CONSIDERATIONS ON THE HYDROLYSIS OF VEGETAL TISSUES

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1 CONSIDERATIONS ON THE HYDROLYSIS OF VEGETAL TISSUES TH. MALUTAN and GH. ROZMARIN "Gh.Asachi" Technical University of Jassy Received January 12, 1995 In this paper our researches concerning the behaviour of vegetal tissue under acidic conditions are presented.one establish the reactivity of oxygen atoms from model compounds of polisaccharides is also estimated the rate constants of autocatalytic hydrolysis reactions in case of hardwoods (beech, poplar) and corb cobs. The complex chemical composition of the vegetal tissues suggests varions posibilities for their chemical processing, for the obtainment of maximum yields with both sugars and furfurolic products, compared with wood's main compounds. The most advantageous method for a complex turning to good account of the vegetal biomass is the total hydrolysis of the constituent polysaccharides, resulting in their transformation into monosaccharides.as to the potential content of nutritive energetic substances, wood and its byproducts represent the basic raw materials for its saccharification. 1 Out of the polyoses of cereals, worth mentioning are xylanes, known as favourizing their assimilation, assuring the energetic resources of the animal organism and stimulating the physiological processes and the quality of food and fodder products, too. Two main types of polysaccharide hydrolysis processes - assuming their conversion up to monoses - are considered, namely: enzymatic or fermentative hydrolysis, and acid or catalytic hydrolysis. 2 As a function of the catalyst's nature, temperature, hydraulic regime, ratio liquid / solid, the following hydrolysis methods may be applied: autocatalytic (wood prehydrolysis, wood steaming - without introducing any catalyst from the outside), fermentative (enzymatic hydrolysis), acid-catalyzed (with mineral or organic catalysts introduced from outside, applications in industrial percolysis being possible) and acid activated methods (sulfite bleaching, action of the concentrated sulphuric acid or of sulphuric anhydride). As to the reaction rate, special mention should be made of the following procedures: temperature regimes below 100 o C (requiring no autoclaves, and favourizing monosaccharides' protection), pentosic hydrolysis (temperature of 140 o C), hexozic

2 hydrolysis ( o C) - the most commonly applied industrial method - and hydrolysis at ultrahigh temperatures ( O C ), characterized by very fast reaction rates. 2 The hydrolysis of native fibrous materials is a slow process, especially when subjected to enzymatic actions, destruction being influenced by the characteristics of primary structure of the chemical compounds, and by cellulose's crystallinity, as well; more than that, the lignin surrounding cellulosic microfibers represents, indeed, a physical barrier limiting the structural accessibility of the vegetal material. Mention should be also made of the fact that, together with lignin, celluloses form wood's amorphous matrix, within which the compounds are involved in very tight physico - chemical links. During hydrolytic treatments, the components of wood's matrix behave differently, as depending on the applied regime. Study of the polysaccharides' reactivity under conditions of acid hydrolysis - using quantum mechanics methods -, performed on model compounds, 3 led to the following series, comparatively with the nucleophilicity of the oxygen atoms: β-d-glucopiranose: O intracyclic > HO (3) > HO (2) > HO (6) 4-O-methylglucoside glucopiranose: O intracyclic > HO (2) > HO (3) > HO (6) cellobiose: O intracyclic > O extracyclic > HO (3) > HO (2) > HO (6) By the application of the same techniques, 3,4 the reactivity order of the oxygen atoms from the lateral chain of the CLPZ complexes (i.e., lignin - polysaccharide complexes), under acidic conditions, has been establieshed: O - C α > O - C β > O - C γ Another direction of study is devoted to the confirmation of the polysaccharides' hydrolysis mechanism, on applynig the MNDO (Molecular Neglect of Diatomic Overlap) routine, permitting to establish an elementary mechanism assuming formation of a cyclic carbocation, in which the charge migrates between the carbon atoms (C1) and the intracyclic oxygen from the anhydroglucopiranose ring. 5

3 The results of these investigations offer an explanations to the differentiated behaviour of wood's main components towards hydrolytic treatments. In order to increasa the yield in reducing substances, a correlation should be established between the main parameters controlling the hydrolytic process, namely: time, temperature, ratio liquid / solid (in the case of wood's autohydrolysis) and the kinetic values charecterizing polysaccharides' hydrolysis reaction. To this end, the kinetics of hemicelluloses' removal to poplar, beech and corn cobs has been studied, under conditions of autocatalytic hydrolysis. 6 The practical observation made was that hemicelluloses' hydrolysis occurs in two stages: a former one, when splitting of the polyosic fragments takes plase, followed by their subsequent dissolving in water, and a latter one - when the dissolved fragments hydrolyse up to monoses and, eventually, are dehydrated to furfural. In the case of hardwoods and of some annual plants, as well, solubilization of the easy - hydrolyzable polysaccharides occurs rapidly (corn cobs) while, for other species (beech, poplar), dissolution is slower. In the conditions considered for the study (T = o C, H = 3, time = min.) the observation was made that hydrolytic removal of pentosans from beech, poplar and corn cobs may modelled kinetically, as a sum of two first order reactions, namely: a rapid and a slower one: P g P u P o F where: P g - pentosans' difficult - to hydrolize fraction; P u - pentosans' easy - to hydrolize fraction; P o - content of pentoses from the solution; F - content of furfural from the solution; k g - rate constant for the hydrolysis reaction of the difficult - to - hydrolize fraction; k u - rate constant for the hydrolysis reaction of the easy - to - hydrolize fraction; k 2 - rate constant for the formation reaction of furfural; The rate constants and the percent values of the difficult - to - hydrolize fraction are presented - comparatively with the total pentose content - Table 1.

4 The rate constant for the fast reaction is closely correlated with the rate constant of the slow reaction, for all species taken into study. The explanation for the different order of magnitute observed with the two constants lies in a different accessibility to the hydrolytic attack and also in the similarity of the hemicelluloses' molecular chains (predominantly O - methyl - glucuronoxylanic ones). It is possible that the slow reaction should be influenced by the intervention of some ligno-polyosic chemical links from the amorphous matrix. The values of the activation energies (127 kj/mol for Fagus silvatica and 120 kj/mol for Populus alba) appear as sensibly higher than those recorder in literature for the hydrolysis performed with clorhidric acid (118 kj/mol) and acetic acid (88 kj/mol) as catalysts. Mathematical modelling, employing the kinetic parameters estimated from experimental data permits to anticipate the yields of products obtained in hydrolytic treatments. 7 In the particular case of hardwoods prehydrolysis with water, some of these yields had the the following values: xylose ( kg/t), furfural ( kg/t),values seen as sensibly modified, as depending on the hydrolysis regime applied and on the initial content of pentosans in wood, the following probable amounts being reached at: kg/t for xylose, kg/t for furfural and kg/t for total reducing substances. The hydrolytic potential of hardwood hemicelluloses, obtained through hydrolysis with water, is correlated with the initial pentosan content in wood.thus, for a thermal regime of 170 o C, ratio liquid/solid = 3 (l/kg), the following correlation equations have be enestablished: Y1 = X; r = Y2 = X; r = Y3 = X; r = where: Y1 - maximum yield in xylose; Y2 - maximum yield in furfural; Y3 - maximum yield in reducing substances; X - initial content of pentosans in wood. The fibrous materials' hydrolytic capacity depends, to a great extent, on their reactivity, on the degree og crystallinity of the cellulosics microfibrils, on their specific surface, on the structure of the amorphous substances and on the accessibility of their components, as well. As already shown, easy-to-hydrolize polysaccharides from the

5 vegetal tissues may be transformed almost wholly in sugars, by a single-stage process of hydrolysis, for either low amounts of catalyst or even in its absence. Nevertheless, the most significant shortcoming of this method lies in the concurrent development of the destruction reactions, which sensibly affects the yield in sugars.a solution would be the utilization of concentrated acids (HCl, H 2 SO 4, HCl gas ) as catalysts, known as acting simultaneously as both agents of polysaccharide dissolution and agents splitting them into cellodextrins and oligosaccharides, or, especially the fact that both reactions occur at cold or at moderate temperatures. In most cases, such activation treatments are applied to celloligninic materials resulted from wood prehydrolysis, after the obtainment of furfural or of hydrolytic lignins. To sum up, the complex turning to good account of the vegetal biomass may be realized by differentiated two steps hydrolysis, as followes: in the former stage - prehydrolysis for a selective removal of polyoses from wood (with sulphuric acid dilued at 170 o C) followed, in the latter stage, by hexozic hydrolysis of the resulted cellolignin, this time at higher temperatures (230 o C), wich influences favourably the yields. References 1 Gh.Rozmarin and V.I.Popa " Tehnologia proceselor hidrolitice si fermentative", Ed.Univ.Tehnice "Gh.Asachi" Iasi, L.T.Fan,M.M.Gharpuray and Y.-H.Lee " Cellulose Hydrolysis", Springer-Verlag, Berlin- Heidelberg, N.Y.-London, Tokyo, Gh.Rozmarin, Gh.Surpateanu and Th.Malutan,Cel. si Hirtie,in press 4 Gh.Rozmarin, Gh.Surpateanu and Th.Malutan, Memoriile Sectiilor Stiintifice ale Academiei, in print 5 Gh.Rozmarin, Gh.Surpateanu and Th.Malutan, Cellulose Chem.Technol., 28, 359 (1994) 6 Gh.Rozmarin, Th.Malutan and M.Barsan, Memoriile Sectiilor Stiintifice ale Academiei, in print 7 Gh.Rozmarin and Th.Malutan, Him.Drev., in print

6 Table 1 Calculated values of the rate constants k u and k g. beech poplar corn cobs Temperature ( o C) P g / P t k u k g Rate constants expressed in min -1

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