Porous Hyper-cross-linked Polymer and Its Use in Biorefining

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Polymer chain model

Polymers are formed by chains made of a repeating unit. Image by Benjah-bmm27

Could we make biomass plant refining not only less-expensive and more efficient, but easier on the environment?

Researchers from Aachen University (Germany) developed a new simple process to make hyper-cross-linked polymers (HCP) without using metal catalysts, with lower environmental impact.

HCPs have high surface area and hydrophobicity; they could, therefore, be used in biorefining, i.e. the separation of hydroxymethyl furfural (an important industrial chemical) from fructose (a renewable source).

Hyper-cross-linked Polymers

Hyper-cross-linked polymers (HCP) are materials formed by several polymeric chains (chains made of a repeating basic unit, a monomer) joined together in a tridimensional (3D) rigid porous structure.

HCPs have many technological applications, which include filtration and separation between different molecules. HCPs with tiny pores of nanometer dimensions are particularly interesting.

what is catalyst

Finding a more efficient catalyst can improve many chemical reactions. Copyright image by Decoded Science, all rights reserved.

HCP Preparation

Science makes HCPs via a polymerization reaction of the monomer using an appropriate catalyst; the most common ones are metal-based compounds such as iron chloride (FeCl3), aluminum chloride (AlCl3) and tin chloride (SnCl4).

When using these catalysts, however, these reactions do not comply with the principles of Green Chemistry; in fact, the processes generate relatively high amounts of waste. Moreover, the reactions have to be performed under a protective nitrogen atmosphere, while the reaction vessels need to be heated first and cooled down successively. All of this also increases the impact of these processes on the environment.

Innovative Synthesis

Because of these problems, scientists have been trying to find alternative, more environmentally-friendly processes to produce Hyper-cross-linked polymers.

Researchers from Aachen University (Germany) made important progresses in this field, as they developed a new method to make HCPs; their results were published in ChemSusChem in August 2015.

In their work the researchers replaced the FeCl3 catalyst with either sulfuric acid (H2SO4) or trifluoromethanesulfonic acid (CF3SO3H).

Simple chemistry

The new process is based on simple chemical reactions. Image by Master Tux

Simpler Process

Decoded Science spoke to Dr. Marcus Rose, one of the researchers involved in the study; he explains the process more in detail.

“To prepare our HCP we used 4,4’-bis-(chloromethyl)-1,1’-biphenyl (BCMBP) and its derivatives as a monomer starting material; we used the “traditional” preparation reaction, with FeCl3. At the same time, however, we performed the polymerization using CF3SO3H or H2SO4 as catalysts; both reactions were pretty simple, as they did not need a protective atmosphere.

We performed the CF3SO3H reaction at room temperature while the H2SO4 reaction needed some heating (about 80 oC); however, we had to maintain this temperature only for 45 minutes; the reaction with FeCl3, on the other hand, needed this temperature for about 18 hours.  

So the processes we developed are both simple, and they require much less energy than the conventional one; because of this, they could be easily scaled up at industrial level, with reasonable costs.”

Comparable Properties

According to Dr. Rose, the HCP prepared with these methods showed comparable, if not better, properties.

“The characteristics of the HCPs depended on the monomer we used for the synthesis. Overall, however, their properties were preserved, despite the preparation through a different route. The HCP prepared from BCMBP using CF3SO3H, for instance, showed very high surface area, comparable to that of the HCP prepared with the traditional method (1842 vs. 1874 m2 g-1. In addition, when CF3SO3H was used, the material was more hydrophobic.”

Use in Biorefining

The hydrophobic character is an important feature for these materials. As Dr. Rose said:

“One of the uses of HCPs can be in biorefining, to separate some of the apolar hydrophobic product(s) from the polar hydrophilic reaction phase.

An example of this is the production of 5-hydroxy methyl furfural (HMF), an important chemical in many industrial processes, from renewable sources – i.e. fructose present in biomass/plants. The separation of HMF from the reaction mixture is a crucial step, which greatly affects the efficiency and sustainability of the whole process.

The use of a hydrophobic porous solid material for a liquid-solid adsorption is an interesting possible solution.”

difference between adsorb absorb

That’s not a typo – adsorb is a word that non-chemistry majors may confuse with absorb, but the two processes are not the same. Copyright image by Decoded Science, all rights reserved

Better Selectivity

Dr. Rose and his colleagues tested the HCPs to separate HMF from fructose; their results were very interesting.

“We saw that the material prepared using CF3SO3H had a very high selectivity – i.e. it adsorbed mainly HMF and almost no fructose; in fact the ratio between the HMF and the fructose adsorbed (a) was much higher than that for the conventional material – 22.8 vs. 8.2.

For the material made with H2SO4, the HMF adsorbed was lower, due to the lower surface area. The material, however, was still more selective than the conventional one, as the a value was 11.3.”

More Sustainable HCP Production

The research of Dr. Rose and coworkers showed that it is possible to make hyper-cross-linked polymers (HCPs) through processes with a smaller impact on the environment.

Due to their high hydrophobicity, these HCPs could be used in the biorefining industry; in this way, they could make the production of some key chemical compounds more sustainable for the environment.

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