DOSY NMR: A Versatile Analytical Chromatographic Tool for

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DOSY NMR: A Versatile Analytical Chromatographic Tool for Lignocellulosic Biomass Conversion Wenzhi Ge, Jennifer Hongyang Zhang, Christian Marcus Pedersen, Tingting Zhao, Fen Yue, Chunyan Chen, Pengfei Wang, Yingxiong Wang, and Yan Qiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01259 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Sustainable Chemistry & Engineering

DOSY NMR: A Versatile Analytical Chromatographic Tool for Lignocellulosic Biomass Conversion Wenzhi Ge,a,b Jennifer Hongyang Zhang,c Christian Marcus Pedersen,d Tingting Zhao,a Fen Yue,a Chunyan Chen,e Pengfei Wang,a Yingxiong Wang,* e Yan Qiao * a

a

Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of

Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China b

University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing

100049, People's Republic of China c

School of Chemistry, University of Edinburgh, Joseph Black Building, David

Brewster Road, Edinburgh, Scotland EH9 3FJ d

Department of Chemistry, University of Copenhagen, Universitetsparken 5,

DK-2100 Copenhagen, Denmark e

Shanxi Engineering Research Center of Biorefinery, Institute of Coal Chemistry,

Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People's Republic of China

Corresponding Author E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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Keywords:

NMR,

DOSY,

Mixture

analysis,

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Lignocellulosic

Biomass,

Biorefinery

ABSTRACT: The diffusion ordered NMR spectroscopy (DOSY) protocol for the analysis of reaction mixture of lignocellulosic biomass conversion has been developed and

investigated

systematically.

Model

reaction

mixtures

from

cellulose,

hemicellulose and lignin conversion, real reaction mixtures of sucrose and glucose dehydration, were facilely separated and assigned in the diffusion dimension without any prior separation or isolation. The shift reagent, EuFOD, was successfully utilized to increase the difference in diffusion and thereby resolution in lignin degradation model. DOSY NMR offers an easy and robust method for the structure identification and reaction mixture separation in biomass conversion.

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Introduction

With the concern about the energy crisis and environmental problems, there is an urgent demand for the sustainable bioresource for chemicals and liquid fuels.1 Lignocellulosic biomass, chitin biomass and algae biomass have been attracted a prevalent interest recently.2-6 Among these biomasses, lignocellulosic biomass is the most abundant resource, and represents the most promising candidate to produce fuels.7 Moreover, lignocellulosic biomass is an excellent alternative for production of valuable chemicals (especially organic molecules with various oxygen-containing functional groups) through catalytic conversion. Thus, converting lignocellulosic biomass has attracted much attention in recent years both in academia and industry.

Various catalytic chemical conversions are involved in lignocellulosic biomass biorefinery. For example, cellulose, which is the major component (40~55%) of lignocellulosic biomass feedstock by weight, can be hydrolyzed into glucose under appropriate catalytic condition.8 Glucose and other C6 sugars, such as fructose, can be further dehydrated to yield chemicals, such as 5-hydroxymethylfurfural (5-HMF), levulinic acid (LA) and formic acid (FA), which then serve as platform molecules for further organic synthesis. As an example, 5-HMF and LA can be converted into value added

chemicals

and/or

liquid

fuels,

such

as

2,5-diformylfuran

(DFF),

2,5-dimethylfuran (DMF), 2,5-furandicarboxylic acid (FDCA) and γ-valerolactone (GVL).9 The hemicellulose fraction, accounting for 15~50% of the lignocellulosic biomass by weight,10 can be upgraded to furfural and its derivatives such as furfuryl

3



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alcohol or furoic acid.11-13 Lignin, which makes up 14~40% of the lignocellulosic biomass by weight,14 is rich in aromatic functionality.15-16 Lignin has received great attention as a sustainable precursor for basic aromatic building blocks, especially phenols, which are currently obtained from fossil-based feedstock.

For the biomass conversion, these reactions mentioned above could produce various products, which will results in complicated analysis. Currently this has been overcomed by using various separation and characterization methods such as high performance liquid chromatography (HPLC)17-18 and gas chromatography (GC).19-20 However, because of the chemical heterogeneity of biomass conversion products, the chromatograms are complicated, and numerous analytical standards and expensive instrumentation is required for proper interpretation. As an example in reactant analysis of mono- and oligosaccharides, the Shodex sugar column has been used in connection with refractive index detector. While for the product analysis and yield calculation of 5-HMF and furfural, a C18 analytical HPLC column in combination with an ultra-violet (UV) detector is required.21-22 Despite the use of specialized purification methods and detection, this setup of analytical tools for 5-HMF gives a poorer performance for separating other products, such as the organic acid LA and FA. Thus, ionic chromatography and/or derivatization prior to analysis are often needed for these rehydration products of the 5-HMF. Therefore, it is crucial for the development of the field to improve the analytic methods for lignocellulosic biomass conversion.

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Nuclear magnetic resonance (NMR) is a well-established analytical method used routinely in all branches of chemical research. Indeed, the NMR approach can provide crucial information on structures and reaction mechanisms of biomass and intermediates, as well as quantify the final product yield.23-25 Advanced in situ NMR has been exploited to monitor the biomass conversion processes and used to acquire molecular level information under the real reaction conditions. Our research group has previously demonstrated its power in the biorefinery by the utilization of NMR to study the degradation of lignocellulosic biomass, inulin biomass and chitin biomass.26-29 Unfortunately, the commonly employed one dimensional (1D) NMR, especially 1H NMR, is limited for complex mixture analysis, because of the spectral complexity, narrow spectra width and closely spaced signals, which commonly causes the signals to overlap. Therefore, extensive utilization of the 1D NMR methods in biomass conversion study is often confounded by unclear interpretation.

Recently, diffusion ordered spectroscopy (DOSY), a pseudo two-dimensional (2D) NMR, has been introduced to overcome the analytical limitation of 1D NMR.30-32 This technique aims to measure diffusion coefficients (D), which is closely related with the molecular weight and shape according to the Stokes-Einstein equation,33

D=

݇ܶ 6ߨߟ‫ݎ‬௦

in which k is the Boltzmann constant, T is the temperature, η is the viscosity of

5



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the liquid, and rs is the hydrodynamic radius of the molecule. Thus, the NMR spectra of individual components of the complex mixtures could be resolved based on their molecular weights and diffusion properties. The successful separation of signals in diffusion dimension could provide an interesting and powerful analytical tool to identify the components of a complex sample mixture, and sometimes even obtain their aggregation status in solution.34-36 Presently, DOSY technique has been used for complex and heterogeneous mixtures analysis, such as supermolecular samples,37 food chemistry.38 It has also been applied for measuring host-guest complex formation39-40 or study intermediates in solution.41 However, there has been no reported study of using DOSY in the lignocellulosic biomass biorefinery. We believe an obvious advantage of DOSY for biorefinery is that, it not only reveals the number of components in the reaction mixture, but it also identifies their chemical structures in one single, rapid experiment. These possible advantages are beyond the scope of the above mentioned GC or HPLC techniques, where separation of such mixtures is not straightforward and identification requires the use of standard compounds. In this paper, we apply and optimize 1H DOSY NMR to directly analyze several model reaction mixtures, as well as real reaction mixtures of sucrose and glucose dehydration from lignocellulosic biomass biorefinery. This can be achieved directly on the mixture, without any chromatographic isolation of components. Furthermore, we show that the shift reagent, EuFOD, enhance the separation effectively in the diffusion dimension. We believe that the presented DOSY NMR approach will pave the way for more efficient studies in biorefinery. 6


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Experimental

Chemicals

Sucrose (analytical grade, 99.8%), cellobiose (analytical grade, 98%), D-xylose (analytical grade, 99.8%), D-glucose (analytical grade, 99.5%), D-fructose (analytical grade, 99.5%), levulinic acid (98%), formic acid (88%, aqueous solution), γ-valerolactone (98%) and furfural (99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. 5-Hydroxymethylfurfural (99%), 2,5-diformylfuran (98%), 2,5-dimethylfuran (99%), 2,5-furandicarboxylic acid (97%), phenol (analytical grade), guaiacol (99%) and 2,6-dimethoxyphenol (98%) were purchased from Aladdin Reagent Company (Shanghai). EuFOD (99%) was obtained from Sigma-Aldrich. Dimethyl sulfoxide-d6 (DMSO-d6, 99.8 atom% D), chloroform-d (CDCl3, 99.8 atom% D) and deuterium oxide (D2O, 99.8 atom% D) were supplied by J&K Scientific Ltd. All chemicals were used without further purification.

NMR experiments

NMR spectra were acquired on a Bruker AV-III 400 MHz NMR spectrometer (9.39 T), using a 5 mm PABBO BB/19F-1H/D probe with z gradient coil producing a maximum gradient strength of 0.50 T m−1. The temperature was calibrated using the NMR temperature standards according to the manuals of Bruker (4% CH3OH in CD3OD for low temperature, 80% ethylene glycol in DMSO-d6 for high temperature). The “doped water (GdCl3 in D2O)” was used as a standard for the gradient strength calibration. The samples contain 1% w/w concentration for each compound were 7



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prepared in 400 µL deuterium solvents for DOSY measurement. 1H NMR was obtained at frequencies of 400.13 MHz. DOSY experiments were performed with the Bruker standard bipolar pulse longitudinal eddy current delay (BPPLED, as shown in Figure S1) pulse sequence. For each DOSY-NMR experiment, 32 BPPLED spectra with 32K date points were collected. The diffusion time (∆) was 100 ms. The duration of the pulse field gradient (δ/2) was adjusted in a range of 600~2000 µs in order to obtain 2%~5% residual signal with the maximum gradient strength. The delay for gradient recovery was 0.2 ms and the eddy current delay was 5 ms. The gradient strength was incremented in 32 steps from 2% to 95% of its maximum value in a linear ramp. All the measurements were performed at 25 °C and at a gas flow rate of 400 lph without sample spinning. After Fourier transformation and baseline correction, the diffusion dimension was processed using Bruker Topspin 3.1 software.

Model mixtures for DOSY NMR

Several model mixtures were designed and prepared in order to evaluate the scope of the DOSY method. Realistic and representative reaction equations for biomass conversion and the analyses chosen (reactants and products) for 1H DOSY experiments are presented in Figure 1.

8


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Hemicellulose

Lignin

Cellulose

Figure 1. Chemical reaction and model compounds for the three main components (namely cellulose, hemicellulose and lignin) of lignocellulosic biomass.

To check the potential utilization of this DOSY tool in practical applications, we designed two kinds of mixtures to simulate the conversion of sucrose and cellobiose to 5-HMF and LA. For the conversion of sucrose (reactant) to the products, i.e. 9



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glucose, fructose, 5-HMF, LA and FA, these were added into sucrose solution.26 Similarly, glucose, 5-HMF, LA and FA, as the products, were mixed with cellobiose (reactant) solution to mimic the reaction mixture, when hydrolyzing cellulose. To assistance the spectral assignment, the 1H NMR of authentic samples were measured and spectra are listed in Figure S2~S7. Besides, gaussian fits of these investigated mixtures, from which diffusion data was generated, were smoothed and displayed in Figure S8~S18.

Results and Discussion

Because hemicellulose upgrading researches are mainly focused on the xylose dehydration to furfural,

1

xylose and corresponding furfural product were initially

investigated in DMSO-d6, and different D values for xylose and furfural were obtained. As shown in Figure 2a, the mixture of compounds was separated well in the diffusion dimension. The upper line corresponds to xylose, and the bottom line is furfural (Figure 2a). The result clarifies that the small product molecule, furfural (MW = 96), diffuses faster (D = 5.31×10-10 m2s-1) than the larger heavier reactant, xylose (MW = 150) (D = 2.79×10-10 m2s-1). In addition, 5-HMF (MW = 126), which is the dehydration product of hexoses, has a molecular structure similar to furfural. It is therefore interesting to apply the DOSY technique to separate these when mixed. Figure 2b shows the DOSY-spectrum and it is clear from this that 5-HMF and furfural are well separated in diffusion dimension.

When comparing Figure 2a with Figure 2b, a notable experimental phenomenon 10


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for H2O was observed. Small solvent molecules usually diffuse faster than the solute molecules, so their DOSY peaks always appear below the others in the diffusion dimension as shown in Figure 1b (D = 7.31×10-10 m2s-1). However, the trace amount of H2O brought by the solvent DMSO-d6 is diffusing slower in the presence of xylose (see Figure 1a (D = 2.88×10-10 m2s-1)). This is due to a stronger interaction between the xylose and H2O, which can be attributed to hydrogen bonding between hydroxyl groups in the xylose and H2O.

(b)

(a)

D-xylose

5-HMF H2O

furfural furfural

DMSO H2O

DMSO

Figure 2. 1H DOSY spectra of mixtures: xylose and furfural (a); furfural and 5-HMF (b). Having established the feasibility of 1H DOSY tool for xylose conversion, it was next applied to lignin, which is rich in aromatic functionalities, and consists of three primary units, namely coniferyl, sinapyl and p-coumaryl alcohol.42 43 In our study, the aromatic compounds, phenol, guaiacol and 2,6-dimethoxyphenol (Figure 1), were selected as the model compounds of lignin degradation, and the obtained 1H-DOSY 11



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spectra are presented in Figure 3. Unfortunately, guaiacol and 2,6-dimethoxyphenol were not well separated in the diffusion dimension, especially the methoxy groups were overlapping (Figure 3a). Enlightened by the above mentioned H2O D change due to the interaction between molecules (Figure 2) and Nilsson’s work on EuFOD, a commonly used shift reagent in NMR spectroscopy, this reagent was employed to enhance the diffusion resolutions of the lignin model compounds.35 An amazing effect was observed, showing that all three components were separated with when applying EuFOD. The signals for guaiacol and 2,6-dimethoxyphenol, which overlapped (see Fig 3a), could now be distinguished and assigned (Figure 3b). The above results are originated from the different interaction strength between each of these three molecules with EuFOD. The guaiacol and 2,6-dimethoxyphenol, which have two and three oxygen atoms, results in different Lewis acid (EuFOD) - Lewis base (oxygen of the substrate) interaction strength. The signals of the methoxy groups are now well-resolved in the diffusion dimension. The upper line corresponds to 2,6-dimethoxyphenol according to the 1H spectra of authentic samples (Figure S3). Since two methoxy groups interact more tightly with EuFOD, it presents the slowest diffusion rate. The middle line refers to guaiacol, and the bottom line matches phenol. Phenol diffuses faster than guaiacol owing to the weaker interaction between its single hydroxyl group and EuFOD. Practically, the 1H signal of EuFOD did not overlap with the signals of investigated components. Therefore, the EuFOD assists to separate and identify these compounds of lignin valorization by DOSY.

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(a) (a)

(b) (b)

2,6-dimethoxyphenol

2,6-dimethoxyphenol

guaiacol

guaiacol

phenol

phenol

Figure 3. 1H DOSY spectra of the phenol, guaiacol and 2,6-dimethoxyphenol in the absence (a) and presence (b) of EuFOD (10 mM), CDCl3 as the solvent.

Cellulose is a homopolymer consisting of β-D-glucopyranose units linked via 1,4-β-glycosidic bonds. It can be deconstructed into its monosaccharide unit, glucose, which is a commonly building block for biomass conversion. Glucose or fructose can be convert into 5-HMF through dehydration reaction under acidic conditions.44 DOSY measurements of three kinds of mixtures, which mimicked the typical reaction involved in the cellulose conversion, were performed under the optimized NMR acquisition conditions (Figure 4). The results showed that mixtures could be resolved successfully due to their different D values. Figure 4a shows the 1H DOSY spectrum of the mixture of glucose, 5-HMF, LA and FA in D2O. These four compounds, together with the HDO, were completely separated in the diffusion dimension. Similar results were obtained for the mixture of fructose, 5-HMF, LA and FA to simulated the conversion of fructose (Figure 4b). 13



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(b)

(a)

glucose

fructose 5-HMF

5-HMF levulinic acid

levulinic acid formic acid

formic acid

HDO

HDO

(d)

(c) (c) levulinic acid

FDCA 5-HMF

γ-valerolactone DFF

DMF

HDO

DMSO

Figure 4. DOSY spectra of three mixtures simulated the reactions of cellulose conversion: (a) glucose, 5-HMF, LA and FA; (b) fructose, 5-HMF, LA and FA; (c) LA and GVL, (d) 5-HMF, DFF, DMF and FDCA. Reduction of LA to GVL has attracted significant interest due to GVL’s properties as a good solvent and reaction medium, together with its use as liquid fuel or fuel additive.45-46 The mixture of LA and GVL was examined and listed in Figure 4c. Interestedly, one signal of GVL (4.74 ppm in Figure 4c), which was adjacent to 14


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the residual proton signal (HDO, 4.70 ppm), diffuses much faster than the other signal originating from GVL, and its DOSY peak appeared above the others as shown in Figure 4c. The solvent signal is clearly much more intense and diffused faster than that of the solute (GVL). So the signal of investigated components seems to be interfered by the HDO signal. Hence, the D for this signal of GVL (4.74 ppm) was larger than theoretical value. Besides, the oxygen-contained liquid fuels and valued added chemicals derived from 5-HMF, such as 2,5-diformylfuran (DFF), 2,5-dimethylfuran (DMF), 2,5-furandicarboxylic acid (FDCA) were also separated in the diffusion dimension successfully.47

We designed two kinds of mixtures to simulate the conversion of sucrose and cellobiose to 5-HMF and LA. Well separated signals for these reactants and products were observed in the D2O (Fig. 5a & 5b). Remarkably, our DOSY tool even work efficiently for these three carbohydrates: sucrose (D = 3.09×10-10 m2s-1), glucose (D = 3.94×10-10 m2s-1) and fructose (D = 3.46×10-10 m2s-1). It is necessary to highlight the successful separation of glucose and fructose, which have the exact same molecular weight, a very similar chemical structure and therefore hydrodynamic radius. Although the spectral overlapping seriously complicates the analysis in the carbohydrate region, the selected protons of sucrose (5.29 ppm) and glucose (5.10 & 4.53 ppm) were clearly separated in the diffusion dimension. Hence, we could unambiguously assign that the upper line refer to sucrose, the middle line corresponds to fructose, and the bottom line to glucose (Fig. 5b). Similar result was obtained for the model mixture simulating cellulose conversion (As shown in Fig. 5c & 5d). 15



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(a)

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(b)

sucrose

5-HMF levulinic acid

fructose

glucose

formic acid

HDO

(c)

(d)

5-HMF cellobiose levulinic acid formic acid glucose

HDO

Figure 5. DOSY spectra of two kinds of mixtures to simulate the conversion of sucrose and cellobiose to 5-HMF and LA. (a) sucrose, fructose, glucose, 5-HMF, LA and FA; (b) the enlarged view of the carbohydrate region in (a) which shows the separation of sucrose, fructose and glucose; (c) cellobiose, glucose, 5-HMF, LA and FA; (d) the enlarged view of the carbohydrate region in (c) which dedicates the separation of cellobiose and glucose.

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Finally, the practical application of DOSY in real reaction mixtures was examined. Reaction mixture for the conversion of glucose in 67 wt% ZnCl2,48 and reaction mixture for the conversion of sucrose in DMSO were separated by DOSY. The obtained 2D spectra in Figure S19 & S20 proved that DOSY is a robust method and it also work well for the real mixtures of monosaccharide (glucose) and disaccharide (sucrose) conversion reactions.

Conclusion For the biomass conversion study and application, the reaction mixture analysis is always a complex and tedious work, when using the available chromatographic methods. This experimental work has developed the DOSY technique as a tool for directly analyzing crude biorefinery compound mixtures efficiently. Under the optimized DOSY conditions, reaction mixtures for three major components of lignocellulosic biomass, i.e. cellulose, hemicellulose and lignin, can be separated and assigned in the diffusion dimension without any prior separation or isolation. Because of the prevalent of liquid state NMR spectrometer (hardware) and easy access operation process for the DOSY program (software), the DOSY NMR method, described here, could be extended from present lignocellulosic biomass to other related field of biorefinery in the future, such as the chitin biomass and algae biomass conversion.

AUTHOR INFORMATION

Corresponding Author: [email protected]; [email protected] 17



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Notes

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Major State Basic Research Development Program of China (973 Program) (2012CB215305) and the Natural Science Foundation for Youths of Shanxi (2013021008-7). Yan QIAO thanks the Chinese Academy of Sciences (2013YC002) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2011137) for financial support. Christian Marcus Pedersen acknowledges the CAS President’s International Fellowship Initiative (2015VMB052). We are appreciating the helpful discussion from Juan Lv and Junfeng Xiang for paper preparation.

Supplementary Information

The spectra of the authentic samples, diffusion curves and real biomass conversion mixtures separation are provided. This information is available free of charge via the Internet.

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For Table of Contents Use Only.

Title: DOSY NMR: A Versatile Analytical Chromatographic Tool for Lignocellulosic Biomass Conversion

Authors: Wenzhi Ge, Jennifer Hongyang Zhang, Christian Marcus Pedersen, Tingting Zhao, Fen Yue, Chunyan Chen, Pengfei Wang, Yingxiong Wang, Yan Qiao

Synopsis: The diffusion ordered NMR spectroscopy (DOSY) protocol for the analysis of reaction mixture of lignocellulosic biomass conversion has been developed and investigated systematically.

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DOSY NMR: A Versatile Analytical Chromatographic Tool for

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