Catalysts and Enzymes in Biofuel Production

Biofuels Catalysts

Introduction

A critical mitigation strategy for the impact of fossil fuels on the environment is using biofuel catalysts to make biofuels from renewable sources for transportation. The problem is that biofuel production costs are high, and this nascent industry cannot compete with oil prices without subsidies. Many processes can produce biofuels, broadly categorized as biochemical and chemical. Biochemical processes utilize proteins called enzymes (biological catalysts). Chemical processes use chemical catalysts or heat to process raw materials into fuels. New technologies are invented and tested in laboratories regularly. However, making them industry-ready requires development work; the process/ chemical/ enzyme may not be appropriate because of cost. We describe some of the latest technological advances that can make biofuel production more economical.

Biochemical Processes

Biodiesel: The enzymes lipase and phospholipase are the significant players in biodiesel. 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 converts phospholipids to diacylglycerol, which becomes a substrate for the lipase. Conventional processes using methanol and catalysts must remove the FFAs and the phospholipids before reactions to improve the biodiesel quality. 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 ten years, intense investigations of issues surrounding the utilization of cellulosic feedstocks for biofuel production have been conducted. Although this process can theoretically utilize the tons of biomass available from farming and dedicated feedstocks, major conversion problems are encountered. One is the cost of the enzymes needed to deconstruct the cellulose and hemicellulose into usable sugar streams. Researchers have pursued several approaches, including changing the structure of the cell walls to lower digestion difficulty, 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 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 require a massive production infrastructure. 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 include multifunctional enzymes and combined bioprocessing organisms, which can decompose and ferment plant polymers into biofuels.

Chemical Catalysts

Chemical catalysis, like biofuel catalysts, has been the method of choice as biofuel catalysts for the efficient production of transportation fuels from fossil carbon sources. Hence, it is naturally 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: The first step is often a destructive biomass pretreatment with solid 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 technique dramatically improves the efficiency of enzymatic upgrading. Likely, 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 biofuel production’s complexity and cost. 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 contacting the biomass with the catalyst using solvent or pyrolysis volatilization.

Multifunctional catalyst systems that combine acids and metals are required to perform these tasks. It is challenging 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 that promote conversion at low temperatures are highly desirable.

Zeolite catalysts have revolutionized petroleum processing, so unsurprisingly, they have received much attention as potential biomass catalysts. ZSM-5 is the best commercial zeolite 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 increasing oxygenate conversion.

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

Zeolite catalysts have 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 output after accounting for the oxygen loss is 50%.  However, using 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 temperatures (375-450^oC).

Developing a process based on this type of metal on solid acid would require high-pressure systems. 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 on biofuel catalysts provide exciting leads for future work. However, these systems must 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.

For over 25 years, Lee Enterprises Consulting has assisted companies and investors with issues relating to bioenergy, biofuels, biomaterials and chemicals, biotechnologies, and feedstocks.   With over 150 consultants, we have the diverse expertise and geographical reach to assist in virtually any bioeconomy project worldwide. These seasoned professionals average over 30 years of industry experience.  Our ability to assemble these professionals into multidisciplinary teams allows us to fully integrate a project’s technical, scientific, and regulatory aspects, and combine them with years of hands-on experience.  Please take a look at our experts and the services we provide.  You will note that most of our experts are also available for ancillary engagements and advice for specialty engagements like serving as expert witnesses in litigation matters.  Call us at 1+ (501) 833-8511 or email us for more information.

See Enzyme Technologies, Enzyme Experts, and the Use of Enzymes in Textiles.