Green Chemical Syntheses and Processes - American Chemical Society

Chapter 16

Environmentally-Benign Liquid-Phase Acetone Condensation Process Using Novel Heterogeneous Catalysts 1

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A.A.Nikolopoulos , B.W-L.Jang , R. Subramanian, J. J. Spivey , D. J. Olsen , T. J. Devon , and R. D. Culp 3

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Research Triangle Institute,P.O.Box 12194, Research Triangle Park,NC27709-2194 Eastman Chemical Company,P.O.Box 1972, Kingsport, TN 37662-5150 Eastman Chemical Company,P.O.Box 7444, Longview,TX75607-7444

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The condensation/hydrogenation of acetone to methyl isobutyl ketone (MIBK) was studied on a series of hydrotalcite-supported noble metal catalysts in a liquid-phase batch microreactor at 118°C and 400 psig. The 0.1wt.% Pd/HTC gave the highest acetone conversion (38%) and selectivity to MIBK (82%). The HTC catalyzes the condensation of acetone to diacetone alcohol (DAA) and its subsequent dehydration to mesityl oxide (MO), whereas the noble metal selectively hydrogenates MO to MIBK. A multi-functionalcatalyst is required for this acetone-to-MIBK reaction.

There is a clear economic incentive to develop heterogeneous catalysts to replace conventional homogeneous catalysts in many industrial processes. Efforts to formulate highly active and selective solid catalysts have been the subject of intense research. One such process of interest is the condensation of alcohols, aldehydes and ketones, known as aldol condensation, which is used industrially for the synthesis of a series of products used extensively in various industries (/). The liquid-phase condensation of acetone is an important industrial process for the synthesis of a series of commodity chemicals like diacetone alcohol (DAA), mesityl oxide (MO) and methyl isobutyl ketone (MIBK) (/). MIBK is extensively 4

Current address: Precious Metals Division, Johnson Matthey, 2001 Nolte Drive, West Deptford, NJ 08066 Corresponding author

194

© 2000 American Chemical Society In Green Chemical Syntheses and Processes; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

195 used as a solvent in paints, resins and coatings, as well as an extracting agent in the production of antibiotics and lubricating oils (2). Over 210 million lb/yr of MIBK are curently produced in the United States (3). A schematic of the major reaction pathways in the acetone condensation process is given in Figure 1.

CH3COOH + (CH ) C = CH Acetic acid Isobutene 3 2

u +CM V« UH3 j μ CH3-C-CH3 v. - (CH ) -C-CH - C-CH ^= Acetone Diacetone alcohol ι II + CH3-C- CH OH • Ο OH I II I (CH ) -C-CH -C-CH -C-(CH ) Triacetone diatcohol 3

2

2

1

3

3

3

2

2

2

3

Γ • Isophorone

4

2

3

3 2

0

3

I°

Ο Hydrogen — 1 II \1 ' II : (CH ) C=CH-C-CH C-CHg + HsO- ^(CH ) CHCH C-CH Mesityt oxide + Water MIBK Ο j Hydrogen II + CH -C- CH MtBC

3


01 BK

A

ο

Mesilylene -

2

3

2

3

3

2

Phorone

Figure 1. Major reaction pathways in the acetone condensation process (Ref.l).

Aldol condensation reactions (and acetone condensation in particular) are practiced industrially using homogeneous liquid-based catalysts, primarily sodium and calcium hydroxide. Such processes generate significant wastewater streams that must be neutralized and properly disposed, resulting in substantial cost. The product separation and waste disposal steps are both energy- and cost-intensive, adding significantly to the selling price of the product MIBK. An active and selective heterogeneous catalyst would, in principle, eliminate this waste and reduce the cost significantly. Therefore, the development of suitable heterogeneous catalysts for aldol condensation would make this process more economically attractive and environmentally benign. A number of solid base catalysts have been reported in the literature to be active for acetone condensation. These include alkali oxides (Na 0, K 0 , Cs 0) (/), alkaline earth oxides (MgO, CaO, BaO) (1,4-6), transition metal oxides (7) and phosphates (8-11), ion-exchange resins (12), zeolites (13) and clay minerals and hydrotalcites (HTC) (14,15). A suitable catalyst for the acetone-to-MIBK reaction must have several properties: the condensation of acetone to DAA is catalyzed by either basic or acidic sites, the dehydration of DAA to MO is acid-catalyzed, and the selective hydrogénation of MO to MIBK requires appropriate metal sites (7,8). The reaction is further complicated by thermodynamic equilibrium limitations, as indicated in Table I. The condensation/dehydration of acetone to MO is limited to about 20% conversion at 120°C (16). However, there is no equilibrium limitation to the overall acetone-to-MIBK reaction. This, coupled with the possibility of numerous thermodynamically favorable side reactions that are also acid/base-catalyzed (Fig. 1), suggests the need to balance the acid/base and hydrogénation properties of the selected catalyst. 2

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In Green Chemical Syntheses and Processes; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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196 Table I. Thermodynamic Equilibrium Properties of the Acetone Condensation/Hydrogénation Reaction Reaction

2 CHJCOCHJ ο

(CH ) =CHCOCHj + H 0 3

(Acetone)

2

2

2

3

2

(CH ) CHCH COCH 3

(Mesityl Oxide)


2 CH3COCH3 + H (Acetone)

-5.2

+3.3

-14.5

-12.1

-19.7

-8.8

(Mesityl Oxide)

(CH ) =CHCOCH + H ο 3

ΔΗ400Κ AG400K (kcal/mol) (kcal/mol)

2

2

3

(MIBK) 2

0 (CH ) CHCH COCH + H 0 3

2

2

3

2

(MIBK)

One promising catalyst formulation consists of a hydrotalcite (Mg-Al hydroxide) support, possessing both basic and acidic sites, loaded with a dispersed noble metal (like Pt or Pd), known to have good hydrogénation activity. Indeed, Pt and Pd catalysts have been reported to selectively hydrogenate the C=C bond of unsaturated ketones (as opposed to the C=0 bond) to produce saturated ketones (6,17). Pd- and Pt-supported hydrotalcites are currently receiving increasing attention as promising catalysts for aldol condensation reactions (18-20). The objective of this research is to develop a fundamental understanding of acetone condensation on noble-metal supported HTC catalysts and to examine the relationship between the physical and chemical properties of these catalysts and their activity/selectivity for the condensation of ketones. This study could result in demonstrating the feasibility of such an economically and environmentally sound process for the industrial liquid-phase aldol condensation.

Experimental

Catalyst Synthesis and Characterization A hydrotalcite (HTC, Mg-Al hydroxide, Mg/Al = 1.8) powder (ID# 95-194-HT001, sample 570, obtained from LaRoche Industries Inc.) was used as the support for this study. Aqueous solutions of PdCl and H PtCl of various concentrations (calculated to form catalysts with nominal metal loadings of 0.1, 0.7, and 1.5 wt.%) were used for the wet impregnation of the HTC support. The impregnated catalysts were dried at 120°C for 2 hours, calcined at 350°C for 2 hours, and then crushed and sieved to particle size of 20 to 40 mesh (370-840 μπι). Prior to reaction, the samples were reduced ex-situ in flowing hydrogen (heating to 230°C at 5°C/min, holding for 1.5 hours, then heating to 450°C at 5°C/min and holding for 8 hours). 2

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197 The surface area of the catalysts was measured by conventional BET methods (nitrogen physisorption at -196°C using a Quanta Chrome-NOVA 1000 instrument). The actual metal loading was measured by inductively-coupled plasma / optical emission spectroscopy (ICP/OES). The acidity and basicity of the synthesized catalysts were measured by N H and C 0 thermoprogrammed desorption, respectively, using an AMI-100 (Zeton-Altamira, Pittsburgh, PA) characterization system. The catalyst samples were reduced in 10% H /Ar at 450°C for 8 hours, followed by treatment in 10% NH /He or 10% C0 /He at 35°C and then by desorption up to 400°C with a heating rate of 10°C/min. 3

2

2


3

2

Reaction System and Procedure The reaction system consisted of the reactor, the gas control panel, and an online data acquisition system. The reactor was a high-pressure 50-ml vessel equipped with a 900-W electric heater, and a high-temperature, high-pressure (HTHP) microRobinson catalyst basket (Autoclave Engineers Inc., Erie, PA), with a 50-mesh opening and 8.4-ml nominal internal volume (Figure 2). The liquid in the reactor was stirred using a magnedrive impeller, the speed of which was controlled by a motor (Saftronics non-regenerative DC Drive) and indicated by a digital tachometer. The reactor headspace pressure was monitored using an analog pressure gauge and a pressure transducer (OMEGA) providing input to a digital pressure meter. The reactor was rated for 5000 psig (340 atm) at 650°F (343°C). It was also equipped with a rupture disk (Autoclave Engineers), rated at 5375 psig at 72°F. The reactor and heater temperatures were continuously monitored by two thermocouples and were digitally displayed. The gas control panel was used for controlling the flow rates of the feed gases to the reactor (by manual metering valves), and measurement by electronic mass flow meters (Brooks 5860). A flow totalizer measured the flow and also displayed the cumulative flow (at STP). The gas control panel and the reactor were located in a fume hood. The data from the flowmeters, pressure transducers, thermocouples, and the digital tachometer were collected by a data acquisition interface (OMEGA Multiscan 1200) and recorded on-line. A typical experiment involved loading ca. 2.7 g of catalyst (reduced ex-situ) into the catalyst basket, which was then placed into the reactor vessel, and adding 20 ml of acetone. The reactor was purged with N to remove the oxygen. It was then pressurized (by closing the reactor vent valve) in a 25% H : 75% N blend to 350 psig while heating to the required reaction temperature and stirring continuously at 200 rpm. This continuous stirring minimized diffusion limitations between the bulk liquid phase and the catalyst particles. After reaching the desired reaction conditions (350 psig and typically 118°C), the N inlet was shut off, while the H inlet was kept open and the reaction pressure was increased to 400 psig. Any loss in pressure, attributed to the consumption of H by the MO hydrogénation to MIBK or the undesired hydrogénation of acetone to isopropanol (IPA), was compensated by feeding H to maintain a constant reactant composition (3:1 N /H ratio). 2

2

2

2

2

2

2

2

2


198 The experiment was typically carried out for 5 hours under these conditions. Upon completion, the H was shut off, the reactor was cooled down, and the liquid sample was collected and analyzed. Analysis was performed in a GC (HP 6890) with FID and a capillary column (HP-1 methyl siloxane) for separation of compounds. Methanol was used as an internal standard. 2

Filter

AE n c Valve vaive

·—~»


Pressure Gauge

2_ Reactor Inlet

Filter

AE Valve

MFC Model 5841

Filter

Dome Pressure

N out of Dome +—[X] 2

-

BPR

*

Pressure Gauge

IFilter Γ

L

-txj—I Reactor Outlet

Vent ToGC f for Analysis ^ — | X } (gas samples)

To Wet Test Meter/ Bubble Flowmeter

N e e ( J | e

J

Vaive "2 Flow

Pressure ' ,

w

(

>

W

a

y

Valve R

e a c t o r

Temp. T

Micrometering Valve (liquid samples)

Data Acquisition System Figure 2. Schematic of the liquid-phase acetone condensation reaction system.


199 Results and Discussion

Catalyst Characterization The physicochemical properties of the catalysts are reported in Table II. A quantitative estimate of the acidity and the basicity of the catalysts, expressed in terms of the amounts of desorbed N H and C 0 per g of catalyst, is also given in this table. The N H desorption data showed no clear correlation between acidity and metal loading for either of the two types of samples. This observation suggests that the HTC support possesses a high concentration of acid sites which appeared to be unaffected by the metal impregnation process. On the other hand, the basicity of the examined catalysts appeared to have a strong negative correlation with metal loading, as shown in Table II. The temperature profiles for the desorption of C 0 from the parent HTC and selected Pd/HTC and Pt/HTC samples are shown in Figures 3a and 3b, respectively. The desorption profile of the non-impregnated, calcined HTC sample (included in both Figures 3a and 3b for the sake of convenience) showed two desorption peaks, one at the low temperature of ca. 100°C, and one at ca. 235°C, corresponding to weakly- and strongly-basic sites, respectively. 3

2


3

2

Table II. Physicochemical Properties of Acetone Condensation Catalysts Catalyst

0

Metal loading (wt.%)

-

HTC

Surface areci

Acidity

Basicityd

(mVg)

(μηίθΐ/g)

(μηιοΐ/g)

254

980

550

3

0

0.1% Pd/HTC

0.13

81

1350

312

0.7% Pd/HTC

0.69

92

NM

NM

1.5% Pd/HTC

1.53

59

1270

162

0.1% Pt/HTC

0.14

147

1020

234

0.7% Pt/HTC

0.67

146

NM

NM

1.5% Pt/HTC

1.47

150

1450

105

a

Measured by ICP/OES on calcined samples Measured by N2 physisorption at -196°C (BET method) on calcined samples Measured by NH3 desorption (up to 400°C by 10°C/min), of samples reduced at 450°C for 8 h and treated with 10% NH3/He at 35°C Measured by CO2 desorption (up to 400°C by 10°C/min), of samples reduced at 450°C for 8 h and treated with 10% C02/He at 35°C NM: Acidity/basicity properties of these intermediate-loaded catalysts were not measured b

c

d



200

Temperature ( ° C )

Figure 3 b. CO2 TPD profiles of HTC and Pt/HTC catalysts.


201 The C 0 desorption profiles of the Pd/HTC and Pt/HTC samples showed moderately smaller low-temperature desorption peaks (ca. 100°C), which became progressively smaller at higher metal loadings. A more dramatic decrease was observed for the high-temperature desorption peaks of the impregnated samples as compared to the parent HTC sample. In fact, the high temperature peak of the high loading samples had essentially disappeared. It can therefore be concluded that the impregnation process resulted in a significant decrease in the basic character of the supported HTC catalysts. 2


Batch reactor experiments The activity and selectivity of the HTC-supported Pd and Pt catalysts for the condensation of acetone, expressed in terms of reactant (acetone) conversion and selectivity to DAA, MO, MIBK, IPA, as well as other by-products is shown in Table III. The homogeneous reaction (i.e., no catalyst present) was also examined for comparison purposes. The results indicate that there was minimal acetone conversion in the absence of catalyst, with the direct hydrogénation of acetone to IPA being the only reaction. This result was not unexpected since, as indicated above, the MIBK formation requires multifunctional catalysis: acidity/basicity for condensation, acidity for dehydration, and metal sites for selective hydrogénation (7,8).

Table III. Catalyst Activity and Selectivity for Acetone Condensation (T=118°C,P=400 psig) Selectivity (mol%)

Conv. Catalyst

(%)

DAA

MO

MIBK

IPA

DMH

DIBK

HGL

>99.9

0

0

0

0

0

0

0

No catalyst

3.1

0

0

0

HTC

19.6

14.4

84.9

0.6

0.1% Pd/HTC

38.0

14.3

0

82.2

0.6

1.2

1.8

0

0.7% Pd/HTC

29.7

11.8

0

71.7

14.7

0.6

1.0

0

1.5% Pd/HTC

28.2

9.0

0

61.8

27.7

0.5

0.8

0.2

32.6

0.4

0.8

0

52.5

0.3

0.5

0

56.8

0.2

0.3

0.3

0.1% Pt/HTC

34.0

12.0

0

52.4

0.7% Pt/HTC

32.7

6.0

0

38.1

1.5% Pt/HTC DAA: MO: MIBK: IPA: DMH: DIBK: HGL:

30.8

6.6

0

33.6

diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) mesityl oxide (4-methyl-3-penten-2-one) methyl isobutyl ketone (4-methyl-2-pentanone) isopropanol (2-propanol) dimethyl heptanone (4,6-dimethyl-2-heptanone) diisobutyl ketone (2,6-dimethyl-4-heptanone) hexylene glycol (2-methyl-2,4-pentanediol)



202 The parent HTC sample (with no Pd or Pt) catalyzed the condensation of acetone to DAA (implying adequate basicity of the HTC catalyst), as well as the subsequent dehydration of DAA to MO (implying adequate acidity of this catalyst). Maximum (equilibrium) conversion of acetone was obtained under the given experimental conditions. The selectivity to MO was 85%, and no other higher condensation products were detected. The formation of MIBK was minimal, as expected, due to the absence of hydrogénation sites. Both series of HTC-supported noble metal catalysts showed enhanced activity (expressed as acetone conversion) and improved selectivity to MIBK compared to the parent HTC sample. The acetone conversion was clearly greater than that in the absence of hydrogénation, due to a shift in the equilibrium-limited acetone-to-MO condensation reaction by direct removal of the product MO (via its hydrogénation to MIBK). No MO was detected at the end of the experiment, suggesting that these catalysts were very active for MO hydrogénation. The main reaction products were MIBK, IPA (formed by the undesirable direct hydrogénation of acetone) and the condensation intermediate DAA. Minor amounts (less than 2 mol%) of other higher condensation products (such as DMH and DIBK) were formed. Also, it is interesting to note that no measurable amounts of methyl isobutyl carbinol (MIBC, see Fig. 1), which is formed by the unselective hydrogénation of the C=0 bond, were detected on any of the examined catalysts. This result is a clear indication of the selective C=C bond hydrogénation of these HTC catalysts. A comparison of the activity and selectivity of the Pd- and Pt-supported HTC catalysts (Table III) indicated that both sets of catalysts exhibited similar overall acetone conversions, with a maximum conversion of 38% obtained on the 0.1% Pd/HTC sample. This catalyst also showed the highest selectivity to MIBK (82%), therefore resulting in maximum MIBK yield (31%). The acetone conversion, as well as the DAA and MIBK selectivity, decreased slightly, but consistently, with higher metal loading for both Pd/HTC and Pt/HTC catalysts. A corresponding increase in IPA selectivity with metal loading showed that higher metal loadings favor the direct hydrogénation of acetone, due to the apparently slower rate of the acetone-to-DAA-to-MO-to-MIBK reaction. In addition, the hydrogénation of MO to MIBK was found to be complete in all cases since essentially no MO was detected in the products. These observations support the hypothesis that MO hydrogénation is not the rate-determining step in MIBK synthesis under the given experimental conditions. The increasing hydrogénation activity of the catalysts with high metal loadings can be more clearly seen in the plot of IPA and MIBK selectivity vs. metal loading, shown in Figure 4. A higher, metal loading favored the direct hydrogénation of acetone to IPA in comparison to its condensation/dehydration, which would lead to the formation of MIBK. Apparently a minimal metal loading (0.1wt.% or less) is required to accomplish complete MO hydrogénation to MIBK while minimizing IPA formation. The acidity of the metal-containing catalysts was greater than that of the HTC, and their basicity was less (Table II). This is likely due to the applied preparation procedure (presence of chloride precursors). Despite the observed variation in acidity


203

and basicity, no clear correlation between catalytic activity or selectivity and either of these pysicochemical properties of the examined catalysts was determined. Apparently the HTC-supported catalysts have at least the required basicity for the acetone condensation to DAA and the required acidity for the dehydration of DAA to MO.


100

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

Metal loading (wt%)

Figure 4. Effect of metal loading on the selectivity ofPd/HTC and Pt/HTC towards MIBK and IP A; T=118°C; P=400psig.

The effect of reaction temperature on the formation of MIBK was also examined. An increase in temperature resulted in increased MIBK yield for the 0.1% Pd/HTC, as shown in Figure 5. The three data points at 118°C correspond to replicate experiments and are presented as an indication of the reproducibility of the activity measurements. Interestingly, this increase in reaction temperature resulted in decreasing selectivity for the major by-products DAA and IPA, and increasing selectivity to MIBK and to heavier by-products (DMH, DIBK). However, extensive formation of heavier (C +) by-products at elevated temperatures could result in blocking the active sites and thus decrease the activity for MIBK formation (12). Consequently, the choice of the appropriate reaction conditions may be a critical factor for optimizing the activity of a given catalytic system for the formation of MIBK. 9


204 50 45 .


40 .

5 0-1 90

.

, 100

,

, 110

.

, 120

.

, 130

,

1 140

Temperature (°C) Figure 5. Effect of temperature on the MIBK yield ofO. lwt. % Pd/HTC; P=400 psig

Conclusions The results of this study clearly demonstrate that the single-step synthesis of MIBK from acetone condensation is feasible on noble-metal-supported hydrotalcites. This process offers significant advantages over the 3-step homogeneously-catalyzed acetone-to-MIBK process currently practiced. It is more efficient, giving higher acetone conversion (by overcoming reaction equilibrium limitations on acetone condensation) and high MIBK selectivity. Also, it is more environmentally benign, since it does not generate any significant amounts of strongly-basic waste streams. Finally, it is more economically attractive, by eliminating the cost-intensive product separation and waste disposal stepsfromthe overall MIBK formation process. The examined noble-metal supported HTC catalysts showed up to 35% yield to MIBK with minimal by-product formation (mainly IPA). Pd was found to be more selective to MIBK than Pt at every loading, despite similar acetone conversions for Pd/HTC and Pt/HTC catalysts. Activity is apparently dictated mainly by the acidity/basicity of the catalyst, which are properties of the support rather than the metal itself. The highest MIBK yield was obtained on the 0.1% Pd/HTC catalyst, indicating that minimal hydrogénation activity is required for the MO-to-MIBK reaction. Higher hydrogénation activity (higher metal loading, Pt instead of Pd) resulted in enhanced selectivity to IPA. Although acidity is critical for the DAA dehydration to MO, the HTC support appeared to possess at least the required acidity for the MO formation. Also, basicity


205 appears to be important for acetone condensation to DAA (the initial step in the reaction sequence), and the HTC support was observed to satisfy this requirement, thus giving maximum (equilibrium) conversion under the examined conditions. However, a direct effect of basicity on the activity and selectivity performance of the metal/HTC samples is rather ambiguous, due to its link with the varying metal loading of the examined HTC catalysts. An increase in reaction temperature favored the formation of MIBK and moderately shifted the product selectivity towards heavier (C +) condensation products. Selection of the appropriate reaction conditions may be critical for maximizing the activity and selectivity for MIBK formation. Downloaded by COLUMBIA UNIV on September 20, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0767.ch016

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Disclaimer Although the research described in this article has been funded wholly or in part by the United States Environmental Protection Agency through grant R-825331 to RTI, it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.

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Watanabe, Y.; Matsumura, Y.; Izumi, Y.; Mizutani, Y. Bull. Chem. Soc. Japan 1974, 47, p. 2922. Watanabe, Y.; Matsumura, Y.; Izumi, Y.; Mizutani, Y. J. Catal. 1975, 40, p. 76. Watanabe, Y.; Matsumura, Y.; Izumi, Y.; Mizutani, Y. Bull. Chem. Soc. Japan 1977, 50, p. 1539. Pittman, C. U., Jr.; Liang, Y. F. J. Org. Chem. 1980, 45, p. 5048. Hattori, H. Chem. Rev. 1995, 95, p. 537. Reichle, W. T. J. Catal. 1985, 94, p. 547. Rao, K. K.; Gravelle, M.; Valente, J. S.; Figueras, F. J. Catal. 1998, 173, p. 115. Klein, F. G.; Banchero, J. T. Ind. Eng. Chem. 1956, 48(8), p. 1278. Kizling, M. B.; Bigey, C.; Touroude, R. Appl. Catal. A 1996, 135, p. L13. Chen, Y. Z.; Hwang, C. M.; Liaw, C. W. Appl. Catal. A 1998, 169, p. 207. Narayanan, S.; Krishna, K. Appl. Catal. A 1998, 174, p. 221. Chen, Y. Z.; Liaw, C. W.; Lee, L. I. Appl. Catal. A 1999, 177, p. 1. In Green Chemical Syntheses and Processes; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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