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Solid performance

Date: 2019-11-15 15:43:58.0
Author: Jon Evans

 

Danger: acid

Liquid acids are central to the production of both bioethanol and biodiesel. In bioethanol production, they catalyze the breakdown of biomass – and can also catalyze the conversion of cellulose to fermentable sugars, although enzymes are generally preferred for this task; in biodiesel production, they catalyze the transesterification reaction between methanol and fats or oils.

The use of liquid acids as catalysts is far from ideal, though: not only are they hazardous and polluting, but they can also be difficult to separate from the resulting products. This is inspiring scientists to try to develop solid versions of these acid catalysts, but they have so far struggled to create solid acid catalysts that are as effective as the liquid versions. Now, in two recent papers in the Journal of Chemical Technology and Biotechnology, separate teams of Chinese scientists report two novel approaches for enhancing the activity of solid acid catalysts, one for biodiesel production and one for bioethanol production.

The main reason why solid active catalysts aren’t as effective as liquid acids is that it’s obviously easier for a liquid acid to mix with the substrate, whether biomass or fats, enhancing the interaction between them. This can be partially offset by formulating the solid acid catalyst as tiny particles or porous materials, thereby giving them a large comparative surface area for interacting with the substrate.

As such, many solid acid catalysts are based on carbon-based materials such as mesoporous carbon, and carbon nanotubes and microspheres, which are usually produced by heating plant-based material under specific conditions. The resulting carbon-based material is then exposed to some variety of sulfuric acid to cover its surface in catalytic sulfonic acid (–SO3H) groups. Unfortunately, this modification process can reduce the porosity of the material while often only producing a thin, unstable covering of sulfonic acid groups, resulting in a solid acid catalyst that isn’t very effective or long-lasting.

To try to overcome these problems, Hong Yuan and his team at North Minzu University in Yinchuan decided to start with microspheres of tin dioxide (SnO2), which they then heated with glucose to coat them in a layer of carbon. Next, they covered these composite microspheres in sulfonic acid groups by either heating them with concentrated sulfuric acid or reacting them with a sulfanilic acid diazonium salt.

In both cases, this produced composite microspheres with lots of pores, and thus a high surface area, covered in a thick, stable layer of sulfonic acid groups. As the scientists report in their paper, these microspheres could catalyze the transesterification reaction between waste frying oil and methanol to produce biodiesel with a yield of 80–85%, which the scientists call a “remarkable catalytic performance”.

Another way to enhance the interaction between a solid acid catalyst and its substrate is to cover the catalyst with molecules that actively bind with the substrate. This was the approach taken by Qing Cao and his team at Taiyuan University of Technology when developing a solid acid catalyst for breaking down cellulose into sugars. They decided to base this catalyst on a carbon-based material called a magnetic mesocarbon microbead, which they had developed in a previous study by reacting coal tar pitch, a cheap by-product of coal processing, with iron oxide (Fe3O4). The advantage of using a magnetic material is that it should be easy to separate from the sugars.

To make the material catalytic, Cao and his team took the usual approach of covering its surface with sulfonic acid groups, which they did by heating it with chlorosulfonic acid. Before this, however, they reacted the material with aluminum chloride, in order to also cover the surface with chloride groups, which readily bind with cellulose.

As Cao and his team report in their paper, when they tested this solid acid catalyst on cellulose, they found that it had a high catalytic activity, able to convert the cellulose into various sugars, including glucose, with a yield of up to 69%. Furthermore, it could easily be separated from the sugars with a permanent magnet and re-used, with the yield only dropping to 60% after the catalyst had been used six times.


The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.


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