What Needs to be Analyzed and Why?

DMITRY MURZIN and BJARNE HOLMBOM

3.1 INTRODUCTION

Today, the use of biomass is considered a promising way to diminish nega­tive environmental impact. Moreover, in some future scenarios, renewable raw materials are thought to be able to replace finite mineral-oil-based raw materials before 2050 [1]. This means that new synthetic routes, which should desirably adhere to the principles of green chemistry [2], need to be developed for the production of chemicals.

Lignocellulosic biomass, as a renewable source of energy and chemi­cals, has attracted a lot of attention recently [3-10]. Wood biomass consists of cellulose (40-50%), lignin (3-10%), hemicelluloses (15-30%) and a variety of extractives (1-10%). Cellulose is a linear polymer of D-glu — copyranose and can contain up to 10,000 units (C6H10O5), connected by glycosidic ether bonds, while the molecular mass for hemicelluloses is

Murzin D and Holmbom B (2013). “Analytical Approaches in the Catalytic Transformation of Bio­mass: What Needs to be Analyzed and Why?” in Catalysis for the Conversion of Biomass and Its Derivatives, Behrens M andDatye AK (Eds.), ISBN: 9783844242829, Reprinted with permisison from the authors.

lower. Hemicelluloses have a more heterogeneous structure than cellulose, consisting mainly of five-carbon (xylose, arabinose) and six-carbon sugars (galactose, glucose and mannose). Contrary to cellulose lignin is a co- niferyl alcohol polymer with coumaryl, coniferyl and sinapyl alcohols as monomers, which are heavily cross-linked, leading to complex structures of large lignin molecules [11].

Chemical treatment of lignocellulosic biomass in general, and wood in particular, can have several targets. One of the options is delignifi- cation of the biomass leading to cellulose and some residual hemicel — luloses, which are further applied in the production of paper or board, or derivatives of cellulose. Thermal (or catalytic) treatment of biomass, e. g., thermal or catalytic pyrolysis, is a route to bio-based synthesis gas and biofuels [12]. Depolymerization results in the formation of low — molecular-mass components (sugars, phenols, furfural, various aromatic and aliphatic hydrocarbons, etc.), e. g., unique building blocks for further chemical synthesis.

Wood biomass contains many valuable raw materials for producing fine and specialty chemicals (Figure 1). These raw materials are carbohy­drates, fatty acids, terpenoids and polyphenols, such as stilbenes, lignans, flavonoids and tannins. Some of them can be exuded directly from living trees, while others are extracted and purified via chemical methods.

In this context, applications of catalytic reagents, which are superior to stoichiometric reagents producing stoichiometric amounts of wastes, are worth mentioning. Well-known benefits in using heterogeneous catalysts are associated with easy catalyst separation, regeneration and reuse, as well as relatively low prices compared to homogeneous catalysts. The re­search regarding catalytic transformations of different wood-derived com­pounds is currently very active [13].

Because of the complexity associated with the processing of biomass per se or the transformation of biomass-derived chemicals, in-depth chemical analysis of all components and their reactions is difficult to perform. Therefore, most analytical methods will be a result of a com­promise between information depth and available resources. It is also obvious that in industrial processes only a limited number of rather fast analytical methods could be utilized since a large number of samples should be processed.

FIGURE 1: Chemical by-products from the forest industry

To have in-depth and molecular-level understanding of the chemical reactions occurring during the transformation of biomass not only ad­vanced analytical methods are required, but additionally, a broad spectrum of these methods needs to be applied. Let us consider, for example, the catalytic conversion of cellulose [14-17] in the presence of hydrogen lead­ing to sugar alcohols. During such a depolymerization reaction not only the concentration of carbohydrates and other products in the liquid phase should be measured, but also the crystallinity of cellulose, its morphology, molecular mass distribution and presence of sugar oligomers. The analysis is even more complicated if in this reaction wood is used directly instead of cellulose.

Analytical techniques have made a tremendous progress in recent years giving a possibility to utilize a wide range of modern instrumental methods, including advanced chromatography, microscopy and spectros­copy. It is apparently clear that all the methods currently available cannot
be treated in this review, thus a rational selection of them was done by the authors based on their experience, with an understanding that it might not cover all the analytical methods presently utilized in catalytic transforma­tions of biomass-derived chemicals, but focuses mostly on chromatography.