Elsevier

Journal of Catalysis

Volume 338, June 2016, Pages 329-339
Journal of Catalysis

Niobium oxides: Correlation of acidity with structure and catalytic performance in sucrose conversion to 5-hydroxymethylfurfural

https://doi.org/10.1016/j.jcat.2016.03.007Get rights and content

Highlights

  • Niobium oxides with varying acidities are compared systematically.

  • Acidity-activity relations established for the catalytic conversion of sugars to HMF.

  • New reaction mechanistic details of sucrose, glucose and fructose conversion to HMF.

  • New insights into structure-acidity relations.

  • Conversion of relatively cheap sucrose to HMF of interest for renewable energy sector.

Abstract

The conversion of sugars to 5-hydroxymethylfurfural (HMF) over solid acids in water represents an environmentally and separation-friendly route to an important platform molecule. In particular, the conversion of sucrose attracts increasing attention because it is cheaper and more widely available than glucose and fructose. Sucrose can undergo rapid hydrolysis to the two monosaccharides, however conversion mechanisms and interactions with solid acids remain unclear. Here, it is shown that niobium oxides possess Brønsted acid (BA) and Lewis acid (LA) sites of tunable quantity and strength, dependent on their structure and morphology. By systematically studying these acid catalysts, it is revealed for the first time that both acid type and strength are significant for the sugar conversion: Fructose reaction is catalyzed by BA, with weaker BA sites being more selective toward HMF. Glucose conversion to HMF involves an additional isomerization step to fructose, which can be catalyzed by both LA and strong BA but LA is more efficient. Sucrose is shown to be easily hydrolyzed into glucose and fructose under the reaction conditions and HMF is formed from the further conversion of the two sugars. It is demonstrated that mesoporous niobium oxide gives the highest HMF yield for sucrose conversion among all niobium oxides due to balanced BA and LA sites with appropriate acid strengths.

Introduction

Fossil fuels are currently the main source of both energy and platform molecules for the chemical industry. Rapidly decreasing reserves and growing economic demands raise the need for alternative sources. Biomass is a desirable choice as there is no net carbon emission upon its use. Extensive research over past years has led to the establishment of possible routes to convert biomass to fuels and process chemicals [1], [2], [3], [4]. 5-hydroxymethylfurfural (HMF), derived from sugar molecules, is one of the most promising platform molecules. Various acid catalysts, both homogeneous and heterogeneous, have been employed to convert sugar molecules to HMF. Including the use of different solvents, reaction conditions and types of sugar, large amounts of data have been accumulated [5], [6], [7], [8], [9], [10]. Disregarding the type of catalysts used, it has been found that HMF yield differs significantly with the type of sugar molecule involved. Fructose is generally found to be more reactive and selective toward HMF than glucose. Although these two sugar molecules are isomers to each other and the same number of water molecules is lost during the HMF formation [6], [11], it is not yet clear why there is a significant difference in their intrinsic reaction rate. Two main reaction pathways for their HMF conversions have been proposed in the literature [5], [6]. The acyclic pathway proceeds via open chain intermediates such as 1,2-enediol and 3-deoxyglucos-2-ene common to both glucose and fructose [12], [13]. The cyclic pathway on the other hand, proceeds via closed ring intermediates to HMF from fructose. Glucose conversion involves an extra isomerization step to fructose prior to HMF (Scheme 1) [14], [15], [16], [17], [18], [19]. It has been claimed that catalysts with Lewis (LA) and/or Brønsted acid (BA) sites are needed for both sugar conversions, however there is no agreement among researchers which type and strength of acid are required. The mechanisms remain unclear regarding the two forms of sugars and their relationship [20], [21]. In practice, sucrose, a dimer composed of glucose and fructose, is of main industrial interest because it is cheaper and more widely available than its components [22]. Although homogeneous acid catalysts, such as organic acids and ionic liquids, can give good yields and selectivities for sugar conversion, extensive product separation procedures make heterogeneous catalysts more attractive. Similarly, separation difficulties are encountered with many yield-enhancing organic solvents employed to extract HMF from aqueous phase. Besides, the use of water as solvent is environmentally friendlier [5], [7], [8]. In order to make sucrose conversion scalable and sustainable we may need to consider its catalytic and process features in an industrial and environmental context. This requires an in-depth understanding of the underlying reaction mechanisms. However, the conversion of sucrose to HMF in comparison with glucose and fructose has not been investigated in depth yet. Our approach to this challenge is to carry out a systematic study of the conversion of glucose, fructose and sucrose to HMF in water over a variety of structurally related solid niobium oxides. Mechanistically, the conversion of sucrose is closely linked to that of its components glucose and fructose, making it worthwhile to study all three sugars comparatively. Niobium oxides are solid acids consisting of interconnected NbO6 octahedra with varying extents of structural distortions and defects. Both BA and LA sites have been identified on niobium oxides, with acidic strength dependent on structure and environment, but the detailed relationship between structure and acidity is not clear [23], [24], [25], [26], [27]. In general, BA sites on metal oxides are associated with strongly acidic hydroxyl groups acting as proton donors and LA sites with exposed oxygen-deficient cations acting as electron pair acceptors [28], [29], [30], [31]. Due to their strongly acidic character, niobium oxides have been studied previously for sugar conversion, for example, de Souza et al. obtained 28% HMF from glucose in water over NbOH and Carlini et al. achieved 29% HMF from fructose in water over phosphoric acid treated NbOH [10], [20]. However, the catalytic mechanisms have not been studied in detail in the aforesaid works, which could help to rationalize the reaction yields.

We are therefore interested in developing niobium oxide as a solid catalyst for the conversion of sucrose to HMF based on a deeper understanding of reaction mechanisms and catalytic features. Using TMP (trimethylphosphine) 31P NMR we have been able to measure the acid properties of different niobium oxides more accurately than previous measurement techniques such as CO, pyridine FTIR and NH3 TPD [32]. The detailed information obtained, in combination with structural analysis by Raman, TEM and XRD, enables us to gain new insights into how acidity correlates with structure in this compound group. Further, catalytic conversions over the niobium oxides for glucose, fructose and sucrose to HMF have been compared systematically. This offers some unique mechanistic insights into the sugar conversion, by correlating acid type and strength with fundamental reaction steps. We are able to demonstrate that mesoporous niobium oxide among all the different types of niobium oxide catalysts studied, gives the highest HMF yield from sucrose due to the balanced BA and LA sites of its structure with appropriate acidic strengths. Overall, we believe this study offers some new essential information, which will lead to a more rational design of solid acid catalysts for sugar conversion.

Section snippets

Experimental

A variety of niobium oxides, ranging from bulk to multi- to monolayer structures has been synthesized according to reported methods. So far no comparative study of these compounds has been conducted, neither regarding structural aspects nor acid catalytic properties. The bulk material HNb3O8 has been prepared by a conventional solid-state reaction of the precursors K2CO3 and Nb2O5, followed by proton exchange. Few layer niobium oxide (hy-Nb) and monolayer niobium oxide (hy-Nb-TEOA) have been

Characterization of catalyst acidity and structure

Different forms of niobium oxide give different surface areas, as shown by BET of nitrogen adsorption measurements (Fig. 1). Similarly large variations are found regarding acidity. By comparing the TMP NMR acidity (Fig. 2) and Hammett indicator measurements over the different forms of niobium oxide (Table 1), it can be seen that TMP NMR gives more detailed information. By means of 31P chemical shift values, BA and LA are easy to differentiate and variations in acid strength of both sites can be

Conclusions

In summary, we have used a combined approach of structural and catalytic performance studies to elucidate the mechanistic details of sugar to HMF conversion over niobium oxides. Using TMP NMR as a tool to analyze surface acidity gives much more detailed information than previous measurement techniques. By further performing TEM, surface area measurements, XRD and Raman spectroscopy, we have been able to gain new insights into structure and acidity relationships. LA sites appear to originate

Outlook

Sucrose is shown to be rapidly hydrolyzed into its components, glucose and fructose, under the reaction conditions. Therefore its efficient further conversion to HMF requires a combined catalytic approach, with both LA and weak BA sites present, in order to convert both glucose and fructose to HMF. Thus, mesoporous niobium oxide from the tested catalysts gives the highest HMF yield in sucrose conversion. So far the BA strength of solid acid catalysts, even when using water as the solvent, is

Acknowledgments

This research is financially supported jointly by EPSRC (UK) and SINOPEC (China).

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