974
Ind. Eng. Chem. Res. 1995,34, 974-980
Catalysts and Supports for Conversion of Lactic Acid to Acrylic Acid and 2,3=Pentanedione Garry C. Gunter, Robert H. Langford, James E. Jackson2 and Dennis J. Miller* Department of Chemical Engineering, Michigan State Uniuersity, East Lansing, Michigan 48824
The catalytic conversion of lactic acid over various sodium salt catalysts and support materials has been carried out to identify potential catalystlsupport combinations for 2,3-pentanedione and acrylic acid production. Low surface area, pure silica is the best support for suppressing undesirable side reactions to acetaldehyde and propanoic acid, which are favored over high surface area (microporous) or surface acidic materials. The best catalysts for 2,3-pentanedione and acrylic acid formation are the sodium salts of group IV and group V oxides, with sodium arsenate giving a 2,3-pentanedione yield of 25% and combined selectivity to acrylic acid and 2,3-pentanedione of 83% at 300 "C and 0.5 MPa total pressure.
Introduction
0
New fermentation-based technologies to produce lactic acid from starch hydrolysates are making this optically active, bifunctional molecule into a viable feedstock for chemicals production. With both hydroxyl and carboxylic acid functions, lactic acid and its derivatives offer novel routes to a variety of products. The primary pathways of interest in this study, those of vapor-phase conversion over supported salt catalysts, are shown in Figure 1. The combination of low lactic acid cost, reduction of petroleum use, and low toxicity of the catalysts used makes processes based on these pathways attractive from an environmental standpoint. Previous studies, most of which are in the patent literature, illustrate that simple inorganic salts are surprisingly good catalysts for converting lactic acid to valuable products. Most work has focused on conversion to acrylic acid; the competing decarbonylation and decarboxylation to acetaldehyde make attainment of high selectivity challenging. One of the first studies reported (Holmen, 1958) described the conversion of lactic acid and lactates to acrylic acid over phosphate and sulfate catalysts, achieving a selectivity of 68% at 400 "C. While other feedstock-catalyst combinations have been studied, including ammonium lactate over Alp04 (Paperizos et al., 1988)and lactic acid over NaH2POdNaHC03 (Sawicki, 19881, none have led to significant improvements over Holmen's results. Lactic acid conversion in supercritical water has been investigated (Mok et al., 1989) using simple acids and bases as catalysts. They identified three primary reaction pathways in supercritical water: (1)decarbonylation to acetaldehyde, CO, and water, (2) decarboxylation to acetaldehyde, C02, and Ha, and (3)dehydration to acrylic acid. Lira and McCrackin (1993) achieved 58% selectivity to acrylic acid from lactic acid by adding Na2HPO4 to near-supercritical water at 310 bar and 360 "C. The phosphate salt catalyst was observed to have little positive effect on the rate of acrylic acid formation, but it did effectively suppress the competing decarbonylation and decarboxylation reactions. In the liquid phase, conversion of lactic acid in the presence of group VI11 metal complexes such as [PtH(PEt3)# produces acrylic, propanoic, P-hydroxypro-
* To whom all correspondence should be addressed.
Department of Chemistry,Michigan State University,East Lansing, MI 48824. +
0888-5885/95I2634-0974$09.00lO
+
(I,
co
+ H~O
+ COP + H2
pz Acetaldehyde
Dehydration
L Acid Lactic
O
H
II
0
Acid
I
Reduction
+"202
Condensation
t
CO, + 2H20
0 2,3-Pentanedione
Figure 1. Primary chemical conversion pathways of lactic acid.
panoic, pyruvic, and acetic acids (Ode11et al., 19851, with acrylic acid yields less than 5% at all conditions. Our prior study of vapor-phase conversion at 0.5 MPa total pressure over supported sodium phosphate catalysts (Gunter et al., 1994) led to the discovery of 2,3pentanedione from lactic acid. This condensation pathway opens additional possibilities for chemical production from lactic acid, as 2,3-pentanedione can undergo further condensation and reaction to a vafiety of products. Both conversion and selectivity to desirable products (2,3-pentanedione and acrylic acid) increase with increasing basicity of the phosphates (Na3P04 > Na2HP04 > NaH2P04). Over Na3P04 on silica-alumina, acrylic acid selectivities of 36% were observed at 350 "C and short (0.6 s) contact times, while 2,3pentanedione selectivities of 56% were observed at lower temperatures (280-320 " C )and longer contact time (3 s). We proposed a mechanism for 2,3-pentanedione formation in which the phosphate combines with the acyl group of lactic acid to form an acyl phosphate intermediate which undergoes condensation to the final product. To better understand the formation of 2,3-pentanedione and further enhance desirable product selectivities, we report in this paper catalytic reactions of lactic acid over sodium salts of several inorganic oxyacids analogous to the phosphates already examined. These catalyst salts have been chosen with central atoms adjacent in the periodic table to identify trends in catalytic activity with molecular structure; salts are in the
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 975 sodium form t o the extent possible to minimize effects of the cation. In addition to examining different catalysts, we have investigated different support materials to determine the influence of both pore structure and chemical makeup on product distribution. Our goal in this work is to identify catalyst-support combinations that give good selectivities to desired products while minimizing undesirable formation of acetaldehyde.
Experimental Section Feedstock. Two sources of lactic acid were used in these studies. DL-Lactic acid (Aldrich, 85% aqueous solution) and L-(-)-lactic acid (Purac, Inc., 88%aqueous solution) were diluted to 34 wt % with HPLC grade water. Higher feed concentrations led to plugging of the reactor feed line and excessive catalyst coking, and lower concentrations gave low product formation rates. Both acids contained small quantities of acetic and propanoic acid (
