2.3.1 Gasification

Organic chemicals can be produced through the gasification of coal. Because of the large domestic reserves of coal on a worldwide basis, this feedstock option is one of the strongest for chemical production in the long term. The technology associated with the gasification of coal [and other carbonaceous feedstocks is well understood (Chadeesingh, 2011)], having been used in Germany in WWII and in South Africa to produce liquid fuels in combination with Fischer-Tropsch processing.

The typical gasification process starts with the production of synthesis gas (a mixture of carbon monoxide and hydrogen) in the gasifier:

$\begin{array}{l}{\mathrm{C}}_{\mathrm{coal}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{C}\mathrm{O}\\ \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\mathrm{C}\mathrm{O}}_2\end{array}$

Water-gas shift reaction:

$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2$

A generic oxygen gasification system comprises the following main steps: (1) reaction of the coal with oxygen or steam at 1000 + psi, (2) quench with water to remove particles and cool the synthesis gas, (3) application of the water-gas-shift reaction to produce hydrogen and carbon dioxide, and (4) application of gas cleaning technology by the use of physical solvents for the simultaneous removal of hydrogen sulfide and carbon dioxide (Mokhatab et al., 2006; Speight, 2007, 2013, 2014a, b). The solvents are recovered by depressurization and the hydrogen sulfide is converted to sulfur in a two-step process by first heating in oxygen to produce sulfur dioxide which then reacts further with hydrogen sulfide to produce sulfur and steam. Further catalysis increases the production of hydrogen by the reaction of carbon monoxide to carbon dioxide in the presence of water to produce additional hydrogen. The Fischer-Tropsch process can be used to produce alkanes, which are building blocks for many large-volume chemicals.

In addition to the production of synthesis gas, other products of coal gasification which may find use in the organic chemicals industry are (1) producer gas, which is a low Btu gas obtained from a coal gasifier (fixed-bed) upon introduction of air instead of oxygen into the fuel bed—the composition of the producer gas is approximately 28% v/v carbon monoxide, 55% v/v nitrogen, 12% v/v hydrogen, and 5% v/v methane with some carbon dioxide, (2) water-gas, which is a medium Btu gas that is produced by the introduction of steam into the hot fuel bed of the gasifier—the composition of the gas is approximately 50% v/v hydrogen and 40% v/v carbon monoxide with small amounts of nitrogen and carbon dioxide, (3) town gas, which is a medium Btu gas that is produced in the coke ovens and has the approximate composition: 55% v/v hydrogen, 27% v/v methane, 6% v/v carbon monoxide, 10% v/v nitrogen, and 2% v/v carbon dioxide—carbon monoxide can be removed from the gas by catalytic treatment with steam to produce carbon dioxide and hydrogen, and (4) synthetic natural gas (SNG), which is methane obtained from the reaction of carbon monoxide or carbon with hydrogen—depending on the methane concentration, the heating value can be in the range of high-Btu gases.

Because any organic carbonaceous material can be gasified (Speight, 2013, 2014b), existing gasifier designs can be adapted to use any type of coal as gasifier feed. Thus, coal characteristics (and other feedstock characteristics) do not offer insurmountable obstacles to its use for the production of synthesis gas as a first step to chemicals or fuel production (Speight, 2014b).

2.5.1 Thermochemical Gasification

Thermochemical gasification is one of several thermochemical conversion processes which also include combustion and pyrolysis and serve to convert carbonaceous feedstock to gases, liquids, and solid products.

Combustion is the complete conversion (in the presence of oxygen) to carbon dioxide, water, and energy. Combustion is also a means of converting waste to energy and involves oxidation of the fuel for the production of heat at elevated temperatures without generating useful intermediate fuel gases, liquids, or solids. Combustion typically employs an excess of the oxidizer (air) to ensure maximum fuel conversion and the products of combustion processes include heat, oxidized species (such as carbon dioxide and water), products of incomplete combustion (such as carbon monoxide and hydrocarbons), other reaction products (most as pollutants), and ash from any inorganic mineral matter (or organo-minerals) in the feedstock.

Pyrolysis is the thermal degradation of a material usually without the addition of any air or oxygen. The process is similar to gasification but generally optimized for the production of fuel liquids or pyrolysis oils (sometimes called biooils if biomass feedstock is used). Pyrolysis also produces gases and a solid char product. Pyrolysis liquids can be used directly (e.g., as boiler fuel and in some stationary engines) or refined for higher quality uses such as organic chemicals, gasoline, diesel fuel, and other products.

Gasification typically refers to conversion in an oxygen- or air-deficient environment to produce fuel gases (such as synthesis gas). The fuel gases are principally carbon monoxide, hydrogen, methane, and lighter hydrocarbons, but depending on the process used, can contain significant amounts of carbon dioxide and nitrogen, the latter mostly from air. Gasification processes also produce liquid products (tars, oils, and other condensates) and solids (char, ash) from solid feedstocks. The combustion of gasification-derived fuel gases generates the same categories of products as direct combustion of solids, but pollution control and conversion efficiencies may be improved. Synthesis gases can produce fuel products and other chemicals by chemical reactions such as Fischer-Tropsch synthesis (Chadeesingh, 2011).

Synthesis gas for commodity chemical production can be derived from any carbonaceous feedstock, such as coal, petroleum residua, or biomass. Issues such as the production of clean syngas via biomass thermochemical processing are similar to issues associated with coal gasification. Gasification of biomass can take place under slightly milder conditions than coal gasification (800–1000°C at 20–30 bar instead of 1400°C at 20–70 bar). Biomass (feedlot and chicken litter) can also be combined with a coal-syngas feedstock (Priyadarsan et al., 2004, 2005).

However, using biomass for thermochemical conversion raised several issues and some pretreatment is necessary and is unique to gasification of biomass. Biomass has a large water content that must be removed before gasification. Also, biomass components (alkali metals, halides, sulfur compounds, and tars) have a significant potential to poison downstream noble metal catalysts used in production of syngas and chemicals (Ragauskas et al., 2006). Technologies have been developed to handle these impurities, but they add to the complexity and cost of the gasification process. In addition, because production of biomass requires a large land base, feedstocks are diffuse, and manufacturing is distributed (e.g., forest pulp mills). Hence, additional methods as required by the character of the feedstock, such as feedstock densification or on-site drying prior to shipping, will be required to achieve economies of scale.

2.5.2 Sugar Fermentation

Fermentation is a metabolic process that converts sugar to acid derivatives, various gases, or alcohol. Another definition is that fermentation is the chemical process by which molecules such as glucose (C₆H₁₂O₆) are decomposed anaerobically. The process can involve complete decomposition of the glucose to carbon dioxide and water (+ energy) or can be adapted to produce ethanol (ethyl alcohol + energy):

$\begin{array}{l}{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to \left[\mathrm{fermentation}\right]\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{energy}\\ {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to \left[\mathrm{fermentation}\right]\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+\mathrm{energy}\end{array}$

The presence of oxygen during the fermentation offers routes to a suite of other chemical products.

Thus, fermentation offers a pathway for the use of biomass as a means of producing organic chemicals either directly or organic chemicals as feedstocks for the production of other chemicals. In fact, the fermentation of sugars has been the focus of much recent investigation because of the chemical character and properties of the products, which include: (1) the presence of functional groups that give rise to building block chemicals, (2) the facile adaptation to petrochemical pathways, and (3) the performance of the production process, which is a well-tested operation.

Chemical transformation of sugars can take place by oxidation (such as during the decomposition of polysaccharides of which starch is an example), oxidative dehydration (of the six-carbon sugars), hydrogenation (sometimes acid-catalyzed) of cellulose derivatives and sugar derivatives, acid amination, and esterification of oils. These products can be further converted by chemical means to derivatives by: (1) oxidation, which tends to be less important for biomass as these compounds are already oxidized, (2) by hydrogenation, (3) by dehydration, (4) by bond cleavage, and (5) by direct polymerization. Biological reactions to derivatives can also be achieved with the advantages that they are generally enzymatic and so very selective, and may go directly from sugar to the end product.

2.5.3 Nonsugar Fermentation

Annual crops such as corn that have a high sugar yield are typically difficult to grow, and need fertilization, irrigation, herbicides, and pesticides. In addition, these crops as well as waste biomass have been engineered for food production, rather than for efficiency of photosynthesis, or energy, or chemicals production (Ragauskas et al., 2006). Use of cellulose from waste biomass would allow raw material to come from corn stalks (Thanakoses et al., 2003a), wood, bagasse from sugarcane (Thanakoses et al., 2003b), and even animal products (if proteins can be used) as well as various types of waste (Chan and Holtzapple, 2003; Aiello-Mazzarri et al., 2005, 2006).

Conversion of cellulose to fuel and hydrocarbons is a multistage process. The biomass must first be physically or chemically broken down, to separate the cellulose from other components, such as lignin. Pretreatment issues dominate cellulose and lignin processing, and often involve acid- or base-catalyzed hydrolysis to facilitate enzymatic breakdown. The costs of pretreatment are high, but some suggest that few major technical improvements in the chemical or acid processing are possible. However, pretreatment is an active area of research, one example being investigation of improvements in lime pretreatment (e.g., Chang and Holtzapple, 2000; Chang et al., 2001a, b).

Once isolated, the next step is the breakdown of cellulose to form sugars. The natural rotting process, facilitated by bacteria or fungi, is slow. Enzymatic hydrolysis increases the rate of the process, but the production of by-products is a problem when the processes of oxidation produce aldehydes and acids as well as sugars. Once produced, these sugars can be fermented and processed by conventional means. Hence, the key needs for fermentation of biomass include bacteria that break down cellulose quickly. Other process goals include the use of different conversion pathways (Lynd, 1996). In addition, separation processes are essential in handling of the varied and variable biofeedstocks which includes separation of cellulose from lignin and other plant materials and separation of the by-products from the product after fermentation is complete. A separation step in itself may allow production of a value-added chemical, such as the production of xylitol [CH₂OH(CHOH)₃CH₂OH, a polyalcohol or sugar alcohol] from the pretreatment of cellulose.

2.5.4 Pyrolysis

A third means of extracting chemicals from biomass feedstocks is by means of pyrolysis (Speight, 2011c). The biomass feedstocks can be wood wastes, bark, or other forest products and the resulting products are biooils which are comprised of oxygenated organic compounds, and water. Pyrolysis is complex, incorporating both evaporation and combustion of a chemically unknown fuel. The process involves (1) heating and vaporization of water, (2) separation of the volatile components, and (3) formation of a porous char or cenosphere from the high-boiling nonvolatile components. Unlike direct combustion, pyrolysis occurs at high temperatures, on the order of 400°C (750°F).

High temperature pyrolysis has been suggested as a more effective method of black liquor gasification than traditional lower temperature approaches because tarry residues are less of a problem. Black liquor is the waste product from the Kraft process when digesting pulpwood into paper pulp, removing lignin, hemicellulose derivatives, and other extractives from the wood to free the cellulose fibers. At temperatures in the range of 700°C and 1000°C (1290–1830°F) the process yields tar products, semivolatile products, and nonvolatile char that were identified from the heating of black liquor (Sricharoenchaikul et al., 2002). In addition, in organic chemicals such as single benzene ring derivatives up to pyrene and fluoranthene (4-ring compounds) derivatives are produced.

3.6.1 Alkylation

Alkylation is the introduction of an alkyl group into an organic compound by substitution or addition. There are six types of alkylation reaction: (1) substitution for hydrogen bound to carbon, such as ethylbenzene (C₆H₅C₂H₅) from benzene (C₆H₆) and ethylene (CH₂CH₂), (2) substitution for hydrogen attached to nitrogen, (3) substitution for hydrogen in a hydroxyl group of an alcohol or phenol, and (4) addition to a tertiary amine to form a quaternary ammonium compound. Acylation reactions that fall outside of the context of this text—such as the addition to a metal to form a carbon-metal bond as well as additions to sulfur or silicon—are not included in this list. The greatest use of the alkylation process is in refineries for the production of alkylates that are used as a blending stock to produce gasoline (Speight, 2014a,b). Other major alkylation products include ethylbenzene (C₆H₅CH₂CH₃), cumene [C₆H₅CH(CH₃)₂], and linear alkylbenzene derivatives [C₆H₅(CH₂)_nCH₃, where n is typically 3 or greater].

Alkylation is commonly carried out in liquid phase at temperatures higher than 200°C (390°F) at above atmospheric pressures. Sometimes vapor phase alkylation is more effective. Alkylation agents are usually olefins, alcohols, alkyl sulfates, or alkyl halides and the typical catalysts are hydrofluoric acid (HF), sulfuric acid (H₂SO₄), or phosphoric acid (H₃PO₄, which is also known as orthophosphoric acid or phosphoric(V) acid). Higher process temperatures cause the expected lowering of product specificity and increased by-product formation. Some more recent alkylation processes (such as used for the production of ethylbenzene and cumene) use zeolite catalysts as they can be more efficient and may have lower emissions. Lewis acids, such as aluminum chloride (AlCl₃) or boron trifluoride (BF₃), may also be used as catalysts.

There are environmental issues, related to the emission of volatile organic compounds (VOCs), which arise during alkylation processes and which include although based on data for the production of ethylbenzene, cumene, and linear alkylbenzene, the emission of VOCs from alkylation reactions tend to be low compared to the emission of VOCs from other refinery processes. However, the by-products and waste disposal of alkyl halides and sulfate derivatives can be problematic.

3.6.2 Ammonolysis

Ammonolysis is the process of forming amines using, as amminating agents, ammonia or primary and secondary amines. These reactions may also include hydroammonolysis in which amines are formed directly from carbonyl compounds using an ammonia-hydrogen mixture and a hydrogenation catalyst. The four main ammonolysis reaction types are: (1) double decomposition in which the ammonia (NH₃) is split into the amino function (NH₂) which becomes part of the amine, and a hydrogen atom which reacts with a radical that is being substituted, (2) dehydration, in which the ammonia reacts to produce water and amines, (3) simple addition, in which both fragments of the ammonia molecule (NH₂ and H) become part of the new amine, and (4) multiple activity, in which ammonia reacts with the amine products to form secondary amine derivatives and tertiary amine derivatives. Nevertheless, typically, the major products of ammonolysis are carbamic acid (H₂NCOOH), ethanolamine derivatives (HOCH₂CH₂NHR), and alkylamine (RNH₂) derivatives.

In the case of the production of ethanolamine, the emission of VOCs arising from the reactors is minimal although there tends to be waste gases associated with any distillation. Any off-gas containing ammonia or amine derivatives is washed or incinerated in order to avoid odor problems and any hydrogen cyanide that is produced during the production of acrylonitrile (CH₂CHCN) can may be recovered.

In the case of water contamination, unreacted ammonia can be recovered from alkaline effluents by steam stripping and recycled back to the process. Ammonia remaining in the effluent can be neutralized with sulfuric acid to produce a precipitate of ammonium sulfate [(NH₄)₂SO₄] that can be separated for use as fertilizer or biologically treated. In addition, wastewater containing impurities such as methanol and amine derivatives can be disposed of either by incineration or by biological treatment. Solid waste materials from base of the stripping unit are incinerated.

3.6.3 Ammoxidation

In the organic chemicals industry, ammoxidation is a process for the production of nitrile derivatives involving the use of ammonia and oxygen—the usual feedstock is an alkene. The process is an important application in the production of acrylonitrile (CH₂CHCN) and process involves the gas phase oxidation of olefins, such as propylene with ammonia in the presence of oxygen and vanadium or molybdenum based catalysts:

$2{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}═{\mathrm{C}\mathrm{H}}_2+2{\mathrm{N}\mathrm{H}}_3+3{\mathrm{O}}_2\to 2{\mathrm{C}\mathrm{H}}_2═\mathrm{CHCN}+6{\mathrm{H}}_2\mathrm{O}$

Acetonitrile (CH₃CN) is a by-product of this process. This colorless liquid is the simplest organic nitrile and is used as a polar aprotic solvent in organic synthesis and in the purification of butadiene. Acetonitrile is also a common two-carbon building block in the synthesis of organic chemicals—the most important applications are in the production of acrylic fibers, thermoplastics, and adiponitrile (NCCH₂CH₂CH₂CH₂CN, a viscous, colorless liquid, an important precursor to the polymer Nylon-6,6), as well as specialty polymers.

3.6.4 Carbonylation

Carbonylation (carboxylation) is the combination of an organic compound with carbon monoxide and carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic compounds. The carbonylation reaction is used to make aldehydes and alcohols containing one additional carbon atom—the major product involves the introduction of the carbonyl group (> CO) into the starting materials to produce aldehydes (CHO), carboxylic acids (CO₂H), and esters (CO₂R). In the laboratory and in industry, carbonylation reactions are the basis of two main types of reactions, hydroformylation and Reppe Chemistry.

The hydroformylation reaction involves the addition of both carbon monoxide and hydrogen to unsaturated organic compounds, usually alkene derivatives to produce aldehyde derivatives:

$\mathrm{R}\mathrm{C}\mathrm{H}═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2+\mathrm{C}\mathrm{O}\to {\mathrm{R}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{H}\mathrm{O}$

The reaction requires the presence of metal catalysts to bond the carbon monoxide and the hydrogen to the olefin. The hydroformylation (“oxo” process) is a variant where olefins are reacted with carbon monoxide and hydrogen (“synthesis gas”) in the presence of a cobalt or rhodium catalyst (such as in the production of n-butyraldehyde from propylene):

${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2+\mathrm{C}\mathrm{O}\to {\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{H}\mathrm{O}$

On the other hand, the Reppe reaction involves the addition of carbon monoxide and an acidic hydrogen donor with the organic substrate. Large-scale applications of this type of carbonylation are the conversion of methanol to acetic acid:

${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+\mathrm{C}\mathrm{O}\to {\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}}_2\mathrm{H}$

In a related hydrocarboxylation, alkene derivatives are converted to carboxylic acid derivative in the presence of metal catalysts:

$\mathrm{R}\mathrm{C}\mathrm{H}═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{O}\to {\mathrm{R}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_2\mathrm{H}$

For example, as part of the industrial synthesis of ibuprofen, a benzyl alcohol derivative is converted to the corresponding carboxylic acids by way of palladium-catalyzed carbonylation reaction:

$\mathrm{ArCH}\left({\mathrm{C}\mathrm{H}}_3\right)\mathrm{O}\mathrm{H}+\mathrm{C}\mathrm{O}\to \mathrm{ArCH}\left({\mathrm{C}\mathrm{H}}_3\right){\mathrm{C}\mathrm{O}}_2\mathrm{H}$

Also, propanoic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl [nickel tetracarbonyl, Ni(CO)₄] as the catalyst:

${\mathrm{C}\mathrm{H}}_2═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{O}\to {\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_2\mathrm{H}$

Hydroesterification is similar to hydrocarboxylation, but uses alcohol derivatives instead of water. Other related industrially oriented reactions include the Koch reaction, which involves the addition of carbon monoxide to unsaturated compounds in the presence of a catalyst, such as in the production of the important intermediate, glycolic acid:

$\underset{\mathrm{Formaldehyde}}{\mathrm{HCHO}}+\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{Glycolic}\mathrm{acid}}{{\mathrm{HOCH}}_2{\mathrm{C}\mathrm{O}}_2\mathrm{H}}$

Alkyl, benzyl, vinyl, aryl, and allyl halides can also be carbonylated in the presence carbon monoxide and suitable catalysts such as manganese (Mn), iron (Ni), or nickel (Ni).

In terms of environmental issues of the carbonylation processes, the processes typically generate vent streams containing VOCs in addition to carbon dioxide as well as other nonVOCs. Residual gas is recovered and used as fuel or flared. In addition, heavy metals (from the catalyst) must be removed from wastewater prior to biological treatment. The solid waste is typically the spent catalysts but caution is advised when planning the disposal process because of the potential for adsorbed material, which might be leached from the solid during periods of heavy rain, melting snow, or acid rain.

Waste: Spent catalysts.

3.6.5 Condensation

A condensation reaction, is a chemical reaction in which two reactants (each reactant contains a functional group) combine to form a higher molecular weight product together with the loss of a lower molecular weight species, such as water, hydrogen chloride, methanal, or acetic acid—typically, the most common lower molecular weight species is water.

A common type of industrial condensation reaction is a condensation polymerization reaction in which a series of condensation steps takes place whereby monomers or monomer chains add to each other to form longer chains.

$\underset{\mathrm{D}\mathrm{i}-\mathrm{acid}}{{\mathrm{HOOCR}}^1\mathrm{COOH}}+\underset{\mathrm{Diamine}}{{\mathrm{H}}_2{\mathrm{N}\mathrm{R}}^2{\mathrm{N}\mathrm{H}}_2}\to –\underset{\mathrm{Polyamide}}{\left({\mathrm{C}\mathrm{O}\mathrm{R}}^1{\mathrm{CONHR}}^2\mathrm{N}\mathrm{H}\right)}{–}_n+n{\mathrm{H}}_2\mathrm{O}$

In this reaction, R¹ and R² are alkyl or aryl moieties and the CONH moiety is the amid bond. The reaction is also known as step-growth polymerization and is used in processes such as the synthesis of polyesters or nylons. This reaction may be either (1) homopolymerization such as in the use of a single monomer with two different end groups that condense or (2) copolymerization of two comonomers with the necessary functional groups on each of the monomers (as shown in the equation above). Small molecules are usually liberated in these condensation steps, unlike polyaddition reactions such as in the polymerization of ethylene which is an addition reaction and there is no elimination of lower molecular weight by-products:

$n{\mathrm{C}\mathrm{H}}_2═{\mathrm{C}\mathrm{H}}_2\to –\left({\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\right){–}_n$

Environmental issues that arise during the use of condensation processes are usually limited to reactor emissions that are generally small and can be mitigated in a combustion unit. In addition, waste water volumes are generally low and the effluents mainly consist of reaction water if recycling after phase separation is not possible. The effluent is composed of high-boiling components (condensation products/by-products) that often show moderate or poor biodegradability, and low-boiling components that typically are more susceptible to biodegradation.

3.6.6 Dealkylation

Dealkylation is a chemical process through which alkyl groups are removed from a given compound such as the dealkylation of toluene to produce benzene or the conversion of 1,2,4-trimethylbenzene to xylene:

$\begin{array}{l}{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{C}\mathrm{H}}_3+{\mathrm{H}}_2\to {\mathrm{C}}_6{\mathrm{H}}_6+{\mathrm{C}\mathrm{H}}_4\\ 1,2,4-{\mathrm{C}}_6{\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_3\right)}_3+{\mathrm{H}}_2\to {\mathrm{C}}_6{\mathrm{H}}_4{\left({\mathrm{C}\mathrm{H}}_3\right)}_2+{\mathrm{C}\mathrm{H}}_4\end{array}$

When hydrogen is used (above equation) the process is known as hydrodealkylation. The hydrodealkylation process typically requires a high temperature and a high pressure as well as the presence of catalyst—the catalyst is predominantly a transition metal-containing catalyst using metal derivatives of chromium or molybdenum.

The dealkylation process is a common process in the organic chemicals industry, especially in industries such as crude oil refining and the pharmaceuticals industry. In fact, in the pharmaceuticals industry dealkylation can lead to activation of certain compounds and can also promote better absorption and efficacy of the pharmaceutical. The dealkylation process is also frequently employed by manufacturers of fertilizers and pesticides.

Many dealkylation processes use oxidative dealkylation (also known as O-dealkylation) in which an oxide is used to assist in removal of the alkyl group from the organic substrate through a reduction-oxidation reaction (redox reaction).

3.6.7 Dehydration

The dehydration of organic chemicals is a decomposition reaction in which a new compound is formed by the expulsion of water. The dehydration process is a subprocess of the condensation process that requires a catalyst. Examples of common dehydrating reactions include:

Reactions	Equations
Conversion of alcohols to ethers	2ROH → ROR + H2O
Conversion of alcohols to alkene	RCH2CHOH-R → RCHCHR + H2O
Conversion of carboxylic acids to acid anhydrides	2RCOOH → (RCO)2O + H2O
Conversion of amides to nitriles	RCONH2 → RCN + H2O

The reverse of a dehydration reaction is a hydration reaction in which water is added to a substrate such as an olefin.

Dehydration:

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\to {\mathrm{C}\mathrm{H}}_2═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$

Hydration:

${\mathrm{C}\mathrm{H}}_2═{\mathrm{C}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$

Typical dehydrating agents used in organic synthesis include concentrated sulfuric acid (H₂SO₄), concentrated phosphoric acid (H₃PO₄), and aluminum oxide (Al₂O₃). In the related condensation reaction, water is released from two different reactants.

3.6.8 Dehydrogenation

Dehydrogenation is the process by which hydrogen is removed from an organic compound to form a new chemical (e.g., to convert saturated into unsaturated compounds). It is used to produce aldehydes and ketones by the dehydrogenation of alcohols. Important products include acetone, cyclohexanone, methyl ethyl ketone, and styrene.

Dehydrogenation is most important in the refinery cracking process, where saturated hydrocarbons are converted into olefins (Speight, 2014a,b, 2016). The process is applied to appropriate hydrocarbon feedstocks (e.g., naphtha) in order to produce the very large volumes of ethylene, propylene, butene derivatives, and butadiene derivatives that are required as feedstocks for the organic chemicals industry.

Cracking (thermal decomposition) of organic compounds may be achieved by thermal (noncatalytic) processes or by catalytic processes and provides a process to convert higher boiling fractions into saturated, nonlinear paraffinic compounds, naphthenes, and aromatics. The concentration of olefin derivatives in the product stream is very low, so this method is more useful for the preparation of blending stocks (such as naphtha and kerosene) for the production of fuels (gasoline and diesel fuel, respectively). Olefin derivatives are more widely produced by the steam cracking of petroleum fractions—in the process a hydrocarbon stream is heated, mixed with steam and, depending on the feedstock, further heated to a cracking-temperature on the order of 600–650°C (1110–1200°F). The conversion of saturated hydrocarbon streams to unsaturated compounds is highly endothermic, and so the process requires a high energy input. High-temperature cracking is also used to produce pyrolysis gasoline from naphtha, gas oil, or high-boiling refinery streams.

Environmental issues that arise from the of dehydrogenation processes include the potential for hydrogen-rich vent streams that are produced as a result of the process but which can be employed as a hydrogen feedstock for other processes or as a refinery fuel. Any volatile hydrocarbons that occur in purge and vent gases will require collection and treatment and can be combined with beneficial energy production. On the other hand, quench water, dilution steam, decoking water, and flare water discharges will require treatment and wastewater streams with a high content of pollutants will require pretreatment prior to acceptance in a biological degradation plant.

3.6.9 Esterification

Esterification typically involves the formation of esters from an organic acid and an alcohol. Esters often have a characteristic pleasant, fruity odor which makes them appropriates for extensive use in the fragrance and flavor industry.

The most common method of esterification is the reaction of a concentrated alcohol and a concentrated carboxylic acid with the elimination of water:

$\underset{\mathrm{Acid}}{{\mathrm{R}}_1{\mathrm{C}\mathrm{O}}_2\mathrm{H}}+\underset{\mathrm{Alcohol}}{{\mathrm{R}}_2\mathrm{O}\mathrm{H}}\leftrightarrow \underset{\mathrm{Ester}}{{\mathrm{R}}_1{\mathrm{C}\mathrm{O}}_2{\mathrm{R}}_2}+{\mathrm{H}}_2\mathrm{O}$

In this equation, R₁ and R₂ are alkyl moieties. Only strong carboxylic acids react sufficiently quickly without a catalyst, so a strong mineral acid (such as sulfuric acid or hydrogen chloride) must usually be added to aid the reaction. Acid anhydrides are also used, e.g., in dialkyl phthalate production. The sulfonic acid group can be bound chemically to a polymeric material and so cation exchangers, such as sulfonated polystyrene, enable esterification under mild conditions.

The equilibrium of the reaction can be shifted to the ester by increasing the concentration of one of the reactants, usually the alcohol. In production scale esterification the reaction mixture is refluxed until all the condensation water is formed, and the water or the ester product is continuously removed from the equilibrium by distillation. The main products from esterification reactions are dimethyl terephthalate, ethyl acrylate, methyl acrylate, and ethyl acetate which have considerable economic importance in many applications (such as for fibers, films, adhesives and plastics). Some volatile esters are used as aromatic materials in perfumes, cosmetics, and foods.

Also, the reaction of alcohol derivatives with carboxylic acid derivatives is not the only process for producing ester derivatives. Alcohols react with acyl chlorides (acid chlorides) and acid anhydrides to produce esters:

$\begin{array}{l}{\mathrm{R}}_1\mathrm{COCl}+{\mathrm{R}}_2\mathrm{O}\mathrm{H}\to {\mathrm{R}}_1{\mathrm{C}\mathrm{O}}_2{\mathrm{R}}_2+\mathrm{H}\mathrm{C}\mathrm{l}\\ {\left({\mathrm{R}}_1\mathrm{C}\mathrm{O}\right)}_2\mathrm{O}+{\mathrm{R}}_2\mathrm{O}\mathrm{H}\to {\mathrm{R}}_1{\mathrm{C}\mathrm{O}}_2{\mathrm{R}}_2+{\mathrm{R}}_1{\mathrm{C}\mathrm{O}}_2\mathrm{H}\end{array}$

Again, in these equations R₁ and R₂ are alkyl moieties. The reactions are irreversible, simplifying and driving the process to completion. Since acyl chlorides and acid anhydrides also react with water, anhydrous conditions are preferred. The analogous acylation of amine derivatives to yield amide derivatives are less sensitive because amines are stronger nucleophiles and react more rapidly than does water.

Finally, ethylene, acetic acid, and oxygen react (in the presence of palladium-based catalysts) to yield vinyl acetate:

$2{\mathrm{C}\mathrm{H}}_2═{\mathrm{C}\mathrm{H}}_2+2{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}}_2\mathrm{H}+{\mathrm{O}}_2\to 2{\mathrm{C}}_2{\mathrm{H}}_3{\mathrm{O}}_2{\mathrm{C}\mathrm{C}\mathrm{H}}_3+2{\mathrm{H}}_2\mathrm{O}$

Direct routes (such as the alcohol-acid reaction) to this same ester are not possible because vinyl alcohol (CH₂CHOH) is unstable and has the propensity under normal conditions (ambient temperature and ambient pressure) to convert to acetaldehyde (CH₃CHO).

Environmental issues related to the esterification process relate to solvent vapor which can be collected and treated (such as by incineration, adsorption). The generation of aqueous effluents is not extensive since water is the only by-product of the esterification reaction. In addition, the choice of solid polymer based ion exchange resins for wastewater treatment avoids the need for extensive treatment facilities. Most esters possess low toxicity because they are easily hydrolyzed on contact with water or moist air, and so the properties of the acid and alcohol components are more important. Furthermore, waste streams can be reduced by recovering (and reusing) any organic solvents, water, and alcohol components. Any wastes from waste water treatment can be incinerated (high-boiling wastes) or recovered by distillation for reuse (low-boiling wastes).

3.6.10 Halogenation

Generally, halogenation is the reaction of a halogen with an alkane in which the introduction of halogen atoms occurs into the organic molecule by an addition reaction or by a substitution reaction. In organic synthesis this may involve the addition of molecular halogens: chlorine, bromine, iodine, or fluorine (Cl₂, Br₂, I_2, or F₂) or hydrohalogenation using: hydrogen chloride, hydrogen bromide, hydrogen iodide, or hydrogen fluoride (HCl, HBr, HI, or HF) to carbon-carbon double bonds. Substitution reactions involve replacing hydrogen atoms in olefin derivatives, paraffin derivatives, or aromatic derivatives with halogen atoms. Chlorination is the most important industrial halogenation reaction and chlorinated organic products include chlorinated aromatic derivatives, chlorinated methane derivatives, and chlorinated ethane derivatives but caution is advised since toxicity issues will demand additional control measures. Fluorination is used almost exclusively in the manufacture of fluorocarbons.

Several pathways exist for the halogenation of organic compounds, including free radical halogenation, electrophilic halogenation, and the halogen addition reaction. For example, saturated hydrocarbon derivatives (alkanes) typically do not add halogens but undergo free radical halogenation which involves the substitution of a hydrogen atom (or hydrogen atoms) by a halogen atom (or halogen atoms). The chemistry of the halogenation of alkane derivatives is usually determined by the relative weakness of the available carbon-hydrogen (CH) bonds. Free radical halogenation is used for the industrial production of chlorinated methane derivatives:

${\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{l}}_2\to {\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{l}+\mathrm{H}\mathrm{C}\mathrm{l}$

Rearrangement often accompanies such free radical reactions. On the other hand, unsaturated compounds, especially alkene derivatives and alkyne derivatives add halogens across the unsaturated bond:

${\mathrm{R}}^1\mathrm{C}\mathrm{H}═{\mathrm{C}\mathrm{H}\mathrm{R}}^2+{\mathrm{X}}_2\to {\mathrm{R}}^1{\mathrm{CHXCHXR}}^2$

However, aromatic compounds are subject to electrophilic halogenation but addition to the ring system can occur under extreme conations:

${\mathrm{R}\mathrm{C}}_6{\mathrm{H}}_5+{\mathrm{X}}_2\to {\mathrm{R}\mathrm{C}}_6{\mathrm{H}}_4\mathrm{X}+\mathrm{H}\mathrm{X}$

The ease of the reaction is influenced by the halogen—fluorine and chlorine are more electrophilic and, as a result, are more aggressive halogenating agents. On the other hand, bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is least reactive halogenating agent of the halogens. Accordingly, the ease of hydrogenolysis (removal of the halogen with hydrogen as HX) follows the reverse trend: iodine is most easily removed from organic compounds and organo fluorine compounds are most stable organo-halogen compounds.

Environmental issues of halogenation processes involve the treatment of waste gases which requires a distinction between acidic streams, reaction gases, and neutral waste streams. Air streams from tanks, distillation columns, and process vents can be collected and treated using such techniques as low temperature condensation or incineration. However, the treatment of acid gas streams is more complex because any equipment in contact with acid gases and water must be constructed from acid-resistant materials or internally coated to prevent corrosion (Speight, 2014c). The halogen content of the waste gas may represent a valuable raw material and pollution control techniques offer an opportunity for its recovery and reuse (either as hydrogen-halogen or aqueous solutions). The techniques may include: (1) product recovery (by vapor stripping of liquid streams followed by recycling to the process), (2) scrubbing the acid gas with an easily halogenated compound preferably a raw material used in the process, (3) absorbing the acid gas in water to give aqueous acid which is often followed by caustic scrubbing for environmental protection, (4) washing out organic constituents with organic solvents, and (5) condensing out organic by-products for use as feedstock in another process (Mokhatab et al., 2006; Speight, 2007, 2014a).

Environmental issues also arise with wastewater streams because the biological degradability (biodegradability) of halogenated hydrocarbons (especially aromatic derivatives) decreases as the halogen content increases. Only chlorinated hydrocarbon derivatives with a low degree of chlorination are degradable in biological waste water treatment plants but only if the concentration of the chlorinated hydrocarbon derivatives does not exceed certain levels. Prior to biological treatment, wastewater containing chlorinated organic compounds usually requires preliminary purification by stripping, extraction, and adsorption (using activated carbon or suitable polymeric resins). Wastewater contamination can be substantially reduced by avoiding the water quenching of reaction gases to separate hydrogen chloride (for example in the production of chlorinated ethane derivatives and chlorinated ethylene derivatives).

Finally, solid waste materials as a result of the halogenation process may arise from sources such as reactor residues or spent catalyst. Incineration is a common method for destruction of the organic components of the solid wastes but considerable attention must be paid to incineration conditions in order to avoid the formation of dioxins. If incineration is used, there is the need for an efficient flue gas scrubbing operation.

3.6.11 Hydrogenation

Catalytic hydrogenation refers to the addition of hydrogen to an organic molecule in the presence of a catalyst. The process can involve direct addition of hydrogen to the double bond of an unsaturated molecule; amine formation by the replacement of oxygen in nitrogen-containing compounds; and alcohol production by addition to aldehydes and ketones. These reactions are used to readily reduce many functional groups; often under mild conditions and with high selectivity.

Reactant	Product
Alkene: R2CCR′2	Alkane: R2CHCHR′2
Alkyne: RCCR	Alkene: cis-RHCCHR′
Aldehyde: RCHO	Primary alcohol: RCH2OH
Ketone: R2CO	Secondary alcohol: R2CHOH
Ester: RCO2R′	Mixed alcohols: RCH2OH + R′OH
Imine: RR′CNR″	Amine: RR′CHNHR″
Amide: RC(O)NR′2	Amine: RCH2NR′2
Nitrile: RCN	Primary amine: RCH2NH2
Nitro: RNO2	Amine: RNH2

Hydrogenation is an exothermic reaction and the equilibrium usually lies far towards the hydrogenated product under most operating temperatures. It is used to produce a wide variety of chemicals such as cyclohexane, aniline, n-butyl alcohol, hexamethylene diamine, as well as ethyl hexanol, and important isocyanate derivatives such as TDI and methylene diphenyl isocyanate both of which are used to produce urethane derivatives and thence urethane polymers.

Hydrogenation catalysts may be heterogeneous or homogeneous—heterogeneous catalysts are solids and form a distinct phase in the gases or liquids. Many metals and metal oxides have general hydrogenation activity—nickel, copper, cobalt, chromium, zinc, iron, and the platinum group are among the elements most frequently used as commercial hydrogenation catalysts.

Generally, the emission of VOCs for hydrogenation processes is relatively low since hydrogen-rich vent streams are typically sent to combustion units. The main issues with hydrogen are likely to arise from sulfur impurities in the process feedstocks or from the dust and ash by-products of the hydrogen production itself. Small quantities of sulfur compounds (such as sulfur dioxide, SO₂, and hydrogen sulfide, H₂S) can be absorbed in dilute caustic solutions or adsorbed on activated charcoal as part of a gas cleaning operation while larger quantities would probably have to be converted to liquid sulfur or to solid sulfur (Mokhatab et al., 2006; Speight, 2007).

The hydrogenation of oxygenated compounds may generate water, which ends up as waste water, but volume of wastewater produced from hydrogenation reactions is not excessive. Moreover, the products often show good biodegradability and low toxicity whereas aniline compounds (from a hydrodenitrogenation process) will need disposal measures that are additional to the bio treatment technologies. In addition, the spent catalysts may be sent to disposal or may be treated for reclamation of any precious metals.

3.6.12 Hydrolysis

Hydrolysis involves the reaction of an organic chemical with water to form two or more new substances and usually means the cleavage of chemical bonds by the addition of water. In fact, Hydrolysis can be the reverse of a condensation reaction in which two molecules join together into a larger one and eject a water molecule. Thus hydrolysis adds water to break down, whereas condensation builds up by removing water. Hydration is the process variant where water reacts with a compound without causing its decomposition. These routes are used in the manufacture of alcohols (e.g., ethanol), glycols (e.g., ethylene glycol, propylene glycol), and propylene oxide.

Acid-base-catalyzed hydrolysis reactions and processes are very common—one example is the hydrolysis of ester derivatives or amide derivatives. The hydrolysis reaction occurs when the nucleophilic reactant (a nucleus-seeking agent, e.g., water or hydroxyl ion) attacks the carbon of the carbonyl group of the ester or the amide using an aqueous base medium since hydroxyl ions are better nucleophiles than polar molecules such as water. In acidic medium, the carbonyl group becomes protonated, and this leads to a much easier nucleophilic attack. The products for both hydrolyses are carboxylic acid derivatives. The oldest commercially practiced example of ester hydrolysis is the saponification reaction which results in the formation of soap and involves the hydrolysis of a triglyceride (fat) with an aqueous base such as sodium hydroxide (NaOH). During the process, glycerol (CH₂OHCHOHCH₂OH) is formed:

The carboxylic acids react with the base, converting them to salts.

In the hydrolysis process, there are generally low-to-no emission of VOCs emanating from the reactor and, in most cases, the products of the hydrolysis process and the hydration process are biodegradable.

3.6.13 Nitration

The nitration process is a general class of chemical process for the introduction of a nitro group (NO₂) into an organic chemical compound. The term is also applied (somewhat incorrectly) to the different process of forming nitrate esters between alcohol derivatives and nitric acid, as occurs in the synthesis of nitroglycerin. The difference between the resulting structure of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a nonoxygen atom, typically carbon or another nitrogen atom, whereas in nitrate esters, also called organic nitrates, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom. There are many major industrial applications of nitration in the strict sense; the most important by volume are for the production of nitroaromatic compounds such as nitrobenzene. Nitration reactions are notably used for the production of explosives, for example the conversion of toluene to TNT (2,4,6-trinitrotoluene).

However, explosives aside, the nitro compounds are of wide importance as chemical intermediates and precursors.

Thus, nitration involves the replacement of a hydrogen atom (in an organic compound) with one or more nitro groups (NO₂). By-products may be unavoidable due to the high reaction temperatures and the highly oxidizing environment, although many nitration reactions are carried out at low temperature for safety reasons. The nitration reaction can be carried out with aliphatic compounds (to produce nitroparaffin derivatives) but the nitration of aromatics is more commercially important (to produce explosives and propellants such as nitrobenzene and nitrotoluene derivatives). This is effected with nitric acid (HNO₃) or, in the case of aromatic nitration reactions, a mixture of nitric and sulfuric acids. Nitration is used in the first step of TDI production.

Environmental issues of nitration processes (excluding the more obvious potential explosive properties) relate to the occurrence of acid vapors (largely nitric or sulfuric acid) from the reaction and quenching as well as any unreacted nitrating agent arising from the use of an excess of the agent to carry the nitration reaction to completion. There is also the potential for the emission of VOCs as well as other gas streams that contain the various oxides of nitrogen. In terms of water pollutants, the nitration of aromatic feedstocks may produce large quantities of waste mixed acid that requires neutralization and disposal, or recovery (e.g., by distillation) and reuse. The products and by-products of the nitration process often are slow to biodegrade (if they are at all biodegradable) and toxic, so additional treatment of the waste products (such as extraction or incineration of aqueous wastes) may be required.

3.6.14 Oxidation

The term oxidation includes many different processes, but in general it describes the addition of one or more oxygen atoms to a compound. Catalytic oxidation process are processes that utilize catalysts to enhance the oxidation reaction—typical oxidation catalysts are metal oxides and metal carboxylates. The catalysis of the oxidation process occurs by the use of both heterogeneous catalysis and homogeneous catalysis (Table 3.8). In the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, redox-active oxides of iron, vanadium, and molybdenum. In many cases, catalysts are modified with the suitable choice (from an extensive list) of additives or promoters that enhance the reaction rate or the product selectivity.

The important homogeneous catalysts for the oxidation of organic compounds are the carboxylic acid salts of cobalt, iron, and manganese. To confer good solubility in the organic solvent, these catalysts are often derived from naphthenic acids and ethylhexanoic acid which are highly lipophilic. These catalysts initiate radical chain reactions that produce organic radicals that combine with oxygen to give hydroperoxide intermediates. Generally, the selectivity of oxidation is determined by the bond energy—for example, benzylic carbon hydrogen (C6H5CHH) bonds are replaced by oxygen faster than aromatic carbon hydrogen (C6H5H) bonds.

Common applications of the process involve oxidation of organic compounds by the oxygen in air. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy. An illustrative catalytic oxidation is the conversion of methanol to the more valuable compound formaldehyde using aerial oxygen:

$2{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{O}}_2\to 2\mathrm{HCHO}+2{\mathrm{H}}_2\mathrm{O}$

This conversion is very slow in the absence of catalysts.

Atmospheric oxygen is by far the most important, and the cheapest, oxidizing agent although the inert nitrogen component will dilute products and generate waste gas streams. Other oxidizing agents include nitric acid, sulfuric acid, oleum, hydrogen peroxide, organic peroxides, and pure oxygen. In general terms, organic materials can be oxidized either by heterolytic or homolytic reactions, or by catalytic reactions (where the oxidizing agent is reduced and then reoxidized). Heterogeneous catalysts based on noble metals play a dominant role in industrial scale oxidations and an important example is the silver catalyzed gas phase reaction between ethylene and oxygen to form ethylene oxide. Ethylene is still the only olefin that can be directly oxidized to the corresponding epoxide with high selectivity. Other important industrial oxidation processes are the production of acetic acid, formaldehyde, phenol, acrylic acid, acetone, and adipic acid. Oxidation reactions are exothermic and heat can be reused in the process to generate steam or to preheat other component streams. Fire and explosion risks exist with heterogeneously catalyzed direct oxidation processes (e.g., ethylene oxide process) and reactions involving concentrated hydrogen peroxide or organic peroxides.

In terms of the environmental aspects of the oxidation process, the oxidation of organic compounds produces a number of by-products (including water) and wastes from partial and complete oxidation. In the organic chemical industry, such compounds as aldehydes, ketones, acids, and alcohols are often the final products of partial oxidation of hydrocarbons. Careful control of partial oxidation reactions is usually required to prevent the material from oxidizing to a greater degree than desired as this produces carbon dioxide and many undesirable gaseous, liquid, or semisolid toxic by-products.

In addition, the emissions of volatile organics can arise from losses of unreacted feed, by-products, and products such as aldehydes and acids. Carbon dioxide is an ever-present by-product in the oxidation of organic compounds since it is difficult (if not impossible in some cases) to prevent the complete oxidation of some carbon. Aldehyde derivatives (especially formaldehyde, HCHO) require strict handling to minimize exposure and this limits atmospheric emissions. Acid gases usually require removal from waste streams. Also, to enable biological degradation in a wastewater treatment plant it will be necessary to neutralize any acidic components and to remove/destroy any chlorinated species that may inhibit biological activity.

3.6.15 Oxyacetylation

Acetylation is a reaction that introduces an acetyl functional group (acetoxy group, CH₃CO) into an organic chemical compound—namely the substitution of the acetyl group for a hydrogen atom—while deacetylation is the removal of an acetyl group from an organic chemical compound. Thus, oxyacetylation involves the addition of oxygen and an acetyl group to an olefin to produce an unsaturated acetate ester. It is used to produce vinyl acetate from ethylene, acetic acid, and oxygen.

3.6.16 Reforming

Reforming is the decomposition (cracking) of hydrocarbon gases or low octane petroleum fractions by heat and pressure. Catalytic reforming is used to convert naphtha (having low octane ratings) into a high-octane liquid product (reformate) which is a premium blending stock for the production of high-octane gasoline. The process converts low-octane linear hydrocarbons (paraffins) into branched alkanes (isoparaffins) and cycloalkane derivatives, which are then partially dehydrogenated to produce high-octane aromatic hydrocarbon derivatives. The dehydrogenation also produces significant amounts of by-product hydrogen, which is used in other refinery processes such as hydrocracking. A side reaction in the reforming process is hydrogenolysis, which produces low-boiling hydrocarbons, such as methane, ethane, propane, and butanes. The nature of the final product is influenced by the source (and composition) of the feedstock. The four major catalytic reforming reactions are:

(1) The dehydrogenation of cycloalkane derivative (naphthenes) to aromatics:

(2) The isomerization of normal paraffin derivatives to isoparaffin derivatives:

(3) The dehydrogenation and aromatization of paraffin derivatives to aromatic products (known as dehydrocyclization): as exemplified in the conversion of normal heptane to toluene, as shown below:

(4) The hydrocarbon of paraffin derivatives into lower molecular weight products

$\underset{n-\mathrm{Heptane}}{{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_3}\to \underset{\mathrm{Isopentane}}{{\left({\mathrm{C}\mathrm{H}}_3\right)}_2{\mathrm{CHCH}}_2{\mathrm{C}\mathrm{H}}_3}+\underset{\mathrm{Ethane}}{{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_3}$

The hydrocracking of paraffin derivatives is only one of the above four major reforming reactions that consumes hydrogen. The isomerization of normal paraffins does not consume or produce hydrogen but, moreover, both the dehydrogenation of naphthene derivatives and the dehydrocyclization of paraffin derivatives produce hydrogen. In many petroleum refineries, the net hydrogen produced in catalytic reforming supplies a significant part of the hydrogen used elsewhere in the refinery (for example, in hydrodesulfurization processes).

In a variation of the reforming process, the steam reforming is a process for producing hydrogen, carbon monoxide, or other useful products from hydrocarbon feedstocks such as natural gas, which is predominantly methane, hence the alternate name for the process: steam-methane reforming. The conversion is achieved in a reactor (reformer) in which the methane reacts at high temperature with the steam. At high temperatures (700–1100°C, 1290–2010°F) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield carbon monoxide and hydrogen:

${\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2$

Thus:

${\mathrm{C}}_n{\mathrm{H}}_m+n{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \left(n+\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\right){\mathrm{H}}_2+n\mathrm{C}\mathrm{O}$

Additional hydrogen can be recovered by a lower-temperature gas-shift reaction with the carbon dioxide produced, in the presence of a copper-based or iron-based catalyst:

Water-gas shift reaction:

$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2$

The first reaction is strongly endothermic (consumes heat) while the second reaction is mildly exothermic (produces heat).

3.6.17 Sulfonation

Sulfonation is the process by which a sulfonic acid group (or corresponding salt or sulfonyl halide) is attached to a carbon atom and the process is used to produce detergents (by sulfonating mixed linear alkyl benzenes with sulfur trioxide or oleum). In the process, a hydrogen atom on an aromatic ring is replaced by a sulfonic acid functional group by an electrophilic aromatic substitution reaction:

Thus, the general equation for sulfonation of the aromatic ring is:

$\mathrm{A}\mathrm{r}\mathrm{H}+\underset{\mathrm{Oleum}}{{\mathrm{H}}_2{\mathrm{SO}}_4}\to {\mathrm{ArSO}}_3\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$

The most widely used sulfonating agent for linear alkylbenzenes is oleum (fuming sulfuric acid—a solution of sulfur trioxide in sulfuric acid, H₂SO₄·SO₃). Sulfuric acid alone is effective in sulfonating the benzene ring but the acid content of the sulfuric acid (solution) must be in excess of 75% v/v. The excess sulfur trioxide in oleum removes the water of reaction thereby preventing the sulfuric acid from descending below the minimum acid content level for an efficient reaction and promotes higher yields of the desired product. Separating the product sulfonates from the reaction mixture is often difficult—the mother liquor (the solution remaining after the reaction) after product separation raises an environmental issue.

The acid vapor from the reaction and quenching as well as unreacted sulfonating agent arising from the excess use to drive the reaction, pose serious disposal problems. Also, acidic wastewater from the reactor and dilute acidic wash waters (from washing the product on the filter) require neutralization. In addition, the filtrate from the separation stage is contaminated with unreacted raw material and acid. Finally, oleum is an extremely strong oxidizing agent and produces tar by-products that also require disposal.