Resumen de Catalizadores para biorrefinería obtención de furfural y su transformación a productos de condensación aldólica
Irantzu Sádaba Zubiri
Nowadays, the sustainability of chemical production processes is one of the most important objectives in the chemical industry. For the majority of its existence, abundant and cheap fossil feedstocks were available. However, their continuous exploitation together with an ever-increasing demand clearly indicates that the price of petroleum feedstock will keep on rising in the near future. This will certainly affect the chemical industry, which will shift gradually towards renewable feedstocks. A new concept is being developed, a biorefinery, in which, biomass is used as feedstock in order to accomplish the same goals as in traditional refineries: production of chemicals, fuels, energy, and even food.
Carbohydrates are one of the most abundant components in biomass. Sugars, in the form of mono- and disaccharides, are readily available from various biomass sources, including crop wastes and constitute a useful raw material for the production of versatile chemicals. In particular, lignocellulosic pentose monosaccharides present in hemicelluloses such as xylose are nowadays catalytically dehydrated into furfural, which is a very versatile compound. In the context of the lignocellulosic-based biorefineries, furfural is acquiring increasing importance as one of the most outstanding building blocks.1 It is one of the compounds in the `top 10+4¿ revised list of bio-based product opportunities from carbohydrates. It can be used in the production of biofuels (as we will see ahead) and bio-based chemicals.
However, the type of catalysts used in these processes has a crucial effect on the sustainability of the process. Nowadays inorganic acid solutions in water are used for the industrial process. However the use of heterogeneous catalysts is preferred over homogeneous ones to prevent expensive separation and purification processes downstream, corrosion problems in industrial equipments and to decrease the negative environmental impact.
This thesis deals with the search of sustainable catalysts for furfural valorization processes. Two different reactions related to the valorization of furfural have been selected: (i) the dehydration of xylose to furfural and (ii) its aldol condensation with acetone to form larger molecules. These aldol products can be subsequently used as such in various applications or converted to fuels by using hydrogenation-dehydration processes. Both of them use water as a solvent, which can be a threat to the stability of the catalyst, so specific attention has been paid to the stability, leaching and reutilization issues. The catalysts were characterized by different techniques and tested in the reactions. The characterization allows a better understanding of the catalytic activity of the solid and can provide insights about the nature and intrinsic activity of the catalytic sites In the first place, results package 1 (RP 1) covers the research on the dehydration of xylose to furfural. Two different acid catalysts were explored. The reaction was first investigated using vanadium phosphates (VPO) as catalysts. Deeper investigation was conducted on orthorhombic vanadyl pyrophosphate (VO)2P2O7, that could be recycled by simply separating the solid acid from the reaction mixture by centrifugation, and no drop in catalytic activity and furfural yields was observed in consecutive 4 h batch runs. However, detailed catalytic characterization studies revealed that the vanadyl pyrophosphate acts as a source of active water soluble species leached from the solid.
Following in this RP1, water-soluble polystyrene modified with sulfonic groups was tested in this reaction. This polymer can be obtained from polystyrene via sulfonation and it is thus a method of reclamation of plastic waste. Excellent results of activity and recyclability were found, although the process required a non conventional ultrafiltration step. In order to overcome this drawback a simple method consisting on a heterogeneization of the polyelectrolyte by its entrapment in a sol-gel silica network was developed. The presence of aminopropyl-funtionalized silica was found to be essential to provide stability by the formation of a strong interaction between the polymer and the support. In this case, the catalyst was successfully reutilized during three consecutive catalytic cycles.
In results package 2 (RP 2), the aldol condensation reaction of furfural and acetone was studied. Solid basic catalysts based on mixed oxides Mg-Zr were tested in the reaction. Two different preparation methods were selected: coprecipitation and alcogel routes. The first one, the coprecipitation method is described in chapter 6.
Chapter 7 is devoted to alcogel methodology. Following these methodologies, two series of catalysts varying the composition were studied and structure-activity relationships were established. Additionally the likely causes of deactivation were also investigated. Finally, the last chapter covers the study of the influence of the activation temperature on the catalytic activity and the chemical and textural properties of the solids prepared by these two routes. In this chapter it is shown that the calcination step is essential to activate the catalyst, irrespective of the preparation method used.
