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Cellulose hydrolysis in acidified LiBr molten salt hydrate (MSH) media Weihua Deng, James R Kennedy, George Tsilomelekis, Weiqing Zheng, and Vladimiros Nikolakis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00757 • Publication Date (Web): 24 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015
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Cellulose hydrolysis in acidified LiBr molten salt hydrate (MSH) media Weihua Deng, James R. Kennedy, George Tsilomelekis, Weiqing Zheng, Vladimiros Nikolakis* Catalysis Center for Energy Innovation, Dept. of Chemical & Biomolecular Engineering, University of Delaware, Newark, DE *Email:
[email protected] Abstract We screened nine acidified molten salt hydrates (solutions with water to salt molar ratio equal or less than the coordination number of the cation) as reaction media for cellulose hydrolysis, and we found that cellulose can be efficiently hydrolyzed in LiBr acidified MSH under mild conditions (> 90% yield to water soluble products in 0.05M H2SO4 at 85°C for 30 minutes). The effect of various factors (temperature, acid and initial cellulose concentrations) on the kinetics of hydrolysis reaction was also investigated. At the lowest temperatures examined (70, 85oC) low amounts of degradation products have been observed, and glucose appears to be in equilibrium with its dimers and possibly other oligomers. Higher temperatures (100 - 115 oC) enhanced the formation of degradation products (organic acids and humins). Analysis of the kinetic data indicate that hydrolysis rates are first order in cellulose and in H2SO4 concentration, and the initial hydrolysis rates have an apparent activation energy ~123kJ/mol. X-ray diffraction, SEM and FTIR were also used to study cellulose’s structural/morphological changes upon treatment in the LiBr MSH media, in an attempt to understand the effects of the cellulose – salt interaction. Analysis of the data indicates that the enhancement of the hydrolysis rates can be attributed to the enhancement of the acidity of reaction media through synergistic effect of dilute acid and
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MSH, the breaking of crystalline structure through swelling, and the interaction of the salt with cellulose chains affecting the conformation and flexibility of the glycosidic bonds. Keywords: vibrational spectroscopy, Raman, FTIR, biomass, green chemicals, biofuels 1. Introduction The desire to decrease our dependence on fossil fuels and the need to minimize greenhouse gas emissions have motivated research for the development of new, alternative and sustainable technologies to produce energy, fuels and valuable chemicals1-6. In this respect, non-edible lignocellulosic biomass is a promising renewable feedstock since it is abundant, does not directly compete with the food chain, and can lead to nearly carbon-free processes with concomitant reduction in CO2 emissions. Furthermore, it has been estimated that the potentially available quantities of lignocellulosic materials in the US are of the same order of magnitude as the annual crude oil demands7, thus it has the potential to replace a significant fraction of petroleum based fuels and chemicals8. As a result, the development of efficient processes for cellulose hydrolysis has attracted a lot of attention9-12 so that it could become a source of valuable glucose. Cellulose is a homopolysacharide of β-D-anhydro-glucopyranose units (AGU) connected through β-(1-4) glycosidic linkages and it could be found in several crystal polymorphs or as amorphous solid. The existence of an extensive intra- and inter- chain hydrogen bond network among cellulose chains could explain its chemical stability and low solubility in water. A schematic of the hydrogen bond network of type I cellulose (which has been used in this study) is depicted in Scheme 1. Type I cellulose consist of planar sheets of hydrogen bonded cellulose chains, stacked together via van der Waals forces13. Cellulose hydrolysis to glucose (ignoring the end groups of each chain) can be described with the following reaction: + → Several processes have been developed for non-enzymatic cellulose hydrolysis since the early 1920’s. In many cases, the Brønsted acidity that catalyzes the cleavage of the β-(1-4) glycosidic bonds is provided by dilute, concentrated or even gaseous inorganic acids such as HCl, H2SO4, 2 ACS Paragon Plus Environment
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HF etc.14,
15
. Depending on the acid concentration a broad range of temperatures has been
employed (form very high (>170oC) to as low as room temperature). However, all these processes suffer from major disadvantages such as high energy consumption16, reactor corrosion17, catalyst recycling and separation issues as well as formation of degradation products (e.g. humins) especially under severe conditions (either at high temperature or with concentrated acids). As a result, the development of efficient cellulose hydrolysis procedures is still a challenge. Solubilization of cellulose in solvents such as N-methylmorpholine- N-oxide (NMNO), Cadoxen18 and ionic liquids19-21, are other alternatives that have attracted the interest of the scientific community. Solid acids have also been tested22 in this respect, however technical challenges, such as low activity and catalyst deactivation as well as maximizing the contact surface area between the two solids still need to be overcome.
Scheme 1: Schematic of inter- and intra- chain hydrogen bond network in type I cellulose The utilization of acidified inorganic molten salt hydrates (AMSH) as cellulose solvents/ modifiers is known to promote cellulose hydrolysis under mild conditions (low acid concentration / temperature) and could help overcome the aforementioned impediments and constraints (for a summary of previous studies see Table 1). Molten salt hydrates have a water to salt molar ratio equal or less than the coordination number of the cation (i.e. all the water molecules are in the inner coordination sphere of the cation), and as a result they are typically
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highly concentrated brines (Csalt> ~50wt%). Depending on the properties of the constituent ions different salts might dissolve, swell, decompose or have no effect on cellulose.23, 24 For example, salts of small hard cations with soft polarizable anions are typically good cellulose solvents. The ability to dissolve cellulose mainly depends on the acidity of the salt, the water content of the melts as well as the structure of the cation coordination sphere. Whereas much effort has been invested in understanding the ability of the molten salt hydrates in swelling and/or dissolving cellulose
23-25
, only few studies investigated the enhancement of cellulose hydrolysis in molten
salt hydrates26-36. Table 1 summarizes representative experimental conditions as well as the glucose yield of the reported studies. Concentrated metal halide solutions with cations such as Li+, Na+ and Zn2+ and halide anions such as Cl- and Br- have typically shown the highest enhancement effects when used as reaction media. The formation of a zinc-cellulose complex during the pretreatment procedure has been claimed to be the key factor for making cellulose more susceptible to hydrolysis in the case of ZnCl232,
33
. Recently, Pan et al. reported34 that
concentrated LiBr solutions are capable of converting biomass to either a mixture of furan adducts (in LiBr/acetone media) or a pentose / hexose sugar mixture (in aqueous media) with high yields. As reported by Almeida et al.
37
, apart from glucose, important chemical
intermediates such as isosorbide can be produced directly from cellulose in a two-step one pot approach by combining hydrolysis in molten ZnCl2 hydrate and hydrogenation over Ru/C catalyst. It must be emphasized that the role of the salt in the molten salt hydrates is different from that in dilute solutions where it primarily exhibits catalytic activity in reactions relevant to biomass conversion. For example, dilute solutions of metal chlorides in water are known to catalyze hexose and pentose isomerization reactions.38-45 Furthermore, dilute solutions of metal chlorides in ionic liquids appear to be active for the hydrolysis of the 1-4 glycosidic bonds.46 The potential of using molten salt hydrates for treating real biomass feedstock (albeit several challenges that need to be faced) has been demonstrated in several publications.26, 34, 36 However, the development of an efficient process requires addressing issues related to utilization of the hydrolyzate in the downstream processes as well as identifying efficient ways for catalyst and 4 ACS Paragon Plus Environment
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molten salt hydrate recycling. In this respect processes like salt extraction in organic solvents, extraction of sugars using boronic acid, carbohydrate precipitation by the addition of an antisolvent are some methods that have been examined.34 Furthermore, the use of certain molten salt hydrates could completely break the azeotrope of water and HCl and could facilitate the recovery of a volatile homogeneous acid from the hydrolyzate mixture.26 Another possible approach is sugar upgrading to other valuable chemicals in the molten salt hydrate solution or to chemicals that can easily be removed from the solvent. For example, a process that involves polysaccharide hydrolysis in an acidified molten salt hydrate, followed by hydrogenation of sugars to polyols followed by their dehydration to a various anhydropolyols has recently been disclosed.36 It is commonly accepted that structural changes of cellulose due to swelling or dissolution play an important role on the enhancement of the hydrolysis rates. However, this argument alone cannot explain the differences in activity when different molten salt hydrates, that swell or dissolve cellulose, are used. Differences in the cellulose conformation in different molten salt hydrate media, some of which are easier to hydrolyze, is one possible explanation. The effect of molten salt hydrate on the solution acidity is another possible contribution that has not been systematically explored. In this manuscript we try to elucidate the roles of the molten salt hydrates in cellulose hydrolysis. LiBr MSH has been used as a representative example based on screening of nine molten salt hydrates. In particular, we study the effects of temperature, water/salt molar ratio and acid concentration on hydrolysis kinetics. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) have been used to identify the effect of treatment with LiBr MSH to particle crystallinity and morphology. FTIR has been extensively used in the past to gain insights into the chemical functionality, crystalline transformation47 and hydrogen bond network48 of different cellulose structures or to in-situ monitor the glycosidic bond cleavage49, and is also used here as a tool that can help us understand the interaction of cellulose with the LiBr MSH.
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Table 1. Cellulose hydrolysis in acidified molten salt hydrate media T [oC]
Time [min]
Glucose yield Reference
5-6
125
20-30
Quantitative conversion to glucose
31
25-40
15-25
60-90
10-20 min
>85
26
HCl
6M
N/A
90
4
N/A
26
6M
HCl
7M
N/A
90
4
N/A
26
LiCl
Saturated solution
HCl
1-4 M
~10
100
1-5
30-100
27
LiCl
1M to Saturated solution
HCl
4M
~10
100
9-45
18-100
27
LiBr
45
HCl
0.25-2
7-15
120-140
1-2 hours
approx. 90% to furan adducts
50
LiBr
60
HCl
3
15-60
42 wt%**
34
ZnCl2
60-70
HCl
0.1% w/v
85
20hr
91.5%
32
ZnCl2
75
HCl
0.5M ZnCl2/cel lulose =18 (w/w)
70
2 hr
99.5% soluble sugars
33
NaCl
0.05M
HCl
0.01M
N/A
170-200 microwa ve
Several minutes
80%
28
NaCl
30
oxalic and maleic acids
0.1M
20g l-1 αcellulose
105
2 hours
0.18g l-1 soluble oligomers
29
NaCl
20
NafionSAC 13
2-3
3-5
170-200
5 days
70% levulinic acid
30
Salt*
Csalt [wt%]
Acid
CaCl2
55
HCl
0.1
CaCl2
5-15
HCl
CaCl2
1.64M
LiCl
Cacid Ccellulose [wt%] [wt%]
