g a l a c t oglucomannans and arabinog l u c u ro n ox y l a n s .
almost all the individual compounds formed from biomass during conversion processes such as pyrolysis and liquefaction. However, many of the compounds in biomass-derived liquid processes such as those described in Chapter 3 are not hydrocarbons; these compounds require
development of procedures different from those developed for petroleum.
6 . 2 . T H E R M O DY N A M I C S
Major innovations in biorefining will require the identification of new processes, many of these being catalytic in nature. If we follow the lead of the of the petroleum and petrochemical industries, these new processes will be invented by scientists and engineers working with reaction mechanisms, reaction networks, and novel reactors. In addition, these developments will require an understanding of how reactants, products, and intermediates interact with catalyst surfaces, partition between the two phases of a biphasic reactor, are distributed
between the gas and liquid (or solvent) phases, or are extracted into specific solvents in separation processes. At present, however, this vital
information about the physical properties of biomass-derived molecules is lacking. This dearth of information impedes not only the discovery of new processes, but also the design, operation, optimization and control of emerging technologies. The unprecedented efficiencies achieved by the petroleum and petrochemical refineries of today would not have been achieved without detailed process analyses, wherein intermediates and heat generated in one process are used in other
processes, such that the most effective utilization of the petroleum feed is achieved, both on an atom- efficiency and energy basis. Moreover, the success of the petroleum and petrochemical industries has been achieved through the development and continuous improvement of new chemical
processes, most of these processes being catalytic in nature. In this respect, the formulation of a new process or the elucidation of the reaction
chemistry for an existing process typically involves consideration, at the molecular level, of reaction mechanisms and/or reaction networks. Such analyses also depend on having knowledge of the physical properties of the reactants, products, and importantly the potential reaction intermediates. Likewise, to advance the field of biofuels production and provide a solid foundation for further work, it is essential that theorists develop new approaches for prediction of the thermodynamic properties (enthalpy, entropy, heat capacity) of biomass-derived molecules in the gas and liquid states, as well as in various solvents. This work should provide not only reliable thermodynamic data for specific biomass-derived molecules of perceived importance in a biorefinery, but should also yield new methods that can be used effectively by practitioners to estimate the thermodynamic properties of new compounds that may appear, for example, in reaction mechanisms, reaction networks, and process flow sheets for new chemical processes. It is important to note, however, that the vast knowledge on the physical properties of molecules now available from the past decades of research and development in the petroleum and
petrochemical industries deals primarily with hydrocarbon molecules that have limited oxygen functionality. Furthermore, most of this information deals with hydrocarbons in the gas phase or
perhaps as non-polar liquids. Unfortunately, many of the reactants, intermediates and even some of the products involved in the processing of biomass- derived feeds are highly oxygenated species, and these molecules may well be processed most effectively in water or in polar solvents. In short, the starting point for any systematic discovery of a new process or for the implementation of an emerging process in a biorefinery is an analysis of the overall
thermodynamics, independent of whether the process is a chemical transformation or a
separation step. It is for this reason that the lack of thermodynamic data for the biomass-derived reactants, products and intermediates presents a major impediment to further developments in biorefining.
6 . 3 . C H E M I CAL REACTION
ENGINEERING
6.3.1 Identification of P rototypical Reactants (model compounds)Much has been learned about catalytic processing in petroleum refining as a result of experimentation with individual compounds that are representative of a larger class of compounds; these are usually referred to as model compounds. Examples are isobutane in catalytic cracking and thiophene in hydrodesulfurization of gas oil. Extensive testing is required to distinguish good from unsatisfactory model compounds, because no model is without limitations in representing a larger group of compounds.
The application of this approach to biomass- derived feedstocks is still in its infancy. The model compounds identified so far have barely been investigated as reactants in conversions intended to mimic biomass conversion. Work is needed to identify appropriate model compounds in biomass- derived feedstocks. A good example of the kind of studies that need to be undertaken is a series of model compounds that were identified as pertinent to the upgrading of pyrolysis oil to hydrocarbons [Gayubo,Aguayo et al., 2004a, 2004b, 2005]. Certain compounds have been shown to be valuable for investigations related to petroleum refining because, being more than representative in their reactivities, they are key components of the
reactive compounds in the feedstock and,
therefore, need to be converted for the product to meet the required purity standards. An example is 4,6-dimethyldibenzothiophene, which must be converted in large measure for the fuel to meet sulfur content specifications.
Work is needed to identify these kinds of key compounds in biomass-derived feedstocks for various upgrading processes.
6.3.2 Lumping of Biomolecules
A useful methodology for representation of complex multicomponent feedstocks in petroleum refining technology is called lumping, whereby a group of compounds that are chemically similar are represented as if they were a single (perhaps fictitious) compound chosen to approximate their reactivity characteristics. The enormous
simplification of this procedure (representing, for example, hundreds of compounds in a petroleum residuum with tens of lumps), which is based on an extensive set of analytical results characterizing the feeds and products, makes it possible to accurately predict the reactivities of various feedstocks (even those not yet tested) and the properties of their refining products.
Successful application of the lumping methods to biomass-derived feedstocks will require thorough analysis of these feedstocks and the products of their conversion, primarily by mass spectrometry. It will also require a more fundamental
understanding of the reactions that occur in biomass upgrading, which may be obtained in part from investigations of representative model compounds, as described above.
Successful identification of model compounds for the upgrading of biomass-derived feedstocks will also require extensive experimentation so that
candidate models can be improved by comparison with experiment. The work will need to be extended to a range of specific catalysts so that generalizations can be made about fundamental catalyst types, such as acids and metals.
6.3.3 Characterization of Activities of Prototypical Cat a l y s t s
At this early stage of understanding biomass conversion, there is only fragmentary knowledge about how various feedstocks (or the model compounds representing them) are converted in the presence of different catalysts. To achieve accurate predictions of the best catalysts and reaction conditions for candidate biomass- conversion processes, it will be necessary to generate data that demonstrate the conversion of individual model compounds with catalysts that may be considered protypical, such as acidic zeolites, supported metals, and bimetallic catalysts consisting of metals on acidic supports. Additional work will provide further improved ideas about which catalysts constitute the best prototypes. The initial choices should be based on performance data, and candidate catalysts that are low in activity, poor in selectivity, or poor in stability should be excluded. As testing proceeds, the data should indicate catalyst stability in longer-term tests in flow systems.
Since biomass contains much more oxygen than fossil fuels such as coal, oil and natural gas, catalytic conversion of this renewable resource will
necessarily involve processing in aqueous phase systems. Consequently, many of the heterogeneous catalysts that have been developed for hydrocarbon processing are inappropriate for conversion of biomass in aqueous media. Moreover, the chemical reactions and mechanisms involved in the
transformation of biomass to transportation fuels are poorly understood.
Transformation of biomass-derived molecules to transportation fuels will involve the removal of oxygen while maintaining, as much as possible, the carbon and hydrogen inventory of the molecules. These transformations necessarily include the selective manipulation of C-C, C-O and C-H bonds. Heterogeneous catalysts and enzymes must be developed to selectively carry out the desired reactions while minimizing the unproductive formation of CO2 and heavy tars.
The data used to identify prototypical catalysts should include activity and selectivity data, as well as enough information to determine the reaction networks of well-chosen model compounds as reactants. Determination of the networks will require analysis to identify reaction intermediates and experiments with these intermediates as feeds. Experiments should also be done to identify classes of compounds in feedstocks that are significant inhibitors of the various reactions, as well as components (even impurities) that may be catalyst poisons. This work should include long-term tests (some with model compounds as reactants and some with full feedstocks) to determine catalyst deactivation. Analysis of used catalysts will be needed to understand the mechanisms of catalyst deactivation.
Ultimately, data characterizing catalyst performance should be reduced to include fundamental reaction kinetics, with rates of reactions expressed as turnover frequencies. Such determinations require characterization of the catalyst to quantify the numbers of catalytic sites (see section 6.5).
6.3.4 Reactor Engineering
Chemical reactor engineering crosscuts all of the areas through which chemical catalysis will address the conversion of biologically derived feedstocks for fuels production. Basic studies will be required to provide the foundation for implementing these
emerging technologies. Mechanisms will need to be determined, if not in atomic detail, at least in the ability to determine kinetic reaction pathways. Once determined, these mechanisms will provide a framework for optimization of yields and
selectivities of conversion processes in practical reactors. Only after the complex details of each reaction step are worked out can these processes be compared, and eventually analyzed in scaled-up processes.
Real feedstocks, as opposed to laboratory-grade feeds, often comprise mixtures of reagents, transformed in parallel reaction networks; the formation of biodiesel by the transesterification of vegetable oil is an example.Transesterification is the reaction of triglyceride (or other esters) with alcohols to produce alkyl esters (biodiesel) and glycerol typically in the presence of acid or base catalysts.The oils contain many glycerol tri-esters from C13-C19 and each transition of tri- to di- to mono- ester could require individual kinetic expressions, all of which would have to be solved simultaneously. It is far simpler to assume that each of the tri-esters would react with similar rates and to lump these together.This might work well for this sequence, while for other more complex reaction mixtures the rates for specific species, or groups of species, could be considerably different. Therefore, it may be advantageous to identify and classify the reactants and identify those for which the overall rates of reaction are crucial to process success. (In cellulosic liquefaction for example, a class of lignin might prove to be most difficult to solubilize by a specific approach.) The identification of the fundamental reaction classes (lumps) can be determined experimentally by labeling. A similar approach is employed in the petrochemical field and in polymerization kinetic analyses.
In addition to influencing the overall yields, reactor configurations can provide a method for the
collection of basic kinetic data. Stirred tank or plug flow reactor configurations appear as the ideal extremes of mixing. However, many of the
reactions involved in the transformation of bio- derived fuels are multiphase. Mixing between the gas, liquid, and solid (e.g. the requirement for the addition of gas to the liquid phase during
hydrogenolysis) phases and at the interfaces then controls overall reactivity.