Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: An analytical study
Introduction
The liquid fraction resulting from pyrolysis of biomass (e.g. bio-oil) has long been investigated as a potential feedstock to obtain chemicals from renewable resources [1]. Cellulose composes the bulk of vegetable biomass and may constitute a stereochemically pure reservoir to draw optically active reagents for the synthesis of bio-active substances and building-blocks for valuable chemicals. The plethora of organic compounds produced from pyrolysis of cellulose [2], [3], [4], [5], [6], [7], also comprises anhydrosugars retaining the structural record of the original stereochemistry and thus existing in pure enantiomeric forms. Examples of chiral anhydrosugars isolated from cellulose-derived tar are depicted in Fig. 1. Their structures encompass different chemical functionalities, e.g. polyhydroxy (LGA), α,β-enone (LGO) and hydroxylactone (LAC), this latter easily convertible into a chiral hydroxycarboxylic acid by hydrolysis [8]. Levoglucosan (LGA) and levoglucosenone (LGO) have been widely explored as chiral intermediates and catalysts in organic synthesis [9].
However, the huge chemical complexity and low thermal stability of the pyrolytic oil, into which target compounds may occur at relatively low concentrations, hamper their isolation by simple separative methods (e.g. distillation, solvent fractionation). Catalytic pyrolysis, whereby suitable catalysts are added to the feed in order to drive the course of pyrolysis towards the compounds of interest, could be envisaged as a potential means to increase conversion meanwhile reducing undesired by-products. In fact, inorganic additives, even impurities, have a strong influence on the pyrolytic behaviour of cellulose affecting the production of pyrolysis products. Simple metal salts, such as NaCl, suppress the yields of LGA favouring dehydration and charring reactions [10]. Treating cellulose with strong protic acids favours dehydration reactions depressing the yields of LGA [11], while appropriate levels of H3PO4 enhance the formation of LGO [12], [13]. LAC was isolated from pyrolysis of cellulose mixed with the Lewis acid zinc chloride [8]. DGP is a minor pyrolysis product of cellulose, and the presence of acids may promote both its formation and decomposition to LGO [14].
Active solids employed in the field of catalysis, mostly the hydrogen form of ZSM- and Y-type zeolites, have been applied in downstream processes for upgrading the bio-oil into less oxygenated fractions [1], [15], [16], [17], [18]. The behaviour of the more recent nanomaterials has been less investigated, nevertheless there are evidences for a peculiar activity of nanopowder metal oxides under pyrolytic conditions [19].
The variety of active solids commercially available and the need for rapid testing prior to time-consuming experiments with bench reactors and scale-up operations, make analytical pyrolysis a valuable approach for a rapid and reliable evaluation of their performance. There are studies in the literature attesting the validity of analytical pyrolysis for assessing the effect of different catalysts on the composition of pyrolytic vapours produced by organic materials [20], [21].
The present study was aimed at exploring the ability of zeolites and nanopowder metal oxides mixed with cellulose to increase the yields of chiral anhydrosugars formed upon pyrolysis. To this purpose, an analytical procedure based on off-line pyrolysis, previously applied for quantitative measurements of polycyclic aromatic hydrocarbons evolved from cellulose and other organic materials [22], was optimised for the analysis of anhydrosugars.
Section snippets
Materials
Nanopowder oxides of magnesium (MgO), aluminium (Al2O3), silicon (SiO2), silicon–titanium (titanium silicate, TiSiO4) and titanium–aluminium (aluminium titanate, TiO2·Al2O3) as well as aluminium titanate powder were purchased from Sigma–Aldrich. The following zeolites were used: H-Y (CBV901 Zeolyst international), NH4-ZSM-5 (CBV8014 Zeolyst international), NH4-Y (Sigma–Aldrich). High purity microgranular cellulose, erythritol were from Sigma–Aldrich, levoglucosan from Fluka. Levoglucosenone was
Qualitative analysis. Tentative identification of LAC
The chromatogram obtained from off-line pyrolysis of pure cellulose is shown at top of Fig. 2. The structural assignment for the principal peaks was accomplished by comparison with GC–MS characteristics reported in the literature [3], [5], [6]. A singular case resulted to be compound # 8. The mass spectrum of this pyrolysis product, reported in Fig. 3a, exhibited its base peak at m/z 43, intense ions at m/z 57, 69, 70 and 85, as well as characteristic radical cations at m/z 116 and 96. The
Conclusions
Despite some limitations inherent to the system (small sample size, low yields, mass transfer problems) possibly responsible for the observed uncertainty (R.S.D. larger than 10%), the analytical approach developed in this study was proved to be satisfactory for the purpose of evaluating the activity of catalysts in the pyrolysis of cellulose on a sound quantitative base.
Although zeolites were capable to modify the distribution of anhydrosugars, they resulted unable to increase their production
Acknowledgments
This research was financed by Provincia di Ravenna and University of Bologna (CIRSA) within the project “Thermochemical conversion of biomass for the production of bio-materials”. CT wish to thank Provincia di Ravenna for granting his fellowship.
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