Elsevier

Molecular Catalysis

Volume 514, September 2021, 111765
Molecular Catalysis

Efficient and selective oxidation of 5-hydroxymethylfurfural catalyzed by metal porphyrin supported by alkaline lignin: Solvent optimization and catalyst loading

https://doi.org/10.1016/j.mcat.2021.111765Get rights and content

Highlights

  • Ambient pressure and less sodium hydroxide were used, demonstrating the sustainability of the process.

  • Agglomeration of metalloporphyrin could be suppressed due to the existence of lignin with larger steric hindrance.

  • Self-assembly process of deprotected lignin affords the catalyst with large specific surface area and high recoverability.

  • Larger O* amount and better dispersion of active oxygen species in supported catalyst promoted the improvement in catalytic activity and the decrease in the activation energy.

Abstract

Cobalt (II)-meso-tetra(4-carboxyphenyl) porphyrin supported by deprotected lignin was synthesized through the covalent linkage between carboxy groups in the porphyrin and hydroxyl groups in the lignin. The resulting supporting catalysts were applied in the selective oxidation of 5-hydroxymethylfurfural. Through solvent optimization, 5-hydroxymethyl-2-furancarboxylic acid and 2,5-furandicarboxylic acid could be selectively obtained. Steric effect of lignin prevented unwanted aggregation-caused quenching, avoiding catalyst deactivation. Thus, supported catalysts displayed better catalytic efficiency compared free metal porphyrin catalyst. Unlike the excessive use of sodium hydroxide in the oxidation of HMF catalyzed by metal catalysts, an equivalent amount of sodium hydroxide was used in this oxidation process. Meanwhile, lower activation energy was detected when metal porphyrin supported by deprotected lignin was used. By adjusting the pH in the reaction, supported catalysts could be reused. The catalytic activity remained stable during the recycling experiments.

Introduction

Fossil fuels from oil, gas and coal are the predominant source for chemicals and energy. Although the utilization of fossil fuels accelerates economic and social development, some serious problems, such as resource depletion [1], global warming and air pollution [2,3], are gradually highlighted with the rapid consumption of fossil fuels. Among various renewable energies, biomass occupies the dominant position [4,5] due to abundant reserves and low price. Through a series of chemical transformations, biomass resources can be transformed into a variety of high-added chemicals and fuels, demonstrating tremendous potential to be the alternative for traditional fossil sources [6,7]. Generally, complex chemical reactions should be applied in the functionalization of petroleum molecules to introduce some functional groups [8]. However, inherent functional groups in the biomass simplify the chemical transformation. Thus, it is advisable to utilize biomass directly to obtain value-added chemicals.

5-Hydroxymethylfurfural (HMF), has been recognized as a biomass substrate compound due to the extensive application of its end products in the fuels, additives and functional polymers [9]. The hydroxyl group and formyl group in the furan ring endow high flexibility in the oxidation reaction, affording various oxidation products, such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 2,5furandicarboxylic acid (FDCA) [10]. Among these oxidation products, HMFCA and FDCA displayed potential applications in the functional polymers [11] through polymerization reaction due to the existence of carboxyl groups and hydroxyl groups. In addition, HMFCA also demonstrated excellent medicinal properties, such as antitumor activity [12]. Due to the molecular similarity to terephthalic acid, FDCA can be used as a monomer in the production of bio-based polymers and composites [13]. Bio-based polyesters from FDCA showed better performance compared with petroleum-based polyesters from terephthalic acid. Meanwhile, FDCA can also serves as versatile synthetic intermediates in plasticizers, pharmaceuticals and coordination polymers [14]. Thus, the exploration for more efficient methods for the preparation and utilization of HMFCA and FDCA has been attracting widespread attention from both academia and industry currently.

Typically, the synthesis of FDCA consists of two oxidation process including the in-situ generation of oxidation intermediates and the oxidation of intermediates. Both intermediates (DFF and HMFCA) can be further oxidized to 5-formyl-2-furancarboxylic acid (FFCA), which is finally transformed to FDCA. The synthetic route may be categorized into two classes according to the intermediates involved (Scheme 1) [15]. Various catalytic systems have been applied in this oxidation reaction, such as heterogeneous catalysts, homogeneous catalysts, and bio-catalysts [16]. Oxygen or air are normally preferred for HMF oxidation due to the broad availability, low price, and benignity to the environment [17,18]. Unlike the synthesis of FDCA, selective oxidation of the aldehyde group in HMF with reserved alcohol group is required in the synthesis of HMFCA. Generally, noble metal catalysts were used in the selective oxidation of HMF, affording HMFCA [19]. Other catalysts, such as immobilized molybdenum complex and photocatalysts, were also applied in the selective oxidation of HMF with a yield of 90-95% [20,21]. Similarly, metal catalysts (Au, Pd, Pt and Ru et al) occupied the main position in the oxidation of HMF for FDCA [[22], [23], [24], [25]]. Meanwhile, pressurized molecular oxygen was the main oxidant in the oxidation of HMF for FDCA [26,27]. However, the application of metal catalysts increased the cost. Pressurized gas enlarged the risk of oxidation reaction. Thus, the development of effective, inexpensive, recyclable, and non-precious metal-based heterogeneous catalysts for the oxidation of HMF is still challenging and of enormous demands [15].

Porphyrins are biologically important pigments that accomplish versatile functions in biological systems, such as oxygen transport, photosynthesis, and enzyme catalytic centers [28]. The coordination of the metal ions into the porphyrin center affords metal porphyrin with multiple redox states, demonstrating impressive catalytic activity in broad chemical reactions, including oxidation, cycloaddition, and reduction [29]. Although metalloporphyrins are widely used in chemical catalysis, they are rarely used in the conversion of HMF. At present, only Lang chang et al. reported that HMF was selectively converted into FDCA with a yield up to 91% over Merrifield resin supported Co (II)-meso-tetra(4-pyridyl)-porphyrin catalyst [30]. However, strong oxidant tert-Butyl hydroperoxide was used in the oxidation. In addition, the highest yield of FDCA can be obtained only when the reaction temperature reaches 100 °C. In the oxidation process catalyzed by metalloporphyrins under oxygen atmosphere, oxidative decomposition, self-polymerization and deactivation of metalloporphyrins are ubiquitous. Non-covalent inter-molecular forces, such as π-π stacking aggregates, cause the risk of aggregation-caused quenching, resulting in decreased catalytic activity [31]. Thus, chemical transformations are necessary for maintaining the catalytic activity of metalloporphyrins.

Lignin, a heterogeneous phenylpropanoid polymer, is currently the only feasible renewable feedstock for aromatic compounds. Due to the high abundance in aromatic ring, lignin displayed high steric hindrance. The combination of lignin and metalloporphyrins prevents aggregation and the related aggregation-caused quenching [31] through kinetic study. Cobalt (II)-meso-tetra(4-carboxyphenyl) porphyrin supported by Deprotected Lignin (DL-CoTCPP) was synthesized from alkali lignin by chemical bonding porphyrin. The catalyst was prepared and used for the oxidation of HMF into HMFCA and FDCA. To our knowledge, this is the first article to selectively prepare HMFCA and FDCA by regulating solvents. Meanwhile, reaction kinetics was also conducted to analyzed the oxidation process. The activation energies of supported catalysts and free catalysts were determined to reveal the role of lignin. Catalyst recovery process was designed to improve the sustainability. Catalyst recycling experiments were carried out to evaluate the availability of reusing.

Section snippets

Materials

Pyrrole, methyl p-formylbenzoate, triphenylphosphine and diethyl azodicarboxylate, all metal chloride salts were reagent grade and provided by Aladdin Corp. (Shanghai, China). HMF was purchased from Beijing Chemicals Co., Ltd. (Beijing, China). Oxidation products of HMF (FDCA and HMFCA) were analytical grade and supplied by J&K Chemical Co., Ltd., (Beijing, China). Alkaline lignin was provided by Xiangjiang Paper Industry Co., Ltd. (Hunan, China). Alkaline lignin was acidified and purified

Catalyst characterization

The procedure for preparing DL-CoTCPP was illustrated in Scheme 2. Firstly, TCPP was prepared by reacting pyrrole with methyl p-formylbenzoate in the presence of propanoic acid. Phenolic and aliphatic hydroxyl groups in Deprotected Lignin could react with carboxylic groups in the TCPP to form ester bonds, affording DL-TCPP [31]. The coordination between Co2+ and TCPP was realized through the reaction of Co(OAc)2 with TCPP at reflux temperature, yielding DL-CoTCPP. The possible structure of

Conclusions

In conclusion, a novel supported metal porphyrin was developed for the oxidation of HMF into HMFCA and FDCA. The catalyst can selectively oxidize HMF to prepare HMFCA and FDCA by solvent selection. Under optimized reaction conditions, the catalyst demonstrated excellent catalytic performance with HMFCA yield of 94.7% and FDCA yield of 71.24%. High temperature, high pressure, high concentration alkaline were not necessary in the oxidation process. Air or oxygen at ambient pressure was used as

CRediT authorship contribution statement

Yuchen Zhu: Visualization, Investigation. Hao Wu: Conceptualization, Methodology, Data curation, Writing – original draft. Zheng Fang: Project administration. Xiaobing Yang: . Kai Guo: . Wei He: Writing – review & editing, Supervision, Funding acquisition.

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.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Acknowledgment

The research has been supported by The National Natural Science Foundation of China (21776130 and 21908094); The State Key Laboratory of Materials-Oriented Chemical Engineering (KL19-01); The Top-notch Academic Programs Project of Jangsu Higher Education Institutions.

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