HETEROGENEOUS CATALYSIS FOR THE SYNTHESIS OF BIOPRODUCTS

Dottorato di Ricerca in Chimica Industriale (XXVII Ciclo) Report finale Dottorando: Dott. Matteo Mariani Tutor: Dott.ssa Laura Santagostini Co-Tutor: Dott. ssa Nicoletta Ravasio Heterogeneous catalysis for the synthesis of bioproducts Before the discovery of crude oil, in 19th century, society was dependent only on biomass to answer to its energy request. Petroleum discovery provided a source of inexpensive material for energy production, that helped to industrialize the world and improved the standards of living. Unfortunately petroleum sources are waning, due to the growing request especially from the emerging countries, like India, China and Brazil. Moreover the crude oil usage creates very burdensome pollution problems, related to the emission of greenhouse gases. One possible answer to this important challenge could be the exploitation of biomass resources to produce not only energy, but also chemicals and liquid fuels in a sustainable way. In fact biomass is a renewable feedstock, that can be also obtained from agricultural and forest wastes. BIOLUBRICANTS Lubricants cover a large part of the worldwide chemicals market, and their consumption is estimated to be around 40 million metric tonnes per year. Automotive and hydraulics are the largest group of sold and used lubricant in the world. Unfortunately about 50% of all sold lubricants are loss in environment, resulting in severe contamination of soil, groundwater and air [1]. As a result, there has been an increasing demand of biolubricants, derived from vegetable oils. This kind of materials are biodegradable, and permit to limit the environmental pollution [2]. Vegetable oils have technical properties suitable for their use in lubricants formulation, but one significant problem is their low thermal stability, that depends on the presence of H atoms located in the beta position of ester groups, thus making the glycerol esters susceptible to elimination reaction and subsequent degradation of the native molecule. That is why so called hindered esters, that is esters of fatty acids with alcohols without H atoms in beta position, are preferred for lubricants that have to withstand high temperature and pressures [3]. The synthesis of these esters is based on the esterification reaction between an acid, derived from vegetable oils, and a polyol, like trimethylolpropane (TMP) or pentaerithrol (PE). The reaction is usually carried out with a homogeneous acidic catalyst (e.g., p-toluenesulfonic acid, mineral acids) that requires neutralization and washing steps:. In order to make the reaction greener it would be better to use heterogeneous catalysis and to avoid in particular the use of huge amounts of water. Heterogeneous catalysts provide simpler and cheaper separation processes, reduced wastes production, in this case inorganic salts, and in some cases can be recycled. Scheme 1: Esterification reaction between TMP and a carboxylic acid I tested in the esterification of fatty acids with polyols a series of amorphous mixed oxides, namely Silica zirconia silica alumina and silica titania, with surface area ranging from 300 to 500 m2/g and porosity in the range of mesoporousity. Results are summed up in Table 1 Polyol Fatty Acid Cat (%) Exp. Cond. t (h) Conv (%) TMP Nonanoic SiO2-ZrO2 (2,5%) 5% exc TMP 6 99 TMP Nonanoic SiO2-TiO2 (2,5%) 5% exc TMP 6 94 TMP Nonanoic SiO2-Al2O3 (2,5%) 5% exc TMP 6 94 TMP Caprilic SiO2-ZrO2 (2,5%) 5% exc TMP 6 98 TMP Caprilic SiO2-TiO2 (2,5%) 5% exc TMP 6 92 TMP Caprilic SiO2-Al2O3 (2,5%) 5% exc TMP 6 92 TMP Oleic SiO2-ZrO2 (2,5%) 5% exc TMP 6 98 TMP Oleic SiO2-TiO2 (2,5%) 5% exc TMP 6 95 TMP Oleic SiO2-Al2O3 (2,5%) 5% exc TMP 6 89 TMP Oleic SiO2-TiO2 (2,5%) stoichiometric 6 87 NPG Oleic SiO2-ZrO2 (2,5%) 5% exc fatty acid 6 92 PE Oleic SiO2-ZrO2 (2,5%) 5% exc fatty acid 6 99 TMP Oleic SiO2-ZrO2 (10%) 5% exc fatty acid 6 >99 TMP Oleic SiO2-ZrO2 (5%) 5% exc fatty acid 6 >99 TMP Oleic SiO2-ZrO2 (2,5%) 5% exc fatty acid 6 99 TMP Oleic Sn ossalate (0,04%) 5% exc TMP 6 89 Table 1: Esterification reactions Reactions were carried out at 200°C without any solvent under a weak nitrogen flow and a Claisen condenser, to remove the water produced in the esterification reaction. Not only activity was comparable with that obtained under the same exp condition in the presence of SnO, but selectivity was higher, giving oils with excellent physical properties. The catalyst could be removed very easily and reused up to 6 times without any re-activation treatment. MONOGLYCERIDES Monoglycerides are a very important class of compound for industry; in fact a lot of products of daily use contain monoglycerides. The long chain fatty acid monoesters of glycerol are valuable compounds with wide applications as emulsifier in food, pharmaceutics, cosmetics, and detergent industries [4]. While short chain fatty acids monoglycerides are used as antibacterial and antibiotic agents in feed. Therefore production of biocompatible monoglycerides gain great importance. Also the synthesis of these compounds starting from renewable raw materials (glycerol from biodiesel production and vegetable oils) is much more important and greener. The industrial synthesis foresee the transesterification of glycerol with triglycerides or fatty acid methyl esters (FAMEs) or, alternatively, the direct esterification of glycerol with free fatty acids (FFAs). Transesterification is usually catalyzed by homogeneous basic catalyst (e.g., KOH, NaOH, Ca(OH)2), but this route has the drawback to produce large amount of soaps that do not permit an easy separation of products. Heterogeneous basic catalysts offer many advantages, like catalyst separation and recycling; the most used ones being MgO and hydrotalcites [5]. On the other hand, esterification needs acidic catalysts, such as mineral or organic ones, but, once again, heterogeneous catalysis offers much more advantages. Acidic oxides and zeolites are the most used catalysts to perform this kind of reaction. In this part of my work, I focused my attention on esterification of glycerol with oleic (table 2) and valeric acid (table 3) in molar ratio 1:1 with glycerol, using different types of silicas as heterogeneous catalysts (2.5 % or 5% by weight with respect to the acid). The reaction temperature was 150°C with valeric acid and 200 °C with oleic acid, in a three necked flask, equipped with Claisen condenser and a bubbler for nitrogen flux. The reaction time was 6 hours, and the catalysts were not activated. Residual acidity was determined by titration with NaOH 0.1M in diethyl ether:ethanol solution 2:1, with phenolphthalein as indicator. Selectivity was determined by GC analysis using an Agilent 6890N GC equipped with a CP-Sil 8 CB column. ENTRY CATALYST ACIDITY CONVERSION SELECTIVITY MG 1 SiO2-TiO2 2.3% 0.9% 99.1% 49% 2 SiO2-Al2O3 135 3.6% 96.4% 77.8% 3 SiO2-ZrO2 4.7% 3.4% 96.6% 76.9% 4 SiO2-Al2O3 0.6% 2.9% 97.1% 37.1% Table 2: Esterification of glycerol with oleic acid with different solid catalysts Conversion of these reaction are excellent, and selectivity in monoglycerides are high for entries 2 and 3, that represent a good compromise between conversion and selectivity. ENTRY CATALYST ACIDITY CONVERSION SELECTIVITY MG 1a Mesoporous silica 8.2% 91.8% 31.35% 2 a SiO2-TiO2 2.3% 6.6% 93.4% 0.4% 3 SiO2-Al2O3 135 19% 81% 72.2% 4 SiO2-TiO2 2.3% 15% 85% 74.3% 5 SiO2-Al2O3 0.6% 16.5% 83.5% 64.5% 6 Mesoporous silica 19% 81% 64.1% a 170°C and 5% by weight of catalyst with respect to acid Table 3: Esterification of glycerol with valeric acid with different solid catalysts As you can see from the table 3 temperature have a dramatic effect on both conversion and selectivity of these reactions. The two reactions performed at 170°C (entry 1 and 2) give good conversion, but a scarce selectivity in monoglycerides. The other reaction shows a little bit lower conversion, but the selectivity is much improved. The catalyst amount don’t have a significant effect on the reaction, so we decided to lower it. CELLULOSE Cellulose consist of a linear polysaccharide with -1,4 linkages of D-glucopyranose monomers. Cellulose is a crystalline material, with a very large amount of hydrogen bonds that reinforce the structure. Since cellulose is the principal constituent of the terrestrial biomass, the attention of chemists fell on this molecule. In fact from cellulose we can extract a lot of interesting molecules, that can be used like biofuels (levulinic esters and methyl-tetrahydrofuran), chemicals (HMF, organic acids, glycols) and sugars [6]. Many method can be used to depolimerize cellulose and to obtain the searched products: two of them are hydrolysis and hydrogenation. The first reaction requests acidic catalysts to broken the glycoside linkages, the latter needs the presence of hydrogen and a transition metal to perform hydrogenation and give reduced products (xylitol, sorbitol and sorbitan). For hydrolysis homogeneous catalysts, like sulphuric acid, can be used but heterogeneous acid catalysts permit to perform a greener process, while in hydrolysis-hydrogenation a solid catalyst are needed. Our research group set up the preparation of a copper oxide catalyst with very interesting characteristics for cellulose hydrolysis and hydrogenation [7]. This CuO/SiO2 catalyst, produced by chemisorption-hydrolysis, present a unexpected acidity, that unsupported copper oxide doesn’t show. This fact was confirmed by pyridine absorption spectra, that shows typical bands of Lewis sites (1453 and 1611 cm-1) also at 200°C. The acidity can be ascribed to the very high dispersion of copper oxide over the support, confirmed by HRTEM analysis (Fig 1). Figure 1: HRTEM and particle distribution of CuO/silica This copper catalyst permits to perform hydrolysis, but also the hydrogenation in an one pot reaction. Testing different supports and the relative copper catalysts, we found some interesting features. For example: the catalyzed reaction was much more selective than the non-catalyzed. Also the copper catalysts, especially CuO/Si, CuO/SiTi and CuO/SiZr shows a sharp increase in glucose and levulinic acid formation. Figure 2: conversion and selectivity for cellulose hydrolisys (A) bare supports and (B) copper catalysts CuO/SiAl represent a singular case, because it shows the higher conversion and selectivity in lactic acid. This is due to the strong Lewis acidity of the support, leading to the formation of ionic copper species with a marked Lewis acidity. This trend is confirmed by a series of catalytic tests with different CuO/SiAl catalysts, with a different copper loading. The lactic acid selectivity grows linearly with the copper content, reaching a maximum at 8% of Cu. This trend is confirmed by the pyridine absorption spectra, that shows a progressive growth of the band ascribed to the pyridinium ion bonded to Lewis acid sites [8]. Figure 3: Conversion and selectivity with CuO/SiAl with different Cu loading We carried out this kind of reactions in a hastelloy Parr autoclave, with 1,5 g of -cellulose and 0.8 g of catalyst in 30 ml of distilled water. The temperature was 180°C, and we used an overpressure of 4 atm of nitrogen. In the case of hydrogenation we tried different hydrogen pressures. The reaction time is 24 hours. The products analysis were performed with an Agilent 1200 Series HPLC equipped with UV and RID detectors and a MetaCarb H Plus column (eluent H2SO4 0.0085 N in MQ water). To determine the conversion in water soluble product I used a Shimadzu TOC-L total organic carbon analyzer. LACTOSE Lactose is a disaccharide formed by one D-glucose unit, linked by a -1,4 bond, to one D-galactose unit. Milk whey is constituted by about 70% of its dry weight by lactose. Whey is the principal waste of dairy industry and constituted a big environmental problem. In fact its disposal is very exigent concerning the chemical and biochemical demand [9]. Its recovering and reuse became newsworthy for countries with huge cheeses production. There are some reaction for exploit lactose and obtain important industrial products, the principal are oxidation, to lactobionic acid, reduction, to obtain lactitol and hydrolysis to obtain glucose and fructose. We think to hydrolyse lactose to obtain the two sugars and then to reduce them to sorbitol and dulcitol. Sorbitol in particular is very interesting for food industry, in fact it is used widely in sugar-free candy and chewingum. Other uses can be in pharmaceutical as excipients. With our catalyst Cu/SiO2 is possible to hydrolyse and reduce, in one-pot reaction, the lactose to obtain sorbitol and dulcitol. All the reaction were carried on in a hastelloy Parr autoclave, with 1 g of -lactose and different loading of catalyst in 40 ml of distilled water. The temperature was 180°C, and we used an overpressure of 30 atm of hydrogen. The reaction time is 8 hours. The products analysis were performed with an Agilent 1200 Series HPLC equipped with UV and RID detectors and a MetaCarb H Plus column (eluent H2SO4 0.0085 N in MQ water). We tried 2 different copper loading: 8% and 16%, and the best one is the latter. Also influence of the support was investigated, using different kind of silica and mixed oxides. The mixed oxides (SiAl and SiZr) results inactive in hydrogenation, but active in hydrolysis. Between silicas the most active are MP04300, Trysil 300 and SiO2 from FLUKA. These 3 catalysts gives selectivity in hydrogenated compounds up to 90%. Surprisingly Trysil gives bad results. Regard to the amount of catalyst we started with 40% by weight by respect to the lactose, but we obtained same results with only 20% by weight. Oxide Surface area (m2/g) PV (mL/g) DP (Å) SiO2 480 0,75 60 SiO2-Al2O3 135 485 0,79 33 SiO2-TiO2 297 1,26 84 SiO2-Al2O3 0,6% 488 1,43 117 SiO2-ZrO2 421 2,38 181 SiO2 MP04300 723 0,66 38 CuO/ SiO2 (8% Cu) 363 0,68 78 CuO/SiO2-Al2O3 135(8% Cu) 412 0,75 37 CuO/SiO2-TiO2(8% Cu) 318 1,01 64 CuO/ SiO2 MP04300 ((% Cu) 299 0,55 73 Table 4: porosimetric data for the used supports and relative copper catalysts In the table there are listed the porosimetric data for the supports used in the reactions.