Inaki Gandarias and Pedro Luis Arias

Additional information is available at the end of the chapter http://dx. doi. org/10.5772/52581

1. Introduction

In a future sustainable scenario a progressive transition by the chemical and energy indus­tries towards renewable feedstock will become compulsory. Energy demand is expected to grow by more than 50% by 2035 [1], with most of this increase in demand emerging from developing nations. Clearly, increasing demand from finite petroleum resources cannot be a satisfactory policy for the long term. The transition to a more renewable production system is now underway; however, this transition needs more research and investment in new tech­nologies to be feasible.

Biomass appears as the only renewable source for liquid fuels and most commodity chemi­cals [2]. This is the reason why, in the near future, bio-refineries in which biomass is catalyti — cally converted to pharmaceuticals, agricultural chemicals, plastics and transportation fuels will take the place of petrochemical plants [3]. Indeed, biomass represents 77.4% of global renewable energy supply [4]. Current technologies to produce liquid fuels from biomass are typically multistep and energy-intensive processes, including the production of ethanol by fermentation of biomass derived glucose [5],bio-oils by fast pyrolysis or high pressure lique­faction of biomass [6,7], polyols and alkanes from hydrogenolysis of biomass derived sorbi­tol [8],and biodiesel from vegetable oils [9].Biomass can also be gasified to produce CO and H2(synthesis gas), which can be further processed to produce methanol or liquid alkanes through Fischer-Tropsch synthesis [10].

The so-called "First Generation" biofuels, such as sugarcane ethanol in Brazil, corn ethanol in US, oilseed rape biodiesel in Germany, and palm oil biodiesel in Malaysia, already present mature commercial markets and well developed technologies. Nonetheless, there is a world­wide increasing awareness against the use of edible oils and seeds to generate transporta­tion fuels, and critical voices have aroused questioning the actual sustainability of these

"First Generation" biofuels. In fact, nowadays 95 % of biodiesel is made from edible oil [9]. This means that possible food resources are being used as automotive fuels when some part of the World’s population is suffering from hunger. Therefore, large-scale production of bio­diesel from edible oils may bring about a global imbalance in the food supply market. An­other significant concern of using "First Generation" technologies is the deforestation and the destruction of ecosystems. Indeed, the expansion of oil-crop plantations for biofuel pro­duction on a large scale has caused deforestation in countries such as Malaysia, Indonesia and Brazil because more and more forest has been cleared for plantation purposes. In addi­tion to this, in developing countries energy crops are powerful competitors for scarce water resources [11].

Being the non-edible portion of the plant and the most abundant source of biomass, ligno — cellulosic biomass materials are attracting growing attention as sustainable and renewable energy sources. The so-called "Second Generation" technologies for the production of fuels and chemicals can use a wide range of lignocellulosic biomass residues such as agricultural, industrial, and forest wastes, and also energy crops (willow, switchgrass) that do not com­pete with food crops for available land. The average composition of lignocellulosic material is as follows: 50% cellulose, 25% hemicellulose, and 20% lignin [12]. Cellulose is a linear pol­ysaccharide with p-1,4 linkages of D-glucopyranose monomers (Figure 1). Hemicellulose is a more complex polymer containing five different sugar monomers: five carbon sugars (xy­lose and arabinose) and six carbon sugars (galactose, glucose, and mannose). Lignin is a highly branched aromatic polymer, that consists of an irregular array of variously bonded "hydroxy-" and "methoxy-" substitutedphenylpropane units. Lignin is mainly found in woody biomass. Lignocellulosic materials can be converted into liquid fuels by three pri­mary routes, including (i) syngas production by gasification, (ii) bio-oil production by pyrol­ysis or liquefaction, and (iii) acid hydrolysis reactions [13].

In the pyrolysis process, biomass feedstock is heated in the absence of oxygen, forming a gaseous product, which after cooling condenses. Depending on the operating conditions that are used, pyrolysis processes are known as slow or fast pyrolysis. Fast pyrolysis proc­esses are characterized by high rates of particle heating (heating rate > 1000°C/min) to tem­peratures around 500°C, and rapid cooling of the produced vapors to condense them (vapor residence time 0.5-5s). In order to obtain that fast heating rates, it is essential to use reactors that provide high external heat transfer (such as fluidized bed reactors) and to guarantee an efficient heat transfer through the biomass particle, using biomass particle size of less than 5 mm [7]. Fast pyrolysis produce 60-75 wt% of liquid bio-oil, 15-25 wt% of solid char, and 10-20 wt% of non condensable gases, depending on the feedstock. In slow pyrolysis biomass is heated to around 500°C at much lower heating rates than those used in fast pyrolysis. The vapor residence times are much longer; they vary from 5 min to 30 min. As a consequence of the lower heating rate and of the longer vapor residence time, lower yields to pyrolysis oils and higher yields to char and gas products are obtained (Figure 2). As a result of all this, for bio-oil production from biomass, fast pyrolysis processes are preferred.

Bio-oils are dark-red brown color liquids. They are also known as pyrolysis oils, bio-crude oil, wood oil or liquid wood. Bio-oils usually have higher density, viscosity and oxygen con­tent compared to fuel-oil. While the sulfur and nitrogen content is usually smaller (Table 1). The high oxygen content of bio-oils generates some negative characteristics like low heating value (HV), immiscibility with conventional fuels and high viscosity. A serious problem of bio-oils is their instability during storage, as their viscosity, HV and density are affected. This is because some of the organic compounds present in bio-oils are highly reactive. For instance, ketones, aldehydes and organic acids react to form ethers, acetals and hemiacetals respectively [15]. Therefore, bio-oils need to be upgraded to reduce their oxygen content in order to increase their stability, to be miscible with conventional oil, and to increase their H/C ratio. This upgrading can be carried out through three different routes: (i) catalytic hy­drotreating, usually known as hydrodeoxygenation (HDO), which consists mainly on decar­boxylation, hydrocracking, hydrogenolysis and hydrogenation reactions, (ii) zeolite upgrading or (iii) through esterification reactions. Zeolite upgrading is carried out without external hydrogen sources, and therefore the resulting oil has lower HV and H/C than con­ventional fuels. Esterification can significantly increase the chemical and physical properties of bio-oil, however it requires using high amounts of alcohols, which are highly demanded. Catalytic hydrotreating appears to have the greatest potential to obtain high grade oils which are compatible with the already available infrastructure for fossil fuels.

Not only fuels, but also commodity chemicals are nowadays derived from petroleum-based resources. Commodity chemicals are involved in the production of a wide variety of prod­ucts and thus are an essential and integral part of the modern societies. Hence, in the search for a sustainable scenario, it is crucial to also look towards alternative biorenewable sources for these chemicals. In the case of platform chemicals coming from biomass, such as glucose, levulinic acid, 5-(hydroxyl-methyl furfural), sorbitol, or glycerol, they usually have higher O/C ratio than most commodity chemicals. Therefore, the conversion of these platform chemicals into value-added chemicals usually requires O removal reactions.

This book chapter summarizes the main aspects involved in the catalytic hydrotreating processes for the oxygen removal from bio-oils and from biomass based platform chemicals.