A Sn-Fe bimetallic doped rice husk catalyst, its preparation method and application

Abstract

This invention belongs to the field of biomass chemical conversion and green catalysis technology, specifically a Sn-Fe bimetallic doped rice husk catalyst, its preparation method, and its application. The steps of this invention are as follows: S1, weigh tin and iron sources as precursors and dissolve them in an organic solvent to obtain a metal precursor solution; S2, add rice husk powder to the metal precursor solution and stir to impregnate; then dry at 105℃ to obtain treated rice husk; S3, heat to 450-600℃ under a nitrogen atmosphere, hold at that temperature, and then naturally cool to room temperature to obtain carbonized rice husk; S4, filter and separate the carbonized rice husk, wash thoroughly with deionized water and anhydrous ethanol respectively; finally, dry at 80℃ for 6 hours to obtain the catalyst. In the catalyst prepared by this invention, Sn-Fe / RHC relies on the porous SiO2-carbon framework of rice husk, and uniformly adorns 2-4 nm SnO2 and Fe2O3 with Sn-O-Fe / Sn-O-C anchor points to form adjacent Lewis-Brønsted double acid sites, achieving "stable structure, precise acid sites, and fast mass transfer", thereby endowing the glucose→HMF reaction with high activity, high selectivity and cycle resistance.

Description

Technical Field

[0001] This invention belongs to the field of biomass chemical conversion and green catalysis technology, and particularly relates to a Sn-Fe bimetallic doped rice husk catalyst, its preparation method and application. Background Technology

[0002] 5-Hydroxymethylfurfural (5-HMF) is recognized internationally as one of the most promising bio-based platform compounds, serving as a key precursor for biofuel additives, polyester monomers, and pharmaceutical intermediates, and holding profound significance for replacing petrochemicals and developing renewable chemicals. Existing methods for preparing 5-HMF involve converting glucose to 5-HMF. Specifically, glucose must first be isomerized to fructose under Lewis acid conditions, and then dehydrated to generate 5-HMF under Brønsted acid conditions. However, this method easily generates byproducts such as furfural condensate, levulinic acid, and formic acid, resulting in a 5-HMF yield of only 40%–70% in traditional homogeneous acid catalytic systems, with unstable selectivity. Homogeneous metal salts, represented by CrCl3 and AlCl3, can achieve short-term yields of 60%–70% in ionic liquids or organic high-boiling-point solvents such as dimethyl sulfoxide (DMSO) at around 140°C. However, the catalysts are difficult to recover, ionic liquids are expensive, post-processing is complex, and they cause severe corrosion to equipment, making them unsuitable for industrial application. Alkaline catalysis processes require strict control of the alkali concentration to avoid side reactions and have a significant impact on product separation and purification, resulting in relatively high overall costs.

[0003] The emergence of solid acid catalysts has provided new ideas for industrial applications, and various solid sulfonic acid resins, molecular sieves, and metal oxides have been extensively studied. For example, solid acids such as Nb2O5 and γ-Al2O3-SO3H can achieve 50%–65% 5-HMF yields in aqueous or water / organic two-phase systems and are recyclable. However, these materials have uneven Brønsted / Lewis acid site distribution and limited acid strength, often requiring harsh conditions above 180°C and more than 2 hours to barely achieve moderate yields. They also have high energy consumption and poor catalyst stability, making it difficult to meet the three 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, increasing the preparation cost.

[0004] Chinese patent CN112543754B discloses a two-phase one-pot method using AlCl3 / activated carbon / DMSO / MIBK–2-butanol to synthesize 5-HMF at 110℃–160℃ for 6–12 hours, with a yield of approximately 65%. However, this method is difficult to implement due to its long reaction time, high energy consumption, and complex solvent recovery. Another Chinese patent CN115536618B discloses a method combining microwave hydrothermal and molten salt, using a LiBr·3H2O / GVL system to achieve a 76.3% yield within 30 minutes at 100℃. However, the recovery cost of high-concentration LiBr and GVL is extremely high, and specialized microwave equipment is required, making it unsuitable for widespread application. More innovative are vacuum or solvent-free dehydration technologies, such as Chinese patent CN115806536B, which proposes to prepare 5-HMF by dehydration under conditions of 0.01 to 5 kPa without the need for organic solvents, making it significantly more environmentally friendly. However, this only illustrates the dehydration system of fructose and has not yet been extended to glucose, making it impossible to directly utilize my country's abundant and inexpensive glucose resources.

[0005] In summary, current technologies present a significant contradiction between achieving high yields and low-cost, green 5-HMF: high yields often rely on expensive or highly corrosive homogeneous systems or high-temperature, long-duration solid-phase catalysis; while green, recyclable solid catalysts often sacrifice reactivity and require stringent preparation conditions. Therefore, there is an urgent need for a Sn-Fe bimetallic doped rice husk catalyst, its preparation method, and its applications to address these issues. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a Sn-Fe bimetallic doped rice husk catalyst, its preparation method, and its applications. This catalyst enables the efficient one-step dehydration of glucose to prepare 5-HMF in a mild two-phase system.

[0007] To achieve one of the above objectives, the present invention adopts the following technical solution:

[0008] A Sn-Fe bimetallic doped rice husk catalyst, the specific steps of which are as follows:

[0009] S1. Weigh out tin source and iron source as precursors, and the molar ratio of tin element in tin source to iron element in iron source is (1~5)∶1. Dissolve them in an organic solvent to prepare metal precursor solution.

[0010] S2. Add 5-10g of dried and ground rice husk powder to the metal precursor solution and soak for 4 hours; then dry at 105℃ for 4-12 hours to obtain the treated rice husk.

[0011] S3. The treated rice husks are placed in a tube furnace and heated to 450-600°C at a rate of 2-10°C / min under a nitrogen atmosphere. After holding for 2 hours, the temperature is naturally cooled to room temperature to obtain carbonized rice husks loaded with heavy metal ions.

[0012] S4. The carbonized rice husks were thoroughly washed with deionized water and anhydrous ethanol, respectively. The deionized water was used to remove residual dopants such as SnCl4 and Fe(NO3)3 from the surface, while the anhydrous ethanol was used to wash away the bio-oil on the surface. Finally, the husks were dried to obtain the Sn-Fe bimetallic doped rice husk catalyst (Sn-Fe / RHC).

[0013] Preferably, in step S2, the rice husk powder is obtained by thoroughly washing the rice husks, drying them at 60–100°C for 12–36 hours, and grinding them to a particle size of 80–200 mesh.

[0014] Preferably, in step S1, magnetic stirring is performed at room temperature for 30 to 120 minutes.

[0015] Preferably, the tin source is SnCl4·5H2O, the iron source is Fe(NO3)3·9H2O, and the organic solvent is anhydrous ethanol.

[0016] Preferably, in step S1, SnCl4·5H2O and Fe(NO3)3·9H2O are weighed at a mass ratio of 2.6:1 and added to 30-70 mL of anhydrous ethanol, and stirred evenly to obtain a metal precursor solution.

[0017] Preferably, the total mass of SnCl4·5H2O and Fe(NO3)3·9H2O is 1.55g.

[0018] To achieve the second objective mentioned above, the present invention provides a method for preparing a Sn-Fe bimetallic doped rice husk catalyst. The catalyst uses SiO2 and carbon skeleton of rice husk as a support, and SnO2 nanoparticles and Fe2O3 nanoparticles are generated in situ on the surface of the support. The particle size of both SnO2 nanoparticles and Fe2O3 nanoparticles is less than 10 nm, preferably 2-4 nm.

[0019] To achieve the third objective mentioned above, this invention provides an application of a Sn-Fe bimetallic doped rice husk catalyst that catalyzes the conversion of glucose into 5-hydroxymethylfurfural.

[0020] Preferably, the specific steps for applying the catalyst are as follows:

[0021] (1) Dissolve glucose in water-THF solution to prepare a mixed solution, wherein the mass fraction of glucose in the mixed solution is 5~30 wt%;

[0022] (2) Add Sn-Fe bimetallic doped rice husk catalyst and NaCl solution with a concentration of 5-30 wt% to the mixed solution. The mass of the catalyst is 5-30% of the mass of glucose. The reaction temperature is 170-220℃ and the reaction time is 15-60 minutes to obtain 5-hydroxymethylfurfural (5-HMF).

[0023] Preferably, in the water-THF mixed solution, the volume ratio of water to THF is 1:(1-5).

[0024] 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 5-30 wt%, and the feed rate is 5-40 mL / min.

[0025] The advantages of this invention are:

[0026] (1) This invention utilizes a water-THF / NaCl biphase reaction system to efficiently catalyze the preparation of 5-hydroxymethylfurfural (5-HMF) from glucose. The catalyst uses activated carbon made from rice husks as a support, with Sn and Fe doping forming 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 carrier surface synergistically accelerate the dehydration of fructose to 5-HMF. Furthermore, the aromatic structure on the surface of rice husk activated carbon stabilizes reaction intermediates through π-π interactions, effectively reducing intermediate energy and further improving reaction selectivity and yield. The two-phase system effectively extracts and protects 5-HMF, reducing byproduct formation.

[0027] (2) This invention proposes using 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, which were then combined with water-THF / NaCl biphase extraction to separate 5-HMF in real time, achieving a green catalytic process with a high yield of 94.3% and selectivity of >95% within 40 minutes at atmospheric pressure and 200℃.

[0028] (3) The optimized process conditions of this invention result in a 5-HMF yield of up to 94.3% at 200℃ and 40 minutes of reaction. The catalyst has high stability and can be reused at least 5 times, which is significantly better than existing CrCl3 catalysis and traditional high temperature and high pressure processes. It has advantages such as environmental friendliness and high economic benefits, and has broad application prospects in the fields of biofuels and polymer monomers.

[0029] (4) The present invention also includes the following effects:

[0030] ① High catalytic activity – Achieves a 94.3% 5-HMF yield within 40 minutes at 200℃, far exceeding that of traditional solid and homogeneous systems.

[0031] ② Low cost – Using rice husks, which have an annual output of 41 million tons and almost zero cost, as a carbon source, the raw material cost for catalyst preparation is extremely low.

[0032] ③ Green and environmentally friendly – ​​It abandons high-boiling-point ionic liquids and vacuum equipment, and only uses water / THF two-phase and NaCl. The process is simple and produces less pollution.

[0033] ④ Good stability – After the catalyst is recycled ≥5 times, the yield of 5-HMF decreases by <5%.

[0034] ⑤ Easy to scale up industrially – can be prepared quickly and efficiently under reaction conditions without additional pressure (using a pressure-resistant reactor to achieve a 200℃ self-pressurized system), the catalyst support and solvent can be recycled, and it is highly economical.

[0035] (5) The catalyst prepared in this invention has a specific surface area of ​​200~500 m² / g, and Sn and Fe are uniformly dispersed on the surface of activated carbon in the form of oxides, forming Lewis acid and Brønsted acid bifunctional sites. In the prepared catalyst, Sn-Fe / RHC relies on the porous SiO2-carbon framework of rice husk, and 2~4 nm SnO2 and Fe2O3 are uniformly dotted with Sn-O-Fe / Sn-OC anchor points to form adjacent Lewis-Brønsted bifunctional acid sites, achieving "stable structure, precise acid sites, and fast mass transfer", thereby endowing the glucose→HMF reaction with high activity, high selectivity and cycle resistance. Specifically, Sn 4+ / Fe 3+ 2-4 nm SnO2 and Fe2O3 nanoparticles were generated in situ on the surface of a carbon-silicon framework and "adorned" on the pore walls by Sn-O-Fe or Sn-OC anchor points. Elemental surface scanning showed that Sn, Fe, O, and C were present in a comprehensive and uniform manner without agglomeration.

[0036] This nanoscale, close-proximity arrangement allows Lewis acid sites (Sn) to be formed. 4+ The carbon-SiO2 composite wall, along with Brønsted acid sites (Fe-OH / carbon surface-OH), forms a synergistic "dual acid" center. This center can first isomerize glucose to fructose, followed by a series of dehydrations to generate HMF, thus lowering the energy barrier for side reactions. The carbon-SiO2 composite wall possesses mechanical rigidity and chemical inertness, and can withstand a salting-out environment of 200℃ and 5-30wt% NaCl without collapsing. Attached Figure Description

[0037] Figure 1 This is a SEM image of Embodiment 1 of the present invention.

[0038] Figure 2This is the EDS diagram of Embodiment 1 of the present invention.

[0039] Figure 3 The bar chart shows the yield of glucose to 5-HMF catalyzed by the catalysts prepared in Examples 1-2 and Comparative Examples 1-3 of this invention.

[0040] Figure 4 The bar chart shows the yield of glucose to 5-HMF catalyzed by the catalyst prepared in Example 1 of this invention at different temperatures.

[0041] Figure 5 This is a bar chart showing the yield of the catalyst prepared in Example 1 of the present invention in the conversion of glucose to 5-HMF at 200°C for different time periods. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0043] The raw materials and reagents used in the following examples are:

[0044] Rice husks (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.).

[0045] Example 1

[0046] (1) Rice husk pretreatment

[0047] Take 5g of natural rice husks, wash them repeatedly with deionized water until clear, and dry them in an 80℃ oven for 24 hours; after drying, grind them into 100-mesh powder to obtain pretreated rice husk powder.

[0048] (2) Metal precursor impregnation (10wt% loading)

[0049] Weigh 1.12g of SnCl4·5H2O and 0.43g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 1.55g, corresponding to 5g rice husk powder—the number of moles of metal ions is precisely prepared according to a 10wt% loading), dissolve in 40mL of anhydrous ethanol, and magnetically stir at room temperature for 30min to obtain a metal precursor solution; add 5g of pretreated rice husk powder to the above solution, stir and impregnate at room temperature for 4h, and then dry at 105℃ for 12h to obtain impregnated dried rice husk.

[0050] (3) Nitrogen carbonization and washing

[0051] The soaked and dried rice husks were placed in a tube furnace, and N2 protective gas was introduced. The temperature was increased to 500℃ at 5℃ / min, and held at this temperature for 2 hours before natural cooling. The carbides were removed and washed alternately with deionized water and anhydrous ethanol until no Cl- was found in the filtrate. - and NO3 - The residue was dried at 80℃ for 6 hours to finally obtain the Sn–Fe / RHC-10% bifunctional doped rice husk activated carbon catalyst.

[0052] Example 2

[0053] (1) Rice husk pretreatment

[0054] Take 5g of natural rice husks, wash them repeatedly with deionized water until clear, and dry them in an 80℃ oven for 24 hours; after drying, grind them into 100-mesh powder to obtain pretreated rice husk powder.

[0055] (2) Metal precursor impregnation (20wt% loading)

[0056] Weigh 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 rice husk powder—the number of moles of metal ions is precisely prepared according to a 20 wt% loading), dissolve in 40 mL of anhydrous ethanol, and magnetically stir at room temperature for 30 min to obtain a metal precursor solution; add 5 g of pretreated rice husk powder to the above solution, stir and impregnate at room temperature for 4 h, and then dry at 105 °C for 12 h to obtain impregnated dried rice husk.

[0057] (3) Nitrogen carbonization and washing

[0058] The soaked and dried rice husks were placed in a tube furnace, and N2 protective gas was introduced. The temperature was increased to 500℃ at 5℃ / min, and held at this temperature for 2 hours before natural cooling. The carbides were removed and washed alternately with deionized water and anhydrous ethanol until no Cl- was found in the filtrate. - and NO3 - The residue was dried at 80℃ for 6 hours to finally obtain the Sn–Fe / RHC-20% bifunctional doped rice husk activated carbon catalyst.

[0059] Comparative Example 1

[0060] (1) Rice husk pretreatment

[0061] Take 5g of natural rice husks, wash them repeatedly with deionized water until clear, and dry them in an 80℃ oven for 24 hours; after drying, grind them into 100-mesh powder to obtain pretreated rice husk powder.

[0062] (2) Direct carbonization (without metal loading)

[0063] 5g of pretreated rice husk powder was loaded into a tube furnace, N2 protective gas was introduced, the temperature was raised to 500℃ at 5℃ / min, and the temperature was held for 2 hours before natural cooling to obtain carbonized rice husk charcoal.

[0064] (3) Washing and drying

[0065] The carbonized product was removed and washed alternately with deionized water and anhydrous ethanol until no residual impurities remained in the filtrate. It was then dried in an oven at 80°C for 6 hours to obtain the rice husk activated carbon for comparison (denoted as RHC).

[0066] Comparative Example 2

[0067] (1) Rice husk pretreatment

[0068] Take 5g of natural rice husks, wash them repeatedly with deionized water until clear, and dry them in an 80℃ oven for 24 hours; after drying, grind them into 100-mesh powder to obtain pretreated rice husk powder.

[0069] (2) Metal precursor impregnation (5wt% loading)

[0070] Weigh 0.56g of SnCl4·5H2O and 0.215g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 0.775g, corresponding to 5g rice husk powder—the number of moles of metal ions is precisely prepared according to 5wt%), dissolve in 40mL of anhydrous ethanol, and stir magnetically at room temperature for 30min to obtain a homogeneous metal solution; add 5g of pretreated rice husk powder to the above solution, stir and impregnate at room temperature for 4h, and then dry at 105℃ for 12h to obtain impregnated rice husk.

[0071] (3) Nitrogen carbonization and washing

[0072] The dried impregnation product was placed in a tube furnace, and N2 protective gas was introduced. The temperature was increased to 500℃ at 5℃ / min, held at this temperature for 2 hours, and then allowed to cool naturally. The carbide was removed and washed alternately with deionized water and anhydrous ethanol until no Cl- was found in the filtrate. - and NO3 - The residue was dried at 80℃ for 6 hours to finally obtain the Sn–Fe / RHC-5% bifunctional doped rice husk activated carbon catalyst.

[0073] Comparative Example 3

[0074] (1) Rice husk pretreatment

[0075] Take 5g of natural rice husks, wash them repeatedly with deionized water until clear, and dry them in an 80℃ oven for 24 hours; after drying, grind them into 100-mesh powder to obtain pretreated rice husk powder.

[0076] (2) Metal precursor impregnation (30wt% loading)

[0077] Weigh 3.36g of SnCl4·5H2O and 1.29g of Fe(NO3)3·9H2O (Sn:Fe mass ratio ≈ 2.6:1, total metal precursor mass 4.65g, corresponding to 5g rice husk powder—the number of moles of metal ions is precisely prepared according to a 30wt% loading), dissolve in 40mL of anhydrous ethanol, and magnetically stir at room temperature for 30min to obtain a homogeneous metal solution; add 5g of pretreated rice husk powder to the above solution, stir and impregnate at room temperature for 4h, and then dry at 105℃ for 12h to obtain impregnated rice husk.

[0078] (3) Nitrogen carbonization and washing

[0079] The dried impregnation product was placed in a tube furnace, and N2 protective gas was introduced. The temperature was increased to 500℃ at 5℃ / min, held at this temperature for 2 hours, and then allowed to cool naturally. The carbide was removed and washed alternately with deionized water and anhydrous ethanol until no Cl- was found in the filtrate. - and NO3 - The residue was dried at 80℃ for 6 hours to finally obtain the Sn–Fe / RHC-30% bifunctional doped rice husk activated carbon catalyst (denoted as Sn-Fe / RHC-30%).

[0080] The catalysts prepared in Examples 1-2 and Comparative Examples 1-3 were applied to the catalytic conversion of glucose to 5-hydroxymethylfurfural. The specific steps were as follows:

[0081] (1) Glucose was dissolved in a water-THF solution to prepare a mixed solution, and the mass fraction of glucose in the mixed solution was 20 wt%; the volume ratio of water to THF in the water-THF mixed solution was 1:3.

[0082] (2) Add 45 mL of THF (tetrahydrofuran) and 0.5 g of Sn-Fe / RHC to the quartz-lined reactor, and then inject 0.5 MPa N2 to remove air from the reactor. Once the reactor reaches the set target temperature, use a high-pressure feed pump to inject an aqueous solution containing 20 wt% glucose and 20 wt% NaCl at a controlled feed rate (15 mL / min). When the feeding ends, start timing when the system temperature returns to the set temperature (180-200 °C). The reaction time is 20-40 minutes. After the reaction is completed, immerse the reactor in an ice-water bath to cool it rapidly to room temperature to obtain 5-hydroxymethylfurfural.

[0083] The yields of glucose to 5-hydroxymethylfurfural catalyzed by the catalysts prepared in Examples 1-2 and Comparative Examples 1-3 were tested respectively, and the results are as follows: Figure 3-4 As shown.

[0084] Figure 3 The results show that in a two-phase salting-out system of water:THF = 1:3 and 3g NaCl, Examples 1 and 2, with 0.5g catalyst catalyzing 3g glucose at 180℃ for 30min, achieved high yields of 76.3% and 75.9% of 5-HMF, respectively, while Comparative Examples 1, 2, and 3 yielded only 63.5%, 51.2%, and 57.7%, respectively. This performance difference primarily stems from the carboxyl and phenolic hydroxyl groups retained in the rice husk char, which provide the basic Brønsted acid dehydration activity; more importantly, within the 10-20wt% loading range, Sn... 4+ and Fe 3+ The bifunctional sites are highly dispersed and synergistic: Sn 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 and condensation, providing a strong Lewis acid core to catalyze the glucose → fructose isomerization, and generating ortho-Brønsted acids in an open hydration state, enabling the Lewis-Brønsted bifunctional center to synergistically promote the dehydration of fructose → 5-HMF; Fe 3+ The addition of surface hydroxyl groups provides a mild Brønsted acid, further enhancing selectivity. Simultaneously, THF phase extraction of 5-HMF and NaCl salting-out accelerate phase transfer and avoid side reactions. However, when the metal loading is too low (insufficient active sites) or too high (Sn / Fe aggregation, excessive acidity), these synergistic and dynamic regulation mechanisms are weakened, resulting in a significant decrease in yield. Therefore, a metal loading of 10-20 wt% is the optimal window for achieving high selectivity and high yield of 5-HMF conversion.

[0085] Figure 4Analysis showed that in a two-phase salting-out system with water:THF = 1:3 and 3g NaCl, 3g glucose was catalyzed with 0.5g Sn–Fe / RHC-10wt%, and both temperature and time significantly affected the yield of 5-HMF. Firstly, during the 30-min isothermal period, the yield exhibited a trend of "first sharply increasing and then rapidly decreasing" with temperature: only 11.28% at 160℃, surging to 76.31% at 180℃; reaching a peak of 82.90% at 200℃; when the temperature continued to rise to 220℃ and 240℃, due to the presence of Sn in the system... 4+ Lewis acid sites and Sn–OH / Fe 3+ The imbalance between dehydration and hydrolysis at the Brønsted acid sites, and the easy re-hydrolysis of 5-HMF to formylpropionic acid or condensation into coke in a highly acidic aqueous phase, resulted in a sharp drop in yield to 60.53% and 7.40%, respectively. This indicates that around 200℃ can fully activate the isomerization reaction of glucose → fructose and the dehydration reaction of fructose → 5-HMF, while also suppressing side reactions through immediate extraction in the THF phase and the "salting out" effect of NaCl, making it the optimal temperature window for optimization.

[0086] Figure 5 Further investigation at 200℃ revealed that the 5-HMF yield exhibited a "steady increase – peak – slow decline" pattern over time: the yield was 73.2% at 20 min, increasing to 82.9% at 30 min, reaching 88.3% at 35 min, and peaking at 94.3% at 40 min. When the reaction time exceeded 40 min, the residual strong acid environment promoted further hydrolysis of 5-HMF# into formylpropionic acid or condensation into coke polymers, causing the yield to decline to 88.4% (45 min) and 84.2% (50 min), respectively. This indicates that a reaction time that is too short limits the yield due to incomplete substrate conversion, while a reaction time that is too long results in a decrease in effective yield due to secondary degradation of 5-HMF. Based on a comprehensive analysis of both temperature and time, 200℃ and 35~40min are the optimal process conditions for this catalytic system to achieve the synergistic effect of isomerization-dehydration-instantaneous extraction and effectively suppress side reactions. This can maximize the selectivity and yield of 5-HMF while ensuring complete glucose conversion, thus providing a precise temperature and time control reference for process scale-up and industrial application.

[0087] Figure 1 and Figure 2 The SEM and EDS characterization of the catalyst prepared in Example 1 further confirmed the structural advantages of the catalyst prepared in Example 1. Figure 1 From left to right, these are three SEM images that are progressively enlarged. Figure 1The results show that the sample maintains a blocky porous framework with pore sizes ranging from tens of micrometers to submicrometers. The pore walls are composed of dense carbon sheets with uniformly dispersed nanoscale particles on the surface. No sintering or pore collapse was observed, indicating that the metal and support are firmly bonded and the porous network is well preserved at this loading level. The high-magnification SEM and corresponding EDS surface scan on the far right visualize the elemental distribution: the four mapping images (red, green, blue, and cyan) correspond to C, O, Fe, and Sn (scale bar 1 μm), respectively. The signals of the four elements are uniformly distributed across the field of view, with no obvious enrichment or vacancy areas, indicating that Sn and Fe are highly dispersed at the nanoscale on the carbon wall surface, forming a close synergy with oxygen and the carbon framework. This hierarchical structure provides a rapid mass transfer channel and sufficient reaction sites for the substrate and active center, providing the morphology and elemental distribution basis for the high-selectivity and high-yield conversion of 5-HMF by the catalyst in the 180–200 °C range.

[0088] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. The application of a Sn-Fe bimetallic doped rice husk catalyst, characterized in that, This catalyst catalyzes the conversion of glucose to 5-hydroxymethylfurfural, and the specific steps for preparing the catalyst are as follows: S1. Weigh out tin source and iron source as precursors, and the molar ratio of tin element in tin source to iron element in iron source is (1~5)∶1. Dissolve them in an organic solvent to prepare metal precursor solution. S2. Add 5-10g of dried and ground rice husk powder to the metal precursor solution and soak for 4 hours; then dry at 105℃ for 4-12 hours to obtain the treated rice husk. S3. The treated rice husks are placed in a tube furnace and heated to 450-600°C at a rate of 2-10°C / min under a nitrogen atmosphere. After holding for 1-4 hours, the temperature is naturally cooled to room temperature to obtain carbonized rice husks loaded with heavy metal ions. S4. The carbonized rice husks were washed with deionized water and anhydrous ethanol, respectively; finally, they were dried to obtain the Sn-Fe bimetallic doped rice husk catalyst.

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

3. The application of the Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that: In step S1, the mixture is magnetically stirred at room temperature for 30–120 minutes.

4. The application of the 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 application of the 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 at a mass ratio of 2.6:1 and added to 30-70 mL of anhydrous ethanol. The mixture is stirred until homogeneous to obtain a metal precursor solution.

6. The application of a Sn-Fe bimetallic doped rice husk catalyst as described in any one of claims 1 to 5, characterized in that, The catalyst uses rice husk SiO2 and carbon skeleton as a support, and SnO2 nanoparticles and Fe2O3 nanoparticles are generated in situ on the surface of the support, and the particle size of SnO2 nanoparticles and Fe2O3 nanoparticles is less than 10 nm.

7. The application of the Sn-Fe bimetallic doped rice husk catalyst according to claim 1, characterized in that, The specific steps are as follows: (1) Dissolve glucose in water-THF solution to prepare a mixed solution, wherein the mass fraction of glucose in the mixed solution is 5-30 wt%; (2) Add Sn-Fe bimetallic doped rice husk catalyst and NaCl solution with a concentration of 5-30 wt% to the mixed solution. The mass of the catalyst is 10-20% of the mass of glucose. The reaction temperature is 170-220℃ and the reaction time is 15-60 minutes to obtain 5-hydroxymethylfurfural.

8. The application of the Sn-Fe bimetallic doped rice husk catalyst according to claim 7, characterized in that, In a water-THF mixed solution, the volume ratio of water to THF is 1:(1-5).

9. The application of the Sn-Fe bimetallic doped rice husk catalyst according to claim 7, 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, with a feed concentration of 5 to 30 wt% and a feed rate of 5 to 40 mL / min.

Images