3.0 LIQUID-PHASE CATA LYTIC
P RO C E S S I N G
OVERVIEW: Biomass-derived oxygenates typically have a high-degree of functionality and a low- thermal stability making it difficult to process them in the gas phase where traditional petroleum reactions occur. Catalytic processing in the liquid- phase allows thermally unstable molecules to be selectively converted into a range of fuels and chemicals. Liquid-phase processing has many advantages including higher thermal efficiencies, high reaction rates per reactor volume, and exclusion of an energy intensive distillation step. However, new catalyst and reactor systems need to be designed specifically for conversion of biomass-derived feedstocks in the liquid-phase in order to realize the full potential of this
technology.
3.1 INTRO D U C T I O N
The overarching goal of liquid-phase catalytic processing of biomass-derived compounds (e.g., sugars) is to produce next-generation liquid transportation fuels that: (i) can be used with the existing infrastructure; (ii) do not involve
energetically-intense distillation steps; and (iii) have high rates of production per reactor volume. One strategy for achieving this goal is to convert sugars to liquid alkanes, such that the liquid fuel derived from biomass is identical, with respect to chemical composition and energy density, to the current liquid fuels currently derived from petroleum (e.g., gasoline, diesel, jet fuel). Another strategy is to produce unconventional types of fuels by precisely engineering molecules with well-defined amounts of oxygen such that the desired volatility and combustion properties are optimized without adversely affecting the energy density or the hydrophobicity of the fuel.
3.2 PROCESS DESCRIPTION A N D
R E S U LTING BIOFUELS
Liquid-phase catalytic processing of biomass- derived compounds offers unique opportunities for achieving high yields of specific, and well-defined, liquid fuels from biomass. For example, liquid- phase catalytic processing of sugars is typically carried out at lower temperatures (e.g., 500 K) compared to biomass pyrolysis, liquefaction, or gasification. However, whereas these latter process can operate with complex biomass feedstocks (e.g., containing cellulose, hemicellulose, and lignin components), liquid-phase catalytic processing typically involves feedstocks containing specific biomass-derived compounds, such as sugars or polyols. Thus, an advantage of liquid-phase catalytic processing is that high selectivities and yields to targeted fuel compounds can be achieved, but a disadvantage of such processing is that real biomass feedstocks must be pretreated to prepare a feed solution that for subsequent liquid-phase catalytic processing.
Because of the high level of functionality
(e.g., -OH, -C=O, -COOH groups), biomass feeds have low volatility and high reactivity, and these feeds must typically be processed by liquid-phase technologies. In addition, in view of their
hydrophilic properties, liquid-phase processing of carbohydrate feeds is typically carried out in the aqueous phase, or under biphasic conditions employing an aqueous and an organic phase. In general, a variety of fuels and chemical
intermediates can be produced from these biomass feeds by employing various types of reactions including: hydrolysis, dehydration, isomerization, C-C coupling (e.g., aldol-condensation), reforming, hydrogenation, oxidation, and hydrogenolysis. The heterogeneous catalysts used for these reactions can include acid, base, metal, and metal-oxide catalysts. Several types of reactions typically occur
3.2 Process Description and Resulting Biofuels
3.2 Feedstocks
3.4 Review of Catalytic Reactions for Liquid Phase Processing
3.5 Advantages of Liquid-Phase Processing
3.6 Current Technology Limitations and Research/Development Needs
3.7 Recommendations
3.8 References
PA RT I C I PA N T S :
Scott Auerbach, Paul Blommel, A.A. (Kwesi) Boateng, Douglas Cameron, James A. Dumesic, Frank Gerry, John Holladay, George W. Huber, Christopher Jones, Alexander Katz, Leo Manzer, Simona Marincean, Raul Miranada,Valeria Reed, John Regalbuto, William D. Rhodes, Bob Saxton,
Brent Shanks, Philip H. Steele, James F Stevens, Galen Suppes,Thomas Henry Vanderspurt,Yang Wang, Rosemarie D. Wesson,Ye Xu, Conrad Zhang
the use of multi-functional catalysts.
The production of ethanol by fermentation of glucose is also a liquid-phase process that produces a liquid transportation fuel. This process has been practiced for many years (e.g., by the brewing industry), which has led to its wide-scale
implementation in the production of bioethanol as a transportation fuel. However, fermentation has several disadvantages. One disadvantage is the rather high energy costs associated with the distillation of ethanol from the aqueous solution in which it is produced. Another is the low rates of production per volume of reaction vessel. In contrast to bioethanol, next-generation fuels derived from liquid-phase processing should have higher energy densities per volume and properties that are nearer to those of gasoline and
diesel fuels.
3.3 FEEDSTO C K S
Biomass may be made amenable for liquid phase catalytic processing by a variety of means. First, the raw biomass feedstock may be fed directly to a liquid phase catalytic reactor. In this case, usually relatively simple feeds such as vegetable oils, starch or cellulose are utilized. For processing more complex, lignocellulosic feeds, the biomass is usually pretreated to produce a liquid stream that is amenable to catalytic upgrading. Bio-oils and bio-oil components can also be used as feeds for liquid-phase processing. Bio-oil generation by fast pyrolysis or liquefaction processes has been discussed in detail in Thrust 1. The most common biomass feed sources along with a few of their key advantages and disadvantages are listed below:
CELLULOSEcan be solvated by aqueous acidic,
basic, or ionic liquid mediated processes. Glucose monosaccharides and degradation products can be
sourced from cellulose for liquid phase upgrading. Hemicellulose can be easily liberated from lignocellulose by mild aqueous acid treatment, yielding a mixture of C5 and C6 sugars and degradation products. Pure hemicellulose as a starting feedstock is difficult to obtain.
LIGNINcan be separated from the sugar
fractions of the holocellulose portion of biomass by multiple means including kraft pulping,
Different treatments leave the solid lignin fraction with different degrees of cross-linking and
condensation. Black liquor is a waste stream produced via kraft pulping that contains lignin and several inorganic chemicals from the pulping process. The aromatic rich stream represents a possible feedstock for further catalytic upgrading.
STARCH and sucrose are common food sources
but are also possible feedstocks for fuels. Starch is a glucose polymer that can be treated to yield glucose and degradation products. Sucrose is a simple disaccharide containing a glucose and fructose unit.
Plant and animal GLYCERIDES are amenable to
liquid phase catalytic processing for the synthesis of gasoline and diesel range fuels. Both virgin oils and waste oils can be feedstocks for such
processes. However, the relative amounts of these feeds are too limited to make a large impact on transportation fuels needs in the US.
PROTEINSrepresent another class of renewable
raw materials for liquid phase catalytic processing. However, they do not represent a viable feedstock for fuels, but maybe useful for chemical production.