Effect of zeolites and nanopowder metal oxides on the distribution of chiral anhydrosugars evolved from pyrolysis of cellulose: An analytical study

Abstract

An analytical procedure, employing a commercial heated filament pyrolyser, was utilised for studying the effect of zeolites (H-Y, NH₄-Y and NH₄-ZSM-5 types) and nanopowder metal oxides (SiO₂, Al₂O₃, MgO, TiSiO₄ and Al₂O₃TiO₂) on the pyrolytic production of chiral anhydrosugars from cellulose. Cellulose mixed with catalyst was pyrolysed at 500 °C for 60 s, the evolved products were trapped onto a XAD-2 resin, eluted with acetonitrile and analysed directly, or after trimethylsilylation, by gas chromatography–mass spectrometry (GC–MS). Yields were determined for the following anhydrosugars: levoglucosan (LGA, 1,6-anhydro-β-d-glucopyranose), levoglucosenone (LGO, 6,8-dioxabicyclo[3.2.1]oct-2-en-4-one), 1,4:3,6-dianhydro-β-d-glucopyranose (DGP) and the δ-lactone of 3-hydroxy-5-hydroxymethyltetrahydrofuran-3-carboxylic acid (LAC, 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one). This latter compound, quoted only once in the available literature, was tentatively identified by its GC–MS characteristics. Zeolites and nanopowder metal oxides exhibited a strong influence on the pyrolytic behaviour of cellulose, but whereas zeolites acted to reduce the overall yields of anhydrosugars with respect to pure cellulose, all nanopowders but silicon oxide provided higher yields. LGO and LAC accounted for the larger production of anhydrosugars promoted by aluminium titanate, titanium silicate and aluminium oxide with respect to pure cellulose, while the yields of LGA and DGP remained comparable or even lower. The nanosized characteristics of aluminium titanate, the oxide giving the highest yields of LGO and LAC, were considered a determinant factor for its activity, as powder aluminium titanate resulted ineffective.

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.