Sn-Fe bimetal doped rice hull catalyst and preparation method and application thereof

Abstract

The invention belongs to the technical field of biomass chemical conversion and green catalysis, and particularly relates to a Sn-Fe bimetal rice hull doped catalyst as well as a preparation method and application thereof. The preparation method comprises the following steps: S1, weighing a tin source and an iron source as precursors, and dissolving the tin source and the iron source in an organic solvent to prepare a metal precursor solution; s2, adding rice hull powder into the metal precursor solution, and stirring and dipping; then drying at 105 DEG C to obtain treated rice husks; s3, heating to 450-600 DEG C in a nitrogen atmosphere, keeping the temperature, and naturally cooling to room temperature to obtain carbonized rice husks; s4, performing suction filtration separation on the carbonized rice husks, and fully washing the carbonized rice husks with deionized water and absolute ethyl alcohol respectively; and finally drying at 80 DEG C for 6 hours to obtain the catalyst. In the prepared catalyst, SnO2 with the particle size of 2-4 nm and Fe2O3 are uniformly embellished by Sn-O-Fe / Sn-O-C anchor points to form double acid sites adjacent to Lewis-Bronsted on the basis of a porous SiO2-carbon skeleton of rice husks by Sn-Fe / RHC, so that the effects of stable structure, precise acid sites and fast mass transfer are realized, and the reaction of glucose to HMF is endowed with high activity, high selectivity and circulation resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomass chemical conversion and green catalysis, and in particular relates to a Sn-Fe bimetallic rice husk doped catalyst, a preparation method and an application thereof. Background Art

[0002] 5-Hydroxymethylfurfural (5-HMF) is currently recognized internationally as one of the most promising bio-based platform compounds. It can be used as a key precursor for biofuel additives, polyester monomers, and pharmaceutical intermediates, and has far-reaching significance for replacing petrochemicals and developing renewable chemicals. Existing methods for preparing 5-HMF can convert glucose into 5-HMF. Specifically, glucose must first be isomerized to fructose under Lewis acid conditions and then dehydrated to 5-HMF under Brønsted acid conditions. However, this method easily produces byproducts such as furfural polycondensates, levulinic acid, and formic acid. As a result, the 5-HMF yield of traditional homogeneous acid catalysis systems is only between 40% and 70%, and the selectivity is unstable. Although homogeneous metal salts represented by CrCl3 and AlCl3 can achieve a yield of 60% to 70% in a short time at around 140°C in ionic liquids or organic high-boiling-point solvents such as dimethyl sulfoxide (DMSO), the catalyst is difficult to recover, the cost of ionic liquids is high, the post-processing is complicated, and the equipment is severely corrosive, making them not worth industrial promotion. The alkaline catalytic process requires strict control of the alkali solution concentration to avoid side reactions, and has a great impact on product separation and purification, and the overall comprehensive cost is still relatively high.

[0003] The emergence of solid acid catalysts has provided new ideas for industrial applications, and various types of solid sulfonic acid resins, molecular sieves and metal oxides have been widely studied. For example, solid acids such as Nb2O5 and γ-Al2O3-SO3H can achieve a 50% to 65% 5-HMF yield in aqueous phase or aqueous / organic two-phase system, and are recyclable. However, the Brønsted / Lewis acid sites of these materials are unevenly distributed and the acid strength is limited. They often require harsh conditions of more than 180°C and more than 2 hours to barely achieve a moderate yield. They have high energy consumption and poor catalyst stability, making it difficult to meet the three major requirements of "green, low-carbon, and economical". Although magnetic solid acids such as MHGC-SO3H can be quickly recovered by magnetic attraction, they require an additional sulfuric acid sulfonation process, which increases the preparation cost; Chinese patent publication number CN112543754B discloses a two-phase, one-pot method using AlCl₃ / activated carbon / DMSO / MIBK–2-butanol to synthesize 5-HMF at 110°C–160°C in 6–12 hours with a yield of approximately 65%. However, this method has been difficult to implement due to its long reaction time, high energy consumption, and complex solvent recovery. Another Chinese patent publication number, CN115536618B, discloses a method combining microwave hydrothermal treatment with molten salts, achieving a 76.3% yield in 30 minutes using a LiBr·3H₂O / GVL system at 100°C. However, the practical recovery cost of high-concentration LiBr and GVL is extremely high, and specialized microwave equipment is required, making it unsuitable for general use. Even more innovative is vacuum or solvent-free dehydration technology, such as the Chinese patent publication number CN115806536B, which proposes dehydration to prepare 5-HMF under conditions of 0.01 to 5 kPa. This method does not require an organic solvent and is significantly environmentally friendly. However, it only demonstrates a fructose dehydration system and has not yet been expanded to glucose, making it impossible to directly utilize my country's abundant and cheap glucose resources.

[0004] In summary, current technologies present a clear contradiction between achieving high 5-HMF yields and green, low-cost approaches. High yields often rely on expensive or corrosive homogeneous systems or high-temperature, long-term solid-phase catalysis. Meanwhile, green, recyclable solid catalysts often sacrifice reactivity and require demanding preparation conditions. Therefore, a Sn-Fe bimetallic rice husk-doped catalyst, its preparation method, and its application are urgently needed to address these challenges. Summary of the Invention

[0005] To overcome the above-mentioned deficiencies in the prior art, the present invention provides a Sn-Fe bimetallic doped rice husk catalyst, its preparation method, and application. The catalyst of the present invention achieves one-step dehydration of glucose to efficiently produce 5-HMF in a mild two-phase system.

[0006] To achieve one of the above purposes, the present invention adopts the following technical solutions: A Sn-Fe bimetallic doped rice husk catalyst, comprising the following steps: S1. Weighing a tin source and an iron source as precursors, wherein the molar ratio of the tin element in the tin source to the iron element in the iron source is (1-5):1, and dissolving them in an organic solvent to prepare a metal precursor solution; S2. Add 5-10 g of dried and ground rice husk powder to the metal precursor solution and soak for 4 hours; then dry at 105° C. for 4-12 hours to obtain treated rice husk; S3, placing the treated rice husk in a tube furnace, heating it to 450-600°C at a rate of 2-10°C / min under a nitrogen atmosphere, maintaining it for 2 hours, and then naturally cooling it to room temperature to obtain carbonized rice husk loaded with heavy metal ions; S4. The carbonized rice husk was thoroughly washed with deionized water and anhydrous ethanol, respectively. The deionized water was used to wash away the residual doping substances on the surface, such as SnCl4 and Fe(NO3)3, and the anhydrous ethanol was used to wash the bio-oil on the surface; finally, the carbonized rice husk was dried to obtain a Sn-Fe bimetallic doped rice husk catalyst (Sn-Fe / RHC).

[0007] Preferably, in step S2, the rice husk powder is obtained by thoroughly washing the rice husk, drying it at 60-100° C. for 12-36 hours, and grinding it to a particle size of 80-200 meshes.

[0008] Preferably, in step S1, magnetic stirring is performed at room temperature for 30 to 120 minutes.

[0009] Preferably, the tin source is SnCl4·5H2O, the iron source is Fe(NO3)3·9H2O, and the organic solvent is anhydrous ethanol.

[0010] Preferably, in step S1, SnCl4·5H2O and Fe(NO3)3·9H2O are weighed at a mass ratio of 2.6:1, added to 30-70 mL of anhydrous ethanol, and stirred evenly to prepare a metal precursor solution.

[0011] Preferably, the total mass of SnCl4·5H2O and Fe(NO3)3·9H2O is 1.55 g.

[0012] To achieve the second of the above-mentioned purposes, the present invention provides a catalyst prepared by a method for preparing a Sn-Fe bimetallic rice husk-doped catalyst, wherein the catalyst uses SiO2 and a carbon skeleton of the rice husk as a carrier, and SnO2 nanoparticles and Fe2O3 nanoparticles are in situ generated on the surface of the carrier, and the particle size of the SnO2 nanoparticles and the Fe2O3 nanoparticles are both less than 10 nm, preferably SnO2 nanoparticles and Fe2O3 nanoparticles of 2 to 4 nm.

[0013] To achieve the third of the above objectives, the present invention provides an application of a Sn-Fe bimetallic doped rice husk catalyst, which catalyzes the conversion of glucose into 5-hydroxymethylfurfural.

[0014] Preferably, the specific steps of catalyst application are: (1) Glucose is dissolved in a water-THF solution to prepare a mixed solution, wherein the mass fraction of glucose in the mixed solution is 5-30 wt%; (2) Adding a Sn-Fe bimetallic doped rice husk catalyst and a 5-30 wt% NaCl solution to the mixed solution, wherein the mass of the catalyst is 5-30% of the mass of glucose, the reaction temperature is 170-220 °C, and the reaction time is 15-60 minutes to obtain 5-hydroxymethylfurfural (5-HMF).

[0015] Preferably, in the water-THF mixed solution, the volume ratio of water to THF is 1:(1-5).

[0016] Preferably, the reaction in step (2) is carried out in a continuous flow reactor; a high-pressure feed pump is used for feeding, the feed concentration is 5 to 30 wt%, and the feed rate is 5 to 40 mL / min.

[0017] The advantages of the present invention are: (1) The present invention efficiently catalyzes glucose to produce 5-hydroxymethylfurfural (5-HMF) through a water-THF / NaCl two-phase reaction system. The catalyst uses activated carbon made from rice husk as a carrier, and Sn and Fe are doped to form highly dispersed Lewis acid and Brønsted acid synergistic catalytic centers, wherein Sn 4+ Promotes efficient isomerization of glucose to fructose, Fe 3+ The hydroxyl groups on the support surface synergistically accelerate the dehydration of fructose to 5-HMF. Furthermore, the aromatic structure on the surface of the rice husk activated carbon stabilizes the reaction intermediates through π-π interactions, effectively reducing their energy and further improving reaction selectivity and yield. The biphasic system effectively extracts and protects 5-HMF, minimizing byproduct formation.

[0018] (2) The present invention proposes to use rice husk activated carbon as a low-cost carrier through Sn 4+ / Fe 3+ Bifunctional doping was used to construct Lewis acid-Brønsted acid synergistic catalytic sites, and water-THF / NaCl two-phase extraction was used to separate 5-HMF in real time, achieving a green catalytic process with a high yield of 94.3% and a selectivity of >95% within 40 minutes at normal pressure and 200°C.

[0019] (3) The present invention optimizes the process conditions. At 200°C and a reaction time of 40 minutes, the 5-HMF yield is as high as 94.3%. The catalyst is highly stable and can be reused at least five times. It is significantly superior to existing CrCl3 catalysis and traditional high-temperature and high-pressure processes. It has the advantages of being environmentally friendly and highly economical, and has broad application prospects in the fields of biofuels and polymer monomers.

[0020] (4) The present invention also has the following effects: ① High catalytic activity: 94.3% 5-HMF yield was achieved within 40 minutes at 200°C, far exceeding traditional solid-phase and homogeneous systems.

[0021] ②. Low cost - Using rice husks as the carbon source, which has an annual output of 41 million tons and is almost free of cost, the cost of raw materials for catalyst preparation is extremely low.

[0022] ③. Green and environmentally friendly - abandoning high-boiling-point ionic liquids and vacuum equipment, only using water / THF two-phase and NaCl, the process is simple and less polluting.

[0023] ④ Good stability: After the catalyst is recycled for ≥5 times, the 5-HMF yield decreases by less than 5%.

[0024] ⑤. Easy to scale up industrially - it can be prepared quickly and efficiently without the need for additional pressurization (using a pressure-resistant reactor to achieve a 200°C self-pressurization system), and the catalyst carrier and solvent can be recycled, which is highly economical.

[0025] (5) The specific surface area of the catalyst prepared by the present invention is 200~500m² / g, and Sn and Fe are uniformly dispersed on the surface of the activated carbon in the form of oxides, forming Lewis acid and Brønsted acid dual functional sites. In the prepared catalyst, Sn-Fe / RHC relies on the porous SiO2-carbon skeleton of rice husk, and evenly embellishes 2~4nm SnO2 and Fe2O3 with Sn-O-Fe / Sn-OC anchor points to form adjacent Lewis-Brønsted dual acid sites, achieving "stable structure, precise acid sites, and fast mass transfer", thereby giving the glucose → HMF reaction high activity, high selectivity and cycle resistance. Specifically, Sn 4+ / Fe 3+ SnO2 and Fe2O3 nanoparticles of 2 to 4 nm are in situ generated on the surface of the carbon-silicon skeleton and "dotted" on the pore walls through Sn-O-Fe or Sn-OC anchor points. Elemental surface scanning shows that Sn, Fe, O, and C are comprehensive and uniform without agglomeration.

[0026] This nanoscale, near-neighbor arrangement allows Lewis acid sites (Sn 4+ ) and Brønsted acid sites (Fe-OH / carbon surface -OH), forming a synergistic "dual acid" center that first isomerizes glucose to fructose and then sequentially dehydrates to HMF, reducing the energy barrier for side reactions. The carbon-SiO2 composite wall is mechanically rigid and chemically inert, withstanding salting-out conditions at 200°C and 5-30wt% NaCl without collapse. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the SEM electron microscope image of Example 1 of the present invention.

[0028] Figure 2 This is the EDS diagram of Example 1 of the present invention.

[0029] Figure 3 This is a bar chart showing the yield of glucose converted to 5-HMF using the catalysts prepared in Examples 1-2 and Comparative Examples 1-3 of the present invention.

[0030] Figure 4 This is a bar chart showing the yield of glucose converted into 5-HMF at different temperatures using the catalyst prepared in Example 1 of the present invention.

[0031] Figure 5 The figure is a bar chart showing the yield of 5-HMF when glucose is converted to 5-HMF by the catalyst prepared in Example 1 of the present invention at 200°C at different time periods. DETAILED DESCRIPTION

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of the present invention.

[0033] The raw materials and reagents used in the following examples are: Rice husk (from a farm in Hefei, Anhui), SnCl4·5H2O (99%, Macklin), Fe(NO3)3·9H2O (AR, Sinopharm Chemical Reagent Co., Ltd.), anhydrous ethanol (C2H6O, 99.7%, Sinopharm Chemical Reagent Co., Ltd.), NaCl (analytical grade, Macklin), tetrahydrofuran (THF, AR, Macklin), sulfuric acid (H2SO4, 98%, Sinopharm Chemical Reagent Co., Ltd.), glucose (analytical grade, Sinopharm Chemical Reagent Co., Ltd.).

[0034] Example 1 (1) Rice husk pretreatment 5 g of natural rice husk was taken, washed repeatedly with deionized water until clear, and dried in an oven at 80° C. for 24 h; after drying, it was ground into 100 mesh powder to obtain pretreated rice husk powder.

[0035] (2) Metal precursor impregnation (10wt% loading) 1.12 g of SnCl4·5H2O and 0.43 g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 1.55 g, corresponding to 5 g of rice husk powder - the molar number of metal ions is accurately configured according to 10 wt% loading), were weighed and dissolved in 40 mL of anhydrous ethanol, and magnetically stirred at room temperature for 30 min to prepare a metal precursor solution; 5 g of pretreated rice husk powder was added to the above solution, stirred and immersed at room temperature for 4 h, and then dried at 105 ° C for 12 h to obtain impregnated and dried rice husk.

[0036] (3) Nitrogen carbonization and washing The impregnated and dried rice husks were placed in a tube furnace, N2 protective gas was introduced, and the temperature was raised to 500℃ at 5℃ / min, kept at this temperature for 2h, and then cooled naturally; the carbide was taken out and washed alternately with deionized water and anhydrous ethanol until there was no Cl in the filtrate. - and NO3 -The residue was dried at 80 °C for 6 h to obtain the Sn–Fe / RHC-10% bifunctional doped rice husk activated carbon catalyst.

[0037] Example 2 (1) Rice husk pretreatment 5 g of natural rice husk was taken, washed repeatedly with deionized water until clear, and dried in an oven at 80° C. for 24 h; after drying, it was ground into 100 mesh powder to obtain pretreated rice husk powder.

[0038] (2) Metal precursor impregnation (20wt% loading) 2.24 g of SnCl4·5H2O and 0.86 g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 3.10 g, corresponding to 5 g of rice husk powder - the molar number of metal ions is precisely configured according to 20 wt% loading), were weighed and dissolved in 40 mL of anhydrous ethanol, and magnetically stirred at room temperature for 30 min to prepare a metal precursor solution; 5 g of pretreated rice husk powder was added to the above solution, stirred and immersed at room temperature for 4 h, and then dried at 105 ° C for 12 h to obtain impregnated and dried rice husk.

[0039] (3) Nitrogen carbonization and washing The impregnated and dried rice husks were placed in a tube furnace, N2 protective gas was introduced, and the temperature was raised to 500℃ at 5℃ / min, kept at this temperature for 2h, and then cooled naturally; the carbide was taken out and washed alternately with deionized water and anhydrous ethanol until there was no Cl in the filtrate. - and NO3 - The residue was dried at 80 °C for 6 h to obtain the Sn–Fe / RHC-20% bifunctional doped rice husk activated carbon catalyst.

[0040] Comparative Example 1 (1) Rice husk pretreatment 5 g of natural rice husk was taken, washed repeatedly with deionized water until clear, and dried in an oven at 80° C. for 24 h; after drying, it was ground into 100 mesh powder to obtain pretreated rice husk powder.

[0041] (2) Direct carbonization (no metal loading) 5 g of pretreated rice husk powder was placed in a tubular furnace, N2 protective gas was introduced, the temperature was raised to 500 °C at 5 °C / min, and the mixture was kept at this temperature for 2 h and then naturally cooled to obtain carbonized rice husk charcoal.

[0042] (3) Washing and drying The carbonized product was taken out and washed alternately with deionized water and anhydrous ethanol until there were no residual impurities in the filtrate. It was then dried in an oven at 80°C for 6 h to obtain rice husk activated carbon for comparison (denoted as RHC).

[0043] Comparative Example 2 (1) Rice husk pretreatment 5 g of natural rice husk was taken, washed repeatedly with deionized water until clear, and dried in an oven at 80° C. for 24 h; after drying, it was ground into 100 mesh powder to obtain pretreated rice husk powder.

[0044] (2) Metal precursor impregnation (5wt% loading) Weigh 0.56 g of SnCl4·5H2O and 0.215 g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 0.775 g, corresponding to 5 g of rice husk powder - the molar number of metal ions is accurately configured according to 5 wt% loading), dissolve them in 40 mL of anhydrous ethanol, and magnetically stir at room temperature for 30 min to prepare a uniform metal solution; 5 g of pretreated rice husk powder is added to the above solution, stirred and immersed at room temperature for 4 h, and then dried at 105 ° C for 12 h to obtain the impregnated rice husk.

[0045] (3) Nitrogen carbonization and washing The dried impregnation product was placed in a tube furnace, N2 protective gas was introduced, and the temperature was raised to 500℃ at 5℃ / min. After keeping the temperature for 2h, it was naturally cooled. The carbide was taken out and washed alternately with deionized water and anhydrous ethanol until there was no Cl in the filtrate. - and NO3 - The residue was dried at 80 °C for 6 h to obtain the Sn–Fe / RHC-5% bifunctional doped rice husk activated carbon catalyst.

[0046] Comparative Example 3 (1) Rice husk pretreatment 5 g of natural rice husk was taken, washed repeatedly with deionized water until clear, and dried in an oven at 80° C. for 24 h; after drying, it was ground into 100 mesh powder to obtain pretreated rice husk powder.

[0047] (2) Metal precursor impregnation (30wt% loading) Weigh 3.36 g of SnCl4·5H2O and 1.29 g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 4.65 g, corresponding to 5 g of rice husk powder - the molar number of metal ions is precisely configured according to 30 wt% loading), dissolve them in 40 mL of anhydrous ethanol, and magnetically stir at room temperature for 30 min to prepare a uniform metal solution; add 5 g of pretreated rice husk powder to the above solution, stir and soak at room temperature for 4 h, and then dry at 105 ° C for 12 h to obtain the impregnated rice husk.

[0048] (3) Nitrogen carbonization and washing The dried impregnation product was placed in a tube furnace, N2 protective gas was introduced, and the temperature was raised to 500℃ at 5℃ / min. After keeping the temperature for 2h, it was naturally cooled. The carbide was taken out and washed alternately with deionized water and anhydrous ethanol until there was no Cl in the filtrate. - and NO3- The residue was dried at 80 °C for 6 h to obtain the Sn–Fe / RHC-30% bifunctional doped rice husk activated carbon catalyst (denoted as Sn-Fe / RHC-30%).

[0049] The catalysts prepared in Examples 1-2 and Comparative Examples 1-3 were used to catalyze the conversion of glucose into 5-hydroxymethylfurfural, and the specific steps were as follows: (1) Glucose was dissolved in a water-THF solution to prepare a mixed solution, wherein the mass fraction of glucose in the mixed solution was 20 wt %; in the water-THF mixed solution, the volume ratio of water to THF was 1:3; (2) 45 mL of THF (tetrahydrofuran) and 0.5 g of Sn-Fe / RHC were added to a quartz-lined reactor, and then 0.5 MPa of N2 was injected to remove the air in the reactor. Once the reactor reached the set target temperature, an aqueous solution containing 20 wt% glucose and 20 wt% NaCl was injected at a controlled feed rate (15 mL / min) using a high-pressure feed pump. As the feed was completed, the system waited for the temperature to return to the set temperature (180-200°C) and started timing. The reaction time was 20 to 40 minutes. After the reaction was completed, the reactor was immersed in an ice-water bath and quickly cooled to room temperature to produce 5-hydroxymethylfurfural.

[0050] The yields of glucose converted into 5-hydroxymethylfurfural by the catalysts prepared in Examples 1-2 and Comparative Examples 1-3 were tested respectively. The results are as follows: Figure 3-4 shown.

[0051] Figure 3 The results show that in a two-phase salting-out system with a water:THF ratio of 1:3 and 3g of NaCl, Examples 1 and 2 can achieve high 5-HMF yields of 76.3% and 75.9%, respectively, using 0.5g of catalyst to catalyze the reaction of 3g of glucose at 180°C for 30min. This performance difference is primarily due to the retained carboxyl and phenolic hydroxyl groups in the rice husk charcoal, which provide the foundation for Brønsted acid dehydration activity. More importantly, in the 10-20wt% loading range, Sn 4+ and Fe 3+ The bifunctional sites of Sn are highly dispersed and synergistic: 4+ Take Sn(OSi)4, Sn(OSi)4·(H2O)2, Sn(OSi)3 -Reversible coordination forms such as OH·Si-OH and Sn(OSi)3(OH)2·(H2O) dynamically switch between "hydrolysis-condensation", providing a strong Lewis acid core to catalyze the isomerization of glucose to fructose, and generating ortho-Brønsted acid in the open hydration state, so that the Lewis-Brønsted dual functional center synergistically promotes the dehydration of fructose to 5-HMF; Fe 3+ The surface hydroxyl groups are supplemented with mild Brønsted acid, further improving selectivity. Simultaneously, the THF phase extracts 5-HMF immediately, and NaCl salting out accelerates interphase transfer, preventing side reactions. However, when the loading is too low (insufficient active sites) or too high (Sn / Fe agglomeration, excessive acidity), these synergistic and dynamic controls are weakened, significantly reducing yield. Therefore, a metal loading of 10-20 wt% represents the optimal window for achieving high selectivity and high yield of 5-HMF conversion.

[0052] Figure 4 The analysis showed that in a two-phase salting-out system with a water:THF ratio of 1:3 and 3g NaCl, the temperature and time of 0.5g Sn–Fe / RHC-10wt% catalyzed the reaction of 3g glucose, and the yield of 5-HMF showed a significant effect. First, when the temperature was kept constant for 30min, the yield showed a trend of "first rising sharply and then rapidly falling" with temperature: it was only 11.28% at 160℃, then soared to 76.31% at 180℃, and reached a peak of 82.90% at 200℃. When the temperature continued to rise to 220℃ and 240℃, due to the Sn in the system, the yield of 5-HMF decreased significantly. 4+ Lewis acid sites and Sn–OH / Fe 3+ The Brønsted acid sites exhibit an imbalance between dehydration and hydrolysis, and 5-HMF readily undergoes rehydrolysis to formylpropionic acid or polycondensation to coke in the highly acidic aqueous phase, resulting in a sharp drop in yield to 60.53% and 7.40%, respectively. This indicates that around 200°C, while fully activating the isomerization of glucose to fructose and the dehydration of fructose to 5-HMF, and simultaneously suppressing side reactions through immediate extraction with THF and the "salting-out" effect of NaCl, represents the optimal temperature window for optimization.

[0053] Figure 5Further investigation of the effect of reaction time on yield at 200°C revealed that the 5-HMF yield followed a steady rise-peak-slow decline pattern over time: 73.2% at 20 minutes, rising to 82.9% at 30 minutes, and 88.3% at 35 minutes, with the yield peaking at 94.3% at 40 minutes. When the reaction time exceeded 40 minutes, the residual strong acid environment caused 5-HMF# to further hydrolyze into formyl propionic acid or condense into char polymers, resulting in yields falling to 88.4% (at 45 minutes) and 84.2% (at 50 minutes), respectively. This suggests that too short a reaction time can limit the yield due to incomplete substrate conversion, while too long a reaction time can reduce the effective yield due to secondary degradation of 5-HMF. Comprehensive analysis of the dual factors of temperature and time revealed that 200°C and 35-40 min are the optimal process conditions for this catalytic system to achieve the synergistic three-step isomerization-dehydration-instant extraction while effectively suppressing side reactions. This can maximize the selectivity and yield of 5-HMF while ensuring complete glucose conversion, thereby providing a precise temperature and time control reference for process scale-up and industrial application.

[0054] Figure 1 and Figure 2 The SEM and EDS characterizations of the catalyst prepared in Example 1 further confirm the structural advantages of the catalyst prepared in Example 1. Figure 1 From left to right are three SEM images that are magnified step by step. Figure 1 The sample maintains a bulky, porous framework with pore sizes ranging from tens of microns to submicron diameters. The pore walls are composed of dense carbon sheets, with nanoparticles evenly distributed on the surface. No sintering or pore collapse is observed, demonstrating a strong metal-support bond and the preservation of an intact porous network at this loading level. The high-magnification SEM and corresponding EDS scans on the far right visualize the elemental distribution: the red, green, blue, and cyan maps correspond to C, O, Fe, and Sn, respectively (scale 1μm). The signals for these four elements are evenly distributed across the field of view, with no apparent enrichment or vacancy. This indicates that Sn and Fe are highly dispersed on the carbon wall surface at the nanoscale, forming a tight synergistic relationship with the oxygen and carbon framework. This hierarchical structure provides fast mass transfer channels and ample reaction sites between the substrate and the active centers, providing the catalyst with the morphology and elemental distribution necessary for high selectivity and high yield conversion of 5-HMF in the 180–200°C range.

[0055] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a Sn-Fe bimetallic doped rice husk catalyst, characterized in that: The specific steps are as follows: S1. Weighing a tin source and an iron source as precursors, wherein the molar ratio of the tin element in the tin source to the iron element in the iron source is (1-5):1, and dissolving them in an organic solvent to prepare a metal precursor solution; S2. Add 5-10 g of dried and ground rice husk powder to the metal precursor solution and soak for 4 hours; then dry at 105° C. for 4-12 hours to obtain treated rice husk; S3, placing the treated rice husk in a tube furnace, heating it to 450-600°C at a rate of 2-10°C / min under a nitrogen atmosphere, maintaining it for 1-4 hours, and then naturally cooling it to room temperature to obtain carbonized rice husk loaded with heavy metal ions; S4. Wash the carbonized rice husk with deionized water and anhydrous ethanol respectively; and finally dry it to obtain a Sn-Fe bimetallic doped rice husk catalyst.

2. The method for preparing a Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that: In step S2, rice husk powder is prepared by thoroughly washing rice husks, drying them at 60-100° C. for 12-36 hours, and grinding them to a particle size of 80-200 meshes.

3. The method for preparing a Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that: In step S1, magnetic stirring is performed at room temperature for 30 to 120 minutes.

4. The method for preparing a Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that: The tin source is SnCl4·5H2O, the iron source is Fe(NO3)3·9H2O, and the organic solvent is anhydrous ethanol.

5. The method for preparing a Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that: In step S1, SnCl4·5H2O and Fe(NO3)3·9H2O are weighed in a mass ratio of 2.6:1, added to 30-70 mL of anhydrous ethanol, and stirred evenly to prepare a metal precursor solution.

6. A catalyst prepared by the method for preparing a Sn-Fe bimetallic doped rice husk catalyst according to any one of claims 1 to 5, characterized in that: The catalyst uses SiO2 and carbon skeleton of rice husk as carriers, and SnO2 nanoparticles and Fe2O3 nanoparticles are in situ generated on the surface of the carrier. The particle size of the SnO2 nanoparticles and Fe2O3 nanoparticles are both less than 10nm.

7. Use of the Sn-Fe bimetallic doped rice husk catalyst as claimed in claim 6, characterized in that: The catalyst catalyzes the conversion of glucose into 5-hydroxymethylfurfural.

8. The use of the Sn-Fe bimetallic doped rice husk catalyst according to claim 7, characterized in that: The specific steps are: (1) Glucose is dissolved in a water-THF solution to prepare a mixed solution, wherein the mass fraction of glucose in the mixed solution is 5 to 30 wt%; (2) A Sn-Fe bimetallic catalyst doped with rice husk and a NaCl solution with a concentration of 5-30 wt% were added to the mixed solution. The mass of the catalyst was 10-20% of the mass of glucose. The reaction temperature was 170-220 °C and the reaction time was 15-60 minutes to obtain 5-hydroxymethylfurfural.

9. The use of the Sn-Fe bimetallic doped rice husk catalyst according to claim 8, characterized in that: In the water-THF mixed solution, the volume ratio of water to THF is 1:(1~5).

10. The use of the Sn-Fe bimetallic doped rice husk catalyst according to claim 8, characterized in that: The reaction in step (2) is carried out in a continuous flow reactor; a high-pressure feed pump is used for feeding, the feed concentration is 5 to 30 wt%, and the feed rate is 5 to 40 mL / min.

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