The Technologies For Methylmethacrylate Production

Polymethylmethacrylate production currently amounts to more than 2.8 million tons per year on a worldwide scale, and the average yearly demand has been steadily increasing in recent years by more than 0.2 million tons. In contrast to the majority of bulk chemicals, for which the number of competitive processes is limited to one or two technologies, in the case of methylmethacrylate (MMA), the monomer for polymethylmethacrylate production, there are several different technologies that are currently successfully applied, and others have been claimed or are under investigation. The main characteristics of these technologies are reported in Table 14.1.1

The traditional acetone-cyanohydrin (ACH) process is the most widely used in Europe and North America, while other processes are more used in Asia. In the

Methods and Reagents for Green Chemistry: An Introduction, Edited by Pietro Tundo, Alvise Perosa, and Fulvio Zecchini

Copyright © 2007 John Wiley & Sons, Inc.

TABLE 14.1 A Summary of Processes for the Synthesis of Methylmethacrylate

Process

Raw Materials (without esterification)

Features

Producer

Commercial Processes

ACH (acetone cyanohydrin) New ACH

Isobutene oxidation Isobutene oxid with esterification Isobutene ammoxidation

C2 route

Alpha process Propyne process Isobutane process

Acetone, HCN, H2S04

Acetone, methylformate

Isobutene or i-butyl alcohol, air i-Butyl alcohol, air

Isobutene, air, ammonia, H2S04 Ethylene, CO, H2, H2CO

Toxic reactant

Coproduct disposal Three catalytic steps

No coproducts, no supply of HCN Two catalytic oxidations Second oxidative step integrated with esterification Catalytic ammoxidation to methacrylonitrile, and hydrolysis Coproduct disposal Carbonylation of ethylene to propionaldehyde, and catalytic condensation with formaldehyde

Processes Under Development or Investigation c2h4, co, ch3oh, h2co

C3H4, CO, CH3OH

Catalytic carbonylation of ethylene to methylpropionate, and catalytic condensation with formaldehyde Catalytic carbonylation of propyne One-step reaction, high yield The limit is the supply of propyne One-step catalytic oxidation

Several companies

Mitsubishi Gas

Several in Japan Asahi Kasei

Asahi Kasei BASF

Lucite (to become commercial in 2008)

Shell

Several companies

ACH process, acetone and hydrogen cyanide react to yield acetone cyanohydrin; the latter is then reacted with excess concentrated sulphuric acid to form metha-crylamide sulfate. In a later stage, methacrylamide is treated with excess aqueous methanol; the amide is hydrolyzed and esterified, with formation of a mixture of methylmethacrylate and methacrylic acid. The ACH process offers economical advantages, especially in Europe, where there are large plants in use, most of which have been in operation for decades, but the process also suffers from drawbacks that have been driving the need for the development of alternative technologies. Specifically, the process makes use of HCN, a very toxic reactant. Moreover, difficulties in its acquisition can be met; in fact, HCN is a by-product of propylene ammoxidation, and the integration of acrylonitrile and MMA products requires the two processes to be in balance. Alternatively, HCN can be produced on purpose, but this is feasible only for large production capacities. The second major drawback of the process is the disposal of ammonium bisulphate, the coproduct of the process. Additional costs are necessary for its recovery or pyrolysis.

The ACH process has recently been improved, as stated by Mitsubishi Gas.2 Acetone-cyanohydrin is first hydrolized to 2-hydroxyisobutylamide with an MnO2 catalyst; the amide is then reacted with methylformiate to produce the methyl ester of 2-hydroxyisobutyric acid, with coproduction of formamide (this reaction is catalyzed by Na methoxide). The ester is finally dehydrated with an Na-Y zeolite to methylmethacrylate. Formamide is converted to cyanhydric acid, which is used to produce acetone-cyanohydrin by reaction with acetone. The process is very elegant, since it avoids the coproduction of ammonium bisulphate, and there is no net income of HCN. Problems may derive from the many synthetic steps involved, and from the high energy consumption.

Other technologies, already commercially applied or under development, are summarized in Table 14.1. Among the latter, the method that uses isobutane as the raw material, directly transformed to methacrylic acid by a single oxidative step, is potentially advantageous in terms of energy and economics. The direct synthesis of methacrylic acid by the oxidation of isobutane looks particularly interesting because of (1) the low cost of the raw material, (2) the simplicity of the one-step process, (3) the very low environmental impact, and (4) the absence of inorganic coproducts. Several patents claiming the use of Keggin-type polyoxo-metalates (POMs) as heterogeneous catalysts for this reaction started appearing in the 1980s and 1990s.3-13 Rohm & Haas was the first (1981), to claim the use of P/Mo/(Sb) mixed oxides for isobutane oxidation.3 Even though no reference is given in the patent to POMs, the catalyst compositions claimed are clearly Keggin-type compounds. After this patent, several others that describe the use of modified Keggin-type POMs as catalysts have been issued to Asahi Kasei, Sumitomo Chem, Mitsubishi Rayon, and others.

Papers published in recent years14 -40 have tried to establish relationships between catalytic performance and the chemical-physical features of the POMs. Specifically, most of the attention has been focused on the possibility of improving the conversion of isobutane and the selectivity to methacrylic acid by developing POMs that contain specific transition metal cations. However, it seems that the further development of this process has met major obstacles in the preparation of a POM, which on the one hand, is active and selective, and on the other hand is structurally stable enough to withstand the reaction conditions necessary for the activation of the alkane, and the very high heat of reaction that develops.

14.2 THE SELECTIVE OXIDATION OF ISOBUTANE CATALYZED BY POMs: MAIN PROCESS FEATURES

A peculiarity of the processes described in the patents2-13 is that all of them use isobutane-rich conditions, with isobutane-to-dioxygen molar ratios between 2 (for processes that include a relatively low concentration of inert components) and 0.8, and so closer to the stoichiometric value 0.5 (for those processes where a large amount of inert components is present). This is shown in Figure 14.1, which reports in a triangular diagram the feed composition claimed by the various companies, with reference to the flammability area at room temperature. Low isobutane conversions are achieved in all cases, and recirculation of unconverted isobutane becomes a compulsory choice. For this reason, Sumitomo claimed the oxidation of CO to CO2 (contained in the effluents from the oxidation reactor) in

Ethylene Oxide Flammability Diagram

Figure 14.1 Triangular scheme of composition isobutane/oxygen/inert, showing the flammability area for mixtures at room temperature, and the feed composition claimed by several industrial companies.

02 is the limiting reactant i-C4 is the limiting reactant

Figure 14.1 Triangular scheme of composition isobutane/oxygen/inert, showing the flammability area for mixtures at room temperature, and the feed composition claimed by several industrial companies.

a separate reactor with a supported Pd catalyst, after the condensation of metha-crolein and methacrylic acid.6'7

In all cases steam is present as the main ballast. The role of steam is to decrease the concentration of isobutane and oxygen in the recycle loop and thus keep the reactant mixture outside the flammability region. Water can be easily separated from the other components of the effluent stream, and also plays a positive role in the catalytic performance of POMs. It is also possible that the presence of water favors the surface reconstruction of the Keggin structure, which decomposes during the reaction at high temperature, and also promotes desorption of methacrylic acid, saving it from unselective consecutive reactions.

Under the reaction conditions described in the patents, methacrolein is always present in nonnegligible amounts, and therefore a commercial process necessitates an economical method for recycling methacrolein. The patents assigned to Asahi Chemical Industry12 claim the use of an organic solvent, a mixture of decane, undecane, and dodecane, which can efficiently absorb isobutane and methacrolein from the off-gas, with 99.5% recovery efficiency. Isobutane and methacrolein are then stripped with air and recycled.

Figure 14.2 shows the simplified flow sheet of the process, as reported in patents issued to Sumitomo.1 CO2 is maintained in the recycle loop to act as a ballast component; the desired concentration of CO2 is obtained by combustion of CO, while excess CO2 is separated. Methacrolein is separated and recycled to the oxidation reactor. An overall recycle yield of 52% to methacrylic acid is reported, with a recycle conversion of 96% and a per-pass isobutane conversion of 10%. The heat of reaction produced, mainly deriving from the combustion reaction, is recovered as steam.

Flow Sheets Chemistry
Figure 14.2 Simplified flow sheet for isobutane oxidation to methacrylic acid, as stated by Sumitomo.

Key points that limit the industrialization of the process were recently illustrated by researchers from Sumitomo.38 Since the selectivity to methacrylic acid plus methacrolein typically decreases with temperature as the conversion increases, this implies that the rate of production of useful products increases only until the higher conversion compensates for the fall of selectivity. As a consequence of this, the maximum productivity value is reached at a specified temperature. For instance, when a selectivity of 45% is reached at 22% isobutane conversion, with a residence time of 5.4 s, a temperature of 370°C, and a feed containing 25% isobutane, 25% oxygen, and 15% steam, a productivity equal to 0.72 mmol/h/gcat is obtained, which is one order of magnitude lower than the one needed to make the process industrially viable.38 However, the productivity is limited by the oxygen conversion, the maximum concentration of which is dictated by the flammability limits (see Figure 14.1), and by temperature, since the POM decomposes above 380°C.

Possible solutions to overcome this problem are:38 (1) decrease the residence time; the decrease of conversion is more than compensated by an increase of selectivity (due to the lower extent of methacrylic acid combustion), and in overall the productivity increases; (2) increase the total pressure, while simultaneously increasing both the oxygen and the isobutane partial pressure, as well as the total gas flow (so as to keep a constant contact time in the reactor). A higher pressure also implies smaller reactor volume, and hence lower investment costs. Under these circumstances, productivity as high as 6.4 mmol/h/gcat was reached, which is acceptable for industrial production.38 The additional heat required for the recirculation of unconverted isobutane and for increased pressure would be equalized by the higher heat generated by the reaction.

As concerns the isobutane-to-oxygen molar feed ratio, a value equal to 1 fulfills the requirements for the best catalytic performance: (1) it keeps the mixture outside the flammability area, (2) it develops isobutane-rich conditions, which imply a reducing environment, beneficial from the selectivity point of view, and (3) it approaches the stoichiometric ratio for methacrylic acid formation, in order to reach a higher per-pass isobutane conversion. The use of oxygen-enriched air creates an addition cost, but is necessary to have isobutane partial pressures that are as high as possible, while keeping a feed ratio equal to 1. The use of air would require isobutane and an oxygen partial pressures no higher than 15%.

An alternative process configuration includes the integration of isobutene oxidation with the process for the olefin production. The latter process may be a traditional dehydrogenation technology, or, alternatively, an oxidehydrogenation. In both cases, the integration between the two steps is different, as schematized in Figure 14.3. In the first case,41 the exit stream of the dehydrogenation reactor is fed, after addition of air, to the first oxidation reactor, where isobutene is oxidized to methacrolein with a multimetal molybdate catalyst; the same catalyst is not active in hydrogen oxidation. The expensive separation of isobutane from isobu-tene is eliminated; however, the presence of hydrogen in the oxidation reactor may give rise to enhanced flammability problems. The final oxidation reactor is

Isobutane recycle Purge

Isobutane recycle Purge

Isobutane, steam Air

Isobutane recycle Purge

Isobutane recycle Purge

Isobutane, oxygen Oxygen

Isobutane recycle Purge

Isobutane recycle Purge

Isobutane Isobutylene

Isobutane, steam Air

Figure 14.3 Different strategies for integration of isobutane (oxi)dehydrogenation to iso-butene and isobutene oxidation to methacrolein and to methacrylic acid.

Isobutane, steam Air

Figure 14.3 Different strategies for integration of isobutane (oxi)dehydrogenation to iso-butene and isobutene oxidation to methacrolein and to methacrylic acid.

aimed at the oxidation of methacrolein to methacrylic acid. The outlet stream is treated to recover methacrylic acid by water adsorption, as well as unconverted isobutane and isobutene from other uncondensable gases; the hydrocarbons are finally recycled.

In the second scheme, the alkane is transformed to the olefin by oxidehydro-genation, and the outlet stream is sent to the second oxidation reactor without any intermediate separation.42 Isobutane and isobutene are recycled, together with oxygen, nitrogen, and carbon oxides. Finally, the third scheme differs from the first one in that hydrogen is separated from propane/propylene after the dehydro-genation step, and oxygen is preferably used instead of air in the oxidation reactor.43

14.3 THE SELECTIVE OXIDATION OF ISOBUTANE CATALYZED BY POMs: MAIN CATALYST FEATURES

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases, Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe3+ or Cu2+), and alkali metal ions (preferably Cs+). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst;26,27 it also affects the surface acidity.

According to the Asahi patents,4 in order to be active and selective, the POM has to be characterized by a cubic structure, and some degree of reduction (also achieved by in situ treatment with isobutene at 450C). Other authors claimed the

20_22 32 33

importance of having the Keggin anion in reduced form.20_22,3A33 It is possible that a more reduced catalyst leads to better selectivity of the product of partial oxidation, and is thus less active in full combustion. This might also explain why isobutane-rich conditions are claimed in most cases; since conditions with a high concentration of isobutane are more reducing than isobutane-leaner ones. A partially reduced catalyst can be achieved by preparing compounds that have organic cations, which during thermal treatment are oxidized at the expense of Mo6+.20_22 Another possibility is to prepare compounds that have cations that can exchange electrons with Mo6+ in the anion. For instance, it has been found that the presence of Sb3+ in the compound makes the reduction of Mo6+ to Mo5+ in the Keggin anion possible even under oxidizing conditions at 350_400°C (i.e., in the presence of air or reaction mixtures under hydrocarbon-lean

32,33,35,44

conditions).32,33,35,44 The reduced POM that develops is stabilized toward reoxidation, thus making it possible to maintain the partially reduced state even under oxidizing conditions.

Some of the results obtained with the various compositions for POM-based catalysts, found in the patent and scientific literature, are summarized in Table 14.2. An outstanding result is reported in Ref.40, for which a conversion higher than 20% is reported, with excellent selectivity to methacrylic acid, even in the

TABLE 14.2 Summary of Results Reported in the Scientific and Patent Literature for the Oxidation of Isobutane to Methacrolein and Methacrylic Acid Catalyzed by Keggin-Type POMs

Catalyst

T,°C

T, s

¿-C4/O2/H2O/N2 Molar Ratios

¿-C4H10 Conv.%

Selectivity % MAC + MAA

Reference

HxPMo12SbOy

340

6.1

10/13/30/47

10

20 + 50

3

H^Pi.iMoi2Vi.iCuo.iCsi.iOy

320

3.6

30/15/20/35

10.3

16.3+55.7

4

H^P1.5Mo12ViCuo.2ndo.5Cs1Oy

320

3.6

30/15/20/35

12.8

15.9 + 53.8

11

H^PL5Mo12VOo.o4Cuo.2Bao.2Ko.5Cso.5Oy

320

2.4

10/16.8/10/63.2

16.3

10.0 + 50.1

9

H4PMo!! VO40/Ta2O5

350

2

4/8/0/88

28.5

41 + 13.3

12

HxPi .5MO12V0.5as0.4CS!.gCuo.sOy

320

3.6

26/13/12/49

11.2

11.5 + 53.6

7

HxPi,5MOi2v0.5As0.4Cs! 4ClXo.30y

330

5.4

6.5/15.5/15/63

25

42.6 + 2.5

13

H3.6CU2.0PM011VO40/SiOf

348

b

15.1/29.6/19.7/35.5

13.0

55.6 + 11.5

5

Hi .34CS2.5Ni0.08PMOi 1VO40

340

2

17/33/0/50

31

29 + 8

14

(Pyr)3PMo12O40

300

C

2.2/13.7/33.5/50.6

22.2

51.1 + tr

21

H2.4Csi.6Pi.7Mo11VL104o

349

3.6

26/12/12/50

10.6

37.6 + 7.9

45

HcFeO.i2PVASO.3MOn Oy

370

8.6

29/29/0/42

23.9

70 + 4

40

n,MO| .2V0.5P1 .5 AS0.4CU0.3CS 1 Aoy

366

5.4

25/25/15/35 (Ptot 1.5 atm)

21.8

44.9

38

Abbreviations'. MAC = methacrolein; MAA = methacrylic acid. a Forty-three percent of active phase. fcW/F 2.1 ghmL-1. CW/F 0.1 g min mL_1.

Abbreviations'. MAC = methacrolein; MAA = methacrylic acid. a Forty-three percent of active phase. fcW/F 2.1 ghmL-1. CW/F 0.1 g min mL_1.

to W

absence of steam in the feed. The catalyst described therein is based on P/Mo/V POM, and contains As and Fe as dopants.

14.4 THE MECHANISM OF SELECTIVE ISOBUTANE OXIDATION CATALYZED BY KEGGIN P/Mo POM, AND THE NATURE OF THE ACTIVE SITES

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacro-lein, which is transformed to acetic acid, methacrylic acid, and carbon oxides.30'31 Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion.38,39 Iso-butene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion.39

From a kinetic point of view, the formation of methacrolein and methacrylic acid occurs through two parallel reactions. This has been explained by hypothesizing the mechanism illustrated in Figure 14.4, elaborated starting with reactivity studies on isobutene, methacrolein, and methacrylic acid, and from Fourier h3c pH3

Mo Mo "Mo h3c pH3

Mo Mo "Mo h3c ch2

Methacrylic acid Mi ch3

h2c ^cooh h2c

Methacrolein

h2u ho cho

Hydrogencyanide Mechanism Oxidation

Mo Mo

Figure 14.4 Mechanism of the oxidation of isobutane to methacrylic acid catalyzed by Keggin-type POMs.

Mo Mo

Figure 14.4 Mechanism of the oxidation of isobutane to methacrylic acid catalyzed by Keggin-type POMs.

transform infrared (FT-IR) measurements of adsorbed intermediate species, which develop from the interaction of isobutane and products with the catalyst.30,31 The mechanism involves the initial separation of a H" species at the tertiary C atom of the alkane; this is the rate-limiting step of the reaction. An adsorbed alkoxy species is thus formed, which is then converted to an allylic alkoxy species. A dioxyalkylidene species develops, where the primary carbon atom is connected to the catalyst surface via two C-O-Mo bridges. This intermediate is either transformed to methacrolein (through dissociation of a C -O bond), or to a carboxylate species (via oxidation on a Mo site), which is the precursor of methacrylic acid formation. Therefore, these two products have a common unsaturated intermediate, and from a kinetic point of view, this corresponds to the presence of two parallel reactions starting from either isobutene or isobutane, depending on whether the unsaturated intermediate desorbs into the gas phase to yield the olefin. The reduced catalyst is then reoxidized by oxygen, according to the classic redox mechanism.23

Several aspects of the reaction mechanism still need to be explained. For instance, nonnegligible amounts of C2 compounds (acetic acid, acrolein) are obtained, the formation of which is not satisfactorily explained by reaction mechanisms proposed in the literature.

All Keggin-type POMs exhibit an initial unsteady catalytic behavior, which can last from a few hours up to 100 hours, depending on the composition of the POM and on the method employed for its preparation.32,33 The progressive variation of catalytic performance occurring during this "equilibration" period is shown in Figure 14.5, where the conversion of isobutane and the selectiviy to the products are plotted as functions of the reaction time.33,35 The catalyst was a

Green Chemistry Figur

Figure 14.5 Catalytic performance of (NH4)3PMo12O40 prepared by precipitation at pH < 1 as a function of time-on-stream. T 380°C, t 3.6 s; feed composition: 26% isobutane, 13% O2, 12% H2O, remainder He. Symbols: Isobutane conversion (♦), sel. to methacrylic acid (■), to methacrolein (O), to acetic acid (x), to carbon monoxide (*), and to carbon dioxide (*).

Figure 14.5 Catalytic performance of (NH4)3PMo12O40 prepared by precipitation at pH < 1 as a function of time-on-stream. T 380°C, t 3.6 s; feed composition: 26% isobutane, 13% O2, 12% H2O, remainder He. Symbols: Isobutane conversion (♦), sel. to methacrylic acid (■), to methacrolein (O), to acetic acid (x), to carbon monoxide (*), and to carbon dioxide (*).

POM composed of (NH4)3PMo12O4o; data were collected at a reaction temperature of 380°C, with an isobutane-rich feedstock (26 mol % isobutane, 13% oxygen, 12% steam, remainder helium), and a residence time of 3.6 s. At the very beginning of its lifetime, the fresh POM was completely unselective and inactive. After approximately 100 hours reaction time, it was 6.5% converted, with a selectivity to methacrylic acid of 42% and to methacrolein of 13%. The main by-product was carbon dioxide. Therefore, the equilibration time was necessary for the generation of the active and selective sites.

Along with variations in catalytic performance, the following phenomena occurred, which led to a substantial change in the chemical-physical features of the catalyst:

1. The partial structural decomposition of the POM, as evidenced by (i) the formation of small amounts of crystalline MoO3, and (ii) the change in the cationic composition. In regard to the latter point, the ammonium content in the catalyst decreased, and its place in the cationic position of the POM was occupied by Mo ions. The formation of Mo dimeric species was shown in the downloaded catalysts, which were made of neighboring Mo ions located in the anion and in the countera-nion cationic position.46

2. The progressive increase in the extent of POM reduction, as evidenced by the ultraviolet visible diffuse reflectance (UV-vis DR) spectra of catalysts downloaded after different reaction times during catalyst equilibration.

These phenomena occurred only when isobutane-rich conditions were used. Indeed, when the reaction was carried out under isobutane-lean conditions (e.g., with 1% isobutane in the feed), the partial structural decomposition and the reduction of the POM did not occur, and similarly the changes in catalytic performance also were not observed, but there was a minor change in selectivity to methacrylic acid.32-35 This means that the reduction of the POM was due to the isobutane-rich conditions, and that the structural decomposition was due to the larger amount of reaction heat released at the catalyst surface under these conditions. Overheating of the catalyst particles took place, with temperatures that favored the incipient structural decomposition of the POM.

It was proposed that the increase in activity during the equilibration period was due to the generation of new active sites,34,35 consisting of the Mo species located in the cationic position in the secondary framework of the POM. A similar hypothesis was formulated by other authors for the methacrolein oxidation to methacrylic acid.47,48 More generally, it is currently believed that for exothermic reactions, and specifically for oxidations, the true working state of the POM, does not correspond to its crystalline form.49 The presence of steam and the large amount of heat released provoke an incipient surface decomposition, which leads to the expulsion of the Mo species from the anion as a metastable defective compound is developed. The latter then evolves into an "oligomerized" surface form, which prevents the segregation of binary oxides.

The same is known to occur in the case of V-substituted POMs, in which the V5+ ions, originally located in the primary anion, are then transferred during the reaction into the cationic position of the POM framework, with generation of lacunary or decomposed Keggin units;18,50,51 this phenomenon leads to a considerable increase in catalytic activity.

After a steady catalytic behavior was reached, the catalyst was treated in air at 350°C, in order to reoxidize it. Thereafter, the reaction was run again under isobutane-rich conditions (Figure 14.5), in order to understand the role of the POM reduction level on catalytic performance. The reoxidized catalyst exhibited a selectivity to methacrylic acid that was initially around 20%, and approximately 20-30 hours were necessary to recover the original performance of the equilibrated, reduced catalyst. On the contrary, the activity of the catalyst was almost the same as before the oxidizing treatment. This confirms that a partially reduced POM is intrinsically more selective to methacrylic acid than a fully oxidized one, and that one reason for the progressive increase in selectivity to methacrylic acid that occurs during the equilibration period was the increase in the POM reduction level, as a consequence of the operation under isobutane-rich conditions.

Was this article helpful?

+1 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment