Method for preparing alkyl fuel through oleic acid gas-phase non-hydrogen catalytic decarboxylation

Abstract

The invention relates to a method for preparing alkyl fuel through oleic acid gas-phase non-hydrogen catalytic decarboxylation in the technical field of biomass energy conversion. The method comprises the following steps: injecting an oleic acid liquid raw material into a heating zone of a fixed bed reactor; the catalyst loaded in the fixed bed is a composite magnesium-aluminum oxide, and the molar ratio of magnesium ions to aluminum ions in the composite magnesium-aluminum oxide is (2-6): 1; the reaction temperature in the fixed bed reactor is 400-500 DEG C, the gasified oleic acid and nitrogen penetrate through a catalyst bed layer together, a decarboxylation reaction is carried out in a nitrogen atmosphere, the volume ratio of the oleic acid liquid to the nitrogen is 1: (600-800), and the mass space velocity is 5-6 h <-1 >. The catalytic decarboxylation reaction has the advantages of high stability and high selectivity, the oleic acid decarboxylation rate reaches 99.2% or above, hydrogen is not needed, the process is simple, the product selectivity is high, and the method is suitable for large-scale production of biomass derived fuel.

Description

Technical Field

[0001] This invention relates to the field of biomass energy conversion technology, specifically a method for preparing long-chain hydrocarbon fuels by catalytic gas-phase decarboxylation of fatty acids under non-hydrogen-contaminated conditions. Background Technology

[0002] With the growing global energy demand and the depletion of fossil fuel resources becoming increasingly prominent, the development of renewable, low-carbon liquid fuel technologies has become a research hotspot in the energy sector. Biomass-derived fatty acids (such as animal and vegetable oils, microbial oils, or waste oils) are considered ideal raw materials for preparing hydrocarbon-based fuels due to their carbon chain structure being similar to that of petroleum-based fuels. Traditional biodiesel (fatty acid methyl esters) suffers from problems such as high oxygen content, low calorific value, poor low-temperature fluidity, and insufficient storage stability. However, converting fatty acids into hydrocarbon fuels (C8-C18 alkanes / olefins) through deoxygenation and decarboxylation can significantly improve fuel quality and achieve compatibility with existing petroleum-based fuel infrastructure.

[0003] Early decarboxylation processes largely relied on hydrogen-exposed conditions, achieving deoxygenation through hydrodeoxygenation (HDO) or hydrodecarboxylation (HDC) under high temperature and pressure. However, the high cost of hydrogen, storage and transportation risks, and the use of precious metal catalysts (such as Pt and Pd) limited their economic viability and large-scale application. In recent years, non-hydrogen-exposed catalytic decarboxylation technology has attracted widespread attention due to its advantages such as not requiring external hydrogen, mild reaction conditions, and low equipment investment. This technology involves designing highly efficient heterogeneous catalysts (such as transition metal oxides, carbon-based materials, and molecular sieve-supported catalysts) to directly break the CO bond in the carboxyl group of fatty acids in an inert or weakly reducing atmosphere, releasing CO2 and generating target hydrocarbons. However, existing non-hydrogen-exposed decarboxylation systems still face challenges such as easy carbon deposition and deactivation of catalyst active sites, difficulty in controlling the selectivity of long-chain fatty acid decarboxylation, and unclear reaction pathways and mechanisms, which restrict their industrialization. Therefore, developing highly stable and highly selective non-hydrogen-exposed decarboxylation catalysts and elucidating the reaction network and kinetic mechanisms have become key research directions for promoting the green conversion of fatty acids into high-quality hydrocarbon-based fuels. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing hydrocarbon fuels by non-hydrogen-dependent catalytic decarboxylation of oleic acid in the gas phase, which can overcome the defects of the prior art, make the decarboxylation of long-chain fatty acids selectively controllable, and give the catalytic decarboxylation reaction the advantages of high stability and high selectivity.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing hydrocarbon-based fuels by non-hydrogen-dependent catalytic decarboxylation of oleic acid in the gas phase, comprising the following steps:

[0006] a. Inject liquid oleic acid feedstock into the heating zone of a fixed-bed reactor; the catalyst loaded in the fixed bed is a composite magnesium aluminum oxide, wherein the molar ratio of magnesium ions to aluminum ions in the composite magnesium aluminum oxide is (2-6:)1;

[0007] b. The reaction temperature in the fixed-bed reactor is 400-500℃. After oleic acid vaporizes, it passes through the catalyst bed along with nitrogen gas and undergoes a decarboxylation reaction under a nitrogen atmosphere. The volume ratio of liquid oleic acid to nitrogen gas is 1:(600-800), and the mass hourly space velocity is 5-6 h⁻¹. -1 .

[0008] The composite magnesium aluminum oxide was prepared by the following steps:

[0009] 1) Preparation of hydrotalcite

[0010] A mixed salt solution consisting of divalent soluble magnesium salt and trivalent soluble aluminum salt, and a mixed alkaline solution consisting of alkali and soluble carbonate were prepared separately. Under continuous heating and stirring, the mixed salt solution and the mixed alkaline solution were simultaneously and slowly added dropwise to obtain a white slurry. The slurry was then liquidified, washed with water, and dried to obtain the magnesium aluminum hydrotalcite.

[0011] 2) Calcination

[0012] Heat to 400-600℃, hold for 240-360 minutes, and then cool naturally to room temperature to obtain composite magnesium-aluminum metal oxide.

[0013] In this invention, after calcination, magnesium-aluminum hydrotalcite is converted into magnesium-aluminum composite oxide. Therefore, the magnesium salt and aluminum salt can be selected more broadly. For example, the divalent soluble magnesium salt can be one or more of magnesium nitrates, sulfates, acetates, and chlorides; the trivalent soluble aluminum salt can be one or more of aluminum nitrates, sulfates, acetates, and chlorides; similarly, the alkali can be one or two of sodium hydroxide and potassium hydroxide; and the soluble carbonate can be a potassium salt or a sodium salt.

[0014] Further, the alkali is sodium hydroxide or potassium hydroxide; the soluble carbonate is sodium carbonate or potassium carbonate corresponding to the alkali. The purpose of choosing carbonates is to make the hydrotalcite structure more regular and of higher purity, because carbonate ions have a stronger affinity for hydrotalcite layers than other anions. On the other hand, to maintain the charge balance of the hydrotalcite, the ratio of anions to cations must be certain. Therefore, the aluminum ions [Al...] 3+ ] with carbonate ions in soluble carbonates [CO3] 2- The molar ratio of [Al] is 2:1 because each [Al] 3+ [Mg] replaces 2+ This will impart 1 unit of positive charge to the shelf, requiring 1 / 2 [CO3] 2-[OH-] to neutralize; in alkali [OH-] - The molar amount of [OH]: The total molar amount of magnesium and aluminum ions = (1-2):1, - [Al] prompted 3+ ] and [Mg 2+ They co-precipitate to form positively charged layers.

[0015] Further, in step 1), the crystallization temperature is preferably 80-120℃, and the time is 2-12 hours. In step 2), the heating rate during calcination is preferably 1-20℃ / min.

[0016] During the decarboxylation reaction of oleic acid, the carboxylic acid first combines with MgO in the magnesium-aluminum composite oxide to form magnesium carboxylate and water. The magnesium carboxylate further decomposes to generate carbonyl-containing free radicals and magnesium-containing free radicals. The carbonyl-containing free radicals further decompose to generate the corresponding hydrocarbons and gases; or combine with hydrogen free radicals to form aldehydes, which further decompose to generate alcohols and hydrocarbons; or combine with methyl free radicals to form ketones, which further decompose to generate small-molecule ketones and hydrocarbons. Mg-containing free radicals lose MgCO3 to generate alkyl free radicals, which combine with or lose hydrogen free radicals to generate the corresponding hydrocarbons. Simultaneously, under the action of Al2O3 in the magnesium-aluminum composite oxide, most of the carboxylic acid directly loses its carboxyl group to form CO2 and hydrocarbons, while a small portion directly loses its carbonyl group to form alcohols and CO. Therefore, hydrocarbon fuels can be prepared without high-pressure hydrogen.

[0017] Compared with existing technologies, the advantages of this invention are as follows: The composite magnesium-aluminum oxide prepared by this invention has a high specific surface area. When applied to the gas-phase non-hydrogen-dependent catalytic decarboxylation of oleic acid to prepare hydrocarbon fuels, the decarboxylation of long-chain fatty acids can be selectively controlled, promoting the decarboxylation pathway to generate C17 alkanes. Its catalytic decarboxylation reaction exhibits high stability and high selectivity. The decarboxylation rate of oleic acid reaches over 99.2%. This invention requires no hydrogen, has a simple process, and high product selectivity, making it suitable for the large-scale production of biomass-derived fuels. Attached Figure Description

[0018] Figure 1 Figures show the characterization and performance of magnesium aluminum layered double hydroxide (Mg / Al) with a molar ratio of 4:1 and its composite oxide catalyst. (a) XRD pattern of Mg / Al layered double hydroxide with a molar ratio of 4:1; (b) XRD pattern of Mg / Al composite oxide with a molar ratio of 4:1; (c) SEM image of Mg / Al composite oxide with a molar ratio of 4:1; and (d) Decarboxylation rate of oleic acid under the conditions of Example 1.

[0019] Figure 2Figures show the characterization and performance of magnesium aluminum layered double hydroxide (Mg / Al) with a molar ratio of 3:1 and its composite oxide catalyst. (a) XRD pattern of Mg / Al layered double hydroxide with a molar ratio of 3:1; (b) XRD pattern of Mg / Al composite oxide with a molar ratio of 3:1; (c) SEM image of Mg / Al composite oxide with a molar ratio of 3:1; and (d) Decarboxylation rate of oleic acid under the conditions of Example 2.

[0020] Figure 3 Characterization and performance study of magnesium aluminum layered double hydroxide (Mg / Al) with a molar ratio of 2:1 and its composite oxide catalyst. Figure (a) XRD pattern of magnesium aluminum layered double hydroxide with a molar ratio of 2:1; (b) XRD pattern of magnesium aluminum composite oxide with a molar ratio of 2:1; (c) SEM image of magnesium aluminum composite oxide with a molar ratio of 2:1; (d) Decarboxylation rate of oleic acid under the conditions of Example 3. Detailed Implementation

[0021] Example 1

[0022] Preparation of composite magnesium aluminum oxide catalysts

[0023] Magnesium nitrate hexahydrate and aluminum nitrate nonahydrate were weighed according to a Mg / Al molar ratio of 4:1 and dissolved in 100 ml of water to prepare [Mg 2+ ]+[Al 3+ A mixed solution with a concentration of 1.2 mol / L is prepared, containing 24.615 g (0.096 mol) of magnesium nitrate hexahydrate and 9.0031 g (0.024 mol) of aluminum nitrate nonahydrate. Separately, 5.088 g (0.048 mol) of anhydrous sodium carbonate and 7.68 g (0.192 mol) of sodium hydroxide are weighed out and 100 ml of alkaline solution is prepared. The [OH-] in the alkali... - ] = 1.6([Mg 2+ ]+[Al 3+ The molar amount of [CO3] 2- ] = 2.0[Al 3+ The molar amount of [ ]. Then, 50 ml of deionized water was added to a three-necked flask. The salt solution and alkali solution were simultaneously and slowly added dropwise to the three-necked flask while stirring thoroughly. The pH of the solution was maintained by controlling the dropping rate. After the addition was complete, the solution was crystallized at 80°C for 10 hours, filtered, washed until the pH of the filtrate was around 7, and then dried. The dried magnesium aluminum hydrotalcite was placed in a muffle furnace at a temperature of 500°C, a heating rate of 15°C / min, and a calcination time of 5 hours.

[0024] Oleic acid decarboxylation reaction

[0025] The catalyst was loaded into a fixed-bed reactor, and nitrogen gas was introduced at a flow rate of 70 ml / min. After heating to 500 °C, oleic acid (the volume ratio of liquid oleic acid to nitrogen gas was 1:700) was injected at a rate of 0.1 ml / min. The reaction temperature in the fixed-bed reactor was maintained at 500 °C. After the oleic acid vaporized, it passed through the fixed bed along with the nitrogen gas, and the decarboxylation reaction was carried out under a nitrogen atmosphere with a mass hourly space velocity (HHSV) of 5.34 h⁻¹. -1 The reaction time was 6 hours, and the decarboxylation rate of the product was 99.7%.

[0026] Example 2

[0027] The difference from Example 1 is that the Mg / Al molar ratio is 3:1; the decarboxylation rate of the product analysis is 98.62%.

[0028] Example 3

[0029] The difference from Example 1 is that the Mg / Al molar ratio is 2:1; the decarboxylation rate of the product analysis is 97.35%.

[0030] Example 4

[0031] The difference from Example 1 is that the reaction temperature in the fixed-bed reactor was 425°C, the reaction time was 6 hours, and the decarboxylation rate of the product was 99.3%.

[0032] Example 5

[0033] The difference from Example 1 is that the reaction temperature in the fixed-bed reactor was 550°C, the reaction time was 6 hours, and the decarboxylation rate of the product was 99.23%.

[0034] Based on Example 1, only the molar ratio of magnesium ions to aluminum ions in the composite magnesium-aluminum oxide was changed, without changing the total amount. The molar ratio of magnesium ions to aluminum ions in the composite magnesium-aluminum oxide was screened, as shown in the table below:

[0035] Serial Number molar ratio of magnesium and aluminum ions Decarboxylation rate 1 0.5:1 72.86％ 2 1:1 77.15％ 3 2:1 97.35％ 4 3:1 98.62％ 5 4:1 99.70％ 6 5:1 98.45％ 7 6:1 99.21％ 8 7:1 96.65％

[0036] The results show that the preferred molar ratio of magnesium ions to aluminum ions in the composite magnesium aluminum oxide is (2-6):1; the most preferred ratio is 4:1.

[0037] Based on Example 1, only the temperature was changed while other factors remained constant. The optimal catalytic pyrolysis reaction temperature for composite magnesium aluminum oxide was screened, as shown in the table below:

[0038]

[0039]

[0040] The results show that the optimal temperature for the catalytic pyrolysis reaction of the composite magnesium aluminum oxide is 400-500℃.

[0041] Based on Example 1, the volume ratio of oleic acid liquid to nitrogen gas was screened, and the results are shown in the table below:

[0042]

[0043] The results showed that the optimal volume ratio of oleic acid liquid to nitrogen was 1:(600-800). Insufficient nitrogen reduced reaction efficiency and lowered the conversion rate. Excessive carrier gas flow rate also reduced the conversion rate, as the oleic acid gas was not fully reacted with the magnesium-aluminum composite oxide before being carried out of the reaction tube.

[0044] Based on Example 1, the mass hourly space velocity (MHV) during decarboxylation was screened, and the results are shown in the table below:

[0045] Serial Number <![CDATA[Mass hourly space velocity (h -1 )]]> Decarboxylation rate 1 4 99.85 2 5 99.45％ 3 5.34 99.7％ 4 6 99.25％ 5 8 94.11％ 6 10 93.25％ 7 12 88.36％ 8 15 84.52％

[0046] The results showed that the decarboxylation rate decreased significantly with increasing mass hourly space velocity (MHSV). This is because MHSV is related to both feed flow rate and catalyst dosage, being inversely proportional to catalyst dosage and directly proportional to feed flow rate. Below 5 h⁻¹, the decarboxylation rate decreases significantly. -1 Although the decarboxylation rate is very high, the reaction rate is slow and the economic efficiency is poor; therefore, a mass hourly space velocity (HHSV) of 5-6 h⁻¹ is chosen. -1 This can maximize the effect of the catalyst.

[0047] In this invention, the magnesium salt can be one or more of magnesium nitrates, sulfates, acetates, and chlorides; the aluminum salt can be one or more of aluminum nitrates, sulfates, acetates, and chlorides; the alkali can be one or two of sodium hydroxide and potassium hydroxide; and the soluble carbonate is a potassium or sodium salt. After calcination, it becomes a metal oxide; therefore, the choice of substance has little impact on the result. The calcination temperature can be selected as 400-600℃, the holding time as 240-360 min, and the heating rate during calcination as 1-20℃ / min. The crystallization temperature can be 80-120℃, and the crystallization time as 2-12 hours.

[0048] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for preparing hydrocarbon-based fuels by non-hydrocatalytic decarboxylation of oleic acid in the gas phase, characterized in that, Includes the following steps: a. Inject oleic acid liquid feedstock into the heating zone of a fixed-bed reactor; the catalyst loaded in the fixed bed is a composite magnesium aluminum oxide, wherein the molar ratio of magnesium ions to aluminum ions in the composite magnesium aluminum oxide is (2-6):1; b. The reaction temperature in the fixed-bed reactor is 400-500℃. After oleic acid vaporizes, it passes through the fixed bed along with nitrogen gas and undergoes a decarboxylation reaction under a nitrogen atmosphere. The volume ratio of liquid oleic acid to nitrogen gas is 1:(600-800), and the mass hourly space velocity is 5-6 h⁻¹. -1 .

2. The method for preparing hydrocarbon-based fuels by non-hydrogen-containing catalytic decarboxylation of oleic acid in the gas phase according to claim 1, characterized in that, The composite magnesium aluminum oxide was prepared by the following steps: 1) Preparation of hydrotalcite A mixed salt solution consisting of divalent soluble magnesium salt and trivalent soluble aluminum salt, and a mixed alkaline solution consisting of alkali and soluble carbonate were prepared separately; wherein the molar ratio of magnesium ions to aluminum ions was (2-6):1; under continuous heating and stirring, the mixed salt solution and the mixed alkaline solution were simultaneously and slowly added dropwise to obtain a white slurry; the slurry was then liquidified, washed with water, and dried to obtain the magnesium-aluminum hydrotalcite; 2) Calcination Heat to 400-600℃, hold for 240-360 minutes, and then cool naturally to room temperature to obtain composite magnesium-aluminum metal oxide.

3. The method for preparing hydrocarbon-based fuels by non-hydrocatalytic decarboxylation of oleic acid in the gas phase according to claim 2, characterized in that, The divalent soluble magnesium salt is one or more of magnesium nitrates, sulfates, acetates, and chlorides; the trivalent soluble aluminum salt is one or more of aluminum nitrates, sulfates, acetates, and chlorides; the alkali is one or two of sodium hydroxide and potassium hydroxide; and the soluble carbonate is a potassium salt or a sodium salt.

4. The method for preparing hydrocarbon fuels by non-hydrogenative catalytic decarboxylation of oleic acid in the gas phase according to claim 3, characterized in that, The alkali is sodium hydroxide or potassium hydroxide; the soluble carbonate is sodium carbonate or potassium carbonate corresponding to the alkali, [Al 3+ ] and [CO3 2- The molar ratio of [OH-] is 2:1; the alkali contains [OH-] - The molar amount of magnesium aluminum ions: the total molar amount of magnesium aluminum ions = (1-2):

1.

5. The method for preparing hydrocarbon fuels by non-hydrocatalytic decarboxylation of oleic acid in the gas phase according to claim 2, characterized in that: In step 1), the crystallization temperature is 80-120℃ and the time is 2-12 hours.

6. The method for preparing hydrocarbon fuels by non-hydrogenative catalytic decarboxylation of oleic acid in the gas phase according to claim 2, characterized in that, In step 2), the heating rate during calcination is 1-20℃ / min.

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