New MOF makes plastic manufacturing more energy efficient

RESEARCHERS have developed a metal-organic framework (MOF) containing iron-peroxo (Fe₂(O₂)) sites which could reduce the amount of energy required for extracting ethylene, the key ingredient in polyethylene plastic.

Ethylene is the largest feedstock in the petrochemical industry and 170m t of ethylene was manufactured worldwide in 2016. Ethylene is the molecule required for manufacturing polyethylene, the most common type of plastic. It is used to make most shopping bags and other everyday containers.

Ethylene is usually produced by steam cracking or thermal decomposition of ethane. An important production step is the separation of ethylene from other hydrocarbons in order to obtain polymer-grade ethylene. The bulk of the hydrocarbon mixture is made up of ethylene and ethane and separating those two hydrocarbons is by far the most energy-intensive step. Conventional ethylene separation is carried out via a cryogenic distillation process which uses temperatures below -100°C.

Sorption methods offer an energy efficient separation alternative that can be performed under ambient conditions. Using sorption methods for ethane/ethylene separation instead of the cryogenic process might reduce energy requirements by 20%.

Researchers in the US, China, and the Netherlands have developed a MOF which achieves efficient ethylene/ethane separation. The MOF separates the hydrocarbons by adsorbing ethane through iron-peroxo sites. This is preferable to previous adsorbents that preferentially adsorbed ethylene which then required subsequent desorption for use. Preferential adsorption of ethane instead of ethylene could reduce the energy needed for adsorption-based separation by about 40%.

Preferential ethane binding

Previous research carried out by a team working with the NCNR (Bloch et al. 2012) found that a framework called MOF-74 was good at separating a variety of hydrocarbons, including ethylene. Through open iron (Fe₂) sites MOF-74 can separate ethylene from ethane by ethylene adsorption. This MOF provided a starting point for the researchers.

The group aimed to find a way to bind alkanes, such as ethane, better than alkenes, such as ethylene. Alkanes are saturated hydrocarbons and alkenes are unsaturated hydrocarbons.

The group were inspired by natural metalloenzymes and synthetic compounds that activate the carbon-hydrogen (C-H) bond in alkanes. Activation of the C-H bond is a step required to enable C-H bond cleavage. These enzymes and compounds contain metals bound to oxygen groups. The researchers reasoned that similar oxygen-containing groups might enable better adsorption to ethane than to ethylene.

The researchers used the material developed by Bloch et al. modified to contain iron-peroxo sites. Binding experiments showed that the modified MOF-74 had greater affinity for ethane than ethylene and that interaction between the MOF and ethane is stronger than for previously reported MOFs. The modified MOF-74 was found to have higher selectivity than a wide-range of porous materials, including the previous best-performing MOF.

The researchers also investigated the binding mechanism to understand what allowed preferential binding. “Without the fundamental understanding of the mechanism, no one would believe our results,” Chen said. The experimentation helped the group to understand the interactions involved in achieving adsorption of ethane.

Ethane/ethylene separation

The researchers evaluated the separation performance of the modified MOF and found that it enabled clean and sharp separation of ethane and ethylene. In one adsorption cycle the modified MOF was able to achieve ≥99.99% pure ethylene from 50/50 ethane/ethylene mixtures. This is in comparison to other adsorbents which require four cycles to reach 99.95% (polymer-grade) ethylene purity.

Highly efficient separation was also achieved for the separation of ethylene from 10/90 ethane/ethylene mixtures and 10/87/1/1/1 ethane/ethylene/methane/hydrogen/acetylene mixtures.

Wei Zhou, a senior staff scientist at the National Institute of Standards and Technology (NIST) Centre for Neutron Research (NCNR), said that the MOF might still require additional work. “We proved this route is promising,” Zhou said, “but we’re not claiming our materials perform so well they can’t be improved. Our future goal is to dramatically increase their selectivity. It’s worth pursuing further.”

Bangling Chen, lead researcher and professor at the University of Texas at San Antonio (UTSA), said: “We are working on even cheaper while more stable MOF materials for such a very important separation.”

Science http://doi.org/cwr6