A red mud-based heterojunction solid base photocatalyst and a preparation method and application thereof

Abstract

The present application relates to a kind of red mud-based heterojunction solid base photocatalyst and its preparation method and application.The preparation method includes: (1) red mud powder is washed with water, then acid washing, drying, to obtain red mud carrier;(2) the red mud carrier is immersed in alkali metal compound solution, drying, to obtain solid base load modification red mud;(3) the solid base load modification red mud is dispersed in semiconductor precursor solution, stirring, drying, to obtain red mud-based heterojunction solid base photocatalyst precursor;(4) the red mud-based heterojunction solid base photocatalyst precursor is calcined, to obtain the red mud-based heterojunction solid base photocatalyst.The present application relies on the natural alkalinity of red mud and the advantage of solid waste resource, combined with the technical characteristics of mild low-consumption photocatalysis, and red mud-based heterojunction solid base photocatalyst adapted to microalgae oil transesterification reaction is prepared, and the efficient preparation of microalgae-based biodiesel under normal temperature and pressure visible light conditions is realized.

Description

Technical Field

[0001] This invention relates to the field of biodiesel preparation and solid waste resource utilization technology, specifically to the resource utilization of red mud, and particularly to a red mud-based heterojunction solid alkaline photocatalyst, its preparation method and application. Background Technology

[0002] Biodiesel, a renewable fatty acid methyl ester fuel that can be blended with petrochemical diesel, has become a research hotspot in the field of new energy due to its advantages such as good combustion performance, environmental friendliness, and biodegradability.

[0003] Microalgae oil, as the core raw material of third-generation biofuels, has advantages over first-generation vegetable oils made from grain and oil crops and second-generation waste oils. It has a short growth cycle, does not require arable land, can efficiently absorb CO2 from the air during the growth process, and has environmental benefits such as carbon sequestration and emission reduction. Microalgae raw materials are widely available and can be cultivated on a large scale, enabling continuous and industrialized production of biodiesel. It is the core development direction of biodiesel raw materials in the future.

[0004] Transesterification is the core process for converting microalgae oil into biodiesel, and the performance of the catalyst directly determines the efficiency, energy consumption, and product quality of the transesterification reaction. Traditional transesterification processes often use homogeneous bases such as NaOH and KOH as catalysts. Although these catalysts have high catalytic activity, they suffer from numerous drawbacks, including easy saponification reactions with free fatty acids in the feedstock, leading to difficulties in product separation. Furthermore, they require neutralization and washing after the reaction, generating large amounts of alkaline wastewater. Additionally, the strong alkalinity corrodes production equipment, increasing industrial operation and maintenance costs. These shortcomings make them unsuitable for the development requirements of green chemistry.

[0005] CN102605012A discloses a technical method for improving the yield of biodiesel from microalgae oil using optimal enzymatic catalytic reaction conditions. This method utilizes inexpensive, common industrial lipases and employs optimal enzymatic catalytic reaction conditions to increase the yield of biodiesel from microalgae oil. Under these optimal conditions, the biodiesel yield from the transesterification of microalgae oil can reach over 73%. Furthermore, the catalytic reaction conditions are mild, with minimal environmental pollution, making it safe, environmentally friendly, and cost-effective, thus demonstrating operational feasibility.

[0006] CN102732385A discloses a method for producing biodiesel from microalgal oils using ultrasound-assisted ionic liquid catalysis. The method utilizes ionic liquid as a catalyst and employs ultrasound-assisted ionic liquid catalysis to perform transesterification with methanol or ethanol. The resulting biodiesel yield is greater than 90%, and all indicators meet the current national biodiesel standard GB / T20828-2007. Furthermore, the method improves biodiesel yield, reduces reaction time and temperature, and saves on biodiesel production costs, providing a new method for the industrial production of biodiesel.

[0007] CN102260518A discloses a method for directly producing biodiesel from microalgae oil. The method utilizes a supported nickel-containing catalyst to directly convert the main component of microalgae oil, triglycerides, into oxygen-free aliphatic alkanes without using polluting methanol, strong acids, or strong alkalis. It has the advantages of simple process, clean and environmentally friendly, and low energy consumption. The resulting biodiesel has a high cetane number and calorific value, which is of great practical significance for meeting the growing fuel demand and has good economic and social benefits.

[0008] Current technologies only improve catalysts to increase the conversion rate of microalgae oil, without addressing the requirements of green chemical development. Red mud is a large-scale solid waste generated during alumina production. Its annual output is large, its storage area is vast, and its alkaline oxides easily cause soil and water pollution. The resource utilization of red mud has always been a challenge for the alumina industry. A method that can both achieve the resource utilization of red mud and efficiently catalyze the transesterification of microalgae oil to produce biodiesel under mild reaction conditions would be of great significance. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a red mud-based heterojunction solid alkaline photocatalyst, its preparation method, and its applications. This invention leverages the natural alkalinity of red mud and its advantages in solid waste resource utilization, combined with the mild and low-consumption characteristics of photocatalysis, to prepare a red mud-based heterojunction solid alkaline photocatalyst suitable for microalgal oil transesterification reactions. Through structural design and process optimization of the catalyst, efficient preparation of microalgal-based biodiesel under ambient temperature, pressure, and visible light conditions is achieved. This forms a technological path integrating solid waste resource utilization with the green preparation of microalgal-based biodiesel, providing new ideas and technical support for the industrial development of third-generation biofuels.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] In a first aspect, the present invention provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst, the method comprising:

[0012] (1) The red mud powder is washed with water, then acid washed, and dried to obtain the red mud carrier;

[0013] (2) The red mud carrier is immersed in an alkali metal compound solution and dried to obtain solid alkali-loaded modified red mud;

[0014] (3) The solid alkali-supported modified red mud is dispersed in a semiconductor precursor solution, stirred, and dried to obtain a red mud-based heterojunction solid alkali photocatalyst precursor.

[0015] (4) The red mud-based heterojunction solid alkaline photocatalyst precursor is calcined to obtain the red mud-based heterojunction solid alkaline photocatalyst.

[0016] This invention focuses on the resource utilization of red mud. First, the red mud powder is graded, impurity removed, and modified to optimize the red mud carrier structure. The graded impurity removal includes first removing soluble sodium and potassium salts by washing with water, and then removing oxide impurities that affect catalytic activity by acid washing, while retaining the natural carrier structure and some alkaline sites of the red mud. At the same time, by increasing the specific surface area of ​​the red mud, a porous structure is provided for subsequent loading and composite, thus obtaining the red mud carrier. Then, alkali metal compounds and composite semiconductors are loaded in situ on the red mud carrier to construct a red mud-based heterojunction solid alkali photocatalyst, realizing the synergistic coupling of solid alkali ester exchange sites and photocatalytic activation sites.

[0017] This invention leverages the natural alkalinity and solid waste resource advantages of red mud, combined with the mild and low-consumption characteristics of photocatalysis, to prepare a red mud-based heterojunction solid alkaline photocatalyst suitable for the transesterification reaction of microalgae oil. This catalyst catalyzes the transesterification reaction of microalgae oil and methanol under ambient temperature, pressure, and visible light irradiation. Through photo-alkali synergistic effects, it activates reaction molecules and lowers the transesterification energy barrier, significantly improving the conversion rate of microalgae-based biodiesel. Simultaneously, the heterojunction structure forms an "active site shielding layer," effectively inhibiting the dissolution of residual impurities in the red mud, improving the catalyst's stability and recycling efficiency, ultimately achieving the goal of low-cost, low-energy, and green production of microalgae-based biodiesel.

[0018] Preferably, the solid-liquid ratio of the water washing in step (1) is 1:(5~8).

[0019] Preferably, the water washing temperature in step (1) is 25℃~30℃.

[0020] Preferably, the water washing time in step (1) is 30 min to 60 min.

[0021] Preferably, the solid-liquid ratio of the pickling in step (1) is 1:(4~6).

[0022] Preferably, the pickling temperature in step (1) is 25°C to 30°C.

[0023] Preferably, the pickling time in step (1) is 20 min to 40 min.

[0024] Preferably, the solid-liquid ratio of the red mud carrier to the alkali metal compound solution in step (2) is 1:(5~8).

[0025] Preferably, the immersion temperature in step (2) is 50°C to 60°C.

[0026] Preferably, the soaking time in step (2) is 2h to 4h.

[0027] Preferably, the solid-liquid ratio of the solid alkali-loaded modified red mud to the semiconductor precursor solution in step (3) is 1:(4~5).

[0028] Preferably, the stirring temperature in step (3) is 60℃~70℃.

[0029] Preferably, the stirring time in step (3) is 3h to 5h.

[0030] Preferably, the concentration of the acid solution used for pickling in step (1) is 5wt%~10wt%.

[0031] Preferably, the acid solution used for pickling in step (1) includes any one of hydrochloric acid solution, sulfuric acid solution or nitric acid solution.

[0032] Preferably, the alkali metal compound in the alkali metal compound solution in step (2) includes any one or a combination of at least two of K2CO3, Ca(NO3)2, Na2CO3 or Mg(NO3)2.

[0033] Preferably, the concentration of the alkali metal compound solution in step (2) is 0.5 mol / L to 1 mol / L.

[0034] Preferably, the semiconductor precursor in the semiconductor precursor solution in step (3) includes any one or a combination of at least two of melamine, tetrabutyl titanate, zinc nitrate, or cadmium xanthate.

[0035] Preferably, the concentration of the semiconductor precursor solution in step (3) is 0.1 mol / L to 0.5 mol / L.

[0036] Preferably, the calcination temperature in step (4) is 400℃~600℃.

[0037] Preferably, the calcination time in step (4) is 1.5h to 5h.

[0038] Preferably, the heating rate of the calcination in step (4) is 5℃ / min to 8℃ / min.

[0039] Preferably, the calcination atmosphere in step (4) includes air.

[0040] Preferably, in step (2), the mass of the alkali metal compound in the alkali metal compound solution is 8% to 18% of the mass of the red mud carrier.

[0041] Preferably, in step (3), the mass of the semiconductor precursor in the semiconductor precursor solution is 20% to 35% of the mass of the solid alkali-loaded modified red mud.

[0042] In a second aspect, the present invention provides a red mud-based heterojunction solid alkaline photocatalyst, which is prepared by the preparation method described in the first aspect.

[0043] Thirdly, the present invention provides an application of the red mud-based heterojunction solid alkaline photocatalyst as described in the second aspect, wherein the red mud-based heterojunction solid alkaline photocatalyst is used in the photocatalytic production of microalgae-based biodiesel, and the application method includes:

[0044] The red mud-based heterojunction solid alkaline photocatalyst, refined microalgae oil, and anhydrous methanol described in the second aspect were added to a photocatalytic reactor, stirred, and subjected to photocatalytic transesterification under visible light irradiation to obtain a mixed system; after standing and separation, crude biodiesel was obtained, and the microalgae-based biodiesel was obtained by vacuum distillation.

[0045] Preferably, the molar ratio of the refined microalgae oil to anhydrous ethanol is (5~7):1.

[0046] Preferably, the mass of the red mud-based heterojunction solid alkaline photocatalyst is 3% to 5% of the mass of the static microalgae oil.

[0047] Preferably, the stirring speed is 300 rpm to 500 rpm.

[0048] Preferably, the wavelength of the visible light is 400nm~760nm.

[0049] Preferably, the intensity of the visible light is 100 mW / cm². 2 ~250mW / cm 2 .

[0050] Preferably, the photocatalytic transesterification reaction takes 1.5 h to 4 h.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] (1) This invention focuses on the resource utilization of red mud. First, the red mud powder is graded, impurities are removed, and the structure of the red mud carrier is optimized to obtain a red mud carrier with a large specific surface area and rich pore structure. Then, alkali metal compounds and composite semiconductors are loaded in situ on the red mud carrier to construct a red mud-based heterojunction solid alkali photocatalyst. This realizes the synergistic coupling of solid alkali ester exchange sites and photocatalytic activation sites, which significantly improves the catalytic efficiency. The conversion rate of microalgae-based biodiesel is ≥92%, and the selectivity is ≥98%.

[0053] (2) Based on the natural alkalinity and solid waste resource utilization advantages of red mud, and combined with the mild and low-consumption technical characteristics of photocatalysis, the red mud-based heterojunction solid alkaline photocatalyst prepared can catalyze the transesterification reaction of microalgae oil and methanol under normal temperature and pressure and visible light irradiation. Through the synergistic effect of light and alkali, the reaction molecules are activated and the transesterification energy barrier is reduced, which significantly improves the conversion rate of microalgae-based biodiesel. At the same time, the heterojunction structure forms an "active site shielding layer", which effectively inhibits the dissolution of residual impurities in red mud, improves the stability and recycling efficiency of the catalyst, and ultimately achieves the goal of low-cost, low-energy consumption and green preparation of microalgae-based biodiesel.

[0054] (3) The red mud-based heterojunction solid alkaline photocatalyst prepared in this invention can be recycled, and after being recycled 8 times, the conversion rate of microalgae-based biodiesel is still above 85%. Attached Figure Description

[0055] Figure 1 This is a diagram of the mixed system obtained after the photocatalytic transesterification reaction in Example 1.

[0056] Figure 2 This is a gas chromatogram of the microalgae-based biodiesel prepared in Example 1. Detailed Implementation

[0057] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0058] The scope of this invention can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form an effective range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this invention. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this invention. In this invention, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0059] In this invention, "a combination of at least two" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or a combination of at least two" means that any one of the listed items can be selected, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this invention. In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" means a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this invention, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0060] In this invention, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond the listed elements unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this invention, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this invention. The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various locations throughout the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this invention can be combined with other embodiments that do not conflict with the technology. The ordinal numbers "first," "second," "third," and "fourth," etc., used in the expressions "first aspect," "second aspect," "third aspect," and "fourth aspect" in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly specifying the importance or quantity of the indicated technical features. They serve only as a non-exhaustive enumeration and do not constitute a closed limitation on quantity.

[0061] In this invention, the order in which the steps are written in the methods described in each embodiment does not imply a strict execution order. The actual execution order of each step should be determined based on its function and possible internal logic. Unless otherwise specified, all steps of this invention can be executed in the order they are written, or in any order without technical conflict. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) executed sequentially, or it may include steps (b) and (a) executed sequentially. If the method also includes step (c), then step (c) can be added to the method in any order without conflict, including but not limited to the execution order of steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), etc.

[0062] In this invention, the technical solution characterized by "solid-liquid ratio" uses g as the unit for solid substances and mL as the unit for liquid substances. For example, the solid-liquid ratio of red mud powder to water is 1:3, which can be understood as adding 3 mL of water for every 1 g of red mud powder added.

[0063] In one specific embodiment, the present invention provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst, the preparation method comprising:

[0064] (1) The red mud powder is washed with water, then acid washed, and dried to obtain the red mud carrier;

[0065] (2) The red mud carrier is immersed in an alkali metal compound solution and dried to obtain solid alkali-loaded modified red mud;

[0066] (3) The solid alkali-supported modified red mud is dispersed in a semiconductor precursor solution, stirred, and dried to obtain a red mud-based heterojunction solid alkali photocatalyst precursor.

[0067] (4) The red mud-based heterojunction solid alkaline photocatalyst precursor is calcined to obtain the red mud-based heterojunction solid alkaline photocatalyst.

[0068] The red mud powder used to prepare the red mud carrier in this invention is obtained by drying, grinding, and sieving the raw red mud produced in the alumina industry. For example, the drying temperature is 105℃~110℃, such as 105℃, 106℃, 107℃, 108℃, 109℃ or 110℃. The drying time is not particularly limited, with the aim of fully evaporating the solvent. The sieve mesh size is 150 mesh~300 mesh, such as 150 mesh, 170 mesh, 190 mesh, 200 mesh, 220 mesh, 240 mesh, 260 mesh, 280 mesh or 300 mesh.

[0069] This invention focuses on the resource utilization of red mud. First, the red mud powder is graded, impurity removed, and modified to optimize the red mud carrier structure. The graded impurity removal includes first removing soluble sodium and potassium salts by washing with water, and then removing oxide impurities that affect catalytic activity by acid washing, while retaining the natural carrier structure and some alkaline sites of the red mud. At the same time, by increasing the specific surface area of ​​the red mud, a porous structure is provided for subsequent loading and composite, thus obtaining the red mud carrier. Then, alkali metal compounds and composite semiconductors are loaded in situ on the red mud carrier to construct a red mud-based heterojunction solid alkali photocatalyst, realizing the synergistic coupling of solid alkali ester exchange sites and photocatalytic activation sites.

[0070] This invention leverages the natural alkalinity and solid waste resource advantages of red mud, combined with the mild and low-consumption characteristics of photocatalysis, to prepare a red mud-based heterojunction solid alkaline photocatalyst suitable for microalgae oil transesterification reaction.

[0071] In the red mud-based heterojunction solid alkali photocatalyst provided by this invention, an alkali metal compound is calcined to obtain a solid alkali, and a semiconductor precursor is calcined to obtain a semiconductor in situ composite on a red mud support. Due to the introduction of the semiconductor, under photocatalysis, photogenerated electrons activate methanol and lower the reaction energy barrier, photogenerated holes suppress saponification side reactions, the solid alkali site directionally catalyzes transesterification, and the heterojunction suppresses carrier recombination. The three synergistically improve the reaction efficiency. Moreover, the solid alkali transesterification catalytic site and the semiconductor photocatalytic activation site are tightly coupled through the heterojunction structure, the light response range covers the visible light region of 400nm~760nm, and the photogenerated carrier recombination rate is reduced to below 30%.

[0072] Therefore, this catalyst catalyzes the transesterification reaction of microalgae oil and methanol under ambient temperature, atmospheric pressure, and visible light irradiation. Through the synergistic effect of light and alkali, it activates the reaction molecules and reduces the transesterification energy barrier, significantly improving the conversion rate of microalgae-based biodiesel. At the same time, the heterojunction structure forms an "active site shielding layer," which effectively inhibits the dissolution of residual impurities in red mud, improves the stability and recycling efficiency of the catalyst, and ultimately achieves the goal of low-cost, low-energy consumption, and green production of microalgae-based biodiesel.

[0073] In some embodiments, the solid-liquid ratio of the water washing in step (1) is 1:(5~8), for example, it can be 1:5, 1:6, 1:7 or 1:8.

[0074] In some embodiments, the water washing temperature in step (1) is 25°C to 30°C, for example, it can be 25°C, 26°C, 27°C, 28°C, 29°C or 30°C.

[0075] In some embodiments, the water washing time in step (1) is 30 min to 60 min, for example, it can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min.

[0076] In some embodiments, the solid-liquid ratio of the pickling in step (1) is 1:(4~6), for example, it can be 1:4, 1:4.5, 1:5, 1:5.5 or 1:6.

[0077] In some embodiments, the pickling temperature in step (1) is 25°C to 30°C, for example, it can be 25°C, 26°C, 27°C, 28°C, 29°C or 30°C.

[0078] In some embodiments, the pickling time in step (1) is 20 min to 40 min, for example, it can be 20 min, 25 min, 30 min, 35 min or 40 min.

[0079] In some embodiments, the solid-liquid ratio of the red mud carrier to the alkali metal compound solution in step (2) is 1:(5~8), for example, it can be 1:5, 1:5.5, 1:6, 1:6.5, 1:7, 1:7.5 or 1:8.

[0080] In some embodiments, the immersion temperature in step (2) is 50°C to 60°C, for example, it can be 50°C, 52°C, 54°C, 56°C, 58°C or 60°C.

[0081] In some implementations, the soaking time in step (2) is 2h to 4h, for example, it can be 2h, 2.5h, 3h, 3.5h or 4h.

[0082] In some embodiments, the solid-liquid ratio of the solid alkali-loaded modified red mud to the semiconductor precursor solution in step (3) is 1:(4~5), for example, it can be 1:4, 1:4.2, 1:4.4, 1:4.6, 1:4.8 or 1:5.

[0083] In some embodiments, the stirring temperature in step (3) is 60°C to 70°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C or 70°C.

[0084] In some embodiments, the stirring time in step (3) is 3h to 5h, for example, it can be 3h, 3.5h, 4h, 4.5h or 5h.

[0085] In this invention, by controlling the concentration of the acid solution during the pickling process, and coordinating the pickling temperature and time, a "light" pickling of the water-washed red mud powder is achieved. This removes oxide impurities such as iron oxides and aluminum oxides from the red mud powder while retaining catalytically active components such as iron and titanium, thus maintaining the natural carrier structure of the red mud powder. Furthermore, by removing impurities from the red mud powder through "light" pickling, the specific surface area of ​​the red mud carrier can be increased, and its pore structure can be optimized, providing a foundation for subsequent loading of alkali metal compounds and composite semiconductors.

[0086] In some embodiments, the concentration of the acid solution used for pickling in step (1) is 5wt% to 10wt%, for example, it can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%.

[0087] In some embodiments, the acid solution used for pickling in step (1) includes any one of hydrochloric acid solution, sulfuric acid solution or nitric acid solution.

[0088] In some embodiments, the alkali metal compound in the alkali metal compound solution in step (2) includes any one or a combination of at least two of K2CO3, Ca(NO3)2, Na2CO3, or Mg(NO3)2. In this invention, the alkali metal compound is calcined to obtain K2CO3, CaO, Na2CO3, or MgO, which is then loaded onto the red mud carrier as a solid alkali.

[0089] In some embodiments, the concentration of the alkali metal compound solution in step (2) is 0.5 mol / L to 1 mol / L, for example, it can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L.

[0090] In some embodiments, the semiconductor precursor in the semiconductor precursor solution of step (3) includes any one or a combination of at least two of melamine, tetrabutyl titanate, zinc nitrate, or cadmium xanthate. The semiconductor precursor, after calcination, yields g-C3N4, TiO2, ZnO, or CdS, respectively.

[0091] In some embodiments, the concentration of the semiconductor precursor solution in step (3) is 0.1 mol / L to 0.5 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L or 0.5 mol / L.

[0092] The solvent for the semiconductor precursor solution is not particularly limited; any suitable solvent can be selected according to the type of semiconductor precursor. For example, when the semiconductor precursor is melamine, the solvent includes any one of ethylene glycol, glycerol, or ethanol; when the semiconductor precursor is tetrabutyl titanate, the solvent includes any one of ethanol, isopropanol, or ethyl acetate; when the semiconductor precursor is zinc nitrate, the solvent includes any one of water, methanol, or ethanol; and when the semiconductor precursor is cadmium xanthate, the solvent includes any one of chloroform, benzene, toluene, or ethyl acetate.

[0093] In some embodiments, the calcination temperature in step (4) is 400℃~600℃, for example, it can be 400℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃ or 600℃, preferably 450℃~550℃.

[0094] In some embodiments, the calcination time in step (4) is 1.5h to 5h, for example, it can be 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h or 5h, preferably 2h to 4h.

[0095] In some embodiments, the heating rate of the calcination in step (4) is 5℃ / min to 8℃ / min, for example, it can be 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min or 8℃ / min.

[0096] In some embodiments, the calcination atmosphere in step (4) includes air.

[0097] In some embodiments, the mass of the alkali metal compound in the alkali metal compound solution in step (2) is 8% to 18% of the mass of the red mud carrier, for example, it can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, or 18%. In this invention, when the alkali metal compound in the alkali metal compound solution is K2CO3 and / or Ca(NO3)2, it is preferred that the mass of the alkali metal compound is 8% to 15% of the mass of the red mud carrier; when the alkali metal compound in the alkali metal compound solution is Na2CO3 and / or Mg(NO3)2, it is preferred that the mass of the alkali metal compound is 10% to 18% of the mass of the red mud carrier.

[0098] In some embodiments, the mass of the semiconductor precursor in the semiconductor precursor solution in step (3) is 20% to 35% of the mass of the solid alkali-loaded modified red mud, for example, it can be 20%, 22%, 24%, 26%, 28%, 30%, 31%, 33%, or 35%. When the semiconductor precursor in the semiconductor precursor solution is melamine and / or tetrabutyl titanate, it is preferred that the mass of the semiconductor precursor is 20% to 30% of the mass of the solid alkali-loaded modified red mud. When the semiconductor precursor in the semiconductor precursor solution is zinc nitrate and / or cadmium xanthate, it is preferred that the mass of the semiconductor precursor is 25% to 35% of the mass of the solid alkali-loaded modified red mud.

[0099] In this invention, the drying temperature and time in steps (1) to (3) are not particularly limited. For example, the drying temperature is 80°C to 90°C, and the drying time is not particularly limited, with the aim of fully evaporating the solvent.

[0100] In another specific embodiment, the present invention provides a red mud-based heterojunction solid alkaline photocatalyst, which is prepared by the preparation method described in one of the aforementioned specific embodiments.

[0101] In yet another specific embodiment, the present invention provides an application of the red mud-based heterojunction solid alkaline photocatalyst as described in another specific embodiment above, wherein the red mud-based heterojunction solid alkaline photocatalyst is used for photocatalytic production of microalgae-based biodiesel, and the application method includes:

[0102] The red mud-based heterojunction solid alkaline photocatalyst, refined microalgae oil, and anhydrous methanol described in another specific embodiment were added to a photocatalytic reactor, stirred, and subjected to photocatalytic transesterification under visible light irradiation to obtain a mixed system; after standing and separation, crude biodiesel was obtained, and the microalgae-based biodiesel was obtained by vacuum distillation.

[0103] In this invention, refined microalgae oil is obtained by pretreatment of microalgae oil raw materials, wherein the pretreatment includes:

[0104] Microalgae oil raw material is obtained by removing moisture and impurities through vacuum distillation to obtain refined microalgae oil. The vacuum degree of vacuum distillation is 0.08MPa~0.09MPa, for example, 0.08MPa, 0.082MPa, 0.084MPa, 0.086MPa, 0.088MPa, or 0.09MPa, and the vacuum distillation temperature is 80℃~90℃, for example, 80℃, 82℃, 84℃, 86℃, 88℃, or 90℃.

[0105] The anhydrous methanol used in the photocatalytic transesterification reaction of this invention is obtained by drying and dehydrating methanol using molecular sieves.

[0106] In this invention, the photocatalytic transesterification reaction can be completed at room temperature and pressure. For example, the reaction temperature can be 25°C to 40°C, such as 25°C, 30°C, 35°C or 40°C, and the pressure is one standard atmosphere.

[0107] In some embodiments, the molar ratio of the refined microalgae oil to anhydrous ethanol is (5~7):1, for example, it can be 5:1, 5.5:1, 6:1, 6.5:1 or 7:1.

[0108] In some embodiments, the mass of the red mud-based heterojunction solid alkaline photocatalyst is 3% to 5% of the mass of the stationary microalgae oil, for example, it can be 3%, 3.5%, 4%, 4.5% or 5%.

[0109] In some embodiments, the stirring speed is 300 rpm to 500 rpm, for example, it can be 300 rpm, 350 rpm, 400 rpm, 450 rpm or 500 rpm.

[0110] In some embodiments, the wavelength of the visible light is 400nm to 760nm, for example, it can be 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm or 760nm.

[0111] In some embodiments, the intensity of the visible light is 100 mW / cm². 2 ~250mW / cm2 For example, it could be 100mW / cm 2 120mW / cm 2 140mW / cm 2 160mW / cm 2 180mW / cm 2 200mW / cm 2 210mW / cm 2 230mW / cm 2 Or 250mW / cm 2 .

[0112] In some embodiments, the photocatalytic transesterification reaction takes 1.5 h to 4 h, for example, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h or 4 h.

[0113] In this invention, when the visible light source is an LED visible light lamp, the luminous intensity of the visible light is preferably 100 mW / cm². 2 ~200mW / cm 2 The photocatalytic transesterification reaction takes 2-4 hours. When the visible light source is a xenon lamp that filters out ultraviolet light to simulate sunlight, the preferred visible light intensity is 150 mW / cm². 2 ~250mW / cm 2 The photocatalytic transesterification reaction takes 1.5 h to 3 h.

[0114] In this invention, the mixed system after the photocatalytic transesterification reaction includes crude biodiesel and crude glycerol obtained after transesterification, as well as a red mud-based heterojunction solid alkaline photocatalyst. After standing, the crude biodiesel, with its lower density, floats on the top layer, while the crude glycerol and the red mud-based heterojunction solid alkaline photocatalyst settle at the bottom. The mixture is separated using a separatory funnel, and the bottom product is centrifuged to obtain the used red mud-based heterojunction solid alkaline photocatalyst. After washing with anhydrous ethanol 2 to 3 times and drying at 80°C to 90°C, it can be directly recycled for the next batch of photocatalytic transesterification reaction.

[0115] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0116] Preparation Example 1

[0117] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst, the preparation method comprising:

[0118] (1) The raw red mud obtained from the alumina industry is fully dried at 105℃, ground, and passed through a 200-mesh sieve to obtain red mud powder.

[0119] (2) The red mud powder was washed with water at 28°C for 40 min and dried at 85°C, with a solid-liquid ratio of 1:7 between the red mud powder and deionized water. Then, the red mud powder was acid-washed at 27°C for 30 min with a solid-liquid ratio of 1:5 between the washed red mud powder and an 8wt% hydrochloric acid solution. After acid washing, the mixture was filtered. The filter cake was washed with deionized water until neutral and dried at 85°C to obtain the red mud carrier.

[0120] (3) The red mud carrier and K2CO3 solution were in a solid-liquid ratio of 1:7. The red mud carrier was immersed in a K2CO3 solution with a concentration of 0.8 mol / L and the mass of K2CO3 in the K2CO3 solution was 12% of the mass of the red mud carrier. The immersion was carried out at 55℃ for 3 hours and then dried at 80℃ to obtain solid alkali-loaded modified red mud.

[0121] (4) The solid-liquid ratio of solid alkali-loaded modified red mud to melamine solution is 1:4.5. The solid alkali-loaded modified red mud is dispersed in an ethanol solution of melamine with a concentration of 0.3 mol / L. The mass of melamine in the melamine solution is 30% of the mass of solid alkali-loaded modified red mud. The mixture is stirred at 65°C for 4 hours and then dried at 80°C to obtain a red mud-based heterojunction solid alkali photocatalyst precursor.

[0122] (5) The red mud-based heterojunction solid alkaline photocatalyst precursor was calcined in an air atmosphere. The temperature was first increased to 500°C at a heating rate of 6°C / min and calcined for 3 hours to obtain the red mud-based heterojunction solid alkaline photocatalyst.

[0123] Preparation Example 2

[0124] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst, the preparation method comprising:

[0125] (1) The raw red mud obtained from the alumina industry is dried at 105°C, ground, and passed through a 150-mesh sieve to obtain red mud powder.

[0126] (2) The red mud powder was washed with water at 25°C for 60 min at a solid-liquid ratio of 1:5 to water and dried at 80°C. Then, the red mud powder was acid-washed at 25°C for 40 min at a solid-liquid ratio of 1:4 to 5wt% nitric acid solution. After acid washing, the mixture was filtered. The filter cake was washed with deionized water until neutral and dried at 80°C to obtain the red mud carrier.

[0127] (3) The red mud carrier and Ca(NO3)2 solution were in a solid-liquid ratio of 1:5. The red mud carrier was immersed in a Ca(NO3)2 solution with a concentration of 0.5 mol / L and the mass of Ca(NO3)2 in the Ca(NO3)2 solution was 8% of the mass of the red mud carrier. The immersion was carried out at 50℃ for 4 hours and then dried at 80℃ to obtain solid alkali-loaded modified red mud.

[0128] (4) The solid-liquid ratio of solid alkali-supported modified red mud to tetrabutyl titanate solution is 1:4. The solid alkali-supported modified red mud is dispersed in an ethanol solution of tetrabutyl titanate with a concentration of 0.1 mol / L. The mass of tetrabutyl titanate in the tetrabutyl titanate solution is 20% of the mass of solid alkali-supported modified red mud. The mixture is stirred at 60°C for 3 h and then dried thoroughly at 85°C to obtain a red mud-based heterojunction solid alkali photocatalyst precursor.

[0129] (5) The red mud-based heterojunction solid alkaline photocatalyst precursor was calcined in an air atmosphere. The temperature was first increased to 450°C at a heating rate of 5°C / min and calcined for 4 hours to obtain the red mud-based heterojunction solid alkaline photocatalyst.

[0130] Preparation Example 3

[0131] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst, the preparation method comprising:

[0132] (1) The raw red mud obtained from the alumina industry is dried at 110°C, ground, and passed through a 300-mesh sieve to obtain red mud powder.

[0133] (2) The red mud powder was washed with water at 30°C for 30 minutes at a solid-liquid ratio of 1:8 to water and dried at 80°C. Then, the red mud powder was acid-washed at 30°C for 20 minutes at a solid-liquid ratio of 1:6 to 10wt% hydrochloric acid solution. After acid washing, the mixture was filtered. The filter cake was washed with deionized water until neutral and dried at 80°C to obtain the red mud carrier.

[0134] (3) The red mud carrier and the alkali metal compound solution are in a solid-liquid ratio of 1:8. The red mud carrier is immersed in a Na2CO3 solution with a concentration of 1 mol / L and the mass of Na2CO3 in the Na2CO3 solution is 18% of the mass of the red mud carrier. The immersion is carried out at 60℃ for 2 hours and then dried at 90℃ to obtain solid alkali-loaded modified red mud.

[0135] (4) The solid-liquid ratio of solid alkali-loaded modified red mud to zinc nitrate solution is 1:5. The solid alkali-loaded modified red mud is dispersed in an aqueous solution of zinc nitrate with a concentration of 0.5 mol / L. The mass of zinc nitrate in the zinc nitrate solution is 35% of the mass of solid alkali-loaded modified red mud. The mixture is stirred at 70°C for 3 hours and then dried thoroughly at 80°C to obtain a red mud-based heterojunction solid alkali photocatalyst precursor.

[0136] (5) The precursor of the red mud-based heterojunction solid alkaline photocatalyst was calcined in an air atmosphere. The temperature was first increased to 550°C at a heating rate of 8°C / min and calcined for 2 hours to obtain the red mud-based heterojunction solid alkaline photocatalyst.

[0137] Preparation Example 4

[0138] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for the calcination temperature of 400℃ and the calcination time of 5h in step (5), the preparation method is the same as that in Example 1.

[0139] Preparation Example 5

[0140] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for the calcination temperature of 600℃ and the calcination time of 1.5h in step (5), the preparation method is the same as in Example 1.

[0141] Preparation Example 6

[0142] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for the concentration of the hydrochloric acid solution used for acid washing in step (2) being 15 wt%, the preparation method is the same as in Example 1.

[0143] Preparation Example 7

[0144] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for step (3), in which the mass of K2CO3 in the K2CO3 solution is 5% of the mass of the red mud carrier, the preparation method is the same as in Example 1.

[0145] Preparation Example 8

[0146] This preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for step (2), in which the mass of K2CO3 in the K2CO3 solution is 20% of the mass of the red mud carrier, the preparation method is the same as in Example 1.

[0147] Preparation Example 9

[0148] This preparation example provides a method for preparing a red mud-based heterojunction solid alkali photocatalyst. Except for step (3), in which the mass of the semiconductor precursor in the semiconductor precursor solution is 15% of the mass of the solid alkali-loaded modified red mud, the preparation method is the same as in Example 1.

[0149] Preparation Example 10

[0150] This preparation example provides a method for preparing a red mud-based heterojunction solid alkali photocatalyst. Except for step (3), in which the mass of the semiconductor precursor in the semiconductor precursor solution is 40% of the mass of the solid alkali-loaded modified red mud, the preparation method is the same as in Example 1.

[0151] Comparative Preparation Example 1

[0152] This comparative preparation example provides a method for preparing a red mud-based solid alkali catalyst. The preparation method is the same as that in Example 1 except that step (4) is omitted.

[0153] Comparative Preparation Example 2

[0154] This comparative preparation example provides a method for preparing a red mud-based photocatalyst. The preparation method is the same as that in Example 1 except that step (3) is omitted.

[0155] Comparative preparation example 3

[0156] This comparative preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for step (1), which only involves water washing, the preparation method is the same as in Example 1.

[0157] Comparative preparation example 4

[0158] This comparative preparation example provides a method for preparing a red mud-based heterojunction solid alkaline photocatalyst. Except for step (1), which involves only acid processing, the preparation method is the same as in Example 1.

[0159] The present invention further applies the red mud-based catalysts prepared in all the above embodiments and comparative examples to the catalytic production of microalgae-based biodiesel. The specific application methods are as follows.

[0160] Application Example 1

[0161] This application example provides a method for applying the red mud-based heterojunction solid alkaline photocatalyst prepared in Example 1 to the photocatalytic production of microalgae-based biodiesel. The application method includes:

[0162] (1) Take microalgae oil raw material and remove water and impurities by vacuum distillation at a vacuum degree of 0.08 MPa and a temperature of 80℃ to obtain refined microalgae oil; dry methanol by molecular sieve to obtain anhydrous methanol;

[0163] (2) The red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Example 1, refined microalgae oil, and anhydrous methanol were added to a photocatalytic reactor. The molar ratio of refined microalgae oil to anhydrous ethanol was 6:1, and the mass of the red mud-based heterojunction solid alkaline photocatalyst was 4% of the mass of the settled microalgae oil. The photocatalytic transesterification reaction was carried out at 30°C and one standard atmosphere.

[0164] The stirring speed was set to 400 rpm, and a 560 nm LED visible light source was used as the visible light source with a light intensity of 200 mW / cm². 2 A photocatalytic transesterification reaction was carried out under visible light irradiation. After 3 hours of reaction, a mixed system was obtained; such as Figure 1 As shown, after the mixed system was allowed to stand and separate into layers, the upper layer of clear yellow liquid was crude biodiesel, and the lower layer of black precipitate was crude glycerol and red mud-based heterojunction solid alkaline photocatalyst. The upper layer of crude biodiesel was collected using a separatory funnel and distilled under reduced pressure at a vacuum of 0.085 MPa and a temperature of 65°C to obtain the microalgae-based biodiesel. The bottom layer of product was centrifuged to obtain the used red mud-based heterojunction solid alkaline photocatalyst, which was washed three times with anhydrous ethanol, dried at 80°C, and then directly recycled for the next batch of photocatalytic transesterification reaction.

[0165] according to Figure 2 The gas chromatogram shown demonstrates the successful preparation of microalgae-based biodiesel in this embodiment.

[0166] Application Example 2

[0167] This application example provides a method for applying the red mud-based heterojunction solid alkaline photocatalyst prepared in Preparation Example 2 to the photocatalytic production of microalgae-based biodiesel. The application method includes:

[0168] (1) Take microalgae oil raw material and remove water and impurities by vacuum distillation under vacuum conditions of 0.085 MPa and 90℃ to obtain refined microalgae oil; dry methanol by molecular sieve to obtain anhydrous methanol;

[0169] (2) The red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Example 2, refined microalgae oil, and anhydrous methanol were added to a photocatalytic reactor. The molar ratio of refined microalgae oil to anhydrous ethanol was 5:1, and the mass of the red mud-based heterojunction solid alkaline photocatalyst was 3% of the mass of the settled microalgae oil. The photocatalytic transesterification reaction was carried out at 25°C and one standard atmosphere.

[0170] The stirring speed was set to 300 rpm, and an LED visible light with a wavelength of 400 nm was used as the visible light source with a light intensity of 250 mW / cm². 2The photocatalytic transesterification reaction was carried out under visible light irradiation. After 1.5 h of reaction, a mixed system was obtained. After standing and separating the layers, the upper layer of crude biodiesel was taken using a separatory funnel and distilled under reduced pressure at a vacuum of 0.08 MPa and a temperature of 60 °C to obtain the microalgae-based biodiesel. The bottom product was centrifuged to obtain the used red mud-based heterojunction solid alkaline photocatalyst. It was washed twice with anhydrous ethanol and dried at 85 °C, and then directly recycled for the next batch of photocatalytic transesterification reaction.

[0171] Application Example 3

[0172] This application example provides a method for applying the red mud-based heterojunction solid alkaline photocatalyst prepared in Preparation Example 3 to the photocatalytic production of microalgae-based biodiesel. The application method includes:

[0173] (1) Take microalgae oil raw material and remove water and impurities by vacuum distillation under vacuum conditions of 0.09 MPa and 80℃ to obtain refined microalgae oil; dry methanol by molecular sieve to obtain anhydrous methanol.

[0174] (2) The red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Example 3, refined microalgae oil, and anhydrous methanol were added to a photocatalytic reactor. The molar ratio of refined microalgae oil to anhydrous ethanol was 7:1, and the mass of the red mud-based heterojunction solid alkaline photocatalyst was 5% of the mass of the settled microalgae oil. The photocatalytic transesterification reaction was carried out at 40°C and one standard atmosphere.

[0175] The stirring speed was set to 500 rpm, and a 760 nm LED visible light source was used as the visible light source with a light intensity of 100 mW / cm². 2 The photocatalytic transesterification reaction was carried out under visible light irradiation. After 4 hours of reaction, a mixed system was obtained. After standing and separating the layers, the upper layer of crude biodiesel was taken using a separatory funnel and distilled under reduced pressure at a vacuum of 0.09 MPa and a temperature of 70°C to obtain the microalgae-based biodiesel. The bottom product was centrifuged to obtain the used red mud-based heterojunction solid alkaline photocatalyst. It was washed three times with anhydrous ethanol, dried at 80°C, and directly recycled for the next batch of photocatalytic transesterification reaction.

[0176] Application Examples 4 to 10

[0177] Application Examples 4 to 10 each provide an application method for the photocatalytic production of microalgae-based biodiesel using a red mud-based heterojunction solid alkaline photocatalyst. The application method is the same as that in Application Example 1, except that in step (2), the red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Examples 4 to 10 is used to replace the red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Example 1.

[0178] Compare and contrast application examples 1 to 4

[0179] Comparative Application Examples 1 to 4 each provide an application method for the catalytic production of microalgae-based biodiesel using a red mud-based catalyst. Except for step (2), in which the red mud-based catalyst obtained in Comparative Preparation Examples 1 to 4 is used to replace the red mud-based heterojunction solid alkaline photocatalyst obtained in Preparation Example 1, the application methods are the same as in Application Example 1.

[0180] Performance testing:

[0181] The conversion rate (calculated from the peak area in gas chromatography) and the selectivity for conversion to biodiesel (determined from the peak position) of refined algal oil in all the above application examples and comparative application examples were tested by gas chromatography-mass spectrometry (GC-MS). The test results are shown in Table 1.

[0182] The red mud-based heterojunction solid alkaline photocatalysts obtained by centrifugation in all the above application examples and comparative application examples were washed with anhydrous ethanol and recycled for the next batch of photocatalytic transesterification reactions. The conversion rate and selectivity of refined algal oil when applied to the 8th photocatalytic transesterification reaction were tested. The test results are shown in Table 1.

[0183] Table 1

[0184]

[0185] In summary, this invention leverages the natural alkalinity and solid waste resource advantages of red mud, combined with the mild and low-consumption characteristics of photocatalysis, and utilizes the synergistic coupling of solid alkali ester exchange sites and photocatalytic activation sites to significantly improve catalytic efficiency. The conversion rate of microalgae-based biodiesel is ≥92%, and the selectivity is ≥98%. Furthermore, the "active site shielding layer" formed by the heterojunction structure effectively inhibits the dissolution of residual impurities in red mud, improving the stability and recycling efficiency of the catalyst. Even after 8 cycles, the conversion rate of microalgae-based biodiesel remains above 85%.

[0186] According to the test results of Application Example 1 and Application Example 6, if the concentration of the hydrochloric acid solution used for acid washing in step (1) is too high, while removing oxide impurities, some active metals in the red mud powder will also be dissolved and removed, resulting in a decrease in the number of active sites in the red mud support, which in turn leads to a decrease in the conversion rate of the catalyst.

[0187] Based on the test results of Application Examples 1, 7, and 10, maintaining the content of solid alkali and semiconductor loaded on the red mud support within a suitable range is beneficial to the synergistic effect of solid alkali transesterification active sites and photocatalytic effect, which activates reaction molecules, reduces transesterification energy barriers, and thus improves the conversion rate of microalgae-based biodiesel.

[0188] Based on the test results of Application Example 1 and Comparative Application Example 1, although transesterification can occur if only a solid alkali is loaded, the reaction relies solely on the thermal catalysis of the solid alkali without visible light-assisted synergistic activation, resulting in a high reaction energy barrier. Under normal temperature and pressure conditions, the catalytic activity is significantly reduced. Furthermore, without the active site shielding layer formed by the semiconductor heterojunction, impurities in the red mud easily dissolve and cover the active sites, leading to a significant decrease in the initial conversion rate and even more pronounced activity decay after repeated use. Moreover, without photogenerated holes to inhibit saponification side reactions, the product selectivity is far lower than that of the red mud-based heterojunction solid alkali photocatalyst based on the synergistic effect of light and alkali provided in this invention.

[0189] Based on the test results of Application Example 1 and Comparative Application Example 2, although a weak transesterification reaction can occur under light if only semiconductors are loaded, the lack of directional catalytic effect of solid base active sites results in a single catalytic site and extremely low transesterification catalytic efficiency. Relying solely on photogenerated charge carriers to activate reaction molecules, it is impossible to efficiently catalyze the transesterification process of microalgae oil. The reaction rate is slow and the conversion rate is low at room temperature and pressure. At the same time, without the synergy of solid base sites, the overall stability of the catalyst is poor, and the activity is rapidly lost after recycling. The product selectivity is also far lower than that of the red mud-based heterojunction solid base photocatalyst provided by this invention.

[0190] Based on the test results of Application Example 1, Comparative Application Examples 3 and 4, if the red mud powder is not washed with water and acid simultaneously, soluble impurities and oxide impurities in the red mud powder cannot be fully removed, the pore structure of the red mud carrier cannot be effectively optimized, and the active sites are exposed, which is not conducive to the in-situ loading of subsequent solid alkali and semiconductors. As a result, the conversion rate and selectivity of the red mud-based catalyst for the preparation of microalgae-based biodiesel decrease.

[0191] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a red mud-based heterojunction solid alkaline photocatalyst, characterized in that, The preparation method includes: (1) The red mud powder is washed with water, then acid washed, and dried to obtain the red mud carrier; (2) The red mud carrier is immersed in an alkali metal compound solution and dried to obtain solid alkali-loaded modified red mud; (3) The solid alkali-supported modified red mud is dispersed in a semiconductor precursor solution, stirred, and dried to obtain a red mud-based heterojunction solid alkali photocatalyst precursor. (4) The red mud-based heterojunction solid alkaline photocatalyst precursor is calcined to obtain the red mud-based heterojunction solid alkaline photocatalyst.

2. The preparation method according to claim 1, characterized in that, The solid-liquid ratio of the water used for washing in step (1) is 1:(5~8); And / or, the water washing temperature in step (1) is 25℃~30℃, and the time is 30min~60min; And / or, the solid-liquid ratio of the pickling in step (1) is 1:(4~6); And / or, the pickling temperature in step (1) is 25℃~30℃ and the time is 20min~40min.

3. The preparation method according to claim 1, characterized in that, The solid-liquid ratio of the red mud carrier to the alkali metal compound solution in step (2) is 1:(5~8); And / or, the immersion temperature in step (2) is 50℃~60℃ and the time is 2h~4h; And / or, the solid-liquid ratio of the solid alkali-loaded modified red mud to the semiconductor precursor solution in step (3) is 1:(4~5); And / or, the stirring temperature in step (3) is 60℃~70℃ and the stirring time is 3h~5h.

4. The preparation method according to claim 1, characterized in that, The concentration of the acid solution used for pickling in step (1) is 5wt%~10wt%; And / or, the acid solution used for pickling in step (1) includes any one of hydrochloric acid solution, sulfuric acid solution or nitric acid solution; And / or, the alkali metal compound in the alkali metal compound solution in step (2) includes any one or a combination of at least two of K2CO3, Ca(NO3)2, Na2CO3 or Mg(NO3)2; And / or, the concentration of the alkali metal compound solution in step (2) is 0.5 mol / L to 1 mol / L; And / or, the semiconductor precursor in the semiconductor precursor solution of step (3) includes any one or a combination of at least two of melamine, tetrabutyl titanate, zinc nitrate or cadmium xanthate; And / or, the concentration of the semiconductor precursor solution in step (3) is 0.1 mol / L to 0.5 mol / L.

5. The preparation method according to claim 1, characterized in that, The calcination temperature in step (4) is 400℃~600℃; And / or, the calcination time in step (4) is 1.5h to 5h; And / or, the heating rate of the calcination in step (4) is 5℃ / min~8℃ / min; And / or, the calcination atmosphere in step (4) includes air.

6. The preparation method according to claim 1, characterized in that, In step (2), the mass of the alkali metal compound in the alkali metal compound solution is 8% to 18% of the mass of the red mud carrier; And / or, in step (3), the mass of the semiconductor precursor in the semiconductor precursor solution is 20% to 35% of the mass of the solid alkali-loaded modified red mud.

7. A red mud-based heterojunction solid alkaline photocatalyst, characterized in that, The red mud-based heterojunction solid alkaline photocatalyst is prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the red mud-based heterojunction solid alkaline photocatalyst as described in claim 7, characterized in that, The red mud-based heterojunction solid alkaline photocatalyst is applied to the photocatalytic production of microalgae-based biodiesel, and the application method includes: The red mud-based heterojunction solid alkaline photocatalyst of claim 7, refined microalgae oil and anhydrous methanol were added to a photocatalytic reactor, stirred and subjected to photocatalytic transesterification under visible light irradiation to obtain a mixed system; after standing and separation, crude biodiesel was obtained, and the microalgae-based biodiesel was obtained by vacuum distillation.

9. The application of the red mud-based heterojunction solid alkaline photocatalyst as described in claim 8, characterized in that, The molar ratio of the refined microalgae oil to anhydrous ethanol is (5~7):1; And / or, the mass of the red mud-based heterojunction solid alkaline photocatalyst is 3% to 5% of the mass of the static microalgae oil.

10. The application of the red mud-based heterojunction solid alkaline photocatalyst as described in claim 8, characterized in that, The stirring speed is 300 rpm to 500 rpm; And / or, the wavelength of the visible light is 400nm~760nm; And / or, the intensity of the visible light is 100 mW / cm². 2 ~250mW / cm 2 ; And / or, the photocatalytic transesterification reaction takes 1.5 h to 4 h.

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