Recycle and extraction: cornerstones for an efficient conversion of

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Recycle and extraction: cornerstones for an efficient conversion of cellulose into 5-hydroxymethylfurfural in ionic liquids Cinzia Chiappe, Maria Jesus Rodriguez Douton, Andrea Mezzetta, Christian S Pomelli, Giulio Assanelli, and Alberto de Angelis ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00875 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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ACS Sustainable Chemistry & Engineering

Recycle and extraction: cornerstones for an efficient conversion of cellulose into 5hydroxymethylfurfural in ionic liquids Cinzia Chiappe,*a Maria J. Rodriguez Douton,a Andrea Mezzetta,a Christian S. Pomelli,a Giulio Assanellib and Alberto R. de Angelis*b. AUTHOR ADDRESS a

Dipartimento di Farmacia, via Bonanno 33, Università di Pisa, 56126 Pisa, Italy.

[email protected] b

ENI Downstream R&D Development, Operations and Technology, S. Donato Milanese (Mi),

Italy. [email protected]. KEYWORDS: cellulose, ionic liquids, HMF, metal catalyst, Brønsted acids.

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]ClCrCl3, [BMIM]Cl-FeCl3, [BMIM]Cl-[MIMC4SO3H][HSO4] and the not yet investigated

ACS Paragon Plus Environment

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[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.

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 5hydroxymethylfurfural (HMF), furfural, 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 have been published reporting more or less efficient conditions to obtain HMF from simple carbohydrates, such as glucose or fructose, or from lignocellulosic biomass. Metal


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chlorides,3-4

or

less

common

organometallic

catalysts,5

dissolved

in

1-butyl-3-

methylimidazolium 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%). Zeolites,8,9 or associations (mixtures?) 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-(4-sulfonic acid)butyl-3-methylimidazolium methylimidazolium

acetate

methylsulfate

([EMIM][OAc]),14

[MIMC4SO3H][CH3SO3] or

the

dual-core

in

sulfonic

1-ethyl-3acid

[bis-

C3SO3HMIM][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 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


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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 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 onepot 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 bio-refinery contexts, including the transformation in new bio-based 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 sideproduct 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.


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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 grams, 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.

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 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 affect 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


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neutralization

reactions:

tetramethylguanidinium

oxalate,

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[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 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 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)

Scheme 1. Tested ionic liquids.


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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 levulinic acid (LA), although in modest yield (20%), was [TMG][HSO4]-H2SO4. Table 1. Cellulose dissolution in ILs (wt%). Cellulose MCC filter paper

[TMG] [OAc] 15 9

[TMG] [HC2O4] 17 11

[TMG] [HSO4] 13 11

[BMIM] [OAc] 16 11

[BMIM]Cl 20 15

Thus, we decided to compare the catalytic activity of two Brønsted acidic ILs ([MIMC4SO3H][HSO4], [MepyrrC4SO3H][HSO4]) and several metal salts (CrCl3, CuCl2, ZnCl2, FeCl3) under similar conditions, including also the not yet investigated titanium(IV) oxysulfate (TiOSO4). Initially, reactions were carried out in [BMIM]Cl (30 g), relatively less expensive than [BMIM][OAc], containing the selected catalyst using microcrystalline cellulose as feedstock (3 g). At the end of the reaction, water was added to the reaction mixture and the formed solid, separated by centrifugation, was washed with water and dried in an oven. This solid is reported in Table 2 as humins. The liquid phase was extracted with ethyl acetate or methylisobutyl ketone (MIBK) and the recovered reaction product was analyzed by NMR. In accordance with literature data,18 MIBK resulted the best solvent for HMF extraction, being able to yield, after addition of a protic solvent to the reaction mixture, a near complete recovery of the desired product (> 90%, determined through blank experiments using HMF and [BMIM]Cl). It is noteworthy that the


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addition of a protic solvent (water) before extraction was a conditio sine qua non to isolate HMF. The strong H-bond between the HMF hydroxyl proton and the IL anion (Cl-) makes indeed impossible, in the absence of a specie able to solvate the IL anion, the extraction of this organic species. Subsequently, for each catalytic system several reaction parameters (temperature, reaction time, catalyst and cellulose load) have been selectively varied (Supplementary Material). In Table 2, the conditions affording the higher yield in HMF are reported for each catalytic system. When using [BMIM]Cl, CrCl3 results the most efficient catalyst, assuring a yield of HMF (as isolated compound), around 80%. This is one of the higher values reported in the literature obtained however after an accurate selection of the reaction conditions showing as these latter can significantly affect the reaction behaviour. Remarkable results (67%, isolated products yield) have been obtained also using FeCl3, even if in this case HMF is not the sole reaction product; acid 4-hydroxymethylfuranoic was indeed isolated together with HMF (7:3, HMF: 4hydroxymethylfuranoic acid (HMFA) ratio). The ability of FeCl3 to catalyze the subsequent oxidation of HMF under the employed reaction conditions appears interesting although, at the moment, behind the scope of our investigation. On the other hand, with the exception of TiOSO4 that allowed to isolate HMF in ca. 40% yield, all the other investigated metal salts (Table 2, entries 1-5) gave generally very low (if any) amounts of HMF. It is also noteworthy that, whereas H3BO3 resulted a very poor catalyst (Table 2 entry 8), stronger acidic ILs were able to give HMF yields up to 59% (Table 2 entries 6,7). On these latter systems, the effect of water dosage was also evaluated and, in particular, when [MIMC4SO3H][HSO4] was used as catalyst, amounts of water comparable to [BMIM]Cl were added (Supplementary Material).


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Finally, the possibility to use combinations of Lewis and Brønsted acids (Table 2, entries 9-14), or of two ILs (Table 2, entry 17), as well as to employ tetramethylguanidinium-based ILs (Table 2, entries 15-16), was tested. Table 2. Cellulose (3 g) degradation in IL (30 g) in the presence of catalyst(s)

1

IL

H2O (g)

Catalyst (wt %)a

T (°C)

Tim e (h)

HMF yield (mol %)b

Humins yield (wt %)

[BMIM]Cl

-----

CrCl3, (1)

120

5

80 ± 3

1±2

c

15 ± 3

2

[BMIM]Cl

-----

FeCl3, (1)

130

4

67± 3

3

[BMIM]Cl

-----

TiOSO4, (1)

130

3

38± 2

32 ± 2

4

[BMIM]Cl

1.5

CuCl2, (1)

125

5

2 ± 0.5

13 ± 1

5

[BMIM]Cl

-----

ZnCl2, (1.2)

130

5

Nd

98 ± 2d

6

[BMIM]Cl

-----

[MIMC4SO3H][HSO4], (2.2)

130

4

59 ± 2

22 ± 3

7

[BMIM]Cl

-----

[MepyrrC4SO3H][HSO4], (2)

130

5

48 ± 2

42 ± 2

8

[BMIM]Cl

-----

H3BO3, (3)

130

7

10± 1

52 ± 3

9

[BMIM]Cl

-----

H3BO3, (2)

CrCl3, (0.8)

120

3

41 ± 3

2± 1

3.7

H2SO4, (1.9)

FeCl3, (2.2)

180

5

Nd

57 ± 5

f

10

[BMIM]Cl

11

[BMIM]Cl

3.2

H2SO4, (1.5)

CrCl3, (1.5)

180

5

Nd

53 ± 5

12

[BMIM]Cl

-----


TiOSO4, (1)

125

5

10 ± 1

96 ± 3

13

[BMIM]Cl

-----

[MIMC4SO3H][HSO4], (2)

CrCl3, (0.8)

130

4

50 ± 3

8±1

14

[BMIM]Cl

1.5


MnCl2, (1) CuCl2, (0.2)

125

5

Nd

62 ± 3

15

[TMG][HSO4]g

3.5

160

7

95% pure.

ACKNOWLEDGMENT Financial support from Eni group is gratefully acknowledged.

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Synopsis


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This paper reports a comparative study on the direct transformation of cellulose into HMF using Lewis and/or Brønsted acids in ILs.


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Recycle and extraction: cornerstones for an efficient conversion of

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