JUAN CARLOS SERRANO-RUIZ and JAMES A. DUMESIC

4.1 INTRODUCTION

Society has reached high levels of development during the last century. This progress, however, has been achieved at the expense of extensive consumption of natural resources, such as petroleum, natural gas and coal. These fossil fuel resources took millions of years to be formed, and they are currently being consumed at a rate that is orders of magnitude higher than their natural regeneration cycle, making them non-renewable sources of energy. The most recent data available for world energy consumption indicate that society still remains highly dependent on fossil fuels at the present time. For example in 2008, fossil fuels supplied 85% of the total energy consumed in the US, [1] and almost 80% of the energy produced in the European Union. [2] These fossil fuel resources are used to provide energy for various sectors of society (i. e., residential, commercial, indus-

Catalytic Routes for the Conversion of Biomass into Liquid Hydrocarbon Transportation Fuels. © Serrano-Ruiz JC and Dumesic JA. Energy & Environmental Science 4,83 (2011), DOI: 10.1039/ c0ee00436g. Reproduced from Energy & Environmental Science with permission from The Royal Society of Chemistry.

trial, transportation and electrical power), among which the transporta­tion sector is the largest and fastest growing energy sector, responsible for almost one third of the total energy consumed in the world. Moreover, a large fraction of the energy for the transportation sector (96%) is currently derived from petroleum. [3]

Three important issues are associated with the large-scale utilization of fossil fuels: availability, global warming and uneven geographic dis­tribution of reserves. Fossil fuels are finite and, as indicated above, their current consumption rate is higher than their corresponding regeneration rate, leading inevitably to depletion. Projections for the near future indi­cate that world energy consumption will increase by 35% over the next 20 years to meet the growing demand of industrialized countries and the rapid development of emerging economies, [4] and world demand for petroleum will raise by 30%, reaching 111 millions of barrels per day in 2035. [5] Taking into account these forecasts and current data of proven reserves, it has been estimated that oil, natural gas and coal will be depleted within the next 40, 60 and 120 years, respectively. [6] In the case of petroleum, many researchers predict a more dramatic situation and estimate that the global production of oil will reach a maximum in the year 2020 and decay thereafter. [7]

Global warming is, possibly, the most dramatic and known collateral effect produced by the massive utilization of fossil fuels. [8] Fossil fuels are transformed into energy by means of combustion reactions, leading to net emissions of CO2, a strong greenhouse gas, into the atmosphere. Ac­cordingly, the extraction of fossil fuels for energy production has allowed a large part of the carbon stored in the earth for millions of years to be released in just a few decades.

Fossil fuels reserves are not equally distributed around the world. The Middle-East countries control the 60% of the oil reserves and the 41% of natural gas supplies, and only three countries (US, China and Russia) ac­count for 60% of the world recoverable coal reserves. [4] This situation can lead to economic instabilities, requires the transportation of fossil fuel resources over long distances, and can cause political and security prob­lems worldwide.

The issues outlined above, inherently associated with fossil fuels, sug­gest that society requires new sources of energy to ensure progress and protect the environment for future generations. These new sources of en­ergy should: (i) have the potential to effectively replace fossil fuels in the current energy production system and (ii) be renewable, well distributed around the world, and not contribute to the accumulation of greenhouse gases into the atmosphere. In this respect, natural resources such as solar energy, wind, hydroelectric power, geothermal activity, and biomass meet these requirements. Unlike fossil fuels, they are abundant and allow the development of zero-carbon or carbonneutral technologies, thus contrib­uting to mitigation of global warming effects. Substitution of fossil fuel — based technologies for those derived from renewable sources is currently spurred by various governments, [9,10] and it will be done progressively and selectively. Thus, while solar, wind, hydroelectric, and geothermal have been proposed as excellent alternatives to coal and natural gas for heat and electricity production in stationary power applications, [11,12] biomass is the only sustainable source of organic carbon currently avail­able on earth, [13] and it is considered to be an ideal substitute for petro­leum in the production of fuels, chemicals and carbon-based materials. [14,15] However, when designing strategies for potential replacement of crude oil by biomass, it is important to note that the petrochemical industry currently consumes three quarters of the crude oil to cover the demand for liquid hydrocarbon fuels of the transportation sector, whereas only a small fraction of the petroleum is utilized in the synthesis of industrial chemicals and other derivatives. [16] Consequently, an effective implementation of biomass in the current energy system will necessarily involve the develop­ment of new technologies for the large-scale production of biofuels.

At the present time, two biomass-derived fuels (so-called first genera­tion of biofuels) have been successfully implemented in the transporta­tion sector: biodiesel (a mixture of long-chain alkyl esters produced by transesterifi cation of vegetable oils with methanol) and ethanol (produced by bacterial fermentation of corn and sugar cane-derived sugars). The pen­etration of these liquid biofuels in the transportation sector is still very weak, and in 2005 they represented only 2% of the total transportation energy. [3] However, the important environmental and economic benefits derived from their large-scale utilization will stimulate society to progres­sively increase reliance on biofuels. Thus, according to projections by the International Energy Agency, the world biofuel production will increase from the current level of 1.9 million of barrels per day (mbd) in 2010 to 5.9 mbd by 2030, which represents 6.3% of the world conventional fuels production. [4] Unlike petroleum-based fuels, liquid biofuels are consid­ered carbon neutral since CO2 produced during fuel combustion is con­sumed by subsequent biomass regrowth. [17] Furthermore, recent stud­ies indicate that the use of liquid biofuels produced domestically would strengthen economies by reducing the dependence of foreign oil and by creating new well-paid jobs in different sectors such as agricultural, forest management and oil industries. [18]