Butadiene: Properties, Reactions, Production and Uses

9. Linear Dimerization and Trimerization; Telomerization: Under the influence of Ni, Co, Pd, or Fe catalysts, butadiene undergoes linear dimerization or trimerization. The resulting linear oligomers, separated by chain length, offer diverse compounds, such as 1,3,7-octatriene.

Telomerization reaction is the di- and oligomerization of a 1,3-diene under addition of a nucleophilic compound. It is catalyzed by Pd, Ni, or Pt and yields a broad spectrum of products depending on the nucleophile used.

10. Hydrogenation: Selective hydrogenation of butadiene generates 1-butene or 2-butene, and careful adjustment of reaction conditions ensures sufficient conversion without over-hydrogenation to n-butane. Commercial processes from companies like Axens, UOP, and Lummus provide tailored approaches to maximize the yield of specific butene isomers.

11. Formation of Complexes: Butadiene forms complexes with various metal compounds, such as Cu(I) salts. While historically used in the extraction of butadiene, modern extractive distillation techniques have largely replaced this method. Complexes with other transition metals like Fe, Ni, Co, Pd, and Pt are well-established, contributing to mechanistic studies in alpha-olefin polymerization catalysis.

3. Production of Butadiene

Historically, diverse methods existed for butadiene production. Today, four main pathways remain:

1. Naphtha Cracking: This dominant route captures butadiene as a byproduct of ethylene production.
2. Butane/Butene Dehydrogenation: This process plays a minor, campaign-based role depending on feedstock and butadiene prices.
3. Ethanol Production: This method has diminished in importance.
4. Acetylene Production: This route is no longer commercially viable.

Emerging bio-based approaches are under exploration.

The economics of butadiene production are heavily influenced by the coupled production of ethylene and propylene. When these C2 and C3 olefins are inexpensive, dedicated butadiene production from butenes and butane becomes more attractive.

3.1. Production From Acetylene

Historically, two pathways were employed to produce butadiene from acetylene. Both are now obsolete due to the energy-intensive and expensive nature of acetylene production. Only the initial steps of the Reppe process, leading to 1,4-butanediol and tetrahydrofuran, remain commercially relevant.

3.2. Production From Ethanol

The IPATJEW and OSTROMISLENSKY process, subsequently modified by LEBEDEW, once served as a key source of butadiene, especially in Germany and Eastern Europe during World War II. This process is illustrated by the reaction:

2 CH₃CH₂OH → CH₂=CHCH=CH₂ + 2 H₂O + H₂

and operated at temperatures ranging from 370 to 450 °C using MgO – SiO₂ or SiO₂ – Al₂O₃ catalysts, it achieves selectivities of up to 70 %.

While no longer competitive for large-scale production, it may still be viable in regions with cheap ethanol or surplus agricultural alcohol.

3.3. Dehydrogenation of Butane and Butenes

The dehydrogenation of butane and butenes to produce butadiene presents unique challenges due to its endothermic nature. To maximize yield and selectivity, several key considerations must be addressed:

Le Chatelier’s Principle: Lowering the partial pressure of the desired products, butadiene and hydrogen, drives the reaction forward. This can be achieved through vacuum operation or by adding steam to the reaction mixture.

Benefits of Steam:

1. Steam helps mitigate catalyst deactivation by preventing coke formation on the catalyst surface.
2. The endothermic nature of the dehydrogenation reaction requires external heat input. Steam readily condenses, releasing significant heat to drive the reaction.
3. The condensed steam can be easily separated from the product stream, ensuring minimal contamination.

While higher temperatures favor butadiene formation, they also increase the risk of undesirable side reactions such as cracking, isomerization, and polymerization. Finding the optimal temperature balance between yield and selectivity is crucial.

Compounds susceptible to the aforementioned side reactions are removed before the dehydrogenation process to minimize their influence.

As the dehydrogenation reaction does not proceed to completion, the product stream requires separation to isolate butadiene. Unreacted starting materials are recycled back into the process to maximize efficiency.

By carefully controlling these aspects, the dehydrogenation of butane and butenes can be optimized for efficient and selective butadiene production.

3.3.1. Dehydrogenation of n-Butane

Two prominent methods exist for dehydrogenating n-butane to butadiene:

1. Houdry-Catadiene Process:

This established one-step process operates commercially since 1943. Multiple packed-bed reactors, arranged parallel, are cycled between reaction and regeneration phases. An aluminum oxide catalyst containing 20% chromium oxide facilitates the dehydrogenation of pure n-butane or its mixture with n-butenes at 550-650 °C and 10-25 kPa.

High temperatures favor butadiene production but also generate undesirable byproducts like C1-C3 hydrocarbons, hydrogen, and catalyst-fouling coke.

After each reaction cycle (5-15 minutes), the reactor undergoes regeneration: burning the coke deposit releases heat, stored in the catalyst and inert material, for the next reaction phase.

This process yields a 15-18% butadiene concentration at the reactor outlet. Subsequent recovery steps, including quenching, compression, stripping, and separation, increase butadiene concentration to 30-50%. From 1000 tons of n-butane, roughly 550 tons of butadiene are obtained. Pure butadiene can be isolated by extractive distillation.

2. Phillips Petroleum Process:

This two-step process offers advantages like longer catalyst life and higher butadiene yields.

1. Dehydrogenation to Butene: n-Butane undergoes catalytic dehydrogenation to butene at 600°C and 1 bar on a Cr₂O₃-Na₂O-Al₂O₃ catalyst.
2. Butene Separation: n-Butenes are extracted using auxiliaries like acetone, acetonitrile, or furfural.
3. Butadiene Formation: Separated n-butenes are dehydrogenated to butadiene in an isothermal tubular reactor. The reactor is heated by flue gas to 600 °C and operates at 1 bar pressure with superheated steam addition over a Fe₂O₃-K₂O-Al₂O₃ catalyst.
4. Butadiene Recovery and Purification: Extractive distillation with the aforementioned auxiliaries recovers and purifies butadiene.

The Phillips process boasts longer running times (no catalyst regeneration) and a 65 % butadiene yield based on n-butane, demonstrating its efficiency and improved selectivity compared to the Houdry-Catadiene process.

3.3.2. Dehydrogenation of n-Butenes

n-Butenes, commonly found in cracked gasoline, olefin production, and LPG dehydrogenation, are often present as part of a C4 hydrocarbon mixture. While C4 paraffins can be separated from C4 olefins via extractive distillation (as in butadiene recovery), separating isobutene and n-butene poses a unique challenge due to their close boiling points.

Several methods address this separation hurdle:

1. BASF Process: Utilizes 40-45% H2SO4 to selectively convert isobutene to tert-butanol, allowing easier separation.

2. Isobutene can be converted to methyl tert-butyl ether or isobutene oligomers for further processing and separation.

3. Distillation with Hydroisomerization: Processes like Axens’ Isopure and CD-Tech’s CD DeIB combine distillation with hydroisomerization of 1-butene to 2-butene. Since 2-butene exhibits a distinct boiling point from isobutene, distillation becomes feasible.

4. Dow Process: Employs dehydrogenation of n-butenes with steam over a Ca/Ni phosphate catalyst stabilized with Cr₂O₃. Superheated steam provides the necessary heat for the reaction, achieving 50% butene conversion and up to 90% butadiene selectivity.

5. Parallel Reactor Approach: As the Dow process requires catalyst regeneration after 15 minutes, parallel reactors are employed, enabling continuous operation.

Similar processes have also been developed by Shell and Phillips Petroleum, showcasing the diverse approaches available for n-butene dehydrogenation and butadiene production.

3.3.3. Oxidative Dehydrogenation of n-Butenes

Oxidative dehydrogenation (ODH) offers several advantages over conventional dehydrogenation processes for butadiene production. By removing hydrogen from the equilibrium through oxidation, ODH significantly improves conversion and selectivity:

2 C₄H₈ + O₂ → 2 C₄H₆ + 2 H₂O

This exothermic reaction provides heat for the endothermic dehydrogenation, further reducing energy requirements. Additionally, the presence of oxygen and steam helps minimize catalyst coking.

Historical ODH Processes:

• Oxo-D Process (Petro-Tex): This process, employed since 1965, utilizes a ferrite catalyst and operates under mild pressure. The advantages include low steam and heating energy consumption, high conversion/selectivity, long catalyst life, and no regeneration requirement.