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Why only Ionic Liquids with Unsaturated Heterocyclic Cations can dissolve Cellulose: A Simulation Study Yao Li, Xiaomin Liu, Yaqin Zhang, Kun Jiang, Jianji Wang, and Suo-Jiang Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00073 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017
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Why only Ionic Liquids with Unsaturated Heterocyclic Cations can dissolve Cellulose: A Simulation Study Yao Lia,b, Xiaomin Liu*,a, Yaqin Zhanga,b, Kun Jianga,b, Jianji Wangc, and Suojiang Zhang*,a a Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 1 North Second Street, Zhongguancun, Beijing, 100190, China. b College of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19 A Yuquan Road, Beijing 100049, P.R. China. c Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reactions, Henan Normal University, 46# East of Construction Road, Xinxiang, Henan 453007, P. R. China *
[email protected],
[email protected] Abstract In recent years, ionic liquids (ILs) have become a promising solvent for cellulose pretreatment in biorefinery. However, almost all the ILs that can dissolve cellulose have an unsaturated heterocyclic cationic structure, while the ILs with cations of saturated ring can hardly dissolve cellulose. To reveal the underlying mechanism, 4 kinds of ILs composed of unsaturated and saturated cations (1-butyl-3-methylimidazolium,
1-butylpyridinium,
1-butyl-1-methylpyrrolidinium,
and
1-butyl-1-methylpiperidinium) and acetate anion were explored as the solvents for a cellulose bunch by molecular dynamics simulation. It is shown that cellulose bunch was only dissolved in the ILs containing unsaturated heterocyclic ring of cations due to two aspects. One is the structure factor: the π electron delocalization of unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds with cellulose. The other is the dynamic effect: the larger volume of cations with saturated heterocyclic ring result in a slow transfer of both cations and anions, which is not beneficial to the dissolution of cellulose. Keywords: cellulose, ionic liquids, hydrogen bonds, electrostatic interaction, diffusion coefficient
1. Introduction In the past few years, green and renewable energy sources have been developed rapidly to face the global challenge of environment and energy crisis. Cellulose is one of the richest biopolymers in the world and it is regarded as the most promising resource for biofuel and biochemicals1.
Consisting
of
repeated
glucopyranose
units
linked
by
covalent
β-1,4-glycosidic bonds, cellulose is a linear condensation polymer with degrees of
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polymerization (DP) from 100 to 200002-3. The polymers (usually 36 chains) are stacked together to form a highly ordered structure called cellulose microfibril4, in which there are intensive intra and interchain hydrogen bonds (H-bonds) network5. The highly ordered structure along with the H-bonds network makes cellulose recalcitrant to dissolve in water and common organic solvents6, which hampers the efficient utilization of the abundant material. Consisting of organic cations and smaller inorganic or organic anions, ionic liquids (ILs) have been widely investigated these years. Their unique properties such as negligible vapor pressure, low melting point and high thermal stability7-8 make ILs possible to be greener alternatives to traditional volatile organic solvents. In 2002, Swatloskis and co-workers found that several ILs are capable of dissolving cellulose under mild conditions9-10. Nowadays the IL process has become one of the most promising biomass pretreatment techniques. However, some drawbacks still exist such as high viscosity and high cost of ILs as well as high energy consumption in their recycle11-12. Thus it is in urgent need to understand the dissolving process and the exact role of cations and anions in the interaction with cellulose13 in order to design cheaper and less viscous IL solvents for cellulose dissolution. Solubility tests14-15, NMR studies16-18 and computer simulations19-24 have been widely carried out to improve our knowledge of cellulose dissolution in ILs. It is recognized that H-bonds between anions and hydroxyl groups of cellulose are most important for cellulose dissolution, and the stronger the anions basicity, the greater the solubility of cellulose in ILs25. Molecular dynamics (MD) simulations of crystalline cellulose dissolution in ILs also revealed the underlying dissolving mechanism26-29. While how anions interact with cellulose is easier to study from both experiments and simulations, the role of cations is still unclear. Some researchers regarded cations as a supportive participator because their had a slight dependence on carbohydrate content
16-17
13
C NMR only
. Some simulation results19,
21, 28
suggested that the interactions between cations and cellulose were mainly hydrophobic interactions. However, supported by NMR results14,
30-31
, some people held the opposite
opinion that cations are also important for the dissolution. In addition, there is another important factor that has been neglected in the mechanism studies of cellulose dissolution by ILs. Table 1 collects some solubility data of cellulose in several ILs. In experiments12, 25 nearly all the ILs that could dissolve cellulose possess an unsaturated heterocyclic structure in their cations, such as imidazolium and pyridinium, while those ILs with the saturated non-aromatic cyclic structure in their cations, such as pyrrolidinium, piperidinium and cholinium, failed to dissolve cellulose, even paired with the most efficient anions chloride and acetate. This implies that the unsaturated heterocyclic
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structure of cations is a prerequisite factor for the dissolution of cellulose. Up to now, there is no clear explanation for these experimental results despite of their importance. To solve this problem, the structures of ILs and cellulose and ILs-cellulose interactions should be revealed. Computational chemistry techniques like MD simulation provide the space-time scale inaccessible by experiments, which may be possible to elucidate the underlying reasons. Table 1. Cellulose solubility in several common seen ILs. The data are taken from two review articles12, 25. Entry
ILs
1
Structure of ILs
Cellulose type
Solubility(g/mol)
[Emim]Ac
Avicel
25.5
2
[Bmim]Ac
Avicel
23.8
3
[MM(EtOH)NH]Ac
Avicel
