Sources of the Alkanes | 烷烃的来源

📚 Sources of the Alkanes | 烷烃的来源

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They form the backbone of the petrochemical industry, serving as fuels, solvents, and feedstocks for the synthesis of countless materials. A clear understanding of where alkanes come from — both naturally and through industrial processes — is essential for any A-Level Chemistry student examining fossil fuel chemistry, energy, and sustainability.

烷烃是通式为 CₙH₂ₙ₊₂ 的饱和烃。它们构成了石化工业的支柱，可用作燃料、溶剂以及无数材料合成的原料。清晰理解烷烃的来源——天然形成和工业过程——对于任何学习化石燃料化学、能源与可持续发展的 A-Level 化学学生而言都至关重要。

1. Introduction to Alkanes and their Importance | 烷烃简介及其重要性

Alkanes consist solely of carbon and hydrogen atoms linked by single covalent bonds, giving them the descriptor ‘saturated’. Their non-polar nature and strong C–C and C–H bonds make them relatively unreactive, except under conditions of combustion or free-radical substitution. Despite this limited reactivity, alkanes are the world’s primary energy source, powering vehicles, heating homes and generating electricity.

烷烃仅由碳原子和氢原子通过单共价键连接而成，因此被称为“饱和”烃。它们的非极性和牢固的 C–C、C–H 键使其在燃烧或自由基取代条件之外相对稳定。尽管反应性有限，烷烃仍是全球主要能源，驱动车辆、供暖和发电。

The lower alkanes such as methane and ethane are gases at room temperature, while larger homologues are liquids or waxy solids. Their physical properties underpin the separation techniques used in industry, particularly fractional distillation. Alkanes also act as the starting material for producing alkenes, polymers, alcohols and many other valuable chemicals.

低碳烷烃如甲烷和乙烷在室温下为气体，而较大的同系物是液体或蜡状固体。它们的物理性质支撑着工业分离技术，尤其是分馏。烷烃同时也是生产烯烃、聚合物、醇以及许多其他重要化学品的起始原料。

2. Crude Oil as the Primary Source | 原油——主要来源

Crude oil, or petroleum, is a complex mixture of hydrocarbons formed over millions of years from the remains of marine organisms subjected to heat and pressure. It is the single most important natural source of alkanes, typically containing straight-chain, branched and cyclic alkanes, along with varying amounts of aromatic hydrocarbons and sulfur compounds.

原油，即石油，是数百万年前海洋生物遗骸在热和压力作用下形成的复杂烃类混合物。它是最重要的烷烃天然来源，通常含有直链、支链和环状烷烃，以及不同含量的芳香烃和硫化物。

The composition of crude oil varies depending on its geographical origin, but alkanes from methane (C₁) up to chains of 40 carbons or more can be present. To make this mixture useful, it must be separated into fractions with narrower boiling ranges, a process carried out in an oil refinery.

原油因产地不同而组分各异，但可包含从甲烷（C₁）到 40 碳以上链长的烷烃。为使这一混合物变得有用，必须将其分离为沸程较窄的馏分，这一过程在炼油厂中完成。

3. Natural Gas | 天然气

Natural gas is a gaseous mixture found trapped in porous rock formations underground, often in association with crude oil deposits or in isolated gas fields. The chief component is methane (≥ 80 %), accompanied by smaller quantities of ethane, propane, butane and traces of pentane and hexane.

天然气是困于地下多孔岩层中的气体混合物，常与原油矿床伴生或存在于独立气田中。其主要组分是甲烷（≥ 80 %），并伴有少量乙烷、丙烷、丁烷以及痕量戊烷和己烷。

After extraction, natural gas is processed to remove water, hydrogen sulfide, carbon dioxide and heavier hydrocarbons. The purified methane-rich gas is used for domestic heating, cooking and electricity generation. Propane and butane are often liquified to create LPG (liquefied petroleum gas), valued for its portability.

开采后，天然气经处理去除水、硫化氢、二氧化碳和较重的烃类。提纯后的富甲烷气用于家用取暖、烹饪和发电。丙烷和丁烷常被液化制成 LPG（液化石油气），因其便携性而备受重视。

4. Fractional Distillation of Crude Oil | 原油的分馏

Alkanes are separated industrially by fractional distillation, which exploits differences in boiling points arising from varying chain length and molecular mass. The crude oil is heated in a furnace to around 400 °C and passed into a tall fractionating column where it vaporises. As the vapour rises, it cools, and hydrocarbons condense on horizontal trays at different heights, each tray collecting a distinct fraction.

烷烃的工业分离采用分馏，利用链长和分子质量不同导致的沸点差异。原油在炉中加热至约 400 °C，然后进入一座高大的分馏塔中汽化。蒸汽上升时逐渐冷却，烃类在不同高度的水平塔盘上凝结，每一塔盘收集一个特定馏分。

Petroleum gas (C₁–C₄, boiling below 20 °C): used as fuel gas and LPG | 石油气（C₁–C₄，沸点低于 20 °C）：用作燃气和 LPG
Gasoline (petrol) (C₅–C₁₀, 40–100 °C): motor fuel | 汽油（C₅–C₁₀，40–100 °C）：车用燃料
Naphtha (C₆–C₁₂, 60–180 °C): feedstock for cracking and reforming | 石脑油（C₆–C₁₂，60–180 °C）：裂化与重整原料
Kerosene (paraffin) (C₁₀–C₁₆, 150–240 °C): aviation fuel, heating | 煤油（C₁₀–C₁₆，150–240 °C）：航空燃料、取暖
Diesel oil (C₁₄–C₂₀, 220–300 °C): diesel engines | 柴油（C₁₄–C₂₀，220–300 °C）：柴油发动机
Lubricating oil, fuel oil (C₂₀–C₄₀, >300 °C): lubricants, ship fuel | 润滑油、燃料油（C₂₀–C₄₀，>300 °C）：润滑剂、船用燃料
Bitumen (residue): road surfacing, roofing | 沥青（残渣）：铺路、屋面

Larger, heavier fractions may undergo further processing, such as vacuum distillation, to avoid cracking at high temperatures. The yield of each fraction is influenced by the demand patterns in global markets, prompting refineries to interconvert fractions using techniques like cracking and reforming.

更大更重的馏分可能会进行真空蒸馏等进一步处理，以避免在高温下裂解。各馏分的产率受全球市场需求模式影响，促使炼油厂利用裂化和重整等技术对馏分进行互相转化。

5. Cracking: Breaking Larger Alkanes | 裂化：分解大分子烷烃

Cracking is the process of breaking long-chain alkanes into smaller, more valuable hydrocarbons. Heavier fractions such as fuel oil are converted into petrol-range molecules and alkenes, which are in high demand for polymer production.

裂化是将长链烷烃断裂为较小、更有价值烃类的过程。燃料油等重质馏分被转化为汽油范围的分子和烯烃，而后者是聚合物制造中需求量很大的原料。

Thermal cracking uses high temperatures (400–900 °C) and pressures (up to 70 atm). Under these conditions, carbon–carbon bonds break homolytically to form free radicals, leading to a mixture of smaller alkanes and alkenes. Steam cracking is a variant that uses steam as a diluent and can produce ethene and propene in high yields.

热裂化使用高温（400–900 °C）和高压（最高 70 atm）。在这些条件下，碳—碳键均裂形成自由基，最终产生较小烷烃和烯烃的混合物。蒸汽裂化是其中一种变体，以蒸汽为稀释剂，能以高产率生产乙烯和丙烯。

Catalytic cracking operates at lower pressures (1–2 atm) and moderate temperatures (about 500 °C) using a zeolite catalyst. The acidic sites on the catalyst promote carbocation formation, which rearranges to give branched alkanes, cycloalkanes and aromatic compounds. This process raises the octane rating of the petrol fraction.

催化裂化在较低压力（1–2 atm）和中等温度（约 500 °C）下，使用沸石催化剂进行操作。催化剂表面酸性位促进碳正离子生成，后者重排得到支链烷烃、环烷烃和芳香族化合物。这一过程能提高汽油馏分的辛烷值。

C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆ (example of thermal cracking)
C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆（热裂化示例）

6. Reforming: Converting Straight-Chain to Branched and Cyclic Alkanes | 重整：将直链烷烃转化为支链和环烷烃

Reforming, often called catalytic reforming, targets the naphtha fraction (C₆–C₁₂) to improve its suitability as petrol. Straight-chain alkanes have low octane numbers and cause engine knocking; reforming converts them into branched-chain isomers, cycloalkanes and aromatics, all of which burn more smoothly.

重整（常称催化重整）针对石脑油馏分（C₆–C₁₂）进行处理，以提高其作为汽油的适用性。直链烷烃辛烷值低，易引起发动机爆震；重整将其转化为支链异构体、环烷烃和芳烃，这些组分燃烧更平稳。

The process employs a bifunctional catalyst, typically platinum on acidic alumina. Metal sites catalyse dehydrogenation and hydrogenation, while acidic sites promote isomerisation and cyclisation. For instance, hexane can be isomerised to 2-methylpentane or cyclised to cyclohexane, which can then be dehydrogenated to benzene.

该过程使用双功能催化剂，通常是将铂负载在酸性氧化铝上。金属位点催化脱氢和加氢，而酸性位点促进异构化和环化。例如，己烷可异构化为 2-甲基戊烷，或环化生成环己烷，再脱氢生成苯。

A typical reaction sequence: n-C₆H₁₄ → (isomerisation) → (CH₃)₂CHCH₂CH₂CH₃ → (cyclisation) → cyclohexane → (dehydrogenation) → C₆H₆ + 3H₂. This increases the yield of hydrogen, a valuable refinery by-product used in processes such as hydrocracking and hydrodesulfurisation.

典型反应序列：n-C₆H₁₄ →（异构化）→ (CH₃)₂CHCH₂CH₂CH₃ →（环化）→ 环己烷 →（脱氢）→ C₆H₆ + 3H₂。这产生更多的氢气，氢气是炼厂宝贵的副产物，可用于加氢裂化和加氢脱硫等过程。

7. Coal as a Source of Alkanes | 煤作为烷烃来源

Although less significant than petroleum today, coal remains a historical and niche source of alkanes. High-temperature destructive distillation of coal (carbonisation) produces coke, coal tar and coal gas. Coal tar contains a complex mixture of aromatic compounds, but some alkanes and cycloalkanes are also present.

虽然目前重要性不如石油，煤仍是烷烃的历史性和特殊来源。煤的高温分解蒸馏（碳化）会生成焦炭、煤焦油和煤气。煤焦油含有复杂的芳香族化合物混合物，但也存在一些烷烃和环烷烃。

More importantly, coal can be gasified by reacting it with steam and oxygen at high temperature, producing synthesis gas (syngas), a mixture of CO and H₂: C(s) + H₂O(g) → CO(g) + H₂(g). Syngas can then be converted catalytically into alkanes through the Fischer–Tropsch process, effectively making coal an indirect source of liquid fuels.

更重要的是，煤可通过高温下与蒸汽和氧气反应进行气化，生成合成气（syngas），即 CO 和 H₂ 的混合物：C(s) + H₂O(g) → CO(g) + H₂(g)。合成气随后可通过费托合成催化转化为烷烃，使得煤实际上成为液体燃料的间接来源。

8. Synthetic Methods: Fischer–Tropsch Process | 合成方法：费托合成

The Fischer–Tropsch (FT) process converts synthesis gas into a range of straight-chain alkanes, alkenes and oxygenates using iron or cobalt catalysts. The reaction can be represented simply as: nCO + (2n+1)H₂ → CₙH₂ₙ₊₂ + nH₂O. It is widely employed in countries with limited access to crude oil but abundant coal or natural gas.

费托（FT）合成使用铁或钴催化剂将合成气转化为一系列直链烷烃、烯烃和含氧化合物。反应可简单表示为：nCO + (2n+1)H₂ → CₙH₂ₙ₊₂ + nH₂O。该技术广泛应用于原油匮乏但煤或天然气资源丰富的国家。

Operating conditions vary: high-temperature FT (about 300–350 °C) favours shorter-chain hydrocarbons for petrol, while low-temperature FT (about 200–240 °C) gives longer wax-like chains that can be hydrocracked into diesel. The primary products often undergo further upgrading to meet fuel specifications.

操作条件有所不同：高温 FT（约 300–350 °C）倾向于生成较短链的汽油组分，而低温 FT（约 200–240 °C）会产生较长的蜡状链，这些长链可通过加氢裂化制成柴油。初级产物通常需进一步提质以满足燃料标准。

9. Hydrogenation of Alkenes | 烯烃加氢

Alkanes can be prepared on an industrial and laboratory scale by the catalytic hydrogenation of alkenes. This is an addition reaction in which hydrogen adds across the C=C double bond in the presence of a metal catalyst such as finely divided nickel, palladium or platinum.

烷烃可在工业和实验室规模通过烯烃的催化加氢制备。这是一种加成反应，氢气在细粉状镍、钯或铂等金属催化剂存在下加成到 C=C 双键上。

A simple illustration is the conversion of ethene to ethane: C₂H₄(g) + H₂(g) → C₂H₆(g) (Ni, 150–200 °C). In an oil refinery, hydrogenation is used to saturate alkenes derived from cracking, producing stable alkane blends for petrol or to prepare pure alkane samples for further synthesis.

一个简单示例是乙烯转变为乙烷：C₂H₄(g) + H₂(g) → C₂H₆(g)（Ni，150–200 °C）。在炼油厂中，加氢用于饱和裂化产生的烯烃，生成稳定的烷烃混合物用于汽油，或制备纯烷烃样品供进一步合成使用。

The reaction is exothermic (ΔH negative) and occurs optimally at moderate temperatures; too high a temperature can shift the equilibrium unfavourably and lead to cracking. Alkene hydrogenation is also a key step in the hardening of vegetable oils, where liquid unsaturated chains become semi-solid alkanes.

该反应放热（ΔH 为负），在中等温度下进行最佳；温度过高会不利地移动平衡并导致裂化。烯烃加氢也是植物油硬化的关键步骤，在此过程中液态不饱和链转变为半固态烷烃。

10. Decarboxylation of Carboxylic Acid Salts | 羧酸盐脱羧

In a classical laboratory preparation, an alkane can be obtained by heating the sodium salt of a carboxylic acid with soda lime (a mixture of NaOH and CaO). The reaction removes the carboxyl group as sodium carbonate, leaving a hydrocarbon with one carbon fewer than the parent acid.

在经典实验室制备中，可通过将羧酸钠盐与碱石灰（NaOH 和 CaO 的混合物）共热来获得烷烃。该反应将羧基以碳酸钠的形式脱去，生成比母体酸少一个碳原子的烃。

The general equation is: RCOONa + NaOH → R–H + Na₂CO₃, with CaO acting as a drying agent and catalyst to soften the reaction conditions. For example, sodium ethanoate yields methane: CH₃COONa + NaOH → CH₄ + Na₂CO₃. This method is particularly useful for preparing small-chain alkanes such as methane and ethane in a school laboratory setting.

通用方程式为：RCOONa + NaOH → R–H + Na₂CO₃，CaO 充当干燥剂和催化剂以缓和反应条件。例如，乙酸钠生成甲烷：CH₃COONa + NaOH → CH₄ + Na₂CO₃。此方法尤其适合在学校实验室环境中制备甲烷、乙烷等小分子烷烃。

11. Environmental and Economic Considerations | 环境与经济考量

The dominance of fossil-derived alkanes raises critical sustainability issues. Combustion of alkane fuels releases large quantities of CO₂, contributing to climate change, while extraction and transport carry risks of oil spills and methane leaks. Furthermore, crude oil and natural gas are finite resources, subject to price volatility and geopolitical tensions.

化石来源烷烃的主导地位引发了关键的可持续性问题。烷烃燃料的燃烧释放大量 CO₂，导致气候变化，而开采和运输也伴随着石油泄漏和甲烷逸散的风险。此外，原油和天然气是有限资源，易受价格波动和地缘政治局势影响。

Economic pressures have driven innovation in refinery processes such as cracking and reforming to maximise the yield of high-demand fractions from each barrel of oil. However, these processes themselves consume energy and generate CO₂, highlighting the trade-off between efficiency and environmental footprint.

经济压力推动了裂化和重整等炼油工艺的创新，以最大限度地提高每桶石油中高需求馏分的产率。但这些工艺本身也消耗能源并产生 CO₂，凸显了效率与环境足迹之间的权衡。

12. Summary and Future Outlook | 总结与展望

The main sources of alkanes — crude oil, natural gas and coal — remain central to global energy supply, but their non-renewable nature is encouraging the search for alternatives. Advances in catalytic processes, carbon capture and storage, and the integration of biomass-derived syngas into Fischer–Tropsch plants are shaping a more diversified future.

烷烃的主要来源——原油、天然气和煤——仍是全球能源供应的核心，但其不可再生性正在推动替代方案的探索。催化工艺的进步、碳捕集与封存以及生物质衍生合成气融入费托合成工厂，正在塑造一个更加多元化的未来。

Understanding the chemical principles behind each alkane source, from fractional distillation to reforming and synthetic routes, equips chemistry students with the knowledge to appreciate both the strengths and the limitations of our current energy system. As the world shifts towards renewables, the chemistry of alkanes will continue to evolve, integrating greener methods and circular carbon strategies.

理解从分馏到重整以及合成路线等烷烃每个来源背后的化学原理，使化学专业学生具备既认识当前能源体系优势、又了解其局限性的知识。随着世界向可再生能源转型，烷烃化学将继续发展，融入更绿色的方法和碳循环战略。

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Sources of the Alkanes | 烷烃的来源