DOSY Plus Selective TOCSY: An Efficient NMR Combination for

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DOSY plus selective TOCSY: an efficient NMR combination for analyzing hydrogenation/hydrogenolysis mixtures of biomass-derived platform compounds Zexiang Lyu, Fen-e Gao, Lizhu Wen, Kemeng Shi, Minjun Ma, Chunju Li, Yingxiong Wang, and Yan Qiao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03992 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Energy & Fuels

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DOSY plus selective TOCSY: an efficient NMR combination for analyzing hydrogenation/hydrogenolysis mixtures of biomass-derived platform compounds

5

Zexiang Lyu,ab Fen-e Gao,c Lizhu Wen,b Kemeng Shi,b Minjun Ma,b Chunju Li,a Yingxiong

6

Wang,*b Yan Qiao*b

7

a

8

Republic of China

9

b

1 2 3

Department of Chemistry, College of Science, Shanghai University, Shanghai 200444, People’s

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

10

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

11

c

12

China

Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People's Republic of

1

ACS Paragon Plus Environment

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ABSTRACT:

14

Analyzing the mixtures obtained from hydrogenation or hydrogenolysis reactions of

15

biomass-derived platform chemicals is a challenging work. With the development and

16

improvement of NMR techniques, the NMR spectrometer proves to be an alternative and

17

highly efficient equipment for the rapid analysis of complex mixtures without the need for

18

tedious purification. Herein, diffusion-ordered spectroscopy (DOSY) is applied in analyzing

19

four

20

hydrogenation/hydrogenolysis reactions of biomass-derived platform chemicals. The results

21

show that the DOSY technique can pseudo separated most components in the model mixtures.

22

1D selective gradient TOCSY technique is used as a supporting tool in the cases where the

23

DOSY technique cannot provide a clear distinguish between the components of the mixtures.

24

This is generally a problem when components in the mixture have very similar diffusion

25

coefficients or severe overlap of peaks. The results show that DOSY and 1D selective gradient

26

TOCSY techniques is a strong combination for complex mixture analyses.

27

model

mixtures,

which

consist

of

the

reactants

and

products

from

1. INTRODUCTION

28

Biomass is a widely available and economical renewable resource, which has become

29

increasingly more attractive for decades.1 Conversion of biomass and biomass-derived platform

30

chemicals into valuable added chemicals and fuels is considered as one of the most promising

31

ways to out phase the fossil energy and solve the environment crisis.2 However, there are several 2


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significant issues, which should be concerned before developing these biomass-derived platform

33

compounds.3 Commonly, an overabundance of oxygen atoms in structure of biomass molecules

34

results in high boiling points, low energy-density and undesired reactivity of many biomass

35

derived platform compounds, which render them unappealing as liquid fuel.4 Therefore,

36

processes of removing oxygen are necessary. To do this, numerous kinds of reactions, including

37

dehydration,5 hydrogenation,6 decarbonylation/decarboxylation7 and C-O hydrogenolysis8, are

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employed. Among these reactions, catalytic hydrogenation and hydrogenolysis are two of the

39

most attractive methods.9,

40

biomass-derived platform compounds are already widely used, the development is hampered by

41

the tedious and challenging analytical methods.

10

Although the hydrogenation/hydrogenolysis reactions of

42

Gas chromatography (GC) and high-performance liquid chromatography (HPLC) are two of

43

the analytical methods used to analyze mixtures.11 Most of the reaction mixtures obtained from

44

hydrogenation/hydrogenolysis processes of biomass-derived platform compounds can be

45

analyzed qualitatively and quantitatively by GC and HPLC. However, these two methods are

46

insufficient and unreliable as the reaction mixtures from the hydrogenation/hydrogenolysis

47

reactions contain components with low volatility, high reactivity, low thermal stability and even

48

similar polarity. For example, a sample analyzed with GC commonly needs derivatization by e.g.

49

silylation. Although HPLC could avoid derivatization by adjusting the polarity of mobile phases,

50

it is often not sufficient, which is especially the case when new peaks are found in the

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chromatograph. Moreover, both GC and HPLC cannot provide detailed structural information of

52

byproducts. Therefore, developing an alternative analysis method taking advantage of other 3


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analytical equipment is highly desired.

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Nuclear magnetic resonance (NMR) spectroscopy is an outstanding analytical tool for

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characterization the structure of organic compounds including biomass-derived molecules.12, 13

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With the development of NMR techniques, both pure compounds and mixtures can be

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characterized efficiently. Recently, diffusion-ordered spectroscopy (DOSY) has been introduced

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into various systems and shown its power in mixture analysis.14, 15 In this technique, the diffusion

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coefficient (D) of a certain compound in diluted solution follows the Stokes-Einstein equation:16

‫=ܦ‬

݇ܶ 6ߨߟ‫ݎ‬௦

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

61

is the hydrodynamic radius of the molecule. The diffusion coefficients of each signal in a

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spectrum can be calculated by conducting a series of pulsed field gradient (PFG) experiments.17,

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18

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mixture can be assigned quickly in diffusion dimension.19, 20 Our research group had successfully

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applied this technique in lignocellulosic biomass biorefinery systems, and several model as well

66

as genuine reaction mixtures of sucrose and glucose dehydration reaction have been studied.21

With a pseudo two-dimensional spectrum generated, signals of the different components in

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However, DOSY NMR may give misleading signals when having compounds with

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overlapped peaks in a relative narrow range of the 1H DOSY spectrum. In addition, DOSY NMR

69

has its limitations when resolving compounds with very similar or even identical diffusion 4


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coefficients. To solve these issues, matrix-assisted DOSY has developed. In this method,

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chromatographic supports, surfactants, polymers or other matrices are added and the resolution

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of signals in the diffusion dimension can thereby be improved dramatically.

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our studies, 1D selective gradient total correlation spectroscopy (TOCSY) is also found to be a

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powerful method, which complement the DOSY technique.25 A sub-spectrum of the components

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in the mixture can be extracted from the spectrum of the highly complex mixture if the mixing

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time (a key parameter in 1D selective gradient TOCSY experiment) is suitable to transfer the

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magnetization to all nuclei of the same spin system.26, 27 It is technique is adequate for analyzing

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biomass-derived platform compounds of hydrogenation/hydrogenolysis reaction because these

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molecules generally contain complete proton spin systems.28 Therefore, 1D selective gradient

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TOCSY can be used in combination with the DOSY technique to identify the overlapped signals.

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In

this

paper,

four

representative

model

mixtures,

22-24

which

As reported in

simulate

the

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hydrogenation/hydrogenolysis reactions of biomass-derived platform compounds, were chosen

83

and studied by 1H diffusion-ordered NMR spectroscopy. The diffusion processes of each sample

84

are discussed below. Considering some of the components in the sample may have overlapped

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signals, which would complicate the identification of the constituents in the mixture, 1D

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selective gradient TOCSY was applied for further detection. This combination of NMR

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techniques

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hydrogenation/hydrogenolysis reactions of the biomass-derived platform compounds.

will

contribute

to

the

analysis

of

5


mixtures

obtained

from

the

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2. EXPERIMENTAL SECTION

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2.1. Chemicals

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Phenol (analytical grade, 99.5%), cyclohexanol (analytical grade, 99%), cyclohexanone

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(analytical grade, 99.5%), cyclohexene (analytical grade, 99.7%), cyclohexane (analytical grade,

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99.5%), dimethyl oxalate (DMO, analytical grade, 99%), furfural (FAL, analytical grade, 99%),

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furfural

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2-methyltetrahydrofuran (2-MTHF, 99.5%) were obtained from Aladdin Reagent Company

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(Shanghai).

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2,5-dihydroxymethylfuran (DHMF, 95%), 2,5-bishydroxymethyl tetrahydrofuran (DHMTHF,

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95%),

99

2,5-dimethylfuran (DMF, 99%), 2,5-dimethyltetrahydrofuran (DMTHF, 95%), dimethyl

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sulfoxide-d6 (DMSO-d6, 99.8 atom% D) and deuterium oxide (D2O, 99.8 atom% D) were

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purchased from J&K Scientific Ltd. Ethylene glycol (EG, analytical grade, 99%) and anhydrous

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ethanol (analytical grade, 99%) were supplied by Tianli Chemical Reagent Co., Ltd. All

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chemicals were used without further purification.

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2.2. NMR experiments

alcohol

(FOL,

Methyl

5-methyl

analytical

glycolate

furfural

grade,

(MG,

(5-MF,

98%),

98%),

98%),

2-methylfuran

(2-MF,

5-hydroxymethylfurfural

5-methyl

furfurylalcohol

98%)

(5-HMF,

(5-MFA,

and

99%),

97%),

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All experiments were conducted on a Bruker AV-III 400 MHz NMR spectrometer (9.39 T)

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equipped with a 5 mm PABBO BB/19F-1H/D probe with z gradient coil producing a maximum 6


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gradient strength of 0.50 T·m-1. The calibrations of temperature and gradient strength were

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performed before the NMR experiments according to the manual of Bruker. The temperature

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calibration was performed with 4% CH3OH in CD3OD for the lower temperature range

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(181.2-300 K) and 80% ethylene glycol in DMSO-d6 for higher temperatures (300-380K). The

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instruction of temperature calibration in Topspin 3.1 software is “calctemp”. The “doped water

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(GdCl3 in D2O)” was used in the gradient strength calibration. The AU program dosy was used to

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calculate and store the absolute gradient strength values. All the experiments were conducted at

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298 K and at a gas flow rate of 400 lph without sample spinning. 1H NMR was obtained at

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frequencies of 400.13 MHz. The Bruker standard bipolar pulse longitudinal eddy current delay

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(BPPLED) pulse sequence was used for the DOSY measurements. Each DOSY NMR

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experiment collected 16 BPPLED spectra with 32K data points. Diffusion time (∆) was set at

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100 ms. The duration of the pulse field gradient (δ/2) was adjusted between 600 to 800 µs to

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acquire 2%~5% residual signal with the maximum gradient strength. The delay for gradient

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recovery was 0.2 ms and the eddy current delay was 5 ms. The gradient strength was

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incremented in 16 steps from 2% to 95% of its maximum value in a linear ramp. The data was

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processed with Bruker Topspin 3.1 software, and the DOSY plots were obtained by Dynamics

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Center 2.2.4 software.

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1D selective gradient TOCSY experiments were performed using a pulse sequence named

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SELMLGP in Bruker Topspin 3.1, which consists of recycling delay, a radio-frequency pulse 7


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and a MLEV17 sequence for mixing and acquisition time. The spectra of the 1D selective

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gradient TOCSY experiments were collected by varying the mixing time between 80 to 150 ms.

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2.3. Computational details

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Calculations have been performed using the B3LYP29-32 density functional method as

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implemented in the Gaussian0933 program package. Geometries were optimized using the

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6-311+g (d,p) all-electron basis set. Frequencies were calculated at the same level as the

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geometry optimization to ensure that the stationary points found were in fact minima (no

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imaginary frequency was found) on the potential energy surfaces. Solvation effects were also

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calculated as single points at the same level as the geometry optimization with the CPCM

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method.34,

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DHMTHF since it is the solvent used in the experiments.

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2.4. Model mixtures for analysis

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The parameter of DMSO was used to optimize the structure of DHMF and

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Four representative hydrogenation/hydrogenolysis reaction mixtures (Scheme 1) containing

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biomass-derived platform chemicals were detected. First, two mixtures consisting of the reactant

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and the products of 5-HMF hydrogenation/hydrogenolysis reactions, were analyzed (Scheme 1a).

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5-HMF, an important platform chemical obtained from renewable biomass resources, can

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generate various useful products through hydrogenation/hydrogenolysis since it contains many

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functional groups such as C=O, C-O, C=C and a furan ring.36-38 At lower temperatures, 8


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hydrogenation of C=C bond of 5-HMF is the dominating process. DHMTHF is obtained as the

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product via hydrogenation and DHMF as an intermediate compound. Furanic fuels, such as DMF

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and DMTHF, are obtained through hydrogenolysis at higher temperatures. Secondly, a furfural

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hydrogenation/hydrogenolysis model mixture was prepared. Furfural is a promising

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lignocellulosic material containing C=C and C=O functional groups. It can be converted into

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special oxygen-containing chemicals through the hydrogenation of the C=C groups from the

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furan rings, or the hydrogenation and hydrogenolysis of C=O group.39 Herein, a representative

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reaction mixture of furfural is analyzed. The model mixture contained four compounds including

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FAL as the reactant, FOL, 2-MF and 2-MTHF as the products (Scheme 1b).40 The third biomass

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system studied is phenolic bio-oil, which can be converted into valuable chemicals such as

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saturated naphthene.41,42 Phenol hydrogenation was selected and analyzed as a representative

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reaction (Scheme 1c). The routes consists of four steps and can produce cyclohexanone,

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cyclohexanol, cyclohexene and cyclohexane.43 The sample simulating the conversion of DMO to

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ethanol is the last model mixture (Scheme 1d).44 This process is a cascade reaction including the

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stepwise hydrogenation of ester groups to ethylene glycol and the following hydrogenolysis of

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C-O bonds to ethanol.45. Mixtures containing DMO, methyl glycolate, EG and ethanol were

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analyzed by DOSY technique. The 1H NMR spectra of these mixtures are shown in Figures

161

S1-S5.

9


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162 163

Scheme 1. The hydrogenation/hydrogenolysis reaction routes of selected biomass-derived

164

platform molecules. 10


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3. RESULTS AND DISCUSSION

As described in the above section, the hydrogenation of 5-HMF can be done at higher

167

temperatures or at lower temperatures (Scheme 1a), with different final products, i.e.

168

and DHMTHF respectively.36, 37 Mixtures composed of the reactant and products of these two

169

reaction paths were prepared and analyzed by 1 DOSY technique separately. The compounds in

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the model mixture are separated well in the diffusion dimension (Figure 1a and 1b). Although the

171

molecular weight of DHMF and DHMTHF is similar, the DOSY signal of them can clearly be

172

distinguished (Figure 1a). Surprisingly, in spite of the molecular weight of DHMTHF being

173

bigger than DHMF, the diffusion coefficient of DHMTHF (MWDHMTHF = 132, DDHMTHF =

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3.123×10-9 m2/s) is a little bigger than DHMF (MWDHMF = 128, DDHMF = 3.038×10-9 m2/s),

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which means that DHMTHF diffuses faster than DHMF in DMSO-d6. Figure 1b displays the

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DOSY spectrum of the 5-HMF hydrogenolysis route at higher temperature. The signals of

177

5-HMF, DHMF and 5-MFA in the mixture are separated well in the diffusion dimension.

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However, the signals of DMF and DMTHF cannot be distinguished clearly due to their similar

179

diffusion coefficients (DDMF = 5.685×10-10 m2/s, DDMTHF = 5.571×10-10 m2/s), i.e. a further

180

discrimination by 1D selective gradient TOCSY is required. Both DMF and DMTHF have

181

structures with coherent five-membered rings, so they are suitable for 1D selective gradient

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TOCSY experiment. The peaks at 1.12 ppm and 2.20 ppm were selected to excite, and the 1D

183

selective gradient TOCSY spectra of DMTHF and DMF were obtained (Figure 1c). It is obvious 11


DMTHF

1

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that the excited signals of DMF and DMTHF are according to their standard 1H NMR spectra,

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which further confirm the presence of DMF and DMTHF. The signals at 2.50 ppm and 3.30 ppm

186

in the 1H NMR spectra belong to DMSO and water respectively. They are not observed in the

187

corresponding 1D selective gradient TOCSY spectra.

188

According to Stokes-Einstein equation, a reason for the diffusion coefficient of DHMTHF

189

being bigger than the one obtained for DHMF could be due to the different hydrodynamic radius

190

of these two molecules. Although both of DHMTHF and DHMF have two hydroxyl groups,

191

DHMF has C=C bonds, which DHMTHF does not contain. It is reasonable to deduce that the

192

rigid structure of DHMF hinders its two hydroxyl groups to form intramolecular hydrogen bonds,

193

whereas DHMTHF shows the ability to constitute intramolecular hydrogen bonds, resulting in a

194

smaller hydrodynamic radius of DHMTHF. The diffusion coefficient is inversely proportional to

195

the hydrodynamic radius of the molecule as described by Stokes-Einstein equation.46 Therefore,

196

the bigger the hydrodynamic is, the smaller diffusion coefficient the molecule show. In addition,

197

the existence of intramolecular hydrogen bonds makes the DHMTHF molecule less polar

198

compared with DHMF, which results in a poorer interaction with the solvent (DMSO-d6) and

199

hence a faster diffusion. DFT calculations have additional been employed to support this

200

rationalization about the intramolecular hydrogen bonds in DHMF and DHMTHF (Figure 1d).

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After the optimization, the calculated hydrogen bond distance of DHMTHF is 1.915 Å. The

202

calculation on the distance of hydroxyl groups in DHMF did not give any result, which indicates 12


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that DHMF cannot form intramolecular hydrogen bonds. The computational results are in

204

agreement and support our results obtained by DOSY spectroscopy well.

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Interestingly, an unknown signal marked with a pink circle (δ= 5.18 ppm) appeared in the

206

mid of the DOSY spectrum (Figure 1b), it is proposed that it belongs to the signals of hydroxyl

207

groups, according to litterature.47 Further support of this was made through adding a drop of

208

deuterated water in the mixture sample followed by conducting a 1H NMR experiment. The

209

signals diminished (Figure S6) due to the exchange of H and D atoms, which indicates that they

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are indeed hydroxyl groups. In Figure 1a, the signals of hydroxyl groups are not shown because

211

the intensity of these peaks are too small, being below the threshold of peak picking when

212

processing the DOSY data.

13


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214

14


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216 217

Figure 1. The 1H DOSY spectra of 5-HMF hydrogenation/hydrogenolysis by the two routes (a, 15


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b), the sub-spectrum of 1D selective gradient TOCSY (c), and the optimized structure of DHMF

219

and DHMTHF (d). DMSO-d6 is the solvent, and the experiment was performed at 298 K. In

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Figure 1d, red balls are oxygen atoms, gray balls are carbon atoms and white balls are hydrogen

221

atoms.

222

Secondly, a usual reaction mixture of furfural hydrogenation, consisting of FAL, FOL,

223

2-MF and 2-MTHF, was analyzed by 1H DOSY technique (Figure 2a). The components in the

224

mixture were distributed well in the diffusion dimension according to their molecular weights.

225

FOL (MWFOL = 98, DFOL = 4.678×10-9 m2/s) diffused slowest due to its biggest molecular weight.

226

FAL (MWFAL = 96, DFAL = 6.014×10-9 m2/s) diffused slightly faster than FOL. Even though the

227

molecular weights of FAL and FOL are very close, the DOSY technique can directly distinguish

228

between them because FOL has a hydroxyl group and interact stronger with the solvent

229

DMSO-d6.48 2-MF (MW2-MF = 82, D2-MF = 7.057×10-9 m2/s) has a larger diffusion coefficient

230

because the molecular weight of 2-MF is smaller than 2-MTHF (MW2-MTHF = 86, D2-MTHF =

231

6.931×10-9 m2/s). In praxis, it is difficult to assign the signals of 2-MF and 2-MTHF if only

232

based on the DOSY spectrum, since their diffusion coefficients are to close. So 1D selective

233

gradient TOCSY experiments were applied to distinguish between 2-MF and 2-MTHF. The

234

peaks between 1.10 and 1.14 ppm and the peak at 2.24 ppm were selectively excited. Resonances

235

of 2-MF and 2-MTHF appeared in the corresponding sub-spectra (Figure 2b). The signal at 2.50

236

ppm and 3.33 ppm in the 1H NMR spectra of 2-MF and 2-MTHF belong to DMSO and H2O. 16


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238 239

Figure 2. The 1H DOSY spectrum of furfural hydrogenation/hydrogenolysis mixture (a) and the 17


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1D selective gradient TOCSY sub-spectra of 2-MF and 2-MTHF (b), DMSO-d6 as the solvent.

241

The temperature was 298 K. The lightning in panel (b) label the peaks excited in 2-MF and

242

2-MTHF.

243

Figure 3a is the 1H DOSY spectrum of the phenol hydrogenation reaction mixture in

244

DMSO-d6. The five components are separated well in the diffusion dimension. From the top to

245

bottom, the signals are assigned to phenol (MWphenol = 94, Dphenol = 4.714×10-10 m2/s),

246

cyclohexanol (MWcyclohexanol = 100, Dcyclohexanol = 5.178×10-10 m2/s), cyclohexanone

247

(MWcyclohexanone = 98, Dcyclohexanone = 6.439×10-10 m2/s), cyclohexane (MWcyclohexane = 84,

248

Dcyclohexane = 6.905×10-10 m2/s) and cyclohexene (MWcyclohexene = 82, Dcyclohexene = 7.140×10-9

249

m2/s). Phenol and cyclohexanol diffused slowest because they both have relative higher

250

molecular weights as well as hydroxyl groups in structure. The hydrogen bonds between solvent

251

DMSO-d6 and these two analytes make them have a smaller diffusion coefficient.48 The

252

molecular weight of cyclohexanol is bigger than phenol, while it diffused faster than phenol,

253

probably because the interaction between phenol and DMSO-d6 is stronger than cyclohexanol.

254

The acidity of phenol is stronger than cyclohexanol, and makes the hydrogen bonding between

255

the phenol and the oxygen atom in DMSO-d6 stronger.49

256

The signals between 1.40 ppm and 1.95 ppm of cyclohexanol and cyclohexanone in 1H

257

DOSY spectrum are overlapped (Figure 3a, circled in a red frame). The severe overlaps generate

18


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misleading signals in the DOSY spectrum and hence confuse the assignment of signals. Indeed,

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it is well known that the measured diffusion coefficient, of compounds with overlapping signals,

260

results in a weighted average of those overlapped protons.50 In order to further demonstrate the

261

existence of cyclohexanol and cyclohexanone in mixture, 1D selective gradient TOCSY

262

experiment was again employed, and the corresponding sub-spectra are shown in Figure 3b. The

263

1D selective gradient TOCSY sub-spectra of cyclohexanol and cyclohexanone are the same as

264

their standard 1H NMR spectra. Notably, the signal of the hydroxyl group (δ= 4.41 ppm, Figure

265

3b) in cyclohexanol can also be observed in the sub-spectrum of cyclohexanol. The signal at 2.50

266

ppm and 3.33 ppm in the 1H NMR spectra of cyclohexanol and cyclohexanone belong to DMSO

267

and H2O.

268 19


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269 270

Figure 3. The 1H DOSY spectrum of phenol hydrogenation/hydrogenolysis mixture (a), and the

271

1D selective gradient TOCSY sub-spectra of cyclohexanone and cyclohexanol (b), DMSO-d6 as

272

the solvent. The selected peaks of cyclohexanone and cyclohexanol are pointed with lightning.

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The experiment temperature is 298 K.

274

Finally, the reactant and products of the DMO hydrogenolysis reaction were analyzed.

275

Notably, this mixture cannot be separated well using DMSO-d6 (Figure S7a) or CDCl3 as the

276

solvents (Figure S7b). In DMSO-d6, EG (MWEG = 62, DEG = 5.044×10-10 m2/s) and MG (DMG =

277

5.202×10-10 m2/s, MWMG = 90) have similar diffusion coefficient and hence cannot be separated

278

in the diffusion dimension. In CDCl3, these five compounds diffused in the following sequence

20


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(from the slowest to fastest): EG (MWEG = 62, DEG = 1.614×10-9 m2/s), DMO (MWDMO = 118,

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DDMO = 1.672×10-9 m2/s,), MG (MWMG = 90, DMG = 1.801×10-9 m2/s) and ethanol (MWethanol =

281

46, Dethanol = 2.235×10-9 m2/s). However, signals of ethanol are not in the same diffusion

282

dimension because the signal at 3.63 ppm partially overlap with the signal of EG. The signal of

283

the hydroxyl group at 2.68 ppm does also disturb the assignment. When the solvent was changed

284

from DMSO-d6 or CDCl3 to methanol-d4, a well separated DOSY spectrum for DMO

285

hydrogenation mixture was obtained (Figure 4). Four substances are displayed in the diffusion

286

dimension in the following order: EG (DEG = 1.435×10-9 m2/s, MWEG = 62), MG (DDMO =

287

1.625×10-9 m2/s, MWDMO = 90), ethanol (Dethanol = 1.748×10-9 m2/s, MWethanol = 46) and DMO

288

(DDMO = 1.825×10-9 m2/s, MWDMO = 118). The diffusion coefficients of these components do not

289

have a simple relation to their molecular weights. For instance, EG possesses a small molecular

290

weight (MWEG = 62), while it diffused slowest. DMO possesses a bigger molecular weight

291

(MWDMO = 90), but it diffused fastest.

292

The reasons for the pseudo separation in a DOSY plot are multifarious besides the

293

molecular weight, such as size, shape, binding phenomena, and molecular interactions of the

294

observed species contributes.50 As reported, methanol-d4 is more suitable as solvent than

295

DMSO-d6 or CDCl3 for molecules with different numbers of hydroxyl groups,25 because

296

methanol-d4 is a protic solvent and hence accepts and donates hydrogen bonds.51,

297

solvation intensities between the solutes and methanol-d4 are the main reason for the 21


52

Different

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298

pseudo-separation in DOSY spectrum. Although the molecular weight of DMO is the biggest,

299

EG, MG, and ethanol diffuse slower than DMO in methanol-d4 because they contain a diverse

300

number of hydroxyl groups and form stronger hydrogen bonds with methanol-d4.53 EG has two

301

hydroxyl groups and diffuses slowest. Both MG and ethanol contain one hydroxyl group in their

302

structures, but MG possesses a bigger molecular weight, so it diffuses slower than ethanol.

303 The

1

304

Figure

305

hydrogenation/hydrogenolysis mixture. The experiment temperature is 298 K.

306

4. CONCLUSIONS

307

4.

H

DOSY

spectrum

(methanol-d4

as

the

solvent)

of

DMO

Several model mixtures, which consisted of the reactant and products of the 22


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hydrogenation/hydrogenolysis reactions of biomass-derived platform compounds, are pseudo

309

separated by DOSY technique. The results indicated that DOSY is a suitable method for

310

analyzing most of these mixtures without any pre-treatment or purification required. In general,

311

the differences in molecular weights of the components in mixtures dominate the pseudo

312

separation in the diffusion dimension. The structure of molecules as well as hydrogen bonding

313

play an important role for enhancing the diffusion resolution. When the signals overlap in the

314

plots, or the compounds in mixture have similar diffusion coefficients, the 1D selective gradient

315

TOCSY is an additional tool for further identification. With the continuous refinement of the

316

advanced NMR techniques, NMR is becoming an optional method for mixture analysis.

317

AUTHOR INFORMATION

318

Corresponding Author

319

*E-mail: [email protected]; [email protected]

320

Notes

321

The authors declare no competing financial interest.

322

ACKNOWLEDGEMENTS

323

The authors thank for National Natural Science Foundation of China (U1710106) and the

324

Key Research and Development Program of Shanxi Province (international cooperation) 23


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Page 24 of 29

325

(201703D421041) for financial support. We thank Christian Marcus Pedersen for help with

326

preparing the manuscript.

327

ABBREVIATIONS

328

NMR, nuclear magnetic resonance; DOSY, diffusion-ordered spectroscopy; TOCSY, total

329

correlation spectroscopy; GC, Gas chromatography; HPLC, high-performance liquid

330

chromatography;

331

5-hydroxymethylfurfural; DHMF, 2,5-dihydroxymethylfuran; DHMTHF, 2,5-bishydroxymethyl

332

tetrahydrofuran;

333

2,5-dimethylfuran; DMTHF, 2,5-dimethyltetrahydrofuran; FAL, furfural； FOL, furfural alcohol;

334

2-MF, 2-methylfuran；2-MTHF, 2-methyltetrahydrofuran; DMSO-d6, dimethyl sulfoxide-d6;

335

D2O, deuterium oxide; EG, Ethylene glycol; MG, Methyl glycolate; BPPLED, bipolar pulse

336

longitudinal eddy current delay.

PFG,

5-MF,

pulsed

field

5-methyl

gradient;

furfural;

DMO,

5-MFA,

dimethyl

5-methyl

337

24


oxalate;

furfurylalcohol;

5-HMF,

DMF,

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Synopsis: A great NMR tool, DOSY plus selective TOCSY, is investigated detailed for analyzing hydrogenation/hydrogenolysis reaction mixtures of biomass-derived platform compounds. 529x211mm (96 x 96 DPI)


DOSY Plus Selective TOCSY: An Efficient NMR Combination for

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