Bases and Basic Materials in Industrial and ... - ACS Publications

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Ind. Eng. Chem. Res. 2009, 48, 6486–6511

Bases and Basic Materials in Industrial and Environmental Chemistry: A Review of Commercial Processes Guido Busca*

Ind. Eng. Chem. Res. 2009.48:6486-6511. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/21/19. For personal use only.

Laboratorio di Chimica delle Superfici e Catalisi Industriale, Dipartimento di Ingegneria Chimica e di Processo “G.B. Bonino”, UniVersita di GenoVa, Piazzale Kennedy, I-16129 GenoVa, Italy

The practical application of liquids and solid bases in industrial and environmental processes is reviewed. Liquid bases include water solutions of metal hydroxides such as caustics and potash, amines, phosphorus compounds and other molecular bases, solutions and slurries of alkali and alkaline-earth carbonates, organic solutions of organometallics, and superbases. Solid bases include basic clays, alkaline-earth oxides, alkali metals containing oxides and zeolites, synthetic hydrotalcites, rare-earth oxides and mixed oxides, impregnated carbons, and supported alkali metals. Some health and safety considerations are discussed, together with process features. Conclusions about the future use of bases and the perspective of a more massive application of solid bases are offered. 1. Introduction Acid-base interactions are, together with redox exchanges, the most relevant phenomena in all fields of chemistry. Acids find extensive application in various fields of industrial and technological chemistry,1,2 such as metallurgy, ceramurgy, and fertilizer technology. They also have important applications in organic3 and hydrocarbon chemistry4 as catalysts,5 as well as in some environmental processes. Similar applications also involve basic compounds. Given that several air and water pollutants are acidic in nature, bases also find a very extensive application in environmental protection processes.6 The development of more efficient, safer, and more environmentally friendly chemical technologies is a major need of humanity, in particular at the beginning of the third millennium, when the hydrocarbon era apparently is approaching its end and the use of renewable raw materials needs to be increased and made more efficient. This review focuses on the characteristics of basic compounds and materials largely used in industrial and technological chemistry. Our contribution is intended to emphasize the link between chemical knowledge about acid-base interactions and engineering of the processes and their environmental impact. This work represents an overview to evaluate the role of bases in environmental prevention and pollution and possible ways of limiting noxious effects from their use. 2. Bases and Basic Materials: A Short Summary Among the several definitions of basicity (and acidity) proposed in the literature, those of Arrhenius, Brønsted, and Lewis are the most useful. According to the definition of S. A. Arrhenius,7 a base is any species that is able to release hydroxide ions in water solution. This approach implies metal hydroxides as typical basic compounds M(OH)n(s) a Mn+(aq) + nOH-(aq)

(1)

assuming that they completely dissociate in water solution. According to J. M. Brønsted8 (and T. M. Lowry,9 who gave a similar but less precise definition), a base is any species that is * Tel.: int-39-010-3536024. Fax: int-39-010-3536028. E-mail: [email protected].

capable of combining with protons. In this view, acid-base interactions consist in the equilibrium exchange of a proton from an acid HA to a base B, generating the conjugate base of HA, A-, plus the conjugate acid of B, HB+ HA + B a A- + HB+

(2)

where B can be neutral or anionic and HB+ can be positively charged or neutral or even anionic. A broader spectrum for bases is considered in the Brønsted definition than in the Arrhenius definition. In fact, bases according to the Brønsted definition, now include all molecular bases (B neutral) and also the anions obtained by solution of salts characterized by basic hydrolysis (where B is anionic; see Table 1). The basic strength in dilute water solutions is currently mostly described in terms of Brønsted basicity, by the shift of the dissociation equilibrium:10 HB+ a H+ + B

(3)

KHB ) [H+][B]/[HB+]

(4)

pKHB ) -log KHB

(5)

where the molar concentration of each species is used as an approximation of its activity. KHB is the acidity constant (Ka) of the conjugate acid HB+ of the base B. As largely discussed in the literature, the basicity of a molecule B is strongly affected by the solvation of HB+ (and, to a lesser extent, also of B) and by the delocalization of the cationic charge of HB+. These factors are affected by the presence of different substituents (electron-donating or electronwithdrawing); by steric hindrance and strain; and finally by conformational effects, which influence the overall electron distributions within B and HB+. Basicity in water solution is buffered by the acidity of water itself: only bases having a basic strength lower or only moderately higher than that of the hydroxide anion can exist as such with some significant concentration in water solution. Instead, bases much stronger than the hydroxide anion can exist in nonaqueous and nonprotic solvents, as long as there are no protons (or other acidic species of the Lewis type; see below) to neutralize them.

10.1021/ie801878d CCC: $40.75  2009 American Chemical Society Published on Web 06/10/2009

Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009

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Table 1. Basic Species Applied in the Liquid Phase examples metal hydroxides salts with basic hydrolysis

acetates hydrogencarbonates phenates carbonates ammonia

sodium sodium sodium sodium sodium

hydroxide acetate bicarbonate phenate carbonate

aliphatic amines molecular bases imines/amines phosphines polyphosphazenes alkali metal alkoxides alkali amide organometallics

primary secondary tertiary

aromatic heterocyclic guanidine trimethylphosphine t-BuP4 sodium ethoxide sodium amide

alkali enolate alkali dienolate alkali acetylide alkyl alkali metal

Based on experiments concerning dissociation or exchange reactions in the presence of strong bases and correlations among data, the water pKa scale can be enlarged to values that are not actually measurable in water. The pioneering work of Conant and Wheland11 was the first attempt to assign pKa values to a number of extremely weak organic acids, including hydrocarbons. McEwen12 extended the study to include a greater number of acids and improved the method quantitatively by using colorimetric, spectroscopic, and polarimetric methods to determine the positions of their equilibria. With a slightly different but complementary approach, Hammett13 suggested suitable acidity functions that were applicable to the characterization of superbases. To measure their relative abilities to ionize weak indicator acids HA, the H- acidity function was defined as H- ) pKa - log([HA]/[A-])

(6)

where pKa is the negative logarithm of the thermodynamic ionization constant of the indicator acid in water and [HA]/ [A-] is the measured ionization ratio of the indicator. The function H- measures the ability of the solution to remove a proton from the acid and thereby enables the strength of weak acids to be measured, kinetic mechanistic studies to be interpreted, and the physicochemical composition of solutions to be investigated.14,15 In recent years, several “superbasic” molecules, characterized by very extensive delocalization of the cationic charge of their protonated conjugate forms, have been synthesized. Polyphosphazenes, proazaphosphatranes, and polyguanidines are superbasic molecules. Other extremely basic species are alkali alkoxides, alkali amides, and organometallics such as alkyl lithium compounds. In Table 1, these species are summarized. Basicity also exists in the gas phase. Experimentally evaluated or calculated gas-phase parameters allowing the determination of basicity in the gas phase are proton affinity (PA) and gasphase basicity (GB).16 PA and GB correspond to the changes in enthalpy (∆H) and Gibbs free energy (∆G), respectively, for the reaction BH+(g) a B(g) + H+(g)

(7)

defined at a finite temperature, usually 298 K. In this case, solvation effects do not exist, so that “pure basicity” can be evaluated.

basic species

pKHB

OH CH3-COO[HCO3]C6H5O[CO3]2NH3 n-C5H11NH2 (H2C)5NH (C2H5)3N C6H5NH2 H5C5N (H2N)2CdNH (H3C)3P [(tC4H9N)3PdN]3PdN(tC4H9) CH3CH2OH2N(CH3COCH2)[HC(CO2C2H5)2]HC≡Cs CH3CH2(CH3)CHs

15.74 4.80 6.37 10.00 10.25 9.24 10.64 11.12 10.75 4.63 5.20 13.6 8.65 30.6 16.0 33 20 13.5 25 >50

-

n-butylamine piperidine triethylamine aniline pyridine

ethoxide anion amide ion acetone anion diethylmalonate anion acetylide anion butyl anion

In the same year (1923), G. N. Lewis17 proposed a different approach, defining a base as any species containing an electron pair that can be donated to form a dative or coordination bond. The Lewis-type acid-base interaction can consequently be denoted as B:AL a δ+B f ALδ-

(8)

This definition is independent of water as the reaction medium and is even more general than the previous ones. In the Lewis approach, the definition of an acid (any species having available empty orbitals, such as protons and also metal cations and other cationic species) is broader than in the Brønsted approach (protic species only). In contrast, Lewis bases and Brønsted bases coincide. In fact, all species with available electron pairs are both Lewis and Brønsted bases. Nucleophilicity is a property closely related to basicity, having particular relevance in the field of organic chemistry.18 Nucleophiles are electron-rich chemical species, having a full n-type orbital, that can react with elecrophilic centers (i.e., electronpoor carbon atoms), allowing for the formation of a new bond. Nucleophiles are always Brønsted and Lewis basic species as well. Even if a ranking of nucleophilicity is based on kinetics more than on thermodynamics, it follows the ranking of Brønsted basicity, at least when the attacking atoms is the same. This is not always true for nucleophiles having different attacking atoms. Strictly speaking, electrophilic centers are not Lewis acidic, because they do not have available empty orbitals, but rather generate empty orbitals by rehybridization or substitution. Species other than those containing electrophilic C atoms have similar behavior: this is the case, for example, for SO2 and SO3. Although the typical application of bases is in the liquid state, basicity also exists at the solid state, that is, at the surfaces of solids at solid-solid, solid-liquid, and solid-gas interfaces. In fact, solid-state reactions (such as sintering19 and the formation of salts or mixed oxides), the adsorption of chemical species on solids from solutions or from the gas phase, and heterogeneous basic catalysis all involve acid-base interactions at solid surfaces. A summary of the widely investigated basic solids is provided in Table 2. Characterization and evaluation of the basicity of solid surfaces is not an easy matter. Several chemical and spectroscopic techniques have recently been developed to do this. Several publications and reviews have been

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Table 2. Families of Solids with Basic Properties basic metal oxide families alkali on metal oxides alkaline-earth oxides alkali and alkaline-earth zeolites transition-metal, rare-earth, and higher-valency oxides hydrotalcites, calcined hydrotalcites, and spinels perovskites beta-aluminas solid metal hydroxides and carbonates basic clays (limestone, dolomite, magnesite, sepiolite, olivine) metal nitrides, sulfides, carbides, phosphides, supported metal fluorides activated carbons and impregnated activated carbons anionic exchange resins organic bases grafted on microporous or mesoporous metal oxides and other organo-inorganic solids supported or solid alkali and alkaline-earth metals or organometallics

devoted to the characterization and use of basic solids, mostly in relation to their use as heterogeneous catalysts, but also in relation to their adsorption properties and to solid and melt reactivity.20-26 In fact, acid-base interactions, mostly of the Lewis type, are also determinant in the chemistry of salt melts, such as in the production of glasses,26,27 as well as in geochemistry and soil chemistry.24 In this review, we do not address the details of solid surface basicity characterization and evaluation. In Figure 1, the structure of the surface basic sites of magnesium oxide, the most deeply investigated basic solid, are schematized according to Chisallet et al..28 The basic sites are constituted by coordinatively unsaturated oxide anions exposed on corners, edges, steps, or terraces on the surface of the cubic rock-salt-type MgO crystals. Unavoidably, coordinatively unsaturated Mg2+ ions are also present nearby, thus justifying an acid-base rather than a purely basic activity. We limit ourselves to an overview of the use of solid bases as alternatives or in comparison to the use of liquid bases in practical industrial processes. 3. Basic Compounds in Industrial and Environmental Processes 3.1. Sodium Hydroxide. Solid anhydrous sodium hydroxide (NaOH) is a polymorphic material. The orthorhombic phase, stable at room temperature, converts at 241 °C into the monoclinic phase. At 293 °C, the cubic rock-salt-type phase is formed. Anhydrous NaOH melts at 318 °C.29 Liquid NaOH boils at 1378 °C. In the solid state, several hydrated forms of NaOH · xH2O, with x ) 1, 3.5, 4, 5, and 7, exist.30 Water and sodium hydroxide monohydrate form a eutectic composition at ∼20% NaOH, melting near -30 °C. In this system, there are two maxima in the melting point corresponding to NaOH ·

Figure 1. Structure of surface basic sites on the surface of MgO nanocrystallites. (Reprinted with permission from ref 28. Copyright 2006, American Chemical Society.)

3.5H2O (i.e., ∼39% NaOH, Tm ≈ 15.5 °C) and NaOH · H2O (i.e., ∼69% NaOH, Tm ≈ 62 °C). The melting point of a commercial 50% soda solution is near 15 °C.31 NaOH has a very high solubility in water (1115 g/L, which corresponds to 52.7% soda solution, at 25 °C). At 100 °C, 3.17 kg of NaOH is soluble in 1 L of water (i.e., 76% NaOH water solution). Soda solutions are poorly volatile quite dense liquids: the density of the commercial 50% NaOH solution is 1.540 g/mL at 0 °C and 1.469 g/mL at 100 °C. The partial pressure of water over this solution at 20 °C is 0.9 Torr. In practice, sodium hydroxide is the most soluble hydroxide on a molar basis, at least at high temperature (KOH is slightly more soluble at room temperature), and consequently, it allows for the production of the most basic hot water solutions. Being predominantly produced from NaCl brines through the electrolytic “chloralkali” process,32,33 it is also quite an inexpensive base in the market. These are good reasons for its very extensive use in both industry and household applications. Next, we briefly describe some of the many industrial processes using sodium hydroxide. In Figure 2, the configurations of vessels/reactors used in some of these processes are schematized. 3.1.1. Caustic Fusion in Metallurgical Processes. Fusion in the presence of soda is used in some metallurgical processes to destroy rocks and separate elements. This is the case, for example, in the treatment of zircon minerals (zirconium silicate, ZrSiO4) at 650 °C with solid soda to produce sodium zirconate (Na2ZrO3) and sodium metasilicate. A successive treatment with water allows for the separation of insoluble hydrous zirconia.34 A similar procedure is used to recover diamonds (unreactive with soda) from rocks.35 3.1.2. Sodium Hydroxide Solutions for Neutralization of Acid Water Streams and for the Production of Basic Solutions in Industrial Chemistry and Metallurgy. Several industrial, petrochemical, and metallurgical processes use acids, for example, as the catalysts. Neutralization might be needed, for example, prior to the disposal of spent acid solutions. Sodium hydroxide can be used for this purpose. Strong basic solutions are also needed in several industrial processes, such as for “caustic leaching” of metal ions in wet metallurgy. This is the case, for example, for the Bayer process used in the production of alumina. The aluminum hydroxide based minerals bauxite, boehmite, gibbsite, and diaspore can be dissolved in NaOH solutions (110-250 °C, 100-260 g/L NaOH 36) under moderate hydrothermal conditions. Whereas most other components of the mineral are quite inert in the process, alumina dissolves as sodium aluminate. Silica, which does dissolve as well, subsequently forms a nearly insoluble compound. These features permit the formation of a sodium aluminate solution, physical separation of the impurities, and precipitation of pure Al(OH)3 from the cooled solution after neutralization. 3.1.3. Sodium Hydroxide Solutions for the Production of Sodium Organic Salts. The reaction of sodium hydroxide solutions with organic acids allows for the production of their sodium salts. This is a common practice in the field of detergent and soap production. In fact, anionic detergents, including soaps, are mostly alkali salts of organic acids. Sodium hydroxide is used in the two-step production of soaps from fatty acids,37 as well as in the production of linear alkylbenzene sulfonate (LAS) salts from the corresponding sulfonic acids for spray-dried detergents for household laundry.38 In the neutralization of alkylbenzenesulfonic acid, in general, only as much water is added as is necessary to form a flowable 50% paste. In the continuous process, this is effected by circulating a large stream


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Figure 2. Schematics of vessels/reactors for applications with caustic streams: (a) three-stage washing column for gases (Merox), (b) columns for prewashing and oxidation of sulfide compounds in liquid fuels (Merox), (c) static mixer and coalescer for caustic washing of alkylated gasoline (STRATCO-Dupont), (d) fiber-film contactor and coalescer for caustic washing (Merichem), (e) reactor for biodiesel synthesis (Lurgi), (f) semibatch spray loop reactor for alcohol polyethoxylation, (g) first-stage stirred-tank reactor for epoxy resin synthesis (Asahi) from bisphenol A (BPA) and epychloridrine (ECH), (h) second-stage continuous stirred-tank reactor for epoxy resin synthesis (Asahi).

of already neutralized product and adding sulfonic acid and sodium hydroxide to this product stream, just in front of the mixing pump.39 Similarly, alkyl sulfate salts and olefin sulfate salts are produced from the corresponding acids. 3.1.4. Sodium Hydroxide Solutions for the Extraction of Acidic Components from Organic Solutions. 3.1.4.1. Sodium Hydroxide Solutions for the Washing of Organic Streams from Mineral Acid Impurities. Organic solutions that are effluents of liquid-acid-catalyzed processes might contain traces of acids as well as of acid-soluble oils (i.e., compounds produced by the direct reaction of the organic compounds with the acid) and must be purified by neutralization. This is the case for alkylate gasoline, which is the product of isobutane alkylation with olefins (mostly isobutene), which is a key process in most refineries.5 Some commercial processes for aliphatic alkylation use concentrated (>90%) sulfuric acid as the catalyst. The reaction is performed at 20-40 °C, 0.3-0.5 MPa, in a triphasic system, with two liquid phases and organic vapors. In the STRATCO process,40 a series of several vigorously stirred “horizontal” contactors are used. The liquid reaction mixture leaves the reactor and is pumped to a settler where the two liquid phases are separated. The organic effluent, after the distillation of unconverted light reactants, is washed first with fresh acid, later with an alkaline solution, and finally with neutral water. Alkaline water washing is carried out at 50-70 °C in a static mixer/coalescer (Figure 2c) to obtain, in addition, the partial hydrolysis of alkyl sulfates, formed in traces as byproducts. In the circulating alkaline water stream, part of the spent alkaline water is withdrawn and substituted by process water with continuous addition of caustic to maintain pH ∼11. Alkaline water washing is also carried out in the production of 1,2-dichloroethane (ethylene dichloride, EDC, the precursor of vinyl chloride monomer), both by the direct chlorination process (i.e., the reaction of ethylene with chlorine in liquid phase) and by the oxychlorination process (i.e., the reaction of

ethylene with hydrogen chloride and oxygen in the gas phase).41 The liquid EDC effluent contains HCl and Cl2 impurities. Washing with dilute caustic allows for the neutralization of residual HCl, as well as the removal of molecular chlorine, which is destroyed by dismutation 2NaOH + Cl2 f NaClO + NaCl + H2O

(9)

The two phases are separated in a settler, where the organic, being heavier (d ) 1.25 g/cm3) than the alkaline water solution, is recovered from the bottom. During this operation, purified EDC is actually saturated with water and must later be dried by distillation of the water/EDC heterogeneous azeotrope, which is recycled back to the settler. Similar washing treatments are performed during other hydrocarbon chlorination processes. The Chlorex process from Merichem extracts chloride impurities from reformate gasoline with caustic.42 Esterifications, such as the production of alkyl phthalates, is mostly performed in the presence of acidic catalysts such as sulfuric acid at 15-200 °C.43 Neutralization with aqueous caustic soda is necessary. However, traces of alkali remain in the organic phase, and therefore, a water wash after the neutralization step is advisible. The mixture is stirred, and the water is then removed by distillation. 3.1.4.2. Caustic Washing for Hydrocarbon Desulfurization. Caustic washing represents an old but still used process allowing for substantial desulfurization and deodorization of hydrocarbon streams in refineries.44-46 It is based on the solubilities of the strongly bad-smelling light mercaptans in concentrated caustic solutions (5-25%), through the formation of sodium mercaptures RSH + NaOH f RSNa + H2O

(10)

as well as on the possibility of oxidizing the mercaptures with air to the almost-odorless disulfides

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2RSHa + 1/2O2 + H2O f RSSR + NaOH

(22)

Similar processes are performed in petrochemical plants to purify light olefins such as ethylene, propylene, and butylenes produced by steam cracking of naphthas. The product of steam cracking is first treated to remove heavy compounds (cracking gasoline) and later typically undergoes a caustic washing. In the ABB Lummus Global SRT cracking technology,47 8-12% sodium hydroxide is circulated over the top of a two- or threestage caustic wash tower. A water wash section is provided on top of the wash tower to remove entrained caustic. After this stage, the acid gas content in the cracked gas is less than 1 ppm. Extraction Merox48 is the trade name of a process from UOP applied to gases, C3 and C4 cuts, liquefied petroleum gas (LPG), and naphtha streams. In the process, the caustic soluble mercaptans are removed in a single, multistage extraction column (Figure 2a) using high-efficiency trays. After Merox extraction, the LPG is low enough in sulfur to be fed directly to alkylation and isomerization processes. A caustic regeneration section converts the extracted dissolved mercaptans into waterinsoluble disulfide oil. The reaction is accelerated to an economically acceptable rate at mild conditions by a proprietary Merox WS catalyst (probably a soluble metal complex49) that is dispersed in the aqueous caustic solution. Thus, disulfides are separated and removed in a settler and sent to fuel or to further processing in a hydrotreater, whereas the regenerated caustic solution is recycled back to the extraction section. In a similar process, Mericat II from Merichem, oxidation of mercaptures is performed with air and a fixed bed of carbonimpregnated catalyst in contact with gasoline in a single reactor.50,51 The Thiolex/Regen process from Merichem51 is also a two-step process allowing for the separation of mercaptans from gaseous hydrocarbons with caustic in a “fiber-film” contactor (Figure 2d), where the two phases disengage, allowing caustic to go to regeneration. Caustic regeneration is peformed through conversion of mercaptures into disulfides with air and a catalyst. Mercaptures are extracted with a solvent in a second fiber-film contactor, and caustic is recylcled to the extraction section. When applied to olefin streams from steam creacking, H2S, CO2, and other acidic oxygenated contaminants are also present.52 Caustic washing allows for the abatement of acidic gases but produces an undesirable polymeric contaminant called “red oil”. This is the result of aldolic condensation of carbonyl compounds and free-radical polymerization reactions. Chemical additives allow for the separation of red oil from spent caustic. Usually, the extraction caustic gradually loses strength through the accumulation of CO2 in the regeneration air supply or of reaction-formed water. However, the caustic retains a high level of alkalinity, making it suitable for reuse in other services, such as prewashing feed for the removal of H2S. The UOP “extraction and sweetening Merox”53 process uses a fixed-bed catalyst, air, and caustic (NaOH) to sweeten kerosene/jet-fuel feedstocks (Figure 2b). Pre- and post-treatment sections are included to ensure that other jet-fuel specifications are met. In this case, the disulfides formed remain in the treated hydrocarbon stream, but with this procedure, the bad smell is strongly reduced. Before flowing to the reactor, the charge stock is passed through a caustic prewash in order to reduce the naphthenic acids present in the kerosene feed. The reactor section of the kerosene/jet-fuel Merox unit consists of a fixed-bed reactor followed by a caustic settler. Air, the source of oxygen, is injected into the feedstock upstream of the reactor. The mixture enters the top of the reactor and percolates downward through

the catalyst bed. The kerosene leaving the caustic settler passes through a water wash, which removes trace quantities of caustic as well as water-soluble surfactants. 3.1.4.3. Caustic Washing of Vegetable Oils. Vegetable oils are treated with caustic to remove free fatty acids.54 All crude vegetable oils destined for human consumption are neutralized to remove free falty acids, albuminous, and mucilaginous matter, and thereafter, they are washed to reduce the soap content of the neutral oil to produce a more stable product. Effective neutralization results in enhanced effectiveness of subsequent steps such as bleaching, hydrogenation, and deodorizing and also results in high yields of a quality product. Neutralization also results in the removal of phosphatides, free fatty acids, and color bodies. Removal of traces of soap and moisture occurs in the washing and drying steps. The neutralization process consists of refining with an excess of 0.1% caustic soda55 and rerefining (whenever required) with two water washes and vacuum drying. In the refining and washing steps, the separation of neutral oil from soapstock and neutral oil from wash water is carried out in one or more high-g supercentrifuges. 3.1.4.4. Removal of Phenols from Organic Streams. Sodium hydroxide solutions allow for the solubilization in water of acidic organic compounds such as phenols and carboxylic acids, as a consequence of the formation of their Na salts. This approach can be applied to remove these compounds from organic mixtures. This is the case for the phenol/acetone synthesis process starting from cumene with the intermediacy of its hydroperoxide, such as the Sunoco/UOP process56 and the ABB Lummus Global process.57 In these processes, both the product acetone and the unreacted reactant cumene to be recycled contain phenol impurities. Both of these streams are purified by washing with caustic solutions, producing sodium phenate, which is later neutralized to recover phenol. In a similar way, phenol can be extracted from “carbolic oil”, namely, a phenol-rich (25%) fraction resulting from the atmospheric distillation of coal tar. The extract is later acidified using CO2 (produced by calcination of limestone), thus reproducing phenol, which is re-extracted using diisobutyl ether. The soda solution is regenerated by reacting the sodium carbonate solution with quicklime.58 3.1.5. Soda Solutions for the Abatement Acidic Pollutants from Gaseous Streams. 3.1.5.1. Abatement of Acid Compounds from Waste Gases. Waste gases arising from chemical processes involving mineral acids can contain dangerous amounts of acid compounds. This is the case for sulfuric acid catalyzed alkylation processes. In the already-cited STRATCO alkyation process,40 12% caustic solution is used to remove acid species from the waste gases arising from the blowdown treatment of spent acid in a six-tray scrubber. Caustic solutions are very active in absorbing CO2 and SO2, such as in combustion gases. However, the reaction is almost irreversible, producing sodium hydrogen carbonate and sulfite solutions. This makes regeneration of the caustic solution almost impossible. In fact, sodium sulfite solution is used in the Wellman-Lord59 absorption-regeneration process to desulfurize combustion gases allowing for the production of concentrated SO2 streams. Similarly, sodium and potassium bicarbonates are used to remove CO2 from gases in regenerative processes (see section 3.4). 3.1.5.2. Abatement of H2S in Natural and Other Contaminated Gases. Excellent removal of H2S from contaminated streams such as natural gas and biogases can be obtained with caustic solutions,60 which result in sulfide-containing solutions


H2O + 2NaOH f Na2S + 2H2O

(12)

The residual sulfur content in the treated gas can be reduced down to 5 ppmv. To obtain a continuous process, sulfide must be removed from the solution without decreasing the caustic concentration. Different methods for regeneration of the contaminated “sulfidic” caustic are possible. The Shell-Paques Thiopaq process uses aerobic bacteria of the Thiobacillus type to convert sulfide into elemental sulfur in a proprietary air-liftloop reactor operating at 30-35 °C and atmospheric pressure.44 The Thiolex/Regen process from Merichem allows for the regeneration of caustic by catalytic oxidation with air and solvent extraction of disulfides.51 Alternatively, oxidant compounds are added continuously to oxidize H2S. The oxidants recommended include chlorine, sodium hypochlorite, calcium hypochlorite, hydrogen peroxide, ozone, sodium nitrite, and potassium permanganate.61 Several processes available in the past have been proposed using iron oxide slurries.62 A solid catalyst such as a mixed oxide in a fixed bed allows for the quite selective conversion of hydrogen sulfide by sodium hypochlorite into sulfate ion, as in the ODORGARD process of ICI.63 The sulfatecontaining solution produced in this way must be disposed as the resulting waste. 3.1.5.3. Abatement of HCl and Cl2. Although abatement of HCl from gases is successfully achieved using fresh water and producing hydrochloric acid, the use of caustic is needed when Cl2 is also present. In the presence of sodium hydroxide, disproportionation of Cl2 occurs, allowing its abatement (eq 9). This is done in the case of EDC synthesis via ethylene oxychlorination.41 3.1.5.4. Removal of Phenol Vapor from Gases. In accordance with the weak acidity of phenol and the high solubility of sodium phenate in water, 3-20% sodium hydroxide is mostly used as the scrubbing agent to remove phenol from gaseous streams in industrial processes43 and to deodorize contaminated air.64 3.1.6. Sodium Hydroxide as a Basic Reactant in Organic Chemistry. Sodium hydroxide reacts as a basic reactant in organic chemistry to remove acids (such as HCl) in elimination reactions. This reaction is performed when elimination by thermal or catalytic cracking is not sufficiently selective to the wanted product. This is the case for the synthesis of vinylidene chloride (1,1-dichloroethylene), a quite significant intermediate in the production of polymers and copolymers. Vinylidene chloride is produced by reaction of 1,1,2-thrichloroethane, which is a byproduct of EDC synthesis, in VCM plants. The reaction is performed continuously using 10-15% sodium hydroxide at 60-110 °C in packed-bed reactors built of nickel alloys.65,66 Life steam is injected to distill the vinylidene chloride. After condensation, vinylidene chloride is washed with alkali and water, dried, and fractionally distilled. Unconverted 1,1,2-trichloroethane is recycled from the appropriate sections. A similar process has been used to produce chloroprene (2-chloro-1,3-butadiene, a significant monomer in the production of elastomers) from 3,4-dichloro-1-butene, with 5-15% caustic soda at 80-110 °C and atmospheric pressure.67 In both cases, high selectivities and high conversions (both >95%) are obtained. Sodium hydroxide also acts as a reactant for the production of epoxy resins from bisphenol A (BPA) and epichlorhydrine (ECH). In the Asahi Denka Kogyo two-step process,68 48% caustic is slowly added to a mixture of BPA and ECH with a large ECH excess in the first-stage stirred-tank batch reactor at ∼100 °C (Figure 2g). This reactor has an external recycle with heating. The ECH/water heterogeneous azeotrope is distilled and split into a condenser-settler, from which ECH is recycled to the reactor. Water is removed, thus shifting the equilibrium. In this stage, the reaction of a BPA molecule with two ECH

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molecules mostly occurs. After an aging period and a second injection of 48% sodium hydroxide, the polymerization is performed in a second reactor with multistage stirred chambers at 80 °C (Figure 2h). 3.1.7. Sodium Hydroxide as a Strong Basic Catalyst. Sodium hydroxide acts as a basic catalyst when it reacts to neutralize an acid and can later be recovered as such. This actually is frequently not completely true because an acidification step might be needed to recover the product. This step finally produces a sodium salt rather than reproducing the hydroxide. Thus, it is not always completely clear whether NaOH acts as a reactant or as a catalyst. Several reactions of industrial interest using liquid bases, mostly NaOH, at least formally as a catalyst can be classified as being used in the fine-chemistry field, thus being performed at small scales with very valuable reactants and products. Some examples of reactions catalyzed by homogeneous bases are reported in Table 3. Several other liquid-base-catalyzed reactions of industrial interest are described in organic chemistry books,3 as well as in refs 53 and 55-60. However, several processes are performed using NaOH as a base catalyst to produce larger-scale industrial organic intermediates.69,70 We summarize a few examples here. 3.1.7.1. Aldol Condensation and Similar Reactions. Quite concentrated caustic solutions (5-30%) produce pH ∼15, with H- values of 15-17. Under these conditions, weakly acidic organic molecules, such as carbonyl compounds with hydrogens in the R positions, can produce enolate anions to a small extent. Further aldol-type condensation reactions and the separation of the product can shift the acid-base equilibrium, allowing a significant conversion of the carbonyl compound. Aldols obtained from carbonyl compounds with more than one R-hydrogen atom can be isolated only at low temperature, because they readily lose water to form R,β-unsaturated carbonyls. This means that heat must be efficiently removed, as these reactions are exothermic. Among the many examples of high industrial interest69,70 are the synthesis of acetaldol by the self-condensation of acetaldehyde, to be further converted into 1,3-butandiene, and the synthesis of diacetone alcohol by the self-condensation of acetone,71 as the first step in the production of methyl isobutyl ketone (MIBK), a valuable industrial solvent. The acetone aldol condensation occurs at 10-20 °C in contact of an alkali solution. Similarly, the aldol condensation of butanal in the presence of caustic gives rise to 2-ethyl-3-hydroxyhexanal at 30 °C and to 2-ethyl-2-hexenal at 80-100 °C (Mitsubishi process). These are the first steps in the production of 2-ethylhexanol, an important alcohol for the synthesis of lubricants such as the ester ethylhexyl phthalate. In the Aldox process from Shell, propylene is converted with CO and hydrogen into butanal and, in one step, is also condensed and hydrogenated to 2-ethylhexanol in the presence of KOH and an oxocatalyst. Mixed condensations with aldehydes that do not have R-hydrogen atoms (such as formaldehyde) are also performed to produce, for example, hydroxypivaldehyde by the reaction of isobutyraldehyde and a 30-37% aqueous solution of formaldehyde at ca. 50 °C and pentaerythritol from formaldehyde and acetaldehyde at 35-65 °C.72 In the fine-chemicals industry, multipurpose reactors and plants are frequently used, allowing different reactions of similar types to be performed sequentially. A typical plant for industrial enol condensation might have two externally cooled stirred-tank swing reactors to allow cleaning and semicontinuous operation (Figure 373). The carbonyl compound to be enolized is premixed with caustic, and the carbonyl compound that should undergo the nucleophilic attack is fed directly in the reactor. The crystallization

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Table 3. Typical Base-Catalyzed Reactions of Industrial Interest, Usually Performed with Liquid Basesa reaction name

reactants (example)

products (example)

typical homogeneous catalyst

aldol condensation Claisen-Schmidt Henry Knoevenagel condensation Wittig Michael addition Bailis-Hilman Guerbet reaction Tishchenko Cannizzaro Meerwein-Ponndorf Oppenheimer cyanoethylation addtion of alcohols to R,βunsaturated carbonyls

heptanal with benzaldehyde 2′-hydroxycetophenone and benzaldehyde nitroalkanes with aldehydes benzyl acetone and ethyl cyanoacetate phosphonium ylide and an aldehyde methyl crotonate aldehyde + R,β-unsaturated carbonyl alcohol + alcohol acetaldehyde formaldehyde + trimethylolacetaldehyde aldehyde + alcohol alcohol + aldehyde acetone + acrylonitrile methanol + crotonaldehyde

jasminadehyde 2′-hydroxy chalcones 2-nitroalkanols 2-cyano-3-methyl-5-phenylpent-2-en-oate internal olefin 3-methyl-2-vinylglutarate allyl alcohol higher alcohol ethylacetate Pentaerythrol (+ formic acid) alcohol + aldehyde aldehyde + alcohol 5-oxo-hexanenitrile 3-methoxy-butanal

NaOH or KOH NaOH, CH3CH2ONa amine amine NaH, t-BuOK, or NaOH alkoxides DABCO t-BuOK + metal complex (CH3CH2O)3Al NaOH, Ca(OH)2 aluminum isopropylate aluminum alkoxides isopropylamine NaOH

a

References 213-220.

with respect to the oil. Methanol excess is used to displace the equilibrium, with a typical methanol-to-triglyceride molar ratio of 6:1. Sodium methoxide attacks the fatty ester, giving rise to an acylic nucleophilic substitution reaction (RCOO)3C3H5 + 3CH3ONa + 3H2O f C3H5(OH)3 + 3RCOOCH3 + 3NaOH

Figure 3. Sketch of a reactive crystallization process for aldol cross condensation. (Reprinted with permission from ref 73. Copyright 2005 Elsevier.)

of the product, whose mass is greater than that of reactants, allows its separation and the displacement of the equilibrium. Interestingly, aldol condensation catalyzed by caustic can also be used to purify organics from contamination. This is what occurs in acetone and phenol synthesis by cumene oxidation.56 The aldehyde impurities are removed from acetone in the FAC (finished acetone column). Caustic is injected into the FAC column to catalyze the condensation of trace aldehydes.The heavier condensation products are less volatile and leave with the FAC bottoms. 3.1.7.2. Production of Biodiesel. Biodiesel is a mixture of methyl esters of fatty acids mostly produced by the transesterification of triglyceride constituents of vegetable oils with methanol in the presence of an alkaline catalyst, mostly sodium hydroxide.74,75 Because the acidities of water and methanol very similar, the methanol/dry sodium hydroxide mixture produces the active nucleophilic reactant, namely, sodium methoxide NaOH + CH3OH f CH3ONa + H2O

(13)

The water content in the mixture should actually be as low as possible. Typically, the NaOH content should be ∼1% by weight

(14)

The reaction occurs at 30-80 °C, under stirring, in a biphasic system because the oil and the methanol/soda solutions almost immiscible. To allow for a single-step process, the triglicerides to be treated should have a very low free fatty acid content, to avoid the formation of soaps. Alternatively, free fatty acids should be esterified in a preliminary step. In the commercial Lurgi biodiesel process, the transesterification reaction is performed in the mixing section of a twostage mixer-settler unit (Figure 2e)76 at atmospheric pressure and ∼60 °C. The subsequent settling section allows for the separation of methyl esters as the light phase from glycerine/ water as the heavy phase. A recycle at the bottom of the reactor allows for the partial recovery of sodium hydroxide. A subsequent countercurrent washing step for the methyl ester removes minute byproduct components and gives a biodiesel that is “ready for use” after the final drying step. The surplus methanol contained in the glycerine/water is removed in a rectification column, which yields methanol in a condition and purity ready for use as a recycle stream to the process. For further glycerine/water purification, additional steps of chemical treatment, evaporation, distillation, and bleaching might optionally follow. A glycerine distillation unit allows pharmaceutical-grade material with a quality of >99.5% to be obtained. The process is highly efficient: 1 kg of raw material yields 1 kg of biodiesel. Low catalyst consumption (5 kg of sodium methylate per ton of diesel fuel produced) is likely obtained with the reactor system, as most of alkali is retained at the bottom of the mixer section. 3.1.7.3. Sodium Hydroxide as the Catalyst of Polyalkoxilation. Polyethoxylation and polypropoxylation reactions are performed industrially to prepare nonionic surfactants and polymers. The starter in the synthesis of a nonionic surfactant is most frequently a fatty alcohol or an alkyl phenol, which is reacted with ethylene oxide or propylene oxide, so as to insert a hydrophilic head in the molecules. The reaction is normally promoted by alkaline catalysts,77 such as NaOH, and is frequently performed in stirred-tank reactors or in recycle reactors such as semibatch spray loop reactors (Figure 2f). A concentration of catalyst of 650 °C) circulating catalyst, continuously moving to the reactor top from the bottom of the regenerator. The catalyst cools again (55 wt % CuO, 21-25 wt % ZnO, and 8-10 wt % Al2O3 in the fresh catalyst, at 200-250 °C and 50-150 bar, in the so-called low-temperature synthesis process,299,300 in multibed cooled tubular reactors. In the older high-temperature process, working at 320-380 °C and 340 bar, more sulfur-resistant catalysts belonging to the ZnO-ZnCr2O4 system were used. A key feature of these catalysts is the ability to adsorb hydrogen dissociatively, with a proton adsorbed on basic oxygen sites, thus producing very active hydride species over cationic sites. These species have been identified as terminal zinc hydrides over hightemperature catalysts,302 whereas over Cu-containing lowtemperature synthesis catalysts, they very likely bridge over two or three metal atoms.303 Both low-temperature and high-temperature methanol synthesis catalysts, when modified by alkali, allow for the production of mixtures of methanol and higher alcohols. These mixtures can be used as octane boosters in gasoline, in contrast to the pure-methanol/gasoline mixture that tends to split into two phases in the presence of moisture. In the Octamix process from Lurgi,300 an alkized Cu-ZnO-Al2O3 or Cu-ZnO-Cr2O3 lowtemperature methanol synthesis catalyst is used at 285-300 °C and 60-90 bar, giving rise to methanol/ethanol/2-propanol/ isobutanol mixtures, whereas in the Snamprogetti MAS process,304,305 K-doped ZnO-ZnCr2O4 catalysts work at 350-400


°C and 100-150 bar, producing essentially a methanol/ isobutanol mixture. This mixture can be used to produce ethers with the Ethermix process.305 In an alternative process, Substifuel from IFP, an alkalized modified Fischer-Tropsch catalyst (alkali-doped Co-Cu-ZnO-Al2O3) is used at 270-320 °C and 60-100 bar, to produce mixtures of linear higher alcohols and methanol.306 Alkali doping certainly promotes C-C bond formation in all cases, but with different mechanisms. In the case of the IFP process, the mechanism is similar to that of Fischer-Tropsch linear hydrocarbon synthesis. In the case of the Lurgi and Snamprogetti processes, aldol condensation of an acetaldehyde-like intermediate is likely.307-309 A further possibility consists in the production of small branched hydrocarbons (mainly isobutane) from syngas 300,310 which occurs with the so-called Isosynthesis processes. They are performed over zirconia or thoria based catalysts at T > 400 °C at 30-60 MPa. Also, in this case, the basicity of the catalyst probably is needed to produce C-C bond formation through an aldol-like mechanism. 3.10.2.4. Methane Oxidative Coupling Catalysts. In the recent past, much work was devoted to the development of the catalysts allowing methane to be used as a chemical reactant. Among these potential applications, much effort has been devoted to the investigation of methanol oxidative coupling,311 to produce ethylene and/or ethane. Typical reaction conditions are temperatures above 700 °C with a significant excess of methane. Most of the catalytic systems allowing some activity and selectivity are strongly basic. MgO catalysts are active, their activity being further enhanced by doping or impurities such as Li+, Na+, and Ca2+. Other active catalysts are rare-earth oxides such as La2O3, Nd2O3, and Sm2O3312 and combinations of alkaline-earth and rare-earth oxides such as SrO/La2O3. Among the reasons for basicity being a requirement are likely limiting the electrophilic reactivity toward the olefin double bond of ethylene, avoiding coking, and possibly achieving the first hydrogen-abstraction step, although heterogeneous/homogeneous radical chemistry is certainly involved in the complex mechanism. Despite the interest in this reaction, the performances obtained have been considered too low for commercialization. Maximum selectivities at 10-15% and 25% conversions were 85% and 80%, respectively. With removal of ethylene and recycling, olefin product yields of more than 70% are possible.312 This reaction can also be considered as a first step for methane aromatization.313 4. Conclusions The data summarized above demonstrate how important the role of basic compounds is in industrial and environmental chemical processes. Basic materials are needed to abate several air pollutants, such as CO2, SO2, NO2, and H2S, that have acidic character. Bases are also very useful for neutralizing liquid acids and catalyzing many specific organic reactions. The development of useful solids to substitute caustic soda, potash, amines, and alkoxides is certainly a very interesting target to limit the production and consumption of these compounds, as well as the production of toxic wastes. A decrease in the consumption of caustic soda could also go in parallel with the limitation of the chemistry of chlorine (coproduced with NaOH in the chloralkali process) suggested by the many environmental concerns about chloro-organic compounds. Because of the toxicity of some of the basic compounds used in industry, such as amines and phosphines and caustic soda

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itself, as well as some risks related to their use, the development of basic solids will improve safety in the chemical industry. Regenerable solid adsorbents are already in use in several processes. However, the development of new solid materials for the abatement of air and water pollutants is an important objective. In this field, we mentioned the development of solids for CO2 sequestration, hot gas cleaning, and biogas purification, as well as new processes for the adsorptive hydrocarbon desulfurization. The use of solid catalysts is certainly very desirable and convenient in the large-scale production of petrochemicals. However, the use of basic solids as catalysts in the finechemicals industry can give rise to problems. In fact, in this field, batch multipurpose tank reactors are mostly used, whose application with liquid mixtures is easy. The use of fixed-bed heterogeneously catalyzed processes in fine chemistry might not always be practical, because of the necessity of using different catalysts (with finely tuned compositions and morphologies) for particular reactions. New reactor concepts could help in this area. Operation in slurry heterogeneous conditions could be easier, however. The development of new processes based on solid bases might be needed in order to develop a “green” industrial chemistry largely based on renewables. Among possible new basic materials are better catalysts for biodiesel production, new adsorbents for bioethanol/water separation, and new catalysts for biomass pyrolysis and biomass steam reforming finally producing “bio-hydrogen”, as well as new processes for the production of biomonomers and biopolymers. However, the investigation of basic materials is perhaps more difficult than for acid ones, as the level of understanding and tuning surface basicity is still at an early stage. It seems that much effort should still be made for a better understanding of basicity in the solid state as a step for the development of new basic materials and new industrial processes based on them. Literature Cited (1) Kirk-Othmer Encyclopedia of Industrial Chemistry, 4th ed.; Wiley: New York, 1998. (2) Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co.: London, 2002. (3) Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic Chemistry; Oxford University Press: New York, 2001. (4) Olah, G. A.; Molna´r, A. Hydrocarbon Chemistry, 2nd ed.; Wiley: New York, 2003. (5) Busca, G. Acid catalysts in industrial hydrocarbon chemistry. Chem. ReV. 2007, 107, 5366–5410. (6) Hocking, M. B. Handbook of Chemical Technology and Pollution Control; Academic Press: San Diego, CA, 1998. (7) Arrhenius, S. A. Recherches sur la conduicibilite´ galvanique des e´lectrolithes, Ph.D. Thesis, University of Uppsala, Uppsala, Sweden, 1884. Arrhenius, S. A. Uber die Dissociation der in Wasser gelo¨sten stoffe. Z. Phys. Chem 1887, 1, 631–648. (8) Brønsted, J. N. Some remarks on the concept of acids and bases. Recl. TraV. Chim. Pays-Bas 1923, 42, 718–728. (9) Lowry, T. M. The uniqueness of hydrogen. Chim. Ind. (London) 1923, 42, 43–47. (10) Atkins, P. N. Physical Chemistry; Oxford University Press: Oxford, U.K., 1997; p 294. (11) Conant, J. B.; Wheland, G. W. The study of extremely weak acids. J. Am. Chem. Soc. 1932, 54, 1212–1221. (12) McEwen, W. K. A Further Study of Extremely Weak Acids. J. Am. Chem. Soc. 1936, 58, 1124–1129. (13) Hammett, L. P. Physical Organic Chemistry; McGraw-Hill: New York, 1940; Chapter IX. (14) Bowden, K. Acidity Functions for Strongly Basic Solutions. Chem. ReV. 1966, 66, 119–131. (15) Koppel, I. A.; Schwesinger, R.; Breuer, T.; Burk, P.; Herodes, K.; Koppel, I.; Leito, I.; Mishima, M. Intrinsic Basicities of Phosphorus Imines and Ylides: A Theoretical Study. J. Phys. Chem. A 2001, 105, 9575–9586.

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ReceiVed for reView December 5, 2008 ReVised manuscript receiVed March 20, 2009 Accepted May 2, 2009 IE801878D

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