3H][HSO4], (2.8) CrCl3, (1)

T (°C)

time (h)

HMF yield (mol %)b

120 130 130 125 130 130 130 130 120 180 180 125

5 4 3 5 5 4 5 7 3 5 5 5

80 ± 3 67 ± 3c 38 ± 2 2 ± 0.5 Nd 59 ± 2 48 ± 2 10 ± 1 41 ± 3 Nd Nd 10 ± 1

1 15 32 13 98 22 42 52 2 57 53 96

130 125

4 5

50 ± 3 Nd

8±1 62 ± 3

160 165 130

7 7 4

95% pure (yield reported in Table 2 and Supplementary Table 1, NMR in the Supporting Information). Experiments were carried out at least in triplicate. The yield of HMF was calculated using the reported15 equation:

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CONCLUSIONS In conclusion, we have shown that the efficiency of the direct transformation of cellulose in HMF is determined not only by the IL structure and catalyst feature but also by the product extraction procedure. This step is not only indeed difficult and tedious but it is able to affect negatively the IL recyclability. Of course, these problems cannot be evidenced when the study is carried out avoiding the separation of the products. Despite the fact that on the basis of the data arising from the first cycle CrCl3 remains the best catalyst for this process, the less sensitivity of TiOSO4 to the presence of residual water, its activity can indeed be easily restored by contact with carbon black, suggesting a higher “robustness”, makes this salt a possible alternative. On the other hand, TiOSO4 shows a comparable activity to CrCl3 when sucrose is used as substrate (70% vs 82%) and even higher in the case of fructose (92% vs 65%), also when more times recycled [BMIM]Cl-catalyst mixtures were used. TiOSO4 could therefore become a more sustainable alternative to many other metal salts in conversion of saccharides (or polysaccharides) in HMF.

HMF yield (%) = ([mHMF(g)/MWHMF] /[mcell (g)/(MWgluc − MWH2O)])× 100 Stability Test. The catalytic system [BMIM]Cl-CrCl3 and the pure [BMIM]Cl have been subjected to a long time stability test. 5.00 g of [BMIM]Cl or 5.00 g of [BMIM]Cl containing 0.05 g of CrCl3 was heated at 140 °C continuously for 80 days, in a closed reaction vessel. After this time, IR and NMR (when possible) of both liquids were identical to those of starting materials. Recycling Experiments. After HMF extraction, the IL mixture was carefully evaporated to remove any residual volatile compound (i.e., residual extraction solvent, added water and eventually formed volatile byproducts). Thus, the recovered liquid phases were used again for cellulose transformation: cellulose was added, and the mixture was maintained at 120 °C for 5 h in the case of [BMIM]ClCrCl3, at 130 °C for 3 h for [BMIM]Cl-TiOSO4, and at 130 °C for 4 h for [BMIM]Cl-[MIMC4SO3H][HSO4]. Formed products (humins and HMF) were recovered and analyzed as reported above. Experiments were carried out in triplicate. Catalyst Activity Assessment. After the cycle no. 8, to verify the catalyst activity of the recovered IL mixtures, after evaporation of the residual solvents, the brown liquids were used as reaction media and catalysts for transformation of glucose, fructose, cellobiose, and sucrose (10% w/w). Reaction mixtures were maintained at 120 °C for 5 h in the case of [BMIM]Cl-CrCl3, at 130 °C for 3 h for [BMIM]ClTiOSO4, and at 130 °C for 4 h for [BMIM]Cl-[MIMC4SO3H][HSO4]. Water was added to the reaction mixture, and the formed solid, separated by centrifugation, was washed with water. This solid was dried in an oven, 12 h at 110 °C, weighed, and analyzed by IR. The liquid phase was extracted with MIBK (5 × 10 mL). From the collected organic phases, dried over MgSO4, after removal of the solvent, the crude product was weighed and analyzed by NMR (Table 3). Experiments were carried out in triplicate. Determination of the Ability of [BMIM]Cl Arising from a Previous Cellulose Dissolution/Recovery Process to Dissolution and Transformation Cellulose. MCC (3 g) was dissolved in [bmim]Cl (30 g), and after 4 h at 130 °C at the resulting solution 10 mL of an antisolvent (water, ethanol, acetone-acetonitrile 1:3) was added. Precipitated cellulose was removed by centrifugation, and to the recovered ILs, after accurate removal of the molecular solvent by evaporation at reduced pressure, the appropriate amount of MCC (10% w/w) was added, and the mixture was heated at 90 °C to dissolve cellulose. To the resulting inhomogeneous mixtures (the previous treatment with water or ethanol drastically reduced the ability of the recovered IL to dissolve cellulose) the catalyst (CrCl3) was added, and the mixtures were maintained at 130 °C for 4 h. Thus, water was added, and the formed solid, separated by centrifugation,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00875. Table S1, cellulose (3 g) degradation in IL (30 g) in the presence of catalyst(s); Figures S1−S4, 1H NMR spectra of the crude extraction mixtures showing the presence of HMF, >95% pure (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Cinzia Chiappe: 0000-0001-6615-908X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Eni group is gratefully acknowledged.

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