Achieving effective fructose-to-5-hydroxymethylfurfural conversion via facile synthesis of large surface phosphate-functionalized porous organic polymers
Graphical abstract
Introduction
With climate change and resource depletion, there is growing interest in replacing fossil resource-based approaches with renewable and sustainable alternatives for the production of chemicals and fuels. Specifically, the conversion of biomass materials such as simple carbohydrates into value-added chemicals has received significant attention for their use as feedstocks for the synthesis of commodity chemicals as well as biofuels [1,2]. Among these, 5-hydroxymethylfurfural (HMF) is a versatile precursor chemical that is listed as one of the top 12 value-added chemicals synthesized from carbohydrates by the U.S. Department of Energy [3]. HMF and its derived products can be used as a starting material for a variety of commodities, including poly-Schiff bases, pharmaceuticals, organic conductors and polymers, and cross-linking agents of polyvinyl alcohol used for batteries [4,5].
To make the overall conversion process environmentally friendly and economically viable, many acid catalysts have been tested to promote a high yield and selective formation of HMF from carbohydrates. Although homogeneous acids such as mineral acids [6,7], ionic liquids [8,9], and organic acids [10] have been investigated, heterogeneous catalysts are often preferred choices for fructose and other biomass conversion owing to their advantages in product separation and ease of handling. A variety of heterogeneous acid catalysts, such as metal salts [11,12], ionic liquid supported metal oxides [13,14], alumina [15], metal carbide [16], functionalized carbon [17], graphene oxide [18,19], and zeolites [20], have been studied to determine their catalytic activities in biofuel production from fructose (Scheme 1) and other biomasses. The physicochemical properties of these heterogeneous catalysts, such as surface acid sites/density, external surface area, and porosity, influence the accessibility and diffusion of fructose to the surface acid sites, thereby influencing the overall catalytic performance. While some of these catalysts may exhibit a high HMF conversion yield, they are often chemically unstable or weakly stable, thereby resulting in limited reuse of the catalyst materials.
Porous organic polymers (POPs) are recently developed as a new class of covalent organic-polymer-based porous materials that can be assembled via the reticular chemistry principle. POPs exhibit exceptional physicochemical stability and can often be tailored to have a high specific surface area and relatively uniform pore structure. These characteristics make them highly promising for various applications, including catalysis [21,22], adsorbents for water and gas contaminants [23,24], energy storage [25], and bio-applications such as drug delivery and phototherapeutics [26]. Furthermore, POPs can be functionalized with specific functional groups via hyper-cross-linked polymerization. This is highly promising for biomass conversion because target Bronsted acids, such as phosphoric acid, sulfonic acid, and hydrochloric acid, can be functionalized as surface acid sites on the POPs at a relatively high density. In this regard, few studies have investigated the applicability of sulfonic acid-functionalized POPs for HMF synthesis [[27], [28], [29]]. While these materials also exhibit effective catalytic activity, catalyst synthesis often requires a harsh chemical environment. Korner et al. investigated the impact of different homogeneous Bronsted acids on the fructose-to-HMF hydrothermal dehydration and reported that phosphoric acids showed much better yields and selectivity for HMF at a similar proton concentration compared with those of sulfonic acid and hydrochloric acid [30]. However, to the extent of our knowledge in heterogeneous catalyst field, there is no existing report on phosphate-functionalized POPs and their catalytic activity for selective conversion of biomass to HMF.
We hypothesized that comprising acidic sites (i.e., phosphate functional groups within the porous organic polymer framework via facile one-pot synthesis can provide an excellent catalytic property for fructose conversion to HMF. Additionally, we anticipated using different phosphate precursors would influence the surface area and mesoporosity of the synthesized POPs and consequently their catalytic activity. Based on these hypotheses, we investigated the physicochemical properties of two phosphate-functionalized POPs, namely B-POP and P-POP, synthesized from crosslinking 1,3,5-triphenylbenzene (TPB) with different precursors, dibenzyl phosphate (DBP) and diphenyl phosphate (DPP) respectively (Scheme 2), and their relationship with catalytic activity and selectivity for fructose conversion to HMF. The specific objectives of this study were 1) to investigate the effect of precursors on physicochemical characteristics of synthesized POPs, 2) to evaluate their catalytic activity and selectivity for HMF formation under different reaction conditions (e.g., solvent type, temperature, catalyst loading, and reaction time) and to identify the optimum conditions, 3) to assess the applicability of phosphate group-functionalized POPs for other biomass compounds, and 4) to test the catalyst stability over extended use.
Section snippets
Chemicals
TPB, DPP, DBP, iron chloride anhydrous, formaldehyde dimethyl acetal (FDA), 1,2-dichloroethane (DCE), dimethyl sulfoxide (DMSO), acetonitrile (ACN), 1,4-dioxane, methanol, ethanol, dichloromethane (DCM), tetrahydrofuran (THF), pyridine, and methanol were obtained from Sigma Aldrich (U.S.A.). Fructose, glucose, cellulose, and sucrose were purchased from Alfa Aesar (U.S.A). All the chemicals were purchased as analytical grade and used as is. N2 gas was purchased from Daehan Special Gas Co., Ltd.,
Synthesis and tailoring physicochemical properties of phosphate-functionalized porous organic polymers (POPs)
The POP catalysts functionalized with phosphate group were synthesized via Friedel-Crafts reaction between aromatic precursor (i.e., 1,3,5-triphenylbenzene) and phosphate-containing cyclic group (i.e., diphenyl phosphate or dibenzyl phosphate) as shown in Scheme 2. Fig. 1 shows the FE-SEM images of synthesized phosphate-functionalized POPs. Both B-POP and P-POP catalysts possessed irregular surface morphology with particle sizes of approximately 1 μm–3 μm. Elemental mapping showed that P along
Conclusions
In summary, two phosphate group-functionalized POPs, namely B-POP and P-POP, were synthesized via a simple one-pot method, and their catalytic activity for fructose conversion to HMF was tested in different solvents of a single medium as well as binary mixtures of DMSO and 1,4-dioxane. B-POP showed better catalytic conversion and selectivity than P-POP with a fructose conversion yield of >99 % and 78 % HMF selectivity within 30 min of reaction time in DMSO at 160 °C. The catalytic activity was
CRediT authorship contribution statement
Seenu Ravi: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Yongju Choi: Writing - review & editing. Jong Kwon Choe: Validation, Writing - review & editing, Supervision, Project administration, 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.
Acknowledgments
This work was supported by the Creative-Pioneering Researchers Program at Seoul National University (SNU) and the National Research Foundation of Korea (NRF-2017R1C1B1003353 and 2020R1C1C1006228). It is also supported by the BK21 PLUS research program of the National Research Foundation of Korea and Institute of Engineering Research at SNU.
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