Solid Acid Catalysts for Industrial Condensations of Ketones and

Structure–activity studies of dodecatungstophosphoric acid impregnated bentonite clay catalyst in hydroxyalkylation of p-cresol. A.C. Garade , V.S. ...
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Ind. Eng. Chem. Res. 2004, 43, 1169-1178

1169

REVIEWS Solid Acid Catalysts for Industrial Condensations of Ketones and Aldehydes with Aromatics A. de Angelis,* P. Ingallina, and C. Perego EniTecnologie, via Maritano 26, I-20097 San Donato Milanese, Italy

The condensation reaction of ketones or aldehydes with aromatics (hydroxyalkylation) is applied in the production of many commodities and fine chemicals. Such a reaction is usually catalyzed by strong mineral acids, which have to be neutralized at the end of the reaction and the resulting salts disposed. In this review, we illustrate the results obtained by several industrial and academic research centers having as their target the substitution of mineral acids by regenerable and nonpolluting solid acid catalysts. In many cases, the results obtained have not allowed for the industrial development of the proposed alternatives, but in one case, the solid-acid-based process (BPA production) has already overcome the traditional one and in a few cases solidacid-based processes seem to have reasonably chances to do the same in a near future. 1. Introduction The condensation of molecules containing carbonyl groups, such as aldehydes and ketones, with aromatic compounds, mainly containing activating groups on the aromatic ring, is a reaction widely applied in the chemical industry to produce two important commodities, bisphenol A and methylendianiline (Table 1). Moreover, this reaction is used in the production of several fine chemicals, most of which are obtained through the condensation of a molecule containing a carbonyl group and two molecules of phenol (Figure 1). 1.1. Mechanism of the Hydroxyalkylation Reaction. The condensation reaction (hydroxyalkylation) of an aromatic ring with an aldehyde or a ketone is a typical aromatic electrophilic substitution. Usually, the reaction is acid catalyzed, and the alkylating species is the carbonium ion1 (Scheme 1a).

This reaction usually does not stop at the first step, and the alcohol initially produced reacts with another molecule of aromatic compounds, giving rise to a diarylation reaction, as illustrated in Scheme 1a. The reactivity of the aldehyde (or ketone) depends strongly on the groups R and R′. Aliphatic aldehydes (R ) aliphatic, R′ ) H) are very reactive, formaldehyde (R ) H, R′ ) H) and short-chain-substituted aldehydes (R ) CH3-, ClCH2-, Cl2CH-, or CCl3-) being the most reactive. Aromatic aldehydes are far less reactive than aliphatic ones, unless one or more electron-attractive groups (e.g., NO2) are present on the aromatic ring.2 Hydroxyalkylation, being an eletrophilic substitution, takes place on aromatic rings with strong activating * To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Products derived from phenol through hydroxyalkylation with different aldehydes and ketones.

Scheme 1

groups such as -OH or -NH2 that are para-ortho orientating. In the presence of very reactive aldehydes, hydroxyalkylation can also be performed on benzene or on aromatic rings with deactivating groups. [For example, trichloroacetaldehyde reacts with chlorobenzene, readily producing 1,1-bis(para-chlorophenyl)-2,2,2 trichloroet-

10.1021/ie030429+ CCC: $27.50 © 2004 American Chemical Society Published on Web 01/28/2004

1170 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Table 1. Main Industrial Products Derived from the Hydroxyalkylation Reaction aromatic

aldehyde or ketone

product

catalyst

phenol phenol phenol phenol phenol phenol aniline benzene o-xylene chlorobenzene

formaldehyde formaldehyde acetone cyclohexanone 2-pyridinaldehyde levulinic acid formaldehyde aloxane acetaldehyde trichloroacetaldehyde

bisphenol F resol and novolac resins bisphenol A (BPA) bisphenol Z bisacodyl derivatives 4,4′-bis(4-hydroxyphenyl) pentanoic acid methylendianiline (MDA) 5,5′ diphenylbarbituric acid 1,1-bis(3,4-dimethylphenyl)ethane (DXE) 1,1-bis(parachlorophenyl)-2,2,2 trichloroethane (DDT)

HCl HCl, H2SO4, or oxalic acid HCl ion exchange resins HCl or H2SO4 H2SO4 HCl HCl oleum (20% SO3) H2SO4 H2SO4

hane (DDT), which was one of the most widely used insecticides around the world.] 1.2. General Features of Industrial Hydroxyalkylation Processes. Industrial hydroxyalkylation processes are usually catalyzed by strong mineral acids (HCl, H2SO4, H3PO4), which are highly corrosive. These acids are dangerous to handle and to transport as they corrode storage and disposal containers; moreover, relevant parts of the plants in which they will be used have to be constructed using expensive special steels that can resist such corrosion. Furthermore, in many processes, the products need to be separated from the acid, and this operation is usually costly. Finally, these acids must often be neutralized at the end of the reaction, giving rise to significant quantities of inorganic salts contaminated by aromatic byproducts that have to be disposed. To avoid these problems, many efforts have been directed at the search for solid acid catalysts, which should be more safe and more environmentally friendly than mineral acids. In addition, solid acid catalysts can be regenerated and reused after the reaction, and in many cases, they show better selectivity than mineral acids toward a specific isomer in a mixture of reaction products. This review illustrates the most important results described in the literature in the condensation of carbonyl groups with aromatic compounds using different solid acid catalysts. The solids acid catalysts investigated as substitutes for mineral acids in hydroxyalkylation reactions are characterized by different compositions and properties and belong to various classes of catalysts such as zeolites,3 heteropolyacids,4 exchange resins,5 zeotypes such as SAPO and AlPO,6 clays,7 fluorinated graphite,8 intermetallic compounds,9 and mesoporous silicaalumina.6 2. Fine Chemicals Production Most of the products summarized in Table 1 are fine chemicals used as drugs and specialities. Even though the production volumes in these cases are relatively small, substitutions of traditional catalysts by solid acids were extensively considered. A new method for the production of benzyl alcohol, a constituent of jasmine oil widely used in perfumery and flavor industries, was proposed10 as a “green” alternative route to current production by benzyl chloride hydrolysis. Benzyl alcohol could be obtained in excellent yield through the condensation of benzene with formaldehyde (Scheme 2a) catalyzed by zeolite ZSM-5 treated with phosphorus or boron. The condensation of 2 mol of benzene with formaldehyde in a biphasic system catalyzed by heteropolyacids (H3PW12O40)11 gives diphenylmethane, used in the fragrance industry and as a stabilizer for polyesters

Scheme 2

(Scheme 2b), in excellent yield. The catalyst can be recycled simply by drying the aqueous phase. The same authors12 claimed a supported strong cationic exchange resin, Aciplex-SiO2, to be an active and selective catalyst; in this case, both the catalyst productivity and resistance to water were increased. p-Vanillol (2-methoxy-3-hydroxybenzyl alcohol), which is an intermediate in the synthesis of vanillin, can be produced in fair yield through the condensation of guaiacol with formaldehyde (Scheme 2c) in the liquid phase, catalyzed by H-mordenite and H-beta zeolites.13 1,1-Bis(3,4-dimethylphenyl)ethane (DXE), which is an intermediate in the production of polyimides and other thermoplastic materials with chemical inertness at high temperature, is industrially obtained by the condensation of acetaldehyde with o-xylene in the presence of a large excess of sulfuric acid (Scheme 2d). Zeolite Y was shown14 to be an excellent catalyst in DXE synthesis with a selectivity to the desired product of more than 95%; furthermore, the catalyst could be easily regenerated at the end of the reaction. The catalyst proved to be effective at a catalyst/acetaldehyde weight ratio of 4.5, but it is not known whether the performance of zeolite Y at a lower catalyst/acetaldehyde weight ratio would still be suitable. 3. Bisphenol Synthesis Bisphenols are formed by the acidic or, under particular reaction conditions, basic condensation of 1 mol of ketone (or aldehyde) with 2 mol of phenols. Aldehydes, and particularly formaldehyde, react with phenols and monosubstituted phenols under acid catalysis to produce complex mixtures of isomers and polynuclear bisphenols.15 The most important product of the condensation of formaldehyde with phenol consists of resol and novolac resins,16 which are widely used for the production of glues and moulding materials, as well as in the painting industry.

Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1171 Scheme 3

Scheme 5

Scheme 4

Scheme 6

In contrast, bisphenol F [bis-(4-hydroxyphenyl)methane], a product of the condensation of 2 mol of phenols with 1 mol of formaldehyde (Scheme 3), has moderate importance as a precursor of liquid bisphenol F resins.17 Bisphenol F is mainly produced in Japan, and its production is estimated to be about 3000 metric tons per year.18 Bisphenol F synthesis is usually performed using HCl as the catalyst. Solid acids, such as ZSM-5 and related zeolites, have reportedly been used as catalysts to produce bisphenol F with good selectivity and complete formaldehyde conversion.19 Recently, a new method of producing bisphenol F with zeolite beta was illustrated.20 The catalyst was very selective, but no data regarding the productivity of the catalyst or its possible regeneration were provided. Aside from reacting phenols with aldehydes, bisphenols of industrial relevance are produced mostly through the condensation of a ketone with phenol. 3.1. Principal Bisphenols Produced and Their Uses. The most important bisphenol produced is bisphenol A (p,p′-isopropylidenediphenol) (BPA), which derives from the acid-catalyzed condensation of 2 mol of phenol with 1 mol of acetone (Scheme 4). World capacity of bisphenol A is 2.67 millions of metric tons (2001) and the four main producers are General electric (25% of world installed capacity), Bayer (16% of world installed capacity), Shell (12% of world installed capacity) and Dow Chemical Corp. (10% of world installed capacity). The world capacity of BPA is increasing with an average annual growth rate, in the past decade, between 6.5 and 7%. Bisphenol A is mostly used for the production of polycarbonates (65% of Bisphenol A produced),21 and epoxy resins. These two polymers employ more than 90% of the Bisphenol A produced; while the remaining 10% is used to produce some fine chemicals. the latest are brominated bisphenols A15 (mainly tetrabromobisphenol A), which are flame retardant, and bisphenol A with alkylic groups on aromatic ring,15 which are used as antioxidant and stabilizers. Other bisphenols of industrial importance are bisphenol Z (Scheme 5-a) and 4,4′ bis(4-hydroxyphenyl) pentanoic acid (Scheme 5-b). Bisphenol Z, which derives from the condensation of 1 mol of cyclohexanone with 2 mol of phenols, is used for the production of polycarbonates films16 and has a worldwide production of about 1360 metric tons per year.22 4,4′-Bis(4-hydroxyphenyl) pentanoic acid is produced by condensing 1 mol of levulinic acid (4-oxopen-

tanoic acid) with 2 mol of phenol and is used to link polyester resins with phenolic resins. 3.2. Industrial Bisphenol A Production. Two industrial processes are used for the production of bisphenol A that differ primarily in terms of the catalyst used: hydrochloric acid or ion-exchange resins,23,24 mainly sulfonic resins. The ion-exchange-resin-catalyzed process requires a treatment of the resin with an aminomercaptan25-27 as a promoter, which is linked to the sulfonic group of the resin through an ionic bond. The positive effect of the promoter is attributed to the formation of a more stable carbonium ion, which can exist in higher concentration and thus alkylates the phenol ring more rapidly.28 In the industrial process, bisphenol A is always produced (Scheme 4) together with sizable quantities of o,p′-bisphenol A (o,p′-BPA/BPA molar ratios up to 1:2); a few percent of trisphenols (BPX), products of the condensation of 1 mol of BPA with 1 mol of acetone and 1 mol of phenol; and low amounts (usually up to 1 mol % of the amount of BPA) of chromans. The processes for BPA production are usually performed in six units: (1) a reactor for the condensation of phenol and acetone; (2) a unit for the crystallization of the bisphenol-phenol adduct; (3) the stripping tower, where the adduct is cracked and phenol is recycled; (4) a unit for the recrystallization of the bisphenol produced; (5) a process unit for the recovery of o,p′-bisphenol A via cracking (Scheme 6); and (6) a unit for the treatment of the wastewaters, which contain inorganic salts and traces of phenol and the reaction products.29 (1) In the reactor (a fixed-bed reactor for the ionexchange-resin process and a continuous stirred tank reactor for HCl-catalyzed process), acetone and phenol react together to produce BPA, along with o,p′-BPA, BPX, and low amounts of other byproducts. The process based on acid resins is a heterogeneous process; therefore, the reaction temperature is higher (90 °C) than in the case of HCl-based process (40-50 °C). Because of corrosion problems, relevant parts of the HCl-catalyzed process plant have to be made of monel alloys (i.e., heat exchangers) or must be glass lined (i.e., reactors). On the other hand, the whole plant for the process based

1172 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004

on ion-exchange resins is made of 304 or 316L stainless steel, which is less expensive.28 This difference has an important influence on the total cost of the plant. In fact, for example, a bisphenol A plant using ionexchange-resin catalyst to produce 200 million pounds per year requires a total project investment of about $101,000,000, compared to about $244,000,000 for a plant using HCl catalyst.28 (2) In the crystallization unit, BPA is separated from the reaction products as the 1:1 molar adduct BPAphenol, which is filtered and washed with pure phenol. No significant differences between the two processes exist in this unit. In the Sinopec-Lummus process,30 the specifically designed crystallization unit produces crystals of large sizes that are easily separated and washed to remove impurities. (3) The crystalline BPA-phenol adduct is then cracked to obtain pure BPA and phenol, which is stripped and recycled. The reaction is thermal, and complete conversion is obtained. (4) In the ion-exchange-resin process, BPA is crystallized once again with a different solvent, usually toluene, to increase its purity. The purity of the product obtained in the Dow High-Purity Generation II process (high-temperature and high-pressure crystallization; water as the solvent) is the highest known to date, with a yellowness index (YI) lower than that of any other BPA produced.28 (5) A special unit for the treatment of o,p′-BPA is needed in the process based on the ion-exchange resin. In contrast, the concentration of o,p′-BPA in the HClbased process is not as high, and such a unit is not needed. In this unit o,p′-BPA is cracked to obtain phenol and p-isopropenylphenol (Scheme 6), which are fed again to the reactor for the synthesis of BPA. (6) The section for the treatment of the wastewater is minimal for the process based on ion-exchange resins, because of the very low amount of wastewaters produced by this kind of process. Larger amounts of wastewaters are produced by the HCl-based process; some quantities of the acid are lost in these wastewaters and have to be neutralized at the end of the process. Most of the HCl is recovered through an extractive distillation treatment using concentrated solutions of calcium chloride. Gaseous HCl is recycled to the BPA synthesis reactor, and calcium chloride solution is concentrated for reuse. Water treatment also has to be performed in the ionexchange-resin process because the aminomercaptan used as a promoter is dissolved in the reaction solution, except for the Dow High-Purity Generation II process. Many ion-exchange resin processes also require a stripping tower for the separation and recycling of unreacted acetone from the BPA reactor. In fact, the process based on HCl as the catalyst results in complete acetone conversion, whereas ion-exchange-resin processes reach only 90-97% acetone conversion. In this respect, the Sinopec-Lummus process30 represents an exception, as complete acetone conversion is obtained. This is because the nitrogen that is fed countercurrent to the stream of the reactants (acetone and phenol) removes the water produced during the reaction. 3.2.1. Solid Acid Catalysts for BPA Production. The first patent in which the synthesis of BPA using solid acid catalysts other than ion-exchange resins is described was granted more than 30 years to Mobil Oil Corporation.31 In this patent, X and Y zeolites were used

Table 2. BPA Synthesis with Heteropolyacidsa,b

catalyst H3PW12O40 H3PW12O40 on MCM-41 Cs2HPW12O40 on MCM-41 H-ZSM-5

selectivity (%) phenol conversion alkylated (%) BPA o,p′-BPA phenols chromans 40.6 34.4

55 50

15 18

28.1

60

10

2

0

1

14 11 7.8 98

5 7 10 0

Reaction conditions: acetone/phenol molar ratio ) 1/3, 160 °C, 6 h. b Reference 34. Note that the mesityl oxide selectivity is not reported in this reference. a

as the solid acid catalyst; the conversion of reactants was poor, between 1 and 6%, and data regarding the selectivity of the reaction were not reported. Non-zeolitic molecular sieves, such as SAPO, ELAPSO, AlPO, MeAPO, FeAPO, and TAPO, were claimed to be active catalysts in BPA synthesis in 1988.32 However, the BPA yield (defined as the product of the conversion of acetone with the selectivity to BPA) obtained using such catalysts was always less than 30%, which is not satisfactory for industrial development. Recently, Texaco patented the use of montmorillonite clay, pretreated with hydrogen fluoride or fluorosulfonic acids and mineral acids, as an active catalyst for BPA synthesis.33 Using catalysts of this type, high acetone conversions (80%) and BPA selectivities (56%) can be obtained, but leaching of the liquid acid can reasonably occur after some time. In fact, such a catalyst is not a truly solid acid but, more likely, an acid-impregnated solid. Heteropolyacids encapsulated into mesoporous molecular sieves, such as MCM-41, were also demonstrated to be active catalysts in BPA synthesis.34 In this case, according to the authors, the heteropolyacid is present as the free acid and as the acidic cesium (Cs2HPW12O40) or ammonium [(NH4)2HPW12O40] salts. The formation of the heteropolyacid salt prevents extraction of the acid from the catalyst and its dissolution in a polar solvent, such as phenol. Formation of the salt had to be carried out in a very specific way to prevent the blockage of the pores. The catalyst based on Cs2HPW12O40 was found to be the best one of this type. In fact, even though the conversion of phenol was lower than with free H3PW12O40 (28.1% vs 40.6%), the selectivity to BPA reached 60%, with more than 10% o,p′-BPA. Unfortunately, the selectivity to chromans, unwanted byproducts, was about 10% (Table 2); this value is more than 10 times the selectivity to chromans (less than 1%) obtained with the ion-exchange-resins-based process. A further problem in the use of this kind of catalyst could be the regeneration of the spent catalyst. In fact, the thermal stability of heteropolyacids and related salts usually does not extend to the temperature (500-550 °C) at which the calcination of a spent catalyst is usually performed to burn out carbonaceous deposits.35,36 In this paper, supported heteropolyacids were compared with zeolite H-ZSM-5, which was claimed to produce only traces of alkylated phenol and some amount of mesityl oxide. This result can be explained by the pore diameter of ZMS-5, which is too narrow for the formation of the bulky products of the condensation of phenol and acetone.34 Recently, large-pore zeolites with spaciousness indexes [an effective measure of the zeolite pores width37,38

Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1173 Scheme 7

Figure 2. Spaciousness indexes of different zeolitic structures (adapted from ref 37). Table 3. BPA Synthesis with Zeolitesa,b catalyst

acetone molar conversion (%)

BPA

beta zeolite Y ERB-1 ZSM-12c

99.9 90.9 81.5 76.4

48.97 38.4 44.2 11.4

selectivity (%) o,p′-BPA BPX chromans 27.3 21.5 21.3 7.9

23.7 40.1 47.3 34.5

Scheme 8

0.01< 0.01< 0.01< -

a Reaction conditions: acetone/phenol molar ratio ) 1/5, 180 °C, 12 h in a closed autoclave. b Reference 39. c Mesityl oxide is 46.22% of the products.

(Figure 2)] greater than 8 were claimed to be active catalysts for BPA synthesis.39 The best results were reported to be obtained with zeolite beta; in this case, complete acetone conversion and selectivities to BPA of 49% and to o,p′-BPA of 27% were reached. The most interesting feature of these zeolites (e.g., beta and ERB-1, which is isostructural with zeolites PSH-3, SSZ-25, and MCM-22) was that no chromans could be detected in the reaction mixture (Table 3). The absence of unwanted chromans could be explained as an effect of the “shape selectivity” of the zeolite pores: possibly, BPA is formed within the pores, whereas the hindrance of chromans (geometric hindrance ≈ 11 Å) is not compatible with the size of the pores (e.g., zeolite beta has pores with openings of dimensions 5.7 × 7.5 and 6.5 × 5.6 Å2). An interesting property of these catalysts is that they can be completely regenerated by treatment with a specific solvent at a temperature preferably higher than the reaction temperature (rejuvenation). In the example reported in the patent,39 the spent catalyst was rejuvenated by treatment with phenol at 280 °C for a period of 8 h. When the catalyst is treated with phenol, or similar compounds, a transalkylation of the compounds formed inside the pores is likely to occur, giving rise to the formation of lower-molecular-weight compounds, which can be dissolved in the solvent. This explanation is in accordance with the observation that the use of a non-transalkylating solvent, such as decane, does not have any rejuvenating effect on the catalyst. After several rejuvenating cycles, a conventional thermal treatment is advisable to restore the catalyst activity. In conclusion, rejuvenation allowed for a delay to the thermal regeneration, limiting possible stresses in the catalyst crystal structure and lowering the energy consumption. 4. Methylenedianiline Synthesis Methylendianiline (MDA), a product of the condensation of 2 mol of aniline with 1 mol of formaldehyde, is used as a precursor, through phosgenation, of methylendiisocianate (MDI) (Scheme 7). During 1999, 1.02 million metric tons of MDI were produced in U.S. and 1.08 million in Western Europe, with an average annual growth rate, in the past 5 years, of 7% in the U.S. and 5% in Western Europe.40

MDI reacts with polyols to produce polyurethans. Polyurethanes are widely used for the production of elastic foams (mattresses, cushions, car seats),41 rigid foams (insulation materials), rigid and flexible moldings with compact skins (window frames, housings, skis, etc.), and engineering moldings with high hardness and elasticity. The industrial production of MDA is performed by adding formaldehyde to stoichiometric amounts of hydrochloric acid and aniline at 60-80 °C in an agitated reactor.42 The reaction mixture is then heated at 100160 °C for about 1 h to complete the condensation. The reaction mixture is then neutralized with an excess of NaOH, producing almost the stoichiometric amount of sodium chloride. The reaction mixture separates into two layers, an organic layer and an aqueous layer that is discharged. The aqueous phase contains mainly sodium chloride and traces of aromatic amines, which are present as contaminants; therefore, a section for the biological treatment of the organic pollutants has to be present. The organic phase is distilled to remove unreacted aniline, and crude MDA is obtained. MDA is obtained together with low amounts of o,p′MDA and o,o′-MDA (Scheme 8) and significant quantities of oligomeric isocianates with a functionalities of 3 and 4 (PMDA). The MDA content in the reaction mixture is about 70-75%, and the sum of the contents of o,p′-MDA and

1174 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Scheme 9

o,o′-MDA is about 3-5%, the remaining part (20-25%) being PMDA. Both MDA and PMDA are valuable intermediates, and their isocianates are industrial products, even though the current price of MDI is higher than that of PMDI. The main unwanted product in MDA synthesis is N-methylated MDA.42 N-methylated MDA derives from the Plo¨chl reaction,43 which consists of a redox reaction of formaldehyde according to

RNH2 + 2CH2O f RNHCH3 + HCOOH N-methylated MDA is a very noxious byproduct in MDA synthesis because it cannot be transformed in MDI through reaction with phosgene (Scheme 9). Having only one isocyanic group per mole, N-methylated MDA interrupts the polymerization chain with polyols, and the free acidity of the N-methylated amine lowers the stability of the polyurethane polymer. The product distribution can be varied by changing the aniline/formaldehyde molar ratio, the HCl content, and the reaction temperature. As the aniline/formaldehyde ratio is increased, the content of MDA in the reaction mixture increases, and the content of PMDA decreases. Unreacted aniline has to be distilled off at the end of the reaction, to avoid the formation of unwanted phenylisocianate, and therefore, increasing the aniline/formaldehyde ratio increases the cost and the volume of the distillation apparatus. The higher the content of hydrochloric acid, the higher the concentration of MDA in product mixture and the ratio of MDA/(o,p′-MDA + o,o′-MDA). However, higher contents of HCl, obviously, also result in higher amounts of sodium hydroxide that must be used at the end of the process to neutralize the HCl and higher amounts of salt produced. The MDA/(o,p′-MDA + o,o′-MDA) ratio is also influenced by the reaction temperature: increasing the reaction temperature increases the concentrations of o,p′-MDA and o,o′-MDA and lowers the concentration of MDA. These two isomers (o,p′-MDA and o,o′-MDA) are added to the polymeric fraction of PMDI, after phosgenation, because their reactivities to polymerization are different from that of MDI. In some cases, significant concentrations of o,p′-MDI (up to 25%) are added to MDI to lower the melting point of the isocyanate and simulate the behavior of tolylene diisocianate (TDI).44-46 4.1. Solid Acid Catalysts for MDA Production. In the MDA process, the substitution of HCl by a solid catalyst is highly desirable, avoiding the production of brines contaminated with aromatic amines, which necessitate special disposal treatments. Furthermore, a solid acid can be regenerated, lowering the total cost of raw materials. Hence, the discovery of a solid acid catalyst for MDA production has been a theme of significant interest for many industrial research centers

since the 1970s, and such interest has surely not decreased even today.47 Except for a unique example of fluorinated graphite,48 which at any rate produces too much N-methylated MDA (1.9-5.9%) and cannot be recycled at the end of the reaction, the catalysts investigated can be divided into four categories: (a) intermetallic compounds, (b) ion-exchange resins, (c) clays and silica-alumina, and (d) zeolites. 4.1.1. Intermetallic Compounds. During the 1980s, researchers from Texaco deeply investigated as catalysts for MDA synthesis a group of compounds that exhibit intermetallic behavior. These compounds are molybdenum aluminide (Mo3Al),49 tungsten borides (WB, W2B, WB5, W2B5), tungsten sulfide (WS2),50 and tungsten silicide (W5Si3).51 All of these compounds are highly stable and noncorrosive, but their acidities are very mild; hence, their activities, even at high temperature (200 °C), are not comparable to that obtained using HCl. In fact, the conversion of formaldehyde to MDA is never complete, and a significant amount of intermediate, 2.9-5.7% of reaction products, is always present. Furthermore, the N-methylated MDA concentration is too high (6.1-8.6% of reaction products50), and the dimeric fraction (MDA, o,p′-MDA, o,o′-MDA) is less than half that of the reaction products (45-48% of the complement being PMDA). 4.1.2. Ion-Exchange Resins. Two kinds of strong cationic exchange resins were studied as catalysts in MDA synthesis: (a) styrene-divinylbenzene containing sulfonic groups52,53 and (b) tetrafluoroethylene-perfluorovinyl ether containing sulfonic groups (Nafion).54 In all cases, complete formaldehyde conversion was obtained at 100-110 °C, when water, a byproduct of the reaction, was removed through distillation from the reaction products. The selectivity to MDA was found to be very high, always more than 80%, with a content of unwanted o,o′-MDA of about 1%. PMDA is obtained in amounts lower than 10%, and higher-molecular-weight condensation products in amounts of less than 1%. However, the productivities reported for these catalysts are not adequate for industrial development. In fact, using styrene-divinylbenzene resin as the catalyst,53 maximum productivities of 4.95 and 2.47 gMDA/gcatalyst were obtained, depending on the ratio of formaldehyde to aniline. On the other hand, the productivity using tetrafluoroethylene-perfluorovinylether resin (Nafion)54 reaches a maximum value of 12 gMDA/gcatalyst. 4.1.3. Clays and Silica-Alumina. Several patents were granted at the end of the 1970s in which either silica-alumina55,56 or clays57-60 were claimed to be active catalysts for MDA synthesis. According to Marquis,55 silica-alumina was used as a cocatalyst together with oxalic acid, but because oxalic acid is claimed as an active catalyst in MDA synthesis, it is hard to determine whether the silica-alumina is a true catalyst itself. According to Marquis and Schulze,56 silica-alumina was used to transalkylate PMDA with aniline to produce MDA, but in this case, because of the high reaction temperature, significant fractions of o,p′(∼20%) and o,o′- (3-4%) MDA were produced. In the late 1970s, Upjohn Company investigated clays and diatomaceous earth as active catalysts in MDA synthesis. The reaction was performed in five steps (Scheme 10): (1) noncatalytic condensation of aniline and formaldehyde to produce aminal (anilinoacetal); (2) separation of the organic phase, containing aminal and

Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1175 Scheme 10

unreacted aniline, from the aqueous phase produced during the reaction; (3) catalytic partial rearrangement of aminal to a mixture of MDA isomers and PMDA at mild temperature (50°C); (4) distillation under vacuum of most of the unreacted aniline from the reaction mixture; and (5) complete catalytic rearrangement of aminal to MDA and PMDA products at 90-110 °C. The reaction was usually performed batchwise, but some examples of fixed-bed continuous reactor tests were also illustrated. Complete aminal conversion was obtained, and the content of MDA isomers in the reaction mixture was between 40 and 60%, with the remainder consisting of PMDA. In this approach, MDA was 73-75% of the MDA isomers, while o,p′-MDA was 22-25% and o,o′-MDA 1-3%. No information regarding N-methylated MDA was provided. The clays showed low resistance to water content; in fact, to be active in the final rearrangement of the aminal (step 5), the water content should not exceed 1500 ppm. No industrial development of these catalysts ever occurred. Recently, mesoporous (pore size between 20 and 500 Å) silica-aluminas were patented as active catalysts for MDA synthesis.61 Also, using mesoporous silicaalumina, the reaction was performed in five steps, as described above. Batch tests were performed by reacting preformed aminal at 150 °C (Table 4). Complete aminal conversion was obtained only using mesoporous silicaalumina, whereas with microporous silica-alumina, only partial conversion was achieved. Using Grace 13110, MDA was the main product obtained with the following product distribution: MDA, 68%; o,p′-MDA, 11%; remainder, mainly trimers (16-18%) and highermolecular-weight products (2-4%). The maximum productivity obtained with these catalysts, 120 gMDA/gcatalyst, was reached when they were tested in a fixed-bed

reactor under continuous operation. The best solvent for the reaction was aniline, but other solvents, such as chlorobenzene or dichlorobenzene, were used as well. The water resistance was improved with respect to clays, and a maximum of 3% in the feed could be tolerated. 4.1.4. Zeolites. Zeolite Y was first claimed to be an active catalyst in MDA synthesis at the end of the 1980s.62 Analogously to the case of clays, the use of a zeolite as a catalyst in MDA synthesis is based on the rearrangement of a preformed aminal (see Scheme 10). The best results were obtained with zeolite Y pretreated with a fluorinated agent or exchanged with various metal ions. In this case, almost complete aminal conversion was achieved (conversion equal to 97.5% of formaldehyde) with a good selectivity to MDA (89% of the dimeric fraction). However, the concentration of partially rearranged intermediate in the reaction mixture remained too high (2-3%), and the maximum water content that could be tolerated in the aminal was 0.1%. In the same period, various zeolites, such as Y zeolites, ZMS-5, and silicalites with isomorphous substitutions with Ti, B, and Fe, were claimed to be active catalysts in MDA synthesis.63 Also in this patent, the complete conversion of aminal to MDA was not obtained, and the residual content of partially rearranged intermediate in the products ranged from 8 to 30%. Recently, a patent64 was issued in which zeolites, active in MDA synthesis, were chosen according to their spaciousness index, which is an effective measure of the width of zeolite pores.37,38 In this patent, zeolites with spaciousness indexes between 2.5 and 19 were said to be the most active in MDA synthesis (Table 5). As already described, the zeolites were used to catalyze the rearrangement of preformed aminal according to the previous illustrated five-step procedure (see section 4.1.3 above). Within this class of zeolites (spaciousness indexes between 2.5 and 19), the best results were obtained using beta (large-pore zeolite with 12 ring openings) and ERB-1 (medium-pore zeolite with 10 ring channel systems and 12 ring pockets on the [001] crystal surface) zeolites. In fact, using beta and ERB-1 zeolites, complete aminal conversion was obtained with no partially rearranged intermediate (Scheme 10). Reaction mixtures contained about 55-60% of MDA and 20-23% of o,p′MDA, with the remainder being composed of trimers and tetramers. Reaction tests were performed both batchwise and in a fixed-bed reactor, with both giving similar products distribution. The maximum productivity (260 gMDA/gzeolite) was obtained with zeolite beta in a fixed-bed reactor; no other examples of testing in a fixedbed reactor were reported for the other zeolites. The water resistance was claimed to be improved compared to the previously patented zeolite, and a maximum of 3% in the feed could be tolerated.

Table 4. MDA Synthesis with Mesoporous Silica-Aluminaa selectivity (%)

b

catalyst

average pore diameter (Å)

surface area (m2/g)

aminal molar conversion (%)

MDA

o,p′-MDA

trimers and tetramers

Grace J639 Grace 13-110 MSA EniTecnologie SAb microporous

206 94 32 13

344 486 885 509

99.9 99.9 99.9 53.2

64 68.29 63.5 6.5

9.8 10.7 10.6 1

26.3 21.0 25.9 45.7

a Reaction conditions: 4 g of aminal, 10 g of aniline (aniline/formaldehyde molar ratio ) 14.75), 250 mg of catalyst, 150 °C, 6 h. Comparative example not claimed in the patent.

1176 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Table 5. MDA Synthesis with Zeolitesa

b

zeolite

spaciousness index

aminal molar conversion (%)

MDA

beta mordenite ERB-1 ZMS-12 zeolite Yb ZSM-51

19 7 8 3 21 1

99.3 98.3 99.9 98.3 83.4 78.8

58.5 71.1 59 54.5 28.8 8

selectivity (%) o,p′-MDA trimers 22.9 15.1 22.2 30.4 3.5 1.9

6 6.9 15.3 10.8 31.4 39

tetramers 9.2 5.1 3.5 2.9 19.9 29.9

a Reaction conditions: 4 g of aminal, 10 g of aniline (aniline/formaldehyde molar ratio ) 14.75), 250 mg of catalyst, 150 °C, 6 h. Comparative examples not claimed in the patent.

Table 6. MDA Synthesis with Modified Beta Zeolitesa silanizing agentb

aminal molar conversion (%)

MDA

TEOS TPOS TBOS OMTS (one cycle) OMTS (two cycles) OMTS (three cycles) untreated zeolite betad

99.9 99.9 99.9 99.9 99.9 99.9 99.3

49.1 54.11 54.8 53.5 57.1 58.1 58.5

selectivity (%) o,p′- + o,o′-MDA trimers 18.6 20.1 20 11.9 10.2 10.1 22.9

23.5 19.7 15.5 15.1 23.8 21.7 6

tetramers

4,4′-MDA/ (2,4′ + 2,2′-MDA)c

8.6 5.7 8.6 16.6 8.4 9.4 9.2

2.6 2.7 2.7 4.5 5.6 5.8 2.2

a Reaction conditions: 4 g of aminal, 10 g of aniline (aniline/formaldehyde molar ratio ) 14.75), 250 mg of catalyst, 150 °C, 6 h. b TEOS, tetraethyl orthosilicate; TBOS, tetrabutyl orthosilicate; TPOS, tetrapropyl orthosilicate; OMTS, octamethyl cyclotetrasiloxane. c Molar ratio. d Comparative examples not claimed in the patent.

Table 7. MDA Synthesis with Modified Beta Zeolitesa selectivity (%) modifying agent

aminal conversion (%)

MDA

o,p′- + o,o′-MDA

trimers and tetramers

4,4′-MDA/ (2,4′ + 2,2′-MDA)b

(NH4)2HPO4 (5 wt %) (NH4)2HPO4 (2 wt %) (NH4)2HPO4 (1 wt %) H3BO3 (4 wt %) H3BO3 (2 wt %) untreated zeolite betac

99.9 99.9 99.9 99.9 99.9 99.3

69.6 69.4 65.6 67.6 73.0 58.5

11.6 18.7 23.0 9.7 14.5 22.9

18.8 11.9 11.4 22.7 12.4 15.2

6 3.7 2.9 6.9 5.0 2.2

a Reaction conditions: 4 g of aminal, 10 g of aniline (aniline/formaldehyde molar ratio ) 14.75), 250 mg of catalyst, 150 °C, 6 h. b Molar ratio. c Comparative examples not claimed in the patent.

At the end of the reaction, partially or completely exhausted zeolitic catalysts have to be regenerated, and the regeneration is usually performed by burning the reaction products present on the catalyst in air or oxygen. The product distribution obtained using zeolite beta64 implies a higher amount of o,p′-MDA than in the product distribution achieved using HCl as the catalyst (22.8% vs 3-5%). This is mainly due to the lower reaction temperature of the HCl-based process42 compared to that used for zeolite catalysts. To solve this possible disadvantage, it was reported that treatment of the surface of the zeolites with silylating reagents65 is effective in decreasing the o,p′-MDA content and increasing the MDA content in the reaction products (Table 6). By treating the chosen zeolite either with organic silicon compounds of general formula Si(OX)4 or especially with octamethylcyclotetrasiloxane (OMTS), the o,p′-MDA content could be sharply reduced (Figure 3) without lowering the aminal conversion or the catalyst life. Two possible reasonable explanations for the increased selectivity to MDA are the inertness of the external nonselective surface of the zeolites and the narrowing of the pore mouth, which modifies the diffusional resistances experienced by different molecules.66 It is also well-known that the silylation of the zeolite surface modifies the shape selectivity of the zeolite67 in many reactions such as alkylation68 and disproportionation69 of toluene.

Figure 3. Influence of different silanizing reagents on the 4,4′MDA/(2,4′-MDA + 2,2′-MDA) molar ratio (T ) 160 °C, aniline/ formaldehyde molar ratio ) 14.75).

Another method shown to be successful in modifying the shape selectivity of zeolites is treatment with phosporous and boron compounds.70-75 Upon treatment of the zeolite with boric acid (H3BO3) or with orthophosphoric acid (H3PO4) as free acids or as ammonia salts,76 an evident shape-selectivity effect of the zeolite toward p,p′-MDA isomer was observed also in the synthesis of MDA. The use of boric acid or phosphoric acid deriva-

Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1177

tives implies important advantages with respect to silylating agents: (1) Water can be used as a solvent. (2) The price of these reactants is very low. (3) The effect of shape selectivity in MDA production is even higher than that obtained treating zeolites with silylating agents (Table 7). In fact, by treating zeolite beta with OMTS three times (best results with silanizing agents), a 4,4′-MDA/ (2,4′-MDA + 2,2′-MDA) molar ratio of 5.77 was reached, whereas with a single treatment with 4% of H3BO3, a 4,4′-MDA/(2,4′-MDA + 2,2′-MDA) molar ratio of 6.94 was obtained. As in the case of silylated zeolites, this effect could be due to the narrowing of the pore mouth, probably caused by the deposition of polyphosphoric compounds and polymeric boric anhydride. 5. Conclusions Many efforts have been directed toward the substitution of strong mineral acids by nonpolluting solid acid catalysts in ketone and aldehyde condensations. The results obtained in such efforts differ depending on the reaction studied. In the production of fine chemicals, solid acids catalysts have not gone beyond the laboratory stage of investigation. The same is true for the production of bisphenol F and related compounds. In contrast, the production of bisphenol A is currently performed industrially with solid acid catalysts, namely, sulfonic resins. Still, also in this field, research is ongoing to find better substitutes for nonregenerable sulfonic resins, and some promising results have been obtained with zeolites. In the production of MDA, which is currently performed only with strong mineral acids, good efforts have been made toward obtaining new processes based on solid acids catalysts. Several different kinds of catalysts have been patented as useful substitutes for hydrochloric acid, and also in this case, zeolites seem to be the most promising alternatives. Literature Cited (1) March, J. Advanced Organic Chemistry; McGraw-Hill: New York, 1977. (2) Migrdichian, V. Organic Synthesis; Reinhold Corp.: New York, 1957. (3) Corma, A. Inorganic solid acids and their use in acidcatalyzed hydrocarbon reactions. Chem. Rev. 1995, 95, 559-614. (4) Okuhara, T.; Mizuno, N.; Misono, M. Catalytic chemistry of heteropoly compounds. Adv. Catal. 1996, 41, 113-251. (5) Chakrabarti, A.; Sharma, M. M. Cationic ion exchange resins as catalyst. React. Polym. 1993, 20, 1-45. (6) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 23732419. (7) Vaccari, A. Preparation and catalytic properties of cationic and anionic clays. Catal. Today 1998, 41, 53-71. (8) Nakajima, T. Fluorine-Carbon and Fluoride-Carbon Materials; Marcel Dekker: New York, 1994. (9) Westbrook, J. H.; Fleischer, R. L. Intermetallic Compounds; J. Wiley and Sons: New York, 2000. (10) Wu, X. Alkylation of Benzene with Formaldehyde over ZSM-5. J. Catal. 1999, 184, 294-297. (11) Okuhara, T. New catalytic functions of heteropoly compounds as solid acids. Catal. Today 2002, 167-176. (12) Hou, Z.; Okuhara, T. Catalytic synthesis of diphenylmethane from benzene and formalin with water-tolerant solid acids. Appl. Catal. A 2001, 206, 147-155. (13) Cavani, F.; Corrado, M.; Mezzogori, R. A note on the role of methanol in the homogeneous and heterogeneous acid-catalysed

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Received for review May 15, 2003 Revised manuscript received December 16, 2003 Accepted December 17, 2003 IE030429+