545517 A New, Sustainable Visible-Light-Driven Methanol to Ethylene Glycol (MTEG) Process

Monday, June 3, 2019: 3:21 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Shunji Xie1, Zebin Shen1, Jiao Deng1, Pu Guo1, Qinghong Zhang1, Haikun Zhang1, Chao Ma2, Zheng Jiang3, Jun Cheng1, Dehui Deng4 and Ye Wang1, (1)Xiamen University, Xiamen, China, (2)Hunan University, Hunan, China, (3)Shanghai Synchrotron Radiation Facility, Shanghai, China, (4)Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

1. Introduction

Methanol can be derived from a variety of carbon resources via synthesis gas process, such as natural gas or shale gas, coal, biomass and carbon dioxide, and is an abundant and renewable one-carbon (C1) building block.1 Several processes such as methanol-to-olefins (MTO) and methanol-to-gasoline (MTG) have been developed for the  C−C coupling of methanol into chemicals, but these processes usually show limited selectivity to a specific product.2 Traditionally, the conversion of methanol involves the activation of its O−H or C−O bond. The system that can selectively activate the unreactive C−H bond of methanol with hydroxyl group intact and form C−C bond is rare.3 Here, we present the first visible-light-driven dehydrogenative coupling of methanol to ethylene glycol (MTEG) process (Eq. 1).4

2CH3OH ¨ HOCH2CH2OH + H2               (1)

2. Experimental

Photocatalytic reactions were carried out in a sealed quartz reactor. Reaction conditions: solution, 76 wt% CH3OH + 24 wt% H2O, 5.0 cm3; atmosphere, N2; light source, 300 W Xe lamp.

3. Results & Discussion

Eq. 1 cannot proceed via conventional catalysis because of the thermodynamic limitation. Photocatalysis is a promising strategy for C−C coupling reactions. We investigated many typical semiconductor photocatalysts for the conversion of CH3OH. However, over most semiconductors examined, instead of EG, HCHO was formed as a major carbon-based product (Table 1), suggesting that the O−H bond in CH3OH is easier to be activated. We discovered that a few metal sulfides such as ZnS and CdS could catalyze the formation of EG, and CdS showed better selectivity under visible-light irradiation. Then, we investigated the effect of various co-catalysts. We found that the loading of MoS2 nanofoam onto CdS nanorods could remarkably enhance the formation rate of EG. The rates of EG and H2 formations over MoS2-foam/CdS were about 24 and 16 times higher than those over CdS alone (Table 1). We performed characterizations and found that MoS2 nanofoam with abundant edge sites accelerates the photocatalytic activity for EG formation by both providing H2-evolution active sites and enhancing the transfer of photogenerated electrons and hole (Figure 1). The mechanistic studies reveal that CdS-based photocatalyst is quite unique in the preferential activation of the C−H bond in methanol without affecting the O−H group, forming EG via •CH2OH radical intermediate. High selectivity (90%) and yield (16%) of EG were obtained by designing a process-intensified reactor with EG separation capability.

Table 1. Catalytic performances of typical semiconductor photocatalysts for the conversion of methanol.

Catalyst

Formation rate (mmol gcat-1 h-1)

e- / h+

Selectivity (%)

EG

HCHO

HCOOH

CO

CO2

H2

CH4

EG

HCHO

HCOOH

UV-Vis light

TiO2

0

1.6

0.11

0.16

0.042

2.0

0.053

0.91

0

84

5.6

ZnO

0

3.0

0.038

0.23

0.028

3.1

0.14

0.90

0

91

1.2

g-C3N4

0

0.79

0.33

0.11

0

1.5

0.039

0.92

0

64

27

ZnS

1.3

2.2

0.067

0.083

0

3.4

0.087

0.92

54

43

1.3

Visible light

ZnS

0

0

0

0

0

0

0

Cu2O

0

0.46

0

0

0

0.42

0

0.91

0

100

0

Bi2S3

0

0.13

0.017

0.023

0

0.19

0

0.91

0

77

10

CuS

0

0.11

0.013

0

0

0.13

0

1.0

0

89

11

CdS

0.46

0.38

0

0

0

0.75

0

0.90

71

29

0

MoS2-foam/CdS

11

2.5

0

0

0

12

0

0.92

90

10

0

Figure 1. Structural properties of the MoS2-foam/CdS catalyst. a, TEM image. b, High-resolution HAADF-STEM image. c, Schematic illustration of MoS2-foam/CdS for photocatalytic synthesis of EG and H2 from CH3OH.

4. Conclusions

We have discovered that the CdS-based photocatalytic system is quite unique in the preferential activation of the C−H bond in methanol for subsequent coupling to EG. The loading of MoS2 significantly improves the formation of EG. The present visible light-driven MTEG process not only offers an atom-efficient method for the synthesis of EG under mild conditions but also opens up a new avenue for preferential C−H bond activation without affecting other functional groups in the same molecule.

References

1.        G. A. Olah, Angew. Chem. Int. Ed. 52, 104-107 (2013).

2.        U. Olsbye, S. Svelle, M. Bjørgen, P. Beato, Ton V. W. Janssens, F. Joensen, S. Bordiga, K. P. Lillerud, Angew. Chem. Int. Ed. 51, 5810-5831 (2012).

3.        S. Zhang, F. Zhang, Y. Tu, Chem. Soc. Rev. 40, 1937-1949 (2011).

4.        S. Xie, Z. Shen, J. Deng, P. Guo, Q. Zhang, H. Zhang, C. Ma, Z. Jiang, J. Cheng, D. Deng, Y. Wang, Nat. Commun. 9, 1181 (2018).


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