Lumpur, Malaysia. Keywords: Catalytic oxidation; Platform chemicals; Nanoporous gold; Green catalysts.

Transcription

1 Advanced Materials Research Vol. 925 (2014) pp Online available since 2014/Apr/17 at (2014) Trans Tech Publications, Switzerland doi: / Green Catalytic Approach for the Synthesis of Platform Chemicals from Palm Tree Lignin Sharifah Bee Abd Hamid 1,a, Mariom Zamila Shilpy 1,b and Md. Eaqub Ali 1,c 1 Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya, Kuala Lumpur, Malaysia a sharifahbee@um.edu.my, b shilpy_bio_ru@yahoo.com, c eaqubali@um.edu.my Keywords: Catalytic oxidation; Platform chemicals; Nanoporous gold; Green catalysts. Abstract Lignin is the second most abundant naturally occurring macromolecule found in plant cellwall, vascular components and woody stems. It is the largest renewable source of aromatic biopolymer. However, lignin is recalcitrant to be broken down by most chemicals. This is because of its complicated heterogeneous molecular structure. However, lignin depolymerization has huge potentials for the synthesis of a number of useful chemicals, perfumes and pharmaceuticals and toiletries. The oxidation products of lignin are important precursors for pulp/paper and food industries, synthetic thin films. Vanillin, veratryl aldehyde and para-benzoquinone are the oxidation products of lignin. These chemicals are the precursors of optically active alcohol, ketone, violuric acid and benzaldehyde. However, the oxidation of biolignin has been remaining a challenging task. Green catalytic approaches might be an interesting solution for the selective depolymerization of lignin into various platform chemicals. Metal oxide/silica supported nanoporous gold has received strong attention as green catalyst for the transformation of various natural polymers. Mesoporous metal oxide/silica provide enlarged surfaces for the breakdown of C-C, C-H and C-OH bonds. This paper has reviewed various green catalytic approaches for the control depolymerization of biolignin into platform chemicals. Introduction Lignin is a Latin word originated from lignum, meaning wood, described by Payen in 1838, and chemically defined by Schulze in It is a hydrophilic substance present in plant cell walls. It is linked by chemically and physically with the other matrix components of the cell walls such as cellulose and hemicelluloses [1]. This linking results increased impermeability, mechanical strength and rigidity of the plant cell walls; it also gives the cells a greater resistance to microbial attacks. Hence, the concentration of lignin in the middle lamella and the primary wall is higher than the concentration in the secondary wall [2]. The lignin oligomers lack a defined primary structure, rather it represents random phenyl-propanoid (C9) polyphenols, which are mainly linked by arylglycerol ether bonds between phenolic para-coumaryl alcohol (H-type), coniferyl alcohol (Gtype), and sinapyl alcohol (S-type) units [3,4]. Lignin is chemically recalcitrant to breakdown by most chemicals because of its complicated heterogeneous structure. Catalytic oxidation of lignin is important pathway for the synthesis of a number of essential drugs, chemicals, perfumes and pharmaceuticals. Recently, many catalysts have been used for lignin oxidation. One of the powerful and promising oxidative catalysts is methyltrioxo rhenium, which is environmental friendly but it suffers from the lack of selectivity. On the other hand, cobalt sulphosalen is active catalyst for oxidation of phenolic and non-phenolic lignin compound. However, it is not stable and also its activity decreases with pyridine. Again, Molebdovanado phosphat polyanion is another potential catalyst but its mechanism of oxidative degradation is unknown. Mn(iv)-Me4DTNE complex catalyst is environmentally friendly because of the formation of large amount of chloro-organics. This paper proposes silica supported gold nanocatalysts, which is one of the best catalyst and used selective oxidation of lignin to produce platform chemicals using TBHP as an oxidant. Platform chemicals are increasingly being considered as a one of sources for the production of All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /05/14,15:29:50)

2 Advanced Materials Research Vol fuels, energy, chemicals and food industry. It has versatile functional groups in organic chemistry and key intermediates for a number of products. Thus their selective synthesis in high yields from oxidation of lignin would be tremendously important in various chemical processes. The high activity of gold catalysts demonstrates that gold can be the catalyst of choice for a number of chemical reactions such as selective oxidations and hydrogenations of organic substrates [5]. Synthesis of Gold Catalyst The gold catalyst on different mesoporous silica materials (MSM) can be prepared with direct synthesis method, which is also known as co-condensation or one-pot synthesis.vtes as functionalizing agent of Au/SBA-15 catalysts can prepared by a one-pot method, using a TEOS:VTES and HAuCl4 solution as gold precursor according to Wu et al. [6]. After stirring and hydrothermal treatment, the solid was filtered, washed and dried, prior to template removal by extraction with ethanol. The resulting catalyst retained the ordered mesostructure of SBA-15, and the AuNPs (~5 nm), present as metallic AuO, were anchored and evenly dispersed in the functionalized SBA-15. The high activity and selectivity for the solvent-free selective oxidation of cyclohexane using this catalyst indicated that VTES may be a good functional group to prepare Au/MSM catalysts by direct synthesis. Candidate Catalyst Methyltrioxo rhenium: Methyltrioxo rhenium (MTO) is the simplest organometallic compound containing Re(VII) which activate the truly oxidizing speices, moleculer oxygen or hydrogen peroxide. Activation of hydrogen peroxide happens with the formation of two peroxorheneum intermediates, a mono-peroxo η 2 complex and bis-peroxo η 2 -complex, both stability and reactivity of these species completely depends on the reaction condition. The transfer of oxygen from these peroxo-complexes to the substrate that happens in a concerted mechanism which includes a butterfly-like transition state [7]. MTO is able in mild conditions to activate H 2 O 2 for the oxidation of substituted phenols and methoxybenzene derivatives to corresponding benzoquinones, some of which are characterized by important biological activities [8,9]. β-ether 4-hydroxy-3-methoxy benzoic acid Hydroxyketone 2,6-dimethoxyphenol Muconolactone Fig. 1, MeReO 3 /H 2 O 2 oxidation of a β-0-4 aryl ether lignin model compound In some cases, products of over-oxidation and successive ring opening of the benzoquinone moiety, such as muconic acid derivatives and di-lactones, were also observed (for an example of oxidative cleavage of natural phenols with MTO [10]). Recently, studies on the oxidation of both lignan and neolignan derivatives with MTO and H 2 O 2 have been performed to evaluate the potentiality of this catalyst system in the oxidative functionalization of lignin [11]. The oxidation of lignin and lignin model compounds was successively performed with heterogeneous rhenium catalysts based on the heterogenation of MTO on low cost, no toxic and easily available polymeric support. However, this catalyst has lack of selectivity. Salen complexes: Salen complexes of various transition metals [M(salen)] are widely used in organic chemistry to oxidise a great variety of substrates by activated molecular oxygen or hydrogen peroxide [12]. Especially cobalt salen ([Co(salen)]) complexes, which had been shown to be compatible with aqueous reaction media [13], were successfully used in various studies on oxidative lignin transformations. The oxidation proceeds mechanistically via the initial formation of a phenoxy-radical, which reacts with molecular oxygen to ultimately form oxidized lignin model compounds[14 17].

3 64 Micro/Nano Science and Engineering (β-0-ether) Arylglycerol-β-aryl ether Fig. 2, Co(salen)/O 2 oxidation of phenolic and non-phenolic β-0-4 aryl ether lignin model compound. Polyoxometalates (POMs): Polyoxometalates can activate hydrogen peroxide and molecular oxygen for the oxidative valorisation of lignin and its various model compounds, and the procedures developed are reported as green alternatives for the oxidative valorisation of lignin. The POM-based oxidation proceeds via different pathways for phenolic and non-phenolic substrates. Phenolic substrates like 60 react supposedly under formation of a phenoxy radical, that is immediately further oxidised by a second POM equivalent to form a cyclohexadienyl cation, leading to the depolymerisation products shown in, mainly quinines and benzoic acid derivatives. Both phenolic and non-phenolic substrates were also shown to react under successive oxidation of the benzylic position. Phenolic substrate Phenol Quinone Fig. 3. Polyoxometalates (POM)-mediated oxidative valorization of a phenolic lignin model compound Potential Chemical from Lignin: aldehyde ketone vanillin phenol Lignin Cinnamic acid Catechol Quinone Toluene Benzene Xylene Acetic acid Fig. 4. Lignin to platform chemicals.

4 Advanced Materials Research Vol Application Vanillin production from lignin oxidation is a biomass based process that employs a byproduct of pulp and paper industry and air to obtain a high added value compound. Vanillin is the major flavor constituent of vanilla. It has a wide range of application in food industry and perfumery. Vanillin is also useful in the synthesis of several pharmaceuticals chemicals. Low level of lignin and modified lignin can yield high performance concrete strength aid, concrete grinding aid and reduced damage of building external wall caused by moisture and acid rain. Lignin provides thermal protection tostyrene, butadiene, rubber polymer, polypropylene, polycaprolactam and also acts as free radical scavengers. Phenols prepared by taking lignin reacting with a H-supplying solvent deploymerization provides routes to Cresols, Catechols, Resorsinols, Quinones, Vanillin, Guaiacols. Lignin enhances the performances of energy storage devices. Alkaline fragmented/purified lignin mined with diesel using surfactants/emulsifies. Conclusion Gold nanoclusters synthesized on various substrates have demonstrated extraordinary catalytic activity in a wide number of chemical conversions. However, they are not been applied for lignin oxidation to platform chemicals. Since the chemical treatments are environmentally unfriendly and biological process are extremely slow, the development of gold catalysts on various substrates would find a wide ranging applications in lignin conversion chemistry. Acknowledgement The authors wish to thank the financial support from the University of Malaya through HIR- MOHE project no. UM.C/HIR/MOHE/SC/32 to Prof. S. Bee. References [1] N.G Lewis, S. Sarkanen, editors, Lignin and Lignan Biosynthesis, ACS symposium series 697, Washington, DC: American Chemical Society, 697 (1998). [2] Helsinki: Gummerus Kirjapaino Technical Association of the Pulp and paper Industry, EUCEPA, editor. Proceedings of the 8th international symposium on wood and pulping chemistry; European polymer journal 49 (2013) [3] E. Adler, Structural elements of lignin: early review, Ind Eng Chem 49(9) (1957) [4] John Wiley & Sons, Lignin: Kirk-Othmer encyclopedia of chemical technology, European polymer journal, 49 (2013) [5] Haruta, M. Kobayashi, T. Sano, H. Yamada, N. Novel gold catalysts for the oxidation of carbon-monoxide at a temperature far below 0 C, Chem. Lett, 16 (1987) [6] Wu, P.P. Bai, P. Loh, K.P. Zhao, X.S. Au nanoparticles dispersed on functionalized mesoporous silica for selective oxidation of cyclohexane, Catal. Today, 158 (2010) [7] W. Adam, WA. Herrmann, CH. Saha-Moller, M. Shimadzu, Oxidation of methoxybenzenes to p-benzoquinones catalyzed by methyltrioxorhenium(vii), J Mol Catal A: Chem 97(1) (1995) [8] W. Adam, W.A. Herrmann, C.R. Saha-Moller, M. Shimadzu, J. Mol. Catal. A, 97 (1995) [9] R. Amorati, G.F. Pedulli, L. Valmigli, O.A. Attanasi, P. Filippone, C. Fiorucci, et al., J. Chem. Soc., Perkin Trans, 2 (2001) [10] R. Saladino, E. Mincione, O.A. Attanasi, P. Filippone, Pure Appl. Chem, 75 (2003) [11]D.C.Ayres, J.D. Loike, Lignans: Chemical, Biological and Clinical Properties, Cambridge University Press, [12] P.L. Nayak, Biodegradable polymers: opportunities and chanllenges, Macrom Chem Phys, 39(3) (1999)

5 66 Micro/Nano Science and Engineering [13] A. Haikarailen, J. Sipila, P. Pietikainen, A. Pajunen, I. Mutikainen. Synthesis and characterization of bulky salen-type complexes of Co,Cu, Fe, Mn and Ni with amphiphilic solubility properties, J ChemSoc, Dalton Trans, 7 (2001) [14] L. Zoia, C. Canevali, M. Orlandi, E.L. Tolppa, J. Sipila, F. Morazzoni, Radical foramtion on TMP fibers and related lignin chemicalchanges, BioRes, 3(1) (2008) [15] E. Bolzacchini, L.B. Chiavetto, C. Canevali, F. Morazzoni, M. Orlandi, B Rindone, Oxidation of propenoidic phenols catalyzed by N,N -ethylene bis(salicylideneiminato)cobalt(ll)[cosalen]: reactivity and spectroscopic studies, J Mol Catal 112(3) (1996) [16] E. Bolzacchini, C. Canevali, F. Morazzoni, M. Orlandi, B. Rindone, R. Scotti, Spectromagnetic investigation of the active species in thoxidation of propenoidic phenols catalysed by [N, N - bis(salicylidene)-ethane-1,2-diaminato]cobalt(ii), J Chem Soc, Dalton Trans, 24 (1997) [17] C. Canevali, M. Orlandi, L. Pardi, B. Rindone, R. Scotti, J. Sipila, et al,oxidative degradation of monomeric and dimeric phenylpropanoids: reactivity and mechanistic investigation, Chem Soc, Dalton Trans, 15 ( 2002)

6 Micro/Nano Science and Engineering / Green Catalytic Approach for the Synthesis of Platform Chemicals from Palm Tree Lignin /