Elsevier

Journal of Catalysis

Volume 388, August 2020, Pages 11-19
Journal of Catalysis

Selectivity enhancement in the g-C3N4-catalyzed conversion of glucose to gluconic acid and glucaric acid by modification of cobalt thioporphyrazine

https://doi.org/10.1016/j.jcat.2020.04.027Get rights and content

Highlights

  • g-C3N4/CoPz composite has high activity in oxidation of glucose to organic acid.

  • CoPz modification can enhance the separation of photogenerated carriers for g-C3N4.

  • CoPz modification can reduce the adsorption of organic acid on catalyst surface.

Abstract

g-C3N4/CoPz (cobalt tetra(2,3-bis(butylthio)maleonitrile)porphyrazine) composite was found to be highly photocatalytic active in oxidation of glucose to gluconic acid and glucaric acid using H2O2 as the oxidant under mild conditions. Under the optimal conditions, the total selectivity of gluconic acid and glucaric acid reached 79.4% with 52.1% of glucose conversion under xenon lamp irradiation within 20 min. The surface modification of g-C3N4 by CoPz displayed a significantly enhanced photocatalytic activity and improved selectivity to gluconic acid and glucaric acid in glucose oxidation in comparison to pure g-C3N4, this was attributed to the synergistic effect between g-C3N4 and CoPz. The influence of the CoPz content, H2O2 addition, glucose concentration and catalyst amount on the glucose oxidation were further evaluated and discussed. g-C3N4/CoPz composite exhibited more efficient separation of the photogenerated carriers in comparison to the individual components, which was responsible for its enhanced photocatalytic activity. The CoPz modification can enhance the adsorption of glucose and reduce the adsorption of organic acids on the surface of catalyst, more importantly, the presence of CoPz can reduce the formation of strong and nonselective hydroxyl radical (·OH), thus obtaining high selectivity of gluconic acid and glucaric acid. In addition, recyclability experiments revealed that the g-C3N4/CoPz composite photocatalyst was stable after five cycles with no apparent loss in activity or selectivity.

Introduction

Biomass as the widely available and renewable resource on earth is a promising alternative to fossil-based resources for producing value-added chemicals or other energy products [1], [2]. Glucose is the unit of C6 carbohydrates, which is the main component of biomass. The oxidation of glucose into gluconic acid and glucaric acid is the important transformation reaction. Gluconic acid is an important intermediate in pharmaceutical and food industries [3], [4], [5]. Glucaric acid is one of the top 12 platform chemicals from biomass, which can be used to produce nylons, plastics and food additives [6], [7], [8]. Currently, gluconic acid is manufactured by the enzymatic oxidation of glucose, and the production route of glucaric acid mainly involves the oxidation of glucose using nitric acid or bleaching agents [4], [8], [9], [10], [11]. However, these traditional processes have a few drawbacks in practical production, for example, the biochemical process suffers from the problems of slow reaction rate and the difficulty in separation of free enzymes, and chemical method has the disadvantages of toxic substances and waste products [12], [13], [14].

Owing to the problems of traditional processes for glucose oxidation, an alternative eco-friendly technology based on the use of clean oxidants (e.g., O2, H2O2) by virtue of heterogeneous photocatalysis is currently receiving considerable attention, because heterogeneous photocatalytic transformation can be ideally driven by sunlight under mild conditions. For example, much research effort has been devoted to developing photocatalytic oxidation of glucose using TiO2-based heterogeneous photocatalyst, photocatalytic oxidation of glucose can provide a wide range of value-added products containing carboxylic acids and other oxygenated hydrocarbons [15], [16], [17], [18], [19]. Nevertheless, it is rare to acquire desirable selectivity for photocatalytic oxidation of glucose to gluconic acid and glucaric acid in the presence of only water as the solvent. Gluconic acid and glucaric acid are the oxidation products of glucose without the cleavage of the C6 chain. Although heterogeneous photocatalysis provides a mild route for glucose conversion, controlling selectivity in oxidation of glucose at C1 position or C1/C6 positions is still quite difficult because of the multifunctional structure of glucose molecule. Therefore, it is highly desirable to explore new photocatalytic system for efficient transformation of glucose to gluconic acid and glucaric acid in water.

Graphite-like carbon nitride (g-C3N4) is a promising photocatalytic material with the advantages of metal-free, nontoxicity, low cost, suitable band gap and high structural stability under both thermal and photochemical conditions [20], [21], [22], [23], [24], [25]. The conduction band and the valence band of g-C3N4 are −1.3 V and 1.4 V at pH 7 versus the normal hydrogen electrode, respectively [26]. As seen from these band characteristics of g-C3N4, light-excited electrons in the conduction band of g-C3N4 possess a large thermodynamic driving force to reduce molecular oxygen, but the moderate oxidation potential of photogenerated holes is inadequate to oxidize the surface hydroxyl group to strong and nonselective hydroxyl radical (·OH). These features of g-C3N4 suggest that it can act as a gentle photocatalyst for selective organic transformations in aqueous media. Indeed, it has been reported that g-C3N4 is an excellent photocatalyst for selective organic transformations, such as benzene into phenol [27], alcohol into aldehyde [26], amine into imine [28], sulphide into sulfoxide and 5-hydroxymethyl-2-furfural into 2,5-furandicarboxyaldehyde [29], [30]. Meanwhile, g-C3N4 can also serve as the support material to immobilize metal nanoparticles. For example, Pd nanoparticles supported on mesoporous g-C3N4 can prompt the Suzuki coupling reaction under mild conditions, mesoporous g-C3N4 acts as the photocatalyst and Pd acts as the coupling catalyst [31]. However, like many other semiconductor photocatalysts, the main disadvantages of g-C3N4 are the low visible-light utilization efficiency and the high recombination rate of photogenerated charges [32], [33], [34], [35]. To address the aforementioned problems, various strategies have been employed to construct g-C3N4 based composite photocatalysts [36], [37], [38], [39], [40], [41], [42]. Among them, coupling g-C3N4 with metalloporphyrin is an efficient way to enhance the utilization efficiency of visible light and the separation efficiency of photogenerated charges, because metalloporphyrin possesses wide visible light absorption region and good electron donating properties [37], [38], [39], [43]. Metallothioporphyrzine (MPz) bearing sulfur-containing groups in the periphery of porphyrin macrocycle exhibits distinct optical and electronic properties in comparison to its porphyrin counterpart, which has been acted as the excellent candidate in photocatalytic fields [44], [45]. In our previous work, many attempts aimed the MPz coupling with semiconductors, such as ZnO and SnO2 [46], [47]. These reports reveal that the modification of MPz can effectively improve the visible light response and the photocatalytic efficiency of semiconductor based photocatalysts.

Considering the advantages of g-C3N4 and MPz, a hybrid material composed of g-C3N4 and MPz might serve as a suitable photocatalyst for facilitating organic transformation in a broader fashion. Herein, the g-C3N4/CoPz (cobalt tetra(2,3-bis(butylthio)maleonitrile)porphyrazine) composite was prepared through an effective mixing method, then the g-C3N4/CoPz composite as a photocatalyst was used for the selective photocatalytic oxidation of glucose in water. The results showed that g-C3N4/CoPz composite had a remarkably enhanced photocatalytic activity of glucose conversion in water using H2O2 as the oxidant, owing to the synergistic effects of two components. More importantly, the modification of CoPz can markedly enhance the selectivity of gluconic acid and glucaric acid, 79.4% of total selectivity to gluconic acid and glucaric acid at 52.1% of glucose conversion was obtained by photocatalysis using g-C3N4/CoPz composite photocatalyst in water.

Section snippets

Material and instruments

All chemicals used in this study were purchased from commercial suppliers and used without any further purification. Ultraviolet-visible (UV-vis) spectra measurements were performed on a Shimazu UV-2600 UV-vis spectrometer. UV--vis diffuse reflectance spectra (UV-vis DRS) measurements were also carried out using the Shimazu UV-2600 UV-vis spectrometer equipped with a diffuse reflectance attachment with an integrating sphere. The crystalline structure of the samples was determined by X-ray

Catalyst characterization

The light-absorption properties of the photocatalysts were measured using UV–vis DRS, as shown in Fig. 1. The pure g-C3N4 shows an absorption edge at about 450 nm. After coupling g-C3N4 with CoPz to form g-C3N4/CoPz composite, the visible light absorption of the composites are enhanced and the absorption edges also show a slight bathochromic shift. A strong absorption peak located at 654 nm ascribed to the Q-band of CoPz was observed in the g-C3N4/CoPz composites, and its absorption intensity

Conclusions

The g-C3N4/CoPz composite was achieved by CoPz immobilized onto the surface of g-C3N4 with an effective mixing method, which showed a high photocatalytic activity towards the oxidation of glucose to gluconic acid and glucaric acid using H2O2 as the oxidant under mild conditions. The experimental results indicated that there existed an interaction between the g-C3N4 and CoPz in the g-C3N4/CoPz composite. CoPz modification improved the visible light response and the separation efficiency of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21772237) and Natural Science Foundation of Hubei Province (No. 2018CFB494).

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