Immobilizing enzymes for future fuels

Biofuels could provide a sustainable alternative to fossil fuels that are currently used to produce gasoline and diesel but their availability is hampered by the need for cheaper and more reliable production methods.

James Palmer reports on new techniques being developed at Louisiana Tech University that could lead to billion dollar savings.

According to the US Energy Information Administration, world energy usage in the transportation sector is second only to requirements in the industrial sector.

Liquid fuels dominate the energy usage in transportation. And, the high power density of chemical energy stored in liquid fuels and the convenient storage and filling of these fuels ensures that they will remain players in the transportation sector for the foreseeable future. Biofuels are seen as sustainable alternatives of fossil fuels that are currently used to produce gasoline and diesel.

Ethanol power

Ethanol is viewed as an obvious near-term solution for gasoline powered vehicles.

The automotive industry has the most experience with ethanol of all biofuels, with worldwide blending of 10 per cent or greater common in many countries and so-called Flex-Fuel Vehicles able to operate on 85 per cent ethanol.

Countries such as Brazil have a history of producing ethanol from sugar cane for these needs. The United States utilizes primarily corn for similar purposes. In both cases, the sugars from these plants are fermented to produce ethanol and then purified to produce fuel grade ethanol.

The challenge with both sources is the competition of fuel needs with food and the requirement of relatively valuable feedstock for the purposes of fuel. As such, an intense effort is underway worldwide to harness cellulosic raw materials to produce ethanol.

Breaking cellulose

Cellulose is a natural polymer of sugar molecules. If this polymer can be broken apart, then the resulting sugar can be fermented and processed similarly to traditional ethanol processes. If cellulosic materials can be utilized, then feedstock such as wood, switch-grass, or other waste biomass can be utilized to produce ethanol.

There are a variety of methods being explored by researchers to break apart these polymers without damaging the structure to useless byproducts. One of the more promising methods is to use enzymes to break apart the cellulose.

Enzymes are naturally occurring catalysts that will facilitate a chemical reaction to occur. Unfortunately enzymes are very specialized in the type of substrates they will react with and they are very expensive.

Companies such as Genecor and Novozyme genetically engineer enzymes to increase their catalytic activity on cellulose and are working hard to reduce their cost. These companies typically produce a soup of enzymes that will operate on a variety of different types of cellulose to produce sugars.

Enzymes are most effective if they are allowed to be “free” in the solution of the reaction. Unfortunately, the high cost of the enzyme makes it a challenge to produce an economically viable process when all of the expensive enzymes are lost in each batch of cellulose that is reacted to sugars.

Researchers throughout the years have immobilized enzymes onto surfaces to allow the enzymes to be separated from the product and reused several times.

Enzymes do lose their catalytic activity over time as their shape changes (they lose their conformation). Therefore, an entrapment method that is to be used industrially must allow for new enzymes to be reconstituted onto the surface. Prior immobilization techniques such as entrapment where the enzyme is simply physically mixed with a plastic do not allow the enzyme to be easily reconstituted.

The mixture of enzymes utilized in biofuels also presents a unique challenge. A number of researchers use specialized chemistry to covalently bond the enzyme to a surface. The use of these self-assembled monolayers is typically tailored to a very specific site on the enzyme.

Layer by Layer

Researchers at Louisiana Tech University are utilizing the layer-by-layer (LbL) nano self-assembly technique to avoid many of these challenges. The LbL technique can be easily reconstituted onto a surface and the nature of the electrostatic interactions allows a variety of different enzymes to be immobilized effectively onto a surface.

The LbL nano self-assembly technique uses charged polymers to coat successive layers onto a surface.

The process is described as being similar to Velcro. If a surface has one type of Velcro, for instance the fabric portion, and a large number of small pieces of the complementary hook portion are poured onto the surface then a thin layer will stick to one another. If the surface is held up and shaken, excess amount of the small hook pieces will fall down leaving the thin layer that now comprises of only the hook portion exposed. Next, a large number of small pieces of the fabric portion can be poured onto the surface and will again bond to one another in a thin layer. Excess amount of the fabric portion that did not attach to the surface can be shaken off and the process can be repeated. In this way, nano scale layers of catalysts can be built up on a surface.

Researchers at Louisiana Tech have demonstrated that enzyme layers can be built up with a complementary charged polymer such as polyethylenimine (PEI). With small molecule reactants, such as urea or organophosphorous pesticides, a linear increase in number of enzyme layers results in a corresponding increase in catalytic activity. Twenty bilayers of enzymes are twice as active as ten layers, and forty layers of enzymes are four times as active as ten layers. However a solid such as cellulose does not have the same mobility through the bilayers and therefore the first few bilayers on a surface appears to provide the most benefit.

As mentioned earlier, enzymes do lose a significant amount of their apparent catalytic activity when immobilized on a surface. This is an apparent decrease because the enzyme is typically as active but the reactant and enzyme have more difficulty coming together (a mass transfer resistance rather than a decrease in the enzymes kinetics).

An order of magnitude decrease in apparent kinetics is not unusual for immobilized enzymes. Regardless, if the enzyme can be successfully reused several times, immobilizing the enzyme can provide significant economic benefits to the overall system.

Cost savings

Estimates of impact were performed using data from a June 2002 NREL technical report NREL/TP-510-324388 “Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis of Corn Stover”.

In the 2002 report, a 20 year project life with a 10 per cent discount rate was assumed. The reactor costs were estimated to be approximately $3 million dollars (installed) while the annual cost of the cellulose was estimated to be $7 million dollars. If one assumes that the immobilized enzyme allows ten batches of ethanol to be produced, but requires a reaction time ten times longer than that estimated in the original 2002 report; the Net Present Value of the immobilized enzyme for a single ethanol plant would result in a savings of approximately $32 million dollars. The United States has a goal of achieving 16 billion gallons of ethanol by 2022. The total Net Present Value of cost savings for all of these plants if immobilized enzymes were used would be approximately $7.5 billion!

Immobilized enzymes

Clearly research work on immobilized enzymes is synergistic with activities to improve enzymes. The vision of this project is to coat the enzymes on low cost polymer beads that can be easily separated after a single batch of cellulose has been hydrolyzed into sugars.

Charged polymers such as polystyrene can be easily coated with the LbL process, but researchers at Tech have even coated neutral polymers such as the cheaper polyethylene.

There are several desirable factors that must be considered when choosing a carrier to be used for the immobilized enzyme. The carrier must facilitate removal from the reaction mixture, it must be easy to mix throughout a large tank, it must be physically strong enough to resist breaking apart in a mixing environment, and finally it must be cheap enough to be used on a large scale. Simple plastic beads fulfill these requirements, especially the ease of mixing as the buoyancy of the polymer can be modified through a variety of means.

Researchers at Tech have demonstrated that the LbL process can be performed on a completely automated pumping system which could easily be scaled-up for a commercial application.

The LbL nano self-assembly process is therefore poised to play a significant role in enhancing the economic viability of the cellulosic ethanol process.

Enzyme producers are gaining experience in fuels, but these companies have plans to diversify into a variety of products that they can produce. In the future, enzymes maybe used to produce a variety of other bio-chemicals for purposes other than fuels.

Effective immobilization methods that can be easily regenerated in a commercial environment may enable these expensive and specialized enzymes to become prevalent.

The synergy between nanotechnology, biotechnology, and traditional sciences are combining to help face the world energy challenges to ensure that our future 100 years are as dynamic and productive as our parents and grand-parents.

Dr James Palmer is an associate professor of engineering at Louisiana Tech University.