**1. Introduction**

The significantly growing demand for energy driven by an ever increasing global population and the strong economic growth in developing countries, the potential threats to the environment associated with the utilization of non-renewable fossil resources (oil, coal and natural gas) and difficulties with exploitation of the dwindling reserves have stimulated the research for alternative feedstocks that can be used for production of fuels and chemicals [1–5]. Biomass is one of the more promising and attractive alternative feedstocks, given its general abundance and wide-ranging availability in Nature and human society [1]. In addition, biomass is the only renewable carbon resource which can serve as a feedstock for the various carbon-containing chemicals and fuels that our society relies on [6].

5-Hydroxymethylfurfural (HMF), being a furan derivative, has been recognized as an important compound in our foods and as a versatile platform molecule derived from biomass for bulk chemicals and fuels production [7]. HMF can be used as biomass substrate compound for the production of a plethora of end products such as biofuels, biodegradable plastics, additives, macromolecules and

functional polymers via different reactions such as oxidation, hydration, hydrogenation, etherification and decarbonylation (Scheme 1a) [8,9]. In this review, we focus on one particular route, namely, the oxidation of HMF to a value-added chemical, 2,5-furandicarboxylic acid (FDCA). HMF can be produced from the dehydration of C6 carbohydrates or direct transformation of cellulose, involving several steps including catalytic hydrolysis of cellulose to glucose, followed by isomerization of glucose to form fructose, and finally dehydration of fructose to produce HMF. Acid sites are normally required for the production of HMF, and the formation of a multitude of other components, i.e., formic acid, levulinic acid and humin, is also observed during the production of HMF, which hampers the purity and yield of HMF [10].

**Scheme 1.** Possible value-added platform chemicals from (**a**) HMF and (**b**) FDCA (reproduced from [11] with permission from Wiley-VCH, copyright 2019).

FDCA, being a promising molecule in the furan family, contains a multifunctional cyclic structure with two carboxylic acid groups attached at the *para* positions of the furan ring. It is listed on the top in all biomass-derived value-added chemicals by the US Department of Energy [12]. FDCA is considered stable, even at high temperatures, because of its high melting point (342 ◦C) due to which it is not easily soluble in common organic and inorganic solvents [13]. Scheme 1b summarizes the possible value-added chemicals which can be derived from FDCA in different reaction systems. FDCA is actually used as a starting material for the synthesis of a new class of bioderived polymers such as polyethylene 2,5-furandicaboxylate (PEF) [14,15]. PEF displays a series of excellent properties (thermal, chemical and mechanical resistance as well as easy depolymerization in Nature), compared to its petrochemical counterpart polyethylene terephthalate (PET) [16,17]. PET is the most common thermoplastic polymer material of the polyester family, and widely used in containers for foods and liquids, thermoforming for manufacturing, fibers for clothing, etc. [18]. The potential of substituting PET by this new bio-polyester PEF has stimulated grea<sup>t</sup> research efforts on this topic [19–22].

The production of FDCA has been studied since the 19th century. Firstly in 1876, Fittig et al successfully prepared FDCA by conducting the dehydration reaction of mucic acid over an acidic catalyst [23]. Concisely, aqueous hydrobromic acid (HBr) which acts both as catalyst and solvent was reacted with mucic acid to produce FDCA. Later, different dehydrating agents were also applied, along with some modifications to ge<sup>t</sup> higher efficiency in the dehydration process but this process is limited because of the severe reaction conditions i.e., high temperature (>120 ◦C), >20 h reaction time, the use of highly concentrated acids and less FDCA selectivity as well as moderate yield (<50%) [24]. Later HMF emerged as a promising biomass feedstock to produce FDCA. Direct oxidation of HMF is a simple method for the efficient and economical production of bio-based FDCA. Scheme 2 presents the general reaction scheme for the production of FDCA by catalytic oxidation of HMF monomer. In the typical reaction pathway, the synthesis of FDCA through catalytic oxidation of HMF proceeds initially through the selective oxidation of the hydroxyl group of HMF to produce 2,5-diformylfuran (DFF) (Scheme 2, Path 1), or via oxidation of the aldehyde group to form 5-hydroxymethyl-2-furan carboxylic acid (HMFCA) (Scheme 2, Path 2). Both intermediates are then further oxidized to 5-formyl-2-furancarboxylic acid (FFCA), which is finally transformed to FDCA [25]. Several reaction systems with different oxidants are used to activate the oxygen species for catalytic oxidation of HMF such as oxygen, air, H2O2, and KMnO4 [26]. Oxygen or air are normally preferred for HMF oxidation due to the broad availability, low price, and benignity to the environment.

**Scheme 2.** Reaction scheme for catalytic oxidation of HMF into FDCA.

Figure 1 depicts the total number of research publications on HMF and FDCA per year from 2000 to 2019. The past few years in particular have seen a significant increase in the number of publications on HMF and FDCA chemistry. Due to maximum product separation capability, oxidation of HMF is considered as advantageous process to produce FDCA in an economical way [27].

**Figure 1.** Total number of reported publications on HMF and FDCA chemistry per year from 2000 to 2019 (topic keywords searched in *Web of Science*: "5-hydroxymethylfurfural HMF" and "2,5-furandicarboxylic acid FDCA").

Various catalytic systems involving heterogeneous catalysts [28], homogeneous catalysts [29], and bio-catalysts [30] have been reported for this process in aqueous media as well as organic and biphasic systems [31]. Electrochemical [32] and photocatalytic [33] processes have also been studied for HMF oxidation to FDCA. Owing to the good recycling capability and stability, heterogeneous catalysts are the most studied for this process. In this review, only metal-based solid catalysts will be discussed, which thus excludes the other catalytic systems, i.e., homogeneous catalysts and bio-catalysts. We first highlight the recent developments and current state of art of the application of metal-based heterogeneous catalysts for the catalytic aerobic oxidation of HMF to FDCA. The emphasis of this review is thus put on comparing the catalytic performance of different catalysts categorized by metal, with specific examples using molecular oxygen (O2) as oxidant and without using base additives. Reaction mechanisms of the aerobic oxidation of HMF to FDCA are also demonstrated in detail. Finally, conclusions are provided and perspectives referring to further development of the catalysts for the practical production are also highlighted, especially in designing e fficient catalysts for green and cost-e ffective catalytic oxidation of HMF to FDCA.

#### **2. Noble Metal Catalysts for FDCA Production**

Oxidation can be catalyzed by metal sites in heterogeneous catalysis. Various noble metals such as gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), and rhodium (Rh) catalysts have been reported for the catalytic oxidation of HMF to FDCA. The choice of molecular oxygen (O2) as oxidant offers advantages of availability and benignity to the environment, in accordance with the concept of "green chemistry". As oxygen is not easy to activate, supported noble metal catalysts are the main heterogeneous catalysts used for the catalytic aerobic oxidation of HMF to FDCA. We will first summarize the recent advances in applying noble metal catalysts for the oxidation of HMF to FDCA under relatively mild conditions.
