Which aromatic will it be today?

Besides cellulose and hemicellulose one of the other main components in plants is the biopolymer lignin. It accounts for around 20 percent of plant biomass and as much as 40 percent of the energy contained therein. The energy content of one kilogram of lignin is comparable with that of coal. However, whereas fuels like bioethanol from cellulose have been a reality for a long time now, energy-rich lignin is often seen merely as troublesome waste. For instance, in the paper industry lignin is simply incinerated at the end of the production chain.

There are, however, ways of making high quality energy sources and basic chemicals from lignin. Today lignin is the only renewable source of aromatic hydrocarbons, also known as aromatics, which are used both as energy sources and as feedstock for plastics and man-made fibre production. The pathway from lignins to these aromatics normally involves pyrolysis oil.

During pyrolysis the lignin is converted without the admixture of further substances, in contrast to incineration or gasification. Pyrolysis oil is, therefore, obtained from lignin simply by heating without oxygen in the atmosphere. Unfortunately, the oil produced in this way is highly corrosive and difficult to store because of its high oxygen content. That’s why pyrolysis oil in its raw state can scarcely be used as fuel at all. Furthermore, it cannot be readily mixed with other liquid fuels. Use as an energy source is dependent on the oil being deoxygenated.

Up to now pyrolysis oil has not been broken down until after pyrolysis in a complicated separation procedure into its oxygen-free components, including aromatics. PSI researchers have now examined a number of catalysts that can be used to extract the derivatives of pyrolysis oil already during lignin pyrolysis, i.e. in one step.

Objective achieved in one step thanks to catalysis

Deoxygenation during pyrolysis is more efficient but it does require the use of a suitable catalyst. However, the search for selective catalysts for lignin pyrolysis, i.e. catalysts that enable the extraction of a specific oxygen-free product in as pure a form as possible, has not been particularly successful despite the efforts of numerous research groups around the world.

To this end, PSI researchers have taken a closer look at a number of catalysts based on zeolites. Zeolites are minerals that are mainly composed of aluminium, silica and oxygen. Chemists call them “solid acids”. The designation stems from the fact that zeolites, like certain acids, tend to release hydrogen nuclei (protons) which readily combine with oxygen to form water. Hence, zeolites help to strip oxygen from the pyrolysis oil components and rebind this oxygen in water.

Pure aromatics extracted with the help of zeolites

In their experiments the researchers observed that catalytic fast lignin pyrolysis over zeolites generates a very high yield of desirable oxygen-free substances, including liquid aromatics. The total yield was particularly high for the H-USY zeolite (ultrastable Y zeolite), a synthetically manufactured zeolite which is used in the harnessing of petrol and diesel from crude oil. The selectivity of catalytic conversion was also important for the scientists. This means the extent to which the catalyst used is suitable for the selective production of a specific product. This selectivity is relevant because it enables the generation of a very pure product in one step. Further separation phases are largely unnecessary. The H-USY zeolite has also proven its worth in terms of selectivity. It can be used to produce a group of important aromatics called BTX (benzene, toluene, xylene) in a highly selective manner. The entire product yield consisted in this case of approximately 40 percent BTX aromatics.

The selectivity was even better for the valuable BTX aromatics when the zeolites were impregnated with transition metals like nickel and then used as the catalyst. With the synthetically manufactured H-ZSM5 zeolite as the carrier for the nickel atoms, selectivity to the BTX aromatics was around 45 percent of the product yield. However, the total liquid yield did fall, too. The researchers likewise examined the catalytic effect of transition metal oxides. Here, too, they succeeded in increasing the selectivity to valuable lignin products. For instance with molybdenum oxide the selectivity to vanillin, the main component in vanilla aroma, could be pushed up to around 30 percent.

Even higher selectivity is definitely possible

The PSI scientists do not as yet fully understand the reasons for the elevated selectivity, in some cases, of the catalysts they examined. They do, however, think this selectivity could be enhanced even further through changes to the synthetically manufactured zeolites. In this context, the size of the pores in the zeolite crystals plays a very important role. If the pore size is adjusted to the size of the molecules created during the reactions, then the selectivity to this molecule is markedly increased.

Previous work at PSI has, however, already demonstrated that it is, in principle, possible to extract valuable products from lignin in just one step. One day this biomass resource, that has been underestimated up to now, could turn out to be more than a troublesome waste product of the paper industry.

Text: Leonid Leiva