polymers

Article Pressure Reduction Enhancing the Production of 5- from Glucose in Aqueous Phase Catalysis System

Ke Ke 1, Hairui Ji 2, Xiaoning Shen 1, Fangong Kong 3 and Bo Li 1,3,*

1 State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China; [email protected] (K.K.); [email protected] (X.S.) 2 School of Light Industry Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 300175, China; [email protected] 3 State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology (Shandong Academy of Sciences), Jinan 300175, China; [email protected] * Correspondence: [email protected]; Tel.: +86-137-5182-6829

Abstract: 5-hydroxymethylfurfural (HMF) obtained from biomass is an important platform chemical for the next generation of plastics and biofuel production. Although industrialized, the high yield of HMF in aqueous systems was rarely achieved. The main problem is that HMF tends to form byproducts when co-adsorbed with water at acid sites. In this study, the pressure was reduced to improve the maximum yield of HMF from 9.3 to 35.2% (at 190 ◦C in 60 min) in a glucose aqueous

 solution. The mechanism here involved water boiling as caused by pressure reduction, which in  turn promoted the desorption of HMF from the solid catalyst, thereby inhibiting the side reaction of

Citation: Ke, K.; Ji, H.; Shen, X.; HMF. Furthermore, the solid catalysts could be reused three times without a significant loss of their Kong, F.; Li, B. Pressure Reduction catalytic activity. Overall, this work provides an effective strategy to improve the yield of HMF in Enhancing the Production of water over heterogeneous catalysts in practice. 5-Hydroxymethylfurfural from Glucose in Aqueous Phase Catalysis Keywords: glucose; 5-hydroxylmethyfurfural; aqueous phase; heterogeneous catalysts; pressure reduction System. Polymers 2021, 13, 2096. https://doi.org/10.3390/polym13132096

Academic Editors: Sylvain Caillol 1. Introduction and Cédric Delattre Biomass is the most abundant renewable resource in the world, with the advantages of low pollution, wide distribution, and renewability [1]. The development and utilization Received: 17 May 2021 of biomass is an effective way to alleviate global energy problems [2–6]; in addition to this Accepted: 10 June 2021 Published: 25 June 2021 aspect, some polymers can be prepared from biomass-based platform compounds. The interest in the preparation of polymeric materials based on biomass resources is growing.

Publisher’s Note: MDPI stays neutral Undoubtedly, polyethylene terephthalate (PET) is the one of the largest volume industrial with regard to jurisdictional claims in polymers. However, PET is not easy to degrade, which in turn causes environmental published maps and institutional affil- pollution and can damage human health. Recently, researchers designed a biodegradable iations. plastic (PEF) synthesized by replacing terephthalic acid with furan-2,5-dicarboxylic acid (FDCA) for replacing PET [7–10]. FDCA, the oxidation product of HMF, is the key point to produce PEF [11–15]. Therefore, it is very necessary to explore HMF in order to produce FDCA for furthering the polymeric industry. There are many raw materials for the production of HMF such as glucose, fructose, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. sucrose, and . Literature has indicated that it is easiest to produce HMF for fruc- This article is an open access article tose [16,17], but glucose should be the most suitable raw material to prepare HMF, as it distributed under the terms and is a cheaper and more abundant material. The glucose–HMF conversion path involved conditions of the Creative Commons isomerizing glucose to fructose over a Lewis acid, followed by dehydrating fructose to Attribution (CC BY) license (https:// form HMF with Brønsted acid [18–20]. It has been reported in literature that homogeneous creativecommons.org/licenses/by/ catalysts could effectively catalyze glucose to obtain HMF [21–26]. However, the disad- 4.0/). vantages of homogeneous catalysts, including corrosion, environmental pollution, and

Polymers 2021, 13, 2096. https://doi.org/10.3390/polym13132096 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2096 2 of 12

difficulty of recycling, limit their application and spur the development of heterogeneous catalysts [27–30]. Moreover, solvents such as organic solvents, biphasic systems, and ionic liquid are the focus in the HMF production [31–35], but they have many problems. DMSO, a typical organic solvent, is hard to separate from HMF because of it having a high boiling point and it being expensive and environmentally unfriendly. Biphasic systems cannot avoid the use of toxic solvents, and the viscosity of the bulk of ionic liquids is too high to mass transfer in reactions. Water has the properties of a low boiling point, low viscos- ity, and high environmental protection. The use of heterogeneous catalysts to catalyze biomass to produce HMF in water meets the requirements of industrial production such as environmental protection and the easy separation of products. Zirconia (ZrO2) was reported to possess acid properties, which makes it a good choice as a catalyst and catalyst support [36]. For the first time, ZrO2 was used for catalyzing glucose to produce HMF in water, with the yield being 10.0% [37]. Chareonlimkun et al. [38] revealed that the mechanism of using sulfated zirconia (SO4–ZrO2) can obtain a higher HMF yield than ZrO2. The reason is that ZrO2 acts as Lewis acid sites to promote the isomerization of glucose to fructose [39], and Brønsted acid sites introduced by acidification further promote the dehydration of fructose to HMF [40]. It was indicated that the yield of HMF was less than 30.0% from glucose in water. The main reason is that HMF is an unstable intermediate that is prone to side reactions [41–43]. Side reactions mainly include two routes: (a) the rehydration reaction between HMF and water is to produce organic acids such as levulinic acid and , and (b) the condensation reaction between HMF and sugars or other products is to form soluble polymer or insoluble humins. Hence, suppressing the occurrence of side reactions is an important method for increasing the yield of HMF. There are very few reports about inhibiting the unwanted reactions of HMF in water. Recently, a method to increase the yield of HMF by adding a low-boiling solvent to the water–ionic liquid system was discovered [34]. The research considered how the bubbles produced by boiling promoted the rate of interphase mass-transfer of HMF in reactions, which increased the extraction ratio of HMF in ionic liquid and avoided the side reactions of HMF. Based on the inspiration of this theory, this work proposes a method for pressure reduction to promote the production of HMF from glucose with a heterogeneous catalyst in aqueous solution. Here, a heterogeneous catalyst is designed according to the acid sites required for the production of HMF from glucose. Then, pressure is reduced as a way to increase the production of HMF, and its mechanism is discussed. Moreover, the effects of temperature, time, and the recycling potential of catalysts are investigated.

2. Materials and Methods 2.1. Materials Glucose (99.5%), (99.0%), and (99.5%) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). HMF (99%, GR), ZrOCl2·8H2O (99.9%), levulinic acid (99.0%), formic acid (99.0%), and fructose (99.0%) were purchased from Macklin Reagent Co. Ltd. (Shanghai, China). NaOH (98%) was purchased from Fuchen Chemical Reagent Co. Ltd. (Tianjin, China). Boric acid (99.5%) was provided by Chinasun Specialty Products Co. Ltd. (Changshu, China). (98%) was supplied from Chemical Reagent Co. (Guangzhou, China).

2.2. Catalyst Preparation 2− We followed the typical approach of preparing a SO4 /B2O3/ZrO2 catalyst by the coprecipitation and acidification method. ZrOCl2·8H2O and sodium hydroxide were dissolved in deionized water. The sodium hydroxide solution was added dropwise to the ZrOCl2·8H2O solution to adjust the pH to 9–10. Then, the precipitate was left to age for 5 h before separation and being washed with ethanol until no white precipitate was detected ◦ with 0.1 M AgNO3 (aq). The final sample was obtained after oven-drying at 110 C for Polymers 2021, 13, 2096 3 of 12

6 h. After this, 0.7 M boric acid (aq) was added to the obtained ZrO(OH)2 with magnetic stirring over a period of approximately 6 h. After five cycles of separation, washing and redispersion with ethanol, the powder obtained was oven-dried at 80 ◦C for 24 h and then calcined in air at 550 ◦C for 4 h. The obtained B2O3/ZrO2 was immersed in 1 M H2SO4 with stirring for 6 h. The suspension was then washed, dried and calcined as above.

2.3. Catalyst Characterization The sample was ground for XRD. XRD spectra were recorded on a XRD-6000 X-ray diffractometer (PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation (40 kV voltage, 40 mA tube current). Diffraction patterns were recorded within a 2θ range from 20◦ to 60◦. XPS spectra were recorded by an Axis Ultra DLD (Kratos Analytical Shimadzu Group Company, Manchester, UK) to detect the elements of Zr, S, B, and O. The data were calibrated by the C1s signal (284.4 eV). The sample was ground with KBr at the ratio of 1:100 and then pressed into disks for FTIR. FTIR spectra in the region of framework vibration (400–4000 cm−1) were recorded with a Tensor 27 infrared spectrometer (Bruker, Bremen, Germany). Acid properties were measured by temperature programmed desorption and pyridine infrared spectroscopy. Temperature programmed desorption of ammonia was performed on a AutoChem1 II 2920 (Micromeritics, Norcross, GA, USA). About 50 mg of ◦ catalyst was saturated with NH3 at 150 C, flushed with He to remove physisorbed gas and then ramped to 650 ◦C at a heating rate of 10 ◦C/min under He flow. The pyridine infrared spectroscopy was determined by Nicolet 6700/TGAQ50 (Thermo Fisher Scientific, Waltham, MA, USA; TA instruments, New Castle, DE, USA). The catalyst was activated at 350 ◦C for 30 min. Pyridine was chemisorbed on the catalyst surface at 100 ◦C. Excess physisorbed pyridine was removed by holding the temperature at 100 ◦C for 15 min.

2.4. Conversion of Glucose to HMF The experiment without pressure reduction was performed as follows: All reactions were carried out in 100 mL cylindrical stainless steel batch reactor (SLM 100, Beijing Century Sen Long experimental apparatus Co., Ltd., Beijing, China). The desired quantity of glucose (0.40 g), distilled water (40 mL), and catalyst (0.20 g) were introduced into the autoclave. Reactions were conducted at 150–190 ◦C with a reaction time range of 30–90 min in 30 min increments. This experiment was taken as the control. The experiment with pressure reduction was performed as follows: The reaction conditions are same as above. The steam was taken out by opening the vent valve slightly every 10 min when the reaction was ongoing (the pressure of the system was reduced due to the process, which is referred to as pressure reduction). It was condensed through the condensing tube for collection. For each experiment, a total of 15–25 mL of exact solution was taken out of the reactor, as shown in Table1. The reactor monitored the reaction time, removed the vial from the heating board, and quenched the reaction in a cool board immediately after reaching the designed time.

2.5. Product Analysis Before the tests, the product was filtered with a 0.22 µm filter. The evaporated sub- stance, products, and residual glucose were determined by Agilent 1100 liquid chromatog- raphy (HPLC). The specific conditions were as follows: chromatographic column, Bio-Rad Aminex HPX-87H column; detector, differential refractive detector (RID); mobile phase, 5 mM sulfuric acid; flow rate, 0.5 mL/min; column temperature, 50 ◦C; detector tempera- ture, 40 ◦C. Figure1 shows the HPLC chromatograph. The yield of products (HMF) and byproducts (levulinic acid, formic acid, levoglucosan, furfural), the conversion of glucose, Polymers 2021, 13, 2096 4 of 12

Polymers 2021, 13, x 4 of 12

extraction efficiency, and yield of humins were calculated by the external standard method. The formulas are as follows: 90 22 mol of product produced in solution Yield of products in the solution (mol%) =30 22 × 100% 170 60 mol of glucose initially charged22 (1) 90 mol of glucose reacted 22.5 Conversion of glucose (mol%) = 30 21× 100% (2) mol of glucose initially charged 180 60 20 electivety of products (mol%) = Yield90 of products × conversion of20 glucose (3) Yield of humins (mol%) = (1 − Selectivity30 of products) × Conversion of20 glucose × 100% 190 60 21.5 (4) 90 22.5 Table 1. The volume of solution in the vessel. 2.5. Product Analysis Temperature (◦C) Time (min) Volume (mL) Before the tests, the product was filtered with a 0.22 µm filter. The evaporated sub- 30 23 stance, products, and residual glucose were determined by Agilent 1100 liquid chroma- tography (HPLC).150 The specific conditions were60 as follows: chromatographic 21.5 column, Bio- Rad Aminex HPX-87H column; detector, differential90 refractive detector 25 (RID); mobile phase, 5 mM sulfuric acid; flow rate, 0.5 mL/30min; column temperature, 50 21 °C ; detector temperature, 40 °C . Figure 1 shows the HPLC chromatograph. The yield of products 160 60 17 (HMF) and byproducts (levulinic acid, formic acid, levoglucosan, furfural), the conversion of glucose, extraction efficiency, and yield 90 of humins were calculated by 22 the external standard method. The formulas are as follows30: 22 170 푚표푙 표푓 푝푟표푑푢푐푡60 푝푟표푑푢푐푒푑 𝑖푛 푠표푙푢푡𝑖표푛 22 푌𝑖푒푙푑 표푓 푝푟표푑푢푐푡푠 𝑖푛 푡ℎ푒 푠표푙푢푡𝑖표푛 (푚표푙%) = × 100% (1) 푚표푙 표푓 푔푙푢푐표푠푒90 𝑖푛𝑖푡𝑖푎푙푙푦 푐ℎ푎푟푔푒푑 22.5 30 21 푚표푙 표푓 푔푙푢푐표푠푒 푟푒푎푐푡푒푑 퐶표푛푣푒푟푠𝑖표푛 표푓 푔푙푢푐표푠푒 (푚표푙%) = 60× 100% 20 (2) 180 푚표푙 표푓 푔푙푢푐표푠푒 𝑖푛𝑖푡𝑖푎푙푙푦 푐ℎ푎푟푔푒푑 90 20 푒푙푒푐푡𝑖푣푒푡푦 표푓 푝푟표푑푢푐푡푠 (푚표푙%) = 푌𝑖푒푙푑 표푓 푝푟표푑푢푐푡푠 ×30푐표푛푣푒푟푠𝑖표푛 표푓 푔푙푢푐표푠푒 20 (3) 190 60 21.5 (4) 90 22.5 푌𝑖푒푙푑 표푓 ℎ푢푚𝑖푛푠 (푚표푙%) = (1 − 푆푒푙푒푐푡𝑖푣𝑖푡푦 표푓 푝푟표푑푢푐푡푠) × 퐶표푛푣푒푟푠𝑖표푛 표푓 푔푙푢푐표푠푒 × 100% (5)

80,000 8.73

) 60,000

a.u.

(

40,000

9.50 20,000 34.12

Peak intensity intensity Peak

0

0 10 20 30 40 50 60 Time (min)

Figure 1. HPLC chromatograph (8.73 min: glucose; 9.50 min: fructose; 34.12 min: HMF). Figure 1. HPLC chromatograph (8.73 min: glucose; 9.50 min: fructose; 34.12 min: HMF).

3. Results 3.1. Catalyst Characterization

Figure 2a shows the XRD pattern of SO42−/B2O3/ZrO2. From this, it can be seen that the XRD pattern of the catalysts exhibited characteristic diffraction peaks of tetragonal

Polymers 2021, 13, x 5 of 12

zirconia (t-ZrO2) and monoclinic zirconia (m-ZrO2). The typical diffraction peaks corre- sponded to the (111), (002), (200) crystal planes of t-ZrO2, which were consistent with the PDF card (JCPDS No. 414-0534). The following crystal planes, i.e., (110), (−111), (−102), (−112), (202), (220), and (−202), confirmed m-ZrO2, which were consistent with the PDF card (JCPDS No. 07-0343). There was no characteristic peak of B2O3 in the sample, indicat- ing that B2O3 may be highly dispersed or amorphous [44]tr. Figure 2b shows the FT-IR spectra of SO42−/B2O3/ZrO2. It can be seen from the figure that there were typical characteristic absorption peaks at 3403 cm−1, 1623 cm−1, 1450–1400 cm−1, 1150–1050 cm−1, 866 cm−1, and 501 cm−1. Among them, the bands at 1623 cm−1 and 3403 cm−1 were attributable to the O–H tensile vibration of adsorbed water on the surface and the H–O–H flexural vibration of bound water [45,46]. The band at 501 cm−1 belonged to the Zr–O vibration. The characteristic peaks at 1150–1050 cm−1 represented the for- mation of BO4 units [46], while the band at 1450–1400 cm−1 and 866 cm−1 represented the formation of BO3 units [44]. The absorption peak at 987–1137 cm−1 was attributed to the stretching vibration peak of S=O. Figure 2e shows the XPS of SO42−/B2O3/ZrO2. The binding energies at 163.9, 179.7, and 527.1 eV corresponded to S 2p, B 1s, and O 1s, respectively. The Zr spectrum showed two peaks at 178.9 and 181.2 eV, which belonged to the Zr 3d5/2 and Zr 3d3/2 spin-orbitals of Zr4+, respectively. All the above results indicated that the required SO42−/B2O3/ZrO2 was successfully prepared. Polymers 2021, 13, 2096 Generally, the adsorption of base molecules (e.g., pyridine or ammonia) combined5 of 12 with binding spectroscopy is a common technique, which was used to determine the na- ture of the surface active sites in solid acids [47]. The acidity on the surface of catalysts were determined by means of temperature programmed desorption (NH3-TPD) (shown in3. Figure Results 2c). The acid sites based on desorption temperature were divided into two cate- gories:3.1. Catalyst weak Characterizationacid (150–250 °C ), and medium acid (250–400 °C ). The amount of surface acid 2− was 1.78Figure mmol/g.2a shows Among the them, XRD patternthe amount of SO of 4weak/B acid2O3/ZrO and medium2. From strong this, it acid can reached be seen 0.74that mmol/g the XRD and pattern 1.04 mmol/g, of the catalysts respectively. exhibited The acidic characteristic properties diffraction of SO42−/B peaks2O3/ZrO of2 tetrag- were investigatedonal zirconia by (t-ZrO FTIR2 spectra) and monoclinic after pyridine zirconia chemisorp (m-ZrOtion,2). as The shown typical in diffractionFigure 2d. There peaks werecorresponded three bands to thein 155 (111),0–1400 (002), cm (200)−1. The crystal peak planesat 1440 ofcm t-ZrO−1 indicated2, which the were presence consistent of a Lewiswith the acid. PDF The card one (JCPDS at 1530 No. cm 414-0534).−1 was attributed The following to the C crystal-C stretching planes, i.e.,vibration (110), of (− pyr-111), idium(−102), ion (− and112), has (202), been (220), used and for (the−202), identificatio confirmedn of m-ZrO desorption2, which of werea Brønsted consistent acid. withAn- otherthe PDF peak card at (JCPDS1512 cm No.−1 was 07-0343). assigned There to wasthe nocharacteristic characteristic peak peak of ofthe B 2coO3-adsorptionin the sample, of pyridineindicating molecules that B2O 3atmay the Brønsted be highly acid dispersed sites and or amorphousthe Lewis acid [44 ].sites [48,49].

a b 3403 1623 11371050 e O1s Zr3d B1s 5/2

(111) Zr3d3/2

Zr3d5

(220) Zr3d

）

)

)

)

Intensity (a.u.) Intensity

Indensity (a.u.) Indensity

Indensity (a.u.) Indensity

Transmittance (%) Transmittance

002

-202

-111

(110)

(

(200)

-112

( (202)

(-102) ( （ S2p 1400 987866501

a 20 25 30 35 40 45 50 55 4000 3000 2000 1000 100 150 200 250 300 350 400 450 500 550 600 174 176 178 180 182 184 186 188 190 2θ (°) Wavenumber (cm-1) Binding energy (eV) Binding energy (eV) d c BrΦnsted acid Lewis acid S2p O1s

Intensity(a.u.)

Indensity (a.u.) Indensity

Indensity (a.u.) Indensity

TCD TCD signal (a.u.)

BrΦnsted and Lewis acid

100 200 300 400 500 600 700 1550 1500 1450 1400 156 158 160 162 164 166 168 170 172 174 522 524 526 528 530 532 534 536 538 540 542 -1 Temperature (℃) Wavenumber (cm ) Binding energy (eV) Binding energy (eV) FigurFiguree 2. Acid properties and structure characterization of of catalysts. ( (aa)) XRD XRD pattern; pattern; ( (bb)) FTIR FTIR spectra; spectra; ( (c)) temperature programmed desorption; ( d) FTIR spectra of pyridine chemisorption; (e) XPS pattern.

2− Figure2b shows the FT-IR spectra of SO 4 /B2O3/ZrO2. It can be seen from the figure that there were typical characteristic absorption peaks at 3403 cm−1, 1623 cm−1,

1450–1400 cm−1, 1150–1050 cm−1, 866 cm−1, and 501 cm−1. Among them, the bands at 1623 cm−1 and 3403 cm−1 were attributable to the O–H tensile vibration of adsorbed water on the surface and the H–O–H flexural vibration of bound water [45,46]. The band at 501 cm−1 belonged to the Zr–O vibration. The characteristic peaks at 1150–1050 cm−1 represented the −1 −1 formation of BO4 units [46], while the band at 1450–1400 cm and 866 cm represented −1 the formation of BO3 units [44]. The absorption peak at 987–1137 cm was attributed to the 2− stretching vibration peak of S=O. Figure2e shows the XPS of SO 4 /B2O3/ZrO2. The binding energies at 163.9, 179.7, and 527.1 eV corresponded to S 2p, B 1s, and O 1s, respectively. The Zr spectrum showed two peaks at 178.9 and 181.2 eV, which belonged to the Zr 3d5/2 and Zr 3d3/2 spin-orbitals of Zr4+, respectively. All the above results indicated that the required 2− SO4 /B2O3/ZrO2 was successfully prepared. Generally, the adsorption of base molecules (e.g., pyridine or ammonia) combined with binding spectroscopy is a common technique, which was used to determine the nature of the surface active sites in solid acids [47]. The acidity on the surface of catalysts were determined by means of temperature programmed desorption (NH3-TPD) (shown in Figure2c). The acid sites based on desorption temperature were divided into two categories: weak acid (150–250 ◦C), and medium acid (250–400 ◦C). The amount of surface acid was 1.78 mmol/g. Among them, the amount of weak acid and medium strong acid reached 2− 0.74 mmol/g and 1.04 mmol/g, respectively. The acidic properties of SO4 /B2O3/ZrO2 were investigated by FTIR spectra after pyridine chemisorption, as shown in Figure2d. Polymers 2021, 13, 2096 6 of 12

There were three bands in 1550–1400 cm−1. The peak at 1440 cm−1 indicated the presence of a Lewis acid. The one at 1530 cm−1 was attributed to the C-C stretching vibration of pyridium ion and has been used for the identification of desorption of a Brønsted acid. Polymers 2021, 13, x 6 of 12 Another peak at 1512 cm−1 was assigned to the characteristic peak of the co-adsorption of pyridine molecules at the Brønsted acid sites and the Lewis acid sites [48,49].

− 3.2. Conversion of GlucoseGlucose to HMF by SO --/B/B2O2O3/ZrO3/ZrO2 2 Figure3 3 showed showed the the results results that that the the influence influence of variousof various temperature temperature and and time time on the on 2− 2− transformationthe transformation of glucose of glucose to HMF to overHMF SO over4 /BSO24O/B3/ZrO2O3/ZrO2. The2. The conversion conversion of glucose of glucose was greatlywas greatly affected affected by temperature. by temperature. Glucose Glucose conversion conversion increased increased from from 45.9 to45.9 60.3% to 6 with0.3% anwith increase an increase in the in temperaturethe temperature from from 150 1 to50 190to 1◦9C0 °C in in 30 3 min.0 min However,. However, the the productionproduction of HMF was was not not sensitive sensitive to to reaction reaction temperature temperature:: HMF HMF yield yield increased increased little little bit bitfrom from 0.0 0.0to 7.7 to% 7.7%.. The maximum The maximum yield yieldof HMF of HMFreached reached 9.3% at 9.3% 190 at°C190 in 90◦C min in 90when min the when conver- the conversionsion rate of rateglucose of glucose reached reached 64.6%. 64.6%. The reason The reason for the for low the selectivity low selectivity was that was there that thereare a arelarge a largenumber number of byproducts of byproducts generated. generated. With With an increase an increase of reaction of reaction time time from from 30 30to to90 90min min,, the the HMF HMF conversion conversion increased increased slowly slowly,, b butut the the conversion conversion of of glucose glucose did not follow this trend. Previously, Previously, B B22OO3/ZrO3/ZrO2–Al2–Al2O22 Ocontain2 containinging Brønsted Brønsted and and Lewis Lewis acid acid sites sites was wascon- conductedducted to produce to produce HMF HMF in inDMSO DMSO [44] [44. ].The The yield yield was was 41.2 41.2%,%, much much higher higher than than that ofof 2- SO42-/B/B2O2O3/ZrO3/ZrO2. The2. The main main reason reason was was that that many many side side-reactions-reactions may may happen happen in in water, whilewhile HMFHMF waswas moremore stablestable inin DMSODMSO [[50]50]..

Figure 3. The effect of temperature and time on conversion of glucose to HMF over SO2−42−/B2O3/ZrO2. Figure 3. The effect of temperature and time on conversion of glucose to HMF over SO4 /B2O3/ZrO2.

According to previous reports [[51]51] (as shown in the Figure4 4),), thethe side-reactionside-reaction in-in- cluded into four paths: (1) isomerization of glucose to fructose; (2) dehydration ofof glucose to levoglucosan (LGA), (LGA), furfural furfural (FF) (FF);; (3) (3) hydration hydration of ofHMF HMF to levulinic to levulinic acid acid (LA) (LA) and andfor- formicmic acid acid (FA); (FA); and and (4) condensation (4) condensation of HMF of HMF to humins. to humins. Soluble Soluble byproducts byproducts such as such LGA, as LGA,FF, LA, FF, FA, LA, and FA, fructose and fructose can be candetected be detected by HPLC. by HPLC.It turns Itout turns that outin addition that in addition to unre- toacted unreacted glucose glucose and HMF, and there HMF, was there a small was aamount small amountof fructose of fructosethat was that less wasthan less 10% than. As 10%.can be As seen can in be Table seen 2 in, other Table byproducts2, other byproducts were thought were to thought be insoluble to be insolublehumins produced humins producedby HMF condensation, by HMF condensation, where the where yield was theyield 46.2%. was However 46.2%. However,, there is no there means is no to means quali- totatively qualitatively analyze analyze the main the components main components of humins. of humins.

2− 3.3.Table Conversion 2. Concentration of Glucose of HMF to HMF in the by vapor SO4 . /B2O3/ZrO2 with Pressure Reduction 3.3.1. The Possible Mechanism of Pressure Reduction Temperature (°C ) Time (min) Yield of HMF (%) According to Table2, HMF was not detected30 by HPLC in the vapor,0.062 which meant that HMF remained in the liquid product in a high content. Compared to the experimental 150 60 0.069 pressure with vapor pressure (as can be seen in Figure5), the phenomenon that only water 90 0.075 was evaporated can be explained. If the extracted vapor was 1 or 2 mL, the temperature 30 0.100 and pressure in the kettle would reduce due to the removal of steam. Obviously, the experimental pressure160 was between the saturated60 vapor pressure of water0.152 and HMF. In other words, releasing vapor pressure could90 promote water to boil. 0.150 30 0.140 170 60 0.185 90 0.243 30 0.239 180 60 0.284 90 0.336 190 30 0.405

Polymers 2021, 13, x 7 of 12

Polymers 2021, 13, 2096 7 of 12 60 0.447 90 0.389

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60 0.447 90 0.389 FigureFigure 4.4.Reaction Reaction pathwaypathwayof of glucose.glucose.

Table3.3. Conversion 2. Concentration of Glucose of HMF to HMF in the by vapor. SO42−/B2O3/ZrO2 with Pressure Reduction 3.3.1. The Possible Mechanism of Pressure Reduction Temperature (◦C) Time (min) Yield of HMF (%) According to Table 2, HMF was not detected by HPLC in the vapor, which meant 30 0.062 that HMF remained in the liquid product in a high content. Compared to the experimental pressure with150 vapor pressure (as can be seen60 in Figure 5), the phenomenon 0.069 that only water was evaporated can be explained. If the extracted90 vapor was 1 or 2 mL, 0.075 the temperature and pressure in the kettle would reduce due30 to the removal of steam. Obviously, 0.100 the ex- perimental pressure was between the saturated vapor pressure of water and HMF. In 160 60 0.152 other words, releasing vapor pressure could promote water to boil. 90 0.150 1400 30 0.140 Figure 4. ReactionSaturated pathway vapor pressure of of water glucose . 1200 170Saturated vapor pressure of HMF 60 0.185 Experimental pressure 90 0.243 3.3. Conversion1000 of Glucose to HMF by SO42−/B2O3/ZrO2 with Pressure Reduction

(kPa) 30 0.239 3.3.1. The800 Possible Mechanism of Pressure Reduction 180 60 0.284 According600 to Table 2, HMF was not detected by HPLC in the vapor, which meant that HMF remained in the liquid product in a 90high content. Compared to the 0.336 experimental 400 pressure with vapor pressure (as can be seen in30 Figure 5), the phenomenon 0.405 that only water

was evaporatedVaporpressure can be explained. If the extracted vapor was 1 or 2 mL, the temperature 200 190 60 0.447 and pressure in the kettle would reduce due to the removal of steam. Obviously, the ex- 0 90 0.389 perimental pressure was between the saturated vapor pressure of water and HMF. In 140 150 160 170 180 190 other words, releasing vapor pressure could promote water to boil. Temperature(℃)

1400

Figure 5. SaturatedSaturated vapor pressurevapor of water pressure of water and HMF at different temperatures. 1200 Saturated vapor pressure of HMF Experimental pressure 3.3.2.1000 Conversion of Glucose to HMF with Pressure Reduction

(kPa) As can be seen in the Figure 6, the reaction temperature had a certain influence on 800 the yield of HMF and the conversion of glucose with reducing the pressure. As tempera- ture600 rose, the glucose conversion gradually increased. For example, it rose from 36.1 to

67.3%400 rapidly when the temperature elevated from 150 to 190 °C in 30 min, which indi- cates that a high reaction temperature could improve the mass transfer and heat transfer betweenVaporpressure 200 the substrate and the catalyst. The acid sites of the catalyst in the reaction system contacted0 with the substrate more fully, which promoted the conversion of glucose. The trend of140 HMF150 yield160 was consistent170 180 with190 glucose conversion when it reacted for 30 min and 60 min. At 90Temperature(℃) min, however, the yield of HMF increased and then decreased. For

FigureFigure 5. 5.SaturatedSaturated vapor vapor pressure pressure of of water water and and HMF HMF at at different different temperatures. temperatures.

3.3.2. Conversion of Glucose to HMF with Pressure Reduction As can be seen in the Figure 6, the reaction temperature had a certain influence on the yield of HMF and the conversion of glucose with reducing the pressure. As tempera- ture rose, the glucose conversion gradually increased. For example, it rose from 36.1 to 67.3% rapidly when the temperature elevated from 150 to 190 °C in 30 min, which indi- cates that a high reaction temperature could improve the mass transfer and heat transfer between the substrate and the catalyst. The acid sites of the catalyst in the reaction system contacted with the substrate more fully, which promoted the conversion of glucose. The trend of HMF yield was consistent with glucose conversion when it reacted for 30 min and 60 min. At 90 min, however, the yield of HMF increased and then decreased. For

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3.3.2. Conversion of Glucose to HMF with Pressure Reduction As can be seen in the Figure6, the reaction temperature had a certain influence on the yield of HMF and the conversion of glucose with reducing the pressure. As temperature rose, the glucose conversion gradually increased. For example, it rose from 36.1 to 67.3% rapidly when the temperature elevated from 150 to 190 ◦C in 30 min, which indicates that a high reaction temperature could improve the mass transfer and heat transfer between the substrate and the catalyst. The acid sites of the catalyst in the reaction system contacted Polymers 2021, 13, x 8 of 12 with the substrate more fully, which promoted the conversion of glucose. The trend of HMF yield was consistent with glucose conversion when it reacted for 30 min and 60 min. At 90 min, however, the yield of HMF increased and then decreased. For example, the ◦ ◦ yieldexample, of HMF the yield was 4.2%of HMF at 150 was C4.2% in 30at min.150 °C When in 30 the min temperature. When the temperature rose to 190 roseC, the to ◦ yield190 °C of, HMFthe yield increased of HMF to 31.5%.increased With to the31.5%. reaction With time the reaction increasing time to 90increasing min at 190 to 90C, min the ◦ yieldat 190 of °C HMF, the decreasedyield of HMF to 24.0%. decreased The maximumto 24.0%. The yield maximum reached withinyield reached 60 min atwithin 190 60C wasmin 35.2%.at 190 These°C was results 35.2%. indicate These thatresults the indicate formation that of polymersthe formation species of polymers hinder the species active siteshinder with the prolonging active sites the with reaction prolonging time. Thethe reaction byproducts time in. The the reactionbyproducts solution in the were reaction also quantified.solution were Although also quantified. the catalyst Although prepared the forcatalyst the selective prepared production for the selective of HMF production was not veryof HMF effective, was not this very method effective, of pressure this method reduction of pressure greatly improvedreduction greatly its yield. improved The result its wasyield. better The thanresult that was of better HCl [ 43than], ZnCl that2 of[52 HCl], and [43 cellulose-derived], ZnCl2 [52], and carbonaceouscellulose-derived [53] carbo- in the aqueousnaceous [5 phase.3] in the Therefore, aqueous this phase. method Therefore, can be this widely method used can for be the widely production used for of HMFthe pro- in theduction aqueous of HMF phase in onthe a aqueous solid catalyst. phase on a solid catalyst.

FigureFigure 6.6. TheThe effecteffect of of temperature temperature and and time time on on conversion conversion of glucoseof glucose to HMFto HMF with with pressure pressure reduction re- 2− over SO /B O 2/ZrO− . duction4 over SO2 43 /B2O32/ZrO2. The occurrence of side reactions is inevitable in water. However, the soluble byprod- ucts suchThe occurrence as LGA, FF, of LA, side FA, reactions and fructose is inevitable were not in detected water. However, by HPLC, asthe shown soluble in byprod-Table3 . Nevertheless,ucts such as LGA, the yieldFF, LA, of FA, humin and decreased fructose were from not 46.2 detected to 36.3%. by HPLC The indexes, as shown were in much Table better3. Nevertheless, than that withoutthe yield pressure of humin reduction, decreased which from indicates46.2 to 36.3 that%. The pressure index reductiones were much can effectivelybetter than promoted that without the pressure production reduction of HMF., which To this indicate extent,sby that reducing pressure pressure, reduction it notcan onlyeffectively improved promoted the yield the ofproduction HMF and of the HMF. conversion To this ofextent, fructose, by reducing but it also pressure, reduced it thenot condensationonly improved products the yield of of HMF. HMF Based and the on conversion this result, of the fructose, method but of pressureit also reduce reductiond the havecondensation remarkably products unique of and HMF. environmentally Based on this friendlyresult, the benefits, method including of pressure that reduction the fact thathave the remarkably isolation andunique purification and environment of intermediateally friendly compounds benefits can, including be avoided. that the Firstly, fact thethat catalyst the isolation and humins and purification were filtrated of intermediate by 0.22 µm. compounds Since there can are be no avoided. other byproducts Firstly, the (e.g.,catalyst LA, and FA, humin LGA,s FF, were and filtrat fructose),ed by HMF0.22 µm only. Since need there to be are purified no other by distillationbyproducts from(e.g., unreactedLA, FA, LGA glucose, FF, andand water.fructose) The, HMF HMF only and waterneed areto be distillation purified by from distillation glucose. Finally,from unre- the purifiedacted glucose HMF isand obtained water. fromThe HMF the solution and water above are through distillation a rotary from evaporator. glucose. Finally, the purifiedZhou’s HMF research is obtained [34] showedfrom the that solution the low above boiling through solvent a rotary produced evaporator. bubbles when boiling:Zhou’s bubbles research rose from [34] theshowed bottom that to the upperlow boiling area, promoting solvent produced the upward bub movementbles when ofboiling HMF: tobubbles ionic liquidrose from for inhibitingthe bottom its to side the upper reactions; area this, promoting process also the upward causes the movement reaction systemof HMF to to be ionic agitated, liquid promotingfor inhibiting the its ionic side liquid reactions; with this high process viscosity also to causes mass the transfer. reaction In thissystem paper, to be the agitat mained reason, promoting for using the ionic pressure liquid reduction with high to promoteviscosity theto mass dehydration transfer. ofIn glucosethis paper, toHMF the main is that reason pressure for using reduction pressure promoted reduction the boilingto promote of water the dehydration caused by the of glucose to HMF is that pressure reduction promoted the boiling of water caused by the reduced pressure (as shown in Figure 7). The bubbles produced by boiling causes the dis- turbance of the reaction system, which promoted the rapid desorption of HMF from the acid sites of solid catalyst and further promoted the adsorption of glucose with more acid sites, which improves the conversion of glucose and the yield of HMF. Both methods are to promote the production of HMF by solvent boiling, and the yield of HMF is increased by more than three times. However, the methods used in this study are more environ- mentally friendly and economical. This is an efficient method to produce HMF by avoid- ing the use of organic solvents.

Polymers 2021, 13, 2096 9 of 12

reduced pressure (as shown in Figure7). The bubbles produced by boiling causes the disturbance of the reaction system, which promoted the rapid desorption of HMF from the acid sites of solid catalyst and further promoted the adsorption of glucose with more acid sites, which improves the conversion of glucose and the yield of HMF. Both methods are to promote the production of HMF by solvent boiling, and the yield of HMF is increased by more than three times. However, the methods used in this study are more environmentally Polymers 2021, 13, x 9 of 12 friendly and economical. This is an efficient method to produce HMF by avoiding the use Polymers 2021, 13, x of organic solvents. 9 of 12

TableTable 3 3.. The byproducts in the solution at a temperature of 190 ◦°CC and time of 60 min.min.

TableControlsControls-Yield 3. The byproducts-Yield (%) in the solution at a temperature of 190 °CP Pressure andressure time Reduction of 60 min. --YieldYield (%) Soluble Products Insoluble Products Soluble Products Insoluble Products SolubleControls Products-Yield (%) Insoluble ProductsP SolubleressureProducts Reduction -Yield (%) Insoluble Products LA FA LGA FF Fructose Humins LA FA LGA FF Fructose Humins LASoluble FA LGAProducts FF FructoseInsoluble Humins Products LASoluble FA LGAProducts FF FructoseInsoluble Humins Products 0.0 0.0 0.0 0.0 0.0 36.3 0.0 0.0 0.0 0.0 4.2 46.2 LA 0.0FA 0.0LGA 0.0FF 0.0Fructose 0.0 Humins 36.3 LA 0.0FA 0.0LGA 0.0FF 0.0Fructose 4.2 Humins 46.2 0.0 0.0 0.0 0.0 0.0 36.3 0.0 0.0 0.0 0.0 4.2 46.2

Figure 7. Mechanism of pressure reduction to enhance catalytic efficiency. Figure 7. Mechanism of pressure reduction to enhance catalytic efficiency. Figure3.4. 7. ReuseMechanism Potential of pressure of Catalysts reduction to enhance catalytic efficiency. 3.4. Reuse Potential of Catalysts The stability of SO42−−B2O3/ZrO2 catalyst was investigated with pressure reduction. 3.4. Reuse Potential of Catalysts2 − The resultsThe stability are shown of SO in4 Figure−B2O 83./ZrO In the2 secondcatalyst reuse, was investigated HMF yield withdecreased pressure slightly reduction. from The stability of SO42−−B2O3/ZrO2 catalyst was investigated with pressure reduction. 35.2The resultsto 34.1%. are In shown the third in Figure reuse,8. Inthe the yield second of HMF reuse, decreased HMF yield to decreased 24.5%, which slightly may from be 35.2due The results are shown in Figure 8. In the second reuse, HMF yield decreased slightly from toto the 34.1%. pore In structure the third of reuse, the catalyst the yield, which of HMF blocked decreased byproduct to 24.5%, humus which and may other be duesolid to sub- the 35.2 to 34.1%. In the third reuse, the yield of HMF decreased to 24.5%, which may be due stances.pore structure of the catalyst, which blocked byproduct humus and other solid substances. to the pore structure of the catalyst, which blocked byproduct humus and other solid sub-

stances.40

40 35

35 30

30 25

25 20

20 15

HMF yield (%) HMF yield 15 10

HMF yield (%) HMF yield 10 5

5 0 1 2 3 0 Reuse times 1 2 3 2− Figure 8. 2− − Figure 8. ReuseReuse potential potentialReuse times of of SO SO4 4−B2OB3/ZrO2O3/ZrO2: 0.402 :g 0.40glucose, g glucose, 0.20 g catalysts, 0.20 g catalysts, 40 mL water, 40 mL at water, 190 °C at, ◦ for190 60C, min for. 60 min. Figure 8. Reuse potential of SO42−−B2O3/ZrO2: 0.40 g glucose, 0.20 g catalysts, 40 mL water, at 190 °C , for 604. min Conclusion. s 4. ConclusionThe methods of using pressure reduction in the procedure of catalytic conversion with solid acid in aqueous system is an effective strategy to increase HMF yield. The possible The method of using pressure reduction in the procedure of catalytic conversion with mechanism is to use the bubbles produced by boiling, as these bubbles can cause turbu- solid acid in aqueous system is an effective strategy to increase HMF yield. The possible lence in the solution, in turn promoting the diffusion and transfer of HMF from the solid mechanism is to use the bubbles produced by boiling, as these bubbles can cause turbu- catalyst and avoiding further side reactions at acid sites. Under the same conditions, the lence in the solution, in turn promoting the diffusion and transfer of HMF from the solid yield of HMF can be increased from 9.3 to 35.2% (at 190 °C for 60 min) when reducing catalyst and avoiding further side reactions at acid sites. Under the same conditions, the yield of HMF can be increased from 9.3 to 35.2% (at 190 °C for 60 min) when reducing

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4. Conclusions The method of using pressure reduction in the procedure of catalytic conversion with solid acid in aqueous system is an effective strategy to increase HMF yield. The possible mechanism is to use the bubbles produced by boiling, as these bubbles can cause turbulence in the solution, in turn promoting the diffusion and transfer of HMF from the solid catalyst and avoiding further side reactions at acid sites. Under the same conditions, the yield of HMF can be increased from 9.3 to 35.2% (at 190 ◦C for 60 min) when reducing pressure. In 2− addition, the SO4 −B2O3/ZrO2 catalysts can also keep stable, and the yield of HMF with the third recycling of this catalyst can also reach 24.5%.

Author Contributions: Writing—original draft preparation, K.K.; writing—review and editing, H.J.; writing—review and editing, X.S.; writing—review and editing, F.K.; writing—review and editing, B.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (grant num- ber 31470607); Natural Science Foundation of Guangdong Province (grant number 2021A1515011012); Foundation of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences (grant number GZKF202033). Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Demirbas, A. Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues. Prog. Energy Combust. 2005, 31, 171–192. [CrossRef] 2. Chheda, J.N.; Dumesic, J.A. An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived . Catal Today 2007, 123, 59–70. [CrossRef] 3. Halliday, G.A.; Young, R.J.; Grushin, V.V. One-pot, two-step, practical catalytic synthesis of 2,5-diformylfuran from fructose. Org. Lett. 2003, 5, 2003–2005. [CrossRef] 4. Mohan, D.; Pittman, C.U.; Steele, P.H. Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuel 2006, 20, 848–889. [CrossRef] 5. Roman-Leshkov, Y.; Barrett, C.J.; Liu, Z.Y.; Dumesic, J.A. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature 2007, 447, U982–U985. [CrossRef][PubMed] 6. Sun, Y.; Cheng, J.Y. Hydrolysis of lignocellulosic materials for ethanol production: A review. Bioresour. Technol. 2002, 83, 1–11. [CrossRef] 7. Casanova, O.; Iborra, S.; Corma, A. Biomass into chemicals: One pot-base free oxidative esterification of 5-hydroxymethyl-2- furfural into 2,5-dimethylfuroate with gold on nanoparticulated ceria. J. Catal. 2009, 265, 109–116. [CrossRef] 8. Papadopoulos, L.; Terzopoulou, Z.; Bikiaris, D.N.; Patsiaoura, D.; Chrissafis, K.; Papageorgiou, D.G.; Papageorgiou, G.Z. Synthesis and Characterization of In-Situ-Prepared Nanocomposites Based on Poly(Propylene 2,5-Furan Dicarboxylate) and Aluminosilicate Clays. Polymers 2018, 9, 937. [CrossRef] 9. Chi, D.; Liu, F.; Na, H.; Chen, J.; Hao, C.; Zhu, J. Poly(neopentyl glycol 2,5-furandicarboxylate): A Promising Hard Segment for the Development of Bio-based Thermoplastic Poly(ether-ester) Elastomer with High Performance. ACS Sustain. Chem. Eng. 2018, 6, 9893–9902. [CrossRef] 10. Eerhart, A.J.J.E.; Faaij, A.P.C.; Patel, M.K. Replacing fossil based PET with biobased PEF; process analysis, energy and GHG balance. Energy Environ. Sci. 2012, 5, 6407–6422. [CrossRef] 11. Bozell, J.J.; Petersen, G.R. Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy’s “Top 10” revisited. Green Chem. 2010, 12, 539–554. [CrossRef] 12. Casanova, O.; Iborra, S.; Corma, A. Biomass into Chemicals: Aerobic Oxidation of 5-Hydroxymethyl-2-furfural into 2,5- Furandicarboxylic Acid with Gold Nanoparticle Catalysts. ChemSusChem 2009, 2, 1138–1144. [CrossRef][PubMed] 13. Yi, G.S.; Teong, S.P.; Zhang, Y.G. Base-free conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over a Ru/C catalyst. Green Chem. 2016, 18, 979–983. [CrossRef] 14. Koopman, F.; Wierckx, N.; de Winde, J.H.; Ruijssenaars, H.J. Efficient whole-cell biotransformation of 5-(hydroxymethyl)furfural into FDCA, 2,5-furandicarboxylic acid. Bioresour. Technol. 2010, 101, 6291–6296. [CrossRef][PubMed] Polymers 2021, 13, 2096 11 of 12

15. Sajid, M.; Zhao, X.B.; Liu, D.H. Production of 2,5-furandicarboxylic acid (FDCA) from 5-hydroxymethylfurfural (HMF): Recent progress focusing on the chemical-catalytic routes. Green Chem. 2018, 20, 5427–5453. [CrossRef] 16. Guo, F.; Fang, Z.; Zhou, T.J. Conversion of fructose and glucose into 5-hydroxymethylfurfural with lignin-derived carbonaceous catalyst under microwave irradiation in dimethyl sulfoxide-ionic liquid mixtures. Bioresour. Technol. 2012, 112, 313–318. [CrossRef] [PubMed] 17. Fan, C.Y.; Guan, H.Y.; Zhang, H.; Wang, J.H.; Wang, S.T.; Wang, X.H. Conversion of fructose and glucose into 5- hydroxymethylfurfural catalyzed by a solid heteropolyacid salt. Biomass Bioenergy 2011, 35, 2659–2665. [CrossRef] 18. Kooyman, C.; Vellenga, K.; De Wilt, H.G.J. The isomerization of d-glucose into d-fructose in aqueous alkaline solutions. Carbohyd. Res. 1977, 54, 33–44. [CrossRef] 19. Angyal, S.J. The Lobry de Bruyn-Alberda van Ekenstein transformation and related reactions. Top. Curr. Chem. 2001, 215, 1–14. 20. Korner, P.; Jung, D.; Kruse, A. The effect of different Bronsted acids on the hydrothermal conversion of fructose to HMF. Green Chem. 2018, 20, 2231–2241. [CrossRef] 21. Cai, C.M.; Nagane, N.; Kumar, R.; Wyman, C.E. Coupling metal halides with a co-solvent to produce furfural and 5-HMF at high yields directly from lignocellulosic biomass as an integrated biofuels strategy. Green Chem. 2014, 16, 3819–3829. [CrossRef] 22. Chheda, J.N.; Roman-Leshkov, Y.; Dumesic, J.A. Production of 5-hydroxymethylfurfural and furfural by dehydration of biomass- derived mono- and poly-saccharides. Green Chem. 2007, 9, 342–350. [CrossRef] 23. Zhao, H.B.; Holladay, J.E.; Brown, H.; Zhang, Z.C. Metal chlorides in ionic liquid solvents convert sugars to 5-hydroxymethylfurfural. Science 2007, 316, 1597–1600. [CrossRef] 24. Hu, L.; Sun, Y.; Lin, L. Efficient Conversion of Glucose into 5-Hydroxymethylfurfural by Chromium(III) Chloride in Inexpensive Ionic Liquid. Ind. Eng. Chem. Res. 2012, 51, 1099–1104. [CrossRef] 25. Yuan, Z.; Xu, C.B.; Cheng, S.N.; Leitch, M. Catalytic conversion of glucose to 5-hydroxymethyl furfural using inexpensive co-catalysts and solvents. Carbohyd. Res. 2011, 346, 2019–2023. [CrossRef][PubMed] 26. Choudhary, V.; Mushrif, S.H.; Ho, C.; Anderko, A.; Nikolakis, V.; Marinkovic, N.S.; Frenkel, A.I.; Sandler, S.I.; Vlachos, D.G. Insights into the Interplay of Lewis and Bronsted Acid Catalysts in Glucose and Fructose Conversion to 5-(Hydroxymethyl)furfural and Levulinic Acid in Aqueous Media. J. Am. Chem. Soc. 2013, 135, 3997–4006. [CrossRef] 27. Thombal, R.S.; Jadhav, V.H. Biomass derived beta-cyclodextrin-SO3 H carbonaceous solid acid catalyst for catalytic conversion of carbohydrates to 5-hydroxymethylfurfural. Appl. Catal. A Gen. 2015, 499, 213–216. [CrossRef] 28. Chen, D.W.; Liang, F.B.; Feng, D.X.; Xian, M.; Zhang, H.B.; Liu, H.Z.; Du, F.L. An efficient route from reproducible glucose to 5-hydroxymethylfurfural catalyzed by porous coordination polymer heterogeneous catalysts. Chem. Eng. J. 2016, 300, 177–184. [CrossRef] 29. Xu, Q.; Zhu, Z.; Tian, Y.K.; Deng, J.; Shi, J.; Fu, Y. Sn-MCM-41 as Efficient Catalyst for the Conversion of Glucose into 5- Hydroxymethylfurfural in Ionic Liquids. Bioresources 2014, 9, 303–315. [CrossRef] 30. Moreno-Recio, M.; Santamaria-Gonzalez, J.; Maireles-Torres, P. Bronsted and Lewis acid ZSM-5 for the catalytic dehydration of glucose into 5-hydroxymethylfurfural. Chem. Eng. J. 2016, 303, 22–30. [CrossRef] 31. Liu, H.; Wang, H.W.; Li, Y.; Yang, W.; Song, C.H.; Li, H.M.; Zhu, W.S.; Jiang, W. Glucose dehydration to 5-hydroxymethylfurfural in ionic liquid over Cr3+-modified ion exchange resin. RSC Adv. 2015, 5, 9290–9297. [CrossRef] 32. Teong, S.P.; Yi, G.S.; Zeng, H.Q.; Zhang, Y.G. The effects of emulsion on sugar dehydration to 5-hydroxymethylfurfural in a biphasic system. Green Chem. 2015, 17, 3751–3755. [CrossRef] 33. He, R.; Huang, X.L.; Zhao, P.; Han, B.; Wu, T.H.; Wu, Y. The Synthesis of 5-Hydroxymethylfurfural from Glucose in Biphasic System by Phosphotungstic Acidified Titanium-Zirconium Dioxide. Waste Biomass Valoriz. 2018, 9, 657–668. [CrossRef] 34. Zhou, J.X.; Xia, Z.; Huang, T.Y.; Yan, P.F.; Xu, W.J.; Xu, Z.W.; Wang, J.J.; Zhang, Z.C. An ionic liquid-organics-water ternary biphasic system enhances the 5-hydroxymethylfurfural yield in catalytic conversion of glucose at high concentrations. Green Chem. 2015, 17, 4206–4216. [CrossRef] 35. Nikolla, E.; Roman-Leshkov, Y.; Moliner, M.; Davis, M.E. “One-Pot” Synthesis of 5-(Hydroxymethyl)furfural from Carbohydrates using Tin-Beta . ACS Catal. 2011, 1, 408–410. [CrossRef] 36. Reddy, B.M.; Khan, A. Recent advances on TiO2-ZrO2 mixed oxides as catalysts and catalyst supports. Catal. Rev. 2005, 47, 257–296. [CrossRef] 37. Watanabe, M.; Aizawa, Y.; Iida, T.; Nishimura, R.; Inomata, H. Catalytic glucose and fructose conversions with TiO2 and ZrO2 in water at 473 K: Relationship between reactivity and acid-base property determined by TPD measurement. Appl. Catal. A Gen. 2005, 295, 150–156. [CrossRef] 38. Chareonlimkun, A.; Champreda, V.; Shotipruk, A.; Laosiripojana, N. Reactions of C-5 and C-6-sugars, cellulose, and lignocellulose under hot compressed water (HCW) in the presence of heterogeneous acid catalysts. Fuel 2010, 89, 2873–2880. [CrossRef] 39. Watanabe, M.; Aizawa, Y.; Iida, T.; Aida, T.M.; Levy, C.; Sue, K.; Inomata, H. Glucose reactions with acid and base catalysts in hot compressed water at 473 K. Carbohyd. Res. 2005, 340, 1925–1930. [CrossRef] 40. Osatiashtiani, A.; Lee, A.F.; Brown, D.R.; Melero, J.A.; Morales, G.; Wilson, K. Bifunctional SO4/ZrO2 catalysts for 5- hydroxymethylfufural (5-HMF) production from glucose. Catal. Sci. Technol. 2014, 4, 333–342. [CrossRef] 41. Fachri, B.A.; Abdilla, R.M.; van de Bovenkamp, H.H.; Rasrendra, C.B.; Heeres, H.J. Experimental and Kinetic Modeling Studies on the Sulfuric Acid Catalyzed Conversion of D-Fructose to 5-Hydroxymethylfurfural and Levulinic Acid in Water. ACS Sustain. Chem. Eng. 2015, 3, 3024–3034. [CrossRef] Polymers 2021, 13, 2096 12 of 12

42. Girisuta, B.; Janssen, L.P.B.M.; Heeres, H.J. A kinetic study on the decomposition of 5-hydroxymethylfurfural into levulinic acid. Green Chem. 2006, 8.[CrossRef] 43. Weingarten, R.; Cho, J.; Xing, R.; Conner, W.C., Jr.; Huber, G.W. Kinetics and reaction engineering of levulinic acid production from aqueous glucose solutions. ChemSusChem 2012, 5, 1280–1290. [CrossRef][PubMed] 44. Han, B.; Zhao, P.; He, R.; Wu, T.; Wu, Y. Catalytic Conversion of Glucose to 5-Hydroxymethyfurfural Over B2O3 Supported Solid Acids Catalysts. Waste Biomass Valoriz. 2018, 9, 2181–2190. [CrossRef] 45. Sharma, P.; Sundaram, M.M.; Singh, D.; Ahuja, R. Highly Energetic and Stable Gadolinium/Bismuth Molybdate with a Fast Reactive Species, Redox Mechanism of Aqueous Electrolyte. ACS Appl. Energy Mater. 2020, 3, 12385–12399. [CrossRef] 46. Mao, D.S.; Lu, G.Z.; Chen, Q.L. Deactivation and regeneration of the B2O3/TiO2-ZrO2 catalyst in the vapor phase Beckmann rearrangement of cyclohexanone oxime. J. Mol. Catal. A Chem. 2005, 240, 164–171. [CrossRef] 47. Yin, F.; Blumenfeld, A.L.; Gruver, V.; Fripiat, J.J. NH3 as a probe molecule for NMR and IR study of zeolite catalyst acidity. J. Phys. Chem. B 1997, 101, 1824–1830. [CrossRef] 48. Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 2373–2419. [CrossRef] 49. Minakshi, M.; Sharma, N.; Ralph, D.; Appadoo, D.; Nallathamby, K. Synthesis and Characterization of Li(Co0.5Ni0.5)PO4 Cathode for Li-Ion Aqueous Battery Applications. Electrochem. Solid-State Lett. 2011, 14.[CrossRef] 50. Tsilomelekis, G.; Josephson, T.R.; Nikolakis, V.; Caratzoulas, S. Origin of 5-Hydroxymethylfurfural Stability in Water/Dimethyl Sulfoxide Mixtures. ChemSusChem 2014, 7, 117–126. [CrossRef] 51. He, J.Y.; Liu, M.J.; Huang, K.F.; Walker, T.W.; Maravelias, C.T.; Dumesic, J.A.; Huber, G.W. Production of levoglucosenone and 5-hydroxymethylfurfural from cellulose in polar aprotic solvent-water mixtures. Green Chem. 2017, 19, 3642–3653. [CrossRef] 52. Deng, T.S.; Cui, X.J.; Qi, Y.Q.; Wang, Y.X.; Hou, X.L.; Zhu, Y.L. Conversion of carbohydrates into 5-hydroxymethylfurfural catalyzed by ZnCl2 in water. Chem. Commun. 2012, 48, 5494–5496. [CrossRef][PubMed] 53. Hu, L.; Zhao, G.; Tang, X.; Wu, Z.; Xu, J.X.; Lin, L.; Liu, S.J. Catalytic conversion of carbohydrates into 5-hydroxymethylfurfural over cellulose-derived carbonaceous catalyst in ionic liquid. Bioresour. Technol. 2013, 148, 501–507. [CrossRef][PubMed]