ADAM F. LEE

1.1 INTRODUCTION

Sustainability, in essence the development of methodologies to meet the needs of the present without compromising those of future generations has become a watchword for modern society, with developed and devel­oping nations and multinational corporations promoting international re­search programmes into sustainable food, energy, materials and even city planning. In the context of energy and materials (specifically synthetic chemicals), despite significant growth in proven and predicted fossil fuel reserves over the next two decades, notably heavy crude oil, tar sands, deepwater wells, and shale oil and gas, there are great uncertainties in the economics of their exploitation via current extraction methodologies,

Catalysing Sustainable Fuel and Chemical Synthesis. © Lee AF. Applied Petrochemical Research 4,1 (2014), doi: 10.1007/s13203-014-0056-z. Licensed under Creative Commons Attribution License, http://creativecommons. org/licenses/by/3.0.

and crucially, an increasing proportion of such carbon resources (estimates vary between 65 and 80 % [1-3]) cannot be burned without breaching the UNFCC targets for a 2°C increase in mean global temperature relative to the pre-industrial level [4, 5]. There is clearly a tightrope to walk between meeting rising energy demands, predicted to rise 50 % globally by 2040 [6] and the requirement to mitigate current CO2 emissions and hence cli­mate change. The quest for sustainable resources to meet the demands of a rapidly rising global population represents one of this century’s grand challenges [7, 8].

While many alternative sources of renewable energy have the poten­tial to meet future energy demands for stationary power generation, bio­mass offers the most readily implemented, low cost solution to a drop-in transportation fuel for blending with/replacing conventional diesel [9] via carbohydrate hydrodeoxygenation (HDO) or lipid transesterification illus­trated in Scheme 1. First generation bio-based fuels derived from edible plant materials received much criticism over the attendant competition be­tween land usage for fuel crops versus traditional agricultural cultivation [10]. Deforestation practices, notably in Indonesia, wherein vast tracts of rainforest and peat land are being cleared to support palm oil plantations have also provoked controversy [11]. To be considered sustainable, sec­ond generation bio-based fuels and chemicals are sought that use biomass sourced from non-edible components of crops, such as stems, leaves and husks or cellulose from agricultural or forestry waste. Alternative non-food crops such as switchgrass or Jatropha curcas [12], which require minimal cultivation and do not compete with traditional arable land or drive de­forestation, are other potential candidate biofuel feedstocks. There is also growing interest in extracting bio-oils from aquatic biomass, which can yield 80-180 times the annual volume of oil per hectare than that obtained from plants [13]. Approximately 9 % of transportation energy needs are predicted to be met via liquid bio-fuels by 2030 [14]. While the abundance of land and aquatic biomass, and particularly of agricultural, forestry and industrial waste, is driving the search for technologies to transform ligno — cellulose into fuels and chemical, energy and atom-efficient processes to isolate lignin and hemicellulose from the more tractable cellulose compo­nent, remain to be identified [15]. Thermal pyrolysis offers one avenue by which to obtain transportation fuels, and wherein catalysis will undoubt-

SCHEME 1: Chemical conversion routes for the co-production of chemicals and transportation fuels from biomass edly play a significant role in both pyrolysis of raw biomass and subse­quent upgrading of bio-oils via deoxygenation and carbon chain growth. Catalytic depolymerisation of lignin may also unlock opportunities for the production of phenolics and related aromatic compounds for fine chemical and pharmaceutical applications [16].

Biodiesel is a clean burning and biodegradable fuel which, when de­rived from non-food plant or algal oils or animal fats, is viewed as a viable alternative (or additive) to current petroleum-derived diesel [17]. Com­mercial biodiesel is currently synthesised via liquid base-catalysed trans­esterification of C14-C20 triacylglyceride (TAG) components of lipids with C1-C2 alcohols [18-21] into fatty acid methyl esters (FAMEs) which con­stitute biodiesel as shown in Scheme 2, alongside glycerol as a potentially valuable by-product [22]. While the use of higher (e. g. C4) alcohols is also possible [23], and advantageous in respect of producing a less polar and corrosive FAME [24] with reduced cloud and pour points [25], the current high cost of longer chain alcohols, and difficulties associated with sepa­rating the heavier FAME product from unreacted alcohol and glycerol, remain problematic. Unfortunately, homogeneous acid and base catalysts can corrode reactors and engine manifolds, and their removal from the resulting biofuel is particularly problematic and energy intensive, requir­ing aqueous quench and neutralisation steps which result in the formation of stable emulsions and soaps [9, 26, 27]. Such homogeneous approaches also yield the glycerine by-product, of significant potential value to the pharmaceutical and cosmetic industries, in a dilute aqueous phase con­taminated by inorganic salts. Heterogeneous catalysis has a rich history of facilitating energy efficient selective molecular transformations and con­tributes to 90 % of chemical manufacturing processes and to more than 20 % of all industrial products [28, 29]. While catalysis has long played a piv­otal role in petroleum refining and petrochemistry, in a post-petroleum era, it will face new challenges as an enabling technology to overcoming the engineering and scientific barriers to economically feasible routes to bio­fuels. The utility of solid base and acid catalysts for biodiesel production has been extensively reviewed [20, 30-33], wherein they offer improved process efficiency by eliminating the need for quenching steps, allowing continuous operation [34], and enhancing the purity of the glycerol by­product. Technical advances in catalyst and reactor design remain essen­tial to utilise non-food based feedstocks and thereby ensure that biodiesel remains a key player in the renewable energy sector for the 21st century. Select pertinent developments in tailoring the nanostructure of solid acid and base catalysts for TAG transesterification to FAMEs and the related esterification of free fatty acid (FFAs) impurities common in bio-oil feed­stocks are therefore discussed herein.

Biomass also offers the only non-fossil fuel route to organic molecules for the manufacture of bulk, fine and speciality chemicals and polymers [35] required to meet societal demands for advanced materials [8, 36]. The production of such highly functional molecules, whether derived from petroleum feedstocks, requires chemoselective transformations in which e. g. specific heteroatoms or functional groups are incorporated or removed without compromising the underpinning molecular properties. The selective oxidation (selox) of alcohols, carbohydrates and related a, P-unsaturated substrates represent an important class of reactions that

SCHEME 2: Carbon cycle for biodiesel production from renewable bio-oils via catalytic transesterification

underpin the synthesis of valuable chemical intermediates [37, 38]. The scientific, technological and commercial importance of green chemistry presents a significant challenge to traditional selox methods, which pre­viously employed hazardous and toxic stoichiometric oxidants including permanganates, chromates and peroxides, with concomitant poor atom ef­ficiencies and requiring energy-intensive separation steps to obtain the de­sired carbonyl or acid product. Alternative heterogeneous catalysts utilis­ing oxygen or air as the oxidant offer vastly improved activity, selectivity and overall atom efficiency in alcohol selox (Scheme 3), but are particu­larly demanding due to the requirement to activate molecular oxygen and C-O bonds in close proximity at a surface in a solid-liquid-gas environ­ment [39-41], and must also be scalable in terms of both catalyst synthe­sis and implementation. For example, continuous flow microreactors have been implemented in both homogeneous and heterogeneous aerobic selox, providing facile catalyst recovery from feedstreams for the latter [42, 43],
but their scale-up/out requires complex manifolding to ensure adequate oxygen dissolution uniform reactant mixing and delivery [44, 45]. Ef­forts to overcome mass transport and solubility issues inherent to 3-phase catalysed oxidations have centred around the use of supercritical carbon dioxide to facilitate rapid diffusion of substrates to and products from the active catalyst site at modest temperatures [46] affording enhanced turn­over frequencies (TOFs), selectivity and on-stream performance versus conventional batch operation in liquid organic solvents [47-51].

The past decade has seen significant progress in understanding the fun­damental mode of action of Platinum Group Metal heterogeneous cata­lysts for aerobic selox and the associated reaction pathways and deactiva­tion processes [41]. This insight has been aided by advances in analytical methodologies, notably the development of in situ or operando (under working conditions) spectroscopic [52-54] /microscopic [55-58] tools able to provide quantitative, spatio-temporal information on structure — function relations of solid catalysts in the liquid and vapour phase. Parallel improvements in inorganic synthetic protocols offer finer control over pre­parative methods to direct the nanostructure (composition, morphology, size, valence and support architecture) of palladium catalysts [59-61] and thereby enhance activity, selectivity and lifetime in an informed manner.

Ultimately, heterogeneous catalysts may offer significant advantages over homogeneous analogues in respect of initial catalyst cost, product separa­tion, and metal recovery and recyclability [62]. Catalyst development can thus no longer be considered simply a matter of reaction kinetics, but as a clean technology wherein all aspects of process design, such as solvent selection, batch/flow operation, catalyst recovery and waste production and disposal are balanced [63]. The efficacy of Platinum Group Metals (PGMs) surfaces towards the liquid phase oxidation of alcohols has been known for over 50 years [64], and the development of heterogeneous plati­num selox catalysts (and more recently coinage metals such as gold [65, 66]) the subject of recent reviews [39, 67-69] hence only palladium selox catalysis is described herein.