Studies in Surface Science and Catalysis
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Glyoxylase biomimics: zeolite catalyzed conversion of triosesK. P. F. Janssen a , J. S. Paul b , B. F. Sels a and P. A. Jacobs aaMicrobial and Molecular Systems, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium. Tel: +3216321468; Fax: +3216321998; E-mail: kris.janssen@biw.kuleuven.be bFlanders Materials Centre, Technologiepark 903, B-9052 Zwijnaarde, BelgiumABSTRACTDirect conversion of biomass derived trioses such as dihydroxyacetone (DHA) and glyceraldehyde (G LA) to alkyl lactates was carried out using alcohols over various Y type zeolite materials. The conversion takes place through a pyruvic aldehyde intermediate which subsequently undergoes esterification. The screened catalysts can be grouped based on their ability to either form methylglyoxal dialkyl acetal or alkyl lactate. The discussed catalysis shows strong resemblance with the biological lactic acid synthesis routes where glyoxylases effect the reaction. Zeolites may therefore be considered as functional biomimics of these enzymes.1. INTRODUCTIONThe development of effective methods for the production of lactic acid from biomass has been of great industrial interest in recent years. Because of their unique physical and biological properties this Į-hydroxy acid and its esters can be employed in anything ranging from the production of cosmetics to the manufacture of biodegradable plastics. Despite some obvious drawbacks, fermentative production of lactic acid from dextrose remains the principal method for industrial lactic acid production up until now. Various chemical methods that exist for the production of these compounds are not put to practical use for a number of reasons.Scheme 1: Reaction of trioses with alcohols that produce the corresponding alkyl lactates or methylglyoxal derivatives.1222From Zeolites to Porous MOF Materials – the 40th Anniversaryof InternationalZeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors)© 2007 Elsevier B.V. All rights reserved.Homogeneous CrCl 3•6H 2O [1,2], although an effective catalyst for the formation of n -butyl lactates, is not only highly toxic. It requires long reaction times, and is very specific in its requirement for the employed alcohol. A more recently reported method for the preparation of alkyl lactates from the biomass derived dihydroxyacetone (DHA) (Scheme 1) catalyzed by tin halides [2] is much more promising in this respect, but the fact remains that homogeneous systems will always suffer from the need for difficult and lengthy product workup in order to remove the catalyst from the reaction mixture. Therefore effective heterogeneous catalyst systems for the production of both lactic acid esters and methyl glyoxal derived diacetals are highly desirable.2. EXPERIMENTAL2.1. Catalyst preparationIn the present work both commercially available USY samples as well as ion exchanged zeolite samples were used. The commercial samples were either used directly or calcined according to following standard procedure (5 K/min to 723 K for 1.5 h). After calcination, allsamples were kept at 373 K up to the moment of reaction.Fig. 1. Schematic overview of the automated preparation and catalytic screening of ion exchanged YzeolitesScreening the parameter space involved in the synthesis of ion exchanged zeolite materials in order to optimize catalyst performance may prove very difficult and time consuming due to the many unit operations that are typically necessary for their preparation. Therefore it would be highly beneficial if the synthesis and catalytic screening process could be parallelized and automated. Even though numerous accounts exist in literature where heterogeneous solid catalysts have been prepared using high-throughput methodologies, be it via sol-gel, precipitation or even wet impregnation techniques [3] many technical difficulties remain with these approaches. For instance, minimizing loss of catalyst material while transferring the sample between different synthesis/washing steps or prior to catalytic testing [4] and maintaining overall homogeneity of the sample during the handling and preparation of the catalyst may prove exceedingly difficult. For these reasons an automated liquid- and solid-handling workstation was used to prepare catalyst materials on a small scale in this work. The system is highly flexible, i.e., with regards to the recipients used, and displays high accuracy and reproducibility.12231224Standard NaY (Si/Al, Zeocat) with a CEC of 3.29 meq/g equilib was used. This zeolite was kept in a controlled atmosphere over saturated NH4Cl solution. In order to study the influence of the ion exchange degree on the catalytic performance 0.2 g of this NaY equilib was exchanged with different amounts of a 0.132 eq./l stock solution such that 500 ȝl of this solution theoretically corresponds to 10% of the CEC. After addition of the stock solution, distilled water was added to obtain a total volume of 5 ml in order to allow proper stirring during the exchange procedure on such a small scale. After 15 h all samples were washed three times in an automated fashion with distilled water to remove excess ionic species and the samples were subsequently dried at 353 K for 6h and calcined according to the previously described procedure.2.2. Reaction mixture preparationIn a typical experiment, 5 ml of an ethanol based mixture containing the DHA substrate (0.4 M) and 1,4-dioxane (internal standard) was added to each sample of the library plate containing 24 samples with 0.2 g of catalyst per sample.2.3. Catalytic screening and analysisAfter reaction at 90 °C for a well defined period individual samples or the entire library are taken from the liquid handler heating bay and left to cool after which a small sample of the reactant mixture for each sample is transferred to a fresh library. All sample vials are closed with a septum cap and the libraries are placed directly on the GC (Thermo Finnigan TraceGC, RTX-5 column, 30m, 0,32 I.D., 25 ȝm) with a programmable CTC autosampler for analysis.3.RESULTS AND DISCUSSION3.1. Conversion of DHAD to lactic acid ethylestersA series of different Y and ultrastable Y zeolites (USY) were tested as catalysts in the present work. The results for the catalytic conversion of DHA dimer to the ethyl ester of lactic acid after a reaction time of 6 hrs. under conditions as described above are summarized in Tables 1 and 2.Table 1Screening of ZF series zeolitesConv. to ethylester (%)Conv. to diacetal (%)non Calc. Calc. non Calc. Calc.ZF 110 1 18 3 74ZF 210 41 65 16 28ZF 220 23 55 13 18ZF 510 6 58 11 30From these single point results it can be seen that overall, zeolite ZF 210 after calcination generates the highest conversions of DHA dimer to ethyl lactate (65%), immediately followed by zeolite CBV 600 which leads to a conversion of 60% to ethyl lactate.When looking more closely at the results for both separate series of zeolite materials it can be seen that for the ZF series of zeolites, ZF 110 is the starting NH4Y material from which all subsequent materials are derived through hydrothermal and/or acid leaching treatment. This material displays almost no activity prior to activation at 723K. After1225 calcination however, the ammonium leads to a large amount of relatively weak Brønsted acid sites in the form of H+ ions at the exchange positions and subsequently a significant increase in conversion to methylglyoxal ethyl diacetal, from 3% to 74%, can be seen. Together with results obtained with mineral acids in the presence of DHA and alcohol [5] this observation seems to indicate that pure Brønsted acid zeolites are capable of catalyzing the pathway 1 and 2 in the overall conversion of DHA to alkyl lactates (Scheme 1).The other samples from the series, notably steamed ZF 210 and ZF 220 and ZF 510 which underwent an additional mild acid leaching after steam treatment all show a remarkably different selectivity. Although some methylglyoxal diacetal (Scheme 1, 1) is still formed, the main product is the ethyl ester of lactic acid (Scheme 1, 2) in this case. Apparently, steam-treated USY zeolites more favorably catalyze reaction routes 1 and 3. Table 2Screening of CBV series zeolitesConv. to ethylester (%) Conv. to diacetal (%)non Calc. Calc. non Calc. Calc.CBV 500 6 52 16 46CBV 600 59 59 10 18CBV 712 38 54 22 21CBV 720 20 33 45 46CBV 760 3 3 29 34CBV 780 0 9 14 20 When the previous results are compared with the materials from the CBV series it is clear that CBV 600 before and after calcination and CBV 500, CBV 712 after calcination lead to similar selectivity patterns as is the case with ZF 210. Again these materials are all USY type materials produced through hydrothermal treatment of a parent NH4Y material.The displayed catalytic behavior of the studied materials (Table 1 and 2) appears to be strongly influenced by the pretreatments these materials underwent during their production. All materials, which are expected to contain a high amount of extra-framework aluminum (EFAL) species, created through steaming of the parent zeolite, display a marked preference for the formation of lactic acid ethyl ester, while zeolites with mainly Brønsted acidity favor alkyl glyoxal diacetal. At present it is not exactly clear what type of EFAL species contributes to the formation of alkyl lactic acid. Further mechanistic and physicochemical investigations into the exact nature of the species and effects involved are certainly necessary.3.2. Zeolite catalyzed acetalization of DHADWhen DHA dimer is reacted with ethanol in the presence of an untreated NaY zeolite, there is absolutely no activity, even after prolonged reaction times. The same can be said for MgNaY, CaNaY and CoNaY materials prepared by ion exchange procedures. However, as can be seen in figure 1, LaNaY and CeNaY materials appear very active and selective catalysts for the conversion of DHA to methyl glyoxal diacetals.It is apparent from these results that a high degree of exchange is necessary to obtain a catalyst which displays high conversions of the DHA dimer substrate and that both LaY and CeY display very similar behavior. It is remarkable that both materials display a Brønsted acidic character under the employed conditions much like the behavior of a typical HY material prepared by thermal activation of NH4Y. Highly exchanged La and CeY materials1226even show higher conversion of DHA to methylglyoxal, yielding more than 90% of the desired product, with almost no formation of lactic acid ester.4.CONCLUSIONIn conclusion, we have achieved a heterogeneous system for the effective conversion of DHA dimer with ethanol to either methylglyoxal derivatives (Scheme 1, 1) or ethyl lactate (Scheme 1, 2) in which zeolites are the active catalysts. It could be shown that catalyst materials can be grouped according to their ability to preferably catalyse either of the two possible reaction pathways.Steam treated USY materials, which are expected to contain high amounts of EFAL species, show a high affinity for the formation of lactic acid ester products (Scheme 1, path 1+3). The exact nature of the EFAL species responsible for the observed selectivity and the mechanism by which it catalyses the conversion of DHAD remains to be identified and further efforts will focus on elucidating the mechanisms.Microporous rare earth exchanged NaY zeolites effectively catalyze the reaction between dihydroxyacetone dimer and ethanol to produce diethyl acetals of methylglyoxal at 90 °C (Scheme 1, path 1+2). CeY zeolite was revealed to be the most efficient catalyst for the presented reaction, followed by LaY. The presented results confirms for a liquid phase reaction that REY zeolites display an acidity very similar to classic Brønsted acids such as sulphuric acid or acidic ion exchange resins. REY materials are clearly better catalysts than classic Brønsted acid zeolites such as USY.In conclusion we have developed a mild, efficient and environmentally benign procedure for the preparation of acetals and alkyl lactic acids of glycerol derived carbonyl compounds using heterogeneous, zeolite-based reusable catalysts.ACKNOWLEDGEMENTSK.J. wishes to thank Flamac (Flanders Materials Centre) for financial support.1227 REFERENCES[1] J. Eriksen and O. Mønsted, Transition Met. Chem., 23 (1998) 783[2] Y. Hayashi and Y. Sasaki, Chem. Commun., (2005) 2717-2718[3] F. Schüth and D. Demuth, Chem. Ing. Tech., 78 (2006) 851.[4] C. Hoffmann, H.W. Schmidt and F. Schüth, J. Catal., 198 (2001) 348[5] S. K. Gupja, GB1473782 (1975)。
