Hf-based metal organic frameworks as bifunctional catalysts for the one-pot conversion of furfural to γ-valerolactone

Highlights

• One-pot conversion of furfural to γ-valerolactone (GVL) was achieved over bifunctional Hf-based metal organic frameworks for the first time.

• The catalysts with both Brønsted and Lewis acid properties offer an efficient way to tandem cascade reactions from furfural to GVL.

• A higher yield of γ-valerolactone was obtained using a mixture of primary alcohol and secondary alcohol as hydrogen donors comparing to single secondary alcohol as hydrogen donor.

Abstract

One-pot conversion of furfural to the target product γ-valerolactone (GVL) is a challenging and meaningful part of biomass exploitation. Development of heterogeneous catalysts with both Brønsted and Lewis acid properties has proved to be promising and useful because they offer an efficient way to tandem cascade reactions from furfural to GVL. Herein, we successfully synthesized sulfated DUT-67(Hf), a novel bifunctional catalyst, via a post-synthetic modification method. The prepared sulfated DUT-67(Hf) was characterized by XRD, SEM, TEM, N₂ adsorption-desorption, Elemental (N, C, H, S) analyses, situ infrared spectra of pyridine adsorptions, XPS, FT-IR, TG, and NH₃-TPD. The acidity of the catalysts could be adjusted by submersion in different concentrations of aqueous sulfuric acid, giving 0.01 mol/L sulfuric acid for 0.42 mmol/g acidity to 0.1 mol/L sulfuric acid for 2.16 mmol/g acidity. Sulfated DUT-67(Hf) possessed by 0.06 mol/L aqueous sulfuric acid exhibited optimal catalytic activity and showed an 87.1% yield of GVL under the conditions of 180 °C after 24 h of reaction. Moreover, the mechanism of introducing Brønsted acid sites into DUT-67(Hf) and the better hydrogen donors was also investigated through contrasting experiments.

Introduction

Because of the gradual exhaustion of fossil resources and the increasing concentration on the global climate change, enormous efforts have been devoted to searching for an efficient method to reduce reliance on fossil resources. Biomass, which is inexpensive, renewable and abundant in nature, is a sustainable raw feedstock to produce liquid fuels and valuable chemicals that are conventionally derived from fossil resource [1,2]. Among the various renewable resources, lignocellulosic biomass, consisting of cellulose (40–50%), hemicellulose (20–35%) and lignin (15–25%), is the most available and logical raw feedstock to yield fuels, fuel additives and various kinds of high-value-added chemicals [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. As one of the most important industrial products derived from hemicellulose, furfural (FUR), a colorless, sweet-smelling, mobile liquid, is currently deemed a potential platform chemical [12,13]. A great diversity of value-added chemicals could be obtained from furfural such as furfuryl alcohol (FA), furfuryl alkyl ethers (FEs), levulinic acid (LA) and γ-valerolactone (GVL) [[14], [15], [16]]. Among those chemicals, γ-valerolactone has been regarded as a valuable platform chemical and serves numerous purposes. GVL can be converted to butene when catalyzed by SiO₂-Al₂O₃, the resulting butene is further applied to produce liquid C₈-C₁₆ alkenes [17]. GVL can also be used as a blending agent in conventional petrol and as a food additive [17,18]. Additionally, people have made use of GVL as a solvent for biomass processing, which led to significant progress in product yields and a more convenient method for producing biomass-derived chemicals [19].

One-pot conversion of furfural to GVL involves complex cascade reactions [10,16,20]. Roman-Leshkov et al. firstly demonstrated the one-pot conversion from furfural to GVL through sequential catalytic transfer hydrogenation (CTH) reaction and hydrolysis reactions. They achieved it over the physical mixture of Zr-Beta zeolite and Al-MFI-ns using 2-butanol as a hydrogen donor resulting in a high GVL yield of 78% after 48 h at 393 K [16]. The cooperation of Lewis and Brønst acid sites catalysts plays an important role in the cascade reaction from furfural to GVL, in which Zr-Beta zeolite supplies Lewis acids to catalyze the CTH reaction and Al-MFI provides Brønsted acids to catalyze the ring-opening reaction. Fan et al. recently showed that using the combination of Au/ZrO₂ and H-ZSM-5 as the catalyst, a high yield of GVL from furfural up to 77.5% could be achieved [21]. Antunes et al. reported that Sn Al-containing zeolite beta catalyst is also active for the one-pot conversion of furfural to GVL using isopropyl alcohol as a hydrogen donor [22]. Li et al. used meso-Zr-Al-beta zeolite as a robust catalyst for the one-pot conversion of furfural to GVL and a high GVL yield of 90% could be obtained using isopropyl alcohol as a hydrogen donor after 24 h at 393 K [23]. Winoto et al. reported on the one-pot transformation of furfural to GVL using heteropolyacid supported on Zr-Beta zeolite as catalyst, a GVL yield of 70% at 433 K after 24 h could be obtained [24]. Furthermore, the one-pot conversion of furfural to GVL could be carried out over ZrO₂-SBA-15 [25].

The overall reaction pathway is depicted in Scheme 1. Firstly, the conversion of furfural to furfuryl alcohol and its ether can be carried out through a catalytic hydrogenation reaction using isopropyl alcohol as the hydrogen donor catalyzed by the Lewis acid. Then the conversion of furfuryl alcohol and its ether to isopropyl levulinate can be achieved through ring-opening reactions with Brønsted acid sites. Isopropyl levulinate is converted to 4-hydroxy pentanoate isopropyl ester (4-HPPE) when catalyzed by Lewis acid sites. Then, the target product GVL is obtained by lactonization of 4-HPPE. Although each individual reaction has been intensively investigated in terms of various heterogeneous catalysts and optimal conditions, a single heterogeneous catalyst that can efficiently catalyze one-pot conversion of furfural to GVL is highly desirable to reduce the cost of equipment and improve efficiency of producing GVL.

Metal-organic frameworks (MOFs) have been developed for a broad variety of applications such as gas sorption, condensation, membrane, cycloaddition, alkylation, condensation reactions, carbene reactions, isomerization, energy storage and catalysis [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. As highly crystalline porous materials, MOFs are synthesized by using inorganic metal ions or metal-containing clusters as nodes and organic moieties as linkers through coordination bonds [38]. Large surface areas, tunable structures and physicochemical properties are acknowledged as significant advantages of metal-organic frameworks (MOFs) [39]. From the viewpoint of catalytic science, large surface areas and accessible porosities will provide a greater number of active sites for the reactant. Sizable channels or cages contribute to reagents diffusion, which makes the active sites more accessible to the reactant [28].

Among the numerous series of metal-organic frameworks (MOFs), Zr/Hf-based MOFs, which possess a wide range of structure types, excellent thermal abilities and chemical stability, have been used for the CTH reaction of carbonyl compounds such as furfural, levulinic acid and its ester [39,40]. From 2016, Valekar et al. first applied MOF-808 to the catalytic transfer hydrogenation reaction of ethyl levulinate and obtained a satisfactory yield [41]. Then, Rojas-Buzo et al. synthesized Hf-MOF-808 and applied it to the Meerwein–Ponndorf–Verley reduction of several carbonyl compounds with excellent yields [42]. Kuwahara et al. synthesized sulfonic acid-functionalized UiO-66 used for the catalytic transfer hydrogenation of levulinic acid and its ester [43]. With these advantageous properties, Zr/Hf-based MOFs are expected to be applicable catalysts for biomass transformation [40]. However, using bifunctional MOFs to catalyze one-pot conversion of furfural to GVL has not yet been realized.

Inspired by the pioneer work [44,45] and our previous work [46], we report the synthesis and characterization of bifunctional metal-organic frameworks (MOFs) sulfated DUT-67(Hf) and its application to one-pot conversion from furfural to GVL. By introducing sulfate into DUT-67(Hf), the prepared catalysts exhibit robust Brønsted acidity, which allows Lewis acid and Brønsted acid sites within a single catalyst. The synergy of Lewis acidity and Brønsted acidity enables the prepared catalysts to catalyze the cascade reactions from furfural to GVL efficiently [22]. To our knowledge, prior to this article, no report has been focused on one-pot conversion of furfural to GVL using bifunctional metal-organic frameworks (MOFs).