
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39223202
71406
10.1038/s41598-024-71406-2
Article
Halloysite functionalized with dendritic moiety containing vitamin B1 hydrochloride as a bio-based catalyst for the synthesis of 5-hydroxymthylfurfural
Yaghoubi Soheila 1
Sadjadi Samahe s.sadjadi@ippi.ac.ir

2
Heravi Majid 1
1 https://ror.org/013cdqc34 grid.411354.6 0000 0001 0097 6984 Department of Chemistry, School of Physic and Chemistry, Alzahra University, PO Box 1993891176, Vanak, Tehran, Iran
2 https://ror.org/01a79sw46 grid.419412.b 0000 0001 1016 0356 Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, PO Box 14975-112, Tehran, Iran
2 9 2024
2 9 2024
2024
14 203812 2 2024
27 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Using halloysite clay and vitamin B1 hydrochloride, a novel acidic halloysite-dendrimer catalytic composite has been developed for conversion of fructose to 5-hydroxymthylfurfural. To grow the dendritic moiety on halloysite, it was first functionalized and then reacted with melamine, epichlorohydrin and vitamin B1 hydrochloride respectively. Then, the resulting composite was treated with ZnCl2 to furnish Lewis acid sites. Use of vitamin B1 as the cationic moiety of ionic liquid obviated use of toxic chemicals and resulted in more environmentally friendly composite. Similarly, dendritic moiety of generation 2 was also grafted on halloysite and the activity of both catalysts for conversion of fructose to 5-hydroxymthylfurfural was investigated to disclose the role of dendrimer generation. For the best catalytic composite, the reaction variables were optimized via RSM and it was revealed that use of 0.035 g catalyst per 0.1 g fructose at 95 °C furnished HMF in 96% yield in 105 min. Turnover numbers (TONs) and frequencies (TOFs) were estimated to be 10,130 and 5788 h−1, respectively. Kinetic studies also underlined that Ea was 22.85 kJ/mol. The thermodynamic parameters of ΔH≠, ΔS≠ and ΔG≠, were calculated to be 23 kJ/mol, − 129.2 J/mol and 72.14 kJ/mol, respectively. Notably, the catalyst exhibited good recyclability and hot filtration approved heterogeneous nature of catalysis.

Keywords

5-Hydroxymthylfurfural
Halloysite
Dendrimer
Vitamin B1 hydrochloride
Subject terms

Chemistry
Catalysis
Catalyst synthesis
Heterogeneous catalysis
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

In attempt to respond to the increasing energy demands and environmental concerns, use of renewable energies, such as biofuels has garnered tremendous attention1–3. To industrialize biofuels, it is imperative to develop ones with comparative energy densities with conventional fuels. In this regard, furan-based biofuels, such as 2,5-dimethylfuran (DMF), which are obtained from non-edible lignocellulosic biomass4–6 are deemed as promising solutions7. The most feasible route to achieve DMF is conversion of carbohydrates, such as fructose to 5-hydroxymthylfurfural (HMF)8–12, followed by hydrodeoxygenation to DMF. Notably, HMF application13–15 is not limited to the synthesis of DMF, but many value-added chemicals and solvents16–18. This makes HMF as a key furanic compounds and myriad attempts have been dedicated to disclose efficient protocols for its synthesis. As conversion of carbohydrates to HMF requires acidic catalyst, development of a heterogeneous acidic catalyst is considered as the main challenge of HMF production. So far, multifarious acidic catalysts, such as heteropolyacids19, H-form zeolites20 and acidic ionic liquids21 have been utilized for this chemical conversion. More recently, use of bio-based compounds22 has been introduced to develop natural-based catalysts for HMF production. In this line, use of halloysite clay (Hal)23,24 that is an aluminosilicate clay with unique tubular morphology and opposite electric charge on both surfaces as a supporting material is of great interest25–27. Interestingly, Hal has instinct acidic feature that renders it a potent candidate for developing acidic catalysts28. Notably, the acidity of pristine Hal is not adequate for promoting fructose dehydration to HMF. Therefore, it is imperative to introduce some acidic active sites on Hal, which can be achieved through chemical modification29. Among various functional groups, dendrimers that are well-ordered, hyper branched, 3-D macromolecules, composed of layers of repeating branches30–32 are potential ones33,34. The merits of denderimers are that they can be directly grafted on various supporting materials via covalent approaches providing a confined microenvironment, which can serve as a nanoreactor32,35. On the other hand, it is possible to control the features of dendrimer by adjusting its generation and components. It is worth noting that the ending groups of dendrimers can also be altered by introduction of various functionalities. As an example, ionic liquids (ILs), which are catalytic species, composed of organic cation and organic/inorganic anions can be incorporated into dendrimer to form dendritic catalysts. Mostly, ILs with aromatic cationic moiety, such as imidazolium or thiazolium are more active than their aliphatic counterparts. Despite the outstanding features of ILs, use of possibly toxic chemicals in their backbones makes them less environmentally friendly. To address this issue, use of bio-based compounds for the synthesis of ILs has been suggested. It is worth noting that some bio-based compounds suffer from drawbacks, such as less thermal stability and high sensitivity, which limit their use under harsh conditions. Hence, it is necessary to enhance their stability. As an approach to circumvent this issue, bio-based compounds can be immobilized on stable supporting materials to devise bio-based catalysts for uses under relatively mild reaction conditions.

In attempt to develop a bio-based acidic catalyst for dehydration of fructose to HMF, in this work we wish to present a novel acidic composite of Hal and dendrimer. More accurately, dendriemrs with vitamin B1 hydrochloride ending groups of generation 1 and 2 were grown on functionalized Hal and then treated with ZnCl2 to furnish Lewis acid sites, Scheme 1. Then, the efficiency of the as-prepared catalysts, G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 for the HMF synthesis was evaluated to disclose the role of dendrimer generation in the catalytic activity. Next, the reaction conditions was optimized for the best catalyst via Response Surface Method, RSM. Recycling and Hot-filtration tests were also conducted to elucidate the recyclability and the nature of the catalyst. To the best of our knowledge, it is the first time that covalent composite of Hal and dendritic vitamin B1-based IL is reported. In fact, this novel catalyst benefits from the features of both Hal, which is an acidic clay, and acidic bio-based IL. Incorporation of this acidic bio-based IL in dendritic moiety also increases the number of ILs and consequently the catalytic active sites, which allows efficient dehydration of fructose under relatively mild conditions. Notably, the covalent attachment of dendritic moiety on Hal also contribute to the stability of the catalyst and its high recyclability. Also, use of vitamin B1 for the synthesis of IL, led to the formation of bio-based IL, which is less toxic and more environmentally friendly.Scheme 1 Schematic presentation of synthetic rout of the catalysts.

Result and discussion

Characterization

To affirm successful formation of G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2, their FTIR spectra were recorded and compared with that of Hal. As depicted in Fig. 1A, the main characteristic absorbance bands of pristine Hal are observed at 537 cm−1 (Al–O–Si vibration), 743 cm−1 (stretching vibration of Si–O), 794 cm−1 (symmetric stretching of Si–O), 1030 cm−1 (Si–O stretching), 1104 cm−1 (perpendicular Si–O–Si stretching), 1654 cm−1 (weak stretching and bending vibrations of water), 3697 cm−1 and 3623 cm−1 (internal –OH). All of these characteristic bands are observable in the FTIR spectra of both G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2, underscoring the fact that the multi-step process used for the introduction of the dendritic moiety on Hal did not demolish its structure. Precise comparison of the FTIR spectra of the catalysts and Hal also implied the appearance of some new bands in the catalysts. In more detail, the absorbance band in 1697 and 2914 cm−1 are related to the –C = N and –CH2 functionalities, which are indicative of conjugation of dendritic functionality. Notably, the stretching band related to Zn–Cl (511 cm−1) overlapped with the that of Hal36.Fig. 1 (a) FTIR spectra of Hal, G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2, (B) XRD patterns Hal and G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2.

XRD was also exploited to explore the crystalline structure of the catalysts and examine the stability of Hal in the course of grafting of dendritic moiety. As displayed in Fig. 1B, Hal characteristic peaks appeared at 2θ = 19.8°, 24.3°, 26.5°, 38.4°, 55.3°, 62.6°, 73.8° and 77.3° (JCPDS No. 29–1487)37–39. Upon introduction of dendritic moiety of generation 1, apart from Hal peaks, broadening of the peaks in the range of 2θ = 15.4–24.8° can be observed, which is ascribed to the amorphous dendritic moiety. Similarly, G2-Hal-MET/ZnCl2 XRD pattern exhibited Hal peaks, implying the structural stability of Hal and some broadening, which makes the pattern distinguishable from pristine Hal.

The morphology of two as-synthesized catalysts, i.e. G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2 has been investigated via SEM. The results underlined that in both samples, the Hal tubes were detectable, confirming that Hal instinct morphology has been maintained in the course of conjugation of dendrimer. Comparison of the SEM images of the two catalysts, Fig. 2A and B, underscored that G2-Hal-MET/ZnCl2 exhibited more compact morphology and more aggregates are observable in this sample compared to G1-Hal-MET/ZnCl2. Indeed, as in G2-Hal-MET/ZnCl2 the dendritic moiety is of generation two, it can form more amorphous aggregates.Fig. 2 SEM images of (A) G1-Hal-MET/ZnCl2 and (B) G2-Hal-MET/ZnCl2 and (C) EDS analysis of G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2.

EDS analysis of G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 was also conducted to approve incorporation of the dendritic moiety. As displayed in Fig. 2C, both catalysts showed the presence of carbon, oxygen, nitrogen, chlorine, aluminum, silicon, zinc and sulfur. Among the aforementioned elements, aluminum, silicon and oxygen are representative of Hal, while the presence of carbon, sulfur, oxygen and nitrogen is a proof for incorporation of the dendritic moiety. Observation of chlorine and zinc atoms is also indicative of formation of Lewis acid. Although EDS analysis is a semi-quantitative one, comparison of the weight and atomic percents of the present atoms in G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 clearly underlined that these values for carbon, nitrogen, chlorine, zinc and sulfur, which can be indicative of functional moiety on Hal are higher for G2-Hal-MET/ZnCl2, confirming formation of generation II of dendrimer in this catalyst. Dispersion of the elements present in G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2 was studied by elemental mapping analysis, Fig. 3. According to the results, the elements related to dendritic moiety have been dispersed uniformly. Similarly, the results approved homogeneous dispersion of chlorine and zinc elements in both catalysts.Fig. 3 Elemental mapping analysis of (A) G1-Hal-MET/ZnCl2 and (B) G2-Hal-MET/ZnCl2.

TG analysis was also exploited to confirm grafting of the dendritic moiety on Hal. As the comparison of thermograms of G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 with that of Hal shows, Fig. 4, both as-prepared catalysts showed lower thermal stability than pristine Hal, which is a proof for the presence of less-thermally stable organic moiety in those samples. In fact, in both G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2, apart from the weight losses of Hal, which are detected at 150 °C and 500 °C additional weight loss stage at 270–400 °C was observed that is due to the decomposition of dendritic moiety. More accurately, in G1-Hal-MET/ZnCl2 an additional weight loss (14 wt%) was observed in the range of 250–400 °C, which is due to the decomposition of organic moiety (melamine, epoxy and vitamin B1). In the case of G2-Hal-MET/ZnCl2, more weight loss steps were detected, Fig. 4. In this thermogram, the weight loss at 250 °C is attributed to the decomposition of melamine and epoxy group, while the one observed in 350 and 470 °C can be ascribed to the decomposition of second generation of dendritic moiety. It is also worth noting that comparison of thermograms of G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 implied that this weight loss in G2-Hal-MET/ZnCl2 is more pronounced and corresponded 33 wt%, while in G1-Hal-MET/ZnCl2 this value is only 14 wt%. This outcome approves successful formation of the second generation of dendrimer in G2-Hal-MET/ZnCl2.Fig. 4 Right: Thermograms of Hal, G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2 and DTG curve of G1-Hal-MET/ZnCl2.

The XPS plot (Fig. 5a) clearly shows the presence of peaks associated with carbon, chlorine, nitrogen, oxygen, silicon, aluminum, sulfur, and zinc. The C1s spectrum depicted in (Fig. 5b) reveals binding energies of 284.4 eV (corresponding to C–N and C–C), 285.5 eV (corresponding to C–O and C=N), and 286.1 eV (corresponding to C-N). A high-resolution O 1 s profile, Fig. 5c was deconvoluted into the peaks at 531.8, 532.5, and 531.1 eV, which are related to O 1s, SiO2, and Al2O3. Deconvolution of Cl 2p, Fig. 5d, resulted in peaks at 198.7, 198.8, 198.9, 200.5 eV which are related to Cl 2p, Cl 2p3 and Zn–Cl, Cl 2p1. According to the deconvoluted high-resolution S 2p (Fig. 5e), the peaks at 165.1, 166.6, and 164.8 eV that are related to S, SO, have been detected. The deconvoluted high-resolution Si 2p spectrum, Fig. 5f, showed the presence of peaks at 99.3 eV, 102.6 eV, and 103.3 eV, which indicate Si, SiO, and SiO2. High-resolution Al 2p spectrum, Fig. 5g, can be deconvoluted into the peaks at 72.9 eV, 72.7 eV, 74.3 eV, and 74.4 eV, which are representative of Al (2p1, 2p3), Al2O3, Al2O3-n H2O, and Al2SiO5 respectively. High-resolution N 1s, Fig. 5h, can be deconvoluted into the peaks at 398.4, 397.1, 397.6, 400.2, and 403.2 eV. Deconvoluted high-resolution Zn 2p (Fig. 5I), into the peaks at 1021.8, 1021.7, 1044.8 eV, which are related to Zn 2p1.Fig. 5 (a) XPS analysis of G2-Hal-MET/ZnCl2 (b) spectrum of C 1 s, (c) spectrum of O1s, (d) spectrum of Cl 2p, (e) spectrum of S 2p, (f) spectrum of Si 2p, (g) spectrum of Al 2p, (h) spectrum of N 1 s, and (I) spectrum of Zn, 2p.

Considering the importance of textural properties of the catalyst, specific surface area (SBET) and total pore volume (Vp) of G2-Hal-MET/ZnCl2 were also measured. Moreover, to shed light into the effect of functionalization of these properties, SBET and Vp of the catalyst were compared with those of pristine Hal. According to the results, the specific surface area of Hal was 52 m2 g−1. This value for G2-Hal-MET/ZnCl2 was ~ 10 m2 g−1, implying coverage of outer surface of Hal with dendritic moiety. Similarly, Vp value of Hal (0.12 cm3 g−1) decreased to 0.09 cm3 g−1 upon introduction of dendritic moiety. Total acidity of G2-Hal-MET/ZnCl2 was also measured using NH3-TPD as 7.34 mmol/g cat.

Comparison of the catalytic activity of G2-Hal-MET/ZnCl2 and G1-Hal-MET/ZnCl2

In this study, two catalysts, G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2 have been prepared, in which the generation of the dendritic moiety was different. In fact, it was believed that in G2-Hal-MET/ZnCl2 that the number of vitamin B1 hydrochloride functionality is higher, the chance of conjugation of ZnCl2, which acts as a Lewis acid is higher and consequently, this catalyst exhibits superior catalytic activity compared to G1-Hal-MET/ZnCl2. To verify this assumption, the catalytic activity of both catalysts for dehydration of fructose to HMF at 100 °C in DMSO as solvent using 0.03 g of catalyst was examined and compared. The outcomes indicated that under the aforesaid reaction conditions, G1-Hal-MET/ZnCl2 led to the formation of HMF in 80% yield, while, G2-Hal-MET/ZnCl2 resulted in 89% HMF, confirming the superiority of G2-Hal-MET/ZnCl2. This issue is due to the presence of higher acidic sites on G2-Hal-MET/ZnCl2.

Optimization of the reaction conditions

As RSM is a precise method that not only can consider the effects of reaction parameter, but also the interactions among them, it was selected to optimize the reaction variables, including catalyst loading, reaction temperature and time have been optimized to achieve the highest HMF yield. To this aim, some initial experiments have been conducted to obtain the suitable range of each variable, Table 1. Then, using a quadratic model the statistic studies was performed. The outcomes of analysis of variance (ANOVA) are summarized in Table 2. RSM calculations also furnished Eq. (1), which includes various parameters with positive and negative coefficients, which indicate their synergistic or antagonistic effects on the HMF yield. More precisely, A, AC and BC parameters with positive coefficients have synergistic effects and ultimately increase the HMF yield, while B, C, AB, A2, B2 and C2 parameters with negative coefficients have antagonistic effects and decrease the yield of HMF.Table 1 The initial tests performed for finding the range of reaction variables for RSM.

Entry	Catalyst (wt%)	Time (min)	Temp. °C	Yield (%)	
1	15	100	120	25	
2	25	100	120	53.15	
3	25	90	130	57	
4	35	90	120	79.8	
5	40	90	120	63.9	
6	35	95	110	88.4	
7	25	95	110	67.6	
8	35	95	105	100	
9	40	95	105	73	
10	35	95	105	95	
11	40	70	100	71	

Table 2 ANOVA results of response surface method using a quadratic model.

Source	Sum of squares	df	Mean square	F-value	p-value	
Model	11,931.94	9	1325.77	22.49	 < 0.0001	
A-Time	5.84	1	5.84	0.0992	0.7593	
B-Tem	10.26	1	10.26	0.1740	0.6854	
C-Cat	123.38	1	123.38	2.09	0.1786	
AB	171.40	1	171.40	2.91	0.1189	
AC	676.57	1	676.57	11.48	0.0069	
BC	821.54	1	821.54	13.94	0.0039	
A2	0.2665	1	0.2665	0.0045	0.9477	
B2	1051.43	1	1051.43	17.84	0.0018	
C2	9441.27	1	9441.27	160.19	 < 0.0001	

Furthermore, the coefficients implies that the order of the magnitude of the influence of the parameters are as follow:

C2 > BC > AC > AB > B2 > C > B > A > A2.1 HMFYield(\% )=+95.76+0.6044A-0.8006B-2.78C-4.63AB+9.20AC+10.13BC-0.1030A2-6.47B2-19.38C2

A: Temperature, B: Time, C: Catalyst amount

By looking at the coefficients in Eq. (1), it can be seen that certain terms like C2, BC, AC, and AB have a greater impact compared to terms like A2 and other powers. For instance, the coefficient for C2 being − 19.38 is the most negative among all coefficients, indicating a significant influence of this variable in the model. This means that changes in the value of C2 lead to substantial variations in the final result, in this case, the HMF yield (%). Similarly, observing larger values for coefficients of variables like BC, AC, and AB highlights the high importance of these variables in affecting the model's outcome. On the other hand, the coefficient for A2, being − 0.1030 and having the smallest negative value, suggests less pronounced impact of A2 compared to other variables.

According to the results, the Model F-value was 22.49, which implies that the model is significant. There is only a 0.01% chance that a F-value of this large could occur due to noise. Also, P-values less than 0.0500 indicates that the model terms are significant. In this case, AC, BC, B2, and C2 are significant model terms. Values greater than 0.1000 indicate the model terms are not significant. The Lack of Fit F-value of 1.90 implies that the Lack of Fit is not significant relative to the pure error.

The R2 value indicates the degree of correlation between the model predictions and the actual results. Here, R2 has a value of 0.910, indicating a good fit between the model predictions and the actual results. Additionally, the Adjusted R2 and Predicted R2 values are 0.90 and 0.728, respectively. Additionally, the 3D graphs (Figs. 6) illustrate the effects of various reaction parameters (temperature, time, and catalyst loading) on the HMF yield. In more detail, in (Fig. 6A), 3D surface plot of the interaction between temperature and time shows that the maximum HMF yield is achieved at 105 °C and 95 min.Fig. 6 (A) 3D surface plot of the interaction between temperature and time amount for HMF yield, (B) 3D surface plot of the interaction between temperature and catalyst amount for HMF yield and (C) A 3D surface plot illustrating the interaction between reaction time and catalyst.

The results obtained from the 3D surface plot in (Fig. 6B), which illustrates the interaction between temperature and catalyst loading on HMF yield, suggest that increasing the catalyst loading up to 0.035 g resulted in the highest HMF yield. Moreover, the optimal reaction temperature was found to be 105 °C.

The 3D surface plot in (Fig. 6C) shows the effect of varying reaction times and catalyst amounts on the HMF yield. As the reaction time increases, the HMF yield increases up to a certain point and then reaches a plateau. Similarly, increasing the catalyst amount initially leads to an increase in HMF yield, but beyond a certain point, further increase in catalyst amount does not result in a significant increase in HMF yield. According to the RSM results, the optimal reaction conditions for maximum HMF yield were achieved using 0.035 g catalyst, at a temperature of 105 °C, and a reaction time of 95 min.

Recyclability of the catalyst

As recyclability of a heterogeneous catalyst can impact its applicability and potential large-scale use, it is imperative to study this feature of the catalyst. The results of the examination of the recyclability of catalyst for fructose dehydration under the optimal reaction conditions are presented in Fig. 7A. Gratifyingly, the results implied that the catalyst maintained its activity for the second run of the reaction and for the third and fourth runs of the reaction only 1% loss of the catalytic activity was detected. Moreover, recycling of the catalyst for fifth run led to only 3% decrement of HMF yield, which underlined high recyclability of the catalyst. Noteworthy, recycling of the catalyst for sixth and seventh runs led to more pronounced loss of the activity and HMF was achieved in 75% yield upon seventh run of the reaction. The structural stability of the recycled catalyst was studied by recording its FTIR spectrum and comparing it with that of fresh one. As displayed in Fig. 7B, the two spectra are very similar and the spectrum of the recycled catalyst exhibited all of the characteristic bands of the catalyst.Fig. 7 (A) Recyclability of the catalyst for the dehydration of fructose to HMF under optimum reaction condition, (B) Comparison of FTIR spectra of fresh and recycled catalysts and (C) The result of hot filtration test of G2-Hal-MET/ZnCl2.

Hot filtration test

In heterogeneous catalysis, two distinct routs can be conceived. In the first rout, the catalytic active species remained heterogeneous during the reaction, while in the second rout, leaching of catalytic species in the reaction media and their re-deposition on the catalyst is probable. To distinguish the nature of catalysis, hot filtration test is mainly applied. In this test, the reaction is halted after a short period of time and then the catalyst is removed and the reaction continued in the absence of the catalyst. It is expected that in the case of true heterogeneous catalysis, no improvement of reaction yield occur upon catalyst removal, while in other case, the presence of the leached species can promote the reaction even after its separation. Gratifyingly, the results of hot filtration of G2-Hal-MET/ZnCl2, Fig. 7C, for dehydration of fructose to HMF under the optimal conditions underlined the true heterogeneous nature of the catalysis.

Kinetic study

Based on the literature, conversion of fructose to HMF occurs via a first-order process40,41 and the reaction rate constant, k, value and is dependent on temperature. The rate of reaction is calculated using Eq. (2).42 In this equation, “r” is the reaction speed, which is the equal to the change in the reactant or product relative to time.2 -rfructose=+dHMFdt=-dfructosedt=+Kfructose

Using X as a conversion rate, Eq. (3) can be achieved.3 fructose=fructose0×1-X

Furthermore, combination of the so-called equations can give Eq. (4).

To calculate k value, the target reaction was conducted at four different temperatures, 90, 94, 98 and 100 °C, and then the plot of − Ln(1-X) vs time was obtained, in which k can be estimated from the slope of the line, Figure S1.4 -ln1-X=kt+C

The results in Figure S2 underlined that there is a linear relationship between k value and temperature and upon increment of reaction temperature, k value increased. This value for temperature of 90, 94, 98, and 100 °C was obtained as 89.1.33, 99.1.1, and 23.2, respectively.

Having k value in hand, activation energy, Ea, was also estimated using Arrhenius Equation, Eq. (5). In this equation R stands for universal gas constant that is 8.314 J/(mol K). As displayed in Figure, S3 plotting of lnk vs 1000/T(K) leads to the line that its slope is equal to − Ea/R. According to the calculations, Ea for the dehydration of fructose to HMF was 22.85 kJ/mol, indicating that using G2-Hal-MET/ZnCl2 as a catalyst, the activation energy barrier was relatively low and conversion of fructose could proceed at moderate temperatures, which is attractive from industrial viewpoint.5 lnk=-EaRT+lnA

Thermodynamic study

To calculate thermodynamic parameters, i.e. activation enthalpy ΔH≠, activation entropy ΔS≠, and activation Gibbs energy ΔG≠, Eqs. (6 and 7) were employed.

In Eq. (6), kb and h represent the Boltzmann constant (1.38 × 10-23j.K-1) and Planck's constant (6.626 × 10-36j.s) respectively43.6 k=kbThe-ΔG≠RT

7 ΔG≠=ΔH≠-TΔS≠

Combining Eqs. (6 and 7, Eq. 8) can be derived. Using this new equation and plotting Ln k/T vs 1000/T (Figure S3), The ΔH≠ (kJ/mol) and ΔS≠ (J/mol) were estimated from the slope -ΔH≠R and the intercept Lnkbh+ΔS≠R of the line respectively43. Having ΔH≠ and ΔS≠, ΔG≠, was calculated using Eq. (7) as 90.92 kJ/mol.8 LnkT=-ΔH≠R1T+lnkbh+ΔS≠R

Comparative study

Dehydration of fructose to HMF is such an important chemical process that has been focused by many research groups and multifarious catalysts have been promoted for this catalysts. To elucidate whether the performance of G2-Hal-MET/ZnCl2 is comparable with previously developed catalysts, its activity under the obtained optimal conditions was compared with some of the randomly selected catalysts, listed in Table 3. As displayed, a broad range of catalysts from metal–organic frameworks (MOF) to heteropolyacids has been applied for this conversion. Notably, compared to G2-Hal-MET/ZnCl2, some catalysts, such as SBA-SO3H and Si-3-IL-HSO4 led to moderate HMF yields. The comparison of the reaction yields also indicated that the catalytic activity of G2-Hal-MET/ZnCl2 is superior to some other catalysts, such as Mesoporous TiO2, Fe3O4@SiO2-SO3H and PS-Tet-SO3H. G2-Hal-MET/ZnCl2 also exhibited comparable catalytic activity compared to some other dendritic catalysts, such as SO3H-dendrimer-SiO2@Fe3O4 and Cell-G3-SO3H. It is worth noting that this comparison cannot be deemed as an accurate comparison due to the difference of the reaction conditions and it is not claimed that G2-Hal-MET/ZnCl2 is the best catalyst for the conversion of fructose to HMF, however, this comparison just can give a sense of efficiency of this catalyst.Table 3 Comparison of the activity of some catalysts for conversion of fructose to HMF.

Entry	Catalyst	Time (min)	Temp. (ºC)	Yield (%)	References	
1	G2-Hal-MET/ZnCl2	105	95	96	–	
2	Si-3-IL-HSO4	30	130	63.0	44	
3	FPILa	60	120	88.3	45	
4	SBA-SO3H	70	160	55	46	
5	Mesoporous TiO2	2	130	82.3	47	
6	PS-Tet-SO3H b	120	150	86.85	48	
7	PW12-ILs-C4-HNSc	120	100	93.7	49	
8	MIL-101(Cr)-SO3H	60	120	90	42	
9	L-Proline derived ionic liquids	50	90	73.6	50	
10	SBA-15-SO3H-10	60	120	 ~ 81	51	
11	Fe3O4@SiO2-SO3H	120	100	93.1	52	
12	Cell-G3-SO3Hd	45	110	96	53	
13	SO3H-dendrimer-SiO2@Fe3O4	60	100	92	54	
14	Hal/k-Cr/PAAe	35	100	97.9	22	
15	Hal-IL	100	100	98.5	55	
16	Hal-IMI-SO3H	73	85	88	28	
aBifunctional polymeric ionic liquids (Immobilizing Cr3+ with SO3H-functionalized solid polymeric ionic liquids).

bPolystyrene functionalized 5-amino-1H tetrazole (PS-Tet).

cHeteropolyacids immobilized ionic liquid-modified organosilica hollow nanosphere.

dThird-generation dendrimer on the surface of 3-mercaptopropyl-modified cellulose.

eComposite of Halloysite and polymer and k-carrageenan.

Reaction mechanism

The plausible mechanism of dehydration of fructose to 5-HMF is depicted in Fig. 8. Initially, adsorption of fructose on the surface of G2-Hal-MET/ZnCl2 through non-covalent interactions brings it close to the main catalytic sites. As illustrated, the reaction proceeds by activation of fructose through coordination of G2-Hal-MET/ZnCl2 to remove water and form an enolic intermediate, which then tautomerizes to form more stable keto-form. Afterwards, loss of the second molecule of water occurs to give 5-HMF.Fig. 8 Plausible reaction mechanism.

Conclusion

In this study, two catalysts, G1-Hal-MET/ZnCl2 and G2-Hal-MET/ZnCl2, were prepared by growing dendritic moiety with vitamin B1 hydrochloride ending groups of generations 1 and 2 on Hal, followed by treatment with ZnCl2. The catalysts were characterized and their catalytic activity for conversion of fructose to HMF was compared to disclose the role of generation of the dendritic moiety in the activity of the catalyst. The results underpinned that the catalytic activity of G2-Hal-MET/ZnCl2 was superior compared to G1-Hal-MET/ZnCl2. Finding G2-Hal-MET/ZnCl2 as the catalyst of choice, the reaction conditions were optimized via RSM and it was found that use of 0.035 g catalyst per 0.1 g fructose at 95 °C furnished HMF in 96% yield in 105 min. Turnover numbers (TONs) and frequencies (TOFs) were estimated to be 10,130 and 5788 h−1, respectively. Notably, recycling test confirmed the recyclability of the catalyst and true heterogeneous nature of the catalysis was approved through hot-filtration method. Kinetic studies also implied that Ea was 22.85 kJ/mol. Furthermore, the thermodynamic parameters of ΔH≠, ΔS≠ and Gibbs free energy were calculated to be 23 kJ/mol, − 129.2 J/mol and 72.14 kJ/mol, respectively.

Experimental

Chemicals and solvents

Halloysite (Hal), (3-chloropropyl) triethoxysilane (CPTES, 95%), epichlorohydrin (EP, 92.5%), triethylamine (99.5%), ethanol (> 99%), dimethyl sulfoxide (DMSO, > 99%), fructose (> 99%), toluene (> 99%), thiamine hydrochloride (vitamin B1 hydrochloride) (99%), zinc chloride (ZnCl2 > 98%) and melamine (99%) all provided from Sigma-Aldrich, were used in the synthesis of the catalysts and examining their catalytic activity.

Synthesis of the catalyst

Preparation of Cl-functionalized Hal: (Hal-Cl)

A homogeneous suspension was prepared by addition of Hal (4 g) into toluene (80 mL), which was stirred for 10 min. Then, CPTES (4 mL) was added and the mixture was refluxed for 24 h, at 110 °C. At the end, the resulting precipitate, Hal-Cl, was obtained by centrifugation, washed multiple times with toluene, and dried at 70 °C for 24 h.

Conjugation of melamine: synthesis of Hal-M

To synthesize Hal-M, a solution of melamine (6 mmol) in DMSO (100 mL) was prepared and then, Hal-Cl (5 g) was added and the resulting mixture was vigorously stirred and heated up to 80 °C for 48 h under an argon atmosphere. After the end of the reaction, the product, Hal-M, was collected by centrifugation, washed several times with ethanol and toluene, and dried overnight at room temperature.

Reaction with epichlorohydrin: synthesis of Hal-ME

To prepare the target compound, Hal-ME, the as- synthesized Hal-M (3 g) was suspended in triethylamine (15 mL) and stirred at room temperature for approximately 30 min. A solution of EP (4 mL) was then added to the stirring mixture in a dropwise manner. The resulting reaction mixture was heated to 60 °C and maintained at that temperature for 24 h. After completion of the reaction, the resulting solid was separated by centrifugation, and washed multiple times with distilled H2O (3 × 20 mL) and EtOH (3 × 20 mL), respectively, to remove any impurities. Finally, the purified solid was dried in a vacuum oven at 60 °C for 24 h, to yield the desired product, Hal-ME. After obtaining the solid product, it was divided it into two equal portions, one used as the starting material for the next synthetic step for the preparing G2-Hal-MET/ZnCl2 catalyst. The remaining half of the product was applied for the synthesis of G1-Hal-MET/ZnCl2 catalyst.

Incorporation of vitamin B1 hydrochloride: synthesis of Hal-MET

To prepare Hal-MET, vitamin B1 hydrochloride (5 mmol, 1.5 g) was added to the stirring suspension of Hal-ME in ethanol and further stirred for 48 h at room temperature. Upon completion of the reaction, the resulting solid was separated by centrifugation, washed several times with distilled H2O (3 × 20 mL) and EtOH (3 × 20 mL) and dried in a vacuum oven at 50 °C overnight.

Preparation of G1-Hal-MET/ZnCl2

First, zinc chloride (4 mmol) was dissolved in (20 mL) of H2O at room temperature and the resulting solution was stirred for 10 min to ensure complete dissolution. Next, Hal-MET (2.5 g) was added to the solution and the obtained mixture was reflexed for 48 h. Upon completion of the reaction, the resulting white solid was separated from the aqueous medium by centrifugation, washed thoroughly with distilled H2O and EtOH, and dried under vacuum at 50 °C for 24 h.

Preparation of G2-Hal-MET/ZnCl2

To prepare G2-Hal-MET/ZnCl2, melamine (2 mmol) was added to Hal-ME and the resulting mixture was stirred at 80 °C for 48 h under an argon atmosphere. Upon completion of the reaction, the resulting precipitate was separated from the mixture by centrifugation using the previously described method. Afterwards, EP (4 mmol) was added to the mixture, and the resulting solution was stirred at ambient temperature for 24 h. The solution was then purified using the same method as described before, which involved multiple items of washing with distilled water and ethanol, followed by drying under vacuum at 50 °C for 24 h. In the next step, the obtained solid (4 mmol) was mixed with vitamin B1 hydrochloride in distilled water and the suspension was stirred for 24 h. At the end of the reaction, the solid was filtered, rinsed several times with distilled water and dried at ambient temperature overnight. In the final stage, the obtained G2-Hal-MET was treated with zinc chloride through the same procedure that has been described for the synthesis of G1-Hal-MET/ZnCl2.

Characterization of the catalyst

X-ray diffraction (XRD) was conducted using a Rigaku Ultima instrument using Cu-Kα radiation. Fourier-transform infrared spectroscopy (FT-IR) was performed via a BRUKER TENSOR 35 spectrophotometer using potassium bromide (KBr) pellets and a scan time of 1 s and a spectral resolution of 2 cm−1. Thermogravimetric analysis (TGA) was done under an O2 atmosphere with a ramp rate of 10 °C/min using a METTLER TOLEDO instrument. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) as well as elemental mapping was performed using a VEGAII TESCAN device equipped with a QX2 RONTEC energy dispersive X-ray analyzer. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a VG-Microtech Multilab 3000 spectrometer with an Al anode. The specific surface areas (SBET) of the samples were estimated using the Brunauer–Emmett–Teller (BET) method using BELSORP MINI II, BEL device with pre-heating at 140 °C for 3 h. The acidity of the catalyst was estimated by the temperature-programmed desorption of ammonia (NH3-TPD), using Chemisorption Analyzer, NanoSORD, with heating ramp rate of 20 °C min−1.

Dehydration of fructose to HMF

To dehydrate fructose to HMF, the catalyst (0.035 g) was added to a stirring solution of fructose (0.1 g) in DMSO (5 mL) and the mixture was heated up to 95 °C for 105 min. Upon completion of the reaction, the catalyst was collected via centrifugation and recovered by washing with DMSO repeatedly and drying in an oven at 60 °C overnight.

Measuring HMF yield

Agilent 1200 Series apparatus equipped with a Brisa LC2 C18 column (5 µm, 25 × 0.46) operated at 35 °C based on the external standard was employed for performing HPLC. The eluent for HPLC was a mixture of acetonitrile (CH3CN) and H2O with a volumetric ratio of 40:60 with flow a rate of 1 mL min−1. The yield of HMF was simply measured through Eq. (9), where Mole (I) is the initial mole of fructose.9 HMFyield%=MoleHMFMoleI×100%

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71406-2.

Author contributions

S.Y.: Formal analysis; data curation; methodology; visualization; investigation; writing—original draft. S.S.: conceptualization; funding acquisition; project administration; resources; supervision; writing—review & editing. M.H.: Funding acquisition; resources; investigation.

Funding

This research has been supported by the National Natural Science Foundation of China (Grant No. 52161145405), Iran National Science Foundation (INSF) under project No. 4000580. The authors also appreciate the partial supports of Iran Polymer and Petrochemical Institute and Al-zahra University.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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