Catalysts and Enzymes in Biofuel Production

Elizabeth Hood, PhD

By Elizabeth E. Hood, PhD and Lorenz Bauer, PhD, Lee Enterprises Consulting, Inc.

Special to The Digest

Introduction

An important mitigation strategy for the impact of fossil fuels on the environment is to use biofuels from renewable sources for transportation. The problem is that the cost of biofuel production is high and this nascent industry cannot compete with oil prices without subsidies. Biofuels can be produced by a number of processes, broadly categorized as biochemical and chemical. Biochemical processes utilize proteins called enzymes (biological catalysts).

Lorenz Bauer, PhD

Chemical processes utilize chemical catalysts and/or heat to process raw materials into fuels. New technologies are invented and tested in laboratories on a regular basis. However, to make them industry-ready requires development work and still the process/ chemical/ enzyme may not be appropriate because of cost. We describe some of the latest advances in these technologies that have the potential to make biofuel production more economical.

Biochemical Processes

Biodiesel: For biodiesel, the enzymes lipase and phospholipase are the major players. A few companies are commercializing biodiesel produced with enzymatic processes. Lipase converts the free fatty acids (FFA) and triacylglycerol to fatty acid methyl esters—the main product comprising biodiesel. The phospholipase is responsible for converting phospholipids to diacylglycerol, which becomes a substrate for the lipase. Conventional processes using methanol and catalysts must remove the FFAs and the phospholipids prior to reactions to improve the quality of the biodiesel. Because the enzymes can utilize these as substrates, the yield is higher and the process saves chemical waste. Enzymes are known and available but the cost is often too high for them to be used on all feedstocks, particularly clean plant oils. One way of extending the life and thus lowering the cost of the enzymes is to immobilize them on a solid substrate to enable multiple cycles of use. Another solution is to produce them in a more cost efficient system or to improve their activity.

Cellulosic ethanol: For biomass conversion, cellulases are key for the digestion of cellulose into glucose for fermentation into biofuels. Over the past 10 years, intense investigations of issues surrounding the utilization of cellulosic feedstocks for biofuel production have been conducted. Although in theory this process can utilize the tons of biomass available from farming and dedicated feedstocks, major problems of conversion are encountered. One is the cost of the enzymes needed for deconstruction of the cellulose and hemicellulose into usable sugar streams. Several approaches have been pursued by researchers including changing the structure of the cell walls to lower the difficulty of digestion, finding better enzymes, using combined digestion and fermentation, and finding better pretreatment technologies to prepare the feedstock for the enzymes.

Enzyme cost: Pretreated biomass is deconstructed with mixtures of enzymes. To use enzymes cost-effectively, the estimated cost of the enzymes should be $0.10 per gallon of biofuel (NREL estimate). For the past 15 years, intense research on enzyme production platforms has yielded fungal enzyme mixtures that do not meet these cost requirements and in fact also require a huge infrastructure for production. A relatively new technology utilizes genetically engineered plant seeds (primarily maize) to accumulate industrial enzymes. At scale, enzymes from this system can be less expensive to produce and formulate because of low requirements for capital infrastructure. Although the plant seed production system is more cost-competitive, it has not been tested at scale for efficacy. Other research efforts are in multifunctional enzymes and combined bioprocessing organisms, the latter of which can decompose plant polymers as well as ferment them into biofuels.

Chemical Catalysts

Chemical catalysis has been the method of choice for the efficient production of transportation fuels from fossil carbon sources so it is natural that it is a mainstay of biomass conversion technology. Many of the routes to biomass transformation involve several steps including depolymerization followed by separation and upgrading processes.

Pretreatment and Deconstruction: Often the first step is a destructive pretreatment of biomass with a strong acid and base hydrolysis. Recently, researchers have been developing an ammonia-based AFEX™ method. The ammonia treatment separates carbohydrates from lignin and opens up the structure. The method greatly improves the efficiency of enzymatic upgrading. It is likely that other downstream catalytic approaches will also be facilitated by pretreatment.

Multifunctional Catalysts: The products from depolymerization include highly oxygenated compounds and light gases that are unsuitable for use as fuels. These materials need to be upgraded by separate processes. However, the required separations and waste products from pretreatment steps increase the complexity and the cost of biofuel production. These requirements have led researchers to search for heterogeneous catalysts that can directly convert biomass to liquid fuels.

Combining biomass depolymerization and upgrading to liquids by deoxygenation in a single step is particularly attractive. Reforming light gases into liquid products is also desirable. An additional catalyst function is controlling the reactivity of intermediate products to prevent recombination into tars and high molecular weight molecules. The direct conversion requires a method for contacting the biomass with the catalyst either by the use of solvent or volatilization by pyrolysis.

Multifunctional catalyst systems that combine acid and metals are required to perform all of these tasks. It is difficult to balance the activity components of these systems to produce the optimum results. Higher temperatures promote gasification to low value carbon oxides and acids, which require additional process steps to reform them to liquid fuels. Long contact times can allow recombination reactions. The most common approach to this problem is to perform reactions in different zones without interstage separation. Catalysts which promote conversion at low temperature are highly desirable.

Zeolite catalysts have revolutionized petroleum processing so it is not surprising they have received a lot of attention as potential biomass catalysts. ZSM-5 has proved to be the best of the commercial zeolites because of its deoxygenation activity, selectivity to lower molecular weight aromatics, and low coking properties. Recently attention has turned to the effects of adding lower-cost base metals to the zeolite to improve conversion and aromatic selectivity. Ni catalysts have received particular attention. A recent example is adding Ni to ZSM-5 to increase the yield of aromatic hydrocarbons while simultaneously increasing the conversion of oxygenates.

The effect of loading Zr, Co and Fe modified ZSM-5 as catalyst for treating the vapor phase from the pyrolysis of sawdust is being investigated. The effects of biomass pretreatments were also explored. It was found that the combined pretreatments and use of an Fe-ZSM-5 greatly improved aromatic yields compared to reaction with unmodified saw dust and ZSM-5.

Zeolite catalysts have some disadvantages in terms of limited hydrothermal stability and a propensity for the micropores to plug with coke or other deposits. Other solid catalysts are being investigated. Direct hydrodeoxygenation of raw woods into liquid alkanes with mass yields up to 28.1 wt% over a multifunctional Pt/NbOPO4 catalyst has been reported. These yields are particularly impressive because the theoretical yield after accounting for the oxygen loss is 50%. However, the use of precious metals may price this approach out of the market. Only slightly lower yields have been achieved using a commercial NiMo hydrotreating catalyst in a pressurized bubbling fluidized bed at high temperature (375-450^oC).

Developing a process based on this type metal on solid acid would require high-pressure systems. Even lower cost metals, like Ni or Mo, would significantly increase catalyst costs. A catalyst transport and regeneration method will likely be necessary.

The Bottom Line

Literature reports of catalysts provide interesting leads for future work. However, these systems need to be evaluated using commercial feedstocks at a reasonable scale for extended periods. The real potential of these new catalysts needs to be judged in the context in of an integrated biofuel plant and includes a techno economic analysis of the entire process. Enzymes can be advantageous because they are used at ambient temperatures and in water-based solvents. Lee Enterprise Consulting has the consultants with the commercial experience to evaluate the potential of these new materials and approaches.