Elsevier

Catalysis Today

Volume 324, 1 March 2019, Pages 66-72
Catalysis Today

Direct chemical conversion of xylan into furfural over sulfonated graphene oxide

https://doi.org/10.1016/j.cattod.2018.07.002Get rights and content

Highlights

  • GO-SO3H was found to be greater for the production of furfural (87%) from xylan without undesired char formation.

  • GO-SO3H was offered its reusability without significant loss in catalytic activity.

  • Oxy-functional groups in GO-SO3H strengthen the -SO3H and helps to enhance the substrate sorption capacity.

Abstract

Graphene oxide (GO) grafted with –SO3H group giving a Brønsted acidic function was successfully applied for selective conversion of xylan to furfural (FA), which can be utilized as a platform chemical for a variety of value-added derivatives. The sulfonated graphene oxide (GO–SO3H) was, for the first time, found to be superior for the selective production of FA with 86% yield from xylan in water as a green solvent. GO–SO3H was successfully recycled after the reaction, and retained its catalytic activity without significant reduction. It is noteworthy that the Brønsted acid site acts as a key role in the production of furfural from xylan. Additionally, the presence of oxy-functional groups not only strengthen grafting of –SO3H with GO which probably prevented leaching into the reaction medium, but also helps to enhance the sorption capacity of substrate on GO-SO3H.

Introduction

Utilization of renewable biomass for the production of chemicals has attracted growing interest to bio-based industries in light of decreasing fossil fuel resources and increasing emission of greenhouse gases [[1], [2], [3]]. The gradual depletion of fossil resources and growing demand for energy, have induced the search for alternative renewable resources. Hemicellulose is a polymer of pentose sugars (xylose, arabinose, etc.), and is extremely abundant and easily available worldwide, as is cellulose. Xylan which is a homopolymer of xylose units and the most common hemicellulose is usually derived from hardwoods and grasses [3]. It is noteworthy that FA is the dehydration derivative of both xylose and xylan and it has been identified as a very promising platform chemical for the production of value-added chemicals such as γ-valerolactone, furfural alcohol, FDCA, and methyltetrahydrofuran [3]. In 1921, FA was first industrially produced from xylan using concentrated sulfuric acid in pressurized steam by Quaker Oats [3,4]. At present, most industrial FA production is based on the chemical conversion of pentose sugars using homogenous catalyst systems [5], of which the yield is very similar in all processes (50% of theoretical yield). However, these processes are not sustainable considering most aspects of green chemistry point of view.

In practice, the development of technology related to the direct conversion of xylan into chemicals is more challenging and would be very beneficial in economical ways [4]. In the last decade, significant efforts have been made by researchers to develop efficient catalytic process to for the production of FA, but most of these studies have only focused on the use of Brønsted acid catalysts (rather than Lewis acids) in the conversion of pentose sugars to FA [4]. Moreover, the use of homogenous catalysts can create serious environmental and economic issues [4]. It is noteworthy to mention that heterogeneous catalysis does have a number of benefits over homogenous catalysts due to easy reusability from a reaction mixture, which is an important consideration for industrial manufacturing processes. Solid-acid catalysts were suggested as an alternative, eco-friendly route, even though the FA yields are often low. Furthermore, an excess of catalyst is often used due to the weak sorption behavior of such catalysts [[5], [6], [7], [8], [9]]. For example, Brønsted acid catalysts, such as Nafion Sac [], MCM-41-based functionalized materials [], and zeolites [] as well as sulfated and sulfonated materials [7d–7f], were investigated as possible catalysts for such conversions [7]. In most of the studies (using both homogenous and heterogeneous catalysts), the yield of FA was often found to be lower than desired due to formation of side products such as polymers and humins [[7], [8], [9], [10]]. In addition, these heterogeneous catalysts based processes required high activation energy (30–32 kcal/mol), which resulted in harsh reaction conditions to activate the complex polymeric structure of xylan [3,4,10].

Meanwhile, Graphene oxide (GO) forms graphene sheets that contain high densities of hydrophilic functional groups, including hydroxyl, carboxyl, and epoxy groups [11]. These hydrophilic functional groups provide grafting sites to incorporate the sulfonic acid groups acted as a Brønsted acid site through sulfonation [12]. Here, we report a new SO3H-functionalized GO (GO–SO3H) prepared by easy sulfonation for the one step chemical conversion of xylane to furfural. Remarkably, higher furfural yield of 86% was obtained by catalytic amount of GO–SO3H catalyst (10.0% of initial feed) in xylan decomposition. In addition, synergetic effects of surface single bondOH group and Brønsted acid sites in GO-SO3H are investigated by the surface grafting of methoxytrimethylsilane (MTS) for selective xylane decomposition to furfural. For comparison, the activity of a sulfonated activated carbon (AC–SO3H) is also reported.

Section snippets

Materials

All chemicals (analytical grade) were purchased from Sigma-Aldrich and used without further purification. In this study, Xylan (>90%) from birch wood was used as the feedstock. Graphene oxide (BET surface area 248 m2/g; N-BARO TECH, South Korea), and activated carbon (BET surface area 1150 m2/g; Duksan Chemicals, South Korea) were used.

Material preparation

The procedure followed for the synthesis of GO–SO3H was reported elsewhere [12]. The solid acid catalyst, GO–SO3H was prepared by sulfonation of GO using

Material characterizations

As discussed above, details of the synthesis procedure for the sulfonation of GO and AC were reported in a previous article [12], in which the structural and physicochemical properties of the synthesized materials were thoroughly investigated. These detailed investigations revealed that GO–SO3H and AC–SO3H are quite different in structural properties, acid amount, and acid type (Brønsted and Lewis). The physicochemical properties of the catalysts tested are presented in Table 1. The BET surface

Conclusions

GO–SO3H was found to be a very promising candidate for the selective synthesis of FA from initial xylan from biomass. The presence of Brønsted acid (SO3H) sites together with other oxy functional groups, are very important in such types of biomass conversion processes. Moreover, this can help to strengthen the –SO3H bonding and not allow leaching into the reaction mixture. GO–SO3H showed excellent catalytic performance and produced a maximum yield of 86.4% FA from xylan without any char

Acknowledgements

This work was supported by the Institutional Research Program of KRICT (SI-1802-02), and the Next Generation Carbon Upcycling Project (2017M1A2A2042517) by the National Research Foundation of Korea. PPU is kindly grateful to the Korea Research Fellowship Program by the National Research Foundation of Korea (2017H1D3A1A02053077).

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    Although KOH-activated carbon can not be successfully sulfonated by concentrated H2SO4, sulfonated KOH-activated carbon with high catalytic performance for furans production can be mildly prepared using sulfanilic acid and isoamyl nitrite as the sulfonating agent and oxidizing agent (Zhang et al., 2020b). The use of sulfonated graphene oxide (SGO) afforded a higher furfural yield (86%) for aqueous degradation of xylan than H2SO4 (59%) and sulfonated activated carbon (64.2%) (Upare et al., 2019). The nanostructure and oxygen-containing functional groups of SGO made the acidic sites more accessible to the substrate.

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