The chemistry and technology of furfural production in modern lignocellulose-feedstock biorefineries (2011)

This dissertation deals with biorefinery technology development, i.e. with the development of sustainable industrial methods aimed at the production of chemicals, fuels, heat and power from lignocellulosic biomass. This work is particularly focused on the production of furfural from hemicellulose-derived pentoses. The possibility of producing materials, chemicals, and fuels from biomass has a long history. Unfortunately fossil resources, in particular oil, have dominated last century both in the chemical and energy sectors due to their enormous availability at relatively low cost. Nowadays, bio-based chemicals and fuels are gaining a much more favorable competitive position, not only by virtue of high oil prices and the forecasts on their future availability, but also because of the increasing public environmental awareness. Although the use of biomass for energy, and in particular for biofuels production, has been greatly challenged recently in terms of real benefits to the environment, the technological advances in the biorefinery field offer many reasons to believe that, in the near future, biomass could be definitely the sustainable alternative to fossil resources in the chemical and also fuels industries. By a careful review of the scientific literature, and observing the recent research trends, it is noticeable that a relatively short list of high-potential biomass-derived building blocks has the potential for replacing or substituting fossil resources in nearly every industrial field. The broad family of furan compounds represents certainly an intriguing selection of biomass derivatives for many industrial applications. Furfural is nowadays the only starting material for the production of nearly all the furan compounds. Furfural industry exists since almost a century, but it is nowadays facing a major renovation challenge in order to meet the global trend toward bio-based products, and the consequent increased demand for furfural and its derivatives. The majority of current furfural production is still based on more or less modified versions of the original Quaker Oats process (1921). For reasons that can be related to their limited technological evolution, the production processes in use today generally suffer from low yields (around 50%), besides significant economical and environmental concerns. All these reasons hindered the expansion and modernization of the furfural industry below its actual potential. A profound technological development is a priority in order to upgrade furfural to a large-volume bio-based commodity. The integrated production of furfural within modern biorefineries is a big opportunity, and it will most probably represent the next cornerstone in the development of furfural industry. In Chapter 1 the opportunities offered by the modern biorefinery in the broader context, and the importance of furan compounds are highlighted. In this context the enormous potential of furfural and its derivatives, both in the chemical and energy sector, is discussed. Recent advances in furfural technology are summarized, both regarding furfural synthesis and applications, eventually stating the motivation behind this dissertation, and the main achievements herein contained. In Chapter 2 the experimental methods used in this work are carefully described. A new lab-scale titanium reactor was built in order to investigate several aspects related to the furfural formation and related reactions, and to enable liquid phase reactions under a relatively broad range of pressure, temperature and pH conditions. This test rig has allowed most of the experimental work behind this dissertation, and thus it is thoroughly described in this chapter in all the relevant aspects typical of chemical reactor engineering. The analytical and experimental methods employed in the several experimental campaigns described in this dissertation are also extensively described. Chapter 3 concerns the reaction kinetics of furfural formation. Even considering the number of relevant works on the topic of furfural formation in acidic media, a general expression for the reaction kinetics, its dependence on the acid nature and concentration, and the potential effect of other species present in solution, is yet to be defined. Results of reaction kinetics studies related to furfural formation from xylose, xylose side reactions, and furfural destruction in acidic aqueous media are thus studied and reported. In Chapter 4 some particular aspects of the chemistry of xylose reaction into furfural are addressed with the aim to clarify the reaction mechanism and to define new green catalytic pathways for its production. Specifically the reduction of mineral acids utilization is addressed by the introduction of alternative catalysts. In this sense the effect of chloride salts in dilute acidic solutions at temperatures between 170 and 200 °C is described. Results indicate the Cl? ions to promote the formation of the 1,2-enediol from the acyclic form of xylose, and thus the subsequent acid catalysed dehydration to furfural. For this reason the presence of Cl? ions led to significant improvements with respect to the H2SO4 base case. The addition of NaCl to a 50mM HCl aqueous solution (0.18 wt%) allows to attain 90% selectivity to furfural. Among the salts tested FeCl3 shows very interesting preliminary results, producing exceptionally high xylose reaction rates. Starting from the results discussed in chapter 4 on the effects of Cl? ions on furfural formation in aqueous acid solution, the general effect of different halides is addressed in Chapter 5. Experimental results show the halides to influence at least two distinct steps in the reaction leading from xylose to furfural under acidic conditions, via different mechanisms. The nucleophilicity of the halides appears to be critical for the dehydration, but not for the initial enolization reaction. By combining different halides synergic effects become evident resulting in very high selectivities and furfural yields. In Chapter 6 the combined production of hemicellulose-derived carbohydrates and an upgraded solid residue from wheat straw using a dilute-acid pretreatment at mild temperature is described. Dilute aqueous HCl solutions were studied at temperatures of 100 and 120 °C, and they were compared to dilute FeCl3 under the same conditions. Comparable yields of soluble sugars and acetic acid were obtained, affording an almost complete removal of pentoses when using 200mM aqueous solutions at 120 °C. The solid residues of pretreatment were characterized showing a preserved crystallinity of the cellulose, and a almost complete removal of ash forming matter other than Si. Results showed upgraded characteristic of the residues for thermal conversion applications compared to the untreated wheat straw. Chapter 7 deals with the industrial processes for the production of furfural, describing in particular an innovative process patented by Delft University of Technology and based on the results contained in this dissertation. As already mentioned, the integrated production of furfural within modern biorefineries will most probably represent the next cornerstone in the development of furfural industry. The innovative process concept described in this chapter is aimed at an economically viable and environmentally sound furfural production, with reduced energy and chemicals consumption. The evaluation of process economics shows encouraging results compared to existing processes, making this concept economically attractive. Finally, in Chapter 8 main concluding remarks are provided, as well as recommendations for future research.